Treg Cells Survive and Thrive in Inhospitable Environments
Authors: Katarzyna M. Grzes, Cameron S. Field, Edward J. Pearce
Journal: Cell Metabolism (2017)

Immune responses are dangerous by nature and require regulation to prevent inflammatory and/or autoimmune sequelae and allow healing. CD4+Foxp3+ T cells (Treg cells) play a crucial role in this process, and in this edition of Cell Metabolism, Angelin et al., 2017Angelin A. Gil-de-Gómez L. Dahiya S. Jiao J. Guo L. Levine M.H. Wang Z. Quinn 3rd, W.J. Kopinski P.K. Wang L. et al.Cell Metab. 2017; 25 (this issue): 1282-1293Abstract Full Text Full Text PDF PubMed Scopus (539) Google Scholar describe how these cells are metabolically adapted to the job. Immune responses are dangerous by nature and require regulation to prevent inflammatory and/or autoimmune sequelae and allow healing. CD4+Foxp3+ T cells (Treg cells) play a crucial role in this process, and in this edition of Cell Metabolism, Angelin et al., 2017Angelin A. Gil-de-Gómez L. Dahiya S. Jiao J. Guo L. Levine M.H. Wang Z. Quinn 3rd, W.J. Kopinski P.K. Wang L. et al.Cell Metab. 2017; 25 (this issue): 1282-1293Abstract Full Text Full Text PDF PubMed Scopus (539) Google Scholar describe how these cells are metabolically adapted to the job. Activated immune cells display fate-associated metabolic differences that are linked to function. Relevant to this discussion, effector CD4+ T cells (Teff cells), which are at the sharp end of the immune response, primarily utilize aerobic glycolysis, whereas Treg cells, which are critical for immune homeostasis because they suppress immune responses, primarily use oxidative phosphorylation (OXPHOS). Broadly speaking, inhibition of the dominant metabolic pathways in these cells inhibits their specific functions. As immune responses and associated inflammation progress, and/or as tumors grow or infections take hold, extracellular tissue conditions can change such that glucose is depleted and lactate accumulates, a direct consequence of increased aerobic glycolysis at the site. These conditions themselves tend to inhibit Teff function (Scharping et al., 2016Scharping N.E. Menk A.V. Moreci R.S. Whetstone R.D. Dadey R.E. Watkins S.C. Ferris R.L. Delgoffe G.M. Immunity. 2016; 45: 374-388Abstract Full Text Full Text PDF PubMed Scopus (371) Google Scholar), but what about Treg cells? In this issue of Cell Metabolism, Angelin et al. propose that the transcription factor Foxp3 reprograms the metabolic state of CD4+ T cells, allowing them to maintain their suppressive functions in environments with low-glucose and high-lactate concentrations (Angelin et al., 2017Angelin A. Gil-de-Gómez L. Dahiya S. Jiao J. Guo L. Levine M.H. Wang Z. Quinn 3rd, W.J. Kopinski P.K. Wang L. et al.Cell Metab. 2017; 25 (this issue): 1282-1293Abstract Full Text Full Text PDF PubMed Scopus (539) Google Scholar). This report is one of three recent papers to show that Foxp3 plays a major role in regulating the expression of metabolic pathway genes in Treg cells (Gerriets et al., 2016Gerriets V.A. Kishton R.J. Johnson M.O. Cohen S. Siska P.J. Nichols A.G. Warmoes M.O. de Cubas A.A. MacIver N.J. Locasale J.W. et al.Nat. Immunol. 2016; 17: 1459-1466Crossref PubMed Scopus (304) Google Scholar, Howie et al., 2017Howie D. Cobbold S.P. Adams E. Ten Bokum A. Necula A.S. Zhang W. Huang H. Roberts D.J. Thomas B. Hester S.S. et al.JCI Insight. 2017; 2: e89160Crossref PubMed Scopus (107) Google Scholar). Treg cells proliferate in inflammatory conditions, a process supported by glycolysis (Gerriets et al., 2016Gerriets V.A. Kishton R.J. Johnson M.O. Cohen S. Siska P.J. Nichols A.G. Warmoes M.O. de Cubas A.A. MacIver N.J. Locasale J.W. et al.Nat. Immunol. 2016; 17: 1459-1466Crossref PubMed Scopus (304) Google Scholar). However, during this proliferative phase Foxp3 expression is suppressed and Treg cells exhibit impaired regulatory function (Gerriets et al., 2016Gerriets V.A. Kishton R.J. Johnson M.O. Cohen S. Siska P.J. Nichols A.G. Warmoes M.O. de Cubas A.A. MacIver N.J. Locasale J.W. et al.Nat. Immunol. 2016; 17: 1459-1466Crossref PubMed Scopus (304) Google Scholar). Nevertheless, increased expression of Foxp3 leads to the inhibition of aerobic glycolysis and redirects cellular metabolism toward OXPHOS, ultimately restoring suppressive function (Gerriets et al., 2016Gerriets V.A. Kishton R.J. Johnson M.O. Cohen S. Siska P.J. Nichols A.G. Warmoes M.O. de Cubas A.A. MacIver N.J. Locasale J.W. et al.Nat. Immunol. 2016; 17: 1459-1466Crossref PubMed Scopus (304) Google Scholar). Here, Angelin et al. show that the ability of Treg cells to suppress the proliferation of Teff cells relies particularly on the intact function of complex I of the electron transport chain (ETC). In this context, they find that Treg cells deficient in Nd6, a component of Complex I, lose suppressive activity. In low glucose, Treg cells maintain redox balance by oxidizing NADH to NAD+ through the coupled action of the tricarboxylic acid (TCA) cycle and the ETC (a process that is supported by their ability to oxidize fatty acids to support OXPHOS (Howie et al., 2017Howie D. Cobbold S.P. Adams E. Ten Bokum A. Necula A.S. Zhang W. Huang H. Roberts D.J. Thomas B. Hester S.S. et al.JCI Insight. 2017; 2: e89160Crossref PubMed Scopus (107) Google Scholar, Michalek et al., 2011Michalek R.D. Gerriets V.A. Jacobs S.R. Macintyre A.N. MacIver N.J. Mason E.F. Sullivan S.A. Nichols A.G. Rathmell J.C. J. Immunol. 2011; 186: 3299-3303Crossref PubMed Scopus (1312) Google Scholar). In contrast, Teff cells, which rely on aerobic glycolysis to regenerate NAD+, face a redox imbalance in low glucose. This is critical for these cells since, among other things, GAPDH, a central regulator of glycolysis, is NAD+ dependent. In the absence of sufficient NAD+, glycolysis cannot proceed. The effects of such an event on Teff function are illustrated in the paper by the use of heptelidic acid, an inhibitor of GAPDH, which inhibits Teff proliferation but has no effect on Treg cell suppressive function. Thus the work by Angelin et al. adds weight to the consensus that OXPHOS is critical for Treg function and illuminates the process by adding details regarding the relative roles of different ETC components and of OXPHOS versus glycolysis in maintaining redox balance under glucose-restricted conditions. How then does Foxp3 expression alter cellular metabolism and promote OXPHOS? Angelin et al. reveal that Foxp3 represses enactment of the c-Myc-dependent transcriptional response that underpins Teff cell development. Following activation in Teff cells, c-Myc expression is induced, allowing metabolic reprogramming for aerobic glycolysis and glutaminolysis (Preston et al., 2015Preston G.C. Sinclair L.V. Kaskar A. Hukelmann J.L. Navarro M.N. Ferrero I. MacDonald H.R. Cowling V.H. Cantrell D.A. EMBO J. 2015; 34: 2008-2024Crossref PubMed Scopus (96) Google Scholar, Wang et al., 2011Wang R. Dillon C.P. Shi L.Z. Milasta S. Carter R. Finkelstein D. McCormick L.L. Fitzgerald P. Chi H. Munger J. Green D.R. Immunity. 2011; 35: 871-882Abstract Full Text Full Text PDF PubMed Scopus (1316) Google Scholar). The authors show that Foxp3 effectively suppresses c-Myc expression through a process that correlates with its binding to the Myc TATA box. The study also hints at the fact that Foxp3 acetylation, a process that the authors have shown in previous reports promotes OXPHOS and Treg function, favors Foxp3 association with the Myc promoter, although this issue is left somewhat unresolved. These results begin to provide an explanation for how glycolysis is regulated in Treg cells and are in agreement with earlier studies showing that expression of the glucose transporter GLUT1 is downregulated by Foxp3 (Gerriets et al., 2016Gerriets V.A. Kishton R.J. Johnson M.O. Cohen S. Siska P.J. Nichols A.G. Warmoes M.O. de Cubas A.A. MacIver N.J. Locasale J.W. et al.Nat. Immunol. 2016; 17: 1459-1466Crossref PubMed Scopus (304) Google Scholar, Michalek et al., 2011Michalek R.D. Gerriets V.A. Jacobs S.R. Macintyre A.N. MacIver N.J. Mason E.F. Sullivan S.A. Nichols A.G. Rathmell J.C. J. Immunol. 2011; 186: 3299-3303Crossref PubMed Scopus (1312) Google Scholar). Teff function is reduced within the tumor microenvironment, which often has higher lactate concentrations compared to lymph nodes due to the adoption of aerobic glycolysis by tumor cells (Scharping et al., 2016Scharping N.E. Menk A.V. Moreci R.S. Whetstone R.D. Dadey R.E. Watkins S.C. Ferris R.L. Delgoffe G.M. Immunity. 2016; 45: 374-388Abstract Full Text Full Text PDF PubMed Scopus (371) Google Scholar). Angelin et al. confirm that increased lactate concentrations are able to suppress Teff proliferation and show that they have no effect on Treg cell proliferation or function under conditions where glucose is limiting. In these conditions, both Treg and Teff cells oxidize lactate to pyruvate at similar rates. However, Treg cells sustain their proliferation, development, and importantly their suppressive functions under these high-lactate concentrations in vitro and in vivo. Further studies revealed that lactate dehydrogenase (LDH), an enzyme that reversibly catalyzes the conversion of pyruvate to lactate, plays a major role in suppressing Teff but not Treg cell functions. This is the result of LDH favoring the reduction of NAD+ to NADH under low-glucose, high-lactate conditions, rather than the opposite under high-glucose, low-lactate conditions. In Treg cells this change in function related to redox balance has less of an effect because NAD+ is being generated by strong NADH oxidation in mitochondria (Figure 1). The idea that OXPHOS allows Treg maintenance and function in tissues with limited glucose concentrations or high lactate concentrations is reinforced by other studies where Treg cells have been shown to play a crucial role in the repair of renal ischemia-reperfusion injury (Gandolfo et al., 2009Gandolfo M.T. Jang H.R. Bagnasco S.M. Ko G.J. Agreda P. Satpute S.R. Crow M.T. King L.S. Rabb H. Kidney Int. 2009; 76: 717-729Abstract Full Text Full Text PDF PubMed Scopus (222) Google Scholar) or muscle damage (Burzyn et al., 2013Burzyn D. Kuswanto W. Kolodin D. Shadrach J.L. Cerletti M. Jang Y. Sefik E. Tan T.G. Wagers A.J. Benoist C. Mathis D. Cell. 2013; 155: 1282-1295Abstract Full Text Full Text PDF PubMed Scopus (738) Google Scholar). Indeed, these findings may help explain why CD8+ effector T cell dysfunction can be rescued in the tumor microenvironment by overexpression of PGC-1α within effector T cells and enforcement of OXPHOS (Scharping et al., 2016Scharping N.E. Menk A.V. Moreci R.S. Whetstone R.D. Dadey R.E. Watkins S.C. Ferris R.L. Delgoffe G.M. Immunity. 2016; 45: 374-388Abstract Full Text Full Text PDF PubMed Scopus (371) Google Scholar). The findings from Angelin et al. provide an evolved view of how Treg cells are able to function within environmental conditions that negatively affect Teff metabolism. As befits an interesting new study, the work by Angelin et al. raises important questions that remain to be answered. For example, the paper leaves open the issue of how binding of Foxp3 to the TATA box of Myc suppresses transcription of this gene. Moreover, it raises interesting questions about the relative expression of c-Myc versus Foxp3 during Treg cell development. For example, is c-Myc expressed early and then suppressed by Foxp3, and is this process dynamic to allow the type of reciprocal cycling between glycolysis and OXPHOS that supports proliferative expansion followed by maximal suppressive function within Treg cell populations? Finally, how do the findings in mouse Treg cells described in the paper translate to human Treg cells, where glycolysis has been shown to be important for suppressive function (De Rosa et al., 2015De Rosa V. Galgani M. Porcellini A. Colamatteo A. Santopaolo M. Zuchegna C. Romano A. De Simone S. Procaccini C. La Rocca C. et al.Nat. Immunol. 2015; 16: 1174-1184Crossref PubMed Scopus (247) Google Scholar)? Foxp3 Reprograms T Cell Metabolism to Function in Low-Glucose, High-Lactate EnvironmentsAngelin et al.Cell MetabolismApril 13, 2017In BriefAngelin et al. report that the Treg transcription factor Foxp3 controls metabolic adaptations including a downregulation of Myc and glycolysis, an induction of oxidative phosphorylation, and an increased NAD:NADH ratio. These metabolic changes allow Tregs to thrive in low-glucose, high-lactate environments, thus maintaining their immunosuppressive function under metabolically challenging conditions. Full-Text PDF Open Archive