Targeting glutamine metabolism enhances tumor specific immunity by inhibiting the generation of MDSCs and reprogramming tumor associated macrophages

Myeloid cells comprise a major component of the Tumor Microenvironment (TME) promoting tumor growth and immune evasion. By employing a novel small molecule inhibitor of glutamine metabolism not only were we able to inhibit tumor growth but we markedly inhibited the generation and recruitment of Myeloid Derived Suppressor Cells (MDSC). Targeting tumor glutamine metabolism led to a decrease in CSF-3 and hence recruitment of MDSC as well immunogenic cell death leading to an increase in inflammatory Tumor Associated Macrophages (TAMs). Alternatively, inhibiting glutamine metabolism of the MDSC themselves led to activation induced cell death and conversion of MDSC to inflammatory macrophages. Surprisingly, blocking glutamine metabolism also inhibited IDO expression of both the tumor and myeloid derived cells leading to a marked decrease in kynurenine levels. This in turn inhibited the development of metastasis and further enhanced anti-tumor immunity. Indeed, targeting glutamine metabolism rendered checkpoint blockade-resistant tumors susceptible to immunotherapy. Overall, our studies define an intimate interplay between the unique metabolism of tumors and the metabolism of suppressive immune cells.