We have shown that Chymotrypsin-like Elastase 1 (CELA1) is expressed in human lung, neutralized by alpha-1 antitrypsin (AAT), and important for emphysema in a mouse model of AAT-deficiency. Here, we show a role for CELA1 in late lung remodeling using human specimens and mouse models. Methods: Studies were approved by CCHMC IRB and IACUC. In mice, emphysema following 2 units of tracheal PPE was assessed at 21, 42, and 84 days and in aged 72-75 week-old WT and Cela1-/-lungs. Lungs of non-lung organ donors, COPD, and aged healthy subjects (n=38 total) were used for PCR, IHC, IF, PLISH, enzymatic assays, and stretch-binding assays. DESeq2 of major airway, small airway, and BAL mRNA-seq was performed. Results: In the mouse PPE model, Cela1 mRNA and protein were increased 3-fold at 21 and 42 days post-PPE (p<0.05). Cela1-/-and WT mice had similar emphysema at 21-days, but WT demonstrated progressive emphysema at 42 and 84-days (p<0.01) while Cela1-/-did not (p<0.01 vs WT). PLISH demonstrated Cela1 mRNA in the epithelium of small conducting airways. Cela1-/-mice had less age-related airspace enlargement than WT (p<0.05). CELA1 protein was increased 3-fold in COPD specimens compared to healthy (p<0.05), and PLISH showed CELA1 mRNA in human conducting airway epithelium. Comparing mRNA-seq of large airway, conducting airway, and BAL, CELA1 mRNA was present only in small conducing airway of COPD and controls subjects. PCA showed CELA1-expressing small airway specimens of normal and COPD clustered with enrichment in secretory pathways. In healthy and COPD specimens, CELA1 mRNA levels correlated with MMP12, but not MMP2, MMP8, MMP9, MMP14, PRTN3, CTSG, or ELANE. Among these mRNAs, only CELA1 and MMP12 correlated with lung elastolytic, gelatinase, and proteinase activity, and only CELA1 was significant (p<0.001). anti-CELA1 antibody and serine proteinase inhibition reduced the elastolytic activity of CELA1 mRNA-high lung homogenate. CELA1 mRNA levels increased exponentially with age, and WB of aged and young lung showed reduced AAT neutralized CELA1 in aged, smoker lung. Biaxial stretching of human lung sections using fluorophore-labeled CELA1 and albumin showed 20-fold increased binding of CELA1 but not albumin with stretch (p<0.05) in healthy, but not COPD lung. Conclusions: We propose a 3-hit model for CELA1mediated airspace destruction in human emphysema. 1-CELA1 expression in small conducting airways increases with age, 2-smoking reduces AAT-neutralization of CELA1, and 3-increased strain from disruption of adjacent alveolar units allows CELA1 binding to lung elastin leading to feed-forward alveolar destruction.