Recombinant alpha-chains of type IV collagen demonstrate that the amino terminal of the Goodpasture autoantigen is crucial for antibody recognition

Autoimmunity in Anti-Glomerular Basement Membrane Disease: A Review of Mechanisms and Prospects for Immunotherapy

Anti-glomerular basement membrane (anti-GBM) disease is an organ-specific autoimmune disorder characterized by autoantibodies against the glomerular and alveolar basement membranes, leading to rapidly progressive glomerulonephritis and severe alveolar hemorrhage. The noncollagenous domain of the α3 chain of type IV collagen, α3(IV)NC1, contains the main target autoantigen in this disease. Epitope mapping studies of α3(IV)NC1 have identified several nephritogenic epitopes and critical residues that bind to autoantibodies and trigger anti-GBM disease. The discovery of novel target antigens has revealed the heterogeneous nature of this disease. In addition, both epitope spreading and mimicry have been implicated in the pathogenesis of anti-GBM disease. Epitope spreading refers to the development of autoimmunity to new autoepitopes, thus worsening disease progression, whereas epitope mimicry, which occurs via sharing of critical residues with microbial peptides, can initiate autoimmunity. An understanding of these autoimmune responses may open opportunities to explore potential new therapeutic approaches for this disease. We review how current advances in epitope mapping, identification of novel autoantigens, and the phenomena of epitope spreading and mimicry have heightened the understanding of autoimmunity in the pathogenesis of anti-GBM disease, and we discuss prospects for immunotherapy.

Role of CD8+ T cells in crescentic glomerulonephritis

Crescentic glomerulonephritis (cGN) comprises three main types according to the pathogenesis and immunofluorescence patterns: anti-glomerular basement membrane antibody cGN, vasculitis-associated cGN and post-infectious immune complex cGN. In this brief review of the immune-pathogenesis of cGN, the focus is mainly on the role of CD8+ T cells in the progression of cGN. Under control conditions, Bowman's capsule (BC) provides a protected immunological niche by preventing access of cytotoxic CD8+ T cells to Bowman's space and thereby podocytes. Even in experimental nephrotoxic nephritis, leukocytes accumulate around the glomeruli, but remain outside of BC, as long as the latter remains intact. However, when and where breaches in BC occur, the inflammatory cells can gain access to and destroy podocytes, thus converting cGN into rapidly progressive glomerulonephritis (RPGN). These conclusions also apply to human cGN, where biopsies show that loss of BC integrity is associated with RPGN and progression to end-stage kidney disease. We propose a two-hit hypothesis for the role of cytotoxic CD8+ T cells in the progression of cGN. The initial insult occurs in response to the immune complex formation or deposition, resulting in local capillary and podocyte injury (first hit). The injured podocytes release neo-epitopes, eventually causing T-cell activation and migration to the glomerulus. Upon generation of breaches in BC, macrophages and CD8+ T cells can now gain access to the glomerular space and destroy neo-epitope expressing podocytes (second hit), resulting in RPGN. While further investigation will be required to test this hypothesis, future therapeutic trials should consider targeting of CD8+ T cells in the therapy of progressive cGN.

Autoimmunity to the alpha 3 chain of type IV collagen in glomerulonephritis is triggered by 'autoantigen complementarity'

'Autoantigen complementarity' is a theory proposing that the initiator of an autoimmune response is not necessarily the autoantigen or its molecular mimic, but may instead be a peptide that is 'antisense/complementary' to the autoantigen. We investigated whether such complementary proteins play a role in the immunopathogenesis of autoimmune glomerulonephritis. Experimental autoimmune glomerulonephritis, a model of anti-glomerular basement membrane (GBM) disease, can be induced in Wistar Kyoto (WKY) rats by immunization with the α3 chain of type IV collagen. In this study, WKY rats were immunized with a complementary α3 peptide (c-α3-Gly) comprised of amino acids that 'complement' the well characterized epitope on α3(IV)NC1, pCol(24-38). Within 8 weeks post-immunization, these animals developed cresentic glomerulonephritis, similar to pCol(24-38)-immunized rats, while animals immunized with scrambled peptide were normal. Anti-idiotypic antibodies to epitopes from c-α3-Gly-immunized animals were shown to be specific for α3 protein, binding in a region containing sense pCol(24-38) sequence. Interestingly, anti-complementary α3 antibodies were identified in sera from patients with anti-GBM disease, suggesting a role for 'autoantigen complementarity' in immunopathogenesis of the human disease. This work supports the idea that autoimmune glomerulonephritis can be initiated through an immune response against a peptide that is anti-sense or complementary to the autoantigen. The implications of this discovery may be far reaching, and other autoimmune diseases could be due to responses to these once unsuspected 'complementary' antigens.

Association of epitope spreading of antiglomerular basement membrane antibodies and kidney injury

BACKGROUND AND OBJECTIVES

Antiglomerular basement membrane autoantibodies are pathogenic in antiglomerular basement membrane disease with two major epitopes, E(A) and E(B), on α3 chain of type IV collagen. This study investigated the epitope spectrum of antiglomerular basement membrane autoantibodies, aiming to identify the association between epitope specificity and kidney injury.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

All 108 patients with antiglomerular basement membrane disease and complete clinical data were divided into three groups according to renal dysfunction: mild group (n=20) with serum creatitine≤1.5 mg/dl; moderate group (n=22) with serum creatinine=1.5-6.8 mg/dl; severe group (n=66) with serum creatitine≥6.8 mg/dl. Epitope spectrums of antibodies were determined by ELISA, and their associations with kidney damage were analyzed. Sequential serum samples in 40 patients were examined during disease courses.

RESULTS

E(A) and E(B) were recognized in 79.6% and 72.2% of patients, respectively. E(A) and E(B) reactions were the lowest in the mild group and higher in the moderate group (E(A): 35.0% versus 81.8%, P=0.002; E(B): 15.0% versus 68.2%, P=0.001). They were the highest in the severe group (E(A): 92.4%, P=0.31; E(B): 90.9%, P=0.02). Close association was observed between renal injury and E(A) and E(B) reactions. Multivariate Cox regression analysis showed that E(B) reaction was an independent risk factor for renal failure (hazard ratio=6.91, P=0.02). The recognition for non-E(AB) remained low among groups. No augmentation of epitope spectrum was shown in serial serum samples.

CONCLUSIONS

Intramolecular epitope spreading might occur before the onset of human antiglomerular basement membrane disease. The autoimmunity to E(A) and E(B), especially E(B), was crucial for kidney dysfunction.

Genetic susceptibility to experimental autoimmune glomerulonephritis in the Wistar Kyoto rat

In experimental autoimmune glomerulonephritis (EAG), a model of Goodpasture's disease, Wistar Kyoto (WKY) rats immunized with collagenase-solubilized glomerular basement membrane (GBM) or the recombinant NC1 domain of the α3 chain of type IV collagen [α3(IV)NC1] develop anti-GBM antibodies and focal necrotizing glomerulonephritis with crescent formation. However, Lewis (LEW) rats, which share the same major histocompatibility complex (MHC) haplotype, are resistant to EAG development. A genome-wide linkage analysis of backcrossed animals with EAG revealed a major quantitative trait locus (QTL) on rat chromosome 13 (LOD = 3.9) linked to the percentage of glomerular crescents. To investigate the role of this QTL in EAG induction, reciprocal congenic rats were generated (LEW.WCrgn1 congenic and WKY.LCrgn1 congenic), immunized with recombinant rat α3(IV)NC1, and assessed for EAG development. WKY.LCrgn1 rats showed a marked reduction in albuminuria, severity of crescentic nephritis, and number of glomerular macrophages compared with WKY controls. No reduction in antibody levels was observed. However, LEW.WCrgn1 rats were resistant to EAG development, as were LEW controls. Macrophage activation in vitro was assessed in parental and congenic rat bone marrow-derived macrophages (BMDMs). WKY.LCrgn1 BMDMs showed a significant reduction in Fc receptor-mediated oxidative burst, phagocytosis of opsonised polystyrene beads, and LPS-induced levels of MCP-1 secretion and iNOS mRNA expression compared with WKY rats. These results confirm the importance of Crgn1 on chromosome 13 in EAG susceptibility, mediated partly through differences in Fc receptor-mediated macrophage activation.

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.

Mucosal tolerance induced by an immunodominant peptide from rat alpha3(IV)NC1 in established experimental autoimmune glomerulonephritis

Experimental autoimmune glomerulonephritis (EAG), an animal model of Goodpasture's disease, can be induced in Wistar Kyoto (WKY) rats by immunization with the noncollagenous domain of the alpha 3 chain of type IV collagen, alpha3(IV)NC1. Recent studies have identified an immunodominant peptide, pCol (24-38), from the N-terminus of rat alpha3(IV)NC1; this peptide contains the major B- and T-cell epitopes in EAG and can induce crescentic nephritis. In this study, we investigated the mechanisms of mucosal tolerance in EAG by examining the effects of the nasal administration of this peptide after the onset of disease. A dose-dependent effect was observed: a dose of 300 microg had no effect, a dose of 1000 microg resulted in a moderate reduction in EAG severity, and a dose of 3000 microg produced a marked reduction in EAG severity accompanied by diminished antigen-specific, T-cell proliferative responses. These results demonstrate that mucosal tolerance in EAG can be induced by nasal administration of an immunodominant peptide from the N-terminus of alpha3(IV)NC1 and should be of value in designing new therapeutic strategies for patients with Goodpasture's disease and other autoimmune disorders.

Identification of a nephritogenic immunodominant B and T cell epitope in experimental autoimmune glomerulonephritis

Experimental autoimmune glomerulonephritis (EAG) can be induced in Wistar Kyoto (WKY) rats by immunization with the non-collagenous domain (NC1) of the alpha 3 chain of type IV collagen, alpha3(IV)NC1. In patients with Goodpasture's disease, the major B cell epitope is located at the N-terminus of alpha3(IV)NC1. In order to investigate whether B and T cell responses in EAG are directed towards immunodominant peptides within the same region of rat alpha3(IV)NC1, we immunized WKY rats with recombinant rat alpha3(IV)NC1 (positive control) and five 15-mer overlapping synthetic peptides from the N-terminus of rat alpha3(IV)NC1: pCol(17-31), pCol(24-38), pCol(31-45), pCol(38-52) and pCol(45-59). Positive control animals immunized with alpha3(IV)NC1 produced an antibody response directed towards alpha3(IV)NC1 and pCol(24-38). Splenic T cells from these animals proliferated in response to alpha3(IV)NC1 and pCol(24-38). No significant antibody or T cell responses were observed to the other peptides examined. Animals immunized with pCol(24-38) developed linear deposits of immunoglobulin G on the glomerular basement membrane, albuminuria and focal necrotizing glomerulonephritis with crescent formation by week 6 after immunization. Circulating antibodies from these animals recognized pCol(24-38) and alpha3(IV)NC1, and their T cells proliferated in response to pCol(24-38) and alpha3(IV)NC1. Animals immunized with the other peptides developed no significant immune response to alpha3(IV)NC1 and no disease. In conclusion, these results demonstrate that a 15-mer peptide from the N-terminus of alpha3(IV)NC1 [pCol(24-38)] is recognized by B and T cells from rats immunized with recombinant alpha3(IV)NC1, and that the same peptide is capable of inducing crescentic glomerulonephritis. Identification of this immunodominant peptide will be of value in designing new therapeutic strategies for inducing mucosal tolerance in EAG, which may be applicable to patients with glomerulonephritis.

Epitope Spreading and Autoimmune Glomerulonephritis in Rats Induced by a T Cell Epitope of Goodpasture’s Antigen

An amino-terminal region of alpha3 chain of type IV collagen noncollagenous domain [alpha3(IV)NC1] that induces experimental autoimmune glomerulonephritis (EAG) in rats has been identified. Only recombinant antigens that contain a nine-amino acid (AA) span of alpha3(IV)NC1, consistent with a T cell epitope, could induce EAG. It was hypothesized that synthetic peptides of this region should induce EAG. Human and rat peptides of this region were synthesized and rats were immunized to define the nephritogenic epitope. A 13-AA rat peptide induced EAG with proteinuria, decreased renal function, and glomerular basement membrane (GBM)-bound deposits in half of the rats. This peptide induces lymph node cell proliferation and development of antibodies to epitopes of alpha3(IV)NC1 external to the peptide immunogen. Carboxy-terminal extension to 21 amino acids results in all rats' demonstrating anti-GBM antibody and severe EAG. Asparagine at position 19 is critical for EAG induction. None of the 50 rats that were immunized with peptide that contained human sequence with isoleucine at position 19 developed EAG, whereas rat sequence with asparagine 19 induced EAG. Truncation of amino terminal AA of the peptide aborts EAG induction. These studies demonstrate that a T cell epitope of alpha3(IV)NC1 induces lymph node cell proliferation, EAG, and intramolecular epitope spreading; that the length of this peptide influences the formation of anti-GBM antibody; and that the presence of asparagine at position 19 of the peptide is critical to disease induction.

The fine specificity and cytokine profile of T-helper cells responsive to the alpha3 chain of type IV collagen in Goodpasture's disease

Goodpasture's disease is a severe nephritis characterized by autoantibodies to the alpha3 chain of type IV collagen, alpha3(IV)NC1, in the glomerular basement membrane. The disease is very strongly associated with HLA-DR15, the affinities of alpha3(IV)NC1 peptides for DR15 are known, and elution experiments have identified major naturally processed sequences. Here, the fine specificity and cytokine profile of alpha3(IV)NC1-reactive T cells from patients with Goodpasture's disease is defined. Peripheral blood mononuclear cells from patients at diagnosis proliferated in response to significantly more peptides (chi(2) = 8.6, P = 0.004) from a panel spanning the sequence of alpha3(IV)NC1 than did those from control DR15-positive donors and were highly focused (P = 0.0002, binomial distribution) on two peptides, alpha3(71-90) and alpha3(131-150). Some peptides induced interferon-gamma, but none induced IL-4. Resolution of disease was accompanied by a striking deviation of the responses from proliferation to secretion of the T-regulatory cytokine IL-10, and addition of neutralizing antibody confirmed that such IL-10 production was suppressive. The affinity of the peptides for DR15 molecules was positively correlated (chi(2) = 14.6, P = 0.00067) with the ability to elicit proliferation. However, unlike foreign antigens, this hierarchy is not due to responses against the major naturally processed peptides, which rarely stimulated proliferation and which have only intermediate affinity for DR15 molecules. It is inferred that the helper response to alpha3(IV)NC1 in Goodpasture's disease is dominated by epitopes that are normally inefficiently presented because of processing constraints.

Immunodominant epitopes of α3(IV)NC1 induce autoimmune glomerulonephritis in rats

BACKGROUND

The major Goodpasture antibody binding epitopes have been localized to the amino-terminal third of the noncollagenous domain (NC1) of the alpha3 chain of type IV collagen [alpha3(IV)NC1]. The present study determined whether the same epitopes induce glomerulonephritis in rats.

METHODS

We immunized Wistar Kyoto (WKY) rats with human alpha3(IV)/alpha1(IV)NC1 chimeric proteins or full-length recombinant alpha3(IV)NC1 (alpha3732). Chimeric protein constructs were thirds of alpha3(IV)NC1 (CP333) replaced by corresponding sequences of homologous nonreactive alpha1(IV)NC1 (CP111). All chimeric proteins contained 30 amino acids of type X collagen at the amino terminus except alpha3732. Two other constructs, T195 EA (EA) and T194 EB (EB), were entirely alpha1(IV)NC1, except for antibody-immunodominant amino acids from the first and second thirds of alpha3(IV)NC1.

RESULTS

Construct immunized animals developed specific antibody responses to recombinant proteins and native human, bovine and rat NC1. CP311 immunized rats, as well as alpha3732 rats, had glomerular IgG, fibrin, and glomerulonephritis with proteinuria by 3 weeks. CP331 produced more severe disease, comparable to positive controls. CP111 produced no disease. EA, but not EB, induced severe glomerulonephritis. Half-dose each of EA plus EB induced disease identical to full-dose EA alone.

CONCLUSION

The amino third of alpha3(IV)NC1 which contains the major epitope for Goodpasture antibody binding, also induces glomerulonephritis in rats. The middle third of alpha3(IV)NC1 does not induce glomerulonephritis but appears to enhance disease with the amino terminal third. Finally, the presence of the collagen X leader sequence appears to convey greater nephritogenicity. These studies suggest that not only the nephritogenic epitope itself, but flanking sequences and the conformational context of the nephritogenic epitope may influence its ability to cause glomerulonephritis.

Point mutations of single amino acids abolish ability of alpha3 NC1 domain to elicit experimental autoimmune glomerulonephritis in rats

We previously showed concordance between Goodpasture syndrome antibody binding and production of experimental glomerulonephritis using human chimeric proteins. We now examine a more limited amino-terminal region of alpha3(IV) non-collagenous domain (NC1) and the impact of single amino acid (AA) mutations of this region on glomerulonephritis induction. Rats were immunized with collagenase-solubilized glomerular basement membrane (csGBM), D3, an alpha1(IV)NC1 chimeric protein with 69 AA of alpha3(IV)NC1 (binds Goodpasture sera), D4, the D3 construct shortened by 4 AA (non-binding), P9, P10, single AA mutants (non-binding), and S2, alpha1(IV)NC1 with 9 AA of alpha3(IV)NC1 (binding). All rats immunized with csGBM and S2 and 50% of D3 rats developed glomerulonephritis. csGBM rats had intense GBM-bound IgG deposits, but S2 and D3 rats had minimal deposits. None of the D4, P9, or P10 rats developed glomerulonephritis. Lymphocytes from nephritic rats proliferated with csGBM, S2, and D3, but not with D4, P9, or P10. Discrete segments of alpha3(IV)NC1 within the alpha1(IV)NC1 backbone can induce glomerulonephritis. Single AA mutations within that epitope render the antigen unresponsive to Goodpasture sera and incapable of inducing glomerulonephritis. These studies support the concordance of glomerulonephritis inductivity and Goodpasture serum binding. Further, they define a critical limited AA sequence within alpha3(IV)NC1 of nine or fewer AA, which confers nephritogenicity to the nonnephritogenic alpha1(IV)NC1 without in vivo antibody binding. This region may be a T-cell epitope responsible for induction of glomerulonephritis in this model in rats and Goodpasture syndrome in man.

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.

Synthetic peptides of Goodpasture's antigen in antiglomerular basement membrane nephritis in rats

Goodpasture's syndrome (GPS) is an autoimmune disease characterized by pulmonary hemorrhage, glomerulonephritis and anti-glomerular basement membrane (GBM) antibodies. The alpha(3) noncollagenous domain (NC1) of type IV collagen [alpha(3)(IV)] is the pathogen. The disease is T-cell-dependent; thus linear peptides initiate the autoimmune process. Studies in a rat model of GPS, experimental autoimmune glomerulonephritis (EAG), have shown that the carboxy-terminal 36 amino acids (purportedly the pathogenic epitope) are not responsible for disease induction. More recent studies implicate the amino terminus of alpha(3)(IV)NC1. Finding the nephritogenic epitope(s) is crucial in the understanding of the disease and for treatment. Because alpha(3)(IV)NC1 contains the antigens that induce GN in rats and human beings, we hypothesized that regions of the alpha(3)(IV)NC1 other than the carboxy terminus were responsible for disease. We investigated overlapping peptides spanning the entire NC1 domain of the alpha(3)(IV) chain N-terminal to the 36-mer (Goodpasture epitope) using the EAG rat model. Most peptides elicited antibody responses exclusively to themselves but not to native GBM. T-cells from GBM-immunized rats proliferated in vitro after stimulation with peptides 6, 8, 14, and 15, 24-mer and 23-mer. Fifteen percent of peptide 8 and peptide 14 rats had mild glomerulonephritis. In none of the animals immunized with other peptides did glomerulonephritis develop. These data suggest that conformation-dependent sites, posttranslational modification, multiple epitopes, concomitant antibody formation, or other disturbances are important in the ability of alpha(3)(IV)NC1 to induce EAG in rats and may also be important in the induction of GPS in human beings.

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.

In Goodpasture's disease, CD4(+) T cells escape thymic deletion and are reactive with the autoantigen alpha3(IV)NC1

Goodpasture's disease is characterized by rapidly progressive glomerulonephritis and pulmonary hemorrhage, in association with circulating and deposited anti-glomerular basement membrane antibodies that recognize the alpha3 chain of type IV collagen [alpha3(IV)NC1] (known as the Goodpasture antigen). Unlike many other autoimmune diseases, recurrences are rare. In experimental models and human studies, both humoral and cellular mechanisms have been demonstrated to be involved in disease pathogenesis. However, there are few data on the characteristics of the autoreactive T cells or the mechanisms of tolerance to the autoantigen in human patients. It was demonstrated, using immunohistochemical analyses and reverse transcription-PCR, that the Goodpasture antigen is expressed in normal human thymus. Using limiting dilution analyses, the frequencies of circulating autoreactive T cells in patients and control subjects were assessed. During acute disease, there were increased frequencies of CD4(+) T cells reactive with alpha3(IV)NC1 (ranging from 1:6300 to 1:65,000), which decreased with time. There was a significant difference between patients during their acute disease phase and control subjects with respect to the frequency index for alpha3(IV)NC1-specific CD4(+) T cells (P < 0.05, Mann Whitney U test). The decrease in autoreactive CD4(+) T-cell numbers during recovery may be the reason why recurrences are infrequent and may explain the loss of pathogenic autoantibodies with time, because of a lack of T-cell help.

Hydrophobic amino acid residues are critical for the immunodominant epitope of the Goodpasture autoantigen. A molecular basis for the cryptic nature of the epitope

Goodpasture (GP) autoimmune disease is caused by autoantibodies to type IV collagen that bind to the glomerular basement membrane, causing rapidly progressing glomerulonephritis. The immunodominant GP(A) autoepitope is encompassed by residues 17-31 (the E(A) region) within the noncollagenous (NC1) domain of the alpha 3(IV) chain. The GP epitope is cryptic in the NC1 hexamer complex that occurs in the type IV collagen network found in tissues and inaccessible to autoantibodies unless the hexamer dissociates. In contrast, the epitope for the Mab3 monoclonal antibody is also located within the E(A) region, but is fully accessible in the hexamer complex. In this study, the identity of residues that compose the GP(A) autoepitope was determined, and the molecular basis of its cryptic nature was explored. This was achieved using site-directed mutagenesis to exchange the alpha3(IV) residues in the E(A) region with the corresponding residues of the homologous but non-immunoreactive alpha1(IV) NC1 domain and then comparing the reactivity of the mutated chimeras with GP(A) and Mab3 antibodies. It was shown that three hydrophobic residues (Ala(18), Ile(19), and Val(27)) and Pro(28) are critical for the GP(A) autoepitope, whereas two hydrophilic residues (Ser(21) and Ser(31)) along with Pro(28) are critical for the Mab3 epitope. These results suggest that the cryptic nature of the GP(A) autoepitope is the result of quaternary interactions of the alpha 3, alpha 4, and alpha 5 NC1 domains of the hexamer complex that bury the one or more hydrophobic residues. These findings provide critical information for understanding the etiology and pathogenesis of the disease as well as for designing drugs that would mimic the epitope and thus block the binding of GP autoantibodies to autoantigen.

Expression and characterization of recombinant rat alpha 3(IV)NC1 and its use in induction of experimental autoimmune glomerulonephritis

BACKGROUND

The autoantigen in Goodpasture's disease is known to be the non-collagenous domain of the alpha3 chain of type IV collagen, alpha 3(IV)NC1. There is mounting evidence that alpha 3(IV)NC1 is also a target of autoimmunity in experimental autoimmune glomerulonephritis (EAG). Sado et al. [Kidney Int 1998; 53, 664-671] have reported that recombinant human alpha 3(IV)NC1 and alpha4(IV)NC1 are nephritogenic in WKY rats. We have proposed that immunization with homologous antigen is more appropriate for detailed investigation of autoimmunity in EAG.

METHODS

To this end, we have cloned and sequenced rat alpha 3(IV)NC1 and expressed it in COS-7 cells. Recombinant rat alpha 3(IV)NC1, secreted into the COS-7 cell supernatant, was purified on an anti-M2 FLAG affinity column and characterized by western blotting. Recombinant antigen was then used to immunize WKY rats, in order to induce EAG.

RESULTS

The recombinant material was antigenic as judged by binding to sera from patients with Goodpasture's disease and a mAb to alpha 3(IV)NC1. Immunization of WKY rats (n=5), with recombinant rat alpha 3(IV)NC1 in FCA at a dose of 1 mg/kg resulted in circulating anti-GBM antibodies directed towards alpha 3(IV)NC1, linear deposits of IgG on the GBM, albuminuria, deposits of fibrin in the glomeruli, severe focal necrotizing glomerulonephritis with crescent formation, and glomerular influx of CD8+ T cells and macrophages. Western blot analysis demonstrated that sera from these rats bound strongly to recombinant rat alpha 3(IV)NC1, as well as to collagenase-solubilized human and rat GBM. The pattern of binding was indistinguishable from that of sera from patients with Goodpasture's disease.

CONCLUSIONS

This purified recombinant rat alpha 3(IV)NC1, which is both antigenic and nephritogenic, will be of value in analysing autoimmune responses in experimental anti-GBM disease.

Molecular properties of the Goodpasture epitope

Goodpasture disease fulfils all criteria for a classical autoimmune disease, where autoantibodies targeted against the non-collagenous domain of the alpha3-chain of collagen IV initiates an inflammatory destruction of the basement membrane in kidney glomeruli and lung alveoli. This leads to a rapidly progressive glomerulonephritis and severe pulmonary hemorrhage. Previous studies have indicated a limited epitope for the toxic antibodies in the N-terminal part of the non-collagenous domain. The epitope has been partially characterized by recreating the epitope in the non-reactive alpha1-chain by exchanging nine residues to the corresponding ones of alpha3. In this study we have investigated to what extent each of these amino acids contribute to the antibody binding in different patient sera. The results show that seven of the nine substitutions are enough to get an epitope that is recognized equally well as the native alpha3-chain by all sera from 20 clinically verified Goodpasture patients. Furthermore, the patient sera reactivity against the different recombinant chains used in the study are very similar, with some minor exceptions, strongly supporting a highly defined and restricted epitope. We are convinced that the restriction of the epitope is of significant importance for the understanding of the etiology of the disease. Thereby also making every step on the way to characterization of the epitope a crucial step on the way to specific therapy for the disease.

Reactive oxygen species expose cryptic epitopes associated with autoimmune goodpasture syndrome

Goodpasture syndrome is an autoimmune disease of the kidneys and lungs mediated by antibodies and T-cells directed to cryptic epitopes hidden within basement membrane hexamers rich in alpha3 non-collagenous globular (NC1) domains of type IV collagen. These epitopes are normally invisible to the immune system, but this privilege can be obviated by chemical modification. Endogenous drivers of immune activation consequent to the loss of privilege have long been suspected. We have examined the ability of reactive oxygen species (ROS) to expose Goodpasture epitopes buried within NC1 hexamers obtained from renal glomeruli abundant in alpha3(IV) NC1 domains. For some hexameric epitopes, like the Goodpasture epitopes, exposure to ROS specifically enhanced recognition by Goodpasture antibodies in a sequential and time-dependent fashion; control binding of epitopes to alpha3(IV) alloantibodies from renal transplant recipients with Alport syndrome was decreased, whereas epitope binding to heterologous antibodies recognizing all alpha3 NC1 epitopes remained the same. Inhibitors of hydrogen peroxide and hydroxyl radical scavengers were capable of attenuating the effects of ROS in cells and kidney by 30-50%, respectively, thereby keeping the Goodpasture epitopes largely concealed when compared with a 70% maximum inhibition by iron chelators. Hydrogen peroxide administration to rodents was sufficient to expose Goodpasture epitope in vivo and initiate autoantibody production. Our findings collectively suggest that ROS can alter the hexameric structure of type IV collagen to expose or destroy selectively immunologic epitopes embedded in basement membrane. The reasons for autoimmunity in Goodpasture syndrome may lie in an age-dependent deterioration in inhibitor function modulating oxidative damage to structural molecules. ROS therefore may play an important role in shaping post-translational epitope diversity or neoantigen formation in organ tissues.

The goodpasture autoantigen. Identification of multiple cryptic epitopes on the NC1 domain of the alpha3(IV) collagen chain

Goodpasture (GP) disease is an autoimmune disorder in which autoantibodies against the alpha3(IV) chain of type IV collagen bind to the glomerular and alveolar basement membranes, causing progressive glomerulonephritis and pulmonary hemorrhage. Two major conformational epitope regions have been identified on the noncollagenous domain of type IV collagen (NC1 domain) of the alpha3(IV) chain as residues 17-31 (E(A)) and 127-141 (E(B)) (Netzer, K.-O. et al. (1999) J. Biol. Chem. 274, 11267-11274). To determine whether these regions are two distinct epitopes or form a single epitope, three GP sera were fractionated by affinity chromatography on immobilized NC1 chimeras containing the E(A) and/or the E(B) region. Four subpopulations of GP antibodies with distinct epitope specificity for the alpha3(IV)NC1 domain were thus separated and characterized. They were designated GP(A), GP(B), GP(AB), and GP(X), to reflect their reactivity with E(A) only, E(B) only, both regions, and neither, respectively. Hence, regions E(A) and E(B) encompass critical amino acids that constitute three distinct epitopes for GP(A), GP(B), and GP(AB) antibodies, respectively, whereas the epitope for GP(X) antibodies is located in a different unknown region. The GP(A) antibodies were consistently immunodominant, accounting for 60-65% of the total immunoreactivity to alpha3(IV)NC1; thus, they probably play a major role in pathogenesis. Regions E(A) and E(B) are held in close proximity because they jointly form the epitope for Mab3, a monoclonal antibody that competes for binding with GP autoantibodies. All GP epitopes are sequestered in the hexamer configuration of the NC1 domain found in tissues and are inaccessible for antibody binding unless dissociation of the hexamer occurs, suggesting a possible mechanism for etiology of GP disease. GP antibodies have the capacity to extract alpha3(IV)NC1 monomers, but not dimers, from native human glomerular basement membrane hexamers, a property that may be of fundamental importance for the pathogenesis of the disease.

Goodpasture disease. Characterization of a single conformational epitope as the target of pathogenic autoantibodies

Goodpasture disease is a prototype autoimmune disease characterized by the formation of autoantibodies against the heterotrimeric basement membrane collagen type IV, which causes a rapidly progressive glomerulonephritis. The pathogenic antibody response is directed to the non-collagenous (NC1) domain of the alpha3 chain of type IV collagen (alpha3(IV)NC1), but not to the homologous region of the alpha1(IV)NC1. To identify the conformation-dependent immunodominant epitope on the alpha3(IV)NC1, a variety of recombinant NC1 domains were constructed by replacing single residues of alpha3(IV) with the corresponding amino acids from the nonreactive alpha1(IV) chain. Replacement mutations were identified that completely destroyed the Goodpasture epitope in the alpha3(IV) chain. Based on the identification of these critical positions, the epitope was finally reconstructed within the frame of the alpha1(IV) chain. The substitution of nine discontinuous positions in the alpha1(IV)NC1 with amino acid residues from the alpha3 chain resulted in a recombinant construct that was recognized by all patients' sera (n = 20) but by none of the sera from healthy controls (n = 10). This provides, for the first time, the molecular characterization of a single immunodominant conformational epitope recognized by pathogenic autoantibodies in a human autoimmune disease, representing the basis for the development of new epitope-specific strategies in the treatment of Goodpasture disease.

The goodpasture autoantigen. Mapping the major conformational epitope(s) of alpha3(IV) collagen to residues 17-31 and 127-141 of the NC1 domain

The Goodpasture (GP) autoantigen has been identified as the alpha3(IV) collagen chain, one of six homologous chains designated alpha1-alpha6 that comprise type IV collagen (Hudson, B. G., Reeders, S. T., and Tryggvason, K. (1993) J. Biol. Chem. 268, 26033-26036). In this study, chimeric proteins were used to map the location of the major conformational, disulfide bond-dependent GP autoepitope(s) that has been previously localized to the noncollagenous (NC1) domain of alpha3(IV) chain. Fourteen alpha1/alpha3 NC1 chimeras were constructed by substituting one or more short sequences of alpha3(IV)NC1 at the corresponding positions in the non-immunoreactive alpha1(IV)NC1 domain and expressed in mammalian cells for proper folding. The interaction between the chimeras and eight GP sera was assessed by both direct and inhibition enzyme-linked immunosorbent assay. Two chimeras, C2 containing residues 17-31 of alpha3(IV)NC1 and C6 containing residues 127-141 of alpha3(IV)NC1, bound autoantibodies, as did combination chimeras containing these regions. The epitope(s) that encompasses these sequences is immunodominant, showing strong reactivity with all GP sera and accounting for 50-90% of the autoantibody reactivity toward alpha3(IV)NC1. The conformational nature of the epitope(s) in the C2 and C6 chimeras was established by reduction of the disulfide bonds and by PEPSCAN analysis of overlapping 12-mer peptides derived from alpha1- and alpha3(IV)NC1 sequences. The amino acid sequences 17-31 and 127-141 in alpha3(IV)NC1 have thus been shown to contain the critical residues of one or two disulfide bond-dependent conformational autoepitopes that bind GP autoantibodies.