The tetracycline antibiotic minocycline alters inflammatory cytokine production post-TBI and post-AIS, thus improving functional and histological outcomes. Bye et al. examined minocycline in a closed head injury model of TBI in mice and observed that administration of minocycline acutely decreased lesion volume and functional deficits at 1 day post-injury [227]. In a study by Yang et al. examining minocycline administration post-AIS in spontaneously hypertensive rats, infarct size and degree of tissue loss/damage in the ischemic hemispheres were reduced as seen via magnetic resonance imaging and apparent diffusion coefficient mapping at two and 4 weeks post-AIS [228]. Minocycline treatment also reduced AIS-induced levels of TNF-α and IL-1β, and increased levels of TGF-β, IL-10, anti-inflammatory M2 microglia/ macrophage markers, as well as cerebral perfusion [228]. Small clinical studies of minocycline in AIS have shown a decrease in IL-6 levels at 24 h post-AIS [229] and have proven safety in AIS patients both with and without tPA administration [230]. Lampl et al. showed improvements in functional recovery as measured by the NIH stroke scale (NIHSS), modified Rankin scale (mRS) and Barthel index(BI), in patients started on 200 mg of minocycline for 5 days within 6–24 h of stroke onset [231]. Srivastava et al., utilizing the same treatment paradigm as Lampl et al., found improved mRS and BI scores at 3 months post-AIS in those treated with minocycline [232]. The same treatment paradigm was also utilized by Amiri-Nikpour et al. who reported improved NIHSS scores at 30, 60 and 90 days post-AIS in male patients receiving minocycline [233]. Kohler et al. utilized lower doses of minocycline (five 100 mg doses) and saw no improvement in NIHSS at 7 days nor in the mRS or BI at 90 days post-AIS [234].

Minocycline administration in multiple adult animal models of TBI has been shown to decrease activation of microglia, improve functional behavioral deficits such as spatial memory deficits and post-TBI anxiety [235], and decrease caspase activation and markers of neuroinflammation and damage [236]. In a study examining the use of minocycline in a blast injury model of TBI in rats, researchers noted a decrease in post-TBI anxiety via the elevated plus maze at 46 days post-injury accompanied by decreased corticosterone levels and improvements in spatial memory via Barnes maze testing post-injury as late as 47 days post-TBI [237]. Minocycline administration also decreased inflammatory and neuron and glial injury-associated markers c-reactive protein, monocyte-chemotactic protein-1, neuron-specific enolase, S100 β, tau, and neurofilament H in this study [237].

In contrast to findings in adult TBI models examining minocycline, in a neonatal model of TBI, minocycline did not improve functional deficits and actually worsened microglial activation and neurodegeneration, highlighting the influence of age on TBI pathophysiology and therapeutic selection [238]. In contrast, in a study of minocycline in neonatal ischemia, microglial activation was not affected, but lesion volume was reduced [239]. Minocycline was also shown to reduce apoptosis and excitotoxicity in neonatal hypoxic-ischemic injury in rats, suggesting a more extensive use for minocycline in ischemic pathologies [240].

To establish the use of minocycline in focal embolic stroke with comorbidities, Type 1 diabetic rats underwent embolic stroke and were given minocycline with or without tPA. It was observed that compared with treatments of saline or tPA alone, minocycline plus tPA combination therapy significantly reduced brain infarction, intracerebral hemorrhage, and hemispheric swelling at 24 h after stroke. The combination also significantly suppressed stroke-induced elevations in plasma levels of MMP-9 and IL-1β up to 24 h after stroke (57).

#### **4.2. Biological response modifiers in TBI and AIS**

IL-1R antagonists are currently being examined in AIS and TBI both pre-clinically as discussed above and clinically. A meta-analysis analyzing IL-1R antagonists in rodent models of stroke, reported an overall decrease in infarct volumes and an improvement in functional outcomes [241]. A recent cross-laboratory study of subcutaneous IL-1R antagonist treatment in AIS also found consistent decreases in neurologic deficits and lesion volume across preclinical models of AIS in multiple laboratories [121]. Pradillo et al. demonstrated that post-AIS subcutaneous administration of IL-1Ra in old rats with comorbidities and in young rats increased neurogenesis and functional outcomes in both populations [242]. IL-1R antagonists have also been proven pre-clinically to reach therapeutic levels via intranasal administration, decreasing IL-1β and TNF-α levels post-stroke in a rat model [243]. IL-1R antagonists compete with IL-1β and IL-1α for IL-1R binding, preventing downstream pro-inflammatory cascades from being activated, thus exerting a neuroprotective effect. Emsley et al. showed the safety of a recombinant human IL-1R antagonist administered post-AIS which also incidentally showed lower levels of IL-6 and peripheral inflammation, in addition to improved clinical outcomes [244]. A phase II clinical trial examining the use of subcutaneous IL-1R antagonist, anakinra, initially administered within 6 h post-stroke with repeat dosing every 12 h for a total of 6 injections over 72 h (ISRCTN74236229) has recently been completed. In TBI, as previously stated, Helmy et al. showed the safety of IL-1R antagonists, but actually reported an increase in the pro-inflammatory M1 microglia phenotype, acknowledging the variation in the immune response depending on the mechanism of injury [109]. The off-label perispinal administration of the anti-TNF-α monoclonal antibody, etanercept in post-stroke cognitive dysfunction and TBI have demonstrated benefit as well [245]. Studies by Chio et al. have demonstrated that post-TBI i.p. administration of etanercept in the fluid percussion model of brain injury in the rat improves motor deficits at 7 days and increases markers of neurogenesis [246], decreases acute TBI-induced rises in glutamate, the lactate/pyruvate ratio, and improves injury-induced motor deficits [247], cognitive deficits in the passive avoidance task [248], and the severity of ischemia at 3 days post-injury [247, 248]. A major barrier to the efficacy of etanercept as a TNF-α inhibitor for AIS and TBI is its poor BBB permeability due to its large size. Therefore, formulations of TNF decoy receptors with better BBB penetrance have been designed and tested in both TBI and AIS. Sumbria et al. show the utility of cTfRMAb-TNFR fusion protein (carboxy terminal transferrin receptor monoclonal antibody-TNF receptor), a genetically engineered monoclonal antibody against the mouse transferrin receptor found in the BBB linked to the TNF receptor to achieve whole molecule transfer into the brain, post-AIS. The administration of intravenous cTfRMAb-TNFR at 45 min post-AIS in mice resulted in significant decreases in subcortical, cortical, and hemispheric stroke volume, in addition to a 54% reduction in neurologic deficits at 24 h and 7 days post-AIS [249]. Clausen et al. examined the use of a dominant-negative inhibitor of soluble TNF called XPro1595 versus etanercept which inhibits both soluble and transmembrane TNF, in a mouse model of AIS. While infarct size was not affected, XPro1595 administration resulted in a decrease in granulocyte influx into the infarct, and improvements in motor and somatosensory function as measured by symmetrical grip strength, rotarod performance, and horizontal rod slip testing [250]. Simultaneously, the acute phase response in the liver was decreased by etanercept administration, indicating the importance of transmembrane TNF-α on the peripheral immune response [250]. TNF-α inhibitors, 3,6′ dithiothalidomide, TNF-α binding protein, and pentoxifylline have also been explored to achieve adequate TNF-α inhibition post-brain injury [69, 72, 251] and post-AIS [252–255], demonstrating improved histologic and behavioral outcomes. More recently, a third generation thalidomide, pomalidomide, was also shown to decrease neuronal cell death and curb neuroinflammation via a large decrease in TNF-α concentration, in addition to improving motor and sensory functional deficits when administered as late as 5 h post-TBI in rats [256]. Additional biologic TNF-α inhibitors, such as infliximab and adalimumab, currently approved for the treatment of inflammatory and autoimmune diseases, such as rheumatoid arthritis, may prove beneficial in AIS and TBI. B-cells have been detected in the brain post-injury due to increased BBB permeability, permitting their infiltration into the brain parenchyma, and have been shown to mediate inflammation in an animal model of stroke resulting in delayed post-stroke cognitive impairment [257]. An analog of the biologic B-cell inhibitor drug, Rituximab, has been shown to decrease B-cell infiltration across the BBB and improve post-stroke cognitive impairment when administered to mice at 5 days post-stroke with biweekly doses for 7 weeks [257].

#### **4.3. Hormonal modulation**

In contrast to findings in adult TBI models examining minocycline, in a neonatal model of TBI, minocycline did not improve functional deficits and actually worsened microglial activation and neurodegeneration, highlighting the influence of age on TBI pathophysiology and therapeutic selection [238]. In contrast, in a study of minocycline in neonatal ischemia, microglial activation was not affected, but lesion volume was reduced [239]. Minocycline was also shown to reduce apoptosis and excitotoxicity in neonatal hypoxic-ischemic injury in rats, suggesting

To establish the use of minocycline in focal embolic stroke with comorbidities, Type 1 diabetic rats underwent embolic stroke and were given minocycline with or without tPA. It was observed that compared with treatments of saline or tPA alone, minocycline plus tPA combination therapy significantly reduced brain infarction, intracerebral hemorrhage, and hemispheric swelling at 24 h after stroke. The combination also significantly suppressed stroke-induced elevations in plasma levels of MMP-9 and IL-1β up to 24 h after stroke (57).

IL-1R antagonists are currently being examined in AIS and TBI both pre-clinically as discussed above and clinically. A meta-analysis analyzing IL-1R antagonists in rodent models of stroke, reported an overall decrease in infarct volumes and an improvement in functional outcomes [241]. A recent cross-laboratory study of subcutaneous IL-1R antagonist treatment in AIS also found consistent decreases in neurologic deficits and lesion volume across preclinical models of AIS in multiple laboratories [121]. Pradillo et al. demonstrated that post-AIS subcutaneous administration of IL-1Ra in old rats with comorbidities and in young rats increased neurogenesis and functional outcomes in both populations [242]. IL-1R antagonists have also been proven pre-clinically to reach therapeutic levels via intranasal administration, decreasing IL-1β and TNF-α levels post-stroke in a rat model [243]. IL-1R antagonists compete with IL-1β and IL-1α for IL-1R binding, preventing downstream pro-inflammatory cascades from being activated, thus exerting a neuroprotective effect. Emsley et al. showed the safety of a recombinant human IL-1R antagonist administered post-AIS which also incidentally showed lower levels of IL-6 and peripheral inflammation, in addition to improved clinical outcomes [244]. A phase II clinical trial examining the use of subcutaneous IL-1R antagonist, anakinra, initially administered within 6 h post-stroke with repeat dosing every 12 h for a total of 6 injections over 72 h (ISRCTN74236229) has recently been completed. In TBI, as previously stated, Helmy et al. showed the safety of IL-1R antagonists, but actually reported an increase in the pro-inflammatory M1 microglia phenotype, acknowledging the variation in the immune response depending on the mechanism of injury [109]. The off-label perispinal administration of the anti-TNF-α monoclonal antibody, etanercept in post-stroke cognitive dysfunction and TBI have demonstrated benefit as well [245]. Studies by Chio et al. have demonstrated that post-TBI i.p. administration of etanercept in the fluid percussion model of brain injury in the rat improves motor deficits at 7 days and increases markers of neurogenesis [246], decreases acute TBI-induced rises in glutamate, the lactate/pyruvate ratio, and improves injury-induced motor deficits [247], cognitive deficits in the passive avoidance task [248], and the severity of ischemia at 3 days

a more extensive use for minocycline in ischemic pathologies [240].

**4.2. Biological response modifiers in TBI and AIS**

234 Mechanisms of Neuroinflammation

Hormonal intervention also impacts post-injury cytokine expression and has been examined in AIS and TBI. Pre-clinically, progesterone administration post-TBI and post-AIS has been shown to decrease edema, lesion size, excitotoxicity, apoptosis, free radical production, microglial activation and pro-inflammatory cytokines like TNF-α and IL-1β, while promoting remyelination and short-term functional preservation [258, 259]. However, these benefits have not been able to translate clinically. While the pilot phase II ProTECT trial (progesterone for traumatic brain injury, experimental clinical treatment) and a randomized controlled trial of progesterone in severe TBI in China showed improved outcomes and decreased mortality in TBI patients, the larger phase III SyNAPSe trial (study of a neuroprotective agent, progesterone in severe traumatic brain injury) and phase III PROTECT trial showed no benefit [260–263]. Progesterone has not been tested clinically in AIS patients.

#### **4.4. Omega-3 fatty acids**

Administration of omega-3 fatty acids in pre-clinical animal studies of TBI and AIS have been demonstrated to decrease post-injury neuronal death, curb increases in pro-inflammatory cytokine IL-1β and caspase-1 and functional deficits post-TBI [264]. In TBI, pre-injury administration of omega-3 fatty acids in rats' diet and via gavage resulted in decreases in neuronal cell death, edema, lesion volume, caspase-1, IL-18, IL-6 and IL-1β, as well as functional deficits in MWM and beam balance; these effects were largely mediated via G-protein coupled receptor 40 (GPR40), as blockage of this receptor reversed these benefits [264]. In patients, fish consumption has been correlated to decreased cerebrovascular disease risk, but omega-3 fatty acid supplementation alone has not been associated with decreased risk [265]. However, in a study of spontaneously hypertensive rats at increased risk for stroke, glucose utilization and cerebral perfusion were improved with omega-3 fatty acid administration [266]. A clinical trial in Japan examining statin therapy in combination with the omega-3 fatty acid, eicosapentaenoic acid, versus statin therapy alone, demonstrated a decreased incidence of recurrent stroke of 20% within 5 years [267].

#### **4.5. N-acetylcysteine**

N-acetylcysteine has been shown to have anti-inflammatory and antioxidant actions, increasing glutathione synthesis to scavenge ROS, decreasing IL-1β and TNF-α levels in brain injury, decreasing caspase-3 levels, and shifting microglia towards M2 anti-inflammatory phenotypes [268–271].

A pre-clinical experiment evaluating the modulation of oxidative stress with N-acetylcysteine and selenium treatments in TBI demonstrated that use of these treatments affected the oxidant and antioxidant, pro- and anti-inflammatory cytokines balance in rats by both downregulating IL-1β, a pro-inflammatory cytokine, and up-regulating IL-4, an anti-inflammatory cytokine [272]. N-acetylcysteine administered 30 min post-TBI in rats resulted in improved MWM performance to near sham-injured levels [273]. When administered 60 min post-weight drop injury in mice, functional deficits in novel object recognition and Y-maze were significantly improved [273]. In combination with minocycline, N-acetylcysteine has been shown to act synergistically to reduce TBI-induced demyelination, augment M2 microglia activation in white matter and modulate TBI-induced neuroinflammation, increasing microglial activation yet decreasing the number of injury-induced phagocytic CD68+ macrophages in the corpus callosum. The combination also improves learning and long-term retention in the active place avoidance task [274]. Clinically, N-acetylcysteine has been tested in a double-blind, placebo controlled study of blast-induced mild TBI; mild TBI symptoms included balance dysfunction, headache, hearing loss, neurocognitive dysfunction and confusion. Patients receiving N-acetylcysteine within 24 h of blast injury had fewer to none of these symptoms on day 7 post-treatment with 86% of treated patients experiencing complete symptom resolution by day 7 versus 42% in those receiving placebo [275].

N-acetylcysteine has also shown efficacy in AIS. In a rat model of AIS, N-acetylcysteine administration reduced infarct volume, apoptosis, as well as TNF-α, IL-1β, and iNOS expression [276]. Functionally, rats receiving N-acetylcysteine had improved motor function [275]. Additional studies showed reduced levels of AIS-induced hippocampal cell death with N-acetylcysteine administration [277] and decreased ischemia evoked levels of Nuclear factor kappaB [271].

#### **4.6. Cannabinoids**

progesterone in severe traumatic brain injury) and phase III PROTECT trial showed no ben-

Administration of omega-3 fatty acids in pre-clinical animal studies of TBI and AIS have been demonstrated to decrease post-injury neuronal death, curb increases in pro-inflammatory cytokine IL-1β and caspase-1 and functional deficits post-TBI [264]. In TBI, pre-injury administration of omega-3 fatty acids in rats' diet and via gavage resulted in decreases in neuronal cell death, edema, lesion volume, caspase-1, IL-18, IL-6 and IL-1β, as well as functional deficits in MWM and beam balance; these effects were largely mediated via G-protein coupled receptor 40 (GPR40), as blockage of this receptor reversed these benefits [264]. In patients, fish consumption has been correlated to decreased cerebrovascular disease risk, but omega-3 fatty acid supplementation alone has not been associated with decreased risk [265]. However, in a study of spontaneously hypertensive rats at increased risk for stroke, glucose utilization and cerebral perfusion were improved with omega-3 fatty acid administration [266]. A clinical trial in Japan examining statin therapy in combination with the omega-3 fatty acid, eicosapentaenoic acid, versus statin therapy alone, demonstrated a decreased incidence of recurrent

N-acetylcysteine has been shown to have anti-inflammatory and antioxidant actions, increasing glutathione synthesis to scavenge ROS, decreasing IL-1β and TNF-α levels in brain injury, decreasing caspase-3 levels, and shifting microglia towards M2 anti-inflammatory pheno-

A pre-clinical experiment evaluating the modulation of oxidative stress with N-acetylcysteine and selenium treatments in TBI demonstrated that use of these treatments affected the oxidant and antioxidant, pro- and anti-inflammatory cytokines balance in rats by both downregulating IL-1β, a pro-inflammatory cytokine, and up-regulating IL-4, an anti-inflammatory cytokine [272]. N-acetylcysteine administered 30 min post-TBI in rats resulted in improved MWM performance to near sham-injured levels [273]. When administered 60 min post-weight drop injury in mice, functional deficits in novel object recognition and Y-maze were significantly improved [273]. In combination with minocycline, N-acetylcysteine has been shown to act synergistically to reduce TBI-induced demyelination, augment M2 microglia activation in white matter and modulate TBI-induced neuroinflammation, increasing microglial activation yet decreasing the number of injury-induced phagocytic CD68+ macrophages in the corpus callosum. The combination also improves learning and long-term retention in the active place avoidance task [274]. Clinically, N-acetylcysteine has been tested in a double-blind, placebo controlled study of blast-induced mild TBI; mild TBI symptoms included balance dysfunction, headache, hearing loss, neurocognitive dysfunction and confusion. Patients receiving N-acetylcysteine within 24 h of blast injury had fewer to none of these symptoms on day 7 post-treatment with 86% of treated patients experiencing complete symptom resolution by

efit [260–263]. Progesterone has not been tested clinically in AIS patients.

**4.4. Omega-3 fatty acids**

236 Mechanisms of Neuroinflammation

stroke of 20% within 5 years [267].

day 7 versus 42% in those receiving placebo [275].

**4.5. N-acetylcysteine**

types [268–271].

In a closed head injury model of TBI in rats, Dexanabinol (HU-211), a synthetic cannabinoid that inhibits TNF-α as well as NMDA receptors and free radical proliferation has been shown to reduce BBB breakdown, edema, and functional deficits [278]. Dexanabinol has also been shown to improve deficits in animal models of AIS [279]. In TBI patients, a phase II clinical trial examining the administration of i.v Dexanabinol within 6 h post-injury was observed to improve rises in intracranial pressure 2–3 days post-injury with non-significant increases in Glasgow outcome scores at 6 months in patients with more severe injuries at presentation [280]. However, in the larger phase III trial of Dexanabinol for severe TBI, no benefits were seen, indicating the need for larger sample sizes and well-defined exclusion criteria and outcome measures to detect true effects [281]. Despite the failure of Dexanabinol, other cytokinemodulating cannabinoids continue to be evaluated and show potential utility in TBI and AIS. The endocannabinoid, 2-arachidonoylglycerol, has been shown to decrease pro-inflammatory cytokine expression, decrease breakdown of the BBB and post-injury edema pre-clinically in a closed head injury model of TBI [282, 283].

#### **5. Brain inflammation and brain injury–conclusions**

This chapter has focused on the role of select cytokines in the pathophysiology of TBI and AIS. By reviewing pre-clinical and clinical studies from the TBI and AIS literature, one can identify hypotheses that have been successfully confirmed in patient populations and identify gaps in the translation of pre-clinical observations to the hospital wards. In translating concepts from preclinical studies one must be cognizant of the variability in the patient populations examined. For example, the impact of the TNF-α polymorphisms identified in Caucasians and Asians would likely impact the efficacy of TNF-α targeting treatments in these populations. Common cytokine gene polymorphisms should continue to be studied in detail to understand variations in the AIS and TBI-induced inflammatory responses in different patient groups [284]. Various mechanisms of TBI ranging from blunt injury to blast versus penetrating injury also induce distinct cytokine responses as described above. Therefore pre-clinical trials of therapeutics as well as the spatiotemporal characterization of cytokine expression should examine multiple modalities of injury and ischemic insults (embolic versus thrombotic). Effects noted consistently across multiple models of TBI and AIS are more likely to translate clinically.

The pathophysiology of AIS and TBI have significant overlap with similar cytokine roles postinjury. Using genetic knockout studies, the particular role of cytokines in these conditions has

**Figure 3. Summary of cytokines reviewed**. Knockout or inhibition of TNF-α, IL-1β, IFN-γ, and IL-18 improved outcomes in AIS and TBI. TNF-α can induce cell survival and proliferation, but has been demonstrated to also induce apoptosis via TNFR2 and TNFR1. IFN-γ can induce neuro- and oligodendro-genesis at low concentrations, while higher concentrations cause leukocyte adhesion, arteriosclerosis, and increase the ratio of Th1:Th2 cells via upregulation of IP-10. IFN-γ, IL-18, and IL-1β promote TNF-α expression. IL-18 causes apoptosis, MMP and PMN release and activation, while IL-1β promotes heat shock protein release and leukocyte adhesion. In contrast, addition or overexpression of IL-10, IL-33, IL-6, and IL-4 improves outcomes. IL-6 decreases TNF-α and IL-1β, while promoting angiogenesis and astrogliosis. IL-33 leads to iNOS release and anti-inflammatory Th2 cells and microglia, in addition to the promotion of IL-10 and IL-4. IL-10 decreases brain injury and the cytokines IL-1, IL-8, IL-6, TNF-α, and IFN-γ, while inhibiting nitric oxide, reactive oxygen species, and leukocyte adhesion, as well as apoptosis and caspase-3. IL-4 increases BDNF, anti-inflammatory microglia, and decreases TNF-α expression. Abbreviations: BDNF, brain-derived neurotrophic factor, IP-10, interferon gamma-induced protein 10 (i.e. C-X-C motif chemokine 10), iNOS, inducible nitric oxide synthase, MMP, metalloproteinase, NO, nitric oxide, PMN, polymorphonuclear cells, Th1 cell, T-helper cell type 1, Th2 cell, T-helper cell type 2, TNFR2, TNF-α receptor 2, TNFR1, TNF-α receptor 1, ROS, reactive oxygen species.

been well defined. However, modulation of cytokine expression just prior to injury, rather than from conception is more informative to the molecule's role in AIS and TBI. Knockout or inhibition of TNF-α, IL-1β, IFN-γ, and IL-18 improved outcomes (see **Figure 3**). In contrast, addition or overexpression of IL-10, IL-33, IL-6, and IL-4 improves AIS and TBI outcomes (see **Figure 3**).

Treatments blocking pro-inflammatory and upregulating anti-inflammatory cytokines through receptor inhibition, synthesis inhibition versus induction, neutralizing antibodies, and inflammatory response modulators have shown promise. The ratio of pro-versus anti-inflammatory cytokines, as well as baseline cytokine levels may allow one to gauge the overall progression of injury in AIS and TBI to better intervene or establish prognoses. Computational modeling of cytokine patterns and interactions in AIS and TBI of different severities with or without co-morbidities or genetic predispositions may help to further predict how changing one or more variables can impact overall pathophysiology and prognosis.

#### **Author details**

Rachelle Dugue, Manan Nath, Andrew Dugue and Frank C. Barone\*

\*Address all correspondence to<EMAIL_ADDRESS>
Departments of Neurology and Physiology & Pharmacology, SUNY Downstate Medical Center, Brooklyn, NY, USA

#### **References**

been well defined. However, modulation of cytokine expression just prior to injury, rather than from conception is more informative to the molecule's role in AIS and TBI. Knockout or inhibition of TNF-α, IL-1β, IFN-γ, and IL-18 improved outcomes (see **Figure 3**). In contrast, addition or overexpression of IL-10, IL-33, IL-6, and IL-4 improves AIS and TBI outcomes (see

**Figure 3. Summary of cytokines reviewed**. Knockout or inhibition of TNF-α, IL-1β, IFN-γ, and IL-18 improved outcomes in AIS and TBI. TNF-α can induce cell survival and proliferation, but has been demonstrated to also induce apoptosis via TNFR2 and TNFR1. IFN-γ can induce neuro- and oligodendro-genesis at low concentrations, while higher concentrations cause leukocyte adhesion, arteriosclerosis, and increase the ratio of Th1:Th2 cells via upregulation of IP-10. IFN-γ, IL-18, and IL-1β promote TNF-α expression. IL-18 causes apoptosis, MMP and PMN release and activation, while IL-1β promotes heat shock protein release and leukocyte adhesion. In contrast, addition or overexpression of IL-10, IL-33, IL-6, and IL-4 improves outcomes. IL-6 decreases TNF-α and IL-1β, while promoting angiogenesis and astrogliosis. IL-33 leads to iNOS release and anti-inflammatory Th2 cells and microglia, in addition to the promotion of IL-10 and IL-4. IL-10 decreases brain injury and the cytokines IL-1, IL-8, IL-6, TNF-α, and IFN-γ, while inhibiting nitric oxide, reactive oxygen species, and leukocyte adhesion, as well as apoptosis and caspase-3. IL-4 increases BDNF, anti-inflammatory microglia, and decreases TNF-α expression. Abbreviations: BDNF, brain-derived neurotrophic factor, IP-10, interferon gamma-induced protein 10 (i.e. C-X-C motif chemokine 10), iNOS, inducible nitric oxide synthase, MMP, metalloproteinase, NO, nitric oxide, PMN, polymorphonuclear cells, Th1 cell, T-helper cell type 1, Th2 cell, T-helper cell type 2, TNFR2, TNF-α receptor 2, TNFR1, TNF-α receptor 1, ROS, reactive oxygen

Treatments blocking pro-inflammatory and upregulating anti-inflammatory cytokines through receptor inhibition, synthesis inhibition versus induction, neutralizing antibodies, and

**Figure 3**).

species.

238 Mechanisms of Neuroinflammation

[24] Spalletta G, Cravello L, Imperiale F, Salani F, Bossú P, Picchetto L, Cao M, Rasura M, Pazzelli F, Orzi F. Neuropsychiatric symptoms and interleukin-6 serum levels in acute stroke. The Journal of Neuropsychiatry and clinical Neurosciences. 2013;**25**:255-263

[11] Feigin VL, Forouzanfar MH, Krishnamurthi R, Mensah GA, Connor M, Bennett DA, Moran AE, Sacco RL, Anderson L, Truelsen T. Global and regional burden of stroke during 1990-2010: Findings from the Global Burden of Disease Study 2010. The Lancet.

[12] Mozaffarian D, Benjamin EJ, Go AS, Arnett DK, Blaha MJ, Cushman M, Das SR, de Ferranti S, Després J-p, Fullerton HJ. Executive summary: Heart disease and stroke statistics-2016 update: A report from the American Heart Association. Circulation. 2016;

[14] Feigin VL, Lawes CM, Bennett DA, Anderson CS. Stroke epidemiology: A review of population-based studies of incidence, prevalence, and case-fatality in the late 20th cen-

[15] Iadecola C, Anrather J. The immunology of stroke: From mechanisms to translation.

[16] Shichita T, Ito M, Morita R, Komai K, Noguchi Y, Ooboshi H, Koshida R, Takahashi S, Kodama T, Yoshimura A. MAFB prevents excess inflammation after ischemic stroke by accelerating clearance of damage signals through MSR1. Nature Medicine.

[17] Deb P, Sharma S, Hassan K. Pathophysiologic mechanisms of acute ischemic stroke: An overview with emphasis on therapeutic significance beyond thrombolysis.

[18] Garden D, Granger D. Pathophysiology of ischemia-reperfusion injury. The Journal of

[19] Swardfager W, Herrmann N, Andreazza A, Swartz R, Khan M, Black S, Lanctôt K. Poststroke neuropsychiatric symptoms: Relationships with IL-17 and oxidative stress.

[20] Swardfager W, Winer DA, Herrmann N, Winer S, Lanctôt KL. Interleukin-17 in poststroke neurodegeneration. Neuroscience & Biobehavioral Reviews. 2013;**37**:436-447 [21] Ormstad H, Aass HCD, Amthor K-F, Lund-Sørensen N, Sandvik L. Serum cytokine and glucose levels as predictors of poststroke fatigue in acute ischemic stroke patients.

[22] Pascoe MC, Crewther SG, Carey LM, Crewther DP. Inflammation and depression: Why poststroke depression may be the norm and not the exception. International Journal of

[23] Wohleb ES, Patterson JM, Sharma V, Quan N, Godbout JP, Sheridan JF. Knockdown of interleukin-1 receptor type-1 on endothelial cells attenuated stress-induced neuroinflammation and prevented anxiety-like behavior. Journal of Neuroscience.

BioMed Research International. Article ID:<PHONE_NUMBER>;**2014**:1-6

[13] Corbyn Z. A growing global burden. Nature. 2014;**510**:S2

tury. The Lancet Neurology. 2003;**2**:43-53

Nature Medicine. 2011;**17**:796-808

Pathophysiology. 2010;**17**:197-218

Journal of Neurology. 2011;**258**:670-676

Pathology. 2000;**190**:255

Stroke. 2011;**6**:128-135

2014;**34**:2583-2591

2014;**383**:245-255

240 Mechanisms of Neuroinflammation

2017;**23**:723-732

**133**:447

[39] Mosser DM. The many faces of macrophage activation. Journal of Leukocyte Biology.

[40] Filiano AJ, Gadani SP, Kipnis J. How and why do T cells and their derived cytokines affect the injured and healthy brain? Nature Reviews Neuroscience. 2017;**18**:375-384

[41] Fassbender K, Rossol S, Kammer T, Daffertshofer M, Wirth S, Dollman M, Hennerici M. Proinflammatory cytokines in serum of patients with acute cerebral ischemia: Kinetics of secretion and relation to the extent of brain damage and outcome of disease. Journal

[42] Tarkowski E, Rosengren L, Blomstrand C, Wikkelsö C, Jensen C, Ekholm S, Tarkowski A. Intrathecal release of pro- and anti-inflammatory cytokines during stroke. Clinical &

[43] Vila N, Castillo J, Dávalos A, Chamorro A. Proinflammatory cytokines and early neuro-

[44] Acalovschi D, Wiest T, Hartmann M, Farahmi M, Mansmann U, Auffarth GU, Grau AJ, Green FR, Grond-Ginsbach C, Schwaninger M. Multiple levels of regulation of the inter-

[45] Smith CJ, Emsley HC, Gavin CM, Georgiou RF, Vail A, Barberan EM, Del Zoppo GJ, Hallenbeck JM, Rothwell NJ, Hopkins SJ. Peak plasma interleukin-6 and other peripheral markers of inflammation in the first week of ischaemic stroke correlate with brain infarct volume, stroke severity and long-term outcome. BMC Neurology. 2004;**4**:2 [46] Di Napoli M, Schwaninger M, Cappelli R, Ceccarelli E, Di Gianfilippo G, Donati C, Emsley HC, Forconi S, Hopkins SJ, Masotti L. Evaluation of C-reactive protein measurement for assessing the risk and prognosis in ischemic stroke. Stroke. 2005;**36**:

[47] Sotgiu S, Zanda B, Marchetti B, Fois ML, Arru G, Pes GM, Salaris F, Arru A, Pirisi A, Rosati G. Inflammatory biomarkers in blood of patients with acute brain ischemia.

[48] Smith CJ, Emsley HC, Vail A, Georgiou RF, Rothwell NJ, Tyrrell PJ, Hopkins SJ. Variability of the systemic acute phase response after ischemic stroke. Journal of the

[49] Nagyőszi P, Nyúl-Tóth Á, Fazakas C, Wilhelm I, Kozma M, Molnár J, Haskó J, Krizbai IA Regulation of NOD-like receptors and inflammasome activation in cerebral endothe-

[50] Walsh JG, Muruve DA, Power C. Inflammasomes in the CNS. Nature Reviews

[51] Abulafia DP, de Rivero Vaccari JP, Lozano JD, Lotocki G, Keane RW, Dietrich WD. Inhibition of the inflammasome complex reduces the inflammatory response after thromboembolic stroke in mice. Journal of Cerebral Blood Flow & Metabolism.

logical worsening in ischemic stroke. Stroke. 2000;**31**:2325-2329

2003;**73**:209-212

242 Mechanisms of Neuroinflammation

1316-1329

of the Neurological Sciences. 1994;**122**:135-139

Experimental Immunology. 1997;**110**:492-499

leukin-6 system in stroke. Stroke. 2003;**34**:1864-1869

European Journal of Neurology. 2006;**13**:505-513

lial cells. Journal of Neurochemistry. 2015;**135**:551-564

Neurological Sciences. 2006;**251**:77-81

Neuroscience. 2014;**15**:84-97

2009;**29**:534-544

[77] Stahel PF, Kariya K, Shohami E, Barnum SR, Eugster H-P, Trentz O, Kossmann T, Morganti-Kossmann MC. Intracerebral complement C5a receptor (CD88) expression is regulated by TNF and lymphotoxin-α following closed head injury in mice. Journal of Neuroimmunology. 2000;**109**:164-172

[65] Ziebell JM, Morganti-Kossmann MC. Involvement of pro-and anti-inflammatory cytokines and chemokines in the pathophysiology of traumatic brain injury. Neuro-

[66] Taupin V, Toulmond S, Serrano A, Benavides J, Zavala F. Increase in IL-6, IL-1 and TNF levels in rat brain following traumatic lesion: Influence of pre-and post-traumatic treatment with Ro5 4864, a peripheral-type (p site) benzodiazepine ligand. Journal of

[67] Shohami E, Novikov M, Bass R, Yamin A, Gallily R. Closed head injury triggers early production of TNF-α and IL-6 by brain tissue. Journal of Cerebral Blood Flow &

[68] Knoblach SM, Fan L, Faden AI. Early neuronal expression of tumor necrosis factor-α after experimental brain injury contributes to neurological impairment. Journal of

[69] Baratz R, Tweedie D, Rubovitch V, Luo W, Yoon JS, Hoffer BJ, Greig NH, Pick CG. Tumor necrosis factor-α synthesis inhibitor, 3, 6'-dithiothalidomide, reverses behavioral impairments induced by minimal traumatic brain injury in mice. Journal of Neuro-

[70] Csuka E, Morganti-Kossmann MC, Lenzlinger PM, Joller H, Trentz O, Kossmann T. IL-10 levels in cerebrospinal fluid and serum of patients with severe traumatic brain injury: Relationship to IL-6, TNF-α, TGF-β1 and blood-brain barrier function. Journal of

[71] Frugier T, Morganti-Kossmann MC, O'Reilly D, McLean CA. In situ detection of inflammatory mediators in post mortem human brain tissue after traumatic injury. Journal of

[72] Shohami E, Bass R, Wallach D, Yamin A, Gallily R. Inhibition of tumor necrosis factor alpha (TNF-α) activity in rat brain is associated with cerebroprotection after closed head

[73] Scherbel U, Raghupathi R, Nakamura M, Saatman KE, Trojanowski JQ, Neugebauer E, Marino MW, McIntosh TK. Differential acute and chronic responses of tumor necrosis factor-deficient mice to experimental brain injury. Proceedings of the National Academy

[74] Bachstetter AD, Rowe RK, Kaneko M, Goulding D, Lifshitz J, Van Eldik LJ. The p38α MAPK regulates microglial responsiveness to diffuse traumatic brain injury. Journal of

[75] Pavlovski D, Thundyil J, Monk PN, Wetsel RA, Taylor SM, Woodruff TM. Generation of complement component C5a by ischemic neurons promotes neuronal apoptosis. The

[76] Van Beek J, Bernaudin M, Petit E, Gasque P, Nouvelot A, MacKenzie ET, Fontaine M. Expression of receptors for complement anaphylatoxins C3a and C5a following permanent focal cerebral ischemia in the mouse. Experimental Neurology. 2000;**161**:373-382

injury. Journal of Cerebral Blood Flow & Metabolism. 1996;**16**:378-384

therapeutics. 2010;**7**:22-30

244 Mechanisms of Neuroinflammation

Neuroimmunology. 1993;**42**:177-185

Neuroimmunology. 1999;**95**:115-125

Neuroimmunology. 1999;**101**:211-221

Metabolism. 1994;**14**:615-619

chemistry. 2011;**118**:1032-1042

Neurotrauma. 2010;**27**:497-507

of Sciences. 1999;**96**:8721-8726

Neuroscience. 2013;**33**:6143-6153

FASEB Journal. 2012;**26**:3680-3690

[104] Tehranian R, Andell-Jonsson S, Beni SM, Yatsiv I, Shohami E, Bartfai T, Lundkvist J, Iverfeldt K. Improved recovery and delayed cytokine induction after closed head injury in mice with central overexpression of the secreted isoform of the interleukin-1 receptor antagonist. Journal of Neurotrauma. 2002;**19**:939-951

[90] Pan W, Ding Y, Yu Y, Ohtaki H, Nakamachi T, Kastin AJ. Stroke upregulates TNFα transport across the blood-brain barrier. Experimental Neurology. 2006;**198**:222-233 [91] Um J-Y, An N-H, Kim H-M. TNF-α and TNF-β gene polymorphisms in cerebral infarc-

[92] Kumar P, Misra S, Kumar A, Pandit AK, Chakravarty K, Prasad K. Association between tumor necrosis factor-α (-238G/A and-308G/A) gene polymorphisms and risk of isch-

[93] Waters RJ, Murray GD, Teasdale GM, Stewart J, Day I, Lee RJ, Nicoll JA. Cytokine gene polymorphisms and outcome after traumatic brain injury. Journal of Neurotrauma.

[94] Allan SM, Tyrrell PJ, Rothwell NJ. Interleukin-1 and neuronal injury. Nature Reviews

[96] Swaroop S, Sengupta N, Suryawanshi AR, Adlakha YK, Basu A. HSP60 plays a regulatory role in IL-1β-induced microglial inflammation via TLR4-p38 MAPK axis. Journal

[97] Rooker S, Jander S, Reempts JV, Stoll G, Jorens PG, Borgers M, Verlooy J. Spatiotemporal pattern of neuroinflammation after impact-acceleration closed head injury in the rat.

[98] Kamm K, VanderKolk W, Lawrence C, Jonker M, Davis AT. The effect of traumatic brain injury upon the concentration and expression of interleukin-1β and interleu-

[99] Knoblach SM, Faden AI. Cortical interleukin-1β elevation after traumatic brain injury in the rat: No effect of two selective antagonists on motor recovery. Neuroscience Letters.

[100] Fan L, Young PR, Barone FC, Feuerstein GZ, Smith DH, McIntosh TK.  Molecular

[101] Lu K-T, Wang Y-W, Yang J-T, Yang Y-L, Chen H-I. Effect of interleukin-1 on traumatic brain injury-induced damage to hippocampal neurons. Journal of Neurotrauma.

[102] Lawrence CB, Allan SM, Rothwell NJ. Interleukin-1β and the interleukin-1 receptor antagonist act in the striatum to modify excitotoxic brain damage in the rat. European

[103] Yang G-Y, Zhao Y-J, Davidson BL, Betz AL.  Brain Research.

kin-10 in the rat. Journal of Trauma and Acute Care Surgery. 2006;**60**:152-157

[95] Dinarello CA. Biologic basis for interleukin-1 in disease. Blood. 1996;**87**:2095-2147

tion. Journal of Molecular Neuroscience. 2003;**21**:167-171

Mediators of Inflammation. Article ID:90123 2006;**2006**: 1-6

emic stroke: A meta-analysis. Pulse. 2015;**3**:217-228

2013;**30**:1710-1716

246 Mechanisms of Neuroinflammation

2000;**289**:5-8

2005;**22**:885-895

1997;**751**:181-188

Immunology. 2005;**5**:629

of Neuroinflammation. 2016;**13**:27

Brain Research. 1995;**30**:125-130

Journal of Neuroscience. 1998;**10**:1188-1195

[127] Sheu J-R, Chen Z-C, Jayakumar T, Chou D-S, Yen T-L, Lee H-N, Pan S-H, Hsia C-H, Yang C-H, Hsieh C-Y. A novel indication of platonin, a therapeutic immunomodulating medicine, on neuroprotection against ischemic stroke in mice. Scientific Reports. Article:42277 2017;**7**:1-14

[115] Park MJ, Sohrabji F. The histone deacetylase inhibitor, sodium butyrate, exhibits neuroprotective effects for ischemic stroke in middle-aged female rats. Journal of

[116] Pearson VL, Rothwell NJ, Toulmond S. Excitotoxic brain damage in the rat induces interleukin-1β protein in microglia and astrocytes: Correlation with the progression of

[117] Boxer PA, Bigge CF. Mechanisms of neuronal cell injury/death and targets for drug

[118] Allan S, Lawrence C, Rothwell N. Interleukin-1β and interleukin-1 receptor antagonist do not affect glutamate release or calcium entry in rat striatal synaptosomes. Molecular

[119] Clausen BH, Lambertsen KL, Babcock AA, Holm TH, Dagnaes-Hansen F, Finsen B. Interleukin-1beta and tumor necrosis factor-alpha are expressed by different subsets of microglia and macrophages after ischemic stroke in mice. Journal of Neuroinflammation.

[120] Shreeniwas R, Koga S, Karakurum M, Pinsky D, Kaiser E, Brett J, Wolitzky B, Norton C, Plocinski J, Benjamin W. Hypoxia-mediated induction of endothelial cell interleukin-1 alpha. An autocrine mechanism promoting expression of leukocyte adhesion molecules

[121] Maysami S, Wong R, Pradillo JM, Denes A, Dhungana H, Malm T, Koistinaho J, Orset C, Rahman M, Rubio M. A cross-laboratory preclinical study on the effectiveness of interleukin-1 receptor antagonist in stroke. Journal of Cerebral Blood Flow & Metabolism.

[122] Relton JK, Rothwell NJ. Interleukin-1 receptor antagonist inhibits ischaemic and excitotoxic neuronal damage in the rat. Brain Research Bulletin. 1992;**29**:243-246

[123] Hara H, Friedlander RM, Gagliardini V, Ayata C, Fink K, Huang Z, Shimizu-Sasamata M, Yuan J, Moskowitz MA. Inhibition of interleukin 1β converting enzyme family proteases reduces ischemic and excitotoxic neuronal damage. Proceedings of the National

[124] Loddick SA, MacKenzie A, Rothwell NJ. An ICE inhibitor, z-VAD-DCB attenuates isch-

[125] Friedlander RM, Gagliardini V, Hara H, Fink KB, Li W, MacDonald G, Fishman MC, Greenberg AH, Moskowitz MA, Yuan J. Expression of a dominant negative mutant of interleukin-1β converting enzyme in transgenic mice prevents neuronal cell death induced by trophic factor withdrawal and ischemic brain injury. Journal of Experimental

[126] Lin R, Cai J, Kostuk EW, Rosenwasser R, Iacovitti L. Fumarate modulates the immune/ inflammatory response and rescues nerve cells and neurological function after stroke

aemic brain damage in the rat. Neuroreport. 1996;**7**:1465-1468

in rats. Journal of Neuroinflammation. 2016;**13**:269

on the vessel surface. Journal of Clinical Investigation. 1992;**90**:2333

Neuroinflammation. 2016;**13**:300

cell death. Glia. 1999;**25**:311-323

Psychiatry. 1998;**3**:178-182

2008;**5**:46

248 Mechanisms of Neuroinflammation

2016;**36**:596-605

Academy of Sciences. 1997;**94**:2007-2012

Medicine. 1997;**185**:933-940

intervention. Drug Discovery Today. 1997;**2**:219-228

[152] Yilmaz G, Arumugam TV, Stokes KY, Granger DN. Role of T lymphocytes and interferon-γ in ischemic stroke. Circulation. 2006;**113**:2105-2112

[139] Mallat Z, Corbaz A, Scoazec A, Besnard S, Lesèche G, Chvatchko Y, Tedgui A. Expression of interleukin-18 in human atherosclerotic plaques and relation to plaque instability.

[140] Yuen C-M, Chiu C-A, Chang L-T, Liou C-W, Lu C-H, Youssef AA, Yip H-K. Level and value of interleukin-18 after acute ischemic stroke. Circulation Journal. 2007;**71**:1691-1696

[141] Welsh P, Lowe GD, Chalmers J, Campbell DJ, Rumley A, Neal BC, MacMahon SW, Woodward M. Associations of proinflammatory cytokines with the risk of recurrent

[142] Rosenman SJ, Shrikant P, Dubb L, Benveniste EN, Ransohoff RM. Cytokine-induced expression of vascular cell adhesion molecule-1 (VCAM-1) by astrocytes and astrocy-

[143] Shaked I, Tchoresh D, Gersner R, Meiri G, Mordechai S, Xiao X, Hart R, Schwartz M. Protective autoimmunity: Interferon-γ enables microglia to remove glutamate without

evoking inflammatory mediators. Journal of Neurochemistry. 2005;**92**:997-1009 [144] Kawanokuchi J, Mizuno T, Takeuchi H, Kato H, Wang J, Mitsuma N, Suzumura A. Production of interferon-y by microglia. Multiple Sclerosis Journal. 2006;**12**:558-564

[145] Holmin S, Höjeberg B. In situ detection of intracerebral cytokine expression after

[146] Yan EB, Satgunaseelan L, Paul E, Bye N, Nguyen P, Agyapomaa D, Kossmann T, Rosenfeld JV, Morganti-Kossmann MC. Post-traumatic hypoxia is associated with prolonged cerebral cytokine production, higher serum biomarker levels, and poor outcome in patients with severe traumatic brain injury. Journal of Neurotrauma. 2014;**31**:618-629

[147] Holmin S, Schalling M, Höjeberg B, Sandberg Nordqvist A-C, Skeftruna A-K, Mathiesen T. Delayed cytokine expression in rat brain following experimental contusion. Journal

[148] Wei HH, Lu X-CM, Shear DA, Waghray A, Yao C, Tortella FC, Dave JR. NNZ-2566 treatment inhibits neuroinflammation and pro-inflammatory cytokine expression induced by experimental penetrating ballistic-like brain injury in rats. Journal of

[149] Williams AJ, Wei HH, Dave JR, Tortella FC. Acute and delayed neuroinflammatory response following experimental penetrating ballistic brain injury in the rat. Journal of

[150] Lagraoui M, Latoche JR, Cartwright NG, Sukumar G, Dalgard CL, Schaefer BC. Controlled cortical impact and craniotomy induce strikingly similar profiles of inflammatory gene expression, but with distinct kinetics. Frontiers in Neurology. Article: 155

[151] Tomimoto H, Akiguchi I, Wakita H, Kinoshita A, Ikemoto A, Nakamura S, Kimura J. Glial expression of cytokines in the brains of cerebrovascular disease patients. Acta

toma cell lines. The Journal of Immunology. 1995;**154**:1888-1899

human brain contusion. Neuroscience Letters. 2004;**369**:108-114

Circulation. 2001;**104**:1598-1603

250 Mechanisms of Neuroinflammation

stroke. Stroke. 2008;**39**:2226-2230

of Neurosurgery. 1997;**86**:493-504

Neuroinflammation. 2009;**6**:19

Neuroinflammation. 2007;**4**:17

Neuropathologica. 1996;**92**:281-287

2012;**3**:1-14

[177] Shichita T, Ago T, Kamouchi M, Kitazono T, Yoshimura A, Ooboshi H. Novel therapeutic strategies targeting innate immune responses and early inflammation after stroke. Journal of Neurochemistry. 2012;**123**:29-38

[165] Knoblach SM, Faden AI. Interleukin-10 improves outcome and alters proinflammatory cytokine expression after experimental traumatic brain injury. Experimental

[166] Kremlev SG, Palmer C. Interleukin-10 inhibits endotoxin-induced pro-inflammatory cytokines in microglial cell cultures. Journal of Neuroimmunology. 2005;**162**:71-80

[167] de Waal Malefyt R, Abrams J, Bennett B, Figdor CG, De Vries JE. Interleukin 10 (IL-10) inhibits cytokine synthesis by human monocytes: An autoregulatory role of IL-10 pro-

[168] Mizuno T, Sawada M, Marunouchi T, Suzumura A. Production of interleukin-10 by mouse glial cells in culture. Biochemical and Biophysical Research Communications.

[169] Bell-Temin H, Culver-Cochran AE, Chaput D, Carlson CM, Kuehl M, Burkhardt BR, Bickford PC, Liu B, Stevens SM. Novel molecular insights into classical and alternative activation states of microglia as revealed by stable isotope labeling by amino acids in cell culture (SILAC)-based proteomics. Molecular & Cellular Proteomics. 2015;**14**:

[170] Colton CA. Heterogeneity of microglial activation in the innate immune response in the

[171] Xu H, Wang Z, Li J, Wu H, Peng Y, Fan L, Chen J, Gu C, Yan F, Wang L. The polarization states of microglia in TBI: A new paradigm for pharmacological intervention. Neural

[172] Bell MJ, Kochanek PM, Doughty LA, Carcillo JA, Adelson PD, Clark RS, Wisniewski SR, Whalen MJ, Dekosky ST. Interleukin-6 and interleukin-10 in cerebrospinal fluid after severe traumatic brain injury in children. Journal of Neurotrauma. 1997;**14**:

[173] Schneider Soares FM, Menezes de Souza N, Libório Schwarzbold M, Paim Diaz A, Costa Nunes J, Hohl A, Nunes Abreu da Silva P, Vieira J, Lisboa de Souza R, Moré Bertotti M, et al. Interleukin-10 is an independent biomarker of severe traumatic brain

[174] Yang Y, Liu H, Zhang H, Ye Q, Wang J, Yang B, Mao L, Zhu W, Leak RK, Xiao B. ST2/ IL-33-dependent microglial response limits acute ischemic brain injury. Journal of

[175] Pérez-de Puig I, Miró F, Salas-Perdomo A, Bonfill-Teixidor E, Ferrer-Ferrer M, Márquez-Kisinousky L, Planas AM. IL-10 deficiency exacerbates the brain inflammatory response to permanent ischemia without preventing resolution of the lesion. Journal of Cerebral

[176] Allaman I, Belanger M, Magistretti PJ. Astrocyte-neuron metabolic relationships: For

brain. Journal of Neuroimmune Pharmacology. 2009;**4**:399-418

injury prognosis. Neuroimmunomodulation. 2012;**19**:377-385

better and for worse. Trends in Neurosciences. 2011;**34**:76-87

Plasticity. Article ID: 5405104 2017;**2017**:1-11

Neuroscience. 2017:3233-3216

Blood Flow & Metabolism. 2013;**33**:1955-1966

duced by monocytes. Journal of Experimental Medicine. 1991;**174**:1209-1220

Neurology. 1998;**153**:143-151

252 Mechanisms of Neuroinflammation

1994;**205**:1907-1915

3173-3184

451-457

[205] Tilg H, Trehu E, Atkins MB, Dinarello CA, Mier JW. Interleukin-6 (IL-6) as an antiinflammatory cytokine: Induction of circulating IL-1 receptor antagonist and soluble tumor necrosis factor receptor p55. Blood. 1994;**83**:113-118

[190] Qian L, Yuanshao L, Wensi H, Yulei Z, Xiaoli C, Brian W, Wanli Z, Zhengyi C, Jie X, Wenhui Z. Serum IL-33 is a novel diagnostic and prognostic biomarker in acute isch-

[191] Liu J, Xing Y, Gao Y, Zhou C. Changes in serum interleukin-33 levels in patients with acute cerebral infarction. Journal of Clinical Neuroscience. 2014;**21**:298-300

[192] Luzina IG, Keegan AD, Heller NM, Rook GA, Shea-Donohue T, Atamas SP. Regulation of inflammation by interleukin-4: A review of "alternatives". Journal of Leukocyte

[193] Derecki NC, Cardani AN, Yang CH, Quinnies KM, Crihfield A, Lynch KR, Kipnis J. Regulation of learning and memory by meningeal immunity: A key role for IL-4.

[194] Gadani SP, Cronk JC, Norris GT, Kipnis J. IL-4 in the brain: A cytokine to remember.

[195] Ansari MA. Temporal profile of M1 and M2 responses in the hippocampus following early 24h of neurotrauma. Journal of the Neurological Sciences. 2015;**357**:41-49

[196] Butovsky O, Ziv Y, Schwartz A, Landa G, Talpalar AE, Pluchino S, Martino G, Schwartz M. Microglia activated by IL-4 or IFN-γ differentially induce neurogenesis and oligodendrogenesis from adult stem/progenitor cells. Molecular and Cellular Neuroscience.

[197] Zhou X, Spittau B, Krieglstein K. TGFβ signalling plays an important role in IL4 induced alternative activation of microglia. Journal of Neuroinflammation. 2012;**9**:210

[198] Liu X, Liu J, Zhao S, Zhang H, Cai W, Cai M, Ji X, Leak RK, Gao Y, Chen J. Interleukin-4 is essential for microglia/macrophage M2 polarization and long-term recovery after

[199] García-Berrocoso T, Giralt D, Bustamante A, Llombart V, Rubiera M, Penalba A, Boada C, Espadaler M, Molina C, Montaner J. Role of beta-defensin 2 and interleukin-4 recep-

tor as stroke outcome biomarkers. Journal of Neurochemistry. 2014;**129**:463-472 [200] Helmy A, De Simoni M-G, Guilfoyle MR, Carpenter KL, Hutchinson PJ. Cytokines and innate inflammation in the pathogenesis of human traumatic brain injury. Progress in

[201] Burda JE, Sofroniew MV. Reactive gliosis and the multicellular response to CNS dam-

[202] Erta M, Quintana A, Hidalgo J. Interleukin-6, a major cytokine in the central nervous

[203] Collingridge GL, Peineau S. Strippers reveal their depressing secrets: Removing AMPA

[204] Schindler R, Mancilla J, Endres S, Ghorbani R, Clark S, Dinarello CA. Correlations and interactions in the production of interleukin-6 (IL-6), IL-1, and tumor necrosis factor (TNF) in human blood mononuclear cells: IL-6 suppresses IL-1 and TNF. Blood. 1990;**75**:40-47

system. International Journal of Biological Sciences. 2012;**8**:1254-1266

emic stroke. Aging and Disease. 2016;**7**:614

Journal of Experimental Medicine. 2010;**207**:1067-1080

The Journal of Immunology. 2012;**189**:4213-4219

cerebral ischemia. Stroke. 2016;**47**:498-504

age and disease. Neuron. 2014;**81**:229-248

Neurobiology. 2011;**95**:352-372

receptors. Neuron. 2014;**82**:3-6

Biology. 2012;**92**:753-764

254 Mechanisms of Neuroinflammation

2006;**31**:149-160

[231] Lampl Y, Boaz M, Gilad R, Lorberboym M, Dabby R, Rapoport A, Anca-Hershkowitz M, Sadeh M. Minocycline treatment in acute stroke an open-label, evaluator-blinded study. Neurology. 2007;**69**:1404-1410

[218] Quintana A, Molinero A, Borup R, Nielsen FC, Campbell IL, Penkowa M, Hidalgo J. Effect of astrocyte-targeted production of IL-6 on traumatic brain injury and its impact

[219] Woiciechowsky C, Schöning B, Cobanov J, Lanksch WR, Volk H-D, Döcke W-D. Early IL-6 plasma concentrations correlate with severity of brain injury and pneumonia in brain-injured patients. Journal of Trauma and Acute Care Surgery. 2002;**52**:339-345 [220] McClain C, Cohen D, Phyllips R, Ott L, Young B. Increased plasma and ventricular interleukin-6 levels in head injured patients. The Journal of Laboratory and Clinical

[221] Ikonomidis I, Andreotti F, Economou E, Stefanadis C, Toutouzas P, Nihoyannopoulos P. Increased proinflammatory cytokines in patients with chronic stable angina and

[222] Dalla Libera AL, Regner A, De Paoli J, Centenaro L, Martins TT, Simon D. IL-6 polymorphism associated with fatal outcome in patients with severe traumatic brain injury.

[223] Lambertsen KL, Biber K, Finsen B. Inflammatory cytokines in experimental and human

[224] Loddick SA, Turnbull AV, Rothwell NJ. Cerebral interleukin-6 is neuroprotective during permanent focal cerebral ischemia in the rat. Journal of Cerebral Blood Flow &

[225] Herrmann O, Tarabin V, Suzuki S, Attigah N, Coserea I, Schneider A, Vogel J, Prinz S, Schwab S, Monyer H. Regulation of body temperature and neuroprotection by endogenous interleukin-6 in cerebral ischemia. Journal of Cerebral Blood Flow & Metabolism.

[226] Ding D. Endovascular mechanical thrombectomy for acute ischemic stroke: A new

[227] Bye N, Habgood MD, Callaway JK, Malakooti N, Potter A, Kossmann T, Morganti-Kossmann MC. Transient neuroprotection by minocycline following traumatic brain injury is associated with attenuated microglial activation but no changes in cell apopto-

[228] Yang Y, Salayandia VM, Thompson JF, Yang LY, Estrada EY, Yang Y. Attenuation of acute stroke injury in rat brain by minocycline promotes blood-brain barrier remodeling and alternative microglia/macrophage activation during recovery. Journal of

[229] Switzer JA, Sikora A, Ergul A, Waller JL, Hess DC, Fagan SC. Minocycline prevents IL-6 increase after acute ischemic stroke. Translational Stroke Research. 2012;**3**:364-368

[230] Fagan SC, Waller JL, Nichols FT, Edwards DJ, Pettigrew LC, Clark WM, Hall CE, Switzer JA, Ergul A, Hess DC. Minocycline to Improve Neurologic Outcome in Stroke

sis or neutrophil infiltration. Experimental Neurology. 2007;**204**:220-233

stroke. Journal of Cerebral Blood Flow & Metabolism. 2012;**32**:1677-1698

their reduction by aspirin. Circulation. 1999;**100**:793-798

on the cortical transcriptome. Developmental Neurobiology. 2008;**68**:195-208

Medicine.1991;**118**:225-230

256 Mechanisms of Neuroinflammation

Brain Injury. 2011;**25**:365-369

Metabolism. 1998;**18**:176-179

Neuroinflammation. 2015;**12**:26

(MINOS). Stroke. 2010;**41**:2283-2287

standard of care. Journal of Stroke. 2015;**17**:123

2003;**23**:406-415

[256] Wang J-Y, Huang Y-N, Chiu C-C, Tweedie D, Luo W, Pick CG, Chou S-Y, Luo Y, Hoffer BJ, Greig NH. Pomalidomide mitigates neuronal loss, neuroinflammation, and behavioral impairments induced by traumatic brain injury in rat. Journal of Neuroinflammation. 2016;**13**:168

[244] Emsley H, Smith C, Georgiou R, Vail A, Hopkins S, Rothwell N, Tyrrell P. A randomised phase II study of interleukin-1 receptor antagonist in acute stroke patients. Journal of

[245] Tobinick E, Kim NM, Reyzin G, Rodriguez-Romanacce H, DePuy V. Selective TNF inhibition for chronic stroke and traumatic brain injury: An observational study involving 629 consecutive patients treated with perispinal etanercept.  Mediators of inflammation. Article ID:<PHONE_NUMBER>;**2013**:1-9

[247] Chio C-C, Chang C-H, Wang C-C, Cheong C-U, Chao C-M, Cheng B-C, Yang C-Z, Chang C-P. Etanercept attenuates traumatic brain injury in rats by reducing early

microglial expression of tumor necrosis factor-α. BMC Neuroscience. 2013;**14**:33

[248] Chio C-C, Lin J-W, Chang M-W, Wang C-C, Kuo J-R, Yang C-Z, Chang C-P. Therapeutic evaluation of etanercept in a model of traumatic brain injury. Journal of Neurochemistry.

[249] Sumbria RK, Boado RJ, Pardridge WM. Brain protection from stroke with intravenous TNFα decoy receptor-trojan horse fusion protein. Journal of Cerebral Blood Flow &

[250] Clausen BH, Degn M, Martin NA, Couch Y, Karimi L, Ormhøj M, Mortensen M-LB, Gredal HB, Gardiner C, Sargent II. Systemically administered anti-TNF therapy ameliorates functional outcomes after focal cerebral ischemia. Journal of Neuroinflammation.

[251] Baratz R, Tweedie D, Wang J-Y, Rubovitch V, Luo W, Hoffer BJ, Greig NH, Pick CG. Transiently lowering tumor necrosis factor-α synthesis ameliorates neuronal cell loss and cognitive impairments induced by minimal traumatic brain injury in mice. Journal

[252] Yoon JS, Lee J-H, Tweedie D, Mughal MR, Chigurupati S, Greig NH, Mattson MP. 3, 6'-dithiothalidomide improves experimental stroke outcome by suppressing neuroin-

[253] de Barros Bruno R, Marques TF, Batista TMT, Lima JCSS, de Arruda KG, Lima PFSF, da Silva Santos N, Cunha GM, Vitor HVN, de Barros Viana GS. Pentoxifylline treatment improves neurological and neurochemical deficits in rats subjected to transient brain

[254] Vakili A, Mojarrad S, Akhavan MM, Rashidy-Pour A. Pentoxifylline attenuates TNF-α protein levels and brain edema following temporary focal cerebral ischemia in rats.

[255] Iwata N, Takayama H, Xuan M, Kamiuchi S, Matsuzaki H, Okazaki M, Hibino Y. Effects of etanercept against transient cerebral ischemia in diabetic rats. BioMed Research

flammation. Journal of Neuroscience Research. 2013;**91**:671-680

Neurology, Neurosurgery & Psychiatry. 2005;**76**:1366-1372

1051-1070

258 Mechanisms of Neuroinflammation

2010;**115**:921-929

2014;**11**:203

Metabolism. 2012;**32**:1933-1938

of Neuroinflammation. 2015;**12**:45

ischemia. Brain Research. 2009;**1260**:55-64

International. Article ID:<PHONE_NUMBER>;**2015**:1-10

Brain Research. 2011;**1377**:119-125

[280] Knoller N, Levi L, Shoshan I, Reichenthal E, Razon N, Rappaport ZH, Biegon A. Dexanabinol (HU-211) in the treatment of severe closed head injury: A randomized, placebo-controlled, phase II clinical trial. Critical Care Medicine. 2002;**30**:548-554

[268] Chen G, Shi J, Hu Z, Hang C. Inhibitory effect on cerebral inflammatory response following traumatic brain injury in rats: A potential neuroprotective mechanism of

[269] Hicdonmez T, Kanter M, Tiryaki M, Parsak T, Cobanoglu S. Neuroprotective effects of N-acetylcysteine on experimental closed head trauma in rats. Neurochemical Research.

[270] Choi S-H, Aid S, Kim H-W, Jackson SH, Bosetti F. Inhibition of NADPH oxidase promotes alternative and anti-inflammatory microglial activation during neuroinflamma-

[271] Shen W-H, Zhang C-Y, Zhang G-Y. Antioxidants attenuate reperfusion injury after global brain ischemia through inhibiting nuclear factor-kappa B activity in rats. Acta

[272] Şenol N, Nazıroğlu M, Yürüker V. N-acetylcysteine and selenium modulate oxidative stress, antioxidant vitamin and cytokine values in traumatic brain injury-induced rats.

[273] Eakin K, Baratz-Goldstein R, Pick CG, Zindel O, Balaban CD, Hoffer ME, Lockwood M, Miller J, Hoffer BJ. Efficacy of N-acetyl cysteine in traumatic brain injury. PloS One.

[274] Haber M, Baki SGA, Grin'kina NM, Irizarry R, Ershova A, Orsi S, Grill RJ, Dash P, Bergold PJ. Minocycline plus N-acetylcysteine synergize to modulate inflammation and prevent cognitive and memory deficits in a rat model of mild traumatic brain

[275] Hoffer ME, Balaban C, Slade MD, Tsao JW, Hoffer B. Amelioration of acute sequelae of blast induced mild traumatic brain injury by N-acetyl cysteine: A double-blind, placebo

[276] Khan M, Sekhon B, Jatana M, Giri S, Gilg AG, Sekhon C, Singh I, Singh AK. Administration of N-acetylcysteine after focal cerebral ischemia protects brain and reduces inflammation in a rat model of experimental stroke. Journal of Neuroscience

[277] Knuckey NW, Palm D, Primiano M, Epstein MH, Johanson CE.  Journal of Neuroimmunology. 1997;**72**:169-177

[279] Lavie G, Teicher A, Shohami E. Dexanabinol (HU-211) improves long-term outcome in rats subjected to transient focal brain ischemia. Abstracts - Society for Neuroscience.

tion. Journal of Neurochemistry. 2012;**120**:292-301

Pharmacologica Sinica. 2003;**24**:1125-1130

Neurochemical Research. 2014;**39**:685-692

injury. Experimental Neurology. 2013;**249**:169-177

controlled study. PloS One. 2013;**8**:e54163

Research. 2004;**76**:519-527

Stroke. 1995;**26**:305-311

2000;**30**:771

N-acetylcysteine. Mediators of Inflammation. Article ID:<PHONE_NUMBER>;**2008**:1-8

2006;**31**:473-481

260 Mechanisms of Neuroinflammation

2014;**9**:e90617

### **Role of Kynurenine Pathway in Glioblastoma**

Gustavo Ignacio Vázquez Cervantes, Nora Karen Olascoaga Arellano, Daniela Ramírez Ortega, Alelí Salazar Ramiro, Dinora F. González Esquivel, Camilo Ríos, Benjamín Pineda Olvera and Verónica Pérez de la Cruz

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68939

#### **Abstract**

In brain, the tryptophan degradation products through the kynurenine pathway exhibit neuromodulatory and inflammatory effects and have been related to the progression of neurodegenerative disorders, furthermore, their protagonism on the modulation of immune response and in cancer development has been reported. The immunosup‐ pressive role of kynurenines has been described on glioblastoma models. In patients, the elevated activity of indoleamine-2,3-dioxygenase (IDO) such as the increase of kyn‐ urenine/tryptophan ratio have been also reported, suggesting that activation of kynuren‐ ine pathway is present during glioblastoma formation and can be related with tumor progression. The importance of the kynurenine pathway during cancer development has encouraged recent studies to the use of IDO inhibitors as a therapeutic strategy for treatment of breast, lung and ovarian cancer, until to get its use in clinical trials. IDO inhibitors also have been used in in vitro and in vivo models of glioblastoma showing promising results. The effect of kynurenines on glioblastoma offer a new perspective about the tryptophan metabolism during cancer. Due to the relevance of the kynurenine pathway in brain homeostasis, immunomodulation and cancer, we discuss the relevance of the kynurenine pathway on the development of glioblastoma multiforme as well as a possible molecular target for glioblastoma treatment.

**Keywords:** glioblastoma, tryptophan catabolism, immune response, immunoedition

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1. Introduction**

Tryptophan (Trp) is considered as an essential amino acid, which is required for life and growth but is not synthesized by the organism. Although approximately 1% of trypto‐ phan dietary intake is used for protein synthesis, the rest 99% is metabolized through the 5‐hydroxyindole pathway for the production of melatonin and serotonin, decarboxylated to tryptamine, transaminated to indol-3-pyruvic acid, or oxidized by the so-named kynurenine pathway (KP), for the new synthesis of NAD+ and NADP+ (**Figure 1**) [1–3].

The KP degrades over 95% of whole tryptophan intake; its rate-limiting enzymes, tryptophan 2,3-dioxygenase (TDO), and indoleamine 2,3-dioxygenase (IDO), catalyze the cleavage of the tryptophan pyrrole ring to produce N-formylkynurenine [1, 4]. Among mammals, TDO is mainly expressed in liver and only uses l-tryptophan as a specific substrate; instead of that, IDO is expressed in extrahepatic tissues, such as brain, lung, kidney, and immune cells [1, 4]. Furthermore, IDO's substrate span is wider than TDO's, being d‐ or l‐tryptophan, d‐ or l-hydroxytryptophan, tryptamine, serotonin, and melatonine available for the catalytic activ‐ ity of IDO [5]. An IDO isoenzyme has been reported, IDO2 is expressed in the murine kid‐ ney, liver, and reproductive system, and the gene encoding IDO2 is adjacent to the IDO gene and has similar affinity for substrates than IDO [6, 7]. N-formylkynurenine is transformed

**Figure 1.** Tryptophan catabolism occurs by different pathways. Ninety-five percent of the dietary uptake of tryptophan is catabolized mainly by the kynurenine pathway. About 3% is decarboxylated to tryptamine or transaminated to indol-3-pyruvic acid, 1% is intended for serotonin and melatonin synthesis, and resting 1% is for protein synthesis.

**Figure 2.** The kynurenine pathway.

**1. Introduction**

264 Mechanisms of Neuroinflammation

Tryptophan (Trp) is considered as an essential amino acid, which is required for life and growth but is not synthesized by the organism. Although approximately 1% of trypto‐ phan dietary intake is used for protein synthesis, the rest 99% is metabolized through the 5‐hydroxyindole pathway for the production of melatonin and serotonin, decarboxylated to tryptamine, transaminated to indol-3-pyruvic acid, or oxidized by the so-named kynurenine

The KP degrades over 95% of whole tryptophan intake; its rate-limiting enzymes, tryptophan 2,3-dioxygenase (TDO), and indoleamine 2,3-dioxygenase (IDO), catalyze the cleavage of the tryptophan pyrrole ring to produce N-formylkynurenine [1, 4]. Among mammals, TDO is mainly expressed in liver and only uses l-tryptophan as a specific substrate; instead of that, IDO is expressed in extrahepatic tissues, such as brain, lung, kidney, and immune cells [1, 4]. Furthermore, IDO's substrate span is wider than TDO's, being d‐ or l‐tryptophan, d‐ or l-hydroxytryptophan, tryptamine, serotonin, and melatonine available for the catalytic activ‐ ity of IDO [5]. An IDO isoenzyme has been reported, IDO2 is expressed in the murine kid‐ ney, liver, and reproductive system, and the gene encoding IDO2 is adjacent to the IDO gene and has similar affinity for substrates than IDO [6, 7]. N-formylkynurenine is transformed

**Figure 1.** Tryptophan catabolism occurs by different pathways. Ninety-five percent of the dietary uptake of tryptophan is catabolized mainly by the kynurenine pathway. About 3% is decarboxylated to tryptamine or transaminated to indol-3-pyruvic acid, 1% is intended for serotonin and melatonin synthesis, and resting 1% is for protein synthesis.

pathway (KP), for the new synthesis of NAD+ and NADP+ (**Figure 1**) [1–3].

**Table 1.** Enzyme kinetics parameters for the kynurenine pathway enzymes.

#### **2. Role of kynurenine pathway in brain**

to l-kynurenine (l-kyn) by the arylformidase (AFMID); kynurenine aminotransferases (KAT I, KAT II), and kynureninase (KYNU), with pyridoxal-5′-phosphate as a cofactor, as well as kynurenine monooxygenase (KMO), which uses FAD as a cofactor, transform l‐kyn into kynurenic acid (Kyna), anthranilic acid (AA), or 3-hydroxykynurenine (3-HK), respectively [8–10]. KMO is located in mitochondrial outer membrane and it has been shown to have high affinity by the substrate suggesting that KMO metabolizes more l‐kyn than other enzymes [11]. KATs and KYNU can also use 3-HK as a substrate to produce xanthurenic acid (Xanth) or 3-hydroxyanthranilic acid (3HAA) [8, 12]. Oxidation of 3HAA by the 3HAA dioxygen‐ ase (3HAADO) produces 2-amino-3-carboxymuconate semialdehyde (Acms) which is trans‐ formed by non-enzymatic dehydration into quinolinic acid (Quin), or into picolinic acid (Pic) by the Acms decarboxylase (ACMSD). Finally, Quin phosphoribosyltransferase (QPRT) uses Quin as a substrate for the formation of nicotinamine-adenine-mononucleotide (NaMN), the nicotinamine-adenine-dinucleotide (NAD+) precursor (**Figure 2**) [13, 14]. Enzyme kinetics of the whole KP have been determined, giving a wide perspective of the kynurenine pathway

and its functionality [13], enzyme kinetics data of the KP are summarized in **Table 1**.

 Enzyme kinetics parameters for the kynurenine pathway enzymes.

3HAADO 3-hydroxyanthranilic acid

266 Mechanisms of Neuroinflammation

ACMSD 2-amino-3-carboxy

Although NAD+ is a final product of the KP, many of the intermediary metabolites, the socalled kynurenines, have demonstrated to exert many effects on cellular homeostasis and bioenergetics resulting on changes in organism behavior or immune response. In brain, Trp is transported through the blood‐brain barrier by the large neutral amino acids transporter [27]. The imbalance on the levels of its metabolites (kynurenines) has been related with the progression of neurodegenerative disorders, such as Huntington, Alzheimer, and Parkinson due to their redox nature and their capacity to exert neuromodulatory mechanisms [28].

On a redox perspective, both 3-HK and 3-HAA represent a double-edged sword. Reports show that 3-HK is able to promote cell death due to the increase in reactive oxygen species (ROS) production; also, 3-HK interaction with metallic ions promotes protein aggregation as well as cataract formation and toxicity in neuronal cultures without region selectivity [29–32] point‐ ing this kynurenine as a pro-oxidant and cytotoxic compound. In the same way, 3-HAA is able to cause protein damage due to interaction with metals producing OH•; furthermore, this metabolite causes oxidative phosphorylation uncoupling and decreased oxygen consumption [31, 33]. On the other hand, there are reports pointing to the antioxidant and protective side of these molecules, demonstrating a 3-HK-induced decrease of oxidative stress parameters; also, it has been observed that 3-HK acts as a free radical scavenger of radicals like O2 •− [34, 35]; the antioxidant activity of 3-HAA also has been reported showing free radical scavenging and ROS production decrease [36]. Finally, the protective effect of 3-HK and 3-HAA on the inhib‐ ited mitochondrial electron transport chain, as electron carriers, also has been reported [37].

On the other hand, Kyna is the only endogenous antagonist to N-methyl-d-aspartate (NMDA) receptors known so far, it inhibits the α7-nicotinic receptors, showing anticonvulsant effects on murine models as well as in human patients [38]. Recently, it was found that Kyna is a free radical scavenger of radicals, such as superoxide anion (O2 •−), hydroxyl radical (OH•), and peroxynitrite (ONOO−), besides it is able to reduce oxidative damage caused by pro-oxidants, so it is considered an endogenous antioxidant [39]. Furthermore, it has been shown that Kyna binds selectively to GPR35 receptor [40]; Kyna-mediated activation of GPR35 leads to intracel‐ lular Ca2+ influx, inositol phosphate production and, attenuates LPS-induced TNF-α release and reduces acetic acid-induced pain, suggesting that GPR35 could be mediating Kyna excit‐ atory, anti-inflammatory, and antinociceptive functions [40–42]. Contributing with the antiinflammatory response, Kyna also has been described as an agonist of the intracellular aryl hydrocarbon receptor (AHR); AHR translocation into the nucleus modulates the production of proinflammatory mediators and, as discussed below, inhibits T immune responses [43]. Because of Kyna is unable to cross the blood-brain barrier, systemic l‐kyn administration has been used to show the protective effect of Kyna in 6-hydroxidopamine-induced Parkinson disease models, hippocampal β-amyloid, and glutamate toxicity [44–46].

Contrasting with Kyna, Quin has been described as an agonist of NMDA receptors; in fact, it has been demonstrated that Quin is a pro-convulsive agent which generates mitochon‐ drial progressive dysfunction by reducing activity of respiratory complexes II and III [47], increases ROS production and lipid peroxidation in presence of Fe2+ [48], induces Ca2+ cel‐ lular influx increase, release of glutamate, and apoptosis [49–51]. Recently, the decrease in autophagy inhibition by Beclin-1 reported in human astrocytes and neurons has been attrib‐ uted to Quin toxicity [52]. Because of the selective damage that exerts in striatal spiny neurons with γ-amino butyric acid and substance P, Quin has also been a good model to mimic early symptoms of Huntington's disease [53–55]. In addition, increases in Quin concentrations were observed in the cerebral cortex of macaques infected with retroviruses, particularly those with local inflammatory lesions [56]. Moreover, Quin levels are increased in cerebrospinal fluid in the acquired immunodeficiency syndrome and in HIV patients; also in humans, Quin levels are elevated after traumatic brain injury [57].

#### **2.1. Inflammatory process and kynurenine pathway components**

Moreover, kynurenine pathway has been related to inflammatory processes. It has been shown that IDO, KMO, and KYNU expression is strongly induced by proinflammatory cyto‐ kines, mainly interferon‐ɣ (IFN-ɣ), interleukin 1 (IL1), interleukin 17 (IL17), or the tumor necrosis factor α (TNF-α) during bacterial and viral infections in immune privileged tissues, such as brain [58, 59]. Thus, activation of the KP by proinflammatory molecules inhibits bacte‐ rial proliferation through tryptophan depletion, avoids autoimmune responses by limiting T cell activation and recruiting of immune regulatory cells [60–62]. The immunoregulatory role of the KP could act as a double-edged sword, meanwhile it is inhibiting pathogen growth and regulating immune responses to avoid an autoimmune damage, the same activation during cancer supports tumor immune evasion and promotes tumor growth and invasiveness.

Beyond their role on neurodegenerative disorders, kynurenines have also been related with cancer progression in more than one way. It has been described that IDO expression is high in all thyroid carcinomas [63]. Moreover, l-kyn and Quin promote neoplastic cell proliferation *in vitro* and the formation of larger tumors *in vivo*, in a colorectal cancer model, through the Wnt/β-catenin signaling, despite of the ablation of IDO [64]. l‐kyn modulates the repairing enzyme DNA polymerase kappa, protecting tumor cells from DNA damage and propitiating genomic instability [65]. Also, Quin produced by microglial cells in glioma models confers tumor tolerance to oxidative stress due to the production of NAD+, promotes cell prolifera‐ tion by the modulation of the fibroblast growth factor-1 (FGF-1) release, and it is elevated in children with central nervous system tumors [66–68]. Kyna also promotes glioma cell prolif‐ eration through FGF-1 release [67]. The high expression and activity of the KP in the tumor microenvironment will be reflected on the maintenance of the NAD+/NADH supply, NAD+ and its reduced form NADH participate in several metabolic, redox, and stress response signals, providing favorable growing conditions for cancer cells. Finally, IDO expression and activity as well as the kynurenines promote cancer development by affecting the host's immune responsiveness. The effects of the KP on cancer immunosuppressive mechanisms will be detailed below.

#### **2.2. Cancer immunoedition**

All the cancer cells must possess a series of characters that allow them to develop tumors, these "hallmarks" let cancer cells to rapidly grow, evade cell death, migrate through the organism and colonize new tissues, modify their metabolic program, and to avoid immune destruction [69].

Before the establishment and development of a tumor, cancer cells must overcome elements of the immune system of the organism that can recognize, induce cellular signals, and finally, destroy exogenous agents or defective cells within the organism. In the early twentieth century, Paul Ehrlich formulated the idea that the immune system of an organism is able to recognize, destroy, and then protect against tumor cells. Later in the mid-twentieth century, the "immu‐ nosurveillance" hypothesis was then postulated by Sir Macfarlane Burnet and Lewis Thomas, based on this original idea and in experimental evidence of that, mouse lacking of interferon-γ responsiveness or with adaptive immunity defects, were susceptible to develop cancer [69, 70]. More recent evidence has demonstrated that the immune system not only protects against can‐ cer progression, immune cells, and signaling but also promotes tumor cell establishment, and furthermore, cancer cells modulate immune mechanisms favoring tumor progression, trans‐ forming the idea of "immunosurveillance" into the "cancer immunoediting" hypothesis [69–70].

The cancer immunoediting hypothesis postulates that cancer cells and immune system cre‐ ate a set of dynamical interactions during the formation of a tumor; these interactions are arranged in three stages of the "cancer immunoediting," or "The three E's hypothesis" pro‐ cess named: elimination, equilibrium, and escape [70]. During the elimination phase, cancer cells are recognized by the immune system and cytotoxic reactions induce cancer cell death; in the equilibrium phase, the cytotoxic mechanisms of the immune system act as selective pressure on tumor cells; cancer cells that survive, remain proliferating and establishing the tumor. Finally, at the escape phase, cancer cells are able to evade and suppress the immune mechanisms leading to the formation of tumors [70–72].

Cancer immunoediting mechanisms could be given by genetic alterations, proper of the can‐ cer cells, which induce overexpression and secretion of suppressive molecules, such as the transforming growth factor-β, the vascular endothelial growth factor, prostaglandin E2, and the soluble MHCI-related gene A; cancer cells also recruit immune regulatory cells like T regu‐ latory cells (Tregs), myeloid-derived suppressor cells (MDSC), and tolerogenic dendritic cells (TDC); and finally, by induction of immune checkpoint, inhibitory molecules, such as PD-L1 [73]. A second type of immunoediting is carried out by cytotoxic T cells of the own adaptive immune system, while exerting their protective role against cancer, T cells release cytokines like IFN-γ which can also induce the production of immune suppressive molecules by tumor cells, during the equilibrium phase of the "cancer immunoediting" [73]. Remarkably, IFN-γ is a strong inducer of IDO expression and activity which leads tryptophan metabolism through the KP in tumor microenvironment, and then to the triggering of the kynurenines' immuno‐ suppressive effects [73, 74]. In cultured human glioma, stimulation with IFN-γ significantly increased the expression of IDO-1, IDO-2, kynureninase, and kynurenine hydroxylase, which potentiate the KP; whereas significantly decreased 2-amino-3-carboxymuconate semialde‐ hyde decarboxylase (ACMSD) and kynurenine aminotransferase-I (KAT-I), which reduce the neuroprotective metabolites [75].

#### *2.2.1. The kynurenines in cancer immunoediting*

autophagy inhibition by Beclin-1 reported in human astrocytes and neurons has been attrib‐ uted to Quin toxicity [52]. Because of the selective damage that exerts in striatal spiny neurons with γ-amino butyric acid and substance P, Quin has also been a good model to mimic early symptoms of Huntington's disease [53–55]. In addition, increases in Quin concentrations were observed in the cerebral cortex of macaques infected with retroviruses, particularly those with local inflammatory lesions [56]. Moreover, Quin levels are increased in cerebrospinal fluid in the acquired immunodeficiency syndrome and in HIV patients; also in humans, Quin levels

Moreover, kynurenine pathway has been related to inflammatory processes. It has been shown that IDO, KMO, and KYNU expression is strongly induced by proinflammatory cyto‐ kines, mainly interferon‐ɣ (IFN-ɣ), interleukin 1 (IL1), interleukin 17 (IL17), or the tumor necrosis factor α (TNF-α) during bacterial and viral infections in immune privileged tissues, such as brain [58, 59]. Thus, activation of the KP by proinflammatory molecules inhibits bacte‐ rial proliferation through tryptophan depletion, avoids autoimmune responses by limiting T cell activation and recruiting of immune regulatory cells [60–62]. The immunoregulatory role of the KP could act as a double-edged sword, meanwhile it is inhibiting pathogen growth and regulating immune responses to avoid an autoimmune damage, the same activation during cancer supports tumor immune evasion and promotes tumor growth and invasiveness.

Beyond their role on neurodegenerative disorders, kynurenines have also been related with cancer progression in more than one way. It has been described that IDO expression is high in all thyroid carcinomas [63]. Moreover, l-kyn and Quin promote neoplastic cell proliferation *in vitro* and the formation of larger tumors *in vivo*, in a colorectal cancer model, through the Wnt/β-catenin signaling, despite of the ablation of IDO [64]. l‐kyn modulates the repairing enzyme DNA polymerase kappa, protecting tumor cells from DNA damage and propitiating genomic instability [65]. Also, Quin produced by microglial cells in glioma models confers tumor tolerance to oxidative stress due to the production of NAD+, promotes cell prolifera‐ tion by the modulation of the fibroblast growth factor-1 (FGF-1) release, and it is elevated in children with central nervous system tumors [66–68]. Kyna also promotes glioma cell prolif‐ eration through FGF-1 release [67]. The high expression and activity of the KP in the tumor microenvironment will be reflected on the maintenance of the NAD+/NADH supply, NAD+ and its reduced form NADH participate in several metabolic, redox, and stress response signals, providing favorable growing conditions for cancer cells. Finally, IDO expression and activity as well as the kynurenines promote cancer development by affecting the host's immune responsiveness. The effects of the KP on cancer immunosuppressive mechanisms

All the cancer cells must possess a series of characters that allow them to develop tumors, these "hallmarks" let cancer cells to rapidly grow, evade cell death, migrate through the organism and colonize new tissues, modify their metabolic program, and to avoid immune destruction [69].

are elevated after traumatic brain injury [57].

268 Mechanisms of Neuroinflammation

will be detailed below.

**2.2. Cancer immunoedition**

**2.1. Inflammatory process and kynurenine pathway components**

As mentioned above, the own immune response against tumors could propitiate the forma‐ tion of an immunosuppressive microenvironment, one of these mechanisms is carried out by IFN-γ and other proinflammatory cytokines secreted by cytotoxic lymphocytes which strongly induce IDO expression. Beyond the increase of IDO levels in tumor cells, the forma‐ tion and accumulation of different kynurenines have demonstrated immunosuppressive and immunoinhibitory effects [76].

Recent works have demonstrated that tryptophan starvation is able to inhibit T cell and mac‐ rophage cell viability and proliferation [77–80]. Furthermore, KP metabolites have also shown direct effects as immunosuppressive molecules. l-kyn is able to activate the aryl hydrocarbon receptor (AHR) and thus to promote the generation of Tregs from naive T cells [81]. Moreover, there are reports showing that 3-HAA, 3-HK, and Quin promote Treg generation and that 3-HAA and 3-HK block T cell activation and induce cell death of CD4+ T and CD8+ T cells as well as of natural killer cells (NK) and B lymphocytes [82–86]. All these reports settle down the idea that immunosuppressive mechanisms carried out through the KP activation may be due to tryptophan deprivation of the tumor microenvironment or secondly, by the direct effect of the kynurenines on Treg recruitment and the impact on viability of effector T cells. Finally, both tryptophan depletion and the presence of the KP intermediates could enhance an immunosuppressive environment. It has been showed that l-kyn, Pic, 3-HK, 3-HAA, and Quin reduced cell viability and induced apoptosis on CD4+ in absence of tryptophan [82, 83]; besides at lower concentrations were able to increase Treg-mediated immunotolerance [84].

It is relevant to note that in some cases, the kynurenines are not produced by cancer cells but by other components of the tumor microenvironment, such as dendritic cells (DCs) or mesenchymal stem cells and also by microglial cells in the case of brain tumors [83, 87, 88]. Regarding dendritic cells expressing IDO, they can suppress effective immune responses through diverse strategies: by inhibiting the proliferation and effector functions of cytotoxic cells, such as NK and CD8+ T lymphocytes, as well as plasma cells; by inducing of CD4+ CD25+ FOXP3+ regulatory T cells from naive CD4+ cells; and by triggering immunosuppressive activity in adjacent IDO-express‐ ing DCs [89]. Possible implication of IDO2 in cancer immunosuppression has been suggested due to reports showing its ability to mobilize the nuclear factor interleukin 6 inhibitor, LIP [6].

Thus, locating the IDO expression and the kynurenines in the "Three E's" context, it would be in this way: IFNɣ, TNFɑ, and proinflammatory cytokines secreted by cytotoxic lymphocytes, while killing recognized tumor cells during Elimination and Equilibrium phases, induce IDO expression of tumor cells, those that survived to immune response, and of other infiltrating cells, such as DCs and macrophages. Tumor cells expressing IDO, deplete tryptophan from the tumor microenvironment and begin to produce kynurenines and NAD+; tryptophan depletion from, and release of kynurenines to the tumor microenvironment, inhibit T cells and NK prolif‐ eration; induce apoptosis, while promote differentiation and proliferation of Tregs, and recruit‐ ing of DCs and macrophages. NAD+ production allows cancer cells to overcome DNA damage and oxidative stresses, promoving their proliferation during the Escape phase.

Because of the importance of the KP in brain and the relationship of tryptophan catabolism with immune modulation during cancer, researchers have begun to clarify the role of the acti‐ vation of this pathway on glioblastoma progression for further development of new strategies to treat this illness.

#### **2.3. The glioblastoma multiforme**

by IFN-γ and other proinflammatory cytokines secreted by cytotoxic lymphocytes which strongly induce IDO expression. Beyond the increase of IDO levels in tumor cells, the forma‐ tion and accumulation of different kynurenines have demonstrated immunosuppressive and

Recent works have demonstrated that tryptophan starvation is able to inhibit T cell and mac‐ rophage cell viability and proliferation [77–80]. Furthermore, KP metabolites have also shown direct effects as immunosuppressive molecules. l-kyn is able to activate the aryl hydrocarbon receptor (AHR) and thus to promote the generation of Tregs from naive T cells [81]. Moreover, there are reports showing that 3-HAA, 3-HK, and Quin promote Treg generation and that 3-HAA and 3-HK block T cell activation and induce cell death of CD4+ T and CD8+ T cells as well as of natural killer cells (NK) and B lymphocytes [82–86]. All these reports settle down the idea that immunosuppressive mechanisms carried out through the KP activation may be due to tryptophan deprivation of the tumor microenvironment or secondly, by the direct effect of the kynurenines on Treg recruitment and the impact on viability of effector T cells. Finally, both tryptophan depletion and the presence of the KP intermediates could enhance an immunosuppressive environment. It has been showed that l-kyn, Pic, 3-HK, 3-HAA, and Quin reduced cell viability and induced apoptosis on CD4+ in absence of tryptophan [82, 83]; besides at lower concentrations were able to increase Treg-mediated immunotolerance [84].

It is relevant to note that in some cases, the kynurenines are not produced by cancer cells but by other components of the tumor microenvironment, such as dendritic cells (DCs) or mesenchymal stem cells and also by microglial cells in the case of brain tumors [83, 87, 88]. Regarding dendritic cells expressing IDO, they can suppress effective immune responses through diverse strategies: by inhibiting the proliferation and effector functions of cytotoxic cells, such as NK and CD8+ T lymphocytes, as well as plasma cells; by inducing of CD4+ CD25+ FOXP3+ regulatory T cells from naive CD4+ cells; and by triggering immunosuppressive activity in adjacent IDO-express‐ ing DCs [89]. Possible implication of IDO2 in cancer immunosuppression has been suggested due to reports showing its ability to mobilize the nuclear factor interleukin 6 inhibitor, LIP [6].

Thus, locating the IDO expression and the kynurenines in the "Three E's" context, it would be in this way: IFNɣ, TNFɑ, and proinflammatory cytokines secreted by cytotoxic lymphocytes, while killing recognized tumor cells during Elimination and Equilibrium phases, induce IDO expression of tumor cells, those that survived to immune response, and of other infiltrating cells, such as DCs and macrophages. Tumor cells expressing IDO, deplete tryptophan from the tumor microenvironment and begin to produce kynurenines and NAD+; tryptophan depletion from, and release of kynurenines to the tumor microenvironment, inhibit T cells and NK prolif‐ eration; induce apoptosis, while promote differentiation and proliferation of Tregs, and recruit‐ ing of DCs and macrophages. NAD+ production allows cancer cells to overcome DNA damage

Because of the importance of the KP in brain and the relationship of tryptophan catabolism with immune modulation during cancer, researchers have begun to clarify the role of the acti‐ vation of this pathway on glioblastoma progression for further development of new strategies

and oxidative stresses, promoving their proliferation during the Escape phase.

immunoinhibitory effects [76].

270 Mechanisms of Neuroinflammation

to treat this illness.

Astrocytes are non-excitable nervous cells of great importance for the physiological maintenance that exert on neurons, by regulating ions and excitatory amino‐acids concentrations on synaptic regions, forming the brain-blood-barrier (BBB)-mediating neuron-blood stream signaling and regulating the transport of nutrients into and out of the brain [90, 91]. Beyond this, astrocytes can be differentiated on a wide span of morphological, physiological, and genomic subtypes [90–91].

Deregulation of cell proliferation and uncontrolled proliferation caused by mutations of cer‐ tain genes lead astrocytes to transform into astrocytomas. Astrocytomas have been classified into four subtypes depending on the rate of proliferation and the grade of malignancy: grade I pilocytic astrocytomas are non-invasive tumors, they represent the less malignant of astrocy‐ tomas; diffuse astrocytomas are grade II tumors that tend to invade peripheral tissues but their growth rate is slow; anaplastic astrocytomas grow faster than diffuse astrocytomas and tend to form tentacle-like projections into surrounding tissues, instead of grade III anaplastic astro‐ cytomas, their frequency is low; finally, glioblastomas (GBM), also known as glioblastoma multiforme, are grade IV tumors, these are the most malignant of astrocytomas [92]. GBM can be originated *de novo* from mature astrocytes, primary GBM, or from a less malignant astrocytoma, secondary GBM; they represent 15.1% of all brain primary tumors and 55.1% of all gliomas [93].

Incidence rate of GBM is 3.19 per 100,000 people with a median age at diagnosis of 64 years old for primary GBM, and 45 years old for secondary GBM, the median survival overall for diagnosed and treated GBM ranges between 12 and 14 months [94, 95]. Furthermore, GBM represent 2.8% of brain tumors during childhood [96]; however, its frequency is lower in adults, high-grade astrocytic tumor incidence rate is 0.85 per 100,000 people being more fre‐ quent in children between 5 and 9 years old, also, the median survival overall is over 43 months [97–99]. The most common treatment for GBM is surgical resection of the tumor, followed by temozolomide-based chemotherapy, and 5000–6000 Gy radiotherapy which pro‐ longs the lifespan 202 weeks [100–102]. Despite the treatment, the survival of afflicted patients is no longer than 14 months and only 5% of the patients have a survival rate of 5 years. It is believed that the lack of response of GBM to the multimodal treatment is due to multiple fac‐ tors that combined, make GBM resistance and aggressiveness.

A major character among GBM is a loss-of-function mutation of Retinoblastoma (Rb) gene, a cell cycle regulator protein at the phase G to phase S checkpoint, that also regulates apop‐ tosis and differentiation, which occurs on 77% of GBM [103, 104]. Instead of that, another genetic mutations and genomic expression patterns differ among GBM, integrated genomic analysis originated four types of GBM named: proneural, associated with PDGFRA, IDH1, and TP53 mutations; neural, expressing neuronal markers, such as NEFL, GABRA1, SYT1, and SLC12A5; classical, showing amplifications on EGFR expression and deletions of Ink4a/ ARF locus; and mesenchymal, characterized by overexpression of CHI3L1 and MET so as deletion of NF1 [103]. Several other mutations in key oncogenic signaling pathways, such as the receptor tyrosine kinase (RTK)/RAS/PI3K, p53 pathways, lead to uncontrolled tumor cell proliferation, genomic instability, and resistance to therapeutic strategies.

GBM may have a wide cellular diversity, more than tumor cells which are polygonal or spindled small sized with big nuclei, GBM-initiating cells (also called glioma stem cells) are responsible for cell proliferation and renewal of GBM cells, also confer chemo- and radioresistance; infiltrat‐ ing multinucleated cells, such as lymphocytes, macrophages, and neutrophils, and cells with lipid vacuoles are present in these tumors [105]. Because GBM are highly angiogenic tumors, blood vessels surrounded by pericytes are also present in tumors. GBM also show necrotic regions among the central body tumor and some other necrotic foci in outer regions of the tumor [105, 106]. In addition to all these mechanisms, the GBM has the ability to create an immuno‐ suppressive environment that prevents the response lead by the immune system against GBM.

#### *2.3.1. Immunosuppressive environment in GBM*

Historically, the brain has been considered an immune privileged organ due to the inability of reject exo-grafts and the lack of response in case of infection [107], as well as, absence of a normal response of the lymphatic system and the extremely distinctiveness of antigenpresenting cells (APCs) in brain tissue [108, 109]. However, diverse studies have recently found that there is not an absolute isolation of the rest of the organism; moreover, within the dural sinuses is located a lymphatic system that connect with the deep cervical lymph nodes, able of taking immune cells and macromolecules into the CNS [108].

Even with this lymphatic system, the access to CNS is highly regulated, mostly by the BBB [110].  However, it has been observed that when a GBM appears, this tumor easily disrupts the BBB due to this abnormal structure, which has as consequence the free-crossing of a more varied group of substances into the CNS [111] and also cells of the immune system.

GBM is infiltrated by several cell types, such as monocyte-derived cells, microglia, and activated T cells. These immune cells are recruited by chemotaxis to the tumor and together with molecules secreted by GBM itself, such as IL-10, prostaglandins, and TGF-β1 (**Table 2**), playing a major role in the immunosuppression of the tumor [112–113]. Specifically, several studies have found that the infiltration of T cells subtypes is higher in GBM than any other CNS tumor [114], comprising between 10 and 30% of cells within the tumor mass [115–116]. Due to the production of immu‐ nosuppressive cytokines, inhibition of T cell proliferation, activation of Tregs and hypoxia, an immunosuppressive environment, is made in GBM, which can be enhanced by the activation of the KP.

Because of the high immunosuppressive conditions in the GBM, new therapies have begun to being developed, focusing on antitumoral immune response for improvement of GBM patients. Thus, epidermal growth factor receptor (EGFR) variant III-specific peptide vaccina‐ tion and autologous tumor lysate-DC vaccination have been tested with successful results on phase I and phase II clinical trials [117–120]. However, a better understanding of the immuno‐ suppressive mechanisms occurring during GBM development, including those carried out by IDO activity and the kynurenines, could impulse the furtherance of new tools and strategies for GBM treatment.

**Table 2.** Molecules implicated in the immunosuppression of GBM.

GBM may have a wide cellular diversity, more than tumor cells which are polygonal or spindled small sized with big nuclei, GBM-initiating cells (also called glioma stem cells) are responsible for cell proliferation and renewal of GBM cells, also confer chemo- and radioresistance; infiltrat‐ ing multinucleated cells, such as lymphocytes, macrophages, and neutrophils, and cells with lipid vacuoles are present in these tumors [105]. Because GBM are highly angiogenic tumors, blood vessels surrounded by pericytes are also present in tumors. GBM also show necrotic regions among the central body tumor and some other necrotic foci in outer regions of the tumor [105, 106]. In addition to all these mechanisms, the GBM has the ability to create an immuno‐ suppressive environment that prevents the response lead by the immune system against GBM.

Historically, the brain has been considered an immune privileged organ due to the inability of reject exo-grafts and the lack of response in case of infection [107], as well as, absence of a normal response of the lymphatic system and the extremely distinctiveness of antigenpresenting cells (APCs) in brain tissue [108, 109]. However, diverse studies have recently found that there is not an absolute isolation of the rest of the organism; moreover, within the dural sinuses is located a lymphatic system that connect with the deep cervical lymph

Even with this lymphatic system, the access to CNS is highly regulated, mostly by the BBB [110].  However, it has been observed that when a GBM appears, this tumor easily disrupts the BBB due to this abnormal structure, which has as consequence the free-crossing of a more varied group of substances into the CNS

GBM is infiltrated by several cell types, such as monocyte-derived cells, microglia, and activated T cells. These immune cells are recruited by chemotaxis to the tumor and together with molecules secreted by GBM itself, such as IL-10, prostaglandins, and TGF-β1 (**Table 2**), playing a major role in the immunosuppression of the tumor [112–113]. Specifically, several studies have found that the infiltration of T cells subtypes is higher in GBM than any other CNS tumor [114], comprising between 10 and 30% of cells within the tumor mass [115–116]. Due to the production of immu‐ nosuppressive cytokines, inhibition of T cell proliferation, activation of Tregs and hypoxia, an immunosuppressive environment, is made in GBM, which can be enhanced by the activation of

Because of the high immunosuppressive conditions in the GBM, new therapies have begun to being developed, focusing on antitumoral immune response for improvement of GBM patients. Thus, epidermal growth factor receptor (EGFR) variant III-specific peptide vaccina‐ tion and autologous tumor lysate-DC vaccination have been tested with successful results on phase I and phase II clinical trials [117–120]. However, a better understanding of the immuno‐ suppressive mechanisms occurring during GBM development, including those carried out by IDO activity and the kynurenines, could impulse the furtherance of new tools and strategies for

nodes, able of taking immune cells and macromolecules into the CNS [108].

*2.3.1. Immunosuppressive environment in GBM*

272 Mechanisms of Neuroinflammation

[111] and also cells of the immune system.

the KP.

GBM treatment.

#### *2.3.2. Implications of kynurenine pathway in GBM development*

Kynurenine pathway enzymes are overexpressed in GBM. Upregulated expression of these enzymes has been related to severity of gliomas and patient survival. The IDO expression in 343 glioma specimens and correlated to patient survival has been analyzed [134], this expression was associated with poor prognosis and was more frequently observed in high-grade glioma. Moreover, in an orthotopic GL261 cell tumor model was shown that IDO-competent brain tumors increased the recruitment of immunosuppressive Tregs, and decreased the frequency of CD8+ T cells compared with IDO-deficient brain tumors [134]. Another study, including 75 gliomas, showed higher expression of IDO in malignant gliomas compared with low-grade gliomas. Moreover, stronger IDO expression was associated with shortened survival time in GBM patients [135]. Additionally, it has been reported that glioma cells have increased mRNA expression of IDO1 and IDO2 compared with human fetal astrocytes and human adult astro‐ cytes cultures as well as decreased Kats mRNA expression, and these expression enhanced when cells were stimulated with IFNɣ [75]; the same work also reported increased Kyn/Trp ratios and decreased Kyna/Kyn ratios in the plasma of glioblastoma patients compared with healthy people [75]. The levels of Trp and Kyn were evaluated in GBM patients before and after tumor resection. Interestingly, both Trp and Kyn significantly decreased after 48 h and 10 weeks after surgery. In addition, patients with a high-ratio Kyn/Trp had worse mean overall survival compared with patients with lower ratio [136].

The significantly increased expression of IDO in cancer cells and in the tumor microenviron‐ ment has arisen as a promising target for GBM treatment, based on the inhibition of IDO as a potential therapy for cancer patients. Among the first strategies, is the systemic adminis‐ tration of 1-methyl tryptophan (1-MT), a competitive inhibitor of the IDO, which delayed tumor development in a murine model of Lewis lung carcinoma [137]. Subsequently, it was described that the expression of IDO in immunogenic cells (P815B) avoided their rejection by preimmunized mice, an effect that was associated with the inhibition of specific T cell response [138]. This effect was partially reversed when preimmunized mice were adminis‐ tered with 1-MT [138]. Due to the results observed in animal models, 1-MT has been tested and approved on phase I clinical trials in patients with solid and solid metastatic neoplasms, lung, ovarian, Fallopian tube, and breast cancer showing disease stabilization in most cases [89, 139–141].

Concerning to GBM, at least three clinical trials involving IDO inhibitors are in recruiting status (Feb. 2017), but the use of 1-MT on *in vivo* GBM models has shown that when is used in com‐ bination with classical chemotherapeutic agents, such as temozolomide or cyclophosphamide, and additional radiotherapy, increases the overall survival as well as reduces the tumor volume [142, 143]. Additionally to this effect, inhibition of IDO has also shown increased complement component C3 deposition on the tumor region, which is necessary for tumor volume reduction [142], as well as increased CD4+ and CD8+ T cell population from animal spleens, treated with 1-MT [143], indicating that IDO inhibition rescues the host's immune activation.

Despite reports pointing that 1-MT has a greater affinity for IDO2 than for IDO [6, 7], it is of mention that the majority of reports about the targeting of the KP for cancer treatment are focused on IDO inhibition. But IDO is just one of the enzymes that fuel this catabolic route, so the better understanding of the roles of both IDO2 and TDO in cancer development and immunosuppression would result in better ways for combating cancer [144].

As mentioned above, in most cases, IDO expression as well as kynurenines production and immunosuppressive mechanisms are carried out by infiltrating tumor cells. For GBM, microg‐ lial cells are an important source of kynurenines so, the microglial depletion could be another important strategy for tumor mitigation. Studies focusing in microglial depletion through the use of CSF-1 receptor inhibitors have shown prolonged mice survival and enhanced antitu‐ moral immune response when animals were treated in combination with DC vaccination [145, 146]. The effect of microglial elimination on IDO expression and on the levels of KP metabo‐ lites, still remains to be elucidated, but the immune response reported with the use of the CSF-1 receptor inhibitors settle down some clues about what could happen in the KP context.

Thus, the use of IDO inhibitors and microglial elimination in combination with classical che‐ motherapy and radiotherapy in GBM murine models, as well as the clinical trials that have tested 1-MT in different kinds of solid and metastatic tumors has shown promising results for targeting IDO and the KP against GBM.

#### **2.4. Concluding remarks**

expression of IDO1 and IDO2 compared with human fetal astrocytes and human adult astro‐ cytes cultures as well as decreased Kats mRNA expression, and these expression enhanced when cells were stimulated with IFNɣ [75]; the same work also reported increased Kyn/Trp ratios and decreased Kyna/Kyn ratios in the plasma of glioblastoma patients compared with healthy people [75]. The levels of Trp and Kyn were evaluated in GBM patients before and after tumor resection. Interestingly, both Trp and Kyn significantly decreased after 48 h and 10 weeks after surgery. In addition, patients with a high-ratio Kyn/Trp had worse mean overall

The significantly increased expression of IDO in cancer cells and in the tumor microenviron‐ ment has arisen as a promising target for GBM treatment, based on the inhibition of IDO as a potential therapy for cancer patients. Among the first strategies, is the systemic adminis‐ tration of 1-methyl tryptophan (1-MT), a competitive inhibitor of the IDO, which delayed tumor development in a murine model of Lewis lung carcinoma [137]. Subsequently, it was described that the expression of IDO in immunogenic cells (P815B) avoided their rejection by preimmunized mice, an effect that was associated with the inhibition of specific T cell response [138]. This effect was partially reversed when preimmunized mice were adminis‐ tered with 1-MT [138]. Due to the results observed in animal models, 1-MT has been tested and approved on phase I clinical trials in patients with solid and solid metastatic neoplasms, lung, ovarian, Fallopian tube, and breast cancer showing disease stabilization in most cases

Concerning to GBM, at least three clinical trials involving IDO inhibitors are in recruiting status (Feb. 2017), but the use of 1-MT on *in vivo* GBM models has shown that when is used in com‐ bination with classical chemotherapeutic agents, such as temozolomide or cyclophosphamide, and additional radiotherapy, increases the overall survival as well as reduces the tumor volume [142, 143]. Additionally to this effect, inhibition of IDO has also shown increased complement component C3 deposition on the tumor region, which is necessary for tumor volume reduction [142], as well as increased CD4+ and CD8+ T cell population from animal spleens, treated with

Despite reports pointing that 1-MT has a greater affinity for IDO2 than for IDO [6, 7], it is of mention that the majority of reports about the targeting of the KP for cancer treatment are focused on IDO inhibition. But IDO is just one of the enzymes that fuel this catabolic route, so the better understanding of the roles of both IDO2 and TDO in cancer development and

As mentioned above, in most cases, IDO expression as well as kynurenines production and immunosuppressive mechanisms are carried out by infiltrating tumor cells. For GBM, microg‐ lial cells are an important source of kynurenines so, the microglial depletion could be another important strategy for tumor mitigation. Studies focusing in microglial depletion through the use of CSF-1 receptor inhibitors have shown prolonged mice survival and enhanced antitu‐ moral immune response when animals were treated in combination with DC vaccination [145, 146]. The effect of microglial elimination on IDO expression and on the levels of KP metabo‐ lites, still remains to be elucidated, but the immune response reported with the use of the CSF-1 receptor inhibitors settle down some clues about what could happen in the KP context.

1-MT [143], indicating that IDO inhibition rescues the host's immune activation.

immunosuppression would result in better ways for combating cancer [144].

survival compared with patients with lower ratio [136].

[89, 139–141].

274 Mechanisms of Neuroinflammation

GBM is an astrocyte-derived tumor, the most common of brain tumors, characterized by its high proliferation rates, genomic instability, and invasiveness, as well as to being a high immu‐ nosuppressive tumor. The nature of GBM results in a rapid expansion into the surrounding tissues, associated with poor prognosis and patient low survival despite classical treatment of these tumors, tumor extirpation, temozolomide therapy, and radiotherapy, which only pro‐ longs median survival over 2 months.

In this chapter, we have discussed the importance of the tryptophan catabolism through the KP during GBM development. Activation of this catabolic route provides cancer cells a constant

**Figure 3.** Global scheme of the role of IDO expression and the kynurenine pathway on glioblastoma development. During glioblastoma formation, a population of tumor cells is recognized and eliminated by the host immune system (cytotoxic T lymphocytes, helper T lymphocytes, and natural killer cells). While eliminating the recognized tumor cells, cytotoxic T lymphocytes secrete IF Ny, TNFα, and other proinflammatory cytokines which strongly induce IDO expression and the kynurenine pathway activation in survivor glioblastoma cells and tumor infiltrating cells (dendritic cells and tumor-associated macrophages). Glioblastoma IDO expressing cells maintain a rich NAD+/NADH pool which provides redox balance, energy supply, and active DNA repairing mechanisms, facilitating genomic instability and tumor growth; also in glioblastoma cells, l-kyn also contributes to genomic instability by modulating DNA polymerase κ. In the tumor microenvironment, released l-kyn promotes Treg differentiation from naïve T cells. While 3-HAA, 3-HK, and Quin promote Treg recruitment and inhibit both cytotoxic and helper T lymphocytes activation and proliferation; and also promote T lymphocytes, natural killer cells, and nontumor cells apoptosis, promoting more invasiveness. Microenvironmental tryptophan depletion caused by high tryptophan uptake by IDO expressing cells enhances the kynurenines effect against T cell activation proliferation. Finally, the use of IDO inhibitor 1-methyl-tryptophan rises as a promising strategy for glioblastoma treatment.

supply of NAD+ and promotes the expression of damaged DNA repairing enzymes, which allow cancer cells to have an active metabolism, and to bypass redox stress and DNA damage pressure (**Figure 3**).

Furthermore, IDO expression and elevated Kyn/Trp ratios are related with poor prognosis and low survival of GBM patients. The expression of key enzymes of the KP is induced by stimulation of cytokines secreted during antitumoral immune responses, after this, trypto‐ phan depletion and kynurenines production inhibit effector CD4+ and CD8+ T cell and NK cell proliferation, meanwhile promote Treg differentiation and MDSC infiltration into the tumor, attenuating antitumor responses. It is of note that IDO expression and immunosuppressive mechanisms are not carried out by cancer cells alone, but tumor infiltrating cells are also able to synthesize IDO and to promote an immunosuppressive environment (**Figure 3**).

The results of the use of IDO inhibitors, like 1-MT, in *in vivo* GBM models and in phase I and phase II clinical trials against lung, breast, and ovarian cancer, arise them as a promising strat‐ egy for combating GBM. However, detailed information about the role of key enzymes of the KP, KYNU, KAT, KMO, and QPRT, could provide more information for the development of more truthful treatments against GBM.

#### **Conflict of interests**

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the chapter.

#### **Acknowledgements**

This work was supported by CONACYT Grant 262010.

#### **Author details**

Gustavo Ignacio Vázquez Cervantes1,2, Nora Karen Olascoaga Arellano3 , Daniela Ramírez Ortega1 , Alelí Salazar Ramiro3 , Dinora F. González Esquivel1 , Camilo Ríos1 , Benjamín Pineda Olvera3 and Verónica Pérez de la Cruz1 \*

\*Address all correspondence to<EMAIL_ADDRESS>
1 Departamento de Neuroquímica, Instituto Nacional de Neurología y Neurocirugía "Manuel Velasco Suárez", Mexico

2 Doctoral Program in Biological Sciences, Universidad Nacional Autónoma de México (UNAM), Mexico

3 Laboratorio de Neuroinmunología, Instituto Nacional de Neurología y Neurocirugía "Manuel Velasco Suarez", Mexico

#### **References**

supply of NAD+ and promotes the expression of damaged DNA repairing enzymes, which allow cancer cells to have an active metabolism, and to bypass redox stress and DNA damage

Furthermore, IDO expression and elevated Kyn/Trp ratios are related with poor prognosis and low survival of GBM patients. The expression of key enzymes of the KP is induced by stimulation of cytokines secreted during antitumoral immune responses, after this, trypto‐ phan depletion and kynurenines production inhibit effector CD4+ and CD8+ T cell and NK cell proliferation, meanwhile promote Treg differentiation and MDSC infiltration into the tumor, attenuating antitumor responses. It is of note that IDO expression and immunosuppressive mechanisms are not carried out by cancer cells alone, but tumor infiltrating cells are also able

The results of the use of IDO inhibitors, like 1-MT, in *in vivo* GBM models and in phase I and phase II clinical trials against lung, breast, and ovarian cancer, arise them as a promising strat‐ egy for combating GBM. However, detailed information about the role of key enzymes of the KP, KYNU, KAT, KMO, and QPRT, could provide more information for the development of

The authors report no conflicts of interest. The authors alone are responsible for the content

, Daniela Ramírez

, Benjamín Pineda

, Camilo Ríos1

to synthesize IDO and to promote an immunosuppressive environment (**Figure 3**).

pressure (**Figure 3**).

276 Mechanisms of Neuroinflammation

more truthful treatments against GBM.

This work was supported by CONACYT Grant 262010.

Gustavo Ignacio Vázquez Cervantes1,2, Nora Karen Olascoaga Arellano3

\*

, Dinora F. González Esquivel1

1 Departamento de Neuroquímica, Instituto Nacional de Neurología y Neurocirugía "Manuel

2 Doctoral Program in Biological Sciences, Universidad Nacional Autónoma de México

3 Laboratorio de Neuroinmunología, Instituto Nacional de Neurología y Neurocirugía

**Conflict of interests**

and writing of the chapter.

**Acknowledgements**

, Alelí Salazar Ramiro3

and Verónica Pérez de la Cruz1

\*Address all correspondence to<EMAIL_ADDRESS>
**Author details**

Velasco Suárez", Mexico

"Manuel Velasco Suarez", Mexico

(UNAM), Mexico

Ortega1

Olvera3

[27] Boado RJ, Li JY, Nagaya M, Zhang C, Pardridge WM. Selective expression of the large neutral amino acid transporter at the blood-brain barrier. Procedures of the National Academy of Sciences of the United States of America. 1999;**96**(21):12079-12084

[14] Badawy AA. Tryptophan availability for kynurenine pathway metabolism across the life span: Control mechanisms and focus on aging, exercise, diet and nutritional supplements. Neuropharmacology. 2017;**112**(Pt B):248-263. DOI: 10.1016/j.neuropharm.2015.11.015

[15] Kudo Y, Boyd CA. Human placental indoleamine 2,3-dioxygenase: Cellular localization and characterization of an enzyme preventing fetal rejection. Biochimica et Biophysica

[16] Basran J, Rafice SA, Chauhan N, Efimov I, Cheesman MR, Ghamsari L, Raven EL. A kinetic, spectroscopic, and redox study of human tryptophan 2,3-dioxygenase. Biochemistry.

[17] Shinohara R, Ishiguro I. The purification and properties of formamidase from rat liver.

[18] Krisch K, Arndt R, Junge W, Menge U, Michelssen K. Purification and some properties of for‐ mamidase from rat and pig liver. Acta Vitaminologica et Enzymologica. 1975;**29**(1-6):302-306

[19] Han Q, Li J, Li J. pH dependence, substrate specificity and inhibition of human kynuren‐ ine aminotransferase I. European Journal of Biochemistry. 2004;**271**(23-24):4804-4814.

[20] Han Q, Cai T, Tagle DA, Robinson H, Li J. Substrate specificity and structure of human aminoadipate aminotransferase/kynurenine aminotransferase II. Bioscience Reports.

[21] Han Q, Robinson H, Cai T, Tagle DA, Li J. Biochemical and structural properties of mouse kynurenine aminotransferase III. Molecular and Cellular Biology. 2009;**29**(3):784-

[22] Han Q, Calvo E, Marinotti O, Fang J, Rizzi M, James AA, Li J. Analysis of the wild-type and mutant genes encoding the enzyme kynurenine monooxygenase of the yellow fever

[23] Lima S, Kristoforov R, Momany C, Phillips RS. Crystal structure of Homo sapiens kyn‐

[24] Malherbe P, Köhler C, Da Prada M, Lang G, Kiefer V, Schwarcz R, Lahm HW, Cesura AM. Molecular cloning and functional expression of human 3-hydroxyanthranilic-acid

[25] Pucci L, Perozzi S, Cimadamore F, Orsomando G, Raffaelli N. Tissue expression and bio‐ chemical characterization of human 2-amino 3-carboxymuconate 6-semialdehyde decar‐ boxylase, a key enzyme in tryptophan catabolism. The FEBS Journal. 2007;**274**(3):827-840.

[26] Liu H, Woznica K, Catton G, Crawford A, Botting N, Naismith JH. Structural and kinetic characterization of quinolinate phosphoribosyltransferase (hQPRTase) from Homo sapi‐ ens. Journal of Molecular Biology. 2007;**373**(3):755-763. DOI: 10.1016/j.jmb.2007.08.043

mosquito, Aedes aegypti. Insect Molecular Biology. 2003;**12**(5):483-490

ureninase. Biochemistry. 2008;**46**(10):2735-2744. DOI: 10.1021/bi0616697

dioxygenase. The Journal of Biological Chemistry. 1994;**29**(19):13792-13797

Acta. 2000;**1500**(1):119-124

278 Mechanisms of Neuroinflammation

DOI: 10.1186/1471-2091-11-19

793. DOI: 10.1128/MCB.01272-08

DOI: 10.1111/j.1742-4658.2007.05635.x

2008;**47**(16):4752-4760. DOI: 10.1021/bi702393b

2008;**28**(4):205-215. DOI: 10.1042/BSR20080085

Biochimica et Biophysica Acta. 1970;**198**(2):324-331

Pineda B, Torres-Ramos M, Santamaría A, La Cruz VP. On the antioxidant properties of kynurenic acid: Free radical scavenging activity and inhibition of oxidative stress. Neurotoxicology and Teratology. 2011;**33**(5):538-347. DOI: 10.1016/j.ntt.2011.07.002

[50] Bordelon YM, Chesselet MF, Nelson D, Welsh F, Erecińska M. Energetic dysfunction in quinolinic acid-lesioned rat striatum. Journal of Neurochemistry. 1997;**69**(4):1629-1639

Pineda B, Torres-Ramos M, Santamaría A, La Cruz VP. On the antioxidant properties of kynurenic acid: Free radical scavenging activity and inhibition of oxidative stress. Neurotoxicology and Teratology. 2011;**33**(5):538-347. DOI: 10.1016/j.ntt.2011.07.002

[40] Wang J, Simonavicius N, Wu X, Swaminath G, Reagan J, Tian H, Ling L. Kynurenic acid as a ligand for orphan G protein-coupled receptor GPR35. The Journal of Biological

[41] Cosi C, Mannaioni G, Cozzi A, Carlà V, Sili M, Cavone L, Maratea D, Moroni F. G-protein coupled receptor 35 (GPR35) activation and inflammatory pain: Studies on the antinoci‐ ceptive effects of kynurenic acid and zaprinast. Neuropharmacology. 2011;**60**(7-8):1227-

[42] Berlinguer-Palmini R, Masi A, Narducci R, Cavone L, Maratea D, Cozzi A, Sili M, Moroni F, Mannaioni G. GPR35 activation reduces Ca2+ transients and contributes to the kyn‐ urenic acid-dependent reduction of synaptic activity at CA3-CA1 synapses. PloS One.

[43] Moroni F1, Cozzi A, Sili M, Mannaioni G. Kynurenic acid: a metabolite with multiple actions and multiple targets in brain and periphery. Journal of Neural Transmission.

[44] Kumar A, Babu GN. *In vivo* neuroprotective effects of peripheral kynurenine on acute neurotoxicity induced by glutamate in rat cerebral cortex. Neurochemical Research.

[45] Carrillo-Mora P, Méndez-Cuesta LA, Pérez-De La Cruz V, Fortoul-van Der Goes TI, Santamaría A. Protective effect of systemic L-kynurenine and probenecid administra‐ tion on behavioural and morphological alterations induced by toxic soluble amyloid beta (25-35) in rat hippocampus. Behavioural Brain Research. 2010;**210**(2):240-250. DOI:

[46] Silva-Adaya D, Pérez-De La Cruz V, Villeda-Hernández J, Carrillo-Mora P, González-Herrera IG, García E, Colín-Barenque L, Pedraza-Chaverrí J, Santamaría A. Protective effect of L-kynurenine and probenecid on 6-hydroxydopamine-induced striatal toxicity in rats: Implications of modulating kynurenate as a protective strategy. Neurotoxicology

[47] Ribeiro CA, Grando V, Dutra Filho CS, Wannmacher CM, Wajner M. Evidence that quinolinic acid severely impairs energy metabolism through activation of NMDA recep‐ tors in striatum from developing rats. Journal of Neurochemistry. 2006;**99**(6):1531-1542

[48] Goda K, Kishimoto R, Shimizu S, Hamane Y, Ueda M. Quinolinic acid and active oxy‐ gens. Possible contribution of active Oxygens during cell death in the brain. Advances in

[49] Moroni F, Lombardi G, Moneti G, Aldinio C. The excitotoxin quinolinic acid is present in the brain of several mammals and its cortical content increases during the aging process.

and Teratology. 2011;**33**(2):303-312. DOI: 10.1016/j.ntt.2010.10.002

Experimental Medicine and Biology. 1996;**398**:247-254

Neuroscience Letters. 1984;**47**(1):51-55

Chemistry. 2006;**281**(31):22021-22028. DOI: 10.1074/jbc.M603503200

1231. DOI: 10.1016/j.neuropharm.2010.11.014

280 Mechanisms of Neuroinflammation

2013;**8**(11):e82180. DOI: 10.1371/journal.pone.0082180

2012;**119**(2):133-139. DOI: 10.1007/s00702-011-0763-x

2010;**35**(4):636-644. DOI: 10.1007/s11064-009-0114-6

10.1016/j.bbr.2010.02.041

[74] van Baren N, Van den Eynde BJ. Tumoral immune resistance mediated by enzymes that degrade tryptophan. Cancer Immunology Research. 2015;**3**(9):978-985. DOI: 10.1158/ 2326-6066.CIR-15-0095

[62] Routy JP, Routy B, Graziani GM, Mehraj V. The kynurenine pathway is a Double-Edged sword in Immune-Privileged sites and in cancer: Implications for immunotherapy. International Journal of Tryptophan Research. 2016;**9**:67-77. DOI: 10.4137/IJTR.S38355

[63] Moretti S, Menicali E, Voce P, Morelli S, Cantarelli S, Sponziello M, Colella R, Fallarino F, Orabona C, Alunno A, de Biase D, Bini V, Mameli MG, Filetti S, Gerli R, Macchiarulo A, Melillo RM, Tallini G, Santoro M, Puccetti P, Avenia N, Puxeddu E. Indoleamine 2,3-dioxygenase 1 (IDO1) is up-regulated in thyroid carcinoma and drives the devel‐ opment of an immunosuppressant tumor microenvironment. The Journal of Clinical

Endocrinology and Metabolism. 2014;**99**(5):E832-E840. DOI: 10.1210/jc.2013-3351 [64] Thaker AI, Rao MS, Bishnupuri KS, Kerr TA, Foster L, Marinshaw JM, Newberry RD, Stenson WF, Ciorba MA. IDO1 metabolites activate β-catenin signaling to promote can‐ cer cell proliferation and colon tumorigenesis in mice. Gastroenterology. 2013;**145**(2):416-

[65] Bostian AC, Eoff RL. Aberrant kynurenine signaling modulates DNA replication stress factors and promotes genomic instability in gliomas. Chemical Research in Toxicology.

[66] Heyes MP, Saito K, Milstien S, Schiff SJ. Quinolinic acid in tumors, hemorrhage and bacterial infections of the central nervous system in children. Journal of the Neurological

[67] Di Serio C, Cozzi A, Angeli I, Doria L, Micucci I, Pellerito S, Mirone P, Masotti G, Moroni F, Tarantini F. Kynurenic acid inhibits the release of the neurotrophic fibroblast growth factor (FGF)-1 and enhances proliferation of glia cells, *in vitro*. Cellular and Molecular

[68] Sahm F, Oezen I, Opitz CA, Radlwimmer B, von Deimling A, Ahrendt T, Adams S, Bode HB, Guillemin GJ, Wick W, Platten M. The endogenous tryptophan metabolite and NAD+ precursor quinolinic acid confers resistance of gliomas to oxidative stress. Cancer

[69] Pecorino L. 2012. Molecular Biology of Cancer. Mechanisms, Targets and Therapeutics.

[70] Rabinovich GA, Gabrilovich D, Sotomayor EM. Immunosuppressive strategies that are mediated by tumor cells. Annual Review of Immunology. 2007;**25**:267-296. DOI: 10.1146/

[71] Dunn GP, Bruce AT, Ikeda H, Old LJ, Schreiber RD. Cancer immunoediting: From immunosurveillance to tumor escape. Nature Immunology. 2002;**3**(11):991-998. DOI:

[72] Dunn GP, Old LJ, Schreiber RD. The three Es of cancer immunoediting. Annual Review of Immunology. 2004;**22**:329-360. DOI: 10.1146/annurev.immunol.22.012703.104803 [73] Yaguchi T, Kawakami Y. Cancer-induced heterogeneous immunosuppressive tumor microenvironments and their personalized modulation. International Immunology.

425. DOI: 10.1053/j.gastro.2013.05.002

282 Mechanisms of Neuroinflammation

Sciences. 1995;**133**(1-2):112-118

3rd ed. Italy: Oxford University Press

annurev.immunol.25.022106.141609

2016;**28**(8):393-399. DOI: 10.1093/intimm/dxw030

10.1038/ni1102-991

2016;**29**(9):1369-1380. DOI: 10.1021/acs.chemrestox.6b00255

Neurobiology. 2005;**25**(6): 981-993. DOI: 10.1007/s10571-005-8469-y

Research. 2013;**73**(11):3225-3234. DOI: 10.1158/0008-5472.CAN-12-3831

allograft survival. Investigative Ophthalmology and Visual Science. 2011;**52**(5):2640- 2648. DOI: 10.1167/iovs.10-5793