Human Metapneumovirus M2-2 Protein Acts as a Negative Regulator of Alpha Interferon Production by Plasmacytoid Dendritic Cells

Human metapneumovirus (HMPV) has the ability to inhibit Toll-like receptor 7 (TLR7)-and TLR9-dependent alpha interferon (IFN-alpha) production by plasmacytoid dendritic cells (pDCs). However, the inhibition mechanism remains largely unknown. To identify viral proteins responsible for this inhibition, we performed a screening of HMPV open reading frames (ORFs) for the ability to block TLR7/9dependent signaling reconstituted in HEK293T cells by transfection with myeloid differentiation factor 88 (MyD88), tumor necrosis factor receptor-associated factor 6 (TRAF6), IKK alpha, and IFN regulatory factor 7 (IRF7). This screening demonstrated that the M2-2 protein was the most potent inhibitor of TLR7/9-dependent IFN-alpha induction. A recombinant HMPV in which the M2-2 ORF was silenced indeed induced greater IFN-alpha production by human pDCs than wild-type HMPV did. Immunoprecipitation experiments showed direct physical association of the M2-2 protein with the inhibitory domain (ID) of IRF7. As a natural consequence of this, transfection of IRF7 lacking the ID, a constitutively active mutant, resulted in activation of the IFN-alpha promoter even in the presence of M2-2. Bioluminescence resonance energy transfer assays and split Renilla luciferase complementation assays revealed that M2-2 inhibited MyD88/TRAF6/IKK alpha-induced homodimerization of IRF7. In contrast, expression of the M2-2 protein did not result in inhibition of IPS-1-induced homodimerization and resultant activation of IRF7. This indicates that inhibition of MyD88/TRAF6/IKK alpha induced IRF7 homodimerization does not result from a steric effect of M2-2 binding. Instead, it was found that M2-2 inhibited MyD88/TRAF6/IKK alpha -induced phosphorylation of IRF7 on Ser477. These results suggest that M2-2 blocks TLR7/9-dependent IFN-alpha induction by preventing IRF7 homodimerization, possibly through its effects on the phosphorylation status of IRF7. IMPORTANCE The family Paramyxoviridae is divided into two subfamilies, the Paramyxovirinae and the Pneumovirinae. Members of the subfamily Paramyxovirinae have the ability to inhibit TLR7/9-dependent IFN-alpha production, and the underlying inhibition mechanism has been intensively studied. In contrast, little is known about how members of the subfamily Pneumovirinae regulate IFN-alpha production by pDCs. We identified the M2-2 protein of HMPV, a member of the subfamily Pneumovirinae, as a negative regulator of IFN-alpha production by pDCs and uncovered the underlying mechanism. This study explains in part why the M2-2 knockout recombinant HMPV is attenuated and further suggests that M2-2 is a potential target for HMPV therapy.