Role of type I interferon signaling in human metapneumovirus pathogenesis and control of viral replication.

Human metapneumovirus (HMPV) is a leading cause of acute respiratory illness. CD8(+) T cells are critical for clearing viral infection, yet recent evidence shows that HMPV and other respiratory viruses induce CD8(+) T cell impairment via PD-1-PD-L1 signaling. We sought to understand the role of type I interferon (IFN) in the innate and adaptive immune responses to HMPV by using a mouse model lacking IFN signaling. Although HMPV titers were higher in the absence of type I IFN, virus was nonetheless cleared and mice were less ill, indicating that type I IFN is not required to resolve HMPV infection but contributes to pathogenesis. Further, despite lower levels of the inhibitory ligand PD-L1 in mice lacking type I IFN, CD8(+) T cells were more impaired in these mice than in WT mice. Our data suggest that specific antigen-presenting cell subsets and the inhibitory receptor Tim-3 may contribute to CD8(+) T cell impairment.