miR-155 induction is a marker of murine norovirus infection but does not contribute to control of replication in vivo

Background: Due to their role in fine-tuning cellular protein expression, microRNAs both promote viral replication and contribute to antiviral responses, for a range of viruses. The interactions between norovirus and the microRNA machinery have not yet been studied. Here, we investigated the changes that occur in microRNA expression during murine norovirus (MNV) infection. Methods: Using RT-qPCR-based arrays, we analysed changes in miRNA expression during infection with the acute strain MNV-1 in two permissive cell lines, a murine macrophage cell line, RAW264.7, and a murine microglial cell line, BV-2. By RT-qPCR, we further confirmed and analysed the changes in miR-155 expression in the infected cell lines, bone-marrow derived macrophage, and tissues harvested from mice infected with the persistent strain MNV-3. Using miR-155 knockout (KO) mice, we investigated whether loss of miR-155 affected viral replication and pathogenesis during persistent MNV-3 infection in vivo and monitored development of a serum IgG response by ELISA. Results: We identified cell-specific panels of miRNAs whose expression were increased or decreased during infection. Only two miRNAs, miR-687 and miR-155, were induced in both cell lines. miR-155, implicated in innate immunity, was also upregulated in bone-marrow derived macrophage and infected tissues. MNV-3 established a persistent infection in miR-155 knockout (KO) mice, with comparable levels of secreted virus and tissue replication observed as for wildtype mice. However, serum anti-MNV IgG levels were significantly reduced in miR-155 KO mice compared to wildtype mice. Conclusions: We have identified a panel of miRNAs whose expression changes with MNV infection. miR-155 induction is a marker of MNV infection in vitro and in vivo, however it does not contribute to the control of persistent infections in vivo. This finding suggests that the immune defects associated with miR-155 deletion, such as lower serum IgG levels, are also not important for control of persistent MNV-3 infection.