A MONOCLONAL-ANTIBODY (SZ21) SPECIFIC FOR PLATELET GPIIIA DISTINGUISHES P1(A1) FROM P1(A2)

Neonatal alloimmune thrombocytopenia (NATP) and posttransfusion purpur a (PTP) are acquired bleeding disorders caused by alloimmune thrombocy topenia. In most cases, the thrombocytopenia is due to an alloantibody directed against the platelet glycoprotein IIb-IIIa (GPIIb-IIIa) comp lex. During the course of routine studies on the role of GPIIb-IIIa in inherited and acquired bleeding and thrombotic disorders, we unexpect edly identified an individual whose platelets reacted by nonreduced We stern blot analysis with anti-GPIIIa polyclonal antisera, but did not react with a commercially available monoclonal antibody (SZ21) specifi c for GPIIIa. We screened all 14 GPIIIa exons for possible nucleotide changes;which might alter amino acids and found variations in only exo ns 3 and 10. Nucleotide sequencing revealed that only the exon 3 alter ation changed the predicted amino acid sequence. This variation was ca used by homozygosity for the uncommon p1(A2) allele of the GPIIIa gene . Platelets from two additional unrelated normal individuals known to be homozygous for p1(A2) also lacked reactivity with SZ21 by Western b lot. Using flow cytometry with intact platelets, we observed a markedl y reduced binding of SZ21 to platelets with the pl(A2) genotype. Scatc hard analyses indicated that SZ21 bound to p1(A1/A1) platelets with a Kd of approximate to 8.26x 10(-10) M, and to pl(A2/A2) platelets with a Kd of approximate to 5.58x10(-9) M. Thus, we have characterized a re adily available monoclonal antibody able to distinguish between the tw o P1(A) alleles of the GPIIIa gene. Because incompatibility for this p latelet polymorphism is the most common cause of neonatal alloimmune t hrombocytopenia and posttransfusion purpura, and because platelet immu nophenotyping reagents lack specificity and are not easily available, this monoclonal antibody could facilitate the management of patients w ith these disorders.