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.

HEPARIN‐INDUCED THROMBOCYTOPENIA IN DIALYSIS: The Pathophysiology of Immune‐mediated Heparin‐induced Thrombocytopenia

“…Rhodes et al (3) first suggested that HIT was an immune‐mediated process when he showed that the IgG fraction of patients' sera resulted in in vitro aggregation of platelets in the presence of heparin. Kelton et al (37) reported that the platelet release reaction could be inhibited by the Fc portion of normal IgG, by patient F(ab′) 2 fragments, and by a monoclonal antibody (IV.3) to the platelet Fc receptor (FcγRII) indicating that the platelet Fc receptor plays a key role in the process. Dextran sulfate or salmon sperm DNA can substitute for heparin in vitro, suggesting that heparin cannot be the antigen, but rather is exposing a neo‐antigen on the platelet surface that serves as the primary antigen (38).…”

The platelet serotonin‐release assay

“…A key property of “HIT antibodies” is their ability to activate platelets both in vitro and in vivo . This occurs when anti‐PF4/heparin antibodies of IgG class bind to platelet Fcγ receptors (IgG receptors) . Since the PF4/heparin‐IgG complexes are multimolecular , two or more Fcγ receptors cross‐link, which is a strong platelet activation stimulus that results in platelet shape change and aggregation, α‐granule and dense‐granule release, and formation of procoagulant, platelet‐derived microparticles .…”

The transfusion‐related acute lung injury controversy: lessons from heparin‐induced thrombocytopenia

“…Activation of monocytes by HLA Class II antibodies parallels findings of a role of monocytes in HIT . In addition, the evolving notion of an important role of Fc receptors in TRALI again parallels the important role of Fc receptors in HIT …”