Choline metabolism underpins macrophage IL-4 polarization and RELMa 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 naïve 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 RELMa (encoded by Retnla) protein content and secretion and caused a dramatic reprogramming toward glycolytic metabolism. To better understand the physiological implications of these observations, naïve or mice infected with intestinal helminths Heligmosomoides polygyrus or Nippostrongylus brasiliensis were treated with the choline kinase a inhibitor, RSM-932A, to limit choline metabolism in vivo. Pharmacological inhibition of choline metabolism lowered RELMa 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, but there were no differences in intestinal worm count nor differences in N. brasiliensis parasite burden. Together, these data demonstrate that choline metabolism is required for macrophage RELMa induction, metabolic programming, and peritoneal immune homeostasis, which could have important implications in the context of other models of infection or cancer immunity. Overall design: To investigate the requirement for choline metabolism during macrophage polarization, we treated bone marrow-derived macrophages with inhibitors of choline uptake or choline kinase and polarized with IL-4. We used cells grown in DMEM with L929 culture supernatant at day 7 from three female 12-week-old C57BL/6J mice. Samples were sent to Novogene for RNA-seq.