Folate metabolism in tumor-associated macrophages drives immunosuppressive function to promote tumor growth

TAMs promote tumor growth, inhibit effector lymphocytes, and induce resistance to immune checkpoint inhibitors (ICI). Therapies designed to deplete TAMs have had little clinical success, and strategies to repolarize immunosuppressive TAMs toward pro-inflammatory states have struggled with systemic toxicity.

The study identifies that the combination of low folate in the tumor microenvironment and TAM dependency on the high-affinity folate receptor beta (FRβ) for folate uptake creates a unique metabolic dependency required for TAM immunosuppressive function. FRβ, encoded by Folr2, is uniquely expressed on myeloid cells and upregulated on TAMs; FRβ+ TAMs exhibit an immunosuppressive phenotype in melanoma and are associated with worse clinical outcomes and resistance to ICI in melanoma patients.

The authors generated a novel Folr2-/- mouse model to study why TAMs express a unique folate receptor. Tumor growth was slowed in the absence of Folr2 in a T-cell-dependent manner, and Folr2-/- TAMs underwent dramatic in vivo repolarization to pro-inflammatory states, resulting in increased cytotoxic T cell and NK cell infiltration into tumors.

Metabolomics performed on melanoma tumors and adjacent normal tissue from warm autopsy patient specimens showed that folate is low in the tumor microenvironment. The authors could increase tumor growth in Folr2-/- mice by raising serum folate to supraphysiologic concentrations with a high folate diet.

Integrated metabolomics and transcriptomic analysis demonstrated that Folr2-/- macrophages have impaired folate uptake and 1C metabolism-based reduction of oxidized glutathione under low folate conditions, resulting in increased mitochondrial reactive oxygen species (ROS) in Folr2-/- cells. Excess ROS causes leakage of mitochondrial DNA into the cytoplasm, activating cGAS-STING signaling and promoting pro-inflammatory macrophage polarization through TBK1 and NF-κB.

Together, the data demonstrate that TAM expression of FRβ promotes immunosuppressive functions by maintaining folate uptake in the low folate tumor microenvironment. The findings suggest FRβ is a novel metabolic checkpoint on TAMs.