Mitogenic activation of phosphoinositide turnover and DNA synthesis in murine CD4⁺8⁺ thymocytes

Eighty percent of the lymphoid cells in the murine thymus are premature CD4⁺8⁺ (double positive) thymocytes. The vast majority of the double-positive cells do not maturate and die in the thymus. Although these cells are subjected to thymic selection processes, their activation competence has been an enigma. We have separated out CD4⁺8⁺ cells and studied their early and late responses to several mitogens. Concanavalin A, anti-CD3 (145-2C11) or anti-Thy1 (G7) monoclonal antibodies enhanced phosphoinositide turnover in double-positive thymocytes. However, DNA synthesis in the mitogen-stimulated cells was only accomplished if IL-2 or IL-4 was added. Alternatively, DNA synthesis could be induced by the calcium ionophore A23187 and phorbol myristate acetate (PMA). The latter mode of activation did not require the addition of exogenous lymphokines. CD4⁺8⁺ thymocytes did not secrete IL-2 or IL-4 following activation by either mitogens or A23187 and PMA. These findings demonstrate that CD4⁺8⁺ thymocytes resemble mature T cells in their ability to respond with DNA synthesis when activated by T-cell mitogens and IL-4 as well as IL-2. The results also delineate the difference between receptor mediated mitogenesis and pharmacological stimulation.