Metabolic regulation of forkhead box P3 alternative splicing isoforms and their impact on health and disease

Forkhead Box P3 (FOXP3) is crucial for the development and suppressive function of human regulatory T cells (Tregs). There are two predominant FOXP3 splicing isoforms in healthy humans, the full-length isoform and the isoform lacking exon 2, with different functions and regulation mechanisms. FOXP3 splicing isoforms show distinct abilities in the cofactor interaction and the nuclear translocation, resulting in different effects on the differentiation, cytokine secretion, suppressive function, linage stability, and environmental adaptation of Tregs. The balance of FOXP3 splicing isoforms is related to autoimmune diseases, inflammatory diseases, and cancers. In response to environmental challenges, FOXP3 transcription and splicing can be finely regulated by T cell antigen receptor stimulation, glycolysis, fatty acid oxidation, and reactive oxygen species, with various signaling pathways involved. Strategies targeting energy metabolism and FOXP3 splicing isoforms in Tregs may provide potential new approaches for the treatment of autoimmune diseases, inflammatory diseases, and cancers. In this review, we summarize recent discoveries about the FOXP3 splicing isoforms and address the metabolic regulation and specific functions of FOXP3 splicing isoforms in Tregs.

Mettl14 sustains FOXP3 expression to promote the differentiation and functions of induced-regulatory T cells via the mTOR signaling pathway

Induced regulatory T cell (iTregs) can be generated in vitro. Thus, iTregs-based therapeutics are receiving increased attention for their potential to treat autoimmune diseases and prevent transplant rejection. However, iTregs fail to maintain FoxP3 expression and suppressive activity, which limits their clinical application. Increasing lines of evidence suggest that methyltransferase-like 14 (METTL14), a critical component of the m6A writer complex, regulates the stability and function of the Treg cells. However, beyond meeting the epigenetic modification of Treg cells, whether Mettl14 plays a role in the fate determination of iTregs is unclear. Here, we systemically investigated the potential function of METTL14 in iTregs differentiation and regulatory activity. In our study, iTregs were generated from CD4⁺ naïve T cells under iTreg-polarizing conditions, we found that the expression of METTL14 was increased in iTregs compared with CD4⁺ naïve T cells. Subsequently, the expression of METTL14 was knocked down by siRNA-METTL14 interference in CD4⁺ naïve T cells and cultured under iTreg-polarizing conditions. According to the results, Mettl14 deficiency resulted in the disruption of iTregs differentiation evidenced by the limited FoxP3 expression. Meanwhile, inflammatory cytokines such as IFN-γ and IL-17a were upregulated in cultured iTregs. We next determined the functional change in METTL14-deficient iTregs. The results of the colitis development in Rag1^-/- mice and CFSE assays revealed that loss of METTL14 significantly compromised the suppressive function of iTregs in vivo and in vitro. We further checked the altered signaling pathway in METTL14-deficient iTregs. We found that reduced METTL14 leads to activation of the mTOR pathway with increased p-mTOR and p-p70S6K, which are known to modulate the suppressive function of iTregs. In conclusion, our study revealed that Mettl14 plays a critical role in the development and suppressive function of iTregs in vitro and could thus serve as a regulatory element for stabilizing iTregs in cell-based therapy.

Human CD4+CD25+CD226- Tregs Demonstrate Increased Purity, Lineage Stability, and Suppressive Capacity Versus CD4+CD25+CD127lo/- Tregs for Adoptive Cell Therapy

Regulatory T cell (Treg) adoptive cell therapy (ACT) represents an emerging strategy for restoring immune tolerance in autoimmune diseases. Tregs are commonly purified using a CD4⁺CD25⁺CD127^lo/- gating strategy, which yields a mixed population: 1) cells expressing the transcription factors, FOXP3 and Helios, that canonically define lineage stable thymic Tregs and 2) unstable FOXP3⁺Helios^- Tregs. Our prior work identified the autoimmune disease risk-associated locus and costimulatory molecule, CD226, as being highly expressed not only on effector T cells but also, interferon-γ (IFN-γ) producing peripheral Tregs (pTreg). Thus, we sought to determine whether isolating Tregs with a CD4⁺CD25⁺CD226^- strategy yields a population with increased purity and suppressive capacity relative to CD4⁺CD25⁺CD127^lo/- cells. After 14d of culture, expanded CD4⁺CD25⁺CD226^- cells displayed a decreased proportion of pTregs relative to CD4⁺CD25⁺CD127^lo/- cells, as measured by FOXP3⁺Helios^- expression and the epigenetic signature at the FOXP3 Treg-specific demethylated region (TSDR). Furthermore, CD226^- Tregs exhibited decreased production of the effector cytokines, IFN-γ, TNF, and IL-17A, along with increased expression of the immunoregulatory cytokine, TGF-β1. Lastly, CD226^- Tregs demonstrated increased in vitro suppressive capacity as compared to their CD127^lo/- counterparts. These data suggest that the exclusion of CD226-expressing cells during Treg sorting yields a population with increased purity, lineage stability, and suppressive capabilities, which may benefit Treg ACT for the treatment of autoimmune diseases.

Co-Expression of FOXP3FL and FOXP3Δ2 Isoforms Is Required for Optimal Treg-Like Cell Phenotypes and Suppressive Function

FOXP3 is the master transcription factor in both murine and human FOXP3⁺ regulatory T cells (Tregs), a T-cell subset with a central role in controlling immune responses. Loss of the functional Foxp3 protein in scurfy mice leads to acute early-onset lethal lymphoproliferation. Similarly, pathogenic FOXP3 mutations in humans lead to immunodysregulation, polyendocrinopathy, enteropathy, and X-linked (IPEX) syndrome, which are characterized by systemic autoimmunity that typically begins in the first year of life. However, although pathogenic FOXP3 mutations lead to overlapping phenotypic consequences in both systems, FOXP3 in human Tregs, but not mouse, is expressed as two predominant isoforms, the full length (FOXP3FL) and the alternatively spliced isoform, delta 2 (FOXP3Δ2). Here, using CRISPR/Cas9 to generate FOXP3 knockout CD4⁺ T cells (FOXP3KO^GFP CD4+ T cells), we restore the expression of each isoform by lentiviral gene transfer to delineate their functional roles in human Tregs. When compared to FOXP3FL or FOXP3Δ2 alone, or double transduction of the same isoform, co-expression of FOXP3FL and FOXP3Δ2 induced the highest overall FOXP3 protein expression in FOXP3KO^GFP CD4+ T cells. This condition, in turn, led to optimal acquisition of Treg-like cell phenotypes including downregulation of cytokines, such as IL-17, and increased suppressive function. Our data confirm that co-expression of FOXP3FL and FOXP3Δ2 leads to optimal Treg-like cell function and supports the need to maintain the expression of both when engineering therapeutics designed to restore FOXP3 function in otherwise deficient cells.

Metabolic Pathways Involved in Regulatory T Cell Functionality

Regulatory T cells (Treg) are well-known for their immune regulatory potential and are essential for maintaining immune homeostasis. The rationale of Treg-based immunotherapy for treating autoimmunity and transplant rejection is to tip the immune balance of effector T cell-mediated immune activation and Treg-mediated immune inhibition in favor of Treg cells, either through endogenous Treg expansion strategies or adoptive transfer of ex vivo expanded Treg. Compelling evidence indicates that Treg show properties of phenotypic heterogeneity and instability, which has caused considerable debate in the field regarding their correct use. Consequently, for further optimization of Treg-based immunotherapy, it is vital to further our understanding of Treg proliferative, migratory, and suppressive behavior. It is increasingly appreciated that the functional profile of immune cells is highly dependent on their metabolic state. Although the metabolic profiles of effector T cells are progressively understood, little is known on Treg in this respect. The objective of this review is to outline the current knowledge of human Treg metabolic profiles associated with the regulation of Treg functionality. As such information on human Treg is still limited, where information was lacking, we included insightful findings from mouse studies. To assess the available evidence on metabolic pathways involved in Treg functionality, PubMed, and Embase were searched for articles in English indexed before April 28th, 2019 using “regulatory T lymphocyte,” “cell metabolism,” “cell proliferation,” “migration,” “suppressor function,” and related search terms. Removal of duplicates and search of the references was performed manually. We discerned that while glycolysis fuels the biosynthetic and bioenergetic needs necessary for proliferation and migration of human Treg, suppressive capacity is mainly maintained by oxidative metabolism. Based on the knowledge of metabolic differences between Treg and non-Treg cells, we additionally discuss and propose ways of how human Treg metabolism could be exploited for the betterment of tolerance-inducing therapies.

Bcl10 is required for the development and suppressive function of Foxp3+ regulatory T cells

Foxp3⁺ regulatory T (Treg) cells play a critical role in peripheral tolerance. Bcl10, acting as a scaffolding protein in the Carma1-Bcl10-Malt1 (CBM) complex, has a critical role in TCR-induced signaling, leading to NF-κB activation and is required for T-cell activation. The role of Bcl10 in conventional T (Tconv) cells has been well characterized; however, the role of Bcl10 in the development of Treg cells and the maintenance of the suppressive function and identity of these cells has not been well characterized. In this study, we found that Bcl10 was required for not only the development but also the function of Treg cells. After deleting Bcl10 in T cells, we found that the development of Treg cells was significantly impaired. When Bcl10 was specifically deleted in mature Treg cells, the suppressive function of the Treg cells was impaired, leading to lethal autoimmunity in Bcl10^fl/flFoxp3^cre mice. Consistently, in contrast to WT Treg cells, Bcl10-deficient Treg cells could not protect Rag1-deficient mice from T-cell transfer-induced colitis. Furthermore, Bcl10-deficient Treg cells downregulated the expression of a series of Treg-cell effector and suppressive genes and decreased effector Treg-cell populations. Moreover, Bcl10-deficient Treg cells were converted into IFNγ-producing proinflammatory cells with increased expression of the transcription factors T-bet and HIF-1α. Together, our study results provide genetic evidence, indicating that Bcl10 is required for the development and function of Treg cells.

Increased miR-142-3p Expression Might Explain Reduced Regulatory T Cell Function in Granulomatosis With Polyangiitis

Objectives: Regulatory T cells (Tregs) are frequently functionally impaired in patients with granulomatosis with polyangiitis (GPA). However, the mechanism underlying their impaired function is unknown. Here, we hypothesized that Treg dysfunction in GPA is due to altered microRNA (miRNA) expression. Methods: RNA isolated from FACS-sorted memory (_M) Tregs (CD4⁺CD45RO⁺CD25⁺CD127^-) of 8 healthy controls (HCs) and 8 GPA patients without treatment was subjected to miRNA microarray analysis. Five differentially expressed miRNAs were validated in a larger cohort by reverse transcriptase quantitative polymerase chain reaction (RT-qPCR). An miRNA target gene database search revealed targets that were tested with RT-qPCR in _MTregs from patients and HCs. cAMP levels were measured using flow cytometry. Results: Microarray analysis revealed 19 differentially expressed miRNAs, of which miR-142-3p was confirmed to be significantly upregulated in _MTregs from GPA patients compared to those from HCs (1.9-fold, p = 0.03). In vitro overexpression of miR-142-3p lowered the suppressive capacity of _MTregs (2.1-fold, p = 0.03), and miR-142-3p expression correlated negatively with the suppressive capacity (rho = -0.446, p = 0.04). Overexpression of miR-142-3p significantly decreased cAMP levels (p = 0.02) and tended to decrease the mRNA levels of a predicted target gene, adenylate cyclase 9 (ADCY9; p = 0.06). In comparison to those from HCs, _MTregs from GPA patients had lower ADCY9 mRNA levels (2-fold, p = 0.008) and produced significantly less cAMP after stimulation. Importantly, induction of cAMP production in miR-142-3p overexpressed _MTregs by forskolin restored their suppressive function in vitro. Conclusion: Overexpression of miR-142-3p in _MTregs from GPA patients might cause functional impairment by targeting ADCY9, which leads to the suppression of cAMP production.

Enzymatic Activity of HPGD in Treg Cells Suppresses Tconv Cells to Maintain Adipose Tissue Homeostasis and Prevent Metabolic Dysfunction

Regulatory T cells (Treg cells) are important for preventing autoimmunity and maintaining tissue homeostasis, but whether Treg cells can adopt tissue- or immune-context-specific suppressive mechanisms is unclear. Here, we found that the enzyme hydroxyprostaglandin dehydrogenase (HPGD), which catabolizes prostaglandin E₂ (PGE₂) into the metabolite 15-keto PGE₂, was highly expressed in Treg cells, particularly those in visceral adipose tissue (VAT). Nuclear receptor peroxisome proliferator-activated receptor-γ (PPARγ)-induced HPGD expression in VAT Treg cells, and consequential Treg-cell-mediated generation of 15-keto PGE₂ suppressed conventional T cell activation and proliferation. Conditional deletion of Hpgd in mouse Treg cells resulted in the accumulation of functionally impaired Treg cells specifically in VAT, causing local inflammation and systemic insulin resistance. Consistent with this mechanism, humans with type 2 diabetes showed decreased HPGD expression in Treg cells. These data indicate that HPGD-mediated suppression is a tissue- and context-dependent suppressive mechanism used by Treg cells to maintain adipose tissue homeostasis.

Novel Effector Phenotype of Tim-3+ Regulatory T Cells Leads to Enhanced Suppressive Function in Head and Neck Cancer Patients

Purpose: Regulatory T (Treg) cells are important suppressive cells among tumor-infiltrating lymphocytes (TIL). Treg cells express the well-known immune checkpoint receptor PD-1, which is reported to mark "exhausted" Treg with lower suppressive function. T-cell immunoglobulin mucin (Tim)-3, a negative regulator of Th1 immunity, is expressed by a sizeable fraction of TIL Tregs, but the functional status of Tim-3+ Tregs remains unclear.Experimental Design: CD4+CTLA-4+CD25high Treg cells were sorted from freshly excised head and neck squamous cell carcinoma (HNSCC) TIL based on Tim-3 expression. Functional and phenotypic features of these Tim-3+ and Tim-3- TIL Tregs were tested by in vitro suppression assays and multi-color flow cytometry. Gene-expression profiling and NanoString analysis of Tim-3+ TIL Treg were performed. A murine HNSCC tumor model was used to test the effect of anti-PD-1 immunotherapy on Tim-3+ Treg.Results: Despite high PD-1 expression, Tim-3+ TIL Treg displayed a greater capacity to inhibit naïve T-cell proliferation than Tim-3- Treg. Tim-3+ Treg from human HNSCC TIL also displayed an effector-like phenotype, with more robust expression of CTLA-4, PD-1, CD39, and IFN-γ receptor. Exogenous IFN-γ treatment could partially reverse the suppressive function of Tim-3+ TIL Treg. Anti-PD-1 immunotherapy downregulated Tim-3 expression on Tregs isolated from murine HNSCC tumors, and this treatment reversed the suppressive function of HNSCC TIL Tregs.Conclusions: Tim-3+ Treg are functionally and phenotypically distinct in HNSCC TIL, and are highly effective at inhibiting T-cell proliferation despite high PD-1 expression. IFN-γ induced by anti-PD-1 immunotherapy may be beneficial by reversing Tim-3+ Treg suppression. Clin Cancer Res; 24(18); 4529-38. ©2018 AACR.

Generation of Human Breg-Like Phenotype with Regulatory Function In Vitro with Bacteria-Derived Oligodeoxynucleotides

Regulatory B cells (Bregs) participate in auto-tolerance maintenance and immune homeostasis. Despite their impact on many diseases and due to the difficulty to define them, knowledge about their origin and their physiological inducers is still unclear. The incomplete understanding about the generation of Bregs and their limited numbers in periphery make it difficult to develop Breg-based therapy. Therefore, identifying factors that promote their development would allow their ex-vivo production in order to create new immunotherapy. This project aims to test the capacity of several cytokines (Interleukin 1-beta (IL-1β), Granulocyte Macrophage Colony-Stimulating Factor (GM-CSF), and Cluster of differentiation 40 ligand (CD40L)) and bacteria-derived oligodeoxynucleotides (CpG-ODN), alone or in combination, to generate B cells with regulatory phenotype and function. We have demonstrated that the Breg-associated phenotypes were heterogeneous between one and other stimulation conditions. However, the expression of other markers related to Bregs such as IL-10, CD80, CD86, CD71, Programmed cell death-1 (PD-1), and Programmed death-ligand 1 (PD-L1) was increased when cells were stimulated with CpG alone or in combination. Moreover, stimulated B cells presented a suppressive function on autologous activated peripheral blood mononuclear cells (PBMC) proliferation. Therefore, this work is the first step to demonstrate the feasibility to induce functional Breg-like cells in vitro and will then facilitate the way to produce Breg-like cells as a potential future cellular therapy.

Immunoregulatory role of IL-35 in T cells of patients with rheumatoid arthritis

OBJECTIVE

IL-35 is the most recently identified member of the IL-12 family. It consists of EBV-induced gene 3 (EBI3) and IL-12α chain p35. We investigated whether IL-35 enhances the in vitro immunosuppressive function of peripheral blood isolated from patients with RA.

METHODS

Peripheral blood was harvested from 17 active and 10 inactive RA patients and IL-35 concentrations were quantified using an ELISA. An expression vector containing IL-35 with a FLAG tag at the carboxyl-terminus was constructed by covalently linking EBI3 and IL-12α (p35). The function of IL-35 was then evaluated in a suppression assay using T cells isolated from human RA patients with CD2, CD3 and CD28 antibodies.

RESULTS

Serum IL-35 levels and the number of Treg were decreased significantly in patients with active RA. There was a significant correlation between serum IL-35 and the 28-joint DAS with ESR (DAS28-ESR) in patients with active RA. IL-35 treatment enhanced the regulatory function, suppressing the levels of inflammatory cytokines such as IL-17 and IFN-γ and the cellular growth of effector T cells stimulated by conjugation with CD2, CD3 and CD28.

CONCLUSION

These data revealed that IL-35 might suppress T cell activation during the peripheral immune responses of RA. Therefore our data suggest that IL-35 might have multiple therapeutic targets.