Unlike conventional MHC class I and class II molecules, CD1 proteins present lipid or glycolipid antigens to T cells. Group 1 CD1 proteins present microbial lipid antigens to diverse T cells, thereby significantly expanding the ability of the adaptive immune system to recognize and respond to pathogens. The group 2 CD1 proteins however, do not interact with diverse T cells and have instead been shown to interact with a unique subset of T cells, the NKT cells. Upon activation, NKT cells promptly secrete large amounts of cytokines, which may both link and regulate innate and adaptive immunity. The unique features of CD1, which lead to the selection, and activation of these CD1-restricted T cells remain largely unknown. This proposal seeks to examine the effects of CD1 expression levels and tissue distribution on the development of CD1-restricted T cells which will determine if novel or conventional mechanisms govern the selection of this unique population of T cells. First, we will use transgenic mice that express high levels of CD1 to examine how CD1 surface density affects the outcomes of NKT cell development. We will also attempt to purify ligands that associate with CD1 from the thymus and spleen of CD1 transgenic mice to determine if tissue-specific ligands exist as the ligands involved in this selection and activation still remain unknown. Secondly, two lines of the CD1 congenic mice, expressing CD1 allele derived from M. castaneus and M. spretus, will be used to study the effect of CD1 polymorphism on antigen presentation and T cell development. In addition, the frequencies and TCR repertoire of the alloreactive CD1-restricted T cells in response to skin graft from CD1-congenic mice will be determined. A final aim is to examine the molecular mechanisms that regulate the unique expression of CD1 which are likely to be important for the selection/development of NKT cells. We will characterize the promoter region of mCD1.1 and mCD1.2 to identify regulatory elements that are involved in the constitutive and tissue-specific expression of CD1. Collectively, these studies will provide new insight into which characteristic of CD1 contributes to its unique capacity to select and activate NKT cells. These studies may lead to novel ways to manipulate the immune response mediated by CD1-restricted T cells.