Tumor-derived cytokines induce bone marrow suppressor cells that mediate immunosuppression through transforming growth factor beta.

Normal bone marrow cells become immunosuppressive when cultured with supernatants of metastatic Lewis lung carcinoma (LLC-LN7) cells. The suppressor-inducing activities in the LLC-LN7 supernatants are interleukin-3 and granulocyte/macrophage-colony-stimulating factor. In the present study, the mechanisms by which these induced suppressor cells (LLCsup-BM) mediate their immunosuppression were investigated. The suppression by LLCsup-BM of splenic concanavalin CA blastogenesis was not dependent on cell contact since immunosuppression occurred regardless of whether the LLCsup-BM were separated from the responder spleen cells by a permeable membrane or if the LLCsup-BM were cocultured with the spleen cells. Culture supernatants of LLCsup-BM also inhibited T cell blastogenesis, being more suppressive than were supernatants of control bone marrow cells, which had been precultured with medium. The suppression by the soluble inhibitors elaborated from the LLCsup-BM was not restricted to the inhibition of T cell function as the supernatants also inhibited the natural killer activity of normal spleen cells. Studies to determine the identity of the suppressive activity produced by the LLCsup-BM showed increased levels of transforming growth factor beta (TGF beta) in their supernatants. Immunosuppressive bone marrow and spleen cells obtained from mice bearing metastatic LLC-LN7 tumors also secreted more TGF beta than did the cells obtained from normal mice. When anti-TGF beta antibodies were added to the LLCsup-BM supernatants, the suppressive activity was diminished. These results suggest that the LLCsup-BM mediate at least part of their immunosuppression through production of TGF beta.