Enhancement of cytotoxic T lymphocyte growth from spleens of P815-tumor-bearing host mice with mafosfamide.

Mafosfamide (Mafo) is an analog of cyclophosphamide that does not require hepatic activation and therefore has in vitro activity. The present study was conducted to determine the effects of in vitro treatment with Mafo on the generation and growth of cytotoxic T lymphocytes (CTL) from tumor-bearing host mice (TBH). In contrast to early (day-11) TBH splenocytes, splenocytes from late (days 18-20) P815 TBH mice suppress the in vitro generation of CTL. Treatment of late TBH splenocytes in vitro with 5-15 microM Mafo resulted in a reduced ability of these cells to suppress in vitro CTL generation. Treatment of late TBH splenocytes with 10 microM Mafo also inhibited their ability to suppress adoptive immunotherapy of intradermal tumors with immune splenocytes. These doses of Mafo were selectively toxic to the suppressive effects of late TBH splenocytes, since treatment of early TBH splenocytes with 1-10 microM Mafo did not significantly inhibit CTL generation. Spleen cells from early (days 10-12) TBH mice, carried in long-term in vitro sensitization cultures in the presence of tumor cells and 20 U/ml human recombinant interleukin-2, did not increase in cell number over time. However, when pretreated with 3 microM Mafo, this population of tumor-sensitized lymphocytes demonstrated 450-fold growth over 6 weeks as compared to the static cell numbers for the untreated controls. High levels of tumor-specific cytolytic activity were maintained in these expanded cells. These results suggest that Mafo pretreatment markedly and selectively inhibits suppressor cells that limit long-term expansion of splenic CTL in culture and inhibit adoptive immunotherapy of solid tumors.