Hypofractionated Radiation Therapy Induces an Immune-Mediated Abscopal Effect When Combined with Anti-PD-1 Antibody

Abstract

Anti tumor check-point immunotherapy could be synergized with local RT.However, various fraction regimens may lead to different immune response.In this study, we tested the immune response under different RT fraction regimens and chose the best fraction regimen to combine with anti-PD-1 immunotherapy. 4T1 murine breast tumor cells were implanted on left and right thigh of BALB/C mice as primary and secondary tumor. Five different fraction SBRT(24Gy×1,8Gy×3,8Gy×1-2Gy×8 or 2Gy×12f) were delivered on primary tumor.Immune response was detected in bilateral tumors, bilateral regional drainage lymph nodes, and spleen, including CD4+T cells, CD8+T cells, CD8+IFN-γ+T cells, and myeloid-derived suppressor cell (MDSC). Then, the best fraction regimen RT was chosen to combine with human anti-PD-1 mAb(HR301210),on humanPD-1 gene knocked-in mice. Relative local and systematic immune response was also detected. No obvious abscopal effect was observed under different fraction RT alone. Though each of the RT regimens achieved growth delay of the irradiated tumor but not the contralateral tumor, we found 24Gy×1 and 8Gy×3 fraction regimens delayed the primary tumor growth more evident than 8Gy×1-2Gy×8 and 2Gy×12f fraction regimens(P＜0.05 and P＜0.05). Hypofractionated radiotherapy(HRT) regimens including 24Gy×1 and 8Gy×3 achieved significantly higher CD8+ tumor infiltrating lymphocytes (TILs)(P＜0.001 and P＜0.05) in bilateral tumors, higher CD4+TILs (P＜0.001 and P＜0.001) in regional drainage lymph nodes of the primary tumor,higher CD4+T cells(P>0.05 and P＜0.05) and CD8 + IFN-γ + T cells (P>0.05 and P>0.05) in the spleen, and lower MDSC (P＜0.001and P＜0.001) in the spleenon 5 days after RT. However, the tendency of immune response stimulation at 48 hours RT was not as obvious as 5 days after RT. We only found CD4+T cells in regional drainage lymph nodes of the primary tumor were evidently higher of all RT groups than control group. In the next study, on day 20 after MC38 colorectal cancer cells injection, we observed primary tumor regression（P＜0.001) and remarkable contralateral tumor regression（P＜0.001)outside the radiation field in 8Gy×3 combined with PD-1-blocking mAb group, the so-called abscopal effect, whichnot occurred in other three groups.So far, we observed that different treatment regimens lead to different immune response. In systemic immune response, compared with single RT or single IT, RT combined with IT increased CD4+T cells（P＜0.05) and CD8+T cells（P＜0.05), decreased MDSC（P＜0.05) in spleen.In local immune response, the tendancy was not as obvious as systemic immune response.More detailed results from ongoing works remain to be further analyzed. Antitumor immune response could be activated by HRTbetter than CRT, which were mostly mediated by CD4+, CD8+T cells and MDSCs.Evident abscopal effect was found after combining SBRT withanti-PD-1 mAb.