The mode of action of the calcium ionophore A23187 on T cell proliferation. I. The ionophore does not replace lymphokines but acts via induction of IL-2 production on IL-2 responsive cells.

The two compounds, the calcium ionophore A23187 and phorbol myristate acetate (PMA), which are not mitogenic for mouse thymocytes, induce proliferation when added in combination to thymocyte cultures. Short exposure to PMA renders the cells responsive to IL-2, added exogeneously. The cells rendered IL-2-responsive by PMA are enriched in the more mature peanut agglutinin (PNA)-negative population. The ionophore does not render PNA-negative thymocytes IL-2-responsive, but induces proliferation of PMA-pulsed PNA-negative thymocytes. PMA-pulsed PNA-negative thymocytes proliferating in response to either exogeneous IL-2 or ionophore express IL-2 receptors. However, the ionophore does not mimic IL-2 action but acts indirectly by induction of both IL-2 production and of IL-2 receptor expression via IL-2. This view is based on the following findings: 1) IL-2 could be detected in supernatants derived from PMA-preactivated thymocytes incubated with the ionophore; 2) The IL-2-induced proliferation, as well as the ionophore-induced proliferation, was specifically blocked by a monoclonal anti-IL-2-receptor antibody; 3) The proliferation induced by exogeneous IL-2, as well as that induced by the ionophore, could be specifically inhibited by metabolically blocked T lymphoblasts carrying IL-2 receptors competing with the responder cells for the available IL-2 added or produced in the system.