Splenic immune effector and suppressor cells in mice bearing a growing plasmacytoma.

The occurrence of tumor immunity in mice bearing a growing plasmacytoma (PC) was studied by local adoptive transfer assay. Spleen cells from mice with a large PC but not with a nonpalpable or small PC retarded and often inhibited entirely the growth of homologous PC. They occasionally caused late regression of growing homologous and heterologous PC. The individual tumor-specific immune effector cells were found to be radiosensitive, non-adherent, Thy 1-positive T-cells. They were resistant to treatment of mice with high-dose cyclophosphamide (CTX). Their generation, however, was abrogated by low-dose CTX and irradiation at the early stage of their development. Spleen cells from PC-bearing mice treated with high-dose CTX were enhanced in their effector activity, suggesting the co-existence of CTX-sensitive suppressor cells. The suppressor cells could be demonstrated in spleens of mice bearing a heterologous PC and were found also to be radiosensitive, non-adherent, Thy 1-positive T-cells. Their generation was blocked by treatment of mice with CTX during the early stage of PC development. These findings provide evidence for the occurrence of immune effector T-cells in mice with a growing PC. This immunity appears to be down-regulated by CTX-sensitive suppressor cells.