Selecting the T cell receptor repertoire

Dysregulated RASGRP1 expression through RUNX1 mediated transcription promotes autoimmunity

RasGRP1 is a Ras guanine nucleotide exchange factor, and an essential regulator of lymphocyte receptor signaling. In mice, Rasgrp1 deletion results in defective T lymphocyte development. RASGRP1-deficient patients suffer from immune deficiency, and the RASGRP1 gene has been linked to autoimmunity. However, how RasGRP1 levels are regulated, and if RasGRP1 dosage alterations contribute to autoimmunity remains unknown. We demonstrate that diminished Rasgrp1 expression caused defective T lymphocyte selection in C57BL/6 mice, and that the severity of inflammatory disease inversely correlates with Rasgrp1 expression levels. In patients with autoimmunity, active inflammation correlated with decreased RASGRP1 levels in CD4+ T cells. By analyzing H3K27 acetylation profiles in human T cells, we identified a RASGRP1 enhancer that harbors autoimmunity-associated SNPs. CRISPR-Cas9 disruption of this enhancer caused lower RasGRP1 expression, and decreased binding of RUNX1 and CBFB transcription factors. Analyzing patients with autoimmunity, we detected reduced RUNX1 expression in CD4+ T cells. Lastly, we mechanistically link RUNX1 to transcriptional regulation of RASGRP1 to reveal a key circuit regulating RasGRP1 expression, which is vital to prevent inflammatory disease.

Regulating the regulators: Is introduction of an antigen-specific approach in regulatory T cells the next step to treat autoimmunity?

In autoimmunity, the important and fragile balance between immunity and tolerance is disturbed, resulting in abnormal immune responses to the body's own tissues and cells. CD4⁺CD25^hiFoxP3⁺ regulatory T cells (Tregs) induce peripheral tolerance in vivo by means of direct cell-cell contact and release of soluble factors, or indirectly through antigen-presenting cells (APC), thereby controlling auto-reactive effector T cells. Based on these unique capacities of Tregs, preclinical studies delivered proof-of-principle for the clinical use of Tregs for the treatment of autoimmune diseases. To date, the first clinical trials using ex vivo expanded polyclonal Tregs have been completed. These pioneering studies demonstrate the feasibility of generating large numbers of polyclonal Tregs in a good manufacturing practices (GMP)-compliant manner, and that infusion of Tregs is well tolerated by patients with no evidence of general immunosuppression. Nonetheless, only modest clinical results were observed, arguing that a more antigen-specific approach might be needed to foster a durable patient-specific clinical cell therapy without the risk for general immunosuppression. In this review, we discuss current knowledge, applications and future goals of adoptive immune-modulatory Treg therapy for the treatment of autoimmune disease and transplant rejection. We describe the key advances and prospects of the potential use of T cell receptor (TCR)- and chimeric antigen receptor (CAR)-engineered Tregs in future clinical applications. These approaches could deliver the long-awaited breakthrough in stopping undesired autoimmune responses and transplant rejections.

One, No One, and One Hundred Thousand: T Regulatory Cells' Multiple Identities in Neuroimmunity

As the Nobel laureate Luigi Pirandello wrote in his novels, identities can be evanescent. Although a quarter of a century has passed since regulatory T cells (Treg) were first described, new studies continue to reveal surprising and contradictory features of this lymphocyte subset. Treg cells are the core of the immunological workforce engaged in the restraint of autoimmune or inflammatory reactions, and their characterization has revealed substantial heterogeneity and complexity in the phenotype and gene expression profiles, proving them to be a most versatile and adaptive cell type, as exemplified by their plasticity in fine-tuning immune responses. Defects in Treg function are associated with several autoimmune diseases, including multiple sclerosis, which is caused by an inappropriate immune reaction toward brain components; conversely, the beneficial effects of immunomodulating therapies on disease progression have been shown to partly act upon the biology of these cells. Both in animals and in humans the pool of circulating Treg cells is a mixture of natural (nTregs) and peripherally-induced Treg (pTregs). Particularly in humans, circulating Treg cells can be phenotypically subdivided into different subpopulations, which so far are not well-characterized, particularly in the context of autoimmunity. Recently, Treg cells have been rediscovered as mediators of tissue healing, and have also shown to be involved in organ homeostasis. Moreover, stability of the Treg lineage has recently been addressed by several conflicting reports, and immune-suppressive abilities of these cells have been shown to be dynamically regulated, particularly in inflammatory conditions, adding further levels of complexity to the study of this cell subset. Finally, Treg cells exert their suppressive function through different mechanisms, some of which—such as their ectoenzymatic activity—are particularly relevant in CNS autoimmunity. Here, we will review the phenotypically and functionally discernible Treg cell subpopulations in health and in multiple sclerosis, touching also upon the effects on this cell type of immunomodulatory drugs used for the treatment of this disease.

Control of Intra-Thymic αβ T Cell Selection and Maturation by H3K27 Methylation and Demethylation

In addition to transcription factor binding, the dynamics of DNA modifications (methylation) and chromatin structure are essential contributors to the control of transcription in eukaryotes. Research in the past few years has emphasized the importance of histone H3 methylation at lysine 27 for lineage specific gene repression, demonstrated that deposition of this mark at specific genes is subject to differentiation-induced changes during development, and identified enzymatic activities, methyl transferases and demethylases, that control these changes. The present review discusses the importance of these mechanisms during intrathymic αβ T cell selection and late differentiation.

TCR tuning of T cell subsets

After selection in the thymus, the post-thymic T cell compartments comprise heterogenous subsets of naive and memory T cells that make continuous T cell receptor (TCR) contact with self-ligands bound to major histocompatibility complex (MHC) molecules. T cell recognition of self-MHC ligands elicits covert TCR signaling and is particularly important for controlling survival of naive T cells. Such tonic TCR signaling is tightly controlled and maintains the cells in a quiescent state to avoid autoimmunity. Here, we review how naive and memory T cells are differentially tuned and wired for TCR sensitivity to self and foreign ligands.

Brucella Peptide Cross-Reactive Major Histocompatibility Complex Class I Presentation Activates SIINFEKL-Specific T Cell Receptor-Expressing T Cells

Brucella spp. are intracellular pathogenic bacteria remarkable in their ability to escape immune surveillance and therefore inflict a state of chronic disease within the host. To enable further immune response studies, Brucella was engineered to express the well-characterized chicken ovalbumin (OVA). Surprisingly, we found that CD8 T cells bearing T cell receptors (TCR) nominally specific for the OVA peptide SIINFEKL (OT-1) reacted to parental Brucella-infected targets as well as OVA-expressing Brucella variants in cytotoxicity assays. Furthermore, splenocytes from Brucella-immunized mice produced gamma interferon (IFN-γ) and exhibited cytotoxicity in response to SIINFEKL-pulsed target cells.To determine if the SIINFEKL-reactive OT-1 TCR could be cross-reacting to Brucella peptides, we searched the Brucella proteome using an algorithm to generate a list of near-neighbor nonamer peptides that would bind to H2K^b. Selecting five Brucella peptide candidates, along with controls, we verified that several of these peptides mimicked SIINFEKL, resulting in T cell activation through the “SIINFEKL-specific” TCR. Activation was dependent on peptide concentration as well as sequence. Our results underscore the complexity and ubiquity of cross-reactivity in T cell recognition. This cross-reactivity may enable microbes such as Brucella to escape immune surveillance by presenting peptides similar to those of the host and may also lead to the activation of autoreactive T cells.

A signal integration model of thymic selection and natural regulatory T cell commitment

The extent of TCR self-reactivity is the basis for selection of a functional and self-tolerant T cell repertoire and is quantified by repeated engagement of TCRs with a diverse pool of self-peptides complexed with self-MHC molecules. The strength of a TCR signal depends on the binding properties of a TCR to the peptide and the MHC, but it is not clear how the specificity to both components drives fate decisions. In this study, we propose a TCR signal-integration model of thymic selection that describes how thymocytes decide among distinct fates, not only based on a single TCR-ligand interaction, but taking into account the TCR stimulation history. These fates are separated based on sustained accumulated signals for positive selection and transient peak signals for negative selection. This spans up the cells into a two-dimensional space where they are either neglected, positively selected, negatively selected, or selected as natural regulatory T cells (nTregs). We show that the dynamics of the integrated signal can serve as a successful basis for extracting specificity of thymocytes to MHC and detecting the existence of cognate self-peptide-MHC. It allows to select a self-MHC-biased and self-peptide-tolerant T cell repertoire. Furthermore, nTregs in the model are enriched with MHC-specific TCRs. This allows nTregs to be more sensitive to activation and more cross-reactive than conventional T cells. This study provides a mechanistic model showing that time integration of TCR-mediated signals, as opposed to single-cell interaction events, is needed to gain a full view on the properties emerging from thymic selection.

Mechanistic basis of immunotherapies for type 1 diabetes mellitus

Type 1 diabetes (T1D) is an autoimmune disease for which there is no cure. The pancreatic beta cells are the source of insulin that keeps blood glucose normal. When susceptible individuals develop T1D, their beta cells are destroyed by autoimmune T lymphocytes and no longer produce insulin. T1D patients therefore depend on daily insulin injections for survival. Gene therapy in T1D aims at the induction of new islets to replace those that have been destroyed by autoimmunity. A major goal of T1D research is to restore functional beta cell mass while eliminating diabetogenic T cells in the hope of achieving insulin independence. Multiple therapeutic strategies for the generation of new beta cells have been under intense investigations. However, newly formed beta cells would be immediately destroyed by diabetogenic T cells. Therefore, successful islet induction therapy must be supported by potent immunotherapy that will protect the newly formed beta cells. Herein, we will summarize the current information on immunotherapies that aim at modifying T cell response to beta cells. We will first outline the immune mechanisms that underlie T1D development and progression and review the scientific background and rationale for specific modes of immunotherapy. Numerous clinical trials using antigen-specific strategies and immune-modifying drugs have been published, though most have proved too toxic or have failed to provide long-term beta cell protection. To develop an effective immunotherapy, there must be a continued effort on defining the molecular basis that underlies T cell response to pancreatic islet antigens in T1D.

Diabetogenic T lymphocytes in human Type 1 diabetes

The field of Type 1 diabetes research has been quick to embrace the era of translational medicine in the recent epoch. Building upon some 30 years of intense immunological research, the past decade has been marked by a series of clinical trials designed to evaluate the potential beneficial effects of a range of immune intervention and prevention strategies [1(••),2-5]. At the heart of Type 1 diabetes is an autoimmune process, the consequence of which is immune-mediated destruction of islet β-cells. Although understanding the pathogenesis of islet autoimmunity is critical, there are also good reasons to focus research onto the β-cell destructive process itself. Measuring preservation of function of insulin-producing cells is currently the best means available to evaluate potential beneficial effects of immunotherapy, but there is an urgent need to discover and monitor immunological correlates of this β-cell destructive process. Whilst the best approach to intervention and prevention has yet to emerge, it is logical that future attempts to intelligently design therapeutics for Type 1 diabetes will need to be predicated on a clear understanding of the process of β-cell destruction and the immune components involved. For these reasons, this review will focus on the role of diabetogenic T lymphocytes in this disease-defining event.

TCR affinity for self-ligands influences the development and function of encephalitogenic T cells

The specificity and affinity of self-reactive T cells is likely to impact the development of autoimmune-disease causing T cells in the thymus as well as their function in the periphery. We identified a naturally occurring, low affinity variant of an MBP Ac1-11/I-A^u specific TCR that is known to induce EAE. Thymocytes in mice carrying the transgenes for this low affinity TCR were poorly positively selected, as compared to their high affinity TCR expressing counterparts. Nonetheless, CD4 T cells bearing the low affinity TCR accumulated in the periphery of the mice. Unlike mice expressing the high affinity TCR, these mice very rarely developed disease. However, if endogenous TCR expression was eliminated by breeding to RAG1 deficient mice, 100% of the mice carrying either the high or the low affinity versions of the TCR developed EAE. Intriguingly, while the incidence of EAE increased, the age of onset of disease in both mice was identical. These data suggest disease onset occurs during a short window of mouse development.

Restricted autoantigen recognition associated with deletional and adaptive regulatory mechanisms

Autoimmune diabetes (T1D) is characterized by CD4(+) T cell reactivity to a variety of islet-associated Ags. At-risk individuals, genetically predisposed to T1D, often have similar T cell reactivity, but nevertheless fail to progress to clinically overt disease. To study the immune tolerance and regulatory environment permissive for such autoreactive T cells, we expressed TCR transgenes derived from two autoreactive human T cells, 4.13 and 164, in HLA-DR4 transgenic mice on a C57BL/6-derived "diabetes-resistant" background. Both TCR are responsive to an immunodominant epitope of glutamic acid decarboxylase 65(555-567), which is identical in sequence between humans and mice, is restricted by HLA-DR4, and is a naturally processed self Ag associated with T1D. Although both TCR use the identical Valpha and Vbeta genes, differing only in CDR3, we found stark differences in the mechanisms utilized in vivo in the maintenance of immune tolerance. A combination of thymic deletion (negative selection), TCR down-regulation, and peripheral activation-induced cell death dominated the phenotype of 164 T cells, which nevertheless still maintain their Ag responsiveness in the periphery. In contrast, 4.13 T cells are much less influenced by central and deletional tolerance mechanisms, and instead display a peripheral immune deviation including differentiation into IL-10-secreting Tr1 cells. These findings indicate a distinct set of regulatory alternatives for autoreactive T cells, even within a single highly restricted HLA-peptide-TCR recognition profile.

A role for CD8 in the developmental tuning of antigen recognition and CD3 conformational change

TCR engagement by peptide-MHC class I (pMHC) ligands induces a conformational change (Deltac) in CD3 (CD3Deltac) that contributes to T cell signaling. We found that when this interaction took place between primary T lineage cells and APCs, the CD8 coreceptor was required to generate CD3Deltac. Interestingly, neither enhancement of Ag binding strength nor Src kinase signaling explained this coreceptor activity. Furthermore, Ag-induced CD3Deltac was developmentally attenuated by the increase in sialylation that accompanies T cell maturation and limits CD8 activity. Thus, both weak and strong ligands induced CD3Deltac in preselection thymocytes, but only strong ligands were effective in mature T cells. We propose that CD8 participation in the TCR/pMHC interaction can physically regulate CD3Deltac induction by "translating" productive Ag encounter from the TCR to the CD3 complex. This suggests one mechanism by which the developmentally regulated variation in CD8 sialylation may contribute to the developmental tuning of T cell sensitivity.

T cells as a self-referential, sensory organ

In light of recent data showing that both helper and cytotoxic T cells can detect even a single molecule of an agonist peptide-MHC, alphabeta T cells are clearly a type of sensory cell, comparable to any in the nervous system. In addition, endogenous (self) peptides bound to MHCs are not just important for thymic selection, but also play an integral role in T cell activation in the response to foreign antigens. With the multitude of specificities available to most T cells, they can thus be considered as a sensory organ, trained on self-peptide-MHCs and primed to detect nonself.

CD4+CD25+Regulatory T Cell Selection

Accumulating evidence indicates that regulatory T cells play a crucial role in preventing autoimmunity. To examine the processes by which regulatory CD4(+) T cells are produced during immune repertoire formation, we have developed transgenic mice that express the influenza virus hemagglutinin (HA) and coexpress major histocompatibility complex class II-restricted T cell receptors (TCRs) with varying affinities for the HA-derived CD4(+) T cell determinant S1. We show that interactions with a single self-peptide can induce thymocytes bearing an autoreactive TCR to undergo selection to become CD4(+) CD25(+) regulatory T cells, and that thymocytes bearing TCRs with low affinity for S1 do not undergo selection into this pathway. We show that CD4(+) thymocytes with identical specificity for the S1 self-peptide can undergo overt deletion versus abundant selection to become CD4(+) CD25(+) regulatory T cells in response to variations in expression of the S1 self-peptide in different lineages of HA transgenic mice. We also show that CD4(+) CD25(+) T cells proliferate in response to their selecting self-peptide in the periphery. Moreover, they do not proliferate in response to lymphopenia in the absence of the selecting self-peptide, reflecting a low level of expression of the high-affinity receptor for IL-7 (CD127) relative to conventional CD4(+) T cells. These studies are determining how specificity for self-peptides directs the thymic selection and peripheral expansion of CD4(+) CD25(+) regulatory T cells. Moreover, the differing responsiveness of CD4(+) CD25(+) regulatory T cells to cytokine- versus self-peptide-mediated signals may direct their accumulation to sites where the self-peptide is expressed.

Stem cell therapy for autoimmune disease: overview of concepts from the Snowbird 2002 tolerance and tissue regeneration meeting

Hematopoietic stem cell transplantation as a treatment for autoimmune disease began in 1996 and has subsequently spread worldwide. In Europe phase III trials have opened, while in America phase III trials are being designed and funded by the National Institutes of Health. On 6 June 2002, clinicians and scientists from around the world met at Snowbird, Utah to discuss the results and future directions of stem cell therapy for autoimmune diseases. What follows are general concepts from chairpersons of this meeting.

[The major histocompatibility complex as self-referential]

In order to effectively perceive the huge diversity of antigenic determinants to which it is confronted, the immune system uses referring internal images from self. The major histocompatibility complex constitutes the main reference to self, essential to the operating system of T and NK lymphocytes. It is involved in shaping the T operating repertoire in the thymus, where each differentiating thymocyte, with its TCR interacting with the MHC-peptide self complexes exposed by thymic presenting cells, should answer to both questions: Is it really necessary (positive selection) Is not dangerous (negative selection)? Once in periphery, naive T lymphocytes will undergo an homeostatic control helping their survival through the same contacts between TCR and MHC-peptide self complex than those which allowed the thymic positive selection. In a more hypothetic way, it is possible that contact between a T lymphocyte and the rare foreign MHC-peptide complexes spread at the surface of antigen presenting cell, is not sufficient to initiate its activation. Some arguments exist to involve the MHC-peptide self complexes themselves in the activation process.Finally, the MHC also constitutes a quality referential for the NK lymphocytes. When a somatic cell, infected by a virus or transformed, repress the expression of one or more of its class I HLA alleles, this absence of the self is perceived by the NK lymphocytes which proceed to its elimination through cytolysis. This disposition to sanction the non-self is also used for therapeutic purpose in the case of a non HLA identical allogenic hematopoïetic stem cells graft.

SLE - hematopoietic stem cell transplantation for systemic lupus erythematosus

Hematopoietic stem cell transplantation was first reported for patients with systemic lupus erythematosus in 1997. The procedure has since been performed worldwide including in Europe, in Brazil, and in China. A National Institutes of Health-funded phase III clinical trial of hematopoietic stem cell transplantation for refractory systemic lupus erythematosus is anticipated to begin in 2003. Encouraging responses are raising new hope about the role of adult hematopoietic stem cells in systemic lupus erythematosus.

Protective effect of DRB1 locus against type 2 diabetes mellitus in Mexican Mestizos

The aim of the study was to investigate the participation of human leukocyte antigen (HLA) class II alleles in the expression of type 2 diabetic and in nondiabetic subjects with and without family history of diabetes. The purpose was to evaluate any HLA association and to look for different patterns of insulin resistance and insulin secretion, comparing subjects with a low probability of developing diabetes, as a result of their family history. We recruited 87 healthy subjects without family history of diabetes, 48 healthy subjects with family history, and 47 type 2 diabetic patients. All of them were Mexican Mestizos of central Mexico. Using a standard 75-g oral glucose tolerance test, insulin resistance was determined and insulin secretion was assessed with the HOMA model. DRB1, DQA1 and DQB1 alleles were typed using polymerase chain reaction-sequence-specific oligonucleotide probe (PCR-SSOP) and sequence specific primers (PCR-SSP). Nondiabetic subjects had similar HOMA-IR and DeltaI 30/DeltaG 30 index (HOMA). A significant decreased frequency of DRB1*0403 (p = 0.01; odds ratio [OR] = 0.20) was demonstrated in type 2 diabetic patients, and DRB1*0701 (p = 0.02; OR = 0.17) in nondiabetics with family history of diabetes. These alleles associated with protection against type 2 diabetes, share glutamic acid at position-74 and were previously demonstrated to contribute to protection against type I diabetes.

Mutational spectral analysis at the HPRT locus in healthy children

There is growing evidence linking somatic mutational events during fetal development and childhood to an increasing number of multifactorial human diseases. Despite this, little is known about the relationship between endogenous and environmentally induced exogenous mutations during human development. Here we describe a comparative spectral analysis of somatic mutations at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) reporter gene locus in healthy children. We observed an age-specific decrease in the proportion of large alterations and a corresponding increase in the proportion of small alterations with increasing age following birth (P<0.001). The age specific decrease in the proportion of large alterations (67-30%) was mainly due to a decrease in the proportion of aberrant variable (V), diversity (D) and joining (J) (V(D)J) recombinase mediated HPRT deletions (P<0.001). The increase in the proportion of small alterations with age (28-64%) was associated with an increase in transversions from 8% in children at the late stages of fetal development to 31% in children 12-16 years old (P=0.003). Transitions decreased with age, especially at CpG dinucleotides (P=0.010), as transversions increased (P=0.009). These patterns of mutations provide insight into important spontaneous, genotoxic, and site-specific recombinational somatic mutational events associated with the age-specific development of human disease in children as well as adults.

Induction of tolerance in autoimmune diseases by hematopoietic stem cell transplantation: getting closer to a cure?

Hematopoietic stem cells (HSCs) are the earliest cells of the immune system, giving rise to B and T lymphocytes, monocytes, tissue macrophages, and dendritic cells. In animal models, adoptive transfer of HSCs, depending on circumstances, may cause, prevent, or cure autoimmune diseases. Clinical trials have reported early remission of otherwise refractory autoimmune disorders after either autologous or allogeneic hematopoietic stem cell transplantation (HSCT). By percentage of transplantations performed, autoimmune diseases are the most rapidly expanding indication for stem cell transplantation. Although numerous editorials or commentaries have been previously published, no prior review has focused on the immunology of transplantation tolerance or development of phase 3 autoimmune HSCT trials. Results from current trials suggest that mobilization of HSCs, conditioning regimen, eligibility and exclusion criteria, toxicity, outcome, source of stem cells, and posttransplantation follow-up need to be disease specific. HSCT-induced remission of an autoimmune disease allows for a prospective analysis of events involved in immune tolerance not available in cross-sectional studies.

Negative regulation of CD4 gene expression by a HES-1-c-Myb complex

Expression of the CD4 gene is tightly controlled throughout thymopoiesis. The downregulation of CD4 gene expression in CD4(-) CD8(-) and CD4(-) CD8(+) T lymphocytes is controlled by a transcriptional silencer located in the first intron of the CD4 locus. Here, we determine that the c-Myb transcription factor binds to a functional site in the CD4 silencer. As c-Myb is also required for CD4 promoter function, these data indicate that depending on the context, c-Myb plays both positive and negative roles in the control of CD4 gene expression. Interestingly, a second CD4 silencer-binding factor, HES-1, binds to c-Myb in vivo and induces it to become a transcriptional repressor. We propose that the recruitment of HES-1 and c-Myb to the silencer leads to the formation of a multifactor complex that induces silencer function and repression of CD4 gene expression.

Thymic selection of CD4+CD25+ regulatory T cells induced by an agonist self-peptide

Despite accumulating evidence that regulatory T cells play a crucial role in preventing autoimmunity, the processes underlying their generation during immune repertoire formation are unknown. We show here that interactions with a single self-peptide can induce thymocytes that bear an autoreactive T cell receptor (TCR) to undergo selection to become CD4+CD25+ regulatory T cells. Selection of CD4+CD25+ thymocytes appears to require a TCR with high affinity for a self peptide because thymocytes that bear TCRs with low affinity do not undergo selection into this pathway. Our findings indicate that specificity for self-peptides directs the selection of CD4+CD25+ regulatory thymocytes by a process that is distinct from positive selection and deletion.

A physiological ligand of positive selection is recognized as a weak agonist

Positive selection is a process that ensures that peripheral T cells express TCR that are self-MHC restricted. This process occurs in the thymus and requires both self-MHC and self-peptides. We have recently established a TCR transgenic (TCR(trans)(+)) mouse model using the C10.4 TCR restricted to the MHC class Ib molecule, H2-M3. Having defined H2-M3 as the positively selecting MHC molecule, the severely limited number of H2-M3 binding peptides allowed us to characterize a mitochondrial NADH dehydrogenase subunit 1-derived 9-mer peptide as the physiological ligand of positive selection. Here, we demonstrate that the NADH dehydrogenase subunit 1 self-peptide is seen by mature C10.4 TCR(trans)(+) T cells as a weak agonist and induces positive selection at a defined concentration range. We also found that the full-length cognate peptide, a strong agonist for mature C10.4 TCR(trans)(+) T cells, initiated positive selection, albeit at significantly lower concentrations. At increased peptide concentrations, and thus increased epitope densities, either peptide only induced the development of partially functional T cells. We conclude that successful positive selection only proceeded at a defined, yet fairly narrow window of avidity.

A physiological ligand of positive selection is seen with high specificity

Positive selection is a process that ensures that peripheral T cells express TCR that are restricted to self-MHC molecules. This process requires both self-MHC and self-peptides. We have recently established a TCR transgenic mouse model (C10.4 TCRtrans+) in which the transgenic TCR was selected on the nonclassical MHC class Ib molecule H2-M3 in conjunction with a physiologically occurring peptide derived from the mitochondrial NADH-dehydrogenase subunit 1 gene (9-mer peptide). Here, the specificity of positive selection of C10.4 TCRtrans+ T cells was examined using a fetal thymic organ culture system. We demonstrated that at low peptide concentrations, shortening the NADH-dehydrogenase subunit 1 gene 9-mer peptide or mutating its surface-exposed side chains severely impaired its ability to induce positive selection. We concluded that under physiological conditions positive selection of C10.4 TCRtrans+ T cells was highly specific and occurred at low epitope densities.

Modulation of immune function by dietary lectins in rheumatoid arthritis

Despite the almost universal clinical observation that inflammation of the gut is frequently associated with inflammation of the joints and vice versa, the nature of this relationship remains elusive. In the present review, we provide evidence for how the interaction of dietary lectins with enterocytes and lymphocytes may facilitate the translocation of both dietary and gut-derived pathogenic antigens to peripheral tissues, which in turn causes persistent peripheral antigenic stimulation. In genetically susceptible individuals, this antigenic stimulation may ultimately result in the expression of overt rheumatoid arthritis (RA) via molecular mimicry, a process whereby foreign peptides, similar in structure to endogenous peptides, may cause antibodies or T-lymphocytes to cross-react with both foreign and endogenous peptides and thereby break immunological tolerance. By eliminating dietary elements, particularly lectins, which adversely influence both enterocyte and lymphocyte structure and function, it is proposed that the peripheral antigenic stimulus (both pathogenic and dietary) will be reduced and thereby result in a diminution of disease symptoms in certain patients with RA.

Positive selection of an H2-M3 restricted T cell receptor

Thymocytes are positively selected for alphabeta T cell antigen receptors (TCR) that recognize antigen in conjunction with self-major histocompatibility complex (MHC) molecules. MHC bound peptides participate in positive selection; however, their role has remained controversial. A TCR transgenic mouse was established using a TCR restricted to the MHC class Ib molecule, H2-M3. Having defined H2-M3 as the positively selecting MHC molecule, the severely limited number of H2-M3 binding peptides allowed us to characterize an NADH dehydrogenase subunit 1 (ND1)-derived peptide as the physiological ligand of positive selection. This peptide bears no apparent sequence homology to the cognate peptide, is expressed ubiquitously, and yet does not interfere with peripheral T cells. Our studies also suggest that positive selection becomes promiscuous at high epitope densities.

Overexpression of the Helix-Loop-Helix protein Id2 blocks T cell development at multiple stages

The Id proteins are inhibitors of basic-Helix-Loop-Helix transcription factor function that have been implicated in the control of cell differentiation and proliferation. To study the role of Id proteins in the control of T cell development, we generated transgenic mice that overexpress the Id2 protein in thymocytes. We detect a significant expansion of the early CD4(-)CD8(+)TCR(-) thymocyte stage and a depletion of the thymocytes of the subsequent developmental stages. These data indicate that the overexpression of Id2 leads to a stage-specific developmental block early in thymopoiesis. In addition, progeny mice from five of the six Id2 transgenic founder lines succumb to aggressive T cell hyperproliferation that resembles lymphoma. Thus, overexpression of the Id2 protein has profound effects on T cell development and oncogenesis, consistent with the hypothesis that the bHLH proteins play critical roles in these processes.

c-Myb is essential for early T cell development

The c-Myb transcription factor is important for fetal hematopoiesis and has been proposed to mediate later stages of lymphocyte development. Using homozygous null c-Myb/Rag1 chimeric mice, we have determined that c-Myb plays an important role in the differentiation of macrophages and lymphocytes from precursor stem cells. We also determine that deletion of c-Myb leads to a complete block in early T cell development just before the oligopotent thymocyte matures into the definitive T cell precursor. These data indicate that c-Myb plays an important role at multiple stages of hematopoiesis and is required at an early stage of T cell development.

Expression of autoimmune disease-related antigens by cells of the immune system

The process of thymic selection is critical for the generation of the mature T-cell repertoire, yet the nature of the self-peptides that serve this function is not known. Several studies suggest that tissue-specific auto-antigens are expressed in the thymus. We initiated this study to examine the expression of a panel of auto-antigens related to several autoimmune diseases in the thymus, peripheral lymphoid organs, and various cell lines. We looked for the expression of these antigens by reverse transcriptase-polymerase chain reaction, fluorescence-activated cell sorter (FACS) analysis, immunoblotting, and immunoprecipitation. We found that in the thymus there is evidence for the expression of a wide variety of disease-related self-antigens including myelin antigens, insulin, cardiac myosin, and retinal S antigen. By FACS analysis, several monoclonal anti-myelin basic protein antibodies were found to bind to immune cells. In Western blotting, we could find in the thymus and other lymphoid organs the expression of myelin basic protein, proteolipid protein, and cyclic nucleotide phosphodiesterase; in contrast, the staining for myelin oligodendrocyte glycoprotein, microtubule-associated Tau protein, and insulin were negative in these organs. The results of these studies confirm that there is evidence for the expression of a variety of auto-antigens in the immune system, both at the mRNA and protein levels, potentially enabling them to participate in the process of thymic education.

Thymocyte selection in Vav and IRF-1 gene-deficient mice

T cells undergo a defined program of phenotypic and genetic changes during differentiation within the thymus. These changes define commitment of T-cell receptor (TCR) gamma delta and TCR alpha beta cells and lineage differentiation into CD4+ T helper and CD8+ cytotoxic T cells. T-cell differentiation and selection in the thymus constitute a tightly co-ordinated multistep journey through a network that can be envisaged as a three-dimensional informational highway made up of stromal cells and extracellular matrix molecules. This intrathymic journey is controlled by information exchange, with thymocytes depending on two-way cellular interactions with thymic stromal cells in order to receive essential signals for maturation and selection. Genetic inactivation of surface receptors, signal transduction molecules, and transcription factors using homologous recombination has provided novel insight into the signaling cascades that relay surface receptor engagement to gene transcription and subsequent progression of the developmental program. In this review we discuss molecular mechanisms of T lymphocyte development in mice that harbour genetic mutations in the guanine nucleotide exchange factor Vav and the interferon regulatory transcription factor 1 (IRF-1). We also propose a novel model of T-cell selection based on TCR alpha chain-directed signals for allelic exclusion and TCR alpha-based selection for single receptor usage.

Vbeta4(+) T cells promote clearance of infection in murine pulmonary histoplasmosis

T cells are essential for controlling infection with Histoplasma capsulatum. Because the T cell receptor is vital for transducing the biological activities of these cells, we sought to determine if exposure to this fungus induced an alteration in the Vbeta repertoire in lungs of C57BL/6 mice infected intranasally. Vbeta2(+) cells were elevated on day 3 after infection; Vbeta4(+) cells were higher than controls on days 7, 10, and 14 after infection. Vbeta10(+) cells were increased on days 14 and 21, and Vbeta11(+) exceeded controls only on day 14. We investigated the clonality and function of Vbeta4(+) cells because their expansion transpired during the critical time of infection, that is, when cellular immunity is activated. Sequence analysis demonstrated preferential use of a restricted set of sequences in the complementarity-determining region 3. Elimination of Vbeta4(+) cells from mice impaired their ability to resolve infection. In contrast, depletion of Vbeta7(+) cells, the abundance of which was similar to that of Vbeta4(+), did not alter elimination of the fungus. The identification of clonotypes of Vbeta4(+) cells suggests that a few antigenic determinants may drive proliferation of this subset, which is necessary for optimal clearance.

Probing degeneracy in T-cell recognition using peptide combinatorial libraries

Recent studies have demonstrated flexibility of the T-cell receptor (TCR) with respect to recognition of peptide bound to self major histocompatibility complex (MHC). With the introduction of peptide combinatorial libraries, it has become possible to dissect the extent of degeneracy in T-cell recognition. On the basis of these novel findings, Bernhard Hemmer and colleagues propose a conceptual framework for lymphocyte selection and survival and the occurrence of autoimmunity.

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-self-antigen. 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 antigen-specific immune responses will provide important insights into mechanisms by which autoimmunity might be induced by viruses bearing structural similarities with self-antigens.

Naturally Occurring Low Affinity Peptide/MHC Class I Ligands Can Mediate Negative Selection and T Cell Activation

The affinity/avidity model for T cell development postulates that ligands with high affinity for the TCR are efficient in negative selection, whereas those with lower affinity/avidity favor positive selection. Using the 2C TCR transgenic model, we evaluated the efficacy of ligands with widely differing affinity for the TCR (3 × 103 to 2 × 106 M−1) in mediating thymocyte deletion. The relative affinities of the 2C TCR for the p2Ca/Ld, dEV-8/Kb, p2Ca-A3/Ld, and p2Ca/Kb ligands are approximately 1000:50:10:1, respectively. Here we show, using an in vitro assay, that the deletion of 2C CD4+CD8+ thymocytes is mediated not only by p2Ca/Ld, but also by the lower affinity ligands dEV-8/Kb, p2Ca-A3/Ld, and p2Ca/Kb, albeit at relatively higher peptide concentrations. Deletion mediated by low affinity ligands required CD8, whereas high affinity ligand-mediated deletion was CD8 independent. The p2Ca/Kb and dEV-8/Kb ligands are naturally occurring in H-2b mice, and others have shown that p2Ca/Kb can induce the maturation of CD4−CD8+2C-TCRhigh thymocytes in fetal thymic organ culture. In this study we showed that in addition to deletion, the p2Ca/Kb and dEV-8/Kb ligands, in the presence of exogenous IL-2, induced mature 2C T cell proliferation, albeit at a lower level than that induced by the high affinity p2Ca/Ld ligand. Thus, the same low affinity ligands that can effect negative selection and, in the case of p2Ca/Kb, the maturation of CD8 single-positive thymocytes can also induce the activation of mature CD8 T cells.

Autoimmunity caused by disruption of central T cell tolerance. A murine model of drug-induced lupus

A side effect of therapy with procainamide and numerous other medications is a lupus-like syndrome characterized by autoantibodies directed against denatured DNA and the (H2A-H2B)-DNA subunit of chromatin. We tested the possibility that an effect of lupus-inducing drugs on central T cell tolerance underlies these phenomena. Two intrathymic injections of procainamide-hydroxylamine (PAHA), a reactive metabolite of procainamide, resulted in prompt production of IgM antidenatured DNA antibodies in C57BL/6xDBA/2 F1 mice. Subsequently, IgG antichromatin antibodies began to appear in the serum 3 wk after the second injection and were sustained for several months. Specificity, inhibition and blocking studies demonstrated that the PAHA-induced antibodies showed remarkable specificity to the (H2A-H2B)-DNA complex. No evidence for polyclonal B cell activation could be detected based on enumeration of Ig-secreting B cells and serum Ig levels, suggesting that a clonally restricted autoimmune response was induced by intrathymic PAHA. The IgG isotype of the antichromatin antibodies indicated involvement of T cell help, and proliferative responses of splenocytes to oligonucleosomes increased up to 100-fold. As little as 5 microM PAHA led to a 10-fold T cell proliferative response to chromatin in short term organ culture of neonatal thymi. We suggest that PAHA interferes with self-tolerance mechanisms accompanying T cell maturation in the thymus, resulting in the emergence of chromatin-reactive T cells followed by humoral autoimmunity.

Self-peptide ligands affect T cell recognition of the homologous influenza A matrix virus peptide M.58-66: modification of the HLA-A2.1/peptide complex structure and T cell antagonism

The cytotoxic T lymphocyte (CTL) response directed against the immunodominant peptide M.58-66 from the matrix of influenza A virus presented by the HLA-A2.1 molecule is characterized by a restricted T cell repertoire. This limitation may be due to selective pressure induced by endogenous homologous ligands responsible for both positive and negative selection in the thymus and partial activation in peripheral T cell responses. We have used three self-protein-derived peptides homologous to M.58-66 to study their HLA-A2.1 binding capacity and recognition by M.58-66-specific HLA-A2.1-restricted CTLs. We show that they antagonize M.58-66-reactive T cells, presumably by the formation of altered HLA-A2.1 complex conformations. The results are discussed with reference to the role of endogenous ligands homologous to antigenic peptides in T cell repertoire selection, tolerance, and overall regulation of the immune response.

Identification of a naturally occurring ligand for thymic positive selection

In the thymus, positive and negative selection shape the T cell repertoire. It has previously been shown that positive selection, like negative selection, is the result of the interaction of the TCR with self-peptides bound to MHC. However, little is known about the number or nature of the self-peptide ligands that mediate positive selection in vivo. We devised a novel assay with enhanced sensitivity for low affinity TCR ligands to identify self-peptides that may be biologically relevant. At least eight K(b)-bound self-peptides were detected by this assay using thymocytes bearing the OT-I TCR (specific for OVAp/K(b)). The sequence of one of these peptides was determined using the recently developed technique of membrane preconcentration-capillary electrophoresis-tandem mass spectrometry. This peptide, CP alpha1, has limited sequence similarity to OVAp, yet was found to induce positive selection of OT-I thymocytes in fetal thymic organ culture.

The agonist-antagonist balance in positive selection

Peptide antigens expressed in the thymus, in combination with self major histocompatibility complex molecules play a crucial role in thymocyte selection and shaping of the mature T-cell repertoire. Here, it is proposed that a single thymocyte may be exposed to numerous different peptide ligands as it matures, such that its fate is determined by the sum of signals produced by these interactions.

Split tolerance to the MHC class I molecule H-2Dd in animals transgenic for its soluble analog

To determine whether the function of MHC molecules in tolerance and education is related to cell surface expression, we have produced two strains of transgenic mice in the C57Bl/6 background that express soluble analogs of the H-2D(d) class I protein. The transgenes were stably integrated and genetically transmitted in a Mendelian fashion. Messenger RNA for the hybrid genes was detected in all tissues analyzed in a class I-like pattern of expression, with the highest levels in lymphoid tissues. All mice bearing the transgenes expressed relatively high levels (0.1 mg/ml) of the encoded protein in their serum as assessed by Western blotting and enzyme-linked immunosorbent assay (ELISA). Gel filtration chromatography showed that the soluble H-2D(d) protein exists as a heterodimer with beta2-microglobulin and as higher order multimers in serum. Lymphoid cells from the transgenic mice showed no cell surface expression of the soluble class I protein in indirect immunofluorescence assays. Splenocytes from two independently derived transgenic lines generated primary cytotoxic and proliferative responses directed against membrane H-2D(d) antigens. Mice of both strains rejected tail skin from donors that differed from the B6 background at the H-2D(d) locus only, but with delayed kinetics compared to nontransgenic littermate controls. Mice expressing the transgenic protein on immunization did not produce antibodies that recognized soluble H-2D(d) in ELISA, whereas B6 mice generated strong antibody responses to challenge with splenocytes bearing cell surface H-2D(d). Thus, transgenic mice expressing soluble H-2D(d) were partially tolerant to stimulation by membrane-bound H-2D(d). As with the activation of T-cells, the induction and maintenance of immunologic tolerance apparently displayed different requirements depending upon the T-cell subpopulation involved.

Major histocompatibility complex recognition by immune receptors: differences among T cell receptor versus antibody interactions with the VSV8/H-2Kb complex

The surface residues of the VSV8/Kb complex important for recognition by N15 and N26 alphabeta T cell receptors (TCR) were mapped by mutational analysis and compared to each other and with epitopes of well-characterized Kb specific monoclonal antibodies (mAb). Three features of immune receptor recognition emerge. First, the footprints of the two TCR on VSV8/Kb are similar with more than 80 % overlap between sites. Given that only 8 of 14 surface exposed VSV8/Kb residues identified as critical for TCR interaction are in common, the chemical basis of the N15 and N26 interactions is nevertheless distinct. Second, the cognate peptide is a major focus of TCR recognition: mutation at any of the three exposed side chains (at p1, p4 or p6) abrogates interaction of both TCR as measured by functional T cell activation. Third, in contrast to TCR, mAb bind to discrete segments on the periphery of the alpha1 and/or alpha2 helices without orientational restriction. These findings suggest that unlike soluble antibodies, surface membrane receptor-ligand interactions on opposing cells (i.e. TCR-peptide/ MHC, CD8-MHC) limit the orientational freedom of the TCR in the immune recognition process.

Mouse CD1-specific NK1 T cells: development, specificity, and function

NK1 T cells are a specialized population of alpha/beta T cells that coexpress receptors of the NK lineage and have the unique potential to very rapidly secrete large amounts of cytokines, providing early help for effector cells and regulating the Th1 or Th2 differentiation of some immune responses. NK1 T cells express a restricted TCR repertoire made of an invariant TCR alpha chain, V alpha 14-J alpha 281, associated with polyclonal V beta 8, V beta 7, and V beta 2 TCR beta chains. NK1 T cells recognize the products of the conserved family of MHC class I-like CD1 genes, apparently in the absence of foreign antigens. Thus, this novel regulatory pathway, which straddles the innate and the adaptive immune systems, is unique in that its activation may not require associative recognition of antigen. Here, we review the specificity and function of mouse NK1 T cells, and we discuss the relationship of this lineage to mainstream T cells and NK cells.

Increased interleukin 4 and immunoglobulin E production in transgenic mice overexpressing NK1 T cells

Natural Killer (NK)1.1+ (NK1) T cells are a specialized subset of alpha/beta T cells that coexpress surface receptors that are normally associated with the NK cell lineage of the innate immune system. On recognition of the conserved, major histocompatibility complex class I-like CD1 molecule, these cells are able to release explosive bursts of interleukin 4 (IL-4), a cytokine that promotes the T helper type 2 (Th2) effector class of an immune response. A unique feature of their T cell receptor (TCR) repertoire is the expression of an invariant TCR alpha chain, V alpha 14-J alpha 281, and of a restricted but polyclonal set of V beta gene families, V beta 8, V beta 7, and V beta 2. Here, we show that transgenic expression of this TCR alpha chain during thymic development is sufficient information to bias the differentiation of mainstream thymocytes towards the NK1 developmental pathway. It markedly increases the frequency of cells with the NK1 pattern of T cell differentiation and also has drastic consequences for the selection of the V beta repertoire. Transgenic CD4 cells exhibited a 10-100-fold increase in IL-4 production on mitogen stimulation in vitro and in vivo, and baseline levels of the Th2-controlled serum immunoglobulin isotypes, IgE and IgG1, were also selectively elevated in vivo.

T-cell repertoire: political correctness in the immune system

Recent studies of T-cell development in various types of mutant mouse shed new light on the relative roles in T-cell selection of antigen versus MHC molecule recognition by T-cell receptors.

Recognition of out-of-frame major histocompatibility complex class I-restricted epitopes in vivo

In the course of constructing a recombinant vaccinia virus encoding the influenza A nucleoprotein (NP) gene preceded by the hemagglutinin leader sequence, we isolated a single base-pair deletion mutant which gave rise to L+NP(1-159) in which only the first 159 amino acids were in frame. Despite this, when we infected target cells, we found that the point mutant was able to sensitize them for lysis not only by cytotoxic T cells recognizing residues 50-58 (the in-frame portion), but also by CTL to epitopes which are downstream of the mutation (366-374 and 378-386). Furthermore, normal C57BL/6 mice can be primed with the frameshift NP to recognize the immunodominant Db-restricted epitope 366-374 (which is out of frame). Experiments in which the mutant gene product was processed in the endoplasmic reticulum of target cells suggested that the apparent suppression occurred during polypeptide extension.

Apoptosis--molecular mechanisms and biomedical implications

Apoptosis is a distinct form of cell death of importance in tissue development and homeostasis and in several diseases. This review summarizes current knowledge about the regulation and molecular mechanisms of apoptosis and discusses the potential role of disregulated apoptosis in several major diseases. Finally, we speculate that modulation of apoptosis may be a target in future drug therapy.

Lowering the tone: mechanisms of immunodominance among epitopes with low affinity for MHC

Past studies of immunodominance among T-cell epitopes have focused on peptides with high affinity for their restriction element, assuming epitopes of low affinity to be immunologically irrelevant. Here, Paul Fairchild and David Wraith challenge this assumption by reviewing evidence that such peptides may contribute to T-cell repertoire selection and autoimmune disease, and suggest approaches to immunotherapy based on exploitation of these peptides.