SUMMARY Methicillin-resistant Staphylococcus aureus (MRSA) is the leading cause of hospital-acquired infection worldwide, causing approximately 14 million infections annually in the United States alone. Increasing antibiotic resistance observed with this pathogen highlights a compelling need to identify new therapeutic targets to treat MRSA infections. Although directly targeting pathogens with antibiotics has been a successful approach for treating infections, many pathogens, including MRSA, eventually become resistant to these drugs. As an alternative, immunomodulatory strategies to enhance host defenses, such as those shown to be effective against cancer cells, have the potential for treating drug-resistant pathogen infections. The Abuaita lab recently showed that the endoplasmic reticulum (ER) stress sensor, IRE1α, is a key component in innate immune defense against MRSA in macrophages, neutrophils, and in a murine skin abscess model of infection. This work found that IRE1α controls macrophage and neutrophil antimicrobial functions by enhancing production of inflammatory molecules including mitochondrial reactive oxygen species (MitoROS), neutrophil extracellular traps (NETs), and IL-1β, all of which are essential for resolving MRSA infection in vivo. In the lung, ER stress occurs during infection, air pollutant inhalation, and during the development of many pulmonary diseases such as Idiopathic Pulmonary Fibrosis and Asthma. It is not well understood how IRE1α is involved in the progression and resolution of lung diseases. As the lung is a vital organ, macrophages and neutrophils must adequately tune their responses to ensure effective antimicrobial function without excessive tissue damage that could inhibit gas exchange. Therefore, investigation of IRE1α-mediated stress responses during lung infection is expected to yield valuable mechanistic insight into the regulation of pulmonary host defenses. The primary goal of this proposal is to elucidate the role of IRE1α in pulmonary host defenses during MRSA infection. The central hypothesis is that infection triggers IRE1α signaling, which enhances lung innate immune effector functions, includes bactericidal activity and production of inflammatory mediators. The following Aims are designed to test this hypothesis: 1) Characterize the requirement of IRE1α activation and MitoROS generation to lung macrophage inflammatory responses and 2) Elucidate whether the IRE1α circuit aids or impedes innate immune defense against pulmonary MRSA infection. Completing this study will define the ER stress response regulatory network in lung host defense and will lay the groundwork for further studies to reveal cellular stress targets for potential antimicrobial immunotherapies.