Characterization of Receptors Involved in Heparin Antibody Complex Mediated Induction of Tissue Factor Expression in Monocytes.

Heparin-induced thrombocytopenia (HIT) is a major cause of drug-induced thrombocytopenia and occurs in 1–5% of individuals exposed to heparin. Paradoxically, 30–50% of individuals with HIT develop thrombosis. The mechanism of thrombosis in HIT is poorly understood. We recently reported that HIT antibody complexes induce tissue factor (TF) expression in monocytes and result in the release of TF-positive microparticles (MPs). The mechanism by which HIT antibody complexes induce monocyte TF has not been established. The objective of this study is to characterize the receptors involved in HIT antibody complex mediated induction of TF expression in monocytes. As HIT antibody complex mediated activation of platelets is dependent on the FcgRIIA receptor, we evaluated the role of the FcgRII receptor in the induction of monocyte TF by HIT antibody complexes. We also evaluated the role of toll like receptor-4 (TLR4) and the platelet factor 4 (PF4) chemokine receptor CXCR3 in this process.

The combination of heparin, PF4 and the murine monoclonal PF4/heparin-specific antibody KKO has been shown to cause activation of platelets and monocytes, and mimic HIT in vitro. Peripheral blood mononuclear cells (PBMCs) from healthy volunteers were pre-incubated for 30 min at 37°C with an inhibitory antibody to the FcgRII receptor (IV.3); anti-CXCR2, 3, or 4 antibodies; anti-TLR4 antibody; or mouse-IgG (mIgG) control. Following pre-incubation with antibodies for 30 minutes, heparin (1U/mL), PF4 (10μg/mL), and KKO (100μg/mL) – together referred to as the HIT antibody complex – were added. Heat-aggregated mIgG and LPS were used as positive controls for the FcgRII and TLR4 receptors, respectively. Following a 6-hour incubation, PBMCs were pelleted by centrifugation and MPs were isolated from the supernatant. The procoagulant activity (PCA) of PBMCs and MPs was measured using clotting assays performed in the presence of the anti-TF antibody HTF-1 or control antibody. TF dependent PCA was calculated by reference to a standard curve generated using relipidated recombinant TF.

Incubation of PBMCs with heat aggregated mIgG for 6 hours resulted in significant induction of cellular TF (345 +/− 36 pg/10⁶ cells) which was blocked by 30 min pre-incubation with the antibody IV.3 (146 +/− 17 pg/10⁶ cells, N=3, p<.003). However, pre-incubation with IV.3 had no significant effect on TF induction (140 +/− 5 pg/10⁶ cells) associated with the HIT antibody complex when compared to control mIgG (110 +/− 18 pg/10⁶ cells, N=3, p<0.11). PBMCs incubated with HIT antibody complexes in the presence of a TLR-4 antibody showed less TF activity (52 +/− 4 pg/10⁶ cells) compared to control mIgG (80 +/− 10 pg/10⁶ cells N=3, p<0.025). A similar, partial inhibition of TF activity was also observed in PBMCs incubated with LPS in the presence of an anti-TLR4 antibody (121 +/− 3 pg/10⁶) compared with a control antibody (89 +/− 2 pg/10⁶, N=3, p<.0013). Experiments with a more effective inhibitor of TLR4 are in progress. PBMCs incubated with the HIT antibody complexes in the presence of an anti-CXCR3 antibody showed less TF activity (36 +/− 7 pg/mL) compared to control mIgG (118 +/− 15 pg/10⁶ cells, N=3, p<0.004). Antibodies against CXCR2 and CXCR4 did not have any significant effect on TF induction. Measurement of MP TF activity mirrored the results described above. Using flow cytometry and an anti-CXCR3 antibody labeled with FITC, we found that 5% (± 0.5%) of monocytes expressed CXCR3 (N=3), which is consistent with the reported literature.

These data suggest that induction of TF in monocytes by HIT antibody complexes is not mediated by the FcgRII receptor. This is contrary to the mechanism of platelet activation by these antibody complexes, which is an FcgRIIa dependent process. We found that TLR4 plays a role in HIT antibody complex mediated induction of TF in monocytes and blocking TLR4 led to a 30% decrease in TF activity. On the other hand, CXCR3 appeared to play a more significant role with blockade of CXCR3 leading to a 70% decrease in TF activity. Further characterization of the role of these receptors in HIT antibody complex mediated induction of TF expression in monocytes is required. We speculate that the extent of CXCR3 and TLR4 expression in monocytes may influence the susceptibility to developing thrombotic complications in HIT.

No relevant conflicts of interest to declare.