HIT is induced by IgG antibodies recognizing neoepitopes on the positively charged
chemokine PF4 within PF4–polyanion complexes (Figure 1FIGURE 1Pathogenesis of Heparin-Induced Thrombocytopenia.).12-14 The resulting immune complexes cross-link Fcγ receptors on platelets (Fcγ RIIa)15 and monocytes (Fcγ RI),16-18 thus activating them. Further enhanced by the alteration of endothelial cells,19 the activation of platelets and monocytes increases thrombin generation. Increased thrombin, not thrombocytopenia, causes the clinical problems.
In addition to binding heparin, PF4 binds other polyanions, such as nucleic acids20 and lipopolysaccharides on bacteria.21 This phenomenon may explain cases of spontaneous HIT after major surgery (causing DNA, RNA, or glycosaminoglycan release) or bacterial infection. An interesting concept22 is that conformationally changed23 PF4 in complex with nonheparin polyanions (e.g., on the bacterial surface) induces primary immunization. These PF4–polyanion complexes serve as a danger signal and result in the rapid generation of IgG antibodies, which facilitate opsonization and phagocytosis of PF4-coated bacteria, even against PF4-labeled pathogens that the immune system has not encountered before. This mechanism, however, results in HIT when it is misdirected22 during heparin treatment as a secondary immune reaction toward platelets coated with PF4–heparin complexes, which results in early production (between day 5and day 14) of high-titer IgG antibodies. Anti–PF4–heparin antibodies are produced by B cells (probably marginal-zone B cells),24 which can mediate a transient antibody response.