SELECTIVE ADHESION OF IMMATURE THYMOCYTES TO BONE-MARROW STROMAL CELLS - RELEVANCE TO T-CELL LYMPHOPOIESIS

We investigated the interactions between the bone marrow microenvironm ent and T cell populations at different stages of maturation. Thymocyt es were seeded onto confluent layers of bone marrow stromal cell lines (MBA-13 or 14F1.1). Within a few hours two main thymocyte populations were observed; one remained in the liquid phase and the other adhered to the stromal cells. After 24 hours of culture, most of the adhering cells expressed the phenotype of the precursors, double negative (DN) CD4(-)CD8(-), or of immature thymocytes, double positive (DP) CD4(+)C D8(+). The number of adhering DN cells did not change during the time of the culture, whereas that of the DP declined. The CD4(+)CD8(-) or C D4(-)CD8(+) cells did not adhere to any significant extent. The expres sion of CD3 antigen on adherent thymocytes was lower than that on nona dherent ones. Sorted thymocytes at a high level of purification (>96%) were cultured over stromal layer and, after 24 hours, 60% of the DN o r 22% of the DP cells were found to adhere to the stroma. The culture medium was replaced every 24 hours or after 48 hours; no significant c hange was noted in the number of adhering DN and DP cells. The reappea rance of immature T cells in the liquid phase suggested proliferation of this cell type. Thus, early thymocytes, phenotypically characterize d as DN and DP, preferentially adhere to bone marrow stromal cells. Th is in vitro phenomenon may represent the function of the BM stroma as an extrathymic site of T cell lymphopoiesis.