Human anti‐PlEl antibody recognizes epitopes associated with the alpha subunit of platelet glycoprotein Ib

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Abstract
Together, a platelet-reactive antibody in the serum of a polytransfused patient (proband) and a platelet-reactive antibody in the serum of a mother of an infant with neonatal thrombocytopenia have served to establish the diallelic, platelet-specific alloantigen system. PlE. We now provide evidence that the platelet-specific antibody in the serum of the proband, anti-PlE1, recognizes epitopes associated with the alpha subunit of glycoprotein (GP) Ib. By 51Cr release, platelets from two of three patients with the Bernard-Soulier syndrome (BSS) responded sub-normally to anti-PlE1, and the apparently normal response of platelets from the last BSS patient was attributable to anti-HLA-A2 antibodies in the proband serum. These results suggested that the PlE1 antigen is associated with the GPIb complex (glycoproteins Ib + IX) known to be absent from BSS platelets. This possibility was confirmed by ELISA using the purified GPIb complex or glycocalicin, the N-terminal fragment of GPIb alpha produced by proteolysis with endogenous platelet calpain, as solid-phase antigen. Anti-PlE1 antibody bound specifically to both the GPIb complex and glycocalicin. 3H-labelled platelet membrane glycoproteins with apparent molecular weights of 130k, 25k, and 21k (under reduced conditions) corresponding to GPIb alpha GPIb beta, and GPlX were immunoprecipitated by anti-PlE1 plasma. Finally, at a titre of 1:16, anti-PlE1 completely inhibited ristocetin-induced platelet agglutination, a propety of platelets mediated by GPIb.