Glutamine is required for M1-like polarization in response to Mycobacterium tuberculosis infection
Authors: Jiang Qingkui , Qiu Yunping , Kurland Irwin J. , Drlica Karl , Subbian Selvakumar , Tyagi Sanjay , Shi Lanbo 
Published: January 2022

In response to Mycobacterium tuberculosis infection, macrophages mount early proinflammatory and antimicrobial responses similar to those observed in M1 macrophages classically activated by LPS and IFN-γ. A metabolic reprogramming to HIF-1-mediated uptake of glucose and its metabolism by glycolysis is required for M1-like polarization, but little is known about other metabolic programs driving M1-like polarization during M. tuberculosis infection. Identification and quantification of labeling patterns of U13C glutamine and U13C glucose-derived metabolites demonstrated that glutamine, rather than glucose, is catabolized in both the oxidative and reductive TCA cycle of M1-like macrophages, thereby generating signaling molecules that include succinate, biosynthetic precursors such as aspartate, and the antimicrobial metabolite itaconate. This conclusion is corroborated by diminished M1 polarization via chemical inhibition of glutaminase (GLS), the key enzyme of the glutaminolysis pathway, and by genetic deletion of GLS in infected macrophages. Furthermore, characterization of the labeling distribution pattern of U15N glutamine in M1-like macrophages indicates that glutamine serves as a nitrogen source for the synthesis of intermediates of purine and pyrimidine metabolism plus amino acids including aspartate. Thus, the catabolism of glutamine, as an integral component of metabolic reprogramming in activating macrophages, fulfills the cellular demand for bioenergetic and biosynthetic precursors of M1-like macrophages. Knowledge of these new immunometabolic features of M1-like macrophages is expected to advance the development of host-directed therapies that will enhance bacterial clearance and prevent immunopathology during tuberculosis.

Recent advances in immunometabolism have stimulated increasing interest in understanding the specific cellular metabolic states of immune cells associated with the various disease states of tuberculosis. As the primary target of Mycobacterium tuberculosis (Mtb) infection, macrophages play essential roles in dictating the progression and final outcome of infection. Previous studies, including our own, show that the upregulation of hypoxia-inducible-factor 1 alpha (HIF-1α) and a metabolic reprogramming to the Warburg effect-like state are general features of the host immune cell response to Mtb infection. They are also critical for macrophage polarization to the M1-like phenotype characterized by high-level expression of proinflammatory and antimicrobial molecules against Mtb infection. However, our knowledge about the immunometabolic features of M1-like macrophages is poor. Using widely targeted small metabolite (WTSM) screening (600+ small polar metabolites) and stable isotope tracing of U13 glutamine, U13 glucose, and N15 glutamine, as well as therapeutic and genetic approaches, we report that, in addition to elevated glucose catabolism by glycolysis, glutamine serves as an important carbon and nitrogen source for the generation of building blocks, signaling molecules, and antimicrobial metabolite during macrophage polarization to the M1-like phenotype. The study adds novel insights into the immunometabolic properties of Mtb-infected macrophages.