Eradication of mouse melanoma by combined treatment with recombinant human interleukin 2 and recombinant murine interferon-gamma

Successful immunotherapy of early s.c. or i.p. (B16) melanoma in syngeneic C57BL/6 (B6) mice was achieved with s.c. peri-lesional injections (for s.c. tumors) or i.p. injections (for i.p. tumors) of recombinant human interleukin 2 (rIL-2) and recombinant murine interferon-gamma (rIFN-gamma). Over a 28-day period, rIL-2 and rIFN-gamma were injected 14 times. Results with this combination were additive with s.c. tumors and synergistic with i.p. tumors. Treatment with 6,250 U-25,000 U of rIL-2 and 2 micrograms of rIFN-gamma began 1-3 days after s.c. inoculation of melanoma. On day 50, 87% (72/83) of mice thus treated were completely free of tumor. None of the 78 control mice (tumor + buffer) survived. Of mice receiving either rIL-2 or rIFN-gamma alone, 59% (47/79) and 53% (44/83), respectively, were tumor-free. I.p. tumors were also eradicated by i.p. injections of rIL-2 (50,000 U) with rIFN-gamma (5, 10, and 15 micrograms) as judged by absence of tumor in 81% (21/26) of mice autopsied between days 45 and 65. No control mice survived, and only 17% (2/12) and 20% (6/30) given either rIL-2 or rIFN-gamma separately (i.p.) were tumor-free. Doses of rIFN-gamma from 1-4 micrograms were more beneficial in eliminating 1-day s.c. melanomas than were higher doses, and local s.c. treatment was far superior to distant or systemic treatment. Non-adherent peritoneal or splenic cells from mice bearing 6-day-old i.p. melanomas and treated with one or both lymphokines on days 3 and 4 were used in cytotoxicity assays in vitro. Significant cytotoxicity against cultured melanoma cells was displayed by cells harvested from lymphokine-treated mice, but there was no evidence of the synergism observed with combination treatment of i.p. tumors in vivo. rIFN-gamma inhibited proliferation of melanoma cells in vitro, whereas rIL-2 stimulated proliferation at 1,000 U/ml. Plating efficiency was increased by at least 30% by culture with 100 U or 1,000 U of rIL-2/ml and both concentrations neutralized the inhibitory effect of 0.05 ng/ml of IFN-gamma, but not of 0.5 or 5.0 ng/ml.