Monocytes are a Favored Target for Surface Platelet Factor 4 (PF4) Antigenic Complex Formation in Heparin-Induced Thrombocytopenia: New Insights into the Thrombotic Risk in HIT Lubica Rauova, Gowthami Arepally, Douglas Cines and Mortimer Poncz HIT is a drug-induced autoimmune thrombocytopenia caused by antibodies to heparin/PF4 complexes that predispose to thrombotic complications. The studies described below examine how monocytes (Mo) may contribute to the thrombotic risk. We demonstrated previously that glycosaminoglycans (GAG) on the surface of platelets bind PF4, forming complexes that are recognized by HIT antibody, leading to platelet activation via the platelet FcγRIIA receptor in vitro and thrombocytopenia/thrombosis in vivo. However, heparin not only induces antibodies to develop against the PF4/GAG surface antigenic complexes, but also rapidly removes the same PF4/GAG complexes from the platelet surface, which may limit the likelihood of developing HIT and help limit its duration. This led us to study the involvement of Mo, which are a rich potential source of tissue factor and are known to be activated in HIT. Moreover, unlike platelets, which are coated with GAG composed almost entirely of chrondroitin sulfate (CS), Mo also express heparan sulfate, which has the capacity to bind PF4 with greater avidity and be resistant to the effect of plasma heparin. We found that Mo bind PF4 with greater avidity than platelets and higher concentrations of UFH are needed to remove PF4/GAG complexes and reduce the binding of a HIT monoclonal antibody KKO. In contrast to platelets, dissociation of PF4/GAG complexes from monocytes requires heparinases in addition to chondroitinases. In addition, macrophages GAG undergo hypersulfation during inflammation. Because clinical studies have shown inflammation predisposes to HIT, we examined the binding of KKO to unstimulated and bacterial lipopolysaccharide (LPS, E. coli serotype 011) stimulated cultured macrophages. Macrophages were derived from primary human Mo or murine bone marrow, cultured in the presence of M-CSF and stimulated with 0–500 ng/mL of LPS for 72 hrs. LPS increased KKO binding in the presence of PF4 2.7±0.7-fold compared to unstimulated cells (p<0.002) and the stimulated cells required ~2-fold higher concentrations of heparin to remove surface PF4/GAG complexes. Addition of [35S]sulfate during the last 24 hrs of incubation lead to a 4.1±0.1-fold increase in the incorporation of 35S into surface GAG after LPS stimulation (p<0.0001). These results provide important insights into the potential role of Mo in the prothrombotic sequelae of HIT. Compared to platelets, Mo are relatively resistant to “antigen down-regulation” by heparin and are more likely to bind anti-PF4/GAG HIT antibodies and become activated. The relative resistance of Mo to the dissociation PF4/GAG complexes from the cell surface also suggest a role in the development of Delayed-Onset HIT after heparin withdrawal.

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