Regulatory potential and control of Foxp3 expression in newborn CD4+ T cells

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

Lymphopenia, Lymphopenia-Induced Proliferation, and Autoimmunity

Immune homeostasis is a tightly regulated system that is critical for defense against invasion by foreign pathogens and protection from self-reactivity for the survival of an individual. How the defects in this system might result in autoimmunity is discussed in this review. Reduced lymphocyte number, termed lymphopenia, can mediate lymphopenia-induced proliferation (LIP) to maintain peripheral lymphocyte numbers. LIP not only occurs in normal physiological conditions but also correlates with autoimmunity. Of note, lymphopenia is also a typical marker of immune aging, consistent with the fact that not only the autoimmunity increases in the elderly, but also autoimmune diseases (ADs) show characteristics of immune aging. Here, we discuss the types and rates of LIP in normal and autoimmune conditions, as well as the coronavirus disease 2019 in the context of LIP. Importantly, although the causative role of LIP has been demonstrated in the development of type 1 diabetes and rheumatoid arthritis, a two-hit model has suggested that the factors other than lymphopenia are required to mediate the loss of control over homeostasis to result in ADs. Interestingly, these factors may be, if not totally, related to the function/number of regulatory T cells which are key modulators to protect from self-reactivity. In this review, we summarize the important roles of lymphopenia/LIP and the Treg cells in various autoimmune conditions, thereby highlighting them as key therapeutic targets for autoimmunity treatments.

From stability to dynamics: understanding molecular mechanisms of regulatory T cells through Foxp3 transcriptional dynamics

Studies on regulatory T cells (T_reg) have focused on thymic T_reg as a stable lineage of immunosuppressive T cells, the differentiation of which is controlled by the transcription factor forkhead box protein 3 (Foxp3). This lineage perspective, however, may constrain hypotheses regarding the role of Foxp3 and T_regin vivo, particularly in clinical settings and immunotherapy development. In this review, we synthesize a new perspective on the role of Foxp3 as a dynamically expressed gene, and thereby revisit the molecular mechanisms for the transcriptional regulation of Foxp3. In particular, we introduce a recent advancement in the study of Foxp3‐mediated T cell regulation through the development of the Timer of cell kinetics and activity (Tocky) system, and show that the investigation of Foxp3 transcriptional dynamics can reveal temporal changes in the differentiation and function of T_regin vivo. We highlight the role of Foxp3 as a gene downstream of T cell receptor (TCR) signalling and show that temporally persistent TCR signals initiate Foxp3 transcription in self‐reactive thymocytes. In addition, we feature the autoregulatory transcriptional circuit for the Foxp3 gene as a mechanism for consolidating T_reg differentiation and activating their suppressive functions. Furthermore, we explore the potential mechanisms behind the dynamic regulation of epigenetic modifications and chromatin architecture for Foxp3 transcription. Lastly, we discuss the clinical relevance of temporal changes in the differentiation and activation of T_reg.

A temporally dynamic Foxp3 autoregulatory transcriptional circuit controls the effector Treg programme

Regulatory T cells (Treg) are negative regulators of the immune response; however, it is poorly understood whether and how Foxp3 transcription is induced and regulated in the periphery during T‐cell responses. Using Foxp3‐Timer of cell kinetics and activity (Tocky) mice, which report real‐time Foxp3 expression, we show that the flux of new Foxp3 expressors and the rate of Foxp3 transcription are increased during inflammation. These persistent dynamics of Foxp3 transcription determine the effector Treg programme and are dependent on a Foxp3 autoregulatory transcriptional circuit. Persistent Foxp3 transcriptional activity controls the expression of coinhibitory molecules, including CTLA‐4 and effector Treg signature genes. Using RNA‐seq, we identify two groups of surface proteins based on their relationship to the temporal dynamics of Foxp3 transcription, and we show proof of principle for the manipulation of Foxp3 dynamics by immunotherapy: new Foxp3 flux is promoted by anti‐TNFRII antibody, and high‐frequency Foxp3 expressors are targeted by anti‐OX40 antibody. Collectively, our study dissects time‐dependent mechanisms behind Foxp3‐driven T‐cell regulation and establishes the Foxp3‐Tocky system as a tool to investigate the mechanisms behind T‐cell immunotherapies.

Regulatory T cells may participate in Helicobacter pylori persistence in gastric MALT lymphoma: lessons from an animal model

It has been postulated that the emergence of autoimmune gastritis in neonatal thymectomised (d3Tx) BALB/c mice may be a consequence of post-surgery deficit in Tregs. In this study, previously obtained samples from d3Tx mice were used in order to determine whether thymectomy creates a deficit in this T cell subset thereby allowing the emergence of autoimmune phenomena as a prerequisite for GML. The splenic Treg reserve and the local recruitment of these cells in the gastric mucosa were investigated using complementary molecular and immunohistochemistry approaches. Higher Foxp3/CD3 ratios were found in the spleen of non-infected d3Tx mice compared to non-thymectomised (NTx) controls. These results indicate a relative enrichment of Tregs following thymectomy in adult mice. The absence of Treg depletion in d3Tx mice is in line with the absence of auto-immune gastritis in non-infected d3Tx mice. Higher levels of T cell and Treg infiltration were also found in the stomach of GML-developing d3Tx mice versus NTx mice. Surprisingly, inflammatory scores inversely correlated with the bacterial inoculum. The presence of a small Treg containing compartment among gastric biopsies of GML developing d3Tx mice may play a role in perseverance of a minimal bacterial numbers thereby maintaining an antigen-dependent stimulation and proliferation.

Mechanisms of diabetic autoimmunity: II--Is diabetes a central or peripheral disorder of effector and regulatory cells?

Two competing hypotheses aiming to explain the onset of autoimmune reactions are discussed in the context of genetic and environmental predisposition to type 1 diabetes (T1D). The first hypothesis has evolved along characterization of the mechanisms of self-discrimination and attributes diabetic autoimmunity to escape of reactive T cells from central regulation in the thymus. The second considers frequent occurrence of autoimmune reactions within the immune homunculus, which are adequately suppressed by regulatory T cells originating from the thymus, and occasionally, insufficient suppression results in autoimmunity. Besides thymic dysfunction, deregulation of both effector and suppressor cells can in fact result from homeostatic aberrations at the peripheral level during initial stages of evolution of adaptive immunity. Pathogenic cells sensitized in the islets are efficiently expanded in the target tissue and pancreatic lymph nodes of lymphopenic neonates. In parallel, the same mechanisms of peripheral sensitization contribute to tolerization through education of naïve/effector T cells and expansion of regulatory T cells. Experimental evidence presented for each individual mechanism implies that T1D may result from a primary effector or suppressor immune abnormality. Disturbed self-tolerance leading to T1D may well result from peripheral deregulation of innate and adaptive immunity, with variable contribution of central thymic dysfunction.

Regulatory T Cells Control Th2-Dominant Murine Autoimmune Gastritis

Pernicious anemia and gastric carcinoma are serious sequelae of autoimmune gastritis (AIG). Our study indicates that in adult C57BL/6-DEREG mice expressing a transgenic diphtheria toxin receptor under the Foxp3 promoter, transient regulatory T cell (Treg) depletion results in long-lasting AIG associated with both H(+)K(+)ATPase and intrinsic factor autoantibody responses. Although functional Tregs emerge over time during AIG occurrence, the effector T cells rapidly become less susceptible to Treg-mediated suppression. Whereas previous studies have implicated dysregulated Th1 cell responses in AIG pathogenesis, eosinophils have been detected in gastric biopsy specimens from patients with AIG. Indeed, AIG in DEREG mice is associated with strong Th2 cell responses, including dominant IgG1 autoantibodies, elevated serum IgE, increased Th2 cytokine production, and eosinophil infiltration in the stomach-draining lymph nodes. In addition, the stomachs exhibit severe mucosal and muscular hypertrophy, parietal cell loss, mucinous epithelial cell metaplasia, and massive eosinophilic inflammation. Notably, the Th2 responses and gastritis severity are significantly ameliorated in IL-4- or eosinophil-deficient mice. Furthermore, expansion of both Th2-promoting IFN regulatory factor 4(+) programmed death ligand 2(+) dendritic cells and ILT3(+) rebounded Tregs was detected after transient Treg depletion. Collectively, these data suggest that Tregs maintain physiological tolerance to clinically relevant gastric autoantigens, and Th2 responses can be a pathogenic mechanism in AIG.

Deletion of CTLA-4 on regulatory T cells during adulthood leads to resistance to autoimmunity

Paterson et al. demonstrate that, in contrast to CTLA-4 germline knockout mice, conditional deletion on T reg cells during adulthood confers protection from EAE and does not increase resistance to tumors.

Cytotoxic T lymphocyte antigen-4 (CTLA-4) is an essential negative regulator of T cell responses. Germline Ctla4 deficiency is lethal, making investigation of the function of CTLA-4 on mature T cells challenging. To elucidate the function of CTLA-4 on mature T cells, we have conditionally ablated Ctla4 in adult mice. We show that, in contrast to germline knockout mice, deletion of Ctla4 during adulthood does not precipitate systemic autoimmunity, but surprisingly confers protection from experimental autoimmune encephalomyelitis (EAE) and does not lead to increased resistance to MC38 tumors. Deletion of Ctla4 during adulthood was accompanied by activation and expansion of both conventional CD4⁺Foxp3⁻ (T conv) and regulatory Foxp3⁺ (T reg cells) T cell subsets; however, deletion of CTLA-4 on T reg cells was necessary and sufficient for protection from EAE. CTLA-4 deleted T reg cells remained functionally suppressive. Deletion of Ctla4 on T reg cells alone or on all adult T cells led to major changes in the Ctla4 sufficient T conv cell compartment, including up-regulation of immunoinhibitory molecules IL-10, LAG-3 and PD-1, thereby providing a compensatory immunosuppressive mechanism. Collectively, our findings point to a profound role for CTLA-4 on T reg cells in limiting their peripheral expansion and activation, thereby regulating the phenotype and function of T conv cells.

Impaired expansion of regulatory T cells in a neonatal thymectomy-induced autoimmune mouse model

Neonatal thymectomy in certain mouse strains is known to induce organ-specific autoimmunity due to impaired functions of T cells, including Foxp3(+) regulatory T (Treg) cells in the thymus. The precise mechanism underlying the induction of autoimmunity by neonatal thymectomy remains unclear. One possibility is that depletion of Treg cells breaks down peripheral tolerance. We examined the functions of Treg cells by using a murine Sjögren syndrome model of NFS/sld mice that underwent neonatal thymectomy. The ratio of Treg cells to effector memory phenotype T cells in thymectomy mice was significantly lower than that of nonthymectomy mice. In addition, in vitro induction of peripherally induced Treg cells by transforming growth factor-β (TGF-β) using naive T cells from Sjögren syndrome model mice was severely impaired. The mRNA expression of TGF-β receptor I and II and Smad3 and -4 in the TGF-β-induced signal transduction pathway of Treg cells in this Sjögren syndrome model were lower than those of control mice. In addition, Treg cells in this Sjögren syndrome model exhibited an interferon-γ-producing Th1-like phenotype that resembled effector T cells. In conclusion, these results suggest that abnormal expansion and differentiation of Treg cells and inflammatory cytokines produced by Treg cells contribute to the development of autoimmunity.

Controversies concerning thymus-derived regulatory T cells: fundamental issues and a new perspective

Thymus-derived regulatory T cells (Tregs) are considered to be a distinct T-cell lineage that is genetically programmed and specialised for immunosuppression. This perspective is based on the key evidence that CD25⁺ Tregs emigrate to neonatal spleen a few days later than other T cells and that thymectomy of 3-day-old mice depletes Tregs only, causing autoimmune diseases. Although widely believed, the evidence has never been reproduced as originally reported, and some studies indicate that Tregs exist in neonates. Thus we examine the consequences of the controversial evidence, revisit the fundamental issues of Tregs and thereby reveal the overlooked relationship of T-cell activation and Foxp3-mediated control of the T-cell system. Here we provide a new model of Tregs and Foxp3, a feedback control perspective, which views Tregs as a component of the system that controls T-cell activation, rather than as a distinct genetically programmed lineage. This perspective provides new insights into the roles of self-reactivity, T cell–antigen-presenting cell interaction and T-cell activation in Foxp3-mediated immune regulation.

Immune tolerance. Regulatory T cells generated early in life play a distinct role in maintaining self-tolerance

Aire is an important regulator of immunological tolerance, operating in a minute subset of thymic stromal cells to induce transcripts encoding peptides that guide T cell selection. Expression of Aire during a perinatal age window is necessary and sufficient to prevent the multiorgan autoimmunity characteristic of Aire-deficient mice. We report that Aire promotes the perinatal generation of a distinct compartment of Foxp3(+)CD4(+) regulatory T (Treg) cells, which stably persists in adult mice. This population has a role in maintaining self-tolerance, a transcriptome and an activation profile distinguishable from those of Tregs produced in adults. Underlying the distinct Treg populations are age-dependent, Aire-independent differences in the processing and presentation of thymic stromal-cell peptides, resulting in different T cell receptor repertoires. Our findings expand the notion of a developmentally layered immune system.

Appropriate development of the liver Treg compartment is modulated by the microbiota and requires TGF-β and MyD88

Neither the early postnatal development of the liver Treg compartment nor the factors that regulate its development has been characterized. We compared the early developmental patterns of Treg cell accumulation in murine liver, thymus, and spleen. A FoxP3^EGFP reporter mouse was employed to identify Treg cells. Mononuclear cells were isolated from organs postnatally, stained for CD4, and examined by flow cytometry to enumerate FoxP3⁺CD4^hi cells. To assess roles for TGF-β1, MyD88, and TLR2, gene-specific knockout pups were generated from heterozygous breeders. To test the role of commensal bacteria, pregnant dams were administered antibiotics during gestation and after parturition. The pattern of appearance of Treg cells differed in liver, spleen, and thymus. Notably, at 1-2 weeks, the frequency of CD4^hi FoxP3⁺ T cells in liver exceeded that in spleen by 1.5- to 2-fold. The relative increase in liver Treg frequency was transient and was dependent upon TGF-β1 and MyD88, but not TLR2, and was abrogated by antibiotic treatment. A relative increase in liver Treg frequency occurs approximately 1-2 weeks after parturition that appears to be driven by colonization of the intestine with commensal bacteria and is mediated by a pathway that requires TGF-β1 and MyD88, but not TLR2.

Neonatal thymectomy favors Helicobacter pylori-promoted gastric mucosa-associated lymphoid tissue lymphoma lesions in BALB/c mice

Neonatal thymectomy in BALB/c mice has been described as a model of gastric mucosa-associated lymphoid tissue (MALT) lymphoma (GML). By using this experimental system, we screened, for the first time to our knowledge, Helicobacter pylori GML-associated strains for their capacity to promote disease. A cohort of BALB/c mice underwent thymectomy at day 3 after birth (d3Tx). Successful thymic ablation was evaluated by the degree of lymphopenia in blood samples collected at 4 weeks of age. d3Tx and non-thymectomized controls were infected with either GML strains (B38 or B47) or control strains (SS1 or TN2GF4). Gastric samples collected at 6, 12, and 18 months after infection were studied for bacteria content, and submitted to histological, immunochemical, molecular, and immunological analyses. Severe gastric inflammation was only observed in d3Tx mice. In these animals, the gastric lamina propria was infiltrated with lymphoid cells organized in follicles composed of B cells with few infiltrating T cells. PCR of D/J IgH gene segments proved the monoclonality of infiltrating B cells, which strongly correlated with the presence of lymphoepithelial lesions. B-cell infiltrates were particularly prominent in mice infected with the B47-GML strain. No pathological changes were detected in noninfected d3Tx mice. We identified new H. pylori isolates adapted to the mouse stomach with high potential of GML development, which is only revealed in hosts rendered lymphopenic by neonatal thymic ablation.

Homeostasis and function of regulatory T cells (Tregs) in vivo: lessons from TCR-transgenic Tregs

The identification of CD25 and subsequently Forkhead box protein 3 (Foxp3) as markers for regulatory T cells (Tregs) has revolutionized our ability to explore this population experimentally. In a similar vein, our understanding of antigen-specific Treg responses in vivo owes much to the fortuitous generation of T-cell receptor (TCR)-transgenic Tregs. This has permitted tracking of Tregs with a defined specificity in vivo, facilitating analysis of how encounter with cognate antigen shapes Treg homeostasis and function. Here, we review the key lessons learned from a decade of analysis of TCR-transgenic Tregs and set this in the broader context of general progress in the field. Use of TCR-transgenic Tregs has led to an appreciation that Tregs are a highly dynamic proliferative population in vivo, rather than an anergic population as they were initially portrayed. It is now clear that Treg homeostasis is positively regulated by encounter with self-antigen expressed on peripheral tissues, which is likely to be relevant to the phenomenon of peripheral repertoire reshaping that has been described for Tregs and the observation that the Treg TCR specificities vary by anatomical location. Substantial evidence has also accumulated to support the role of CD28 costimulation and interleukin-2 in Treg homeostasis. The availability of TCR-transgenic Tregs has enabled analysis of Treg populations that are sufficient or deficient in particular genes, without the comparison being confounded by repertoire alterations. This approach has yielded insights into genes required for Treg function in vivo, with particular progress being made on the role of ctla-4 in this context. As the prospect of manipulating Treg populations in the clinic becomes reality, a full appreciation of the rules governing their homeostasis will prove increasingly important.

T cells generated in the absence of a thoracic thymus fail to establish homeostasis

Cervical thymus mimics the thoracic thymus in supporting T-cell development and exists in a subset of mice and humans. Importantly, it remains unknown whether the cervical thymus can generate T cells that are self-tolerant in the complete absence of signals from the thoracic thymus. Using a fetal liver reconstitution model in thoracic thymectomized RAG(-/-) mice, we found that T cells could be generated without contribution from the thoracic thymus. However, these mice had decreased T cells, increased proportions of effector memory T cells and Treg phenotype cells, increased serum IgG1/2b, and increased frequency of T cells expressing IFN-γ, IL-17 or IL-10. Half of the mice that received a thoracic thymectomy and fetal liver cells, unlike sham surgery controls, developed substantial morbidity with age. Disease was associated with lymphopenia-driven activation rather than inherent defects in the cervical thymus, as both thoracic and cervical thymocytes could generate disease in lymphopenic recipients. Administration of the homeostatic cytokine IL-7 caused a rapid, transient increase in T-cell numbers and reduced the time to disease onset. Together the data suggests that the cervical thymus can function in the complete absence of the thoracic thymus; however, the T cells generated do not establish homeostasis.

Premature CD4+ T cell aging and its contribution to lymphopenia-induced proliferation of memory cells in autoimmune-prone non-obese diabetic mice

Lymphopenia-induced proliferation (LIP), a mechanism to maintain a constant number of T cells in circulation, occurs in both normal aging and autoimmune disease. The incidence of most autoimmune diseases increases with age, and premature CD4(+) T cell aging has been reported in several autoimmune diseases. In this study, we tested the hypothesis that premature CD4(+) T cell aging can cause autoimmune disease by examining whether premature CD4(+) T cell aging exists and causes LIP in our mouse model. Non-obese diabetic (NOD) mice were used because, in addition to Treg defects, the LIP of T cells has been shown to plays a causative role in the development of insulin-dependent diabetes mellitus (IDDM) in these mice. We found that with advancing age, NOD mice exhibited an accelerated decrease in the number of CD4(+) T cells due to the loss of naïve cells. This was accompanied by an increase in the percentage of memory cells, leading to a reduced naïve/memory ratio. In addition, both the percentage of CD28(+) cells in CD4(+) T cells and IL-2 production decreased, while the percentage of FAS(+)CD44(+) increased, suggesting that NOD mice exhibit premature CD4(+) T cell aging. This process preferentially contributed to LIP of memory cells. Therefore, our results suggest that premature CD4(+) T cell aging underlies the development of IDDM in NOD mice. Given that CD28 and IL-2 play important roles in Treg function, the relationships between premature CD4(+) T cell aging and lymphopenia as well as Treg defects in autoimmune-prone NOD mice are proposed.

Disruption of TSC1/2 signaling complex reveals a checkpoint governing thymic CD4+ CD25+ Foxp3+ regulatory T-cell development in mice

Thymic-derived CD4(+)CD25(+)Foxp3(+) natural regulatory T (nTreg) cells are essential for the maintenance of peripheral immune tolerance. Signaling pathways that drive immature thymic progenitors to differentiate into CD4(+)CD25(+)Foxp3(+) nTreg cells need to be elucidated. The precise role of the TSC1/2 complex, a critical negative regulator of mammalian target of rapamycin (mTOR), in thymic CD4(+)CD25(+)Foxp3(+) nTreg-cell development remains elusive. In the present study, we found that the percentage and cell number of thymic CD4(+)CD25(+)Foxp3(+) nTreg cells were significantly increased in T-cell-specific TSC1-knockout (TSC1KO) mice. Nevertheless, the levels of CD4(+)CD25(+)Foxp3(-) nTreg precursors in TSC1KO thymus were indistinguishable from those in wild-type mice. TSC1KO CD4(+)CD25(+)Foxp3(+) nTreg cells showed normal cell death but enhanced proliferative response to IL-2 in a STAT5-dependent manner. Rapamycin (Rapa) treatment failed to rescue but rather increased the frequency of CD4(+)CD25(+)Foxp3(+) nTreg cells in TSC1KO and RictorKO mice. The percentage and cell number of thymic CD4(+)CD25(+)Foxp3(+) nTreg cells were significantly increased in T-cell-specific RictorKO mice but not in PtenKO mice. Collectively, our studies suggest that TSC1 plays an important role in regulating thymic CD4(+)CD25(+)Foxp3(+) nTreg-cell development via a Rapa-resistant and mTORC2-dependent signaling pathway.

CD4+CD25+ Regulatory T cell ontogeny and preferential migration to the cecal tonsils in chickens

Thymic CD4(+)CD25(+) cells have regulatory-T-cell-like properties in chickens. This study examined the ontogeny of CD4(+)CD25(+) cells in the thymus and in peripheral compartments in chickens. CD4(+)CD25(+) cells started to appear in the thymus at day 15 of incubation (E15), although at low percentages. Expressed as a percentage of CD4(+) cells, CD4(+)CD25(+) cells increased (P<0.01) from 1.7% at E20 to 7.3% at 0 d post-hatch (D0). CD4(+)CD25(+) cells did not appear in the spleen or cecal tonsils of embryos. Expressed as a percentage of CD4(+) cells, CD4(+)CD25(+) cells increased (P<0.01) from 0% at D0 to 27% at D1 in cecal tonsils and from 0% at D0 to 11% at D1 in the spleen. Expressed as a percentage of all mononuclear cells, cecal tonsils at D1 had approximately 3.5-fold higher percentage of CD4(+)CD25(+) cells than the spleen at D1. CD4(+)CD25(+) cells from cecal tonsils of chicks at D1 were suppressive. CD4(+)CD25(+) cells from D0 thymus, when injected back into MHC-compatible chicks, migrated to cecal tonsils and lungs and were detected until 10 d post-injection. CD4(+)CD25(+) cells from cecal tonsils had a higher (P = 0.01) relative amount of CCR9 mRNA than CD4(+)CD25(+) cells from the thymus. It could be concluded that in chickens CD4(+)CD25(+) cells migrate from the thymus immediately post-hatch and preferentially colonize the gut associated lymphoid tissues. CD4(+)CD25(+) cells' preferential migration to cecal tonsils is likely directed through the CCR9 pathway in chickens.

Potent immunosuppression by a bivalent molecule binding to CD200R and TGF-betaR

BACKGROUND

The novel immunosuppressive molecule, CD200, has been reported to induce immunoregulation after interaction with its receptor(s), CD200R(s), in part at least through augmented induction of regulatory T-cell populations. Independent studies have also described increased expression of indoleamine-2,3-dioxygenase after CD200R triggering, whereas others have provided evidence that TGF-beta is important for the induction or function of many populations of regulatory T cells. We have asked whether a hybrid molecule in which a soluble fusion protein containing CD200, CD200Fc, was linked to TGF-beta through a glycine linker (Gly6) functions as a superior immunosuppressant molecule when compared with CD200Fc or TGF-beta alone, or in combination.

METHODS

The hybrid molecule CD200FcGly6TGF-beta was expressed by transient transfection in CHO cells and purified over a protein A column. Functional activity of this and recombinant CD200Fc or TGF-beta alone were assessed in mixed leukocyte cultures (MLCs) and in skin graft rejection in vivo.

RESULTS

Immunosuppression mediated by CD200FcGly6TGF-beta is dependent on both functional CD200 and TGF-beta moieties, as indicated by inhibition of suppression using anti-CD200 or anti-TGF-beta antibodies. Using as responder cells, using antigen-presenting cell from mice with a deletion of the CD200R gene and responder T cells from mice with siRNA-mediated suppression of expression of the TGF-betaII receptor, we show that suppression follows binding to TGF-betaRII on T cells, and CD200R1 on antigen-presenting cells. Indoleamine-2,3-dioxygenase inhibitors did not attenuate suppression by CD200FcGly6TGF-beta.

CONCLUSION

CD200FcGly6TGF-beta is a potent immunosuppressant in vivo and in vitro.

Lymphocyte proliferation in immune-mediated diseases

Defects in T cell homeostatic mechanisms can result in T cell lymphopenia, defined as decreased numbers of lymphocytes. Lymphopenia results in homeostatic proliferation in order to maintain T cell homeostasis. It has been proposed that homeostatic proliferation can expand the pool of autoreactive T cells that promote autoimmunity, and indeed recent studies have further substantiated this observation in both animal models and humans. Conversely, homeostatic proliferation can promote tumor immunity by allowing tumor-specific T cells to accumulate. In this review, we discuss how the outcome of homeostatic proliferation can function both in a deleterious manner in autoimmunity and a beneficial way in tumor immunity. We also discuss the roles of various cytokines and T regulatory cells that control homeostatic proliferation.

Neonatal tolerance revisited: a perinatal window for Aire control of autoimmunity

There has long been conceptual and experimental support for, but also challenges to, the notion that the initial period of the immune system's development is particularly important for the establishment of tolerance to self. The display of self-antigens by thymic epithelial cells is key to inducing tolerance in the T lymphocyte compartment, a process enhanced by the Aire transcription factor. Using a doxycycline-regulated transgene to target Aire expression to the thymic epithelium, complementing the Aire knockout in a temporally controlled manner, we find that Aire is essential in the perinatal period to prevent the multiorgan autoimmunity that is typical of Aire deficiency. Surprisingly, Aire could be shut down soon thereafter and remain off for long periods, with few deleterious consequences. The lymphopenic state present in neonates was a factor in this dichotomy because inducing lymphopenia during Aire turnoff in adults recreated the disease, which, conversely, could be ameliorated by supplementing neonates with adult lymphocytes. In short, Aire expression during the perinatal period is both necessary and sufficient to induce long-lasting tolerance and avoid autoimmunity. Aire-controlled mechanisms of central tolerance are largely dispensable in the adult, as a previously tolerized T cell pool can buffer newly generated autoreactive T cells that might emerge.

Pathogenic effector T cell enrichment overcomes regulatory T cell control and generates autoimmune gastritis

Regulatory T cells (Treg) deficiency leads to a severe, systemic, and lethal disease, as showed in immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome patients, and scurfy mouse. Postneonatal thymectomy autoimmune gastritis has also been attributed to the absence of Tregs. In this case however, disease is mild, organ-specific, and, more important, it is not an obligatory outcome. We addressed this paradox comparing T cell compartments in gastritis-susceptible and resistant animals. We found that neonatal thymectomy-induced gastritis is not caused by the absence of Tregs. Instead of this, it is the presence of gastritogenic T cell clones that determines susceptibility to disease. The expansion of such clones under lymphopenic conditions results in a reduced Treg:effector T cell ratio that is not enough to control gastritis development. Finally, the presence of gastritogenic clones is determined by the amount of gastric Ag expressed in the neonatal thymus, emphasizing the importance of effector repertoire variability, present even in genetically identical subjects, to organ-specific autoimmune disease susceptibility.

(Xeno)estrogen regulation of food allergy

Food allergy and other types of allergies are becoming epidemic in both the developed and developing countries. A large amount of information is available in literature that (xeno)estrogens can regulate the immune response in general, and the development of allergy in particular; however, the effect of (xeno)estrogens on food allergy is basically unknown. With increasing use of xenobiotics worldwide, chemicals with estrogenic activity have been accumulating in our environment. This review has summarized the current literature relating to the topic (xeno)estrogen regulation of food allergy. The effect of (xeno)estrogens on enterocytes, proteases for protein hydrolysis, dendritic cells and T-regulatory cells in the gastrointestinal tract has been discussed. Finally, considering the current confusion in literature regarding the effect of phytoestrogen genistein on the immune system, a brief discussion has been included for its effect on T(H)1-T(H)2 polarization, and possibly food allergy in its relation to windows of exposure. Sufficient evidences exist to support the notion that (xeno)estrogens can regulate food allergy, with the developmental periods more sensitive. Further clinical and animal studies are needed to determine the causal relationship between the exposure of (xeno)estrogens and incidence of food allergy, and the underlying mechanisms.

Cutting edge: Autoimmune disease in day 3 thymectomized mice is actively controlled by endogenous disease-specific regulatory T cells

Female B6AF1 mice thymectomized on day 3 (d3tx) develop autoimmune ovarian disease (AOD) and dacryoadenitis. It has been hypothesized that d3tx breaks tolerance by depleting late ontogeny regulatory T cells (Treg). We now report that Treg greatly expand over effector T cells in d3tx mice and adoptively suppress autoimmune disease in d3tx recipients. In the d3tx donors, Treg from ovarian lymph nodes (LN) preferentially suppress AOD and Treg from lacrimal gland LN preferentially suppress dacryoadenitis, suggesting they are strategically positioned for disease control. Indeed, the autologous disease in d3tx mice is dramatically enhanced by in vivo depletion of endogenous Treg. Moreover, normal 3-day-old mice possess Treg that suppress AOD and autoimmune gastritis as efficiently as adult cells. Thus, d3tx mice possess disease-relevant Treg of presumed neonatal origin. They accumulate in the regional LN and actively inhibit concurrent autoimmune disease; however, they cannot fully prevent autoimmune disease development.

Early increases in superantigen-specific Foxp3+ regulatory T cells during mouse mammary tumor virus infection

Mouse mammary tumor virus (MMTV) is a milk-borne betaretrovirus that has developed strategies to exploit and subvert the host immune system. Here, we show in a natural model of MMTV infection that the virus causes early and progressive increases in superantigen (SAg)-specific Foxp3(+) regulatory T cells (T(reg)) in Peyer's patches (PP). These increases were shown to be dependent on the presence of dendritic cells. CD4(+) CD25(+) T cells from the PP of infected mice preferentially suppress the proliferative response of T cells to SAg-expressing antigen-presenting cells ex vivo. We investigated the influence of the depletion of CD25(+) cells at different stages of the infection. When CD25(+) cells were depleted before MMTV infection, an increase in the number of PP SAg-cognate Foxp3(-) T cells was found at day 6 of infection. Since the SAg response is associated with viral amplification, the possibility exists that T(reg) cells attenuate the increase in viral load at the beginning of the infection. In contrast, depletion of CD25(+) cells once the initial SAg response has developed caused a lower viral load, suggesting that at later stages T(reg) cells may favor viral persistence. Thus, our results indicated that T(reg) cells play an important and complex role during MMTV infection.

T regulatory cells contribute to the attenuated primary CD8+ and CD4+ T cell responses to herpes simplex virus type 2 in neonatal mice

The first weeks of life are characterized by immune tolerance and increased susceptibility to intracellular pathogens. The neonatal adaptive response to HSV is attenuated compared with adult control models in humans and mice. T Regulatory cells (Tregs) control autoimmunity and excessive immune responses to infection. We therefore compared Treg responses in the draining lymph nodes (LN) of HSV-infected neonatal and adult C57BL/6 mice with the effect of Treg depletion/inactivation by anti-CD25 (PC61) treatment before infection on Ag-specific T cell effector responses at this site. There was a small, but significant increase in the frequency of CD4(+)Foxp3(+) Tregs at day 3 postinfection (p.i.) in the LN of neonatal and adult mice, compared with age-matched mock-infected controls. Depletion of Tregs before HSV infection significantly enhanced HSV-specific CD8(+) T cell cytotoxicity in vivo, cell number, activation, and granzyme B expression 4 days p.i. only in neonatal mice, and significantly enhanced CD8(+) and CD4(+) T cell IFN-gamma responses in both infected adults and neonates. Treg depletion also reduced the titer of infectious virus in the draining LN and nervous system of infected neonates on days 2 and 3 p.i. Treg suppression of the neonatal CTL response p.i. with HSV was associated with increased expression of TGF-beta in the draining LN at day 4 p.i. compared with uninfected neonates, but IL-10 was increased in infected adults alone. These experiments support the notion that the newborn primary T cell effector responses to HSV are suppressed by Tregs.

Thymic production of human FOXP3(+) regulatory T cells is stable but does not correlate with peripheral FOXP3 expression

In humans functionally mature FOXP3(+) regulatory T (Treg) cells can be found already in the fetus, but the kinetics of their maturation is still unknown. Here, we show that from birth to until 10 years of age the thymic production of FOXP3(+) Treg cells is very stable and correlates with T-lymphopoiesis in general. The level of FOXP3 expression in the blood was also very stable, even when children and adults were compared, but there was no correlation between thymic and peripheral FOXP3 levels. Analysis of the cell cycle-associated marker Ki67 showed that a substantial fraction of peripheral FOXP3(+) cells is dividing. This characteristic was obtained in the periphery, since it was not observed in thymic CD4(+) FOXP3(+) cells. These data suggest that the thymic output of human Treg cells is intrinsically stable, while in the periphery the increased rate of proliferation severs the connection between production and homeostatic maintenance of the FOXP3(+) Treg cell population.

Two genetic loci independently confer susceptibility to autoimmune gastritis

Autoimmune gastritis is a CD4+ T cell-mediated disease induced in genetically susceptible mice by thymectomy on the third day after birth. Previous linkage analysis indicated that Gasa1 and Gasa2, the major susceptibility loci for gastritis, are located on mouse chromosome 4. Here we verified these linkage data by showing that BALB.B6 congenic mice, in which the distal approximately 40 Mb of chromosome 4 was replaced by C57BL/6 DNA, were resistant to autoimmune gastritis. Analysis of further BALB.B6 congenic strains demonstrated that Gasa1 and Gasa2 can act independently to cause full expression of susceptibility to autoimmune disease. Gasa1 and Gasa2 are located between D4Mit352-D4Mit204 and D4Mit343-telomere, respectively. Numerical differences in Foxp3+ regulatory T cells were apparent between the BALB/c and congenic strains, but it is unlikely that this phenotype accounted for differences in autoimmune susceptibility. The positions of Gasa1 and Gasa2 correspond closely to the positions of Idd11 and Idd9, two autoimmune diabetes susceptibility loci in nonobese diabetic (NOD), mice and this prompted us to examine autoimmune gastritis in NOD mice. After neonatal thymectomy, NOD mice developed autoimmune gastritis, albeit at a slightly lower incidence and severity of disease than in BALB/c mice. Diabetes-resistant congenic NOD.B6 mice, harbouring a B6-derived interval encompassing the Gasa1/2-Idd9/11 loci, demonstrated a slight reduction in the incidence of autoimmune gastritis. This reduction was not significant compared with the reduction observed in BALB.B6 congenic mice, suggesting a difference in the genetic aetiology of autoimmune gastritis in NOD and BALB mice.

Heterogeneity in the CD4 T Cell Compartment and the Variability of Neonatal Immune Responsiveness

Over the past decade, it has become clear that T cell immune responses in both murine and human neonates are very heterogeneous, running the gamut from poor or deviant responsiveness to mature, adult-like inflammatory function. How this variability arises is not well understood but there is now a great deal of information suggesting that differences in the T cell compartments in neonates vs adults play important roles. A number of cell types or processes are qualitatively or quantitatively different in the neonate. These include (a) alternate epigenetic programs at the Th2 cytokine locus, (b) enhanced homeostatic proliferation, (c) a relative abundance of fetal-origin cells, (d) a greater representation of recent thymic emigrants, (e) high proportions of potentially self-reactive cells, (f) a developmental delay in the production of regulatory T cells, and (g) cells bearing TCR with limited N region diversity. Different conditions of antigen exposure may lead to different environmental signals that promote the selective responsiveness of one or more of these populations. Therefore, the variability of neonatal responses may be a function of the heterogeneous nature of the responding T cell population. In this review, we will describe these various subpopulations in detail and speculate as to the manner in which they could contribute to the heterogeneity of neonatal immune responses.

When three is not a crowd: a Crossregulation model of the dynamics and repertoire selection of regulatory CD4+ T cells

Regulatory CD4(+) T cells, enriched in the CD25 pool of healthy individuals, mediate natural tolerance and prevent autoimmune diseases. Despite their fundamental and potential clinical significance, regulatory T (T(R)) cells have not yet been incorporated in a coherent theory of the immune system. This article reviews experimental evidence and theoretical arguments supporting a model of T(R) cell dynamics, uncovering some of its most relevant biological implications. According to this model, the persistence and expansion of T(R) cell populations depend strictly on specific interactions they make with antigen-presenting cells (APCs) and conventional effector T (T(E)) cells. This three-partner crossregulation imposes that T(R) cells feed on the specific autoimmune activities they suppress, with implications ranging from their interactions with other cells to their repertoire selection in the periphery and in the thymus, and to the relationship between these cells and the innate immune system. These implications stem from the basic prediction that the peripheral dynamics sort the CD4(+) T-cell repertoire into two subsets: a less diverse set of small clones of autoreactive effector and regulatory cells that regulate each other's growth, and a more diverse set of barely autoreactive T(E) cell clones, whose expansion is limited only by APC availability. It is argued that such partitioning of the repertoire sets the ground for self-non-self discrimination.

Severe FOXP3+ and naïve T lymphopenia in a non-IPEX form of autoimmune enteropathy combined with an immunodeficiency

BACKGROUND & AIMS

Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome is the best-characterized form of a rare entity called autoimmune enteropathy (AIE). IPEX syndrome is due to mutations in the FOXP3 gene, a transcription factor essential for the development and function of the natural regulatory CD25(+)CD4(+) T cells. We studied a female patient with a polyautoimmune AIE syndrome resembling a mild form of IPEX syndrome but associated with recurrent bacterial infections and mild hypogammaglobulinemia. We hypothesized that this syndrome combined a deficit of FOXP3(+) cells and other lymphocyte populations.

METHODS

We analyzed the major lymphocyte subsets and the FOXP3(+) regulatory system in blood samples obtained during the 2-year period that followed the last autoimmune manifestation.

RESULTS

The patient had severe naïve T lymphopenia and a major deficit of FOXP3(+)CD4(+) T cells, both in circulation and in the highly inflamed intestinal mucosa, but mutations in the FOXP3 locus were excluded. The blood FOXP3(+) pool was devoid of CD25(high) cells, but the few regulatory CD25(+) cells were functional. Intrinsic defects in the expression of CD25, FOXP3, and interleukin 2 were excluded. Upon activation, a small subset of cells, presumably committed to regulatory function, sustained expression of CD25 and FOXP3.

CONCLUSIONS

Peripheral T lymphopenia of both naïve and natural regulatory T cells might be the consequence of defective thymic production or the short life span of exported T cells. This case sheds new light in the etiology of autoimmune manifestations in T-cell immunodeficiencies and in the heterogeneity of AIE.

Targeting of CD25 and glucocorticoid-induced TNF receptor family-related gene-expressing T cells differentially modulates asthma risk in offspring of asthmatic and normal mother mice

Immunological mechanisms leading to increased asthma susceptibility in early life remain obscure. In this study, we examined the effects of neonatal Ab treatments targeting T cell populations on the development of an asthma syndrome. We used a model of increased asthma susceptibility where offspring of asthmatic BALB/c mother mice are more prone (than normal pups) to develop the disease. Neonatal pretreatment of naive pups with mAb directed against the IL-2Ralpha chain (CD25), the costimulatory molecule glucocorticoid-induced TNFR family related gene, and the inhibitory molecule CTLA-4 elicited contrasting effects in offspring depending on the mother's asthma status. Specifically, neonatal CD25(high) T cell depletion stimulated asthma susceptibility in normal offspring whereas it ameliorated the condition of pups born of asthmatic mothers. Conversely, glucocorticoid-induced TNFR family related gene ligation as a primary signal reduced the spleen cellularity and largely abrogated asthma susceptibility in asthma-prone offspring, without inducing disease in normal pups. Striking changes in Th1/Th2 cytokine levels, especially IL-4, followed mAb pretreatment and were consistent with the impact on asthma susceptibility. These results point to major differences in neonatal T cell population and responsiveness related to maternal asthma history. Interventions that temporarily remove and/or inactivate specific T cell subsets may therefore prove useful to attenuate early life asthma susceptibility and prevent the development of Th2-driven allergic airway disease.

The role of the transcription factor Foxp3 in the development of regulatory T cells

Early studies of mice subjected to neonatal thymectomy and analyses of adoptive T-cell transfer into lymphopenic hosts led to the identification of a specialized subset of regulatory CD4+ T cells capable of suppressing various manifestations of autoimmunity. Recently, a combination of genetic, molecular, and traditional cellular approaches provided novel powerful means to investigate the biology of these cells. Here, we review earlier and current work from our laboratory, establishing a dedicated function for the transcription factor Foxp3 in the process of regulatory T-cell lineage commitment and a role for TCR- and cytokine-mediated signals in regulation of Foxp3 expression.

Developmental regulation of Foxp3 expression during ontogeny

Thymectomy of neonatal mice can result in the development of autoimmune pathology. It has been proposed that thymic output of regulatory T (T reg) cells is delayed during ontogeny and that the development of autoimmune disease in neonatally thymectomized mice is caused by the escape of self-reactive T cells before thymectomy without accompanying T reg cells. However, the kinetics of T reg cell production within the thymus during ontogeny has not been assessed. We demonstrate that the development of Foxp3-expressing T reg cells is substantially delayed relative to nonregulatory thymocytes during ontogeny. Based on our data, we speculate that induction of Foxp3 in developing thymocytes and, thus, commitment to the T reg cell lineage is facilitated by a signal largely associated with the thymic medulla.

Delayed functional maturation of natural regulatory T cells in the medulla of postnatal thymus: role of TSLP

BACKGROUND

Generation of functional (CD4+)(CD8-)CD25+ regulatory T cells (Treg) in the murine thymus depends on FoxP3. Removal of the thymus from neonatal mice has been shown to result in a multiple organ autoimmune disease phenotype that can be prevented by introducing the FoxP3+ Treg population to the animal. It has therefore, been proposed that functional FoxP3+ Treg cells are not made in the neonatal thymus; however, it remains unclear when and where functional (FoxP3+)(CD4+)(CD8-)CD25+ thymocytes are generated in postnatal thymus.

RESULTS

We report that neither FoxP3 mRNA nor protein is expressed in (CD4+)(CD8-)CD25+, or (CD4+)(CD8-)CD25- thymocytes until 3-4 days post birth, despite the presence of mature (CD4+)(CD8-)CD25+/- thymocytes in the thymus by 1-2 days after birth. (FoxP3-)(CD4+)(CD8-)CD25+ thymocytes from day 2 newborn mice show no Treg activity. Interestingly, we are able to detect low numbers of FoxP3+ thymocytes dispersed throughout the medullary region of the thymus as early as 3-4 days post birth. Expression of FoxP3 is induced in embryonic day 17 fetal thymus organ culture (FTOC) after 4-6 days of in vitro culture. Treatment of FTOCs with thymic stromal derived lymphopoietin (TSLP) enhanced expression of FoxP3, and blocking the TSLP receptor reduces FoxP3 expression in FTOC. Furthermore, TSLP stimulates FoxP3 expression in purified (CD4+)CD8- thymocytes, but not in (CD4+)CD8+, (CD4-)CD8+ and (CD4-)CD8- thymocytes.

CONCLUSION

Expression of FoxP3 or Treg maturation is ontogenically distinct and kinetically delayed from the generation of (CD4+)(CD8-)CD25+ or (CD4+)(CD8-)CD25- thymocytes in the postnatal thymus. TSLP produced from medullary thymic epithelia cells (mTEC) contributes to the expression of FoxP3 and the maturation of natural regulatory T cells. Overall, these results suggest that the development of Treg cells requires paracrine signaling during late stages of thymocyte maturation that is distinct from signaling during positive or negative selection.

Are regulatory T-cells linked with aging?

There is increasing evidence for an active and 'dominant' tolerance mediated by regulatory T-cells. Out of these CD4+ 'naturally occurring' regulatory T-cells (TREGs) are currently the main research focus in this field. TREGs exert their suppressive function in vitro in a contact-dependent manner and preferentially express high levels of CD25 and the forkhead and winged-helix family transcription factor forkhead box P3 (FOXP3). Age-related increment of the prevalences of CD4+ CD25(hi) TREGs were described controversially, and whether such changes explain immune dysfunction in the elderly is still unclear. During aging thymic TREG output may decrease with significant loss of thymic capacity to generate new T-cells, and TREG homeostasis has been shown to be sustained by alternative pathways like peripheral generation of TREGs. An imbalance of TREG homeostasis would then predispose to immune dysfunction in aged individuals explaining their higher risk of immune-mediated diseases, cancer or infections.

Imbalance of regulatory T cells in human autoimmune diseases

The breakdown of mechanisms assuring the recognition of self and non-self is a hallmark feature of autoimmune diseases. In the past 10 years, there has been a steadily increasing interest in a subpopulation of regulatory T cells, which exert their suppressive function in vitro in a contact-dependent manner and preferentially express high levels of CD25 and forkhead and winged-helix family transcription factor forkhead box P3 (FOXP3) (TREGs). Recent findings of changed prevalences and functional efficiencies indicate that these TREGs play a unique role in autoimmune diseases. Clinical findings in patients with mutated FOXP3 genes and a specific polymorphism in the promotor region of FOXP3 also support the role of FOXP3 as a 'master control gene' in the development and functioning of TREGs. Both altered generation of TREGs and insufficient suppression of inflammation in autoimmune diseases are considered to be crucial for the initiation and perpetuation of disease. TREG-related somatic cell therapy is considered as an intriguing new intervention to approach autoimmune diseases.

Thymic commitment of regulatory T cells is a pathway of TCR-dependent selection that isolates repertoires undergoing positive or negative selection

The seminal work of Le Douarin and colleagues (Ohki et al. 1987; Ohki et al. 1988; Salaun et al. 1990; Coutinho et al. 1993) first demonstrated that peripheral tissue-specific tolerance is centrally established in the thymus, by epithelial stromal cells (TEC). Subsequent experiments have shown that TEC-tolerance is dominant and mediated by CD4 regulatory T cells (Treg) that are generated intrathymically by recognition of antigens expressed on TECs (Modigliani et al. 1995; Modigliani et al. 1996a). From these and other observations, in 1996 Modigliani and colleagues derived a general model for the establishment and maintenance of natural tolerance (MM96) (Modigliani et al. 1996b), with two central propositions: (1) T cell receptor (TCR)-dependent sorting of emergent repertoires generates TEC-specific Treg displaying the highest TCR self-affinities below deletion thresholds, thus isolating repertoires undergoing positive and negative selection; (2) Treg are intrathymically committed (and activated) for a unique differentiative pathway with regulatory effector functions. The model explained the embryonic/perinatal time window of natural tolerance acquisition, by developmental programs determining (1) TCR multireactivity, (2) the cellular composition in the thymic stroma (relative abundance of epithelial vs hemopoietic cells), and (3) the dynamics of peripheral lymphocyte pools, built by accumulation of recent thymic emigrants (RTE) that remain recruitable to regulatory functions. We discuss here the MM96 in the light of recent results demonstrating the promiscuous expression of tissue-specific antigens by medullary TECs (Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004) and indicating that Treg represent a unique differentiative pathway (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003), which is adopted by CD4 T cells with high avidity for TEC-antigens (Bensinger et al. 2001; Jordan et al. 2001; Apostolou et al. 2002). In the likelihood that autoimmune diseases (AID) result from Treg deficits, some of which might have a thymic origin, we also speculate on therapeutic strategies aiming at selectively stimulating their de novo production or peripheral function, within recent findings on Treg responses to inflammation (Caramalho et al. 2003; Lopes-Carvalho et al., submitted, Caramalho et al., submitted). In short, the MM96 argued that natural tolerance is dominant, established and maintained by the activity of Treg, which are selected upon high-affinity recognition of self-ligands on TECs, and committed intrathymically to a unique differentiative pathway geared to anti-inflammatory and antiproliferative effector functions. By postulating the intrathymic deletion of self-reactivities on hemopoietic stromal cells (THC), together with the inability of peripheral resident lymphocytes to engage in the regulatory pathway, the MM96 simultaneously explained the maintenance of responsiveness to non-self in a context of suppression mediating dominant self-tolerance. The major difficulty of the MM96 is related to the apparent tissue specificity of Treg repertoires generated intrathymically. This difficulty has now been principally solved by the work of Hanahan, Kyewski and others (Jolicoeur et al. 1994; Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004), demonstrating the selective expression of a variety of tissue-specific antigens by TECs, in topological patterns that are compatible with the MM96, but difficult to conciliate with recessive tolerance models (Kappler et al. 1987; Kisielow et al. 1988). While the developmentally regulated multireactivity of TCR repertoires (Gavin and Bevan 1995), as well as the peripheral recruitment of Treg among RTE (Modigliani et al. 1996a) might add to this process, it would seem that the establishment of tissue-specific tolerance essentially stems from the "promiscuous expression of tissue antigens" by TEC. The findings of AID resulting from natural mutations (reviewed in Pitkanen and Peterson 2003) or the targeted inactivation (Anderson et al. 2002; Ramsey et al. 2002) of the AIRE transcription factor that regulates promiscuous gene expression on TECs support this conclusion. The observations on the correlation of natural or forced expression of the Foxp3 transcription factor in CD4 T cells with Treg phenotype and function (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003) provided support for the MM96 contention that Treg represent a unique differentiative pathway that is naturally established inside the thymus. Furthermore, Caton and colleagues (Jordan et al. 2001), as well as several other groups (Bensinger et al. 2001; Apostolou et al. 2002), have provided direct evidence for our postulate that Treg are selected among differentiating CD4 T cells with high affinity for ligands expressed on TECs (Modigliani et al. 1996b). Finally, the demonstration by Caramalho et al. that Treg express innate immunity receptors (Caramalho et al. 2003) and respond to pro-inflammatory signals and products of inflammation (Caramalho et al., submitted) brought about a new understanding on the peripheral regulation of Treg function. Together with the observation that Treg also respond to ongoing activities of "naïve/effector" T cells--possibly through the IL-2 produced in these conditions--these findings explain the participation of Treg in all immune responses (Onizuka et al. 1999; Shimizu et al. 1999; Annacker et al. 2001; Curotto de Lafaille et al. 2001; Almeida et al. 2002; Shevach 2002; Bach and Francois Bach 2003; Wood and Sakaguchi 2003; Mittrucker and Kaufmann 2004; Sakaguchi 2004), beyond their fundamental role in ensuring self-tolerance (e.g., Modigliani et al. 1996a; Shevach 2000; Hori et al. 2003; Sakaguchi 2004; Thompson and Powrie 2004). Thus, anti-inflammatory and anti-proliferative Treg are amplified by signals that promote or mediate inflammation and proliferation, accounting for the quality control of responses (Coutinho et al. 2001). In turn, such natural regulation of Treg by immune responses to non-self may well explain the alarming epidemiology of allergic and AID in wealthy societies (Wills-Karp et al. 2001; Bach 2002; Yazdanbakhsh et al. 2002), where a variety of childhood infections have become rare or absent. Thus, it is plausible that Treg were evolutionarily set by a given density of infectious agents in the environment. With hindsight, it is not too surprising that natural Treg performance falls once hygiene, vaccination, and antibiotics suddenly (i.e., 100 years) plunged infectious density to below some critical physiological threshold. As the immune system is not adapted to modern clean conditions of postnatal development, clinical immunologists must now deal with frequent Treg deficiencies (allergies and AID) for which they have no curative or rational treatments. It is essential, therefore, that basic immunologists concentrate on strategies to selectively stimulate the production, survival, and activity of this set of lymphocytes that is instrumental in preventing immune pathology. We have argued that the culprit of this inability of basic research to solve major clinical problems has been the self-righteousness of recessive tolerance champions, from Ehrlich to some of our contemporaries. It is ironical, however, that none of us--including the heretic opponents of horror autotoxicus--had understood that self-tolerance, or its robustness at least, is in part determined by the frequency and intensity of the responses to non-self. In the evolution of ideas on immunological tolerance, the time might be ripe for some kinds of synthesis. First, conventional theory reduced self-tolerance to negative selection and microbial defense to positive selection, while the MM96 solution was the precise opposite: positive selection of autoreactivities for self-tolerance (Treg) and negative selection (of Treg) for ridding responses. In contrast, it would now appear that positive and negative selection of autoreactive T cells are both necessary to establish either self-tolerance or competence to eliminate microbes, two processes that actually reinforce each other in the maintenance of self-integrity. Second, V-region recognition has generally been held responsible for specific discrimination between what should be either tolerated or eliminated from the organism. In contrast again, it would now seem that both processes of self-tolerance and microbial defense (self/non-self discrimination) also operate on the basis of evolutionarily ancient, germ-line-encoded innate, nonspecific receptors (Medzhitov and Janeway 2000) capable of a coarse level of self/non-self discrimination (Coutinho 1975). It could thus be interesting to revisit notions of cooperativity between V-regions and such mitogen receptors, both in single cell functions (Coutinho et al. 1974) and in the system's evolution (Coutinho 1975, 1980) as well. After all, major transitions in evolution were cooperative (Maynard-Smith and Szathmary 1995).

Failure of CD25+ T cells from lupus-prone mice to suppress lupus glomerulonephritis and sialoadenitis

The development of organ-specific autoimmune diseases in mice thymectomized on day 3 of life (d3tx mice) can be prevented by transferring CD4(+)CD25(+) T cells from syngeneic, normal adult mice. Using a d3tx model, we asked whether CD4(+)CD25(+) T cell deficiency contributes to glomerulonephritis (GN) in lupus-prone mice. New Zealand Mixed 2328 (NZM2328) mice spontaneously develop autoantibodies to dsDNA and female-dominant, fatal GN. After d3tx, both male and female NZM2328 mice developed 1) accelerated dsDNA autoantibody response, 2) early onset and severe proliferative GN with massive mesangial immune complexes, and 3) autoimmune disease of the thyroid, lacrimal gland, and salivary gland. The d3tx male mice also developed autoimmune prostatitis. The transfer of CD25(+) cells from 6-wk-old asymptomatic NZM2328 donors effectively suppressed dsDNA autoantibody and the development of autoimmune diseases, with the exception of proliferative lupus GN and sialoadenitis. This finding indicates that NZM2328 lupus mice have a selective deficiency in T cells that regulates the development of lupus GN and sialoadenitis. After d3tx, the proliferative GN of female mice progressed to fatal GN, but largely regressed in the male, thereby revealing a checkpoint in lupus GN progression that depends on gender.

Numbers of -expressing CD4CD25 T cells do not correlate with the establishment of long-term tolerance after allogeneic stem cell transplantation

OBJECTIVE

Regulatory CD4 T cells that express high levels of CD25 play a vital role in the maintenance of tolerance to self antigens and are required for the induction of nonresponsiveness to alloantigens. The long-term CD4+CD25high T-cell reconstitution after allogeneic stem cell transplantation is unknown. Here, we evaluated whether recovery of this T-cell subset might be linked to the establishment of full donor/recipient tolerance.

METHODS

The frequency of CD4+CD25high T cells was determined by Fluorescence Activated Cell Sorter (FACS) analysis in 31 patients, with a mean follow-up of more than 31 months posttransplant. The expression levels of Foxp3 mRNA were assessed by quantitative real-time polymerase chain reaction (RT-PCR).

RESULTS

Patients with or without graft-versus-host disease (GvHD) had significant and persistent CD4 T-cell lymphopenia. The relative frequency of CD25high cells and the expression levels of FoxP3 mRNA within this subset were similar between all patients and healthy controls. No significant difference was found in the number of Foxp3-expressing CD4+CD25high T cells in patients with or without GvHD. Finally, younger age and absence of previous GvHD were significantly linked to CD4+CD25high T-cell recovery.

CONCLUSION

The low number of Foxp3-expressing CD4+CD25high T cells in grafted patients is not a specific default of this compartment but a consequence of global CD4 T-cell lymphopenia after allogeneic stem cell transplantation. Moreover, levels of Foxp3 mRNA in the CD25+ T-cell compartment do not allow predicting the development of GvHD in the long term.

A well adapted regulatory contrivance: regulatory T cell development and the forkhead family transcription factor Foxp3

The random generation of antigen receptors in developing lymphocytes results in a considerable risk of autoimmunity. Regulatory T cells (T(reg) cells) act in a dominant, trans-acting way to actively suppress immune activation and maintain immune tolerance. Here, we discuss the principal advances in our understanding of the molecular mechanisms of T(reg) cell development and function with particular emphasis on the forkhead transcription factor Foxp3. Accumulating evidence suggests that T(reg) cells represent a dedicated T cell lineage and that Foxp3 functions as the T(reg) cell lineage specification factor. The aggressive early-onset lymphoproliferative syndrome resulting from Foxp3 deficiency identifies T(reg) cells as vital mediators of immunological tolerance to self and Foxp3 as the mediator of the genetic mechanism of dominant tolerance.

Prenatal tolerance--a role for regulatory T cells?

Regulatory T cells (TR cells) play a major role in controlling immune self reactivity. However, little is known about their occurrence and functions in early developmental stages. In this issue of the European Journal of Immunology, Cupedo et al. report the presence of functional CD4+CD25+ TR cells in the human fetus. In contrast to previous studies, the analysis is performed on fetal thymus, spleen and lymph node samples in addition to cord blood cells. Interestingly, TR cells are present in all these organs from 14 weeks of gestation, along with FoxP3 (forkhead box protein 3) RNA, a marker for naturally arising TR cells. The fetal TR cells show, however, phenotypic differences depending on their location, possibly because of variations in their activation state. The emergence of TR cells so early in fetal development raises a number of questions about the mechanisms of self reactivity and tolerance in the prenatal stages, which may have important implications for our understanding of childhood pathologies.

Foxp3+ CD25- CD4 T cells constitute a reservoir of committed regulatory cells that regain CD25 expression upon homeostatic expansion

Expression of the IL-2 receptor alpha chain (CD25) by peripheral CD4 T cells follows cellular activation. However, CD25 expression by CD4 cells is widely used as a marker to identify regulatory T cells (T(R)), although cells with regulatory properties are also found in the CD4+CD25- subset. By using in vivo functional assays and Foxp3 expression as a faithful marker of T(R) differentiation, we have evaluated the requirements for CD25 expression by peripheral T(R). We first show that in vivo depletion of CD25+ cells prevents the development of spontaneous encephalomyelitis in recombination-activating gene (RAG)-deficient anti-myelin basic protein T cell antigen receptor (TCR) transgenic mice, and allows disease induction in otherwise healthy RAG-competent transgenic mice. Similar treatment in normal thymectomized animals is followed by the fast recovery of a normal number of CD25+ T(R). Consistently, Foxp3-expressing T(R) encompassed in the CD25- cell population convert to CD25+ after homeostatic expansion and are selectable by IL-2 in vitro. Surface expression of CD25 on T(R) is controlled by the activity of conventional CD4 cells and is fully labile because it can be lost and regained without affecting the functional potential of the cells. These findings reveal that Foxp3-expressing CD25- cells constitute a peripheral reservoir of differentiated T(R), recruited to the CD25+ pool upon homeostatic expansion and/or activation. This analysis, together with the notion that physiological commitment of T(R) takes place exclusively in the thymus should help for the interpretation of experiments assessing peripheral T(R) differentiation from naive CD4 T cells, defined as CD25-.