Japanese encephalitis virus (JEV) has become one of the most common causes of viral encephalitis through the South-East Asia. Indeed, more than 60% of the world population inhabits Japanese encephalitis, and the virus is currently spreading to previou...
Japanese encephalitis virus (JEV) has become one of the most common causes of viral encephalitis through the South-East Asia. Indeed, more than 60% of the world population inhabits Japanese encephalitis, and the virus is currently spreading to previously unaffected regions, such as Indonesia, Pakistan, and northern areas of Australia. Japanese encephalitis is a mosquito-borne zoonotic viral disease of the central nervous system caused by JEV, a member of the genus Flavivirus, that is symptomatically, genetically, and ecologically similar to West Nile virus and Dengue virus. It is estimated that 30,000 to 50,000 cases of JE occur each year, resulting in 10,000 to 15,000 deaths, but this actual number of cases might be underestimated. However, the exact pathogenesis of JEV-associated disease in human and mice has not yet been completely elucidated. Also, how does the innate immune response contribute to viral encephalitis and regulate adaptive immunity in vivo is still debatable. The present study on virus induced encephalitis was designed and performed to address all the questions, and ultimately to provide information on JE immunotherapeutics.
According to our study results, the different triggering of Toll-like receptor signal array, a major innate immune receptor recognizing viral antigens strikingly regulates the contrast outcome of viral encephalitis caused by JEV. Notably, TLR3-/- mice were highly susceptible to viral encephalitis marked by enhanced viral replication, early recruitment of inflammatory CD11b+Ly6Chigh monocytes and depreciation of blood-brain barrier permeability, whereas TLR4-/- mice showed enhanced resistance to viral encephalitis with reduced immunopathological phenomena, compared to wild-type mice. Infection of cultured BMDC with JEV showed that TLR3-/- BMDC was more permissive to viral replication and showed diminished type I IFN defense. In contrast, TLR4-/- BMDC showed markedly enhanced innate defense of type I IFNs, thereby leading to reduced viral replication. Furthermore, TLR4-/- mice elicited enhanced JEV-specific CD4+ and CD8+ T cell responses as observed by higher number of IFN-?? and TNF-??-producing CD4+ and CD8+ T cells. One more interesting finding of our study is that the early expansion of CD4+CD25+Foxp3+ regulatory T cells by JEV infection was dependent on TLR4 signal pathway, thereby regulating early immune responses that might contribute to the restriction of early viral replication. Our results suggest that JEV affects CD4+CD25+Foxp3+ Treg homeostasis through TLR4 signal pathway.
Moreover, we explored the contribution of CD4+ Th subsets, Th1, Th2, Th17 as well as Foxp3+ Treg cells, to neurological disorder during the progression of JE. Sub-lethal infection with JEV induced activation of CD4+ and CD8+ T cells with peak levels at 3 days post-infection, whereas lethal infection provided activation of CD4+ and CD8+ T cells peaked at 5 days post-infection. In particular, mice showing neurological disorder had higher proportion of activated CD4+ and CD8+ T cells than non-paralyzed mice, despite lymphopenia. Moreover, altered proportion of IL-17-producing Th17 and CD4+Foxp3+ Treg cells was observed between paralyzed and non-paralyzed mice. Interestingly, while CD4+Foxp3+ Tregs that were adoptively transferred 2 days prior to infection made the recipients vulnerable to JE, CD4+Foxp3+ Treg cells that were adoptively transferred 2 days after infection provided resistance to JE, which suggesting that CD4+Foxp3+ Treg cells elicited dual-phased roles during the progression of neurological disorder caused by JEV infection. This dual-phased role of CD4+Foxp3+ Tregs was further confirmed by using Foxp3-diphtheria toxin receptor (DTR) knock-in mice that CD4+Foxp3+ Tregs can be depleted with infection of diphtheria toxin (DT). Therefore, our results suggest that the balanced regulation between CD4+ Th subsets during the progression of JE affect the outcome of disease, thereby providing useful information to JE therapeutics and prognosis.