Platelet glycoprotein II_b-III_a (α_IIbβ₃ 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 × 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.