Cellular and molecular signaling towards T cell immunological self-tolerance.

The binding of a cognate antigen to T cell receptor (TCR) complex triggers a series of intracellular events controlling T cell activation, proliferation, and differentiation. Upon TCR engagement, different negative regulatory feedback mechanisms are rapidly activated to counterbalance T cell activation, thus preventing excessive signal propagation and promoting the induction of immunological self-tolerance. Both positive and negative regulatory processes are tightly controlled to ensure the effective elimination of foreign antigens while limiting surrounding tissue damage and autoimmunity. In this context, signals deriving from co-stimulatory molecules (i.e., CD80, CD86), co-inhibitory receptors (PD-1, CTLA-4), the tyrosine phosphatase CD45 and cytokines such as IL-2 synergize with TCR-derived signals to guide T cell fate and differentiation. The balance of these mechanisms is also crucial for the generation of CD4+ Foxp3+ regulatory T cells, a cellular subset involved in the control of immunological self-tolerance. This review provides an overview of the most relevant pathways induced by TCR activation combined with those derived from co-stimulatory and co-inhibitory molecules implicated in the cell-intrinsic modulation of T cell activation. In addition to the latter, we dissected mechanisms responsible for T cell-mediated suppression of immune cell activation through regulatory T cell generation, homeostasis, and effector functions. We also discuss how imbalanced signaling derived from TCR and accessory molecules can contribute to autoimmune disease pathogenesis.

Thymic mimetic cells function beyond self-tolerance.

Development of immunocompetent T cells in the thymus is required for effective defence against all types of pathogens, including viruses, bacteria and fungi. To this end, T cells undergo a very strict educational program in the thymus, during which both non-functional and self-reactive T cell clones are eliminated by means of positive and negative selection1.Thymic epithelial cells (TECs) have an indispensable role in these processes, and previous studies have shown the notable heterogeneity of these cells2-7. Here, using multiomic analysis, we provide further insights into the functional and developmental diversity of TECs in mice, and reveal a detailed atlas of the TEC compartment according to cell transcriptional states and chromatin landscapes. Our analysis highlights unconventional TEC subsets that are similar to functionally well-defined parenchymal populations, including endocrine cells, microfold cells and myocytes. By focusing on the endocrine and microfold TEC populations, we show that endocrine TECs require Insm1 for their development and are crucial to maintaining thymus cellularity in a ghrelin-dependent manner; by contrast, microfold TECs require Spib for their development and are essential for the generation of thymic IgA+ plasma cells. Collectively, our study reveals that medullary TECs have the potential to differentiate into various types of molecularly distinct and functionally defined cells, which not only contribute to the induction of central tolerance, but also regulate the homeostasis of other thymus-resident populations.

Redox regulation of age-associated defects in generation and maintenance of T cell self-tolerance and immunity to foreign antigens.

Thymic atrophy reduces naive T cell production and contributes to increased susceptibility to viral infection with age. Expression of tissue-restricted antigen (TRA) genes also declines with age and has been thought to increase autoimmune disease susceptibility. We find that diminished expression of a model TRA gene in aged thymic stromal cells correlates with impaired clonal deletion of cognate T cells recognizing an autoantigen involved in atherosclerosis. Clonal deletion in the polyclonal thymocyte population is also perturbed. Distinct age-associated defects in the generation of antigen-specific T cells include a conspicuous decline in generation of T cells recognizing an immunodominant influenza epitope. Increased catalase activity delays thymic atrophy, and here, we show that it mitigates declining production of influenza-specific T cells and their frequency in lung after infection, but does not reverse declines in TRA expression or efficient negative selection. These results reveal important considerations for strategies to restore thymic function.

Self-Centered Function of Adaptive Immunity in Regulation of Immune Responses and in Tolerance.

The search for common mechanisms underlying the pathogenesis of chronic inflammatory conditions has crystalized the concept of continuous dual resetting of the immune repertoire (CDR) as a basic principle of the immune system function. Consequently, outlined was the first dynamic comprehensive picture of the immune system function. The goal of this study is to elaborate on regulation of immune responses and mechanisms of tolerance, particularly focusing on adaptive immunity. It is well established that the T/B cell repertoire is selected and maintained based on interactions with self. However, their activation also requires interaction with a self-specific major histocompatibility complex (MHC) "code," i.e., the context of MHC molecules. Therefore, not only repertoire selection and maintenance but also the T/B cell activation and function are self-centered. Thus, adaptive effectors may be primarily focused on the state of self and maintenance of integrity of the self, and only to a certain degree on elimination of the foreign. As examples of such function are used immunologically poorly understood MHC-disparate settings typical for transplantation and pregnancy. Transplantation represents an extreme setting of strong systemic compartment-level adaptive/MHC-restricted immune responses. Described are clinically identified conditions for operational tolerance of MHC-disparate tissues/living systems in allotransplantation, which are in line with the CDR-proposed self-centered regulatory role of T/B cells. In contrast, normal pregnancy is coexistence of semiallogeneic or entirely allogeneic mother and fetus, but without alloreactivity akin to transplantation settings. Presented data support the notion that maintenance of pregnancy is a process that relies predominantly on innate/MHC-independent immune mechanisms. By the inception of hemotrophic stage of pregnancy (second and third trimester), both mother and child are individual living systems, with established adaptive immune repertoires. Although mother-fetus interactions at that point become indirect systemic compartment-level communications, their interactions throughout gestation remain within the innate realm of molecular-level adaptations.

Self-mediated positive selection of T cells sets an obstacle to the recognition of nonself.

Adaptive immune recognition is mediated by the binding of peptide-human leukocyte antigen complexes by T cells. Positive selection of T cells in the thymus is a fundamental step in the generation of a responding T cell repertoire: only those T cells survive that recognize human peptides presented on the surface of cortical thymic epithelial cells. We propose that while this step is essential for optimal immune function, the process results in a defective T cell repertoire because it is mediated by self-peptides. To test our hypothesis, we focused on amino acid motifs of peptides in contact with T cell receptors. We found that motifs rarely or not found in the human proteome are unlikely to be recognized by the immune system just like the ones that are not expressed in cortical thymic epithelial cells or not presented on their surface. Peptides carrying such motifs were especially dissimilar to human proteins. Importantly, we present our main findings on two independent T cell activation datasets and directly demonstrate the absence of naïve T cells in the repertoire of healthy individuals. We also show that T cell cross-reactivity is unable to compensate for the absence of positively selected T cells. Additionally, we show that the proposed mechanism could influence the risk for different infectious diseases. In sum, our results suggest a side effect of T cell positive selection, which could explain the nonresponsiveness to many nonself peptides and could improve the understanding of adaptive immune recognition.

Preclinical models of arthritis for studying immunotherapy and immune tolerance.

Increasingly earlier identification of individuals at high risk of rheumatoid arthritis (RA) (eg, with autoantibodies and mild symptoms) improves the feasibility of preventing or curing disease. The use of antigen-specific immunotherapies to reinstate immunological self-tolerance represent a highly attractive strategy due to their potential to induce disease resolution, in contrast to existing approaches that require long-term treatment of underlying symptoms.Preclinical animal models have been used to understand disease mechanisms and to evaluate novel immunotherapeutic approaches. However, models are required to understand critical processes supporting disease development such as the breach of self-tolerance that triggers autoimmunity and the progression from asymptomatic autoimmunity to joint pain and bone loss. These models would also be useful in evaluating the response to treatment in the pre-RA period.This review proposes that focusing on immune processes contributing to initial disease induction rather than end-stage pathological consequences is essential to allow development and evaluation of novel immunotherapies for early intervention. We will describe and critique existing models in arthritis and the broader field of autoimmunity that may fulfil these criteria. We will also identify key gaps in our ability to study these processes in animal models, to highlight where further research should be targeted.

Strength and Numbers: The Role of Affinity and Avidity in the 'Quality' of T Cell Tolerance.

The ability of T cells to identify foreign antigens and mount an efficient immune response while limiting activation upon recognition of self and self-associated peptides is critical. Multiple tolerance mechanisms work in concert to prevent the generation and activation of self-reactive T cells. T cell tolerance is tightly regulated, as defects in these processes can lead to devastating disease; a wide variety of autoimmune diseases and, more recently, adverse immune-related events associated with checkpoint blockade immunotherapy have been linked to a breakdown in T cell tolerance. The quantity and quality of antigen receptor signaling depend on a variety of parameters that include T cell receptor affinity and avidity for peptide. Autoreactive T cell fate choices (e.g., deletion, anergy, regulatory T cell development) are highly dependent on the strength of T cell receptor interactions with self-peptide. However, less is known about how differences in the strength of T cell receptor signaling during differentiation influences the 'function' and persistence of anergic and regulatory T cell populations. Here, we review the literature on this subject and discuss the clinical implications of how T cell receptor signal strength influences the 'quality' of anergic and regulatory T cell populations.

Molecular mechanisms of nonself nucleic acid recognition by the innate immune system.

Nucleic acids (NAs) represent one of the most important classes of molecules recognized by the innate immune system. However, NAs are not limited to pathogens, but are also present within the host. As such, the immune system has evolved an elaborate set of pathogen recognition receptors (PRRs) that employ various strategies to recognize distinct types of NAs, while reliably distinguishing between self and nonself. The here-employed strategies encompass the positioning of NA-sensing PRRs in certain subcellular compartments that potentially come in contact with pathogens but not host NAs, the existence of counterregulatory measures that keep endogenous NAs below a certain threshold, and also the specific identification of certain nonself patterns. Here, we review recent advances in the molecular mechanisms of NA recognition by TLRs, RLRs, and the cGAS-STING axis. We highlight the differences in NA-PRR interfaces that confer specificity and selectivity toward an NA ligand, as well as the NA-dependent induced conformational changes required for signal transduction.

Adapting T Cell Receptor Ligand Discrimination Capability via LAT.

Self- and non-self ligand discrimination is a core principle underlying T cell-mediated immunity. Mature αß T cells can respond to a foreign peptide ligand presented by major histocompatibility complex molecules (pMHCs) on antigen presenting cells, on a background of continuously sensed self-pMHCs. How αß T cells can properly balance high sensitivity and high specificity to foreign pMHCs, while surrounded by a sea of self-peptide ligands is not well understood. Such discrimination cannot be explained solely by the affinity parameters of T cell antigen receptor (TCR) and pMHC interaction. In this review, we will discuss how T cell ligand discrimination may be molecularly defined by events downstream of the TCR-pMHC interaction. We will discuss new evidence in support of the kinetic proofreading model of TCR ligand discrimination, and in particular how the kinetics of specific phosphorylation sites within the adaptor protein linker for activation of T cells (LAT) determine the outcome of TCR signaling. In addition, we will discuss emerging data regarding how some kinases, including ZAP-70 and LCK, may possess scaffolding functions to more efficiently direct their kinase activities.

Taking regulatory T cells into medicine.

Regulatory T cells (Tregs) are indispensable for the establishment and maintenance of immunological self-tolerance. Their genetic anomalies or variations in function are causative of various monogenic and polygenic autoimmune diseases. Treg-based reestablishment of self-tolerance is envisioned to cure autoimmune diseases in the clinic.

Distinct Foxp3 enhancer elements coordinate development, maintenance, and function of regulatory T cells.

The transcription factor Foxp3 plays crucial roles for Treg cell development and function. Conserved non-coding sequences (CNSs) at the Foxp3 locus control Foxp3 transcription, but how they developmentally contribute to Treg cell lineage specification remains obscure. Here, we show that among Foxp3 CNSs, the promoter-upstream CNS0 and the intergenic CNS3, which bind distinct transcription factors, were activated at early stages of thymocyte differentiation prior to Foxp3 promoter activation, with sequential genomic looping bridging these regions and the promoter. While deletion of either CNS0 or CNS3 partially compromised thymic Treg cell generation, deletion of both completely abrogated the generation and impaired the stability of Foxp3 expression in residual Treg cells. As a result, CNS0 and CNS3 double-deleted mice succumbed to lethal systemic autoimmunity and inflammation. Thus, hierarchical and coordinated activation of Foxp3 CNS0 and CNS3 initiates and stabilizes Foxp3 gene expression, thereby crucially controlling Treg cell development, maintenance, and consequently immunological self-tolerance.

The historical postulate: Is it the basis, at the level of the system, for self-nonself discrimination?

Burnet envisaged the early presence of self-antigens in development, or 'the history' of an animal, ablates the animal's ability to immunologically respond against them. Lederberg added the idea that the continuous presence of self-antigens is required to maintain tolerance throughout life. We refer to Lederberg's proposal as 'The Historical Postulate'. The mechanism of central tolerance, as now understood, is consistent with The Historical Postulate. Some observations, reflecting peripheral tolerance, appear inconsistent with this postulate. For example, some foreign peripheral tissues, grafted onto an animal before the immune system develops, can be rejected as the immune system matures. The original two-signal model of lymphocyte activation was proposed in part because it accounted for peripheral tolerance in a manner consistent with The Historical Postulate. We proposed that lymphocyte activation required antigen-mediated lymphocyte cooperation, whereas antigen would inactivate lymphocytes when insufficient in number to achieve activation. We argue here that the exceptions to The Historical Postulate can be explained by the two-signal model of lymphocyte activation: they reflect the existence of greater numbers of lymphocytes specific for these antigens than for natural peripheral antigens, and so are outside the physiological limits important in selecting through evolution this mechanism of peripheral tolerance. We argue that a consideration of whether The Historical Postulate is valid is important, even if only valid within certain understandable limits. The currently popular DAMP model of CD4 T cell activation is, strictly speaking and in a manner we discuss, in violation of this postulate.

HLA genotyping meets response to immune checkpoint inhibitors prediction: A story just started.

The implementation of the immune checkpoint blockade as a therapeutic option in contemporary oncology is one of the significant immunological achievements in the last century. Constantly accumulating evidence suggests that the response to immune checkpoint inhibitors (ICIs) is not universal. Therefore, it is critical to identify determinants for response, resistance and adverse effects of immune checkpoint therapy that could be developed as prognostic and predictive markers. Recent large scale analyses of cancer genome data revealed the key role of HLA class I and class II molecules in cancer immunoediting, and it appears that HLA diversity can predict response to ICIs. In the present review, we summarize the emerging data on the role of HLA germline variations as a marker for response to ICIs.

Tregs self-organize into a computing ecosystem and implement a sophisticated optimization algorithm for mediating immune response.

Regulatory T cells (Tregs) play a crucial role in mediating immune response. Yet an algorithmic understanding of the role of Tregs in adaptive immunity remains lacking. Here, we present a biophysically realistic model of Treg-mediated self-tolerance in which Tregs bind to self-antigens and locally inhibit the proliferation of nearby activated T cells. By exploiting a duality between ecological dynamics and constrained optimization, we show that Tregs tile the potential antigen space while simultaneously minimizing the overlap between Treg activation profiles. We find that for sufficiently high Treg diversity, Treg-mediated self-tolerance is robust to fluctuations in self-antigen concentrations but lowering the Treg diversity results in a sharp transition-related to the Gardner transition in perceptrons-to a regime where changes in self-antigen concentrations can result in an autoimmune response. We propose an experimental test of this transition in immune-deficient mice and discuss potential implications for autoimmune diseases.

Influence of Innate Immunity on Immune Tolerance.

This review mainly focuses on the mechanisms of peripheral immune tolerance within the perspectives of innate immunity. Healthy immune response requires balanced interaction of the highly specialized elements of immunity within a harmony. Innate immunity supported by microbial pattern recognition receptors, physical anatomical barriers and soluble effectors stands as the first line of defense against non-self-antigens. Innate receptors recognize major classes of pathogens and trigger immediate immune/inflammatory responses. The decisive action has been the key issue in skewing of immune reactivity to a pathogen or to tolerate self- and non-self-antigens. Non-responsiveness to self- or to harmless foreign antigens with means of multiple mechanisms is known as immune tolerance; a non-inflammatory, non-proliferative and suppressive response linked to suppressor molecules as CTLA-4 and cytokines like IL-10, TGF-ß and IL-35, and also to non-inflammatory blocking antibody isotypes as IgG4. Regulatory cells ascertain both induction and maintenance of peripheral tolerance. Allergic diseases, autoimmunity and transplant rejection are the best illustrations of immune tolerance loss. Adaptive immunity responsible for both establishment and maintenance of a long-lasting immune responsiveness is mainly fine-tuned by actions of innate immunity. Better understanding of the relationship between innate immunity and immune tolerance is a prerequisite both for better understanding of pathogenesis of tolerance-related diseases and also for development of novel therapeutic options. CONCLUSION: Recent evidences point the important roles of innate immunity for establishment of immune tolerance with decisive role in central mechanisms. In a peremptory way, a 'balanced tolerance' is essential for the survival.

Proenkephalin+ regulatory T cells expanded by ultraviolet B exposure maintain skin homeostasis with a healing function.

Regulatory T (Treg) cells, expressing CD25 (interleukin-2 receptor α chain) and Foxp3 transcription factor, maintain immunological self-tolerance and suppress various immune responses. Here we report a feature of skin Treg cells expanded by ultraviolet B (UVB) exposure. We found that skin Treg cells possessing a healing function are expanded by UVB exposure with the expression of an endogenous opioid precursor, proenkephalin (PENK). Upon UVB exposure, skin Treg cells were expanded with a unique TCR repertoire. Also, they highly expressed a distinctive set of genes enriched in "wound healing involved in inflammatory responses" and the "neuropeptide signaling pathway," as indicated by the high expression of Penk. We found that not only was PENK expression at the protein level detected in the UVB-expanded skin Treg (UVB-skin Treg) cells, but that a PENK-derived neuropeptide, methionine enkephalin (Met-ENK), from Treg cells promoted the outgrowth of epidermal keratinocytes in an ex vivo skin explant assay. Notably, UVB-skin Treg cells also promoted wound healing in an in vivo wound closure assay. In addition, UVB-skin Treg cells produced amphiregulin (AREG), which plays a key role in Treg-mediated tissue repair. Identification of a unique function of PENK+ UVB-skin Treg cells provides a mechanism for maintaining skin homeostasis.

Novel MHC-Independent αßTCRs Specific for CD48, CD102, and CD155 Self-Proteins and Their Selection in the Thymus.

MHC-independent αßTCRs (TCRs) recognize conformational epitopes on native self-proteins and arise in mice lacking both MHC and CD4/CD8 coreceptor proteins. Although naturally generated in the thymus, these TCRs resemble re-engineered therapeutic chimeric antigen receptor (CAR) T cells in their specificity for MHC-independent ligands. Here we identify naturally arising MHC-independent TCRs reactive to three native self-proteins (CD48, CD102, and CD155) involved in cell adhesion. We report that naturally arising MHC-independent TCRs require high affinity TCR-ligand engagements in the thymus to signal positive selection and that high affinity positive selection generates a peripheral TCR repertoire with limited diversity and increased self-reactivity. We conclude that the affinity of TCR-ligand engagements required to signal positive selection in the thymus inversely determines the diversity and self-tolerance of the mature TCR repertoire that is selected.

T follicular regulatory cells: Guardians of the germinal centre?

It is a central tenet of the clonal selection theory, that lymphocyte repertoires are tolerized to self-antigens during their ontogeny. Germinal centres are the sites in secondary lymphoid tissues where B cells undergo affinity maturation and class-switching to produce high-affinity antibodies. This process is crucial, both in our ability to mount protective humoral responses to infections and to vaccinations, but it is also involved in untoward reactions to self-antigens, which underlie autoimmunity. The process of affinity maturation poses a significant challenge to tolerance, as the random nature of somatic hypermutation can introduce novel reactivities. Therefore, it has been a long-standing idea that mechanisms must exist which limit the emergence of autoreactivity at the germinal centre level. One of these mechanisms is the requirement for linked recognition, which imposes on B cells a dependence on centrally tolerant T follicular helper cells. However, as linked recognition can be bypassed by adduct formation of autoantigenic complexes, it has been an appealing notion that there should be an additional layer of dominant mechanisms regulating emergence of autoreactive specificities. About a decade ago, this notion was addressed by the discovery of a novel subset of T regulatory cells localizing to the germinal centre and regulating germinal centre B-cell responses. Here, we detail the progress that has been made towards characterizing this T follicular regulatory cell subset and understanding the functions of these 'guardians of the germinal centre'.

Influence of Androgens on Immunity to Self and Foreign: Effects on Immunity and Cancer.

It is well-known that sex hormones can directly and indirectly influence immune cell function. Different studies support a suppressive role of androgens on different components of the immune system by decreasing antibody production, T cell proliferation, NK cytotoxicity, and stimulating the production of anti-inflammatory cytokines. Androgen receptors have also been detected in many different cells of hematopoietic origin leading to direct effects of their ligands on the development and function of the immune system. The immunosuppressive properties of androgens could contribute to gender dimorphisms in autoimmune and infectious disease and thereby also hamper immune surveillance of tumors. Consistently, females generally are more prone to autoimmunity, while relatively less susceptible to infections, and have lower incidence and mortality of the majority of cancers compared to males. Some studies show that androgen deprivation therapy (ADT) can induce expansion of naïve T cells and increase T-cell responses. Emerging clinical data also reveal that ADT might enhance the efficacy of various immunotherapies including immune checkpoint blockade. In this review, we will discuss the potential role of androgens and their receptors in the immune responses in the context of different diseases. A particular focus will be on cancer, highlighting the effect of androgens on immune surveillance, tumor biology and on the efficacy of anti-cancer therapies including emerging immune therapies.

Intestinal Delivery of Proinsulin and IL-10 via Lactococcus lactis Combined With Low-Dose Anti-CD3 Restores Tolerance Outside the Window of Acute Type 1 Diabetes Diagnosis.

A combination treatment (CT) of proinsulin and IL-10 orally delivered via genetically modified Lactococcus lactis bacteria combined with low-dose anti-CD3 (aCD3) therapy successfully restores glucose homeostasis in newly diagnosed non-obese diabetic (NOD) mice. Tolerance is accompanied by the accumulation of Foxp3+ regulatory T cells (Tregs) in the pancreas. To test the potential of this therapy outside the window of acute diabetes diagnosis, we substituted autoimmune diabetic mice, with disease duration varying between 4 and 53 days, with syngeneic islets at the time of therapy initiation. Untreated islet recipients consistently showed disease recurrence after 8.2 ± 0.7 days, while 32% of aCD3-treated and 48% of CT-treated mice remained normoglycemic until 6 weeks after therapy initiation (P < 0.001 vs. untreated controls for both treatments, P < 0.05 CT vs. aCD3 therapy). However, mice that were diabetic for more than 2 weeks before treatment initiation were less efficient at maintaining normoglycemia than those treated within 2 weeks of diabetes diagnosis, particularly in the aCD3-treated group. The complete elimination of endogenous beta cell mass with alloxan at the time of diabetes diagnosis pointed toward the significance of continuous feeding of the islet antigen proinsulin at the time of aCD3 therapy for treatment success. The CT providing proinsulin protected 69% of mice, compared to 33% when an irrelevant antigen (ovalbumin) was combined with aCD3 therapy, or to 27% with aCD3 therapy alone. Sustained tolerance was accompanied with a reduction of IGRP+CD8+ autoreactive T cells and an increase in insulin-reactive (InsB12-20 or InsB13-2) Foxp3+CD4+ Tregs, with a specific accumulation of Foxp3+ Tregs around the insulin-containing islet grafts after CT with proinsulin. The combination of proinsulin and IL-10 via oral Lactococcus lactis with low-dose aCD3 therapy can restore tolerance to beta cells in autoimmune diabetic mice, also when therapy is started outside the window of acute diabetes diagnosis, providing persistence of insulin-containing islets or prolonged beta cell function.