POSITIVE SELECTION OF THYMOCYTES INVOLVES SUSTAINED INTERACTIONS WITHTHE THYMIC MICROENVIRONMENT

CD4(+)8(+) cortical thymocytes are critically dependent upon interacti on with the thymic epithelium to undergo positive selection and matura tion into single-positive CD4(+) or CD8(+) cells. Here we investigate further the nature of this interaction and provide evidence that posit ive selection requires sustained, rather than ''single hit,'' interact ion with thymic stromal cells. We also show that calcineurin-mediated signaling in thymocytes is required for the initial stages of positive selection, but is not essential throughout the period of thymocyte de pendence on stromal cell contact during positive selection. In additio n, we show that double-positive thymocytes that have initiated positiv e selection (CD69(+)4(+)8(+)) and newly generated single-positive (CD6 9(+)4(+)) cells differ markedly in response to the same stimulus throu gh the TCR. The former undergo deletion, whereas the latter proliferat e, indicating that a critical change in response to TCR ligation occur s within the narrow developmental window between these two stages.