Where Mitochondria Meet Autoimmunity: The Treg Cell Link

Although a crucial role for mitochondrial metabolism in controlling T regulatory (Treg) cell function has been recognized, its contribution during autoimmunity has not yet been fully elucidated. In this issue of Cell Metabolism, Alissafi and colleagues demonstrate that during autoimmunity, Treg cell functional alterations associate with mitochondrial oxidative stress, dysfunctional mitophagy, and enhanced DNA damage response, culminating with their cell death. Although a crucial role for mitochondrial metabolism in controlling T regulatory (Treg) cell function has been recognized, its contribution during autoimmunity has not yet been fully elucidated. In this issue of Cell Metabolism, Alissafi and colleagues demonstrate that during autoimmunity, Treg cell functional alterations associate with mitochondrial oxidative stress, dysfunctional mitophagy, and enhanced DNA damage response, culminating with their cell death. CD4+CD25+FOXP3+ T regulatory (Treg) cells are a specific cellular subset, known for their immunoregulatory role, and their reduced number and/or compromised function have been detected in a wide range of autoimmune disorders (Dominguez-Villar and Hafler, 2018Dominguez-Villar M. Hafler D.A. Regulatory T cells in autoimmune disease.Nat. Immunol. 2018; 19: 665-673Crossref PubMed Scopus (212) Google Scholar; Carbone et al., 2014Carbone F. De Rosa V. Carrieri P.B. Montella S. Bruzzese D. Porcellini A. Procaccini C. La Cava A. Matarese G. Regulatory T cell proliferative potential is impaired in human autoimmune disease.Nat. Med. 2014; 20: 69-74Crossref PubMed Scopus (148) Google Scholar). Several studies have shown that distinct metabolic assets are associated with specific immune cell fate and function. Indeed, the field of immunometabolism has gained importance in the last ten years, because intracellular metabolism and metabolites can directly influence immune cells, modulating their activity according to specific microenvironmental cues. It has also been recognized that intracellular metabolism exerts a major impact on Treg function, proliferation, migration, and even their fate determination. Indeed, the mitochondrion, the powerhouse of the cell, represents a centrally positioned hub for production of energy, and recently it has been studied for its action beyond bioenergetics, such as the modulation of immune response. Importantly, deregulation of mitochondrial function increases intracellular oxidative stress, induces autophagy, and mediates cellular damage, leading to altered homeostasis in immune cells (Rambold and Pearce, 2018Rambold A.S. Pearce E.L. Mitochondrial Dynamics at the Interface of Immune Cell Metabolism and Function.Trends Immunol. 2018; 39: 6-18Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar). Therefore, the maintenance of mitochondrial network integrity and activity is crucial for the maintenance of immune cell balance and proper function. In the context of Treg cell immunobiology, mitochondrial metabolism has been shown to support their immunosuppressive functions (Weinberg et al., 2019Weinberg S.E. Singer B.D. Steinert E.M. Martinez C.A. Mehta M.M. Martínez-Reyes I. Gao P. Helmin K.A. Abdala-Valencia H. Sena L.A. et al.Mitochondrial complex III is essential for suppressive function of regulatory T cells.Nature. 2019; 565: 495-499Crossref PubMed Scopus (136) Google Scholar) and their stability (Beier et al., 2015Beier U.H. Angelin A. Akimova T. Wang L. Liu Y. Xiao H. Koike M.A. Hancock S.A. Bhatti T.R. Han R. et al.Essential role of mitochondrial energy metabolism in Foxp3+ T-regulatory cell function and allograft survival.FASEB J. 2015; 29: 2315-2326Crossref PubMed Scopus (116) Google Scholar), while also allowing their survival in lactate-rich environments (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.Foxp3 Reprograms T Cell Metabolism to Function in Low-Glucose, High-Lactate Environments.Cell Metab. 2017; 25: 1282-1293.e7Abstract Full Text Full Text PDF PubMed Scopus (322) Google Scholar). However, the molecular mechanism involved in the metabolic control of Treg cell function during autoimmune conditions has not yet been fully clarified. In a new paper by Alissafi et al., 2020Alissafi T. Kalafati L. Lazari M. Filia A. Kloukina I. Manifava M. Lim J.H. Alexaki V.I. Ktistakis N.T. Doskas T. et al.Mitochondrial Oxidative Damage Underlies Regulatory T Cell Defects in Autoimmunity.Cell Metab. 2020; 32 (this issue): 591-604Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar in this issue of Cell Metabolism, the authors identified a metabolic reprogramming of Treg cells in patients with autoimmune diseases (such as multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematosus). Indeed, through high-throughput transcriptomic analyses (namely, RNA-seq), the authors found several alterations in Treg cells from autoimmune subjects, as compared to healthy individuals, especially in terms of altered expression of genes connected with oxidative stress, DNA damage response, mitochondrial dysfunction, and cell-death-related pathways. They confirmed these findings in vivo in mice with experimental autoimmune encephalomyelitis (EAE) whose Treg cells displayed enhanced mitochondrial oxidative stress with a concomitant increase in mitochondrial mass, mitochondrial DNA content, enhanced accumulation of damaged mitochondria, and increased mitochondrial reactive oxygen species (mtROS) production (Figure 1). Starting from this evidence, the authors dissected the cellular basis underlying mitochondrial dysfunction in Treg cells from autoimmune mice and analyzed the link between autophagy and mitophagy, a specialized form of autophagy, which targets mitochondria to lysosomes for their degradation (Figure 1). Interestingly, they found profound defects in Treg cell lysosomal functions (in terms of degradation), affecting the completion of mitophagy during autoimmune response. These lysosomal alterations exacerbated mitochondrial oxidative stress and promoted a DNA damage response (DDR), leading to Treg cell death (Figure 1). To obtain insight into the possible molecular mechanism linking lysosomal-mediated mitophagy defects with the increased oxidative stress in Treg cells, the authors performed very elegant experiments, including the generation of a mouse model with ablation of Atg5 (an essential gene for mitophagy), specifically in Treg cells. The characterization of these mice highlighted enhanced DNA damage, excessive mtROS production, and reduced survival of Treg cells, thus suggesting that impaired lysosomal function is responsible for the increased mitochondrial oxidative stress in Treg cells. Finally, the authors proposed a possible therapeutic approach to overcome the functional deficit in Treg cells, consisting of in vivo treatment with MitoTEMPO (a mitochondria-specific superoxide scavenger). This treatment has proven to be very useful in reducing mtROS production, decreasing DNA damage and consequently Treg apoptosis, while also restoring lysosomal functions. In the treated mice, all these events were associated with reduced EAE symptoms and with an increased recruitment of Treg cells and a drastic reduction of pro-pathogenic Th1 and Th17 cells within the spinal cord. Overall, the study by Alissafi et al. helps to shed light on a previously unrecognized mechanism of Treg cell alteration during autoimmunity, highlighting the role of mitochondrial oxidative stress as a crucial determinant of Treg cell fate. These data are in line with recent reports showing that Treg cell-specific ablation of mitochondrial respiratory chain complex III results in the development of fatal inflammatory disease (Weinberg et al., 2019Weinberg S.E. Singer B.D. Steinert E.M. Martinez C.A. Mehta M.M. Martínez-Reyes I. Gao P. Helmin K.A. Abdala-Valencia H. Sena L.A. et al.Mitochondrial complex III is essential for suppressive function of regulatory T cells.Nature. 2019; 565: 495-499Crossref PubMed Scopus (136) Google Scholar) and that deletion of mitochondrial transcription factor A (Tfam), crucial for mitochondrial respiration and mitochondrial DNA replication, impairs Treg cell maintenance in tumors (Fu et al., 2019Fu Z. Ye J. Dean J.W. Bostick J.W. Weinberg S.E. Xiong L. Oliff K.N. Chen Z.E. Avram D. Chandel N.S. Zhou L. Requirement of Mitochondrial Transcription Factor A in Tissue-Resident Regulatory T Cell Maintenance and Function.Cell Rep. 2019; 28: 159-171.e4, e154Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). The results of this study further extend those findings, demonstrating that mitochondrial dysfunction is a hallmark of Treg cell alteration that has been observed in autoimmunity, while also providing the cellular and molecular mechanisms accounting for such phenomena. In a broader context, this works well in a scenario in which modulation of Treg cell number and/or function is of considerable therapeutic interest, in inhibiting autoimmune disorders and transplantation on one hand and boosting antitumor immune response on the other. In particular, this study adds another piece in the complex puzzle of metabolic regulation of Treg cell function and highlights the possibility that manipulating cellular metabolism, and specifically mitochondrial oxidative phosphorylation (OXPHOS), could provide a promising therapeutic intervention for immune-mediated disorders through modulation of Treg cell activity. Further in-depth studies are surely needed to explore this issue and determine how complex, multisystemic responses can regulate the balance between immunological self-tolerance and autoimmunity. In this context, a few key issues remain unresolved, including the role of mitochondria in the epigenetic control of Foxp3 expression. Specifically, how mitochondrial activity can modulate histone modifications, recruit transcription factors to the Foxp3 promoter and regulatory regions, as well as involve microRNAs in Foxp3 expression need to be further investigated. Also, the upstream mechanisms driving mitochondrial dysfunction in Treg cells are still unknown and are important, because characterizing them would be of great help in monitoring Treg cell activity. Further, because glucose metabolism has been shown to be pivotal in the induction of human Treg cells (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.Glycolysis controls the induction of human regulatory T cells by modulating the expression of FOXP3 exon 2 splicing variants.Nat. Immunol. 2015; 16: 1174-1184Crossref PubMed Scopus (186) Google Scholar) and in the control of their migration (Kishore et al., 2017Kishore M. Cheung K.C.P. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Regulatory T Cell Migration Is Dependent on Glucokinase-Mediated Glycolysis.Immunity. 2017; 47: 875-889.e10Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar), it would be important to dissect how mitochondrial metabolism is related to glycolysis in the control of Treg cell and Foxp3 expression and whether this cross-talk might be altered in autoimmunity. Additionally, another conundrum that warrants additional investigation is the role of several nutrients and environmental factors in modulating mitochondrial metabolism in Treg cells. Indeed, future work could aim at defining how environmental-specific nutrients orchestrate Treg cell function by affecting their stability and plasticity, thus balancing immune responses via a nutritional approach. Finally, it remains to be explored in clinical studies whether drugs affecting mitochondrial metabolism might have beneficial effects in the context of human autoimmunity (e.g., Moccia et al., 2019Moccia M. Capacchione A. Lanzillo R. Carbone F. Micillo T. Perna F. De Rosa A. Carotenuto A. Albero R. Matarese G. et al.Coenzyme Q10 supplementation reduces peripheral oxidative stress and inflammation in interferon-β1a-treated multiple sclerosis.Ther. Adv. Neurol. Disorder. 2019; 12 (1756286418819074)Google Scholar) by selectively modulating Treg cell activity in order to realize the full biological and therapeutic potential of the findings highlighted here (Alissafi et al., 2020Alissafi T. Kalafati L. Lazari M. Filia A. Kloukina I. Manifava M. Lim J.H. Alexaki V.I. Ktistakis N.T. Doskas T. et al.Mitochondrial Oxidative Damage Underlies Regulatory T Cell Defects in Autoimmunity.Cell Metab. 2020; 32 (this issue): 591-604Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar). G.M. is funded by grants from Fondazione Italiana Sclerosi Multipla (FISM, nos. 2016/R/18 and 2018/S/5 ) and Progetti di Rilevante Interesse Nazionale (PRIN, 2017 K55HLC 001 ). C.P. is supported by the Italian Ministry of Health ( GR-2016-02363749 and GR-2018-12366154 ). Mitochondrial Oxidative Damage Underlies Regulatory T Cell Defects in AutoimmunityAlissafi et al.Cell MetabolismJuly 31, 2020In BriefHerein, Alissafi et al. reveal that in Tregs during autoimmunity there is elevated mitochondrial oxidative stress, which induces a DNA damage response and cell death. Using a mouse model, they show that mitochondrial ROS scavenging in Tregs ameliorates autoimmune responses. These findings define new Treg checkpoints in autoimmune diseases. Full-Text PDF Open Archive