Platelet glycoprotein II(b)-III(a) (α(IIb)β₃ integrin) confers fibrinogen- and activation-dependent aggregation on heterologous cells

To analyze molecular mechanisms of platelet aggregation, we have studied the aggregation of Chinese hamster ovary (CHO) cells expressing between 1 and 4 x 10⁵ recombinant human glycoprotein (GP) II(b)-III(a) molecules per cell (A5 cells). These cells aggregated as measured by the disappearance of single cells during rotary agitation. Aggregation was dependent on the presence of extracellular fibrinogen (~500 nmol/L) and divalent cations, and required prior activation of the GPII(b)-III(a). A synthetic peptide (GRGDSP) and monoclonal anti-GPII(b)-III(a) antibody (2G12) that block platelet aggregation also blocked aggregation of these cells. Parent CHO cells or those expressing recombinant GPII(b)-III(a) containing a point mutation that causes variant thrombasthenia both failed to aggregate when stimulated in the presence of fibrinogen. These data show that GPII(b)-III(a) is the only unique platelet surface component required for aggregation.