Central tolerance and autoimmune diseases]

Central tolerance is established by the repertoire selection of immature T lymphocytes in the thymus, avoiding autoimmune responses to self-antigens. Differential ligand-TCR interactions that result in positive and negative selection initiate differential intracellular signals that, in turn, lead to the survival-or-death decision of immature thymocytes. TCR signal dysregulation due to the mutation of ZAP-70 or defective apoptosis of autoreactive thymocytes due to the deficiency of pro-apoptotic protein Bim impair tolerance and cause autoimmunity. Thymic repertoire selection also induces the development of CD25(+)CD4(+) regulatory T cells, which play important roles for maintaining peripheral tolerance. Furthermore, the establishment of central tolerance requires the development of thymic medulla that is mediated by the activation of NF-kappaB signaling pathway, promiscuous expression of tissue-specific self-antigens by medullary epithelial cells that is regulated by AIRE, and cortex-to-medulla migration of developing thymocytes that is regulated by CCR7-mediated chemokine signals.     

The potential for induction of autoimmune disease by a randomly-mutated self-antigen

The pathology of most autoimmune diseases is well described. However, the exact event that triggers the onset of the inflammatory cascade leading to disease is less certain and most autoimmune diseases are complex idiopathic diseases with no single gene known to be causative. In many cases, a relation to an infectious disease is described, and it is thought that microbes can play a direct role in induction of autoimmunity, for instance by molecular mimicry or bystander activation of autoreactive T cells. In contrast, less attention has been given to the possibility that modified self-antigens can be immunogenic and lead to autoimmunity against wildtype self-antigens. In theory, modified self-antigens can arise by random errors and mutations during protein synthesis and would be recognized as foreign antigens by na?ve B and T lymphocytes. Here, it is postulated that the initial auto-antigen is not a germline self-antigen, but rather a mutated self-antigen. This mutated self-antigen might interfere with peripheral tolerance if presented to the immune system during an infection. The infection lead to bystander activation of na?ve T and B cells with specificity for mutated self-antigen and this can lead to epitopespreading in which T and B cells with specificity for wildtype self-antigens are activated as a result of general inflammation.     

Breakdown of T cell tolerance and autoimmunity in primary immunodeficiency--lessons learned from monogenic disorders in mice and men

A key feature of the immune system is the capacity to monitor and control infections from non-self pathogens while maintaining tolerance to self-antigens. Primary immunodeficiencies (PID) are characterized by an increased susceptibility to infections, often associated with aberrant inflammatory responses and a concomitant high prevalence of autoimmunity. Autoimmunity in PID raises a conundrum: How can an immune system fail to respond to non-self pathogens while reacting vigorously to self-antigens? Recent advances from studies of PID patients and related animal models have revealed the crucial role of Aire-induced expression of self-antigens for deletion of autoreactive T cells in the thymus (central tolerance). Moreover, lessons from PID have provided unequivocal evidence for the essential role of regulatory T cells in suppressing autoreactive T cells in the periphery. Finally, findings from PID have broadened our understanding of how homeostatic proliferation and increased load or decreased clearance of apoptotic cells and non-self pathogens can lead to breakdown of peripheral tolerance.     

Immune recognition of influenza hemagglutinin as a viral and a neo-self-antigen

To analyze mechanisms governing tolerance and autoimmunity to self-antigens, we have generated lineages of transgenic mice that express the influenza virus PR8 hemagglutinin (HA) as a neo-selfantigen. By comparing the HA-specific T and B cell responses that can be induced in HA Tg mice with those that are induced in non-Tg (BALB/c) mice, the specificity and genetic basis with which tolerance is induced to the HA has been examined. This article summarizes studies using lineages of HA Tg mice that express different forms and amounts of the HA under the control of the SV40 promoter/enhancer. Our studies have revealed that specific subsets of HA-specific T and B cells are negatively selected from the primary repertoires of HA Tg mice. However, substantial populations of HA-specific T and B cells evade negative selection and can be activated by virus immunization. Understanding the capacity of these autoreactive lymphocytes to differentiate and participate in antigenspecific immune responses will provide important insights into mechanisms by which autoimmunity might be induced by viruses bearing structural similarities with self-antigens.     

Antigen presentation and self-nonself discrimination

Self-nonself discrimination is primarily carried out by T cells. Since the ligand recognized by T cells is a complex formed by antigenic peptides bound to MHC molecules, positive and negative selection of T lymphocytes must be based on the recognition of complexes formed by self-peptides bound to MHC molecules. This requires that self-antigens are continuously processed, bound by MHC molecules, and presented to T cells under conditions inducing both positive selection of T cells potentially able to recognize foreign antigens and negative selection, either by physical deletion or functional inactivation, of potentially autoreactive T cells. Self-nonself discrimination is not confined to intrathymic development of T lymphocytes, but it is a continuing process among peripheral T cells. Accordingly, autoimmunity is induced when self-antigens, or foreign antigens cross-reactive with self antigens, bound to MHC molecules, are presented under conditions able to activate self-reactive T cells. Based on these premises, a way of interfering with the induction of autoimmune diseases could rely on blocking the MHC binding site presenting the autoantigen.     

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’.     

Idiotypes and autoimmunity

The reports discussed above have increased our knowledge of idiotypes, mainly with respect to additional CRI on autoantibodies and to a relatively new aspect of 'pathogenic idiotypes'. It is obvious that much remains to be discovered about the normal role of idiotypes and how they might be involved in the pathogenesis of autoimmune disorders. The relationship of the idiotypic network to tolerance is a matter of speculation; tolerance implies the ability to distinguish between 'foreign' and self-antigens and it is important to remember that antibody V regions are also self-antigens. Several mechanisms have been proposed to explain immunological tolerance. Originally, it was envisaged that the repertoires of both T and B lymphocytes were in some way purged of potentially self-reactive clones. However, it is now evident that self-reactive lymphocytes do exist but are normally held under control. Finally, it may be that certain self-antigens are simply never exposed to immune surveillance. It seems that the control of self-reactive lymphocytes is central to the question of tolerance. In the absence of autoimmune disease, autoantibodies can be produced, for example, after many infectious diseases or vaccinations. However, this type of perturbation of the immune system is associated with the short-term production of low titres of low-affinity antibodies, generally of the IgM isotype, which are thought to represent germline gene products. Homeostasis is soon re-established, possibly by regulatory T cells interacting with autoreactive B cells through their idiotypic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Peripheral T-cell tolerance: the contribution of permissive T-cell migration into parenchymal tissues of the neonate

Summary: T lymphocytes with self-destructive capacity are often found in healthy individuals, indicating efficient control mechanisms chat prevent chronic autoimmune deseases. Since naive T lymphocytes do not circulate through extralymphoid tissues the concept has emerged that peripheral T cells ignore tissue-,specific antigens unless they are presented by professional antigen-presenting cells in the lymphoid compartments. However, this view pays attention only to experiments performed in adult animals. This report reviews the evidence that tissues of neonatal mice, in contrast to adults, exhibit high accessibility for naive T cells, thereby allowing the direct contact with tissue-specific self-antigens on parenchymal cells during neonatal life and tolerance induction to such self-antigens. In mouse bone marrow chimeras generated at different ages, recent thymic emigrants were tolerized to a major histocompatibility class I antigen expressed on keratinocytes only during a neonatal period and not during adulthood. Blockade of T-cell migration neonatally prevented tolerance induction. The neonatally induced tolerance is maintained during adulthood, apparantly by a dominant regulatory mechanism. Thus, parenchymal cells and T-cell migration in the neonate contribute to the control of autoreactive T cells.     

Neonatal tolerance to a Th2-mediated autoimmune disease generates CD8+ Tc1 regulatory cells

Immunological tolerance can be achieved in animals by exposure of newborn to a foreign antigen. Depending on the dose and timing of the antigenic challenge, tolerance has been reported to result in clonal deletion, anergy or active suppression. In this latter case, regulatory T cells prevent autoimmunity by suppressing the reactivity of pathogenic self-reactive T cells. We have previously reported the generation of a neonatal, mercury-specific, and dominant tolerance to autoimmunity induced by mercury salts in rats. Chronic exposure to mercury salts can lead to SLE-like autoimmune responses, mediated by autoreactive CD4+ Th2 cells, that regulate and are followed by a resistant state mediated by protective CD8+ T cells. The aim of the study was to compare the resistance to the neonatal tolerance to mercury disease, and to further characterize the CD8+ T cells endowed with regulatory capacity in the neonatal tolerance model. We report here that resistance to mercury disease is long lasting and not mercury-specific, suggesting that different CD8+ T cells are involved in resistance and neonatal tolerance, and that regulatory CD8+ Tc1 cells generated in tolerance are required to control the CD8- cell population from developing Th2-mediated autoimmunity. Upon mercury recall, CD8+ CD45RC(high) T cells, that represent the Tc1 subset in the rat, expanded and were polarized towards IFNgamma production. Interestingly, identical results were obtained with the CD8+ CD25+T cell population. Substantial amounts of FasL gene expression were detected in CD8+ T lymphocytes upon recall with the tolerogen. AICD may be one of the regulatory mechanisms used by these regulatory CD8+ Tc1 cells that control neonatal tolerance to a Th2-mediated autoimmune disorder.     

Spotlight on the role of hormonal factors in the emergence of autoreactive B-lymphocytes

Pathogenic autoimmunity requires a combination of inherited and acquired factors. In as much as hormones influence the sexual dimorphism of the immune system, it is possible that they can initiate or accelerate an autoimmune process, and contribute to gender-biased autoimmune disorders. Not only natural hormones, but also endocrine disruptors, such as environmental estrogens, may act in conjunction with other factors to override immune tolerance to self-antigens. In lupus, murine and human studies demonstrate that female sex hormones are implicated in disease pathogenesis. In the B cell compartment, both prolactin and estrogen are immunomodulators that affect maturation, selection and antibody secretion. Their impact may be based on their capacity to allow autoreactive B cells to escape the normal mechanisms of tolerance and to accumulate in sufficient numbers to cause clinically apparent disease. Both hormones lead to the survival and activation of autoreactive B cells, but they skew B cell maturation towards different directions, with prolactin inducing T cell-dependent autoreactive follicular B cells and estrogen eliciting T cell-independent autoreactive marginal zone B cells. Differential modulation of the cytokine milieu by hormones may also affect the development and activation of specific mature B cell subsets. This novel insight suggests that targeted manipulation of these pathways may represent a promising avenue in the treatment of lupus and other gender-biased autoimmune diseases.     

The role of B lymphocytes in the progression from autoimmunity to autoimmune disease

Autoimmunity, defined as the presence of autoreactive T and/or B lymphocytes in the periphery, is a frequent and probably even physiological condition. It is mainly caused by the fact that the central tolerance mechanisms, which are responsible for counter-selection of autoreactive lymphocytes, are not perfect and thus a limited number of these autoreactive cells can mature and enter the periphery. Nonetheless, autoreactive cells do not lead automatically to autoimmune disease as evidenced by a multitude of experimental and human data sets. Interestingly, the progression from autoimmunity to autoimmune disease is not only determined by the degree of central tolerance leakage and thus the amount of autoreactive lymphocytes in the periphery, but also by peripheral mechanism of activation and control of the autoreactive cells. In this review, we discuss the contribution of peripheral B lymphocytes in this process, ranging from activation of T cells and epitope spreading to control of the autoimmune process by regulatory mechanisms. We also discuss the parallels with the role of B cells in the induction and control of alloimmunity in the context of organ transplantation, as more precise knowledge of the pathogenic antigens and time of initiation of the immune response in allo- versus auto-immunity allows better dissection of the exact role of B cells. Since peripheral mechanisms may be easier to modulate than central tolerance, a more thorough understanding of the role of peripheral B cells in the progression from autoimmunity to autoimmune disease may open new avenues for treatment and prevention of autoimmune disorders.     

Neonatal tolerant immunity for vaccination against autoimmunity

Autoimmunity arises when the immune system no longer tolerates self and precipitates lymphocyte reactivity against our own antigens. Although the developing T cell repertoire is constantly purging, self-recognition events do exist when such tight control is evaded and autoreactive lymphocytes escape the thymus (the sites of T cell development) and migrate to the periphery. Upon activation these autoreactive cells may exert aggressive behavior toward one's own tissues and organs leading to autoimmune disease. Multiple sclerosis, Rheumatoid arthritis, and type I diabetes are autoimmune diseases mediated by autoreactive T cells. A logical approach to prevent such autoimmunity would be to reprogram those lymphocytes to tolerate the self antigen. Injection of antigen at the neonatal stage promotes a state of tolerance such that successive encounter with antigen does not precipitate aggressive reactions. The mechanism underlying neonatal tolerance involves priming of T cells whose effector functions do not cause inflammatory reactions upon recognition of antigen but rather induce protective immunity. This form of tolerant immunity provides an attractive strategy for vaccination against autoimmunity. Herein, it is shown that neonatal exposure to a self-peptide-immunoglobulin chimera drives a tolerant immunity toward the self-peptide and protects against the autoimmune disease, experimental allergic encephalomyelitis.     

Self-tolerance and the composition of T cell repertoire

T cell recognition of self-major histocompatibility complexpeptide complexes dictates the composition of the T cell receptor repertoire. Research projects in our laboratory deal with the mechanisms that regulate the composition of the repertoire specific for self-antigens and the defects that can result in autoimmunity. Two different types of disease models are under investigation: juvenile (type 1) diabetes and cancer. Both of these diseases are impacted by the presence of anti-self CD8 cells, yet in opposite ways. By understanding the mechanisms of peripheral tolerance and the reasons they fail in autoimmunity, we may learn how to prevent undersirable autoimmunity and how to encourage an autoimmune response when it is needed to eliminate tumor cells.     

CD3+CD4-CD8- alphabeta-TCR+ T cell as immune regulatory cell

Down-regulation of immune responses by regulatory T cells is one of the major mechanisms involved in the induction of tolerance to self- and alloantigens as demonstrated in a number of models of transplantation and autoimmunity. It is clear that regulatory T cells consist of different subsets. Recently a novel subset of antigen-specific alphabeta-TCR+ CD4-CD8- (double negative, DN) regulatory T cells has been found to be able to inhibit the function of the CD8+ T cells carrying the same T cell receptor specificity and prevent the rejection of skin allografts. Identification of the DN regulatory T cells and their novel mechanism of suppression can help us to understand how donor-specific transplantation tolerance can be achieved and to explain how tolerance to self-antigens can be maintained in the periphery.     

Nanoparticle‐based approaches to immune tolerance for the treatment of autoimmune diseases

Autoimmune diseases are caused by antigenically complex immune responses of the adaptive and innate immune system against specific cells, tissues or organs. Antigen‐specific approaches for induction of immune tolerance in autoimmunity, based on the use of antigenic peptides or proteins, have failed to deliver the desired therapeutic results in clinical trials. These approaches, which are largely relying on triggering clonal anergy and/or deletion of defined autoreactive specificities, do not address the overwhelming antigenic, molecular, and cellular complexity of most autoimmune diseases, which involve various immune cells and ever‐growing repertoires of antigenic epitopes on numerous self‐antigens. Advances in the field of regulatory T‐cell (Treg) biology have suggested that Treg cells might be able to afford dominant tolerance provided they are properly activated and expanded in vivo. More recently, nanotechnology has introduced novel technical advances capable of modulating immune responses. Here, we review nanoparticle‐based approaches designed to induce immune tolerance, ranging from approaches that primarily trigger clonal T‐cell anergy or deletion to approaches that trigger Treg cell formation and expansion from autoreactive T‐cell effectors. We will also highlight the therapeutic potential and positive outcomes in numerous experimental models of autoimmunity.     

Cross-presentation: inducing CD8 T cell immunity and tolerance

The term cross-presentation denotes the presentation of exogenous (extracellular) antigens to T cells, particularly CD8 T cells. It permits professional antigen-presenting cells which have collected antigens in nonlymphoid tissues to activate naive CD8 T cells in the secondary lymphatic compartment. Thus it allows CD8 T cells to scan nonlymphoid tissues for pathogens without the need of migrating there themselves and may hence be critical for immune responses to tissue-tropic viruses. It may also be essential in the immune response to nonlymphoid tumors. In contrast to the induction of immunogenic responses to pathogens, cross-presentation of self-antigens leads to CD8 T cell tolerance by deletion of autoreactive CD8 T cells. The precise way in which the immune system distinguishes self from foreign is not known, but modification in the cross-presenting antigen-presenting cell, such as that achieved by CD4 T cell help or inflammatory signals, may play a critical role in this process. If the dose of the self-antigen or the avidity of the T cell receptor is too low, cross-presentation fails to remove autoreactive CD8 T cells. Ignoring the self-antigen, these cells recirculate through the secondary lymphatics, unless they are activated, for example, by a cross-reactive virus. Then autoimmunity may be triggered.     

Neonatal Tolerant Immunity for Vaccination against Autoimmunity

Autoimmunity arises when the immune system no longer tolerates self and precipitates lymphocyte reactivity against our own antigens. Although the developing T cell repertoire is constantly purging, self-recognition events do exist when such tight control is evaded and autoreactive lymphocytes escape the thymus (the sites of T cell development) and migrate to the periphery. Upon activation these autoreactive cells may exert aggressive behavior toward one's own tissues and organs leading to autoimmune disease. Multiple sclerosis, Rheumatoid arthritis, and type I diabetes are autoimmune diseases mediated by autoreactive T cells. A logical approach to prevent such autoimmunity would be to reprogram those lymphocytes to tolerate the self antigen. Injection of antigen at the neonatal stage promotes a state of tolerance such that successive encounter with antigen does not precipitate aggressive reactions. The mechanism underlying neonatal tolerance involves priming of T cells whose effector functions do not cause inflammatory reactions upon recognition of antigen but rather induce protective immunity. This form of tolerant immunity provides an attractive strategy for vaccination against autoimmunity. Herein, it is shown that neonatal exposure to a self-peptide-immunoglobulin chimera drives a tolerant immunity toward the self-peptide and protects against the autoimmune disease, experimental allergic encephalomyelitis.     

The physiological role of regulatory T cells in the prevention of autoimmunity: generation, specificity and mode of action

Until recently, the traditional view was that tolerance to self antigens was maintained by a combination of physical deletion, in the thymus, and functional deletion, in the periphery, of autoreactive T cells. There is now, however, abundant evidence that the normal T cell repertoire contains overtly autoreactive T cells whose pathogenic potential is held in check by the activity of a distinct subset of peripheral T cells, the so-called regulatory or suppressor T cells. This article examines data from one model of rodent autoimmunity, where autoimmune pathology develops following thymectomy and irradiation of normal laboratory rats, which characterize the development and function of these so-called regulatory T cells.     

Human NKT cells direct the differentiation of myeloid APCs that regulate T cell responses via expression of programmed cell death ligands

NKT cells are innate lymphocytes that can recognize self or foreign lipids presented by CD1d molecules. NKT cells have been shown to inhibit the development of autoimmunity in murine model systems, however, the pathways by which they foster immune tolerance remain poorly understood. Here we show that autoreactive human NKT cells stimulate monocytes to differentiate into myeloid APCs that have a regulatory phenotype characterized by poor conjugate formation with T cells. The NKT cell instructed myeloid APCs show elevated expression of the inhibitory ligand PD-L2, and blocking PD-L1 and PD-L2 during interactions of the APCs with T cells results in improved cluster formation and significantly increased T cell proliferative responses. The elevated expression of PD-L molecules on NKT-instructed APCs appears to result from exposure to extracellular ATP that is produced during NKT-monocyte interactions, and blocking purinergic signaling during monocyte differentiation results in APCs that form clusters with T cells and stimulate their proliferation. Finally, we show that human monocytes and NKT cells that are injected into immunodeficient mice co-localize together in spleen and liver, and after 3 days in vivo in the presence of NKT cells a fraction of the myeloid cells have upregulated markers associated with differentiation into professional APCs. These results suggest that autoreactive human NKT cells may promote tolerance by inducing the differentiation of regulatory myeloid APCs that limit T cell proliferation through expression of PD-L molecules.