Self-representation in the thymus: an extended view

Too dangerous to ignore: self-tolerance and the control of ignorant autoreactive T cells

Self-reactive T cells often escape thymic negative selection and are released into the periphery. While many of these T cells are tolerized by peripheral deletion or anergy, a proportion persists in a naïve (or ignorant) state. Self-ignorant T cells are probably one of the greatest threats to the maintenance of self-tolerance, as inadvertent activation of these cells may provoke autoimmune pathology. Nevertheless, despite the presence of self-ignorant T cells within most individuals, the majority of people fail to develop autoimmunity. The means by which self-ignorant T cells are silenced by the immune system remains a major issue within the tolerance field and it has received surprisingly little attention within the literature. In this review, we first summarize the factors that allow such cells to persist in a self-ignorant state, with a particular focus on the role of self-antigen dose. We next consider the conditions under which such self-reactive cells may become activated and speculate on how the immune system is able to prevent such cells from precipitating autoimmune disease.

Promiscuous gene expression patterns in single medullary thymic epithelial cells argue for a stochastic mechanism

Promiscuous expression of tissue-restricted autoantigens in medullary thymic epithelial cells (mTECs) imposes central T cell tolerance. The molecular regulation of this unusual gene expression is not understood, in particular its delineation from cell lineage-specific gene expression control remains unclear. Here, we compared the expression profile of the casein gene locus in mTECs and mammary gland epithelial cells by single cell PCR. Mammary gland cells showed highly correlated intra- and interchromosomal coexpression of milk proteins (the casein genes, lactalbumin-alpha and whey acidic protein) and one of its transcriptional regulators (Elf5). In contrast, coexpression of these genes in mature CD80(hi) mTECs was rarely observed and no pattern of gene expression in individual mTECs was discernible. The apparent stochastic expression pattern of genes within the casein locus, the lower mRNA levels compared with mammary gland cells in conjunction with frequent coexpression of insulin in single mTECs clearly delineates the molecular mechanism(s) of promiscuous gene expression from cell lineage-specific gene control.

Induction of immune tolerance to facilitate beta cell regeneration in type 1 diabetes

A definitive cure for type 1 diabetes is currently being pursued with enormous effort by the scientific community. Different strategies are followed to restore physiologic production of insulin in diabetic patients. Restoration of self-tolerance remains the milestone that must be reached in order to move a step further and recover a cell source capable of independent and functional insulin production. Multiple strategies aimed at modulation of both central and peripheral immunity must be considered. Promising results now show that the immune system can be modulated in a way that acquisition of a "diabetes-suppressive" phenotype is possible. Once self-tolerance is achieved, reversal of the disease may be obtained by simply allowing physiologic rescue and/or regeneration of the beta cells to take place. Given that these outcomes have already been confirmed in humans, refinement of existing protocols along with novel methods adapted to T1DM reversal will allow translation into clinical trials.

Peptides presented in vivo by HLA-DR in thyroid autoimmunity

The association of the major histocompatibility complex (MHC) genes with autoimmune diseases together with the ectopic expression of class II molecules by epithelial cells of the target tissue gives to these molecules a central role in the pathogenesis of the disease, in its regulation and in the persistence of the immune response in situ. HLA-DR molecules expressed by thyroid follicular cells in thyroid autoimmune diseases are compact molecules stably associated with peptides. The nature of these peptides is of vital importance in the understanding of the disease, since these MHC-II-peptide complexes are going to be recognized by both effector and regulatory T cells in situ. In this chapter, we review the current state of the analysis of naturally processed peptides presented by MHC class II molecules in the context of autoimmunity and we discuss our data of natural HLA-DR ligands eluted from Graves' disease affected thyroid glands, from where autoantigen-derived peptides have been identified.

How promiscuity promotes tolerance: the case of myasthenia gravis

Central T cell tolerance in the thymus plays a nonredundant role in preventing organ-specific autoimmunity. This role is largely attributable to promiscuous expression of tissue-restricted auto-antigens in medullary thymic epithelial cells (mTECs). The nascent T cell repertoire is exhaustively screened for potential autoreactivity against these ligands presented by mTECs. Failure of this screening process can result in organ-specific autoimmune diseases affecting single or multiple organs. Here we discuss how promiscuous expression of the acetylcholine receptor alpha-chain in the human thymus can potentially affect self-tolerance toward this well characterized auto-antigen and thus influence the disease course of myasthenia gravis.

Autoimmune regulator initiates the expression of promiscuous genes in thymic epithelial cells

The expression of peripheral antigens in the thymus, known as promiscuous gene expression, has been implicated in T cell tolerance and autoimmunity. Here we identified thymic epithelial cells (TECs) as the main cell type that expresses a diverse range of tissue-restricted antigens (TRAs). The TECs of a common autoimmune (non-obese diabetic [NOD]) mouse model express much lower levels of an autoimmune regulator (Aire) and TRAs than normal (Balb/c) TECs. Transfection of an Aire plasmid led to increased levels of TRA expression in cultured TECs from Balb/c and NOD mice; an increase that was enhanced by the presence of thymocytes. These data show that Aire initiates promiscuous gene expression in TECs, and that this function might be under thymocyte control.

Shaping of the autoreactive regulatory T cell repertoire by thymic cortical positive selection

The main function of regulatory T lymphocytes is to keep autoimmune responses at bay. Accordingly, it has been firmly established that the repertoire of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) is enriched in autospecific cells. Differences in thymic-positive and/or -negative selection may account for selection of the qualitatively distinct regulatory and conventional T cell (Tconv) repertoires. It has previously been shown that precursors for Tregs are less sensitive to negative selection than Tconv precursors. Studies with TCR/ligand doubly transgenic mice suggested that an agonist ligand might induce positive selection of Treg (but not Tconv) cells. However, massive deletion of Tconv (but not Treg) cell precursors observed in these mice renders interpretation of such data problematic and a potential role for positive selection in generation of the autospecific Treg repertoire has remained therefore incompletely understood. To study this important unresolved issue and circumvent use of TCR/ligand-transgenic mice, we have developed transgenic mice expressing a single MHC class II/peptide ligand on positively selecting thymic cortical epithelial cells. We found that functional Treg (but not Tconv) cells specific for the single ligand were preferentially selected from the naturally diverse repertoire of immature precursors. Our data therefore demonstrate that thymic cortical positive selection of regulatory and Tconv precursors is governed by distinct rules and that it plays an important role in shaping the autoreactive Treg repertoire.

Thymic generation and regeneration: a new paradigm for establishing clinical tolerance of stem cell-based therapies

Tolerance to tissue-engineering products is a major obstacle hindering the clinical application of this rapidly advancing technology. Manipulation of central tolerance, by establishing thymus chimerism of both donor and host-derived haemopoietic cells (haemopoietic stem cell transplant--HSCT), should purge any T cells reactive to potential donor organ or tissue transplant. A functional thymus, however, is required to induce chimerism and repopulate the peripheral T cell pool, but age-related thymic atrophy and damage caused by ablative conditioning regimes significantly reduce thymic function and increase incident of infection-dependent morbidity and mortality. Thus rejuvenation of the thymus alongside HSCT may potentiate the use of this strategy in the clinic. In addition, the use of thymic epithelial progenitor cell technology may allow growth of ex vivo thymic tissue for use in clinical situations of immunodeficiency as well as in establishing tolerance to tissue/organ products derived from the same source.

Dendritic cells in the kidney

Dendritic cells (DC) in nonlymphoid organs function at the crossroads of innate and adaptive immunity, self-tolerance, and tissue homeostasis. This review provides an overview of the study of DC in the kidney, tracing its history leading to the current knowledge of the origins, migration, and function of renal DC. Together, these studies suggest that renal DC play a critical role in the health and disease of the kidney, opening the way to direct targeting of renal DC for therapeutic benefit.

An IRF8-binding promoter variant and AIRE control CHRNA1 promiscuous expression in thymus

Promiscuous expression of tissue-restricted auto-antigens in the thymus imposes T-cell tolerance and provides protection from autoimmune diseases. Promiscuous expression of a set of self-antigens occurs in medullary thymic epithelial cells and is partly controlled by the autoimmune regulator (AIRE), a nuclear protein for which loss-of-function mutations cause the type 1 autoimmune polyendocrine syndrome. However, additional factors must be involved in the regulation of this promiscuous expression. Here we describe a mechanism controlling thymic transcription of a prototypic tissue-restricted human auto-antigen gene, CHRNA1. This gene encodes the alpha-subunit of the muscle acetylcholine receptor, which is the main target of pathogenic auto-antibodies in autoimmune myasthenia gravis. On re-sequencing the CHRNA1 gene, we identified a functional bi-allelic variant in the promoter that is associated with early onset of disease in two independent human populations (France and United Kingdom). We show that this variant prevents binding of interferon regulatory factor 8 (IRF8) and abrogates CHRNA1 promoter activity in thymic epithelial cells in vitro. Notably, both the CHRNA1 promoter variant and AIRE modulate CHRNA1 messenger RNA levels in human medullary thymic epithelial cells ex vivo and also in a transactivation assay. These findings reveal a critical function of AIRE and the interferon signalling pathway in regulating quantitative expression of this auto-antigen in the thymus, suggesting that together they set the threshold for self-tolerance versus autoimmunity.

T cell tolerance to the skin: a central role for central tolerance

T cell tolerance to self-antigens is believed to be achieved in a two-step process. The first step, called central tolerance, takes place in the thymus. The second step takes place outside the thymus in secondary lymphoid organs. One may ask why two mechanisms are needed to insure T cell tolerance. These two mechanisms share redundant functions and dysfunctions, leading to T cell-mediated autoimmune syndromes. By reviewing the literature on relevant animal models for T cell tolerance and our own recent findings, we are providing evidences that only central tolerance is acting for the skin.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Transcriptional basis of lymphocyte tolerance

Signaling through lymphocyte antigen receptors has the potential to initiate several distinct outcomes: proliferation, differentiation, apoptosis, or functional unresponsiveness. Expansion and differentiation of effector T cells is required for defense against foreign antigens, whereas functional unresponsiveness, termed anergy, is a cell-intrinsic mechanism that contributes to peripheral self-tolerance. Other mechanisms of peripheral tolerance include the 'dominant' tolerance imposed by regulatory T cells and immunosuppression mediated by interleukin-10 and transforming growth factor-beta. T- and B-cell antigen receptor ligation induces an increase in intracellular calcium levels as well as activating additional signaling pathways that are further potentiated by costimulatory receptors. In this review, we argue that cell-intrinsic programs of peripheral anergy and tolerance are imposed by sustained calcium signaling in lymphocytes. We address in particular the role of the calcium-dependent transcription factor nuclear factor for activation of T cells, which is activated by antigen receptor stimulation and, depending on the presence or absence of input from its transcriptional partner, activator protein-1, dictates two distinct transcriptional programs: activation or tolerance.

Regulation of peripheral T cell tolerance by the E3 ubiquitin ligase Cbl-b

The family of the Casitas B-lineage Lymphoma (Cbl) proteins, c-Cbl, Cbl-b, and Cbl-3, function as E3 ubiquitin ligases and molecular adaptors. In particular, Cbl-b acts as a gatekeeper in T cell activation that controls activation thresholds and the requirement for co-stimulation. Loss of Cbl-b expression renders animals susceptible to antigen-triggered autoimmunity suggesting that Cbl-b is a key autoimmunity gene. In addition, Cbl-b plays a critical role in T cell anergy and escape from regulatory T cells (Treg) suppression. Modulation of Cbl-b might provide us with a unique opportunity for future immune treatment of human disorders such as autoimmunity, immunodeficiency, or cancer.

When three is not a crowd: a Crossregulation model of the dynamics and repertoire selection of regulatory CD4+ T cells

Regulatory CD4(+) T cells, enriched in the CD25 pool of healthy individuals, mediate natural tolerance and prevent autoimmune diseases. Despite their fundamental and potential clinical significance, regulatory T (T(R)) cells have not yet been incorporated in a coherent theory of the immune system. This article reviews experimental evidence and theoretical arguments supporting a model of T(R) cell dynamics, uncovering some of its most relevant biological implications. According to this model, the persistence and expansion of T(R) cell populations depend strictly on specific interactions they make with antigen-presenting cells (APCs) and conventional effector T (T(E)) cells. This three-partner crossregulation imposes that T(R) cells feed on the specific autoimmune activities they suppress, with implications ranging from their interactions with other cells to their repertoire selection in the periphery and in the thymus, and to the relationship between these cells and the innate immune system. These implications stem from the basic prediction that the peripheral dynamics sort the CD4(+) T-cell repertoire into two subsets: a less diverse set of small clones of autoreactive effector and regulatory cells that regulate each other's growth, and a more diverse set of barely autoreactive T(E) cell clones, whose expansion is limited only by APC availability. It is argued that such partitioning of the repertoire sets the ground for self-non-self discrimination.

Heterogeneity of thymic dendritic cells

Thymus is the site of generation and selection of T-lymphocytes. It also contains phenotypically and functionally distinct dendritic cell (DC) populations, including conventional DC (cDC) and plasmacytoid DC (pDC). Thymic cDC are heterogeneous and contain two subsets: a major subset derived from the precursors within thymus, and a minor subset presumably of extrathymic origin. Increasing evidence suggest that thymic cDC can cross-present self-antigens to developing thymocytes and play an important role in thymocyte negative selection and central tolerance induction. Thymic pDC can produce type-I interferon upon appropriate activation. However, their role in a steady state thymus is currently unclear.

Expression of Tumor-Associated Differentiation Antigens, MUC1 Glycoforms and CEA, in Human Thymic Epithelial Cells: Implications for Self-Tolerance and Tumor Therapy

Expression of tissue-restricted self-antigens in the thymus, termed promiscuous gene expression, imposes T cell tolerance and protects from autoimmune diseases. This antigen pool also includes various types of tumor-associated antigens (TAA) previously thought to be secluded from the immune system. The scope of promiscuous gene expression has been defined by mRNA analysis at the global level of isolated medullary thymic epithelial cells (mTECs). Information at the protein level on the frequency of mTECs expressing a given antigen, on coexpression patterns, and post-translational modifications is largely missing. We report here promiscuous expression at the protein level of two TAA, MUC1 and CEA, in situ and in purified human mTECs. Both antigens are expressed in 1% to 3% of mTECs, either individually or coexpressed in the same cell. Using a panel of anti-MUC1 monoclonal antibodies recognizing different post-translational modifications, i.e., glycoforms of MUC1, we show that only fully glycosylated forms of MUC1 and the differentiation-dependent glycoforms were detected on mTECs, but not the cancer-associated glycoforms. Our findings imply that MUC1 and CEA are amenable to central tolerance induction, which might, however, be incomplete in case of tumor cell-restricted MUC1 glycoforms. Knowledge of these subtleties in promiscuous gene expression may, in the future, assist the selection of T cell tumor vaccines for clinical trials.

The same genomic region conditions clonal deletion and clonal deviation to the CD8alphaalpha and regulatory T cell lineages in NOD versus C57BL/6 mice

Clonal deviation is a mechanism by which immature thymocytes expressing a self-reactive T cell antigen receptor (TCR) are rescued from clonal deletion by adopting an alternative differentiation pathway resistant to apoptosis. Here, we confirm and generalize previous indications that genetic alleles in NOD mice condition ineffective clonal deviation toward the CD8alphaalpha lineage, a peculiar population of TCRalphabeta lymphocytes that electively colonizes the intraepithelial lymphocyte pool in the gut. Thymic selection of CD8alphaalpha cells was very age-dependent, occurring almost exclusively in the postnatal period. Fewer CD8alphaalpha cells were found in the thymus and intraepithelial lymphocytes of BDC2.5 TCR transgenic mice on the NOD than on the C57BL/6 (B6) background; this paucity extended to standard NOD mice, albeit to a lesser extent. CD8alphaalpha cells resided in the BDC2.5 pancreatic infiltrate, and they were more abundant on the B6 than the NOD background, correlating with aggressivity of the lesion. A (B6(g7) x NOD)F(2) intercross in agonist-challenged BDC2.5 fetal thymic organ cultures demonstrated the existence of a major quantitative trait locus on chromosome 3, coincident with an interval associated with resistance to clonal deletion. A replicate linkage confirmed these positions and showed that the same region also controls clonal deviation toward the CD4(+)FoxP3(+) regulatory T cell lineage. That clonal deviation toward the CD8alphaalpha and regulatory T cell pathways share genetic control further highlights the similarities between these two "rescue lineages," consistent with an immunoregulatory role for CD8alphaalpha cells.

The evolutionary history of lymphoid organs

Lymphoid organs are important regulators of lymphocyte development and immune responses. During vertebrate evolution, primary lymphoid organs appeared earlier than secondary lymphoid organs. Among the sites of primary lymphopoiesis during evolution and ontogeny, those for B cell differentiation have differed considerably, although they often have had myelolymphatic characteristics. In contrast, only a single site for T cell differentiation has occurred, exclusively the thymus. Based on those observations and the known features of variable-diversity-joining gene recombination, we propose a model for the successive specification of different lymphocyte lineages during vertebrate evolution. According to our model, T cells were the first lymphocytes to acquire variable-diversity-joining-type receptors, and the thymus was the first lymphoid organ to evolve in vertebrates to deal with potentially autoreactive, somatically diversified T cell receptors.

Potential functional role of plasmacytoid dendritic cells in cancer immunity

Plasmacytoid dendritic cells (pDCs), as well as myeloid dendritic cells (mDCs), have a dual role not only in initiating immune responses but also in inducing tolerance to exogenous and endogenous antigens. Tumour antigens originate from endogenous self-antigens, which are poorly immunogenic and also subject to change during tumour progression. In general, tumour antigens derived from apoptotic cells are captured by immature mDCs, antigen presentation by which is most likely to result in immune tolerance. In contrast, tumour antigens may be taken up by pDCs through Toll-like receptor 9 (TLR9) via receptor-mediated endocytosis. TLR9-dependent activation of pDCs results in the secretion of pro-inflammatory cytokines such as interleukin (IL)-12 and type I interferons (IFNs) through a MyD88-dependent pathway. Type I IFNs also activate mDCs for T-cell priming. Although pDCs recruited to the tumour site are implicated in facilitating tumour growth via immune suppression, they can be released from the tumour as a result of cell death caused by primary systemic chemotherapy, and can then be activated through TLR9. Thus, synergistically with mDCs, pDCs may also play a crucial role in mediating cancer immunity. In this review, the potential functional duality and plasticity of pDCs mediated by TLR9 ligation in cancer immunity will be discussed.

Aire-dependent alterations in medullary thymic epithelium indicate a role for Aire in thymic epithelial differentiation

The prevalent view of thymic epithelial differentiation and Aire activity holds that Aire functions in terminally differentiated medullary thymic epithelial cells (MTECs) to derepress the expression of structural tissue-restricted Ags, including pancreatic endocrine hormones. An alternative view of these processes has proposed that Aire functions to regulate the differentiation of immature thymic epithelial cells, thereby affecting tissue-restricted Ag expression and negative selection. In this study, we demonstrate that Aire impacts several aspects of murine MTECs and provide support for this second model. Expression of transcription factors associated with developmental plasticity of progenitor cells, Nanog, Oct4, and Sox2, by MTECs was Aire dependent. Similarly, the transcription factors that regulate pancreatic development and the expression of pancreatic hormones are also expressed by wild-type MTECs in an Aire-dependent manner. The altered transcriptional profiles in Aire-deficient MTECs were accompanied by changes in the organization and composition of the medullary epithelial compartment, including a reduction in the medullary compartment defined by keratin (K) 14 expression, altered patterns of K5 and K8 expression, and more prominent epithelial cysts. These findings implicate Aire in the regulation of MTEC differentiation and the organization of the medullary thymic compartment and are compatible with a role for Aire in thymic epithelium differentiation.

Medullary thymic epithelial cells expressing Aire represent a unique lineage derived from cells expressing claudin

The autoimmune regulator Aire is expressed in a small proportion of medullary thymic epithelial cells (mTECs) and is crucial in the induction of central T cell tolerance. The origin and development of Aire(+) mTECs, however, are not well understood. Here we demonstrate that the tight-junction components claudin-3 and claudin-4 (Cld3,4) were 'preferentially' expressed in Aire(+) mTECs. In early ontogeny, Cld3,4(hi) TECs derived from the most apical layer of the stratified thymic anlage first expressed known mTEC markers such as UEA-1 ligand and MTS10. We provide evidence that such Cld3,4(hi) UEA-1(+) TECs represented the initial progenitors specified for Aire(+) mTECs, whose development crucially required NF-kappaB-inducing kinase and the adaptor molecule TRAF6. Our results suggest that Aire(+) mTECs represent terminally differentiated cells in a unique lineage arising during thymic organogenesis.

Evaluation of polymorphic splicing in the mechanism of the association of the insulin gene with diabetes

The association of type 1 diabetes with the insulin gene (IDDM2 locus) has been mapped to a short haplotype encompassing two single nucleotide polymorphisms (SNPs) in perfect linkage disequilibrium (r(2) = 1) with each other and with the two allele classes at the variable number of tandem repeats (VNTR) polymorphism upstream of the transcription site. Although it is believed that the genetic effect is mediated through transcriptional effects of the VNTR, an alternative mechanism has been recently proposed: In transfected cells, the common A allele at one of the SNPs (-23A-->T, in relation to the translation-initiation codon) weakens the splicing of intron 1, resulting in a minor ( approximately 15% of total RNA) transcript with a longer 5' untranslated region and sixfold enhanced translational efficiency. The purpose of our study was to confirm these findings in RNA from normal human pancreas and thymus. We report that pancreas does contain the alternative transcript in an allele-dependent manner but at a very low proportion (<5% of total INS mRNA). We believe that this level would have a minor, if any, biological effect involved in the mechanism of the IDDM2 locus.

Spontaneous CD8 T cell responses against the melanocyte differentiation antigen RAB38/NY-MEL-1 in melanoma patients

The melanocyte differentiation Ag RAB38/NY-MEL-1 was identified by serological expression cloning (SEREX) and is expressed in the vast majority of melanoma lesions. The immunogenicity of RAB38/NY-MEL-1 has been corroborated previously by the frequent occurrence of specific Ab responses in melanoma patients. To elucidate potential CD8 T cell responses, we applied in vitro sensitization with overlapping peptides spanning the RAB38/NY-MEL-1 protein sequence and the reverse immunology approach. The identified peptide RAB38/NY-MEL-1(50-58) exhibited a marked response in ELISPOT assays after in vitro sensitization of CD8 T cells from HLA-A *0201(+) melanoma patients. In vitro digestion assays using purified proteasomes provided evidence of natural processing of RAB38/NY-MEL-1(50-58) peptide. Accordingly, monoclonal RAB38/NY-MEL-1(50-58)-specific T cell populations were capable of specifically recognizing HLA-A2(+) melanoma cell lines expressing RAB38/NY-MEL-1. Applying fluorescent HLA-A2/RAB38/NY-MEL-1(50-58) multimeric constructs, we were able to document a spontaneously developed memory/effector CD8 T cell response against this peptide in a melanoma patient. To elucidate the Ag-processing pathway, we demonstrate that RAB38/NY-MEL-1(50-58) is produced efficiently by the standard proteasome and the immunoproteasome. In addition to the identification of a RAB38/NY-MEL-1-derived immunogenic CD8 T cell epitope, this study is instrumental for both the onset and monitoring of future RAB38/NY-MEL-1-based vaccination trials.

Transplantation of bone marrow genetically engineered to express proinsulin II protects against autoimmune insulitis in NOD mice

BACKGROUND

Type 1 diabetes (T1D) is a T-cell-dependent autoimmune disease resulting from destructive inflammation (insulitis) of the insulin-producing pancreatic beta-cells. Transgenic expression of proinsulin II by a MHC class II promoter or transfer of bone marrow from these transgenic mice protects NOD mice from insulitis and diabetes. We assessed the feasibility of gene therapy in the NOD mouse as an approach to treat T1D by ex vivo genetic manipulation of normal hematopoietic stem cells (HSCs) with proinsulin II followed by transfer to recipient mice.

METHODS

HSCs were isolated from 6-8-week-old NOD female mice and transduced in vitro with retrovirus encoding enhanced green fluorescent protein (EGFP) and either proinsulin II or control autoantigen. Additional control groups included mice transferred with non-manipulated bone marrow and mice which did not receive bone marrow transfer. EGFP-sorted or non-sorted HSCs were transferred into pre-conditioned 3-4-week-old female NOD mice and insulitis was assessed 8 weeks post-transfer.

RESULTS

Chimerism was established in all major lymphoid tissues, ranging from 5-15% in non-sorted bone marrow transplants to 20-45% in EGFP-sorted bone marrow transplants. The incidence and degree of insulitis was significantly reduced in mice receiving proinsulin II bone marrow compared to controls. However, the incidence of sialitis in mice receiving proinsulin II bone marrow and control mice was not altered, indicating protection from insulitis was antigen specific.

CONCLUSIONS

We show for the first time that ex vivo genetic manipulation of HSCs to express proinsulin II followed by transplantation to NOD mice can establish molecular chimerism and protect from destructive insulitis in an antigen-specific manner.

Derivation, culture, and characterization of thymic epithelial cell lines

The major histocompatibility complex (MHC)-restricted presentation of self-peptides, generated from tissue-specific antigens, by thymic epithelial cells (TECs) is essential for development of central tolerance and for generation of the regulatory T-cell repertoire in the thymus. However, the mechanisms by which self-peptides are generated in and presented by TECs have not been well defined. To elucidate the processes involved in MHC class II-restricted presentation of self-peptides by TECs, cortical and medullary TEC lines may be established from C57BL/6 mouse thymi. Localization of a variety of molecules, including the MHC class II molecules critically involved in the presentation of antigen by TECs, may be investigated by both immunofluores-cence microscopy and Western blotting analyses. Our own studies using these approaches have demonstrated that such molecules are localized in the H2-DM+ lysosomal compartments isolated from both cortical and medullary TECs.

NF-kappaB2 is required for the control of autoimmunity by regulating the development of medullary thymic epithelial cells

Medullary thymic epithelial cells function as antigen-presenting cells in negative selection of self-reactive T cell clones, a process essential for the establishment of central self-tolerance. These cells mirror peripheral tissues through promiscuous expression of a diverse set of tissue-restricted self-antigens. The genes and signaling pathways that regulate the development of medullary thymic epithelial cells are not fully understood. Here we show that mice deficient in NF-kappaB2, a member of the NF-kappaB family, display a marked reduction in the number of mature medullary thymic epithelial cells that express CD80 and bind the lectin Ulex europaeus agglutinin-1, leading to a significant decrease in the extent of promiscuous gene expression in the thymus of NF-kappaB2(-/-) mice. Moreover, NF-kappaB2(-/-) mice manifest autoimmunity characterized by multiorgan infiltration of activated T cells and high levels of autoantibodies to multiple organs. A subpopulation of the mice also develops immune complex glomerulonephritis. These findings identify a physiological function of NF-kappaB2 in the development of medullary thymic epithelial cells and, thus, the control of self-tolerance induction.

The spreading of HIV-1 infection in the human organism is caused by fractalkine trafficking of the infected lymphocytes—a review, hypothesis and implications for treatment

The reviews on HIV-1/AIDS [1-8] highlighted the mechanism by which HIV-1 virions utilize dendritic cells (DCs) for transport from the genitals, the portal of virus infection, to the draining lymph nodes where DCs carry HIV-1 virions and present viral antigens by HLA class I and II to CD4(+) T cells. Interaction of the T cells with viral antigens presented by HLA class II molecules polarizes them to become Th2 cells, the targets of HIV-1 infection and producers of HIV-1 progeny virions. The T cells which interact with viral antigen presented by HLA class I polarize to become Th1 cells, which stimulate the CD8(+) T cell precursors to develop into antiviral cytotoxic T cells. In addition, HIV-1 virions shed gp120 glycoprotein molecules which bind to IgE immunoglobulin molecules bound to FCepsilonRI+ innate system cells (basophils, mast cells and monocytes) and induce them to release large amounts of Th2 cytokines (IL-4, IL-5, IL-10, IL-13), thereby creating an allergy-like condition. The present review attempts to define the role of chemokine receptors like CCR5 and CXCR4, and especially fractalkine receptor CX3CR1 in the trafficking of lymphocytes in healthy individuals and HIV-1/AIDS patients. The role of chemokine receptors as co-receptors for HIV-1 virion gp120 glycoprotein has been defined, but the role of fractalkine and fractalkine receptor has been clarified only recently [9-19]. In healthy individuals fractalkine is expressed by blood vessel endothelial cells and the CX3CR1 receptors are expressed on leukocytes that migrate in the peripheral blood in the direction of increased fractalkine concentration. In HIV-1/AIDS patients the virus-infected CD4(+) Th2 cells migrate to organs that harbor the adaptive immune system cells in the thymus, genitals, gastrointestinal tract, and to the brain. A most significant finding which revealed the importance of the human CX3CR1 gene expression to the progression of the infection to the stage of AIDS was recently reported by Faure and collaborators [20, 21] who showed that the delayed or rapid progression to AIDS was affected in HIV-1-infected individuals who had inherited a fractalkine receptor gene with the polymorphisms V249I or T280M, respectively, located in the sixth and seventh transmembrane domains of CX3CR1 protein. The T280M mutation in the CX3CR1 gene caused a rapid progression to AIDS, while in patients with the V249I mutation progression to AIDS was much slower. These studies led to the idea that it might be possible to slow or prevent HIV-1/AIDS progression in HIV-1 patients by treating them with fractalkine antagonists that will bind to and inhibit the activity of the fractalkine receptor. It is hypothesized that treatment of HIV-1/AIDS patients with a combination of fractalkine antagonists, IL-4 antagonist IL-4delta2 and the adjuvant CpG ODN induced release of type I IFN from PDF, and may inhibit HIV-1 infection, especially in HAART-treated patients infected with drug-resistant HIV-1 mutants due to prevention of the availability of immune cells needed for the viral evasion of the immune response. The hypothesis implies that the advantage of the suggested mode of treatment of HIV-1-infected people is prevention of cellular processes that are used by the viral protein to cause immunodeficiency, and prevention of HIV-1 replication without induction of resistant mutants.

Regulation of the T cell response

The T cell branch of the immune system can respond to a virtually infinite variety of exogenous antigens, thus including the possibility of self-antigen recognition and dangerous autoimmune reactions. Therefore, regulatory mechanisms operate both during ontogeny within the thymus and after birth in the periphery. The control of self-reactive T cells occurs through a process of negative selection that results in apoptosis of T cells showing high affinity for self-peptides expressed at the thymic level by means of promiscuous gene expression. Self-reactive T cells escaped to negative selection are controlled in the periphery by other regulatory mechanisms, the most important being natural Foxp3+ T regulatory (Treg) cells. Regulation is also required to control excessive effector T cell responses against exogenous antigens, when they become dangerous for the body. Three types of effector T cells have been recognized: T helper 1 (Th1) cells, which are protective against intracellular bacteria; Th2 cells, which play some role in the protection against nematodes, but are responsible for allergic reactions; Th17 cells, which are probably effective in the protection against extracellular bacteria, but also play a role in the amplification of autoimmune disorders. Abnormal or excessive Th effector responses are regulated by different mechanisms. Redirection or immune deviation of Th1- or Th2-dominated responses is provided by cytokines [interferon-gamma (IFN-gamma) vs. interleukin-4 (IL-4)] produced by the same cell types and by the CXCR3-binding chemokines CXCL4 and CXCL10. Moreover, both Th1 and Th2 responses can be suppressed by adaptive Treg cells through contact-dependent mechanisms and/or the production of IL-10 and transforming growth factor-beta (TGF-beta). Finally, TGF-beta1 can promote the development of both Th17 effector and adaptive Treg cells, while the contemporaneous production of IL-6 contributes to the development of Th17 cells, but inhibits Treg cells. The development of Th17 cells is also down-regulated by IL-4 produced by Th2 cells and by IFN-gamma produced by Th1 cells.

The role of CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) in regulatory T-cell biology

The profound influence of CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) on T-cell immunity has been known for over a decade, yet the precise roles played by these molecules still continue to emerge. Initially viewed as molecules that provide cell-intrinsic costimulatory and inhibitory signals, recent evidence suggests that both CD28 and CTLA-4 are also important in the homeostasis and function of a population of suppressive cells, termed regulatory T cells (Tregs). Here we review the main features of the CD28 and CTLA-4 system and examine how these impact upon Treg biology.

Cell death and thymic export during fetal life

In the fetus the peripheral T cell pool expands as the fetus grows, but the mechanisms that regulate T cell homeostasis during fetal life are unknown. Here, we show that the peripheral T cell pool in the sheep fetus is established by the export from the fetal thymus of twice as many CD8+ as CD4+ thymic emigrants every day. Clonal deletion of CD4+ thymocytes in the fetal thymus appeared to be more stringent than was the case for CD8+ thymocytes because only 1 in 35 single-positive CD4 (SPCD4) thymocytes was exported from the thymus whereas the majority (2/3) of the single-positive CD8 (SPCD8) thymocytes were exported from the fetal thymus each day. Furthermore, within the thymus, the number of apoptotic SPCD4 thymocytes was 40 times greater than the number of apoptotic SPCD8 thymocytes. A tissue-specific migration of CD8+ emigrants localizing in the spleen was also established in the fetus in contrast to CD4+ emigrants, which migrated randomly to spleen and LN.

Trafficking on serpentines: molecular insight on how maturating T cells find their winding paths in the thymus

Maintenance of the peripheral T-cell pool throughout the life requires uninterrupted generation of T cells. The majority of peripheral T cells are generated in the thymus. However, the thymus does not contain hematopoietic progenitors with unlimited self-renewing potential, and continuous production of T cells requires importation of such progenitors from the bone marrow into the thymus. Thymus-homing progenitors enter the thymus and subsequently migrate throughout distinct intrathymic microenvironments while differentiating into mature T cells. At each step of this scheduled journey, developing thymocytes interact intimately with the local stroma, which allow them to proceed to the next stage of their differentiation and maturation program. Undoubtedly, thymocyte/stroma interactions are instrumental for both thymocytes and stroma, because only their ongoing interplay generates and maintains a fully operational thymus, able to guarantee unimpaired T-cell supply. Therefore, proper T-cell generation intrinsically involves polarized cell migration during both adult life and embryogenesis when the thymus primordium develops into a functional thymus. The molecular mechanisms controlling cell migration during thymus development and postnatal T-cell differentiation are beginning to be defined. This review focuses on recent data regarding the role of cell migration in both colonization of the fetal thymus and T-cell development during postnatal life in mice.

Central tolerance: good but imperfect

T-cell development is a highly coordinated process that depends on interactions between thymocytes, thymic epithelium, and bone marrow (BM)-derived dendritic cells (DCs). Before entering the peripheral T-cell pool, thymocytes are subject to negative selection, a process that eliminates (or deletes) T cells with high affinity toward self-antigens and therefore promotes self-tolerance. These self-antigens include those that are broadly expressed ubiquitous antigens and those whose expression is restricted to a few tissues, tissue-specific antigens (TSAs). Expression of TSAs in the thymus is mostly a property of medullary thymic epithelial cells (mTECs), and because these cells may be less capable than BM-derived DCs at mediating negative selection to ubiquitous antigens, we investigated the roles of both of these cell types in tolerance to TSAs. Here, we review our studies in which we found that mTECs were competent mediators of negative selection to a subset of TSA-reactive T cells, while thymic DCs extend the range of TSA-reactive T cells that undergo negative selection by capturing TSAs from mTECs. In addition, we recently investigated the efficiency of central tolerance to TSA during ontogeny, and we report that this process was less efficient in neonates than adult animals.

Establishment and functioning of intrathymic microenvironments

The thymus supports the production of self-tolerant T cells from immature precursors. Studying the mechanisms regulating the establishment and maintenance of stromal microenvironments within the thymus therefore is essential to our understanding of T-cell production and ultimately immune system functioning. Despite our ability to phenotypically define stromal cell compartments of the thymus, the mechanisms regulating their development and the ways by which they influence T-cell precursors are still unclear. Here, we review recent findings and highlight unresolved issues relating to the development and functioning of thymic stromal cells.

The alternative NF-kappaB pathway from biochemistry to biology: pitfalls and promises for future drug development

The past two decades have led to a tremendous work on the transcription factor NF-kappaB and its molecular mechanisms of activation. The nuclear translocation of NF-kappaB is controlled by two main pathways: the classical and the alternative NF-kappaB pathways. The classical NF-kappaB pathway activates the IKK complex that controls the inducible degradation of most IkappaB family members that are IkappaBalpha, IkappaBbeta, IkappaBvarepsilon and p105. The alternative NF-kappaB pathway induces p100 processing and p52 generation through the activation of at least two kinases, which are NIK and IKKalpha. Genetic studies have shown that IKKgamma is dispensable for the alternative pathway, which suggests the existence of an alternative IKKalpha-containing complex. It is noteworthy that activation of particular p52 heterodimers like p52/RelB requires solely the alternative pathway while activation of p52/p65 or p52/c-Rel involves a "hybrid pathway". Among others, LTbetaR, BAFF-R, CD40 and RANK have the ability to induce the alternative pathway. The latter plays some roles in biological functions controlled by these receptors, which are the development of secondary lymphoid organs, the proliferation, survival and maturation of B cell, and the osteoclastogenesis. Exacerbated activation of the alternative pathway is potentially associated to a wide range of disorders like rheumatoid arthritis, ulcerative colitis or B cell lymphomas. Therefore, inhibitors of the alternative pathway could be valuable tools for the treatment of inflammatory disorders and cancers.

Clonal deletion of thymocytes by circulating dendritic cells homing to the thymus

Dendritic cell (DC) presentation of self antigen to thymocytes is essential to the establishment of central tolerance. We show here that circulating DCs were recruited to the thymic medulla through a three-step adhesion cascade involving P-selectin, interactions of the integrin VLA-4 with its ligand VCAM-1, and pertussis toxin-sensitive chemoattractant signaling. Ovalbumin-specific OT-II thymocytes were selectively deleted after intravenous injection of antigen-loaded exogenous DCs. We documented migration of endogenous DCs to the thymus in parabiotic mice and after painting mouse skin with fluorescein isothiocyanate. Antibody to VLA-4 blocked the accumulation of peripheral tissue-derived DCs in the thymus and also inhibited the deletion of OT-II thymocytes in mice expressing membrane-bound ovalbumin in cardiac myocytes. These findings identify a migratory route by which peripheral DCs may contribute to central tolerance.

Expression profiling of autoimmune regulator AIRE mRNA in a comprehensive set of human normal and neoplastic tissues

Defects in the autoimmune regulator (AIRE) gene cause the monogenic autoimmune disease autoimmune polyendocrinopathy syndrome type 1 (APS-1), which is characterized by a loss of self-tolerance to multiple organs. In concordance with its role in immune tolerance, AIRE is strongly expressed in medullary thymic epithelial cells (mTECs). Data on mechanisms controlling AIRE activation and the expression of this gene in other tissues are fragmentary and controversial. We report here AIRE mRNA expression profiling of a large set of normal human tissues and cells, tumor specimen and methylation deficient cell lines. On this broad data basis we found that AIRE mRNA expression is confined to mTECs in thymus and to lymph node tissue and that DNA hypomethylation contributes to transcriptional control of this gene.

Onset of promiscuous gene expression in murine fetal thymus organ culture

T-cell differentiation and induction of tolerance to self-antigens occurs mainly in the thymus. Thymic stromal cells, specifically medullary thymic epithelial cells, express a diverse set of genes encoding parenchymal organ-specific proteins. This phenomenon has been termed promiscuous gene expression (PGE) and has been implicated in preventing organ-specific autoimmunity by inducing T-cell tolerance to self antigens. Early thymopoiesis and the critical factors involved in T-cell differentiation can be reproduced in vitro by murine fetal thymus organ culture (FTOC), which mimics the natural thymic microenvironment. To evaluate the occurrence of PGE in FTOC, gene expression profiling during in vitro thymic development in BALB/c mice was performed using a set of nylon cDNA microarrays containing 9216 sequences. The statistical analysis of the microarray data (sam program) revealed the temporal repression and induction of 57 parenchymal and seven lymphoid organ-specific genes. Most of the genes analysed are repressed during early thymic development (15-17 days post-coitum). The expression of the autoimmune regulator (AIRE) gene at 16 days post-coitum marks the onset of PGE. This precedes the induction of parenchymal organ genes during the late developmental phase at 20 days post-coitum. The mechanism of T-cell tolerance induction begins during fetal development and continues into adulthood. Our findings are significant because they show a fine demarcation of PGE onset, which plays a central role in induction of T-cell tolerance.

Tolerance to proinsulin-2 is due to radioresistant thymic cells

Proinsulin is a key Ag in type 1 diabetes, but the mechanisms regulating proinsulin immune tolerance are unknown. We have shown that preproinsulin-2 gene-deficient mice (proins-2(-/-)) are intolerant to proinsulin-2. In this study, we analyzed the mechanisms underlying T cell-mediated tolerance to proinsulin-2 in 129/Sv nonautoimmune mice. The expression of one proinsulin-2 allele, whatever its parental origin, was sufficient to maintain tolerance. The site of proinsulin-2 expression relevant to tolerance was evaluated in thymus and bone marrow chimeras. CD4+ T cell reactivity to proinsulin-2 was independent of proinsulin-2 expression in radiation-sensitive bone marrow-derived cells. A wt thymus restored tolerance in proins-2(-/-) mice. Conversely, the absence of the preproinsulin-2 gene in radioresistant thymic cells was sufficient to break tolerance. Although chimeric animals had proinsulin-2-reactive CD4+ T cells in their peripheral repertoire, they displayed no insulitis or insulin Abs, suggesting additional protective mechanisms. In a model involving transfer to immunodeficient (CD3epsilon(-/-)) mice, naive and proinsulin-2-primed CD4+ T cells were not activated, but could be activated by immunization regardless of whether the recipient mice expressed proinsulin-2. Furthermore, we could not identify a role for putative specific T cells regulating proinsulin-2-reactive CD4+ T in transfer experiments. Thus, proinsulin-2 gene expression by radioresistant thymic epithelial cells is involved in the induction of self-tolerance, and additional factors are required to induce islet abnormalities.

Effects of protein aggregates: an immunologic perspective

The capacity of protein aggregates to enhance immune responses to the monomeric form of the protein has been known for over a half-century. Despite the clear connection between protein aggregates and antibody mediated adverse events in treatment with early therapeutic protein products such as intravenous immune globulin (IVIG) and human growth hormone, surprisingly little is known about the nature of the aggregate species responsible for such effects. This review focuses on a framework for understanding how aggregate species potentially interact with the immune system to enhance immune responses, garnered from basic immunologic research. Thus, protein antigens presented in a highly arrayed structure, such as might be found in large nondenatured aggregate species, are highly potent in inducing antibody responses even in the absence of T-cell help. Their potency may relate to the ability of multivalent protein species to extensively cross-link B-cell receptor, which (1) activates B cells via Bt kinases to proliferate, and (2) targets protein to class II major histocompatibility complex (MHC)-loading compartments, efficiently eliciting T-cell help for antibody responses. The review further focuses on protein aggregates as they affect an immunogenicity risk assessment, the use of animal models and studies in uncovering effects of protein aggregates, and changes in product manufacture and packaging that may affect generation of protein aggregates.

Quality control in self/nonself discrimination

Unicellular eukaryotes primarily employ self/nonself discrimination to avoid self-mating, whereas multicellular organisms also use self/nonself discrimination in immune defense. Recent advances in understanding self/nonself discrimination in eukaryotes shed new light on the emergence of the most sophisticated self/nonself discrimination system known, the antigen receptors employed in the adaptive immune system.

Processing of tumor-associated antigen by the proteasomes of dendritic cells controls in vivo T-cell responses

Dendritic cells are unique in their capacity to process antigens and prime naive CD8(+) T cells. Contrary to most cells, which express the standard proteasomes, dendritic cells express immunoproteasomes constitutively. The melanoma-associated protein Melan-A(MART1) contains an HLA-A2-restricted peptide that is poorly processed by melanoma cells expressing immunoproteasomes in vitro. Here, we show that the expression of Melan-A in dendritic cells fails to elicit T-cell responses in vitro and in vivo because it is not processed by the proteasomes of dendritic cells. In contrast, dendritic cells lacking immunoproteasomes induce strong anti-Melan-A T-cell responses in vitro and in vivo. These results suggest that the inefficient processing of self-antigens, such as Melan-A, by the immunoproteasomes of professional antigen-presenting cells prevents the induction of antitumor T-cell responses in vivo.

Evidence from autoimmune thyroiditis of skewed X-chromosome inactivation in female predisposition to autoimmunity

The etiologic factors in the development of autoimmune thyroid diseases (AITDs) are not fully understood. We investigated the role of skewed X-chromosome inactivation (XCI) mosaicism in female predisposition to AITDs. One hundred and ten female AITDs patients (81 Hashimoto's thyroiditis (HT), 29 Graves' disease (GD)), and 160 female controls were analyzed for the androgen receptor locus by the HpaII/polymerase chain reaction assay to assess XCI patterns in DNA extracted from peripheral blood cells. In addition, thyroid biopsy, buccal mucosa, and hair follicle specimens were obtained from five patients whose blood revealed an extremely skewed pattern of XCI, and the analysis was repeated. Skewed XCI was observed in DNA from peripheral blood cells in 28 of 83 informative patients (34%) as compared with 10 of 124 informative controls (8%, P<0.0001). Extreme skewing was present in 16 patients (19%), but only in three controls (2.4%, P<0.0001). The buccal mucosa, and although less marked, the thyroid specimens also showed skewing. Analysis of two familial cases showed that only the affected individuals demonstrate skewed XCI patterns. Based on these results, skewed XCI mosaicism may play a significant role in the pathogenesis of AITDs.

Evolutionary conservation of neuropeptide expression in the thymus of different species

Evidence suggests that the immune and neuroendocrine systems cross talk by sharing ligands and receptors. Hormones and neuropeptides produced by the neuroendocrine system often modulate the function of lymphoid organs and immune cells. We have previously reported the intrathymic expression of somatostatin (SOM) in the mouse and that several neuropeptides, most notably calcitonin-gene-related peptide (CGRP), neuropeptide Y (NPY), SOM and substance P (SP), can modulate thymocyte development. However, little is known about the intrathymic expression of these neuropeptides either in the mouse or in other species. Moreover, a comparative analysis of the expression of these molecules would highlight the evolutionary importance of intrathymic neuroendocrine interactions in T-cell development. We have studied the expression of different neuropeptides in the thymus of zebrafish, Xenopus, avians, rodent, porcine, equine and human by immunohistochemistry and reverse transcription-polymerase chain reaction. We found that CGRP, NPY, SOM, SP and vasointestinal polypeptide (VIP) are expressed in the thymus of all species investigated. The thymic location of many of these neuropeptides was conserved and appears to be within the stromal compartments. Interestingly, in the avian thymus the expression of CGRP, SOM and SP appears to change depending on the age of the tissue. These findings suggest that neuropeptides may play an important role in T-cell development and provide further evidence of cross talk between the immune and neuroendocrine systems.

A central role for central tolerance

Recent elucidation of the role of central tolerance in preventing organ-specific autoimmunity has changed our concepts of self/nonself discrimination. This paradigmatic shift is largely attributable to the discovery of promiscuous expression of tissue-restricted self-antigens (TRAs) by medullary thymic epithelial cells (mTECs). TRA expression in mTECs mirrors virtually all tissues of the body, irrespective of developmental or spatio-temporal expression patterns. This review summarizes current knowledge on the cellular and molecular regulation of TRA expression in mTECs, outlines relevant mechanisms of antigen presentation and modes of tolerance induction, and discusses implications for the pathogenesis of autoimmune diseases and other biological processes such as fertility, pregnancy, puberty, and tumor defense.

The level of thymic expression of RPE65 inversely correlates with its capacity to induce experimental autoimmune uveitis (EAU) in different rodent strains

We have previously shown that immunization with RPE65 produces in rats of four strains a severe inflammatory eye disease, designated experimental autoimmune uveitis (EAU). Here, we examined the uveitogenicity of RPE65 in six strains of mice. Only one strain, C57Bl/6, was found to develop consistently moderate levels of EAU, whereas other strains (BALB/c, B10.A, B10.BR, B10.RIII, C57BL/10J) were found to be essentially resistant to disease induced by RPE65. Analysis of the expression of RPE65 mRNA in thymi of the six mouse strains revealed detectable levels of the transcript in all strains, but with remarkable quantitative differences, with the lowest levels seen in thymi of C57Bl/6 mice, the only strain susceptible to RPE65-induced EAU. Moreover, unlike the finding with the mice, no RPE65 mRNA was detected in thymi of any of the four rat strains (Lewis, BN, F344, SHR) all of which are susceptible to the disease. These data thus indicate that the susceptibility to RPE65-induced EAU is inversely related to the thymic expression of the molecule. The data also suggest that this disease can be induced only in mice in which thymic expression of RPE65 is sufficiently low to allow the escape from deletion of T-cells with the adequate capacity to initiate the pathogenic immune response.

Naturally arising Foxp3-expressing CD25+CD4+ regulatory T cells in self-tolerance and autoimmune disease

Naturally arising CD25+CD4+ regulatory T cells, which express the transcription factor Foxp3, play key roles in the maintenance of immunologic self-tolerance and negative control of a variety of physiological and pathological immune responses. The majority of them are produced by the normal thymus as a functionally mature T cell subpopulation specialized for suppressive function. Their generation is in part genetically and developmentally controlled. Genetically determined or environmentally induced abnormality in CD25+CD4+ regulatory T cell development, maintenance, and function can be a cause of autoimmune disease in humans.