Demonstration of the heterogeneity of epitopes of the platelet-specific alloantigen, Baka.

It is well known that the platelet-specific alloantigen, Baka is carried on glycoprotein (GP) IIb, but little is known about the biochemical characteristics of its epitopes. To clarify the characteristics of the epitopes, we examined the interaction of four anti-Baka sera (Yam, Lin, Kl and MO) with their epitopes, either with or without modifications by sodium dodecyl sulphate (SDS) and/or neuraminidase. By immunoprecipitation, all four antisera bound to the intact GP IIb/IIIa complex from a Baka-positive subject. In contrast, immunoblotting demonstrated that Yam, Lin and Kl bound to SDS-denatured GP IIb, while MO did not. When blotted GP IIb was treated with neuraminidase, Yam and Lin did not bind to desialylated GP IIb, while Kl still did. When the purified GP IIb/IIIa complex or washed platelets were treated first with neuraminidase followed by immunoblotting, the molecular weight of GP IIb decreased from 145 kD to 138 kD; Yam did not bind to desialylated GP IIb, but Kl did. Furthermore, to eliminate the effect of SDS, we examined the interaction of Yam and Lin with neuraminidase-treated platelets using flow cytometry. The results were the same as those obtained using immunoblotting. Our results thus demonstrate that the expression of the Baka epitopes is not uniform and that sialic acid contributes to the expression of some actual allogenic epitopes.