Cellular basis of immunologic interactions in adoptive T cell therapy of established metastases from a syngeneic murine sarcoma

The adoptive transfer of specifically sensitized T lymphocytes can effectively mediate the regression of established local and metastatic tumors. Previous experiments using the weakly immunogenic MCA 105 sarcoma indicated that cellular interactions between transferred L3T4+ helper and Lyt-2+ cytotoxic immune T cells were necessary for mediating tumor regression. In this study, the kinetics of T-T cell interactions were analyzed by in vivo depletion of T cell subsets with mAb. The anti-tumor efficacy of transferred immune cells was abrogated by in vivo administration of either L3T4 or Lyt-2 mAb on the day of cellular therapy. However, if mAb were given 3 days after the transfer of immune cells, depletion of Lyt-2+ but not L3T4+ cells abrogated anti-tumor efficacy. T cell depletion on day 6 after transfer of immune cells had no adverse effect on tumor regression, indicating the period required for T cell reactivity in vivo. Furthermore, depletion of Ia+ cells by in vivo mAb treatment abrogated the anti-tumor efficacy of immune cells. It is thus hypothesized that there are two distinct but sequential phases of in vivo T cell interactions leading to the regression of established tumors after adoptive immunotherapy. An initial "helper/inducer" phase apparently requires the interaction of L3T4+ immune cells and the tumor Ag involving Ia+ cells. The inducement of L3T4+ cell activation is to provide helper function via the secretion of IL-2. The second phase designated as an "effector phase" involves differentiation of immune Lyt-2+ cells under the influence of IL-2 secreted during the helper/inducer phase for generation of mature Lyt-2+ effector cells. To further support the hypothesis of a two-phase process we have examined the phenotype and kinetics of tumor regression mediated by effector cells generated by secondary in vitro sensitization (IVS). Although the IVS cells were generated from fresh MCA 105 immune spleen cells, their anti-tumor efficacy was mediated solely by Lyt-2+ lymphocytes. Kinetic studies revealed that the in vivo requirement of IVS Lyt-2+ effector cells to mediate tumor regression was less than 3 days, and the anti-tumor reactivity of these cells was not affected by in vivo depletion of Ia+ cells. Thus, the IVS reaction is likely representative of the in vivo counterpart of the helper/inducer phase leading to the generation of mature Lyt-2+ immune effector cells. Tumor regression after transfer of Lyt-2+ cells generated in IVS therefore required a relatively shorter period of time than that required after the transfer of fresh noncultured MCA 105 immune spleen cells.