Interactions of Foot-and-Mouth Disease Virus with Soluble Bovine α V β 3 and α V β 6 Integrins

Abstract

At least four members of the integrin family of receptors, α _V β ₁ , α _V β ₃ , α _V β ₆ , and α _V β ₈ , have been identified as receptors for foot-and-mouth disease virus (FMDV) in vitro. Our investigators have recently shown that the efficiency of receptor usage appears to be related to the viral serotype and may be influenced by structural differences on the viral surface (H. Duque and B. Baxt, J. Virol. 77: 2500-2511, 2003). To further examine these differences, we generated soluble α _V β ₃ and α _V β ₆ integrins. cDNA plasmids encoding the individual complete integrin α _V , β ₃ , and β ₆ subunits were used to amplify sequences encoding the subunits' signal peptide and ectodomain, resulting in subunits lacking transmembrane and cytoplasmic domains. COS-1 cells were transfected with plasmids encoding the soluble α _V subunit and either the soluble β ₃ or β ₆ subunit and labeled with [ ³⁵ S]methionine-cysteine. Complete subunit heterodimeric integrins were secreted into the medium, as determined by radioimmunoprecipitation with specific monoclonal and polyclonal antibodies. For the examination of the integrins' biological activities, stable cell lines producing the soluble integrins were generated in HEK 293A cells. In the presence of divalent cations, soluble α _V β ₆ bound to representatives of type A or O viruses, immobilized on plastic dishes, and significantly inhibited viral replication, as determined by plaque reduction assays. In contrast, soluble α _V β ₃ was unable to bind to immobilized virus of either serotype; however, virus bound to the immobilized integrin, suggesting that FMDV binding to α _V β ₃ is a low-affinity interaction. In addition, soluble α _V β ₃ did not neutralize virus infectivity. Incubation of soluble α _V β ₆ with labeled type A ₁₂ or O ₁ resulted in a significant inhibition of virus adsorption to BHK cells, while soluble α _V β ₃ caused a low (20 to 30%), but consistent, inhibition of virus adsorption. Virus incubated with soluble α _V β ₆ had a lower sedimentation rate than native virus on sucrose density gradients, but the particles retained all of their structural proteins and still contained bound integrin and, therefore, were not exhibiting characteristics of a picornavirus A particle.