Phenotype of stromal cell-associated thymocytes in situ is compatible with selection of the T cell repertoire at an "immature" stage of thymic T cell differentiation.

Murine thymocytes interacting with cortical macrophages, cortical epithelial cells and medullary dendritic cells in situ express T cell receptors at low to intermediate density. They co-express the lineage markers Lyt-2 and L3T4 and are not enriched for cells expressing high-density interleukin 2 and lymph node homing receptors. Thus, recognition of stromal cells in situ in distinct thymic microenvironments occurs at a common stage of T cell maturation which is phenotypically intermediate between intrathymic precursor cells and mature medullary-type thymocytes. The surface phenotype of dendritic cell-associated thymocytes indicates the presence of thymocytes with a "cortical" phenotype within the antigen-exposed ("nonsterile") phase of T cell maturation in the medulla.