Integrin alphaVbeta3 Binds to the RGD motif of glycoprotein B of Kaposi's sarcoma-associated herpesvirus and functions as an RGD-dependent entry receptor

Kaposi's sarcoma-associated herpesvirus (KSHV) envelope-associated glycoprotein B (gB) is involved in the initial steps of binding to host cells during KSHV infection. gB contains an RGD motif reported to bind the integrin α₃β₁ during virus entry. Although the ligand specificity of α₃β₁ has been controversial, current literature indicates that α₃β₁ ligand recognition is independent of RGD. We compared α₃β₁ to the RGD-binding integrin, α_Vβ₃, for binding to envelope-associated gB and a gB(RGD) peptide. Adhesion assays demonstrated that β₃-CHO cells overexpressing α_Vβ₃ specifically bound gB(RGD), whereas α₃-CHO cells overexpressing α₃β₁ did not. Function-blocking antibodies to α_Vβ₃ inhibited the adhesion of HT1080 fibrosarcoma cells to gB(RGD), while antibodies to α₃β₁ did not. Using affinity-purified integrins and confocal microscopy, α_Vβ₃ bound to gB(RGD) and KSHV virions, demonstrating direct receptor-ligand interactions. Specific α_Vβ₃ antagonists, including cyclic and dicyclic RGD peptides and α_Vβ₃ function-blocking antibodies, inhibited KSHV infection by 70 to 80%. Keratinocytes from α₃-null mice lacking α₃β₁ were fully competent for infection by KSHV, and reconstitution of α₃β₁ function by transfection with α₃ cDNA reduced KSHV infectivity from 74% to 55%. Additional inhibitory effects of α₃β₁ on the cell surface expression of α_Vβ₃ and on α_Vβ₃-mediated adhesion of α₃-CHO cells overexpressing α₃β₁ were detected, consistent with previous reports of transdominant inhibition of α_Vβ₃ function by α₃β₁. These observations may explain previous reports of an inhibition of KSHV infection by soluble α₃β₁. Our studies demonstrate that α_Vβ₃ is a cellular receptor mediating both the cell adhesion and entry of KSHV into target cells through binding the virion-associated gB(RGD).