Enhanced Anticancer Immunity Relieves Hypoxia in Tumors Treated with a Combination of Anti-PD-1 and Radiation Therapies

A combination of anti-programmed death-1 (anti-PD-1) immunotherapy and radiation therapy (RT) has shown good clinical outcomes. Previously, we had reported that enhanced anticancer immune response can relieve the hypoxia in tumors after treatment with anti-PD-1 agents. In this study, we investigated the relationship between tumor hypoxia and anticancer immunity in the combination therapy. An anti-PD-1 antibody was administered to mice with CT26 tumor on days 0 and 3. Tumors were irradiated with X-rays on days 0, 1, 2, and 3. Hypoxia in tumors was non-invasively examined with a hypoxia positron emission tomography (PET) probe, [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO). The percentages of immune and hypoxic cells were analyzed using flow cytometry with pimonidazole. One day after four cycles of irradiation (day 4), no change was observed in the numbers of CD8⁺ T cells and hypoxic cells within the tumor in the combination therapy group. On day 10, a later timepoint, the numbers of CD8⁺ T cells was increased. The number of hypoxic cells was decreased, but [¹⁸F]FMISO accumulation in the tumors was unchanged. Previous studies had shown that CD8⁺ T cells remodel tumor vasculature in anti-PD-1 therapy, leading to the reduction in hypoxia. Our results also suggest that the recruited CD8⁺ T cells reduced hypoxia in tumors irradiated with X-rays, although [¹⁸F]FMISO-PET did not indicate these changes. In conclusion, the combination therapy with anti-PD-1 antibody and RT reduced the hypoxic levels in tumors in association with the increased infiltration of CD8⁺ T cells.