traumatic brain injury ( tbi ) is characterized by intracranial damage resulting from an external force and can be the product of physical insults such as puncture , blunt impact , or blast ( maas et al . , 2008 ) .
tbi affects 1.7 million people annually , presenting a significant economic burden , and is particularly prevalent in military casualties , where an increase in explosive warfare has led to a parallel rise in tbi occurrence ( okie , 2005 ; acosta et al .
tbi can be classified as mild , moderate , or severe ( lozano et al . , 2015 ) .
determining the severity of tbi is commonly accomplished in humans with the glasgow coma scale ( gcs ) , a simple questionnaire used to evaluate the patient 's degree of consciousness in conjunction with medical imaging techniques ( lozano et al . , 2015 ) .
associated symptoms such as headache , dizziness , fatigue , or nausea may be short - lived in the case of mild tbi , while in more severe instances cognitive symptoms may progress chronically to resemble neurodegenerative diseases such as alzheimer 's disease ( ad ) and parkinson 's disease ( pd ) ( lozano et al . ,
epidemiological data also suggest that sufferers of tbi are at an increased risk for developing ad and pd later in life through poorly understood mechanisms ( lozano et al . , 2015 ) .
the acute phase consists of the immediate damage produced from the insult , and the chronic phase may extend for years and is defined by spreading inflammation , cell death , and neural dysfunction that is triggered by the primary injury ( werner and engelhard , 2007 ; acosta et al .
the expanded distribution of pro - inflammatory molecules , reactive species , and other damaging byproducts from the primary injury site causes a progressive wave of cell damage , forming a region of dead and endangered cells called the penumbra ( zhao et al .
this region poses the greatest threat of chronic symptom development , but also points to a significant opportunity for cell rescue ( zhao et al . , 2005 ;
once considered to be an acute injury only , our understanding of tbi as possessing this chronic aspect has revealed the opportunity of a greater time frame for therapeutic intervention ( acosta et al .
the chronic phase of tbi is marked by a multitude of complex metabolic , immune , and cellular responses which are seen primarily in tissue adjacent to the injury site , but can also spread to distal regions of the brain ( kumar and loane , 2012 ; lozano et al . ,
while these responses are seen universally in tbi , researchers have shown that the specific pathology and intricacies of the injury response are heterogeneous , and vary depending on the nature and location of the tbi ( pabon et al . , 2016 ) .
the immediate damage that occurs during a tbi includes necrosis , excitotoxicity , and mutilation of local neurons , microglial activation , and vascular cells - none of which present a significant opportunity for intervention ( kumar and loane , 2012 ) .
instead , recent research has focused on the multiple aspects of chronic tbi as a means of reducing the secondary cell death that ensues after the initial acute phase ( lozano et al . ,
in particular , the neuroinflammatory aspect of tbi physiopathology has been explored as a target for preventing secondary cell death and symptom progression ( lozano et al . , 2015 ) .
inflammation plays a dual role in the brain after a tbi ; in the acute phase , inflammation has been shown to be neuroprotective , while aberrant inflammation throughout the chronic phase is a key factor in perpetuating cell death ( lozano et al . , 2015 ) .
sequestration of the chronic inflammation has been proven to be neuroprotective ( lozano et al . , 2015 ) .
the transition from protective inflammation to degenerative inflammation - as well as the large number of competing pro - inflammatory molecules ( i.e. , tumor necrosis factor ( tnf- ) and interleukin ( il-1 ) and anti - inflammatory molecules ( i.e. , il-10 and tgf- ) - make for a complex pathology ( lozano et al . , 2015 ) . the neurodegenerative inflammation and edema seen after tbi
is linked to the dysfunction of the blood - brain barrier ( bbb ) that occurs after the insult ( shlosberg et al . , 2010 ;
the protective effects of the bbb are contingent on the presence of uninterrupted , selective endothelial cells which moderate the entrance of blood - borne particles ( neuwelt et al . , 2011 ) . additionally , the health of these endothelial cells and the fidelity of the bbb is intimately connected to the health of supportive astrocytes .
the mechanical force from the tbi compromises this fidelity , allowing a host of exogenous proteins such as albumin , fibrinogen , and thrombin , as well as peripheral immune cells , to enter the brain parenchyma ( shlosberg et al .
, 2010 ; neuwelt et al . , 2011 ; lozano et al . , 2015 ) .
these invading substances trigger the activation of microglia within the injured brain region , causing an immune response that persists for as long as the bbb remains compromised ( shlosberg et al . , 2010 ;
initially , the microglial activation is a protective response , but the activation can become excessive and self - perpetuating over time ( lozano et al . , 2015 ) .
additionally , the invading peripheral cells have been shown to release pro - inflammatory mediators such as cytokines , chemokines , prostaglandins , and free radicals , serving to progressively exacerbate the inflammatory process as well as increasing the permeability of the bbb ( lozano et al . , 2015 ) .
this elevated permeability to large molecules and cells shifts the osmotic pressure within the brain , causing edema and an increase in intracranial pressure ( neuwelt et al .
damage to the bbb is a continuing issue , as increased levels of molecules which damage tight junctions such as matrix metallopeptidase-9 ( mmp-9 ) and vascular endothelial growth factor ( vegf ) occur post - tbi and impede the recovery of the bbb ( guo et al . , 1989 ;
the immune response is further complicated by differentiation of environment - sensitive microglia into multiple phenotypes ( lozano et al . , 2015 ) .
depending on the microenvironment that the microglia is exposed to , the cell may assume an m1 , pro - inflammatory , phenotype or an m2 , anti - inflammatory phenotype that attenuates the pro - inflammatory m1 microglia ( lozano et al . , 2015 ) .
possible therapeutic techniques aim to minimize harmful chronic inflammation , while not preventing acute - phase neuroprotective inflammation , by developing means of downregulating pro - inflammatory molecules , upregulating anti - inflammatory molecules , and aiding in bbb repair ( lozano et al . , 2015 ) . the only defense against the primary injury caused by a tbi is prevention with the use of safety equipment such as helmets and seatbelts ( hirschenfang et al .
the window of opportunity for potential therapies lies within the period of secondary cell death which may last for days , months , or years ( iida et al . ,
of the different secondary death mechanisms , neuro - inflammation provides the greatest potential for intervention due to its delayed onset .
glutamate excitotoxicity for example has a much narrower range for intervention as it is an immediate response and may normalize within 120 hours after insult ( matsumoto et al . , 1986 ) .
based on the length of time required for enough inflammatory cells to migrate and induce secondary cell death , the onset of damaging neuroinflammmation is a slower process ( offit et al . , 1986 ) .
neuroinflammation may also persist as a chronic consequence of tbi and this secondary effect is closely linked to many neurodegenerative diseases such as dementia pugilistica ( dp ) , ad , pd and other pathologies ( colmano and gross , 1971 ) .
while targeting neuroinflammation is a reasonable approach to sequestering secondary cell death in tbi , it should be noted that neuroinflammation also has beneficial effects .
previous studies have shown that high doses of anti - inflammatory drugs administered after tbi led to worse outcomes due to the loss of neuroprotective effects in addition to the existing harmful effects ( ziebell and morganti - kossmann , 2010 ) .
several stand - alone anti - inflammatory drugs have been tested for efficacy in tbi treatment .
minocycline is a tetracycline derivative known to have both neuroprotective and anti - inflammatory properties ( kumar and loane , 2012 ) .
minocycline is a prime candidate for clinical trials due to its ability to cross the bbb in addition to its safety for human use as declared by the us food and drug administration ( saatman et al . , 2008 ) .
this drug minimizes the release of proinflammatory cytokines and chemokines , in conjunction with other mediators of inflammation , decreases nitric oxide by directly inhibiting the overactivation and proliferation of microglia cells ( homsi et al . , 2009 ; kovesdi et al . ,
2012 ) . reducing and inactivating microglial cells is key as it attributes to the decline in cytokines il-1 ,
il-6 , and matrix metallopeptidase 9 ( mmp-9 ) , all of which facilitate the proinflammatory response ( homsi et al . , 2009 ; ziebell and morganti - kossmann , 2010 ;
the use of minocycline in animal models has illustrated a significant reduction in inflammation and tissue damage , thereby improving outcomes ( homsi et al .
however , results of other studies contradict this , suggesting there is no beneficial effects of minocycline on tbi .
these opposing outcomes are a predicted result of deviations in dosage and administration intervals , calling for further investigation ( homsi et al . , 2009 ; kelso et al . , 2011 )
melatonin is a hormone produced in the pineal gland and is currently being explored as a stand - alone drug due to its neuroprotective characteristics ( lozano et al . , 2015 ) .
it is an enzyme that easily passes through cell membranes due to it lipophilic properties .
the mechanism of action for its anti - inflammatory effects are by way of inhibiting microglial activation and reducing pro - inflammatory cytokine secretion , such as il-1 and tnf- ( wang et al . , 2013 ; ding et al . , 2014a )
the effects of melatonin as a tbi therapeutic drug vary , similar to the results of minocycline administration .
lower brain edema and reduced cortical neural degeneration was displayed in successful trials , implying improvement of cognitive deficits ( ding et al . , 2014b ) .
however , there was no significant cognitive enhancements in experimental models , the likely cause being attributed to dosage ( kelso et al . , 2011 ) .
while melatonin is a prospective therapy to sequester secondary damage caused by inflammation , additional studies are necessary to address the long - term feasibility , safety , and efficacy of melatonin in multiple models of tbi ( hirschenfang et al . , 1968 ;
statins are a well - known group of drugs used to treat high cholesterol that have additionally shown neuroprotective and anti - inflammatory effects in a mouse model of subarachnoid hemorrhage ( uekawa et al . , 2014 ) .
statins inhibits the signaling pathways of toll - like receptor 4 , nuclear factor b ( nf-b ) activation , and some small g - proteins , all of which contribute to the reduction of microglia activation ( loane and faden , 2010 ; wang et al . , 2014 ) .
by additionally inhibiting epidermal growth factor receptors that play a part in astrogliosis , statins also decrease astrocyte activation
( wu et al . , 2010 ) . without microglia and astrocytes contributing their proinflammatory effects
, there is a reduction in the expression of proinflammatory cytokines il-1 and tnf- , as well as intracellular and intercellular adhesion molecules , and consequently an overall sequestration of the neuroinflammatory process ( loane and faden , 2010 ; wu et al . , 2010 ; uekawa et al . , 2014 ) .
in tbi models accompanied by statin administration , results have displayed an increase in neuronal survival , growth , and differentiation and a lessening of functional impairment ( loane and faden , 2010 ) .
minor improvements in amnesia and disorientation ( as assessed by the galveston orientation amnesia test ) were achieved after rosuvastatin was given over a 10 day period to tbi patients within a clinical trial ( tapia - perez et al . , 2008 ) .
statins are an already well established class of drugs that are tolerated by patients , with occasional mild side effects that are distinct and can be monitored without difficulty .
statins show promise in preclinical for other neurological deficits such as stroke , intracerebral hemorrhage , but more extensive laboratory studies in tbi models are needed to improve clinical outcomes ( hirschenfang et al . , 1968 ; lozano et al . , 2015 ) .
currently , there remain safety and efficacy optimization hurdles about the use of anti - inflammatory pharmacotherapy for the treatment of tbi .
the use of these drugs has demonstrated a notable reduction in the inflammatory response ; however , it is crucial to further investigate the applicability of these treatment options in a clinical setting . as previously mentioned , concerns regarding dosage and long - term outcomes warrant further investigation of these drugs prior to their clinical use .
stem cells are undifferentiated cells that have the potential to regenerate damaged tissue secondary to their ability to proliferate numerously , differentiate into multiple cell lines , and provide restorative resources to surviving cells ( antonucci et al . , 2014 ;
preclinical studies have demonstrated the remarkable regenerative ability of stem cells to transform into newly differentiated neurons following tbi ( antonucci et al . , 2012 ; rodrigues et al . , 2012 ; liu et al . , 2013 ; acosta et al . , 2014 ; tajiri et al .
following brain injury , stem cell therapy shows notable potential in sequestering neural cell death and a prolonged inflammatory response , which results in increased recovery in both cognitive and motor function ( acosta et al . , 2014 ; de la pena et al . , 2014 ) .
the hallmark of stem cells dampening the tbi - induced inflammatory response lies within their mechanism of action .
mesenchymal stem cells ( mscs ) possess the ability to migrate to the site of injury and activate cellular effectors of the inflammatory response such as microglia , t lymphocytes , and neutrophils ( borlongan , 2011 ; borlongan et al .
key inflammatory signals , such as tnf- and il-1 , induce mscs secrete an anti - inflammatory protein called tnf--stimulated gene / protein 6 ( tsg-6 ) ( watanabe et al .
, 2013 ; zhang et al . , 2013 ) . the anti - inflammatory role of tsg-6
is carried out by disrupting the inflammatory signaling pathways of both toll - like receptors ( tlrs ) and nf-b ( watanabe et al . , 2013 ; zhang et al . , 2013 )
the inflammatory nf-b signaling cascade is characterized by the production of proinflammatory cytokines by t cells , such as interferon  ( russo et al . , 2011 ) .
interestingly , tsg-6 is able to modulate the activity of t cells to instead produce anti - inflammatory cytokines , such as il-4 ( russo et al . , 2011 ) .
once transplanted , stem cells are faced with the struggle of surviving in the hostile environment at the site of injury ( dela pena et al . , 2014 ) .
certain factors , such as granulocyte - colony stimulating factor ( g - csf ) , can be introduced alongside stem cells to promote the neuroprotection of the transplanted stem cells ( acosta et al . , 2014 ) . combining stem cells with factors such as g - csf
provide a significant improvement in neurogenesis and a reduction in cell death , when compared to being administered alone ( acosta et al . , 2014 ) .
g - csf , a cytokine , has the inherent capability to reduce brain edema , enable recovery of motor function , and to improve control of glutamate levels ( acosta et al . , 2014 ) .
the mechanism of g - csf utilizes receptor - mediated transport to recruit endogenous stem cells from the bone marrow into the peripheral blood .
these mobilized stem cells can then migrate to the site of injury , where they can synthesize and release growth factors , chemokines , and cytokines that aid in the process of brain tissue repair ( acosta et al . , 2014 ) .
recent studies demonstrate that transplanted stem cells preferentially migrate to the spleen , rather than the brain ( acosta et al . , 2015a ) .
the concept of increased stem cell survival in the spleen over the brain suggests that direct , targeted therapy may not always be the best treatment option ( acosta et al . , 2015a ) .
indeed , studies are investigating the efficacy of systemic delivery of stem cells in tbi and other disease models , such as stroke ( acosta et al .
several types of transplantable cells have been tested in the laboratory , with a few reaching clinical trials , for cell therapy in stroke , including fetal cells , nt2n cells , ctx0e3 , embryonic stem cells , neural stem / progenitor cells , umbilical cord blood , amnion , adipose , and induced pluripotent stem cells ( hara et al . , 2008 ; li et al . , 2008 ;
, 2011 ; liu et al . , 2014 ; de la pena et al . , 2015 ) . primarily due to solid safety profile in other disease indications , preclinical studies and on - going clinical trials
have given special attention to bone marrow and its cellular derivatives ( borlongan et al .
. direct intracerebral implantation and peripheral transplantation , such as intravenous , intra - arterial , and intranasal , have documented the functional benefits of bone marrow - derived stem cells ( borlongan et al . , 2004 ; borlongan , 2011 ; borlongan et al . , 2011 ; prasad et al . , 2014 ; acosta et al . , 2015a ) .
clinical trials have been initiated , and preliminary reports have demonstrated safety , although efficacy warrants additional investigations ( steinberg et al . , 2016 ) .
this concept highlights the need for investigations of combination therapy ( figure 1 and table 1 ) , in order to improve the outcomes of drugs and stem transplantation in tbi and other related brain disorders .
indeed , combining drugs , such as granulocyte - colony stimulating factor ( g - csf ) , erythropoietin ( epo ) , and statins may improve the functional benefits of stem cells towards brain repair in tbi and other related disorders .
in this review , the outlined studies detailed treatment options for tbi that focus on either stand - alone pharmacotherapy or cell therapy
. a single therapeutic approach may not be optimal in reducing inflammation and in initiating the brain repair process after tbi .
it is more likely that combination therapy of anti - inflammatory drugs alongside stem cell transplantation will yield enhanced results that can be translated to the clinical setting .
in addition , most preclinical studies are limited by their use of small animal models for tbi , which may not accurately mimic the clinical scenario in humans .
thus , limited clinical trials of stand - alone therapies , alongside more advanced laboratory studies incorporating optimization of safety and efficacy on dosages and long - term functional assessments of combined pharmacotherapy and stem cell transplantation in large animal models , are required to allow these treatment options to become available at the bedside . 

 the pathologic process of chronic phase traumatic brain injury is associated with spreading inflammation , cell death , and neural dysfunction . 
 it is thought that sequestration of inflammatory mediators can facilitate recovery and promote an environment that fosters cellular regeneration . 
 studies have targeted post - traumatic brain injury inflammation with the use of pharmacotherapy and cell therapy . 
 these therapeutic options are aimed at reducing the edematous and neurodegenerative inflammation that have been associated with compromising the integrity of the blood - brain barrier . 
 although studies have yielded positive results from anti - inflammatory pharmacotherapy and cell therapy individually , emerging research has begun to target inflammation using combination therapy . 
 the joint use of anti - inflammatory drugs alongside stem cell transplantation may provide better clinical outcomes for traumatic brain injury patients . despite the promising results in this field of research , it is important to note that most of the studies mentioned in this review have completed their studies using animal models . 
 translation of this research into a clinical setting will require additional laboratory experiments and larger preclinical trials . 
Traumatic Brain Injury and Inflammation
Anti-inflammation Pharmacotherapy
Stem Cell Therapy for Sequestration of Inflammation
Future Directions