DO PATIENTS WITH THE HEPARIN-INDUCED THROMBOCYTOPENIA SYNDROME HAVE HEPARIN-SPECIFIC ANTIBODIES

Purpose: Patients with the heparin-induced thrombocytopenia syndrome (HIT) have heparin-associated antibodies (HAb+), which, in the presence of heparin, are responsible for platelet activation and aggregation. This study addressed the questions: (1) are the antibodies specific for heparin; and (2) how do the antibodies cause platelet aggregation? Methods: Plasmas from 79 patients with HIT were divided into seven plasma samples: HAb+ plasma sample 1 (24 pooled plasmas); HAb+ plasma sample 2 (50 pooled plasmas); and HAb+ plasma samples 3 through 7 (individual plasmas). Normal patient plasmas were used as controls (HAb-). Results: AH seven HAb+ plasma samples caused platelet aggregation (PLA) in the presence of heparin and formed a precipitation line with heparin in gel immunodiffusion plates (HAb- plasmas did neither). The HAb+ plasma samples reacted with heparin, as determined by immunoprecipitation in sodium dodecylsulfate-polyacrylamide gel, with the production of a band at 50 kd (no band with HAb- plasmas). The plasma samples 1 and 2 were passed over heparin sepharose beads three times; the unabsorbed plasmas produced 3 + PLA, the first effluent produced 2 + PLA, and the second and third effluents produced no PLA. The heparin sepharose beads stained 3+, 2+, and 1+, after the respective passages, with fluorescein-labeled goat sera containing anti-human immunoglobulin G antibody. HAb+ plasma samples were digested with pepsin to separate the F(ab')(2) fragments from the Pc fragments. The F(ab')(2) fragments reacted with heparin as determined by immunoprecipitation in sodium dodecylsulfate-polyacrylamide gel with the production of a band at 25 kd, but did not cause PLA in the presence of heparin. Conclusion: Patients with HIT have heparin-specific antibodies that react with heparin in a classic F(ab')(2) reaction and require the Fc fragment for platelet aggregation.