Metabolic inhibition of glutamate-cysteine ligase increases dendritic cell-mediated anti-tumor immunity in melanoma

Metabolic competition and nutrient restriction in the tumor microenvironment (TME) shape the immune infiltrate in tumors and subsequently tumor immunity. In this study we used the transgenic melanoma mouse model tg(Grm1)EPv, which spontaneously develops melanoma due to the ectopic expression of the metabotropic glutamate receptor 1 (Grm1) in melanocytes to investigate if aberrant glutamate metabolism drives tumor formation, and affects immune cell function. LC-MS/MS-based metabolomic analyses and RNA sequencing revealed changes in glutaminolysis, a glycolytic shift (Warburg effect), and reduced ATP levels in advanced tumors compared with tumor free tissue, suggesting respiratory chain dysfunction. These metabolic changes in the TME are advantageous for the tumor cells and unfavorable for immune cells, such as dendritic cells (DC). Indeed, flow cytometry analysis of myeloid subsets during tumor progression showed a decline in tumor-infiltrating conventional type 2 dendritic cell (cDC2) and macrophages, alongside an increase in neutrophil and monocyte populations in advanced lesions. Interference with glutaminolysis using L-buthionine sulfoximine (BSO), an inhibitor of the glutamylcysteine synthetase depleting cellular glutathione levels, induced immunogenic cell death, namely ferroptosis, in an tg(Grm1)EPv -derived cell line in vitro. Therefore, we evaluated the combination of this inhibitor with immunotherapy as a promising new approach for the treatment of tumors in the tg(Grm1)EPv mouse model. We observed that tumor growth could be delayed in vivo when BSO was combined with a therapy regimen boosting DC numbers and activation. This knowledge can drive the design of novel therapeutic strategies for cancer patients involving potential modification of tumor glutamate metabolism.