PD-1 Blockade Enhances Radiation Therapy-Induced Abscopal Effect

Abstract

Purpose/Objective(s)The abscopal effect is an immune-mediated phenomenon wherein tumor response is observed in nonirradiated areas outside the radiation therapy (RT) field. The paucity of clinical observation of the abscopal effect implies that this RT-induced anti-tumor immunity could be restrained by immunoregulatory mechanisms. PD-1 and its ligand B7-H1 (PD-L1) regulate the effector phase of immune-mediated tumor regression induced by RT. Therefore, we hypothesized that PD-1 blockade might enhance RT-induced abscopal effect.Materials/MethodsA solitary metastatic melanoma model was developed by injecting B16-ova melanoma cell lines into the left posterior flank (primary site) and right anterior flank (distant site) of C57 WT and PD-1 knockout (KO) mice. A single RT dose (15 Gy) was delivered to the primary site, and tumor growth was measured at both primary and distant sites. Animals were sacrificed when the tumor burden caused symptoms. In a similar experiment, monoclonal PD-1 blocking antibody (5 mg/kg) with or without RT was delivered to WT mice, and survival and tumor growth end points were obtained. Anti-melanoma PD-1+ CD8+ T cell population in the tumors was assessed with flow cytometry after restimulation with ova peptide and CD28 ex vivo.ResultsRT significantly increased survival in PD-1 KO mice as compared with WT mice (28 days vs 20 days; p = 0.043). Two PD-1 KO mice treated with RT had a complete response at both primary and distant sites, and a rechallenge did not result in tumor growth. Mean tumor size at the primary site for PD-1 KO and WT mice on day 20 was 29.6 mm2 and 110 mm2, respectively (p = 0.024). The respective numbers for the distant (nonradiated) tumors for PD-1 KO and WT on day 20 were 88.6 mm2 and 218 mm2 (p = 0.028). Similarly, anti-PD-1 therapy with RT resulted in significantly longer survival than PD-1 blockade therapy alone (29 days vs 19 days; p = 0.021). The mean tumor size on day 19 at the primary site was 103 mm2 (anti-PD-1) and 26.2 mm2 (anti-PD-1 + RT) (p = 0.043). Secondary tumor growth was 197.8 mm2 and 88.4 mm2 for PD-1 therapy alone or with RT (p = 0.037). RT alone to the primary tumor had no suppressive effect on the distant tumor growth. Flow analysis confirmed the presence of anti-melanoma PD-1+ CD8+ effector T cells in both primary and distant tumor sites.ConclusionsIn our preclinical models, the addition of RT to PD-1 blockade (PD-1 KO and anti-PD-1 antibody) suppressed tumor growth at both radiated and non-radiated sites, and successfully recapitulated the clinical abscopal effect of RT. Two mice developed anti-melanoma immunity and did not exhibit melanoma growth upon rechallenge. These results provide a rationale for testing RT and PD-1 blockade combination therapy in a clinical setting. Purpose/Objective(s)The abscopal effect is an immune-mediated phenomenon wherein tumor response is observed in nonirradiated areas outside the radiation therapy (RT) field. The paucity of clinical observation of the abscopal effect implies that this RT-induced anti-tumor immunity could be restrained by immunoregulatory mechanisms. PD-1 and its ligand B7-H1 (PD-L1) regulate the effector phase of immune-mediated tumor regression induced by RT. Therefore, we hypothesized that PD-1 blockade might enhance RT-induced abscopal effect. The abscopal effect is an immune-mediated phenomenon wherein tumor response is observed in nonirradiated areas outside the radiation therapy (RT) field. The paucity of clinical observation of the abscopal effect implies that this RT-induced anti-tumor immunity could be restrained by immunoregulatory mechanisms. PD-1 and its ligand B7-H1 (PD-L1) regulate the effector phase of immune-mediated tumor regression induced by RT. Therefore, we hypothesized that PD-1 blockade might enhance RT-induced abscopal effect. Materials/MethodsA solitary metastatic melanoma model was developed by injecting B16-ova melanoma cell lines into the left posterior flank (primary site) and right anterior flank (distant site) of C57 WT and PD-1 knockout (KO) mice. A single RT dose (15 Gy) was delivered to the primary site, and tumor growth was measured at both primary and distant sites. Animals were sacrificed when the tumor burden caused symptoms. In a similar experiment, monoclonal PD-1 blocking antibody (5 mg/kg) with or without RT was delivered to WT mice, and survival and tumor growth end points were obtained. Anti-melanoma PD-1+ CD8+ T cell population in the tumors was assessed with flow cytometry after restimulation with ova peptide and CD28 ex vivo. A solitary metastatic melanoma model was developed by injecting B16-ova melanoma cell lines into the left posterior flank (primary site) and right anterior flank (distant site) of C57 WT and PD-1 knockout (KO) mice. A single RT dose (15 Gy) was delivered to the primary site, and tumor growth was measured at both primary and distant sites. Animals were sacrificed when the tumor burden caused symptoms. In a similar experiment, monoclonal PD-1 blocking antibody (5 mg/kg) with or without RT was delivered to WT mice, and survival and tumor growth end points were obtained. Anti-melanoma PD-1+ CD8+ T cell population in the tumors was assessed with flow cytometry after restimulation with ova peptide and CD28 ex vivo. ResultsRT significantly increased survival in PD-1 KO mice as compared with WT mice (28 days vs 20 days; p = 0.043). Two PD-1 KO mice treated with RT had a complete response at both primary and distant sites, and a rechallenge did not result in tumor growth. Mean tumor size at the primary site for PD-1 KO and WT mice on day 20 was 29.6 mm2 and 110 mm2, respectively (p = 0.024). The respective numbers for the distant (nonradiated) tumors for PD-1 KO and WT on day 20 were 88.6 mm2 and 218 mm2 (p = 0.028). Similarly, anti-PD-1 therapy with RT resulted in significantly longer survival than PD-1 blockade therapy alone (29 days vs 19 days; p = 0.021). The mean tumor size on day 19 at the primary site was 103 mm2 (anti-PD-1) and 26.2 mm2 (anti-PD-1 + RT) (p = 0.043). Secondary tumor growth was 197.8 mm2 and 88.4 mm2 for PD-1 therapy alone or with RT (p = 0.037). RT alone to the primary tumor had no suppressive effect on the distant tumor growth. Flow analysis confirmed the presence of anti-melanoma PD-1+ CD8+ effector T cells in both primary and distant tumor sites. RT significantly increased survival in PD-1 KO mice as compared with WT mice (28 days vs 20 days; p = 0.043). Two PD-1 KO mice treated with RT had a complete response at both primary and distant sites, and a rechallenge did not result in tumor growth. Mean tumor size at the primary site for PD-1 KO and WT mice on day 20 was 29.6 mm2 and 110 mm2, respectively (p = 0.024). The respective numbers for the distant (nonradiated) tumors for PD-1 KO and WT on day 20 were 88.6 mm2 and 218 mm2 (p = 0.028). Similarly, anti-PD-1 therapy with RT resulted in significantly longer survival than PD-1 blockade therapy alone (29 days vs 19 days; p = 0.021). The mean tumor size on day 19 at the primary site was 103 mm2 (anti-PD-1) and 26.2 mm2 (anti-PD-1 + RT) (p = 0.043). Secondary tumor growth was 197.8 mm2 and 88.4 mm2 for PD-1 therapy alone or with RT (p = 0.037). RT alone to the primary tumor had no suppressive effect on the distant tumor growth. Flow analysis confirmed the presence of anti-melanoma PD-1+ CD8+ effector T cells in both primary and distant tumor sites. ConclusionsIn our preclinical models, the addition of RT to PD-1 blockade (PD-1 KO and anti-PD-1 antibody) suppressed tumor growth at both radiated and non-radiated sites, and successfully recapitulated the clinical abscopal effect of RT. Two mice developed anti-melanoma immunity and did not exhibit melanoma growth upon rechallenge. These results provide a rationale for testing RT and PD-1 blockade combination therapy in a clinical setting. In our preclinical models, the addition of RT to PD-1 blockade (PD-1 KO and anti-PD-1 antibody) suppressed tumor growth at both radiated and non-radiated sites, and successfully recapitulated the clinical abscopal effect of RT. Two mice developed anti-melanoma immunity and did not exhibit melanoma growth upon rechallenge. These results provide a rationale for testing RT and PD-1 blockade combination therapy in a clinical setting.