NLRP3 inflammasome and interleukin-1 contributions to COVID-19-associated coagulopathy and immunothrombosis

Immunothrombosis-immune-mediated activation of coagulation-is protective against pathogens, but excessive immunothrombosis can result in pathological thrombosis and multiorgan damage, as in severe coronavirus disease 2019 (COVID-19). The NACHT-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome produces major proinflammatory cytokines of the interleukin (IL)-1 family, IL-1β and IL-18, and induces pyroptotic cell death. Activation of the NLRP3 inflammasome pathway also promotes immunothrombotic programs including release of neutrophil extracellular traps and tissue factor by leukocytes, and prothrombotic responses by platelets and the vascular endothelium. NLRP3 inflammasome activation occurs in patients with COVID-19 pneumonia. In preclinical models, NLRP3 inflammasome pathway blockade restrains COVID-19-like hyperinflammation and pathology. Anakinra, recombinant human IL-1 receptor antagonist, showed safety and efficacy and is approved for the treatment of hypoxaemic COVID-19 patients with early signs of hyperinflammation. The non-selective NLRP3 inhibitor colchicine reduced hospitalization and death in a subgroup of COVID-19 outpatients but is not approved for the treatment of COVID-19. Additional COVID-19 trials testing NLRP3 inflammasome pathway blockers are inconclusive or ongoing. We herein outline the contribution of immunothrombosis to COVID-19-associated coagulopathy, and review preclinical and clinical evidence suggesting an engagement of the NLRP3 inflammasome pathway in the immunothrombotic pathogenesis of COVID-19. We also summarize current efforts to target the NLRP3 inflammasome pathway in COVID-19, and discuss challenges, unmet gaps, and the therapeutic potential that inflammasome-targeted strategies may provide for inflammation-driven thrombotic disorders including COVID-19.

Abstract 123: Genetic Deficiency Of The Ectoenzyme CD73 Increases Neutrophil Extracellular Trap Formation In Patients With ACDC

Background/Purpose: CD73 is a ecto-5′-nucleotidase located on the plasma membrane that generates adenosine from AMP in the extracellular space, and functions as a checkpoint against immune and vascular activation. CD73 inhibitors that eliminate this immune checkpoint blockade are being studied in phase 2/3 cancer clinical trials. We recently identified that lack of CD73 promotes vascular thromboinflammation and neutrophil extracellular trap (NET) formation in mice. The role of CD73 in human NETosis, however, remains to be studied. Arterial calcification due to deficiency of CD73 (ACDC) is a rare, autosomal recessive condition (~20 patients globally) that results in occlusive peripheral vascular disease. We hypothesized that the congenital absence of CD73 and its function as an immune checkpoint would heighten innate immune activation in patients with ACDC.

Methods: To test this hypothesis, six patients with molecularly confirmed ACDC were recruited to the NIH for mechanistic studies. Using purified neutrophils from ACDC patients and healthy controls, spontaneous and LPS-induced NET formation was quantified by both Sytox extracellular DNA (eDNA) labeling and NET-associated MPO activity.

Results: Absence of surface CD73 was confirmed by flow cytometry. Relative to healthy controls, four of six (4/6) patients with ACDC demonstrated increased ex vivo spontaneous NET formation, but not LPS-triggered NETosis. The four ACDC patients with increased spontaneous NETosis had a mean 1.7-fold increase in Sytox-stained eDNA and 2-fold increase in MPO activity compared with contemporary healthy controls.

Conclusion: These data reveal marked, spontaneous NETosis in patients deficient in the ectoenzyme CD73. Comparison with LPS-stimulated neutrophils demonstrates near maximal NETosis at baseline in patients with ACDC, consistent with a model where CD73 tonically suppresses neutrophil activation to maintain vascular and immune homeostasis. Ongoing studies are exploring the mechanism of spontaneous NETosis, thrombin formation, and fibrinolysis in patients with ACDC. Taken together, these data will have relevance to patients with ACDC, and inform the thromboinflammatory risk of cancer patients receiving CD73 inhibitors.

Synthetic ionophores as non-resistant antibiotic adjuvants

Antimicrobial resistance is a world-wide health care crisis. New antimicrobials must both exhibit potency and thwart the ability of bacteria to develop resistance to them. We report the use of synthetic ionophores as a new approach to developing non-resistant antimicrobials and adjuvants. Most studies involving amphiphilic antimicrobials have focused on either developing synthetic amphiphiles that show ion transport, or developing non-cytotoxic analogs of such peptidic amphiphiles as colistin. We have rationally designed, prepared, and evaluated crown ether-based synthetic ionophores (‘hydraphiles’) that show selective ion transport through bilayer membranes and are toxic to bacteria. We report here that hydraphiles exhibit a broad range of antimicrobial properties and that they function as adjuvants in concert with FDA-approved antibiotics against multi-drug resistant (MDR) bacteria. Studies described herein demonstrate that benzyl C₁₄ hydraphile (BC₁₄H) shows high efficacy as an antimicrobial. BC₁₄H, at sub-MIC concentrations, forms aggregates of ∼200 nm that interact with the surface of bacteria. Surface-active BC₁₄H then localizes in the bacterial membranes, which increases their permeability. As a result, antibiotic influx into the bacterial cytosol increases in the presence of BC_nHs. Efflux pump inhibition and accumulation of substrate was also observed, likely due to disruption of the cation gradient. As a result, BC₁₄H recovers the activity of norfloxacin by 128-fold against resistant Staphylococcus aureus. BC₁₄H shows extremely low resistance development and is less cytotoxic than colistin. Overall, synthetic ionophores represent a new scaffold for developing efficient and non-resistant antimicrobial-adjuvants.

Antimicrobial resistance is a world-wide health care crisis.