the earliest host - cell response to viral infection is activation of the type - i interferon 
 ( ifn/ ) system ( see fig .
1 ) , which ultimately 
 leads to the establishment of antiviral responses in infected / neighboring cells and 
 contributes to the shaping of an effective adaptive response [ 55 , 70 ] .
initiation of the ifn 
 response follows recognition of pathogen associated molecular patterns ( pamps ) by 
 germline - encoded pattern - recognition proteins ( prps ) ( e.g. toll - like receptors [ tlrs ] and 
 retinoic acid - inducible gene i [ rigi]-like receptors [ rlrs ] ) [ 55 , 70 ] .
rna virus pamps include 
 double stranded rna ( dsrna ) and uncapped rna with a 5 triphosphate , which are presented by 
 genomic rna or products of transcription / replication ; these variously stimulate the 
 cytoplasmic rlrs rig - i and melanoma differentiation - associated protein 5 ( mda5 ) ( which 
 principally recognize pamps generated within infected cells ) and certain of the 
 transmembrane tlrs ( which detect extracellularly - derived pamps ) [ 6 , 22 , 55 , 70 ] .
rlr signaling proceeds 
 through the adapter protein mitochondrial antiviral - signaling protein ( mavs ) , ultimately 
 activating the kinases , tank - binding kinase 1 ( tbk1 ) and i - kappa b kinase  ( ikk ) ( fig .
1 ) ; an initially distinct signaling pathway from 
 the double stranded rna receptor tlr3 also converges at tbk1/ikk . subsequent phosphorylation / dimerisation of ifn regulatory factor 3 

( irf3 ) in most cell types ( and irf7 in certain immune cells or ifn - primed cells ) precedes 
 nuclear localization leading to ifn/ expression .
nuclear factor - kappa b ( nfkb ) is also 
 activated to form part of the ifn enhanceosome ( transcription factor complex ) , as well as 
 activating the expression of other pro - inflamatory cytokines .
secreted ifn/ subsequently binds to the type - i ifn receptor ( ifnar ) to activate the 
 transcription factors signal transducers and activators of transcription 1 and 2 ( stat1 and 
 2 ) ( fig .
stat1/2 heterodimerization via 
 reciprocal phosphotyrosine - sh2 interactions precedes nuclear translocation and formation 
 with irf9 of the ifn - stimulated gene factor 3 ( isgf3 ) , which upregulates several hundred 
 ifn - stimulated gene ( isg ) products , including antiviral ( e.g. protein kinase r [ pkr ] , 
 tetherin , myxoma virus resistance [ mx ] , promyelocytic leukemia [ pml ] ) and immunostimulatory 
 ( e.g. major histocompatibility complex [ mhc ] ) proteins , to coordinate a potent antiviral 
 response [ 55 , 63 ] .
experiments using genetically modified virus and/or infection of dendritic cells to 
 minimize viral mechanisms that inhibit immune signaling ( see below and fig .
1 ) , have indicated that rabv can induce ifn through pathways 
 dependent on rig - i , mda5 and mavs , but independent of tlr signaling , consistent with 
 increased pathogenicity in mavs - deficient mice [ 16 , 
 22 ] . since pathogenicity is also enhanced in 
 ifnar mice
, this indicates that 
 rabv can induce type i - ifns which are at least partially effective in slowing the onset 
 and/or reducing the severity of symptoms in vivo .
nevertheless , rabv 
 remains a potent killer , indicative of efficient mechanisms to counteract the ifn 
 response .
in common with many other viruses , rabv expresses proteins that can directly inhibit ifn 
 signaling pathways .
collectively termed viral ifn antagonists , which 
 encompass a remarkable array of proteins ( with hundreds of examples in the literature ) , 
 variously reported to target all stages of the ifn response through diverse mechanisms 
 [ 44 , 55 , 
 70 ] .
these mechanisms can be broadly categorized 
 as : ( i ) general inhibition of host gene expression , ( ii ) sequestration / masking of pamps and 
 ( iii ) sequestration / modification of ifn signaling components or isg products .
more generally , mechanisms may be considered as 
 virus - targeted ( where antagonists affect other viral components to 
 minimize pamp production or exposure to prps ) or host - targeted ( where 
 antagonists , often autonomously , form inhibitory interactions with cellular factors ) .
many 
 ifn antagonists are multifunctional with roles in the basic viral life cycle as well as 
 accessory roles in ifn antagonism , which often encompass multiple mechanisms targeting 
 different stages of the response [ 55 , 70 ] .
this is particularly important to rna viruses where 
 limited genome size excludes the expression of dedicated antagonists , resulting in the 
 evolution of accessory ifn - antagonist functions within conserved structural proteins and/or 
 alternative products encoded by conserved genes ( e.g. [ 1 , 11 , 55 , 66 , 70 , 72 ] ) .
this has made analysis of the 
 significance of ifn antagonism in infection / pathogenicity challenging due to potential 
 off - target effects in mutagenic studies ; 
 however , several recent studies of rabv and other viruses indicate critical roles in disease 
 ( see below ) .
rabv p , m and n proteins have ifn antagonist functions that encompass many archetypal 
 features of this class of protein .
p protein is the best defined , with in 
 vitro infection and protein expression studies , indicative of several mechanisms , 
 mediated through a large interactome incorporating host innate immune factors , nuclear 
 trafficking receptors and elements of the cytoskeleton ( reviewed in [ 11 , 31 , 49 , 60 ] ; , summarized below and in 
 figs . 1 and 2 ) .
notably , p protein forms many 
 other interactions with host proteins not obviously associated with immunity , indicative of 
 roles for p as a major  hub  at the virus - host interface ( e.g. [ 17 , 32 , 51 , 56 ] ) .
this is perhaps 
 consistent with a high degree of evolutionary  flexibility  in this non - enzymatic component 
 of the replication machinery , which is encoded by a p gene in all mononegaviruses and is 
 essential to replication , but shows little to no sequence conservation across the mnv order 
 or even between genera of the same family .
the first indications of a role for p protein in immune evasion came from the conzelmann 
 laboratory , through analysis of rabv in which p protein expression was downregulated by 
 translocation of the p gene within the genome . 

although the precise molecular interactions 
 involved in tbk1 inhibition remain unresolved [ 8 , 
 58 ] , it was recently shown that , p proteins of 
 certain wild - type street strains , but not those of representative fixed laboratory strains , 
 have an additional function to inhibit ikk , involving physical interaction with this 
 adapter molecule .
subsequently , the conzelmann and blondel groups identified an interaction of p protein with 
 stat1 and stat2 that inhibits their nuclear translocation and transactivation in response to 
 type - i and type - ii ifns ( [ 9 , 71 ] , fig .
this interaction 
 is conserved among lyssaviruses and notably is strongly dependent on ifn - activation 
 ( phospholylation of stats ) , possibly reflecting a mechanism to ensure that p protein is 
 diverted from replication / transcription function only when required [ 9 , 75 ] .
recently , p protein was 
 also shown to target stat3 , enabling inhibition of il6-dependent signaling , indicative of 
 immune antagonistic roles beyond ifns .
other than the full length p protein ( p or p1 ) , which interacts with l protein in the rdrp 
 complex , rabv p gene expresses four n - terminal truncated isoforms ( p2-p5 ) from four extra 
 in - frame start codons via a ribosomal leaky scanning mechanism .
p2-p5 lack the n - terminal l - binding region , suggestive of 
 alternative functions ( reviewed in ) , including 
 specialized roles in ifn antagonism .
indeed , analysis of rabv in which isoform expression is 
 modulated using an internal ribosomal entry site ( ires ) has suggested that , while p1 is 
 required for efficient replication , p2 has more important roles in suppressing ifn signaling 
 .
protein expression studies have also 
 identified a number of apparently isoform - specific mechanisms of ifn antagonism , suggestive 
 of a multifaceted strategy ( fig .
the mechanisms 
 involved in inhibition of ifn signaling by rabv p proteins are shown .
all of the 
 mechanisms require a physical interaction of p proteins ( p1 , p2 and p3 ) with stats . 

mechanisms directly implicated in pathogenicity 
 are indicated by an asterisk . ) ; this might be required to effect an efficient generalized shutdown of the potent 
 antiviral ifn system or to enable targeting of specific elements / pathways of the ifn 
 response that may be more important during different stages of infection and/or in different 
 cell types .
p1/p2 exploit the cellular trafficking machinery to undergo active nuclear 
 export via an n - terminal nuclear export sequence ( nes ) , thereby effecting strong nuclear 
 exclusion of p1/p2-associated stats ( [ 9 , 23 , 33 , 52 , 71 , 75 , 76 ] , fig .
n - terminal truncation inactivates the 
 nes , activates a nuclear localization sequence ( nls ) and induces association with 
 microtubules [ 7 , 47 , 50 ] ; together with several additional 
 trafficking sequences [ 45 , 46 , 49,50,51,52 ] , this results in p3 distribution to the cytoskeleton and nucleus , 
 enabling cytoskeletal arrest of p3-associated stat1 and a potential intra - nuclear blockade 
 of stat1-dna interaction ( [ 47 , 72 ] , fig .
2 ) . p protein may 
 also target functions of isg products , since p1 and p3 interact with the pml protein . 

although the significance of this interaction is unclear , expression of certain pml isoforms 
 can inhibit infection , suggestive of antiviral functions toward rabv [ 4 , 5 ] .
specific mechanisms of antagonism of ifn signaling by rabv p proteins . the mechanisms 
 involved in inhibition of ifn signaling by rabv p proteins are shown .
all of the 
 mechanisms require a physical interaction of p proteins ( p1 , p2 and p3 ) with stats . 

m protein immune antagonistic activity involves interaction with a novel isoform of nfkb 
 rela protein , relap43 , which was initially identified in a screen for m 
 protein interactors and shown to be a positive regulator of nfkb signaling ( , fig .
m 
 protein interaction inhibited expression of type i ifn , proinflammatory cytokines and 
 antiviral genes irf1 and hiap , consistent with important roles for m - relap43 in immune 
 evasion .
recent data indicate that rabv n protein acts as an ifn antagonist by suppressing ifn 
 induction , although this uses a mechanism distinct from that of p protein , acting upstream 
 of tbk1/irf3 at the level of rig - i ( , fig .
1 ) . notably , this effect was only observed in the 
 context of rabv infection , where the pathogenic rabv strain nishigahara 
 induced lower irf3 activation and expression of type - i ifn / chemokines than the attenuated 
 nishigahara - derivative strain ni - ce .
mutagenic 
 analysis demonstrated that this is due to substitutions in the ni - ce n gene affecting two n 
 protein residues . since single expression of n 
 protein from a plasmid
did not inhibit responses to the dsrna analogue polyi : c or infection 
 by a heterologous virus , this appears to involve a virus - targeted mechanism , potentially due to n protein function in 
 encapsidating the rabv genome / replication products and/or forming cage - like structures 
 around cytoplasmic virus factories ( negri bodies ) [ 30 , 42 ] , thereby inhibiting pamp - prp 
 recognition .
comparative analyses of wild - type street strains and fixed laboratory / attenuated vaccine 
 strains have included the demonstration that the cvs - b2c laboratory strain induces greater 
 inflammatory responses in mice ( including elevated type - i ifn - related gene and 
 pro - inflamatory cytokine expression ) than the street strain of bat origin ( shbrv ) , 
 indicative of greater immuno - evasive function in the latter . similarly , relap3 targeting has been reported to be observed for m protein 
 from wild - type lyssaviruses but not from laboratory / vaccine strains , with analogous results reported for ikk targeting by p protein 
 .
while these data indicate a correlation 
 between ifn antagonist function and pathogenesis , some other findings argue against such a 
 simple relationship , including the report that the cvs - b2c laboratory strain harbors greater 
 capacity to antagonize ifn signaling than a wild - type dog rabv strain drv , which reverse genetic analysis indicated to be due 
 to differences in the ifn - antagonist - encoding p gene of the respective strains .
this is 
 perhaps to be expected , since the attenuated phenotypes of different strains , particularly 
 fixed strains adapted to laboratory animal models and/or cell culture , are likely to have 
 multigenic origin ( e.g. ) ; indeed , previous data 
 suggested that g protein is responsible for the attenuated phenotype of cvs - b2c strain via a 
 mechanism distinct from p protein - mediated immune evasion .
similarly , the behavior of street strains derived from host species , such as 
 bats and dogs , may differ significantly when transferred to laboratory animal models . in 
 this respect
, it is possible that the p protein - dependent difference observed between drv 
 and cvs - b2c in murine neuronal cells   relates to 
 some extent to the adaptation of the latter to rodents since the parental cvs strain was 
 serially passaged in mouse brains .
thus , more direct evidence for key roles of ifn - antagonism in disease has come from reverse 
 genetics studies , which have enabled direct comparison of the pathogenicity of viruses that 
 are homogeneous except for altered expression of , or sequences within , specific 
 ifn - antagonists .
suppression of p protein expression in the sad rabv strain using 
 transcription and translation - based approaches has been shown to cause attenuation 
 in vivo , correlating with reduced capacity to inhibit ifn induction and 
 ifn signaling ( [ 8 , 9 , 36 , 58 ] and see above ) .
the fact that the p - deficient viruses showed more virulent 
 phenotype in ifnar - deficient mice is consistent with an important role of ifn antagonism in 
 the viral pathogenesis ; however , approaches using altered expression of the entire p protein 
 are complicated by potential  off - target  effects due to p protein s function in genome 
 transcription / replication .
furthermore , such approaches can not assess the contribution of 
 particular mechanisms of ifn antagonism .
directed mutagenesis of rabv has enabled such 
 analyses , where deletions within a 10-residue central region of p protein and substitutions 
 in the hydrophobic  w - hole  of the p protein c - terminal domain have been shown to 
 specifically impair antagonism of ifn induction and stat1/2 signaling , respectively [ 58 , 76 ] . 

importantly , defects in replication / transcription function or growth in ifn - incompetent 
 cells were not apparent for viruses carrying either the deletions or the substitutions , but 
 neurovirulence in mice was reduced in both cases , indicating significant roles for 
 antagonism of both ifn induction and signaling in disease .
reverse genetics studies using the nishigahara / ni - ce viruses ( see above ) have also revealed 
 roles in virulence for p protein nuclear trafficking and consequent inhibition of stat1 
 trafficking / signaling [ 23 , 37,38,39 ] .
nishigahara is lethal in mice following intracerebral ( ic ) or 
 intramuscular ( i m ) inoculation , indicative of neurovirulence and neuroinvasiveness , 
 respectively , while the derivative ni - ce strain is non - lethal by either route [ 67 , 78 ] .
although 
 attenuation of ni - ce is multigenic , a 
 significant restoration of pathogenicity in the recombinant ce(nip ) strain , in which the 
 nishigahara p gene is expressed in the ni - ce genetic background , identified p protein as a 
 pathogenic determinant by both inoculation routes [ 23 , 67 , 78 ] .
this correlated with altered viral sensitivity to ifn in neuronal and muscle 
 cell lines , suggesting that circumvention of ifn responses by p protein is important not 
 only to propagation in neurons , but also in muscle cells en route to 
 peripheral nerves [ 23 , 78 ] .
analysis of p protein function in neuronal cells indicated that ni - ce p protein is not 
 defective for suppression of ifn induction or binding to stat1 , but can not prevent stat1 
 nuclear accumulation / signaling due to mutations in the p protein nes that impair nuclear 
 export of p - stat1 complexes .
notably , in muscle 
 cells , ni - ce p protein displayed an additional defect in antagonizing ifn induction , 
 suggestive of differences in ifn responses of these cell types .
this is consistent with the idea that the diverse ifn - antagonistic 
 mechanisms identified for p protein might have developed to enable immune evasion in 
 different cell - types .
the reverse genetics approach using nishigahara / ni - ce chimeric viruses has also highlighted 
 the importance of n protein - dependent inhibition of rig - i signaling in the pathogenicity : 
 the expression of the nishigahara n gene in the ni - ce background [ generating the ce(nin ) 
 strain ] resulted in increased suppression of viral ifn induction and pathogenicity in mice , 
 with decreased ifn responses and increased viral spread observed in the brain [ 38 , 39 ] .
rabv not only impacts on innate immune signaling within infected cells , but also has 
 developed mechanisms to suppress broader adaptive inflammatory responses . in the cns , 
 despite high levels of viral replication in naturally infected humans and animals , a lack of 
 significant infiltration by inflammatory immune cells is a hallmark of pathogenic infection 
 .
several lines of evidence indicate key roles 
 in pathogenesis [ 2 , 74 ] .
.   demonstrated that infection of mouse brain with the pathogenic rabv strain 
 shbrv , only modestly induces expression of chemokines , such as mcp-1 ( ccl2 ) , ip-10 ( cxcl10 ) 
 and rantes ( ccl5 ) , that are critical for leukocyte recruitment to sites of inflammation , 
 while the less pathogenic laboratory strain cvs - b2c induced expression more strongly . 

cytokines including proinflammatory il-6 , and cytokine receptors , were also up - regulated in 
 cvs - b2c - infected brain .
these data indicated a correlation between the ability of rabv to 
 suppress inflammatory responses in the cns and pathogenicity , suggesting that rabv s 
 mechanisms to maintain low levels of chemokines and cytokines and/or their cognate receptors 
 in the infected cns ( including the mechanisms described above ) , suppress inflammation and 
 virus clearance , resulting in a lethal outcome .
similar approaches have provided insights into the mechanisms by which rabv regulates 
 inflammatory cell infiltration of the cns , highlighting a role for the blood - brain barrier 
 ( bbb ) .
in particular , infection with the attenuated cvs - f3 strain increased bbb 
 permeability , which was accompanied by high - levels of expression of several chemokines 
 including ccl2 , ccl5 and cxcl10 in the cerebellum as well as infiltration of cd4 
 t cells and cd19 b cells into this region .
in contrast , infection with pathogenic shbrv maintained bbb integrity and 
 prevented infiltration of immune effectors . 

these data strongly indicated that enhancement of bbb permeability is important for 
 recruitment of immune effectors into the cns and subsequent clearance of attenuated rabv , 
 while pathogenic strains harbor mechanisms to maintain the bbb .
the key role of bbb 
 integrity in the outcome of viral infection was also supported by the finding that in plsjl 
 mice , which show moderate resistance to shbrv infection , reduction of bbb permeability 
 through treatment with a steroid hormone increased the mortality rate of infected mice , 
 while enhancement of bbb permeability by immunization with myelin basic protein resulted in 
 decreased mortality .
it was also reported that , 
 in mouse brain infected with cvs - f3 strain , virus - specific antibody produced in 
 situ after recruitment of b cells into the brain plays a critical role in virus 
 clearance , highlighting the role of the humoral 
 response in the cns .
several studies have begun to delineate the mechanisms by which attenuated rabv infection 
 results in loss of bbb integrity .
phares et al .   reported that in cvs - f3-infected mouse , cd4 t cells , but 
 not cd8 t cells or b cells , play an important role in enhancement of bbb 
 permeability .
the study suggested that ifn- released by cd4 t cells that 
 accumulate in the neurovasculature stimulates neurovascular endothelial cells to produce 
 peroxynitrite ( onoo ) , which directly triggers enhancement of bbb permeability .
a 
 recent study demonstrated that in mouse brain infected with attenuated cvs - b2c strain , 
 cxcl10 produced by infected neurons promotes infiltration of cxcr3 
 cd4 t cells , which can differentiate into ifn--producing th1 cells as well as 
 th17 cells expressing il-17 , an important contributor to disruption of the bbb .
the identification of mechanisms by which attenuated rabv infection results in recruitment 
 of immune effectors via enhanced bbb permeability has provided insights into the strategies 
 used by pathogenic rabv to inhibit inflammatory responses in the cns .
specifically , it is 
 thought that pathogenic rabv minimizes expression of cxcl10 in neurons , thereby preventing 
 infiltration of cxcr3 cd4 t cells ( fig . 3afig .
( a ) infection of neurons by pathogenic but not attenuated rabv suppresses 
 induction of cxcl10 expression , reducing cxcl10-dependent infiltration of 
 cxcr3 cd4 + t cells , thereby preventing transition to il17-producing th17 
 cells that promote bbb permeability and immune cell infiltration .
( b ) rabv infection 
 induces expression of b7-h1 and other immunosuppressive molecules on infected neurons 
 ( and potentially non - infected glia ) , which induce the apoptosis of infiltrating t 
 cells . ) ; this hypothesis is based on the observations that infection of mice by pathogenic 
 shbrv only modestly induces expression of cxcl10 in the mouse brains   and that infection of cultured neuroblastoma with nishigahara induces 
 significantly lower levels of cxcl10 than infection with the attenuated ni - ce .
thus , reduction of cxcl10 expression in infected 
 neurons may be a general strategy used by pathogenic rabv strains to suppress inflammatory 
 responses in the cns .
( a ) infection of neurons by pathogenic but not attenuated rabv suppresses 
 induction of cxcl10 expression , reducing cxcl10-dependent infiltration of 
 cxcr3 cd4 + t cells , thereby preventing transition to il17-producing th17 
 cells that promote bbb permeability and immune cell infiltration .
( b ) rabv infection 
 induces expression of b7-h1 and other immunosuppressive molecules on infected neurons 
 ( and potentially non - infected glia ) , which induce the apoptosis of infiltrating t 
 cells .
interestingly , studies by other groups have suggested that rabv may have evolved 
 additional , distinct strategies to suppress inflammatory t - cell responses in the cns . in 
 contrast to the model above
, it was reported that infection both by pathogenic cvs strain 
 and by attenuated pv strain led to infiltration of cd3 t cells into the cns of 
 infected mice after i m inoculation .
however , by 
 seven days post - inocultion , apoptosis of infiltrating t cells and the resultant 
 disappearance of t cells from the cns were observed in mice infected with cvs , but not with 
 pv . these findings are particularly intriguing as rabv generally does not infect lymphocytes , 
 raising the question of how pathogenic rabv induces t - cell apoptosis .
the most likely 
 scenario appears to relate to the finding that pathogenic rabv infection induces the 
 expression of immunosuppressive molecules , such as b7-h1 , hla - g and fasl , on neurons , which 
 can trigger apoptosis of activated t cells , both in mouse cns and in cultured neurons [ 3 , 27 , 28 , 41 ] .
this 
 indicates that pathogenic rabv can hijack host immunosuppressive mechanisms to destroy 
 infiltrating t cells ( fig .
this hypothesis is 
 directly supported by the finding that a pathogenic cvs strain caused significantly milder 
 symptoms in b7-h1 knockout mice than in wild - type mice .
notably , it was also found that the percentages of cd3 t cells 
 and cd8 t cells in the total t cell infiltrate in the cns of b7-h1 knockout mice 
 were higher than those in the cns of the wild - type mice and that the proportion of apoptotic 
 cd8 t cells was reduced in the cns of the b7-h1 knockout mice .
these findings 
 indicate that the b7-h1-mediated apoptosis of t cells infiltrating the cns fulfills a key 
 role not only in suppression of the inflammatory response , but also in the pathogenesis of 
 rabv .
while the findings above are suggestive of a sophisticated strategy for immune evasion by 
 rabv , there is a discrepancy that needs to be resolved : expression of the immunosuppressive 
 b7-h1 and hla - g , both known isg products , is induced by ifn [ 14 , 27 , 28 ] , but ifn production and signaling is suppressed in neurons infected by 
 pathogenic rabv via the function of n , m and p proteins ( see above ) .
however , even 
 pathogenic rabv infection induces modest but detectable level of ifn in the brain and in 
 cultured neurons [ 37 , 74 ] .
ifn might be sufficient to induce b7-h1 and hla - g expression 
 and to cause destruction of infiltrating t cells .
although it has been speculated that 
 non - infected glia cells play a role in the destruction of t cells in the cns , this remains to be proven experimentally .
current knowledge indicates that rabv effects a potent and multipronged strategy to disable 
 host immunity at many levels , with a growing body of evidence from experiments using 
 genetically modified viruses / hosts indicating critical roles in disease . in ifn - mediated 
 innate immunity ,
the capacity of p protein to interact with and effect mislocalization of 
 stats , and that of p and n proteins to inhibit rlr - driven ifn induction , have been 
 demonstrated to impact on pathogenicity in animals . in adaptive immunity / inflammation of the 
 cns , rabv has a clear capacity to suppress host responses , with two mechanisms suggested in 
 in vivo models , specifically blockage of immune effector infiltration via 
 maintenance of bbb integrity , and induction of t - cell apoptosis ( fig .
firstly , the roles in pathogenicity of a number of ifn - antagonist mechanisms of p 
 protein identified in vitro , such as targeting of pml , stat3 , and 
 microtubules remain unresolved and await research using reverse genetics approaches / animal 
 infection models similar to those that indicated roles in disease for nes - driven p protein 
 nuclear export .
secondly , there are apparently 
 conflicting reports with respect to the roles of ifn signaling / isg expression in infection 
 ( where antiviral and  proviral  roles have been identified ) , as well as the contribution of 
 altered bbb permeability and t - cell apoptosis in the cns to rabv suppression of t - cell 
 immunity [ 28 , 62 ] .
although such observations may relate in part to differences in experimental 
 design / models used , it seems likely that lyssaviruses have evolved diverse mechanisms to 
 effect immune subversion , as indicated by in vitro studies of 
 ifn - antagonism , and that certain mechanisms may predominate in different settings due to 
 factors , such as the viral strains / species examined , cell type infected , stage of infection , 
 infectious dose , etc . for example , a pathogenic strain that only moderately 
 suppresses the expression of ifn and/or cxcl10 in neurons might have evolved mechanisms to 
 induce apoptosis of t cells .
thirdly , in spite of clear evidence that suppression of 
 inflammation in the cns is important to disease , the molecular mechanisms by which the virus 
 achieves this remain undefined , although involvement of p , m or n protein in suppressing 
 signaling during cytokine / chemokine production seems likely .
future research using genetically modified viruses / hosts should help to delineate these 
 issues , but what is clear at present is that mutations impacting viral evasion of immunity 
 can potently impact disease , indicating that such mutants are potentially useful for the 
 generation of attenuated vaccine strains .
in particular , novel mutations affecting immune 
 evasion may be combined with mutations affecting other genes / mechanisms [ 15 , 24 , 38 , 67,68,69 ] to achieve 
 high levels of safety while retaining largely native structure and replication to generate 
 self - adjuventing vaccines ; such advances should greatly enhance deliverability , including 
 providing the potential means to achieve single - dose oral vaccination , thereby overcoming 
 current limitations of multiple - dose / needle injection regimes , as well as providing potent 
 protection in diverse host species .
importantly , such vaccines should not simply be 
  weakened  , but should have the potential to induce stronger immune responses that could 
 generate protection more efficiently .
this promise is yet to be realized , however , as a 
 recent study using a p protein - mutated rabv strain unable to prevent ifn induction could 
 induce protection in orally - vaccinated foxes , but gave no apparent improvement in protection 
 compared with the wild - type strain .
the 
 effectiveness of such mutations in attenuating strains optimized for immunogenicity , or of 
 alternative strategies targeting stat antagonism by p protein , or rig - i - antagonism by n 
 protein , awaits further research .
finally , another clear corollary of the above considerations is that the mechanisms 
 underlying immune evasion may provide targets for therapeutics for rabies .
several molecular 
 interfaces have been shown to be critical to viral inhibition of immune signaling and 
 virulence ( e.g. p protein - stat1 , p - protein - exportin interactions ) .
others have been defined 
 as important in vitro but not confirmed in vivo ( see 
 above ) , and some involve interactions as yet undefined at the molecular level ( e.g. n 
 protein - mediated inhibition of rig - i ) . in most cases ,
however , progress has been made toward 
 elucidating the sites involved by mapping important domains / sequences [ 5 , 38 , 76 ] .
the ultimate outcome of such research would be molecular and 
 structural definition of the interfaces , providing knowledge that might guide the 
 development of inhibitors capable of preventing viral evasion of innate immunity .
until the 
 mechanisms underlying viral modulation of bbb permeability and t - cell apoptosis are 
 resolved , potential pathways to therapeutic targeting of these processes are less clear . 
 however , since viral targeting of innate signaling is probably linked to effects on adaptive 
 immunity , the above mentioned therapeutics may have a dual effect on both arms of the 
 response .
it is also possible that targeting the host to enhance adaptive responses , rather 
 than targeting viral counter - measures might be useful ; indeed , mouse studies indicated that 
 strategies to directly alter bbb permeability can improve outcomes of infection , providing proof - of - principal to assess such 
 possibilities towards antiviral therapies .
 rabies is a zoonotic disease caused by the lyssavirus rabies virus 
 ( rabv ) that can infect most mammals , including humans , where it has a case - fatality rate 
 of almost 100% . 
 although preventable by vaccination , rabies causes c. 59,000 human 
 fatalities every year worldwide . 
 thus , there exists an urgent need to establish an 
 effective therapy and/or improve dissemination of vaccines for humans and animals . 
 these 
 outcomes require greater understanding of the mechanisms of rabv pathogenesis to identify 
 new molecular targets for the development of therapeutics and/or live vaccines with high 
 levels of safety . 
 importantly , a number of studies in recent years have indicated that 
 rabv specifically suppresses host immunity through diverse mechanisms and that this is a 
 key process in pathogenicity . here , 
 we review current understanding of immune modulation 
 by rabv , with an emphasis on its significance to pathogenicity and the potential 
 exploitation of this knowledge to develop new vaccines and antivirals . 
THE IFN RESPONSE AND ITS INDUCTION BY RABV
VIRAL COUNTERMEASURES: IFN ANTAGONISTS
LYSSAVIRUS IFN ANTAGONISTS
ROLES OF IFN ANTAGONISM IN PATHOGENICITY
EVASION OF INFLAMMATORY RESPONSES IN THE CNS
PERSPECTIVES