A New Private Platelet Antigen, Groa, Localized on Glycoprotein IIIa, Involved in Neonatal Alloimmune Thrombocytopenia

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Abstract
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 Sraand Vaa, 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 ‘Groa’ 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 Groaantigen was found to be inherited in an autosomal-codominant fashion. In the MAIPA, we localized the Groaantigen 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 Groaantigen 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 Groaantigen, suggesting a phenotype frequency in the Dutch population of less than 0.01.
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