A NEW PRIVATE PLATELET ANTIGEN, GRO(A), LOCALIZED ON GLYCOPROTEIN IIIA, INVOLVED IN NEONATAL ALLOIMMUNE THROMBOCYTOPENIA

The serum of a Caucasian woman who gave birth to a child with neonatal alloimmune thrombocytopenia contained antibodies directed against a platelet antigen of the newborn. There was no incompatibility for the known platelet alloantigens HPA-1 to HPA-7 or for the private or low-frequency antigens Sr-a and Va(a), between the platelets of the parents. However, crossmatching with the serum of the mother and the platelets of the child and the father was strongly positive, suggesting a new platelet antibody specificity. To investigate the inheritance of the 'Gro(a)' antigen involved, the available family members were tested in the platelet immunofluorescence test (PIFT) and the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay. The Gro(a) antigen was found to be inherited in an autosomal-codominant fashion. In the MAIPA, we localized the Gro(a) antigen on the glycoprotein IIb/IIIa complex (alpha IIb beta 3). The GP IIb/IIIa localization was confirmed in immunoprecipitation studies. In Western blotting experiments, we further localized the Gro(a) antigen on the GP IIIa (beta 3) subunit of the GP IIb/IIIa complex. Until now we have tested approximately 400 unrelated donors. None of these appeared to be positive for the Gro(a) antigen, suggesting a phenotype frequency in the Dutch population of less than 0.01.