dcs are part of the innate immune system and function as sentinels for pathogens at potential sites of invasion ( e.g. , the skin or the gastrointestinal tract ) .
proteins are then converted into peptides which are subsequently presented on major histocompatibility complex ( mhc ) molecules and recognized by t lymphocytes . the dendritic cell drives the immune response depending on the kind of antigen it has taken up .
if the host needs defense against an invaded pathogen , dcs present the antigenic peptides to cytotoxic cd8 t cells and proinflammatory cd4 t helper ( th1 ) cells activating the lymphocytes via direct cell - cell contact and proinflammatory cytokines such as interleukin- ( il- ) 12 or il-6 [ 3 , 4 ] . if autoimmunity or abundant inflammation needs to be dampened dcs interact with regulatory t cells ( treg ) directly and via secretion of anti - inflammatory cytokines such as il-10 or retinoic acid ( ra ) [ 57 ] .
what is often hardly considered in experimental settings is the fact that all processes directing the immune response of our organism take place under deprivation of nutrients and oxygen .
the interplay of dendritic cells and lymphocytes either takes place in severely inflamed tissue or in secondary lymphoid organs .
oxygen distribution in the spleen and lymph nodes is highly variable and hypoxic lymphocytes have been identified in both organs .
the adaptation of cells to reduced oxygen tensions is largely coordinated by hypoxia - inducible factors ( hifs ) which have come into the focus of immunological research during the last ten years .
this review focuses on mouse and human dendritic cells and their maturation and activation under hypoxic conditions . by covering publications on in vitro studies a particular emphasis
is put on hifs as coordinators of the genetic response to hypoxia . the availability of mice with a dc - specific hif-1 k .- o .
now allows a first appreciation of the potential role of hypoxia in dc function in vivo .
hypoxia - inducible factors belong to the family of basic helix - loop - helix proteins .
they consist of one - ( hif-1 , hif-2 , or hif-3 ) and one common -subunit ( hif-1 or arnt , aryl carbon nuclear translocator ) forming the dna - binding transcription factor dimer .
whereas the hif-1 protein is not affected by changes in oxygen tension hif- proteins are only detectable under hypoxic conditions .
mrna is expressed and transcribed in all nucleated cells whereas the expression of hif-2 is more limited .
hif-2 is found especially in endothelial cells but also in immune cells such as macrophages or dendritic cells .
the role of hif-3 remains poorly defined so far , particularly in immune cells , and this review will therefore focus on hif-1 and hif-2. hif- proteins are under tight posttranslational control by oxygen . under normoxia ,
specific prolyl hydroxylase domain containing enzymes ( phd 1 , 2 , and 3 ) use molecular oxygen to immediately hydroxylate distinct prolyl residues of newly synthesized hif--subunits .
hydroxylated hif- ( oh - hif- ) interacts with the von - hippel - lindau protein ( pvhl ) e3 ligase complex that polyubiquinates oh - hif- leading to instantaneous proteasomal degradation under normoxia .
in contrast , under hypoxia , prolyl hydroxylases lack oxygen as a crucial cosubstrate and are reduced in their enzymatic activity .
hif-s can accumulate , translocate into the nucleus , and dimerize with hif-1. the transcription factor complexes hif-1 ( hif-1/hif-1 ) and hif-2 ( hif-2/hif-1 ) recruit cofactors such as p300/cbp ( camp - response element binding protein ) and bind to hypoxia - responsive elements of target gene dna .
in addition to proline hydroxylation , hif-1 and hif-2 can be hydroxylated at an asparagine residue in their c - terminal part .
asparagine hydroxylation is controlled by an oxygen - sensitive asparagyl hydroxylase , termed factor - inhibiting hif- ( fih ) 1 .
hifs have been shown to regulate more than 100 genes that are involved in glucose metabolism , cell death , cell cycle , angiogenesis , and erythropoiesis . during the last decade
it has been recognized that many other factors are able to induce hif- although hif-1 has been studied more thoroughly than hif-2. bacterial lipopolysaccharides ( lps ) are able to induce the nf-b pathway and hif-1 mrna has been shown to be a target of classical nf-b activation by several groups [ 1215 ] .
recently it was shown that hif-1 is one of the essential modulators of the cytokine response to bacterial lps as it is crucial for the synthesis of il-1 .
furthermore , hif-1 protein and mrna expression have been shown to be induced in macrophages purified from wounds .
the authors could show that elevated protein levels hereby depended on the inflammatory cytokine tnf- .
reactive oxygen species ( ros ) have been shown to induce hif-1 as well , although this effect seemed to be time - dependent as external h2o2 induced hif-1 protein in human osteosarcoma cells at early time points but suppressed it later on .
nitric oxide ( no ) has been shown to induce hif-1 in normoxic macrophages stimulating macrophage migration by modulating the actin cytoskeleton via small gtpases .
the role of hif-2 in the function of immune cells has been studied much less .
macrophage no production and expression of costimulatory molecules cd86 and mhcii were unaffected by loss of hif-2 , but imtiyaz and coworkers found profound changes in cytokine mrna expression and protein release after stimulation with lps and interferon  ( ifn ) .
murine bone - marrow - derived macrophages lacking hif-2 showed reduced induction of il-1 , il12p35 , cxcl2 , and il-6 mrna levels under inflammatory hypoxia whereas hypoxia alone if at all only moderately affected mrna expression of these genes . moving to in vivo models the authors found that macrophage hif-2 was required for an adequate immune response to cutaneous and peritoneal irritants . furthermore , loss of macrophage hif-2 prevented infiltration of tumor associated macrophages ( tams ) in models of hepatocellular carcinoma and colitis - associated cancer which inhibited tumor growth .
the role of hif-2 in the function of dendritic cells under hypoxia is almost completely unexplored , although filippi et al .
hif-1 activity has been shown to be induced by a number of viral infections as well .
hepatitis bx protein ( hbx ) increases the stability of hif-1 protein via p42/44 mitogen - activated protein kinases ( mapk ) .
transactivation of hif-1 is also increased as hbx induces the interaction of the transcription factor with cbp , one of the cofactors hifs recruit for dna binding .
kaposi 's sarcoma - associated herpesvirus ( kshv ) and epstein - barr - virus ( ebv ) induce hif-1 protein by increasing the proteasomal degradation of proteins involved in the normoxic degradation of hif-1. lana , kshv latency - associated nuclear antigen , increases the degradation of pvhl whereas the phds 1 and 3 are degraded by proteasomes upon ebv infection .
furthermore , kshv expresses functional hres in the promoter regions of viral genes which could be activated in vitro by binding of either hif-1 or hif-2 . taking these data into consideration , activation of hif-1 ( and potentially hif-2 ) in viral infections seems to bring benefit to the pathogen rather than the host .
dcs are central in coordinating immune responses against pathogens : whenever they register pathogens they pick them up , process the proteins of pathogens , and present typical antigenic peptides to cells of the adaptive immunity . these processes
do not only activate the cell but also induce dc differentiation and maturation and make them migrate towards secondary lymphoid organs .
dc differentiation and maturation are accompanied and at least in part defined by the upregulation of costimulatory molecules such as cd80 and cd86 .
furthermore , mature dendritic cells express high levels of surface mhcii and cd40 and are able to secrete il-12 .
this is important as dcs have to function as antigen presenters and producers of il-12 at the same time to induce differentiation and proliferation of th1 cells .
until now , several groups have analyzed dc function under hypoxic conditions . for human dcs
it has been shown that differentiation of dcs from blood monocytes under hypoxia resulted in a more active phenotype exhibiting a higher ability to stimulate allogeneic t cell responses .
furthermore , hypoxic immature dcs have been shown to downregulate bacterial phagocytosis and exhibit an increased migratory capacity .
several groups have contributed to the current knowledge on the underlying mechanisms : elia et al .   and ricciardi et al .  
have found higher expression of the costimulatory molecules cd80 , hla class ii , and cd86 on the cellular surface of hypoxic immature dcs .
in contrast , costimulatory molecule expression appeared not to be different between mature hypoxic or normoxic dcs .
reported that hypoxic immature like mature dc exhibited higher allo - t cell stimulation than normoxic dcs .
this finding was partly supported by elia et al .   who also found this effect for hypoxic immature dcs .
exposed immature monocyte - derived dcs to hypoxia but could not detect any differences in the expression of costimulatory markers . in turn ,
in contrast observed a hypoxia - mediated differentiation of immature human monocyte - derived dcs .
they exposed the cells for longer time periods to harsher hypoxic conditions than spirig and coworkers and could detect a higher expression of cd40 after hypoxic treatment .
hypoxia also led to higher t cell stimulatory activity of dcs and this effect could be blocked by inhibition of the hif pathway . of note ,
maturation of dcs under hypoxia leads to a shift in the expression of chemokines and chemokine receptors .
while chemokines get downregulated chemokine receptors are upregulated indicating that hypoxia may favor migratory capacity of mature dcs carrying cytokine receptors rather than immunologic functions such as immune cell recruitment [ 29 , 33 ] .
could recently show that short - term hypoxia induces the migratory capacity of immature and mature dcs in an in vitro migration assay and that this effect depends on the expression of hif-1 .
several groups have shown that hypoxia downregulated antigen uptake by immature dcs [ 28 , 30 ] in a manner that seemed to be independent of hif-1 .
these in part divergent reports may in fact be explained by some still existing uncertainties about the actual degree of cellular hypoxia under different experimental conditions .
in addition , the duration of hypoxic exposure will affect the dc response as well as continuous versus intermittent hypoxia .
hypoxia did not change costimulatory molecule expression of mature dcs , but several groups have found other hypoxia - dependent changes in mature dc function . chronic hypoxia promoted the onset of a highly proinflammatory gene expression profile in mature dcs generated from human monocytes .
hypoxic mature dcs thereby showed induced gene expression of cytokines and chemokines that are known to induce endothelial cell survival , recruitment and adhesion of mononuclear phagocytes , and recruitment and activation of predominantly th1/th17 cells .
yang et al .   extended this knowledge and showed that hypoxic mature dcs upregulated the expression of a2b adenosine receptor ( a2bar ) and thereby predominantly induced th2 activation . in 2011 , bosco et al .
found that chronic hypoxia potently induced the cell surface expression of triggering receptor expressed on myeloid cells- ( trem- ) 1 on mature dcs .
the authors illustrated a transient induction depending on the severity of hypoxia and they identified an hre in the promoter region of trem-1 .
additional work of the same group has shown that hypoxia induces trem-1 expression also in immature dcs . trem-1
cross - linking has been associated with an induced release of inflammatory cytokines such as tnf- , il-6 , and chemokines such as cxcl8 , ccl4 , and ccl5 .
in addition , trem-1 cross - linking has been shown to induce the release of il12p70 , a cytokine inducing th1 immune responses .
this would mean that hypoxia does not only favor migration of mature dcs but also modulates their inflammatory repertoire to attract other immune cells and to direct t cell activation .
thus , for human dendritic cells it seems appropriate when bosco and varesio claim a 
examples of how hypoxia affects differentiation and cytokine response of human dendritic cells can be found in table 1 .
reported that murine bone - marrow - derived dendritic cells upregulate cd80 and cd86 after exposure to hypoxia and bacterial lps in an hif-1-dependent manner .
consistent with these findings dcs cultivated under hypoxic conditions were less efficient in antigen uptake as they showed a more mature phenotype .
in contrast , hypoxic treatment ( 24  h ) alone was not able to induce expression of costimulatory molecules on the surface of bmdcs .
this group used a conditional knockdown of hif-1 in bmdcs ( by crossing hif-1 mice with cd11c - cre animals ) rather than a sirna approach and they kept bmdcs under normoxic or hypoxic conditions during the whole process of differentiation from bone marrow cells .
dcs differentiated for six days under hypoxic conditions showed a marked upregulation of cd80 , cd86 , and mhcii but none of these molecules showed an hif-1 dependent regulation .
the work of khler et al . instead revealed changes in cytokine expression of hypoxic dcs which were mainly unaffected by loss of hif-1. only secretion of il-22 seemed to be hif-1 dependent when bmdcs were differentiated under hypoxic conditions .
in addition , only dcs expressing functional hif-1 showed an increase of surface ccr7 after hypoxic differentiation .
further experiments regarding dc stimulation with defined pathogens revealed that bacterial cpgs but not viral poly(i : c ) were able to stabilize hif-1 protein .
concluded that myd88 is essential to induce inflammatory , hif-1-dependent gene transcription ( myd88 is not involved in intracellular tlr3 signaling triggered by poly(i : c ) ) .
one potential target gene of hif-1 in dcs cultivated under inflammatory hypoxia ( hypoxia + lps or hypoxia + cpgs , resp . ) was inducible nitric oxide synthase .
cpgs could stimulate hif-1 protein in bmdcs generated from control and conditional hif-1 knockout animals ( deletion of the dna binding domain of hif-1 under the control of the lyz2-cre - promoter ; bmdc cultures of these mice show knockout in generated dcs , although circulating dendritic cells do not express lyz2-cre - promoter ) .
furthermore , bmdcs lacking functional hif-1 protein showed severe deficiencies in the release of type i interferons after lps stimulation and could not induce a proper t cell activation in an in vitro cd8 lymphocyte activation assay .
last but not least hif-1-induced gene expression has been found to modulate the differentiation of murine plasmacytoid dcs ( pdcs ) .
fms - related tyrosine kinase 3-ligand ( flt-3l ) induced differentiation of pdcs from murine bone marrow cells was dramatically reduced upon hypoxic cultivation and this effect was not found when pdcs were lacking hif-1. inhibitor of dna binding 2 ( id2 ) was identified as suppressor of differentiation .
id2 thereby seemed to be exclusively regulated by hif-1 as a loss of hif-2 in the respective cells could not hinder suppression of differentiation .
examples on how hypoxia affects the differentiation and cytokine response of murine dendritic cells can be found in table 2 .
until now , there are only few published studies that address the role of hifs in dendritic cells in inflammatory settings .
have analyzed the role of hifs for plasmacytoid dc differentiation and found that hif-1 limited pdc generation in the bone marrow . in its absence ,
pdc development was encouraged and numbers of pdc increased . when mice with breast tumors in a pymt - mmtv model were crossed with hif-1 lyz2-cre mice lacking functional hif-1 in myeloid cells
observed markedly enhanced numbers of pdc within the tumors compared with wt controls . whether this affects tumor progression and disease outcome still has to be analyzed .
khler et al .   have assayed the ability of bmdcs differentiated under hypoxic conditions to get recruited to secondary lymphoid organs . for this purpose , they generated differentially labeled bmdcs from hif-1 and hif-1 cd11c - cre mice under normoxic and hypoxic conditions and injected equal amounts of wt and hif-1 k .- o .
they could distinguish how many wt or knockout dcs had migrated towards the popliteal lymph nodes and found that the hypoxic increase in dc migration was hif-1 dependent .
three other models have been used to elucidate the influence of dendritic cell hif expression on their interaction with t cells .
first , vaccination efficacy has been addressed and second two models of inflammation were investigated , namely , leishmania infection and dextran sodium sulfate- ( dss- ) induced murine colitis [ 4648 ] .
bhandari et al . studied hif-1 tie2-cre mice showing knockout not only in endothelial but also in hematopoietic cells .
they vaccinated hif-1 and hif-1 tie2-cre mice with a synthetic ova peptide that specifically induced a cd8 t cell response .
analysis of ifn release by specific t cells in the spleens eight days after vaccination revealed that dendritic cells lacking hif-1 very insufficiently induced t cell activation compared to wt dcs .
in addition to this , hif-1 tie2-cre mice showed reduced titers of specific ova antibodies .
very recent studies used mouse models with dendritic hif-1 knockout in inflammatory settings in vivo .
hammami et al .   analyzed hif-1 and hif-1 cd11c - cre animals in leishmania infection .
they found that ablation of hif-1 in dendritic cells resulted in a more efficient immune response .
this comprised increased production of il-12 , induced expansion of cd8 cells , and higher frequency of short - lived effector cells , a specialized cd8 t cell population that can be found in resolving acute infection .
this could be shown to restrict cd8 t cell expansion as hemizygous hif-1 mice exhibited a significantly higher expansion of cd8 cells .
fully in line with these findings that hif-1 dampens the inflammatory response , but with a different consequence for the outcome of these mice , flck et al . reported that hif-1 is essential for dendritic cells to induce regulatory t cells ( tregs ) in a model of acute dss - colitis .
hif-1 mice and hif-1 cd11c - cre animals received equal amounts of dss , but loss of dendritic hif-1 caused more abundant inflammation and worse outcome for the animals .
mesenteric lymph nodes of hif-1 cd11c - cre animals showed significantly reduced expression of the anti - inflammatory cytokine il-10 and of transforming growth factor- ( tgf- ) both are potent inducers of tregs .
in addition , the authors found a strongly reduced expression of aldehyde dehydrogenase ( aldh ) 1a2 , which is necessary for dendritic cells to catalyze retinal to retinoic acid ( ra ) .
t cells of these lymph nodes in turn showed reduced expression of the retinoic acid receptor ( rar )  and diminished expression of the gut homing markers ccr9 and 47 integrin .
hif-1 deficiency in dcs thus abrogated their ability to induce treg proliferation in secondary lymphoid organs and disrupted treg homing towards the inflamed gut .
apart from the interplay between dendritic cells and t cells predominantly occurring in the mesenteric lymph nodes the authors could also show that the loss of hif-1 by dcs affected the crosstalk between dcs and intestinal epithelial cells ( iecs ) causing iecs to produce more mucins and therefore overcome intestinal barrier damage .
another hint that loss of dendritic hif-1 may affect the crosstalk between dcs and iecs was the finding that only hif-1 mice exhibited increased levels of thymic stromal lymphopoietin receptor ( tslpr ) in dss colitis .
a reduced expression of the respective receptor may drive dcs towards a more inflammatory phenotype .  
figure 1 summarizes the effects of ( inflammatory ) hypoxia and hif-1 knockout on dendritic cells as it has been discussed above .
dendritic cells normally have to function under conditions of inflammatory hypoxia . not only inflammatory stimuli , but also hypoxia profoundly affects their cellular function .
the key transcription factors regulating dcs ' adaptation to conditions of ( inflammatory ) hypoxia are hypoxia - inducible factors ( hifs ) 1 and 2 .
hypoxic culture of immature dcs induces expression of costimulatory molecules such as cd80 , cd86 , and hla class ii / mhcii [ 28 , 29 ] . whether these effects are dependent on hypoxia - inducible factor- ( hif- ) 1 or not is still under debate [ 41 , 42 ] . in contrast , hif-1 is well recognized to affect migratory capacities of human and murine dendritic cells , most likely by shaping the cellular chemokine / chemokine receptor profile [ 21 , 29 ] .
until now , only very few in vivo studies have addressed the role of hifs for dendritic cell function ; however these studies indicate that hif-1 affects ( i ) differentiation of plasmacytoid dendritic cells in the bone marrow , ( ii ) migration of dcs towards secondary lymphoid organs , ( iii ) cd8 t cell activation and release of pathogen - specific antibodies , ( iv ) expansion of cd8 t cells and short - lived effector cells , and ( v ) induction of tregs .
used the identical dendritic cell - specific hif-1 knockout model ( hif-1 cd11c - cre ) in different inflammatory settings . very strikingly , both studies reported findings tending in the same direction but with very different outcomes for the treated animals . in both studies , dendritic cells lacking hif-1 were the more potent inducers of an inflammatory response  with benefits for the outcome in leishmania infection   but abundant inflammation and more severe illness in dss colitis .
clearly , more in vivo studies are needed to better understand the fragile balance between necessary induction of immunity to fight a disease and overactivation of immune cells damaging the host .
hypoxia - inducible factors may thereby be important transcriptional regulators in the balance of the immune status of dendritic cells .
 dendritic cells ( dcs ) are considered as one of the main regulators of immune responses . 
 they collect antigens , process them , and present typical antigenic structures to lymphocytes , thereby inducing an adaptive immune response . 
 all these processes take place under conditions of oxygen shortage ( hypoxia ) which is often not considered in experimental settings . 
 this review highlights how deeply hypoxia modulates human as well as mouse immature and mature dendritic cell functions . 
 it tries to link in vitro results to actual in vivo studies and outlines how hypoxia - mediated shaping of dendritic cells affects the activation of ( innate ) immunity . 
1. Introduction
2. Criteria for This Review
3. Conclusion