Immunochemical characterization of the new platelet alloantigen system Bra/Brb.

We report the immunochemical characterization of the new platelet-specific alloantigens Bra and Brb. Bra antibodies were from mothers of children with neonatal alloimmune thrombocytopenia (NAIT), and anti-Brb was found in the serum of a polytransfused patient. By radioimmunoprecipitation, anti-Bra and anti-Brb precipitated two proteins with apparent relative molecular masses in sodium dodecyl sulphate-polyacrylamide gel electrophoresis of 155,000 and 130,000 under non-reduced conditions, and of 165,000 and 148,000 under reduced conditions. In two-dimensional polyacrylamide gel electrophoresis, the two bands moved with isoelectric points ranging from 5.2 to 5.4 and from 4.5 to 4.7, respectively. These features fulfil previously defined criteria for platelet membrane glycoproteins (GP) Ia and IIa. The results were supported by data obtained by an assay employing monoclonal antibody (mab)-specific immobilization of platelet antigens (MAIPA). By this technique, Bra and Brb antigens could be immobilized by mabs specific for the GP Ia/IIa complex (mab Gi 14) or a mab specific for the very late antigen-2 (mab 12 F1), but not by a mab specific for GP IIb/IIIa complex (mab Gi3). Furthermore, platelets from a thrombasthenic patient with complete absence of GP IIb/IIIa expressed the Brb antigen normally as shown in MAIPA; this antigen could be immunoprecipitated with anti-Brb and was identical to that of normal platelets. This confirms that the antigens of the Br system are not associated with the GP IIb/IIIa complex. By direct binding studies using mabs Gi3 and Gi14, we calculated that 51,500 +/- 3900 molecules of anti-GP IIb/IIIa and 6,470 +/- 500 molecules of anti-GP Ia/IIa were bound per platelet at saturation. Our results provide evidence for the first platelet-specific alloantigen system residing on the GP Ia. The difficulty in detecting anti-Bra and anti-Brb by direct binding assays may be related to the small number of GP Ia/IIa complexes on platelets.