The Goodpasture antigen is expressed in the human thymus

How Thymocyte Deletion in the Cortex May Curtail Antigen-Specific T-Regulatory Cell Development in the Medulla

CD4⁺ T cell responses to self-antigens are pivotal for immunological self-tolerance. Activation of Foxp3^– T-conventional (T-conv) cells can precipitate autoimmune disease, whereas activation of Foxp3⁺ T-regulatory (T-reg) cells is essential to prevent autoimmune disease. This distinction indicates the importance of the thymus in controlling the differentiation of self-reactive CD4⁺ T cells. Thymocytes and thymic antigen-presenting cells (APC) depend on each other for normal maturation and differentiation. In this Hypothesis and Theory article, we propose this mutual dependence dictates which self-antigens induce T-reg cell development in the thymic medulla. We postulate self-reactive CD4⁺ CD8^– thymocytes deliver signals that stabilize and amplify the presentation of their cognate self-antigen by APC in the thymic medulla, thereby seeding a niche for the development of T-reg cells specific for the same self-antigen. By limiting the number of antigen-specific CD4⁺ thymocytes in the medulla, thymocyte deletion in the cortex may impede the formation of medullary T-reg niches containing certain self-antigens. Susceptibility to autoimmune disease may arise from cortical deletion creating a “hole” in the self-antigen repertoire recognized by T-reg cells.

Novel Combination of Surface Markers for the Reliable and Comprehensive Identification of Human Thymic Epithelial Cells by Flow Cytometry: Quantitation and Transcriptional Characterization of Thymic Stroma in a Pediatric Cohort

Thymic epithelial cells (TECs) are essential in supporting the development of mature T cells from hematopoietic progenitor cells and facilitate their lineage-commitment, proliferation, T-cell receptor repertoire selection and maturation. While animal model systems have greatly aided in elucidating the contribution of stromal cells to these intricate processes, human tissue has been more difficult to study, partly due to a lack of suitable surface markers comprehensively defining human TECs. Here, we conducted a flow cytometry based surface marker screen to reliably identify and quantify human TECs and delineate medullary from cortical subsets. These findings were validated by transcriptomic and histologic means. The combination of EpCAM, podoplanin (pdpn), CD49f and CD200 comprehensively identified human TECs and not only allowed their reliable distinction in medullary and cortical subsets but also their detailed quantitation. Transcriptomic profiling of each subset in comparison to fibroblasts and endothelial cells confirmed the identity of the different stromal cell subsets sorted according to the proposed strategy. Our dataset not only demonstrated transcriptional similarities between TEC and cells of mesenchymal origin but furthermore revealed a subset-specific distribution of a specific set of extracellular matrix-related genes in TECs. This indicates that TECs significantly contribute to the distinct compartmentalization - and thus function - of the human thymus. We applied the strategy to quantify TEC subsets in 31 immunologically healthy children, which revealed sex-specific differences of TEC composition early in life. As the distribution of mature CD4- or CD8-single-positive thymocytes was correspondingly altered, the composition of the thymic epithelial compartment may directly impact on the CD4-CD8-lineage choice of thymocytes. We prove that the plain, reliable strategy proposed here to comprehensively identify human TEC subpopulations by flow cytometry based on surface marker expression is suitable to determine their frequency and phenotype in health and disease and allows sorting of live cells for downstream analysis. Its use reaches from a reliable diagnostic tool for thymic biopsies to improved phenotypic characterization of thymic grafts intended for therapeutic use.

Role of regulatory T cells in experimental autoimmune glomerulonephritis

Anti-glomerular basement membrane (anti-GBM) disease is characterized by antibodies and T cells directed against the Goodpasture antigen, the noncollagenous domain of the α3-chain of type IV collagen [α3(IV)NC1] of the GBM. Consequences are the deposition of autoantibodies along the GBM and the development of crescentic glomerulonephritis (GN) with rapid loss of renal function. Forkhead box protein P3 (Foxp3)⁺ regulatory T (Treg) cells are crucial for the maintenance of peripheral tolerance to self-antigens and the prevention of immunopathology. Here, we use the mouse model of experimental autoimmune GN to characterize the role of Treg cells in anti-GBM disease. Immunization of DBA/1 mice with α3(IV)NC1 induced the formation of α3(IV)NC1-specific T cells and antibodies and, after 8-10 wk, the development of crescentic GN. Immunization resulted in increased frequencies of peripheral Treg cells and renal accumulation of these cells in the stage of acute GN. Depletion of Treg cells during immunization led to enhanced generation of α3(IV)NC1-specific antibodies and T cells and to aggravated GN. In contrast, depletion or expansion of the Treg cell population in mice with established autoimmunity had only minor consequences for renal inflammation and did not alter the severity of GN. In conclusion, our results indicate that in anti-GBM disease, Treg cells restrict the induction of autoimmunity against α3(IV)NC1. However, Treg cells are inefficient in preventing crescentic GN after autoimmunity has been established.

Anti-Glomerular Basement Membrane Disease

Anti-glomerular basement membrane (anti-GBM) disease is a rare autoimmune small vessel vasculitis characterized by autoreactivity to antigens in type IV collagen chains expressed in glomerular and alveolar basement membrane. The detection of circulating anti-GBM antibodies, which are shown to be directly pathogenic, is central to disease diagnosis. Clinically, anti-GBM disease usually presents with rapidly progressive glomerulonephritis with or without alveolar hemorrhage. Rapid diagnosis and early treatment are required to prevent mortality and to preserve renal function. Relapse in anti-GBM disease is uncommon. Variant and atypical forms of anti-GBM disease are increasingly recognised.

Antigen-Specificity in the Thymic Development and Peripheral Activity of CD4+FOXP3+ T Regulatory Cells

CD4⁺Foxp3⁺ T regulatory cells (Treg) are essential for the life of the organism, in particular because they protect the host against its own autoaggressive CD4⁺Foxp3^- T lymphocytes (Tconv). Treg distinctively suppress autoaggressive immunity while permitting efficient defense against infectious diseases. This split effect indicates that Treg activity is controlled in an antigen-specific manner. This specificity is achieved first by the formation of the Treg repertoire during their development, and second by their activation in the periphery. This review presents novel information on the antigen-specificity of Treg development in the thymus, and Treg function in the periphery. These aspects have so far remained imprecisely understood due to the lack of knowledge of the actual antigens recognized by Treg during the different steps of their life, so that most previous studies have been performed using artificial antigens. However, recent studies identified some antigens mediating the positive selection of autoreactive Treg in the thymus, and the function of Treg in the periphery in autoimmune and allergic disorders. These investigations emphasized the remarkable specificity of Treg development and function. Indeed, the development of autoreactive Treg in the thymus was found to be mediated by single autoantigens, so that the absence of one antigen led to a dramatic loss of Treg reacting toward that antigen. The specificity of Treg development is important because the constitution of the Treg repertoire, and especially the presence of holes in this repertoire, was found to crucially influence human immunopathology. Indeed, it was found that the development of human immunopathology was permitted by the lack of Treg against the antigens driving the autoimmune or allergic T cell responses rather than by the impairment of Treg activation or function. The specificity of Treg suppression in the periphery is therefore intimately associated with the mechanisms shaping the formation of the Treg repertoire during their development. This novel information refines significantly our understanding of the antigen-specificity of Treg protective function, which is required to envision how these cells distinctively regulate unwanted immune responses as well as for the development of appropriate approaches to optimally harness them therapeutically in autoimmune, malignant, and infectious diseases.

Basement membranes and autoimmune diseases

Basement membrane components are targets of autoimmune attack in diverse diseases that destroy kidneys, lungs, skin, mucous membranes, joints, and other organs in man. Epitopes on collagen and laminin, in particular, are targeted by autoantibodies and T cells in anti-glomerular basement membrane glomerulonephritis, Goodpasture's disease, rheumatoid arthritis, post-lung transplant bronchiolitis obliterans syndrome, and multiple autoimmune dermatoses. This review examines major diseases linked to basement membrane autoreactivity, with a focus on investigations in patients and animal models that advance our understanding of disease pathogenesis. Autoimmunity to glomerular basement membrane type IV is discussed in depth as a prototypic organ-specific autoimmune disease yielding novel insights into the complexity of anti-basement membrane immunity and the roles of genetic and environmental susceptibility.

The HLA-DRB1*15:01-restricted Goodpasture's T cell epitope induces GN

Human anti-glomerular basement membrane (GBM) disease strongly associates with HLA-DRB1*15:01. The target autoantigen in this disease is the noncollagenous domain of the α3 chain of type IV collagen, α3(IV)NC1, but critical early T cell epitopes presented by this human MHC class II molecule are unknown. Here, by immunizing HLA-DRB1*15:01 transgenic mice with whole recombinant α3(IV)NC1 and with overlapping α3(IV)NC1 peptides, we defined a HLA-DRB1*15:01-restricted α3(IV)NC1 T cell epitope (α3136-146) with four critical residues. This peptide was not immunogenic in HLA-DRB1*01:01 transgenic or C57BL/6 mice. The T cell epitope is naturally processed from α3(IV)NC1. CD4(+) T cell clones, generated from HLA-DRB1*15:01 transgenic mice and specific for α3136-146, transferred disease into naive HLA-DRB1*15:01 transgenic mice, evidenced by the development of necrotizing crescentic GN, albuminuria, renal impairment, and accumulation of CD4(+) T cells and macrophages in glomeruli. Because Fcγ receptors are implicated in disease susceptibility, we crossed HLA transgenic mice onto an FcγRIIb-deficient background. Immunization with either α3136-146 or α3(IV)NC1 induced GN in HLA-DRB1*15:01 transgenic FcγRIIb-deficient mice, but HLA-DRB1*01:01 transgenic FcγRIIb-deficient mice were unaffected. Taken together, these results demonstrate that the HLA-DRB1*15:01-restricted T cell epitope α3136-146 can induce T cell responses and injury in anti-GBM GN.

Cutting edge issues in Goodpasture's disease

Goodpasture's disease, or anti-glomerular basement membrane (anti-GBM) disease, is a systemic autoimmune disorder defined by anti-GBM antibody-mediated damage (mainly immunoglobulin G-1) resulting in progressive crescentic glomerulonephritis and, frequently, diffuse pulmonary alveolar hemorrhage. It may be regarded as a "conformeropathy" where the quaternary structure of the α345NC1 hexamer that constitutes GBM undergoes a conformational change, exposing pathogenic epitopes on the α3 and α5 chains, eliciting a pathogenic autoantibody anti-GBM response. Goodpasture's disease accounts for 20% of all patients presenting with a pulmonary-renal syndrome and may be associated with detectable perinuclear antineutrophil cytoplasmic autoantibody positivity in up to a third of patients. Associated triggers may include tobacco smoking, hydrocarbon solvent exposure, and cocaine abuse. Cough, hemoptysis, and dyspnea with fatigue are the commonest presenting features. It is critical to rapidly distinguish Goodpasture's disease from other causes of pulmonary-renal syndromes such as Wegener's granulomatosis. Early and intensive treatment with plasmapheresis and immunosuppression with systemic corticosteroids pending results of diagnostic testing, and later cyclophosphamide, is often beneficial, with 90% of patients surviving the acute presentation of Goodpasture's disease. The need for hemodialysis on initial presentation, a serum creatinine >5 mg/dL, and 50% to 100% crescents on renal biopsy, portend the necessity of long-term hemodialysis. Further elucidation of the molecular pathobiology of Goodpasture's disease, particularly the regulation of involved antigen-specific T cells, may improve early diagnosis, treatment, and outcomes in this rare but potentially lethal autoimmune disorder.

Genetic elimination of α3(IV) collagen fails to rescue anti-collagen B cells

Organ deposition of autoantibodies against the noncollagenous-1 domain of the α3 chain of type IV collagen leads to severe kidney and lung injury in anti-glomerular basement membrane disease. The origin and regulation of these highly pathogenic autoantibodies remains unknown. Anti-α3(IV) collagen B lymphocytes are predicted to mature in vivo ignorant of target antigen because α3(IV) collagen expression is highly tissue restricted and pathogenic epitopes are cryptic. However, a recent analysis of an anti-α3(IV)NC1 collagen autoantibody transgenic mouse model revealed that developing B cells are rapidly silenced by deletion and editing in the bone marrow. To dissect the role of collagen as central tolerogen in this model, we determined B cell fate in autoantibody transgenic mice genetically lacking α3(IV) collagen. We found that absence of the tissue target autoantigen has little impact on the fate of anti-α3(IV)NC1 B cells. This implies a more complex regulatory mechanism for preventing anti-glomerular basement membrane disease than has been previously considered, including the possibility that a second antigen present in bone marrow engages and tolerizes anti-α3(IV)NC1 collagen B cells.

Crescentic glomerulonephritis: new aspects of pathogenesis

This review provides a summary of recent advances in the understanding of crescentic glomerulonephritis, focusing on antineutrophil cytoplasm antibody (ANCA)-associated vasculitis and anti-glomerular basement membrane (anti-GBM) antibody disease. In ANCA-associated vasculitis (AAV), four main conceptual advances are discussed as follows: (1) evidence for the pathogenicity of ANCA, (2) molecular mimicry and the role of infection in AAV, (3) evidence for aberrant T-cell responses and T-cell regulation in AAV, and (4) advances in understanding of genetic predisposition to AAV. In relation to anti-GBM disease we discuss the following: (1) the nature of the Goodpasture autoantigens, (2) T-cell responses and regulation in anti-GBM disease, and (3) human leukocyte antigen and non-human leukocyte antigen genetic associations.

Natural autoantibodies to myeloperoxidase, proteinase 3, and the glomerular basement membrane are present in normal individuals

Anti-neutrophil cytoplasmic antibodies (ANCAs) have a pathogenic role in ANCA-associated vasculitis. The origin of ANCAs and anti-glomerular basement membrane (GBM) antibodies, however, is unknown. In this study, we determined whether natural autoantibodies against myeloperoxidase (MPO), proteinase 3 (PR3), and GBM were present in each of 10 healthy Chinese and Swedish individuals, negative for all three antigens by routine ELISA. Antibodies were purified from isolated IgG by antigen-specific affinity columns. Natural anti-GBM autoantibodies gave a linear staining pattern along the GBM of human renal sections. On ethanol-fixed granulocytes, both natural anti-MPO and anti-PR3 autoantibodies gave cytoplasmic staining. The titers of natural anti-MPO/PR3 autoantibodies were significantly lower than those from patients with vasculitis. In competition ELISA, the binding of natural anti-MPO autoantibodies could be inhibited by MPO, but not by PR3 or noncollagenous domains from type IV collagen. The same specificity results were found for natural anti-PR3 and anti-GBM autoantibodies. Overall, individuals of the Chinese origin had more natural autoantibodies than did those of the Swedish origin, but no other differences were found. Hence, our study shows that healthy individuals have masked circulating, noncross-reactive, antigen-specific natural autoantibodies against MPO, PR3, and GBM in their serum and IgG fractions. Further studies are needed to determine their role if any in the etiology of ANCA-associated vasculitis and anti-GBM disease.

Central tolerance regulates B cells reactive with Goodpasture antigen alpha3(IV)NC1 collagen

Patients and rodents with Goodpasture's syndrome (GPS) develop severe autoimmune crescentic glomerulonephritis, kidney failure, and lung hemorrhage due to binding of pathogenic autoantibodies to the NC1 domain of the alpha3 chain of type IV collagen. Target epitopes are cryptic, normally hidden from circulating Abs by protein-protein interactions and the highly tissue-restricted expression of the alpha3(IV) collagen chain. Based on this limited Ag exposure, it has been suggested that target epitopes are not available as B cell tolerogens. To determine how pathogenic anti-GPS autoantibody responses are regulated, we generated an Ig transgenic (Tg) mouse model that expresses an Ig that binds alpha3(IV)NC1 collagen epitopes recognized by serum IgG of patients with GPS. Phenotypic analysis reveals B cell depletion and L chain editing in Tg mice. To determine the default tolerance phenotype in the absence of receptor editing and endogenous lymphocyte populations, we crossed Tg mice two generations with mice deficient in Rag. Resulting Tg Rag-deficient mice have central B cell deletion. Thus, development of Tg anti-alpha3(IV)NC1 collagen B cells is halted in the bone marrow, at which point the cells are deleted unless rescued by a Rag enzyme-dependent process, such as editing. The central tolerance phenotype implies that tolerizing self-Ag is expressed in bone marrow.

Advances in the pathogenesis of Goodpasture's disease: from epitopes to autoantibodies to effector T cells

Goodpasture's disease, an "organ-specific" autoimmune disease is manifest by rapidly progressive glomerulonephritis and pulmonary hemorrhage. Studies into the pathogenesis of this disease have shed light on the autoantigen (the non-collagenous domain of the alpha3 chain of type IV collagen, alpha3(IV)NC1) and its epitopes, as well as the involvement of autoantibodies and cellular effectors in disease. The discovery of alpha3(IV)NC1 lead to studies that defined the structure and biology of type IV collagen and are defining B and T cell epitopes. Goodpasture autoantibody epitopes are "cryptic" in that they are structurally sequestered by adjacent non-collagenous domains of alpha4 and alpha5 type IV collagen. T cell epitope studies in rats demonstrated that a 13-mer could induce experimental autoimmune glomerulonephritis. T cells from patients with Goodpasture's recognize two epitopes, in regions which are highly susceptible in antigen processing by endosomal proteases. Goodpasture's disease is strongly associated with HLA DRB1 genes, whereby DRB1*1501 confers susceptibility and the DRB1*0701 and DRB1*0101 are dominantly protective. Experimental data implicate both autoantibodies and cell mediated immunity as disease effectors. Observations in humans suggest that regulatory T cells are associated with the development of self-immunoregulation in the convalescent phase of disease.

Mammalian collagen IV

Four decades have passed since the first discovery of collagen IV by Kefalides in 1966. Since then collagen IV has been investigated extensively by a large number of research laboratories around the world. Advances in molecular genetics have resulted in identification of six evolutionary related mammalian genes encoding six different polypeptide chains of collagen IV. The genes are differentially expressed during the embryonic development, providing different tissues with specific collagen IV networks each having unique biochemical properties. Newly translated alpha-chains interact and assemble in the endoplasmic reticulum in a chain-specific fashion and form unique heterotrimers. Unlike most collagens, type IV collagen is an exclusive member of the basement membranes and through a complex inter- and intramolecular interactions form supramolecular networks that influence cell adhesion, migration, and differentiation. Collagen IV is directly involved in a number of genetic and acquired disease such as Alport's and Goodpasture's syndromes. Recent discoveries have also highlighted a new and direct role for collagen IV in the development of rare genetic diseases such as cerebral hemorrhage and porencephaly in infants and hemorrhagic stroke in adults. Years of intensive investigations have resulted in a vast body of information about the structure, function, and biology of collagen IV. In this review article, we will summarize essential findings on the structural and functional relationships of different collagen IV chains and their roles in health and disease.

Expression of pemphigus-autoantigen desmoglein 1 in human thymus

Desmoglein (Dsg) 1 is a transmembrane glycoprotein of the desmosome allowing cell-cell adhesion between keratinocytes, whose expression is restricted to stratified squamous epithelia-like epidermis. Dsg1 is the target autoantigen of pathogenic autoantibodies produced by pemphigus foliaceus and 50% of pemphigus vulgaris patients in a Dsg1-specific T-cell-dependent pathway. Herewith, we show that mRNA of the DSG1 gene is present in normal human thymus and show by quantitative real-time polymerase chain reaction analysis that the expression of DSG1 transcript increases with age. Although immunoblot analysis on human thymus extracts using different anti-Dsg1 antibodies did not allow to detect the protein, we show by double-immunofluorescence assay that Dsg1 is expressed at protein level by CD19+ CD63+ cells located in the medulla. These data provide another illustration of the thymic expression of a tissue-specific autoantigen involved in an organ-specific autoimmune disease, which may participate in the tolerance acquisition and/or regulation of Dsg1-specific T cells.

Autoepitopes and alloepitopes of type IV collagen: role in the molecular pathogenesis of anti-GBM antibody glomerulonephritis

Anti-glomerular basement membrane (anti-GBM) antibodies elicited by autoimmune or alloimmune mechanisms are associated with aggressive forms of rapid progressive glomerulonephritis. Pathogenic anti-GBM autoantibodies and alloantibodies target the noncollagenous (NC1) domains of the alpha3alpha4alpha5(IV) collagen, a major GBM component. In autoimmune anti-GBM glomerulonephritis, a breakdown of immune self-tolerance leads to the activation of autoreactive B and T cells recognizing epitopes within the alpha3NC1 subunit. In the GBM, the conformational epitopes targeted by anti-GBM autoantibodies are structurally sequestered within the alpha3alpha4alpha5NC1 hexamer complex formed upon assembly of collagen IV chains into trimeric molecules and networks. Autoantibodies selectively bind to and dissociate a subset of alpha3alpha4alpha5NC1 hexamers composed of monomer subunits, whereas hexamers containing NC1 dimer subunits are resistant to dissociation by autoantibodies. The crypticity of alpha3NC1 autoepitopes suggests that self-tolerance to alpha3(IV) collagen is broken by structural alterations of the native alpha3alpha4alpha5NC1 hexamer that unmask normally sequestered epitopes, triggering an autoimmune reaction. Post-transplant anti-GBM nephritis in the renal allograft of transplanted Alport patients is mediated by an alloimmune reaction to the NC1 domains of alpha3alpha4alpha5(IV) collagen, present in the allograft GBM but absent from Alport basement membranes. Alloantibodies from patients with autosomal-recessive Alport syndrome predominantly bind to the alpha3NC1 domain, whereas alloantibodies from X-linked Alport patients target preferentially, though not exclusively, epitopes within the alpha5NC1 subunit. The accessibility of the alloantigenic sites within the alpha3alpha4alpha5NC1 hexamers, contrasting with the crypticity of autoantigenic sites, suggest that different molecular forms of alpha3alpha4alpha5(IV) collagen initiate the immunopathogenic responses in the two forms of anti-GBM disease. Advances in elucidating the structure of the GBM antigen and the identification of the pathogenic B and T cell epitopes, along with new insights into the pathogenic mechanisms at cellular and molecular level will facilitate the development of targeted strategies for prevention, detection, and treatment of human anti-GBM antibody glomerulonephritis.

Presentation of the Goodpasture Autoantigen Requires Proteolytic Unlocking Steps That Destroy Prominent T Cell Epitopes

The most abundant autoreactive T cells in patients with Goodpasture's disease are specific for peptides in the autoantigen that have high affinity for the disease-associated HLA class II molecule, DR15. How can such T cells escape self-tolerance mechanisms? This study showed that these peptides are highly susceptible to destruction in the earliest stages of antigen processing, and some must be cleaved for antigen digestion to be possible ("unlocking"). Goodpasture autoantigen [collagen alpha3(IV)NC1; approximately 31 kD] that was incubated with B cell lysosomes was cleaved within a few minutes to form approximately 9- and approximately 22-kD fragments, then increasing quantities of smaller peptides. The processing was completely abrogated by pepstatin A, a specific inhibitor of cathepsin D/E, even though lysosomal extracts contain a rich array of proteases. Purified cathepsin D generated the same major alpha3(IV)NC1 fragments as entire lysosomes, suggesting that cathepsin D cleavages are required to initiate alpha3(IV)NC1 processing. The initial unlocking cleavages destroyed two major self-epitopes, and subsequent preferred cleavages destroyed all of the other T cell epitopes that are recognized by most patients' autoreactive T cells. The responses of T cell clones that are specific for a major disease-associated peptide to antigen-pulsed intact antigen-presenting cells were substantially enhanced by pepstatin A treatment. Therefore, cathepsin D activity significantly diminishes presentation of alpha3(IV)NC1 peptides that are recognized by patients' T cells by destroying the peptides in early processing. These observations can explain why the mature T cell repertoire includes reactivity toward these self-peptides and suggests that a key factor in disease initiation is likely to be a shift in antigen processing.

T cells in crescentic glomerulonephritis

Crescent formation in glomerulonephritis (GN) is a manifestation of severe glomerular injury that usually results in a poor clinical outcome. In humans, crescentic GN is frequently associated with evidence of either systemic or organ-specific autoimmunity. T cells play a major role in initiation of adaptive immune responses that lead to crescentic injury. In experimental models of crescentic GN, Th1 predominant immune responses have been shown to promote crescent formation. Perturbation of regulatory T cell function may contribute to development of autoimmune crescentic GN. The presence of T cells and macrophages in crescentic glomeruli, frequently in the absence of humoral mediators of immunity, suggest a dominant effector role for T cells in crescentic GN. The association of cellular immune mediators with local fibrin deposition implicates cell-mediated "delayed-type hypersensitivity-like" mechanisms in crescent formation. Intrinsic renal cells also contribute to T cell-driven effector mechanisms in crescentic GN, via expression of MHC II and co-stimulatory molecules and by production of chemokines and cytokines that amplify leukocyte recruitment and injury.

Natural autoantibodies against glomerular basement membrane exist in normal human sera

Autoimmunity to glomerular basement membrane (GBM) could induce Goodpasture disease, and natural autoantibodies against GBM in the sera of normal individuals were not reported. The aim of the study was to identify and characterize natural autoantibodies against GBM in normal human sera. Natural anti-GBM autoantibodies were purified from the sera of five healthy persons by affinity chromatography, using purified bovine alpha(IV)non-collagenous (NC1) as solid-phase ligands. Antigen specificity, immunoglobulin G (IgG) subclasses, and antibody avidity of the natural autoantibodies were investigated by enzyme-linked immunosorbant assay (ELISA), Western-blot analysis, indirect immunofluorescence, and antigen-inhibition ELISA, and compared with those of 32 patients with anti-GBM disease. Natural anti-GBM autoantibodies could be purified from IgG fractions of all the five persons, with an average amount of 0.5% of total IgG fractions. Antigen specificity of the natural autoantibodies was identified by blotting to human alpha(IV)NC1, reactivity to recombinant alpha3(IV)NC1, and linear staining along the GBM of normal kidney sections. Titers of the natural autoantibodies were much lower than those of patients (1:60.6 vs 1:993.6, P<0.001). IgG subclasses distribution of the natural autoantibodies was restricted to IgG2 (100%) and IgG4 (100%), while for patients it was mainly IgG1 (93.8%) and IgG4 (90.6%). Avidity of the natural autoantibodies was lower than that of patients, the amount of alpha(IV)NC1 used for 50% inhibition was 1.65 and 0.46 microg, respectively (P<0.05). In conclusion, natural anti-GBM autoantibodies exist in normal human sera. Antibody levels, IgG subclasses, and avidity of the natural autoantibodies were different from those of patients. Fine specificity of the natural autoantibodies needed to be elucidated.

The kinetic stability of MHC class II:peptide complexes is a key parameter that dictates immunodominance

T cell priming to exogenous antigens reflects regulated antigen processing in dendritic cells, subsequent homing to lymph nodes, sustained interactions between T cells and antigen-bearing dendritic cells, and, ultimately, selective T cell activation and differentiation. In this study, we test the hypothesis that an intrinsic property of the class II:peptide complex is a key determinant that dictates the specificity of an emerging CD4 T cell response. We found that immunodominant peptides possess extremely long half-lives with class II molecules (t(1/2) > 150 hr), whereas cryptic peptides displayed half-lives of less than 10 hr. Furthermore, and most importantly, by using a peptide shuttle vector and four independent antigens, we demonstrate a direct, causative relationship between the half-life of peptide epitopes and their immunogenicity in vivo. Taken collectively, our results suggest the half-life of class II:peptide complexes is the primary parameter that dictates the ultimate hierarchy of the elicited T cell response.

Goodpasture autoantibodies unmask cryptic epitopes by selectively dissociating autoantigen complexes lacking structural reinforcement: novel mechanisms for immune privilege and autoimmune pathogenesis

Rapidly progressive glomerulonephritis in Goodpasture disease is mediated by autoantibodies binding to the non-collagenous NC1 domain of alpha3(IV) collagen in the glomerular basement membrane. Goodpasture epitopes in the native autoantigen are cryptic (sequestered) within the NC1 hexamers of the alpha3alpha4alpha5(IV) collagen network. The biochemical mechanism for crypticity and exposure for autoantibody binding is not known. We now report that crypticity is a feature of the quaternary structure of two distinct subsets of alpha3alpha4alpha5(IV) NC1 hexamers: autoantibody-reactive M-hexamers containing only monomer subunits and autoantibody-impenetrable D-hexamers composed of both dimer and monomer subunits. Goodpasture antibodies only breach the quaternary structure of M-hexamers, unmasking the cryptic epitopes, whereas D-hexamers are resistant to autoantibodies under native conditions. The epitopes of D-hexamers are structurally sequestered by dimer reinforcement of the quaternary complex, which represents a new molecular solution for conferring immunologic privilege to a potential autoantigen. Dissociation of non-reinforced M-alpha3alpha4alpha5(IV) hexamers by Goodpasture antibodies is a novel mechanism whereby pathogenic autoantibodies gain access to cryptic B cell epitopes. These findings provide fundamental new insights into immune privilege and the molecular mechanisms underlying the pathogenesis of human autoimmune Goodpasture disease.

Patients with Goodpasture's disease have two normal COL4A3 alleles encoding the NC1 domain of the type IV collagen 3 chain

BACKGROUND

Goodpasture's disease (GP) is a rare but severe disease characterized by anti-glomerular basement membrane antibodies, rapidly progressive glomerulonephritis and lung haemorrhage. The autoantibodies are restricted to a narrow epitope region on the NC1 domain of the alpha 3 chain of type IV collagen. GP is strongly associated with major histocompatibility complex (MHC) allele HLA DRB1-15. Recent research, however, has failed to identify a T-cell epitope with molecular characteristics that explain the relationship between the MHC class II molecule and the autoantibody generation. We hypothesized that an as yet unidentified sequence variant in exons 48-52 of the COL4A3 gene that encodes the NC1 domain of the type IV collagen alpha 3 chain could generate a new peptide sequence that, through interaction with specific MHC class II molecules, would increase the risk of developing GP.

METHODS

All patients previously treated for GP at the Lund and Malmö University Hospitals, who were alive at the time of the study, were asked to participate. DNA was extracted from leukocytes and subjected to genomic tissue typing and sequencing of the COL4A3 gene exons 48-52.

RESULTS

All 15 patients in the study had a nucleotide sequence in the COL4A3 gene encoding a protein identical to GenBank entry NM_000091. HLA D allele distribution was in line with previous publications, showing a strong positive association between HLA DRB1-15, HLA DQB1-6 and GP (P<0.02). Of the 15 GP patients, 73% carried HLA DRB1-15 and 87% carried the HLA DQB1-6 antigen. Corresponding figures for the controls were 27 and 50%.

CONCLUSION

This study effectively falsifies the hypothesis that a minor alteration in the COL4A3 gene could be a major factor in the aetiology of GP. Scandinavian GP patients have an MHC distribution similar to that which has been described previously for Anglo-Saxon patients.

Pathogenesis of Goodpasture syndrome: a molecular perspective

Goodpasture (GP) syndrome is a form of anti-glomerular basement membrane (GBM) disease, in which autoantibodies bind to alpha3(IV) collagen in GBM causing rapidly progressive glomerulonephritis and pulmonary hemorrhage. The conformational GP epitopes have been mapped to 2 regions within the noncollagenous (NC1) domain of the alpha3(IV) chain. Recently, we described the molecular organization of the autoantigen in the native alpha3alpha4alpha5(IV) collagen network of the GBM. The crystal structure of the NC1 domain has revealed how the GP epitopes are sequestered in the native GBM. Further insight into the pathogenesis of disease has been obtained from better animal models. These advances provide a foundation for the development of new specific therapies.

Regulation by CD25+ lymphocytes of autoantigen-specific T-cell responses in Goodpasture's (anti-GBM) disease

BACKGROUND

Goodpasture's, or anti-glomerular basement membrane (GBM), disease is unusual among autoimmune diseases in that it rarely follows a relapsing-remitting course. Moreover, untreated, autoantibodies disappear spontaneously after 1 to 3 years and, following treatment, autoreactive T cells diminish in frequency. This suggests that operational tolerance toward the autoantigen is reestablished. However, the mechanisms underlying this have remained unclear. Recent data have suggested that a population of regulatory T lymphocytes can suppress both autoimmune and alloimmune responses in animal models and are present in normal individuals. However, to date, they have not been demonstrated to play a role in human renal autoimmune disease.

METHODS

We studied the role of regulatory CD25+ cells in suppressing T-cell responses to the Goodpasture autoantigen in nine patients with Goodpasture's disease.

RESULTS

At the time of acute presentation, there was no evidence of a regulatory cell population. However, from 3 months onward a population emerged, capable of suppressing the response to the Goodpasture autoantigen. Following depletion of CD25+ cells, the frequencies of autoreactive-, GBM-, or collagen alpha 3(IV)NC1-specific T cells were significantly increased (P = 0.031 by paired t test), with five of seven (71%) convalescent patients and no acute patients demonstrating regulation.

CONCLUSION

These data demonstrate that, in Goodpasture's disease, regulatory CD25+ T cells play a role in inhibiting the autoimmune response. Their emergence and persistence may underlie the "single hit" nature of this condition. Understanding the conditions required for the development and propagation of these cells would allow development of novel therapeutic strategies for inducing hyporesponsiveness in autoimmune disease.

Granulocyte and monocyte adsorption apheresis in a patient with antiglomerular basement membrane glomerulonephritis and active ulcerative colitis

A 58-year-old woman with Goodpasture syndrome and active ulcerative colitis is described. On admission, the patient had exertional dyspnea, hemoptysis, severe hypertension, and peripheral edema. Her serum levels of urea nitrogen and creatinine were increased, and her hemoglobin concentration was reduced. The patient had a rapidly progressive glomerulonephritis with acute renal failure. She was treated with methylprednisolone, cyclophosphamide, and plasmapheresis but failed to regain renal function. Circulating anti-glomerular basement membrane (anti-GBM) antibody was positive; however, serum antinuclear antibody, proteinase-3-antineutrophil cytoplasm antibody and myeloperoxidase-antineutrophil cytoplasm antibody were negative. Nineteen months after initial presentation, she developed abdominal pain and severe diarrhea. These symptoms did not improve with conventional treatment. Colonoscopy performed after 3 months showed multiple ulcers in the colon. She was diagnosed with ulcerative colitis. She underwent granulocyte and monocyte adsorption apheresis once per week for 5 weeks. At 8 weeks, her symptoms had improved; her stool number was markedly decreased, and the bloody stools and abdominal pain disappeared. These results suggest that granulocyte and monocyte apheresis may be of benefit in the therapy of a patient with ulcerative colitis who previously had Goodpasture syndrome

Immunology of anti-glomerular basement membrane disease

Anti-glomerular basement membrane disease is a form of autoimmune glomerulonephritis often accompanied by lung haemorrhage. It is characterized by circulating and deposited antibodies that bind basement membrane components in the glomerulus and lung alveolus. Since early descriptions of the deposition of immunoglobulin on the glomerular basement membrane, work has focused on the binding properties of the autoantibodies, and this has led to the identification of the autoantigen as the non-collagenous region of the alpha 3 chain of type IV collagen. Despite being thought of as a prototypic antibody mediated autoimmune disease, it is becoming apparent that both humoral and cellular immune mechanisms act in concert to initiate and perpetuate disease. Recent data have shed light on the molecular pathogenesis of anti-glomerular basement membrane disease and provided a more complete framework on which to build our understanding of autoimmune renal disease. This should lead to novel approaches to immunotherapy for patients with glomerulonephritis.