Distinct Epitopes for Anti–Glomerular Basement Membrane Alport Alloantibodies and Goodpasture Autoantibodies within the Noncollagenous Domain of α3(IV) Collagen: A Janus-Faced Antigen

Unleashing the power of complement activation: unraveling renal damage in human anti-glomerular basement membrane disease

Anti-glomerular basement membrane (GBM) disease is a rare but life-threatening autoimmune disorder characterized by rapidly progressive glomerulonephritis with or without pulmonary hemorrhage. Renal biopsies of anti-GBM patients predominantly show linear deposition of IgG and complement component 3 (C3), indicating a close association between antigen-antibody reactions and subsequent complement activation in the pathogenesis of the disease. All three major pathways of complement activation, including the classical, lectin, and alternative pathways, are involved in human anti-GBM disease. Several complement factors, such as C3, C5b-9, and factor B, show a positive correlation with the severity of the renal injury and act as risk factors for renal outcomes. Furthermore, compared to patients with single positivity for anti-GBM antibodies, individuals who are double-seropositive for anti-neutrophil cytoplasmic antibody (ANCA) and anti-GBM antibodies exhibit a unique clinical phenotype that lies between ANCA-associated vasculitis (AAV) and anti-GBM disease. Complement activation may serve as a potential "bridge" for triggering both AAV and anti-GBM conditions. The aim of this article is to provide a comprehensive review of the latest clinical evidence regarding the role of complement activation in anti-GBM disease. Furthermore, potential therapeutic strategies targeting complement components and associated precautions are discussed, to establish a theoretical basis for complement-targeted therapies.

Anti-glomerular basement membrane vasculitis

Antiglomerular basement membrane disease (anti-GBM) is a rare life-threatening autoimmune vasculitis that involves small vessels and it is characterized by circulating autoantibodies directed against type IV collagen antigens expressed in glomerular and alveolar basement membrane. The typical clinical manifestations are the rapidly progressive glomerulonephritis and the alveolar hemorrhage. The diagnosis is usually confirmed by the detection of anti-GBM circulating antibodies. If not rapidly recognized, anti-GBM disease can lead to end stage kidney disease (ESKD). An early diagnosis and prompt treatment with immunosuppressive therapies and plasmapheresis are crucial to prevent a poor outcome. In this review, we discuss the primary form of anti-GBM (the so called Goodpasture syndrome) but also cases associated with other autoimmune diseases such as antineutrophil-cytoplasmic-antibody (ANCA) vasculitis, membranous nephropathy, IgA nephritis and systemic lupus erythematosus (SLE), as well as the few cases of anti-GBM vasculitis complicating kidney transplantation in the Alport syndrome.

Molecular Analysis of Goodpasture's Disease Following Hematopoietic Stem Cell Transplant in a Pediatric Patient, Recalls the Conformeropathy of Wild-Type Anti-GBM Disease

Background: Goodpasture's disease (GP) is mediated by autoantibodies that bind the glomerular and alveolar basement membrane, causing rapidly progressive glomerulonephritis with or without pulmonary hemorrhage. The autoantibodies bind neoepitopes formed upon disruption of the quaternary structure of α345NC1 hexamer, a critical structural domain of α345 collagen IV scaffolds. Hexamer disruption leads to a conformational changes that transitions α3 and α5NC1 subunits into immunogens, however, the trigger remains unknown. This contrasts with another anti-GBM disease, Alports' post-transplant nephritis (APTN), where the pathogenic alloantibody binds directly to native NC1 hexamer. The current report includes the first study of antigenic specificity and allo-incompatability in anti-GBM disease occurring after allogeneic haematopoietic stem cell transplant (HSCT).

Results: The anti-GBM antibodies were found to be directed predominantly against the E_A epitope of the α3 NC1 monomer of collagen IV and developed rapidly in patient serum reaching peak level within 5 weeks. Autoantibody binding to native α345NC1 hexamer was minimal; however, binding was greatly increased upon dissociation of the native hexamer. There were no polymorphic genetic differences between donor and recipient collagen IV genes which would be predicted to cause a significant NC1 conformational change or to provide a target for antibody binding. Both patient and donor possessed the Goodpasture's susceptibility HLA-allele DRB1^*1501.

Conclusions: The current report includes the first in-depth study of allo-incompatability and antigenic specificity in anti-GBM disease occurring after allogeneic haematopoietic stem cell transplant (HSCT). No polymorphic genetic differences were identified between donor and recipient collagen IV genes which would be predicted to provide a target for antibody binding. Furthermore, autoantibody binding to native α345NC1 hexamer was minimal, increasing greatly upon dissociation of the native hexamer, resembling wild-type GP diseases and marking this as the first example of a post-HSCT conformeropathy.

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.

Generation of induced pluripotent stem cells from renal tubular cells of a patient with Alport syndrome

Alport syndrome (AS) is a hereditary disease that leads to kidney failure and is caused by mutations in the COL4A3, COL4A4, and COL4A5 genes that lead to the absence of collagen α3α4α5 (IV) networks in the mature kidney glomerular basement membrane. Approximately 80% of AS is X-linked because of mutations in COL4A5, the gene encoding the alpha 5 chain of type IV collagen. To investigate the pathogenesis of AS at the genetic level, we generated induced pluripotent stem cells (iPSCs) from renal tubular cells of a patient with AS. The successful iPSC generation laid the foundation to master the repair of the COL4A5 gene and to evaluate the differentiation of iPSC into Sertoli cells and the accompanying epigenetic changes at each stage. The generation of iPSCs from AS patients not only confirms that iPSCs could be generated from renal tubular cells, but also provides a novel type of genetic therapy for AS patients. In this study, we generated iPSCs from renal tubular cells via ectopic expression of four transcription factors (Oct4, Sox2, c-myc, and Klf4). According to the human embryonic stem cell (hESC) charter, iPSC formation was confirmed by comparatively analyzing hESC markers via colony morphology, immunohistochemistry, qRT-PCR, flow cytometry, gene expression profiling of the three germ layers, and karyotyping. Our results demonstrated that iPSCs were similar to hESCs with regard to morphology, proliferation, hESC-specific surface marker expression, and differentiation into the cell types of the three germ layers. The efficient generation of iPSCs from the renal tubular cells of an AS patient would provide a novel model to investigate the mechanisms underlying AS and to develop new treatments for AS.

A Case of Alport Syndrome with Posttransplant Antiglomerular Basement Membrane Disease despite Negative Antiglomerular Basement Membrane Antibodies by EIA Treated with Plasmapheresis and Intravenous Immunoglobulin

Posttransplant antiglomerular basement membrane (anti-GBM) disease occurs in approximately 5% of Alport patients and usually ends in irreversible graft failure. Recent research has focused on characterizing the structure of the anti-GBM alloepitope. Here we present a case of a 22-year-old male with end-stage renal disease secondary to Alport syndrome, with a previously failed renal allograft, who received a second deceased-donor kidney transplant. Six days after transplantation, he developed acute kidney injury. The serum anti-GBM IgG was negative by enzyme immunoassay (EIA). On biopsy, he had crescentic glomerulonephritis with linear GBM fixation of IgG. With further analysis by western blotting, we were able to detect antibodies to an unidentified protein from the basement membrane. This patient was treated with plasmapheresis twice per week and monthly intravenous immunoglobulin (IVIG) for a total of five months. At the end of treatment, these unknown antibodies were no longer detected. His renal function improved, and he has not required dialysis. We conclude that anti-GBM disease in patients with Alport Syndrome may be caused by circulating antibodies to other components of the basement membrane that are undetectable by routine anti-GBM EIA and may respond to treatment with plasmapheresis and IVIG.

Developments in purification methods for obtaining and evaluation of collagen derived endogenous angioinhibitors

Collagen constitutes one of the vital components of the basement membrane scaffolds. Non-collagenous domains (NC1) derived from collagens exhibit potent anti-angiogenic properties, thus attaining significance in regulation of angiogenesis promoted diseases. Individual NC1 domains essential for anti-angiogenic evaluations are generally obtained through purification of individual non-collagenous domains, which have undergone steady developments for enhancing the yields, purpose of biological evaluations and solubility based on the nature of different NC1 domains. This review focuses on the method developments in obtaining biologically active NC1 domains and for specific evaluations in different scenarios.

Recurrence of glomerulonephritis after renal transplantation

Recurrence of glomerulonephritis following renal transplantation is considered an important cause of allograft failure. The incidence of recurrence of glomerulonephritis varies widely depending on the definition of recurrence (pathologic recurrence or clinicopathologic recurrence) and the original glomerular disease. Moreover the impact of recurrence of glomerular disease on allograft outcome varies widely between different forms of glomerulonephritis. Whereas IgA nephritis recurs in up to one third of transplanted patients, this is not associated with adverse effects on graft survival. In contrast, recurrent focal segmental glomerulosclerosis and membranoproliferative glomerulopathy have an unfavorable prognosis. Overall, long-term graft survival in patients transplanted for glomerulonephritis is comparable to survival in patients with other causes of ESRD. In recent years, several mechanisms for recurrent disease after transplantation (e.g. PLA2R antibodies in membranous nephropathy and suPAR in FSGS) have been identified, and these findings have helped to elucidate the pathogenesis of glomerular diseases. Although renal transplantation is the treatment of choice for end-stage renal disease as a consequence of glomerulonephritis, further studies are required to develop optimal strategies to prevent, diagnose and treat recurrent glomerular diseases.

Quaternary epitopes of α345(IV) collagen initiate Alport post-transplant anti-GBM nephritis

Alport post-transplant nephritis (APTN) is an aggressive form of anti-glomerular basement membrane disease that targets the allograft in transplanted patients with X-linked Alport syndrome. Alloantibodies develop against the NC1 domain of α5(IV) collagen, which occurs in normal kidneys, including renal allografts, forming distinct α345(IV) and α1256(IV) networks. Here, we studied the roles of these networks as antigens inciting alloimmunity and as targets of nephritogenic alloantibodies in APTN. We found that patients with APTN, but not those without nephritis, produce two kinds of alloantibodies against allogeneic collagen IV. Some alloantibodies targeted alloepitopes within α5NC1 monomers, shared by α345NC1 and α1256NC1 hexamers. Other alloantibodies specifically targeted alloepitopes that depended on the quaternary structure of α345NC1 hexamers. In Col4a5-null mice, immunization with native forms of allogeneic collagen IV exclusively elicited antibodies to quaternary α345NC1 alloepitopes, whereas alloimmunogens lacking native quaternary structure elicited antibodies to shared α5NC1 alloepitopes. These results imply that quaternary epitopes within α345NC1 hexamers may initiate alloimmune responses after transplant in X-linked Alport patients. Thus, α345NC1 hexamers are the culprit alloantigen and primary target of all alloantibodies mediating APTN, whereas α1256NC1 hexamers become secondary targets of anti-α5NC1 alloantibodies. Reliable detection of alloantibodies by immunoassays using α345NC1 hexamers may improve outcomes by facilitating early, accurate diagnosis.

Proteolysis breaks tolerance toward intact α345(IV) collagen, eliciting novel anti-glomerular basement membrane autoantibodies specific for α345NC1 hexamers

Goodpasture disease is an autoimmune kidney disease mediated by autoantibodies against noncollagenous domain 1 (NC1) monomers of α3(IV) collagen that bind to the glomerular basement membrane (GBM), usually causing rapidly progressive glomerulonephritis (GN). We identified a novel type of human IgG4-restricted anti-GBM autoantibodies associated with mild nonprogressive GN, which specifically targeted α345NC1 hexamers but not α3NC1 monomers. The mechanisms eliciting these anti-GBM autoantibodies were investigated in mouse models recapitulating this phenotype. Wild-type and FcγRIIB(-/-) mice immunized with autologous murine GBM NC1 hexamers produced mouse IgG1-restricted autoantibodies specific for α345NC1 hexamers, which bound to the GBM in vivo but did not cause GN. In these mice, intact collagen IV from murine GBM was not immunogenic. However, in Col4a3(-/-) Alport mice, both intact collagen IV and NC1 hexamers from murine GBM elicited IgG Abs specific for α345NC1 hexamers, which were not subclass restricted. As heterologous Ag in COL4A3-humanized mice, murine GBM NC1 hexamers elicited mouse IgG1, IgG2a, and IgG2b autoantibodies specific for α345NC1 hexamers and induced anti-GBM Ab GN. These findings indicate that tolerance toward autologous intact α345(IV) collagen is established in hosts expressing this Ag, even though autoreactive B cells specific for α345NC1 hexamers are not purged from their repertoire. Proteolysis selectively breaches this tolerance by generating autoimmunogenic α345NC1 hexamers. This provides a mechanism eliciting autoantibodies specific for α345NC1 hexamers, which are restricted to noninflammatory IgG subclasses and are nonnephritogenic. In Alport syndrome, lack of tolerance toward α345(IV) collagen promotes production of alloantibodies to α345NC1 hexamers, including proinflammatory IgG subclasses that mediate posttransplant anti-GBM nephritis.

Murine membranous nephropathy: immunization with α3(IV) collagen fragment induces subepithelial immune complexes and FcγR-independent nephrotic syndrome

Membranous nephropathy (MN) is a leading cause of nephrotic syndrome in adults and a significant cause of end-stage renal disease, yet current therapies are nonspecific, toxic, and often ineffective. The development of novel targeted therapies requires a detailed understanding of the pathogenic mechanisms, but progress is hampered by the lack of a robust mouse model of disease. We report that DBA/1 mice as well as congenic FcγRIII(-/-) and FcRγ(-/-) mice immunized with a fragment of α3(IV) collagen developed massive albuminuria and nephrotic syndrome, because of subepithelial deposits of mouse IgG and C3 with corresponding basement membrane reaction and podocyte foot process effacement. The clinical presentation and histopathologic findings were characteristic of MN. Although immunized mice produced genuine anti-α3NC1 autoantibodies that bound to kidney and lung basement membranes, neither crescentic glomerulonephritis nor alveolitis ensued, likely because of the predominance of mouse IgG1 over IgG2a and IgG2b autoantibodies. The ablation of activating IgG Fc receptors did not ameliorate injury, implicating subepithelial deposition of immune complexes and consequent complement activation as a major effector pathway. We have thus established an active model of murine MN. This model, leveraged by the availability of genetically engineered mice and mouse-specific reagents, will be instrumental in studying the pathogenesis of MN and evaluating the efficacy of novel experimental therapies.

Novel targets for immunotherapy in glomerulonephritis

Glomerulonephritis is a common cause of chronic kidney disease and end stage renal failure. Current therapy relies on variably effective, nonspecific and toxic immunosuppression. Recent insights into underlying biology and disease pathogenesis in human glomerulonephritis combined with advances in the fields of inflammation and autoimmunity promise a cadre of novel targeted interventions. This review highlights the therapeutic potential of two antigens, alpha3 (IV)NC1 collagen and podocyte neutral endopeptidase, and two cell signaling and effector molecules, IgG Fc receptors and complement, judged to be particularly amenable to therapeutic manipulation in man. It is anticipated that continued dissection of pathogenesis in the diverse disorders that comprise the glomerulonephritides will provide the basis for individualized disease-specific therapy.

Alport alloantibodies but not Goodpasture autoantibodies induce murine glomerulonephritis: protection by quinary crosslinks locking cryptic α3(IV) collagen autoepitopes in vivo

The noncollagenous (NC1) domains of alpha3alpha4alpha5(IV) collagen in the glomerular basement membrane (GBM) are targets of Goodpasture autoantibodies or Alport posttransplant nephritis alloantibodies mediating rapidly progressive glomerulonephritis. Because the autoepitopes but not the alloepitopes become cryptic upon assembly of alpha3alpha4alpha5NC1 hexamers, we investigated how the accessibility of B cell epitopes in vivo influences the development of glomerulonephritis in mice passively immunized with human anti-GBM Abs. Alport alloantibodies, which bound to native murine alpha3alpha4alpha5NC1 hexamers in vitro, deposited linearly along the mouse GBM in vivo, eliciting crescentic glomerulonephritis in Fcgr2b(-/-) mice susceptible to Ab-mediated inflammation. Goodpasture autoantibodies, which bound to murine alpha3NC1 monomer and dimer subunits but not to native alpha3alpha4alpha5NC1 hexamers in vitro, neither bound to the mouse GBM in vivo nor induced experimental glomerulonephritis. This was due to quinary NC1 crosslinks, recently identified as sulfilimine bonds, which comprehensively locked the cryptic Goodpasture autoepitopes in the mouse GBM. In contrast, non-crosslinked alpha3NC1 subunits were identified as a native target of Goodpasture autoantibodies in the GBM of squirrel monkeys, a species susceptible to Goodpasture autoantibody-mediated nephritis. Thus, crypticity of B cell autoepitopes in tissues uncouples potentially pathogenic autoantibodies from autoimmune disease. Crosslinking of alpha3alpha4alpha5NC1 hexamers represents a novel mechanism averting autoantibody binding and subsequent tissue injury by posttranslational modifications of an autoantigen.

Basement membranes and human disease

In 1990, the role of basement membranes in human disease was established by the identification of COL4A5 mutations in Alport's syndrome. Since then, the number of diseases caused by mutations in basement membrane components has steadily increased as has our understanding of the roles of basement membranes in organ development and function. However, many questions remain as to the molecular and cellular consequences of these mutations and the way in which they lead to the observed disease phenotypes. Despite this, exciting progress has recently been made with potential treatment options for some of these so far incurable diseases.

Rituximab in two cases of Goodpasture's syndrome

We describe the effects of rituximab in two patients with refractory Goodpasture's syndrome. After one single 1000 mg dose administration, rituximab did not appear to improve renal function in either of our cases. Polyoma virus-negative leukoencephalopathy, a probable toxic side effect of rituximab, was seen in our first patient.

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.

Identification of noncollagenous sites encoding specific interactions and quaternary assembly of alpha 3 alpha 4 alpha 5(IV) collagen: implications for Alport gene therapy

Defective assembly of alpha 3 alpha 4 alpha 5(IV) collagen in the glomerular basement membrane causes Alport syndrome, a hereditary glomerulonephritis progressing to end-stage kidney failure. Assembly of collagen IV chains into heterotrimeric molecules and networks is driven by their noncollagenous (NC1) domains, but the sites encoding the specificity of these interactions are not known. To identify the sites directing quaternary assembly of alpha 3 alpha 4 alpha 5(IV) collagen, correctly folded NC1 chimeras were produced, and their interactions with other NC1 monomers were evaluated. All alpha1/alpha 5 chimeras containing alpha 5 NC1 residues 188-227 replicated the ability of alpha 5 NC1 to bind to alpha3NC1 and co-assemble into NC1 hexamers. Conversely, substitution of alpha 5 NC1 residues 188-227 by alpha1NC1 abolished these quaternary interactions. The amino-terminal 58 residues of alpha3NC1 encoded binding to alpha 5 NC1, but this interaction was not sufficient for hexamer co-assembly. Because alpha 5 NC1 residues 188-227 are necessary and sufficient for assembly into alpha 3 alpha 4 alpha 5 NC1 hexamers, whereas the immunodominant alloantigenic sites of alpha 5 NC1 do not encode specific quaternary interactions, the findings provide a basis for the rational design of less immunogenic alpha 5(IV) collagen constructs for the gene therapy of X-linked Alport patients.

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.

A role for collagen IV cross-links in conferring immune privilege to the Goodpasture autoantigen: structural basis for the crypticity of B cell epitopes

The detailed structural basis for the cryptic nature (crypticity) of a B cell epitope harbored by an autoantigen is unknown. Because the immune system may be ignorant of the existence of such "cryptic" epitopes, their exposure could be an important feature in autoimmunity. Here we investigated the structural basis for the crypticity of the epitopes of the Goodpasture autoantigen, the alpha3alpha4alpha5 noncollagenous-1 (NC1) hexamer, a globular domain that connects two triple-helical molecules of the alpha3alpha4alpha5 collagen IV network. The NC1 hexamer occurs in two isoforms as follows: the M-isoform composed of monomer subunits in which the epitopes are accessible to autoantibodies, and the D-isoform composed of both monomer and dimer subunits in which the epitopes are cryptic. The D-isoform was characterized with respect to quaternary structure, as revealed by mass spectrometry of dimer subunits, homology modeling, and molecular dynamics simulation. The results revealed that the D-isoform contains two kinds of cross-links as follows: S-hydroxylysyl-methionine and S-lysyl-methionine cross-links, which stabilize the alpha3alpha5-heterodimers and alpha4alpha4-homodimers, respectively. Construction and analysis of a three-dimensional model of the D-isoform of the alpha3alpha4alpha5 NC1 hexamer revealed that crypticity is a consequence of the following: (a) sequestration of key residues between neighboring subunits that are stabilized by domain-swapping interactions, and (b) by cross-linking of subunits at the trimer-trimer interface, which stabilizes the structural integrity of the NC1 hexamer and protects against binding of autoantibodies. The sequestrated epitopes and cross-linked subunits represent a novel structural mechanism for conferring immune privilege at the level of quaternary structure. Perturbation of the quaternary structure may be a key factor in the etiology of Goodpasture disease.

Natural anti-GBM antibodies from normal human sera recognize α3(IV)NC1 restrictively and recognize the same epitopes as anti-GBM antibodies from patients with anti-GBM disease

Anti-GBM disease is a rare autoimmune condition characterized by autoantibodies targeting the alpha3 chain non-collagen 1 domain of type IV collagen (alpha3(IV)NC1). Recently, we isolated IgG reacting with alpha3(IV)NC1 from normal healthy human sera. The current study examined the antigen and epitope specificity of these natural autoantibodies (NAA) using recombinant human alpha1, 3, 5(IV)NC1 and three constructs expressing, previously defined epitope regions designated E(A), E(B) and S2, in the alpha1(IV)NC1 background. The NAA preparations reacted with recombinant human alpha3(IV)NC1 to the same extent as with purified bovine alpha(IV)NC1, but not with recombinant human alpha1 and alpha5(IV)NC1. NAA preparations recognized the three chimeric proteins (E(A), E(B) and S2) yielding similar absorbance values. We conclude that anti-GBM NAA recognize the same major epitopes as anti-GBM antibodies from patients with Goodpasture's disease.

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.

Mannose Receptor Expression and Function Define a New Population of Murine Dendritic Cells

In vitro the mannose receptor (MR) mediates Ag internalization by dendritic cells (DC) and favors the presentation of mannosylated ligands to T cells. However, in vivo MR seems to play a role not in Ag presentation but in the homeostatic clearance of endogenous ligands, which could have the secondary benefit of reducing the levels of endogenous Ag available for presentation to the adaptive immune system. We have now observed that while MR(+) cells are consistently absent from T cell areas of spleen and mesenteric lymph nodes (LN), peripheral LN of untreated adult mice contain a minor population of MR(+)MHCII(+) in the paracortex. This novel MR(+) cell population can be readily identified by flow cytometry and express markers characteristic of DC. Furthermore, these MR(+) DC-like cells located in T cell areas can be targeted with MR ligands (anti-MR mAb). Numbers of MR(+)MHCII(+) cells in the paracortex are increased upon stimulation of the innate immune system and, accordingly, the amount of anti-MR mAb reaching MR(+)MHCII(+) cells in T cell areas is dramatically enhanced under these conditions. Our results indicate that the MR can act as an Ag-acquisition system in a DC subpopulation restricted to lymphoid organs draining the periphery. Moreover, the effect of TLR agonists on the numbers of these MR(+) DC suggests that the immunogenicity of MR ligands could be under the control of innate stimulation. In accordance with these observations, ligands highly specific for the MR elicit enhanced humoral responses in vivo only when administered in combination with endotoxin.

The alloantigenic sites of alpha3alpha4alpha5(IV) collagen: pathogenic X-linked alport alloantibodies target two accessible conformational epitopes in the alpha5NC1 domain

Anti-glomerular basement membrane (GBM) antibody nephritis is caused by an autoimmune or alloimmune reaction to the NC1 domains of alpha3alpha4alpha5(IV) collagen. Some patients with X-linked Alport syndrome (XLAS) develop post-transplant nephritis mediated by pathogenic anti-GBM alloantibodies to collagen IV chains present in the renal allograft but absent from the tissues of the patient. In this work, the epitopes targeted by alloantibodies from these patients were identified and characterized. All XLAS alloantibodies recognized conformational epitopes in the NC1 domain of alpha5(IV) collagen, which were mapped using chimeric alpha1/alpha5 NC1 domains expressed in mammalian cells. Allograft-eluted alloantibodies mainly targeted two conformational alloepitopes mapping to alpha5NC1 residues 1-45 and 114-168. These regions also encompassed the major epitopes of circulating XLAS alloantibodies, which in some patients additionally targeted alpha5NC1 residues 169-229. Both kidney-eluted and circulating alloantibodies to alpha5NC1 distinctively targeted epitopes accessible in the alpha3alpha4alpha5NC1 hexamers of human GBM, unlike anti-GBM autoantibodies, which targeted sequestered alpha3NC1 epitopes. The results identify two immunodominant alpha5NC1 epitopes as major alloantigenic sites of alpha3alpha4alpha5(IV) collagen specifically implicated in the pathogenesis of post-transplant nephritis in XLAS patients. The contrast between the accessibility of these alloepitopes and the crypticity of autoepitopes indicates that distinct molecular forms of antigen may initiate the immunopathogenic processes in the two forms of anti-GBM disease.

Renal transplantation in patients with Alport syndrome

For pediatric kidney transplant physicians, two aspects of Alport syndrome set the disease apart from other causes of terminal renal failure. First, an understanding of the genetics of Alport syndrome is needed to make appropriate decisions regarding potential related kidney donors to Alport patients requiring renal transplantation. Second, renal transplantation for Alport syndrome may be complicated by post-transplant anti-GBM nephritis, a problem that is nearly unique to this disease. This review discusses these aspects of Alport syndrome and attempts to provide rational recommendations for clinicians.