In vivo detection and partial characterization of effector and suppressor cell populations in spleens of mice with large metastatic fibrosarcomas.

The MC-2 fibrosarcoma, which is a transplantable tumour syngeneic for BALB/c mice, metastasizes to lymph nodes draining subcutaneous inoculation sites, and also to the lungs. T cell-mediated immunity was detected in Winn assays using spleens from excision immunized mice. T cell-mediated anti-tumour immunity was also detected in spleens from mice with small tumours but disappeared as the tumour burden increased. Protective immune spleen cell activity in the Winn assay was inhibited by prior addition of spleen cells from mice with large tumours, causing increased tumour incidence. Splenic metastases occasionally occurred in the MC-2 model, but were not demonstrable by bioassay in any of the experiments detecting splenic suppressor cell activity. In vivo protective activity was restored to advanced-stage tumour-bearer spleens by whole-body ionizing irradiation (0.5 and 2.5 Gy) of donor mice 15 h before sampling. Spleen cells from mice with small tumours remained protective after 1.5, 2.5 and 4.0 Gy of irradiation in vivo. These results are consistent with the properties of radiosensitive suppressor T cells. In contrast to reports in other tumour models, suppressor cells were not detected until late in the course of MC-2 development. This is surprising in view of the aggressively metastatic nature of MC-2. It is postulated that modulation of the anti-tumour immune response by the suppressor cells is associated with metastasis in this tumour model. The late appearance of both suppressor cells and metastatic cells in the spleen may reflect similar processes occurring earlier in regional lymph nodes.