PROJECT SUMMARY/ABSTRACT Th17 cells defend the host against extracellular bacteria by releasing cytokines that recruit and activate a variety of other cell types, and as we discovered by the release of the antimicrobial protein IL-26 that kills bacteria in axenic (i.e. cell-free) cultures. Although Th17 have been implicated in host defense against intracellular bacteria, the mechanism(s) are not known. Because IL-26 is present in humans and not mice, we investigate the role of the Th17 cytokine IL-26 in leprosy, caused by the intracellular bacterium Mycobacterium leprae (mLEP), which provides a unique model to study human immune responses to infection. The disease presents as a spectrum in which the clinical presentation correlates with the immune response to the pathogen. In addition, the skin lesions of leprosy are readily accessible for study. Our preliminary data indicates that IL-26 expression in leprosy lesions significantly correlates with lesions from patients in which bacteria are eliminated over time. In addition, we show that IL-26 enters mLEP-infected macrophages (MΦs), induces autophagy as well as phagolysosomal fusion, colocalizes with the intracellular bacteria and reduces its viability. These data indicate IL-26 provides a mechanism by which Th17 cells contribute to host defense against intracellular bacteria. Our overall hypothesis is therefore that innate activation of Th17 cells leads to secretion of IL-26, which contributes to host defense against mLEP and other intracellular bacteria. Our specific aims are: 1) Determine the mechanism(s) by which an extracellular antimicrobial protein, IL-26 gains access to intracellular pathogens in MΦs, results in an antimicrobial response, 2) Discover if IL-1β, via activation of an IL-1R+ subset of Th17 cells, represents an innate mechanism of host defense against bacterial infection, as well as the role of monocytes/macrophages in producing IL-26; and, 3) Investigate the role of IL- 26 in host defense against intracellular bacteria residing in distinct subcellular compartments. In summary, the proposed studies will provide new insights into the mechanisms by which IL-26 contributes to immunity against intracellular bacteria including the role of IL-26 in skin infection.