Abstract
The present experiments were performed to investigate the possibility of inhibiting tumor growth in vivo with syngeneic lymphocytes sensitized in vitro on monolayers of the tumor under test, and to study the effect of a thymic humoral factor (THF) in this sensitization process. Monolayers of fibrosarcoma cells were used to sensitize spleen cells from syngeneic donors against the tumor. Such sensitized lymphocytes manifested cytotoxic activity against cells of the fibrosarcoma in a microassay measuring tumor‐cell detachment. However, when the sensitized lymphocytes were mixed with the fibrosarcoma cells and injected into syngeneic mice, enhanced tumor growth was observed in vivo. Addition of thymic humoral factor to the cultures during sensitization resulted in increased cytotoxic activity by the lymphocytes in vitro and a reduction in the tumor enhancement caused by these cells when injected in vivo. Enhanced tumor growth occurred when activated lymphocytes of allogeneic as well as syngeneic origin were injected together with the fibrosarcoma cells. Enhancement, which was already apparent when the spleen cells had been sensitized for 24h, could be circumvented by separate administration of lymphocytes and tumor cells. Syngeneic lymphocytes injected systemically after sensitization for 5 days exerted anti‐tumor reactivity against the fibrosarcoma grafted in the foot‐pad of syngeneic mice. Tumor growth was further inhibited by systemic injection of lymphocytes which had been sensitized in the presence of the thymic humoral factor.