T-cell apoptosis and differential human leucocyte antigen class II expression in human thymus

HLA-DQ antibodies in alloimmunity, what makes them different?

PURPOSE OF REVIEW

De novo HLA-DQ antibodies are the most frequently observed after solid-organ allotransplantation; and are associated with the worse adverse graft outcomes compared with all other HLA antibodies. However, the biological explanation for this observation is not yet known. Herein, we examine unique characteristics of alloimmunity directed specifically against HLA-DQ molecules.

RECENT FINDINGS

While investigators attempted to decipher functional properties of HLA class II antigens that may explain their immunogenicity and pathogenicity, most early studies focused on the more expressed molecule - HLA-DR. We here summarize up-to-date literature documenting specific features of HLA-DQ, as compared to other class II HLA antigens. Structural and cell-surface expression differences have been noted on various cell types. Some evidence suggests variations in antigen-presenting function and intracellular activation pathways after antigen/antibody interaction.

SUMMARY

The clinical effects of donor-recipient incompatibility at HLA-DQ, the risk of generating de novo antibodies leading to rejection, and the inferior graft outcomes indicate increased immunogenicity and pathogenicity that is unique to this HLA antigen. Clearly, knowledge generated for HLA-DR cannot be applied interchangeably. Deeper understanding of features unique to HLA-DQ may support the generation of targeted preventive-therapeutic strategies and ultimately improve solid-organ transplant outcomes.

Molecular and cellular characterization of European sea bass CD3ε+ T lymphocytes and their modulation by microalgal feed supplementation

The CD3 coreceptor is a master T cell surface marker, and genes encoding CD3ζ, γδ, and ε chains have been reported in several teleost fish. Here, a complete cDNA sequence of CD3ɛ chain was identified from a sea bass (Dicentrarchus labrax L.) gill transcriptome. Its basal expression was quantified in both lymphoid and non-lymphoid organs of sea bass juveniles with real-time qPCR analysis. After either in vitro stimulation of head kidney leukocytes with the T-cell mitogen phytohaemagglutinin or in vivo stimulation with an orally administered Vibrio anguillarum vaccine, CD3ε expression levels increased in head kidney leukocytes, confirming that CD3ε T cells may play important roles in fish systemic protection against pathogens. Further, three peptides were designed on the CD3ɛ cytoplasmic tail region and employed as immunogens for antibody production in rabbit. One antiserum so obtained, named RACD3/1, immunostained a band of the expected size in a western blot of a sea bass thymocyte lysate. The distribution of CD3ε⁺ lymphocyte population in the lymphoid organs and mucosal tissues was addressed in healthy fish by IHC. In decreasing percentage order, CD3ε⁺ lymphocytes were detected by flow cytometry in thymus, peripheral blood leukocytes, gills, head kidney, gut, and spleen. Finally, a significant in vivo enhancement of CD3ε⁺ T intestinal lymphocytes was found in fish fed on diets in which 100% fish meal was replaced by the microalgae Nannochloropsis sp. biomass. These results indicate that CD3ε⁺ T cells are involved in nutritional immune responses.

Human leukocyte antigen matching in organ transplantation: what we know and how can we make it better (Revisiting the past, improving the future)

PURPOSE OF REVIEW

A renaissance for human leukocyte antigen (HLA) testing emerged with the understanding that donor-specific HLA antibodies play a significant role in long-term allograft survival. This renewed focus on donor/recipient histocompatibility led to a recent quest to decipher antibody responses or, as introduced into the transplantation lexicon, 'HLA-epitope matching'.

RECENT FINDINGS

Whether matching is at the antigen or the epitope level, in-depth understanding of how histo-incompatibility leads to activation of an immune response is required. HLA-DQ donor-specific antibody (DSA) has the highest association with poor graft survival. However, HLA-DQ antigens and antibodies are understudied and significant gaps still exist in understanding the function of HLA-DQ in immune activation. Much of our knowledge about HLA class-II molecules is derived from studies performed on HLA-DR, whether it is crystallography, antigen processing and presentation analysis, or activation of T-cell signal-transduction pathways. Indeed, HLA-DQ molecules are less amenable for laboratory testing, but the limited studies that were performed indicate that HLA-DQ might have, at least to some extent, a different role compared with HLA-DR.

SUMMARY

This review highlights qualities of HLA-DQ that may be associated with different pathways of activating an immune response. Understanding the consequences of such differences may lead to better appreciation and significance of HLA-DQ for matching purposes.

Class II MHC antigen processing in immune tolerance and inflammation

Presentation of peptide antigens by MHC-II proteins is prerequisite to effective CD4 T cell tolerance to self and to recognition of foreign antigens. Antigen uptake and processing pathways as well as expression of the peptide exchange factors HLA-DM and HLA-DO differ among the various professional and non-professional antigen-presenting cells and are modulated by cell developmental state and activation. Recent studies have highlighted the importance of these cell-specific factors in controlling the source and breadth of peptides presented by MHC-II under different conditions. During inflammation, increased presentation of selected self-peptides has implications for maintenance of peripheral tolerance and autoimmunity.

Peptide presentation by HLA-DQ molecules is associated with the development of immune tolerance

HLA class II proteins are important elements of human adaptive immune recognition and are associated with numerous infectious and immune-mediated diseases. These highly variable molecules can be classified into DP, DQ and DR groups. It has been proposed that in contrast with DP and DR, epitope binding by DQ variants rather results in immune tolerance. However, the pieces of evidence are limited and controversial. We found that DQ molecules bind more human epitopes than DR. Pathogen-associated epitopes bound by DQ molecules are more similar to human proteins than the ones bound by DR. Accordingly, DQ molecules bind epitopes of significantly different pathogen species. Moreover, the binding of autoimmunity-associated epitopes by DQ confers protection from autoimmune diseases. Our results suggest a special role of HLA-DQ in immune homeostasis and help to better understand the association of HLA molecules with infectious and autoimmune diseases.

Expression of cell cycle and apoptosis regulators in thymus and thymic epithelial tumors

The human thymus supports the production of self-tolerant T cells with competent and regulatory functions. Various cellular components of the thymic microenvironment such as thymic epithelial cells (TEC) and dendritic cells play essential roles in thymic T cell differentiation. The multiple cellular events occurring during thymic T cell and TEC differentiation involve proteins regulating cell cycle and apoptosis. Dysregulation of the cell cycle and apoptosis networks is involved in the pathogenesis of thymic epithelial tumors (TET) which are divided into two broad categories, thymomas and thymic carcinomas. The present review focuses on the usefulness of the analysis of the expression patterns of major cell cycle and apoptosis regulators in order to gain insight in the histophysiology of thymus and the histopathology, the clinical behavior and the biology of TET.

The Repertoires of Peptides Presented by MHC-II in the Thymus and in Peripheral Tissue: A Clue for Autoimmunity?

T-cell tolerance to self-antigens is established in the thymus through the recognition by developing thymocytes of self-peptide-MHC complexes and induced and maintained in the periphery. Efficient negative selection of auto-reactive T cells in the thymus is dependent on the in situ expression of both ubiquitous and tissue-restricted self-antigens and on the presentation of derived peptides. Weak or inadequate intrathymic expression of self-antigens increases the risk to generate an autoimmune-prone T-cell repertoire. Indeed, even small changes of self-antigen expression in the thymus affect negative selection and increase the predisposition to autoimmunity. Together with other mechanisms, tolerance is maintained in the peripheral lymphoid organs via the recognition by mature T cells of a similar set of self-peptides in homeostatic conditions. However, non-lymphoid peripheral tissue, where organ-specific autoimmunity takes place, often have differential functional processes that may lead to the generation of epitopes that are absent or non-presented in the thymus. These putative differences between peptides presented by MHC molecules in the thymus and in peripheral tissues might be a major key to the initiation and maintenance of autoimmune conditions.

HLA-DO and Its Role in MHC Class II Antigen Presentation

Helper T cells are stimulated to fight infections or diseases upon recognition of peptides from antigens that are processed and presented by the proteins of Major Histocompatibility Complex (MHC) Class II molecules. Degradation of a full protein into small peptide fragments is a lengthy process consisting of many steps and chaperones. Malfunctions during any step of antigen processing could lead to the development of self-reactive T cells or defective immune response to pathogens. Although much has been accomplished regarding how antigens are processed and presented to T cells, many questions still remain unanswered, preventing the design of therapeutics for direct intervention with antigen processing. Here, we review published work on the discovery and function of a MHC class II molecular chaperone, HLA-DO, in human, and its mouse analog H2-O, herein called DO. While DO was originally discovered decades ago, elucidating its function has proven challenging. DO was discovered in association with another chaperone HLA-DM (DM) but unlike DM, its distribution is more tissue specific, and its function more subtle.

Composition of the HLA-DR-associated human thymus peptidome

Major histocompatibility complex class II (MHC-II) molecules bind to and display antigenic peptides on the surface of antigen-presenting cells (APCs). In the absence of infection, MHC-II molecules on APCs present self-peptides and interact with CD4(+) T cells to maintain tolerance and homeostasis. In the thymus, self-peptides bind to MHC-II molecules expressed by defined populations of APCs specialised for the different steps of T-cell selection. Cortical epithelial cells present peptides for positive selection, whereas medullary epithelial cells and dendritic cells are responsible for peptide presentation for negative selection. However, few data are available on the peptides presented by MHC molecules in the thymus. Here, we apply mass spectrometry to analyse and identify MHC-II-associated peptides from five fresh human thymus samples. The data show a diverse self-peptide repertoire, mostly consisting of predicted MHC-II high binders. Despite technical limitations preventing single cell population analyses of peptides, these data constitute the first direct assessment of the HLA-II-bound peptidome and provide insight into how this peptidome is generated and how it drives T-cell repertoire formation.

HLA-DO acts as a substrate mimic to inhibit HLA-DM by a competitive mechanism

MHCII proteins bind peptide antigens in endosomal compartments of antigen-presenting cells. The non-classical MHCII protein HLA-DM chaperones peptide-free MHCII against inactivation and catalyzes peptide exchange on loaded MHCII. Another non-classical MHCII protein, HLA-DO, binds HLA-DM and influences the repertoire of peptides presented by MHCII proteins. However, the mechanism by which HLA-DO functions is unclear. Here we use x-ray crystallography, enzyme kinetics and mutagenesis approaches to investigate human HLA-DO structure and function. In complex with HLA-DM, HLA-DO adopts a classical MHCII structure, with alterations near the alpha subunit 3₁₀ helix. HLA-DO binds to HLA-DM at the same sites implicated in MHCII interaction, and kinetic analysis demonstrates that HLA-DO acts as a competitive inhibitor. These results show that HLA-DO inhibits HLA-DM function by acting as a substrate mimic and place constraints on possible functional roles for HLA-DO in antigen presentation.

DQB1*0602 rather than DRB1*1501 confers susceptibility to multiple sclerosis-like disease induced by proteolipid protein (PLP)

Background

Multiple sclerosis (MS) is associated with pathogenic autoimmunity primarily focused on major CNS-myelin target antigens including myelin basic protein (MBP), proteolipidprotein (PLP), myelin oligodendrocyte protein (MOG). MS is a complex trait whereby the HLA genes, particularly class-II genes of HLA-DR15 haplotype, dominate the genetic contribution to disease-risk. Due to strong linkage disequilibrium in HLA-II region, it has been hard to establish precisely whether the functionally relevant effect derives from the DRB1*1501, DQA1*0102-DQB1*0602, or DRB5*0101 loci of HLA-DR15 haplotype, their combinations, or their epistatic interactions. Nevertheless, most genetic studies have indicated DRB1*1501 as a primary risk factor in MS. Here, we used 'HLA-humanized' mice to discern the potential relative contribution of DRB1*1501 and DQB1*0602 alleles to susceptibility to "humanized" MS-like disease induced by PLP, one of the most prominent and encephalitogenic target-antigens implicated in human MS.

Methods

The HLA-DRB1*1501- and HLA-DQB1*0602-Tg mice (MHC-II^-/-), and control non-HLA-DR15-relevant-Tg mice were immunized with a set of overlapping PLP peptides or with recombinant soluble PLP for induction of "humanized" MS-like disease, as well as for ex-vivo analysis of immunogenic/immunodominant HLA-restricted T-cell epitopes and associated cytokine secretion profile.

Results

PLP autoimmunity in both HLA-DR15-Tg mice was focused on 139-151 and 175-194 epitopes. Strikingly, however, the HLA-DRB1*1501-transgenics were refractory to disease induction by any of the overlapping PLP peptides, while HLA-DQB1*0602 transgenics were susceptible to disease induction by PLP139-151 and PLP175-194 peptides. Although both transgenics responded to both peptides, the PLP139-151- and PLP175-194-reactive T-cells were directed to Th1/Th17 phenotype in DQB1*0602-Tg mice and towards Th2 in DRB1*1501-Tg mice.

Conclusions

While genome studies map a strong MS susceptibility effect to the region of DRB1*1501, our findings offer a rationale for potential involvement of pathogenic DQ6-associated autoimmunity in MS. Moreover, that DQB1*0602, but not DRB1*1501, determines disease-susceptibility to PLP in HLA-transgenics, suggests a potential differential, functional role for DQB1*0602 as a predisposing allele in MS. This, together with previously demonstrated disease-susceptibility to MBP and MOG in DRB1*1501-transgenics, also suggests a differential role for DRB1*1501 and DQB1*0602 depending on target antigen and imply a potential complex 'genotype/target antigen/phenotype' relationship in MS heterogeneity.

Developmental changes in cell proliferation and apoptosis in the normal duck thymus

Cell proliferation and apoptosis in the normal duck thymus during embryonic and post-embryonic development were studied. The flow cytometry assay shows that the level of G(0)/G(1) thymic cell population and the proportion of apoptotic cells increased with age, while the levels of S phase, G(2) + M phase and the proliferating index decreased with age. Proliferation cell nuclear antigen (PCNA) was mainly detected in the nuclei of lymphocytes. The number of PCNA-positive cells in the cortex and medulla significantly decreased with age. Transferase-mediated dUTP nick-end labelling (TUNEL) reaction stained apoptotic bodies in the cytoplasm of macrophages and free apoptotic bodies or nuclei with condensed chromatin in lymphocytes. The number of TUNEL-positive cells in the cortex and medulla markedly increased with age. The amount of proliferation and apoptotic cells in the thymic cortex was higher than that in the medulla. The balance between proliferation and apoptosis in the duck thymus may account for the process of thymic development and involution.

Salmonid T cells assemble in the thymus, spleen and in novel interbranchial lymphoid tissue

In modern bony fishes, or teleost fish, the general lack of leucocyte markers has greatly hampered investigations of the anatomy of the immune system and its reactions involved in inflammatory responses. We have previously reported the cloning and sequencing of the salmon CD3 complex, molecules that are specifically expressed in T cells. Here, we generate and validate sera recognizing a peptide sequence of the CD3ε chain. Flow cytometry analysis revealed high numbers of CD3ε(+) or T cells in the thymus, gill and intestine, whereas lower numbers were detected in the head kidney, spleen and peripheral blood leucocytes. Subsequent morphological analysis showed accumulations of T cells in the thymus and spleen and in the newly discovered gill-located interbranchial lymphoid tissue. In the latter, the T cells are embedded in a meshwork of epithelial cells and in the spleen, they cluster in the white pulp surrounding ellipsoids. The anatomical organization of the salmonid thymic cortex and medulla seems to be composed of three layers consisting of a sub-epithelial medulla-like zone, an intermediate cortex-like zone and finally another cortex-like basal zone. Our study in the salmonid thymus reports a previously non-described tissue organization. In the intestinal tract, abundant T cells were found embedded in the epithelium. In non-lymphoid organs, the presence of T cells was limited. The results show that the interbranchial lymphoid tissue is quantitatively a very important site of T cell aggregation, strategically located to facilitate antigen encounter. The interbranchial lymphoid tissue has no resemblance to previously described lymphoid tissues.

Aged human thymus hassall's corpuscles are immunoreactive for IGF-I and IGF-I receptor

Although Hassall's corpuscles have been proposed to act in both maturation of developing thymocytes and removal of apoptotic cells, their function remains an enigma. The involvement of insulin-like growth factor I (IGF-I) in the local autocrine and paracrine control of T-cell development in human thymus is still unclear. In this study, we investigated the structure and distribution of IGF-I and IGF-I receptor (IGF-IR)-immunopositive Hassall's corpuscles in aged human thymus using bright-field immunohistochemistry and immunoelectron microscopy. We report new immunocytochemical data for the presence of IGF-I/IGF-IR double-immunopositive Hassall's corpuscles in structurally preserved regions of age-involuted thymus and discuss the involvement of these unique thymic components in the local regulation of T-cell development and thymus plasticity during aging by IGF-I/IGF-IR-mediated cell signaling pathway.

Restoration of Thymic Homeostasis in a Tumor-Bearing Host by in Vivo Administration of Medicinal Herb Tinospora Cordifolia

In vivo administration of alcoholic extract of medicinal plant Tinospora cordifolia (TC) to mice bearing a spontaneous T cell lymphoma designated as Dalton's lymphoma prevented tumor growth-dependent regression of thymus. TC was found to augment proliferation of thymocytes with a concomitant decrease in thymocyte apoptosis. It also resulted in a decrease in the number of Hassal's corpuscles. Restoration of thymus homeostasis was caused by TC-dependent augmentation in production of thymocyte growth promoting cytokines Interleukin-2 and Interferon-gamma from thymocytes. TC was found to downregulate thymocyte apoptosis by modulation of Caspase pathway. TC administration retarded tumor growth and prolonged survival of tumor-bearing mice. The possible mechanisms are discussed.

HLA-DQ-regulated T-cell responses to islet cell autoantigens insulin and GAD65

HLA-DQ is strongly associated with genetic predisposition to type 1 diabetes. It is assumed that HLA-DQ molecules exert their effects on the disease via the presentation of peptides from islet autoantigens to CD4(+) T-cells, but little information regarding HLA-DQ-restricted, islet antigen-specific, autoreactive T-cells is available. To investigate the role of HLA-DQ in the immune response to islet autoantigens, we measured T-cell proliferation to insulin and GAD65 in the presence and absence of monoclonal antibodies that block HLA-DQ-mediated antigen presentation in recent-onset type 1 diabetic patients and their siblings. Positive proliferative T-cell responses to GAD65 were observed in 60% of type 1 diabetic patients and 52% of siblings. This proliferation was significantly reduced in the presence of anti-DQ antibody, demonstrating the presence of primed, effector HLA-DQ-restricted T-cell responses to GAD65. Positive proliferative responses to insulin were observed in 25% of type 1 diabetic patients and 10% of siblings. However, blocking HLA-DQ-restricted T-cell responses led to a significant increase in proliferation to insulin, implying the presence of primed suppressive HLA-DQ-restricted T-cell responses to insulin. These results indicate that HLA-DQ acts as a restriction element for both proliferative and suppressor cells, with the relative balance of these cells dependent on the nature of the autoantigen.

HLA class II polymorphisms determine responses to bacterial superantigens

The excessive immunological response triggered by microbial superantigens has been implicated in the etiology of a wide range of human diseases but has been most clearly defined for the staphylococcal and streptococcal toxic shock syndromes. Because MHC class II presentation of superantigens to T cells is not MHC-restricted, the possibility that HLA polymorphisms could influence superantigenicity, and thus clinical susceptibility to the toxicity of individual superantigens, has received little attention. In this study, we demonstrate that binding of streptococcal and staphylococcal superantigens to HLA class II is influenced by allelic differences in class II. For the superantigen streptococcal pyrogenic exotoxin A, class II binding is dependent on DQ alpha-chain polymorphisms such that HLA-DQA1*01 alpha-chains show greater binding than DQA1*03/05 alpha-chains. The functional implications of differential binding on T cell activation were investigated in various experimental systems using human T cells and murine Vbeta8.2 transgenic cells as responders. These studies showed quantitative and qualitative differences resulting from differential HLA-DQ binding. We observed changes in T cell proliferation and cytokine production, and in the Vbeta specific changes in T cell repertoire that have hitherto been regarded as a defining feature of an individual superantigen. Our observations reveal a mechanism for the different outcomes seen following infection by toxigenic bacteria.

Proinsulin expression by Hassall's corpuscles in the mouse thymus

The thymus expresses proinsulin, among many other tissue-specific antigens, and the inheritance of genetically determined low thymic proinsulin expression has been associated with impaired proinsulin-specific autoreactive T-cell tolerance and type 1 diabetes susceptibility. The cellular and molecular biology of proinsulin expression in the thymus remains unknown, and contradictory reports exist regarding the identity of proinsulin-producing cells. Using knock-in mice expressing beta-galactosidase (beta-Gal) under the control of an endogenous insulin promoter, we found that thymic proinsulin and beta-Gal transcripts were detectable at high levels in purified thymic epithelial cells. Immunohistochemical analysis of beta-Gal activity showed that most proinsulin expression can be accounted for by rare medullary epithelial cells of the Hassall's corpuscles. Moreover, flow cytometry analyses of beta-Gal-positive cells showed that only 1-3% of all epithelial cells express proinsulin, and this technique will now provide us with a method for isolating the proinsulin-producing cells in mouse thymus.

Expression of MHC Class II, CD70, CD80, CD86 and pro-inflammatory cytokines is differentially regulated in oral epithelial cells following bacterial challenge

Oral epithelium may play a regulatory role in local immune responses when interacting with bacteria. The present study was undertaken to investigate the effects of selected bacterial pathogens found in periodontal and endodontic infections on oral epithelial cells. Expression of cell surface molecules (major histocompatibility complex (MHC) Class II, CD54, CD70, CD80 and CD86) and secretion of inflammatory cytokines (interleukin (IL)-1beta, IL-6, and tumor necrosis factor (TNF)-alpha) in response to selected bacterial challenge were examined on an immortalized oral epithelial cell line, HOK-18A and a skin epithelial cell line, HaCaT. Actinomyces viscosus, Actinomyces israelii, Fusobacterium nucleatum lipopolysaccharide (LPS) or primary human periradicular exudate from a granuloma were co-cultured with epithelial cells for 4 or 24 h. Subsequently, cell surface expression of MHC Class II, CD54, CD70, CD80 and CD86, along with pro-inflammatory cytokine levels were determined using flow cytometry, ELISA and RT-PCR. Results indicated that the selected oral bacteria have greater effects on oral versus skin epithelial cells. F. nucleatum increased MHC Class II and CD54 (ICAM-1) cell surface expression on HOK-18A and HaCaT cells. A. israelii also had enhancing effects on the expression of CD54 and MHC Class II. A. israelii and LPS induced a 2.8-fold (P < 0.001) and 4.4-fold (P < 0.005) TNF-alpha secretion, respectively, while F. nucleatum and LPS induced a 10-fold (P < 0.0004) and 6-fold (P < 0.01) IL-1beta secretion, respectively by HOK-18A. Interestingly, CD70, CD80, and CD86 were generally decreased upon bacteria and LPS challenge on HOK-18A. The effects of increased MHC Class II and decreased CD70 were also evident with challenge of human periradicular exudate on HOK-18A. The implications of the study are unique in that oral epithelial cells may play both activating and inhibitory roles in the host immune response towards infection by oral bacteria. We introduce a concept of 'dormancy' where the differential expression of key cell surface antigens on oral epithelial cells may keep the recruited immune effector cells in a state of unresponsiveness, thus contributing to the long term quiescent period observed in many periodontal and endodontic lesions.

HLA-DRB1 allele polymorphisms in genetic susceptibility to esophageal carcinoma

AIM: To probe into the genetic susceptibility of HLA-DRB1 alleles to esophageal carcinoma in Han Chinese in Hubei Province.

METHODS: HLA-DRB1 allele polymorphisms were typed by polymerase chain reaction with sequence-specific primers (PCR-SSP) in 42 unrelated patients with esophageal cancer and 136 unrelated normal control subjects and the associated HLA-DRB1 allele was measured by nucleotide sequence analysis with PCR.SAS software was used in statistics.

RESULTS: Allele frequency (AF) of HLA-DRB1*0901 was significantly higher in esophageal carcinoma patients than that in the normal controls (0.2500 vs 0.1397, P = 0.028, the odds ratio 2.053, etiologic fraction 0.1282). After analyzed the allele nucleotide sequence of HLA-DRB1*0901 which approachs to the corresponded exon 2 sequence of the allele in genebank. There was no association between patients and controls in the rested HLA-DRB1 alleles.

CONCLUSION: HLA-DRB1*0901 allele is more common in the patients with esophageal carcinoma than in the healthy controls, which is positively associated with the patients of Hubei Han Chinese. Individuals carrying HLA-DRB1*0901 may be susceptible to esophageal carcinoma.

Cutting edge: identification of the targets of clonal deletion in an unmanipulated thymus

Autoreactive thymocytes can be eliminated by clonal deletion during their development in the thymus. The precise developmental stage(s) at which clonal deletion occurs in a normal thymus has been difficult to assess, in large part because of the absence of a specific marker for TCR-mediated apoptosis. In this report, we reveal that Nur77 expression can be used as a specific marker of clonal deletion in an unmanipulated thymus and directly identify TCRintCD4+CD8+ and semimature CD4+CD8- thymocytes as the principal targets of deletion. These data indicate that clonal deletion normally occurs at a relatively late stage of development, as cells mature from CD4+CD8+ thymocytes to single-positive T cells.

The Goodpasture antigen is expressed in the human thymus

BACKGROUND

Autoimmunity to kidney antigens causes membranous nephropathy and Goodpasture's disease and very likely is pivotal in many other glomerular diseases. We investigated the potential for central tolerance to the best-characterized kidney autoantigen, the NC1 domain of the alpha3 chain of type IV collagen [alpha3(IV)NC1], which is the target of autoimmune attack in Goodpasture's disease.

METHODS

Indirect immunofluorescence on human thymus and polymerase chain reaction (PCR) and Southern blot analysis of cDNA reverse transcribed from RNA extracted from human thymus and kidney.

RESULTS

Indirect immunofluorescence on human thymus demonstrated the presence of alpha3(IV)NC1 in all six thymus samples examined. The homologous collagen IV chain, alpha5(IV)NC1, also was detected with a similar intra-thymic distribution. Strikingly, thymic alpha3 and alpha5 localized around and within Hassall's corpuscles in the thymic medulla, which are structures implicated in T cell apoptosis and possibly negative selection. In contrast, alpha1(IV)NC1 localized to the basement membranes of interlobular septa and blood vessels, as is typical of collagen IV chains situated outside the thymus. Reverse transcription-polymerase chain reaction (RT-PCR) confirmed the presence of mRNA encoding alpha3(IV)NC1 and alpha5(IV)NC1 in thymic tissue establishing that the antigens were likely to have been synthesized locally.

CONCLUSIONS

The results demonstrate that alpha3(IV)NC1 is expressed in the human thymus, and therefore should be available for induction of alpha3(IV)NC1-specific tolerance. This observation has the important implication that patients' alpha3(IV)NC1-specific, autoreactive T cells are more likely to recognize cryptic epitopes that are not adequately presented by thymic antigen-presenting cells (APC) than the major antigen-derived epitopes generally identified by conventional approaches.

Altered major histocompatibility complex class II peptide loading in H2-O-deficient mice

The biosynthesis of MHC class II/peptide complexes involves classical, cell surface MHC products as well as the intracellular component H2-M, required for the removal of invariant chain-derived CLIP and for peptide loading. The function of another intracellular class II heterodimer, H2-O, is the matter of some controversy. The physical association of H2-O with H2-M and co-localization in class II+ vesicles suggest a related function in peptide exchange. Furthermore, the distinctive thymic distribution of H2-O raises the possibility of a specialized role in T cell thymic selection. To investigate the role of H2-O in vivo we generated mice carrying a targeted disruption in the H2-Oa gene. No evidence was obtained for a defect in removal of CLIP. However, the array of endogenous peptides bound by class II was altered and a defect in antigen presentation through H2-A to T cells was seen on the 129/Sv/ C57BL/6 mixed strain background but not in 129/Sv pure strain mice. Furthermore, H2-O-null mice showed enhanced selection of CD4+ single positive thymocytes. The findings indicate that H2-O interacts with H2-M in peptide editing but that the genetic background in which H2-O deficiency is manifest is also important. Overall, the experiments indicate that H2-O/HLA-DO should be regarded as neither up-regulating nor down-regulating the DM-dependent release of CLIP, but as a modulator of peptide editing, determining the presenting cell type specific peptide profile able to retain stability in the class II groove.