Induction and Upregulation by Interleukin 2 of High-affinity Interleukin 2 Receptors on Thymocytes and T Cells

We show that purified recombinant interleukin 2 (rIL-2) alone induces the expression of high- and low-affinity interleukin 2 (IL-2) receptors in vitro on human T cells and thymocytes that have not been activated previously by lectins or other inducing agents. IL-2 receptors are expressed after 24 hr, as determined by the binding of 125I-labeled monoclonal anti-IL-2 receptor antibody 2A3, which binds equally to high- and low-affinity receptors. High-affinity receptors were distinguished from low-affinity receptors by the binding of 125I-labeled IL-2 to T cells and by the proliferative response of thymocytes to IL-2, in concentrations that selectively interact with the high-affinity class of IL-2 receptors. The IL-2-induced proliferation of thymocytes in vitro induced by IL-2 alone is dependent upon the concentration of IL-2 and is inhibited by monoclonal anti-Tac antibody, indicating that the proliferative response is mediated by the binding of IL-2 to the receptors. In addition, we demonstrate that IL-2 augments the number of high-affinity receptors on concanavalin A-activated thymocytes. These results document that IL-2 acts as a hormone that induces the activation of thymocytes and T cells, as evidenced by the de novo induction of biologically active, high-affinity IL-2 receptors. IL-2 also upregulates the expression of high-affinity IL-2 receptors on activated thymocytes. These observations illustrate the biologic importance of the regulatory role of IL-2 in the immune response.