Choline metabolism underpins macrophage IL-4 polarization and RELMα up-regulation in helminth infection

Type 2 cytokines like IL-4 are hallmarks of helminth infection and activate macrophages to limit immunopathology and mediate helminth clearance. In addition to cytokines, nutrients and metabolites critically influence macrophage polarization. Choline is an essential nutrient known to support normal macrophage responses to lipopolysaccharide; however, its function in macrophages polarized by type 2 cytokines is unknown. Using murine IL-4-polarized macrophages, targeted lipidomics revealed significantly elevated levels of phosphatidylcholine, with select changes to other choline-containing lipid species. These changes were supported by the coordinated upregulation of choline transport compared to naive macrophages. Pharmacological inhibition of choline metabolism significantly suppressed several mitochondrial transcripts and dramatically inhibited select IL-4-responsive transcripts, most notably, Retnla. We further confirmed that blocking choline metabolism diminished IL-4-induced RELM alpha (encoded by Retnla) protein content and secretion and caused a dramatic reprogramming toward glycolytic metabolism. To better understand the physiological implications of these observations, naive or mice infected with the intestinal helminth Heligmosomoides polygyrus were treated with the choline kinase alpha inhibitor, RSM-932A, to limit choline metabolism in vivo. Pharmacological inhibition of choline metabolism lowered RELM alpha expression across cell-types and tissues and led to the disappearance of peritoneal macrophages and B-1 lymphocytes and an influx of infiltrating monocytes. The impaired macrophage activation was associated with some loss in optimal immunity to H. polygyrus, with increased egg burden. Together, these data demonstrate that choline metabolism is required for macrophage RELM alpha induction, metabolic programming, and peritoneal immune homeostasis, which could have important implications in the context of other models of infection or cancer immunity. Metabolic factors such as diet can profoundly impact the immune response to pathogens. Choline is an essential nutrient that functions as a precursor to membrane phospholipids. Here, we identify a critical role for choline metabolism in driving IL-4 macrophage polarization and shaping the cellular immune response to intestinal helminth infection. IL-4 polarized macrophages had increased phosphatidylcholine biosynthesis and content. Inhibition of choline metabolism dramatically impaired macrophage activation and metabolism in culture, and in vivo following intestinal helminth infection, which was accompanied by reduced expression of secreted protein RELM alpha, and a change in the immune cellular composition. Overall, these studies identify a previously unrecognized role for the essential nutrient choline in shaping the cellular immune landscape under normal conditions and in response to intestinal helminths.

Automated summary

Type 2 cytokines and phosphatidylcholine play important roles in helminth infection by regulating macrophage immune response and metabolism. Inhibition of phosphatidylcholine metabolism can significantly suppress macrophage activation, alter immune cellular composition, and impair immune response to helminth infection. These findings have important implications for understanding the cellular immune landscape under normal conditions and during intestinal helminth infection.