ABH ANTIGENS ON HUMAN PLATELETS - EXPRESSION ON THE GLYCOSYL PHOSPHATIDYLINOSITOL-ANCHORED PROTEIN CD109

Platelets express alloantigens that are platelet specific (eg, the HPA antigens) and alloantigens that are shared with other blood cells (eg , the ABH antigens). The blood group A and B determinants are expresse d on glycolipids and on some intrinsic platelet membrane glycoproteins . This report characterizes multiple platelet proteins reacting with b lood group antibodies in serum samples from mothers of children born w ith neonatal alloimmune thrombocytopenia. ABH antigens on additional p latelet proteins are identified, including the glycosyl phosphatidylin ositol-anchored protein CD109. The proteins that carry ABH antigens we re identified by using monoclonal antibodies to glycoproteins Ib, IIb/ IIIa, Ia/IIa, CD31, and CD109 and immunoprecipitation/immunoblotting t echniques with monoclonal antibodies to A and B antigens, The maternal serum samples and anti-A and anti-B monoclonal antibodies immunopreci pitated identical radiolabeled platelet proteins including proteins at 220 and 175 kd and proteins with mobilities corresponding to glycopro teins Ib, IIb/IIIa, IV, and V. Treatment of platelets with phosphatidy linositol-specific phospholipase C released into the supernatant a 175 -kd protein that expressed the blood group determinants. This protein comigrated with the glycosyl phosphatidylinositol-anchored protein CD1 09, When platelet proteins were purified by immunoprecipitation with m onoclonal antibodies and then tested by immunoblotting, anti-A reacted with the glycosyl phosphatidylinositol-anchored protein CD109 and to glycoproteins Ib, IIb, IIa, IIIa, and CD31 (PECAM). These results indi cate that structures for modification by glycosyltransferases exist on platelet CD109, which also expresses the Gov alloantigen system. This study indicates that certain platelet proteins express both platelet- specific and blood group antigens that may contribute to platelet tran sfusion refractoriness and to neonatal alloimmune thrombocytopenia.