miR-155 induction in microglial cells suppresses Japanese encephalitis virus replication and negatively modulates innate immune responses

Background: Microglial cells, which are resident macrophages of the central nervous system, play important roles in immune responses and pathogenesis. Japanese encephalitis virus (JEV) is a neurotropic virus that infects microglial cells in brain. Several microRNAs including miR-155 and miR-146a play an important role in defining the microglia inflammatory profile. In this study, we have investigated the effect of miR-155 and miR-146a modulation on JEV infection as well as innate immune responses in human microglial cells. Methods: In vitro studies were performed in JEV-infected human microglial CHME3 cells. miR-155 or miR-146a were overexpressed and total RNA and protein were extracted following JEV-infection. Expression of genes involved in innate immune responses was studied by PCR array, quantitative real-time PCR (qPCR), western blot and Fluorescence activated cell sorter (FACS). JEV replication was monitored by studying the viral RNA by qPCR, protein by western blot, and titres by plaque assay. Results: Overexpression of miR-155 in CHME3 cells resulted in significantly reduced JEV replication whereas miR-146a overexpression had an insignificant effect. Additionally, interferon regulatory factor 8 (IRF8) and complement factor H (CFH) were induced during JEV infection; however, this induction was attenuated in miR-155 overexpressing cells following JEV infection. Further, JEV-induced NF-kB regulated downstream gene expression was attenuated. Interestingly, an increased level of CD45, a negative regulator of microglia activation and a reduced phosphorylated-Signal Transducers and Activators of Transcription (p-STAT1) expression was observed in miR-155 overexpressing cells upon JEV infection. Conclusion: Induction of miR-155 in human microglial cells may negatively modulate JEV-induced innate immune gene expression and may have a beneficial role in limiting JEV replication in human microglial cells.