AGONIST-STIMULATED LIGAND-BINDING BY THE PLATELET INTEGRIN ALPHA-IIB-BETA-3 IN A LYMPHOCYTE EXPRESSION SYSTEM

The ligand binding activity of the platelet integrin alpha IIb beta 3 is initiated by agonist generated intraplatelet signals. We studied this process in vitro by expressing recombinant alpha IIb beta 3 in Epstein Barr virus-immortalized B lymphocytes. We found that phorbol ester stimulation induced the adhesion of lymphocytes expressing alpha IIb beta 3 to immobilized fibrinogen, Moreover, replacement of the transmembrane and cytoplasmic domains of the cu and beta subunits of alpha II beta p3 with those of alpha L beta 2 significantly increased adherence, whereas replacement of only the cytoplasmic domains significantly decreased adherence. This suggests that transmembrane segments are involved in the agonist induced modulation of alpha IIb beta 3 activity. Similar results were seen when the alpha IIb beta 3 activation dependent monoclonal antibody PAC-1 was substituted for immobilized fibrinogen. We also found that the adherence of lymphocytes expressing beta 3 with either of the two alpha IIb/alpha L chimeras was similar to that of cells expressing alpha IIb beta 3, whereas the adherence of cells expressing alpha IIb with either of the two beta 3/beta 2 chimeras was substantially decreased, suggesting that the identity of the cytoplasmic domain of beta 3, but not of alpha IIb, is critical for alpha IIb beta 3 function, This report indicates that B lymphocytes contain signal transduction pathways involving protein kinase C that can increase the ligand binding activity of alpha IIb beta 3 and demonstrates the utility of these cells as an expression system for the study of agonist-stimulated alpha IIb beta 3 function.