Regulatory T-cell differentiation versus clonal deletion of autoreactive thymocytes

Breaking tolerance: the autoimmune aspect of atherosclerosis

Atherosclerotic cardiovascular disease (ASCVD) is a chronic inflammatory disease of the arterial walls and is characterized by the accumulation of lipoproteins that are insufficiently cleared by phagocytes. Following the initiation of atherosclerosis, the pathological progression is accelerated by engagement of the adaptive immune system. Atherosclerosis triggers the breakdown of tolerance to self-components. This loss of tolerance is reflected in defective expression of immune checkpoint molecules, dysfunctional antigen presentation, and aberrations in T cell populations - most notably in regulatory T (Treg) cells - and in the production of autoantibodies. The breakdown of tolerance to self-proteins that is observed in ASCVD may be linked to the conversion of Treg cells to 'exTreg' cells because many Treg cells in ASCVD express T cell receptors that are specific for self-epitopes. Alternatively, or in addition, breakdown of tolerance may trigger the activation of naive T cells, resulting in the clonal expansion of T cell populations with pro-inflammatory and cytotoxic effector phenotypes. In this Perspective, we review the evidence that atherosclerosis is associated with a breakdown of tolerance to self-antigens, discuss possible immunological mechanisms and identify knowledge gaps to map out future research. Rational approaches aimed at re-establishing immune tolerance may become game changers in treating ASCVD and in preventing its downstream sequelae, which include heart attacks and strokes.

SKI Regulates Medullary Thymic Epithelial Cell Differentiation to Control Peripheral T Cell Responses in Mice

The thymus is an important site for the establishment of an appropriate immune response through positive and negative selection of developing T cells. During selection, developing T cells interact with cortical and medullary thymic epithelial cells (TECs), termed cTECs and mTECs, respectively. Using a Foxn1Cre+/-SKIfl/fl mouse model, we found that TEC-specific deletion of SKI reduced the mTEC compartment in the thymus and that tissue-restricted Ag expression in mTECs was altered. This decrease in the medullary area led to a decrease in CD4 thymocyte cellularity; however, mature CD4 cellularity in the spleen remained normal. Interestingly, naive CD4 T cells purified from SKI-deleted mice showed a defect in proliferation in vitro after global TCR stimulation, and these mice were significantly protected from developing experimental autoimmune encephalomyelitis compared with the control mice. Overall, our findings suggest that SKI signaling in the thymus regulates mTEC differentiation and function as well as downstream peripheral T cell responses and provide evidence for targeting SKI in T cell-driven autoimmune diseases such as multiple sclerosis.

Obesity-induced thymic involution and cancer risk

Declining thymic functions associated either with old age (i.e., age-related thymic involution), or with acute involution as a result of stress, infectious disease, or cytoreductive therapies (e.g., chemotherapy/radiotherapy), have been associated with cancer development. A key mechanism underlying such increased cancer risk is the thymus-dependent debilitation of adaptive immunity, which is responsible for orchestrating immunoediting mechanisms and tumor immune surveillance. In the past few years, a blooming set of evidence has intriguingly linked obesity with cancer development and progression. The majority of such studies has focused on obesity-driven chronic inflammation, steroid/sex hormone and adipokine production, and hyperinsulinemia, as principal factors affecting the tumor microenvironment and driving the development of primary malignancy. However, experimental observations about the negative impact of obesity on T cell development and maturation have existed for more than half a century. Here, we critically discuss the molecular and cellular mechanisms of obesity-driven thymic involution as a previously underrepresented intermediary pathology leading to cancer development and progression. This knowledge could be especially relevant in the context of childhood obesity, because impaired thymic function in young individuals leads to immune system abnormalities, and predisposes to various pediatric cancers. A thorough understanding behind the molecular and cellular circuitries governing obesity-induced thymic involution could therefore help towards the rationalized development of targeted thymic regeneration strategies for obese individuals at high risk of cancer development.

Multimodal human thymic profiling reveals trajectories and cellular milieu for T agonist selection

To prevent autoimmunity, thymocytes expressing self-reactive T cell receptors (TCRs) are negatively selected, however, divergence into tolerogenic, agonist selected lineages represent an alternative fate. As thymocyte development, selection, and lineage choices are dependent on spatial context and cell-to-cell interactions, we have performed Cellular Indexing of Transcriptomes and Epitopes by sequencing (CITE-seq) and spatial transcriptomics on paediatric human thymu​​s. Thymocytes expressing markers of strong TCR signalling diverged from the conventional developmental trajectory prior to CD4⁺ or CD8⁺ lineage commitment, while markers of different agonist selected T cell populations (CD8αα(I), CD8αα(II), T_(agonist), T_reg(diff), and T_reg) exhibited variable timing of induction. Expression profiles of chemokines and co-stimulatory molecules, together with spatial localisation, supported that dendritic cells, B cells, and stromal cells contribute to agonist selection, with different subsets influencing thymocytes at specific developmental stages within distinct spatial niches. Understanding factors influencing agonist T cells is needed to benefit from their immunoregulatory effects in clinical use.

The thymic microenvironment gradually modulates the phenotype of thymus-homing peripheral conventional dendritic cells

Background & Aims

Thymic conventional dendritic cells (t‑DCs) are crucial for the development of T cells. A substantial fraction of t‑DCs originates extrathymically and migrates to the thymus. Here, these cells contribute to key processes of central tolerance like the clonal deletion of self‑reactive thymocytes and the generation of regulatory T (Treg) cells. So far, it is only incompletely understood which impact the thymic microenvironment has on thymus‑homing conventional DCs (cDCs), which phenotypic changes occur after the entry of peripheral cDCs into the thymus and which functional properties these modulated cells acquire.

Materials & Methods

In the present study, we mimicked the thymus‑homing of peripheral cDCs by introducing ex vivo isolated splenic cDCs (sp‑DCs) into reaggregated thymic organ cultures (RTOCs).

Results

Already after two days of culture, the transcriptomic profile of sp‑DCs was modulated and had acquired certain key signatures of t‑DCs. The regulated genes included immunomodulatory cytokines and chemokines as well as costimulatory molecules. After four days of culture, sp‑DCs appeared to have at least partially acquired the peculiar Treg cell‐inducing capacity characteristic of t‑DCs.

Discussion & Conclusion

Taken together, our findings indicate that peripheral cDCs possess a high degree of plasticity enabling them to quickly adapt to the thymus‐specific microenvironment. We further provide indirect evidence that thymus‐specific properties such as the efficient induction of Treg cells under homeostatic conditions can be partially transferred to thymus‑homing peripheral cDC subsets.

Sex-specific remodeling of T-cell compartment with aging: Implications for rat susceptibility to central nervous system autoimmune diseases

The incidence of multiple sclerosis (MS) and susceptibility of animals to experimental autoimmune encephalomyelitis (EAE), the most commonly used experimental model of MS, decrease with aging. Generally, autoimmune diseases develop as the ultimate outcome of an imbalance between damaging immune responses against self and regulatory immune responses (keeping the former under control). Thus, in this review the age-related changes possibly underlying this balance were discussed. Specifically, considering the central role of T cells in MS/EAE, the impact of aging on overall functional capacity (reflecting both overall count and individual functional cell properties) of self-reactive conventional T cells (Tcons) and FoxP3+ regulatory T cells (Tregs), as the most potent immunoregulatory/suppressive cells, was analyzed, as well. The analysis encompasses three distinct compartments: thymus (the primary lymphoid organ responsible for the elimination of self-reactive T cells - negative selection and the generation of Tregs, compensating for imperfections of the negative selection), peripheral blood/lymphoid tissues ("afferent" compartment), and brain/spinal cord tissues ("target" compartment). Given that the incidence of MS and susceptibility of animals to EAE are greater in women/females than in age-matched men/males, sex as independent variable was also considered. In conclusion, with aging, sex-specific alterations in the balance of self-reactive Tcons/Tregs are likely to occur not only in the thymus/"afferent" compartment, but also in the "target" compartment, reflecting multifaceted changes in both T-cell types. Their in depth understanding is important not only for envisaging effects of aging, but also for designing interventions to slow-down aging without any adverse effect on incidence of autoimmune diseases.

Thymic Epithelial Cell Alterations and Defective Thymopoiesis Lead to Central and Peripheral Tolerance Perturbation in MHCII Deficiency

Major Histocompatibility Complex (MHC) class II (MHCII) deficiency (MHCII-D), also known as Bare Lymphocyte Syndrome (BLS), is a rare combined immunodeficiency due to mutations in genes regulating expression of MHCII molecules. MHCII deficiency results in impaired cellular and humoral immune responses, leading to severe infections and autoimmunity. Abnormal cross-talk with developing T cells due to the absence of MHCII expression likely leads to defects in thymic epithelial cells (TEC). However, the contribution of TEC alterations to the pathogenesis of this primary immunodeficiency has not been well characterized to date, in particular in regard to immune dysregulation. To this aim, we have performed an in-depth cellular and molecular characterization of TEC in this disease. We observed an overall perturbation of thymic structure and function in both MHCII-/- mice and patients. Transcriptomic and proteomic profiling of murine TEC revealed several alterations. In particular, we demonstrated that impairment of lymphostromal cross-talk in the thymus of MHCII-/- mice affects mTEC maturation and promiscuous gene expression and causes defects of central tolerance. Furthermore, we observed peripheral tolerance impairment, likely due to defective Treg cell generation and/or function and B cell tolerance breakdown. Overall, our findings reveal disease-specific TEC defects resulting in perturbation of central tolerance and limiting the potential benefits of hematopoietic stem cell transplantation in MHCII deficiency.

Thymic atrophy creates holes in Treg-mediated immuno-regulation via impairment of an antigen-specific clone

Age-related thymic atrophy results in reduced output of naïve conventional T (Tcon) cells. However, its impact on regulatory T (Treg) cells is insufficiently understood. Given evidence that thymic Treg (tTreg) cell generation is enhanced in the aged, atrophy thymus and that the aged periphery accumulates peripheral Treg (pTreg) cells, we asked why these Treg cells are unable to effectively attenuate increased autoreactivity-induced chronic inflammation in the elderly. We designed a mock-self-antigen chimera mouse model, in which membrane-bound ovalbumin (mOVA) transgenic mice, bearing a FoxN1-floxed gene for induction of conditional thymic atrophy, received OVA-specific (OT-II) T-cell receptor (TCR) transgenic progenitor cells. The chimeric mice with thymic atrophy exhibited a significant decrease in OVA-specific tTreg and pTreg cells but not polyclonal (pan)-Treg cells. These OVA-specific pTreg cells were significantly less able to suppress OVA-specific stimulation-induced proliferation in vitro and exhibited lower FoxP3 expression. Additionally, we conducted preliminary TCR repertoire diversity sequencing for Treg cells among recent thymic emigrants (RTEs) from Rag^GFP -FoxP3^RFP dual-reporter mice and observed a trend for decreased diversity in mice with thymic atrophy compared to littermates with normal thymus. These data indicate that although the effects of age-related thymic atrophy do not affect pan-Treg generation, certain tissue-specific Treg clones may experience abnormal agonist selection. This, combined with enhanced pan-pTreg cells, may greatly contribute to age-related chronic inflammation, even in the absence of acute autoimmune disease in the elderly.

Regulatory T cells in autoimmune hepatitis: an updated overview

Regulatory T-cells (Tregs) are key players in the maintenance of immune homeostasis by preventing immune responses to self-antigens. Defects in Treg frequency and/or function result in overwhelming CD4 and CD8 T cell immune responses participating in the autoimmune attack. Perpetuation of autoimmune damage is also favored by Treg predisposition to acquire effector cell features upon exposure to a proinflammatory challenge. Treg impairment plays a permissive role in the initiation and perpetuation of autoimmune liver diseases, namely autoimmune hepatitis, primary biliary cholangitis and primary sclerosing cholangitis. In this Review, we outline studies reporting the role of Treg impairment in the pathogenesis of these conditions and discuss methods to restore Treg number and function either by generation/expansion in the test tube or through in vivo expansion upon administration of low dose IL-2. Challenges and caveats of these potential therapeutic strategies are also reviewed and discussed.

Could cross-reactivity rescue Foxp3+ regulatory T cell precursors from thymic deletion?

Thymocytes that bind with high affinity to peptides displayed by MHC class II (pMHC-II) are deleted while low-affinity binders differentiate into naive CD4+ T cells. However, Foxp3+ regulatory T cells (Tregs) seem to defy this binary choice as their precursors require high-affinity interaction with pMHC-II for maturation in the thymus. Here, we rely on the antigen-specific interpretive framework, SPIRAL (Specific ImmunoRegulatory Algorithm), to propose that Tregs escape thymic deletion by forming dyads with IL-2-producing T cells via antigen cross-reactivity. This interpretation reconciles contradictions related to Treg ontogeny in the thymus and their role in modulating antigen-specific immune responses.

Modulatory Effects of Pregnancy on Inflammatory Bowel Disease

The disease course of autoimmune diseases such as rheumatoid arthritis is altered during pregnancy, and a similar modulatory role of pregnancy on inflammatory bowel disease (IBD) has been proposed. Hormonal, immunological, and microbial changes occurring during normal pregnancy may interact with the pathophysiology of IBD. IBD consists of Crohn's disease and ulcerative colitis, and because of genetic, immunological, and microbial differences between these disease entities, they may react differently during pregnancy and should be described separately. This review will address the pregnancy-induced physiological changes and their potential effect on the disease course of ulcerative colitis and Crohn's disease, with emphasis on the modulation of epithelial barrier function and immune profiles by pregnancy hormones, microbial changes, and microchimerism.

Thymic Function Associated With Cancer Development, Relapse, and Antitumor Immunity - A Mini-Review

The thymus is the central lymphoid organ for T cell development, a cradle of T cells, and for central tolerance establishment, an educator of T cells, maintaining homeostatic cellular immunity. T cell immunity is critical to control cancer occurrence, relapse, and antitumor immunity. Evidence on how aberrant thymic function influences cancer remains largely insufficient, however, there has been recent progress. For example, the involuted thymus results in reduced output of naïve T cells and a restricted T cell receptor (TCR) repertoire, inducing immunosenescence and potentially dampening immune surveillance of neoplasia. In addition, the involuted thymus relatively enhances regulatory T (Treg) cell generation. This coupled with age-related accumulation of Treg cells in the periphery, potentially provides a supportive microenvironment for tumors to escape T cell-mediated antitumor responses. Furthermore, acute thymic involution from chemotherapy can create a tumor reservoir, resulting from an inflammatory microenvironment in the thymus, which is suitable for disseminated tumor cells to hide, survive chemotherapy, and become dormant. This may eventually result in cancer metastatic relapse. On the other hand, if thymic involution is wisely taken advantage of, it may be potentially beneficial to antitumor immunity, since the involuted thymus increases output of self-reactive T cells, which may recognize certain tumor-associated self-antigens and enhance antitumor immunity, as demonstrated through depletion of autoimmune regulator (AIRE) gene in the thymus. Herein, we briefly review recent research progression regarding how altered thymic function modifies T cell immunity against tumors.

Contributions of Age-Related Thymic Involution to Immunosenescence and Inflammaging

Immune system aging is characterized by the paradox of immunosenescence (insufficiency) and inflammaging (over-reaction), which incorporate two sides of the same coin, resulting in immune disorder. Immunosenescence refers to disruption in the structural architecture of immune organs and dysfunction in immune responses, resulting from both aged innate and adaptive immunity. Inflammaging, described as a chronic, sterile, systemic inflammatory condition associated with advanced age, is mainly attributed to somatic cellular senescence-associated secretory phenotype (SASP) and age-related autoimmune predisposition. However, the inability to reduce senescent somatic cells (SSCs), because of immunosenescence, exacerbates inflammaging. Age-related adaptive immune system deviations, particularly altered T cell function, are derived from age-related thymic atrophy or involution, a hallmark of thymic aging. Recently, there have been major developments in understanding how age-related thymic involution contributes to inflammaging and immunosenescence at the cellular and molecular levels, including genetic and epigenetic regulation, as well as developments of many potential rejuvenation strategies. Herein, we discuss the research progress uncovering how age-related thymic involution contributes to immunosenescence and inflammaging, as well as their intersection. We also describe how T cell adaptive immunity mediates inflammaging and plays a crucial role in the progression of age-related neurological and cardiovascular diseases, as well as cancer. We then briefly outline the underlying cellular and molecular mechanisms of age-related thymic involution, and finally summarize potential rejuvenation strategies to restore aged thymic function.

Molecular regulatory networks of thymic epithelial cell differentiation

Functional mature T cells are generated in the thymus. Thymic epithelial cells (TECs) provide the essential microenvironment for T cell development and maturation. According to their function and localization, TECs are roughly divided into cortical TECs (cTECs) and medullary TECs (mTECs), which are responsible for positive and negative selection, respectively. This review summarizes the current understanding of TEC biology, the identification of fetal and adult bipotent TEC progenitors, and the signaling pathways that control the development and maturation of TECs. The understanding of the ontogeny, differentiation, maturation and function of cTECs lags behind that of mTECs. Better understanding TEC biology will provide clues about TEC development and the applications of thymus engineering.

Identification of a genomic enhancer that enforces proper apoptosis induction in thymic negative selection

During thymic negative selection, autoreactive thymocytes carrying T cell receptor (TCR) with overtly strong affinity to self-MHC/self-peptide are removed by Bim-dependent apoptosis, but how Bim is specifically regulated to link TCR activation and apoptosis induction is unclear. Here we identify a murine T cell-specific genomic enhancer EBAB (Bub1-Acoxl-Bim), whose deletion leads to accumulation of thymocytes expressing high affinity TCRs. Consistently, EBAB knockout mice have defective negative selection and fail to delete autoreactive thymocytes in various settings, with this defect accompanied by reduced Bim expression and apoptosis induction. By contrast, EBAB is dispensable for maintaining peripheral T cell homeostasis via Bim-dependent pathways. Our data thus implicate EBAB as an important, developmental stage-specific regulator of Bim expression and apoptosis induction to enforce thymic negative selection and suppress autoimmunity. Our study unravels a part of genomic enhancer codes that underlie complex and context-dependent gene regulation in TCR signaling.

Absence of MHC-II expression by lymph node stromal cells results in autoimmunity

MHCII-restricted antigen presentation by lymph node stromal cells is essential for regulatory T-cell proliferation and functions, and for the regulation of autoimmunity.

How lymph node stromal cells (LNSCs) shape peripheral T-cell responses remains unclear. We have previously demonstrated that murine LNSCs, lymphatic endothelial cells (LECs), blood endothelial cells (BECs), and fibroblastic reticular cells (FRCs) use the IFN-γ–inducible promoter IV (pIV) of the MHC class II (MHCII) transactivator CIITA to express MHCII. Here, we show that aging mice (>1 yr old) in which MHCII is abrogated in LNSCs by the selective deletion of pIV exhibit a significant T-cell dysregulation in LNs, including defective Treg and increased effector CD4⁺ and CD8⁺ T-cell frequencies, resulting in enhanced peripheral organ T-cell infiltration and autoantibody production. The proliferation of LN-Tregs interacting with LECs increases following MHCII up-regulation by LECs upon aging or after exposure to IFN-γ, this effect being abolished in mice in which LECs lack MHCII. Overall, our work underpins the importance of LNSCs, particularly LECs, in supporting Tregs and T-cell tolerance.

Comparison of Burnet's clonal selection theory with tumor cell-clone development

Increasing evidence has shown that Darwin's theory of evolution provides vital insights into the emergence and etiology of different types of cancer. On a microscopic scale, cancer stem cells meet the conditions for the Darwinian process of natural selection. In particular, cancer stem cells undergo cell reproduction characterized by the emergence of heritable variability that promotes replication and cell survival. Methods: Evidence from previous studies was gathered to compare Burnet's clonal selection theory with the tumor evolution theory. Results: The findings show that the Darwinian theory offers a general framework for understanding fundamental aspects of cancer. As fundamental theoretical frameworks, Burnet's clonal selection theory and the tumor evolution theory can be used to explain cancer cell evolution and identify the beneficial adaptations that contribute to cell survival in tissue landscapes and tissue ecosystems. Conclusions: In conclusion, this study shows that both Burnet's clonal selection theory and the tumor evolution theory postulate that cancer cells in tissue ecosystems evolve through reiterative processes, such as clonal expansion, clonal selection, and genetic diversification. Therefore, both theories provide insights into the complexities and dynamics of cancer, including its development and progression. Finally, we take into account the occurrence of biologic variation in both tumor cells and lymphocytes. It is important to note that the presence of lymphocyte variations appears to be advantageous in the framework of tumor defense but also dangerous within the framework of autoimmune disease development.

Formation of the Intrathymic Dendritic Cell Pool Requires CCL21-Mediated Recruitment of CCR7+ Progenitors to the Thymus

During αβ T cell development in the thymus, migration of newly selected CD4⁺ and CD8⁺ thymocytes into medullary areas enables tolerance mechanisms to purge the newly selected αβ TCR repertoire of autoreactive specificities. Thymic dendritic cells (DC) play key roles in this process and consist of three distinct subsets that differ in their developmental origins. Thus, plasmacytoid DC and Sirpα⁺ conventional DC type 2 are extrathymically derived and enter into the thymus via their respective expression of the chemokine receptors CCR9 and CCR2. In contrast, although Sirpα⁻ conventional DC type 1 (cDC1) are known to arise intrathymically from immature progenitors, the precise nature of such thymus-colonizing progenitors and the mechanisms controlling their thymus entry are unclear. In this article, we report a selective reduction in thymic cDC1 in mice lacking the chemokine receptor CCR7. In addition, we show that the thymus contains a CD11c⁺MHC class II⁻Sirpα⁻Flt3⁺ cDC progenitor population that expresses CCR7, and that migration of these cells to the thymus is impaired in Ccr7^−/− mice. Moreover, thymic cDC1 defects in Ccr7^−/− mice are mirrored in plt/plt mice, with further analysis of mice individually lacking the CCR7 ligands CCL21Ser (Ccl21a^−/−) or CCL19 (Ccl19^−/−) demonstrating an essential role for CCR7-CCL21Ser during intrathymic cDC1 development. Collectively, our data support a mechanism in which CCR7-CCL21Ser interactions guide the migration of cDC progenitors to the thymus for correct formation of the intrathymic cDC1 pool.

Tuning T Cell Signaling Sensitivity Alters the Behavior of CD4+ T Cells during an Immune Response

Intricate processes in the thymus and periphery help curb the development and activation of autoreactive T cells. The subtle signals that govern these processes are an area of great interest, but tuning TCR sensitivity for the purpose of affecting T cell behavior remains technically challenging. Previously, our laboratory described the derivation of two TCR-transgenic CD4 T cell mouse lines, LLO56 and LLO118, which recognize the same cognate Listeria epitope with the same affinity. Despite the similarity of the two TCRs, LLO56 cells respond poorly in a primary infection whereas LLO118 cells respond robustly. Phenotypic examination of both lines revealed a substantial difference in their surface of expression of CD5, which serves as a dependable readout of the self-reactivity of a cell. We hypothesized that the increased interaction with self by the CD5-high LLO56 was mediated through TCR signaling, and was involved in the characteristic weak primary response of LLO56 to infection. To explore this issue, we generated an inducible knock-in mouse expressing the self-sensitizing voltage-gated sodium channel Scn5a. Overexpression of Scn5a in peripheral T cells via the CD4-Cre promoter resulted in increased TCR-proximal signaling. Further, Scn5a-expressing LLO118 cells, after transfer into BL6 recipient mice, displayed an impaired response during infection relative to wild-type LLO118 cells. In this way, we were able to demonstrate that tuning of TCR sensitivity to self can be used to alter in vivo immune responses. Overall, these studies highlight the critical relationship between TCR-self-pMHC interaction and an immune response to infection.

RelB intrinsically regulates the development and function of medullary thymic epithelial cells

Medullary thymic epithelial cells (mTECs) act as one of the major stromal components in the thymus for selection and maturation of both conventional T cells and non-conventional T cells. Extensive efforts have been spent to understand how mTEC development and function are regulated. Although RelB has been well accepted to be a critical transcriptional factor for mTEC development, the underlying mechanisms still remain largely unclear. In this study, by generating thymic epithelial cell specific RelB deficient mice, we found that epithelial intrinsic RelB is required for mTEC homeostatic proliferation. Mechanistically, RelB regulates the expression of genes involved in cell cycle. Functionally, lack of intrinsic RelB in thymic epithelial cells results in dramatically reduced population of mTECs and impaired development of thymic invariant natural killer T (iNKT) cells and intraepithelial lymphocyte precursors (IELPs). This study thus reveals an epithelial intrinsic role of RelB on mTEC development and function.

Capacity of tTreg generation is not impaired in the atrophied thymus

Postnatal thymic epithelial cell (TEC) homeostatic defect- or natural aging-induced thymic atrophy results in a decline in central T-cell tolerance establishment, which is constituted by thymocyte negative selection and cluster of differentiation (CD) 4⁺ thymic regulatory T (tTreg) cell generation. Emerging evidence shows this decline mainly results from defects in negative selection, but there is insufficient evidence regarding whether tTreg cell generation is also impaired. We mechanistically studied tTreg cell generation in the atrophied thymus by utilizing both postnatal TEC-defective (resulting from FoxN1-floxed conditional knockout [cKO]) and naturally aged mouse models. We found that the capacity of tTreg cell generation was not impaired compared to CD4⁺ thymic conventional T cells, suggesting thymic atrophy positively influences tTreg cell generation. This is potentially attributed to decreased T cell receptor (TCR) signaling strength due to inefficiency in promiscuous expression of self-antigens or presenting a neo-self-antigen by medullary TECs, displaying decreased negative selection-related marker genes (Nur77 and CD5^high) in CD4 single positive (SP) thymocytes. Our results provide evidence that the atrophied thymus attempts to balance the defective negative selection by enhancing tTreg cell generation to maintain central T-cell tolerance in the elderly. Once the balance is broken, age-related diseases could take place.

Chronic inflammation in the elderly is partially attributed to atrophy of the thymus—an organ that regulates the immune system—and in particular the ability of organisms to recognize their own cells—a phenomenon known as central tolerance. Immune central tolerance is established by two processes: first, immune cells that react strongly to self are eliminated in a process called negative selection, and second, thymic regulatory T (tTreg) cells are generated to suppress self-reactive immune reactions. The former has already been reported to be defective in the aged thymus, but whether the generation of new tTreg cells is also impaired has remained unclear. Here, we analyzed the effect of aging on tTreg cell generation and found that the atrophied thymus is still able to make new tTreg cells; indeed, we show that tTreg cell generation capacity is enhanced when compared with other naïve T cells from the same thymus. We conclude that the balance of defective negative selection with enhanced tTreg cell generation may be necessary to avoid autoimmune diseases during aging.

Induced Regulatory T Cells Superimpose Their Suppressive Capacity with Effector T Cells in Lymph Nodes via Antigen-Specific S1p1-Dependent Egress Blockage

Regulatory T cells (Tregs) restrict overexuberant lymphocyte activation. While close proximity between Tregs and their suppression targets is important for optimal inhibition, and literature indicates that draining lymph nodes (LNs) may serve as a prime location for the suppression, signaling details orchestrating this event are not fully characterized. Using a protocol to enable peripheral generation of inducible antigen-specific Tregs (asTregs) to control allergen-induced asthma, we have identified an antigen-specific mechanism that locks asTregs within hilar LNs which in turn suppresses airway inflammation. The suppressive asTregs, upon antigen stimulation in the LN, downregulate sphingosine-1-phosphate receptor 1 egress receptor expression. These asTregs in turn mediate the downregulation of the same receptor on incoming effector T cells. Therefore, asTregs and effector T cells are locked in these draining LNs for prolonged interactions. Disruption of individual steps of this retention sequence abolishes the inflammation controlled by asTregs. Collectively, this study identifies a new requirement of spatial congregation with their suppression targets essential for asTreg functions and suggests therapeutic programs via Treg traffic control.

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.

Deletional tolerance prevents AQP4-directed autoimmunity in mice

Neuromyelitis optica (NMO) is an autoimmune disorder of the central nervous system (CNS) mediated by antibodies to the water channel protein AQP4 expressed in astrocytes. The contribution of AQP4‐specific T cells to the class switch recombination of pathogenic AQP4‐specific antibodies and the inflammation of the blood–brain barrier is incompletely understood, as immunogenic naturally processed T‐cell epitopes of AQP4 are unknown. By immunizing Aqp4^−/− mice with full‐length murine AQP4 protein followed by recall with overlapping peptides, we here identify AQP4(201‐220) as the major immunogenic IA^b‐restricted epitope of AQP4. We show that WT mice do not harbor AQP4(201–220)‐specific T‐cell clones in their natural repertoire due to deletional tolerance. However, immunization with AQP4(201–220) of Rag1^−/− mice reconstituted with the mature T‐cell repertoire of Aqp4^−/− mice elicits an encephalomyelitic syndrome. Similarly to the T‐cell repertoire, the B‐cell repertoire of WT mice is “purged” of AQP4‐specific B cells, and robust serum responses to AQP4 are only mounted in Aqp4^−/− mice. While AQP4(201–220)‐specific T cells alone induce encephalomyelitis, NMO‐specific lesional patterns in the CNS and the retina only occur in the additional presence of anti‐AQP4 antibodies. Thus, failure of deletional T‐cell and B‐cell tolerance against AQP4 is a prerequisite for clinically manifest NMO.

An expanding stage for commensal microbes in host immune regulation

Gastrointestinal commensal microbiota is a concentrated mix of microbial life forms, including bacteria, fungi, archaea and viruses. These life forms are targets of host antimicrobial defense in order to establish a homeostatic symbiosis inside the host. However, they are also instrumental in shaping the functions of our immune system via a diverse set of communication mechanisms. In the gut, T helper 17, regulatory T and B cells are continuously tuned by specific microbial strains and metabolic processes. These cells in return help to establish a mutually beneficial exchange with the gut microbial contents. Imbalances in this symbiosis lead to dysregulations in the host's ability to control infections and the development of autoimmune diseases. In addition, the commensal microbiota has a significant and obligatory role in shaping both gut intrinsic and distal lymphoid organs, casting a large impact on the overall immune landscape in the host. This review discusses the major components of the microbial community in the gut and how its members collectively and individually exert regulatory roles in the host immune system and lymphoid structure development, as well as the functions of several major immune cell types.

Immune senescence: significance of the stromal microenvironment

The immune system undergoes age-associated changes known as immunosenescence, resulting in increased susceptibility to infections, cancers and autoimmunity in the aged. The basis of our understanding of immunosenescence has been derived primarily from studies examining intrinsic defects within many of the cells of the immune system. While these studies have provided insight into the mechanisms of immunosenescence, a picture is now emerging that the stromal microenvironment within lymphoid organs also contributes significantly to the age-associated decline of immune function. These extrinsic defects appear to impact the functional activity of immune cells and may offer a potential target to recover immune activity. Indeed, rejuvenation studies which have targeted the stromal niche have restored immune function in aged successfully, highlighting the impact of the microenvironment towards the aetiology of immunosenescence.

Molecular mechanism and cellular function of MHCII ubiquitination

The major histocompatibility complex class II (MHCII) is ubiquitinated via the evolutionarily conserved lysine in the cytoplasmic tail of the β chain in dendritic cells (DCs) and B cells. The ubiquitination is mediated by the membrane-associated RING-CH1 (MARCH1) ubiquitin ligase although it can be also mediated by the homologous ligase MARCH8 in model cell lines. The ubiquitination promotes MHCII endocytosis and lysosomal sorting that results in a reduction in the level of MHCII at cell surface. Functionally, MHCII ubiquitination serves as a means by which DCs suppress MHCII expression and reduce antigen presentation in response to the immune regulatory cytokine interleukin-10 (IL-10) and regulatory T cells. Recently, additional roles of MHCII ubiquitination have emerged. MHCII ubiquitination promoted DC production of inflammatory cytokines in response to the Toll-like receptor ligands. It also potentiated DC ability to activate antigen-specific naive CD4(+) T cells while limiting the amount of antigens presented at cell surface. Similarly, MHCII ubiquitination promoted DC activation of CD4(+) thymocytes supporting regulatory T-cell development independent of its effect of limiting antigen presentation. Thus, ubiquitination appears to confer MHCII a function independent of presenting antigens by a mechanism yet to be identified.

Characterization of the inflammatory cell infiltrate and expression of costimulatory molecules in chronic echinococcus granulosus infection of the human liver

Background

The local immune responses to chronic echinococcal infections in various organs are largely unknown. Since the liver is the most frequently involved organ in such infections in human we aimed to characterize the inflammatory as well as immune cell infiltrate around hydatid cysts in the liver and compared to common inflammatory processes of the liver.

Method

Surgical samples from the liver of 21 cystic echinococcosis (CE) patients were studied and the distribution of different types of inflammatory and immune cells were determined by immunohistochemistry. Furthermore, expression levels of costimulatory CTLA4, CD28, CD80 and CD86 molecules were measured at RNA level by PCR. Liver biopsy samples from patients with steatohepatitis (SH, n = 11) and chronic hepatitis (CH, n = 11) were used as non-inflammatory and chronic inflammatory controls, respectively. The composition and density of the inflammatory and immune cell infiltrates have been compared by using morphometry.

Results

CD3+ T cells predominated the inflammatory infiltrate in all pathological processes, while in CE samples CD20+ B cells, in CH samples CD68+ macrophages were also frequent. Both myeloperoxidase (MPO) + leukocytes and CD68+ macrophages were found to be significantly decreased in CE as compared to either SH or CH samples. Concerning T cell subtypes, only CD8+ T cells were found to be significantly decreased in SH samples. CD1a + dendritic cells were almost completely missing from CE biopsies unlike in any other sample types. There were no differences detected in the mRNA expression of costimulatory molecules except decreased expression of CD28 in CE samples.

Conclusion

In the hydatid lesions of the liver of chronic echinococcal infections T cell-mediated immunity seems to be impaired as compared to other types of chronic inflammatory processes, suggesting an immunosuppressive role for Echinococcus granulosus, which deserve further attentions.

CD4+ T Cell Tolerance to Tissue-Restricted Self Antigens Is Mediated by Antigen-Specific Regulatory T Cells Rather Than Deletion

Deletion of self-antigen-specific T cells during thymic development provides protection from autoimmunity. However, it is unclear how efficiently this occurs for tissue-restricted self antigens, or how immune tolerance is maintained for self-antigen-specific T cells that routinely escape deletion. Here we show that endogenous CD4+ T cells with specificity for a set of tissue-restricted self antigens were not deleted at all. For pancreatic self antigen, this resulted in an absence of steady-state tolerance, while for the lung and intestine, tolerance was maintained by the enhanced presence of thymically-derived antigen-specific Foxp3+ regulatory T (Treg) cells. Unlike deletional tolerance, Treg cell-mediated tolerance was broken by successive antigen challenges. These findings reveal that for some tissue-restricted self antigens, tolerance relies entirely on nondeletional mechanisms that are less durable than T cell deletion. This might explain why autoimmunity is often tissue-specific, and it offers a rationale for cancer vaccine strategies targeting tissue-restricted tumor antigens.

Forging T-Lymphocyte Identity: Intersecting Networks of Transcriptional Control

T-lymphocyte development branches off from other lymphoid developmental programs through its requirement for sustained environmental signals through the Notch pathway. In the thymus, Notch signaling induces a succession of T-lineage regulatory factors that collectively create the T-cell identity through distinct steps. This process involves both the staged activation of T-cell identity genes and the staged repression of progenitor-cell-inherited regulatory genes once their roles in self-renewal and population expansion are no longer needed. With the recent characterization of innate lymphoid cells (ILCs) that share transcriptional regulation programs extensively with T-cell subsets, T-cell identity can increasingly be seen as defined in modular terms, as the processes selecting and actuating effector function are potentially detachable from the processes generating and selecting clonally unique T-cell receptor structures. The developmental pathways of different classes of T cells and ILCs are distinguished by the numbers of prerequisites of gene rearrangement, selection, and antigen contact before the cells gain access to nearly common regulatory mechanisms for choosing effector function. Here, the major classes of transcription factors that interact with Notch signals during T-lineage specification are discussed in terms of their roles in these programs, the evidence for their spectra of target genes at different stages, and their cross-regulatory and cooperative actions with each other. Specific topics include Notch modulation of PU.1 and GATA-3, PU.1-Notch competition, the relationship between PU.1 and GATA-3, and the roles of E proteins, Bcl11b, and GATA-3 in guiding acquisition of T-cell identity while avoiding redirection to an ILC fate.

The Role of Dendritic Cells in Central Tolerance

Dendritic cells (DCs) play a significant role in establishing self-tolerance through their ability to present self-antigens to developing T cells in the thymus. DCs are predominantly localized in the medullary region of thymus and present a broad range of self-antigens, which include tissue-restricted antigens expressed and transferred from medullary thymic epithelial cells, circulating antigens directly captured by thymic DCs through coticomedullary junction blood vessels, and peripheral tissue antigens captured and transported by peripheral tissue DCs homing to the thymus. When antigen-presenting DCs make a high affinity interaction with antigen-specific thymocytes, this interaction drives the interacting thymocytes to death, a process often referred to as negative selection, which fundamentally blocks the self-reactive thymocytes from differentiating into mature T cells. Alternatively, the interacting thymocytes differentiate into the regulatory T (Treg) cells, a distinct T cell subset with potent immune suppressive activities. The specific mechanisms by which thymic DCs differentiate Treg cells have been proposed by several laboratories. Here, we review the literatures that elucidate the contribution of thymic DCs to negative selection and Treg cell differentiation, and discusses its potential mechanisms and future directions.

Thymic involution perturbs negative selection leading to autoreactive T cells that induce chronic inflammation

Thymic involution and the subsequent amplified release of autoreactive T cells increase the susceptibility toward developing autoimmunity, but whether they induce chronic inflammation with advanced age remains unclear. The presence of chronic low-level proinflammatory factors in elderly individuals (termed inflammaging) is a significant risk factor for morbidity and mortality in virtually every chronic age-related disease. To determine how thymic involution leads to the persistent release and activation of autoreactive T cells capable of inducing inflammaging, we used a Foxn1 conditional knockout mouse model that induces accelerated thymic involution while maintaining a young periphery. We found that thymic involution leads to T cell activation shortly after thymic egress, which is accompanied by a chronic inflammatory phenotype consisting of cellular infiltration into non-lymphoid tissues, increased TNF-α production, and elevated serum IL-6. Autoreactive T cell clones were detected in the periphery of Foxn1 conditional knockout mice. A failure of negative selection, facilitated by decreased expression of Aire rather than impaired regulatory T cell generation, led to autoreactive T cell generation. Furthermore, the young environment can reverse age-related regulatory T cell accumulation in naturally aged mice, but not inflammatory infiltration. Taken together, these findings identify thymic involution and the persistent activation of autoreactive T cells as a contributing source of chronic inflammation (inflammaging).

Inhibins tune the thymocyte selection process by regulating thymic stromal cell differentiation

Inhibins and Activins are members of the TGF-β superfamily that regulate the differentiation of several cell types. These ligands were initially identified as hormones that regulate the hypothalamus-pituitary-gonadal axis; however, increasing evidence has demonstrated that they are key regulators in the immune system. We have previously demonstrated that Inhibins are the main Activin ligands expressed in the murine thymus and that they regulate thymocyte differentiation, promoting the DN3-DN4 transition and the selection of SP thymocytes. As Inhibins are mainly produced by thymic stromal cells, which also express Activin receptors and Smad proteins, we hypothesized that Inhibins might play a role in stromal cell differentiation and function. Here, we demonstrate that, in the absence of Inhibins, thymic conventional dendritic cells display reduced levels of MHC Class II (MHCII) and CD86. In addition, the ratio between cTECs and mTECs was affected, indicating that mTEC differentiation was favoured and cTEC diminished in the absence of Inhibins. These changes appeared to impact thymocyte selection leading to a decreased selection of CD4SP thymocytes and increased generation of natural regulatory T cells. These findings demonstrate that Inhibins tune the T cell selection process by regulating both thymocyte and stromal cell differentiation.

Defective selection of thymic regulatory T cells accompanies autoimmunity and pulmonary infiltrates in Tcra-deficient mice double transgenic for human La/Sjögren's syndrome-B and human La-specific TCR

A human La/Sjögren's syndrome-B (hLa)-specific TCR/hLa neo-self-Ag double-transgenic (Tg) mouse model was developed and used to investigate cellular tolerance and autoimmunity to the ubiquitous RNA-binding La Ag often targeted in systemic lupus erythematosus and Sjögren's syndrome. Extensive thymic clonal deletion of CD4(+) T cells occurred in H-2(k/k) double-Tg mice presenting high levels of the I-E(k)-restricted hLa T cell epitope. In contrast, deletion was less extensive in H-2(k/b) double-Tg mice presenting lower levels of the epitope, and some surviving thymocytes were positively selected as thymic regulatory T cells (tTreg). These mice remained serologically tolerant to hLa and healthy. H-2(k/b) double-Tg mice deficient of all endogenous Tcra genes, a deficiency known to impair Treg development and function, produced IgG anti-hLa autoantibodies and displayed defective tTreg development. These autoimmune mice had interstitial lung disease characterized by lymphocytic aggregates containing Tg T cells with an activated, effector memory phenotype. Salivary gland infiltrates were notably absent. Thus, expression of nuclear hLa Ag induces thymic clonal deletion and tTreg selection, and lymphocytic infiltration of the lung is a consequence of La-specific CD4(+) T cell autoimmunity.

The deubiquitinating enzyme CYLD regulates the differentiation and maturation of thymic medullary epithelial cells

The cross talk between thymocytes and the thymic epithelium is critical for T-cell development and the establishment of central tolerance. Medullary thymic epithelial cells (mTECs) are located in the thymic medulla and mediate the elimination of self-reactive thymocytes, thereby preventing the onset of autoimmunity. Previous studies identified the deubiquitinating enzyme CYLD as a critical regulator of T-cell development by activating proximal T-cell receptor signaling during the transition of double-positive to single-positive thymocytes. Here we evaluated the impact of the naturally occurring short-splice variant of the cyld gene (sCYLD) on the development and maturation of mTECs. We found that thymi of CYLD(ex7/8) mice, solely expressing sCYLD, displayed a reduced number of mature mTECs caused by a developmental block during the transition of immature to mature mTECs. Further, we could demonstrate an impaired negative selection of thymocytes in these mice. Our data demonstrate that inefficient negative selection in the thymus of CYLD(ex7/8) mice result from a defect in mTEC maturation.

The interleukin-15 system suppresses T cell-mediated autoimmunity by regulating negative selection and nT(H)17 cell homeostasis in the thymus

The interleukin-15 (IL-15) system is important for regulating both innate and adaptive immune responses, however, its role in autoimmune disease remained unclear. Here we found that Il15(-/-) and Il15ra(-/-) mice spontaneously developed late-onset autoimmune phenotypes. CD4(+) T cells of the knockout mice showed elevated autoreactivity as demonstrated by the induction of lymphocyte infiltration in the lacrimal and salivary glands when transferred into nude mice. The antigen-presenting cells in the thymic medullary regions expressed IL-15 and IL-15Rα, whose deficiency resulted in insufficient negative selection and elevated number of natural IL-17A-producing CD4(+) thymocytes. These findings reveal previously unknown functions of the IL-15 system in thymocyte development, and thus a new layer of regulation in T cell-mediated autoimmunity.

Molecular mimicry and clonal deletion: A fresh look

In this article, I trace the historic background of clonal deletion and molecular mimicry, two major pillars underlying our present understanding of autoimmunity and autoimmune disease. Clonal deletion originated as a critical element of the clonal selection theory of antibody formation in order to explain tolerance of self. If we did have complete clonal deletion, there would be major voids, the infamous "black holes", in our immune repertoire. For comprehensive, protective adaptive immunity, full deletion is necessarily a rare event. Molecular mimicry, the sharing of epitopes among self and non-self antigens, is extraordinary common and provides the evidence that complete deletion of self-reactive clones is rare. If molecular mimicry were not common, protective adaptive immunity could not be all-encompassing. By taking a fresh look at these two processes together we can envision their evolutionary basis and understand the need for regulatory devices to prevent molecular mimicry from progressing to autoimmune disease.

Positive and negative selection of the T cell repertoire: what thymocytes see (and don't see)

The fate of developing T cells is specified by the interaction of their antigen receptors with self-peptide-MHC complexes that are displayed by thymic antigen-presenting cells (APCs). Various subsets of thymic APCs are strategically positioned in particular thymic microenvironments and they coordinate the selection of a functional and self-tolerant T cell repertoire. In this Review, we discuss the different strategies that these APCs use to sample and process self antigens and to thereby generate partly unique, 'idiosyncratic' peptide-MHC ligandomes. We discuss how the particular composition of the peptide-MHC ligandomes that are presented by specific APC subsets not only shapes the T cell repertoire in the thymus but may also indelibly imprint the behaviour of mature T cells in the periphery.

The chromatin landscape and transcription factors in T cell programming

T cell development from multipotent progenitors to specialized effector subsets of mature T cells is guided by the iterative action of transcription factors. At each stage, transcription factors interact not only with an existing landscape of histone modifications and nucleosome packing, but also with other bound factors, while they modify the landscape for later-arriving factors in ways that fundamentally affect the control of gene expression. This review covers insights from genome-wide analyses of transcription factor binding and resulting chromatin conformation changes that reveal roles of cytokine signaling in effector T cell programming, the ways in which one factor can completely transform the impacts of previously bound factors, and the ways in which the baseline chromatin landscape is established during early T cell lineage commitment.

Transcriptional control of early T and B cell developmental choices

T and B cells share a common somatic gene rearrangement mechanism for assembling the genes that code for their antigen receptors; they also have developmental pathways with many parallels. Shared usage of basic helix-loop-helix E proteins as transcriptional drivers underlies these common features. However, the transcription factor networks in which these E proteins are embedded are different both in membership and in architecture for T and B cell gene regulatory programs. These differences permit lineage commitment decisions to be made in different hierarchical orders. Furthermore, in contrast to B cell gene networks, the T cell gene network architecture for effector differentiation is sufficiently modular so that E protein inputs can be removed. Complete T cell-like effector differentiation can proceed without T cell receptor rearrangement or selection when E proteins are neutralized, yielding natural killer and other innate lymphoid cells.

Insufficient autoantigen presentation and failure of tolerance in a mouse model of rheumatoid arthritis

OBJECTIVE

In the K/BxN mouse model of rheumatoid arthritis, T cells reactive for the self antigen glucose-6-phosphate isomerase (GPI) escape negative selection even though GPI expression is ubiquitous. We sought to determine whether insufficient GPI presentation could account for the failure of negative selection and for the development of arthritis.

METHODS

To increase the antigen presentation of GPI, we generated transgenic mice expressing a membrane-bound form of GPI (mGPI) and crossed them with K/BxN mice. A monoclonal antibody specific for the α-chain of the KRN T cell receptor was generated to examine the fate of GPI-specific T cells.

RESULTS

The mGPI-transgenic mice presented GPI more efficiently and showed a dramatic increase in negative selection and an inhibition of arthritis. Interestingly, thymic negative selection remained incomplete in these mice, and the escaped autoreactive T cells were anergic in the peripheral lymphoid organs, suggesting that enhanced antigen presentation also induces peripheral tolerance. Despite this apparent tolerance induction toward GPI, these mice developed a chronic wasting disease, characterized by colonic inflammation with epithelial dysplasia, as well as a dramatic reduction in Treg cells.

CONCLUSION

These data indicate that insufficient autoantigen expression or presentation results in defects of both central and peripheral tolerance in the K/BxN mice. Our findings also support the idea that insufficient autoantigen levels may underlie the development of autoimmunity.

Development of Gene Transfer for Induction of Antigen-specific Tolerance

Gene replacement therapies, like organ and cell transplantation are likely to introduce neo-antigens that elicit rejection via humoral and/or effector T cell immune responses. Nonetheless, thanks to an ever growing body of pre-clinical studies it is now well accepted that gene transfer protocols can be specifically designed and optimized for induction of antigen-specific immune tolerance. One approach is to specifically express a gene in a tissue with a tolerogenic microenvironment such as the liver or thymus. Another strategy is to transfer a particular gene into hematopoietic stem cells or immunological precursor cells thus educating the immune system to recognize the therapeutic protein as "self". In addition, expression of the therapeutic protein in pro-tolerogenic antigen presenting cells such as immature dendritic cells and B cells has proven to be promising. All three approaches have successfully prevented unwanted immune responses in pre-clinical studies aimed at the treatment of inherited protein deficiencies, e.g. lysosomal storage disorders and hemophilia, and of type I diabetes and multiple sclerosis. In this review we focus on current gene transfer protocols that induce tolerance, including gene delivery vehicles and target tissues, and discuss successes and obstacles in different disease models.

The critical role of STIM1-dependent Ca2+ signalling during T-cell development and activation

T lymphocytes are key cellular effectors of adaptive immunity able to recognize a virtually limitless number of antigenic peptides and mount an immune response. Ca(2+) signals are crucial to the development and activation of T cells and Stromal Interaction Molecule 1 (STIM1) has been identified as a critical modulator of intracellular Ca(2+) levels in T cells. Although the role of STIM1 in T cell activation has been extensively investigated, the role of STIM1 in T cell development has been somewhat controversial. Indeed, deficiencies in STIM1 expression and function lead to both developmental defects associated with the development of autoimmunity yet also interfere with T cell activation leading to severe combined immunodeficiency signifying a multifaceted role of STIM1 in T cell physiology and pathophysiology.

The aryl hydrocarbon receptor and food allergy

The immune system is important for protection against pathogens and malignant cells. However, malfunction of the immune system can also result in detrimental auto-immune diseases, inflammatory diseases, cancers and allergies. The aryl hydrocarbon receptor (AhR), present in numerous tissues and cell subsets, including cells of the immune system, plays an important role in the functioning of the immune system. Activation of the AhR is for example associated with various effects on dendritic cells (DCs), regulatory T cells and the Th1/Th2 cell balance. These cells play a major role in the development of food allergy. Food allergy is an increasing health problem in both humans and animals. Despite the knowledge in risk factors and cellular mechanisms for food allergy, no approved treatments are available yet. Recently, it has been shown that activation of the AhR by dioxin-like compounds suppresses allergic sensitization by suppressing the absolute number of precursor and effector T cells, by preserving CD4(+)CD25(+)Foxp3(+) Treg cells and by affecting DCs and their interaction with effector T cells. Future research should elucidate whether and how AhR activation can be used to interfere in food allergic responses in humans and in animals. This may lead to new prevention strategies and therapeutic possibilities for food allergy.

MARCH1-mediated MHCII ubiquitination promotes dendritic cell selection of natural regulatory T cells

Ubiquitination of MHCII molecules on dendritic cells is essential for the development of natural regulatory T cells

Membrane-associated RING-CH1 (MARCH1) is an E3 ubiquitin ligase that mediates ubiquitination of MHCII in dendritic cells (DCs). MARCH1-mediated MHCII ubiquitination in DCs is known to regulate MHCII surface expression, thereby controlling DC-mediated T cell activation in vitro. However, its role at steady state or in vivo is not clearly understood. Here, we show that MARCH1 deficiency resulted in a substantial reduction in the number of thymus-derived regulatory T cells (T reg cells) in mice. A specific ablation of MHCII ubiquitination also significantly reduced the number of thymic T reg cells. Indeed, DCs deficient in MARCH1 or MHCII ubiquitination both failed to generate antigen-specific T reg cells in vivo and in vitro, although both exhibited an increased capacity for antigen presentation in parallel with the increased surface MHCII. Thus, MARCH1-mediated MHCII ubiquitination in DCs is required for proper production of naturally occurring T reg cells, suggesting a role in balancing immunogenic and regulatory T cell development.

Th17 cell development: from the cradle to the grave

T cells surviving the clonal selection process emigrate from the thymus to the periphery as immature naive T cells. In the periphery, upon activation under specific cytokine milieus, naive T cells adopt specific effector phenotypes, e.g. T-helper 1 (Th1), Th2, or Th17, and acquire diverse functions to control a myriad of pathogens, tissue injuries, and other immunological insults. Interleukin-23 (IL-23) is one of the key cytokines that shapes the development and function of Th17 cells with characteristic expression of retinoic acid receptor-related orphan receptor γ-t (RORγt), IL-17, IL-22, and granulocyte macrophage colony-stimulating factor (GM-CSF). More recently, emerging data suggest that IL-23 also promotes development of 'natural Th17' (nTh17) cells that arise from the thymus, analogous to natural regulatory T cells (nTreg). We are just beginning to understand the unique thymic developmental path of nTh17 cells, which are distinct from antigen-experienced memory Th17 cells. In this review, we explore the differentiation and function of inducible, natural, and memory Th17 subsets, which encompass a broad range of immune functions while maintaining tissue hemostasis, and highlight the participation of IL-23 during the life cycle of Th17 cells.