Human metapneumovirus glycoprotein G inhibits TLR4-dependent signaling in monocyte-derived dendritic cells.

Human metapneumovirus (hMPV) is a major cause of upper and lower respiratory infections in children and adults. Recent work from our group demonstrated that hMPV G glycoprotein is an important virulence factor, responsible for inhibiting innate immune responses in airway epithelial cells. Myeloid dendritic cells (DCs) are potent APCs and play a major role in initiating and modulating the innate and adaptive immune responses. In this study, we found that TLR4 plays a major role in hMPV-induced activation of monocyte-derived DCs (moDCs), as downregulation of its expression by small interfering RNA significantly blocked hMPV-induced chemokine and type I IFN expression. Similar results were found in bone marrow-derived DCs from TLR4-deficient mice. moDCs infected with a virus lacking G protein expression produced higher levels of cytokines and chemokines compared with cells infected with wild-type virus, suggesting that G protein plays an inhibitory role in viral-induced cellular responses. Specifically, G protein affects TLR4-dependent signaling, as infection of moDCs with recombinant hMPV lacking G protein inhibited LPS-induced production of cytokine and chemokines significantly less than did wild-type virus, and treatment of moDCs with purified G protein resulted in a similar inhibition of LPS-dependent signaling. Our results demonstrate that hMPV G protein plays an important role in inhibiting host innate immune responses, likely affecting adaptive responses too.