CD69 expression discriminates MHC-dependent and -independent stages of thymocyte positive selection

In the thymus, phenotypically and functionally mature single positive cells are generated from immature CD4(+)8(+) precursors by a process known as po sitive selection, Although this event is known to involve alpha beta TCR li gation by peptide/MHC complexes expressed on thymic stromal cells, it is cl ear that positive selection is a multistage process involving transition th rough an intermediate CD4(+)8(+)69(+) phase as well as subsequent postselec tion phases. By analyzing the development of preselection CD4(+)8(+)69(-) a nd intermediate CD4(+)8(+)69(+) thymocytes in the presence of MHC class I-d eficient, MHC class II-deficient, and MHC double-deficient thymic stromal c ells, we investigated the role of MHC molecules at three distinct points du ring positive selection, Although the initiation of positive selection is c ritically dependent upon MHC interactions, we find the that later stages of maturation, involving the differentiation of CD4(+)8(-) and CD4(-)8(+) cel ls from CD4(+)8(+)69(+) thymocytes, occur in the absence of MHC molecules, Moreover, an analysis of the postselection. proliferation of newly generate d CD4(+)8(-) and CD4(-)8(+) thymocytes shows that this also occurs independ ently of MHC molecules, Thus, our data provide direct evidence that, althou gh positive selection is a multistage process initiated by TCR-MHC interact ions, continuation of this process and subsequent postselection events are independent of ongoing engagement of the TCR.