Identification of a developmentally regulated phase of postselection expansion driven by thymic epithelium.

To investigate events following the initiation of positive selection, we have used reaggregate organ cultures to follow the maturation of purified CD4+8+69+ thymocytes; these thymocytes represent a subpopulation of thymocytes which have already received positive selection signals. Using a dilution analysis of an FITC-based membrane-binding dye, 5-(and -6)-carboxyfluorescein diacetate succinimidyl ester, to allow a quantitative measure of proliferation, we show that while newly selected CD4+ and CD8+ cells are nondividing, both subsets subsequently undergo a wave of postpositive selection proliferation involving multiple cell divisions. Moreover, in the presence of fetal stromal cells, postselection expansion is more extensive in newborn thymocytes compared with adult thymocytes, suggesting that this phase of expansion is developmentally regulated. We also show that proliferation of CD4+ and CD8+ cells is seen in reaggregates of purified MHC class II+ thymic epithelial cells, while CD4+ and CD8+ cells generated from bcl-2 transgenic CD4+8+69+ thymocytes in the absence of stromal cell support survive but do not proliferate; this observation indicates that MHC class II+ thymic epithelial cells are both necessary and sufficient to mediate this wave of cell division. Finally, the maturation of CD4+8+69+ thymocytes and the subsequent proliferation of CD4+ and CD8+ cells occur in the presence of MHC-mismatched thymic stromal cells, suggesting that the later stages of positive selection and the associated postselection events do not depend on interactions with the same peptide/MHC complexes responsible for initiation.