Monocyte-Derived Dendritic Cells Inhibits TLR4-Dependent Signaling in Human Metapneumovirus Glycoprotein G

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. H uman metapneumovirus (hMPV) is a recently identified RNA virus belonging to the Paramyxoviridae family, which includes several major human and animal pathogens (1). Epidemiological studies indicate that hMPV is a significant human respiratory pathogen with worldwide distribution (2, 3). It is associated with respiratory illnesses in children, adults, and immunocompromised patients, ranging from upper respiratory tract infections to severe bronchiolitis and pneumonia (4–6). Evidence suggests that the virus may also cause repeated infection throughout life (7, 8). TLRs have been shown to be involved in the activation of innate immune responses by recognizing different pathogen-associated molecular patterns (9–13). Their role in virus-triggered cellular signaling is stimulus-and cell type-dependent (reviewed in Ref. 14). After recognition of their own pathogen-associated molecular patterns following viral infection, TLRs trigger intracellular sig-naling pathways that are necessary to the induction of inflammatory cytokines, chemokines, as well as type I IFNs. Respiratory syn-cytial virus (RSV) fusion protein, the envelope proteins of mam-mary tumor virus, and murine leukemia virus have been shown to activate TLR4 (15, 16), but their role in other viral infections, such as hMPV, is not known. Dendritic cells (DCs) play …