TNF-α Acts as an Immunoregulator in the Mouse Brain by Reducing the Incidence of Severe Disease Following Japanese Encephalitis Virus Infection

Abstract

Japanese encephalitis virus (JEV) causes acute central nervous system (CNS) disease in humans, in whom the clinical symptoms vary from febrile illness to meningitis and encephalitis. However, the mechanism of severe encephalitis has not been fully elucidated. In this study, using a mouse model, we investigated the pathogenetic mechanisms that correlate with fatal JEV infection. Following extraneural infection with the JaOArS982 strain of JEV, infected mice exhibited clinical signs ranging from mild to fatal outcome. Comparison of the pathogenetic response between severe and mild cases of JaOArS982-infected mice revealed increased levels of TNF-α in the brains of severe cases. However, unexpectedly, the mortality rate of TNF-α KO mice was significantly increased compared with that of WT mice, indicating that TNF-α plays a protective role against fatal infection. Interestingly, there were no significant differences of viral load in the CNS between WT and TNF-α KO mice. However, exaggerated inflammatory responses were observed in the CNS of TNF-α KO mice. Although these observations were also obtained in IL-10 KO mice, the mortality and enhanced inflammatory responses were more pronounced in TNF-α KO mice. Our findings therefore provide the first evidence that TNF-α has an immunoregulatory effect on pro-inflammatory cytokines in the CNS during JEV infection and consequently protects the animals from fatal disease. Thus, we propose that the increased level of TNF-α in severe cases was the result of severe disease, and secondly that immunopathological effects contribute to severe neuronal degeneration resulting in fatal disease. In future, further elucidation of the immunoregulatory mechanism of TNF-α will be an important priority to enable the development of effective treatment strategies for Japanese encephalitis.

A Single Amino Acid Substitution in the NS2A Protein of Japanese Encephalitis Virus Affects Virus Propagation In Vitro but Not In Vivo

“…Viral loads in the brain were also compared as previously shown (8,12). Unexpectedly, S982_I A23 F A113 D B81 showed survival curves similar to that of the parent S982-IC virus (Fig.…”

“…Several viral and host factors affect disease severity during JEV infection. We previously suggested that the host response, resulting in immunopathological effects, contributes to fatal infections (8). Furthermore, we also demonstrated that the JaOArS982 and JaTH160 strains of JEV exhibited different virulence in mice (8).…”

“…We previously suggested that the host response, resulting in immunopathological effects, contributes to fatal infections (8). Furthermore, we also demonstrated that the JaOArS982 and JaTH160 strains of JEV exhibited different virulence in mice (8). Therefore, a geneticsbased comparison between these strains may provide an effective approach to identify viral factors contributing to severe disease.…”

“…Our previous data showed that viral loads in the CNS of JaTH160-infected mice were significantly higher than those of JaOArS982-infected mice (8). This raised the possibility that virus propagation in neuronal cells is different between JaTH160 and JaOArS982.…”

“…Our previous results showed that following subcutaneous infection with 10 4 PFU of JaTH160, mice showed 100% mortality, whereas JaOArS982 caused approximately 30% mortality in mice (8). We first constructed infectious cDNA clones harboring fulllength genes of JaOArS982 and JaTH160 and produced infectious viruses S982-IC and JaTH-IC from the respective cDNAs (9).…”