PROJECT SUMMARY Significance. Better understanding is required of innate mechanisms and new cell therapies that enhance host resilience against acute lung injury (ALI), and the associated acute respiratory distress syndrome (ARDS). Based on our prior findings, our premise is that gap junctional channels (GJCs) between myeloid cells and the alveolar epithelium (AE) determine host resilience to ALI. Hence, we will first, evaluate the extent to which increasing alveolar GJCs between airway-instilled bone marrow-derived monocytes (BMDMs) and the AE enhances host resilience. Second, we will evaluate the role of GJC-communicated Ca2+ waves, leading to activation of calmodulin kinase kinase-2 (CAMKK2) in the AE, as mechanisms underlying host resilience. Approach. We will study mouse models of ALI, induced by intranasal instillation of LPS (lipopolysaccharide), or of the Gram-negative bacteria, Pseudomonas aeruginosa (PA), an opportunistic pathogen that causes ARDS. We will evaluate the premise by means of real-time confocal microscopy (RCM) of live mouse lungs, as well as other general approaches. In SA1, we will determine the timing of alveolar GJC formation after BMDM instillation, the extent to which the BMDMs convert to AMs, and the impact on ALI as quantified by our reported assays. In SA2, we will quantify real-time Ca2+ responses, CAMKK2 activation and the role the kinase’s target, AMP kinase (AMPK). The studies will involve genetically modified mice, and human lung slices, and consideration of rigor, sex and human relevance. Impact. Our studies will for the first time, reveal the importance of enhancing GJCs between alveolar myeloid cells and the AE as a means to bolstering host resilience to ALI. Application of instilled BMDMs will be understood as novel cell therapy for ALI/ARDS. Outstandingly novel mechanistic understanding will be achieved regarding the pathobiology of host resilience against ALI.