Effects of HIT Antibody Containing Immune Complexes

HIT antibodies bind to PF4-heparin complexes by their F(ab')2 domains (Horne and Alkins, 1996; Newman and Chong, 2000). The predominant immunoglobulin isotype in clinical HIT is IgG (Amiral et al., 1996b; Warkentin et al., 2005b; Juhl et al., 2006). Thus, divalent IgG binding to multimolecular PF4-heparin complexes, leads to the formation of large immune complexes containing HIT-IgG on the platelet surface. The interaction of the HIT-IgG Fc with the platelet FcyIIa receptors leads to cross-linking of these receptors on the same or adjacent platelets, which triggers platelet activation (Kelton et al., 1988, 1994; Chong et al., 1989a) (see Chapter 8). The HIT antibody-mediated platelet activation can be inhibited by a monoclonal antibody specific for the FcyIIa receptor, by high concentrations of Fc fragments derived from normal IgG, and by excess heparin saturating all binding sites on PF4, and thus preventing the formation of multimolecular complexes (Greinacher et al., 1994b; Visentin et al., 1994).

Besides these effects on platelets, polyclonal HIT antibodies bind to endothe-lial cells (Cines et al., 1987; Visentin et al., 1994). The most convincing evidence demonstrating that these antibodies are the same ones that cause platelet activation was provided by classic adsorption-elution experiments (Greinacher et al., 1994a). Purified IgG obtained from sera of HIT patients gave positive reactions in both activation (serotonin release) and antigen (anti-PF4/heparin) assays. This IgG fraction was then incubated with cultured endothelial cells and, after extensive washing, the antibodies were eluted from the endothelial cells by low pH. The eluate again tested positive in both activation and antigen assays. Thus, these experiments showed that the antibodies recognize the same epitope on platelets, endothelial cells, and PF4-heparin complexes coated onto a microtiter plate. It appears most likely that the epitope on endothelial cells comprises surface GAGs (Cines et al., 1987; Greinacher et al., 1994a; Visentin et al., 1994).

In addition to platelet and endothelial cell activation, there is concomitant activation of coagulation, as shown by marked elevations in thrombin-AT complex levels (Warkentin et al., 1997; Greinacher et al., 2000). The simultaneous activation of platelets, endothelium, and coagulation factors is in line with the development of thrombocytopenia combined with thrombosis and disseminated intravascular coagulation in patients with HIT (see Chapter 2).

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