Clonal deletion induced by either radioresistant thymic host cells or lymphohemopoietic donor cells at different stages of class I-restricted T cell ontogeny.

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Abstract
Major histocompatibility complex (MHC) products and self-antigens expressed in the thymus determine the repertoire of mature alpha/beta T cells. While positive selection of self-MHC-restricted T cells is directed by MHC molecules expressed by thymic epithelial cells, negative selection depends to a large extent on self-antigens presented by lymphohemopoietic cells. However, radioresistant components of the thymus also influence negative selection, but it remains controversial whether this is accomplished by clonal deletion, clonal anergy, or other mechanisms. In this study, T cell development in mice expressing a transgenic T cell receptor (TCR) specific for lymphocytic choriomeningitis virus (LCMV) plus H-2Db was analyzed in the presence or absence of the viral antigen. A novel approach to analyze the thymic tissue requirements for negative selection was possible by comparing thymocyte selection in H-2Db versus H-2Dbm13 mice, since the latter allowed positive selection but not LCMV-specific deletion of transgenic TCR-expressing thymocytes. In irradiation bone marrow chimeras expressing the restriction element for negative selection (H-2Db) on host tissue, we show that radioresistant recipient cells in the thymus deleted developing T cells at an early stage of differentiation. In contrast, chimeras expressing H-2Db on lymphohemopoietic donor cells showed clonal deletion at a later stage during ontogeny.