Identification of alpha3, alpha4, and alpha5 chains of type IV collagen as alloantigens for Alport posttransplant anti-glomerular basement membrane antibodies

Renal transplantation in patients with Alport syndrome: patient selection, outcomes, and donor evaluation

Alport syndrome is an inherited disorder of basement membrane collagen IV that frequently results in end-stage renal disease. Patients with Alport syndrome who undergo renal transplantation have generally excellent outcomes. Posttransplant antiglomerular basement membrane nephritis is a rare complication of renal transplantation for Alport syndrome. Because Alport syndrome is a genetic disorder, potential related donors must be carefully evaluated in order to minimize harm.

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.

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.

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.

Goodpasture's disease: molecular architecture of the autoantigen provides clues to etiology and pathogenesis

PURPOSE OF REVIEW

Goodpasture's disease is an autoimmune disorder characterized by the deposition of pathogenic autoantibodies in basement membranes of kidney and lung, which induces rapidly progressive glomerulonephritis and pulmonary hemorrhage. The target antigen is the α3NC1 domain of collagen IV, which is expressed in target organs as an α345 network. Recent studies of specificity and epitopes of Goodpasture's autoantibodies and discovery of novel posttranslational modification of the antigen, a sulfilimine bond, provide further insight into mechanisms of initiation and progression of Goodpasture's disease.

RECENT FINDINGS

Analysis of the specificity of Goodpasture's autoantibodies revealed a distinct subset of circulating and kidney-bound antiα5NC1 antibody, which is associated with loss of kidney function. Structural integrity of the α345NC1 hexamer is stabilized by the novel sulfilimine crosslinks conferring immune privilege to the Goodpasture's autoantigen. Native antibodies may contribute to establishment of immune tolerance to autoantigen. Structural analysis of epitopes for autoantibodies and alloantibodies indicates a critical role of conformational change in the α345NC1 hexamer in eliciting an autoimmune response in Goodpasture's disease.

SUMMARY

Understanding of the quaternary structure of the Goodpasture's autoantigen continues to provide insights into autoimmune mechanisms that serve as a basis for development of novel diagnostic tools and therapies for Goodpasture's disease.

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.

Mapping structural landmarks, ligand binding sites, and missense mutations to the collagen IV heterotrimers predicts major functional domains, novel interactions, and variation in phenotypes in inherited diseases affecting basement membranes

Collagen IV is the major protein found in basement membranes. It comprises three heterotrimers (α1α1α2, α3α4α5, and α5α5α6) that form distinct networks, and are responsible for membrane strength and integrity.We constructed linear maps of the collagen IV heterotrimers ("interactomes") that indicated major structural landmarks, known and predicted ligand-binding sites, and missense mutations, in order to identify functional and disease-associated domains, potential interactions between ligands, and genotype–phenotype relationships. The maps documented more than 30 known ligand-binding sites as well as motifs for integrins, heparin, von Willebrand factor (VWF), decorin, and bone morphogenetic protein (BMP). They predicted functional domains for angiogenesis and haemostasis, and disease domains for autoimmunity, tumor growth and inhibition, infection, and glycation. Cooperative ligand interactions were indicated by binding site proximity, for example, between integrins, matrix metalloproteinases, and heparin. The maps indicated that mutations affecting major ligand-binding sites, for example, for Von Hippel Lindau (VHL) protein in the α1 chain or integrins in the α5 chain, resulted in distinctive phenotypes (Hereditary Angiopathy, Nephropathy, Aneurysms, and muscle Cramps [HANAC] syndrome, and early-onset Alport syndrome, respectively). These maps further our understanding of basement membrane biology and disease, and suggest novel membrane interactions, functions, and therapeutic targets.

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.

Modeling of human anti-GBM antibody-alpha3(IV)NC1 interactions predicts antigenic cross-linking through contact of both heavy chains with repeating epitopes on alpha3(IV)NC1

BACKGROUND/AIMS

Patients with anti-glomerular basement membrane diseases produce pathogenic autoantibodies (autoAb) that deposit in the kidney and initiate severe inflammation. Restricted antigenic specificity of the autoAb against 2 regions (with related sequences) within alpha3(IV)NC1, along with shared idiotypes (i.e. structural determinants), among pathogenic human autoAb suggested that common genetic elements encode the autoAb. The aim of this study was to determine whether the idiotypic relatedness of the autoAb was due to the fact that unique and similar genes were used to encode them, divergent genes were used to produce Ab with similar Ag-binding properties and conformation, or if other mechanisms were operative.

METHODS

The encoding V gene sequences of pathogenic human anti-alpha3(IV)NC1 Ab, derived following immunization of XenoMice which produce human but not murine IgG, with alpha3(IV)NC1 were determined. Predicted conformations of autoAb-alpha3(IV)NC1 interactions were derived using the Ab sequences and molecularmodels of the alpha3(IV)NC1 structure.

RESULTS

The pathogenic Ab were encoded by multiple, common V(H) and V(L) gene families indicating that they were not encoded by a unique subset of genes and that normal individuals have the capacity to produce them. However, modeling of the Ag-Ab interactions suggested that although the contact regions varied for individual Ab, the optimized energy constraints facilitate interaction of both Ab-binding regions with pathogenically relevant epitopes on alpha3(IV)NC1.

CONCLUSIONS

The results suggest that the repetitive nature and relatedness of the alpha3(IV)NC1 antigenic epitopes facilitate cross-linking of pathogenic Ab, in vivo, by allowing both IgG Fab to bind to the basement membrane. This most likely accounts for the high-affinity Ab binding we and others observed among human anti-alpha3(IV)NC1 Ab. Based on these observations, we postulate that this interaction provides for the stability of the Ab interaction, resulting in a high-affinity interaction that serves as an ideal scaffold for optimal FcR engagement and complement activation, thereby accelerating inflammation and contributing to the rapidly progressive nature of this disease.

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.

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.

Alport syndrome and thin basement membrane nephropathy

Both Alport syndrome and thin basement membrane nephropathy (TBMN) can be considered as genetic diseases of the GBM involving the alpha3/alpha4/alpha5 network of type IV collagen. Mutations in any of the COL4A3, COL4A4 or COL4A5 genes can lead to total or partial loss of this network. Males with mutations in the X-linked COL4A5 gene develop Alport syndrome with progressive renal disease and sometimes extra-renal disease. Females who are heterozygous for a COL4A5 mutation are considered to be carriers for X-linked Alport syndrome. Although their clinical course and GBM ultrastructural changes can sometimes mimic TBMN, more often it tends to be more progressive than usually seen in TBMN. Males or females who are heterozygous for COL4A3 or COL4A4 mutations usually manifest as TBMN, with nonprogressive hematuria, while those who are homozygous or combined heterozygotes develop autosomal-recessive Alport syndrome. Thus, individuals with TBMN can be considered to be carriers for autosomal-recessive Alport syndrome, but there remain some exceptions in which patients heterozygous for COL4A3 or COL4A4 mutations develop autosomal-dominant Alport syndrome. Distinguishing between all these groups of patients requires a combination of family history and a renal biopsy for electron microscopic examination of the GBM and immunohistochemical staining of the GBM for the alpha3, alpha4 and alpha5 chains of type IV collagen. Mutational analysis of the COL4A3, COL4A4, and COL4A5 genes, whenever it becomes available, will be a valuable adjunct to the diagnostic workup in these patients. Novel therapeutic approaches may one day provide a treatment or cure for these patients, avoiding the need for transplantation and dialysis.

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.

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.

Glomérulonéphrites extracapillaires

Crescentic glomerulonephritis are characterised by a crescent shaped cellular proliferation that may lead to glomerular destruction. Over 50% of at least 10 analysed glomeruli should be affected. The search for immune deposits by immunofluorescence is an important diagnostic step. Patients present with rapidly progressive glomerulonephritis (RPGN): renal failure, proteinuria and haematuria. Extra-renal symptoms may help diagnosis. Diseases are classified in three groups according to immunofluorescence studies. Group I is characterised by linear deposits along the glomerular basement membrane (GBM) with anti-GBM auto-antibodies responsible for Goodpasture's disease. Group II put together various diseases with immune complex deposits. In group III, no significant immune deposits are found. Those "pauci-immune" glomerulonephritis are secondary to anti-neutrophil cytoplasmic antibodies (ANCA) positive systemic vasculitis, mainly Wegener's granulomatosis and microscopic polyangiitis. Primary glomerulonephritis may also be associated with crescent formation. Treatment is urgently required. Diagnosis is suspected in the context of extra-renal symptoms or immunological abnormalities, and confirmed by a kidney biopsy, that also helps to define prognosis. Apart from some group II glomerulonephritis, the induction treatment is often an association of steroids and cyclophosphamide, with plasma exchange in case of Goodpasture's disease. After remission, a maintenance treatment is required for ANCA-positive vasculitis to prevent relapses. The high rate of opportunistic infections and cancer give the rational for searching less aggressive therapeutic options.

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.

Mechanisms of Disease: alloimmunization in renal diseases

Graft rejection has long been considered the paradigm of renal diseases induced by alloimmunization, particularly alloimmunization directed against HLA antigens. Accumulating evidence indicates that non-HLA immunity also has an important role in clinical transplantation. Targets of alloimmunization include antigens of tubular basement membrane, tubular epithelial cells and endothelial cells. They can be polymorphic allovariants (as shown in the rat) or 'hidden' antigens exposed when the graft is damaged. Alloimmunization can also occur when a person genetically deficient in a renal protein (e.g. the alpha5 (IV) collagen chain in X-linked Alport's syndrome or nephrin in Finnish-type nephrotic syndrome) is transplanted to treat end-stage renal failure. The non-mutated protein in the donor kidney is recognized as a foreign antigen, and the resulting alloimmune response can damage the graft. We have demonstrated that alloimmunity can also affect the native kidney. We have characterized a novel fetomaternal disease in which a genetic defect in the MME gene encoding neutral endopeptidase (NEP) in the mother leads to the development of membranous nephropathy in her fetus (maternal anti-NEP antibodies bind to NEP on fetal podocytes). Our findings raise the possibility that mutations or genetic polyporphisms in MME or other genes expressed by the podocyte are involved in alloimmune-mediated development of membranous nephropathy after kidney or bone marrow transplantation.

Recurrence of proteinuria following renal transplantation in congenital nephrotic syndrome of the Finnish type

We report a Caucasian boy of Italian descent with congenital nephrotic syndrome of the Finnish type (NPHS1, CNF, MIM 256300) who developed recurrence of proteinuria and hypoalbuminemia on the seventh post-operative day following living related renal transplantation from his paternal aunt. The allograft biopsy was normal except for effacement of podocyte foot processes on electron microscopy. He was treated by the substitution of mycophenolate mofetil with cyclophosphamide for 12 weeks, in addition to cyclosporine, prednisone and daclizumab. His proteinuria resolved quickly following the initiation of cyclophosphamide treatment, and he remains in remission 4 years after receiving his transplant. His native and allograft kidneys were evaluated for nephrin expression by immunohistochemistry, DNA analysis for the NPHS1 mutation, serum for the presence of auto-antibodies to nephrin by both enzyme-linked immunosorbent assay (ELISA) and fetal glomeruli immunofluorescence assay, and serum for glomerular permeability to albumin (Palb) activity using a functional in vitro assay for Palb. Nephrin expression was completely absent in the native kidney, while it was decreased in the allograft compared with normal. DNA analysis of the NPHS1 gene revealed mutations 3248G>T and 3250delG in exon 24, causing G1083V and 1084Vfs, respectively, inherited from his father, and 3478C>T in exon 27, that leads to R1160X, inherited from his mother. Serum was negative for auto-antibodies to nephrin. Interestingly, the Palb activity was increased at the time of recurrence of proteinuria following transplantation (Palb 0.73+/-0.10) and remained elevated when retested more than 3 years later (Palb 0.54+/-0.09). This is the first report of increased Palb activity in recurrence of proteinuria following transplantation in NPHS1. We speculate the role of increased Palb activity in the recurrence of proteinuria following transplantation in NPHS1.

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

Alport posttransplantation anti-glomerular basement membrane (GBM) nephritis is mediated by alloantibodies against the noncollagenous (NC1) domains of the alpha3alpha4alpha5(IV) collagen network, which is present in the GBM of the allograft but absent from Alport kidneys. The specificity of kidney-bound anti-GBM alloantibodies from a patient who had autosomal recessive Alport syndrome (ARAS) and developed posttransplantation nephritis was compared with that of Goodpasture autoantibodies from patients with autoimmune anti-GBM disease. Allograft-eluted alloantibodies reacted specifically with alpha3alpha4alpha5 NC1 hexamers, targeting their alpha3NC1 and alpha4NC1 subunits, and recognized a noncontiguous alloepitope formed jointly by the E(A) and E(B) regions of alpha3NC1 domain. In contrast, human Goodpasture autoantibodies recognized the separate E(A) and E(B) autoepitopes of alpha3NC1 but not the composite alloepitope. Molecular modeling of alpha3NC1 revealed that the alloepitope is more accessible within the NC1 hexamers than the partially sequestered Goodpasture autoepitopes. Overall, the specificity of alloantibodies indicated a selective lack of immune tolerance toward the alpha3 and alpha4(IV) collagen chains not expressed in patients with ARAS. Using COL4A3 knockout mice, a model of ARAS, it was shown further that acid-dissociated rather than native alpha3alpha4alpha5 NC1 hexamers elicited murine anti-GBM antibodies most closely resembling human ARAS alloantibodies. In contrast, alpha3NC1 monomers elicited Goodpasture-like murine antibodies, targeting the E(A) and E(B) autoepitopes. Thus, the identity of alpha3NC1 epitopes targeted by anti-GBM antibodies is strongly influenced by the molecular organization of the immunogen. These findings suggest that different isoforms of alpha3(IV) collagen may be implicated in the pathogenesis of ARAS posttransplantation anti-GBM nephritis and Goodpasture disease.

Retransplantation in Alport post-transplant anti-GBM disease

BACKGROUND

Post transplant anti-glomerular basement membrane (GBM) disease affects up to 5% of patients with Alport's syndrome. Defects in the COL4A5 gene are responsible for most cases, and alpha 5(IV)NC1 is the usual target for alloantibodies. Gene deletions are more commonly associated with this complication than are point mutations. The disease is severe in renal allografts and nearly always results in graft loss.

METHODS

Three cases of retransplantation in Alport's syndrome are described here in detail. All cases were started on immunosuppressive therapy early in the course of their disease and one patient (case 2) received pre-emptive anti-T-cell therapy (Campath IH). Anti-GBM antibodies in these cases were investigated by standard anti-GBM enzyme-linked immunosorbent assay (ELISA), by indirect immunofluorescence, and by Western blotting using collagenase-digested human GBM and recombinant type IV collagen NC1 domains made in insect cells.

RESULTS

All cases showed early antibody and complement fixation to human GBM. Target alloantibodies were to alpha 5(IV)NC1 domain predominantly. Cases two and three gave negative results on standard ELISA for anti-GBM antibodies. Pathologic examination revealed crescentic glomerulonephritis, which was rapid in onset in case 1, blunted and less aggressive in case 3, and case 2 developed segmental necrosis without crescent formation. Neutrophilic infiltrates were an early feature in all 3 cases. All cases are compared with a review of all retransplanted cases in the literature.

CONCLUSION

Alport anti-GBM disease is a severe disease in retransplanted patients. Anti-T-cell therapy seemed to modify the pathologic findings but did not prevent graft loss. Longer term plasma exchange and mycophenolate mofetil may attenuate the illness, but in these cases did not prevent graft loss. Western blotting detected alloantibodies to alpha 5(IV) NC1 domain and is more sensitive and specific for this disease than standard ELISAs.

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.

Living related kidney transplantation in a patient with autosomal-recessive Alport syndrome

We discuss a patient with Alport syndrome who received a renal transplant from a donor with thin basement membrane disease. A 30-year-old woman, diagnosed with Alport syndrome on the basis of sensorineural hearing loss, characteristic renal biopsy findings and a family history of microhaematuria, entered chronic haemodialysis therapy. She then received a renal transplant donated from her father, who had sensorineural hearing loss and persistent microhaematuria. On the day of renal transplantation, a 1-h graft biopsy after reperfusion showed thin basement membrane disease. We re-tested the patient's native kidney biopsy specimen by immunohistochemical staining using alpha-chain-specific collagen type IV monoclonal antibodies. There was no expression of collagen type IV alpha3-, alpha4- and alpha5-chain on glomerular basement membrane, but positive staining of alpha5-chain on Bowman's capsular basement membrane was noted. A diagnosis of autosomal-recessive Alport syndrome was made. We concluded that this family might display different phenotypic expressions of the same genotype: one suffered end-stage renal disease and the other thin basement membrane disease.

Antibody-mediated glomerulonephritis in mice: the role of endotoxin, complement and genetic background

Antibody-mediated glomerulonephritis in man may be exacerbated by infection and this effect may be mediated by bacterial endotoxin. There is evidence supporting a role for endotoxin in heterologous nephrotoxic nephritis in rats, but the role of endotoxin in this model in mice has not previously been explored. Previous data in mice on the role of complement in this model are conflicting and this may be due to the mixed genetic background of mice used in these studies. We used the model of heterologous nephrotoxic nephritis in mice and explored the role of endotoxin, complement and genetic background. In this study we show a synergy between antibody and endotoxin in causing a neutrophil influx. We also show that C1q-deficient mice have an increased susceptibility to glomerular inflammation but this is seen only on a mixed 129/Sv x C57BL/6 genetic background. On a C57BL/6 background we did not find any differences in disease susceptibility when wildtype, C1q, factor B or factor B/C2 deficient mice were compared. We also demonstrate that C57BL/6 mice are more susceptible to glomerular inflammation than 129/Sv mice. These results show that endotoxin is required in this model in mice, and that complement does not play a major role in glomerular inflammation in C57BL/6 mice. C1q may play a protective role in mixed-strain 129/Sv x C57BL/6 mice, but the data may also be explained by systematic bias in background genes, as there is a large difference in disease susceptibility between C57BL/6 and 129/Sv mice.

Evidence for immune responses to a self-antigen in lung transplantation: role of type V collagen-specific T cells in the pathogenesis of lung allograft rejection

We have reported that lung allograft rejection involves an immune response to a native protein in the lung, type V collagen (col(V)), and that col(V)-induced oral tolerance prevented acute and chronic rejection. In support of these findings col(V) fragments were detected in allografts during rejection, but not in normal lungs. The purpose of the current study was to isolate and characterize col(V)-specific allograft-infiltrating T cells and to determine their contribution to the rejection response in vivo. Two col(V)-specific T cell lines, LT1 and LT3, were isolated from F344 (RT1(lv1)) rat lung allografts during rejection that occurred after transplantation into WKY (RT1(l)) recipients. Both cell lines, but not normal lung lymphocytes, proliferated in response to col(V). Neither LT1 nor LT3 proliferated in response to alloantigens. LT1 and LT3 were CD4(+)CD25(-) and produced IFN-gamma in response to col(V). Compared with normal CD4(+) T cells, both cell lines expressed a limited V-beta TCR repertoire. Each cell strongly expressed V-beta 9 and 16, but differed in expression of other V-betas. Adoptive transfer of each cell line did not induce pathology in lungs of normal WKY rats. In contrast, adoptive transfer of LT1, but not LT3, caused marked peribronchiolar and perivascular inflammation in isograft (WKY) lungs and abrogated col(V)-induced oral tolerance to allograft (F344) lungs. Collectively, these data show that lung allograft rejection involves both allo- and autoimmune responses, and graft destruction that occurs during the rejection response may expose allograft-infiltrating T cells to potentially antigenic epitopes in col(V).

Renal transplant in patients with Alport's syndrome

We evaluated 52 renal grafts transplanted into 41 patients with a pretransplantation diagnosis of Alport's syndrome. Overall 1-, 5-, and 10-year patient and graft survival rates were 95.1%, 90.2%, and 80.5% and 86.8%, 66%, and 45.3%, respectively. Although 14% of renal graft biopsy specimens examined with immunofluorescent microscopy showed linear glomerular basement membrane (GBM) immunoglobulin G deposits, only 1 of 41 patients (2.4%) or 52 grafts (1.9%) developed posttransplantation anti-GBM disease. The incidence of anti-GBM disease was 3.1% (1 of 32 patients) in a subgroup of male transplant recipients. Our analysis suggests that the incidence of anti-GBM disease in transplant recipients with Alport's syndrome is less than previously reported. In addition, it does not appear that HLA-DR alleles, which predispose to the development of anti-GBM disease in native kidneys, have a role in transplant recipients with Alport's syndrome posttransplantation. However, immunosuppression level may have a pathophysiological role in the development of anti-GBM disease. The majority of grafts in transplant recipients with Alport's syndrome failed because of chronic allograft nephropathy (69% of grafts) and acute rejection (22% of grafts). A history of previous acute rejection was the only factor that significantly affected graft outcome.