A New Private Platelet Antigen, Gro, 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^aand 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^aantigen was found to be inherited in an autosomal‐codominant fashion. In the MAIPA, we localized the Gro^aantigen on the glycoprotein IIb/IIIa complex (αIIbβ3). The GP IIb/IIIa localization was confirmed in immunoprecipitation studies. In Western blotting experiments, we further localized the Gro^aantigen on the GP IIIa (β3) 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^aantigen, suggesting a phenotype frequency in the Dutch population of less than 0.01.