Suppressor T cells in BCG-treated mice interfere with an in vivo specific antitumoral immune response

The interference by BCG in the induction and expression of a specific antitumoral immune reaction was studied in B6 mice, using the in vivo Winn assay and also active immunization. T cells immunized against MCA-induced fibrosarcoma (MC B6-1) transferred together with the tumour cells protected the syngeneic host against tumour take. Pretreatment of normal B6 mice with moderate or high doses of BCG prevented the development of a protective immune response after immunization. Moreover, a single dose of 1 mg, or 2 doses of 0.01 mg BCG, completely eliminated an established antitumour immunity. Suppressor cells are involved in the BCG-induced inhibitory effect; they interfered (1) with the expression of the antitumour response, since their addition to immune T cells in the Winn test resulted in decreased protection and (2) with the induction of the antitumour response, since injection of spleen cells from BCG-treated mice (BCG SpC) into normal mice before immunization inhibited the development of immunity. Treatment of BCG SpC with anti Thy 1.2 and anti Lyt 1.2 antibodies plus complement before injection into normal mice significantly decreased the suppressive activity, showing that the suppressor cells induced by BCG are T cells expressing the Lyt 1+ phenotype. The partial increase in protection obtained after IL-2 administration to BCG-treated mice suggests that the suppressive action of BCG SpC on the IL-2 producing capacity of helper T cells is only one of a number of possible mechanisms of T-cell-mediated suppression.