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.