Human Metapneumovirus Glycoprotein G Inhibits Innate Immune Responses

Abstract

Human metapneumovirus (hMPV) is a leading cause of acute respiratory tract infection in infants, as well as in the elderly and immunocompromised patients. No effective treatment or vaccine for hMPV is currently available. A recombinant hMPV lacking the G protein (rhMPV-ΔG) was recently developed as a potential vaccine candidate and shown to be attenuated in the respiratory tract of a rodent model of infection. The mechanism of its attenuation, as well as the role of G protein in modulation of hMPV-induced cellular responses in vitro, as well as in vivo, is currently unknown. In this study, we found that rhMPV-ΔG-infected airway epithelial cells produced higher levels of chemokines and type I interferon (IFN) compared to cells infected with rhMPV-WT. Infection of airway epithelial cells with rhMPV-ΔG enhanced activation of transcription factors belonging to the nuclear factor (NF)-κB and interferon regulatory factor (IRF) families, as revealed by increased nuclear translocation and/or phosphorylation of these transcription factors. Compared to rhMPV-WT, rhMPV-ΔG also increased IRF- and NF-κB-dependent gene transcription, which was reversely inhibited by G protein expression. Since RNA helicases have been shown to play a fundamental role in initiating viral-induced cellular signaling, we investigated whether retinoic induced gene (RIG)-I was the target of G protein inhibitory activity. We found that indeed G protein associated with RIG-I and inhibited RIG-I-dependent gene transcription, identifying an important mechanism by which hMPV affects innate immune responses. This is the first study investigating the role of hMPV G protein in cellular signaling and identifies G as an important virulence factor, as it inhibits the production of important immune and antiviral mediators by targeting RIG-I, a major intracellular viral RNA sensor. Human metapneumovirus (hMPV), a member of the Paramyxoviridae family, is an important cause of respiratory morbidity throughout life. The contribution of viral-specific proteins to the pathogenesis of hMPV infection and immune evasion is largely unknown. Previous work has suggested that the glycoprotein G of hMPV is not necessary for the process of viral fusion and attachment to host cells, and a recombinant hMPV lacking the G protein (rhMPV-ΔG) shows an attenuated phenotype in the respiratory tract of animal models of infection. Airway epithelial cells, a major component of the innate immune system, are a primary target of hMPV infection. In this study, we show that hMPV G protein functions as a major inhibitory factor of the host antiviral response by blocking production of inducible chemokines and IFN-α/β. A major finding of this work is the demonstration that hMPV G protein interacts with RIG-I, a cytoplasmic viral sensor. As result, hMPV G protein inhibits RIG-I-dependent signaling pathways, including activation of NF-κB and IRF-3, two transcription factors necessary for the synthesis of inflammatory and antiviral cytokines. Understanding the function of hMPV proteins is critical for the future design of effective antiviral therapies and rationale design of vaccine candidates.