Characteristics of the immature cells involved in T cell-mediated enhancement of syngeneic tumor growth.

Enhancement of tumor growth was observed when non-sensitized thymocytes were injected together with tumor cells into syngeneic mice, although this tumor enhancement was less pronounced than that caused by tumor-sensitized T lymphocytes. The cells within the thymus which are responsible for this tumor enhancement were found to be rapidly dividing and to be absent from the thymus a day after cortisone administration. At a longer time interval the cortison-depleted thymus was repopulated by dividing cells which exhibited tumor-enhancing reactivity. The characteristics of these cells suggest that they are in the early stages of thymic processing. The enhancing thymocytes were sensitive to treatment with the thymic humoral factor which functions in T cell maturation, and their enhancing activity was cancelled by such treatment. These results are compatible with our hypothesis that exposure of immature T cells to a tumor stimulus may lead to tumor enhancement whereas interaction between mature T lymphocytes and tumor cells may be required for tumor inhibition.