Positive selection of T cells is a complex developmental process generating long-lived, functionally mature CD4+CD8- and CD4-CD8+ cells from short-lived, immature CD4+CD8+ precursors. The process is initiated in the thymus by interaction of the alpha beta TCR with molecules encoded by the MHC, occurs without cell division, and involves rescue from programmed cell death (PCD), as well as induction of differentiation and maturation of selected precursors. It is unclear whether development of small, positively selected CD4+CD8+ thymocytes (characterized by up-regulated levels of TCR and CD69 molecules) depends on further interactions with MHC molecules and, if so, whether such interactions are required for survival, for maturation, or for both. The involvement of the TCR and/or CD4/CD8 coreceptors in transmitting additional signals is also unknown. We have examined these questions by analyzing survival and differentiation of early (CD4+CD8+TCRhi) and later (CD4-CD8+TCRhi) postselection stages of thymocytes from normal and bcl-2 transgenic mice expressing transgenic, class I MHC-restricted TCR, upon intrathymic transfer into recipients that lacked ligands either for both the TCR and CD8 coreceptor, or for the TCR only. The results provide direct evidence that induction of differentiation of CD4+CD8+ thymocytes by recognition of MHC molecules does not rescue them from PCD and is insufficient to activate the entire maturation program. Both processes require continual engagement of the TCR by positively selecting MHC molecules that, at least in the case of class I MHC-restricted CD4-CD8+ T cells, cannot be substituted by the engagement of coreceptor alone.

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