Human thymocytes do not respond to interleukin-2 after removal of mature “bright” CD5 positive cells

Interleukin-2 receptors (IL-2R) are expressed on minor populations of immature and mature human thymocytes. These studies were designed to determine if immature T cells could respond to the mitogen phytohemagglutinin (PHA-P) plus IL-2 in vitro by increasing the expression of IL-2R and by proliferation. Using monoclonal antibodies to CD5 and magnetic immunobeads we were able to remove all mature, “bright” CD5 + cells from nylon wool-purified thymocytes and to obtain less mature cells which consisted almost completely of cells with the CD4 + CD8 + phenotype. These immature cells were mostly “dim” CD5 + and less than 5% CD5 − and a small percentage expressed the IL-2R. After culture in serum-free medium with PHA-P, these cells showed only a slight increase in the percentage of IL-2R + cells and the addition of IL-2 did not increase the percentage of IL-2R + cells and no proliferation was observed. Unseparated, nylon wool-purified thymocytes contained 14% bright CD5 + cells. These bright CD5 + cells had a mature phenotype of CD4 + CD8 − (52%) and CD4 − CD8 + (27%) cells. A small percentage of these cells were IL-2R + . These bright CD5 + IL-2R + cells were predominantly mature CD4 + CD8 − cells as measured by three-color flow cytometry. After culture with PHA-P and IL-2, the percentage of IL-2R + cells increased and they were now found not only on CD4 + CD8 − but also on CD4 − CD8 + and on CD4 + CD8 + cells. IL-2 plus PHA-P increased proliferation of these cells as compared to those cultured in medium with PHA-P without IL-2. Thus, we show that human immature thymocytes in contrast to mature thymocytes are not responsive to IL-2 as measured by a lack of IL-2R expression and proliferation. These data indicate that mature thymocytes can express a functional high affinity receptor for IL-2 and suggest that immature thymocytes may not possess a (functional) p75 chain of the IL-2R.