The specificity of the syngeneic mixed leukocyte response, a primary anti-I region T cell proliferative response, is determined intrathymically.

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Abstract
In previous studies, the syngeneic MLR of peripheral T cells was shown to be predominantly an I region-restricted function. In this report we show that adult thymocytes are also capable of responding to syngeneic irradiated stimulator cells in a syngeneic MLR, provided that TCGF is added to the culture system. Using this assay, it was possible for the first time to examine the pattern of I region restriction within the thymus itself. Analysis of the thymocyte syngeneic MLR in thymuses from radiation-induced bone marrow chimeras demonstrated that the MHC preference seen in the peripheral T cell population also existed in cells resident within the thymus. Experiments utilizing congenitally athymic mice transplanted with allogeneic thymic grafts demonstrated that both peripheral T cells and thymocytes from such animals displayed a strong preferential proliferation toward stimulator cells bearing thymic-type MHC determinants. The results in the nude model thus demonstrate that the thymus by itself is sufficient to impart such restriction specificity on a developing T cell repertoire. These results are consistent with the notion that the thymus exerts selective pressure on maturing T cell populations that results in a skewing of the T cell repertoire toward the recognition of thymic-type I region products, and that this MHC preference exists before expansion of T cells in the periphery.

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