Human metapneumovirus SH protein promotes JAK1 degradation to impair host IL-6 signaling
Human metapneumovirus (HMPV) is a leading cause of respiratory infections in children, older adults, and those with underlying conditions 1,2,3,4. HMPV must evade immune defenses to replicate successfully; however, the viral proteins used to accomplish this are poorly characterized. The HMPV small hydrophobic (SH) protein has been reported to inhibit signaling through type I and type II interferon (IFN) receptors in vitro, in part by preventing STAT1 phosphorylation5. HMPV infection also inhibits IL-6 signaling. However, the mechanisms by which SH inhibits signaling, and its involvement in IL-6 signaling inhibition are unknown. Here, we used transfection of SH expression plasmids and SH-deleted virus ({Delta}SH) to show that SH is the viral factor responsible for inhibition of IL-6 signaling during HMPV infection. Transfection of SH-expression vectors or infection with wildtype, but not {Delta}SH virus, blocked IL-6 mediated STAT3 activation. Further, JAK1 protein (but not RNA) was significantly reduced in cells infected with wildtype but not {Delta}SH virus. The SH-mediated reduction of JAK1 was partially restored by addition of proteasome inhibitors, suggesting proteasomal degradation of JAK1. Confocal microscopy indicated that infection relocalized JAK1 to viral replication factories. Co-immunoprecipitation showed that SH interacts with JAK1 and ubiquitin, further linking SH to proteasomal degradation machinery. These data indicate that SH inhibits IL-6 and IFN signaling in infected cells in part by promoting proteasomal degradation of JAK1 and that SH is necessary for IL-6 and IFN signaling inhibition in infection. These findings enhance our understanding of the immune evasion mechanisms of an important respiratory pathogen. ImportanceHuman metapneumovirus (HMPV) is a common cause of severe respiratory illness, especially in children and older adults, in whom it is a leading cause of hospitalization. Prior research suggests that severe HMPV infection is driven by a strong immune response to the virus, and especially by inflammatory immune signals like interferons (IFN). HMPV produces a small hydrophobic protein (SH) that is known to block IFN signaling, but the mechanism by which it functions, and its ability to inhibit other important immune signals remains unexplored. This paper demonstrates that SH can inhibit another related immune signal, IL-6, and demonstrates that SH depletes JAKs, critical proteins involved in both IL-6 and IFN signaling. A robust understanding of how HMPV and related viruses interfere with immune signals important for disease could pave the way for future treatments aimed at mitigating severe infections.