Foxp3(+) T cells expressing RORγt represent a stable regulatory T-cell effector lineage with enhanced suppressive capacity during intestinal inflammation

Regionalized Development and Maintenance of the Intestinal Adaptive Immune Landscape

The intestinal immune system has the daunting task of protecting us from pathogenic insults while limiting inflammatory responses against the resident commensal microbiota and providing tolerance to food antigens. This role is particularly impressive when one considers the vast mucosal surface and changing landscape that the intestinal immune system must monitor. In this review, we highlight regional differences in the development and composition of the adaptive immune landscape of the intestine and the impact of local intrinsic and environmental factors that shape this process. To conclude, we review the evidence for a critical window of opportunity for early-life exposures that affect immune development and alter disease susceptibility later in life.

Homeostatic Immunity and the Microbiota

The microbiota plays a fundamental role in the induction, education, and function of the host immune system. In return, the host immune system has evolved multiple means by which to maintain its symbiotic relationship with the microbiota. The maintenance of this dialogue allows the induction of protective responses to pathogens and the utilization of regulatory pathways involved in the sustained tolerance to innocuous antigens. The ability of microbes to set the immunological tone of tissues, both locally and systemically, requires tonic sensing of microbes and complex feedback loops between innate and adaptive components of the immune system. Here we review the dominant cellular mediators of these interactions and discuss emerging themes associated with our current understanding of the homeostatic immunological dialogue between the host and its microbiota.

Altered regulatory T-cell fractions and Helios expression in clinically isolated syndrome: clues to the development of multiple sclerosis

Development of multiple sclerosis (MS) is frequently preceded by an acute or subacute neurological disturbance referred to as clinically isolated syndrome (CIS). The specific immunological disturbances present in CIS remain underexamined. This study analysed peripheral blood mononuclear cells from n=18 treatment-naive individuals with recently diagnosed CIS (<120 days) for disturbances in the phenotype of T regulatory (Treg), follicular T regulatory (Tfr), T helper (Th), follicular T helper (Tfh) and B cells. Relative to healthy controls (n=19), CIS was associated with lower proportions of suppressive CD45RA⁺FoxP3^lo Treg and Tfr cells and greater proportions of non-suppressive CD45RA⁻FoxP3^lo and Th17-like Treg and Tfr. Lower Helios expression (maen fluorescent intensity) was measured across all Treg and Tfr fractions in the CIS group, suggesting less potent regulatory function. Greater frequencies of activated, efficient B-cell helper Tfh subsets and a trend for a higher proportion of IgD⁻CD27⁻ B cells was also detected in the CIS group, characteristics that were positively correlated with Treg and Tfr Helios expression. These results indicate that Treg and Tfr impairment is an early feature in MS.

The Guanine-Nucleotide Exchange Factor Caldag Gefi Fine-Tunes Functional Properties of Regulatory T Cells

Using quantitative phosphopeptide sequencing of unstimulated versus stimulated primary murine Foxp3⁺ regulatory and Foxp3^– conventional T cells (Tregs and Tconv, respectively), we detected a novel and differentially regulated tyrosine phosphorylation site within the C1 domain of the guanine-nucleotide exchange factor CalDAG GEFI. We hypothesized that the Treg-specific and activation-dependent reduced phosphorylation at Y523 allows binding of CalDAG GEFI to diacylglycerol, thereby impacting the formation of a Treg-specific immunological synapse. However, diacylglycerol binding assays of phosphomutant C1 domains of CalDAG GEFI could not confirm this hypothesis. Moreover, CalDAG GEFI^–/– mice displayed normal Treg numbers in thymus and secondary lymphoid organs, and CalDAG GEFI^–/– Tregs showed unaltered in vitro suppressive capacity when compared to CalDAG GEFI^+/+ Tregs. Interestingly, when tested in vivo, CalDAG GEFI^–/– Tregs displayed a slightly reduced suppressive ability in the transfer colitis model when compared to CalDAG GEFI^+/+ Tregs. Additionally, CRISPR-Cas9-generated CalDAG GEFI^–/– Jurkat T cell clones showed reduced adhesion to ICAM-1 and fibronectin when compared to CalDAG GEFI-competent Jurkat T cells. Therefore, we speculate that deficiency in CalDAG GEFI impairs adherence of Tregs to antigen-presenting cells, thereby impeding formation of a fully functional immunological synapse, which finally results in a reduced suppressive potential.

Differential expression of GPR15 on T cells during ulcerative colitis

G protein-coupled receptor 15 (GPR15) was recently highlighted as a colon-homing receptor for murine and human CD4+ T cells. The aim of this study was to explore the functional phenotype of human GPR15+CD4+ T cells, focusing on Tregs and effector T cells (Teffs), and to determine whether GPR15 is the driver for the migration of T cells to the colon during ulcerative colitis (UC). In the peripheral blood, GPR15 was expressed on Tregs and Teffs; both GPR15+ T cell subsets produced less IFN-γ and IL-4 but more IL-17 after stimulation and showed a higher migration activity compared with GPR15-CD4+ T cells. In UC patients, GPR15 expression was increased on Tregs in the peripheral blood but not on Teffs. Interestingly, the expression of GPR15 was significantly enhanced on colonic T cells of UC patients in noninflamed biopsies but not in inflamed biopsies. The differential expression of GPR15 in UC patients was accompanied by a significant reduction of bacterial immunoregulatory metabolites in the feces. In conclusion, GPR15 expression on CD4+ T cells is altered in UC patients, which may have implications for the development of therapeutic approaches to target T cell trafficking to the colon.

Transforming growth factor β: a master regulator of the gut microbiota and immune cell interactions

The relationship between host organisms and their microbiota has co-evolved towards an inter-dependent network of mutualistic interactions. This interplay is particularly well studied in the gastrointestinal tract, where microbiota and host immune cells can modulate each other directly, as well as indirectly, through the production and release of chemical molecules and signals. In this review, we define the functional impact of transforming growth factor-beta (TGF-β) on this complex interplay, especially through its modulation of the activity of local regulatory T cells (Tregs), type 17 helper (Th17) cells, innate lymphoid cells (ILCs) and B cells.

CD40-signalling abrogates induction of RORγt+ Treg cells by intestinal CD103+ DCs and causes fatal colitis

Immune homeostasis in intestinal tissues depends on the generation of regulatory T (Treg) cells. CD103⁺ dendritic cells (DCs) acquire microbiota-derived material from the gut lumen for transport to draining lymph nodes and generation of receptor-related orphan γt⁺ (RORγt⁺) Helios⁻-induced Treg (iTreg) cells. Here we show CD40-signalling as a microbe-independent signal that can induce migration of CD103⁺ DCs from the lamina propria (LP) to the mesenteric lymph nodes. Transgenic mice with constitutive CD11c-specific CD40-signalling have reduced numbers of CD103⁺ DCs in LP and a low frequency of RORγt⁺Helios⁻ iTreg cells, exacerbated inflammatory Th1/Th17 responses, high titres of microbiota-specific immunoglobulins, dysbiosis and fatal colitis, but no pathology is detected in other tissues. Our data demonstrate a CD40-dependent mechanism capable of abrogating iTreg cell induction by DCs, and suggest that the CD40L/CD40-signalling axis might be able to intervene in the generation of new iTreg cells in order to counter-regulate immune suppression to enhance immunity.

CD103⁺ dendritic cells induce iTreg cells to maintain immune balance in the gut, but how CD40-signalling regulates this process is unclear. Here the authors show that mice with constitutive CD11c-specific CD40-signalling have altered CD103⁺ dendritic cell migration, reduced iTreg cell induction, and fatal colitis.

Human IL-6RhiTIGIT- CD4+CD127lowCD25+ T cells display potent in vitro suppressive capacity and a distinct Th17 profile

To date many clinical studies aim to increase the number and/or fitness of CD4⁺ CD127^lowCD25⁺ regulatory T cells (Tregs) in vivo to harness their regulatory potential in the context of treating autoimmune disease. Here, we sought to define the phenotype and function of Tregs expressing the highest levels of IL-6 receptor (IL-6R). We have identified a population of CD4⁺ CD127^lowCD25⁺ TIGIT⁻ T cells distinguished by their elevated IL-6R expression that lacked expression of HELIOS, showed higher CTLA-4 expression, and displayed increased suppressive capacity compared to IL-6R^hiTIGIT⁺ Tregs. IL-6R^hiTIGIT⁻ CD127^lowCD25⁺ T cells contained a majority of cells demethylated at FOXP3 and displayed a Th17 transcriptional signature, including RORC (RORγt) and the capacity of producing both pro- and anti-inflammatory cytokines, such as IL-17, IL-22 and IL-10. We propose that in vivo, in the presence of IL-6-associated inflammation, the suppressive function of CD4⁺ CD127^lowCD25⁺ FOXP3⁺ IL-6R^hiTIGIT⁻ T cells is temporarily disarmed allowing further activation of the effector functions and potential pathogenic tissue damage.

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IL-6R is highly expressed in certain Treg subsets.

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IL-6R^hiTIGIT⁻ CD127^lowCD25⁺ T cells contain a subset of antigen-experienced Tregs with potent suppression capacity.

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IL-6R^hiTIGIT⁻ Tregs display a Th17 transcriptional profile ex vivo, and the capacity to migrate to the gut.

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IL-2 treatment in humans elicits the trafficking and expansion of Tregs in circulation.

RORγt expression in Tregs promotes systemic lupus erythematosus via IL-17 secretion, alteration of Treg phenotype and suppression of Th2 responses

Systemic lupus erythematosus (SLE) is a common autoimmune disorder with a complex and poorly understood immunopathogenesis. However, a pathogenic role for the T helper type 17 (Th17) axis was demonstrated by many studies, while regulatory T cells (T_regs ) were shown to mediate protection. Recently, we and others characterized a novel and independent T cell population expressing both the T_reg characteristic transcription factor forkhead box protein 3 (FoxP3) and the Th17-defining retinoic acid receptor-related orphan nuclear receptor γt (RORγt). Studies in a model of acute glomerulonephritis unveiled potent regulatory, but also proinflammatory, functions of RORγt⁺ FoxP3⁺ T_regs . This bi-functional nature prompted us to suggest the name 'biT_regs '. Importantly, the pathogenic biT_reg effects were dependent upon expression of RORγt. We thus aimed to evaluate the contribution of RORγt⁺ FoxP3⁺ biT_regs to pristane-induced SLE and explored the therapeutic potential of interference with RORγt activation. Our analyses revealed expansion of IL-17 producing biT_regs in a distinctive time-course and organ-specific pattern, coincident with the development of autoimmunity and tissue injury. Importantly, specific ablation of RORγt activation in endogenous biT_regs resulted in significant amelioration of pristane-induced pulmonary vasculitis and lupus nephritis. As potential mechanisms underlying the observed protection, we found that secretion of IL-17 by biT_regs was abrogated completely in FoxP3^Cre  × RORC^fl/fl mice. Furthermore, T_regs showed a more activated phenotype after cell-specific inactivation of RORγt signalling. Finally, and remarkably, biT_regs were found to potently suppress anti-inflammatory Th2 immunity in a RORγt-dependent manner. Our study thus identifies biT_regs as novel players in SLE and advocates RORγt-directed interventions as promising therapeutic strategies.

RORγt, a multitask nuclear receptor at mucosal surfaces

RORγt is a nuclear hormone receptor that has followed an exponential success carrier. Its modest origins as an orphan receptor cloned from human pancreas blossomed within 15 years into a critical regulator of anti-microbial immunity and a major target in the fight against inflammatory pathologies. Here, I review its role as a transcription factor required for the generation of type 3 lymphoid cells, which induce the development of lymphoid tissues, provide resistance of epithelial stem cells to injury, maintain homeostasis with the symbiotic microbiota, orchestrate defense against extracellular microbes, and regulate allergic responses. RORγt is also an intriguing molecule that is regulated by the circadian rhythm and includes cholesterol metabolites as ligands. RORγt therefore links anti-microbial immunity with circadian rhythms and steroids, the logic of which remains to be understood.

The Functional Stability of FOXP3 and RORγt in Treg and Th17 and Their Therapeutic Applications

The balance of CD4⁺CD25⁺FOXP3⁺ regulatory T cells (Tregs) and effector T cells plays a key role in maintaining immune homeostasis, while the imbalance of them is related to many inflammatory diseases in both human and mice. Here we discuss about the plasticity of Tregs and Th17 cells, and the related human diseases resulted from the imbalance of them. Further, we will focus on the mechanisms regulating the plasticity between Tregs and Th17 cells and the potential therapeutic strategies by targeting regulators of the expression and activity of FOXP3 and RORγt or regulators of Treg/Th17 balance in autoimmune diseases, allergy, infection, and cancer.

CD4+ T Cell Fate in Glomerulonephritis: A Tale of Th1, Th17, and Novel Treg Subtypes

Multiple studies have identified CD4⁺ T cells as central players of glomerulonephritis (GN). Cells of the Th1 and Th17 responses cause renal tissue damage, while Tregs mediate protection. Recently, a high degree of plasticity among these T cell lineages was proposed. During inflammation, Th17 cells were shown to have the potential of transdifferentiation into Th1, Th2, or alternatively anti-inflammatory Tr1 cells. Currently available data from studies in GN, however, do not indicate relevant Th17 to Th1 or Th2 conversion, leaving the Th17 cell fate enigmatic. Tregs, on the other hand, were speculated to transdifferentiate into Th17 cells. Again, data from GN do not support this concept. Rather, it seems that previously unrecognized subspecialized effector Treg lineages exist. These include Th1 specific Treg1 as well as Th17 directed Treg17 cells. Furthermore, a bifunctional Treg subpopulation was recently identified in GN, which secrets IL-17 and coexpresses Foxp3 together with the Th17 characteristic transcription factor RORγt. Similarities between these different and highly specialized effector Treg subpopulations with the corresponding T helper effector cell lineages might have resulted in previous misinterpretation as Treg transdifferentiation. In summary, Th17 cells have a relatively stable phenotype during GN, while, in the case of Tregs, currently available data suggest lineage heterogeneity rather than plasticity.

Antigen-Specific Development of Mucosal Foxp3+RORγt+ T Cells from Regulatory T Cell Precursors

Foxp3⁺retinoic acid-related orphan receptor (ROR)γt⁺ T cells have recently been characterized as an immunoregulatory population highly enriched in the colon lamina propria. However, their developmental origin and relationship to RORγt^- regulatory T and Th17 cells remain unclear. In this study, we use a fixed TCRβ system to show that the TCR repertoire of the Foxp3⁺RORγt⁺ population is mostly distinct compared with other colonic T cell subsets. However, of these TCRs, a fraction is also found in the Th17 subset, suggesting that TCR repertoire overlap may contribute to the reported ability of Foxp3⁺RORγt⁺ cells to regulate Th17 immunity. Naive transgenic T cells expressing a Foxp3⁺RORγt⁺-restricted TCR first acquire a Foxp3⁺RORγt^- phenotype before coexpressing RORγt, suggesting that Foxp3⁺RORγt⁺ cell development can occur via an RORγt^- regulatory T cell intermediate.

Context- and Tissue-Specific Regulation of Immunity and Tolerance by Regulatory T Cells

The immune system has evolved to defend the organism against an almost infinite number of pathogens in a locally confined and antigen-specific manner while at the same time preserving tolerance to harmless antigens and self. Regulatory T (Treg) cells essentially contribute to an immunoregulatory network preventing excessive immune responses and immunopathology. There is emerging evidence that Treg cells not only operate in secondary lymphoid tissue but also regulate immune responses directly at the site of inflammation. Hence, the classification of Treg cells might need to be further extended by Treg cell subsets that are functionally and phenotypically polarized by their residency. In this review, we discuss recent findings on these tissue-resident Treg cell subsets and how these cells may operate in a tissue- and context-dependent manner.

Elevated IL-23R Expression and Foxp3+Rorgt+ Cells in Intestinal Mucosa During Acute and Chronic Colitis

Background

IL-23/IL-23R signaling plays a pivotal role during the course of inflammatory bowel diseases (IBD). However, the underlying mechanisms are poorly characterized. Foxp3⁺ regulatory T cells are critical in the maintenance of gut immune homeostasis and therefore are important in preventing the development of IBD. This study was performed to clarify the association between IL-23/IL-23R signaling and Foxp3⁺ regulatory T cells in colitis.

Material/Methods

Acute and chronic mouse colitis models were established by administering mice DSS in drinking water. IL-23R, IL-23, IL-I7, and IFN-γ expression level, as well as regulatory T cell, Th17-, and Th1-related transcription factors Foxp3, RORγt, and T-bet were assayed by real-time PCR. The frequency of Foxp3⁺ RORγt⁺ cells in a Foxp3⁺ cell population in colon mucosa during acute and chronic colitis was evaluated through flow cytometry. The signaling pathway mediated by IL-23R in the colon mucosa from acute colitis mice and chronic colitis mice was monitored by Western blot analysis.

Results

We detected elevated IL-23R, IL-23, and IFN-γ expression in colon mucosa during acute and chronic colitis and found increased IL-17 in acute colitis mice. Transcription factors Foxp3 and T-bet were elevated in colon mucosa during acute and chronic colitis. Phosphorylation of Stat3 was greatly enhanced, indicating the activation of IL-23R function in colitis mice. The percentage of Foxp3⁺ T cells in acute and chronic colitis mice was comparable to control mice, but there was a 2-fold increase of Foxp3⁺ RORγt⁺ cells among the Foxp3⁺ cell population in acute and chronic colitis mice compared to control mice.

Conclusions

These findings indicate that the induction of Foxp3⁺ RORγt⁺ T cells could be enhanced during inflammation in the intestine where IL-23R expression is greatly induced. Our study highlights the importance of IL-23R expression level and the instability of Foxp3⁺ regulatory T cells in the development of inflammatory bowel diseases.

The microbiota in adaptive immune homeostasis and disease

In the mucosa, the immune system's T cells and B cells have position-specific phenotypes and functions that are influenced by the microbiota. These cells play pivotal parts in the maintenance of immune homeostasis by suppressing responses to harmless antigens and by enforcing the integrity of the barrier functions of the gut mucosa. Imbalances in the gut microbiota, known as dysbiosis, can trigger several immune disorders through the activity of T cells that are both near to and distant from the site of their induction. Elucidation of the mechanisms that distinguish between homeostatic and pathogenic microbiota-host interactions could identify therapeutic targets for preventing or modulating inflammatory diseases and for boosting the efficacy of cancer immunotherapy.

Tolerance to the Intestinal Microbiota Mediated by ROR(γt)(+) Cells

Harmless microbes colonizing the gut require the establishment of a well-equilibrated symbiosis between this microbiota and its host. However, the immune system is primed to recognize both conserved microbial patterns and foreign antigens, and therefore developed strong tolerance mechanisms to prevent potential fatal immune reactivity to symbiotic microbes. The transcription factor RAR-related orphan-like γt [ROR(γt); encoded by Rorc] plays a key role in the gut for lymphoid tissue organogenesis, development of innate lymphoid cells type 3 (ILC3s) and proinflammatory type 17 T helper (Th17) cells. Surprisingly, recent research has revealed a contribution of ROR(γt)-expressing cells in a variety of tolerance mechanisms in both the innate and adaptive immune system.

Tissue Tregs

The immune system is responsible for defending an organism against the myriad of microbial invaders it constantly confronts. It has become increasingly clear that the immune system has a second major function: the maintenance of organismal homeostasis. Foxp3(+)CD4(+) regulatory T cells (Tregs) are important contributors to both of these critical activities, defense being the primary purview of Tregs circulating through lymphoid organs, and homeostasis ensured mainly by their counterparts residing in parenchymal tissues. This review focuses on so-called tissue Tregs. We first survey existing information on the phenotype, function, sustaining factors, and human equivalents of the three best-characterized tissue-Treg populations-those operating in visceral adipose tissue, skeletal muscle, and the colonic lamina propria. We then attempt to distill general principles from this body of work-as concerns the provenance, local adaptation, molecular sustenance, and targets of action of tissue Tregs, in particular.

Development and maintenance of intestinal regulatory T cells

Gut-resident forkhead box P3 (FOXP3)(+)CD4(+) regulatory T cells (Treg cells) are distinct from those in other organs and have gut-specific phenotypes and functions. Whereas Treg cells in other organs have T cell receptors (TCRs) specific for self antigens, intestinal Treg cells have a distinct set of TCRs that are specific for intestinal antigens, and these cells have pivotal roles in the suppression of immune responses against harmless dietary antigens and commensal microorganisms. The differentiation, migration and maintenance of intestinal Treg cells are controlled by specific signals from the local environment. In particular, certain members of the microbiota continuously provide antigens and immunoregulatory small molecules that modulate intestinal Treg cells. Understanding the development and the maintenance of intestinal Treg cells provides important insights into disease-relevant host-microorganism interactions.