(a) C57Bl/6 mice (six to seven per group) were seeded subcutaneously with B16 tumors. After 9 days, the mice received an intraperitoneal (IP) injection of PC61 or control immunoglobulin G (IgG). After a further 24 hours, the mice were injected IP with phosphate-buffered saline (PBS) or with recombinant human IL-2 (75,000 U/injection three times a day for 3 days). On the fourth day, a single further injection of IL-2 was given. Two hours after this last injection of IL-2/PBS, the mice received the first of two intravenous (IV) injections of 108 plaque forming units (pfu) of VSV-GFP, followed 24 hours later by the second IV virus injection. (b) The survival of the mice treated as shown is plotted against the days elapsed after tumor seeding. (c) The lungs taken from the mice that had received the treatments described in a or b were examined 7 days after the final injection. Hematoxylin and eosin–stained sections, examined by two independent pathologists, showed no abnormalities in any group. Representative sections of lungs from mice treated with PBS or PC61/IL-2 are shown. (d) The experiment described in b was repeated in two groups of mice, both of which received PC61/IL-2 and VSV. One group also received an injection of the natural killer (NK) cell-depleting asialo GM-1 antibody 24 hours before treatment with PC61 (NK depleted). The survival of the mice treated as shown is plotted against the days elapsed after tumor seeding. GFP, green fluorescent protein; VSV, vesicular stomatitis virus.