The innate immune response represents a critical component of the host defense against viruses and is coordinated at the cellular level by activation of transcription factors that regulate the expression of inducible gene products with antiviral and/or inflammatory activity. As the immune system evolved to fight viral infections, so viruses developed strategies to evade the host immune responses, mainly by targeting the type I interferon system. HMPV is the second most common cause of epidemic respiratory infections in infants and young children and a significant cause of respiratory tract infections in the elderly and immunocompromised patients. The availability of the reverse genetic system for negative sense RNA viruses has allowed the dissection of viral protein functions in viral replication as well as in cellular signaling. As a recently identified virus, little is known about the role of individual hMPV proteins in modulating host cell responses. In this study, we found that hMPV infection of airway epithelial cells, the primary target of hMPV infection 
, induced the secretion of a variety of cytokines and chemokines, as well as type I interferons, whose expression is coordinated by subsets of transcription factors belonging to the NF-κB and IRF families. Surprisingly, deletion of G protein resulted in enhanced production of chemokines and type I interferon (IFN), as well as increased activation of both families of transcription factors. The enhanced responses to rhMPV-ΔG infection were not due to an increased ability of rhMPV-ΔG to replicate, as the accumulation of F protein in infected airway epithelial cells and viral titers were lower in cells infected with rhMPV-ΔG, compared to rhMPV-WT.
Circumvention of the IFN response occurs through different strategies. Two major categories include either direct suppression of IFN production or interference with IFN signaling, through inhibition of the JAK/STAT pathway. The kinetics of enhanced chemokine and IFN production and transcription factor activation in response to rhMPV-ΔG infection, which occurred at early time points of infection, suggested that G protein regulated an early signaling event triggered in response to hMPV infection of airway epithelial cells.
RIG-I and MDA-5 are two RNA helicases that have been recently identified as fundamental sensors of viral infections 
. They recognize single and double-stranded RNA and their engagement triggers a signaling cascade leading to NF-κB and IRF activation, through interaction with the mitochondrial antiviral signaling (MAVS) adaptor molecule 
. Several viral proteins have been shown to be able to disrupt the RIG-I/MDA-5/MAVS signaling pathway by sequestering viral RNA from helicase binding and/or disrupting helicase interaction with downstream signaling molecules or by increasing protein degradation. Influenza A NS1 protein employs the first two mechanisms to block RIG-I-mediated IFN induction 
. Similarly, V proteins of paramyxoviruses have been shown to bind MDA-5 and inhibit IFN-β production  
. On the other hand, poliovirus infection induces MDA-5 degradation, also inhibiting IFN induction 
. HCV 3/4A proteases cleave MAVS at the level of mitochondria insertion, releasing the protein to the cytoplasmic compartment, therefore preventing further transmission of RIG-I-dependent signaling, resulting in the inhibition of host's antiviral responses 
. In this study, we found that hMPV G protein physically interacts with RIG-I and inhibits RIG-I, but not MDA-5 and MAVS-induced IFN-β transcription, justifying the broad inhibitory effect of G protein on activation of NF-κB and IRF transcription factors and induction of antiviral and pro-inflammatory molecules.
The inhibitory effect of hMPV G protein on cellular signaling could be a common feature of surface glycoproteins of enveloped single strand, negative strand RNA viruses. RSV G protein has been shown to modulate cytokine and chemokine production, as infection with a mutant RSV lacking the full-length G protein or the soluble part of G protein (sG) enhanced production of IL-6 and IL-8 in monocytes 
, as well as IL-8 and RANTES secretion and ICAM expression in airway epithelial cells 
. RSV-ΔG also caused more lung inflammation, compared to WT, in a mouse model of infection 
. The G protein of pneumonia virus of mice, a murine relative of RSV, has also been recently identified as an important virulence factor, as viral replication of a recombinant mutant virus lacking G is severely restricted in a BALB/c mouse model of infection 
. Similarly, the surface glycoproteins of hantaviruses, in particular the ones associated with hemorrhagic pulmonary syndrome (HPS), have been shown to affect IRF-3 activation and IFN production, via interaction with RIG-I and TBK-1, a kinase responsible for viral-induced IRF-3 phosphorylation, as well as to inhibit IFN-mediated cellular responses 
hPMV infection is associated with production of inflammatory mediators not only in vitro
but also in vivo 
. The high attenuation of rhMPV-ΔG replication in the lower respiratory tracts of rodent models of infection suggest that it could be developed as a vaccine candidate  
. Our findings suggest that careful investigation is needed in an animal model of infection to address whether rhMPV-ΔG can cause enhance lung inflammation and impaired lung function, as recently shown for RSV lacking the G protein 
In summary, this study provides us with novel information on the role of hMPV G protein in regulating host cell responses. Further studies are needed to determine the exact mechanism by which G protein inhibits RIG-I activation and to define the domains/amino acid residues mediating RIG-I and G interaction.