In recent years a substantial number of findings have been made in the area of immunometabolism, by which we mean the changes in intracellular metabolic pathways in immune cells that alter their function. Here, we provide a brief refresher course on six of the major metabolic pathways involved (specifically, glycolysis, the tricarboxylic acid (TCA) cycle, the pentose phosphate pathway, fatty acid oxidation, fatty acid synthesis and amino acid metabolism), giving specific examples of how precise changes in the metabolites of these pathways shape the immune cell response. What is emerging is a complex interplay between metabolic reprogramming and immunity, which is providing an extra dimension to our understanding of the immune system in health and disease.

Macrophages (MFs) are heterogeneous and metabolically flexible, with metabolism strongly affecting immune activation. A classic response to proinflammatory activation is increased flux through glycolysis with a downregulation of oxidative metabolism, whereas alternative activation is primarily oxidative, which begs the question of whether targeting glucose metabolism is a viable approach to control MF activation. We created a murine model of myeloid-specific glucose transporter GLUT1 (Slc2a1) deletion. Bone marrow-derived MFs (BMDM) from Slc2a1 M2/2 mice failed to uptake glucose and demonstrated reduced glycolysis and pentose phosphate pathway activity. Activated BMDMs displayed elevated metabolism of oleate and glutamine, yet maximal respiratory capacity was blunted in MF lacking GLUT1, demonstrating an incomplete metabolic reprogramming. Slc2a1 M2/2 BMDMs displayed a mixed inflammatory phenotype with reductions of the classically activated pro-and anti-inflammatory markers, yet less oxidative stress. Slc2a1 M2/2 BMDMs had reduced proinflammatory metabolites, whereas metabolites indicative of alternative activation-such as ornithine and polyamines-were greatly elevated in the absence of GLUT1. Adipose tissue MFs of lean Slc2a1 M2/2 mice had increased alternative M2-like activation marker mannose receptor CD206, yet lack of GLUT1 was not a critical mediator in the development of obesity-associated metabolic dysregulation. However, Ldlr 2/2 mice lacking myeloid GLUT1 developed unstable atherosclerotic lesions. Defective phagocytic capacity in Slc2a1 M2/2 BMDMs may have contributed to unstable atheroma formation. Together, our findings suggest that although lack of GLUT1 blunted glycolysis and the pentose phosphate pathway, MF were metabolically flexible enough that inflammatory cytokine release was not dramatically regulated, yet phagocytic defects hindered MF function in chronic diseases.

SUMMARY The hypoxic tumor microenvironment serves as a niche for maintaining the glioma-initiating cells (GICs) that are critical for glioblastoma (GBM) occurrence and recurrence. Here we report that hypoxia-induced miR-215 is vital for reprograming GICs to fit the hypoxic microenvironment via suppressing the expression of an epigenetic regulator KDM1B and modulating activities of multiple pathways. Interestingly, biogenesis of miR-215 and several miRNAs is accelerated post-transcriptionally by hypoxia-inducible factors (HIFs) through HIF-Drosha interaction. Moreover, miR-215 expression correlates inversely with KDM1B while positively with HIF1α and GBM progression in patients. These findings reveal a direct role of HIF in regulating miRNA biogenesis and consequently activating the miR-215-KDM1B-mediated signaling required for GIC adaptation to hypoxia.

Upon stimulation, CD4+ T cells differentiate into effector (Teff) or regulatory (Treg) T cells, the balance of which is crucial to prevent inflammation or autoimmunity. One approach to treat autoimmune disease is to modulate T cell differentiation to favor Treg over Teff cells. We examined the metabolite and metabolic gene expression profiles of the subsets and found that they have distinct metabolic requirements and programs. Teff have a high expression of the glucose transporter GLUT1 as well as glucose metabolism genes and a high rate of glycolysis. In contrast, Treg instead have high expression of mitochondrial metabolism genes and low GLUT1 levels. The effect of metabolic differences to modulate the Teff:Treg balance can be tested directly via genetic modulation of GLUT1. In vitro, T cells from GLUT1 transgenic mice are more inflammatory than wildtype littermates. Alternatively, T cells from GLUT1 knockout mice are unable to increase glucose metabolism upon activation and have fewer Th1, Th2 and Th17 cells. Despite less total CD4+ T cells, these mice have normal natural and inducible Treg numbers. In a mouse model of inflammatory bowel disease (IBD), deletion of GLUT1 in T cells inhibits IBD disease progression. The mice have less CD4+ T cells in the mesenteric lymph nodes and decreased inflammatory cytokine production than wildtype control T cells. Thus, the modulation of glucose metabolism may provide an alternative treatment for inflammatory and autoimmune disease.