DISTINCT DIFFERENTIATIVE STAGES OF CD4(+)CD8(+) THYMOCYTE DEVELOPMENTDEFINED BY THE LACK OF CORECEPTOR BINDING IN POSITIVE SELECTION

Cortical CD4(+)CD8(+) thymocytes mature into CD4(+) or CD8(+) thymocyt es through a process termed positive selection. To better define diffe rentiative stages of CD4(+)CD8(+) thymocyte development in positive se lection, we performed a phenotypic analysis of CD4(+)CD8(+) thymocytes from H-Y mice mated to various genetic backgrounds. We have previousl y shown that coordinate binding of the H-Y TCR and the CD8 coreceptor to the restricting D-b MHC class I molecule is required for the effici ent positive selection of this TCR. In this study we have used TCR, CD 5, and CD45 expression levels as markers for thymocyte maturation. Lac k of CD8/D-b interaction was achieved by introducing a mutation that a brogates CD8 binding in the alpha 3 domain of D-b. We found that the a bsence of coreceptor ligation prevented TCR up-regulation in CD4(+)CD8 (+) thymocytes and resulted in a developmental arrest characterized by low levels of TCR and CD45. We have previously shown that deletion of CD4(+)CD8(+) thymocytes expressing the H-Y TCR is facilitated by CD8 coreceptor ligation. Here we show that expression of the deleting liga nd in the absence of coreceptor ligation caused CD5 up-regulation with out concomitant TCR or CD45 up-regulation in CD4(+)CD8(+) thymocytes. In a beta(2)-microglobulin null background, introduction of the H-Y TC R caused the majority of CD4(+)CD8(+) thymocytes to express an unusual ly low level of the CD5 activation marker, suggesting that a low-affin ity or noncognate TCR/MHC interaction may be required for initial CD5 up-regulation to intermediate levels. Collectively, these observations favor a maturational process in positive selection in which CD5 up-re gulation precedes CD45 and TCR up-regulation.