Suppressor cells prevent host resistance to tumor growth

The role of immunosuppressor cells in preventing host immune rejection of tumor cells is described. The growth of Lewis lung carcinoma (3LL) in syngeneic C57BL mice is accompanied by a weak and transient anti-tumor cytotoxic response that is later on replaced by tumor-enhancing activity. This enhancing activity is correlated with the generation of suppressor cells in 3LL-bearing mice. Such suppressors were demon strated in two ways: (a) Elimination of the hydrocortisone(HC)-sensitive lymphoid population from tumorbearing mice (TBM) resulted in a significant increase in the anti-tumor cytotoxic response and in a marked delay in tumor development. It is assumed that HC inactivates precursors of suppressor lymphocytes whereas the mature suppressor cells themselves are HC-resistant. (b) The increased resistance against the tumor could be partially re-suppressed by restoring the HC-treated TBM with spleen or thymus cells from normal C57BL. Suppression, however, was more pronounced if the resistant mice were restored with spleen or thymus cells from TBM. HC-resistant spleen cells from TBM that appear to be enriched for mature suppressor cells were capable of suppressing in vitro the secondary sensitization of spleen cells from TBM against tumor cells.