Skin dendritic cells (DC) are efficient antigen-presenting cells that are a critical component of the cutaneous immune system. The best characterized skin DC subset is the Langerhans cell (LC) that resides in the epidermis of the skin and is the first DC population to encounter cutaneous pathogens, environmental insults and epidermal neoplasia. Major obstacles to studying the function of skin DC include: the difficulty in accurately identifying skin DC subsets; the inability to test the functional importance of DC subsets in vivo; and the absence of a physiologic model system in which antigen-specific responses to cutaneous antigens can be examined in vivo. We believe that we have overcome some of these obstacles though the development transgenic mice that have a specific, complete and durable absence of LC. Using classic assays of the cutaneous immune response we have shown that contact hypersensitivity (CHS) and rejection of allogeneic skin grafts do not require LC. Instead, we found that CHS responses and rejection of minor-mismatched skin grafts were enhanced in the absence of LC. Moreover, we have also recently discovered markers that identify a novel DC subset in the dermis that is required for efficient CHS responses. We propose to exploit these tools to examine the mechanism of LC-mediated regulation of CHS. We will examine whether LC-mediated regulation occurs during the steady-state or at the time of immunization and whether LC-derived IL-10 or TGF2 are required. We will also compare the expression profile of LC with other skin DC subsets by microarray. Finally, we will generate an antigen-specific infection model by combining an existing model of cutaneous Candidiasis with Candida albicans that we will engineer to express ovalbumin. We will be able to use this model to examine whether LC regulate or promote immune responses to yeast and how LC affect the development of antigen-specific responses.