Human Metapneumovirus Small Hydrophobic Protein Inhibits Interferon Induction in Plasmacytoid Dendritic Cells via TLR7 Signaling Pathway

Human metapneumovirus (hMPV) is a leading cause of upper and lower respiratory tract infections in infants, elderly and immunocompromised patients. hMPV encodes a small hydrophobic (SH) protein thought to be important in immunopathogenesis, although the exact function is unknown. Plasmocytoid dendritic cells represent an important source of IFN produced upon entry of bacterial and viral pathogens. Human pDCs were infected with recombinant hMPV, either wild type (rhMPV-WT) or lacking SH protein expression (rhMPV-ΔSH), followed by measurement of type I IFN in the supernatants by ELISA. IFN secretion was also measured from spleen pDCs from TLR7-/-mice following hMPV infection. HEK293 cells, which stably express TLR7, were transfected with a luciferase-tagged IFN-α4 promoter, MyD88, IKK-α and either TRAF6 or TRAF3 expression plasmids in the presence of SH expression plasmid or empty vector, and treated with recombinant IFN-α to induce TLR7 expression. Increased production of IFN-α and β by rhMPV-ΔSH infected pDCs vs rhMPV-WT infected pDCs suggests that SH protein inhibits type I IFN production in these cells. Compared to wt mice, neither TLR7-/- or MyD88-/- mice produced type I IFNs after hMPV infection. SH expression inhibited TRAF6-, but not TRAF3-dependent, IFN-α4 promoter activation, suggesting that SH targets TRAF6 to inhibit TLR7-induced IFN production. HMPV SH protein inhibits TLR7/MyD88/TRAF6 signaling leading to IFN gene transcription, identifying a novel mechanism by which hPMV SH protein modulates innate immune responses.