Defining an Antigenic Epitope on Platelet Factor 4 Associated With

Heparin-induced thrombocytopenia (HIT) is a potentially serious complication of heparin therapy. Antibodies to plate-let factor 4 (PF4)/heparin complexes have been implicated in the pathogenesis of this disorder, but the antigenic epi-tope(s) on the protein have not been defined. To address this issue, we studied the binding of HIT antibodies to a series of recombinant proteins containing either point mutations in PF4 or chimeras containing various domains of PF4 and the related protein, neutrophil activating peptide-2 (NAP-2). Se-rum samples from 50 patients with a positive 14C-serotonin release assay (14C-SRA) and a clinical diagnosis of HIT and 20 normal controls were studied. HIT antibodies reacted strongly with wild-type (WT) PF4/heparin complexes, but reacted little, if at all, with NAP-2/heparin complexes (optical density [OD]405 5 2.5 and 0.2, respectively). Alanine substitutions a