Affinity modulation of the α_IIbβ₃ integrin (platelet GPIIb-IIIa) is an intrinsic property of the receptor

To analyze the basis of affinity modulation of integrin function, we studied cloned stable Chinese hamster ovary cell lines expressing recombinant integrins of the β₃ family (α_IIbβ₃ and α_vβ₃). Antigenic and peptide recognition specificities of the recombinant receptors resembled those of the native receptors found in platelets or endothelial cells. The α_IIbβ₃-expressing cell line (A5) bound RGD peptides and immobilized fibrinogen (Fg) but not soluble fibrinogen or the activation-specific monoclonal anti-α_IIbβ₃ (PAC1), indicating that it was in the affinity state found on resting platelets. Several platelet agonists failed to alter the affinity state of ("activate") recombinant α_IIbβ₃. The binding of soluble Fg and PAC1, however, was stimulated in both platelets and A5 cells by addition of IgG papain-digestion products (Fab) fragments of certain β₃-specific monoclonal antibodies. These antibodies stimulated PAC1 binding to platelets fixed under conditions rendering them unresponsive to other agonists. Addition of these antibodies to detergent-solubilized α_IIbβ₃ also stimulated specific Fg binding. These data demonstrate that certain anti-β₃ antibodies activate α_IIbβ₃ by acting directly on the receptor, possibly by altering its conformation. Furthermore, they indicate that the activation state of α_IIbβ₃ is a property of the receptor itself rather than of the surrounding cell membrane microenvironment.