T cell tolerance by clonal elimination in the thymus

Biological characteristics of transcription factor RelB in different immune cell types: implications for the treatment of multiple sclerosis

Transcription factor RelB is a member of the nuclear factror-kappa B (NF-κB) family, which plays a crucial role in mediating immune responses. Plenty of studies have demonstrated that RelB actively contributes to lymphoid organ development, dendritic cells maturation and function and T cells differentiation, as well as B cell development and survival. RelB deficiency may cause a variety of immunological disorders in both mice and humans. Multiple sclerosis (MS) is an inflammatory and demyelinating disease of the central nervous system which involves a board of immune cell populations. Thereby, RelB may exert an impact on MS by modulating the functions of dendritic cells and the differentiation of T cells and B cells. Despite intensive research, the role of RelB in MS and its animal model, experimental autoimmune encephalomyelitis, is still unclear. Herein, we give an overview of the biological characters of RelB, summarize the updated knowledge regarding the role of RelB in different cell types that contribute to MS pathogenesis and discuss the potential RelB-targeted therapeutic implications for MS.

A transgenic mouse that spontaneously develops pathogenic TSH receptor antibodies will facilitate study of antigen-specific immunotherapy for human Graves' disease

Graves' hyperthyroidism can be treated but not cured. Antigen-specific immunotherapy would accomplish this goal, for which purpose an animal model is an invaluable tool. Two types of animal models are available. First, pathogenic TSHR antibodies (TSHRAb) can be induced by injecting mice with fibroblasts co-expressing the human TSHR (hTSHR) and MHC class II, or in mammals using plasmid or adenovirus vectors encoding the hTSHR or its A-subunit. Second, a mouse model that spontaneously develops pathogenic TSHRAb resembling those in human disease was recently described. This outcome was accomplished by transgenic intrathyroidal expression of the hTSHR A-subunit in NOD.H2^h4 mice that are genetically predisposed to develop thyroiditis but, without the transgene, do not generate TSHRAb. Recently, novel approaches to antigen-specific immunotherapy have been tested, primarily in the induced model, by injecting TSHR A-subunit protein or cyclic TSHR peptides. T-cell tolerance has also been induced in "humanized" HLA-DR3 mice by injecting synthetic peptides predicted in silico to mimic naturally processed TSHR T-cell epitopes. Indeed, a phase 1 study based on the latter approach has been conducted in humans. In the spontaneous model (hTSHR/NOD.H2^h mice), injection of soluble or nanoparticle-bearing hTSHR A-subunits had the unwanted effect of exacerbating pathogenic TSHRAb levels. A promising avenue for tolerance induction, successful in other conditions and yet to be tested with the TSHR, involves encapsulating the antigen. In conclusion, these studies provide insight into the potential outcome of immunotherapeutic approaches and emphasize the importance of a spontaneous model to test future novel, antigen-specific immunotherapies for Graves' disease.

Immune checkpoint inhibitors for the treatment of MSI-H/MMR-D colorectal cancer and a perspective on resistance mechanisms

Metastatic colorectal cancer (CRC) with a mismatch repair-deficiency (MMR-D)/microsatellite instability-high (MSI-H) phenotype carries unique characteristics such as increased tumour mutational burden and tumour-infiltrating lymphocytes. Studies have shown a sustained clinical response to immune checkpoint inhibitors with dramatic clinical improvement in patients with MSI-H/MMR-D CRC. However, the observed response rates range between 30% and 50% suggesting the existence of intrinsic resistance mechanisms. Moreover, disease progression after an initial positive response to immune checkpoint inhibitor treatment points to acquired resistance mechanisms. In this review article, we discuss the clinical trials that established the efficacy of immune checkpoint inhibitors in patients with MSI-H/MMR-D CRC, consider biomarkers of the immune response and elaborate on potential mechanisms related to intrinsic and acquired resistance. We also provide a perspective on possible future therapeutic approaches that might improve clinical outcomes, particularly in patients with actionable resistance mechanisms.

Antibody Conditioning Enables MHC-Mismatched Hematopoietic Stem Cell Transplants and Organ Graft Tolerance

Hematopoietic cell transplantation can correct hematological and immunological disorders by replacing a diseased blood system with a healthy one, but this currently requires depleting a patient's existing hematopoietic system with toxic and non-specific chemotherapy, radiation, or both. Here we report an antibody-based conditioning protocol with reduced toxicity and enhanced specificity for robust hematopoietic stem cell (HSC) transplantation and engraftment in recipient mice. Host pre-treatment with six monoclonal antibodies targeting CD47, T cells, NK cells, and HSCs followed by donor HSC transplantation enabled stable hematopoietic system reconstitution in recipients with mismatches at half (haploidentical) or all major histocompatibility complex (MHC) genes. This approach allowed tolerance to heart tissue from HSC donor strains in haploidentical recipients, showing potential applications for solid organ transplantation without immune suppression. Fully mismatched chimeric mice developed antibody responses to nominal antigens, showing preserved functional immunity. These findings suggest approaches for transplanting immunologically mismatched HSCs and solid organs with limited toxicity.

Regulation of TSHR Expression in the Thyroid and Thymus May Contribute to TSHR Tolerance Failure in Graves' Disease Patients via Two Distinct Mechanisms

Graves' disease (GD) involves the presence of agonistic auto-antibodies against the thyrotropin receptor (TSHR), which are responsible for the clinical symptoms. While failure of TSHR tolerance is central to GD pathogenesis, the process leading to this failure remains poorly understood. Two mechanisms intimately linked to tolerance have been proposed to explain the association of SNPs located in TSHR intron 1 to GD: (1) differential alternative splicing in the thyroid; and (2) modulation of expression in the thymus. To elucidate the relative contribution to these two mechanisms to GD pathogenesis, we analyzed the level of full-length and ST4 and ST5 isoform expression in the thyroid (n = 49) and thymus (n = 39) glands, and the influence of intron 1-associated SNPs on such expression. The results show that: (1) the level of flTSHR and ST4 expression in the thymus was unexpectedly high (20% that of the thyroid); (2) while flTSHR is the predominant isoform, the levels are similar to ST4 (ratio flTSHR/ST4 = 1.34 in the thyroid and ratio flTSHR/ST4 in the thymus = 1.93); (3) next-generation sequencing confirmed the effect of the TSHR intron 1 polymorphism on TSHR expression in the thymus with a bias of 1.5 ± 0.2 overexpression of the protective allele in the thymus compared to the thyroid; (4) GD-associated intron 1 SNPs did not influence TSHR alternative splicing of ST4 and ST5 in the thyroid and thymus; and (5) three-color confocal imaging showed that TSHR is associated with both thymocytes, macrophages, and dendritic cells in the thymus. Our findings confirm the effect of intron 1 polymorphisms on thymic TSHR expression and we present evidence against an effect on the relative expression of isoforms. The high level of ST4 expression in the thymus and its distribution within the tissue suggest that this would most likely be the isoform that induces central tolerance to TSHR thus omitting most of the hinge and transmembrane portion. The lack of central tolerance to a large portion of TSHR may explain the relatively high frequency of autoimmunity related to TSHR and its clinical consequence, GD.

The presentation of neuroendocrine self-peptides in the thymus: an essential event for individual life and vertebrate survival

Confirming Burnet's early hypothesis, elimination of self‐reactive T cells in the thymus was demonstrated in the late 1980s, and an important question immediately arose about the nature of the self‐peptides expressed in the thymus. Many genes encoding neuroendocrine‐related and tissue‐restricted antigens (TRAs) are transcribed in thymic epithelial cells (TECs). They are then processed for presentation by proteins of the major histocompatibility complex (MHC) expressed by TECs and thymic dendritic cells. MHC presentation of self‐peptides in the thymus programs self‐tolerance by two complementary mechanisms: (1) negative selection of self‐reactive “forbidden” T cell clones starting already in fetal life, and (2) generation of self‐specific thymic regulatory T lymphocytes (tT_reg cells), mainly after birth. Many studies, including the discovery of the transcription factors autoimmune regulator (AIRE) and fasciculation and elongation protein zeta family zinc finger (FEZF2), have shown that a defect in thymus central self‐tolerance is the earliest event promoting autoimmunity. AIRE and FEZF2 control the level of transcription of many neuroendocrine self‐peptides and TRAs in the thymic epithelium. Furthermore, AIRE and FEZF2 mutations are associated with the development of autoimmunity in peripheral organs. The discovery of the intrathymic presentation of self‐peptides has revolutionized our knowledge of immunology and is opening novel avenues for prevention/treatment of autoimmunity.

Breaking tolerance with engineered class I antigen-presenting molecules

Successful efforts to activate T cells capable of recognizing weak cancer-associated self-antigens have employed altered peptide antigens to activate T cell responses capable of cross-reacting on native tumor-associated self. A limitation of this approach is the requirement for detailed knowledge about the altered self-peptide ligands used in these vaccines. In the current study we considered allorecognition as an approach for activating CTL capable of recognizing weak or self-antigens in the context of self-MHC. Nonself antigen-presenting molecules typically contain polymorphisms that influence interactions with the bound peptide and TCR interface. Recognition of these nonself structures results in peptide-dependent alloimmunity. Alloreactive T cells target their inducing alloantigens as well as third-party alloantigens but generally fail to target self-antigens. Certain residues located on the alpha-1/2 domains of class I antigen-presenting molecules primarily interface with TCR. These residues are more conserved within and across species than are residues that determine peptide antigen binding properties. Class I variants designed with amino acid substitutions at key positions within the conserved helical structures are shown to provide strong activating signals to alloreactive CD8 T cells while avoiding changes in naturally bound peptide ligands. Importantly, CTL activated in this manner can break self-tolerance by reacting to self-peptides presented by native MHC. The ability to activate self-tolerant T cells capable of cross-reacting on self-peptide-MHC in vivo represents an approach for inducing autoimmunity, with possible application in cancer vaccines.

Targeting Type 1 Diabetes: Selective Approaches for New Therapies

The clinical onset of type 1 diabetes is characterized by the destruction of the insulin-producing β cells of the pancreas and is caused by autoantigen-induced inflammation (insulitis) of the islets of Langerhans. The current standard of care for type 1 diabetes mellitus patients allows for management of the disease with exogenous insulin, but patients eventually succumb to many chronic complications such as limb amputation, blindness, and kidney failure. New therapeutic approaches now on the horizon are looking beyond glycemic management and are evaluating new strategies from protecting and regenerating endogenous islets to treating the underlying autoimmunity through selective modulation of key immune cell populations. Currently, there are no effective treatments for the autoimmunity that causes the disease, and strategies that aim to delay or prevent the onset of the disease will play an important role in the future of diabetes research. In this review, we summarize many of the key efforts underway that utilize molecular approaches to selectively modulate this disease and look at new therapeutic paradigms that can transform clinical treatment.

RAG gene defects at the verge of immunodeficiency and immune dysregulation

Mutations of the recombinase activating genes (RAG) in humans underlie a broad spectrum of clinical and immunological phenotypes that reflect different degrees of impairment of T- and B-cell development and alterations of mechanisms of central and peripheral tolerance. Recent studies have shown that this phenotypic heterogeneity correlates, albeit imperfectly, with different levels of recombination activity of the mutant RAG proteins. Furthermore, studies in patients and in newly developed animal models carrying hypomorphic RAG mutations have disclosed various mechanisms underlying immune dysregulation in this condition. Careful annotation of clinical outcome and immune reconstitution in RAG-deficient patients who have received hematopoietic stem cell transplantation has shown that progress has been made in the treatment of this disease, but new approaches remain to be tested to improve stem cell engraftment and durable immune reconstitution. Finally, initial attempts have been made to treat RAG deficiency with gene therapy.

Unregulated antigen-presenting cell activation by T cells breaks self tolerance

T cells proliferate vigorously following acute depletion of CD4⁺ Foxp3⁺ T regulatory cells [natural Tregs (nTregs)] and also when naive T cells are transferred to syngeneic, nTreg-deficient Rag1 ^-/- hosts. Here, using mice raised in an antigen-free (AF) environment, we show that proliferation in these two situations is directed to self ligands rather than food or commensal antigens. In both situations, the absence of nTregs elevates B7 expression on host dendritic cells (DCs) and enables a small subset of naive CD4 T cells with high self affinity to respond overtly to host DCs: bidirectional T/DC interaction ensues, leading to progressive DC activation and reciprocal strong proliferation of T cells accompanied by peripheral Treg (pTreg) formation. Likewise, high-affinity CD4 T cells proliferate vigorously and form pTregs when cultured with autologous DCs in vitro in the absence of nTregs: this anti-self response is MHCII/peptide dependent and elicited by the raised level of B7 on cultured DCs. The data support a model in which self tolerance is imposed via modulation of CD28 signaling and explains the pathological effects of superagonistic CD28 antibodies.

Live-Cell FRET Imaging Reveals a Role of Extracellular Signal-Regulated Kinase Activity Dynamics in Thymocyte Motility

Extracellular signal-regulated kinase (ERK) plays critical roles in T cell development in the thymus. Nevertheless, the dynamics of ERK activity and the role of ERK in regulating thymocyte motility remain largely unknown due to technical limitations. To visualize ERK activity in thymocytes, we here developed knockin reporter mice expressing a Förster/fluorescence resonance energy transfer (FRET)-based biosensor for ERK from the ROSA26 locus. Live imaging of thymocytes isolated from the reporter mice revealed that ERK regulates thymocyte motility in a subtype-specific manner. Negative correlation between ERK activity and motility was observed in CD4/CD8 double-positive thymocytes and CD8 single-positive thymocytes, but not in CD4 single-positive thymocytes. Interestingly, however, the temporal deviations of ERK activity from the average correlate with the motility of CD4 single-positive thymocytes. Thus, live-cell FRET imaging will open a window to understanding the dynamic nature and the diverse functions of ERK signaling in T cell biology.

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Mice expressing EKAREV from ROSA26 locus enable ERK activity monitoring in T cells

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ERK activity negatively regulates the motility of thymocytes in the thymus

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Temporal dynamics of ERK activity regulates cell motility of CD4-SP in the medulla

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TCR signal from intercellular association induces ERK activity dynamics in CD4-SP

Thymic tolerance as a key brake on autoimmunity

Although the thymus has long been recognized as a key organ for T cell selection, the intricate details linking these selection events to human autoimmunity have been challenging to decipher. Over the last two decades, there has been rapid progress in understanding the role of thymic tolerance mechanisms in autoimmunity through genetics. Here we review some of the recent progress in understanding key thymic tolerance processes that are critical for preventing autoimmune disease.

Thymic B Cell-Mediated Attack of Thymic Stroma Precedes Type 1 Diabetes Development

Type 1 diabetes (T1D) results from a coordinated autoimmune attack of insulin producing beta cells in the pancreas by the innate and adaptive immune systems, beta cell death being predominantly T cell-mediated. In addition to T cells, peripheral B cells are important in T1D progression. The thymus of mice and man also contains B cells, and lately they have been linked to central tolerance of T cells. The role of thymic B cells in T1D is undefined. Here, we show there are abnormalities in the thymic B cell compartment before beta cell destruction and T1D manifestation. Using non-obese diabetic (NOD) mice, we document that preceding T1D development, there is significant accumulation of thymic B cells-partly through in situ development- and the putative formation of ectopic germinal centers. In addition, in NOD mice we quantify thymic plasma cells and observe in situ binding of immunoglobulins to undefined antigens on a proportion of medullary thymic epithelial cells (mTECs). By contrast, no ectopic germinal centers or pronounced intrathymic autoantibodies are detectable in animals not genetically predisposed to developing T1D. Binding of autoantibodies to thymic stroma correlates with apoptosis of mTECs, including insulin-expressing cells. By contrast, apoptosis of mTECs was decreased by 50% in B cell-deficient NOD mice suggesting intrathymic autoantibodies may selectively target certain mTECs for destruction. Furthermore, we observe that these thymic B cell-associated events correlated with an increased prevalence of premature thymic emigration of T cells. Together, our data suggest that the thymus may be a principal autoimmune target in T1D and contributes to disease progression.

αβ T-cell receptors with a central CDR3 cysteine are enriched in CD8αα intraepithelial lymphocytes and their thymic precursors

The thymus plays a crucial role in immune tolerance by exposing developing T cells (thymocytes) to a myriad of self-antigens. Strong T-cell receptor (TCR) engagement induces tolerance in self-reactive thymocytes by stimulating apoptosis or selection into specialized T-cell lineages, including intestinal TCRαβ⁺ CD8αα⁺ intraepithelial lymphocytes (IEL). TCR-intrinsic amino acid motifs that can be used to predict whether a TCR will be strongly self-reactive remain elusive. Here, a novel TCR sequence alignment approach revealed that T-cell lineages in C57BL/6 mice had divergent usage of cysteine within two positions of the amino acid at the apex of the complementarity-determining region 3 (CDR3) of the TCRα or TCRβ chain. Compared to pre-selection thymocytes, central CDR3 cysteine usage was increased in IEL and Type A IEL precursors (IELp) and markedly decreased in Foxp3⁺ regulatory T cells (T-reg) and naïve T cells. These findings reveal a TCR-intrinsic motif that distinguishes Type A IELp and IEL from T-reg and naïve T cells.

The role of regulatory T cells, interleukin-10 and in vivo scintigraphy in autoimmune and idiopathic diseases - Therapeutic perspectives and prognosis

Previous studies have demonstrated the expression of the CD25 marker on the surface of naturally occurring T cells (Tregs) of mice, which have a self-reactive cellular profile. Recently, expression of other markers that aid in the identification of these cells has been detected in lymphocyte subtypes of individuals suffering of autoimmune and idiopathic diseases, including: CD25, CTLA-4 (cytotoxic T-lymphocyte antigen 4), HLA-DR (human leukocyte antigen) and Interleukin 10 (IL-10), opening new perspectives for a better understanding of an association between such receptors present on the cell surface and the prognosis of autoimmune diseases. The role of these molecules has already been described in the literature for the modulation of the inflammatory response in infectious and parasitic diseases. Thus, the function, phenotype and frequency of expression of the a-chain receptor of IL-2 (CD25) and IL-10 in lymphocyte subtypes were investigated. Murine models have been used to demonstrate a possible correlation between the expression of the CD25 marker (on the surface of CD4 lymphocytes) and the control of self-tolerance mechanisms. These studies provided support for the presentation of a review of the role of cells expressing IL-2, IL-10, HLA-DR and CTLA-4 receptors in the monitoring of immunosuppression in diseases classified as autoimmune, providing perspectives for understanding peripheral regulation mechanisms and the pathophysiology of these diseases in humans. In addition, a therapeutic approach based on the manipulation of the phenotype of these cells and ways of scintigraphically monitoring the manifestations of these diseases by labeling their receptors is discussed as a perspective. In this paper, we have included the description of experiments in ex vivo regulation of IL-10 and synthesis of thio-sugars and poly-sugars to produce radiopharmaceuticals for monitoring inflammation. These experiments may yield benefits for the treatment and prognosis of autoimmune diseases.

Rebooting Human Immunology

Recent progress in both conceptual and technological approaches to human immunology have rejuvenated a field that has long been in the shadow of the inbred mouse model. This is a healthy development both for the clinical relevance of immunology and for the fact that it is a way to gain access to the wealth of phenomenology in the many human diseases that involve the immune system. This is where we are likely to discover new immunological mechanisms and principals, especially those involving genetic heterogeneity or environmental influences that are difficult to model effectively in inbred mice. We also suggest that there are likely to be novel immunological mechanisms in long-lived, less fecund mammals such as human beings since they must remain healthy far longer than short-lived rodents in order for the species to survive.

Retinoblastoma 1 protects T cell maturation from premature apoptosis by inhibiting E2F1

T lymphocytes are key cellular components of an acquired immune system and play essential roles in cell-mediated immunity. T cell development occurs in the thymus where 95% of immature thymocytes are eliminated via apoptosis. It is known that mutation of Zeb1, one of the retinoblastoma 1 (Rb1) target genes, results in a decrease in the number of immature T cells in mice. E2F1, an RB1-interacting protein, has been shown to regulate mature T cell development by interfering with thymocyte apoptosis. However, whether Rb1 regulates thymocyte development in vivo still needs to be further investigated. Here, we use a zebrafish model to investigate the role of Rb1 in T cell development. We show that Rb1-deficient fish exhibit a significant reduction in T cell number during early development that it is attributed to the accelerated apoptosis of immature T cells in a caspase-dependent manner. We further show that E2F1 overexpression could mimic the reduced T lymphocytes phenotype of Rb1 mutants, and E2F1 knockdown could rescue the phenotype in Rb1-deficient mutants. Collectively, our data indicate that the Rb1-E2F1-caspase axis is crucial for protecting immature T cells from apoptosis during early T lymphocyte maturation.

T cell receptor α-chain polymorphic allele frequencies in Caucasians and Polynesians

Restriction length polymorphisms in the variable and constant regions of the T cell receptor alpha-chain were examined in 42 Caucasians, 29 Maoris and 27 Pacific Islanders. Southern blots of Taq I digested DNA were hybridized with the T cell receptor alpha-chain probe pY14. Our results confirm that a 1.4 kb T cell receptor alpha chain-Taq 1 band is allelic to a 0.5 kb band. A significant difference in the frequency of the 1.4 and 0.5 kb alleles of the variable region of the alpha-chain was detected in Caucasians when compared with Maoris or Pacific Islanders (P < 0.0001). No differences in the frequency of the 2.0 and 7.0 kb alleles of the constant region gene were detected between any of the racial groups. These data may be relevant to ethnic differences in susceptibility to immune disorders.

A population of innate myelolymphoblastoid effector cell expanded by inactivation of mTOR complex 1 in mice

Adaptive autoimmunity is restrained by controlling population sizes and pathogenicity of harmful clones, while innate destruction is controlled at effector phase. We report here that deletion of Rptor in mouse hematopoietic stem/progenitor cells causes self-destructive innate immunity by massively increasing the population of previously uncharacterized innate myelolymphoblastoid effector cells (IMLECs). Mouse IMLECs are CD3-B220-NK1.1-Ter119- CD11clow/-CD115-F4/80low/-Gr-1- CD11b+, but surprisingly express high levels of PD-L1. Although they morphologically resemble lymphocytes and actively produce transcripts from Immunoglobulin loci, IMLECs have non-rearranged Ig loci, are phenotypically distinguishable from all known lymphocytes, and have a gene signature that bridges lymphoid and myeloid leukocytes. Rptor deletion unleashes differentiation of IMLECs from common myeloid progenitor cells by reducing expression of Myb. Importantly, IMLECs broadly overexpress pattern-recognition receptors and their expansion causes systemic inflammation in response to Toll-like receptor ligands in mice. Our data unveil a novel leukocyte population and an unrecognized role of Raptor/mTORC1 in innate immune tolerance.

The somatically generated portion of T cell receptor CDR3α contributes to the MHC allele specificity of the T cell receptor

Mature T cells bearing αβ T cell receptors react with foreign antigens bound to alleles of major histocompatibility complex proteins (MHC) that they were exposed to during their development in the thymus, a phenomenon known as positive selection. The structural basis for positive selection has long been debated. Here, using mice expressing one of two different T cell receptor β chains and various MHC alleles, we show that positive selection-induced MHC bias of T cell receptors is affected both by the germline encoded elements of the T cell receptor α and β chain and, surprisingly, dramatically affected by the non germ line encoded portions of CDR3 of the T cell receptor α chain. Thus, in addition to determining specificity for antigen, the non germline encoded elements of T cell receptors may help the proteins cope with the extremely polymorphic nature of major histocompatibility complex products within the species.

Redefining thymus medulla specialization for central tolerance

During αβT cell development, the thymus medulla represents an essential microenvironment for T cell tolerance. This functional specialization is attributed to its typical organized topology consisting of a branching structure that contains medullary thymic epithelial cell (mTEC) networks to support negative selection and Foxp3+ T-regulatory cell (T-reg) development. Here, by performing TEC-specific deletion of the thymus medulla regulator lymphotoxin β receptor (LTβR), we show that thymic tolerance mechanisms operate independently of LTβR-mediated mTEC development and organization. Consistent with this, mTECs continue to express Fezf2 and Aire, regulators of intrathymic self-antigens, and support T-reg development despite loss of LTβR-mediated medulla organogenesis. Moreover, we demonstrate that LTβR controls thymic tolerance by regulating the frequency and makeup of intrathymic dendritic cells (DCs) required for effective thymocyte negative selection. In all, our study demonstrates that thymus medulla specialization for thymic tolerance segregates from medulla organogenesis and instead involves LTβR-mediated regulation of the thymic DC pool.

Thymic function in the regulation of T cells, and molecular mechanisms underlying the modulation of cytokines and stress signaling (Review)

The thymus is critical in establishing and maintaining the appropriate microenvironment for promoting the development and selection of T cells. The function and structure of the thymus gland has been extensively studied, particularly as the thymus serves an important physiological role in the lymphatic system. Numerous studies have investigated the morphological features of thymic involution. Recently, research attention has increasingly been focused on thymic proteins as targets for drug intervention. Omics approaches have yielded novel insights into the thymus and possible drug targets. The present review addresses the signaling and transcriptional functions of the thymus, including the molecular mechanisms underlying the regulatory functions of T cells and their role in the immune system. In addition, the levels of cytokines secreted in the thymus have a significant effect on thymic functions, including thymocyte migration and development, thymic atrophy and thymic recovery. Furthermore, the regulation and molecular mechanisms of stress-mediated thymic atrophy and involution were investigated, with particular emphasis on thymic function as a potential target for drug development and discovery using proteomics.

[History of the thymus: from an "accident of evolution" to the programming of immunological self-tolerance]

This synthesis presents the most important disruptions of conceptions about the thymus since its discovery in Antique Greece. For centuries, the thymus was considered as a vestigial organ, and its role in T-lymphocyte differentiation has been proposed only in the 1960's. Most recent studies attribute to the thymus an essential and unique role in the programming of central immunological self-tolerance. The basal mechanism implicated in this function is the transcription in thymic epithelium of genes encoding precursors of self-antigens. Processing of these latter leads to presentation of self-antigens by the major histocompatibility complex (MHC) machinery expressed by thymic epithelial cells and dendritic cells. During fetal life, this presentation drives negative selection of T-cell clones harboring receptors with high affinity for these MHC/self-antigen complexes. After birth, this presentation also promotes the generation of regulatory T cells specific for these complexes. A number of studies, as well as the identification of Aire and Fezf2 genes, have shown that a thymus dysfunction plays a crucial role in the development of organ-specific autoimmunity.

Alternative Start Sites Downstream of Non-Sense Mutations Drive Antigen Presentation and Tolerance Induction to C-Terminal Epitopes

CTL responses to the transgene product remain an active area of concern for the gene therapy field. A patient's underlying genetic mutation may influence the qualitative nature of these potentially destructive T cell responses. Individuals with a mutation that introduces a premature termination codon (PTC) that prevents synthesis of the full-length peptide are considered more likely to mount a transgene-specific T cell response because of a lack of immune tolerance to C-terminal epitopes as a consequence of absent endogenous Ag presentation. In this article, we demonstrate that a human ornithine transcarbamylase gene containing various PTC-inducing non-sense mutations is able to generate and present epitopes downstream of the termination codon. Generation of these epitopes occurs primarily from alternative translation start sites downstream of the stop codon. Furthermore, we show that expression of these genes from adeno-associated virus vectors in C57BL/6 mice is able to induce peripheral tolerance to epitopes downstream of the PTC. These results suggest that, despite the lack of full-length endogenous protein, patients with PTC-inducing non-sense mutations may still present T cell epitopes downstream of the premature termination site that may render the subject tolerant to wild-type transgene products.

High-level intrathymic thyrotrophin receptor expression in thyroiditis-prone mice protects against the spontaneous generation of pathogenic thyrotrophin receptor autoantibodies

The thyrotrophin receptor (TSHR) A-subunit is the autoantigen targeted by pathogenic autoantibodies that cause Graves' hyperthyroidism, a common autoimmune disease in humans. Previously, we reported that pathogenic TSHR antibodies develop spontaneously in thyroiditis-susceptible non-obese diabetic (NOD).H2h4 mice bearing a human TSHR A-subunit transgene, which is expressed at low levels in both the thyroid and thymus (Lo-expressor transgene). The present study tested recent evidence that high intrathymic TSHR expression protects against the development of pathogenic TSHR antibodies in humans. By successive back-crossing, we transferred to the NOD.H2h4 background a human TSHR A-subunit transgene expressed at high levels in the thyroid and thymus (Hi-expressor transgene). In the sixth back-cross generation (> 98% NOD.H2h4 genome), only transgenic offspring produced spontaneously immunoglobulin (Ig)G class non-pathogenic human TSHR A-subunit antibodies. In contrast, both transgenic and non-transgenic offspring developed antibodies to thyroglobulin and thyroid peroxidase. However, non-pathogenic human TSHR antibody levels in Hi-expressor offspring were lower than in Lo-expressor transgenic mice. Moreover, pathogenic TSHR antibodies, detected by inhibition of TSH binding to the TSHR, only developed in back-cross offspring bearing the Lo-expressor, but not the Hi-expressor, transgene. High versus low expression human TSHR A-subunit in the NOD.H2h4 thymus was not explained by the transgene locations, namely chromosome 2 (127-147 Mb; Hi-expressor) and chromosome 1 (22.9-39.3 Mb; low expressor). Nevertheless, using thyroiditis-prone NOD.H2h4 mice and two transgenic lines, our data support the association from human studies that low intrathymic TSHR expression is associated with susceptibility to developing pathogenic TSHR antibodies, while high intrathymic TSHR expression is protective.

Thymic Epithelial Cells

Intrathymic T cell development is a complex process that depends upon continuous guidance from thymus stromal cell microenvironments. The thymic epithelium within the thymic stroma comprises highly specialized cells with a high degree of anatomic, phenotypic, and functional heterogeneity. These properties are collectively required to bias thymocyte development toward production of self-tolerant and functionally competent T cells. The importance of thymic epithelial cells (TECs) is evidenced by clear links between their dysfunction and multiple diseases where autoimmunity and immunodeficiency are major components. Consequently, TECs are an attractive target for cell therapies to restore effective immune system function. The pathways and molecular regulators that control TEC development are becoming clearer, as are their influences on particular stages of T cell development. Here, we review both historical and the most recent advances in our understanding of the cellular and molecular mechanisms controlling TEC development, function, dysfunction, and regeneration.

[Thymoma and autoimmune diseases]

The association between thymoma and autoimmunity is well known. Besides myasthenia gravis, which is found in 15 to 20% of patients with thymoma, other autoimmune diseases have been reported: erythroblastopenia, systemic lupus erythematosus, inflammatory myopathies, thyroid disorders, Isaac's syndrome or Good's syndrome. More anecdotally, Morvan's syndrome, limbic encephalitis, other autoimmune cytopenias, autoimmune hepatitis, and bullous skin diseases (pemphigus, lichen) have been reported. Autoimmune diseases occur most often before thymectomy, but they can be discovered at the time of surgery or later. Two situations require the systematic investigation of a thymoma: the occurrence of myasthenia gravis or autoimmune erythroblastopenia. Nevertheless, the late onset of systemic lupus erythematosus or the association of several autoimmune manifestations should lead to look for a thymoma. Neither the characteristics of the patients nor the pathological data can predict the occurrence of an autoimmune disease after thymectomy. Thus, thymectomy usefulness in the course of the autoimmune disease, except myasthenia gravis, has not been demonstrated. This seems to indicate the preponderant role of self-reactive T lymphocytes distributed in the peripheral immune system prior to surgery. Given the high infectious morbidity in patients with thymoma, immunoglobulin replacement therapy should be considered in patients with hypogammaglobulinemia who receive immunosuppressive therapy, even in the absence of prior infection.

Cellular and Molecular Mechanisms of Autoimmunity and Lupus Nephritis

Autoimmunity involves immune responses directed against self, which are a result of defective self/foreign distinction of the immune system, leading to proliferation of self-reactive lymphocytes, and is characterized by systemic, as well as tissue-specific, inflammation. Numerous mechanisms operate to ensure the immune tolerance to self-antigens. However, monogenetic defects or genetic variants that weaken immune tolerance render susceptibility to the loss of immune tolerance, which is further triggered by environmental factors. In this review, we discuss the phenomenon of immune tolerance, genetic and environmental factors that influence the immune tolerance, factors that induce autoimmunity such as epigenetic and transcription factors, neutrophil extracellular trap formation, extracellular vesicles, ion channels, and lipid mediators, as well as costimulatory or coinhibitory molecules that contribute to an autoimmune response. Further, we discuss the cellular and molecular mechanisms of autoimmune tissue injury and inflammation during systemic lupus erythematosus and lupus nephritis.

Graves' Disease in Pediatric and Elderly Patients with 22q11.2 Deletion Syndrome

22q11.2 Deletion Syndrome (22qDS) is often complicated by autoimmune diseases. To clarify the causal relationship, we examined the lymphocyte subset distribution and the human leucocyte antigen (HLA) in two female patients (one child and an elderly) with Graves' disease (GD) and 22qDS. Thymus dysgenesis might have contributed to the T-cell imbalance and the lack of negative selection in both cases. Notably, HLA-DR14, a known risk factor for GD in Japanese individuals and the decreased regulatory T-cell numbers that were seen in the pediatric case, may affect the early onset of GD. Central and peripheral tolerance and Th1 cells appeared to be associated with the pathogenesis of GD in 22qDS.

Cytokines and metabolic factors regulate tumoricidal T-cell function during cancer immunotherapy

Recent advances in cancer biology and genetics have fostered precision therapies targeting tumor-specific attributes. Immune-based therapies that elicit cytolytic T cells (CTL) specific for tumor antigens can provide therapeutic benefit to cancer patients, however, cure rates are typically low. This largely results from immunosuppressive mechanisms operating within the tumor microenvironment, many of which inflict metabolic stresses upon CTL. Conversely, immunotherapies can mitigate specific metabolic stressors. For instance, dual costimulation immunotherapy with CD134 (OX40) plus CD137 (4-1BB) agonists appears to mediate tumor control in part by engaging cytokine networks that enable infiltrating CTL to compete for limiting supplies of glucose. Future efforts combining modalities that endow CTL with complimentary metabolic advantages should improve therapeutic efficacies.

Programmed Death-1 Culls Peripheral Accumulation of High-Affinity Autoreactive CD4 T Cells to Protect against Autoimmunity

Self-reactive CD4 T cells are incompletely deleted during thymic development, and their peripheral seeding highlights the need for additional safeguards to avert autoimmunity. Here, we show an essential role for the coinhibitory molecule programmed death-1 (PD-1) in silencing the activation of high-affinity autoreactive CD4 T cells. Each wave of self-reactive CD4 T cells that escapes thymic deletion autonomously upregulates PD-1 to maintain self-tolerance. By tracking the progeny derived from individual autoreactive CD4 T cell clones, we demonstrate that self-reactive cells with the greatest autoimmune threat and highest self-antigen affinity express the most PD-1. Reciprocally, PD-1 deprivation unleashes high-affinity self-reactive CD4 T cells in target tissues to exacerbate neuronal inflammation and autoimmune diabetes. Reliance on PD-1 to actively maintain self-tolerance may explain why exploiting this pathway by cancerous cells and invasive microbes efficiently subverts protective immunity, and why autoimmune side effects can develop after PD-1-neutralizing checkpoint therapies.

Thymic Nurse Cells Participate in Heterotypic Internalization and Repertoire Selection of Immature Thymocytes; Their Removal from the Thymus of Autoimmune Animals May be Important to Disease Etiology

Thymic nurse cells (TNCs) are specialized epithelial cells that reside in the thymic cortex. The initial report of their discovery in 1980 showed TNCs to contain up to 200 thymocytes within specialized vacuoles in their cytoplasm. Much has been reported since that time to determine the function of this heterotypic internalization event that exists between TNCs and developing thymocytes. In this review, we discuss the literature reported that describes the internalization event and the role TNCs play during T cell development in the thymus as well as why these multicellular complexes may be important in inhibiting the development of autoimmune diseases.

Class II major histocompatibility complex mutant mice to study the germ-line bias of T-cell antigen receptors

The interaction of αβ T-cell antigen receptors (TCRs) with peptides bound to MHC molecules lies at the center of adaptive immunity. Whether TCRs have evolved to react with MHC or, instead, processes in the thymus involving coreceptors and other molecules select MHC-specific TCRs de novo from a random repertoire is a longstanding immunological question. Here, using nuclease-targeted mutagenesis, we address this question in vivo by generating three independent lines of knockin mice with single-amino acid mutations of conserved class II MHC amino acids that often are involved in interactions with the germ-line-encoded portions of TCRs. Although the TCR repertoire generated in these mutants is similar in size and diversity to that in WT mice, the evolutionary bias of TCRs for MHC is suggested by a shift and preferential use of some TCR subfamilies over others in mice expressing the mutant class II MHCs. Furthermore, T cells educated on these mutant MHC molecules are alloreactive to each other and to WT cells, and vice versa, suggesting strong functional differences among these repertoires. Taken together, these results highlight both the flexibility of thymic selection and the evolutionary bias of TCRs for MHC.

Immunotherapy for oesophagogastric cancer

INTRODUCTION

Gastric and oesophageal cancers are a pressing global health problem with high mortality rates and poor outcomes for advanced disease. The mainstay of treatment in the palliative setting has traditionally been chemotherapy, which accrues only modest survival benefits. As with other cancer types, there is increasing interest in the use of immunotherapy approaches to improve outcomes.

AREAS COVERED

This paper reviews the aetiological and genetic characteristics of oesophagogastric (OG) cancers relevant to the application of immunotherapy and outlines the historical, present-day and potential future applications of immunotherapy in their management.

EXPERT OPINION

The use of agents targeting the PD1 pathway have led to impressive and durable responses in a minority of OG cancer patients and it would be expected that combinatorial approaches with chemotherapy, radiotherapy and other biological agents will improve responses further. Identification of clinically robust biomarkers is crucial in refining such approaches moving forwards. The application of modern sequencing technology to the development of personalized neoantigen-based vaccines represents an exciting amalgamation of genomics and immunotherapy, with potentially important clinical implications in OG cancer.

Extrinsic allospecific signals of hematopoietic origin dictate iNKT cell lineage-fate decisions during development

Invariant NKT (iNKT) cells are critical to the maintenance of tolerance toward alloantigens encountered during postnatal life pointing to the existence of a process for self-education. However, the impact of developmentally encountered alloantigens in shaping the phenotype and function of iNKT cells has not been described. To better understand this process, the current report examined naïve iNKT cells as they matured in an allogeneic environment. Following the prenatal transfer of fetal hematopoietic cells between age-matched allogeneic murine fetuses, cell-extrinsic signals appeared to dictate allospecific patterns of Ly49 receptor expression and lineage diversity in developing iNKT cells. Regulation for this process arose from cells of hematopoietic origin requiring only rare exposure to facilitate broad changes in developing iNKT cells. These findings highlight surprisingly asymmetric allospecific alterations in iNKT cells as they develop and mature in an allogeneic environment and establish a new paradigm for study of the self-education of iNKT cells.

Cross-Reactivity of TCR Repertoire: Current Concepts, Challenges, and Implication for Allotransplantation

Being able to track donor reactive T cells during the course of organ transplantation is a key to improve the graft survival, to prevent graft dysfunction, and to adapt the immunosuppressive regimen. The attempts of transplant immunologists have been for long hampered by the large size of the alloreactive T cell repertoire. Understanding how self-TCR can interact with allogeneic MHC is a key to critically appraise the different assays available to analyze the TCR Vβ repertoire usage. In this report, we will review conceptually and experimentally the process of cross-reactivity. We will then highlight what can be learned from allotransplantation, a situation of artificial cross-reactivity. Finally, the low- and high-resolution techniques to characterize the TCR Vβ repertoire usage in transplantation will be critically discussed.

Enhancing the safety of antibody-based immunomodulatory cancer therapy without compromising therapeutic benefit: Can we have our cake and eat it too?

INTRODUCTION

Monoclonal antibodies (mAbs) targeting checkpoint inhibitors have demonstrated clinical benefit in treating patients with cancer and have paved the way for additional immune-modulating mAbs such as those targeting costimulatory receptors. The full clinical utility of these agents, however, is hampered by immune-related adverse events (irAEs) that can occur during therapy.

AREAS COVERED

We first provide a general overview of tumor immunity, followed by a review of the two major classes of immunomodulatory mAbs being developed as cancer therapeutics: checkpoint inhibitors and costimulatory receptor agonists. We then discuss therapy-associated adverse events. Finally, we describe in detail the mechanisms driving their therapeutic activity, with an emphasis on interactions between antibody fragment crystallizable (Fc) domains and Fc receptors (FcR).

EXPERT OPINION

Given that Fc-FcR interactions appear critical in facilitating the ability of immunomodulatory mAbs to elicit both therapeutically useful as well as adverse effects, the engineering of mAbs that can effectively engage their targets while limiting interaction with FcRs might represent a promising future avenue for developing the next generation of immune-enhancing tumoricidal agents with increased safety and retention of efficacy.

The Dendritic Cell Major Histocompatibility Complex II (MHC II) Peptidome Derives from a Variety of Processing Pathways and Includes Peptides with a Broad Spectrum of HLA-DM Sensitivity

The repertoire of peptides displayed in vivo by MHC II molecules derives from a wide spectrum of proteins produced by different cell types. Although intracellular endosomal processing in dendritic cells and B cells has been characterized for a few antigens, the overall range of processing pathways responsible for generating the MHC II peptidome are currently unclear. To determine the contribution of non-endosomal processing pathways, we eluted and sequenced over 3000 HLA-DR1-bound peptides presented in vivo by dendritic cells. The processing enzymes were identified by reference to a database of experimentally determined cleavage sites and experimentally validated for four epitopes derived from complement 3, collagen II, thymosin β4, and gelsolin. We determined that self-antigens processed by tissue-specific proteases, including complement, matrix metalloproteases, caspases, and granzymes, and carried by lymph, contribute significantly to the MHC II self-peptidome presented by conventional dendritic cells in vivo. Additionally, the presented peptides exhibited a wide spectrum of binding affinity and HLA-DM susceptibility. The results indicate that the HLA-DR1-restricted self-peptidome presented under physiological conditions derives from a variety of processing pathways. Non-endosomal processing enzymes add to the number of epitopes cleaved by cathepsins, altogether generating a wider peptide repertoire. Taken together with HLA-DM-dependent and-independent loading pathways, this ensures that a broad self-peptidome is presented by dendritic cells. This work brings attention to the role of "self-recognition" as a dynamic interaction between dendritic cells and the metabolic/catabolic activities ongoing in every parenchymal organ as part of tissue growth, remodeling, and physiological apoptosis.

Fezf2 Orchestrates a Thymic Program of Self-Antigen Expression for Immune Tolerance

Self-tolerance to immune reactions is established via promiscuous expression of tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs), leading to the elimination of T cells that respond to self-antigens. The transcriptional regulator Aire has been thought to be sufficient for the induction of TRAs, despite some indications that other factors may promote TRA expression in the thymus. Here, we show that the transcription factor Fezf2 directly regulates various TRA genes in mTECs independently of Aire. Mice lacking Fezf2 in mTECs displayed severe autoimmune symptoms, including the production of autoantibodies and inflammatory cell infiltration targeted to peripheral organs. These responses differed from those detected in Aire-deficient mice. Furthermore, Fezf2 expression and Aire expression are regulated by distinct signaling pathways and promote the expression of different classes of proteins. Thus, two independent factors, Fezf2 and Aire, permit the expression of TRAs in the thymus to ensure immune tolerance.

Remission of systemic lupus erythematosus disease activity with regulatory cytokine interleukin (IL)-35 in Murphy Roths Large (MRL)/lpr mice

The immunological mechanisms mediated by regulatory cytokine interleukin (IL)-35 are unclear in systemic lupus erythematosus (SLE). We investigated the frequency of CD4(+) CD25(+) forkhead box protein 3 (FoxP3)(+) regulatory T (Treg ) and IL-10(+) regulatory B (Breg ) cells and related immunoregulatory mechanisms in a female Murphy Roths Large (MRL)/lpr mouse model of spontaneous lupus-like disease, with or without IL-35 treatment. A remission of histopathology characteristics of lupus flare and nephritis was observed in the MRL/lpr mice upon IL-35 treatment. Accordingly, IL-35 and IL-35 receptor subunits (gp130 and IL-12Rβ2) and cytokines of MRL/lpr and BALB/c mice (normal controls) were measured. The increased anti-inflammatory cytokines and decreased proinflammatory cytokines were possibly associated with the restoration of Treg and Breg frequency in MRL/lpr mice with IL-35 treatment, compared to phosphate-buffered saline (PBS) treatment. mRNA expressions of Treg -related FoxP3, IL-35 subunit (p35 and EBI3) and soluble IL-35 receptor subunit (gp130 and IL12Rβ2) in splenic cells were up-regulated significantly in IL-35-treated mice. Compared with the PBS treatment group, IL-35-treated MRL/lpr mice showed an up-regulation of Treg -related genes and the activation of IL-35-related intracellular Janus kinase/signal transducer and activator of transcription signal pathways, thereby indicating the immunoregulatory role of IL-35 in SLE. These in vivo findings may provide a biochemical basis for further investigation of the regulatory mechanisms of IL-35 for the treatment of autoimmune-mediated inflammation.