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

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 kX) 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 GPIX were immunoprecipitated by anti-PlE1 plasma. Finally, at a titre of 1:16, anti-PlEl completely inhibited ristocetin-induced platelet agglutination, a property of platelets mediated by GPIb.