Instability of the transcription factor Foxp3 leads to the generation of pathogenic memory T cells in vivo. Nat Immunol. 2009;10:1000-1007. [PMID: 19633673 PMCID: PMC2729804 DOI: 10.1038/ni.1774]

Cancer vaccination reprograms regulatory T cells into helper CD4 T cells to promote antitumor CD8 T-cell responses

Evaluation of: Sharma MD, Hou DY, Baban B et al. : Reprogrammed Foxp3+ regulatory T cells provide essential help to support cross-presentation and CD8+ T cell priming in naive mice. Immunity 33, 942–954 (2010). It has been recognized that natural CD4+Foxp3+ Tregs could display a phenotypic and functional plasticity in an inflammatory microenvironment. Following the loss of key transcription factor, Foxp3 and core inhibitory molecules associated with suppression, Tregs are reprogrammed into proinflammatory effector cells in vivo. However, the biological significance of this conversion is elusive. Sharma et al. demonstrate that in response to vaccines containing antigens, IFA and CpG, a large proportion of Tregs are dedifferentiated into Th1-like effector cells, which coexpress CD40L – a key molecule for CD8 help by licensing dendritic cells. Under certain experimental conditions, these reprogrammed Tregs are absolutely essential in helping the differentiation of CD8 T cells primed by antigen cross-presentation pathways. Treg conversion is diminished by tumor-induced indoleamine 2,3-dioxygenase in tumor-bearing mice, and blockade of indoleamine 2,3-dioxygenase activity in vivo is able to rescue Treg reprogramming. Collectively, in response to signaling from innate immune cells, Tregs are rapidly reprogrammed into Th1-like effector cells, which are also capable of providing timely help for antigen-specific CD8 T cells in the early phase of activation, when the traditional cognate help from conventional CD4 T cells has not yet became available.

Continuous activation of the CD122/STAT-5 signaling pathway during selection of antigen-specific regulatory T cells in the murine thymus

Signaling events affecting thymic selection of un-manipulated polyclonal natural CD25(+)foxp3(+) regulatory T cells (nTreg) have not been established ex vivo. Here, we report a higher frequency of phosphorylated STAT-5 (pSTAT-5) in nTreg cells in the adult murine thymus and to a lesser extent in the periphery, compared to other CD4(+)CD8(-) subsets. In the neonatal thymus, the numbers of pSTAT-5(+) cells in CD25(+)foxp3(-) and nTreg cells increased in parallel, suggesting that pSTAT-5(+)CD25(+)foxp3(-) cells might represent the precursors of foxp3(+) regulatory T cells. This "specific" pSTAT-5 expression detected in nTreg cells ex vivo was likely due to a very recent signal given by IL-2/IL-15 cytokines in vivo since (i) it disappeared rapidly if cells were left unstimulated in vitro and (ii) was also observed if total thymocytes were stimulated in vitro with saturating amounts of IL-2 and/or IL-15 but not IL-7. Interestingly, STAT-5 activation upon IL-2 stimulation correlated better with foxp3 and CD122 than with CD25 expression. Finally, we show that expression of an endogenous superantigen strongly affected the early Treg cell repertoire but not the proportion of pSTAT-5(+) cells within this repertoire. Our results reveal that continuous activation of the CD122/STAT-5 signaling pathway characterize regulatory lineage differentiation in the murine thymus.

Phenotypical and functional specialization of FOXP3+ regulatory T cells

Forkhead box P3 (FOXP3)(+) regulatory T (T(Reg)) cells prevent autoimmune disease, maintain immune homeostasis and modulate immune responses during infection. To accomplish these tasks, T(Reg) cell activity is precisely controlled, and this requires T(Reg) cells to alter their migratory, functional and homeostatic properties in response to specific cues in the immune environment. We review progress in understanding the diversity of T(Reg) cells, T(Reg) cell function in different anatomical and inflammatory settings, and the influence of the immune environment on T(Reg) cell activity. We also consider how these factors affect immune-mediated disease in the contexts of infection, autoimmunity, cancer and transplantation.

Epigenetic reprogramming of the RORC locus during in vitro expansion is a distinctive feature of human memory but not naïve Treg

The adoptive transfer of in vitro expanded Treg is a promising treatment option for autoimmune as well as alloantigen-induced diseases. Yet, concerns about the phenotypic and functional stability of Tregs upon in vitro culture command both careful selection of the starting population and thorough characterization of the final cell product. Recently, a high degree of developmental plasticity has been described for murine Treg and Th17 cells. Similarly, IL-17-producing FOXP3(+) cells have been detected among the CD45RA(-) memory-type subpopulation of human Tregs ex vivo. This prompted us to investigate the predisposition of human naïve and memory Tregs to develop into Th17 cells during polyclonal in vitro expansion. Here, we show that stimulation-induced DNA demethylation of RORC, which encodes the lineage-defining transcription factor for Th17 cells, occurs selectively in CD45RA(-) memory-type Tregs, irrespective of their FOXP3 expression level. On the contrary, naïve CD45RA(+) Tregs retain stable CpG methylation across the RORC locus even upon prolonged ex vivo expansion and in consequence show only a marginal tendency to express RORC and develop into IL-17-producing cells. These findings are highly relevant for the generation of therapeutic Treg products.

Report of a joint ESOT and AST meeting: highlights in biologic agents and transplantation

A joint meeting organized by the European (ESOT) and American (AST) Societies of Transplantation occurred in Nice, France, October 1-3, 2010. Focused on emerging use of biologic agents in solid organ transplantation, it served as a venue for state-of-the-art updates in basic immunology and clinical science, with an emphasis on the interrelatedness of the two. This meeting summary is designed to highlight important insights communicated in Nice, offer an overview of novel therapeutics in development, and entice members of all societies to consider attending a second joint symposium, under consideration for 2012.

CD4+CD25+CD127dimFoxp3+ T cells are cytotoxic for human neurons

MS lesions are characterized by destruction of myelin and significant neuronal and axonal loss. Preliminary studies with the use of T(regs) in the mouse model of MS have been extremely encouraging. However, recent studies with human cells have shown the presence of different subpopulations of T cells within the CD4(+)CD25(+)Foxp3(+) T cell phenotype, some of which do not have regulatory functions. These findings suggest a potential difference between mouse and human in the regulatory phenotype. Here, we show that human activated CD4(+)CD25(+)Foxp3(+) T cells are neurotoxic in vitro. These cells expressed high levels of the cytotoxic molecule GrB and had no suppressive effect. On the contrary, they produced IFN-γ and low IL-17, suggesting a shift toward a T(H)1 phenotype. Thus, our data confirm the presence of a nonregulatory cytotoxic subpopulation within the human CD4(+)CD25(+)Foxp3(+) T cells and suggest further studies on the human regulatory phenotype prior to any potential therapeutic application.

Plasticity of human regulatory T cells in healthy subjects and patients with type 1 diabetes

Regulatory T cells (Tregs) constitute an attractive therapeutic target given their essential role in controlling autoimmunity. However, recent animal studies provide evidence for functional heterogeneity and lineage plasticity within the Treg compartment. To understand better the plasticity of human Tregs in the context of type 1 diabetes, we characterized an IFN-γ-competent subset of human CD4(+)CD127(lo/-)CD25(+) Tregs. We measured the frequency of Tregs in the peripheral blood of patients with type 1 diabetes by epigenetic analysis of the Treg-specific demethylated region (TSDR) and the frequency of the IFN-γ(+) subset by flow cytometry. Purified IFN-γ(+) Tregs were assessed for suppressive function, degree of TSDR demethylation, and expression of Treg lineage markers FOXP3 and Helios. The frequency of Tregs in peripheral blood was comparable but the FOXP3(+)IFN-γ(+) fraction was significantly increased in patients with type 1 diabetes compared to healthy controls. Purified IFN-γ(+) Tregs expressed FOXP3 and possessed suppressive activity but lacked Helios expression and were predominately methylated at the TSDR, characteristics of an adaptive Treg. Naive Tregs were capable of upregulating expression of Th1-associated T-bet, CXCR3, and IFN-γ in response to IL-12. Notably, naive, thymic-derived natural Tregs also demonstrated the capacity for Th1 differentiation without concomitant loss of Helios expression or TSDR demethylation.

Induction of transplantation tolerance converts potential effector T cells into graft-protective regulatory T cells

Naturally occurring FOXP3(+) CD4(+) Treg have a crucial role in self-tolerance. The ability to generate similar populations against alloantigens offers the possibility of preventing transplant rejection without indefinite global immunosuppression. Exposure of mice to donor alloantigens combined with anti-CD4 antibody induces operational tolerance to cardiac allografts, and generates Treg that prevent skin and islet allograft rejection in adoptive transfer models. If protocols that generate Treg in vivo are to be developed in the clinical setting it will be important to know the origin of the Treg population and the mechanisms responsible for their generation. In this study, we demonstrate that graft-protective Treg arise in vivo both from naturally occurring FOXP3(+) CD4(+) Treg and from non-regulatory FOXP3(-) CD4(+) cells. Importantly, tolerance induction also inhibits CD4(+) effector cell priming and T cells from tolerant mice have impaired effector function in vitro. Thus, adaptive tolerance induction shapes the immune response to alloantigen by converting potential effector cells into graft-protective Treg and by expanding alloreactive naturally occurring Treg. In relation to clinical tolerance induction, the data indicate that while the generation of alloreactive Treg may be critical for long-term allograft survival without chronic immunosuppression, successful protocols will also require strategies that target potential effector cells.

Host-bacterial symbiosis in health and disease

All animals live in symbiosis. Shaped by eons of co-evolution, host-bacterial associations have developed into prosperous relationships creating mechanisms for mutual benefits to both microbe and host. No better example exists in biology than the astounding numbers of bacteria harbored by the lower gastrointestinal tract of mammals. The mammalian gut represents a complex ecosystem consisting of an extraordinary number of resident commensal bacteria existing in homeostasis with the host's immune system. Most impressive about this relationship may be the concept that the host not only tolerates, but has evolved to require colonization by beneficial microorganisms, known as commensals, for various aspects of immune development and function. The microbiota provides critical signals that promote maturation of immune cells and tissues, leading to protection from infections by pathogens. Gut bacteria also appear to contribute to non-infectious immune disorders such as inflammatory bowel disease and autoimmunity. How the microbiota influences host immune responses is an active area of research with important implications for human health. This review synthesizes emerging findings and concepts that describe the mutualism between the microbiota and mammals, specifically emphasizing the role of gut bacteria in shaping an immune response that mediates the balance between health and disease. Unlocking how beneficial bacteria affect the development of the immune system may lead to novel and natural therapies based on harnessing the immunomodulatory properties of the microbiota.

Human FoxP3(+)CD4(+) regulatory T cells: their knowns and unknowns

Human regulatory T cells (Tregs) expressing the transcription factor FoxP3 play indispensable roles for the maintenance of immunological self-tolerance and immune homeostasis. In this review, we discuss immunological characteristics of human FoxP3(+) Tregs and to what extent they are similar to or different from the murine counterparts. We also discuss important issues that remain to be addressed in the field to better understand the pathophysiology of autoimmune disease and to build solid basis for immunotherapies targeting FoxP3(+) Tregs.

Environmental influences on T regulatory cells in inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by chronic, idiopathic inflammation of the intestine. The disease is thought to result from a combination of genetic and environmental factors which ultimately leads to a mucosal immune system that overreacts to normal constituents of the mucosal microbiota. The inflammation in IBD is primarily mediated by inappropriate production of proinflammatory cytokines by CD4(+) T effector cells, effects that are suppressed by CD4(+) T regulatory cells. Defects in both the function of T regulatory cells, and the ability of T effector cells to be suppressed, have been implicated in IBD. In this review we will discuss environmental factors, including cytokines, vitamins A and D, and commensal bacteria, which influence the phenotype and function of regulatory T cells and thereby alter the course of IBD. We will also discuss how these environmental signals can be manipulated therapeutically in order to improve the function of regulatory T cells and ultimately restore mucosal homeostasis in patients with IBD.

Distinct inflammatory signals have physiologically divergent effects on epigenetic regulation of Foxp3 expression and Treg function

Foxp3 expression in regulatory T cells (Treg) is required for their development and suppressive function. How different inflammatory signals affect Foxp3 chromatin structure, expression and Tregs plasticity are not completely known. In the present study, the Toll-like receptor 2 (TLR2) ligand peptidoglycan inhibited Foxp3 expression in both natural Treg (nTreg) and TGFβ-driven adaptive Treg (aTreg). Inhibition was independent of paracrine Th1, Th2 and Th17 cytokines. PGN-induced T cell-intrinsic TLR2-Myd88-dependent IFR1 expression and induced IRF1 bound to IRF1 response elements (IRF-E) in the Foxp3 promoter and intronic enhancers, and negatively regulated Foxp3 expression. Inflammatory IL-6 and TLR2 signals induced divergent chromatin changes at the Foxp3 locus and regulated Treg suppressor function, and in an islet transplant model resulted in differences in their ability to prolong graft survival. These findings are important for understanding how different inflammatory signals can affect the transplantation tolerance and immunity.

Retinoic acid and rapamycin differentially affect and synergistically promote the ex vivo expansion of natural human T regulatory cells

Natural T regulatory cells (Tregs) are challenging to expand ex vivo, and this has severely hindered in vivo evaluation of their therapeutic potential. All trans retinoic acid (ATRA) plays an important role in mediating immune homeostasis in vivo, and we investigated whether ATRA could be used to promote the ex vivo expansion of Tregs purified from adult human peripheral blood. We found that ATRA helped maintain FOXP3 expression during the expansion process, but this effect was transient and serum-dependent. Furthermore, natural Tregs treated with rapamycin, but not with ATRA, suppressed cytokine production in co-cultured effector T cells. This suppressive activity correlated with the ability of expanded Tregs to induce FOXP3 expression in non-Treg cell populations. Examination of CD45RA+ and CD45RA− Treg subsets revealed that ATRA failed to maintain suppressive activity in either population, but interestingly, Tregs expanded in the presence of both rapamycin and ATRA displayed more suppressive activity and had a more favorable epigenetic status of the FOXP3 gene than Tregs expanded in the presence of rapamycin only. We conclude that while the use of ATRA as a single agent to expand Tregs for human therapy is not warranted, its use in combination with rapamycin may have benefit.

Immunosuppressive mechanisms during viral infectious diseases

For a virus to establish persistence in the host, it has to exploit the host immune system such that the active T-cell responses against the virus are curbed. On the other hand, the goal of the immune system is to clear the virus, following which the immune responses need to be downregulated, by a process known as immunoregulation. There are multiple known immunoregulatory mechanisms that appear to play a role in persistent viral infections. In the recent past, IL-10 and PD-1 have been identified to be playing a significant role in the regulation of antiviral immune responses. The evidence that viruses can escape immunologic attack by taking advantage of the host's immune system is found in LCMV infection of mice and in humans persistently infected with HIV and HCV. The recent observation that the functionally inactive T-cells during chronic viral infections can be made to regain their cytokine secretion and cytolytic abilities is very encouraging. Thus, it would be likely that neutralization negative immune regulation during persistent viral infection would result in the preservation of effector T-cell responses against the virus, thereby resulting in the elimination of the persistent infection.

Adaptive immunity in the host-microbiota dialog

The intestinal tract represents the largest mucosal surface and is a major site of multifaceted interactions between the host mucosal immune system and components of the intestinal microbiota. Host immune responses to the commensal microbiota are tightly controlled and, meanwhile, the microbiota actively shapes intestinal immune responses to itself. Appreciation of these interactions during health and disease may direct therapeutic approaches to a broad range of autoimmune and inflammatory disorders in humans. In this review, we will discuss findings on how the intestinal immune system, especially adaptive immune cells, helps accommodate the large number of resident bacteria, and in turn how the microbiota shapes intestinal immune responses to achieve mutualism.

Transplantation tolerance: Clinical potential of regulatory T cells

The major challenge in transplantation medicine remains long-term allograft acceptance, with preserved allograft function under minimal chronic immunosuppression. To safely achieve the goal of sustained donor-specific T and B cell non-responsiveness, research efforts are now focusing on therapies based on cell subsets with regulatory properties. In particular the transfusion of human regulatory T cells (Treg) is currently being evaluated in phase I/II clinical trials for the treatment of graft versus host disease following hematopoietic stem cell transplantation, and is also under consideration for solid organ transplantation. The purpose of this review is to recapitulate current knowledge on naturally occurring as well as induced human Treg, with emphasis on their specific phenotype, suppressive function and how these cells can be manipulated in vitro and/or in vivo for therapeutic purposes in transplantation medicine. We highlight the potential but also possible limitations of Treg-based strategies to promote long-term allograft survival. It is evident that the bench-to-beside translation of these protocols still requires further understanding of Treg biology. Nevertheless, current data already suggest that Treg therapy alone will not be sufficient and needs to be combined with other immunomodulatory approaches in order to induce allograft tolerance.

Thymic and peripheral differentiation of regulatory T cells

The development of regulatory T (Treg) cells is essential for the maintenance of immune tolerance and homeostasis. Here, we review recent studies that have advanced our understanding of Treg cell differentiation. In the thymus, TCR specificity to self-antigen appears to be a primary determinant for Treg cell lineage commitment, with c-Rel being an important factor that links T cell receptor (TCR) engagement and Foxp3 expression, along with cytokines and costimulatory molecules. It is also clear that postthymic events shape the peripheral Treg cell population. This includes preferential maintenance of Treg cells specific to self-antigens presented in the periphery, as well as the de novo generation of Treg cells from conventional Foxp3(-) T cells. The process of peripheral Treg cell differentiation shares some features with thymic Treg cell development, but there are notable differences. Together, thymic and peripheral Treg cell differentiation appear to generate an "imprint" of both self- and foreign antigens in the peripheral Treg cell population to provide dominant tolerance.

Type 1 Diabetes: Etiology, Immunology, and Therapeutic Strategies

Type 1 diabetes (T1D) is a chronic autoimmune disease in which destruction or damaging of the beta-cells in the islets of Langerhans results in insulin deficiency and hyperglycemia. We only know for sure that autoimmunity is the predominant effector mechanism of T1D, but may not be its primary cause. T1D precipitates in genetically susceptible individuals, very likely as a result of an environmental trigger. Current genetic data point towards the following genes as susceptibility genes: HLA, insulin, PTPN22, IL2Ra, and CTLA4. Epidemiological and other studies suggest a triggering role for enteroviruses, while other microorganisms might provide protection. Efficacious prevention of T1D will require detection of the earliest events in the process. So far, autoantibodies are most widely used as serum biomarker, but T-cell readouts and metabolome studies might strengthen and bring forward diagnosis. Current preventive clinical trials mostly focus on environmental triggers. Therapeutic trials test the efficacy of antigen-specific and antigen-nonspecific immune interventions, but also include restoration of the affected beta-cell mass by islet transplantation, neogenesis and regeneration, and combinations thereof. In this comprehensive review, we explain the genetic, environmental, and immunological data underlying the prevention and intervention strategies to constrain T1D.

CD8 regulates T regulatory cell production of IL-6 and maintains their suppressive phenotype in allergic lung disease

Naturally occurring CD4(+)CD25(+)Foxp3(+) T regulatory cells (nTregs) regulate lung allergic responses through production of IL-10 and TGF-β. nTregs from CD8(-/-) mice failed to suppress lung allergic responses and were characterized by reduced levels of Foxp3, IL-10, and TGF-β, and high levels of IL-6. Administration of anti-IL-6 or anti-IL-6R to wild-type recipients prior to transfer of CD8(-/-) nTregs restored suppression. nTregs from IL-6(-/-) mice were suppressive, but lost this capability if incubated with IL-6 prior to transfer. The importance of CD8 in regulating the production of IL-6 in nTregs was demonstrated by the loss of suppression and increases in IL-6 following transfer of nTregs from wild-type donors depleted of CD8(+) cells. Transfer of nTregs from CD8(-/-) donors reconstituted with CD8(+) T cells was suppressive, and accordingly, IL-6 levels were reduced. These data identify the critical role of CD8-T regulatory cell interactions in regulating the suppressive phenotype of nTregs through control of IL-6 production.

An oscillatory switch in mTOR kinase activity sets regulatory T cell responsiveness

There is a discrepancy between the in vitro anergic state of CD4(+)CD25(hi)FoxP3(+) regulatory T (Treg) cells and their in vivo proliferative capability. The underlying mechanism of this paradox is unknown. Here we show that the anergic state of Treg cells depends on the elevated activity of the mammalian target of rapamycin (mTOR) pathway induced by leptin: a transient inhibition of mTOR with rapamycin, before T cell receptor (TCR) stimulation, made Treg cells highly proliferative in the absence of exogenous interleukin-2 (IL-2). This was a dynamic and oscillatory phenomenon characterized by an early downregulation of the leptin-mTOR pathway followed by an increase in mTOR activation necessary for Treg cell expansion to occur. These data suggest that energy metabolism, through the leptin-mTOR-axis, sets responsiveness of Treg cells that use this information to control immune tolerance and autoimmunity.

Reprogrammed foxp3(+) regulatory T cells provide essential help to support cross-presentation and CD8(+) T cell priming in naive mice

Foxp3(+) regulatory T (Treg) cells can undergo reprogramming into a phenotype expressing proinflammatory cytokines. However, the biologic significance of this conversion remains unclear. We show that large numbers of Treg cells undergo rapid reprogramming into activated T helper cells after vaccination with antigen plus Toll-like receptor 9 (TLR-9) ligand. Helper activity from converted Treg cells proved essential during initial priming of CD8(+) T cells to a new cross-presented antigen. Help from Treg cells was dependent on CD40L, and (unlike help from conventional non-Treg CD4(+) cells) did not require preactivation or prior exposure to antigen. In hosts with established tumors, Treg cell reprogramming was suppressed by tumor-induced indoleamine 2,3-dioxygenase (IDO) and vaccination failed because of lack of help. Treg cell reprogramming, vaccine efficacy, and antitumor CD8(+) T cell responses were restored by pharmacologic inhibition of IDO. Reprogrammed Treg cells can thus participate as previously unrecognized drivers of certain early CD8(+) T cell responses.

Generation of human Th1-like regulatory CD4+ T cells by an intrinsic IFN-γ- and T-bet-dependent pathway

Murine Foxp3(+) Treg have recently been shown to express T-bet, a transcription factor characteristic of Th1 effector cells. A human Treg phenotype equivalent has not been reported. Here, we show that naïve human CD4(+) T cells incubated with low numbers of CD40-activated allogeneic B cells preferentially differentiate into alloantigen-specific CD4(hi) CD25(hi) Treg. These differentiated cells potently suppress effector T-cell responses and express T-bet, IFN-γ, and CXCR3, the features of Th1 effector cells. In contrast, co-culture of naïve CD4(+) T cells with high numbers of allogeneic B cells results in CD4(+) CD25(+) T cells that promote, rather than inhibit, effector T-cell responses, demonstrating the plasticity of CD4(+) T-cell differentiation in response to alloantigen-presenting B cells. The optimal accumulation of CD4(hi) CD25(hi) Treg induced using higher T cell:B cell co-culture ratios was dependent on the expression of T-bet and endogenously produced IFN-γ. Induction of Treg-mediated suppression function in the Treg population was not. As CXCR3 confers the preferential trafficking of T cells to tissue sites of IFN-γ, these human Th1-like Treg might be useful for modulating pathological Th1 effector responses, such as that occurring during graft-versus-host disease or graft rejection.

CD4+ regulatory T cells in solid organ transplantation

PURPOSE OF REVIEW

Solid organ transplantation is the most effective treatment for end-stage organ failure, but the long-term outcomes remain suboptimal. CD4 regulatory T cells (Tregs) are emerging as a potential therapy to facilitate long-term allograft survival. This review provides a general overview of the biology of CD4 Tregs and then goes on to discuss the most relevant and recent experimental and clinical evidence for their therapeutic use in solid organ transplantation.

RECENT FINDINGS

There have been major advances in our understanding of Tregs, including improvements in methods for their isolation and expansion. Experimental models are providing very important data on the in-vitro and in-vivo behavior of Tregs in transplantation, while recent clinical trials of Treg cellular therapy in graft-versus-host disease are offering a valuable insight into the efficacy of Treg adoptive cellular therapy.

SUMMARY

Data in favor of Treg cellular therapy in transplantation are mounting, and we predict that their use in clinical trials is on the horizon.

Transgenic mouse models of multiple sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease affecting the central nervous system (CNS) and a frequent cause of neurological disability in young adults. Multifocal inflammatory lesions in the CNS white matter, demyelination, oligodendrocyte loss, axonal damage, as well as astrogliosis represent the histological hallmarks of the disease. These pathological features of MS can be mimicked, at least in part, using animal models. This review discusses the current concepts of the immune effector mechanisms driving CNS demyelination in murine models. It highlights the fundamental contribution of transgenesis in identifying the mediators and mechanisms involved in the pathophysiology of MS models.

Plasticity of T(reg) cells: is reprogramming of T(reg) cells possible in the presence of FOXP3?

Regulatory T cells (T(reg) cells) are involved in self tolerance, immune homeostasis, prevention of autoimmunity, and suppression of immunity to pathogens or tumors. The forkhead transcription factor FOXP3 is essential for T(reg)-cell development and function as mutations in FOXP3 cause severe autoimmune diseases in mice and humans. Over the last years it has been postulated that FOXP3 expression in T(reg) prevents effector T-cell (T(effector)-cell) lineage commitment, yet several recent studies suggest that the co-existence of effector and regulatory T-cell programs can occur and might help to enable T(reg) cells with properties necessary to exert their function in peripheral tissues. Furthermore, downregulation of FOXP3 in the periphery might help T(reg) cells to lose suppressive functions and gain memory properties with specificity for self-antigens and an effector phenotype including the ability to produce IFN-γ and IL-17. This plasticity might have an impact on their reactivity towards autoimmunity as well as tumors or infections.

Oestrogen-mediated protection of experimental autoimmune encephalomyelitis in the absence of Foxp3+ regulatory T cells implicates compensatory pathways including regulatory B cells

Oestrogen (17β-oestradiol, E₂) is a highly effective treatment for experimental autoimmune encephalomyelitis (EAE) that may potentiate Foxp3+ regulatory T (Treg) cells, which in turn limit the expansion of encephalitogenic T-cell specificities. To determine if Treg cells constitute the major non-redundant protective pathway for E₂, we evaluated E₂ protection of EAE after targeted deletion of Foxp3 expression in Foxp3-DTR mice. Unexpectedly, E₂-treated Foxp3-deficient mice were completely protected against clinical and histological myelin oligodendrocyte glycoprotein (MOG)-35-55 peptide-induced EAE before succumbing to diphtheria toxin-induced mortality. This finding indicated the presence of alternative E₂-dependent EAE-protective pathways that could compensate for the lack of Treg cells. Further investigation revealed that E₂ treatment inhibited proliferation and expression of CCL2 and CXCL2, but enhanced secretion of interleukin-10 (IL-10) and IL-13 by MOG-35-55-specific spleen cells. These changes occurred concomitantly with increased expression of several chemokines and receptors, including CXCL13 and CXCR5, and the negative co-activation molecules, PD-L1 and B7.2, by B cells and dendritic cells. Furthermore, E₂ treatment resulted in higher percentages of spleen and lymph node T cells expressing IL-17, interferon-γ and tumour necrosis factor-α, but with lower expression of CCR6, suggesting sequestration of MOG-35-55 peptide-specific T cells in peripheral immune organs. Taken together, these data suggest that E₂-induced mechanisms that provide protection against EAE in the absence of Foxp3+ Treg cells include induction of regulatory B cells and peripheral sequestration of encephalitogenic T cells.

FOXP3 and RORγt: transcriptional regulation of Treg and Th17

FOXP3(+)CD4(+)CD25(+) Regulatory T (Treg) cells and IL-17 producing helper T cells (Th17) are critical subsets of T cells which play essential roles in immune homeostasis. The Forkhead family transcription factor FOXP3 is predominantly expressed in Treg cells, where the FOXP3 ensemble is essential for Treg cell development and function. As FOXP3 is to Treg cells, the orphan retinoic acid nuclear receptor (ROR) family transcription factor RORγt is essential for Th17 development and function. In this review, we summarize recent progress of our understanding towards the molecular mechanisms underlying the differentiation and function of FOXP3(+) Treg cells and RORγt expressing Th17 cells. These may provide new insights into therapeutic intervention and targeting of human immune-deficient diseases.

IL-17- and IFN-γ-secreting Foxp3+ T cells infiltrate the target tissue in experimental autoimmunity

CD4(+)Foxp3(+) regulatory T cells (Tregs) have been considered crucial in controlling immune system homeostasis, and their derangement is often associated to autoimmunity. Tregs identification is, however, difficult because most markers, including CD25 and Foxp3, are shared by recently activated T cells. We show in this paper that CD4(+)Foxp3(+) T cells are generated in peripheral lymphoid organs on immunization and readily accumulate in the target organ of an autoimmune reaction, together with classical inflammatory cells, constituting up to 50% of infiltrating CD4(+) T cells. Most CD4(+)Foxp3(+) T cells are, however, CD25(-) and express proinflammatory cytokines such as IL-17 and IFN-γ, questioning their suppressive nature. Moreover, in vitro CD4(+) T lymphocytes from naive and autoimmune mice, stimulated to differentiate into Th1, Th2, Th17, and induced Tregs, display early mixed expression of lineage-specific markers. These results clearly point to an unprecedented plasticity of naive CD4(+) T cells, that integrating inflammatory signals may change their fate from the initial lineage commitment to a different functional phenotype.

Myelin-reactive, TGF-β-induced regulatory T cells can be programmed to develop Th1-like effector function but remain less proinflammatory than myelin-reactive Th1 effectors and can suppress pathogenic T cell clonal expansion in vivo

Interest in the use of regulatory T cells (Tregs) as cellular therapeutics has been tempered by reports of naturally occurring Tregs losing Foxp3 expression and producing IL-17, raising concerns over a switch to pathogenic function under inflammatory conditions in vivo. TGF-β-induced Tregs (inducible Tregs [iTregs]), generated in large numbers in response to disease-relevant Ags, represent the most amenable source of therapeutic Tregs. Using Foxp3-reporter T cells recognizing myelin basic protein (MBP), we investigated the capacity of iTregs to produce effector-associated cytokines under proinflammatory cytokine conditions in vitro and whether this translated into proinflammatory function in vivo. In contrast with naturally occurring Tregs, iTregs resisted conversion to an IL-17-producing phenotype but were able to express T-bet and to produce IFN-γ. iTregs initiated their T-bet expression during their in vitro induction, and this was dependent on exposure to IFN-γ. IL-12 reignited iTreg expression of T-bet and further promoted iTreg production of IFN-γ upon secondary stimulation. Despite losing Foxp3 expression and expressing both T-bet and IFN-γ, MBP-responsive IL-12-conditioned iTregs induced only mild CNS inflammation and only when given in high numbers. Furthermore, iTregs retained an ability to suppress naive T cell clonal expansion in vivo and protected against the development of experimental autoimmune encephalomyelitis. Therefore, despite bearing predictive hallmarks of pathogenic effector function, previously Foxp3(+) iTregs have much lower proinflammatory potential than that of MBP-responsive Th1 cells. Our results demonstrate that autoprotective versus autoaggressive functions in iTregs are not simply a binary relationship to be determined by their relative expression of Foxp3 versus T-bet and IFN-γ.

An intrinsic mechanism predisposes Foxp3-expressing regulatory T cells to Th2 conversion in vivo

Naturally occurring regulatory T (nTreg) cells express Foxp3 and were originally discovered as immune suppressors critical for self-tolerance and immune homeostasis. Through yet-to-be-defined mechanisms, nTreg cells were recently shown to convert into proinflammatory cells. Particularly, attenuation of Foxp3 expression led to Th2 conversion of nTreg cells in vivo. In this paper, we demonstrated an nTreg-specific mechanism controlling their Th2 conversion. We found that wild-type nTreg cells expressing reduced levels of Foxp3 but not those expressing no Foxp3 produced the Th2 cytokine IL-4. Intriguingly, IL-4 production by converted nTreg cells is required for Th2 differentiation of coexisting naive CD4 T cells in vivo, suggesting that Th2 conversion of nTreg cells might be critical for directing Th2 immune responses. Th2 conversion of nTreg cells was not due to their inability to become Th1 cells, because IFN-γ was produced by Foxp3-low-expressing cells when IL-4/STAT-6 signaling was abrogated. Surprisingly, however, unlike naive CD4 T cells whose IL-4 production is dependent on STAT-6, Foxp3-low-expressing cells generated IL-4 independent of STAT-6, indicating an intrinsic mechanism that favors nTreg-to-Th2 differentiation. Indeed, compared with naive CD4 T cells, nTreg expressed elevated levels of GATA-3 independent of STAT-6. And GATA-3 was required for nTreg-to-Th2 conversion. Foxp3 may account for this GATA-3 upregulation in nTreg cells, because ectopic expression of Foxp3 preferentially promoted GATA-3 but not T-bet expression. Thus, we have identified an intrinsic mechanism that imposes a Th2/Th1 imbalance and predisposes Foxp3-expressing cells to IL-4 production independent of STAT-6 signaling.

The importance of LAT in the activation, homeostasis, and regulatory function of T cells

LAT (linker for activation of T cells) is a transmembrane adaptor protein that plays an essential role in TCR-mediated signaling and thymocyte development. Because LAT-deficient mice have an early block in thymocyte development, we utilized an inducible system to delete LAT in primary T cells to study LAT function in T cell activation, homeostasis, and survival. Deletion of LAT caused primary T cells to become unresponsive to stimulation from the TCR and impaired T cell homeostatic proliferation and long term survival. Furthermore, deletion of LAT led to reduced expression of Foxp3, CTLA-4, and CD25 in T(reg) cells and impaired their function. Consequently, mice with LAT deleted developed a lymphoproliferative syndrome similar to that in LATY136F mice, although less severe. Our data implicate that LAT has positive and negative roles in the regulation of mature T cells.

Expression of CD39 by human peripheral blood CD4+ CD25+ T cells denotes a regulatory memory phenotype

We have shown that CD39 and CD73 are coexpressed on the surface of murine CD4+ Foxp3+ regulatory T cells (Treg) and generate extracellular adenosine, contributing to Treg immunosuppressive activity. We now describe that CD39, independently of CD73, is expressed by a subset of blood-derived human CD4+ CD25+ CD127lo Treg, defined by robust expression of Foxp3. A further distinct population of CD4+ CD39+ T lymphocytes can be identified, which do not express CD25 and FoxP3 and exhibit the memory effector cellular phenotype. Differential expression of CD25 and CD39 on circulating CD4+ T cells distinguishes between Treg and pathogenic cellular populations that secrete proinflammatory cytokines such as IFNγ and IL-17. These latter cell populations are increased, with a concomitant decrease in the CD4+ CD25+ CD39+ Tregs, in the peripheral blood of patients with renal allograft rejection. We conclude that the ectonucleotidase CD39 is a useful and dynamic lymphocytes surface marker that can be used to identify different peripheral blood T cell-populations to allow tracking of these in health and disease, as in renal allograft rejection.

Inflammation and the balance of Treg and Th17 cells in transplant rejection and tolerance

PURPOSE OF REVIEW

Inflammation of the allograft, occurring as a consequence of hypoxia and ischemia/reperfusion injury, adversely influences short-term and long-term transplant outcomes. Thus far, imbalance of tissue-protective Treg and tissue-destructive Th17 cells has been confirmed in a number of tissue-inflammatory states, including autoimmune disease. Hence, benefits of tilting Treg-Th17 equilibrium toward dominance of Tregs may promote transplant tolerance.

RECENT FINDINGS

Adverse graft inflammation creates extreme resistance to the induction of donor-specific tolerance. Proinflammatory cytokines, when abundantly expressed within the graft and draining lymph nodes, prevent commitment of donor-activated T cells into graft-protective, T-regulatory phenotype, while fostering generation of donor-reactive Th1, Th2 or Th17 effector subsets. In addition, the inflammatory milieu may destabilize the program of both natural and induced Tregs, converting them into inflammatory, effector-like phenotypes. Therefore permanent, Treg-dependent acceptance of an allograft may not be achieved without limiting adverse tissue inflammation.

SUMMARY

Balance of graft-protective regulatory and graft-destructive effector T cells largely depends on the balance of proinflammatory and anti-inflammatory cytokines in the milieu, in which donor-directed T-cell response occurs. In the absence of proinflammatory cytokines, the constitutive expression of TGF-beta may guide recipient T cells into a tissue-protective, pro-tolerant mode. Therefore, targeting adverse tissue inflammation may represent a powerful means to tilt antidonor immunity towards tolerance.

Effector and regulatory T-cell subsets in autoimmunity and tissue inflammation

Many autoimmune diseases are driven by self-reactive T helper cells. Until recently, organ-specific autoimmune diseases were primarily associated with Th1 cells but not Th2 cells. However, the discovery of a number of new effector T-cell subsets, like Th17 and Th9 cells, and regulatory T cells, like Tregs and Tr1 cells, has changed the way we view and understand autoimmunity at cellular and molecular levels. In recent years, IL-17-producing Th17 cells have emerged as major players in autoimmunity. The complicated relationship between Th1 and Th17 cells, as well as the intricate balance between Tregs and Th17 cells, provides a basis for understanding the immunological mechanisms that induce and regulate autoimmunity. Here, we give an overview of the interplay between different effector T-cell subsets and regulatory T-cell subsets, and how they contribute to the development of autoimmunity and tissue inflammation.

Synergistic effect of IL-6 and IL-4 in driving fate revision of natural Foxp3+ regulatory T cells

Expression of forkhead transcription factor Foxp3 defines a distinct lineage of naturally arising regulatory T cells (nTregs) that is segregated from effector CD4(+) T cells during early development in the thymus. It remains elusive whether nTregs can convert into effector cells by turning off their Foxp3 expression and, if so, whether Th17 is a default alternative fate choice. In this report we provide compelling evidence showing that effector T cell-polarizing cytokines IL-6 and IL-4 can act synergistically to induce marked downregulation and inactivation of Foxp3 gene expression in mouse nTregs, and consequently the loss of suppressor phenotype and functions. However, the resulting Foxp3(-) cells are not polarized and do not express IL-17 or other Th17-associated genes. Therefore, nTreg fate revision is not restricted to the Treg-Th17 axis and is likely to represent a rather broad phenomenon with divergent outcomes.

Interleukin-17: friend or foe in atherosclerosis?

PURPOSE OF REVIEW

To summarize the current knowledge on the origin and physiological function of T helper 17 (Th17) and discuss the contrasting results pertaining to the role of Th17 and interleukin-17 (IL-17) in atherosclerosis.

RECENT FINDINGS

Atherosclerosis is a chronic inflammatory arterial disease driven by both innate and adaptive immune responses to modified lipoproteins. Although initial studies have focused on the role of Th1 and Th2 responses in atherosclerosis, more recent findings identified atheroprotective roles for several subsets of regulatory T cells. Th17 represents a new T-cell lineage with important roles in the clearance of pathogenic bacteria and fungi. The increase of Th17 and IL-17 has been recently linked to the pathogenesis of several autoimmune diseases. However, its role in chronic inflammatory diseases such as atherosclerosis remains poorly understood. The few studies available on this topic have generated contrasting results, which could be attributed to different approaches used on various mouse models.

SUMMARY

IL-17 seems to have a modulatory role in atherosclerosis. Future studies are needed to better determine the molecular mechanisms involved in this regulation and examine whether targeting IL-17 pathway will be useful to treat cardiovascular diseases.

Intestinal dendritic cells: their role in bacterial recognition, lymphocyte homing, and intestinal inflammation

Dendritic cells (DCs) play a key role in discriminating between commensal microorganisms and potentially harmful pathogens and in maintaining the balance between tolerance and active immunity. The regulatory role of DC is of particular importance in the gut where the immune system lies in intimate contact with the highly antigenic external environment. Intestinal DC constantly survey the luminal microenvironment. They act as sentinels, acquiring antigens in peripheral tissues before migrating to secondary lymphoid organs to activate naive T cells. They are also sensors, responding to a spectrum of environmental cues by extensive differentiation or maturation. Recent studies have begun to elucidate mechanisms for functional specializations of DC in the intestine that may include the involvement of retinoic acid and transforming growth factor-β. Specialized CD103(+) intestinal DC can promote the differentiation of Foxp3(+) regulatory T cells via a retinoic acid-dependent process. Different DC outcomes are, in part, influenced by their exposure to microbial stimuli. Evidence is also emerging of the close interaction between bacteria, epithelial cells, and DC in the maintenance of intestinal immune homeostasis. Here we review recent advances of functionally specialized intestinal DC and their mechanisms of antigen uptake and recognition. We also discuss the interaction of DC with intestinal microbiota and their ability to orchestrate protective immunity and immune tolerance in the host. Lastly, we describe how DC functions are altered in intestinal inflammation and their emerging potential as a therapeutic target in inflammatory bowel disease.

Neutralization of IL-4 reverses the nonresponsiveness of CD4+ T cells to regulatory T-cell induction in non-responder mouse strains

It is well established that naive cells can be converted by TGF-β into CD4(+)CD25(+) regulatory T (Treg) cells with therapeutic potentials. Likewise, it is shown that all-trans retinoic acid (ATRA) can greatly enhance TGF-β-induced Treg conversion, a phenomenon which has mainly been studied in C57BL/6 mice. Here we show that, although purified naive cells are highly susceptible to Treg generation, total CD4(+) T-cell populations from different mouse strains display significantly different sensitivities to TGF-β/ATRA-induced Treg conversion. The resistance of "non-responder" strains is associated with an enhanced production of IL-4 by memory T cells as well as an increased sensitivity of naive T cells to the action of IL-4. Importantly, neutralization of IL-4 overcomes the differences, thereby enabling TGF-β/ATRA to generate large numbers of functional Treg cells from total CD4(+) T cells in a consistent manner across different mouse strains. Likewise, blockade of IL-4 significantly enhances TGF-β/ATRA-induced Treg conversion from human naive T cells in the presence of memory cells. These results show that the inherent resistance of "non-responder" mouse strains to Treg conversion induced by TGF-β and ATRA can be reverted by neutralization of IL-4 and that inhibiting the action of IL-4 is beneficial or even necessary for consistent inducible Treg generation.

Stability of the regulatory T cell lineage in vivo

Tissue maintenance and homeostasis can be achieved through the replacement of dying cells by differentiating precursors or self-renewal of terminally differentiated cells or relies heavily on cellular longevity in poorly regenerating tissues. Regulatory T cells (T(reg) cells) represent an actively dividing cell population with critical function in suppression of lethal immune-mediated inflammation. The plasticity of T(reg) cells has been actively debated because it could factor importantly in protective immunity or autoimmunity. By using inducible labeling and tracking of T(reg) cell fate in vivo, or transfers of highly purified T(reg) cells, we have demonstrated notable stability of this cell population under physiologic and inflammatory conditions. Our results suggest that self-renewal of mature T(reg) cells serves as a major mechanism of maintenance of the T(reg) cell lineage in adult mice.

T regulatory cells and the control of alloimmunity: from characterisation to clinical application

T regulatory cells (Treg) play an important role in the induction and maintenance of immunological tolerance. Recent findings in experimental transplant models combined with the development of functional reporter mice have opened new avenues to study Treg biology and their therapeutic potential. In particular, recent advances in understanding Treg function and lineage stability revealed unexpected plasticity of this lineage. Nevertheless, pre-clinical and pilot clinical trials using Treg cells as cellular therapies have been initiated suggesting the safety and feasibility of such treatment.

Small RNA regulators of T cell-mediated autoimmunity

BACKGROUND

MicroRNAs (miRNAs) are short singlestranded RNA molecules that regulate gene expression post-transcriptionally. Several hundred miRNAs exist in the mammalian genome and regulate developmental processes, cell cycle, and survival.

METHODS

In this review,we highlight generalmodes of miRNA function and relate them to how such regulation can be beneficial for immune homeostasis and the prevention of autoimmune diseases.We highlight examples of experimentally verified miRNA function and their target genes in the immune system and place them in context of concepts relevant to an understanding of autoimmune pathogenesis. Where available, we refer to clinical correlations. Finally, we speculate how emerging knowledge about miRNA function in the immune system might be used diagnostically and therapeutically.