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 p latelet antigen of the newborn. There was no incompatibility for the k nown platelet alloantigens HPA-1 to HPA-7 or for the private or low-fr equency 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 p latelet antibody specificity. To investigate the inheritance of the 'G ro(a)' antigen involved, the available family members were tested in t he 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 II b/IIIa complex (alpha IIb beta 3). The GP IIb/IIIa localization was co nfirmed in immunoprecipitation studies. In Western blotting experiment s, we further localized the Gro(a) antigen on the GP IIIa (beta 3) sub unit of the GP IIb/IIIa complex. Until now we have tested approximatel y 400 unrelated donors. None of these appeared to be positive for the Gro(a) antigen, suggesting a phenotype frequency in the Dutch populati on of less than 0.01.