Platelet Factor 4 (PF4) Antigenic Complexes on the Macrophage Surface: Implications for the Pathogenesis of Heparin Induced Thrombocytopenia (HIT).

HIT is the most frequent antibody-mediated drug-induced thrombocytopenia and is unique in the associated prevalence of thrombosis. HIT is caused by antibodies to a normal host protein, PF4, complexed to heparin or cellular glycosaminoglycans (GAGs). We have shown that binding of HIT antibodies and a monoclonal HIT-like antibody KKO to platelet surface chondroitin sulfate (CS) follows a bell-shaped curve with respect to PF4 concentration. At external PF4 concentrations below the peak value of <50 μg/mL, heparin dissociates PF4 and thereby decreases surface antigenicity, whereas at PF4 concentrations >50 μg/mL, heparin initially increases antigenicity. Surface GAGs on macrophages differ from those on platelets in several ways, including having a higher affinity for PF4. In addition, the binding of HIT-IgG elicits the elaboration of tissue factor. Therefore, we characterized the binding of HIT IgG antibody to macrophages exposed to different concentrations of PF4. Binding of PF4 to the surface of human/murine macrophages also followed a bell-shaped curve with peak antigenicity at the same PF4 concentration as seen on platelets. However, HIT antigenticity persisted at lower PF4 concentrations. For example at a PF4 concentration of 6 μg/mL, six times as much KKO bound to macrophages as to platelets. PF4-coated monocytes were also more resistant to the abrogation of KKO binding by heparin. At therapeutic concentrations of heparin, HIT antigenicity actually increased on macrophages exposed to optimal concentrations of PF4, while platelet surface antigenicity was nearly eliminated. Enzymatic removal of surface GAGs showed that unlike platelet surfaces, where CS is the major GAG involved in HIT antigenicity, a more complex pattern was seen on macrophages that included heparan sulfates, CS and likely dermatan sulfates. Thus, these studies show that expression of surface HIT antigenic complexes on macrophages develop at much lower concentrations of PF4 than platelets and that these complexes are more resistant to removal by heparin. The enhanced avidity of macrophages for PF4/GAG complexes may also contribute to the persistent risk of thrombosis for at least several days after heparin exposure has been stopped. Whether measurement of antigenic complexes on macrophages would identity patients at higher risk of thrombosis and whether reduction in the risk of thrombosis requires measures directed at the integrity of these complexes in addition to use of direct thrombin inhibitors are under investigation.