IAIP and HMW-HA, but not bikunin, prevent histone-induced cytotoxicity. Cell viability was measured by flow cytometry using PI staining in HL-60 cells incubated with CTH (50 µg/mL) at 37°C for 30 minutes in the presence or absence of (A) IAIP 25 to 100 µg/mL, (B) HA 250 to 500 µg/mL, and (C) bikunin 100 to 500 µg/mL. (D) Cell viability measured as above in EA.hy926 endothelial cell line in the presence or absence of IAIP 100 µg/mL, HA 500 µg/mL, or bikunin 500 µg/mL. (E) Effect of chondroitinase ABC (C-ABC) on cell protective effect of IAIP. HL-60 cells were incubated with histones with or without IAIP pretreated or not with C-ABC (0.05 IU/mg, 18 hours at 37°C) before analysis with flow cytometry. C-ABC alone did not affect cell viability. (F) Polybrene (PB) abolishes the cytoprotective effects of IAIP and HA. HL-60 cells were incubated with histones with or without IAIP or HA pretreated or not with Polybrene (0.5 mg/mL). Polybrene alone does not affect histone cytotoxicity and has no direct effect on cell viability. Data are presented as mean ± standard error of the mean (SEM) of 3 independent experiments using one-way analysis of variance (ANOVA) with Bonferroni’s multicomparison test. ****P < .0001, ***P < .001, and **P < .01 compared with the histone challenge group. NS, not significant.