STING-dependent preventive effect by ablative hyperfractioned radiotherapy and delayed delivery of a CSF-1R inhibitor.

Abstract

e14190 Background: It has been reported that hyperfractioned radiotherapy can bring out abscopal effects when combined with checkpoint inhibitors and radiation-induced myeloid-derived suppressor cells (MDSCs) infiltration can lead to radioresistance. Our prior work has shown that ablative hypofractionated radiotherapy (AHFRT) alone can not trigger significant abscopal effects in three mouse tumor models. The goal of this study was to figure out whether the AHFRT play a preventive role in different tumor models and can be enhanced by a CSF-1R inhibitor. Methods: In experiment exploring the preventive effect of AHFRT, on Day 0, B16/ CT26/ MC38 tumor cells or PBS were inoculated s.c. in the left-side thigh of mice, respectively. Then, control mice and mice with established tumors were irradiated by AHFRT (12Gy*3f*3days). When the AHFRT accomplished, mice randomly divided into different groups and the second tumors inoculated s.c. in the right-side thigh, respectively: 1 day/ 3 days/ 7 days after AHFRT accomplished. Besides, “1 day after AHFRT accomplished” mice, both established MC38/CT26 tumors and PBS controls, were divided into 4 groups: control, a-CSF-1R, AHFRT, AHFRT+a-CSF-1R. And MC38 tumor cells were irradiated. The mice and cells followed for tumor growth and further analyses (mainly flow cytometry, western-blotting and qPCR). Results: We found that AHFRT alone could trigger significant preventive effects in” 7 days after AHFRT accomplished” group in B16 tumor model and “ 1 day/ 3 days/ 7 days after AHFRT accomplished” groups in CT26/ MC38 tumor models. In CT26/ MC38 tumor models, all the second tumors were cured which inoculated in “7 days after AHFRT accomplished” groups because of the preventive effects. And these cured mice keep survival without tumors when rechallenged at 180 days after AHFRT accomplished. AHFRT induced CD11b+CD11c+MHC-II+ antigen presenting cells at the 1st day after AHFRT accomplished and effector CD8+ T cells at 3rd day after AHFRT accomplished. AHFRT could active cGAS, STING, CXCL10 and IRF3 in the irradiated MC38 cells. And with a STING inhibitor, the growth of second tumors lost the preventive effect from AHFRT. And a delayed delivery of a small-molecule CSF-1R inhibitor decreased monocyte-MDSC(CD11b+CD11c-Gr1-) and delay the growth of second tumors inoculated 1 day after AHFRT accomplished. Conclusions: We show thatAHFRT triggers in situ vaccination and subsequent prevention effect. The prevention effect is STING-dependent and can be enhanced by a delayed delivery of CSF-1R inhibitor.