A subset of natural killer cells achieves self-tolerance without expressing inhibitory receptors specific for self-MHC molecules

It is widely believed that self-tolerance of natural killer (NK) cells occurs because each NK cell expresses at least one inhibitory receptor specific for a host major histocompatibility complex (MHC) class I molecule. Here we report that some NK cells lack all known self-MHC-specific inhibitory receptors, yet are nevertheless self-tolerant. These NK cells exhibit a normal cell surface phenotype and some functional activity. However, they respond poorly to class I-deficient normal cells, tumor cells, or cross-linking of stimulatory receptors, suggesting that self-tolerance is established by dampening stimulatory signaling. Thus, self-tolerance of NK cells in normal animals can occur independently of MHC-mediated inhibition, and hyporesponsiveness plays a role in self-tolerance of NK cells, as also proposed for B and T cells.

The common sense of the self-nonself discrimination

The vertebrate immune system was evolutionarily selected to express a large random somatically generated paratopic repertoire coupled to effector mechanisms invented, in large measure, by non-vertebrates. The self-nonself discrimination is determined by Decision 1, the sorting of this repertoire into those specificities (anti-self) which, if expressed, would debilitate the host and those specificities (anti-nonself) which, if not expressed, would result in the death of the host by infection. Decision 1, the sorting of the repertoire, is mediated by a somatic learning process operating epitope-by-epitope that deletes anti-self specificities leaving the residue as anti-nonself. The activation of anti-nonself is the first step on entry into Decision 2, which optimizes the choice and magnitude of the effector class that rids the pathogen without significantly debilitating the host. The principles governing Decision 1, the self-nonself discrimination are analyzed here.

Transplantation tolerance in pediatric recipients: lessons and challenges

Clinical transplantation tolerance has remained an elusive goal in the 50 yr since it was first described in experimental animals. Greater understanding of the molecular mechanisms responsible for allorecognition have allowed for the development of promising immunosuppressive strategies that may bring us closer to reproducible induction of tolerance; consideration of past successes and failures from both clinical and basic science is required to define future challenges facing this field. This article reviews mechanisms of self and transplantation tolerance, translation of basic science research to clinical protocols in animals and human beings, the changing role of immunosuppression, complications following tolerance induction and controversies surrounding the choice of patients for tolerance trials with a focus on issues relevant to pediatric patients. The role of the Immune Tolerance Network is discussed along with realistic goals for tolerance induction in human beings over the next decade.

Do autoantigens define autoimmunity orvice versa?

The basic function of the adaptive immune system is to distinguish self from foreign. The failure of self tolerance can result in autoimmunity, which comes in many forms but still targets a limited selection of the total available autologous determinants. This selectivity must reflect the underlying mechanisms of the autoimmune reaction, as well as the particular features of the autoantigens that are targeted. Here I discuss the overall paradigm of autoimmunity, and what kinds of mechanisms might play a role. It is likely that multiple different pathways are critical in various diseases, and even in a single condition.

Mannosylated PLP(139-151) induces peptide-specific tolerance to experimental autoimmune encephalomyelitis

SJL mice immunized with mannosylated (M-) PLP(139-151) in complete adjuvant do not develop EAE and little CNS mononuclear cell infiltration; other mannosylated peptides were ineffective in this experimental setting. Despite apparently normal T cell responses, M-PLP(139-151)-immunized mice show impaired delayed-type-sensitivity to PLP(139-151) but a normal response to other peptides. After re-immunization with PLP(139-151) in complete adjuvant, these mice are largely tolerant to EAE, show less T cell proliferation and decreased peptide-specific IgG2a. Our data suggest that M-PLP(139-151) induces peptide-specific tolerance to EAE via a mechanism of deletion or impaired migration of encephalitogenic T cells.

Regulatory T cells and organ transplantation

Empirical studies attempting to explain tolerance to transplanted tissues have demonstrated a regulatory role for CD4+ T-cells. We here propose that regulatory T-cells mediating transplantion tolerance comprise two sets which can functionally complement each other. The CD4+CD25+ "natural regulator" arises in the thymus, and is preoccupied with self-antigens expressed at sites of inflammation. The second, comprising both CD4+CD25+ (FoxP3+) and CD4+CD25- Tr1-like cells are induced by persistent danger-free antigen in the periphery. The role of these cells is to moderate immune responses to prevent tissue destruction while allowing microbial elimination.

Mounting evidence for vitamin D as an environmental factor affecting autoimmune disease prevalence

Low vitamin D status has been implicated in the etiology of autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, insulin-dependent diabetes mellitus, and inflammatory bowel disease. The optimal level of vitamin D intake required to support optimal immune function is not known but is likely to be at least that required for healthy bones. Experimentally, vitamin D deficiency results in the increased incidence of autoimmune disease. Mechanistically, the data point to a role for vitamin D in the development of self-tolerance. The vitamin D hormone (1,25-dihydroxy vitamin D(3)) regulates T helper cell (Th1) and dendritic cell function while inducing regulatory T-cell function. The net result is a decrease in the Th1-driven autoimmune response and decreased severity of symptoms. This review discusses the accumulating evidence pointing to a link between vitamin D and autoimmunity. Increased vitamin D intakes might decrease the incidence and severity of autoimmune diseases and the rate of bone fracture.

Silent antibodies

Over the past years, progress has been made in understanding B cells and antibody recognition functions, particularly in the context of autoimmune diseases. In addition to the existence of "natural antibodies", recent studies suggest the existence of immunoglobulins with no apparent specificity that may acquire polyreactivity following a mild denaturation in inflammatory sites. They are called "silent antibodies". Together with related observations on B cell development, selection and signaling, the recent insights are providing clues into our understanding of autoimmunity.

Immunological study of IFNbeta-1a-treated and untreated multiple sclerosis patients: clarifying IFNbeta mechanisms and establishing specific dendritic cell immunotherapy

OBJECTIVES

A comparative immunological evaluation of multiple sclerosis (MS) patients receiving IFNbeta treatment and patients who are not receiving treatment may help clarify IFNbeta neurological mechanisms and lead the way to an effective dendritic cell (DC) immunotherapy. This type of study helps clarify the pathological function of T cells and DCs within the TH1/TH2/TH3 network as well as the specific interactions between TH1/TH2/TH3 cytokines implicated in MS pathological mechanisms and determine the best way of reestablishing the TH1/TH2/TH3 network equilibrium.

METHODS

We studied network interactions between TH1/TH2/TH3 cytokine levels in serum and supernatants of whole blood and CD14+ monocyte-derived DCs in the remission phase of the disease and in correlation to the Expanded Disability Status Scale (EDSS).

RESULTS

We found that TH1 dysregulation results in a disruption of the maturation and activation of dendritic and T cells, and a lack of T-regulating cells for the induction of self-tolerance; IFNbeta mechanisms restore regulation by reestablishing the network balance but fail to resolve the disease completely due to in vivo IL12p70 network interactions leading to the deletion of self-aggressive cells.

CONCLUSIONS

Our results indicate that a specific DC immunotherapy could cure rather than treat MS. The best point to reestablish the normal physiological cycle is at the immature DC stage which can be done in vitro with treated peripheral blood CD14+ cells and used in vivo to stimulate the expansion of specific regulatory T cells.

Regulatory T cells in the establishment and maintenance of self-tolerance: role of the thymic epithelium

The thymus constitutes the microenvironment for T lymphocyte differentiation and acquisition of self-tolerance. Aiming to specify the contributions of the two essential parts of the thymus, namely hemopoietic and epithelial, we have devised experimental models in birds and mice. Chimeric thymuses, xenogeneic in birds and allogeneic in mice, were constructed early in development. In both models we could demonstrate a critical role of the epithelial component of the thymic stroma in induction and maintenance of self-tolerance. These experiments showed that suppression mechanisms are also implicated in these events, strongly suggesting the existence of regulatory T cells in both models. Before these experiments the control of self-tolerance was usually attributed to suppressive cells. However, as the cell phenotypes were not identified, the role of these cells was disregarded. Numerous studies since our investigations argue in favour of regulatory mechanisms. The work we initiated several years ago represents a contribution to our understanding of the two linked and opposite aspects of immune-responded control, namely self-tolerance and autoimmunity.

Regulating self-tolerance by deregulating gene expression

Intrathymic expression of peripheral auto-antigens, termed promiscuous gene expression, extends the scope of central T-cell tolerance to peripheral organs and proves essential for the induction and maintenance of self-tolerance. The purification of antigen-presenting cells has been instrumental in identifying promiscuous gene expression as an inherent property of medullary epithelial cells. The pool of promiscuously expressed genes might encompass up to 10% of the whole genome. The remarkable diversity of this gene pool implies a complex mode of regulation, which cannot be solely explained by the action of a single factor, such as the transcriptional autoimmune regulator AIRE. Co-localization of promiscuously expressed genes in clusters also suggests epigenetic mechanisms (e.g. DNA methylation) to account for this unorthodox gene expression pattern. The identification of the molecular components controlling the expression of tissue-restricted genes in the thymus promises to add valuable new insights into the complex genetic regulation underlying most autoimmune diseases.

CD4 T Lymphocyte–mediated Lung Disease

Although considerable evidence indicates a role for CD4(+) T lymphocytes (T cells) in airway inflammation, little data exist regarding the mechanisms underlying the induction and regulation of CD4(+) T cell reactivity to lung-specific antigens. To dissect the immunologic and molecular mechanisms of CD4(+) T cell dysregulation, reactivity to a self-antigen expressed in the lung of mice bearing a major histocompatibility complex class-II-restricted T cell receptor specific for this antigen was studied. Transgenic mice developed a progressive interstitial pneumonitis characterized by massive lymphocytic and plasmacytic infiltration of interalveolar septa, a clinical picture closely resembling some of the interstitial lung diseases. Pulmonary inflammation reached a plateau state in older mice with prominent formation of lymphoid follicles but reduced interstitial infiltration. Extensive immunologic characterization of self-reactive CD4(+) T cells isolated from the inflamed lung suggested the induction of regulatory T cells in the site of inflammation. Moreover, inflammation was accompanied by broad changes in the gene expression pattern toward a profile partially resembling that of activated, but strikingly, also that of regulatory CD4(+) T cells. Together our data provide important insights into functional and molecular alterations being associated with the induction and/or regulation of T cell-mediated pulmonary inflammation.

Initiation of autoimmunity

It has recently become clear that several factors must coincide for the initiation of autoimmunity. At minimum, these involve a genetic predisposition, naive lymphocytes that can react with autoantigens and a precipitating event that leads to T and/or B cell activation. Inter-individual variations in these factors probably explain the significant complexity associated with autoimmune diseases; however, quantitative issues are also important because clinical disease will manifest only if a sufficient amount of cellular material has been destroyed. Therefore, the presence of autoreactive lymphocytes does not always signify disease; rather, the kinetics of their generation, their resulting numbers and the regulation of their activation and effector functions (destructive versus regulatory) will determine the ultimate outcome and make the difference between subclinical autoimmunity and disease.

Development and function of CD25+CD4+ regulatory T cells

The essential role played by CD25(+)CD4(+) regulatory T cells (T(R) cells) in the control of physiological as well as pathological immunity is now well established, but many aspects of their biology still remain unclear. One of the unresolved issues regards their development: where does this occur, what signals are required, and how do T(R) cells fit into the larger taxonomy of the T-cell family? Recent data has begun to shed light on the development and function of these important cells.

Mouse lysozyme-M knockout mice reveal how the self-determinant hierarchy shapes the T cell repertoire against this circulating self antigen in wild-type mice

We have studied T cell tolerance to defined determinants within ML-M using wild-type (WT; ML-M(+/+)) and LysMcre (ML-M(-/-)) C3H (H-2(k)) mice to determine the relative contribution of ML-M-derived epitopes vs those from other self Ags in selection of the ML-M-specific T cell repertoire. ML-M was totally nonimmunogenic in WT mice, but was rendered immunogenic in LysMcre mice. Most of the response to ML-M in LysMcre mice was directed to the immunodominant determinant region 105-119. This determinant is spontaneously displayed (without adding exogenous ML-M) by macrophages of WT, but not LysMcre, mice and is stimulatory for peptide 105-119 (p105-119)-primed T cells. Moreover, neonatal tolerization of LysMcre mice with p105-119 or ML-M abrogated the T cell response to subsequent challenge with ML-M or p105-119. Furthermore, p95-109 and p110-125 of ML-M were immunogenic in LysMcre mice, but not in WT mice, thereby representing subdominant, tolerance-inducing epitopes of ML-M. As expected, the T cell repertoire to cryptic ML determinants in WT mice was also intact in LysMcre mice. Furthermore, the pattern of response to the related homologue of ML-M, hen eggwhite lysozyme, was similar in these two groups of mice. Thus, several codominant T cell determinants within ML-M contribute significantly to tolerance induction, and the anti-cryptic T cell repertoire to ML-M was positively selected on non-ML-M self ligands. These results reveal that the induction of self tolerance to a multideterminant protein follows the quantitative hierarchy of self-determinant expression and are of relevance in understanding the pathogenesis of autoimmunity.

Antigen localization within the splenic marginal zone restores humoral immune response and IgG class switch in complement C4-deficient mice

Defects of early complement components (C1, C4 and C2) are associated with the development of systemic lupus erythematosus (SLE). The availability of complement knockout mice has increased our knowledge on the role of complement in the regulation of adaptive immunity. An impaired removal of apoptotic bodies, a disturbed clearance of IgG immune complexes (ICs) and an insufficient B-cell regulation via complement receptors CD21/CD35 have been discussed as explanations for the induction of autoimmunity; however, a unifying hypothesis for the loss of B-cell tolerance in the absence of C1 or C4 is still lacking. Using IgM-containing ICs, we observed a significant accumulation of antigen within the splenic marginal zone (MZ) of C4-deficient mice but not in C3-deficient or complement receptors CD21/CD35-deficient mice. The targeting of antigen toward the MZ restored adaptive immunity (antibody response and class switch) in C4-deficient animals. A new explanation for the association of SLE and complement C4 deficiency would be that in the absence of C4, natural antibodies (IgM type) localize more self-antigen toward the MZ so that the auto-antibody response is increased and autoimmune disease ensues. As such, an inadequate localization of self-antigens might be responsible for the annulment of peripheral B-cell tolerance in the absence of C4.

Recognition of the peripheral self by naturally arising CD25+ CD4+ T cell receptors

Naturally arising CD25+ CD4+ regulatory T cells (TR) play an important role in the prevention of autoimmunity. TCR specificity is thought to play a critical role in TR development and function, but the repertoire and specificity of TR TCRs remain largely unknown. We find by sequencing of TRAV14 (Valpha2) TCRalpha chains associated with a transgenic TCRbeta chain that the TRand CD25- CD4+ TCR repertoires are similarly diverse, yet only partially overlapping. Retroviral expression of TCRalpha genes in TCR transgenic RAG-deficient T cells revealed that a high frequency of TCRs derived from CD25+ but not CD25- CD4+ T cells confers the ability to rapidly expand upon transfer into a lymphopenic host. Thus, these data show that a large proportion of naturally arising TR have substantially more efficient interactions with MHC class II bound peptides from the peripheral self than CD25- T cells.

Incomplete tolerance to the tumour-associated antigen MDM2

MDM2 is a tumour-associated antigen widely expressed by normal tissues and over-expressed by many tumours of different origin. We wanted to define the level of immunological tolerance against MDM2 and explore its potential in tumour immunotherapy. Two murine MDM2 epitopes, pMDM100 and pMDM441, differ in their affinity for MHC class I molecules. Previous observations made in vitro suggested that pMDM100, due to its high affinity for K(b), induces a high level of tolerance, whereas tolerance to pMDM441, which binds poorly to D(b), is incomplete. In the present article we test the immunogenicity of these two peptides in vivo. Surprisingly, mice immunized with pMDM100 generated cytotoxic T lymphocytes (CTL) that killed tumour cell lines expressing MDM2 endogenously, indicating the existence of high-avidity CTL specific for a widely expressed protein. However, the response was limited as effector CTL disappeared after continued in vitro stimulation. While immunization with the individual MDM2 peptides did not protect against tumour challenge, mice immunized with both pMDM100 and pMDM441 were partially protected. These observations suggest that targeting of multiple epitopes may be required to vaccinate against tumours expressing elevated levels of CTL-recognized self-proteins.

From the hemangioblast to self-tolerance: a series of innovations gained from studies on the avian embryo

During the last decades of the 20th century, studies on the vertebrate hematopoietic and immune systems have largely been performed, on mammalian models. The mouse has been the preferred material for several cogent reasons: (i) numerous well defined genetic strains are available; (ii) this species has been and still is instrumental in the study of gene activity through transgenesis; and (iii) in vitro culture techniques and in vivo assays for blood cells together with a wide array of antibodies and nucleic acid probes have been developed to investigate the cellular interactions occurring during hematopoiesis and immune reactivity. However, important and fundamental notions have emerged from using another higher vertebrate model, the avian embryo. The distinction among small lymphocytes of two populations, the T and B lymphocytes, endowed with different roles in adaptive immunity and dependant on different environments for their specification, has relied on experiments carried out in birds. The avian model has been critical for the analysis of the origin and traffic of hematopoietic precursor cells. It allowed the demonstration that both hematopoietic and angioblastic lineages arise from a common precursor, a cell whose existence had been proposed but never undoubtedly proven, the hemangioblast. Finally a form of thymus-dependant 'dominant' tolerance was demonstrated on the basis of experiments in the avian embryo, which initiated a large current of studies on 'regulatory T-cells'. Work in this model during the last decades has relied strongly on the construction of chimeras between quail and chick embryos that allowed a refined analysis of cell behaviour during embryogenesis. The novel perception of developmental neuropoiesis and immunopoiesis that followed proved to be largely applicable to lower and higher vertebrates, notably mammals.

E3 ubiquitin ligases as T cell anergy factors

E3 ubiquitin ligases have emerged as key molecular regulators of immune cell function. Three families of proteins with ubiquitin ligase activity have been described (the HECT, RING and U-box proteins), and each may be involved in the regulation of immune responses during infection by targeting specific inhibitory molecules for proteolytic destruction. Several HECT and RING E3 proteins have now also been linked to the induction and maintenance of immune self-tolerance: c-Cbl, Cbl-b, GRAIL, Itch and Nedd4 each negatively regulate T cell growth factor production and proliferation. This review will discuss the relationship between the ubiquitination of select components of the antigen-sensing signaling apparatus in T cells and the development and maintenance of the clonal anergy state.

In vivo cyclophosphamide and IL-2 treatment impedes self-antigen-induced effector CD4 cell tolerization: implications for adoptive immunotherapy

The development of T cell tolerance directed toward tumor-associated Ags can limit the repertoire of functional tumor-reactive T cells, thus impairing the ability of vaccines to elicit effective antitumor immunity. Adoptive immunotherapy strategies using ex vivo expanded tumor-reactive effector T cells can bypass this problem; however, the susceptibility of effector T cells to undergoing tolerization suggests that tolerance might also negatively impact adoptive immunotherapy. Nonetheless, adoptive immunotherapy strategies can be effective, particularly those utilizing the drug cyclophosphamide (CY) and/or exogenous IL-2. In the current study, we used a TCR-transgenic mouse adoptive transfer system to assess whether CY plus IL-2 treatment rescues effector CD4 cell function in the face of tolerizing Ag (i.e., cognate parenchymal self-Ag). CY plus IL-2 treatment not only enhances proliferation and accumulation of effector CD4 cells, but also preserves the ability of these cells to express the effector cytokine IFN-gamma (and to a lesser extent TNF-alpha) in proportion to the level of parenchymal self-Ag expression. When administered individually, CY but not IL-2 can markedly impede tolerization, although their combination is the most effective. Although effector CD4 cells in CY plus IL-2-treated self-Ag-expressing mice eventually succumb to tolerization, this delay results in an increased level of in situ IFN-gamma expression in cognate Ag-expressing parenchymal tissues as well as death via a mechanism that requires direct parenchymal Ag presentation. These results suggest that one potential mechanism by which CY and IL-2 augment adoptive immunotherapy strategies to treat cancer is by impeding the tolerization of tumor-reactive effector T cells.

Naturally arising CD4+ regulatory t cells for immunologic self-tolerance and negative control of immune responses

Naturally occurring CD4+ regulatory T cells, the majority of which express CD25, are engaged in dominant control of self-reactive T cells, contributing to the maintenance of immunologic self-tolerance. Their depletion or functional alteration leads to the development of autoimmune disease in otherwise normal animals. The majority, if not all, of such CD25+CD4+ regulatory T cells are produced by the normal thymus as a functionally distinct and mature subpopulation of T cells. Their repertoire of antigen specificities is as broad as that of naive T cells, and they are capable of recognizing both self and nonself antigens, thus enabling them to control various immune responses. In addition to antigen recognition, signals through various accessory molecules and via cytokines control their activation, expansion, and survival, and tune their suppressive activity. Furthermore, the generation of CD25+CD4+ regulatory T cells in the immune system is at least in part developmentally and genetically controlled. Genetic defects that primarily affect their development or function can indeed be a primary cause of autoimmune and other inflammatory disorders in humans. Based on recent advances in our understanding of the cellular and molecular basis of this T cell-mediated immune regulation, this review discusses how naturally arising CD25+CD4+ regulatory T cells contribute to the maintenance of immunologic self-tolerance and negative control of various immune responses, and how they can be exploited to prevent and treat autoimmune disease, allergy, cancer, and chronic infection, or establish donor-specific transplantation tolerance.