Shaping of the autoreactive T-cell repertoire by a splice variant of self protein expressed in thymic epithelial cells

T cell response in experimental autoimmune encephalomyelitis (EAE): role of self and cross-reactive antigens in shaping, tuning, and regulating the autopathogenic T cell repertoire

T cells that can respond to self-antigens are present in the peripheral immune repertoire of all healthy individuals. Recently we have found that unmanipulated SJL mice that are highly susceptible to EAE also maintain a very high frequency of T cells responding to an encephalitogenic epitope of a myelin antigen proteolipid protein (PLP) 139-151 in the peripheral repertoire. This is not due to lack of expression of myelin antigens in the thymus resulting in escape of PLP 139-151 reactive cells from central tolerance, but is due to expression of a splice variant of PLP named DM20, which lacks the residues 116-150. In spite of this high frequency, the PLP 139-151 reactive cells remain undifferentiated in the periphery and do not induce spontaneous EAE. In contrast, SJL TCR transgenic mice expressing a receptor derived from a pathogenic T cell clone do develop spontaneous disease. This may be because in normal mice, autoreactive cells are kept in check by an alternate PLP 139-151 reactive nonpathogenic repertoire, which maintains a balance that keeps them healthy. If this is the case, selective activation of one repertoire or the other may alter susceptibility to autoimmune disease. Since T cells are generally cross-reactive, besides responding to nonself-antigens, they also maintain significant responses to self-antigens. Based on the PLP 139-151 system, we propose a model in which activation with foreign antigens can result in the generation of pathogenic memory T cells that mediate autoimmunity. We also outline circumstances under which activation of self-reactive T cells with foreign antigens can generate selective tolerance and thus generate protective/regulatory memory against self while still maintaining significant responses against foreign antigens. This provides a mechanism by which the fidelity and specificity of the immune system against foreign antigens is improved without increasing the potential for developing an autoimmune disease.

Peripheral antigen-expressing cells and autoimmunity

There is growing evidence that self-molecules with tissue-restricted expression are also expressed at low levels in the thymus, where such expression may affect the development of self-tolerance. Genetic factors may modulate such expression and, in turn, influence susceptibility to autoimmune responses against specific molecules. Contrasting reports exist regarding the phenotype of the cells that express self-molecules in the thymus. Indeed, both bone marrow derived antigen-presenting cells and thymic epithelial cells were reported to express self-molecules with tissue-restricted expression. Further studies of these cells and the mechanisms by which they exert their putative tolerogenic effects clearly are necessary.

Identification of autoimmune encephalomyelitis-associated common CDR3 sequences by CDR3 spectratyping and subsequent DNA hybridization

In previous studies, we demonstrated that T cell receptor (TCR) Vbeta8.2 and Vbeta10, both of which are frequently used by encephalitogenic T cells, spectratypes expand oligoclonally in spinal cord lesions of Lewis rats with experimental autoimmune encephalomyelitis (EAE) and that the DSSYEQYF and WDGSGNVLYF sequences are predominantly found in the complementarity-determining region 3 (CDR3) of spectratype-derived TCR clones. However, it is unknown whether these CDR3 sequences are used only by Vbeta8.2- and Vbeta10-positive T cells or by encephalitogenic T cells bearing Vbetas other than these Vbetas. The present study was undertaken to address this issue using a new approach, i.e. CDR3 spectratyping and subsequent DNA hybridization with several probes corresponding to various parts of the CDR3 region. Consequently, we found that probes specific for the Vbeta8.2 spectratype-derived Dbeta and Jbeta2.7 hybridized only with the Vbeta8.2 spectratype in acute EAE and with the Vbeta8.2 and Vbeta12 spectratypes in chronic relapsing EAE. Similarly, a probe specific for the Vbeta10 spectratype-derived Dbeta hybridized only with the Vbeta10 spectratype in both acute and chronic relapsing EAE. In contrast, a probe specific for Jbeta1.3 hybridized with several Vbeta spectratypes including Vbeta8.2 and Vbeta10 only during the early stage of the disease. These findings suggest that T cells bearing a few Vbetas with a limited number of the CDR3 sequences are activated in acute and chronic relapsing EAE induced in Lewis rats. Characterization of the CDR3 region of pathogenic TCR by this approach may be of value for the screening of autoimmune disease-associated TCR and for the development of TCR-based specific immunotherapy.

Perinatal blockade of b7-1 and b7-2 inhibits clonal deletion of highly pathogenic autoreactive T cells

A number of in vitro studies have suggested that costimulatory molecules B7-1 and B7-2 and their receptor CD28 can promote clonal deletion, and limited in vivo studies have indicated that CD28 is involved in the clonal deletion of some T cells. However, the significance of B7-mediated clonal deletion in preventing autoimmune diseases has not been studied systematically. Here we report that the perinatal blockade of B7-1 and B7-2 substantially inhibits the clonal deletion of T cells in the thymus and leads to an accumulation of T cells capable of inducing fatal multiorgan inflammation. These results reveal a critical role for costimulatory molecules B7-1 and B7-2 in deleting pathogenic autoreactive T cells in the thymus. The critical role of B7-1 and B7-2 in T cell clonal deletion may explain, at least in part, the paradoxical increase of autoimmune disease in mice deficient for this family of costimulatory molecules, such as cytotoxic T lymphocyte associated molecule 4, CD28, and B7-2. The strong pathogenicity of the self-reactive T cells supports a central hypothesis in immunology, which is that clonal deletion plays an important role in preventing autoimmune diseases.

Cutting edge: Attenuated experimental autoimmune encephalomyelitis in eta-1/osteopontin-deficient mice

Recent studies indicate that early T lymphocyte activation 1 (Eta-1), also known as osteopontin, is a cytokine contributing to the development of Th1 immunity. In the present report, the role of Eta-1 in experimental autoimmune encephalomyelitis (EAE), a disease associated with Th1 immunity, was examined by analysis of disease progression in Eta-1-deficient (Eta-1-/-) mice. Although incidence and onset of peptide-induced EAE were found to be similar in Eta-1-/- and Eta-1+/+ mice, Eta-1-/- mice displayed significantly lower mean maximal clinical score and faster recovery without spontaneous relapses. Accordingly, decreased inflammatory infiltration and demyelination were observed in the spinal cords of Eta-1-/- mice. Furthermore, in comparison to Eta-1+/+, Eta-1-/- CD4+ T cells had reduced expression of IFN-gamma and TNF-alpha upon ex vivo restimulation. Taken together, these results suggest that Eta-1 may sustain autoimmune responses by assisting in maintenance of Th1 immunity during EAE.

Oligodendrocytes expressing exclusively the DM20 isoform of the proteolipid protein gene: myelination and development

Oligodendroglia and Schwann cells synthesize myelin-specific proteins and lipids for the assembly of the highly organized myelin membrane of the motor-sensory axons in the central (CNS) and peripheral nervous system (PNS), respectively, allowing rapid saltatory conduction. The isoforms of the main myelin proteins, the peripheral myelin basic isoproteins (MBP) and the integral proteolipid proteins, PLP and DM20, arise from alternative splicing. Activation of a cryptic splice site in exon III of plp leads to the deletion of 105 bp encoding the PLP-specific 35 amino acid residues within the cytosolic loop 3 of the four-transmembrane domain (TMD) integral membrane protein. To study the different proposed functions of DM20 during the development of oligodendrocytes and in myelination, we targeted the plp locus in embryonic stem cells by homologous recombination by a construct, which allows solely the expression of the DM20 specific exon III sequence. The resulting dm20(only) mouse line expresses exclusively DM20 isoprotein, which is functionally assembled into the membrane, forming a highly ordered and tightly compacted myelin sheath. The truncated cytosolic loop devoid of the PLP-specific 35 amino acid residues, including two thioester groups, had no impact on the periodicity of CNS myelin. In contrast to the PLP/DM20-deficient mouse, mutant CNS of dm20(only) mice showed no axonal swellings and neurodegeneration but a slow punctuated disintegration of the compact layers of the myelin sheath and a rare oligodendrocyte death developing with aging.

Insights into autoimmunity gained from structural analysis of MHC-peptide complexes

The structural and functional properties of HLA-DQ and -DR molecules that confer susceptibility to several common autoimmune diseases, such as type 1 diabetes, rheumatoid arthritis and multiple sclerosis, have been defined. The relevant polymorphisms directly affect interaction with peptides, which provides strong support for the hypothesis that these diseases are peptide-antigen driven. Several studies indicate that structural modifications of peptides can affect MHC class II binding and/or TCR recognition and should be considered in the analysis of T cell responses in autoimmune diseases.

Pre-emptive targeting of the epitope spreading cascade with genetically modified regulatory T cells during autoimmune demyelinating disease

Epitope spreading or endogenous self-priming has been implicated in mediating the progression of autoimmune disease. In the present study we created an immune-deviated, epitope spreading response in SWXJ mice after the onset of experimental autoimmune encephalomyelitis, a prototypic autoimmune animal model widely used in multiple sclerosis research. We established an immunoregulatory spreading repertoire by transferring T cells genetically modified to produce high levels of IL-10 in response to a dominant epitope spreading determinant. Installation of a Th2/Tr1-like spreading repertoire resulted in a marked and prolonged inhibition of disease progression and demyelination characterized by 1) bystander inhibition of the recall response to the priming immunogen, and 2) a Th1-->Tr1 immune-deviated spreading response involving a shift in the source of IL-10 production from the transferred regulatory population to the host-derived, endogenously primed repertoire. Thus, our data provide a rationale for cell-based therapeutic intervention in multiple sclerosis by showing that pre-emptive targeting of the epitope spreading cascade with regulatory T cells effectively induces an immune-deviated spreading response capable of inhibiting ongoing inflammatory autoreactivity and disease progression.

CD4(+) regulatory T cells

There is now compelling evidence that CD4(+) T cells that specialize in the suppression of immune responses play a key role in the control of immune pathology. Recently, there have been a number of reports that have provided information on the generation of CD4(+) regulatory T cells in the thymus and in the periphery. These cells have also been identified in humans, paving the way for analysis of the function of CD4(+) regulatory T cells in immune-mediated disease.

A defect in central tolerance in NOD mice

The predisposition of nonobese diabetic (NOD) mice to develop autoimmune disease is usually attributed to defects in peripheral tolerance mechanisms. Here, evidence is presented that NOD mice display a defect in central tolerance (negative selection) of thymocytes. Impaired central tolerance in NOD mice was most prominent in a population of semi-mature thymocytes found in the medulla. The defect was apparent in vivo as well as in vitro, was independent of IAbetag7 expression and affected both Fas-dependent and Fas-independent pathways of apoptosis; for Fas-dependent apoptosis, the defective tolerance of NOD thymocytes correlated with the strong T cell receptor-mediated up-regulation of caspase 8-homologous FLICE (Fas-associated death-domain-like interleukin 1beta-converting enzyme)-inhibitory protein. In light of these findings, disease onset in NOD mice may reflect defects in central as well as peripheral tolerance.

T- and B-cell responses to myelin oligodendrocyte glycoprotein in experimental autoimmune encephalomyelitis and multiple sclerosis

The identification of myelin oligodendrocyte glycoprotein (MOG) as a target for autoantibody-mediated demyelination in experimental autoimmune encephalomyelitis (EAE) resulted in the re-evaluation of the role of B cell responses to myelin autoantigens in the immunopathogenesis of multiple sclerosis. MOG is a central nervous system specific myelin glycoprotein that is expressed preferentially on the outermost surface of the myelin sheath. Although MOG is only a minor component of CNS myelin it is highly immunogenic, inducing severe EAE in both rodents and primates. In rat and marmoset models of MOG-induced EAE demyelination is antibody-dependent and reproduces the immunopathology seen in many cases of MS. In contrast, in mice inflammation in the CNS can result in demyelination in the absence of a MOG-specific B cell response, although if present this will enhance disease severity and demyelination. Clinical studies indicate that autoimmune responses to MOG are enhanced in many CNS diseases and implicate MOG-specific B cell responses in the immunopathogenesis of multiple sclerosis. This review provides a summary of our current understanding of MOG as a target autoantigen in EAE and MS, and addresses the crucial question as to how immune tolerance to MOG may be maintained in the healthy individual.

The H2-Ab gene influences the severity of experimental allergic encephalomyelitis induced by proteolipoprotein peptide 103-116

Immunization of H2(p) and H2(q) congenic C3H mouse strains with the PLP 103-116 peptide elicited two distinct experimental allergic encephalomyelitis (EAE) disease courses. C3H.Q (H2(q)) mice developed an acute-phase disease with classical ascending paralytic signs whereas C3H.NB (H2(p)) developed a highly variable disease course with symptoms originating from CNS above the spinal chord. C3H.Q lacks functional H2-E molecules and share H2-Aalpha with C3H.NB. To examine if the differences found at positions 85, 86, 88, and 89 in the Abeta-chains account for disease susceptibility, H2(q) mice were made transgenic with the Ab(p) gene. The Ab(p)-transgenic mice on the C3H.Q background developed a more severe disease course, demonstrating the importance of class II. However, the onset was not affected and the disease showed a classical ascending paralysis similar to the C3H.Q suggesting that the observed brain symptoms were related to nonclass II genes. Inhibition studies performed on affinity purified MHC class II molecules indicated that the PLP 103-116 peptide bound to A(p) with slightly higher affinity than to A(q). Both A(q) and A(p) formed long-lived stable complexes (t(1/2)>24 h) with the PLP 103-116 peptide, but a higher amount of the peptide was loaded on to A(p) compared with A(q). An F2 gene segregation experiment, in which the low PLP 103-116 binding A(r) molecule and the high binding A(p) molecule could be compared for the influence on the disease susceptibility, indicated a role for both peptide binding affinity and non-MHC genes. Based on our results, we conclude that the H2-Ab gene controls severity of EAE but not necessarily the onset or type of disease course and that affinity of the disease-promoting peptide for the class II molecule is a critical pathogenic factor.

Promiscuous gene expression in medullary thymic epithelial cells mirrors the peripheral self

Expression of peripheral antigens in the thymus has been implicated in T cell tolerance and autoimmunity. Here we identified medullary thymic epithelial cells as being a unique cell type that expresses a diverse range of tissue-specific antigens. We found that this promiscuous gene expression was a cell-autonomous property of medullary epithelial cells and was maintained during the entire period of thymic T cell output. It may facilitate tolerance induction to self-antigens that would otherwise be temporally or spatially secluded from the immune system. However, the array of promiscuously expressed self-antigens appeared random rather than selected and was not confined to secluded self-antigens.

Lymphostromal interactions in thymic development and function

The generation of a peripheral T-cell pool is essential for normal immune system function. CD4+ and CD8+ T cells are produced most efficiently in the thymus, which provides a complexity of discrete cellular microenvironments. Specialized stromal cells, that make up such microenvironments, influence each stage in the maturation programme of immature T-cell precursors. Progress has recently been made in elucidating events that regulate the development of intrathymic microenvironments, as well as mechanisms of thymocyte differentiation. It is becoming increasingly clear that the generation and maintenance of thymic environments that are capable of supporting efficient T-cell development, requires complex interplay between lymphoid and stromal compartments of the thymus.

Frequency, heterogeneity and encephalitogenicity of T cells specific for myelin oligodendrocyte glycoprotein in naive outbred primates

Auto-reactive T cells present in healthy subjects remain in a state of unresponsiveness, but may trigger autoimmunity under various situations. Although myelin oligodendrocyte glycoprotein (MOG) is a potential target antigen in multiple sclerosis (MS), MOG-reactive T cell responses are present in the blood of both healthy subjects and MS-affected individuals. To investigate the disease-inducing potential and regulation of these autoreactive T cells in healthy outbred populations, we have characterized MOG-reactive T cell clones obtained by limiting dilution from peripheral blood of unimmunized C. jacchus marmosets. We report an extraordinarily high prevalence of circulating MOG-reactive T cells in these naive animals (2.6 +/- 1.4 / 10(5) PBMC), and a broadly diverse repertoire of epitope recognition encompassing at least three regions within the extracellular domain of MOG. Adoptive transfer of a MOG21-40-specific T cell clone resulted in mild clinical experimental allergic encephalomyelitis, characterized pathologically by rare foci of inflammation and minimal demyelination. We conclude that MOG-reactive T cells are present in healthy primates at a highly prevalent frequency, and are potentially capable of triggering central nervous system autoimmunity. Expansion of these autoreactive T cells must be tightly controlled to maintain immune homeostasis in healthy individuals.

The thymus and central tolerance

T-cell differentiation in the thymus generates a peripheral repertoire of mature T cells that mounts strong responses to foreign antigens but is largely unresponsive to self-antigens. This state of specific immunological tolerance to self-components involves both central and peripheral mechanisms. Here we review the process whereby many T cells with potential reactivity for self-antigens are eliminated in the thymus during early T-cell differentiation. This process of central tolerance (negative selection) reflects apoptosis and is a consequence of immature T cells receiving strong intracellular signalling through T-cell receptor (TCR) recognition of peptides bound to major histocompatibility complex (MHC) molecules. Central tolerance occurs mainly in the medullary region of the thymus and depends upon contact with peptide-MHC complexes expressed on bone-marrow-derived antigen-presenting cells (APCs); whether tolerance also occurs in the cortex is still controversial. Tolerance induction requires a combination of TCR ligation and co-stimulatory signals. Co-stimulation reflects interaction between complementary molecules on T cells and APCs and probably involves multiple molecules acting in consort, which may account for why deletion of individual molecules with known or potential co-stimulatory function has little or no effect on central tolerance. The range of self-antigens that induce central tolerance is considerable and, via low-level expression in the thymus, may also include tissue-specific antigens; central tolerance to these latter antigens, however, is likely to be limited to high-affinity T cells, leaving low-affinity cells to escape. Tolerance to alloantigens and the possibility of using central tolerance to promote acceptance of allografts are discussed.

A pathogenic role for myelin-specific CD8(+) T cells in a model for multiple sclerosis

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system (CNS) characterized by plaques of infiltrating CD4(+) and CD8(+) T cells. Studies of MS and experimental autoimmune encephalomyelitis (EAE), an animal model of MS, focus on the contribution of CD4(+) myelin-specific T cells. The role of CD8(+) myelin-specific T cells in mediating EAE or MS has not been described previously. Here, we demonstrate that myelin-specific CD8(+) T cells induce severe CNS autoimmunity in mice. The pathology and clinical symptoms in CD8(+) T cell-mediated CNS autoimmunity demonstrate similarities to MS not seen in myelin-specific CD4(+) T cell-mediated EAE. These data suggest that myelin-specific CD8(+) T cells could function as effector cells in the pathogenesis of MS.

Inducible IL-2 production by dendritic cells revealed by global gene expression analysis

Dendritic cells (DCs) are strong activators of primary T cell responses. Their priming ability is acquired upon encounter with maturation stimuli. To identify the genes that are differentially expressed upon maturation induced by exposure to Gram-negative bacteria, a kinetic study of DC gene expression was done with microarrays representing 11,000 genes and ESTs (expressed sequence tags). Approximately 3000 differentially expressed transcripts were identified. We found that functional interleukin 2 (IL-2) mRNA, which gave rise to IL-2 production, was transiently up-regulated at early time-points after bacterial encounter. In contrast, macrophages did not produce IL-2 upon bacterial stimulation. Thus, IL-2 is an additional key cytokine that confers unique T cell stimulatory capacity to DCs.

Thiopalmitoylation of myelin proteolipid protein epitopes enhances immunogenicity and encephalitogenicity

Proteolipid protein (PLP) is the most abundant protein of CNS myelin, and is posttranslationally acylated by covalent attachment of long chain fatty acids to cysteine residues via a thioester linkage. Two of the acylation sites are within epitopes of PLP that are encephalitogenic in SJL/J mice (PLP(104-117) and PLP(139-151)) and against which increased immune responses have been detected in some multiple sclerosis patients. It is known that attachment of certain types of lipid side chains to peptides can result in their enhanced immunogenicity. The aim of this study was to determine whether thioacylated PLP peptides, as occur in the native protein, are more immunogenic than their nonacylated counterparts, and whether thioacylation influences the development of autoreactivity and experimental autoimmune encephalomyelitis. The results show that in comparison with nonacylated peptides, thioacylated PLP lipopeptides can induce greater T cell and Ab responses to both the acylated and nonacylated peptides. They also enhanced the development and chronicity of experimental autoimmune encephalomyelitis. Synthetic peptides in which the fatty acid was attached via an amide linkage at the N terminus were not encephalitogenic, and they induced greater proportions of CD8+ cells in initial in vitro stimulation. Therefore, the lability and the site of the linkage between the peptide and fatty acid may be important for induction of encephalitogenic CD4+ T cells. These results suggest that immune responses induced by endogenous thioacylated lipopeptides may contribute to the immunopathogenesis of chronic experimental demyelinating diseases and multiple sclerosis.

Autoimmune disease: why and where it occurs

Autoimmune disease is controlled by genetic and environmental factors. Both of these affect susceptibility to autoimmunity at three levels: the overall reactivity of the immune system, the specific antigen and its presentation, and the target issue.

Sampling of complementing self-antigen pools by thymic stromal cells maximizes the scope of central T cell tolerance

Expression of peripheral antigens in the thymus has been implicated in T cell tolerance and autoimmunity, yet the identity of cells involved remains elusive. Here we show that antigen expression in a minor fraction of medullary thymic epithelial cells leads to deletion of specific CD4 T cells. Strikingly, this deletion is not dependent on cross-presentation by hemopoietic antigen-presenting cells, which have been ascribed a predominant role in negative selection. By contrast, when the same antigen enters the thymus via the blood stream, negative selection is strictly dependent on antigen presentation by hemopoietic cells. These findings imply that the (re)-presentation of "self" by thymic stromal cells is non-redundant, and that different thymic antigen-presenting cells instead cover complementing sets of self-antigens, thus maximizing the scope of central tolerance

Normal CNS myelination in transgenic mice overexpressing MHC class I H-2L(d) in oligodendrocytes

In demyelinating diseases, such as multiple sclerosis, an upregulation of MHC class I expression is thought to contribute to oligodendrocyte/myelin damage. In order to investigate potential physiological consequences of upregulated MHC class I expression in oligodendrocytes, we generated transgenic mice that overexpress H-2L(d) under the control of the proteolipid protein (PLP) promoter (PLP-L(d) mice). We focused our studies on the MHC class I molecule H-2L(d), because of its unique intracellular transport characteristics. In the CNS of PLP-L(d) mice, H-2L(d) was expressed by oligodendrocytes. Furthermore, H-2L(d) protein was transported to and expressed on the surface of oligodendrocytes. Most importantly, this upregulation of MHC class I expression in the CNS of PLP-L(d) mice did not by itself result in a de- or dysmyelinating phenotype. These transgenic mice are likely to provide a unique and novel tool for the analysis of potential roles of MHC class I-mediated mechanisms in demyelinating pathologies.

Mice resistant to experimental autoimmune encephalomyelitis have increased thymic expression of myelin basic protein and increased MBP specific T cell tolerance

The relationship between expression of the autoantigens in thymi and susceptibility to autoimmune disease was determined in the experimental autoimmune encephalomyelitis (EAE) model. In two different sets of MHC congenic strains of mice characterized by differential susceptibility to EAE, levels of expression of MBP were shown to be higher in the more resistant strain. These data raised the possibility that more central tolerance to MBP may occur in more resistant strains. Differential tolerance was then evidenced by a decrease in T cell responses to MBP 83-102 in the more resistant strains. Together, these data indicate that the list of non-MHC genes involved in susceptibility to autoimmune disease should include genes which regulate expression of autoantigens in thymi.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Differential splicing of the IA-2 mRNA in pancreas and lymphoid organs as a permissive genetic mechanism for autoimmunity against the IA-2 type 1 diabetes autoantigen

Type 1 diabetes results from the autoimmune destruction of pancreatic beta-cells in genetically susceptible individuals. Growing evidence suggests that genetically determined variation in the expression of self-antigens in thymus may affect the shaping of the T-cell repertoire and susceptibility to autoimmunity. For example, both allelic variation and parent-of-origin effects influence the thymic expression of insulin (a known type 1 diabetes autoantigen), and insulin gene transcription levels in thymus inversely correlate with susceptibility in both humans and transgenic models. It is unclear why patients lose tolerance to IA-2 (insulinoma-associated tyrosine phosphatase-like protein, or islet cell antigen 512 [ICA512]), especially because IA-2 polymorphisms are not associated with type 1 diabetes. We report that alternative splicing determines differential IA-2 expression in islets compared with thymus and spleen. Islets express full-length mRNA and two alternatively spliced transcripts, whereas thymus and spleen exclusively express an alternatively spliced transcript lacking exon 13. This encodes for the transmembrane (TM) and juxta-membrane (JM) domains that comprise several type 1 diabetes target epitopes, supporting the concept that tolerance to IA-2 epitopes not expressed in lymphoid organs may not be achieved. We propose differential splicing as a regulatory mechanism of gene expression playing a permissive role in the development of autoimmune responses to IA-2. Our findings also show that candidate gene expression studies can help in dissecting the complex genetic determinants of a multifactorial disease such as type 1 diabetes.

Dendritic cells derived from patients with multiple sclerosis show high CD1a and low CD86 expression

Dendritic cells (DC) are important antigen presenting cells (APC) and play a major role in initiating and orchestrating immune responses by priming T cells. Little is known about involvement of DC in multiple sclerosis (MS), where auto-aggressive T cells against myelin autoantigens are considered to contribute to inflammation and demyelination in the central nervous system. In this study, we compared phenotype and cytokine secretion of DC from patients with MS, other neurological diseases (OND) and healthy subjects. DC were generated from blood adherent mononuclear cells (MNC) by culture for 7 days with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). The yield and morphology of DC were similar in MS patients and controls. In both, the DC phenotype was that of immature myeloid lineage, comprising CD1a+ and CD11c+. The proportion of CD1a+ DC, being important for presentation of lipid antigens to T cells, was higher in MS patients compared to controls. The proportion of CD86+ DC, a co-stimulatory molecule that is assumed to promote Th2 differentiation, was low in MS. Low proportions of CD86+ DC were only observed in untreated MS patients but not in patients treated with IFN-beta. Production of IL-10 and IL-12 p40 by DC did not differ in MS patients and controls. These findings indicate that alterations of functionally important surface molecules on DC are associated with MS.

Frequencies of neuroantigen-specific T cells in the central nervous system versus the immune periphery during the course of experimental allergic encephalomyelitis

Direct measurements of the frequency and the cytokine signature of the neuroantigen-specific effector cells in experimental allergic encephalomyelitis (EAE) are a continuing challenge. This is true for lymphoid tissues, and more importantly, for the CNS itself. Using enzyme-linked immunospot analysis (ELISPOT) assays, we followed proteolipid protein (PLP) 139--151-specific T cells engaged by active immunization of SJL mice. The total numbers of PLP(139--151)-specific CD4 cells were highest before disease onset. At this time, these cells resided in lymphoid and nonlymphoid tissues, but were not detected in the CNS. While the PLP(139--151)-specific cells reached high frequencies in the CNS during clinical EAE, in absolute numbers, less than 20% of them were present in the target organ, with the majority residing in the periphery throughout all stages of the disease. The numbers of PLP(139--151)-specific cells gradually declined in both compartments with time. While eventually this first wave of effector cells completely disappeared from the CNS, PLP(178--191)-specific cells became engaged, being detected first in the CNS. These data suggest that throughout all stages of EAE, the effector cells in the CNS are recruited from a vast peripheral reservoir, and that the second wave of effector cells is engaged while the first wave undergoes exhaustion.

Age-dependent T cell tolerance and autoimmunity to myelin basic protein

Experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis, is induced by activating a subset of myelin basic protein (MBP)-specific T cells that have escaped tolerance induction. Here, we define the tolerance mechanisms that eliminate the majority of MBP-specific T cells from the periphery. We show that MBP-specific T cells undergo central tolerance mediated by bone marrow-derived antigen-presenting cells presenting exogenously derived MBP epitopes. The efficiency of tolerance is age dependent, reflecting the developmentally regulated expression of MBP. Dependence of tolerance on the amount of MBP expressed in vivo results in an age window of susceptibility to EAE in mice that peaks during puberty. These results suggest that factors regulating expression of self-antigens in vivo can influence susceptibility to autoimmunity.

An unexpected version of horror autotoxicus: anaphylactic shock to a self-peptide

EAE can refer either to experimental autoimmune encephalomyelitis or experimental allergic encephalomyelitis. Although EAE is classically a prototypic T helper 1 (TH1) cell-mediated autoimmune disease, it can also be induced by TH2 cells. Characteristically, the most severe manifestation of allergy, anaphylaxis, is associated with exposure to a foreign antigen that is often derived from medication, insect venom or food. We report here that, after self-tolerance to myelin is destroyed, anaphylaxis may be triggered by a self-antigen, in this case a myelin peptide. "Horror autotoxicus", which was initially described by Ehrlich, may not only include autoimmunity to self, it may also encompass immediate hypersensitivity to self, which leads to shock and rapid death.

P0 glycoprotein peptides 56-71 and 180-199 dose-dependently induce acute and chronic experimental autoimmune neuritis in Lewis rats associated with epitope spreading

Two synthetic peripheral nerve myelin P0 protein peptides, an immunodominant (amino acids 180-199) and a cryptic (amino acids 56-71) one, induced an acute or chronic course of experimental autoimmune neuritis (EAN) in Lewis rats, when given at low dose (50-100 microg/rat) or high dose (250 microg/rat), respectively. Corresponding to the different clinical course, pathological changes and immune responses were found: (1) Onset of clinical signs of P0 peptide 56-71 (P0 56-71) induced EAN was 1-3 days later than in P0 peptide 180-199 (P0 180-199) induced EAN at all immunizing doses, whereas the peak of the disease occurred at a similar time point post immunization (p.i.), i.e. at days 14-16 p.i. in P0 56-71 induced EAN and at day 16 p.i. in P0 180-199 induced EAN. (2) Intramolecular epitope spreading as assessed by delayed type hypersensitivity response occurred in P0 56-71 induced EAN at both low and high antigen doses and in P0 180-199 induced EAN at high antigen dose (250 microg/rat) only. (3) P0 180-199 stimulated higher levels of interferon-gamma production in P0 180-199 induced EAN than in P0 56-71 induced EAN and vice versa. (4) Histopathologic evaluation revealed a similar grade of mononuclear cell infiltration in the sciatic nerves of both types of EAN, but more severe demyelination was found in P0 180-199 induced EAN compared to P0 56-71 induced EAN. The results support the hypothesis that high dose autoantigen immunization induces extensive determinant spreading and chronic course of autoimmune diseases.

Self-antigen-presenting cells expressing diabetes-associated autoantigens exist in both thymus and peripheral lymphoid organs

Recent reports indicate that genes with tissue-restricted expression, including those encoding the type 1 diabetes autoantigens insulin, glutamic acid decarboxylase (GAD), and the tyrosine-phosphatase-like protein IA-2 (or ICA512), are transcribed in the thymus. The reported modulation of diabetes susceptibility by genetically determined differences in thymic insulin levels and studies in transgenic mice provide correlative and functional evidence that thymic expression of peripheral proteins is crucial for immunological self-tolerance. However, there are no specific data about the existence, tissue distribution, phenotype, and function of those cells that express insulin and other self-antigens in the human thymus. We find that the human thymus harbors specialized cells synthesizing (pro)insulin, GAD, and IA-2, mainly localized in the medulla, and we demonstrate such cells also in peripheral lymphoid organs (spleen and lymph nodes). Phenotypic analysis qualifies these cells as antigen-presenting cells (APCs), including both dendritic cells and macrophages. These cells often appear surrounded by apoptotic lymphocytes, both in thymus and spleen, and may therefore be involved in the deletion of autoreactive lymphocytes. Our findings demonstrate the existence of, and define the tissue distribution and phenotype of, a novel subset of APCs expressing self-antigens in human lymphoid organs that appear to be involved in the regulation of self-tolerance throughout life.

The origin and regulation of autopathogenic T cells

A clear understanding of the events surrounding the selection of autoreactive T cells in the thymus and their regulation in the periphery has eluded immunologists for years. However, recent work examining the expression of tissue-specific antigens in the thymus and the biochemistry of disease associated MHC alleles has provided important clues into the generation of the autoreactive T cell repertoire in the thymus. In addition, recent studies focusing on the role of immunoregulatory cytokines and cross-reactive peptide ligands has provided information regarding both the regulation and activation of autoreactive cells in the periphery. An improved understanding of the selection and regulation of autoreactive T cells will undoubtedly aid in the development of strategies for treating autoimmune disease.

Transplantation tolerance and autoimmunity after xenogeneic thymus transplantation

Successful grafting of vascularized xenografts (Xgs) depends on the ability to reliably induce both T cell-independent and -dependent immune tolerance. After temporary NK cell depletion, B cell suppression, and pretransplant infusion of donor Ags, athymic rats simultaneously transplanted with hamster heart and thymus Xgs developed immunocompetent rat-derived T cells that tolerated the hamster Xgs but provoked multiple-organ autoimmunity. The autoimmune syndrome was probably due to an insufficient development of tolerance for some rat organs; for example, it led to thyroiditis in the recipient rat thyroid, but not in simultaneously transplanted donor hamster thyroid. Moreover, grafting a mixed hamster/rat thymic epithelial cell graft could prevent the autoimmune syndrome. These experiments indicate that host-type thymic epithelial cells may be essential for the establishment of complete self-tolerance and that mixed host/donor thymus grafts may induce T cell xenotolerance while maintaining self-tolerance in the recipient.

Early requirement for B cells for development of spontaneous autoimmune thyroiditis in NOD.H-2h4 mice

B cells are known to play an important role in the pathogenesis of several autoimmune diseases. NOD.H-2h4 mice develop spontaneous autoimmune thyroiditis (SAT) and anti-mouse thyroglobulin (MTg) autoantibodies, the levels of which correlate closely with the severity of thyroid lesions. NOD.H-2h4 mice genetically deficient in B cells (NOD.Kmu(null)) or rendered B cell-deficient by treatment from birth with anti-IgM develop minimal SAT. B cells were required some time in the first 4-6 wk after birth, because NOD.Kmu(null) or NOD.H-2h4 mice did not develop SAT when they were reconstituted with B cells as adults. The requirement for B cells was apparently not solely to produce anti-MTg autoantibodies, because passive transfer of anti-MTg Ab did not enable B cell-deficient mice to develop SAT, and mice given B cells as adults produced autoantibodies but did not develop SAT. B cell-deficient mice developed SAT if their T cells developed from bone marrow precursors in the presence of B cells. Because B cells are required early in life and their function cannot be replaced by anti-MTg autoantibodies, B cells may be required for the activation or selection of autoreactive T cells. These autoreactive T cells are apparently unable to respond to Ag if B cells are absent in the first 4-6 wk after birth.

Myelin oligodendrocyte glycoprotein-DNA vaccination induces antibody-mediated autoaggression in experimental autoimmune encephalomyelitis

One strategy to reestablish self tolerance in autoimmune diseases is based on the use of DNA vaccination to induce ectopic expression of the target autoantigen. We assessed the potential of vaccination with a DNA construct encoding the myelin oligodendrocyte glycoprotein (MOG), an important candidate autoantigen in multiple sclerosis, to induce tolerance and protect against experimental autoimmune encephalomyelitis (EAE). Unexpectedly, mice vaccinated with MOG-DNA developed an exacerbated form of EAE when challenged with either MOG or an unrelated encephalitogen, myelin proteolipid protein. We demonstrate that this is due to the inability of DNA vaccination to tolerize the MOG-specific T cell response and to the concomitant induction of a cytopathic MOG-specific autoantibody response, which is pathogenic, enhancing demyelination, inflammation and disease severity. Our data suggest that tolerogenic strategies for autoimmune diseases based on DNA vaccination should be approached with caution, as the outcome is unpredictable.

The role of autoantigens in autoimmune disease

Many autoantigens have been identified in human patients and in rodent models. In numerous experimental settings, these autoantigens or related autoreactive lymphocytes can transfer autoimmunity. Although autoreactivity spreads to new epitopes during the course of disease, single-epitope-specific therapies show considerable efficacy in multi-epitope-induced models of autoimmunity. These observations may indicate that epitope-specific therapies operate at the level of regulating mechanisms of immune tolerance rather than exerting a direct effect on autoreactive T lymphocytes.

Detection of autoreactive T cells in H-2u mice using peptide-MHC multimers

Myelin basic protein (MBP)-specific T cells play a critical role in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), a prototype for T cell-mediated autoimmunity. In PL/J and B10.PL mice (H-2(u) haplotype), the immunodominant epitope of MBP is represented by an N-terminal nonameric peptide, MBP1-9. To date, the MBP1-9-specific T cell repertoire has not been analyzed in quantitative terms. In the present study we demonstrate, using MHC class II tetramers, that 15,000-70,000 self-antigen-specific T(h) cells accumulate in the draining lymph nodes following immunization with spinal cord homogenate or MBP1-9. In contrast, MBP1-9-specific T cells are undetectable in unimmunized H-2(u) mice and represent >60% of the CD4 cells in naive mice transgenic for a TCR specific for this epitope. The results suggest that the extremely low affinity of the N-terminal peptide for I-A(u) does not limit the MBP1-9-specific T cells from expanding into a sizeable pool of autoreactive T cells. Therefore, the primary immune response to MBP1-9 does not differ quantitatively from previously reported CD4(+) T cell responses to foreign antigens.

T- and B-cell nonresponsiveness to self-alphaB-crystallin in SJL mice prevents the induction of experimental allergic encephalomyelitis

The myelin-associated stress protein alphaB-crystallin triggers strong proliferative responses and IFN-gamma production by human T cells and it is considered a candidate autoantigen in multiple sclerosis. In this study we examined the capacity of alphaB-crystallin or peptides derived thereof to induce experimental autoimmune encephalomyelitis (EAE) in SJL mice. Despite extensive efforts to induce EAE using active immunization with whole alphaB-crystallin, using adoptive transfer of lymphocytes or using peptide immunizations, no clinical or histological signs of EAE could be induced. SJL mice were unable to mount proliferative T-cell responses in vitro or delayed-type hypersensitivity responses in vivo to self-alphaB-crystallin. Also, immunization with self-alphaB-crystallin did not lead to any antibody response in SJL mice while bovine alphaB-crystallin triggered clear antibody responses within 1 week. Immunizations with alphaB-crystallin-derived peptides led to the activation of IL-4-producing Th2 cells and only a few IFN-gamma-producing Th1 cells. Peptide-specific T cells showed no cross-reactivity against whole alphaB-crystallin. The inability of SJL mice to mount proinflammatory T-cell responses against self-alphaB-crystallin readily explains the lack of EAE induction by immunization with whole protein or peptides derived from it. T- and B-cell nonresponsiveness is associated with constitutive expression of full-length alphaB-crystallin in both primary and secondary lymphoid organs in SJL mice, which is seen in other mammals as well, but not in humans.

Adrenal autoimmunity: results and developments

Autoimmune Addison's disease (autoimmune adrenalitis) often occurs in autoimmune polyendocrinopathy syndromes APS1 (APECED) and APS2. Although the genetic background and etiology of the two syndromes is remarkably different, they both result in a similar autoimmune destruction of the adrenal cortex. Recently, the defective gene in APS1, AIRE (autoimmune regulator) was identified, whereas in APS2, the major genetic factor remains to be found in the human major histocompatibility complex haplotype (HLA) region. In addition to the genetic factors, the recent findings in genetics and immunity leading to the pathogenesis of adrenal autoimmunity in polyendocrinopathy syndromes are discussed.

Differential splicing of antigen-encoding RNA reduces endogenous epitope presentation that regulates the expansion and cytotoxicity of T cells

The activation of CTLs is dependent on the recognition of MHC-bound peptide present on the surface of APCs. We give evidence in this study that differential splicing of Ag-encoding RNA can decrease the antigenic dose in APCs and regulate the recall of human memory CTLs. Differential splicing of RNA that encoded an immunodominant HLA-B8-restricted CTL epitope of EBV reduced the functional presentation of this epitope, and consequently the in vitro expansion and activity of CTLs, as measured by MHC/peptide-tetramer staining and cytotoxicity assays. The reduced activity of the stimulated CTLs was not only due to lower numbers of Ag-specific CTLs but, surprisingly, was also characterized by decreased cytotoxicity of the CTLs to target cells presenting limiting amounts of the peptide epitope. As indicated by TCR repertoire analysis, the reduction in CTL activity was not caused by stimulation of distinct populations of TCR clonotypes. This study demonstrates how a common eukaryotic posttranscriptional mechanism of gene regulation can modulate the endogenous presentation of Ag and ultimately contribute to the fine tuning of immunological memory cells, which are important in the fight against pathogens and tumors and in autoimmunity.

CD8(+) T-cell autoreactivity to an HLA-B27-restricted self-epitope correlates with ankylosing spondylitis

HLA-B27 is highly associated with ankylosing spondylitis (AS), but the mechanism is unknown. Among the HLA-B27 alleles, B*2709, which differs by one amino acid from the susceptible B*2705, is not associated with the disease. Here, we analyze the reactivity, in patients with AS and in healthy controls carrying the B*2709 or B*2705 alleles, to an EBV epitope derived from LMP2 (236-244) and to a sequence-related self-peptide from vasoactive intestinal peptide receptor 1 (VIP1R 400-408). We found that both B*2705(+) and B*2709(+) subjects possess LMP2 236-244-specific, HLA-B27-restricted T cells, whereas only the B*2705(+) individuals respond significantly to VIP1R 400-408. These results prompted us to compare, by IFN-gamma ELISPOT analysis, the T-cell response to VIP1R 400-408 in patients with AS versus B*2705 healthy controls. The data show that VIP1R 400-408-specific reactivity is a major feature of the patients with AS. These findings show, for the first time to our knowledge, a widespread reactivity in patients with AS against a self-epitope that exhibits some features of a putative "arthritogenic" peptide.

Self-antigen presentation by thymic stromal cells: a subtle division of labor

Self-antigen-MHC complexes expressed by thymic stromal cells serve as ligands for TCR-mediated positive and negative selection, resulting in a self-MHC-restricted, self-tolerant T cell repertoire. It has recently become apparent that thymic stromal cells differ in their accessibility to antigen as well as their ability to process and present antigen. These differences result in the sampling by thymic stromal cells of largely nonoverlapping self-antigen pools and the display of self-peptide profiles specific for each cell type. In conjunction with single or serial cell-cell interactions between thymocytes and stromal cells, such differences in self-antigen display allow for maximal (re)presentation of 'self' in the thymus and optimize the efficacy of positive and negative selection.

Fulminant spontaneous autoimmunity of the central nervous system in mice transgenic for the myelin proteolipid protein-specific T cell receptor

Proteolipid protein (PLP)-139-151 is the dominant encephalitogenic peptide that induces experimental autoimmune encephalomyelitis (EAE) in SJL (H-2(s)) mice. To examine the contribution of T cell receptor (TCR) specificity in the induction of EAE, we generated transgenic mice expressing the rearranged TCR genes from an encephalitogenic or a nonencephalitogenic PLP-139-151/I-A(s)-specific T cell clone. Both types of transgenic lines developed spontaneous EAE, but, remarkably, the lines expressing the TCR from the nonencephalitogenic clone showed increasingly higher frequencies of disease (60-83%) in progressive SJL backcrosses and could not be propagated on the susceptible background. The T cells from the transgenic mice were not tolerized, because they responded vigorously to the antigen in vitro and mediated EAE when the mice were immunized with antigen. Besides being the only description of a TCR transgenic mice for the PLP-139-151/I-A(s) epitope, the results demonstrate that the TCR from a nonencephalitogenic PLP-specific T cell clone can induce autoimmune disease when expressed appropriately in vivo.