PROJECT SUMMARY Acute Respiratory Distress Syndrome (ARDS) is characterized by non-cardiogenic pulmonary edema leading to severe hypoxemia requiring mechanical ventilation. ARDS occurs as a result of acute lung injury (ALI) to pulmonary epithelium and/or endothelium. Using two different murine model of ALI, we found that the type 2- associated cytokine, IL-5, reduces lung edema and protects mice from mortality1. However, the mechanisms by which this cytokine attenuates ALI is not known, and elucidating these mechanisms is a primary goal of my new independent research group. IL-5 has classically been associated with detrimental eosinophilic responses in asthma and atopic disease. However, we found a novel beneficial role for IL-5 in the airways. Surprisingly, treatment with exogenous IL-5 in the airways was sufficient to reduce lung edema and rescue mice from death induced by bleomycin (BLM) challenge. Our preliminary data demonstrates an essential role for endogenous IL- 5 in survival from ALI, as IL-5 receptor-deficient (IL-5Rα-/-) mice have increased edema and death after BLM challenge. No increase in mortality was observed in eosinophil-deficient mice compared to their wild-type littermates, suggesting that IL-5 may act in an eosinophil-independent manner. We confirmed the importance of IL-5 signaling in a second model of ALI induced by LPS and found that IL-5Rα-/- mice had increased lung permeability and neutrophils compared to IL-5Rα+/+ mice. These findings suggest that IL-5 is a critical cytokine for preserving barrier function in the lungs and protecting from ALI-induced mortality. To understand why barrier function is reduced in IL-5Rα-/- mice, we examined structural cells in the lungs after injury and found a defect in epithelial proliferation. Further, we made the surprising discovery that the epithelium itself expressed IL-5Rα. Expression of the IL-5Rα was increased after BLM challenge and highest on dividing Ki67+ epithelial cells. IL- 5Rα mRNA and protein was also expressed in primary human airway epithelial cells (AECs). AEC IL-5Rα is functional since IL-5 treatment of differentiated epithelium in air-liquid interface cultures led to phosphorylation of Erk and Akt. The overall hypothesis of the proposed studies is that IL-5 acts on lung epithelium to reduce the edema and mortality associated with ALI. The effect of IL-5Rα ligation on epithelial cells is completely unknown, and may be a novel pathway by which IL-5 protects against lung injury. These studies will provide essential understanding of the role of IL-5 in lung homeostasis and pathology and may change the paradigm that IL-5 acts solely on lung eosinophils. To test this hypothesis, we will perform the following two Aims. Aim 1: Determine the mechanism by which epithelial cells respond to IL-5 to promote survival from ALI in vivo. Aim 2: Determine the pathways activated by IL-5 in human airway epithelial cells in vitro. Our studies will demonstrate new, heretofore undescribed...