Heparin-induced thrombocytopenia (HIT) is an immune-mediated thrombocytopenic disorder associated with a severe prothrombotic state. We investigated whether neutrophils and neutrophil extracellular traps (NETs) contribute to the development of thrombosis in HIT. Using an endothelialized microfluidic system and a murine passive immunization model, we show that HIT induction leads to increased neutrophil adherence to venous endothelium. In HIT mice, endothelial adherence is enhanced immediately downstream of nascent venous thrombi, after which neutrophils undergo retrograde migration via a CXCR2-dependent mechanism to accumulate into the thrombi. Using a microfluidic system, we found that PF4 binds to NETs, leading them to become compact and DNase resistant. PF4-NET complexes selectively bind HIT antibodies, which further protect them from nuclease digestion. In HIT mice, inhibition of NET formation through Padi4 gene disruption or DNase treatment limited venous thrombus size. PAD4 inactivation did affect arterial thrombi or severity of thrombocytopenia in HIT. Thus, neutrophil activation contributes to the development of venous thrombosis in HIT by enhancing neutrophil-endothelial adhesion and neutrophil clot infiltration, where incorporated PF4-NET-HIT antibody complexes lead to thrombosis propagation. Inhibition of neutrophil endothelial adhesion, prevention of neutrophil chemokine-dependent recruitment of neutrophils to thrombi, or suppression of NET release should be explored as strategies to prevent venous thrombosis in HIT.

When stimulated by infection or inflammation, neutrophils expel NETs, decondensed chromatin coated with histones and antimicrobial proteins that ensnares pathogens but also damages host tissue. Platelet factor 4 (PF4, CXCL4) is a CXC chemokine stored in platelet alpha-granules and released in high concentrations during platelet activation. Tetrameric PF4 has a very high affinity for polyanionic molecules, including DNA, and we have found that PF4 binds and physically compacts NETs, causing them to have increased resistance to endonuclease digestion. Our group has also observed that PF4 expression leads to enhanced survival in a murine model of sepsis. Based on these findings, we chose to investigate whether PF4-mediated NET compaction is protective in endotoxemia. To study PF4-NET interactions, we developed a microfluidic assay in which neutrophils were adhered to fibronectin-coated channels and then stimulated to release NETs with phorbol myristate acetate(PMA). NETs were visualized by staining with the fluorescent nucleic acid stain SYTOX. Changes in NET morphology and fluorescence were quantified in the presence of varying PF4 concentrations. DNase I was then infused through these channels and the extent of digestion was measured. These experiments showed that the presence of PF4 led to NET compaction and decreased NET degradation following DNase infusion. We then performed in vitro studies examining NET-endothelial interactions in which isolated neutrophils were stimulated to release NETs, incubated with buffer alone or buffer containing PF4, and flowed through human endothelial umbilical vein cell (HUVEC) lined microfluidic channels that had been stimulated with tumor necrosis factor (TNF) α. EC viability was assessed 24-hours post NET exposure and revealed that the presence of PF4 protected HUVECs from NET-induced damage. To further investigate PF4-NET interactions in endotoxemia, we conducted in vivo studies using PF4-deficient mice (mPF4-/-) and wildtype (WT) controls injected with lipopolysaccharide (LPS). Plasma NET markers [cell free DNA (cfDNA), citrullinated histones (cit-His), and myeloperoxidase (MPO)] were quantified via ELISA and Western blot at various time points following LPS injection. mPF4-/- mice were also implanted with PF4-containing osmotic pumps and the NET markers were also assessed following LPS exposure. These experiments revealed that compared to WT mice, LPS injected mPF4-/- mice had significantly higher plasma levels of NET components, including cfDNA, cit-His and MPO. When mPF4-/- mice were implanted with PF4-releasing osmotic pumps prior to LPS injection, they had plasma NET component levels comparable to those observed in WT mice. Based on the results of our in vitro and in vivo studies, we propose that PF4 infusion compacts NETs, decreasing their susceptibility to DNAse lysis, and preventing the release of toxic NET degradation products (NDPs) such as cfDNA and cit-His. We posit that PF4-mediated sequestration of NDPs prevents endothelial cell damage in the HUVEC-lined microfluidic model. We believe that the results of our studies in mPF4-/- mice demonstrate that PF4 has a similarly protective effect in vivo, decreasing NET lysis and reducing NDP generation. These findings suggest that in sepsis, the stabilization rather than the lysis of NETs may be therapeutic. Further investigation should be performed to determine if treatment with PF4 or other small positively-charged proteins such as protamine sulfate that can sequester NDPs, may be beneficial the treatment of sepsis. Disclosures No relevant conflicts of interest to declare.

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