No abstract

The capacity of old (18-24 months) C57BL/6 mice to develop an immune reaction against MC-B6-1 fibrosarcoma cells was studied using in vivo adoptive transfer experiments (Winn assay) and in vitro T cell-mediated cytotoxicity test. Anti-tumor immunity was found to decline with age, as indicated by a decreased anti-tumor growth T cell activity. A suppressive activity was also found present in the splenic T cell population of old mice which can inhibit the in vivo generation of immune T cells in young mice. These suppressors, or their precursors, were resistant to cyclophosphamide treatment and were effective only when administered 3 days before the immunization of young mice. These mice developed immune T cells perfectly when the suppressors were administered 3 days after immunization, indicating that suppressors may act at an early phase of T cell activation. The protective activity of T cells in vivo correlated well with the in vitro T cell cytotoxicity for MC-B6-1 tumor cells, as both were depressed in old mice. Exogenous interleukin 2 (IL 2) addition during the 4-day culture period partially restored the low cytotoxic activity of old immunized lymphocytes, suggesting that specific clones were present but that a lack of IL 2 limited their expansion. However, in vivo supplementation with IL 2 administered after immunization did not increase the protection mediated by old immunized T cells but, rather, increased the suppression. This work demonstrates the presence of a T cell suppressive activity in the spleen of old mice but also indicates that precursors of cytotoxic cells are generated by the immunization. It seems that in vitro IL 2 addition increases cytotoxic cells while in vivo IL 2 administration amplifies the development of suppressor cells generated during immunization of aged mice.

Semi-allogeneic somatic hybrid cells derived from the fusion of a C57BL/6 fibrosarcoma (MCB6-1) and A9 cells (C3H origin) were used to immunize C57BL/6 mice against the parental tumor cells. These hybrid cells expressed H-2 histocompatibility antigen of both parental cells (H-2b and H-2k), and failed to produce tumors in normal C57BL/6 mice. A single i.p. injection of hybrid cells induced anti-tumor immunity which could be transferred to normal C57BL/6 recipient mice by immune spleen or peritoneal cells; the efficient cells were T cells, as this activity was completely abrogated by treatment with anti-Thy-1-2 antiserum and complement. Among immune splenic T cells, only the light-density T cells, obtained after fractionation on Percoll gradient, were effective in the transfer of immunity. Immunity induced by the hybrid cells was specific for MCB6-1 parental tumor cells. This immunity could be transferred during two brief periods, 7 to 12 days, and 40 to 50 days, after hybrid cell injection; there appeared to be an intermediate period, 12 to 40 days after immunization, during which no immunity could be transferred. These results suggest a suppressive mechanism implicated during hybrid cell immunization and interacting with the anti-tumor immune response.

No abstract

Antitumor immunity against a fibrosarcoma in C57BL/6 mice was obtained by means of a semi-allogenic somatic hybrid cell derived from the fusion of this C57BL/6 fibrosarcoma (MCB6-1) and A9 cells of C3H origin. In a Winn assay, this immunity could be transferred by T lymphocytes to normal C57BL/6 recipient mice during an early and a late phase after immunization. There appeared to be a transient non-responsive period during which no immunity could be transferred. Injection of cyclophosphamide (CY) into mice before immunization increased the level of immunity during this period, and reconstitution of animals with normal spleen cells abolished the effect of CY. During the non-responsive period, suppressor cells were demonstrated in the spleen: the i.v. transfer of these suppressor cells to normal mice significantly inhibited the induction of antitumor immunity; the suppressive effect was transferred by T lymphocytes of the Lyt-2+ phenotype. No suppressive effect on antitumor protection was observed when suppressor cells were transferred simultaneously with immune T lymphocytes in the Winn assay. From these findings, it appears that T-suppressor cells regulate the antitumor response, interfering with the afferent (induction) arm of the immune response.