The Platelet Antigens CD9, CD42 and Integrin αIIbβIIIa Can be Topographically Associated and Transduce Functionally Similar Signals

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Abstract
Investigation of the specific effects of different mAb known to stimulate platelets (agonist mAb) is complicated by interaction of the Fc portion of these mAb with the platelet Fc gamma RII. This has led to the conclusion that nearly all agonist-mAb-induced activation of platelets is mediated by this receptor. However, the target antigen-mediated signal can be analysed provided that the effects of Fc gamma RII engagement can either be reduced or eliminated. We have therefore blocked platelet Fc gamma RII with IV.3 Fab fragments (an anti-Fc gamma RII mAb), and stimulated the platelets by cross-linking intact agonist mAb with F(ab')2 fragments of an Fc-specific anti-mouse antibody. By analysing functional platelet responses and protein-tyrosine phosphorylation, we found that such non-Fc gamma RII-mediated cross-linking of CD9, CD42 and glycoprotein (gp) IIb/IIIa generates closely similar signals. Since this may indicate molecular associations, we analyzed the surface topography of platelets using the chemical cross-linking agent dithiobis(sulfosuccinimidyl propionate). We found that a proportion of CD9, gpIIb/IIIa and CD42 molecules associate with each other on the platelet surface membrane. Thus, our results suggest that these antigens are able to form a larger molecular complex and induce similar signals. Furthermore, cross-linking of CD9 and CD42 stimulated thrombasthenic platelets completely lacking gpIIb/IIIa. These data therefore indicate that CD9 and CD42 can signal independently of gpIIb/IIIa, and that signals generated by all these molecules may converge on a common pathway.