Characterization of colony-forming cells in the human thymus: evidence for a suppressor mechanism

Activation pathways for thymic cell growth remain partially unknown. Thymocytes generally show low growth responses when cultured in the presence of mitogens, especially when T-cell colony formation by these cells is assayed. In the present work we studied the T-cell colony-forming ability of thymic subsets under phytohemagglutinin (PHA) and/or recombinant interleukin 2 (rIL2) stimulation. Our results confirm the low number of T-cell colonies obtained from human thymus as compared to bone marrow and peripheral blood sources. In contrast, significant colony numbers were observed when thymocytes were depleted of CD8+ cells. This increase was due to the suppression exerted by CD8+ cells on both CD4+- and CD4- -, CD8- -derived colony precursors. This inhibitory mechanism is dose-dependent and cannot be replaced by media conditioned by CD8+ cells. Limiting dilution analysis corroborates agar assays and points to the existence of T-cell colony-forming cells within the double-negative (CD4-, CD8-) thymic subset.