BackgroundCOPD is the third leading cause of death worldwide. Cigarette smoke (CS)-induced chronic inflammation inducing airway remodelling, emphysema and impaired lung function is the primary cause. Effective therapies are urgently needed. Human chymase (hCMA)1 and its orthologue mCMA1/mouse mast cell protease (mMCP)5 are exocytosed from activated mast cells and have adverse roles in numerous disorders, but their role in COPD is unknown.MethodsWe evaluated hCMA1 levels in lung tissues of COPD patients. We usedmmcp5-deficient (^−/−) mice to evaluate this protease's role and potential for therapeutic targeting in CS-induced experimental COPD. In addition, we usedex vivo/in vitrostudies to define mechanisms.ResultsThe levels of hCMA1 mRNA and CMA1⁺mast cells were increased in lung tissues from severe compared to early/mild COPD patients, non-COPD smokers and healthy controls. Degranulated mast cell numbers and mMCP5 protein were increased in lung tissues of wild-type mice with experimental COPD.mmcp5^−/−mice were protected against CS-induced inflammation and macrophage accumulation, airway remodelling, emphysema and impaired lung function in experimental COPD. CS extract challenge of co-cultures of mast cells from wild-type, but notmmcp5^−/−mice with wild-type lung macrophages increased in tumour necrosis factor (TNF)-α release. It also caused the release of CMA1 from human mast cells, and recombinant hCMA-1 induced TNF-α release from human macrophages. Treatment with CMA1 inhibitor potently suppressed these hallmark features of experimental COPD.ConclusionCMA1/mMCP5 promotes the pathogenesis of COPD, in part, by inducing TNF-α expression and release from lung macrophages. Inhibiting hCMA1 may be a novel treatment for COPD.