Competitive binding of a monoclonal antibody SZ-21 with anti-PLA1 antibodies and its potential for clinical application.

Both intact IgG and Fab fragments of a monoclonal antibody SZ-21, directed against platelet glycoprotein (GP) IIIa, inhibited binding of anti-PLA1 antibodies to PLA1-positive platelets in a dose dependent manner. Conversely, the binding capacity of the platelets for SZ-21 was decreased in the presence of anti-PLA1 antibodies. In Western blots, the determinant for SZ-21 was present on GPIIIa from either PLA1 or PLA2 homozygotes, but SZ-21 did not affect the interaction of anti-Yukb alloantibodies with their target antigen on GPIIIa. These results suggest that SZ-21 reacts with an epitope on the GPIIIa molecule very close to but not identical with that for anti-PLA1. The existence of PLA1-reactive alloantibodies in serum could be demonstrated by their competitive effect on the binding of SZ-21.