Utilization of Treg Cells in Solid Organ Transplantation

Iguratimod ameliorates antibody-mediated rejection after renal transplant by modulating the Th17/Treg paradigm

BACKGROUND

Iguratimod (IGU) is widely used in clinical practice due to its stable anti-inflammatory effects. Our previous studies have confirmed that the proportion of Th17/Treg balance in patients taking IGU altered significantly. This study aims to explore the role of IGU in antibody-mediated rejection (ABMR) and its potential mechanisms.

METHODS

We conducted bioinformatics analysis of sequencing data from the GEO database to analyze the abundance of immune cell infiltration in transplanted kidney tissues. In vivo, IGU was intervened in a mice secondary skin transplantation model and a mice kidney transplantation ABMR model, and histological morphology of the grafts were examined by pathological staining, while relevant indicators were determined through qRT-PCR, immunohistochemistry, and enzyme-linked immunosorbent assay, observed T cell differentiation by flow cytometry, and preliminarily assessed the immunosuppressive effect of IGU. In vitro, we established Th17 and Treg cell induction and stimulation differentiation culture systems and added IGU for intervention to explore its effects on their differentiation.

RESULTS

Through bioinformatics analysis, we found that Th17 and Treg may play important roles in the occurrence and development of ABMR. In vivo, we found that IGU could effectively reduce the damage caused by ABMR to the grafts, alleviate the infiltration of inflammatory cells in the graft tissues, and reduce the deposition of C4d in the grafts. Moreover, it is also found that IGU regulated the differentiation of Th17 and Treg cells in the spleen and peripheral blood and reduced the expression of IL-17A in the grafts and serum. In addition, same changes were observed in the induction and differentiation culture system of Th17 and Treg cells in vitro after the addition of IGU.

CONCLUSION

IGU can inhibit the progression of ABMR by regulating the differentiation of Th17 and Treg cells, providing novel insights for optimizing clinical immunosuppressive treatment regimens.

Graft-Specific Regulatory T Cells for Long-Lasting, Local Tolerance Induction

BACKGROUND

Solid organ transplantation is hindered by immune-mediated chronic graft dysfunction and the side effects of immunosuppressive therapy. Regulatory T cells (Tregs) are crucial for modulating immune responses post-transplantation; however, the transfer of polyspecific Tregs alone is insufficient to induce allotolerance in rodent models.

METHODS

To enhance the efficacy of adoptive Treg therapy, we investigated different immune interventions in the recipients. By utilizing an immunogenic skin transplant model and existing transplantation medicine reagents, we facilitated the clinical translation of our findings. Specifically, antigen-specific Tregs were used.

RESULTS

Our study demonstrated that combining the available induction therapies with drug-induced T-cell proliferation due to lymphopenia effectively increased the Treg/T effector ratios. This results in significant Treg accumulation within the graft, leading to long-term tolerance after the transfer of antigen-specific Tregs. Importantly, all the animals achieved operational tolerance, which boosted the presence of adoptively transferred Tregs within the graft.

CONCLUSIONS

This protocol offers a means to establish tolerance by utilizing antigen-specific Tregs. These results have promising implications for future trials involving adoptive Treg therapy in organ transplantation.

Treg-Derived Extracellular Vesicles: Roles in Diseases and Theranostics

Regulatory T cells (Tregs), a subset of CD4+ T cells, are indispensable in maintaining immune self-tolerance and have been utilized in various diseases. Treg-derived extracellular vesicles (Treg-EVs) have been discovered to play an important role in the mechanism of Treg functions. As cell-derived membranous particles, EVs carry multiple bioactive substances that possess tremendous potential for theranostics. Treg-EVs are involved in numerous physiological and pathological processes, carrying proteins and miRNAs inherited from the parental cells. To comprehensively understand the function of Treg-EVs, here we reviewed the classification of Treg-EVs, the active molecules in Treg-EVs, their various applications in diseases, and the existing challenges for Treg-EVs based theranostics. This Review aims to clarify the feasibility and potential of Treg-EVs in diseases and theranostics, facilitating further research and application of Treg-EVs.

Immune modulation in transplant medicine: a comprehensive review of cell therapy applications and future directions

Balancing the immune response after solid organ transplantation (SOT) and vascularized composite allotransplantation (VCA) remains an ongoing clinical challenge. While immunosuppressants can effectively reduce acute rejection rates following transplant surgery, some patients still experience recurrent acute rejection episodes, which in turn may progress to chronic rejection. Furthermore, these immunosuppressive regimens are associated with an increased risk of malignancies and metabolic disorders. Despite significant advancements in the field, these IS related side effects persist as clinical hurdles, emphasizing the need for innovative therapeutic strategies to improve transplant survival and longevity. Cellular therapy, a novel therapeutic approach, has emerged as a potential pathway to promote immune tolerance while minimizing systemic side-effects of standard IS regiments. Various cell types, including chimeric antigen receptor T cells (CAR-T), mesenchymal stromal cells (MSCs), regulatory myeloid cells (RMCs) and regulatory T cells (Tregs), offer unique immunomodulatory properties that may help achieve improved outcomes in transplant patients. This review aims to elucidate the role of cellular therapies, particularly MSCs, T cells, Tregs, RMCs, macrophages, and dendritic cells in SOT and VCA. We explore the immunological features of each cell type, their capacity for immune regulation, and the prospective advantages and obstacles linked to their application in transplant patients. An in-depth outline of the current state of the technology may help SOT and VCA providers refine their perioperative treatment strategies while laying the foundation for further trials that investigate cellular therapeutics in transplantation surgery.

Targeting Macrophages in Organ Transplantation: A Step Toward Personalized Medicine

Organ transplantation remains the most optimal strategy for patients with end-stage organ failure. However, prevailing methods of immunosuppression are marred by adverse side effects, and allograft rejection remains common. It is imperative to identify and comprehensively characterize the cell types involved in allograft rejection, and develop therapies with greater specificity. There is increasing recognition that processes mediating allograft rejection are the result of interactions between innate and adaptive immune cells. Macrophages are heterogeneous innate immune cells with diverse functions that contribute to ischemia-reperfusion injury, acute rejection, and chronic rejection. Macrophages are inflammatory cells capable of innate allorecognition that strengthen their responses to secondary exposures over time via "trained immunity." However, macrophages also adopt immunoregulatory phenotypes and may promote allograft tolerance. In this review, we discuss the roles of macrophages in rejection and tolerance, and detail how macrophage plasticity and polarization influence transplantation outcomes. A comprehensive understanding of macrophages in transplant will guide future personalized approaches to therapies aimed at facilitating tolerance or mitigating the rejection process.

Kidney transplants from elderly donors: what we have learned 20 years after the Crystal City consensus criteria meeting

Based on the current projection of the general population and the combined increase in end-stage kidney disease with age, the number of elderly donors and recipients is increasing, raising crucial questions about how to minimize the discard rate of organs from elderly donors and improve graft and patient outcomes. In 2002, extended criteria donors were the focus of a meeting in Crystal City (VA, USA), with a goal of maximizing the use of organs from deceased donors. Since then, extended criteria donors have progressively contributed to a large number of transplanted grafts worldwide, posing specific issues for allocation systems, recipient management, and therapeutic approaches. This review analyzes what we have learned in the last 20 years about extended criteria donor utilization, the promising innovations in immunosuppressive management, and the molecular pathways involved in the aging process, which constitute potential targets for novel therapies.

CXCR5 + CD8 + T Cell-mediated Suppression of Humoral Alloimmunity and AMR in Mice Is Optimized With mTOR and Impaired With Calcineurin Inhibition

BACKGROUND

Adoptive cellular therapy (ACT) with antibody-suppressor CXCR5 + CD8 + T cells (CD8 + T Ab-supp ) inhibits alloantibody production, antibody-mediated rejection (AMR), and prolongs graft survival in multiple transplant mouse models. However, it is not known how conventional immunosuppressive agents impact the efficacy of CD8 + T Ab-supp ACT.

METHODS

We investigated the efficacy of CD8 + T Ab-supp cell ACT when combined with calcineurin inhibitor (CNi) or mammalian target of rapamycin inhibitor (mTORi) in a murine model of kidney transplant.

RESULTS

ACT-mediated decrease in germinal center B cells, posttransplant alloantibody titer, and amelioration of AMR in high alloantibody-producing CCR5 knockout kidney transplant recipients were impaired when ACT was combined with CNi and enhanced when combined with mTORi. CNi (but not mTORi) reduced ACT-mediated in vivo cytotoxicity of IgG + B cells and was associated with increased quantity of germinal center B cells. Neither CNi nor mTORi treatment impacted the expression of cytotoxic effector molecules (FasL, Lamp1, perforin, granzyme B) by CD8 + T Ab-supp after ACT. Concurrent treatment with CNi (but not mTORi) reduced in vivo proliferation of CD8 + T Ab-supp after ACT. The increase in quantity of splenic CD44 + CXCR5 + CD8 + T cells that occurs after ACT was reduced by concurrent treatment with CNi but not by concurrent treatment with mTORi (dose-dependent).

CONCLUSIONS

Impaired efficacy of ACT by CNi is attributed to reduced persistence and/or expansion of CD8 + T Ab-supp cells after ACT. In contrast, concurrent immunosuppression with mTORi preserves CD8 + T Ab-supp cells quantity, in vivo proliferation, and in vivo cytotoxic effector function after ACT and enhances suppression of humoral alloimmunity and AMR.

Solid organ transplantation and gut microbiota: a review of the potential immunomodulatory properties of short-chain fatty acids in graft maintenance

Transplantation is the treatment of choice for several end-stage organ defects: it considerably improves patient survival and quality of life. However, post-transplant recipients may experience episodes of rejection that can favor or ultimately lead to graft loss. Graft maintenance requires a complex and life-long immunosuppressive treatment. Different immunosuppressive drugs (i.e., calcineurin inhibitors, glucocorticoids, biological immunosuppressive agents, mammalian target of rapamycin inhibitors, and antiproliferative or antimetabolic agents) are used in combination to mitigate the immune response against the allograft. Unfortunately, the use of these antirejection agents may lead to opportunistic infections, metabolic (e.g., post-transplant diabetes mellitus) or cardiovascular (e.g., arterial hypertension) disorders, cancer (e.g., non-Hodgkin lymphoma) and other adverse effects. Lately, immunosuppressive drugs have also been associated with gut microbiome alterations, known as dysbiosis, and were shown to affect gut microbiota-derived short-chain fatty acids (SCFA) production. SCFA play a key immunomodulatory role in physiological conditions, and their impairment in transplant patients could partly counterbalance the effect of immunosuppressive drugs leading to the activation of deleterious pathways and graft rejection. In this review, we will first present an overview of the mechanisms of graft rejection that are prevented by the immunosuppressive protocol. Next, we will explain the dynamic changes of the gut microbiota during transplantation, focusing on SCFA. Finally, we will describe the known functions of SCFA in regulating immune-inflammatory reactions and discuss the impact of SCFA impairment in immunosuppressive drug treated patients.

Chimeric antigen receptor Treg therapy in transplantation

In the quest for more precise and effective organ transplantation therapies, chimeric antigen receptor (CAR) regulatory T cell (Treg) therapies represent a potential cutting-edge advance. This review comprehensively analyses CAR Tregs and how they may address important drawbacks of polyclonal Tregs and conventional immunosuppressants. We examine a growing body of preclinical findings of CAR Treg therapy in transplantation, discuss CAR Treg design specifics, and explore established and attractive new targets in transplantation. In addition, we explore present impediments where future studies will be necessary to determine the efficacy of CAR Tregs in reshaping alloimmune responses and transplant microenvironments to reduce reliance on chemical immunosuppressants. Overall, ongoing studies and trials are crucial for understanding the full scope of CAR Treg therapy in transplantation.

Clinical Manufacturing of Regulatory T Cell Products For Adoptive Cell Therapy and Strategies to Improve Therapeutic Efficacy

Based on successes in preclinical animal transplant models, adoptive cell therapy (ACT) with regulatory T cells (Tregs) is a promising modality to induce allograft tolerance or reduce the use of immunosuppressive drugs to prevent rejection. Extensive work has been done in optimizing the best approach to manufacture Treg cell products for testing in transplant recipients. Collectively, clinical evaluations have demonstrated that large numbers of Tregs can be expanded ex vivo and infused safely. However, these trials have failed to induce robust drug-free tolerance and/or significantly reduce the level of immunosuppression needed to prevent solid organ transplant (SOTx) rejection. Improving Treg therapy effectiveness may require increasing Treg persistence or orchestrating Treg migration to secondary lymphatic tissues or places of inflammation. In this review, we describe current clinical Treg manufacturing methods used for clinical trials. We also highlight current strategies being implemented to improve delivered Treg ACT persistence and migration in preclinical studies.

Comparing the Benefits and Drawbacks of Stem Cell Therapy Based on the Cell Origin or Manipulation Process: Addressing Immunogenicity

Mesenchymal stem cells (MSCs) are effective in treating autoimmune diseases and managing various conditions, such as engraftment of allogeneic islets. Additionally, autologous and HLA-matched allogeneic MSCs can aid in the engraftment of human allogeneic kidneys with or without low doses of tacrolimus, respectively. However, HLA alloantigens are problematic because cell therapy uses more HLA-mismatched allogeneic cells than autologous for convenience and standardization. In particular, HLA-mismatched MSCs showed increased Ag-specific T/B cells and reduced viability faster than HLA-matched MSCs. In CRISPR/Cas9-based cell therapy, Cas9 induce T cell activation in the recipient's immune system. Interestingly, despite their immunogenicity being limited to the cells with foreign Ags, the accumulation of HLA alloantigen-sensitized T/B cells may lead to allograft rejection, suggesting that alloantigens may have a greater scope of adverse effects than foreign Ags. To avoid alloantigen recognition, the β2-microglobulin knockout (B2MKO) system, eliminating class-I MHC, was able to avoid rejection by alloreactive CD8 T cells compared to controls. Moreover, universal donor cells in which both B2M and Class II MHC transactivator (CIITA) were knocked out was more effective in avoiding immune rejection than single KO. However, B2MKO and CIITA KO system remain to be controlled and validated for adverse effects such as the development of tumorigenicity due to deficient Ag recognition by CD8 T and CD4 T cells, respectively. Overall, better HLA-matching or depletion of HLA alloantigens prior to cell therapy can reduce repetitive transplantation through the long-term survival of allogeneic cell therapy, which may be especially important for patients seeking allogeneic transplantation.

Single-cell analysis reveals lysyl oxidase (Lox)+ fibroblast subset involved in cardiac fibrosis of diabetic mice

INTRODUCTION

Myocardial fibrosis and cardiac dysfunction are the main characteristics of diabetic heart disease. However, the molecular mechanisms underlying diabetic myocardial fibrosis remain unclear.

OBJECTIVES

This study aimed to investigate the heterogeneity of cardiac fibroblasts in diabetic mice and its possible mechanism in the development of diabetic myocardial fibrosis.

METHODS

We established a diabetic mouse model by injecting mice with streptozotocin. The overall cell profiles in diabetic hearts were analyzed using single-cell RNA transcriptomic techniques. Cardiac function was evaluated by echocardiography. Cardiac fibrosis was assessed by Masson's trichrome and Sirius red staining. Protein expression was analyzed using Western blotting and immunofluorescence staining.

RESULTS

A total of 11,585 cells were captured in control (Ctrl) and diabetic (DM) hearts. Twelve cell types were identified in this study. The number of fibroblasts was significantly higher in the DM hearts than in the Ctrl group. The fibroblasts were further re-clustered into nine subsets. Interestingly, cluster 4 fibroblasts were significantly increased in diabetic hearts compared with other fibroblast clusters. Lysyl oxidase (Lox) was highly expressed in DM fibroblasts (especially in cluster 4). Beta-aminopropionitrile, a Lox inhibitor, inhibited collagen expression and alleviated cardiac dysfunction in the diabetic group. Lysyl oxidase inhibition also reduced high glucose-induced collagen protein upregulation in primary fibroblasts. Moreover, a TGF-β receptor inhibitor not only prevented an increase in Lox and Col I but also inhibited the phosphorylation of Smad2/3 in fibroblasts.

CONCLUSIONS

This study revealed the heterogeneity of cardiac fibroblasts in diabetic mice for the first time. Fibroblasts with high expression of Lox (cluster 4 fibroblasts) were identified to play a crucial role in fibrosis in diabetic heart disease. The findings of this study may provide a possible therapeutic target for interstitial fibrosis.

Application and Effectiveness of Chinese Medicine in Regulating Immune Checkpoint Pathways

Immunotherapy targeting immune checkpoint molecules has emerged as a key approach in cancer treatment, representing the forefront of antitumor research. However, studies on immune checkpoint molecules have mainly focused on targeted therapies. Chinese medicine (CM) research as a complementary medicine has revealed that immune checkpoint molecules also undergo disease-specific changes in the context of autoimmune diseases. This review article presents a comprehensive analysis of CM studies on immune checkpoint molecules in the last 5 years, with a focus on their role in different diseases and treatment modalities. CM research predominantly utilizes oral administration of herbal plant extracts or acupuncture techniques, which stimulate the immune system by activating specific acupoints through temperature and needling. In this study, we analyzed the modulation and mechanisms of immune checkpoint molecules associated with different coinhibitory and costimulatory molecules, and reviewed the immune functions of related molecules and CM studies in treating autoimmune diseases and tumors. By summarizing the characteristics and research value of CM in regulating immune checkpoint molecules, this review aims to provide a useful reference for future studies in this field.

Regulatory T cells in autoimmune kidney diseases and transplantation

Regulatory T (Treg) cells that express the transcription factor forkhead box protein P3 (FOXP3) are naturally present in the immune system and have roles in the maintenance of immunological self-tolerance and immune system and tissue homeostasis. Treg cells suppress T cell activation, expansion and effector functions by various mechanisms, particularly by controlling the functions of antigen-presenting cells. They can also contribute to tissue repair by suppressing inflammation and facilitating tissue regeneration, for example, via the production of growth factors and the promotion of stem cell differentiation and proliferation. Monogenic anomalies of Treg cells and genetic variations of Treg cell functional molecules can cause or predispose patients to the development of autoimmune diseases and other inflammatory disorders, including kidney diseases. Treg cells can potentially be utilized or targeted to treat immunological diseases and establish transplantation tolerance, for example, by expanding natural Treg cells in vivo using IL-2 or small molecules or by expanding them in vitro for adoptive Treg cell therapy. Efforts are also being made to convert antigen-specific conventional T cells into Treg cells and to generate chimeric antigen receptor Treg cells from natural Treg cells for adoptive Treg cell therapies with the aim of achieving antigen-specific immune suppression and tolerance in the clinic.

Enforced gut homing of murine regulatory T cells reduces early graft-versus-host disease severity

Damage to the gastrointestinal tract following allogeneic hematopoietic stem cell transplantation is a significant contributor to the severity and perpetuation of graft-versus-host disease. In preclinical models and clinical trials, we showed that infusing high numbers of regulatory T cells reduces graft-versus-host disease incidence. Despite no change in in vitro suppressive function, transfer of ex vivo expanded regulatory T cells transduced to overexpress G protein-coupled receptor 15 or C-C motif chemokine receptor 9, specific homing receptors for colon or small intestine, respectively, lessened graft-versus-host disease severity in mice. Increased regulatory T cell frequency and retention within the gastrointestinal tissues of mice that received gut homing T cells correlated with lower inflammation and gut damage early post-transplant, decreased graft-versus-host disease severity, and prolonged survival compared with those receiving control transduced regulatory T cells. These data provide evidence that enforced targeting of ex vivo expanded regulatory T cells to the gastrointestinal tract diminishes gut injury and is associated with decreased graft-versus-host disease severity.

Adjuvant conditioning induces an immunosuppressive milieu that delays alloislet rejection through the expansion of myeloid-derived suppressor cells

Advances in immunosuppression have been relatively stagnant over the past 2 decades, and transplant recipients continue to experience long-term morbidity associated with immunosuppression regimens. Strategies to reduce or eliminate the dosage of immunosuppression medications are needed. We discovered a novel administration strategy using the classic adjuvant alum to condition murine islet transplant recipients, known as adjuvant conditioning (AC), to expand both polymorphonuclear and monocytic myeloid-derived suppressive cells (MDSCs) in vivo. These AC MDSCs potently suppress T cell proliferation when cultured together in vitro. AC MDSCs also facilitate naïve CD4+ T cells to differentiate into regulatory T cells. In addition, we were able to demonstrate a significant delay in alloislet rejection compared with that by saline-treated control following adjuvant treatment in a MDSC-dependent manner. Furthermore, AC MDSCs produce significantly more interleukin (IL)-10 than saline-treated controls, which we demonstrated to be critical for the increased T cell suppressor function of AC MDSCs as well as the observed protective effect of AC against alloislet rejection. Our data suggest that adjuvant-related therapeutics designed to expand MDSCs could be a useful strategy to prevent transplant rejection and curb the use of toxic immunosuppressive regimens currently used in transplant patients.

TIPE2 deficiency prolongs mouse heart allograft survival by inhibiting immature DCs-induced Treg generation

It has been reported that deletion of tumor necrosis factor-α-induced protein-8 like 2 (TNFAIP8L2, TIPE2) facilitates the activation of T-cell receptors. However, the role of TIPE2 in T-cell-mediated acute transplant rejection remains unclear. To illustrate the underlying cellular mechanisms, we transplanted BALB/c hearts into C57BL/6 wild-type C57BL/6 mice or mice deficient for TIPE2 (TIPE2^-/-) and found that TIPE2^-/- recipient mice showed significantly prolonged survival of heart allografts and suppressed maturation of CD11c⁺ dendritic cells (DCs), which largely abolished the activation and proliferation of alloreactive T cells and their cytotoxic activity. TIPE2^-/- DCs increased CD4⁺Foxp3⁺CD127^- Treg generation, likely by inhibiting DCs maturation and CD80 and CD86 expression. Administration of anti-CD25 abolished the allograft survival induced by TIPE2 deficiency. Moreover, TIPE2 deficiency increased IL-10 production in T cells and in recipient serum and allografts. Mechanistic studies revealed that TIPE2^-/- restrained the maturation of DCs via inhibition of PI3K/AKT phosphorylation during alloantigen stimulation. Taken together, TIPE2 deficiency in recipient mice inhibited acute rejection by increasing Tregs generated by immature DCs. Thus, TIPE2 could be a therapeutic target for suppressing rejection in organ transplantation.

Blood unconjugated bilirubin and tacrolimus are negative predictors of specific cellular immunity in kidney transplant recipients after SAR-CoV-2 inactivated vaccination

The immunogenicity of SARS-CoV-2 vaccines is poor in kidney transplant recipients (KTRs). The factors related to poor immunogenicity to vaccination in KTRs are not well defined. Here, observational study demonstrated no severe adverse effects were observed in KTRs and healthy participants (HPs) after first or second dose of SARS-CoV-2 inactivated vaccine. Different from HPs with excellent immunity against SARS-CoV-2, IgG antibodies against S1 subunit of spike protein, receptor-binding domain, and nucleocapsid protein were not effectively induced in a majority of KTRs after the second dose of inactivated vaccine. Specific T cell immunity response was detectable in 40% KTRs after the second dose of inactivated vaccine. KTRs who developed specific T cell immunity were more likely to be female, and have lower levels of total bilirubin, unconjugated bilirubin, and blood tacrolimus concentrations. Multivariate logistic regression analysis found that blood unconjugated bilirubin and tacrolimus concentration were significantly negatively associated with SARS-CoV-2 specific T cell immunity response in KTRs. Altogether, these data suggest compared to humoral immunity, SARS-CoV-2 specific T cell immunity response are more likely to be induced in KTRs after administration of inactivated vaccine. Reduction of unconjugated bilirubin and tacrolimus concentration might benefit specific cellular immunity response in KTRs following vaccination.

Mitochondrial fusion induced by transforming growth factor-β1 serves as a switch that governs the metabolic reprogramming during differentiation of regulatory T cells

Although metabolic reprogramming during the differentiation of regulatory T cells (Treg cells) has been extensively studied, the molecular switch to alter energy metabolism remains undefined. The present study explores the critical role of mitochondrial dynamics in the reprogramming and consequent generation of Treg cells. The results showed that during Treg cell differentiation, mitochondrial fusion but not fission led to elevation of oxygen consumption rate values, facilitation of metabolic reprogramming, and increase of number of Treg cells and expression of Foxp3 in vitro and in vivo. Mechanistically, mitochondrial fusion favored fatty acid oxidation but restricted glycolysis in Treg cells through down-regulating the expression of HIF-1α. Transforming growth factor-β1 (TGF-β1) played a crucial role in the induction of mitochondrial fusion, which activated Smad2/3, promoted the expression of PGC-1α and therefore facilitated the expression of mitochondrial fusion proteins. In conclusion, during Treg cell differentiation, TGF-β1 promotes PGC-1α-mediated mitochondrial fusion, which drives metabolic reprogramming from glycolysis to fatty acid oxidation via suppressing HIF-1α expression, and therefore favors the generation of Treg cells. The signals and proteins involved in mitochondrial fusion are potential therapeutic targets for Treg cell-related diseases.

The aging of the immune system and its implications for transplantation

By the last third of life, most mammals, including humans, exhibit a decline in immune cell numbers, immune organ structure, and immune defense of the organism, commonly known as immunosenescence. This decline leads to clinical manifestations of increased susceptibility to infections, particularly those caused by emerging and reemerging microorganisms, which can reach staggering levels-infection with SARS-CoV-2 has been 270-fold more lethal to older adults over 80 years of age, compared to their 18-39-year-old counterparts. However, while this would be expected to be beneficial to situations where hyporeactivity of the immune system may be desirable, this is not always the case. Here, we discuss the cellular and molecular underpinnings of immunosenescence as they pertain to outcomes of solid organ and hematopoietic transplantation.

Stepwise acquisition of unique epigenetic signatures during differentiation of tissue Treg cells

Regulatory T cells in non-lymphoid tissues are not only critical for maintaining self-tolerance, but are also important for promoting organ homeostasis and tissue repair. It is proposed that the generation of tissue Treg cells is a stepwise, multi-site process, accompanied by extensive epigenome remodeling, finally leading to the acquisition of unique tissue-specific epigenetic signatures. This process is initiated in the thymus, where Treg cells acquire core phenotypic and functional properties, followed by a priming step in secondary lymphoid organs that permits Treg cells to exit the lymphoid organs and seed into non-lymphoid tissues. There, a final specialization process takes place in response to unique microenvironmental cues in the respective tissue. In this review, we will summarize recent findings on this multi-site tissue Treg cell differentiation and highlight the importance of epigenetic remodeling during these stepwise events.