CD25- T cells generate CD25+Foxp3+ regulatory T cells by peripheral expansion

Prognostic significance of regulatory T cells in tumor

Since entering the immunological stage several decades ago, regulatory T cell biology has been realized as fundamentally important in the prevention of autoimmune conditions, induction of transplant tolerance and the immune response to cancer. The role of regulatory T cells in tumor immunobiology is still being elucidated. Currently, regulatory T cells are implicated in the dampening of antitumor T-cell responses both through direct and indirect means. A number of investigators have demonstrated that regulatory T cell density and location may serve as independent prognostic factors in several types of cancer and are alternately detrimental or beneficial to patient survival. In this article, we will review the characteristics and functional phenotype of classical regulatory T cells, describe their distribution and quantification in tumor-bearing hosts and summarize recent studies investigating the prognostic significance of regulatory T cell number and locality in various cancers.

Identification of an immediate Foxp3(-) precursor to Foxp3(+) regulatory T cells in peripheral lymphoid organs of nonmanipulated mice

CD4⁺CD25⁺ regulatory T cells (T reg cells) expressing the transcription factor Foxp3 can be induced from peripheral T cell receptor (TCR) transgenic CD4⁺CD25⁻Foxp3⁻ T cells stimulated with noninflammatory dendritic cells presenting low amounts of agonist cognate antigen. However, limited evidence exists for extra-thymic T reg cell generation from non-TCR transgenic T cells in unmanipulated mice. We compared events early during agonist-driven generation of Foxp3⁺ TCR transgenic T cells to polyclonal CD4⁺ T cell populations in unmanipulated mice. We identified an interleukin-2– and phosphatidylinositol-3-kinase–dependent precommitted Foxp3⁻ precursor to Foxp3⁺ T reg cells in peripheral lymphoid organs. Transforming growth factor β signaling played a minor role in the generation and subsequent differentiation of these T reg precursor cells.

Expansion of regulatory T cells via IL-2/anti-IL-2 mAb complexes suppresses experimental myasthenia

Human autoimmune diseases are often characterized by a relative deficiency in CD4(+)CD25(+) regulatory T cells (Treg). We therefore hypothesized that expansion of Treg can ameliorate autoimmune pathology. We tested this hypothesis in an experimental model for autoimmune myasthenia gravis (MG), a B-cell-mediated disease characterized by auto-Ab directed against the acetylcholine receptor within neuromuscular junctions. We showed that injection of immune complexes composed of the cytokine IL-2 and anti-IL-2 mAb (JES6-1A12) induced an effective and sustained expansion of Treg, via peripheral proliferation of CD4(+)CD25(+)Foxp3(+) cells and peripheral conversion of CD4(+)CD25(-)Foxp3(-) cells. The expanded Treg potently suppressed autoreactive T- and B-cell responses to acetylcholine receptor and attenuated the muscular weakness that is characteristic of MG. Thus, IL-2/anti-IL-2 mAb complexes can expand functional Treg in vivo, providing a potential clinical application of this modality for treatment of MG and other autoimmune disorders.

"Default" generation of neonatal regulatory T cells

CD4(+)Foxp3(+) regulatory T (Treg) cells were shown to control all aspects of immune responses. How these Treg cells develop is not fully defined, especially in neonates during development of the immune system. We studied the induction of Treg cells from neonatal T cells with various TCR stimulatory conditions, because TCR stimulation is required for Treg cell generation. Independent of the types of TCR stimulus and without the addition of exogenous TGF-beta, up to 70% of neonatal CD4(+)Foxp3(-) T cells became CD4(+)Foxp3(+) Treg cells, whereas generally <10% of adult CD4(+)Foxp3(-) T cells became CD4(+)Foxp3(+) Treg cells under the same conditions. These neonatal Treg cells exert suppressive function and display relatively stable Foxp3 expression. Importantly, this ability of Treg cell generation gradually diminishes within 2 wk of birth. Consistent with in vitro findings, the in vivo i.p. injection of anti-CD3 mAb to stimulate T cells also resulted in a >3-fold increase in Treg cells in neonates but not in adults. Furthermore, neonatal or adult Foxp3(-) T cells were adoptively transferred into Rag1(-/-) mice. Twelve days later, the frequency of CD4(+)Foxp3(+) T cells converted from neonatal cells was 6-fold higher than that converted from adult cells. Taken together, neonatal CD4(+) T cells have an intrinsic "default" mechanism to become Treg cells in response to TCR stimulations. This finding provides intriguing implications about neonatal immunity, Treg cell generation, and tolerance establishment early in life.

Regulatory T cells in many flavors control asthma

That regulatory T cells (Tregs) have a crucial role in controlling allergic diseases such as asthma is now undisputed. The cytokines most commonly implicated in Treg-mediated suppression of allergic asthma are transforming growth factor-beta (TGF-beta) and interleukin (IL)-10). In addition to naturally occurring Tregs, adaptive Tregs, induced in response to foreign antigens, have been shown in recent studies. The concept of inducible/adaptive Tregs (iTregs) has considerable significance in preventing asthma if generated early enough in life. This is because cytokines such as IL-4 and IL-6 inhibit Foxp3 induction in naive CD4+ T cells and therefore de novo generation of Tregs can be expected to be less efficient when it is concomitant with effector cell development in response to an allergen. However, if iTregs can be induced, the process of infectious tolerance would facilitate expansion of the iTreg pool as suggested in the recent literature. It is tempting to speculate that there is a window of opportunity in early life in the context of a relatively immature immune system that is permissive for the generation of iTregs specific to a spectrum of allergens that would regulate asthma for lifelong. The focus of this review is the relevance of nTregs and iTregs in controlling asthma from early life into adulthood, the mechanisms underlying Treg function, and the prospects for using our current concepts to harness the full potential of Tregs to limit disease development and progression.

Conversion of Th2 memory cells into Foxp3+ regulatory T cells suppressing Th2-mediated allergic asthma

Genetic and epigenetic programming of T helper (Th) cell subsets during their polarization from naive Th cells establishes long-lived memory Th cells that stably maintain their lineage signatures. However, whether memory Th cells can be redifferentiated into another Th lineage is unclear. In this study, we show that Ag-specific memory Th cells were redifferentiated into Foxp3(+) T cells by TGF-beta when stimulated in the presence of all-trans retinoic acid and rapamycin. The "converted" Foxp3(+) T cells that were derived from Th2 memory cells down-regulated GATA-3 and IRF4 and produced little IL-4, IL-5, and IL-13. Instead, the converted Foxp3(+) T cells suppressed the proliferation and cytokine production of Th2 memory cells. More importantly, the converted Foxp3(+) T cells efficiently accumulated in the airways and significantly suppressed Th2 memory cell-mediated airway hyperreactivity, eosinophilia, and allergen-specific IgE production. Our findings reveal the plasticity of Th2 memory cells and provide a strategy for adoptive immunotherapy for the treatment of allergic diseases.

Bilirubin Promotes De Novo Generation of T Regulatory Cells

We have previously demonstrated that bilirubin administration to the recipient induces tolerance towards islet cell transplants across a complete MHC mismatch in a mouse model. Here we assess the mechanisms of such protection. Bilirubin treatment of recipients improved function of islet allografts by suppressing expressions of proinflammatory and proapoptotic genes in those islets and by increasing Foxp3+ T regulatory (Treg) cells at the site of transplanted islets at various days after transplantation. No prolongation of graft survival was observed in recipients treated with bilirubin when CD4+CD25+ T cells were predepleted from those recipients, indicating that Treg cells are necessary for the protective effect of bilirubin. Adoptive transfer of Treg cells from tolerant mice into Rag1-/- recipients resulted in long-term acceptance of skin allografts in an alloantigen-specific manner, suggesting that Treg cells are sufficient to induce tolerance. In addition, bilirubin treatment promoted de novo generation of Treg cells in Rag1-/- recipients. Thus, bilirubin treatment to the recipients prolongs islet allograft survival via a Treg-dependent manner in which CD4+CD25+ Treg cells are both necessary and sufficient for tolerance induction and graft acceptance. Bilirubin treatment promotes de novo generation of Treg cells that might account for the protective effects of bilirubin given to recipients.

Genomic definition of multiple ex vivo regulatory T cell subphenotypes

Regulatory T (Treg) cells that express the Foxp3 transcription factor are essential for lymphoid homeostasis and immune tolerance to self. Other nonimmunological functions of Treg cells, such as controlling metabolic function in adipose tissue, are also emerging. Treg cells originate primarily in the thymus, but can also be elicited from conventional T cells by in vivo exposure to low-dose antigen or homeostatic expansion or by activation in the presence of TGFbeta in vitro. Treg cells are characterized by a distinct transcriptional signature controlled in part, but not solely, by Foxp3. For a better perspective on transcriptional control in Treg cells, we compared gene expression profiles of a broad panel of Treg cells from various origins or anatomical locations. Treg cells generated by different means form different subphenotypes and were identifiable by particular combinations of transcripts, none of which fully encompassed the entire Treg signature. Molecules involved in Treg cell effector function, chemokine receptors, and the transcription factors that control them were differentially represented in these subphenotypes. Treg cells from the gut proved dissimilar to cells elicited by exposure to TGFbeta in vitro, but instead they resembled a CD103(+)Klrg1(+) subphenotype preferentially generated in response to lymphopenia.

CD4+CD25+ Treg induction by an HSP60-derived peptide SJMHE1 from Schistosoma japonicum is TLR2 dependent

Chronic schistosome infection results in the suppression of host immune responses, allowing long-term schistosome survival and restricting pathology. Current theories suggest that Treg play an important role in this regulation. However, the mechanism of Treg induction during schistosome infection is still unknown. The aim of this study was to determine the mechanism behind the induction of CD4(+)CD25(+) T cells by Schistosoma japonicum HSP60 (SjHSP60)-derived peptide SJMHE1 as well as to elucidate the cellular and molecular basis for the induction of CD4(+)CD25(+) T cells during S. japonicum infection. Mice immunized with SJMHE1 or spleen and LN cells from naïve mice pretreated with SJMHE1 in vitro all displayed an increase in CD4(+)CD25(+) T-cell populations. Release of IL-10 and TGF-beta by SJMHE1 stimulation may contribute to suppression. Adoptively transferred SJMHE1-induced CD4(+)CD25(+) T cells inhibited delayed-type hypersensitivity in BALB/c mice. Additionally, SJMHE1-treated APC were tolerogenic and induced CD4(+) cells to differentiate into suppressive CD4(+)CD25(+) Treg. Furthermore, our data support a role for TLR2 in SJMHE1-mediated CD4(+)CD25(+) Treg induction. These findings provide the basis for a more complete understanding of the S. japonicum-host interactions that contribute to host homeostatic mechanisms, preventing an excessive immune response.

Dendritic cells support homeostatic expansion of Foxp3+ regulatory T cells in Foxp3.LuciDTR mice

Foxp3(+)CD4(+) regulatory T cells (Tregs) are crucial in maintaining self-tolerance and limiting immune responses to pathogens. Shifting the sensitive balance between Tregs and effector T cells requires extensive knowledge of the homeostatic properties of the different T cell populations. For the investigation of Treg homeostatic expansion, we introduce in this study novel BAC transgenic mice, designated Foxp3.LuciDTR, coexpressing enhanced GFP, luciferase for bioluminescence imaging of Tregs, and the diphtheria toxin receptor (DTR) for specific ablation of Tregs. Of several founder lines, Foxp3.LuciDTR-4 mice displayed approximately 95% Treg depletion following injection of DT, resulting in activation of conventional CD4(+) T cells, probably due to lack of control by Tregs. In contrast, Foxp3.LuciDTR-3 mice displayed only approximately 70% Treg depletion without concomitant activation of CD4(+) T cells and represented, therefore, a suitable model to study Treg homeostasis in an environment where other T cell populations were not altered. After depletion, the Treg compartment recovered to its original size in approximately 2 wk. This recovery was mediated in a thymus-independent fashion by homeostatic proliferation of the surviving, nondepleted Tregs. The proliferating Tregs acquired an activated phenotype and maintained their suppressive capacity. Studies involving DT-mediated depletion of dendritic cells in CD11c.DOG mice showed that dendritic cells were required for optimal Treg homeostasis. In addition, IL-2 was identified as an essential factor for homeostatic recovery of the Treg compartment. These results show that Treg homeostasis is specifically regulated by the size of the Treg compartment and is independent of proliferation of conventional T cells.

The many face-lifts of CD4 T helper cells

Recent advances in stem cell research have redefined previous concepts of hematopoietic hierarchy, lineage commitment, and cell fate. The immune system is comprised of several well-defined cell lineages of which many exhibit high levels of plasticity or capacity in changing their phenotype. The CD4 T helper cells provide a peculiar example of apparently defined cell subsets, at times described as lineages, but also highly sensitive to tissue environmental cues that may change their fate. The classical Th1/Th2 CD4 T cell differentiation referred to for many years as the main CD4 T cell fate dichotomy and the later additions of CD4 helper T cell variants, such as T helper 17 (Th17) and induced regulatory T cells (iTreg), have added complexity but also doubts on the accuracy of defining CD4 T cell subsets as fixed T cell lineages.

Regulatory function of cytomegalovirus-specific CD4+CD27-CD28- T cells

CMV infection is characterized by high of frequencies of CD27-CD28- T cells. Here we demonstrate that CMV-specific CD4+CD27-CD28- cells are regulatory T cells (TR). CD4+CD27-CD28- cells sorted from CMV-stimulated PBMC of CMV-seropositive donors inhibited de novo CMV-specific proliferation of autologous PBMC in a dose-dependent fashion. Compared with the entire CMV-stimulated CD4+ T-cell population, higher proportions of CD4+CD27-CD28- TR expressed FoxP3, TGFbeta, granzyme B, perforin, GITR and PD-1, lower proportions expressed CD127 and PD1-L and similar proportions expressed CD25, CTLA4, Fas-L and GITR-L. CMV-CD4+CD27-CD28- TR expanded in response to IL-2, but not to CMV antigenic restimulation. The anti-proliferative effect of CMV-CD4+CD27-CD28- TR significantly decreased after granzyme B or TGFbeta inhibition. The CMV-CD4+CD27-CD28- TR of HIV-infected and uninfected donors had similar phenotypes and anti-proliferative potency, but HIV-infected individuals had higher proportions of CMV-CD4+CD27-CD28- TR. The CMV-CD4+CD27-CD28- TR may contribute to the downregulation of CMV-specific and nonspecific immune responses of CMV-infected individuals.

c-Rel is required for the development of thymic Foxp3+ CD4 regulatory T cells

During thymopoiesis, a unique program of gene expression promotes the development of CD4 regulatory T (T reg) cells. Although Foxp3 maintains a pattern of gene expression necessary for T reg cell function, other transcription factors are emerging as important determinants of T reg cell development. We show that the NF-κB transcription factor c-Rel is highly expressed in thymic T reg cells and that in c-rel^−/− mice, thymic T reg cell numbers are markedly reduced as a result of a T cell–intrinsic defect that is manifest during thymocyte development. Although c-Rel is not essential for TGF-β conversion of peripheral CD4⁺CD25⁻ T cells into CD4⁺Foxp3⁺ cells, it is required for optimal homeostatic expansion of peripheral T reg cells. Despite a lower number of peripheral T reg cells in c-rel^−/− mice, the residual peripheral c-rel^−/− T reg cells express normal levels of Foxp3, display a pattern of cell surface markers and gene expression similar to those of wild-type T reg cells, and effectively suppress effector T cell function in culture and in vivo. Collectively, our results indicate that c-Rel is important for both the thymic development and peripheral homeostatic proliferation of T reg cells.

Allogeneic induced human FOXP3(+)IFN-gamma(+) T cells exhibit selective suppressive capacity

Human induced CD4(+)CD25(+) T cells have been shown to express FOXP3, similar to naturally occurring Treg cells (nTreg). However, the suppressive capacity of these cells is still under debate. The current study was designed to investigate functional characteristics of CD25(+)FOXP3(+) derived from CD25(-) T cells. Stimulation of CD25(-) PBMC with allogeneic PBMC resulted in production of CD4(+)CD25(high) T cells. This process was more rapid and prominent when highly mature DC were used for stimulation. The resultant CD4(+)CD25(high) population concurrently exhibited regulatory markers FOXP3, CTLA-4, GITR, and inflammatory cytokines IL-2 and IFN-gamma. These human-induced FOXP3(+)IFN-gamma(+) T cells were shown, for the first time, to markedly inhibit alloreactive T-cell expansion, similar to nTreg. However, in contrast to nTreg, the induced CD4(+)CD25(+)FOXP3(+) cells did not suppress proliferation against a third party donor stimulus or CMV. This suggested that the cell population possessed a more selective suppressive capacity targeted against the original stimulus only. The induced human CD4(+)CD25(+)FOXP3(+) subset derived from CD25(-) T cells, while expressing inflammatory cytokines, exhibits a suppressive cell contact-dependent effect, restricted against T cells responding to the original stimulus. Such unique properties suggest that these cells are potentially ideal for the use as post-transplant GVH disease prophylaxis.

Retinoic acid can enhance conversion of naive into regulatory T cells independently of secreted cytokines

It has been reported that retinoic acid (RA) enhances regulatory T (T reg) cell conversion by inhibiting the secretion of cytokines that interfere with conversion. This report shows that these conclusions provide a partial explanation at best. First, RA not only interfered with cytokine secretion but also with the ability of these cytokines to inhibit T reg cell conversion of naive T cells. Furthermore, RA enhanced conversion even in the absence of inhibitory cytokines. The latter effect depended on the RA receptor α (RARα) but did not require Smad3, despite the fact that RA enhanced Smad3 expression. The RARα1 isoform was not essential for RA-dependent enhancement of transforming growth factor β–driven conversion, suggesting that conversion can also be mediated by RARα2. Interleukin (IL)-6 strongly reduced RARα expression levels such that a deficiency of the predominant RARα1 isoform leaves too little RARα2 for RA to inhibit the generation of Th17 cells in the presence of IL-6.

Impact of the TCR signal on regulatory T cell homeostasis, function, and trafficking

Signaling through the T cell antigen receptor (TCR) is important for the homeostasis of naïve and memory CD4⁺ T cells. The significance of TCR signaling in regulatory T (Treg) cells has not been systematically addressed. Using an Ox40-cre allele that is prominently expressed in Treg cells, and a conditional null allele of the gene encoding p56^Lck, we have examined the importance of TCR signaling in Treg cells. Inactivation of p56^Lck resulted in abnormal Treg homeostasis characterized by impaired turnover, preferential redistribution to the lymph nodes, loss of suppressive function, and striking changes in gene expression. Abnormal Treg cell homeostasis and function did not reflect the involvement of p56^Lck in CD4 function because these effects were not observed when CD4 expression was inactivated by Ox40-cre.The results make clear multiple aspects of Treg cell homeostasis and phenotype that are dependent on a sustained capacity to signal through the TCR.

The development and function of regulatory T cells

Regulatory T cells (Tregs) are a critical subset of T cells that mediate peripheral tolerance. There are two types of Tregs: natural Tregs, which develop in the thymus, and induced Tregs, which are derived from naive CD4(+) T cells in the periphery. Tregs utilize a variety of mechanisms to suppress the immune response. While Tregs are critical for the peripheral maintenance of potential autoreactive T cells, they can also be detrimental by preventing effective anti-tumor responses and sterilizing immunity against pathogens. In this review, we will discuss the development of natural and induced Tregs as well as the role of Tregs in a variety of disease settings and the mechanisms they utilize for suppression.

Immune manipulation of advanced breast cancer: an interpretative model of the relationship between immune system and tumor cell biology

This review summarizes some recent clinical immunological approaches with cytokines and/or antibodies for therapy of advanced breast cancer. It considers the recent advances in genetics and molecular tumor biology related to impaired immunosurveillance involving cytokines and growth factors to explain clinical results. Evasion of the host immune attack might be induced by the following groups of mechanisms: (a) tumor dependent (genomic instability, HLA class I antigen abnormalities, upregulation of fetal type nonclassical HLA class I molecules, epitope immunodominance, apoptosis inhibition by defective death receptor signaling, apoptosis of activated T cells, tumor cannibalism and constitutive activation of signal transducer, and activator of transcription-3 (Stat 3) and nuclear factor-kappaB (NF-kappaB) signaling); (b) host dependent (CD4+CD25+ regulatory T cells (T reg), CD4+ T cells anergy, Th2 antitumor immunity diversion and myeloid suppressor cells); (c) tumor and host dependent (lack of co-stimulation molecules, immunosuppressive cytokines (vascular endothelial growth factor (VEGF), interleukin (IL)-10, prostaglandin (PG)E2, transforming growth factor (TGF)-beta)). Cytokines and growth factors are involved in virtually all three types of mechanisms. These mechanisms are integrated with the current knowledge of tumor growth and inhibited apoptosis primarily mediated by cytokines and growth factors to propose an interpretation of the relationships among tumor cells, tumor stroma, and tumor-infiltrating lymphocytes. Tumor growth, defective immunorecognition and immunosuppression are the three principal effects considered responsible for immune evasion.

Peripheral induction of tolerance by retinal antigen expression

The contribution of peripheral expression of tissue-specific CNS Ags to the generation of tolerance is uncertain. To study this question, we examined mice transgenic (Tg) for expression of beta-galactosidase (beta gal) on the retinal photoreceptor cell arrestin promoter, in conjunction with TCR Tg mice producing CD4(+) T cells specific for beta gal (beta galTCR). Several strategies were used to test the hypothesis that betagal expressed in the retina supported thymus-independent tolerance and regulatory T cell development. Retinal expression generated an immunoregulatory response that depressed development of immune responses to beta gal following systemic immunization with beta gal. This regulation was transferable to naive mice by CD3(+)4(+)25(+) T cells from naive retinal beta gal(+) donors. Experiments that removed the beta gal(+) retina by enucleation showed that subsequent development of a regulatory response was lost. Adoptive transfer of CD25(-) beta galTCR T cells into retinal beta gal Tg mice on the Rag(-/-) background led to regulatory activity that limited lymphopenia-induced proliferation of beta galTCR T cells in mice with retinal expression of beta gal and inhibited the ear-swelling assay for delayed type hypersensitivity. These results show that retinal expression of very small amounts of a tissue-specific Ag can generate tolerance that includes regulatory T cells.

Expansion of CD4+ CD25+ Foxp3+ T cells by bone marrow-derived dendritic cells

Dendritic cells (DCs) are the most important antigen-presenting cells of the immune system and have a crucial role in T-lymphocyte activation and adaptive immunity initiation. However, DCs have also been implicated in maintaining immunological tolerance. In this study, we evaluated changes in the CD4(+) CD25(+) Foxp3(+) T-cell population after co-culture of lymph node cells from BALB/c mice with syngeneic bone marrow-derived DCs. Our results showed an increase in CD4(+) CD25(+) Foxp3(+) T cells after co-culture which occurred regardless of the activation state of DCs and the presence of allogeneic apoptotic cells; however, it was greater when DCs were immature and were pulsed with the alloantigen. Interestingly, syngeneic apoptotic thymocytes were not as efficient as allogeneic apoptotic cells in expanding the CD4(+) CD25(+) Foxp3(+) T-cell population. In all experimental settings, DCs produced high amounts of transforming growth factor (TGF)-beta. The presence of allogeneic apoptotic cells induced interleukin (IL)-2 production in immature and mature DC cultures. This cytokine was also detected in the supernatants under all experimental conditions and enhanced when immature DCs were pulsed with the alloantigen. CD4(+) CD25(+) Foxp3(+) T-cell expansion during co-culture of lymph node cells with DCs strongly suggested that the presence of alloantigen enhanced the number of regulatory T cells (Tregs) in vitro. Our data also suggest a role for both TGF-beta and IL-2 in the augmentation of the CD4(+) CD25(+) Foxp3(+) population.

Natural and adaptive foxp3+ regulatory T cells: more of the same or a division of labor?

Adaptive Foxp3(+)CD4(+) regulatory T (iTreg) cells develop outside the thymus under subimmunogenic antigen presentation, during chronic inflammation, and during normal homeostasis of the gut. iTreg cells are essential in mucosal immune tolerance and in the control of severe chronic allergic inflammation, and most likely are one of the main barriers to the eradication of tumors. The Foxp3(+) iTreg cell repertoire is drawn from naive conventional CD4(+) T cells, whereas natural Treg (nTreg) cells are selected by high-avidity interactions in the thymus. The full extent of differences and similarities between iTreg and nTreg cells is yet to be defined. We speculate that iTreg cell development is driven by the need to maintain a noninflammatory environment in the gut, to suppress immune responses to environmental and food allergens, and to decrease chronic inflammation, whereas nTreg cells prevent autoimmunity and raise the activation threshold for all immune responses.

More stories on Th17 cells

For more than two decades, immunologists have been using the so-called Th1/Th2 paradigm to explain most of the phenomena related to adaptive immunity. The Th1/Th2 paradigm implied the existence of two different, mutually regulated, CD4(+) T helper subsets: Th1 cells, driving cell-mediated immune responses involved in tissue damage and fighting infection against intracellular parasites; and Th2 cells that mediate IgE production and are particularly involved in eosinophilic inflammation, allergy and clearance of helminthic infections. A third member of the T helper set, IL-17-producing CD4(+) T cells, now called Th17 cells, was recently described as a distinct lineage that does not share developmental pathways with either Th1 or Th2 cells. The Th17 subset has been linked to autoimmune disorders, being able to produce IL-17, IL-17F and IL-21 among other inflammatory cytokines. Interestingly, it has been reported that there is not only a cross-regulation among Th1, Th2 and Th17 effector cells but there is also a dichotomy in the generation of Th17 and T regulatory cells. Therefore, Treg and Th17 effector cells arise in a mutually exclusive fashion, depending on whether they are activated in the presence of TGF-beta or TGF-beta plus inflammatory cytokines such as IL-6. This review will address the discovery of the Th17 cells, and recent progress on their development and regulation.

A central role for induced regulatory T cells in tolerance induction in experimental colitis

In addition to thymus-derived or natural T regulatory (nT(reg)) cells, a second subset of induced T regulatory (iT(reg)) cells arises de novo from conventional CD4(+) T cells in the periphery. The function of iT(reg) cells in tolerance was examined in a CD45RB(high)CD4(+) T cell transfer model of colitis. In situ-generated iT(reg) cells were similar to nT(reg) cells in their capacity to suppress T cell proliferation in vitro and their absence in vivo accelerated bowel disease. Treatment with nT(reg) cells resolved the colitis, but only when iT(reg) cells were also present. Although iT(reg) cells required Foxp3 for suppressive activity and phenotypic stability, their gene expression profile was distinct from the established nT(reg) "genetic signature," indicative of developmental and possibly mechanistic differences. These results identified a functional role for iT(reg) cells in vivo and demonstrated that both iT(reg) and nT(reg) cells can act in concert to maintain tolerance.

Smad7 controls resistance of colitogenic T cells to regulatory T cell-mediated suppression

BACKGROUND & AIMS

Foxp3-expressing regulatory T cells (Tregs) play a key role in the maintenance of the gut immune homeostasis, and an intact transforming growth factor (TGF)-beta signaling is required for their function. In inflammatory bowel disease (IBD), the TGF-beta signaling is impaired because of high expression of the inhibitory molecule Smad7. Although no intrinsic defects in Tregs function have been shown in IBD, it is still unknown whether colitogenic T cells are susceptible to Treg-mediated suppression. In this study, we have investigated whether IBD mucosal CD4+ T cells are resistant to Tregs and whether Smad7 is involved in this process.

METHODS

IBD lamina propria mononuclear cells (LPMC) were cultured with or without Tregs, and proliferation was assessed by flow cytometry. Proliferation of IBD LPMC was also evaluated after Smad7 antisense oligonuclotide treatment. Treg-mediated suppression of T-cell proliferation and proinflammatory cytokine expression was investigated in murine Smad7 transgenic cells. In vivo, the Smad7-dependent resistance of colitogenic naïve T cells to Tregs was studied in the adoptive transfer model of colitis.

RESULTS

IBD LPMC were resistant to Treg-mediated suppression, and this phenomenon was reverted by Smad7 antisense treatment. Consistently, CD4+ T cells isolated from Smad7 transgenic mice showed high proliferation, produced considerable amount of inflammatory cytokines following activation, and induced a severe colitis when transferred in immunodeficient RAG1 knockout mice even in the presence of wild-type Tregs.

CONCLUSIONS

Smad7 makes CD4+ T cells resistant to Tregs-mediated suppression thus fine-tuning their proinflammatory potential.

Tumor necrosis factor-related apoptosis-inducing ligand inhibits experimental autoimmune thyroiditis by the expansion of CD4+CD25+ regulatory T cells

There have been several reports that TNF-related apoptosis-inducing ligand (TRAIL) has the ability to suppress the development of experimental autoimmune diseases, including a mouse model of experimental autoimmune encephalomyelitis, a rabbit model of rheumatoid arthritis, type 1 diabetes mellitus, in mice and experimental autoimmune thyroiditis (EAT) in mice. However, the mechanism underlying TRAIL effect is not well defined. In the present study, we specifically examined TRAIL effects on CD4(+)CD25(+) regulatory T cells. CD4(+)CD25(+) T cells prepared from mouse thyroglobulin (mTg)-immunized CBA/J mice proliferate in the presence of TRAIL and dendritic cells in vitro. These CD4(+)CD25(+) T cells included both CD4(+)CD25(+)CD45RB(Low) (regulatory) and CD4(+)CD25(+)CD45RB(High) (effector) T cells. Our results demonstrated that mTg-immunized mice treated with TRAIL showed significant increases in the number of CD4(+)CD25(+)CD45RB(Low) T cells compared with mice immunized with mTg alone. CD4(+)CD25(+)CD45RB(Low) T cells expressed much higher levels of the forkhead family transcription factor, IL-10, and TGFbeta1 than CD4(+)CD25(+)CD45RB(High) T cells, and these cells can completely suppress the proliferation of the mTg-primed splenocytes in lower concentrations than the unfractionated CD4(+)CD25(+) T cells. Furthermore, transfer of these cells into CBA/J mice prior to mTg-primed splenocyte injection could markedly reduce the frequency and severity of EAT development. CD4(+)CD25(+)CD45RB(Low) T cells were more effective at suppressing histological thyroiditis than unfractionated cells. These results indicated that TRAIL can increase the number of mTg-specific CD4(+)CD25(+)CD45RB(Low) T cells, inhibiting autoimmune responses and preventing the progression of EAT. These findings reveal a novel mechanism by which TRAIL could inhibit autoimmune disease.

Induction of type I IFN is required for overcoming tumor-specific T-cell tolerance after stem cell transplantation

Tumor-specific T-cell tolerance represents one major mechanism of tumor-induced immune evasion. Myeloablative chemotherapy with stem cell transplantation may offer the best chance of achieving a state of minimal residual disease and, thus, minimize tumor-induced immune evasion. However, studies have shown that tumor-specific T-cell tolerance persists after transplantation. Here, we showed that CD4(+)CD25(+) regulatory T (T(Reg)) cells play a critical role in tumor-specific CD8(+) T-cell tolerance after transplantation. Removal of T(Reg) cells from the donor lymphocyte graft did not overcome this tolerance because of rapid conversion of donor CD4(+)CD25(-) T cells into CD4(+)CD25(+)Foxp3(+) T(Reg) cells in recipients after transplantation, and depletion of T(Reg) cells in recipients was necessary for the reversal of tumor-specific tolerance. These results suggest that strategies capable of overcoming T-cell tolerance in recipients are required to promote antitumor immunity after transplantation. Toward this goal, we showed that dendritic cell (DC) vaccines coadministered with the TLR9 ligand, CpG could effectively overcome tumor-specific tolerance, leading to significant prolongation of tumor-free survival after transplantation. We further showed that CpG-induced type I interferon was critical for the reversal of tumor-specific tolerance in vivo. Collectively, these results may suggest effective immunotherapeutic strategies for treating cancer after stem cell transplantation.

From the diet to the nucleus: vitamin A and TGF-beta join efforts at the mucosal interface of the intestine

The vitamin A metabolites, including retinoic acid (RA), form ligands for retinoic acid-related nuclear receptors and together they play pleiotropic roles in various biological processes. Recently, we described that RA also functions as a key modulator of transforming growth factor-beta (TGF-beta)-driven immune deviation, capable of suppressing the differentiation of interleukin-17 secreting T helper cells (T(H)17) and conversely promoting the generation of Foxp3(+) T regulatory (Treg) cells. This review will focus on the role of RA in the reciprocal TGF-beta-driven differentiation of T(H)17 and Treg and on the importance of such regulatory mechanism to control a functional immune system, in particular at the mucosal interface of the intestine.

Antigen-specific peripheral shaping of the natural regulatory T cell population

Although regulatory T (T reg) cells are thought to develop primarily in the thymus, the peripheral events that shape the protective T reg cell population are unclear. We analyzed the peripheral CD4(+) T cell receptor (TCR) repertoire by cellular phenotype and location in mice with a fixed TCRbeta chain. We found that T reg (Foxp3(+)) cells showed a marked skewing of TCR usage by anatomical location in a manner similar to antigen-experienced (CD44(hi)Foxp3(-)) but not naive (CD44(lo)Foxp3(-)) cells, even though CD44(hi) and T reg cells used mostly dissimilar TCRs. This was likely unrelated to peripheral conversion, which we estimate generates only a small percentage of peripheral T reg cells in adults. Conversion was readily observed, however, during the immune response induced by Foxp3(-) cells in lymphopenic hosts. Interestingly, the converted Foxp3(+) and expanded Foxp3(-) TCR repertoires were different, suggesting that generation of Foxp3(+) cells is not an automatic process upon antigen activation of Foxp3(-) T cells. Retroviral expression of these TCRs in primary monoclonal T cells confirmed that conversion did not require prior cellular conditioning. Thus, these data demonstrate that TCR specificity plays a crucial role in the process of peripheral conversion and in shaping the peripheral T reg cell population to the local antigenic landscape.

Regulatory T cells and their role in rheumatic diseases: a potential target for novel therapeutic development

Regulatory T cells have an important role in limiting immune reactions and are essential regulators of self-tolerance. Among them, CD4+CD25high regulatory T cells are the best-described subset. In this article, we summarize current knowledge on the phenotype, function, and development of CD4+CD25high regulatory T cells. We also review the literature on the role of these T cells in rheumatic diseases and discuss the potential for their use in immunotherapy.

B7-1 and B7-2 differentially control peripheral homeostasis of CD4(+)CD25(+)Foxp3(+) regulatory T cells

Targeting the CD28/B7 interaction remains among the most promising approaches to treat transplant rejection. A drawback to this approach is however the observations of decreased numbers of naturally occurring regulatory T cells (Tregs) in CD28(-) or B7-deficient non-obese diabetic (NOD) mice, cells that maintain immunological self tolerance, prevent autoimmunity and control immune responses to transplants. In this study, therefore, we investigated the relative contributions of B7-1 and B7-2, the only known ligands of CD28, on the thymic development and peripheral homeostasis of Tregs. Our data indicates that the absence of both B7-1 and B7-2 result in a dramatic reduction in the frequencies of Tregs in thymus and peripheral tissues of B7-1/B7-2-deficient mice with no apparent changes in the percentage and distribution of conventional T-cell subsets. In addition, neither B7-1 nor B7-2 expression alone is sufficient for the development and peripheral homeostasis of Tregs. Interestingly, while B7-1 and B7-2 equally contribute to thymic development of Tregs, the significant loss in peripheral homeostasis of Tregs is more evident in the absence of B7-2 as compared to B7-1. Consistent with these results we found that B7-2-deficient DCs are less effective than B7-1-deficient DCs in maintaining Tregs in vitro due to their inability to induce IL-2 production by conventional T cells and sustain Tregs expression of CD25. This study provides the first demonstration of relative roles of B7-1 and B7-2 in Tregs homeostasis and suggests that therapeutic approaches designed to selectively interrupt CD28/B7-2 interaction could indeed have measurable impact on sustaining Tregs homeostasis.

Immunological mechanisms and clinical implications of regulatory T cell deficiency in a systemic autoimmune disorder: roles of IL-2 versus IL-15

Regulatory T cell deficiency is evident in patients with lupus, but the casual [corrected] relationship and underlying mechanism leading to Treg deficiency are unclear. We analyzed the Treg profile, induction and functions of Treg in a lupus mouse model. A characteristic age-dependent biphasic change of Treg frequency was observed in the MRL/lpr mice, which developed a spontaneous lupus-like disease. After an early increase, Treg frequency in the peripheral lymphoid organs rapidly declined with age. Functionally, Treg from both young and old MRL/lpr mice were fully competent in suppressing the wild-type MRL/+ T effector cell (Teff) responses. Adoptive transfer of MRL/+ Treg markedly suppressed clinical disease in the MRL/lpr mice. We demonstrated that the reduced Treg frequency was a result of insufficient peripheral Treg expansion due to defective MRL/lpr Teff in IL-2 production, and the associated defects in dendritic cells, which could be fully restored by exogenous IL-2. In the absence of IL-2, MRL/lpr Teff but not MRL/lpr Treg were highly responsive to IL-15 and could expand rapidly due to enhanced IL-15R expression and IL-15 synthesis. These findings thus provide a clear causal relationship and immunological mechanism underlying Treg deficiency and systemic autoimmunity.

Adaptive Foxp3+ regulatory T cell-dependent and -independent control of allergic inflammation

Adaptive Foxp3(+) regulatory T (Treg) cells develop during induction of mucosal tolerance and after immunization. Large numbers of Foxp3(+) T cells have been found in inflamed tissues. We investigated the role of adaptive Foxp3(+) Treg cells in mucosal tolerance and in chronic allergic lung inflammation. We used two strains of mice that are devoid of naturally occurring Treg cells; one is capable of generating adaptive Foxp3(+) Treg cells upon exposure to antigen, whereas the other is deficient in both naturally occurring and adaptive Foxp3(+) Treg cells. We found that adaptive Foxp3(+) Treg cells were essential for establishing mucosal tolerance and for suppressing IL-4 production and lymphoid neogenesis in chronic inflammation, whereas IL-5 production and eosinophilia could be controlled by Foxp3-independent, IFN-gamma-dependent mechanisms. Thus, whereas adaptive Foxp3(+) Treg cells regulate sensitization to allergens and the severity of chronic inflammation, IFN-gamma-producing cells can play a beneficial role in inflammatory conditions involving eosinophils.

T-regulatory cells in systemic lupus erythematosus

Thymus-derived CD4(+)CD25(high)Foxp3(+) T-regulatory cells (Tregs) have an important role in the mechanisms of peripheral immune tolerance and in the prevention of pathogenic autoimmunity through the suppression of proliferation and production of pro-inflammatory cytokines in effector immune cells. Some studies have shown that in systemic lupus erythematosus (SLE) the number of circulating Tregs may be decreased during active disease, and that the extent of such decrease may correlate with severity of the disease. Recent data in murine models of lupus have suggested the possibility to target Tregs for the modulation of SLE, and Treg-based intervention has been proposed as a novel therapeutic mean for a better management of the disease. This review provides an update on the role of Tregs in SLE, discussing new findings in relation to possible targeting of Tregs for immune modulation in lupus.

Recent thymic origin, differentiation, and turnover of regulatory T cells

Regulatory CD4+ T cells (Treg) are essential to maintain self-tolerance. Release of natural Treg from the thymus is believed to commence soon after birth, but it is unclear how many are produced by "conversion" in the periphery, whether numbers are maintained after puberty by general homeostatic mechanisms that regulate lymphocyte numbers, or whether significant numbers are produced by the involuted thymus. To address the origin of Treg in normal adult rats, we focused on recent thymus emigrants (RTE). Approximately 30% of CD4+CD25+forkhead box p3 (Foxp3)+ Treg expressed markers associated with RTE. Following thymectomy, numbers of cells expressing these markers fell by 80% within 30 days. Furthermore, although only approximately 5% of CD4+ single-positive thymocytes expressed Foxp3 within 24 h after intrathymic injection of FITC, more than 30% of the labeled CD4+ RTE were Foxp3+, suggesting that some RTE may acquire Foxp3 in the periphery. Thus, some RTE may acquire Foxp3 rapidly after emigration from the thymus. Treg are dividing rapidly with apparent half-lives of approximately 18 days and approximately 7 days for the CD4+CD25+Foxp3+ and CD4+CD25-Foxp3+ subsets, respectively. The apparently slower turnover of CD4+CD25+Foxp3+ cells is a result of CD4+CD25+Foxp3+ --> CD4+CD25-Foxp3+ conversion, with no loss of regulatory function. Taken together, the data suggest that Treg in adults are relatively short-lived and that their numbers are maintained by rapid cell division and continuous replenishment from the thymus.

Susceptibility of neonatal T cells and adult thymocytes to peripheral tolerance to allogeneic stimuli

We studied the tolerization of neonatal thymocytes (NT), neonatal splenocytes (NS) and adult thymocytes (AT), transferred to syngeneic nude (nu/nu) hosts previously injected with semi-allogeneic splenocytes, without any supportive immunosuppressive treatment. This protocol allows the study of peripheral tolerance in the absence of the thymus. BALB/c neonatal T cells and ATs were able to expand in syngeneic BALB/c nu/nu mice and functionally reconstituted an allogeneic response, rejecting (BALB/c x B6.Ba) F1 splenocytes transferred 3-4 weeks after injection of BALB/c cells. However, if (BALB/c x B6.Ba) F1 cells were injected into BALB/c nude hosts 30 days before transfer of NT, NS or AT cells, the F1 population was preserved and specific tolerance to B6 allografts was established. Furthermore, transfer to lymphopenic F1 nu/nu showed that tolerance could be established only for neonatal populations, showing that unique properties of neonatal T cells allow their tolerization in both lymphopenic and non-lymphopenic conditions, in the absence of suppressive immunotherapy. These results bring empirical support to the possibility of T-cell engraftment in immunodeficient patients showing partial identity with donor major histocompatibility complex (MHC) genes; the manipulation of immunological maturity of donor T cells may be the key for successful reconstitution of immunocompetence without induction of graft-versus-host disease.

CD4+ CD25+ FOXP3+ regulatory T cells increase de novo in kidney transplant patients after immunodepletion with Campath-1H

Campath-1H (Alemtuzumab) is an effective immunodepletion agent used in renal transplantation. To evaluate its influence on T lymphocytes during repletion, we analyzed peripheral blood from Campath-1H-treated renal allograft recipients for the presence of FOXP3(+) regulatory T (Treg) cells. Flow cytometry demonstrated that CD4(+)CD25(+)FOXP3(+) lymphocytes increased significantly within the CD4(+) T-cell population, skewing Treg/Teff (T effector) ratios for up to several years. In contrast, Treg levels in patients treated with anti-CD25 (Basiliximab) and maintained on CsA demonstrated a sustained decrease. The increase in Tregs in Campath-1H treated patients developed independent of maintenance immunosuppression. Importantly, the increase in Tregs was not fully explained by their homeostatic proliferation, increased thymic output, or Treg sparing, suggesting de novo generation/expansion. Consistent with this, in vitro stimulation of PBMCs with Campath-1H, with or without anti-CD3, activation led to an increase in CD4(+)CD25(+)FOXP3(+) cells that had suppressive capabilities. Together, these data suggest that Campath-1H promotes an increase in peripheral Tregs and may act as an intrinsic generator of Tregs in vivo.

Beta-catenin stabilization extends regulatory T cell survival and induces anergy in nonregulatory T cells

Beta-catenin is a central molecule in the Wnt pathway. Expression of a stable form of beta-catenin on CD4+CD25+ regulatory T (T(reg)) cells resulted in a marked enhancement of survival of these cells in vitro. Furthermore, stable beta-catenin-expressing CD4+CD25+ T(reg) cells outcompeted control T(reg) cells in vivo, and the number of T(reg) cells necessary for protection against inflammatory bowel disease could be substantially reduced when stable beta-catenin-expressing CD4+CD25+ T(reg) cells were used instead of control T(reg) cells. Expression of stable beta-catenin on potentially pathogenic CD4+CD25- T cells rendered these cells anergic, and the beta-catenin-mediated induction of anergy occurred even in Foxp3-deficient T cells. Thus, through enhanced survival of existing regulatory T cells, and through induction of unresponsiveness in precursors of T effector cells, beta-catenin stabilization has a powerful effect on the prevention of inflammatory disease.

Enhanced efficacy of regulatory T cell transfer against increasing resistance, by elevated Foxp3 expression induced in arthritic murine hosts

OBJECTIVE

To investigate the efficacy of type II collagen-reactive Foxp3-expressing T cell transfer in suppressing collagen-induced arthritis (CIA) in relation to disease progression.

METHODS

CD3-activated CD4 T cells were retrovirally transduced with the Foxp3 gene, and their in vitro suppressive activity on T cell proliferation was assessed for correlation with Foxp3 levels. To suppress CIA, Foxp3-transduced T cells generated with type II collagen- or ovalbumin (OVA)-pulsed dendritic cells (DCs), which were fractionated by Foxp3 levels, were adoptively transferred to mice at various time points.

RESULTS

The in vitro suppressive activity of Foxp3-transduced cells correlated positively with Foxp3 levels. Type II collagen-reactive, but not OVA-reactive, Foxp3-transduced cells significantly suppressed CIA when they were transferred before immunization, and this suppression was accompanied by decreased anti-type II collagen antibody production. Larger cell numbers were required to suppress CIA when transfer occurred 20 days after immunization, indicating that hosts became resistant to suppression. Transfer of 1 x 10(5) Foxp3(low) cells had only a marginal effect on CIA suppression in immunized hosts, while transfer of Foxp3(high) cells at smaller doses significantly suppressed CIA. Transfer of 1 x 10(5) Foxp3(high) cells after establishment of arthritis attenuated disease progression but did not reverse joint swelling.

CONCLUSION

Resistance to Foxp3-transduced T cells proceeded as CIA progressed, suggesting that late-stage aggressive arthritis is more resistant to regulatory T cell transfer. An elevated expression level of Foxp3 in type II collagen-specific T cells improved their suppressive function in CIA. Thus, transfer of T cells expressing high levels of Foxp3 could be a strategy to overcome the induced resistance to regulatory T cell therapy.

OX40 costimulation turns off Foxp3+ Tregs

OX40 is a recently identified T-cell costimulatory molecule that belongs to the TNF/TNFR superfamily. OX40 can be expressed by both activated T effector cells and Foxp3(+) Tregs. It is well known that OX40 delivers a potent costimulatory signal to T effector cells, but very little is known about the role of OX40 in regulating the suppressor properties of Foxp3(+) Tregs and the de novo generation of new inducible Foxp3(+) Tregs from T effector cells. In the present study, we found, by using a newly created foxp3gfp knockin model, that OX40 was dispensable for the genesis and suppressor functions of naturally arising CD4(+)Foxp3(+) Tregs, but stimulating OX40 on the Foxp3(+) Tregs abrogated their ability to suppress T effector cell proliferation, IFN-gamma production, and T effector cell-mediated allograft rejection. OX40 costimulation did not significantly affect proliferation and survival of the naturally arising Foxp3(+) Tregs, but profoundly inhibited Foxp3 gene expression. Importantly, OX40 costimulation to T effector cells prevented the induction of new inducible Foxp3(+) Tregs from T effector cells. Our study identified OX40 as a key negative regulator of Foxp3(+) Tregs and may have important clinical implications in models of transplantation and autoimmunity.

G protein-coupled receptor 83 is dispensable for the development and function of regulatory T cells

Global analyses of gene expression in regulatory T (Treg) cells, whose development is critically dependent upon the transcription factor Foxp3, have provided many clues as to the molecular mechanisms these cells employ to control immune responses and establish immune tolerance. Through these studies, G protein-coupled receptor 83 (GPR83) was found to be expressed at high levels in Treg-cell populations. However, its function remained unclear. Recently, it has been suggested that GPR83 is involved in the induction of Foxp3 expression in the peripheral nonregulatory Foxp3- CD4 T cells. To examine a role for GPR83 in Treg-cell biology, we generated and characterized GPR83-deficient mice. We have shown that GPR83 abolition does not result in measurable pathology or changes in the numbers or function of Foxp3+ Treg cells. Furthermore, while in vitro analysis suggested a potential involvement of GPR83 in transforming growth factor beta-dependent Foxp3 induction, there was no difference in the ability of nonregulatory GPR83-deficient and nondeficient Foxp3- T cells to acquire Foxp3 expression in vivo. Collectively, our results demonstrate that GPR83 is dispensable for Treg-cell development and function.

Vitamin A metabolites induce gut-homing FoxP3+ regulatory T cells

In this study, we report a novel biological function of vitamin A metabolites in conversion of naive FoxP3- CD4+ T cells into a unique FoxP3+ regulatory T cell subset (termed "retinoid-induced FoxP3+ T cells") in both human and mouse T cells. We found that the major vitamin A metabolite all-trans-retinoic acid induces histone acetylation at the FoxP3 gene promoter and expression of the FoxP3 protein in CD4+ T cells. The induction of retinoid-induced FoxP3+ T cells is mediated by the nuclear retinoic acid receptor alpha and involves T cell activation driven by mucosal dendritic cells and costimulation through CD28. Retinoic acid can promote TGF-beta1-dependent generation of FoxP3+ regulatory T cells but decrease the TGF-beta1- and IL-6-dependent generation of inflammatory Th17 cells in mouse T cells. Retinoid-induced FoxP3+ T cells can efficiently suppress target cells and, thus, have a regulatory function typical for FoxP3+ T cells. A unique cellular feature of these regulatory T cells is their high expression of gut-homing receptors that are important for migration to the mucosal tissues particularly the small intestine. Taken together, these results identify retinoids as positive regulatory factors for generation of gut-homing FoxP3+ T cells.

The effect of HMG-CoA reductase inhibitors on naturally occurring CD4+CD25+ T cells

Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are in widespread use due to their LDL reducing properties and concomitant improvement of clinical outcome in patients with and without preexisting atherosclerosis. Considerable evidence suggests that immune mediated mechanisms play a dominant role in the beneficial effects of statins. Naturally occurring CD4(+)CD25(+) regulatory T cells (Tregs) have a key role in the prevention of various inflammatory and autoimmune disorders by suppressing immune responses. We tested the hypothesis that statins influence the circulating number and the functional properties of Tregs. We studied the effects of in vivo and in vitro statin treatment of human and murine mononuclear cells on the number of Tregs and the expression level of their master transcription regulator, Foxp3. Atorvastatin, but not mevastatin nor pravastatin, treatment of human peripheral blood mononuclear cells (PBMCs) increased the number of CD4(+)CD25(high) cells, and CD4(+)CD25(+)Foxp3(+) cells. These Tregs, induced by atorvastatin, expressed high levels of Foxp3, which correlated with an increased regulatory potential. Furthermore, co-culture studies revealed that atorvastatin induced CD4(+)CD25(+)Foxp3(+) Tregs were derived from peripheral CD4(+)CD25(-)Foxp3(-) cells. Simvastatin and pravastatin treatment in hyperlipidemic subjects increased the number of Tregs. In C57BL/6 mice however, no effect of statins on Tregs was evident. In conclusion, statins appear to significantly influence the peripheral pool of Tregs in humans. This finding may shed light on the mechanisms governing the plaque stabilizing properties of statins.

TCR-based lineage tracing: no evidence for conversion of conventional into regulatory T cells in response to a natural self-antigen in pancreatic islets

Foxp3-expressing regulatory T (T reg) cells derive primarily from selection in the thymus. Yet conversion of mature conventional CD4⁺ T (T conv) cell lymphocytes can be achieved in several conditions, such as transforming growth factor β treatment, homeostatic expansion, or chronic exposure to low-dose antigen. Such conversion might provide a means to generate peripheral tolerance by “converting” potentially damaging T cells that react to self-antigens. We tested this hypothesis in mice transgenic for the BDC2.5 T cell receptor (TCR), which is representative of a diabetogenic specificity that is naturally present in NOD mice and reactive against a pancreatic self-antigen. In the thymus, before any exposure to antigen, clonotype-positive T reg and T conv cells express a second TCRα chain derived from endogenous loci. High-throughput single-cell sequencing of secondary TCRs of the Vα2 family showed their joining CDR3α regions to be very different in T reg and T conv cell thymocytes. These specific CDR3α motifs, thus, provided a “tag” with which to test the actual impact of T conv to T reg cell conversion in response to peripheral self-antigen; should the autoreactive clonotypic TCR induce T conv to T reg cell conversion upon encounter of cognate antigen in the pancreas or draining lymph node, one would expect to detect tag CDR3α motifs from T conv cells in the T reg cell populations. Sequencing large numbers of peripheral BDC⁺Vα2⁺ cells showed that little to no conversion occurs in response to this pancreatic autoantigen.

RA8, a human anti-CD25 antibody against human Treg cells

Although anti-CD25 antibodies exist for clinical use in patients, there is a need for the development of a human Treg antibody that will abrogate the immunosuppressive function of this small but critical T cell subtype. Based upon mounting evidence that the level of Treg cells in the tumor microenvironment correlates with clinical prognosis and stage in man, it appears that Treg cells play an important role in the tumor's ability to overcome host immune responses. In mice, the rat anti-mouse CD25 antibody PC61 causes depletion of CD25-bearing Treg cells both peripherally in lymphatic tissues and in the tumor microenvironment, without inducing symptoms of autoimmunity. A similar antibody, though with the ability to delete Treg cells specifically, would be an important new tool for reversing tumor escape associated with Treg immunosuppression in man. To begin to generate such a reagent, we now describe the development of a human anti-CD25 antibody using a novel yeast display library. The target antigen CD25-Fc was constructed and used for five rounds of selection using a non-immune yeast display library that contained as many as 10(9) single chain variable fragments (scFv). Two unique clones with low K(D) values (RA4 and RA8) were then selected to construct fully human anti-CD25 antibodies (IgG1/kappa) for stable expression. One antibody, RA8, showed excellent binding to human CD25(+) cell lines and to human Treg cells and appears to be an excellent candidate for the generation of a human reagent that may be used in man for the immunotherapy of cancer.