Response of Human Platelets to Activating Monoclonal Antibodies: Importance

Certain monoclonal antibodies (MoAbs) specific for platelet membrane glycoproteins are known t o be capable of activating platelets, and it is generally thought that platelets from normal subjects are equally susceptible t o stimulation by such MoAbs. We found that platelets from 20 normal donors varied significantly in their sensitivity to three lgGl murine MoAbs specific for membrane glycoproteins CD9, GPIV (CD36). and the GPIIb/ llla complex (CD41). respectively. The response of platelets to these MoAbs was blocked by prior addition of MoAb IV.3 specific for the FcyRll receptor, indicating that activation was Fc receptor mediated. Platelets that responded poorly to these MoAbs failed to bind the MoAb 41H.16, specific for the ”responder“ form of FcyRII, but platelets that responded well reacted with this MoAb. The average number of FcyRll receptors on platelets from “responders” and “non-responders“ was approximately the same. However, the number of FcyRll receptors expressed influenced sensitivity of a subgroup of ”responder” platelets to the anti-CD41 MoAb. These platelets were judged on the