301 - Targeting Trem2+ Macrophages Modulates Response to Radiotherapy

Purpose/Objective(s):

The mechanisms underlying radiotherapy-mediated immunosuppression remain incompletely understood. While TREM2+ macrophages have been reported to play a dual role in the tumor immune microenvironment (TIME), their specific function in radiotherapy-induced immunosuppression is unclear. This study aimed to investigate the mechanistic role of TREM2+ macrophages in establishing an immunosuppressive microenvironment following radiotherapy.

Materials/Methods:

Murine tumor models were established using MC38 and LLC cells. The impact of a single 20 Gy fraction of stereotactic body radiotherapy on the TIME was assessed. Tumor-infiltrating immune cells were analyzed via flow cytometry for subset proportions and cytokine secretion. Single-cell RNA sequencing (scRNA-seq) was employed to delineate transcriptional changes and signaling pathways within immune populations. The functional role of TREM2+ macrophages was validated in vivo using Trem2^fl/fl-Lyz2^creconditional knockout mice and with an anti-TREM2 monoclonal antibody (mAb).

Results:

Radiotherapy (20 Gy) inhibited the growth of the irradiated tumor but failed to control distant, non-irradiated tumors. Flow cytometry revealed a significant increase in tumor-infiltrating macrophages post-radiation, which exhibited upregulated expression of Arg1 and IL-10. These macrophages suppressed the proliferation and effector functions (Granzyme B, IFN-?) of CD8+ T cells. Depletion of macrophages reversed this radiotherapy-induced immunosuppressive phenotype. scRNA-seq identified TREM2+ macrophages as a predominant subset, whose presence correlated positively with immunosuppressive markers (Apoe, Pf4, Ms4a7, Fabp5). In vivo and in vitro assays confirmed that TREM2+ macrophages secreted higher levels of Arg1 and IL-10 after radiation, directly inhibiting CD8+ T cell function. Mechanistically, radiation upregulated the expression of fatty acid-binding protein 5 (FABP5) in TREM2+ macrophages, leading to enhanced fatty acid oxidation (FAO) metabolic activity and driving their polarization toward an immunosuppressive phenotype. The FABP5 inhibitor SBFI-26 significantly suppressed Arg1 and IL-10 expression in TREM2+ macrophages and reversed their immunosuppressive function. In vivo, both genetic ablation of TREM2 in macrophages and treatment with an anti-TREM2 mAb enhanced the cytotoxicity of CD8+ T cells, bolstered anti-tumor immunity, and ultimately improved the efficacy of radiotherapy.

Conclusion:

Our findings demonstrate that TREM2+ macrophages, through FABP5-mediated enhancement of FAO, are pivotal mediators of radiotherapy-induced immunosuppression. Targeting TREM2 represents a promising strategy to overcome radiotherapy resistance and improve therapeutic outcomes.