Differential effects of peptide diversity and stromal cell type in positive and negative selection in the thymus

Thymocyte positive selection results in maturation to the single-positive s tage, while negative selection results in death by apoptosis, Although kine tic analyses indicate only 3-5% of CD4(+)8(+) cells reach the single-positi ve stage, the balance of positive and negative selection and the nature and quantity of cells mediating maximal negative selection are uncertain. Here , using a system where the number and type of stromal cells and thymocytes can be controlled, we investigated the maturation of CD4(+)8(+) thymocytes in the presence or absence of thymic epithelium and dendritic cells (DC) fr om wild-type (wt) and H-2M(-/-) mice expressing different peptide arrays. W e find that titration of wt DC into reaggregates of wt epithelium has a dra matic effect on the number of CD4(+) cells generated, with 1% DC causing a maximal 80% reduction, Moreover, while addition of 1% wt DC into cultures o f H-2M(-/-) epithelium causes a 90% reduction in CD4(+) cells, no effect wa s observed when similar numbers of wt thymic epithelium were added. Collect ively, these data provide the first accurate indication of the quantity and quality of stromal cells required for maximal negative selection in the th ymus, demonstrate the importance of peptide diversity in T cell selection, and highlight a large degree of overlap between positive and negative selec tion events.