Macrophages are metabolically heterogeneous within the tumor microenvironment

Macrophages are often prominently present in the tumor microenvironment, where distinct macrophage populations can differentially affect tumor progression. Although metabolism influences macrophage function, studies on the metabolic characteristics of ex vivo tumor-associated macrophage (TAM) subsets are rather limited. Using transcriptomic and metabolic analyses, we now reveal that pro-inflammatory major histocompatibility complex (MHC)-II^hi TAMs display a hampered tricarboxylic acid (TCA) cycle, while reparative MHC-II^lo TAMs show higher oxidative and glycolytic metabolism. Although both TAM subsets rapidly exchange lactate in high-lactate conditions, only MHC-II^lo TAMs use lactate as an additional carbon source. Accordingly, lactate supports the oxidative metabolism in MHC-II^lo TAMs, while it decreases the metabolic activity of MHC-II^hi TAMs. Lactate subtly affects the transcriptome of MHC-II^lo TAMs, increases L-arginine metabolism, and enhances the T cell suppressive capacity of these TAMs. Overall, our data uncover the metabolic intricacies of distinct TAM subsets and identify lactate as a carbon source and metabolic and functional regulator of TAMs.