Heparin-induced thrombocytopenia: laboratory studies

Abstract

This report describes studies into the pathophysiology of heparin- induced thrombocytopenia. The IgG fraction from each of nine patients with heparin-induced thrombocytopenia caused heparin-dependent platelet release of radiolabeled serotonin. Both the Fc and the Fab portions of the IgG molecule were required for the platelet reactivity. The platelet release reaction could be inhibited by the Fc portion of normal human or goat IgG, and patient F(ab')2, but not F(ab')2 from healthy controls. These results suggested that the Fab portion of IgG binds to heparin forming an immune complex and the immune complexes initiate the platelet release reaction by binding to the platelet Fc receptors. To directly challenge this hypothesis, we preincubated the serotonin-labeled platelets with the monoclonal antibody against the platelet Fc receptor (IV.3). This monoclonal antibody completely inhibited the release reaction caused by heparin and patient sera, as well as heat aggregated IgG, but did not block collagen or thrombin- induced platelet release. Heparin-dependent platelet release also could be inhibited in vitro by the addition of monocytes and neutrophils, but not by red cells, presumably because the Fc receptors on the phagocytic cells have a higher binding affinity for IgG complexes than do platelets. Platelets from patients with congenital deficiencies of specific glycoproteins Ib and IX (Bernard-Soulier syndrome) and IIb and IIIa (Glanzmann's thrombasthenia) displayed normal heparin-dependent release indicating that the release reaction did not require the participation of these glycoproteins. These studies indicate that heparin-induced thrombocytopenia is an IgG-heparin immune complex disorder involving both the Fab and Fc portion of the IgG molecule.

Antiphospholipid antibody‐dependent C5b‐9 formation

“…A recent review by Arnout (1996) suggested that a preactivating event may be required for APA-induced platelet activation. As suggested by the author, binding of APA to negatively-charged phospholipids in complex with the appropriate cofactor protein after an initial activation signal could potentiate platelet activation through the platelet Fc receptor, much the same way as HIT-induced platelet activation is thought to be mediated (Kelton et al, 1988;Anderson, 1992). However, recent evidence suggests that HIT-mediated platelet activation, in some cases, proceeds independent of the platelet Fc receptor (Amiral et al, 1992;Horne & Alkins, 1996).…”

Anti‐CD36 autoantibodies in thrombotic thrombocytopenic purpura and other thrombotic disorders: identification of an 85 kD form of CD36 as a target antigen

“…(iii) Heparin-induced thrombocytopenia without thrombosis (Kelton et al, 1988;Chong et al, 1989;Amiral et al, 1992). 10 patients were investigated.…”

Affinity purification of heparin‐dependent antibodies to platelet factor 4 developed in heparin‐induced thrombocytopenia: biological characteristics and effects on platelet activation

“…In most cases, these antibodies are specific to macromolecular complexes of heparin (H) and platelet factor 4 (PF4) (Amiral et al, 1992;Greinacher et al, 1994;Visentin et al, 1994) and are mainly of the IgG isotype. Both the Fc and the Fab portions of HIT IgG are required for the activation of platelets by HIT IgG, which probably involves the binding of immune complexes to platelet FcgRII-A receptors (Kelton et al, 1988). The target epitopes for anti-PF4 antibodies are preferentially exposed in vitro by H-PF4 macromolecular complexes if well-defined concentrations of heparin and PF4 are present, corresponding in solution to about 27 units of unfractionated heparin (UH)/mg of tetrameric PF4 (Greinacher et al, 1994;Amiral et al, 1995).…”