Fig. 1

MDW index modifications in whole EDTA blood samples collected from healthy subjects treated in vitro with 100 µg/mL of histone mixture and 100 µg/mL of histone mixture + 1 µg/mL of LPS, compared to COVID-19 and Sepsis profiles, and mechanistic network of histone actions in sepsis. Sepsis patients was categorized according to Sepsis-2/3 diagnostic criteria; n = 8, mean age 63 ± 13.2 years; median SOFA score of 3, range 2–7; no patient needed for mechanical ventilation or continuous renal replacement therapy. COVID-19 patients had mild/moderate SARS-CoV-2 infection; n = 7, mean age 68 ± 14.4 years; no patient needed for mechanical ventilation. Aliquots of 1 mL of whole blood from each volunteer were exposed to a mixture of commercially available histones (100 µg/mL) (Histone from calf thymus, Sigma), in absence or presence of 1 µg/mL of lipopolysaccharide (LPS) (from Escherichia coli O127:B8 strain, Sigma). The samples, maintained at RT, were analyzed for MDW at 30, 60 and 180 min after careful inversion avoiding sedimentation of blood cells, and processed within 4 h of collection. MDW and routine complete blood cell counts were performed on UniCell DxH900 Hematology Analyzer (Beckman Coulter). The choice of whole blood treatment with 100 µg/mL of a mixture of commercially available human histones is in agreement with the literature evidence suggesting that the concentration of 20 µg/mL of circulating histone H3 was detected in patients with critical COVID-19 [15] and that the same deleterious effects of histone H3 is obtained with five-fold higher concentrations of mixture of histones [3]. The MDW values, scatter plots and blood smears are representative of at least triplicate analyses. Values are plotted as mean ± SEM (**very significant = p: 0.001–0.01; ****extremely significant = p < 0.0001). A MDW modifications after histones and LPS + histone treatments for 3 h in control subjects compared with classical and viral Sepsis. B Time-dependent increases of MDW values (linear regressions: control subjects, Y = 0.01029x + 17.45 r² = 0.4065; histone 100 µg/mL, Y = 0.03751x + 18.06 r² = 0.6995; 100 µg/mL of histone mixture + 1 µg/mL of LPS, Y = 0.06951x + 17.85 r² = 0.9317). C Representative modifications of MDW, blood smears and scatter plots in both classical and viral Sepsis, compared to histone and histone + LPS whole blood treatments. D Schematic representation of a possible predictive/mechanistic network of how circulating histones commonly mediate monocyte alterations in both classic and viral sepsis (METosis, monocyte extracellular traps; TLR, Toll-like receptor; NLRP, NOD-like receptor protein; LPS, lipopolysaccharide; SARS-CoV-2, severe acute respiratory syndrome coronavirus-2; MyD88, myeloid differentiation primary response gene 88; NFkB, nuclear factor kappa-light-chain-enhancer of activated B cells)