Regulation of immune responses aginst the syngeneic ADJ-PC-5 plasmacytoma in BALB-c mice. III. Induction of specific T suppressor cells to the BALB/c plasmacytoma ADJ-PC-5 during early stages of tumorigenesis.

Initial stages of tumour growth are not easily accessible to investigation. Therefore an experimental procedure was developed to mimic tumorigenesis as closely as possible. BALB/c mice received intraperitoneally exponentially increasing numbers of irradiated syngeneic ADJ-PC-5 plasmacytoma cells. The initial injection began with two cells per mouse and according to the generation time of this tumour, subsequent doses were doubled until mice had received up to 10(5) tumour cells. At various stages of treatment, peritoneal exudate cells (PEC) and spleen cells (SC) were tested for either cytotoxicity or specific suppression of induction of a primary in vitro T-cell cytotoxic response (CTL) of BALB/c spleen cells against ADJ-PC-5 plasmacytoma cells. No cytotoxic PEC were found. Instead, PEC from mice in which the final tumour cells number had reached or exceeded 10(3) irradiated ADJ-PC-5 cells, induced complete suppression of this primary in vitro CTL. Specificity was found both in the induction and effector phase of suppression. Specific suppression was mediated by Thy-1.2+ cells and amplified by non-specific suppression through adherent cells. The data arae discussed in context with previous findings on the in vivo immunogenicity and tolerogenicity of the ADJ-PC-5 plasmacytoma. They suggest that induction of T suppressor (Ts) cells might be an early event in tumorigenesis.