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Summary Patients who are severely affected by coronavirus disease 2019 (COVID‐19) may develop a delayed onset ‘cytokine storm’, which includes an increase in interleukin‐6 (IL‐6). This may be followed by a pro‐thrombotic state and increased D‐dimers. It was anticipated that tocilizumab (TCZ), an anti‐IL‐6 receptor monoclonal antibody, would mitigate inflammation and coagulation in patients with COVID‐19. However, clinical trials with TCZ have recorded an increase in D‐dimer levels. In contrast to TCZ, colchicine reduced D‐dimer levels in patients with COVID‐19. To understand how the two anti‐inflammatory agents have diverse effects on D‐dimer levels, we present data from two clinical trials that we performed. In the first trial, TCZ was administered (8 mg/kg) to patients who had a positive polymerase chain reaction test for COVID‐19. In the second trial, colchicine was given (0·5 mg twice a day). We found that TCZ significantly increased IL‐6, α‐Defensin (α‐Def), a pro‐thrombotic peptide, and D‐dimers. In contrast, treatment with colchicine reduced α‐Def and Di‐dimer levels. In vitro studies show that IL‐6 stimulated the release of α‐Def from human neutrophils but in contrast to colchicine, TCZ did not inhibit the stimulatory effect of IL‐6; raising the possibility that the increase in IL‐6 in patients with COVID‐19 treated with TCZ triggers the release of α‐Def, which promotes pro‐thrombotic events reflected in an increase in D‐dimer levels.

Summary The inflammatory response to SARS/CoV‐2 (COVID‐19) infection may contribute to the risk of thromboembolic complications. α‐Defensins, antimicrobial peptides released from activated neutrophils, are anti‐fibrinolytic and prothrombotic in vitro and in mouse models. In this prospective study of 176 patients with COVID‐19 infection, we found that plasma levels of α‐defensins were elevated, tracked with disease progression/mortality or resolution and with plasma levels of interleukin‐6 (IL‐6) and D‐dimers. Immunohistochemistry revealed intense deposition of α‐defensins in lung vasculature and thrombi. IL‐6 stimulated the release of α‐defensins from neutrophils, thereby accelerating coagulation and inhibiting fibrinolysis in human blood, imitating the coagulation pattern in COVID‐19 patients. The procoagulant effect of IL‐6 was inhibited by colchicine, which blocks neutrophil degranulation. These studies describe a link between inflammation and the risk of thromboembolism, and they identify a potential new approach to mitigate this risk in patients with COVID‐19 and potentially in other inflammatory prothrombotic conditions.

Inflammation and thrombosis are integrated, mutually reinforcing processes, but the interregulatory mechanisms are incompletely defined. Here, we examined the contribution of α-defensins (α-defs), antimicrobial proteins released from activated human neutrophils, on clot formation in vitro and in vivo. Activation of the intrinsic pathway of coagulation stimulates release of α-defs from neutrophils. α-Defs accelerate fibrin polymerization, increase fiber density and branching, incorporate into nascent fibrin clots, and impede fibrinolysis in vitro. Transgenic mice (Def++) expressing human α-Def-1 developed larger, occlusive, neutrophil-rich clots after partial inferior vena cava (IVC) ligation than those that formed in wild-type (WT) mice. IVC thrombi extracted from Def++ mice were composed of a fibrin meshwork that was denser and contained a higher proportion of tightly packed compressed polyhedral erythrocytes than those that developed in WT mice. Def++ mice were resistant to thromboprophylaxis with heparin. Inhibiting activation of the intrinsic pathway of coagulation, bone marrow transplantation from WT mice or provision of colchicine to Def++ mice to inhibit neutrophil degranulation decreased plasma levels of α-defs, caused a phenotypic reversion characterized by smaller thrombi comparable to those formed in WT mice, and restored responsiveness to heparin. These data identify α-defs as a potentially important and tractable link between innate immunity and thrombosis.

Atherosclerosis, the predominant cause of death in well-resourced countries, may develop in the presence of plasma lipid levels within the normal range. Inflammation may contribute to lesion development in these individuals, but the underlying mechanisms are not well understood. Transgenic mice expressing α-def-1 released from activated neutrophils develop larger lipid and macrophage-rich lesions in the proximal aortae notwithstanding hypocholesterolemia caused by accelerated clearance of α-def-1/low-density lipoprotein (LDL) complexes from the plasma. The phenotype does not develop when the release of α-def-1 is prevented with colchicine. However, ApoE-/- mice crossed with α-def-1 mice or given exogenous α-def-1 develop smaller aortic lesions associated with reduced plasma cholesterol, suggesting a protective effect of accelerated LDL clearance. Experiments were performed to address this seeming paradox and to determine if α-def-1 might provide a means to lower cholesterol and thereby attenuate atherogenesis. We confirmed that exposing ApoE-/- mice to α-def-1 lowers total plasma cholesterol and decreases lesion size. However, lesion size was larger than in mice with total plasma cholesterol lowered to the same extent by inhibiting its adsorption or by ingesting a low-fat diet. Furthermore, α-def-1 levels correlated independently with lesion size in ApoE-/- mice. These studies show that α-def-1 has competing effects on atherogenesis. Although α-def-1 accelerates LDL clearance from plasma, it also stimulates deposition and retention of LDL in the vasculature, which may contribute to development of atherosclerosis in individuals with normal or even low plasma levels of cholesterol. Inhibiting α-def-1 may attenuate the impact of chronic inflammation on atherosclerotic vascular disease.