How do tissues sense proteolytic stress? 2309437

Abstract Introduction Excess proteolytic activity — from endogenous proteases, microbial toxins, venoms, and industrial enzymes — occurs during inflammation, infection, tissue injury, and genetic disorders (α1-antitrypsin deficiency). These activities contribute to major diseases including COPD, ARDS, rheumatoid arthritis, aneurysm, chronic wounds, and cancer, yet how tissues detect and resist proteolytic stress is poorly understood. Methods To address this, we developed an in vivo lung model using the bacterial protease LasB delivered either once (acute exposure) or three times at 24-hour intervals (repeated exposure). Acute exposure caused marked vascular injury, erythrocyte extravasation with heme release, neutrophil influx, and loss of lung function. Strikingly, repeated exposures produced a resilient phenotype: reduced protein leakage, fewer erythrocytes and neutrophils, and preserved lung function. Results Mechanistic studies using genetic mouse models, transcriptomics, and pharmacology showed this adaptation depended on sensing of oxidative molecules — erythrocytic heme — and activating tissue oxidative stress response. Alveolar macrophages emerged as the principal sensors activating NRF2 transcription factor which induces an antioxidant gene program including heme oxygenase 1 (Hmox1). Hmox1 catabolized heme to bilirubin, a metabolite that can relay stress information to neighboring fibroblasts. In response, fibroblasts produced several protease inhibitors in NRF2-dependent manner that directly bound and inhibited LasB activity, suggesting active resistance to proteolytic stress. Conclusion These results identify tissue-resident macrophages and an NRF2-dependent program as central mediators of lung adaptation to proteolytic stress, revealing a cellular circuit by which tissues detect protease-driven damage and actively restore homeostasis. Funding Source HHMI, FASI, NIH Topic Categories Innate Immune Responses and Host Defense: Cellular Mechanisms (INC)