Infectious Bursal Disease Virus Activates c-Src To Promote α4β1 Integrin-Dependent Viral Entry by Modulating the Downstream Akt-RhoA GTPase-Actin Rearrangement Cascade.

While the entry of infectious bursal disease virus (IBDV) is initiated by the binding of the virus to the two major receptors integrin and HSP90, the signaling events after receptor binding and how they contribute to virus entry remain elusive. We show here that IBDV activates c-Src by inducing the phosphorylation of the Y416 residue in c-Src both in DF-1 chicken fibroblasts and in vivo in the bursa of Fabricius from specific-pathogen-free (SPF) chickens. Importantly, inactivated IBDV fails to stimulate c-Src Y416 phosphorylation, and a very virulent IBDV strain induces a much higher level of c-Src Y416 phosphorylation than does an attenuated strain. Inhibition of c-Src activation by an Src kinase inhibitor or expression of a c-Src dominant negative mutant results in a significant decrease in the internalization of IBDV but has little effect on virus adhesion. Furthermore, short hairpin RNA (shRNA) downregulation of integrin, either the α4 or β1 subunit, but not HSP90 remarkably attenuates IBDV-induced c-Src Y416 phosphorylation, resulting in a decrease in IBDV internalization but not virus adhesion. Moreover, interestingly, inhibition of either c-Src downstream of the phosphatidylinositol 3-kinase (PI3K)/Akt-RhoA signaling cascade or actin rearrangement leads to a significant decrease in IBDV internalization irrespective of the IBDV-induced high levels of c-Src phosphorylation. Cumulatively, our results suggest a novel feed-forward model whereby IBDV activates c-Src for benefiting its cell entry via an integrin-mediated pathway by the activation of downstream PI3K/Akt-RhoA signaling and cytoskeleton actin rearrangement. IMPORTANCE: While IBDV-caused immunosuppression is highly related to viral invasion, the molecular basis of the cellular entry of IBDV remains elusive. In this study, we demonstrate that IBDV activates c-Src by inducing the phosphorylation of the Y416 residue in c-Src to promote virus internalization but not virus adhesion. The ability to induce the level of c-Src Y416 phosphorylation correlates with the pathogenicity of an IBDV strain. IBDV-induced c-Src Y416 activation is α4β1 integrin but not HSP90 dependent and involves the activation of the downstream PI3K/Akt-RhoA GTPase-actin rearrangement cascade. Thus, our findings provide new insights into the IBDV infection process and the potential for c-Src as a candidate target for the development of IBDV therapeutic drugs.