A CLOSE SPATIAL RELATIONSHIP BETWEEN GP IIB-IIIA COMPLEXES AND CD9 ANTIGEN AS DEMONSTRATED BY THE MAIPA TECHNIQUE

CD9 is a well-defined component of the platelet plasma membrane and ha s a copy number almost equivalent to that of glycoprotein (GP) IIb-III a complexes, the aggregation receptor on platelets, It has an apparent molecular mass of 24 kD and is otherwise known as p24, Stimulation of p24 by monoclonal antibodies (MAb) induces platelet aggregation and g ranule release, involves Fc gamma RII, and is mainly mediated through the stimulation of phospholipase C. In accordance with a signalling fu nction, p24 has been reported to associate with small GTP-binding prot eins and to GP IIb-IIIa complexes upon activation, We now report furth er evidence of a strong relationship between p24 and GP IIb-IIIa in pl atelets, Using the MAIPA (monoclonal antibody immobilization of platel et antigens) assay in the screening of human antibodies to platelet gl ycoproteins, we found that GP IIb-IIIa-antibody complexes were almost invariably associated with p24 in the harvested detergent-soluble frac tion of platelet lysates, Thus, associated human antibodies were detec ted following the targeting of either GP IIb-IIIa or p24 by monospecif ic murine monoclonal antibodies (MAbs). This is a point to bear in min d when assessing for antibodies to p24 or GP Ilb-IIIa in immune thromb ocytopenias.