Interleukin-2 gene-transduced human leukemic cells induce major histocompatibility complex-restricted and -unrestricted anti-leukemic effectors in mixed lymphocyte-tumor cultures

To explore the feasibility of designing vaccination protocols in acute leukemia patients with cytokine gene-transduced leukemic cells, we studied in vitro the growth potential of human leukemic cells transduced with the interleukin-2 (IL-2), IL-7, or IL-7 plus IL-2 genes, as well as the capacity of generating both autologous and allogeneic cytotoxic lymphocytes directed against the parental cells. A lymphoblastic T-cell line, ST4, obtained from a patient in long-lasting complete remission, was retrovirally engineered with the IL-2, IL-7, and IL-7 plus IL-2 genes; in addition, clones releasing different amounts of the cytokines were obtained by limiting dilution. Mixed lymphocyte-tumor cultures (MLTCs) were set up with parental or transduced leukemic cells as stimulators and with autologous or allogeneic lymphocytes as responders. When nonirradiated ST4 parental cells or clones producing 80 IU/mL/10(6) cells/72 hours of IL-2 were used as stimulators, the proliferation of leukemic cells was blocked and the transduced leukemic cells were completely cleared from the cultures by day 16; repeated restimulations with IL-2-producing leukemic cells were required to sustain long-term lymphocyte survival. On the contrary, when IL-7- or IL-7-IL-2-producing cells were used as stimulators, only a delay in leukemic cell overgrowth was observed, and lymphocytes were completely cleared from the cultures after day 60. IL-7 production by the different clones ranged between 11 and 36 ng/mL/10(6) cells/72 hours, whereas the highest IL-2-producing IL-7-IL-2 clone released 50 IU/mL/10(6) cells/72 hours of IL-2. When the stimulator efficacy of the highest IL-2-producing clone (ST4/IL-2#A7) was compared with that of exogenous IL-2 plus parental cells, a 7-fold higher amount of exogenous IL-2 was required to achieve the same results obtained with IL-2-producing leukemic cells. Autologous and allogeneic long-term MLTCs (up to 35 days) with ST4/IL-2#A7 as the stimulator were capable of generating cytotoxic effectors equally endowed with both major histocompatibility complex (MHC) class I-unrestricted and -restricted activity against parental ST4 cells. By day 18 of both autologous and allogeneic cultures, a substantial proportion of CD56+ cells was consistently recorded; this was coupled to a predominantly M