Human Platelet-Specific Antigen, Siba, Is Associated With the Molecular Weight Polymorphism of Glycoprotein Ibα

Abstract

AbstractPlatelet-specific antigen Siba has been highly implicated in the pathogenesis of refractoriness to human leukocyte antigen (HLA)-matched platelet transfusions in Japan. We provide evidence that the Siba antigen is located on the glycoprotein (GP) Ibα and has a close association with the molecular weight (mol wt) polymorphism of GPIb. In modified antigen-capture ELISA (MACE), anti-Siba antibody reacted only with GPIb/IX held by a murine anti-GPIb/IX monoclonal antibody (MoAb). The reactivity of anti-Siba antibody to Siba-positive (Siba+) platelets was abolished after they were treated with Serratia marcescens protease. Platelets from 50 healthy volunteers were semiquantitatively phenotyped for Siba antigen by MACE and divided into three distinct groups: strongly positive, positive, and negative. They were also analyzed by sodium dodeyl sulfate-poiyacrylamide gel electrophoresis (SDS-PAGE) and periodic acid-silver staining for mol wt polymorphism of GPIb, phenotyped as A, B, C, or D. Without exception. Siba+ platelets showed larger phenotypes (A or B). Removal of sialic acid from Siba+ platelets did not reduce the binding of anti-Siba. Finally, anti-Siba antibody specifically immunoprecipitated A and B phenotypes of GPIb from Siba+ platelets. Thus, Siba antigen evidently is located in the region of glycocalicin that is present only on the A and B phenotypes of GPIb.