Histone H4 Aggravates Inflammatory Injury Mainly Through TLR4 in Chlorine Gas Induced Acute Respiratory Distress Syndrome

Abstract

Background: Chlorine gas (Cl2) 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 Cl2 is very high, mainly because the pathogenesis of ARDS is still not very clear. Histone H4 has been identified as an important endogenous pro-inflammatory molecule. The present work aimed to study the pathogenic role of histone H4 in Cl2 induced ARDS. Methods: ARDS was induced by Cl2 exposure in male C57BL/6 mice. Circulating histone H4, blood gas, pulmonary edema, endothelial activation, and neutrophil infiltration were measured during acute lung injury. Histone H4 or anti-H4 antibody was administered through tail vein 1h prior to Cl2 exposure to study the pathogenic role of histone H4. The Toll-like receptor 2 knock-out (Tlr2-KO), Tlr4-KO mice and the blocking antibody against TLR1, TLR2, TLR4 or TLR6 were used to explore the mechanism involved in histone H4 mediated injury. Results: Cl2 exposure induced a concentration dependent acute lung injury. The levels of circulating histone H4 were positively correlated with the concentrations of Cl2. Pretreatment with intravenous histone H4 further aggravated lethality rate, blood gas, endothelial activation, and neutrophil infiltration while the anti-H4 antibody showed protective effects. The deficiency of Tlr4 improved lethality rate, blood gas, and pulmonary edema; prevented endothelial and neutrophil activation caused by Cl2 exposure. More importantly, the gene deletion of Tlr4 diminished greatly the effect of histone H4 or anti-H4 antibody observed in the 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 Cl2. TLR4 was closely involved in histone H4 mediated inflammatory injury. The intervention targeted at histone H4 is potentially protective.