T cell recognition of antigenic self-TCR sequences constitutes a distinct peripheral autoregulatory mechanism for limiting inflammatory reactions mediated by Th1 cells directed at tissue-specific antigens such as myelin proteins. Data obtained from our clinical trials using TCR peptides to vaccinate patients with multiple sclerosis (MS) have raised crucial questions regarding the origin and mechanism of action of TCR- specific T cells that will require a return to animal models. Specifically, we have observed that TCR-reactive T cells may acquire properties associated with CD4+CD25+ regulatory T cells (Treg), in addition to their previously documented ability to regulate Th1 cells through the release of IL-10, with properties similar to Th2 or Tr1 cells. These observations raise the fundamental question of whether the TCR-reactive cells represent a single distinct regulatory lineage or whether T cells bearing T cell receptors specific for self TCR determinants can differentiate into different types of regulatory or effector T cells according to their micro- environment. This question has important implications because in the latter case, the autoimmune disease process itself might direct a different distribution of TCR-reactive T cell subtypes than occurs during health, with unknown effects on regulatory function. We thus propose the hypothesis that TCR-specific T cells represent a unique lineage of autoreactive cells that mediate a spectrum of regulatory effects that are dependent on both thymic and peripheral differentiation pathways. To address this hypothesis, we propose to: 1) Determine what are the developmental pathways for CD4+ TCR-specific T cells; 2) Determine what are the governing mechanisms by which TCR-reactive T cells inhibit pathogenic and bystander T cells and prevent experimental autoimmune encephalomyelitis (EAE); and 3) Evaluate the spectrum of TCR-reactive T cell types in HC and in MS patients before and after vaccination and their effects on immune function. We will utilize humanized Tg mice that express HLA-DR2, a known risk factor for MS, that are highly susceptible to EAE induced with myelin oligodendrocyte glycoprotein (MOG)-35-55 peptide. Moreover, in order to more effectively follow pathogenic T cells and evaluate induction of a focused anti-TCR response, we will utilize DR2 mice that also express a human TCR specific for myelin basic protein (MBP)-85-99 peptide. These DR2/TCR+ mice are highly susceptible to EAE induced with the MBP-85-99 peptide, and we further propose to mimic human T cell presentation of self-TCR determinants by producing DR2/CIITA-Tg mice, in which T cells are programmed to over-express class II molecules. Studies in these mice and in mice deficient in the Treg associated Foxp3 gene are crucial for a definitive determination of differences in the protective function of various TCR-reactive subtypes. Results from the animal models will then be translated back to human donors to evaluate the distribution of TCR subtypes present in un-immunized HC and TCR vaccinated MS patients to establish predominant patterns that are associated with clinical benefit.