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A defect in TCR-proximal signaling is a major characteristic of CD4 T cells in systemic lupus erythematosus; however, it is not fully known how defects in TCR signaling lead to lupus-like systemic autoimmunity characterized by germinal center development and autoantibody production against nuclear Ags. In this study, we show that SKG mice, which develop autoimmune arthritis in a BALB/c background due to defective TCR signaling by a Zap70 mutation, develop lupus-like systemic autoimmune disease in the C57BL/6 (B6) background (B6SKG mice). B6SKG mice showed multiorgan inflammation with immune complex deposition and anti-dsDNA Ab production. Follicular helper T cells (Tfh), which help germinal center formation, were spontaneously expanded in B6SKG mice. Th cells secreting IFN-γ or IL-17 and regulatory T cells were also increased in B6SKG mice compared with wild-type B6 mice, with the regulatory T cell subpopulation losing the expression of CD25. Among the factors related to Tfh differentiation, the number of dendritic cells and the expression levels of the costimulatory molecules CD80, CD86, and ICOSL in dendritic cells but not in B cells were specifically increased in wild-type B6 mice compared with BALB/c mice. The inhibition of these costimulatory molecules suppressed Tfh development and lupus-like autoimmunity. Thus, a defect in TCR-proximal signaling leads to lupus-like systemic autoimmunity under the specific genetic background that facilitates Tfh development., (Copyright © 2019 by The American Association of Immunologists, Inc.)

Arthritis in a genetically susceptible SKG strain of mice models a theoretical paradigm wherein autoimmune arthritis arises because of interplay between preexisting autoreactive T cells and environmental stimuli. SKG mice have a point mutation in ZAP-70 that results in attenuated TCR signaling, altered thymic selection, and spontaneous production of autoreactive T cells that cause arthritis following exposure to microbial β-glucans. In this study, we identify Nod2, an innate immune receptor, as a critical suppressor of arthritis in SKG mice. SKG mice deficient in Nod2 (Nod2 -/- SKG) developed a dramatically exacerbated form of arthritis, which was independent of sex and microbiota, but required the skg mutation in T cells. Worsened arthritis in Nod2 -/- SKG mice was accompanied by expansion of Th17 cells, which to some measure coproduced TNF, GM-CSF, and IL-22, along with elevated IL-17A levels within joint synovial fluid. Importantly, neutralization of IL-17A mitigated arthritis in Nod2 -/- SKG mice, indicating that Nod2-mediated protection occurs through suppression of the Th17 response. Nod2 deficiency did not alter regulatory T cell development or function. Instead, Nod2 deficiency resulted in an enhanced fundamental ability of SKG CD4 + T cells (from naive mice) to produce increased levels of IL-17 and to passively transfer arthritis to lymphopenic recipients on a single-cell level. These data reveal a previously unconsidered role for T cell-intrinsic Nod2 as an endogenous negative regulator of Th17 responses and arthritogenic T cells. Based on our findings, future studies aimed at understanding a negative regulatory function of Nod2 within autoreactive T cells could provide novel therapeutic strategies for treatment of patients with arthritis., (Copyright © 2018 by The American Association of Immunologists, Inc.)

Skin dendritic cells (DCs) are divided into several subsets with distinctive functions. This study shows a previously unappreciated role of dermal CD11b-type Langerin - DCs in maintaining immunological self-tolerance after UVB exposure. After UVB exposure, dermal CD11b-type Langerin - DCs upregulated surface CD86 expression, induced proliferation of Foxp3 + regulatory T (Treg) cells without exogenous Ags, and upregulated a set of genes associated with immunological tolerance. This Treg-expansion activity was significantly hampered by CD80/CD86 blockade in vivo. These results indicate that CD11b-type Langerin - DCs from the UVB-exposed skin are specialized to expand Treg cells in the skin, which suppress autoimmunity., (Copyright © 2017 by The American Association of Immunologists, Inc.)

Mechanistic target of rapamycin complex (mTORC)1 integrates intracellular sufficiency of nutrients and regulates various cellular functions. Previous studies using mice with conditional knockout of mTORC1 component proteins (i.e., mTOR, Raptor, and Rheb) gave conflicting results on the roles of mTORC1 in CD4 + T cells. Lamtor1 is the protein that is required for amino acid sensing and activation of mTORC1; however, the roles of Lamtor1 in T cells have not been investigated. In this article, we show that Lamtor1-deficient CD4 + T cells exhibited marked reductions in proliferation, IL-2 production, mTORC1 activity, and expression of purine- and lipid-synthesis genes. Polarization of Th17 cells, but not Th1 and Th2 cells, diminished following the loss of Lamtor1. Accordingly, CD4-Cre -driven Lamtor1-knockout mice exhibited reduced numbers of CD4 + and CD8 + T cells at rest, and they were completely resistant to experimental autoimmune encephalomyelitis. In contrast, genetic ablation of Lamtor1 in Foxp3 + T cells resulted in severe autoimmunity and premature death. Lamtor1-deficient regulatory T cells survived ex vivo as long as wild-type regulatory T cells; however, they exhibited a marked loss of suppressive function and expression of signature molecules, such as CTLA-4. These results indicate that Lamtor1 plays essential roles in CD4 + T cells. Our data suggest that Lamtor1 should be considered a novel therapeutic target in immune systems., (Copyright © 2017 by The American Association of Immunologists, Inc.)

The cytoplasmic phosphatase, protein tyrosine phosphatase nonreceptor type 22 (PTPN22), is a negative regulator of T cell signaling. Genome-wide association studies have shown that single-nucleotide polymorphisms in PTPN22 confer an increased risk of developing multiple autoimmune diseases in humans. The precise function of PTPN22 and how the variant protein contributes to autoimmunity is not well understood. To address this issue, we investigated the effect of PTPN22 deficiency on disease susceptibility in a mouse model of autoimmune arthritis. The SKG mouse expresses a hypomorphic mutant allele of ZAP70, which, upon exposure to fungal Ags, predisposes the mice to a CD4(+) T cell-mediated autoimmune arthritis that closely resembles rheumatoid arthritis in humans. Surprisingly, SKG Ptpn22(-/-) mice developed less severe mannan-induced arthritis compared with SKG mice. Diminution of disease was not due to significant alterations in thymocyte development or repertoire selection in SKG Ptpn22(-/-) mice, even though T cell-mediated signal transduction was improved. Instead, Ptpn22 deficiency appeared to bias CD4 Th cell differentiation away from the Th17 lineage, which is pathogenic in this setting, to a more Th1/T regulatory-focused response. These data show that even small perturbations in TCR signal transduction pathways can have profound consequences on the differentiation of T cell lineages and thus for the development of autoimmune diseases., (Copyright © 2016 The Authors.)

Th-inducing Pox virus and zinc finger/Krüppel-like factor (ThPOK) is a key commitment factor for CD4(+) lineage T cells and is essential for the maintenance of CD4 lineage integrity; thus, the expression of ThPOK has to be tightly controlled. In this article, we demonstrate that Myc-associated zinc finger-related factor (MAZR) and Runt-related transcription factor 1 (Runx1) together repressed ThPOK in preselection double-positive thymocytes, whereas MAZR acted in synergy with Runx3 in the repression of ThPOK in CD8(+) T cells. Furthermore, MAZR-Runx1 and MAZR-Runx3 double-mutant mice showed enhanced derepression of Cd4 in double-negative thymocytes and in CD8(+) T cells in comparison with Runx1 or Runx3 single-deficient mice, respectively, indicating that MAZR modulates Cd4 silencing. Thus, our data demonstrate developmental stage-specific synergistic activities between MAZR and Runx/core-binding factor β (CBFβ) complexes. Finally, retroviral Cre-mediated conditional deletion of MAZR in peripheral CD8(+) T cells led to the derepression of ThPOK, thus showing that MAZR is also part of the molecular machinery that maintains a repressed state of ThPOK in CD8(+) T cells., (Copyright © 2015 by The American Association of Immunologists, Inc.)

Accumulating evidence shows that immunological tolerance induced by Ag administration together with UVB irradiation is dependent on Foxp3(+) CD4(+) regulatory T (Treg) cells. However, the mechanisms by which UVB controls Treg cells in the skin are currently unclear. In this study, we have shown that exposure to UVB induced expansion of Treg cells up to 50-60% of the CD4(+) T cells in the irradiated skin. The Treg cell expansion in the skin lasted for 2 wk after exposure, which contributed to homeostasis of Treg cells in the periphery later. UVB-expanded Treg cells formed clusters with dendritic cells and proliferated in situ. Furthermore, the expanded Treg cells appeared to derive from neuropilin 1(+) thymus-derived Treg (tTreg) cells in the periphery because UVB-expanded Treg cells possessed Treg cell-specific CpG hypomethylation pattern, as seen in tTreg cells. These results collectively indicate that homeostasis of tTreg cells is controlled by UVB exposure in the skin. UVB therapy may be useful for not only inflammatory skin disorders, but also autoimmunity, transplantation, and allergy., (Copyright © 2014 by The American Association of Immunologists, Inc.)

Dietary compounds as well as commensal microbiota contribute to the generation of a unique gut environment. In this study, we report that dietary folic acid (FA) is required for the maintenance of Foxp3+ regulatory T cells (Tregs) in the colon. Deficiency of FA in the diet resulted in marked reduction of Foxp3+ Tregs selectively in the colon. Blockade of folate receptor 4 and treatment with methotrexate, which inhibits folate metabolic pathways, decreased colonic Foxp3+ Tregs. Compared with splenic Tregs, colonic Tregs were more activated to proliferate vigorously and were highly sensitive to apoptosis. In colonic Tregs derived from mice fed with a FA-deficient diet, expression of anti-apoptotic molecules Bcl-2 and Bcl-xL was severely decreased. A general reduction of peripheral Tregs was induced by a neutralizing Ab against IL-2, but a further decrease by additional FA deficiency was observed exclusively in the colon. Mice fed with an FA-deficient diet exhibited higher susceptibility to intestinal inflammation. These findings reveal the previously unappreciated role of dietary FA in promotion of survival of Foxp3+ Tregs that are in a highly activated state in the colon.

The balance between immune activation and suppression must be regulated to maintain immune homeostasis. Tissue macrophages (MΦs) constitute the major cellular subsets of APCs within the body; however, how and what types of resident MΦs are involved in the regulation of immune homeostasis in the peripheral lymphoid tissues are poorly understood. Splenic red pulp MΦ (RPMs) remove self-Ags, such as blood-borne particulates and aged erythrocytes, from the blood. Although many scattered T cells exist in the red pulp of the spleen, little attention has been given to how RPMs prevent harmful T cell immune responses against self-Ags. In this study, we found that murine splenic F4/80(hi)Mac-1(low) MΦs residing in the red pulp showed different expression patterns of surface markers compared with F4/80(+)Mac-1(hi) monocytes/MΦs. Studies with purified cell populations demonstrated that F4/80(hi)Mac-1(low) MΦs regulated CD4(+) T cell responses by producing soluble suppressive factors, including TGF-β and IL-10. Moreover, F4/80(hi)Mac-1(low) MΦs induced the differentiation of naive CD4(+) T cells into functional Foxp3(+) regulatory T cells. Additionally, we found that the differentiation of F4/80(hi)Mac-1(low) MΦs was critically regulated by CSF-1, and in vitro-generated bone marrow-derived MΦs induced by CSF-1 suppressed CD4(+) T cell responses and induced the generation of Foxp3(+) regulatory T cells in vivo. These results suggested that splenic CSF-1-dependent F4/80(hi)Mac-1(low) MΦs are a subpopulation of RPMs and regulate peripheral immune homeostasis.

To test if manipulating TCR complex-mediated signaling (TCR signaling) could treat autoimmune disease, we generated the double SKG Src-like adapter protein (SLAP) knockout (DSSKO) mouse model. The SKG mutation in ZAP70 and SLAP have opposing functions on the regulation of TCR signaling. The combination of these two mutations alters TCR signaling in the context of a defined genetic background, uniform environmental conditions, and a well-characterized signaling disruption. In contrast to SKG mice, DSSKO mice do not develop zymosan-induced chronic autoimmune arthritis. This arthritis prevention is not due to significant alterations in thymocyte development or repertoire selection but instead enhanced numbers of regulatory T cells (Tregs) and decreased numbers of Th17 cells skewing the ratio of Tregs to autoreactive effector T cells. Treg depletion and/or functional blockade led to the development of arthritis in DSSKO mice. In vitro suppression of effector T cell proliferation was also enhanced, demonstrating that DSSKO mice have increased numbers of Tregs with increased function. Understanding how TCR signals influence development, expansion, and function of Tregs in DSSKO mice could advance our ability to manipulate Treg biology to treat ultimately autoimmune disease.

Treatment of tumor-bearing mice with a stimulatory Ab to glucocorticoid-induced TNFR family-related receptor (GITR) has previously been shown to elicit protective T cell responses against poorly immunogenic tumors. However, the role of GITR stimulation on CD8 T cells and the nature of tumor rejection Ags have yet to be determined. In this study, we show that a stimulatory mAb to GITR (clone DTA-1) acts directly on CD8 T cells, but not on CD4(+)CD25(+) regulatory T (T(reg)) cells, in B16 tumor-bearing mice to induce concomitant immunity against secondary B16 tumors, as well as protective memory following surgical excision of the primary tumor. Melanoma growth itself induced GITR expression on tumor-specific CD8 T cells, providing a mechanism whereby these cells may respond to stimulatory anti-GITR. Unexpectedly, in contrast to T(reg) cell depletion therapy with anti-CD4, GITR stimulation induced very weak CD8 T cell responses to melanocyte differentiation Ags expressed by the tumor, and did not induce autoimmune vitiligo. Accordingly, GITR-stimulated hosts that were primed with B16 melanoma rejected B16, but not the unrelated JBRH melanoma, indicating that tumor rejection Ags are tumor-specific rather than shared. In support of this, we show that GITR stimulation induces CD8 T cell responses to a tumor-specific Ag, and that these responses are of higher functional avidity compared with those induced by T(reg) cell depletion. We conclude that stimulation of GITR on effector CD8 T cells results in high-avidity T cell responses to tumor-specific Ags, thereby inducing potent antitumor immunity in the absence of autoimmunity.

The regulation of T cell expansion by TNFR family members plays an important role in determining the magnitude of the immune response to pathogens. As several members of the TNFR family, including glucocorticoid-induced TNFR-related protein (GITR), are found on both regulatory and effector T cells, there is much interest in understanding how their effects on these opposing arms of the immune system affect disease outcome. Whereas much work has focused on the role of GITR on regulatory T cells, little is known about its intrinsic role on effector T cells in an infectious disease context. In this study, we demonstrate that GITR signaling on CD8 T cells leads to TNFR-associated factor (TRAF) 2/5-dependent, TRAF1-independent NF-κB induction, resulting in increased Bcl-x(L). In vivo, GITR on CD8 T cells has a profound effect on CD8 T cell expansion, via effects on T cell survival. Moreover, GITR is required on CD8 T cells for enhancement of influenza-specific CD8 T cell expansion upon administration of agonistic anti-GITR Ab, DTA-1. Remarkably, CD8 T cell-intrinsic GITR is essential for mouse survival during severe, but dispensable during mild respiratory influenza infection. These studies highlight the importance of GITR as a CD8 T cell costimulator during acute viral infection, and argue that despite the similarity among several TNFR family members in inducing T lymphocyte survival, they clearly have nonredundant functions in protection from severe infection.

Mice with a mutation of the zeta-associated protein of 70 kDa gene (skg mutation) are genetically prone to develop autoimmune arthritis, depending on the environment. In a set of mice with the mutation, the amount of zeta-associated protein of 70 kDa protein as well as its tyrosine phosphorylation upon TCR stimulation decreased from +/+, skg/+, skg/skg, to skg/- mice in a stepwise manner. The reduction resulted in graded alterations of thymic positive and negative selection of self-reactive T cells and Foxp3(+) natural regulatory T cells (Tregs) and their respective functions. Consequently, skg/- mice spontaneously developed autoimmune arthritis even in a microbially clean environment, whereas skg/skg mice required stimulation through innate immunity for disease manifestation. After Treg depletion, organ-specific autoimmune diseases, especially autoimmune gastritis, predominantly developed in +/+, at a lesser incidence in skg/+, but not in skg/skg BALB/c mice, which suffered from other autoimmune diseases, especially autoimmune arthritis. In correlation with this change, gastritis-mediating TCR transgenic T cells were positively selected in +/+, less in skg/+, but not in skg/skg BALB/c mice. Similarly, on the genetic background of diabetes-prone NOD mice, diabetes spontaneously developed in +/+, at a lesser incidence in skg/+, but not in skg/skg mice, which instead succumbed to arthritis. Thus, the graded attenuation of TCR signaling alters the repertoire and the function of autoimmune T cells and natural Tregs in a progressive manner. It also changes the dependency of disease development on environmental stimuli. These findings collectively provide a model of how genetic anomaly of T cell signaling contributes to the development of autoimmune disease.

Chronic infectious diseases and cancers are often associated with suboptimal effector T cell responses. Enhancement of T cell costimulatory signals has been extensively studied for cancer immunotherapy but not so for the treatment of infectious disease. The few previous attempts at this strategy using infection models have lacked cellular specificity, with major immunoregulatory mechanisms or innate immune cells also being targeted. In this study, we examined the potential of promoting T cell responses via the glucocorticoid-induced TNF receptor (GITR) family-related protein in a murine model of visceral leishmaniasis. GITR stimulation during established infection markedly improved antiparasitic immunity. This required CD4(+) T cells, TNF, and IFN-gamma, but crucially, was independent of regulatory T (Treg) cells. GITR stimulation enhanced CD4(+) T cell expansion without modulating Treg cell function or protecting conventional CD4(+) T cells from Treg cell suppression. GITR stimulation substantially improved the efficacy of a first-line visceral leishmaniasis drug against both acute hepatic infection and chronic infection in the spleen, demonstrating its potential to improve clinical outcomes. This study identifies a novel strategy to therapeutically enhance CD4(+) T cell-mediated antiparasitic immunity and, importantly, achieves this goal without impairment of Treg cell function.

Dendritic cells (DCs) express the immunoregulatory enzyme IDO in response to certain inflammatory stimuli, but it is unclear whether DCs express this enzyme under steady-state conditions in vivo. In this study, we report that the DCs in mesenteric lymph nodes (MLNs) constitutively express functional IDO, which metabolizes tryptophan to kynurenine. In line with a previous report that regulatory T cells (Tregs) can induce IDO in DCs via the CTLA-4/B7 interaction, a substantial proportion of the MLN DCs were located in juxtaposition to Tregs, whereas this tendency was not observed for splenic DCs, which do not express IDO constitutively. When CTLA-4 was selectively deleted in Tregs, the frequency of IDO-expressing DCs in MLNs decreased significantly, confirming CTLA-4's role in IDO expression by MLN DCs. We also found that the MLN DCs produced CCL22, which can attract Tregs via CCR4, and that the phagocytosis of autologous apoptotic cells induced CCL22 expression in CCL22 mRNA-negative DCs. Mice genetically deficient in the receptor for CCL22, CCR4, showed markedly reduced IDO expression in MLN-DCs, supporting the involvement of the CCL22/CCR4 axis in IDO induction. Together with our previous observation that MLN DCs contain much intracytoplasmic cellular debris in vivo, these results indicate that reciprocal interactions between the DCs and Tregs via both B7/CTLA-4 and CCL22/CCR4 lead to IDO induction in MLN DCs, which may be initiated and/or augmented by the phagocytosis of autologous apoptotic cells by intestinal DCs. Such a mechanism may help induce the specific milieu in MLNs that is required for the induction of oral tolerance.

Autoimmunity is often accompanied by the development of ectopic lymphoid tissues in the target organ, and these tissues have been believed to have close relevance to the severity of the disease. However, the true relationship between the extent of such lymphoid structures and the intensity or type of immune responses mediated by self-reactive T cells has remained unclear. In the present study, we generated transgenic mice expressing TCR from an autoimmune gastritis (AIG)-inducing Th1 cell clone specific for one of the major stomach self-Ags, H(+)/K(+)-ATPase alpha subunit. The transgenic mice spontaneously develop massive lymphoid neogenesis with a highly organized tissue structure in the gastric mucosa, demonstrating Ag-specific, T cell-mediated induction of the lymphoid tissues. Nevertheless, the damage of surrounding tissue and autoantibody production were considerably limited compared with those in typical AIG induced by neonatal thymectomy. Such a moderate pathology is likely due to the locally restricted activation and Th2 skewing of self-reactive T cells, as well as the accumulation of naturally occurring regulatory T cells in the target organ. Altogether, the findings suggest that lymphoid neogenesis in chronic autoimmunity does not simply correlate with the destructive response; rather, the overall activation status of the T cell network, i.e., the balance of self-reactivity and tolerance, in the local environment has an impact.

Glucocorticoid-induced TNF receptor family related protein (GITR) is present on many different cell types. Previous studies have shown that in vivo administration of an anti-GITR agonist mAb (DTA-1) inhibits regulatory T cells (Treg)-dependent suppression and enhances T cell responses. In this study, we show that administration of DTA-1 induces >85% tumor rejection in mice challenged with B16 melanoma. Rejection requires CD4+, CD8+, and NK1.1+ cells and is dependent on IFN-gamma and Fas ligand and independent of perforin. Depletion of Treg via anti-CD25 treatment does not induce B16 rejection, whereas 100% of the mice depleted of CD25+ cells and treated with DTA-1 reject tumors, indicating a predominant role of GITR on effector T cell costimulation rather than on Treg modulation. T cells isolated from DTA-1-treated mice challenged with B16 are specific against B16 and several melanoma differentiation Ags. These mice develop memory against B16, and a small proportion of them develop mild hypopigmentation. Consistent with previous studies showing that GITR stimulation increases Treg proliferation in vitro, we found in our model that GITR stimulation expanded the absolute number of FoxP3+ cells in vivo. Thus, we conclude that overall, GITR stimulation overcomes self-tolerance/ignorance and enhances T cell-mediated antitumor activity with minimal autoimmunity.

Aging is associated with a progressive decline in T cell-mediated immune responses. However, it has been unknown whether regulatory/suppressive CD4 T cells are involved in this decline. Our in vitro analyses revealed that CD4+CD25+ T cells, the well-characterized naturally occurring regulatory/suppressive CD4 T cells, in aged mice are functionally comparable to those in young mice (i.e., anergic and suppressive), although slightly increased in number. In contrast, functional changes to whole CD4+CD25- T cells were pronounced in aged mice, i.e., the majority of aged CD4+CD25- T cells exhibited a significant hyporesponsiveness, and the remaining cells maintained a normal responsiveness. Furthermore, we identified Foxp3 (a transcription factor critical in conferring the regulatory/suppressive function to CD4 T cells)-positive suppressive CD4 T cells among aged hyporesponsive CD4+CD25- T cells. These results suggest that the age-related decline in T cell-mediated immune responses is ascribable to changes in the CD4+CD25- T cell population and not to a functional augmentation of suppressive CD4+CD25+ T cells.

In this study, we investigated the effect of an agonistic mAb (DTA-1) against glucocorticoid-induced TNF receptor (GITR) in a murine model of systemic lupus erythematosus-like chronic graft-vs-host disease (cGVHD). A single dose of DTA-1 inhibited the production of anti-DNA IgG1 autoantibody and the development of glomerulonephritis, typical symptoms of cGVHD. DTA-1-treated mice showed clinical and pathological signs of acute GVHD (aGVHD), such as lymphopenia, loss of body weight, increase of donor cell engraftment, and intestinal damage, indicating that DTA-1 shifted cGVHD toward aGVHD. The conversion of cGVHD to aGVHD occurred because DTA-1 prevented donor CD8+ T cell anergy. Functionally active donor CD8+ T cells produced high levels of IFN-gamma and had an elevated CTL activity against host Ags. In in vitro MLR, anergic responder CD8+ T cells were generated, and DTA-1 stimulated the activation of these anergic CD8+ T cells. We further confirmed in vivo that donor CD8+ T cells, but not donor CD4+ T cells, were responsible for the DTA-1-mediated conversion of cGVHD to aGVHD. These results indicate that donor CD8+ T cell anergy is a restriction factor in the development of aGVHD and that in vivo ligation of GITR prevents CD8+ T cell anergy by activating donor CD8+ T cells that otherwise become anergic. In sum, our data suggest GITR as an important costimulatory molecule regulating cGVHD vs aGVHD and as a target for therapeutic intervention in a variety of related diseases.

Anomalies of naturally occurring CD4+ regulatory T cells (Treg) cause severe autoimmune/inflammatory diseases in humans and rodents. The transcription factor Foxp3 is currently the most specific marker for natural CD4+ Treg, but it would be useful if other Treg markers, particularly cell surface molecules, could be elucidated. We demonstrate in this study that the vast majority of Foxp3-expressing CD4+ T cells (whether CD25+ or CD25-) show constitutive high-level expression of glucocorticoid-induced TNFR family-related gene/protein (GITR). Transfer of T cell or thymocyte suspensions depleted of GITR(high) cells produces in BALB/c nude mice a wider spectrum and more severe forms of autoimmune diseases than does transfer of similar cell suspensions depleted of CD25+ CD4+ T cells only. Notably, mice that receive cells depleted of GITR(high) populations develop severe multiorgan inflammation that includes fatal autoimmune myocarditis resembling giant cell myocarditis in humans, accompanying high-titer anti-myosin autoantibodies. Similar transfer of GITR(high)-depleted cells from prediabetic NOD mice to NOD-SCID mice accelerates the development of diabetes and induces skeletal muscle myositis and other autoimmune/inflammatory diseases. We conclude that GITR(high), Foxp3-expressing natural Treg, containing both CD25+ and CD25- cell populations, contribute to preventing a variety of autoimmune/inflammatory diseases, and depletion of these cells allows the activation of even weak or rare autoreactive T cells yielding widespread severe autoimmune disease. Diseases induced in this way include many which have been suspected of an autoimmune etiology in humans without much evidence. GITR(high), Foxp3-expressing natural Treg represent a potential target for the treatment and prevention of these diseases.

Although it has been shown that CD4(+)CD25(+) regulatory T cells (T(reg)) contribute to long-term graft acceptance, their impact on the effector compartment and the mechanism by which they exert suppression in vivo remain unresolved. Using a CD4(+) TCR transgenic model for graft tolerance, we have unveiled the independent contributions of anergy and active suppression to the fate of immune and tolerant alloreactive T cells in vivo. First, it is shown that anti-CD154-induced tolerance resulted in the abortive expansion of the alloreactive, effector T cell pool. Second, commensurate with reduced expansion, there was a loss of cytokine production, activation marker expression, and absence of memory T cell markers. All these parameters defined the tolerant alloreactive T cells and correlated with the inability to mediate graft rejection. Third, the tolerant alloreactive T cell phenotype that is induced by CD154 was reversed by the in vivo depletion of T(reg). Reversal of the tolerant phenotype was followed by rapid rejection of the allograft. Fourth, in addition to T(reg) depletion, costimulation of the tolerant alloreactive T cells or activation of the APC compartment also reverted alloreactive T cell tolerance and restored an activated phenotype. Finally, it is shown that the suppression is long-lived, and in the absence of anti-CD154 and donor-specific transfusion, these T(reg) can chronically suppress effector cell responses, allowing long-lived graft acceptance.

CD4+CD25+ regulatory T cells (Treg) are potent immunosuppressive cells that are pivotal in the regulation of peripheral tolerance. In this report, we identify granzyme B (GZ-B) as one of the key components of Treg-mediated suppression. Induction of regulatory activity is correlated with the up-regulation of GZ-B expression. Proof of a functional involvement of GZ-B in contact-mediated suppression by Treg is shown by the reduced ability of Treg from GZ-B-/- mice to suppress as efficiently as Treg from WT mice. GZ-B-mediated suppression is perforin independent, because suppression by Treg from perforin-/- and WT is indistinguishable. Additionally, suppression mediated by Treg appears to be mediated, in part, by the induction of apoptosis in the CD4+CD25- effector cell. In summary, GZ-B is one of the key mechanisms through which CD4+CD25+ Treg induce cell contact-mediated suppression.

Autoimmune regulator (AIRE) gene mutation is responsible for the development of organ-specific autoimmune disease with monogenic autosomal recessive inheritance. Although Aire has been considered to regulate the elimination of autoreactive T cells through transcriptional control of tissue-specific Ags in thymic epithelial cells, other mechanisms of AIRE-dependent tolerance remain to be investigated. We have established Aire-deficient mice and examined the mechanisms underlying the breakdown of self-tolerance. The production and/or function of immunoregulatory T cells were retained in the Aire-deficient mice. The mice developed Sjogren's syndrome-like pathologic changes in the exocrine organs, and this was associated with autoimmunity against a ubiquitous protein, alpha-fodrin. Remarkably, transcriptional expression of alpha-fodrin was retained in the Aire-deficient thymus. These results suggest that Aire regulates the survival of autoreactive T cells beyond transcriptional control of self-protein expression in the thymus, at least against this ubiquitous protein. Rather, Aire may regulate the processing and/or presentation of self-proteins so that the maturing T cells can recognize the self-Ags in a form capable of efficiently triggering autoreactive T cells. With the use of inbred Aire-deficient mouse strains, we also demonstrate the presence of some additional factor(s) that determine the target-organ specificity of the autoimmune disease caused by Aire deficiency.

The glucocorticoid-induced TNF receptor (GITR), which is a member of the TNF receptor family, is expressed preferentially at high levels on CD25+CD4+ regulatory T cells and plays a key role in the peripheral tolerance that is mediated by these cells. GITR is also expressed on conventional CD4+ and CD8+ T cells, and its expression is enhanced rapidly after activation. In this report we show that the GITR provides a potent costimulatory signal to both CD25+ and CD25- CD4+ T cells. GITR-mediated stimulation induced by anti-GITR mAb DTA-1 or GITR ligand transfectants efficiently augmented the proliferation of both CD25-CD4+ and CD25+CD4+ T cells under the limited dose of anti-CD3 stimulation. The augmentation of T cell activation was further confirmed by the enhanced cell cycle progression; early induction of the activation Ags, CD69 and CD25; cytokine production, such as IL-2, IFN-gamma, IL-4, and IL-10; anti-CD3-induced redirected cytotoxicity; and intracellular signaling, assessed by translocation of NF-kappaB components. GITR costimulation showed a potent ability to produce high amounts of IL-10, which resulted in counter-regulation of the enhanced proliferative responses. Our results highlight evidence that GITR acts as a potent and unique costimulator for an early CD4+ T cell activation.

Physical contact between thymocytes and the thymic stroma is essential for T cell maturation and shapes the T cell repertoire in the periphery. Stromal elements that control these processes still remain elusive. We used a mouse strain with mutant NF-kappaB-inducing kinase (NIK) to examine the mechanisms underlying the breakdown of self-tolerance. This NIK-mutant strain manifests autoimmunity and disorganized thymic structure with abnormal expression of Rel proteins in the stroma. Production of immunoregulatory T cells that control autoreactive T cells was impaired in NIK-mutant mice. The autoimmune disease seen in NIK-mutant mice was reproduced in athymic nude mice by grafting embryonic thymus from NIK-mutant mice, and this was rescued by supply of exogenous immunoregulatory T cells. Impaired production of immunoregulatory T cells by thymic stroma without normal NIK was associated with altered expression of peripheral tissue-restricted Ags, suggesting an essential role of NIK in the thymic microenvironment in the establishment of central tolerance.

Mutations in the CD40 ligand (CD40L) gene lead to X-linked immunodeficiency with hyper-IgM, which is often associated with autoimmune diseases. To determine the contribution of defective CD40-CD40L interactions to T cell autoreactivity, we reconstituted CD40-CD40L interactions by transferring T cells from CD40-deficient mice to syngenic athymic nude mice and assessed autoimmunity. T cells from CD40-deficient mice triggered autoimmune diseases accompanied with elevations of various autoantibodies, while those from wild-type mice did not. In CD40-deficient mice, the CD25(+) CD45RB(low) CD4(+) subpopulation which regulates T cell autoreactivity was markedly reduced. CD40-deficient APCs failed to induce T regulatory cells 1 producing high levels of an inhibitory cytokine, IL-10 in vitro. Furthermore, autoimmune development was inhibited when T cells from CD40-deficient mice were cotransferred with CD45RB(low) CD4(+) T cells from wild-type mice or with T regulatory cells 1 induced on CD40-expressing APCs. Collectively, our results indicate that CD40-CD40L interactions contribute to negative regulation of T cell autoreactivity and that defective interactions can lead to autoimmunity.

This study shows that removal of a T cell subpopulation can evoke effective tumor immunity in otherwise nonresponding animals. Elimination of CD25-expressing T cells, which constitute 5-10% of peripheral CD4+ T cells in normal naive mice, elicited potent immune responses to syngeneic tumors in vivo and eradicated them. The responses were mediated by tumor-specific CD8+ CTLs and tumor-nonspecific CD4-8- cytotoxic cells akin to NK cells. Furthermore, in vitro culture of CD25+4+ T cell-depleted splenic cell suspensions prepared from tumor-unsensitized normal mice led to spontaneous generation of similar CD4-8- cytotoxic cells capable of killing a broad spectrum of tumors; reconstitution of CD25+4+ T cells inhibited the generation. In this culture, self-reactive CD25-4+ T cells responding to self peptides/class II MHC complexes on APCs spontaneously proliferated upon removal of CD25+4+ T cells, secreting large amounts of IL-2. The IL-2 thus produced appeared to be responsible for the generation of CD4-8- NK cells as lymphokine-activated killer cells, because direct addition of an equivalent amount of IL-2 to the culture of CD4-8- cells generated similar lymphokine-activated killer/NK cells, whereas coculture of normal CD4-8- cells with CD25-4+ T cells from IL-2-deficient mice did not. Thus, removal of immunoregulatory CD25+4+ T cells can abrogate immunological unresponsiveness to syngeneic tumors in vivo and in vitro, leading to spontaneous development of tumor-specific effector cells as well as tumor-nonspecific ones. This novel way of evoking tumor immunity would help to devise effective immunotherapy for cancer in humans.

This study shows that the normal thymus produces immunoregulatory CD25+4+8- thymocytes capable of controlling self-reactive T cells. Transfer of thymocyte suspensions depleted of CD25+4+8- thymocytes, which constitute approximately 5% of steroid-resistant mature CD4+8- thymocytes in normal naive mice, produces various autoimmune diseases in syngeneic athymic nude mice. These CD25+4+8- thymocytes are nonproliferative (anergic) to TCR stimulation in vitro, but potently suppress the proliferation of other CD4+8- or CD4-8+ thymocytes; breakage of their anergic state in vitro by high doses of IL-2 or anti-CD28 Ab simultaneously abrogates their suppressive activity; and transfer of such suppression-abrogated thymocyte suspensions produces autoimmune disease in nude mice. These immunoregulatory CD25+4+8- thymocytes/T cells are functionally distinct from activated CD25+4+ T cells derived from CD25-4+ thymocytes/T cells in that the latter scarcely exhibits suppressive activity in vitro, although both CD25+4+ populations express a similar profile of cell surface markers. Furthermore, the CD25+4+8- thymocytes appear to acquire their anergic and suppressive property through the thymic selection process, since TCR transgenic mice develop similar anergic/suppressive CD25+4+8- thymocytes and CD25+4+ T cells that predominantly express TCRs utilizing endogenous alpha-chains, but RAG-2-deficient TCR transgenic mice do not. These results taken together indicate that anergic/suppressive CD25+4+8- thymocytes and peripheral T cells in normal naive mice may constitute a common T cell lineage functionally and developmentally distinct from other T cells, and that production of this unique immunoregulatory T cell population can be another key function of the thymus in maintaining immunologic self-tolerance.

Neonatal infection of the mouse T lymphotropic virus (MTLV), a member of herpes viridae, causes various organ-specific autoimmune diseases, such as autoimmune gastritis, in selected strains of normal mice. The infection selectively depletes CD4+ T cells in the thymus and periphery for 2-3 wk from 1 wk after infection. Thymectomy 3 wk after neonatal MTLV infection enhances the autoimmune responses and produces autoimmune diseases at higher incidences and in a wider spectrum of organs than MTLV infection alone. On the other hand, inoculation of peripheral CD4+ cells from syngeneic noninfected adult mice prevents the autoimmune development. These autoimmune diseases can be adoptively transferred to syngeneic athymic nude mice by CD4+ T cells. The virus is not detected by bioassay in the organs/tissues damaged by the autoimmune responses. Furthermore, similar autoimmune diseases can be induced in normal mice by manipulating the neonatal thymus/T cells (e.g., by neonatal thymectomy) without virus infection. These results taken together indicate that neonatal MTLV infection elicits autoimmune disease by primarily affecting thymocytes/T cells, not self Ags. It may provoke or enhance thymic production of CD4+ pathogenic self-reactive T cells by altering the thymic clonal deletion mechanism, or reduce the production of CD4+ regulatory T cells controlling self-reactive T cells, or both. The possibility is discussed that other T cell-tropic viruses may cause autoimmunity in humans and animals by affecting the T cell immune system, not the self Ags to be targeted by the autoimmunity.

Approximately 10% of peripheral CD4+ cells and less than 1% of CD8+ cells in normal unimmunized adult mice express the IL-2 receptor alpha-chain (CD25) molecules. When CD4+ cell suspensions prepared from BALB/c nu/+ mice lymph nodes and spleens were depleted of CD25+ cells by specific mAb and C, and then inoculated into BALB/c athymic nude (nu/nu) mice, all recipients spontaneously developed histologically and serologically evident autoimmune diseases (such as thyroiditis, gastritis, insulitis, sialoadenitis, adrenalitis, oophoritis, glomerulonephritis, and polyarthritis); some mice also developed graft-vs-host-like wasting disease. Reconstitution of CD4+CD25+ cells within a limited period after transfer of CD4+CD25- cells prevented these autoimmune developments in a dose-dependent fashion, whereas the reconstitution several days later, or inoculation of an equivalent dose of CD8+ cells, was far less efficient for the prevention. When nu/nu mice were transplanted with allogeneic skins or immunized with xenogeneic proteins at the time of CD25- cell inoculation, they showed significantly heightened immune responses to the skins or proteins, and reconstitution of CD4+CD25+ cells normalized the responses. Taken together, these results indicate that CD4+CD25+ cells contribute to maintaining self-tolerance by down-regulating immune response to self and non-self Ags in an Ag-nonspecific manner, presumably at the T cell activation stage; elimination/reduction of CD4+CD25+ cells relieves this general suppression, thereby not only enhancing immune responses to non-self Ags, but also eliciting autoimmune responses to certain self-Ags. Abnormality of this T cell-mediated mechanism of peripheral tolerance can be a possible cause of various autoimmune diseases.

Ionizing radiation can functionally alter the immune system and break self-tolerance. High dose (42.5 Gy), fractionated (2.5 Gy 17 times) total lymphoid irradiation (TLI) on mice caused various organ-specific autoimmune diseases, such as gastritis, thyroiditis, and orchitis, depending on the radiation dosages, the extent of lymphoid irradiation, and the genetic background of the mouse strains. Radiation-induced tissue damage is not the primary cause of the autoimmune disease because irradiation of the target organs alone failed to elicit the autoimmunity and shielding of the organs from irradiation was unable to prevent it. In contrast, irradiation of both the thymus and the peripheral lymphoid organs/tissues was required for efficient induction of autoimmune disease by TLI. TLI eliminated the majority of mature thymocytes and the peripheral T cells for 1 mo, and inoculation of spleen cell, thymocyte, or bone marrow cell suspensions (prepared from syngeneic nonirradiated mice) within 2 wk after TLI effectively prevented the autoimmune development. Depletion of T cells from the inocula abrogated the preventive activity. CD4+ T cells mediated the autoimmune prevention but CD8+ T cells did not. CD4+ T cells also appeared to mediate the TLI-induced autoimmune disease because CD4+ T cells from disease-bearing TLI mice adoptively transferred the autoimmune disease to syngeneic naive mice. Taken together, these results indicate that high dose, fractionated ionizing radiation on the lymphoid organs/tissues can cause autoimmune disease by affecting the T cell immune system, rather than the target self-Ags, presumably by altering T cell-dependent control of self-reactive T cells.

Germline expression of rearranged TCR alpha-chain transgenes with the Ig H chain enhancer reproducibly elicits T cell-mediated autoimmune disease in the thyroid gland, gastric mucosa, Langerhans islets, salivary gland, ovaries, and testes in selected strains of normal mice. Multiple organs are destroyed in a single transgenic mouse and the same organ in transgenic strains with different MHC background, suggesting the transgene expression can elicit self-reactive T cell clones having different Ag specificities and MHC restrictions. Construction of this autoimmune-inducing TCR alpha EH transgene does not require particular V alpha J alpha gene segments or Ag specificities. Moreover, the autoimmune disease can be adoptively transferred to syngeneic normal mice by T cells expressing endogenous TCR alpha-chains. Taken together, these results indicate that the TCR alpha EH transgene expression does not suppress endogenous alpha-chain gene rearrangement and may trigger the expansion/activation of various self-reactive T cells expressing endogenous TCR alpha- and beta-chains. Furthermore, it appears that the transgene-induced autoimmune T cells are not deleted in the normal thymus or rendered anergic upon contact with the normal target self Ag, but can be controlled by a T cell-dependent mechanism, since transfer of the transgenic bone marrow cells to histocompatible SCID mice produces the same autoimmune disease as in the donors, and the autoimmune development in the SCID mice is effectively prevented by co-transfer of syngeneic nontransgenic T cells. This novel autoimmune model produced by genetic manipulation of the T cell lineage, not the target self Ag or the environment of T cell differentiation/selection, should be useful for elucidating the immunologic and genetic basis of autoimmune disease.

Cyclosporin A (CsA), a potent immunosuppressive drug, caused organ-specific autoimmune disease, such as gastritis with anti-parietal cell autoantibodies or oophoritis with anti-oocyte autoantibodies, in BALB/c mice when the drug was administered daily for 1 wk to newborns. Administration to adult mice did not. CsA abrogated the production of L3T4+ T cells and Lyt-2+ T cells in the thymus. Consequently, these T cells were substantially depleted from the peripheral lymphoid organs, especially when the drug was administered from the day of birth. Autoimmune disease was prevented when CsA-treated newborn mice were inoculated with splenic T cells from normal syngeneic mice. However, removal of the thymus immediately after neonatal CsA treatment produced autoimmune disease with a higher incidence and in a wider spectrum of organs, i.e., thyroiditis, sialoadenitis of the salivary gland, gastritis, insulitis of the endocrine pancreas, adrenalitis, oophoritis, or orchitis. Each autoimmune disease was accompanied by the development of circulating autoantibodies specific for the corresponding organ Ag. Immunopathology of these autoimmune diseases was quite similar to that of human organ-specific autoimmune diseases.