Early Precursor Thymocytes Can Produce Interleukin 2 upon Stimulation with Calcium Ionophore and Phorbol Ester

T-cell precursors were stimulated with a conventional T-cell mitogen or with the calcium ionophore A23187 in order to determine whether pre-T cells acquire the ability to produce interleukin 2 (IL-2) before they acquire the ability to respond to antigen or mitogenic lectins. Immature T cells were obtained by eliminating mouse thymocytes that expressed the Lyt2 and L3T4 cell surface proteins. The remaining Lyt2-, L3T4- cells were stimulated for IL-2 production by using concanavalin A (Con A) or A23187, together with phorbol 12-myristate 13-acetate (PMA). We found that these ``double-negative'' thymocytes were unresponsive to Con A plus PMA but produced substantial amounts of IL-2 when stimulated with A23187 plus PMA. In contrast, both stimulation regimens induced more mature T-lymphocyte populations to produce IL-2. This implies that developing T cells acquire the ability to make IL-2 upon induction before they acquire the ability to be triggered by Con A. Day-15 fetal and cortical thymocytes were also tested for their ability to make IL-2. Both populations failed to synthesize this growth factor, even when stimulated with A23187 and PMA. For cortical thymocytes, this result, together with the finding that A23187 plus PMA fails to activate these cells, suggests that this population is immunologically inert rather than immature. On the other hand, the inability of day-15 fetal thymocytes to produce IL-2 indicates that these T-cell precursors are developmentally distinct from adult Lyt2-, L3T4- thymocytes, which they phenotypically resemble.