Differential expression of a ligand induced binding site (LIBS) by GPIIb-IIIa ligand recognition peptides and...

The glycoprotein (GP) IIb-IIIa complex is an attractive anti-platelet target for the prevention of thrombotic events associated with coronary artery disease. Although GPIIb-IIIa antagonists inhibit GPIIb-IIIa binding to its ligands, the interactions have not been fully clarified, particularly with respect to their ability to induce structural changes in the complex that lead to exposure of neoantigenic epitopes or ligand-induced binding sites (LIBS). In this study we used the anti-LIBS monoclonal antibody (mAb) D3 to further define the activation states of purified active and inactive GPIIb-IIIa. We also compared the data obtained in the purified system to that observed with intact human platelets. Active GPIIb-IIIa expressed significantly greater high-affinity D3 LIBS sites compared to the inactive form. In addition, the ligand recognition peptides RGDS and H12 caused increased expression of the D3 epitope, with RGDS eliciting a much more potent response. The response of the purified GPIIb-IIIa to these peptides paralleled that observed with human platelets. To explore whether the platelet antagonists abciximab, eptifibatide and tirofiban induced expression of the D3 LIBS site, a modified competitive ELISA was developed. Our data indicate that the use of purified GPIIb-IIIa with this ELISA system provides a reproducible approach for exploring the interactions between GPIIb-IIIa and its antagonists. Whereas abciximab caused no detectable increase in the expression of the D3 epitope on purified GPIIb-IIIa, eptifibatide, tirofiban, RGDS, and H12 induced differential expression of the high-affinity LIBS. Studies with intact platelets suggested that abciximab blocked the binding of the D3 and LIBS6 mAbs, and that the pre bound anti-LIBS D3 sterically hindered abciximab binding.