Host cell signaling in response to RSV infection

RSV infection is the most important cause of hospitalization of infants, and is also the major cause for admissions in adults with chronic cardiac disease and pulmonary diseases. So far, there is no effective treatment or vaccine against RSV infection. Intensive studies have been performed to understand mechanisms and consequences of RSV-induced host cell signaling. My dissertation project provides three major contributions to this field.;Firstly, I found that retinoic acid-inducible gene I (RIG-I) and Toll like receptor 3 (TLR3) play distinct roles in mediating RSV-induced innate immune responses. Short interfering RNA (siRNA)-mediated RIG-I "knockdown" significantly inhibited the translocation of nuclear factor-kappaB (NF-kappaB) and interferon response factor 3 (IRF3) to the nucleus at the early phase of viral infection. In contrast, siRNA-mediated TLR3 knockdown did not affect RSV-induced NF-kappaB binding to DNA but block the activating phosphorylation of NF-kappaB/RelA at serine residue 276.;Secondly, I first demonstrated that RIG-I was involved in the activation of NIK·IKKalpha complex, two key noncanonical kinases that induce the protelytic processing of an IkappaBalpha-like inhibitor p100 to p52. I also proved that a fraction of RelA was associated with cytoplasmic p100. In addition, the RSV-induced proteolysis of p100 not only produces p52, but also releases RelA and increases its nuclear translocation. This finding suggested that part of the RelA activation in response to RSV infection was induced by the NIK·IKKalpha complex. Because inhibition of NIK·IKKalpha does not affect RSV replication but reduces inflammatory chemokine production, this protein complex could be a good target for drug treatment of RSV infection.;Thirdly, our previous study has reported the existence of IKKgammaDelta, a splicing variant of IKKgamma, which excludes exon 5. I compared the function of IKKgamma and IKKgammaDelta in response to ssRNA viruses. I found that in contrast to IKKgamma, which is essential for both NF-kappaB and IRF3 activation, IKKgammaDelta failed to activate IRF3 in response to either Sendai virus or respiratory syncytial virus (RSV). However, IKKgammaDelta mediated NF-kappaB activation was intact. I demonstrated that the missing region of IKKgammaDelta made it unable to recruit TANK, a key factor that recruits atypical IKKs to activate the IRF3 pathway.