Platelet activation induced by a murine monoclonal antibody directed against a novel tetra‐span antigen

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Abstract
MAb 14A2.H1 identifies a novel low-abundance platelet surface antigen, PETA-3, which is a member of the tetra-span (TM4) family. This MAb brings about platelet aggregation and mediator release, which is completely inhibitable by prostaglandin E1, and partially inhibitable by aspirin and ketanserin. Platelet activation by MAb 14A2.H1 is dependent on interaction with both the platelet Fc receptor, FcγRII, and the specific antigen as it was prevented by either a blocking MAb to FcγRII (IV.3) or F(ab')2 fragments of 14A2.H1. The extent of platelet activation by the antibody varied considerably between donors, and is believed to reflect the polymorphism of FcγRII. Subaggregating concentrations of 14A2.H1 synergized with other platelet agonists, ADP, adrenaline, collagen and serotonin, indicating signalling via a pathway distinct from these activators. Synergy was also blocked by MAb rv.3, or F(ab')2 fragments of 14A2.H1. The similar low copy number of PETA-3 and FcγRII in the platelet membrane (approximately 1000/platelet). together with the dependence on FcγRII for activation by MAb 14A2.H1, suggests that PETA-3 may be a component of the FcγRII signal transducing complex in platelets.