Autoimmunity is defined by the inability of the immune system to differentiate between healthy body tissue and foreign antigens, thereafter inducing a heightened immune reaction to normal body tissues it would normally ignore. During thymocyte development, T cells undergo a selection process based on the avidity of their individual T cell receptor (TCR) to the self-ligand/major histocompatibility complex (MHC) presented by thymic antigen presenting cells. T cells that express TCRs with strong avidity to self are usually filtered out during this process by negative selection in order to remove potentially hazardous autoreactive T cells. We have recently made a novel observation that in addition to TCR signal, a cohort of molecules, such as CD5, CD6, GITR and CD26 alter the TCR signal potential and contribute to establishing a diverse T cell repertoire. These 'fine-tuning molecules work together in symmetry to establish a signaling threshold during T cell selection, as suggested in our studies using knockout for a set of these molecules. Guided by this novel finding, we posit that a mis-regulation of these molecules results in a positive selection and hypersensitivity of T cells, which contributes to the development of autoimmune disorders. Moreover, preliminary evidence suggests that these molecules continue to be tuned in the periphery and may contribute to T cell hyper-reactivity. We will begin to test this idea in this exploratory proposal. We will begin by analyzing the differential expression of the top twenty candidate 'fine-tuning' molecules in thymic and peripheral T cells of autoimmune mouse models. Using the activating or neutralizing antibodies for the five most differentially expressed molecules, we will determine the function of these 'fine-tuning' molecules in affecting the signaling threshold in autoimmune and normal T cells. We will then determine whether modulating the expression of these molecules (top two candidates in this exploratory grant) in autoimmune T cells will restore normal T cell activation and homeostasis. This study has the potential to reveal autoreactive T cell markers that may serve as early biomarkers as well as candidate therapeutic targets to treat autoimmune pathologies.