Anti-human platelet tetraspanin (CD9) monoclonal antibodies induce platelet integrin aIIb beta 3 activation in a Fc receptor-independent fashion

Objectives To characterize the activation of platelet integrin alpha IIb be ta3 induced by two anti-human platelet tetraspanin monoclonal antibodies (m Abs), HI117 and SJ9A4, and investigate their potential mechanism of action. Methods Using I-125-labeled human fibrinogen (Fg), specific Fg binding to h uman platelets induced by HI117 and SJ9A4 was measured. Results HI117 and SJ9A4 (10 mug/ml and 20 mug/ml) induced specific Fg bindi ng to human platelets, suggesting that the two mAbs evoked activation of pl atelet integrin alpha IIb beta3. Further study indicated that HI117 and SJ9 A4 induced integrin alpha IIb beta3 activation independent of platelet Fc-r eceptors, and that HI117 and SJ9A4-induced integrin alpha IIb beta3 activat ion was inhibited by pretreatment of platelets with sphingosine, aspirin, a pyrase, and/or PGI(2). Conclusions Anti-platelet tetraspanin (CD9) mAbs, HI117 and SJ9A4, can indu ce platelet integrin alpha IIb beta3 activation independent of Fc-receptors . Three signaling pathways, namely thromboxane, secreted ADP, and cAMP path ways, may be involved in the process, with protein kinase C activation pres umably being the common step of the three pathways.