The anti-tumor immune response of radiotherapy combined with anti-CD200R1 in hepatocellular carcinoma.

2525 Background: Various clinical trials demonstrate the efficacy of regional radiotherapy or chemotherapy in augmenting the therapeutic response to PD1/L1 antibody in hepatocellular carcinoma (HCC). However, a significant proportion of HCC patients still exhibit suboptimal responses to these combination therapies. The immune-suppressive tumor microenvironment (TME) poses a formidable barrier to effective immunotherapy. Recent evidences underscore the pivotal role played by myeloid cells in modulating the immunosuppressive TME following regional treatments. Therefore, targeting the myeloid immunosuppressive checkpoint represents a promising strategy for enhancing the efficacy of current local-immune combination therapy. Methods: Initially, single cell RNA sequencing (scRNA-seq) was conducted on tumor infilitrating immune cells (TIICs) of mouse HCC tissues three days post-radiotherapy. Subsequently, the phenotype of tumor-infiltrating CD200 + macrophages was characterized in mouse HCC tissues using flow cytometry. Next, the specific mechanism underlying CD200 up-regulation in tumor-infiltrating macrophages was investigated using wild-type mice and STING KO mice. Furthermore, the chemotactic and interactive mechanisms between CD200 + macrophages and CD200R + myeloid-derived suppressor cells (MDSCs) were investigated through in vitro and in vivo experiments. Finally, the systemic antitumor effect of combined radiotherapy and anti-CD200R treatment was observed in multiple non-immunogenic HCC mouse models. Results: The infiltration level of CD200 + macrophages in HCC tissues significantly increase following local radiotherapy. The accumulation of CD200 + macrophage subset relies on potent STING activation subsequent to radiotherapy. In vitro and in vivo experiments demonstrate the robust capacity of CD200 + macrophages in enhancing myeloid immunosuppression. Mechanistically, CD200 + macrophages recruit CXCR2 + CD200R + MDSC through secretion of CXCL1/3, thereby amplifying the proliferation and immunosuppressive capacity of this specific MDSC subset via interaction between CD200 and CD200R. Targeted blockade of CD200R improve the inhibitory immune microenvironment by suppressing the immunosuppressive capacity of myeloid suppressor cells. Lastly, combined anti-CD200R1 with radiotherapy sensitized the therapeutic effect of PD-1/L1 antibody in multiple non-immunogenic HCC mouse models. Conclusions: Our studies identify CD200 + macrophages as a crucial immune suppressor that is essential for tumor immune resistance following radiotherapy. These findings unveil a novel mechanism of immune evasion mediated by STING activation, thereby offering a potential target for further optimization of local-immune combination therapy in HCC.