Recent observations indicate that cyclosporin A (CsA) blocks TCR initiated signal transduction and thus leads to abnormal TCR repertoire selection. CsA treatment can lead to overt autoimmune disease in two different experimental models. In both these models, the development of disease requires both the presence of autoreactive cells derived from the CsA treated thymus and an immunoregulatory abnormality in peripheral T cells. Preliminary data suggest that CsA inhibits thymic selection of the TCR2 (alphabeta TCR) repertoire, but not thymic maturation of the TCR1 (gammadelta TCR) sublineage. The broad, long-term objectives of these studies are to determine the physiologic function of these different T cell sublineages and to elucidate the nature of the peripheral immunoregulatory activity involved in tolerance. The specific aims are to test three hypotheses. The first hypothesis is that CsA inhibits both positive and negative selection of the TCR2 repertoire in the thymus, but does not affect thymic TCR1 cell maturation. This hypothesis will be tested by determining the effect of CsA on thymic ontogeny utilizing a panel of mAb which recognize different TCRs and associated molecules. These studies will involve analysis of both murine and chicken T cell development. The focus will be on the selection of cells expressing particular Vbeta genes in the mouse and the maturation of the TCR3 subset, a novel alphabeta-like TCR, in the chicken. The second hypothesis is that the TCR1 repertoire is selected in peripheral tissue. This hypothesis will be tested by examination of the development and tissue homing pattern of TCR1 cells expressing particular Vgamma and Vdelta genes. Two experimental approaches will be used: 1) determining if CsA treatment results in inhibition of specific tissue homing patterns characteristic of TCR1 cells which utilize certain Vgamma genes. 2) intrathymic injection of FITC to identify recent thymic emigrants in peripheral tissues and the determining the pattern of Vgamma usage in these cells. The third hypothesis is that in models of CsA induced autoimmune disease, TCR2 cells mediate autoreactivity, while TCR1 cells regulate disease activity. Two different experimental models will studied: 1) neonatal CsA plus thymectomy and 2) lethally irradiated adults rescued with syngeneic BM transplant plus Ca treatment. These models will be established and the frequency and histologic localization of the different T cells sublineages will be characterized by immunohistochemical staining and FACS analysis. In both these models, the presence of "normal" T cells blocks the ability of autoreactive T cells induce actual tissue lesions. The phenotype of both the autoreactive effector cells and the "normal" regulatory cells will be determined using magnetic activated cell sorting and an adoptive transfer protocol.