Central tolerance: Essential for preventing autoimmune disease?

Immune checkpoint modulators in early clinical development for the treatment of type 1 diabetes

INTRODUCTION

Despite the improvements of insulin therapy, people with type 1 diabetes (T1D) still suffer from a decreased quality of life and life expectancy. The search toward a cure for T1D is therefore still a scorching open field of research.

AREAS COVERED

Tackling the immune checkpoint signaling pathways has gained importance in the field of cancer immunotherapy. The same pathways can be targeted in autoimmunity with an opposite principle: to dampen the exaggerated immune response. In this review, we report a comprehensive excursus on the cellular and molecular mechanisms that lead to loss of immunological tolerance, and recent evidence on the role of immune checkpoint molecules in the development of T1D and their potential application for the mitigation of autoimmune diabetes.

EXPERT OPINION

Contrasting results about the efficacy of immune checkpoint modulators for T1D have been published, with very few molecules from preclinical studies eligible for use in humans. The heterogeneous and complex pathophysiology of T1D may explain the conflicting evidence. Designing clinical trials that acknowledge the pathophysiological and clinical complexity of T1D and that forecast the need of simultaneously tackling different disease pathways will be crucial to enhance the benefits which may be gained by such compounds.

Thymic changes as a contributing factor in the increased susceptibility of old Albino Oxford rats to EAE development

The study was aimed to examine putative contribution of thymic involution to ageing-associated increase in susceptibility of Albino Oxford (AO) rats to the development of clinical EAE, and vice versa influence of the disease on the progression of thymic involution. To this end we examined (i) the parameters of thymocyte negative selection efficacy, the thymic generation of CD4+CD25+Foxp3+ T regulatory cells (Tregs) and thymic capacity to instruct/predetermine IL-17-producing T-cell differentiation, and thymopietic efficacy-associated accumulation of "inflammescent" cytotoxic CD28- T cells in the periphery, and (ii) the key underlying mechanisms in young and old non-immunised AO rats and their counterparts immunised for EAE (on the 16^th day post-immunisation when the disease in old rats reached the plateau) using flow cytometry analysis and/or RT-qPCR. It was found that thymic involution impairs: (i) the efficacy of negative selection (by affecting thymocyte expression of CD90, negative regulator of selection threshold and the expression of thymic stromal cell integrity factors) and (ii) Treg generation (by diminishing expression of cytokines supporting their differentiation/maturation). Additionally, the results suggest that thymic involution facilitates CD8+ T-cell differentiation into IL-17-producing cells (previously linked to the development of clinical EAE in old AO rats). Furthermore, they confirmed that ageing-related decrease in thymic T-cell output (as indicated by diminished frequency of recent thymic emigrants in peripheral blood) resulted in the accumulation of CD28- T cells in peripheral blood and, upon immunisation, in the target organ. On the other hand, the development of EAE (most likely by increasing circulatory levels of proinflammatory cytokines) contributed to the decline in thymic output of T cells, including Tregs, and thereby to the progression/maintenance of clinical EAE. Thus, in AO rats thymic involution via multi-layered mechanisms may favour the development of clinically manifested autoimmunity, which, in turn, precipitates the thymus atrophy.

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.

A Few Key Historical Events in the Antibody Field: The Alacritous Antibody

We have coined the term "alacrity" to describe the extraordinary diversity of B cell activation potentials, even among cells in a single B cell clone responding to a single antigen. The discovery of methodologies for B cell culture in limiting dilution allowed scientists to identify the source of cellular heterogeneity among cells of the immune system. Analyses of individual B cells set the stage for more detailed descriptions of the factors that diversify B cell functions, some of which will be expanded upon by partner articles in this B cell issue.

CD28 null CD4 T-cell expansions in autoimmune disease suggest a link with cytomegalovirus infection

Immunosenescence is thought to contribute to the increase of autoimmune diseases in older people. Immunosenescence is often associated with the presence of an expanded population of CD4 T cells lacking expression of CD28 (CD28 ^null). These highly cytotoxic CD4 T cells were isolated from disease-affected tissues in patients with rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, or other chronic inflammatory diseases and their numbers appeared to be linked to disease severity. However, we recently demonstrated that the common herpes virus, cytomegalovirus (CMV), not ageing, is the major driver of this subset of cytotoxic T cells. In this review, we discuss how CMV might potentiate and exacerbate autoimmune disease through the expansion of CD28 ^null CD4 T cells.

An introduction to biomaterial-based strategies for curbing autoimmunity

Recently, scientists have made significant progress in the development of immunotherapeutics that correct aberrant, autoimmune responses. Yet, concerns about the safety, efficacy, and wide scale applicability continue to hinder use of contemporary, immunology-based strategies. There is a clear need for therapies that finely control molecular and cellular elements of the immune system. Biomaterial engineers have taken up this challenge to develop therapeutics with selective spatial and temporal control of immune cells. In this review, we introduce the immunology of autoimmune disorders, survey the current therapeutic strategies for autoimmune diseases, and highlight the ongoing research efforts to engineer the immune system using biomaterials, for positive therapeutic outcomes in treatment of autoimmune disorders.

Thymus and type 1 diabetes: an update

Type 1 diabetes (T1D) is a chronic disease resulting from the selective autoimmune destruction of pancreatic islet β cells. The absence and/or breakdown of immune self-tolerance to islet β cells is now recognized as the essential cause for the development of the diabetogenic autoimmune response. For a long time, a failure in peripheral tolerogenic mechanisms was regarded as the main source of an inappropriate immune process directed against insulin-secreting β cells. While defective peripheral self-tolerance still deserves to be further investigated, the demonstration that all members of the insulin gene family are transcribed in thymic epithelial cells (TECs) of different species under the control of the AutoImmune REgulator (AIRE) gene/protein has highlighted the importance of central self-tolerance to insulin-secreting islet β cells. Moreover, there is now evidence that a primary or acquired failure in thymus-dependent central self-tolerance to β cells plays a primary role in T1D pathogenesis. This novel knowledge is currently translated into the development of innovative tolerogenic/regulatory approaches designed to reprogram the specific immune self-tolerance to islet β cells.

Stability of regulatory T-cell lineage

Regulatory T (Treg) cells expressing the transcription factor Foxp3 constitute a unique T-cell lineage committed to suppressive functions and play a central role in maintaining self-tolerance and immune homeostasis. While their differentiation state is remarkably stable in the face of various perturbations from the extracellular environment, recent studies have also revealed their adaptability to the changing environment; in response to extrinsic cues, Treg cells differentiate further into distinct substates to regulate different classes of immune responses effectively. In contrast, some other recent studies have challenged this notion of a committed Treg cell lineage and suggested that Treg cells might lose their identity and be reprogrammed to various effector helper T cells under certain circumstances, although this issue of environment-induced Treg cell reprogramming remains highly controversial. This review will focus on recent advances in our understanding of how the stability and adaptability of Treg cell lineage is regulated and how it might be perturbed in a changing environment.

Thymic self-antigen expression for the design of a negative/tolerogenic self-vaccine against type 1 diabetes

Before being able to react against infectious non-self-antigens, the immune system has to be educated in the recognition and tolerance of neuroendocrine proteins, and this critical process essentially takes place in the thymus. The development of the autoimmune diabetogenic response results from a thymus dysfunction in programming central self-tolerance to pancreatic insulin-secreting islet β cells, leading to the breakdown of immune homeostasis with an enrichment of islet β cell reactive effector T cells and a deficiency of β cell-specific natural regulatory T cells (nTreg) in the peripheral T-lymphocyte repertoire. Insulin-like growth factor 2 (IGF-2) is the dominant member of the insulin family expressed during fetal life by the thymic epithelium under the control of the autoimmune regulator (AIRE) gene/protein. Based on the close homology and cross-tolerance between insulin, the primary T1D autoantigen, and IGF-2, the dominant self-antigen of the insulin family, a novel type of vaccination, so-called “negative/tolerogenic self-vaccination”, is currently developed for prevention and cure of T1D. If this approach were found to be effective for reprogramming immunological tolerance in T1D, it could pave the way for the design of negative self-vaccines against autoimmune endocrine diseases, as well as other organ-specific autoimmune diseases.

Dysregulation of thymic clonal deletion and the escape of autoreactive T cells

Events ongoing in the thymus are critical for deleting developing thymocytes specific for tissue antigens, and establishing self-tolerance within the T cell compartment. Aberrant thymic negative selection, however, is believed to generate a repertoire with increased self-reactivity, which in turn can contribute to the development of T cell-mediated autoimmunity. In this review, mechanisms that regulate the efficacy of negative selection and influence the deletion of autoreactive thymocytes will be discussed.