Extracellular histones aggravate inflammation in ARDS by promoting alveolar macrophage pyroptosis

Extracellular histones have been discovered to play a pathogenic role in ARDS, but the underlying mechanisms are yet to be fully defined. Alveolar macrophage (AM) is essential for the initiation and progression of lung inflammation; of note, AM pyroptosis has been suggested contributing to ARDS-associated inflammation. Here we aimed to investigate whether extracellular histones promote ARDS by triggering AM pyroptosis. The BALF samples of ARDS patients were collected and AMs were further isolated. Extracellular histones, AM pyroptosis, and pyroptosis-associated mediators were measured. Furthermore, the effects of extracellular histones on AM pyroptosis and the underlying mechanisms were investigated. It showed that extracellular histones were markedly elevated in the BALF of ARDS patients and correlated with the increased AM pyroptosis. ARDS patient’s BALF induced pronounced pyroptosis in cultured human monocytes, which could be prevented by neutralizing extracellular histones with heparin. In addition, exogenous histones induced pyroptosis of MH-S cells in a dose- and time-dependent manner, which acted through the NLRP3 inflammasome signaling pathway. Inhibition of NLRP3 inflammasome signaling substantially reduced cell pyroptosis. In a murine model of LPS-induced ARDS, extracellular histones were increased in the BALF and its increase was associated with enhanced AM pyroptosis and exaggerated lung inflammation. Blockade of extracellular histones or NLPR3 inflammasome equally inhibited macrophage pyroptosis, whereas targeting histones appeared more effective in alleviating lung inflammation. This study suggested that extracellular histones promote AM pyroptosis through NLRP3 inflammasome pathway, which in turn aggravates lung inflammation in ARDS. Pharmacological manipulation of extracellular histones or AM pyroptosis may become promising strategies for the treatment of ARDS.