IL-21 and IRF4: A Complex Partnership in Immune and Metabolic Regulation

Adipose tissue is comprised of a community of different immune cells that contributes to regulation of energy storage and release by adipocytes. The population of various immune cells is dynamic, undergoing phenotypic and compositional changes in response to physiologic (e.g., fasting vs. feeding) and pathologic (e.g., lean vs. obese) stimuli. Cumulative evidence suggests that a regulatory or anti-inflammatory immune phenotype promotes metabolic homeostasis, whereas a proinflammatory response is associated with metabolic dysregulation (1). Accordingly, lean, healthy white adipose tissue (WAT) is home to a population of alternatively activated macrophages (M2s) as well as immune cells that mediate M2 polarization, such as eosinophils, CD4 T-helper type (Th) 2 cells, and regulatory T cells (Tregs) (1). During obesity, WAT is infiltrated by additional classically activated macrophages (M1s), neutrophils, mast cells, and CD8 T cells that release proinflammatory cytokines, thereby sustaining metabolic inflammation, or meta-inflammation (2). Although the changes in immune repertoires associated with different metabolic states are well characterized, mechanisms underlying the switches in immune phenotypes remain unclear. Due to their roles in skewing immune cell responses, multiple cytokines have been suggested to play a role in the development of meta-inflammation in obesity. The first studies to recognize that obesity is associated with inflammation identified tumor necrosis factor-a (TNFa) as a mediator of WAT insulin resistance. TNFa produced by infiltrating M1s triggers the activation of Jun NH2terminal kinase and inhibitor of kB kinase b causing antagonistic phosphorylation of insulin receptor substrates (3). Other proinflammatory cytokines produced upon inflammasome activation, namely interleukin (IL)-1b and IL-18, induce adipose inflammation and suppress insulin response (4). In contrast, Th2 cytokines, including IL-4 and IL-13 released primarily by resident eosinophils and innate lymphoid cells, activate M2 polarization and maintain WAT homeostasis (5). The anti-inflammatory cytokine IL-10 has also been shown to enhance adipose insulin sensitivity (6). Both M2s and Tregs produce IL-10. The discovery of a unique population of adipose tissue resident Tregs that are enriched in IL-10 has spurred interest into their physiologic function in metabolic regulation (7,8). Notably, obesity greatly suppresses the number of Tregs in WAT. In this issue, Fabrizi et al. (9) demonstrate that mice lacking Il-21 are protected against high-fat diet–induced metabolic dysfunction. Il-21 is produced by Th17 cells, which in turn promotes expansion of Th17 cells and inhibits induction of Tregs (10). Mice lacking Il-21 (Il-21 KO) are resistant to high-fat diet–induced weight gain and display improved glucose and insulin tolerance. Not surprisingly, the Treg and M2 populations are increased and metainflammation is suppressed in WAT of Il-21 KO mice. Several cell types are capable of transducing Il-21 signaling through expression of Il-21 receptor (Il-21R). Fabrizi et al. not only show that immune cells within WAT, but also adipocytes express Il-21R. High-fat diets increase mRNA levels of Il-21 by immune cells and Il21R by adipocytes, suggesting that Il-21 may act on adipocytes in a paracrine manner. Consistent with this model, mice lacking Il-21 have smaller adipocytes accompanied by increased expression of transcriptional regulators of oxidative metabolism (Nrf1 and Erra) and fasting responses (forkhead box class O1 [FoxO1] and interferon regulatory factor 4 [Irf4]). Irf4 has been shown to control WAT lipolysis (11). During fasting, Irf4 expression is induced by FoxO1, allowing for Irf4 to drive transcription of multiple lipolytic genes, including Pnpla2 (adipose triglyceride lipase) and Lipe (hormonesensitive lipase) (11). Adipocyte-specific Irf4 knockout mice gain more weight, have larger adipocytes, and exhibit defective adaptive responses to prolonged fasting and cold exposure, conditions that require functional lipolysis.