Respiratory syncytial virus induces RelA release from cytoplasmic 100-kDa NF-κB2 complexes via a novel retinoic acid-inducible gene-I•NF-κB-inducing kinase signaling pathway

Respiratory syncytial virus (RSV) is a primary cause of severe lower respiratory tract infection in children world wide. RSV infects airway epithelial cells, where it activates inflammatory genes via the NF-kappa B pathway. NF-kappa B is controlled by two pathways, a canonical pathway that releases sequestered RelA complexes from the I kappa B alpha inhibitor, and a second, the noncanonical pathway, that releases RelB from the 100-kDa NF-kappa B2 complex. Recently we found that the retinoic acid-inducible gene I (RIG-I) is a major intracellular RSV sensor upstream of the canonical pathway. In this study, we surprisingly found that RIG-I silencing also inhibited p100 processing to 52-kDa NF-kappa B2 (p52), suggesting that RIG-I was functionally upstream of the noncanonical regulatory kinase complex composed of (NIKIKK)-I-.alpha subunits. Co-immunoprecipitation experiments not only demonstrated that NIK associated with RIG-I and its downstream adaptor, mitochondrial antiviral signaling (MAVS), but also showed the association between IKK alpha and MAVS. To further understand the role of the (NIKIKK)-I-.alpha pathway, we compared RSV-induced NF-kappa B activation using wild type, Ikk gamma(-/-), Nik(-/-), and Ikk alpha(-/-)-deficient MEF cells. Interestingly, we found that in canonical pathway-defective Ikk gamma(-/-) cells, RSV induced RelA by liberation from p100 complexes. RSV was still able to activate IP10, Rantes, and Gro beta gene expression in Ikk gamma(-/-) cells, and this induction was inhibited by small interfering RNA-mediated RelA knockdown but not RelB silencing. These data suggest that part of the RelA activation in response to RSV infection was induced by a cross-talk pathway involving the noncanonical (NIKIKK)-I-.alpha complex downstream of RIG-I(.)MAVS. This pathway may be a potential target for RSV treatment.