Histone H4 aggravates inflammatory injury through TLR4 in chlorine gas-induced acute respiratory distress syndrome

BACKGROUND: Chlorine gas (Cl(2)) exposure remains a public health concern in household, occupational, and transportation accidents around the world. The death rate associated with acute respiratory distress syndrome (ARDS) caused by high concentrations of Cl(2) is very high, mainly because the pathogenesis of ARDS remains unclear. Histone H4 has been identified as an important endogenous pro-inflammatory molecule. The present study aimed to examine the pathogenic role of histone H4 in Cl(2)-induced ARDS. METHODS: ARDS was induced by Cl(2) exposure in male C57BL/6 mice. Circulating histone H4, blood gas, pulmonary edema, endothelial activation, and neutrophil infiltration were measured during acute lung injury (ALI). Histone H4 or anti-H4 antibody was administered through the tail vein 1 h prior to Cl(2) exposure to study the pathogenic role of histone H4. Toll-like receptor 2 knock-out (Tlr2-KO) and Tlr4-KO mice were used in conjunction with blocking antibody against TLR1, TLR2, TLR4, or TLR6 to explore the mechanism involved in histone H4-mediated injury. RESULTS: Cl(2) exposure induced a concentration-dependent ALI. The levels of circulating histone H4 were positively correlated with Cl(2) concentrations. Pretreatment with intravenous histone H4 further aggravated lethality rate, blood gas, endothelial activation, and neutrophil infiltration, while anti-H4 antibody showed protective effects. Tlr4 deficiency improved lethality rate, blood gas, and pulmonary edema, and prevented endothelial and neutrophil activation caused by Cl(2) exposure. More importantly, Tlr4 gene deletion greatly diminished the effect of histone H4 or anti-H4 antibody observed in wild-type (WT) mice. The impact of Tlr2 on inflammatory injury was not significant. The role of TLRs was also validated by endothelial activation mediated by histone H4 in vitro. CONCLUSIONS: Circulating histone H4 played a pro-inflammatory role in ARDS caused by Cl(2). TLR4 was closely involved in histone H4-mediated inflammatory injury. Therefore, intervention targeting histone H4 is potentially protective.