Characterization of cytostatic effector lymphocytes during the development of a syngeneic lymphosarcoma in C3H mice: use of monoclonal reagents to identify T-cell subsets.

The development of the in vitro cytostatic capacity of splenic lymphocyte subpopulations from C3H mice carrying the syngeneic Gardner tumor was examined at different times after intramuscular tumor injection. Most mice died between 3 to 6 weeks after tumor injection, while some rejected their tumors or survived longer than 3 months. Cell separation procedures and monoclonal antibodies against T-cell subsets were used to identify the cells responsible in anti-tumor immunity. Cytostatic capacity against tumor cells developed in the T-cell enriched subpopulation of splenocytes 3 days after tumor injection and was partly abrogated by anti-Lyt-1. Effector function of Lyt-2+ T cells and B cells developed later and peaked at around 10 days after tumor injection. Another cell population with cytostatic capacity which was not blocked by anti-Lyt-1, anti-Lyt-2, or anti-Ly-5 was noted to develop early after tumor injection and lacked both T-cell and B-cell markers ("null"). This subpopulation was eluted with T cells from nylon wool columns and comprised up to 50% of the T-enriched fraction of splenocytes in later stages of tumor growth. An interesting characteristic of these "null" cells was susceptibility to T-cell suppression both in early and later stages of tumor growth except in regressor mice which lacked suppressor T cells. The cytostatic capacity of the "null" cells could be restored either by removal of Thy-1+ cells from the T-enriched fraction by panning, or the addition of anti-Thy-1 or F(ab')2 fragments of anti-Thy-1 to the lymphocyte-tumor reaction mixtures. Most mice examined after 10 days of tumor growth were immunosuppressed to varying degrees. Unseparated splenocytes from these mice were not cytostatic but removal of T cells allowed the B cells to exert their cytostatic capacity. A strong underlying B-cell cytostasis was shown to be present in long survivor mice even though their unseparated spleen cells were only weakly cytostatic. T cells did not play a role in the regression of tumors or long-term survival of tumor bearer mice. Splenocytes from regressor mice were strongly cytostatic, their anti-tumor activity residing in the "null" and B-cell populations.