Visualizing the Molecular and Cellular Basis of Heparin Induced Thrombocytopenia.

Abstract 228

Heparin-induced thrombocytopenia (HIT) is an iatrogenic complication of heparin therapy caused by antibodies that recognize the platelet chemokine, Platelet Factor 4 (PF4), complexed to heparin or to cellular glycosaminoglycans (GAG). The origin of the sentinel feature of HIT - high propensity for thrombosis in the setting of mild thrombocytopenia - remains undefined. We have shown previously that monocytes are particularly susceptible to activation by HIT antibody because unlike platelets, whose surface glycosaminoglycans (GAGs) consist almost entirely of chondroitin sulfates (CS), monocytes express heparan and dermatan sulfates that bind to PF4 with greater affinity. Therefore, antigenic complexes form at lower concentrations of PF4 on monocytes than on platelets and are more resistant to removal by exogenous heparin. We now provide 2 lines of visual evidence in support of this model of the prothrombotic state in HIT: 1) We demonstrate by scanning electron microscopy that spheroidal particles indicative of PF4/GAG complexes are present on the surface of human monocytes upon addition of PF4. 2) Thrombus formation has been visualized in a previously described murine HIT model involving the infusion of the HIT-like monoclonal antibody KKO into mice double-transgenic for human PF4 (hPF4+) and FcγRIIA receptor (FcγRIIA+) on their platelets. After KKO infusion, hPF4+/FcγRIIA+ mice developed vigorous and concurrent accumulation of HIT antigenic complexes and fibrin at sites of laser-induced endothelial injury, not present in similarly treated single transgenic mice. Accumulation of PF4 and fibrin at sites of injury was not increased by a polyclonal anti-hPF4 antibody that does not induce thrombocytopenia in hPF4+/FcγRIIA+ mice. Monocytes and/or monocyte microparticles accumulated at sites of laser-induced injury in KKO-infused hPF4+/FcγRIIA+ mice, but not in KKO-infused single transgenic mice or hPF4+/FcγRIIA+ mice infused with an isotype control, beginning 2.5 min after formation of HIT antigenic complexes and fibrin deposition, accumulating preferentially at the upstream end of developing thrombi. These results provide new insights into the potential role of monocytes in the prothrombotic sequelae of HIT. Antigenic complexes form on monocytes exposed to PF4 and accumulate in HIT antibody-induced thrombi. Monocytes appear to reinforce thrombus formation initiated by injured endothelial cells and activated platelets.