Retinoic acid stimulation of the induction of mouse killer T-cells in allogeneic and syngeneic systems.

The ability of retinoic acid (RA), a potent antitumor agent, to stimulate cell-mediated cytotoxicity (CMC) in mice was investigated. Low doses of RA (5-300 micrograms/mouse/day) administered ip into C57BL/6 mice for 5 days daily or for 1--3 months three times a week before immunization in vivo or in vitro with allogeneic BALB/c S194 myeloma cells led to an enhanced cytotoxic activity of their spleen effector cells. Similarly, in a syngeneic situation injection of RA into C57BL/6 or BALB/c mice before in vitro challenge with EL 4 (C57BL/6) or S194 (BALB/c) tumor cells strongly stimulated CMC. The enhanced cytotoxic activity was effected by thymus-derived lymphocytes (T-cells) and specific for the H-2 histocompatibility antigens in the case of the allogeneic sensitization or specific for tumor antigens in the case of the syngeneic sensitization. Because RA had no effect on the effector step of CMC, RA likely enhanced the induction step of T-CMC. The action of RA was antigen-dependent, and it is therefore a true adjuvant rather than a nonspecific stimulator or polyclonal activator of cytotoxic T-cells.