Localization of the platelet-specific HPA-2 (Ko) alloantigens on the N-terminal globular fragment of platelet glycoprotein Ib alpha.
The human platelet-specific alloantigens HPA-2a and HPA-2b (= Kob and Koa) together constitute a biallelic antigen system. The HPA-2 antigens have not, to date, been located on a particular platelet membrane molecule. Here, we describe the localization of these antigens on platelet glycoprotein (GP) Ib alpha. Platelets from two patients with the Bernard-Soulier syndrome (BSS) were HPA-2(a-,b-) in the immunofluorescence test with HPA-2 alloantibodies on chloroquine-treated platelets. With monoclonal antibody (MoAb) immobilization of platelet antigen assay (MAIPA), positive reactions were obtained only when MoAbs against the platelet GPIb/IX complex were used in combination with anti-HPA-2a or -2b alloantibodies and normal donor platelets. By immunoprecipitation under nonreducing and reducing conditions a protein of 160 Kd and 145 Kd, respectively, was precipitated by the anti-HPA-2a serum. A protein migrating identically to this was precipitated by anti-GPIb MoAb. Normal donor platelets became HPA-2(a-,b-) after elastase treatment, suggesting that anti-HPA-2 antibodies bind to the N-terminal elastase-sensitive part of GPIb alpha. Anti-HPA-2a antibodies inhibited the ristocetin-induced agglutination of HPA-2a-positive platelets but not of HPA-2a-negative platelets, indicating that the epitopes recognized by these alloantibodies are localized in the proximity of the von Willebrand-factor-binding domain. Together, these data provide evidence that the HPA-2 alloantigens are located on the N-terminal globular elastase-sensitive part of GPIb alpha. Furthermore, we show that the recently described Siba antigen is probably identical to HPA-2a.
Central Laboratory, Netherlands Red Cross Blood Transfusion Service, Amsterdam.
R W
RW
Kuijpers
W H
WH
Ouwehand
P M
PM
Bleeker
D
D
Christie
A E
AE
von dem Borne
D000911:Antibodies, Monoclonal; D016824:Antigens, Human Platelet / Q000032:analysis* / Q000276:immunology; D002738:Chloroquine / Q000494:pharmacology; D000939:Epitopes / Q000032:analysis; D005455:Fluorescent Antibody Technique; D006801:Humans; D007106:Immune Sera; D007163:Immunosorbent Techniques; D007519:Isoantigens / Q000032:analysis*; D010196:Pancreatic Elastase / Q000494:pharmacology; D010974:Platelet Aggregation / Q000187:drug effects; D010980:Platelet Membrane Glycoproteins / Q000276:immunology*; D012310:Ristocetin / Q000494:pharmacology; D014841:von Willebrand Factor / Q000378:metabolism
D000911:Antibodies, Monoclonal; D016824:Antigens, Human Platelet; D000939:Epitopes; D007106:Immune Sera; D007519:Isoantigens; D010980:Platelet Membrane Glycoproteins; D014841:von Willebrand Factor; D012310:Ristocetin; D002738:Chloroquine; D010196:Pancreatic Elastase