Regulation of Interleukin-2 Synthesis in Human Thymocytes

This study was designed to investigate whether or not human thymocytes can synthesize and respond to interleukin-2 (IL-2). Thymocytes were isolated from thymic sections obtained during cardiac surgery, and immature, heavy (density, 1.070-1.075 g/ml) thymocytes were separated from mature, light (density, less than or equal to 1.065 g/ml) thymocytes by buoyant density centrifugation. Thymocytes were stimulated with concanavalin A (con A), B lymphoblastoid (Bl) cells, phorbol myristate acetate (PMA), or with a combination of two or more inducing agents. Culture supernatants were analyzed for IL-2 activity by measuring the proliferation of an IL-2-dependent cytotoxic T cell line ( CTLL -A11). The response of human thymocytes to endogenous or exogenous (purified) IL-2 was assessed by determining their proliferative activity and their capacity to consume and absorb IL-2. Stimulation with con A in combination with PMA or Bl cells resulted in secretion of IL-2 by dense, immature thymocytes, low-density, mature thymocytes, and unfractionated thymocytes, and in an increase in [3H]thymidine incorporation. IL-2 synthesis preceded the proliferative response and required the continuous presence of con A. It was inhibited by cyclosporin A (CsA), an immunosuppressant which inhibits T cell activation and proliferation. As a result of the inhibition of endogenous IL-2 synthesis, thymocyte proliferation at 120 h of culture was also inhibited. CsA did not inhibit proliferation at 48 h of culture when exogenous IL-2 absorption became evident, but before the onset of the IL-2-dependent proliferative response.