Effect of IL-15 on anti-tumor immune response to radio-immunotherapy through systemic immune activation.

e14641 Background: In this study, we propose to investigate the key role of IL-15 in radio-immunotherapy and its mechanism in regulating systemic immune activation by radio-immunotherapy. And further explore the potential of IL-15 combined with radio-immunotherapy. Methods: LLC5 and MC38 cells were used in this study. Mouse model and co-culture model were constructed to analyze the effect of IL-15 after radio-immunotherapy. Anti-IL-15 antibody was used to clarify the role of IL-15 on radio-immunotherapy. Flow cytometry and multiplex immunofluorescence were used to clarify IL-15Rα lever in macrophage after radio-immunotherapy. The effect of IL-15 in tumor and secondary lymphoid organ(SLO) was observed after macrophage removal. Macrophage IL-15 level were tested by ELISA. Tumor growth, TME, and memory effect after IL-15 combined with radio-immunotherapy were observed. In co-culture model, CD8+ T cell function were tested. Clearance macrophage of mice to determine the role of macrophage in combination therapy. After macrophages co-cultured with tumor cells, cytokine microarray was used to analyze the cytokine secretion of macrophages in combination treatment. Results: IL-15 secretion was significantly upregulated in TME after radio-immunotherapy. IL-15 and IL-15Rα were most enriched on macrophages and IL-15Rα was significantly increased after radio-immunotherapy. IL-15 inhibition significantly attenuated tumor killing and systemic immune activation and resulted in decreased IL-15Rα expression on macrophages. Removal of macrophages resulted in a significant reduction of IL-15 in tumor and spleen. Radio-immunotherapy in the co-culture model induced a significant increase of IL-15 secreted by macrophages.IL-15 combined with radio-immunotherapy can significantly controlled tumor growth, stimulated systemic immune activation, and CD8+ T cells and memory T cells were significantly increased in the circulation of long-term survival mice. In co-culture experiments, combination treatment stimulated proliferation and secretion of IFN-γ+CD8+ T cells. And it stimulated macrophages secrete more chemokines, especially CCL5, recruit more CD8+ T cells aggregate around macrophages. Macrophage clearance and co-culture experiments confirm that the synergistic effect of this combination therapy is mainly dependent on macrophages. Conclusions: IL-15 plays a critical role in systemic immune activation induced by radio-immunotherapy. Macrophages are the main cellular site of IL-15 effect in the microenvironment of tumor and SLO after radio-immunotherapy. IL-15 combined with radio-immunotherapy had a synergistic antitumor effect, and this combination treatment induced a further enhancement of memory T-cell-mediated systemic immune activation. Macrophage-secreted chemokines such as CCL5 played an important role in the antitumor effect of this combination therapy strategy.