In this study, we confirmed that mouse mortality is not dependent on the inoculation dose of JEV, that the increase in the mRNA levels of IFN-I related genes in mouse is suggested to be related to the increase of the dose of inoculated JEV, and that when IFN receptor is incapacitated during infection an inoculation dose-dependent mortality can occur in a mouse. Taken together, these suggest that IFN-I response affects the dose-independent mortality in a mouse model.
In our preliminary experiments, we intravenously injected constant amount of Poly (I:C) (a potent IFN inducer) or exogenous IFNs in mice following JEV infection to examine whether this treatment could provide protection in JEV-infected mice at lower inoculation dose but not at a higher dose. However, apparent protective effect on mortality by this treatment was not observed, and hence dose-independent mortality was not restored (data not shown). It could be due to technical problem that prevented IFN effects to reach local sites of infected tissues, because apparent IFN-I induction was not confirmed in the serum of mice injected with either inoculum (data not shown). However, this kind of approach is important because it may be able to give certain clues for elucidating further the mechanism on dose-independent mortality and thus further improvement of experimental design is required.
IFN-I response of JEV infected mice was initially examined by determining the levels of IFN-α and IFN-β in the serum through ELISA, but our attempt failed even in the mice that showed high mRNA levels of IFN-α and IFN-β in the spleen, e.g. those that received 106 pfu inoculation at 24 hours pi and 102 pfu inoculation at 72 hours pi. It could be due to technical difficulty. Therefore, the mRNA levels which were easier to detect by quantitative real-time RT-PCR were determined instead.
We examined the levels for IRF3, IRF7, IRF9, MDA5, RIG-I and PKR. IRFs play central roles in the induction of IFN-I at the gene transcriptional level . IRF3 and IRF7 have been implicated as positive regulators of IFN-I gene expression induced by virus infections [18, 19], whereas IRF9 constitutes an IFN-stimulated gene factor 3 together with STAT1 and STAT2, and is responsible for the induction of the IRF7 gene . MDA5 and RIG-I function as cytoplasmic sensors of pathogen-associated molecular patterns within viral RNA and their expression is greatly increased with IFN-I exposure following virus infection . They trigger the signal pathway of IRF3 and IRF7 [18, 19]. PKR, an IFN-inducible gene product, binds to viral double-stranded RNA and halts protein synthesis by phosphorylating translation initiation factor eIF2 . It plays an important role for the IFN-I induction, and its activation accompanies IRF3 activation [22, 23]. The upregulation of the mRNA of these IFN-I related genes were observed in the present study in JEV infected mice and these reflected IFN response. A component or components of this response could have been affected following JEV infection at very high dose leading to a dose independent mortality.
Interestingly, it was observed that mRNA levels of IFN-α and IFN-β in the spleen of uninfected mice were somehow higher than those of 102 and 100 pfu-inoculated mice at 24 and 48 hours pi (Figure 2). Although One-way analysis of variance and Tukey’s Multiple Comparison Test used in this study showed no significant differences between them, these observations raised the possibility that low-dose inoculation with JEV might induce suppressive effects on IFN-I mRNA levels at early phase of infection. Further investigations will be required to elucidate this phenomenon.
In our previous and preliminary studies, we tried to detect mRNA of inflammatory genes including IFNs and their associated genes in the brains of JEV-infected mice. However, these mRNA were detected only after 5 days pi and the levels were not significantly different between mice inoculated with different doses. These observations showed patterns of viral loads similar to those shown in Figure 1. Clinical signs in fatal cases were observed after 7–10 days pi, but apparent CNS disease such as paralysis was not exhibited and their clinical signs (e.g. weight loss) were not significantly different between various inoculation doses. In our JEV-infected mouse model, main pathological changes and neuronal damage were observed in brain cortex . The lesions seem to be related to memory deficiency and mental retardation but not paralysis and movement disorder. Thus, it was quite difficult to observe the CNS signs in JEV-infected mice, although lethal encephalitis was observed in dead mice. Encephalitis was a result of neuronal infection and subsequent inflammatory response. Systemic IFN-I response at early phase of infection appears to affect to viral CNS entry and neuronal infection. Therefore, we suggest that interference of inoculation dose-dependence by IFNs occurred in peripheral tissues, and thus subsequent neuronal infection and inflammatory responses including IFNs in the brains were not different between various inoculation doses.
We previously suggested that immunopathogenic responses in addition to high CNS infection contribute to the severe prognoses and we observed variable immune response in individual mouse infected with JEV or TBEV [13, 24]. These data raise the possibility that there may be a variety of acquired immune response, e. g. specific T cell clones affecting either protective or pathogenetic functions in dying and recovering mice. Furthermore, we propose that the mortality following extraneural infection in mice does not simply represent neuroinvasiveness and thus it is difficult to compare pathogenesis by the lethal doses after peripheral inoculation in mouse model. To understand the pathogenic mechanism of flavivirus encephalitis, further elucidation of IFN-response, immunopathological effect, and their correlation will be an important priority to develop effective treatment strategies for flavivirus encephalitis.