OC 66.5 - Neutrophil extracellular trap components differentially induce S100A8/A9 expression on platelets associated with a procoagulant phenotype

Background: Neutrophil Extracellular Traps (NETs) are observed in patients with COVID-19 and sepsis and markers of NETosis correlate with disease severity and thrombosis. S100A8/A9, a damage-associated molecular pattern released during NETosis, supports the formation of procoagulant platelets through GPIbα. S100A8/A9 was also detected in platelets from patients with COVID-19 and sepsis but the mechanism driving its expression and its functional relevance in platelet function are unknown.

Aims: We investigated S100A8/A9 expression on platelet microvesicles from ICU patients with sepsis and COVID19 and its correlation with inflammatory and thrombotic markers. We further investigated the mechanism(s) driving S100A8/A9 expression in platelets and its relationship with platelet phenotype.

Methods: Inflammatory and thrombotic markers in healthy donors, COVID-19 and sepsis patients were measured by Luminex/ELISA. The expression of S100A8/A9, phosphatidylserine, P-selectin, GPIIb/IIIa on platelet microvesicles/platelets was measured by flow cytometry.

Results: Soluble (s) ICAM-1, sVCAM-1, sP-selectin and VWF were increased in patients with COVID-19 and sepsis while sCD40L, CCL5 and sGalectin-3 were significantly higher in ICU patients with COVID-19. Neutrophil activation markers S100A8/A9, S100A12, Lipocalin-2 were increased in both cohorts, but significantly higher in COVID-19 patients and correlated with S100A8/A9- and phosphatidylserine-double positive platelet microvesicles. S100A8/A9 was absent in healthy donor unstimulated platelets, whereas it was differentially upregulated on the surface of platelets stimulated with selective NET components but not with classical platelet agonists. Anti-GPIbα antibody SZ2, which blocks GPIbα -VWF interaction and S100A8/A9-induced phosphatidylserine expression, drastically decreased S100A8/A9 and phosphatidylserine expression on platelets. S100A8/A9-positive platelets display a procoagulant and proinflammatory phenotype.

Conclusion(s): Our results show that high level of NETosis marker correlates with the presence of platelet microvesicles positive for S100A8/A9 and phosphatidylserine. Mechanistically, NET components differentially induce S100A8/A9-expression on the platelet surface, driving in an autocrine way phosphatidylserine exposure in a GPIbα-dependent manner. These data suggest a new mechanism by which NETs induce inflammatory and procoagulant platelets.