RANKL promotes airway inflammation via pro-inflammatory alveolar macrophage activation in COPD

Chronic airway inflammation is a key mechanism involved in the pathogenesis and progression of chronic obstructive pulmonary disease (COPD). Altered activation states of alveolar macrophages (AMs) and the release of inflammatory cytokines represent critical phenotypes in this inflammatory process. Although RANKL is known to participate in immune regulation and cytokine secretion, its potential involvement in pro-inflammatory activation of AMs and the associated contribution to airway inflammation in COPD remains incompletely understood. The purpose of this study is to investigate the RANKL pathway in COPD and to elucidate its role in AM activation and airway inflammation. First, we quantified and localized RANKL and its receptor RANK in lung tissues and assessed CD86-associated pro-inflammatory macrophage activation in bronchoalveolar lavage fluid from COPD patients, smokers, and non-smokers. Next, wild-type mice were exposed to either air or cigarette smoke (CS) for 24 weeks. CS-exposed mice received intraperitoneal injections of either an anti-mouse RANKL monoclonal antibody or a rat IgG2a kappa isotype control antibody; macrophage activation status and airway inflammation were subsequently evaluated. Finally, we investigated the in vitro biological function of RANKL in CS-induced pro-inflammatory macrophage activation and airway inflammation. We found that the expression of both RANKL and RANK, along with enhanced CD86-associated pro-inflammatory macrophage activation, was increased in the lung tissues of COPD patients. In these tissues, RANKL and RANK were localized to AMs. In CS-exposed mice, pro-inflammatory macrophage activation and airway inflammation were significantly increased; however, these effects were ameliorated in CS-exposed mice treated with the anti-RANKL monoclonal antibody. In vitro, cigarette smoke extract (CSE) up-regulated the expression of RANKL and RANK in AMs. AMs responded to CSE and RANKL stimulation by exhibiting pro-inflammatory activation and enhanced cytokine expression. Furthermore, CSE-induced pro-inflammatory macrophage activation and cytokine expression were partially inhibited by the addition of a neutralizing anti-RANKL monoclonal antibody. RANKL contributes to airway inflammation through pro-inflammatory alveolar macrophage activation in COPD. These findings extend current understanding of the role of the RANKL pathway in airway inflammation and highlight it as a potential therapeutic target in COPD.