Type-I interferon response affects an inoculation dose-independent mortality in mice following Japanese encephalitis virus infection.

BACKGROUND: The laboratory mouse model is commonly employed to study the pathogenesis of encephalitic flaviviruses such as Japanese encephalitis virus (JEV). However, it is known that some strains of these viruses do not elicit a typical mortality dose response curve from this organism after peripheral infection and the reason for it has not yet been fully understood. It is suggested that induction of more vigorous Type-I IFN (IFN-I) response might control early virus dissemination following increasing infectious challenge doses of the virus. Thus, the objective of this study was to examine this suggested role of IFN-I in the mortality of mice infected with various doses of JEV. METHODS: Inbred 129 mice and their IFNAR KO (A129) mice were subcutaneously inoculated with 100, 102, 104 or 106 pfu of JaOArS982 strain of JEV. Mice were weighed daily and observed for clinical signs. Virus titers in the brains and spleens of JEV-infected mice were determined by plaque forming assays. The upregulated mRNA levels of genes related to IFN-I response of mice were examined by real-time PCR. RESULTS: The mortality rates of 129 mice infected with JaOArS982 did not significantly increase despite the increase in inoculation dose and no significant difference of viral loads was observed between their brains. However, there was clear elevation of the mRNA levels of interferon regulatory factor (IRF)3, IRF7, IRF9, MDA5 and RIG-I at 24 hours post-infection depending on the inoculation dose. In A129 mice, length of survival days and the viral loads of spleen and brain were observed to be inoculation dose-dependent. CONCLUSIONS: From these results, it is suggested that early IFN-I response elicited by high inoculation doses of JEV provides an anti-viral effect during the early phase of infection. Accordingly, virus replication is counteracted by IFN-I response at each increasing inoculation dose resulting in the interference of impending severe disease course or fatal outcome; hence, this might explain the inoculation dose-independent mortality in mice caused by Japanese encephalitis virus.

TNF-α acts as an immunoregulator in the mouse brain by reducing the incidence of severe disease following Japanese encephalitis virus infection.

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.