PROJECT SUMMARY Langerhans cells (LCs) are the dendritic cells of the skin epidermis that detect physical, chemical and microbial injuries and promote immune responses. They play crucial roles in the immune response to skin allergens, melanoma and other skin cancers as well as infectious agents. Recent studies have shown that LCs are unique among myeloid cells in their development: they arise from embryonic progenitors of the yolk sac and the fetal liver that populate the developing skin before birth. LCs proliferate immediately before birth and during the first days of post-natal life and then self-renew with a slow turnover rate. However, the cytokine(s) responsible for driving the development of LC precursors in the skin remain unknown. Although mouse models in which LCs can be depleted either constitutively or conditionally were recently developed, contrasting roles for LCs in contact hypersensitivity (CHS) responses have been reported in studies using these mice. Thus, whether LCs induce tolerogenic or immunogenic responses to skin allergens remains an open question. In our preliminary data we show for the first time that LC development depends on IL-34, a recently discovered skin-derived cytokine that binds the CSF-1 receptor. Moreover, we demonstrate that IL-34-deficient mice selectively lack LCs and have attenuated CHS responses, with fewer IFN--secreting T cells infiltrating the skin, in comparison to wildtype mice. In this proposal, we will take advantage of our IL-34 knock-in/knock-out mice that express - galactosidase as an IL-34 reporter as well as mice we have generated that carry floxed IL-34 alleles. In specific aim 1 we will investigate when and where IL-34 promotes LC development. We hypothesize that IL- 34 is selectively expressed in the skin, especially during the perinatal period, driving the differentiation of LC progenitors that hae previously migrated from the yolk sac and the fetal liver into the developing skin. In specific aim 2, we will test the hypothesis that LCs are required to induce both IFN--producing (type 1) and IL-4-, IL-5-, IL-13-producing (type 2) T cells specific for skin allergens, depending on the innate response to the allergen and/or adjuvant and the resulting cytokine microenvironment that instructs LCs. Our discovery that IL-34 is essential for LC development and our generation of a novel mouse model lacking LCs affords a unique window of opportunity to advance our knowledge of skin and LC immune function, which will provide the framework for new strategies to treat skin pathologies that target IL-34 and LCs.