Two monoclonal anti-platelet antibodies, 3B2 and 8G11, have been raised that are specific for normal human platelets. 3B2 is unique in that it has decreased reactivity for platelets from 16 patients with autoimmune thrombocytopenic purpura [mean platelet count, 65,000 +/- 6,000 (SEM)]. With 8G11 in an enzyme-linked immunosorbent assay, the mean of the ratios of patient platelet OD to control platelet OD was 0.95 +/- 0.07, whereas with 3B2, the mean of the ratios of patient platelet OD to control OD was 0.24 +/- 0.04, P less than 0.001. With 3B2 the mean of the OD ratios of five patients with autoimmune thrombocytopenic purpura in remission (greater than 150,000 platelets per mm3) compared to controls was 0.80 +/- 0.14. 3B2 did not react with platelets from a patient with Glanzmann's thrombasthenia, in which membranes lack glycoproteins IIb and IIIa (GPIIb and GPIIIa). Platelet membranes were run on crossed immunoelectrophoresis against a rabbit polyclonal anti-human platelet membrane antibody with 125I-labeled purified 3B2 in an intermediate spacer gel. 3B2 reacted with the GPIIb-GPIIIa-Ca2+ complex in the presence of excess Ca2+ and with GPIIb alone in the presence of excess EGTA. When Triton X-100-solubilized platelet membranes were immunoprecipitated with 3B2 plus rabbit anti-mouse IgG, reduced, and run on NaDodSO4/polyacrylamide gel electrophoresis, a single protein band was obtained with a molecular weight of 120,000 (the molecular weight of GPIIb). Thus, the reactivity of monoclonal antibody 3B2 with GPIIb or the GPIIb-GPIIIa-Ca2+ complex appears to be inhibited by the presence of autoantibody on platelets.
