Evidence for Heterogeneity of Glycoprotein lila

Neonatal alloimmune thrombocytopenic purpura associated with a new platelet-specific alloantigen Pen' has been reported. We now provide direct evidence that the Pen' determinant is associated with glycoprotein (GP) IIIa, but that it is distinct from epitopes that define the pIA system. By ELISA wherein monoclonal antibodies specific for GPIIb (Tab) and specific for GPIIIa (AP3) were used to capture and hold antigens from a platelet lysate prepared under conditions that generate free GPIIb and GPIIIa, anti-Pen' reacted with GPIIIa held by AP3 but not with GPIIb held by Tab. In an alternative ELISA where purified GPIIIa from both PlAl-positive and PlAl-negative platelets were used individually as antigen, anti-Pen' reacted with both allelic forms of GPIIIa. By radioimmunoprecipitation, anti-Pen' precipitated a single surface-labeled membrane protein with electrophoretic characteristics in sodium dodecyl sulfate-polyacrylamide gels, under nonreduced or reduced conditions, identical to those of GPIIIa. By fluorocytometry, platelets from several donors, regardless of peA phenotype, bound an amount ofanti-Pen' roughly equivalent to one-half that amount of anti-PlI' bound by PeAl homozygous (Al/Al) platelets and roughly equal to that amount of antiP1Ae bound by p1Al heterozygous (Al/A2) platelets. Using platelets from donors typed homozygous for p1Al and Pen' in a quantitative indirect binding assay, 14-24,000 molecules of anti-Pen' and 41-51,000 molecules ofanti-PlAl were bound per platelet at saturation. Anti-Pen' completely inhibited ADP-induced aggregation of Pen3-positive platelets, regardless of peA phenotype. These results indicate that the Pen' determinant is associated with GPIIIa but distinct from PA.