Distinct Effects of Thymic Epithelial Culture Supernatants on T Cell Properties of Mouse Thymocytes Separated by the Use of Peanut Agglutinin

Supernatants from thymic epithelial cultures (TES), a putative analogue of thymic humoral function, are shown to induce strong cytolytic T lymphocyte (CTL) responses when added to primary mixed lymphocyte cultures (MLC) with thymocytes as responder cells. No enhancing effects of TES are observed with spleen cells or hydrocortisone-resistant thymocytes. These effects of TES on functional T cell parameters are accompanied by a reduction in part of the thymocytes in Thy 1 antigen density and sensitivity to hydrocortisone (HC). Inasmuch as immunocompetent T cells are found only among the HC-resistant thymocytes expressing a low Thy-1 antigen density, these findings suggest a TES-induced maturation process of thymocytes. In addition, we attempted to answer the question of whether the enhancing effects of TES on T cell-proliferative capacity and T cell killer function of thymocytes are due to an effect on previously unresponsive immature T cells or merely to extra stimulation of already responsive cells. Using the peanut agglutinin (PNA)-agglutination technique as a reversible probe, we could now demonstrate that TES affects the T cell-proliferative and CTL responses of the PNA-agglutinating thymocytes only. These findings support the concept that TES induces maturation of immature thymocytes, inasmuch as the PNA-agglutinating cells represent the immunoincompetent cells which are rich in Thy 1 and poor in H-2 antigens and HC sensitive. The PNA-nonagglutinating thymocytes exhibited high reactivity in all the functional assays investigated (like HC-resistant thymocytes), and this could not be further enhanced by TES. Together with the observation that supernatants from nonthymic epithelial cultures or from thymic lymphocyte cultures exhibit no effects on any of the abovementioned parameters, these findings suggest that TES represents a relevant in vitro model for studying thymic humoral function.