In vitro studies with an anti-platelet monoclonal antibody; P256.

P256 is a platelet-specific monoclonal antibody. It has potential as a diagnostic agent for use in thrombus detection by radioimmunoscintigraphy. However, the whole antibody has been reported to cause platelet aggregation. The object of the present study was to determine the effects of the F(ab')2 and Fab' fragments of P256 on human platelets in vitro. Gel-filtered platelets were prepared from citrated venous blood, and aggregation was studied at 37 degrees C using the method of Born. The whole antibody caused aggregation over the concentration range 10(-9)-10(-7) M, with a maximum of 70% aggregation. The F(ab')2 fragment was less potent with a maximum response of only 45%. The monovalent Fab' fragment caused no aggregation at concentrations up to 10(-7) M. However, binding studies with 111In-labelled monovalent Fab' fragment demonstrated saturable displaceable binding (Kd = 9.1 x 10(-9) M). Aggregation induced by the divalent forms of the antibody was partially dependent upon added fibrinogen. It is concluded that the monovalent Fab' fragment of P256 may be used in platelet-labelling studies without risk of inducing aggregation irrespective of concentration. The whole antibody, or its divalent F(ab')2 fragment, may also be used provided their concentrations are kept below 10(-9) M.