Evidence that monoclonal antibodies against CD9 antigen induce specific association between CD9 and the platelet glycoprotein IIb-IIIa complex

Monoclonal antibodies to the CD9 antigen are powerful platelet agonists. We report here the novel finding that the anti-CD9 monoclonal antibodies 50H.19 and ALB₆ promote physical association between CD9 antigen and the glycoprotein IIb-IIIa complex (GPIIb-IIIa) component of the platelet fibrinogen receptor. The monoclonal antibodies do not consistently immunoprecipitate proteins other than CD9 from ¹²⁵I-labeled human platelets even if the platelets are first treated with the homobifunctional cross-linking reagent dithiobis(succinimidyl propionate), indicating that CD9 antigen is not physically associated with other membrane proteins in the resting state. However, the addition of agonistic concentrations of either monoclonal antibody before cross-linking results in the co-precipitation of proteins corresponding in mobility and peptide composition to GPIIb, and GPIIIa. The association of CD9 with the GPIIb-IIIa complex is unaffected by a combination of aspirin and ADP scavengers sufficient to abrogate anti-CD9 monoclonal antibody-induced platelet aggregation, and is therefore not dependent upon thromboxane- and ADP-mediated pathways of intracellular signalling. The specificity of the association is demonstrated by the lack of other co-precipitating major proteins, by the requirement for induction by anti-CD9 monoclonal antibodies, and by the failure to promote reciprocal association with either of the anti-GPIIb-IIIa complex monoclonal antibodies P2 and HuP1-m1a.