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

Abstract

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.