Regulation of immune responses against the syngeneic ADJ-PC-5 plasmacytoma in BALB/c mice. II. Suppression of T-cell cytotoxicity by pretreatment of mice with subimmunogenic doses of tumour cells.

Protective immunity towards a lethal dose of 10(4) living ADJ-PC-5 cells can be induced in 56% of BALB/c mice by treating them with 10(7) irradiated cells. This protection can be modulated by pretreatment of animals with lower numbers of irradiated tumour cells. Repeated injections of 10(1) cells before immunization and challenge with tumour cells lead to tumours in only 16% of the animals, whereas repeated injections of 10(5) cells shift the tumour incidence to 78%. The enhanced tumour incidence is paralleled by suppression of T-cell cytotoxicity which can be transferred by spleen cells into 100 r-irradiated BALB/c mice. The possible significance of induction of specific immunosuppression during the initial stages of tumour growth is discussed. It is stressed that this suppression becomes effective at a dose 10(3) times lower than that which could induce protective immunity.