Host Determinants of Human Metapneumovirus Immunity and Pathogenesis

HMPV is a major cause of lower respiratory infection (LRI) in children worldwide, second only to respiratory syncytial virus (RSV). Although nearly all people are infected with HMPV during childhood, immunity to HMPV is incomplete and re-infections occur throughout life. Hospitalization with HMPV is more likely in persons with underlying conditions such as asthma, chronic obstructive pulmonary disease, HIV, or prematurity. There are no approved antiviral therapies or vaccines. Host or viral determinants of virulence are not known for HMPV. One limitation of many prior studies is that most HMPV research uses one of a few lab-adapted strains that cause minimal disease in mice. We have identified clinical isolates of HMPV that cause severe and fatal disease in mice, providing tools to elucidate mechanisms of pathogenesis. Our preliminary data show that virulent HMPV potently induces types I and III interferon (IFN), and that these cytokines exhibit discordant roles. IFN is critical to initiate and shape adaptive immune responses but can contribute to disease; we aim to define the contribution of types I and III IFN in HMPV. HMPV inhibits type I IFN responses including STAT1 and STAT2 phosphorylation by an unknown mechanism, and HMPV lacks the paramyxovirus V protein or NS1/NS2 of RSV. Several HMPV proteins have been implicated, including G, M2-1, P, and SH. Published data from our group and others suggest SH mediates immune inhibition but a mechanism or specific SH-host protein interaction is unknown. The contribution of other HMPV proteins to virulence has not been defined. We propose to use in vitro and in vivo approaches to address these knowledge gaps. In Aim 1, we will define the contributions of type III IFN (IFN-λ) to HMPV immunity and pathogenesis using global and conditional knockout mice. Aim 2 proposes to identify the cellular target(s) of SH, define interacting domains, and discover the molecular mechanisms of innate immune inhibition by HMPV. We will use transient transfection of tagged mutant SH proteins and viruses with SH mutations in cell and mouse experiments. In Aim 3, will use reverse genetics developed in the lab to generate chimeric viruses and identify viral protein determinants of virulence using established mouse models. Our preliminary data suggest an important role for IFN-λ in HMPV immunity, confirm innate immune inhibition by HMPV SH, and demonstrate the strength of the avirulent and virulent strains to identify viral determinants of virulence. The results of the proposed research will clarify mechanisms of HMPV pathogenesis and provide a blueprint for potential therapeutic avenues and strategies for viral attenuation for vaccines. The findings are likely to be relevant to other respiratory viruses.