Effect of polyamine depletion in vivo by DL-alpha-difluoromethylornithine on functionally distinct populations of tumoricidal effector cells in normal and tumor-bearing mice.

The objective of the present investigation was to examine the effect of in vivo polyamine depletion by DL-alpha-difluoromethylornithine (DFMO), a specific irreversible inhibitor of ornithine decarboxylase, on cell-mediated tumoricidal activity in normal and tumor-bearing (B16 melanoma) mice. DFMO treatment in vivo for 6 days reduced splenic leukocyte polyamine levels and the induction of cytotoxic T-lymphocytes (greater than 50%) in both normal and tumor-bearing mice. However, substantially less inhibition was observed in the ability to generate cytotoxic T-lymphocytes following 18 days of DFMO treatment. In contrast, DFMO treatment for 6 or 18 days did not impair splenic natural cell-mediated cytotoxicity, assessed against natural killer sensitive YAC-1 target cells and natural cytotoxic sensitive WEHI-164 target cells, in normal or tumor-bearing mice. Natural cell-mediated cytotoxicity was not observed against fresh B16 melanoma cells. However, macrophage-mediated tumoricidal activity directed against B16 melanoma cells was augmented 79% following 6 but not 18 days of DFMO treatment. These results demonstrate that DFMO can exert very selective effects on functionally distinct populations of antitumor effector cells in vivo depending upon the schedule of DFMO administration.