Most T cells in the lymphoid tissues express diverse antigen receptors composed of alpha and beta chains which recognize peptide antigens bound to self major histocompatibility complex (MHC) proteins. The lymphoid tissues also contain a small subpopulation of T cells bearing a heterodimer composed of clonally diverse gamma and delta chains. In contrast to the lymphoid tissues, it has been recently demonstrated that several epithelial tissues contain almost exclusively gamma delta T cells. The epithelial gamma delta cells appear to be a distinct subset of T cells and differ from the lymphoid gamma delta and alpha beta T cells in several ways. These cells originate from fetal precursors and express a tissue specific, invariant antigen receptor. We have proposed that the role of these cells may be to recognize damage induced self antigens on neighboring cells and thus provide protection from infection and malignancy in that tissue. The epidermis is the residence of the Thy-1+ dendritic epidermal T cells (dEC) which express a canonical TCR composed of V gamma3 and V delta1 chains. These cells appear to recognize self antigens expressed by skin keratinocytes. We will use these cells as a model in which to address the tissue of antigen specificity of epithelial T cells with invariant gamma delta T cells receptors. Specifically, we propose to: (1) Characterize the distribution and requirements for expression for expression of the keratinocyte antigen and activation of the dEC using phenotypic and functional analyses. (2) Fractionate keratinocyte peptide preparations to determine if the dEC recognize antigen in the form of a discrete peptide. (3) Produce monoclonal antibodies which recognize the keratinocyte molecules responsible for interaction with the dEC. (4) Clone and identify the gene encoding any novel keratinocyte molecules identified using the new monoclonal antibodies. (5) Use fetal thymic organ culture to determine whether selection processes occur in the fetal thymus which are responsible for the generation of the canonical gamma delta receptors. These studies will provide insight into the specificity of epithelial T cells with invariant gamma delta receptors and information about their role in immune surveillance and protection from malignancy.