(809) IL-33 suppresses tumor growth through attenuating Treg-derived chemokines and MDSC recruitment

Introduction/Rationale: CD4+ regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) are key mediators of immunosuppressive tumor microenvironment (TME). However, the crosstalk between these cells has been poorly understood. Our previous data show interleukin-33 (IL-33) treatment suppresses tumor growth, accompanied by a reduced percentage of myeloid cells and an increased proportion of Tregs. Here, we report our findings that IL-33 treatment affects Treg chemokine production relevant to MDSCs recruitment.

Methods: We utilized Foxp3Cre-YFP (TregWT) and Foxp3Cre-YFPIl1rl1fl/fl (TregΔIl1rl1) mice on a C57BL/6 background for subcutaneous B16-F10 melanoma models, with IL-33 treatment (1 µg/mouse/day, i.p.). Flow cytometry was used to analyze tumor-infiltrating immune cells. We evaluated the transcriptomic profile of total CD45+ immune cells in the TME using scRNA-seq and validated the key findings at the protein level via immunofluorescence.

Results: IL-33 treatment failed to suppress tumor growth in TregΔIl1rl1 mice, without affecting the frequency of Tregs out of CD45+ leukocytes in the TME. These data indicated ST2 expression in Tregs is indispensable to the anti-tumor effect of systemic IL-33 treatment. Flow cytometry data showed IL-33 therapy significantly reduced the proportions of both monocytic MDSCs (CD11b+Ly6C+Ly6G-F4/80-, M-MDSCs) and polymorphonuclear MDSCs (CD11b+Ly6G+Ly6Clow, PMN-MDSCs). ScRNA-seq analysis revealed that IL-33 treatment downregulated Tregs chemokine expression, including Cxcl2, Cxcl3 and Ccl3, without alternating gene expression relating to immunosuppressive functions such as Foxp3, Ctla4 and Il10. Lastly, immunofluorescence data further confirmed that CXCL2+ cells were significantly increased within the tumors from TregΔIl1rl1 mice, almost all of which were Tregs.

Conclusion: Our findings suggest that IL-33 exerts its anti-tumor effect by dampening Treg CXCL2 expression, thereby potentially reducing MDSC recruitment in the TME.