Macrophages are metabolically heterogenous 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 metabolomic analyses, we now reveal that pro-inflammatory MHC-IIhi TAMs display a hampered TCA cycle, while reparative MHC-IIlo TAMs show a higher oxidative and glycolytic metabolism. Although both TAM subsets rapidly exchange lactate in high lactate conditions, only MHC-IIlo TAMs use lactate as an additional carbon source. Accordingly, lactate supports the oxidative metabolism in MHC-IIlo TAMs, while it decreased the metabolic activity of MHC-IIhi TAMs. Lactate subtly affected the transcriptome of MHC-IIlo TAMs, increased L-arginine metabolism and enhanced 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. Bulk RNA Seq: MHC-IIhi and MHC-IIlo TAMs derived from 3LL-R tumor-bearing C57Bl/6 mice.Freshly sorted TAM subsets and TAM subsets incubated for 24h in presence or absence of 20mM sodium L-lactate.