Learning the Autoimmune Pathogenesis Through the Study of Aire.

One of the difficulties in studying the pathogenesis of autoimmune diseases is that the disease is multifactorial involving sex, age, MHC, environment, and some genetic factors. Because deficiency of Aire, a transcriptional regulator, is an autoimmune disease caused by a single gene abnormality, Aire is an ideal research target for approaching the enigma of autoimmunity, e.g., the mechanisms underlying Aire deficiency can be studied using genetically modified animals. Nevertheless, the exact mechanisms of the breakdown of self-tolerance due to Aire's dysfunction have not yet been fully clarified. This is due, at least in part, to the lack of information on the exact target genes controlled by Aire. State-of-the-art research infrastructures such as single-cell analysis are now in place to elucidate the essential function of Aire. The knowledge gained through the study of Aire-mediated tolerance should help our understanding of the pathogenesis of autoimmune disease in general.

Thymocyte Development of Humanized Mice Is Promoted by Interactions with Human-Derived Antigen Presenting Cells upon Immunization.

Immune responses in humanized mice are generally inefficient without co-transplantation of human thymus or HLA transgenes. Previously, we generated humanized mice via the intra-bone marrow injection of CD133+ cord blood cells into irradiated adult immunodeficient mice (IBMI-huNSG mice), which could mount functional immune responses against HTLV-1, although the underlying mechanisms were still unknown. Here, we investigated thymocyte development in IBMI-huNSG mice, focusing on the roles of human and mouse MHC restriction. IBMI-huNSG mice had normal developmental profiles but aberrant thymic structures. Surprisingly, the thymic medulla-like regions expanded after immunization due to enhanced thymocyte expansion in association with the increase in HLA-DR+ cells, including CD205+ dendritic cells (DCs). The organ culture of thymus from immunized IBMI-huNSG mice with a neutralizing antibody to HLA-DR showed the HLA-DR-dependent expansion of CD4 single positive thymocytes. Mature peripheral T-cells exhibited alloreactive proliferation when co-cultured with human peripheral blood mononuclear cells. Live imaging of the thymus from immunized IBMI-huNSG mice revealed dynamic adhesive contacts of human-derived thymocytes and DCs accompanied by Rap1 activation. These findings demonstrate that an increase in HLA-DR+ cells by immunization promotes HLA-restricted thymocyte expansion in humanized mice, offering a unique opportunity to generate humanized mice with ease.

Revisiting Aire and tissue-restricted antigens at single-cell resolution.

The thymus is a highly specialized organ that plays an indispensable role in the establishment of self-tolerance, a process characterized by the "education" of developing T-cells. To provide competent T-cells tolerant to self-antigens, medullary thymic epithelial cells (mTECs) orchestrate negative selection by ectopically expressing a wide range of genes, including various tissue-restricted antigens (TRAs). Notably, recent advancements in the high-throughput single-cell analysis have revealed remarkable heterogeneity in mTECs, giving us important clues for dissecting the mechanisms underlying TRA expression. We overview how recent single-cell studies have furthered our understanding of mTECs, with a focus on the role of Aire in inducing mTEC heterogeneity to encompass TRAs.

Facile formation of barium titanium oxyhydride on a titanium hydride surface as an ammonia synthesis catalyst.

Oxyhydrides are promising compounds as supports for ammonia synthesis catalysts because they suppress hydrogen poisoning on the catalyst surface and enhance the ammonia synthesis activity. Herein, we developed a facile method for preparing BaTiO2.5H0.5, a perovskite oxyhydride, on a TiH2 surface via the conventional wet impregnation method using TiH2 and Ba hydroxide. Scanning electron microscopy and high-angle annular dark-field scanning transmission electron microscopy observations revealed that BaTiO2.5H0.5 crystallized as nanoparticles of ca. 100-200 nm on the TiH2 surface. The Ru-loaded catalyst Ru/BaTiO2.5H0.5-TiH2 exhibited 2.46 times higher ammonia synthesis activity (3.05 mmol-NH3 g-1 h-1 at 400 °C) than the benchmark Ru catalyst Ru-Cs/MgO (1.24 mmol-NH3 g-1 h-1 at 400 °C) because of the suppression of hydrogen poisoning. The analysis of reaction orders showed that the effect of suppressing hydrogen poisoning on Ru/BaTiO2.5H0.5-TiH2 was equivalent to that of the reported Ru/BaTiO2.5H0.5 catalyst, thus supporting the formation of BaTiO2.5H0.5 perovskite oxyhydride. This study demonstrated that the selection of appropriate raw materials facilitates the formation of BaTiO2.5H0.5 oxyhydride nanoparticles on the TiH2 surface using the conventional synthesis method.

Cooperative but Distinct Role of Medullary Thymic Epithelial Cells and Dendritic Cells in the Production of Regulatory T Cells in the Thymus.

Regulatory T cells (Tregs) are produced in the thymus to establish self-tolerance, and agonistic stimuli by self-Ags play a pivotal role in this process. Although two types of APCs, medullary thymic epithelial cells (mTECs) and dendritic cells (DCs), are responsible for presenting self-Ags together with costimulatory/cytokine signals, the distinct role of each APC in producing Tregs remains enigmatic. We have approached this issue by depleting the mTECs and DCs using mice expressing diphtheria toxin receptors driven by Aire and CD11c promoters, respectively. Depletion of mTECs showed an effect on Treg production quantitatively and qualitatively more profound than that of DCs followed by the development of distinct organ-specific autoimmune lesions in the hosts. Because self-Ags produced by mTECs are transferable to DCs through a process known as Ag transfer, we monitored the process of Ag transfer using mice expressing GFP from TECs. Although GFP expressed from total TECs was effectively transferred to DCs, GFP expressed from cortical TECs was not, suggesting that mTECs are the predominant source of self-Ags. We also found that GFP expressed not only from mature mTECs but also from immature mTECs was transferred to DCs, suggesting that a broad spectrum of molecules were subjected to Ag transfer during mTEC development. Interestingly, the numbers of recirculating non-Tregs producing IL-2, an important source for Treg expansion in the thymus, were reduced only in the mTEC-depleted mice. These results suggested the cooperative but distinct role of mTECs and DCs in the production of Tregs to avoid autoimmunity.

AIRE illuminates the feature of medullary thymic epithelial cells in thymic carcinoma.

Despite the clear distinction between cortical (cTECs) and medullary thymic epithelial cells (mTECs) in physiology, the cell of origin of thymic carcinomas (TCs) and other thymic epithelial tumors remained enigmatic. We addressed this issue by focusing on AIRE, an mTEC-specific transcriptional regulator that is required for immunological self-tolerance. We found that a large proportion of TCs expressed AIRE with typical nuclear dot morphology by immunohistochemistry. AIRE expression in TCs was supported by the RNA-seq data in the TCGA-THYM database. Furthermore, our bioinformatics approach to the recent single-cell RNA-seq data on human thymi has revealed that TCs hold molecular characteristics of multiple mTEC subpopulations. In contrast, TCs lacked the gene signatures for cTECs. We propose that TCs are tumors derived from mTECs.

Dispensable Role of Aire in CD11c+ Conventional Dendritic Cells for Antigen Presentation and Shaping the Transcriptome.

Aire, the defect of which is responsible for the development of autoimmunity, is predominantly expressed in medullary thymic epithelial cells, and it controls a wide variety of genes, including those of tissue-restricted Ags, for establishing thymic tolerance. Aire is also expressed from APCs in the periphery, called extrathymic Aire-expressing cells (eTACs), and their complementing role to thymic tolerance has been suggested. eTACs are composed of two distinct classes of APCs, conventional dendritic cell (cDC)-type and group 3 innate lymphoid cell (ILC3)-like-type expressing retinoic acid receptor-related orphan receptor γt (RORγt). Although the essential role of Aire in the latter in the Th17-mediated immune response against Candida albicans has been reported, the role of Aire in the cDC-type eTACs for this action has not been examined. Furthermore, the significance of Aire in the production of the transcriptome of the cDC-type eTACs remains unknown. We have approached these issues using a high-fidelity Aire-reporter mouse strain. We found that although the cDC-type eTACs dominated ILC3-like-type eTACs in number and they served as efficient APCs for the immune response against an exogenous Ag as well as for the C. albicans-specific Th17 immune response, loss of Aire in cDC-type eTACs showed no clear effect on these functions. Furthermore, loss of Aire showed no major impact on the transcriptome from cDC-type eTACs. These results suggested that Aire in cDC-type eTACs may not have a cell-intrinsic role in the immune response in contrast to the role of Aire in ILC3-like-type eTACs.

MHC class II inhibits the generation of IL-17A+ Vγ6 γδ T cells in the thymus at perinatal stage.

Vγ6+ γδ T cells develop in the thymus at the perinatal stage and are exclusive IL-17A producers among γδ T cells. The loss of MHC class II led to the expansion of IL-17A+ Vγ6+ γδ T cells in the thymus. Thus, MHC class II in the thymus inhibits the generation of IL-17A+ Vγ6+ γδ T cells.

Development of organ-specific autoimmunity by dysregulated Aire expression.

Deficiency for AIRE/Aire in both humans and mice results in the development of organ-specific autoimmune disease. We tested whether augmented and/or dysregulated AIRE/Aire expression might be also prone to the breakdown of self-tolerance. To define the effect of augmented Aire expression on the development of autoimmunity, antigen-specific clonal deletion and production of clonotypic regulatory T cells (Tregs) in the thymus were examined using mice expressing two additional copies of Aire in a heterozygous state (3xAire-knockin mice: 3xAire-KI). We found that both clonal deletion of autoreactive T cells and production of clonotypic Tregs in the thymus from 3xAire-KI were impaired in a T-cell receptor-transgenic system. Furthermore, 3xAire-KI females showed higher scores of experimental autoimmune encephalomyelitis induced by myelin oligodendrocyte glycoprotein than wild-type littermates, suggesting that augmented Aire expression exacerbates organ-specific autoimmunity under disease-prone conditions. In humans, we found that one patient with amyopathic dermatomyositis showed CD3- CD19- cells expressing AIRE in the peripheral blood before the treatment but not during the remission phase treated with immunosuppressive drugs. Thus, not only loss of function of AIRE/Aire but also augmented and/or dysregulated expression of AIRE/Aire should be considered for the pathogenesis of organ-specific autoimmunity. We suggest that further analyses should be pursued to establish a novel link between organ-specific autoimmune disease and dysregulated AIRE expression in clinical settings.

No Major Impact of Two Homologous Proteins Ly6C1 and Ly6C2 on Immune Homeostasis.

Ly6C comprises two homologous components of Ly6C1 and Ly6C2, and the expression of either of the Ly6C molecules defines unique functional subsets of monocytes. Ly6C is also expressed by other immune cell types, including Aire-expressing medullary thymic epithelial cells. Because the role of Ly6C expression in determining the functional subsets remains unclear, we generated mice deficient for both Ly6C1 and Ly6C2 with CRISPR-Cas9-mediated deletion. Mice deficient for Ly6C1/Ly6C2 showed no major alterations in the subsets and function of monocyte and other immune cells, including the cells involved in the dextran sulfate sodium salt-induced colitis model. By generating the mice deficient for Ly6C1 alone, we have also investigated the expression pattern of Ly6C1 and Ly6C2 in immune cells. Except for medullary thymic epithelial cells and CD4 single-positive T cells, immune cells predominantly expressed Ly6C2. Thus, despite the importance as a marker with a unique differential expression pattern, the Ly6C molecules have no major impact on determining the functional subsets and maintaining immune homeostasis.

Aire suppresses CTLA-4 expression from the thymic stroma to control autoimmunity.

Impaired production of thymic regulatory T cells (Tregs) is implicated in the development of Aire-dependent autoimmunity. Because Tregs require agonistic T cell receptor stimuli by self-antigens to develop, reduced expression of self-antigens from medullary thymic epithelial cells (mTECs) has been considered to play a major role in the reduced Treg production in Aire deficiency. Here, we show that mTECs abnormally express co-inhibitory receptor CTLA-4 if Aire is non-functional. Upon binding with CD80/CD86 ligands expressed on thymic dendritic cells (DCs), the ectopically expressed CTLA-4 from Aire-deficient mTECs removes the CD80/CD86 ligands from the DCs. This attenuates the ability of DCs to provide co-stimulatory signals and to present self-antigens transferred from mTECs, both of which are required for Treg production. Accordingly, impaired production of Tregs and organ-specific autoimmunity in Aire-deficient mice are rescued by the depletion of CTLA-4 expression from mTECs. Our studies illuminate the significance of mTEC-DC interaction coordinated by Aire for the establishment of thymic tolerance.

Enhanced ammonia synthesis activity of Ru-supported cerium-lanthanum oxide induced by Ti substitution forming mesopores.

Ru/Ce0.5La0.5-xTixO1.75+0.5x solid solutions with cubic fluorite structure were successfully synthesized via the polymerized complex method. While the Ti substitution enhanced Ce4+ reducibility by compensating for oxygen vacancies, the reducibility showed no correlation with ammonia synthesis activity. However, Ru/Ce0.5La0.4Ti0.1O1.8 showed the highest activity originating from the facilitated formation of mesopores.

Aire Controls Heterogeneity of Medullary Thymic Epithelial Cells for the Expression of Self-Antigens.

The deficiency of Aire, a transcriptional regulator whose defect results in the development of autoimmunity, is associated with reduced expression of tissue-restricted self-Ags (TRAs) in medullary thymic epithelial cells (mTECs). Although the mechanisms underlying Aire-dependent expression of TRAs need to be explored, the physical identification of the target(s) of Aire has been hampered by the low and promiscuous expression of TRAs. We have tackled this issue by engineering mice with augmented Aire expression. Integration of the transcriptomic data from Aire-augmented and Aire-deficient mTECs revealed that a large proportion of so-called Aire-dependent genes, including those of TRAs, may not be direct transcriptional targets downstream of Aire. Rather, Aire induces TRA expression indirectly through controlling the heterogeneity of mTECs, as revealed by single-cell analyses. In contrast, Ccl25 emerged as a canonical target of Aire, and we verified this both in vitro and in vivo. Our approach has illuminated the Aire's primary targets while distinguishing them from the secondary targets.

Response to Comments on "Aberrant type 1 immunity drives susceptibility to mucosal fungal infections".

Puel and Casanova and Kisand et al. challenge our conclusions that interferonopathy and not IL-17/IL-22 autoantibodies promote candidiasis in autoimmune polyendocrinopathy­candidiasis­ectodermal dystrophy. We acknowledge that conclusive evidence for causation is difficult to obtain in complex human diseases. However, our studies clearly document interferonopathy driving mucosal candidiasis with intact IL-17/IL-22 responses in Aire-deficient mice, with strong corroborative evidence in patients.

Aberrant type 1 immunity drives susceptibility to mucosal fungal infections.

Human monogenic disorders have revealed the critical contribution of type 17 responses in mucosal fungal surveillance. We unexpectedly found that in certain settings, enhanced type 1 immunity rather than defective type 17 responses can promote mucosal fungal infection susceptibility. Notably, in mice and humans with AIRE deficiency, an autoimmune disease characterized by selective susceptibility to mucosal but not systemic fungal infection, mucosal type 17 responses are intact while type 1 responses are exacerbated. These responses promote aberrant interferon-γ (IFN-γ)- and signal transducer and activator of transcription 1 (STAT1)-dependent epithelial barrier defects as well as mucosal fungal infection susceptibility. Concordantly, genetic and pharmacologic inhibition of IFN-γ or Janus kinase (JAK)-STAT signaling ameliorates mucosal fungal disease. Thus, we identify aberrant T cell-dependent, type 1 mucosal inflammation as a critical tissue-specific pathogenic mechanism that promotes mucosal fungal infection susceptibility in mice and humans.

A novel method to identify Post-Aire stages of medullary thymic epithelial cell differentiation.

Autoimmune regulator+ (Aire) medullary thymic epithelial cells (mTECs) play a critical role in tolerance induction. Several studies demonstrated that Aire+ mTECs differentiate further into Post-Aire cells. Yet, the identification of terminal stages of mTEC maturation depends on unique fate-mapping mouse models. Herein, we resolve this limitation by segmenting the mTEChi (MHCIIhi CD80hi ) compartment into mTECA/hi (CD24- Sca1- ), mTECB/hi (CD24+ Sca1- ), and mTECC/hi (CD24+ Sca1+ ). While mTECA/hi included mostly Aire-expressing cells, mTECB/hi contained Aire+ and Aire- cells and mTECC/hi were mainly composed of cells lacking Aire. The differential expression pattern of Aire led us to investigate the precursor-product relationship between these subsets. Strikingly, transcriptomic analysis of mTECA/hi , mTECB/hi , and mTECC/hi sequentially mirrored the specific genetic program of Early-, Late- and Post-Aire mTECs. Corroborating their Post-Aire nature, mTECC/hi downregulated the expression of tissue-restricted antigens, acquired traits of differentiated keratinocytes, and were absent in Aire-deficient mice. Collectively, our findings reveal a new and simple blueprint to survey late stages of mTEC differentiation.

Tissue-specific autoimmunity controlled by Aire in thymic and peripheral tolerance mechanisms.

Tissue-specific autoimmune diseases are assumed to arise through malfunction of two checkpoints for immune tolerance: defective elimination of autoreactive T cells in the thymus and activation of these T cells by corresponding autoantigens in the periphery. However, evidence for this model and the outcome of such alterations in each or both of the tolerance mechanisms have not been sufficiently investigated. We studied these issues by expressing human AIRE (huAIRE) as a modifier of tolerance function in NOD mice wherein the defects of thymic and peripheral tolerance together cause type I diabetes (T1D). Additive huAIRE expression in the thymic stroma had no major impact on the production of diabetogenic T cells in the thymus. In contrast, huAIRE expression in peripheral antigen-presenting cells (APCs) rendered the mice resistant to T1D, while maintaining other tissue-specific autoimmune responses and antibody production against an exogenous protein antigen, because of the loss of Xcr1+ dendritic cells, an essential component for activating diabetogenic T cells in the periphery. These results contrast with our recent demonstration that huAIRE expression in both the thymic stroma and peripheral APCs resulted in the paradoxical development of muscle-specific autoimmunity. Our results reveal that tissue-specific autoimmunity is differentially controlled by a combination of thymic function and peripheral tolerance, which can be manipulated by expression of huAIRE/Aire in each or both of the tolerance mechanisms.

Development of a new monoclonal antibody specific to mouse Vγ6 chain.

There are seven Vγ gene segments in the TCR γ chain loci of mice. We developed monoclonal antibodies (mAbs) specific to the Vγ6 chain (Heilig & Tonegawa nomenclature). By immunizing Vγ4/6 KO mice with complementarity-determining region peptides in Vγ6 chains, we generated three hybridomas. These hybridomas produced mAbs capable of cell surface staining of Vγ6/Vδ1 gene-transfected T-cell line lacking TCR as well as of Vγ1- Vγ4- Vγ5- Vγ7- γδ T cells and the CD3high TCRδint γδ T cells in various organs. The location of Vγ6+ γδ T cells, which peaked in the newborn thymus, was associated with mTEC. In vivo administration of clone 1C10-1F7 mAb impaired protection against Klebsiella pneumoniae infection but ameliorated psoriasis-like dermatitis induced by imiquimod treatment. These new mAbs are useful to elucidate the development, location, and functions of Vγ6 γδ T cells in mice.

Hassall's corpuscles with cellular-senescence features maintain IFNα production through neutrophils and pDC activation in the thymus.

Hassall's corpuscles (HCs) are composed of cornifying, terminally differentiated medullary thymic epithelial cells (mTECs) that are developed under the control of Aire. Here, we demonstrated that HC-mTECs show features of cellular senescence and produce inflammatory cytokines and chemokines including CXCL5, thereby recruiting and activating neutrophils to produce IL-23 in the thymic medulla. We further indicated that thymic plasmacytoid dendritic cells (pDCs) expressing IL-23 receptors constitutively produced Ifna, which plays a role in single positive (SP) cell maturation, in an Il23a-dependent manner. Neutrophil depletion with anti-Ly6G antibody injection resulted in a significant decrease of Ifna expression in the thymic pDCs, suggesting that thymic neutrophil activation underlies the Ifna expression in thymic pDCs in steady state conditions. A New Zealand White mouse strain showing HC hyperplasia exhibited greater numbers and activation of thymic neutrophils and pDCs than B6 mice, whereas Aire-deficient B6 mice with defective HC development and SP thymocyte maturation showed significantly compromised numbers and activation of these cells. These results collectively suggested that HC-mTECs with cell-senescence features initiate a unique cell activation cascade including neutrophils and pDCs leading to the constitutive IFNα expression required for SP T-cell maturation in the thymic medulla.

Aire Controls in Trans the Production of Medullary Thymic Epithelial Cells Expressing Ly-6C/Ly-6G.

Medullary thymic epithelial cells (mTECs), which express a wide range of tissue-restricted Ags (TRAs), contribute to the establishment of self-tolerance by eliminating autoreactive T cells and/or inducing regulatory T cells. Aire controls a diverse set of TRAs within Aire-expressing cells by employing various transcriptional pathways. As Aire has a profound effect on transcriptomes of mTECs, including TRAs not only at the single-cell but also the population level, we suspected that Aire (Aire+ mTECs) might control the cellular composition of the thymic microenvironment. In this study, we confirmed that this is indeed the case by identifying a novel mTEC subset expressing Ly-6 family protein whose production was defective in Aire-deficient thymi. Reaggregated thymic organ culture experiments demonstrated that Aire did not induce the expression of Ly-6C/Ly-6G molecules from mTECs as Aire-dependent TRAs in a cell-intrinsic manner. Instead, Aire+ mTECs functioned in trans to maintain Ly-6C/Ly-6G+ mTECs. Thus, Aire not only controls TRA expression transcriptionally within the cell but also controls the overall composition of mTECs in a cell-extrinsic manner, thereby regulating the transcriptome from mTECs on a global scale.