A Self T Cell Epitope Induces Autoantibody Response: Mechanism for Production of Antibodies to Diverse Glomerular Basement Membrane Antigens

Epitope Spreading in Immune-Mediated Glomerulonephritis: The Expanding Target

Epitope spreading is a critical mechanism driving the progression of autoimmune glomerulonephritis. This phenomenon, where immune responses broaden from a single epitope to encompass additional targets, contributes to the complexity and severity of diseases such as membranous nephropathy (MN), lupus nephritis (LN), and ANCA-associated vasculitis (AAV). In MN, intramolecular spreading within the phospholipase A2 receptor correlates with a worse prognosis, while LN exemplifies both intra- and intermolecular spreading, exacerbating renal involvement. Similarly, ANCA reactivity in AAV highlights the destructive potential of epitope diversification. Understanding these immunological cascades reveals therapeutic opportunities-targeting early epitope spreading could curb disease progression. Despite promising insights, the clinical utility of epitope spreading as a prognostic tool remains debated. This review provides a complete overview of the current evidence, exploring the dual-edged nature of epitope spreading, the intricate immune mechanisms behind it, and its therapeutic implications. By elucidating these dynamics, we aim to pave the way for more precise, targeted interventions in autoimmune glomerular diseases.

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.

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.

Inter-molecular epitope spreading does not lead to extension of autoimmunity beyond target tissue in autoimmune glomerulonephritis

Inter-molecular epitope spreading during autoimmune pathogenesis leads to generation of new pathogenic epitopes on other autoantigens beyond the original one. It raises an important question as whether autoimmunity extends beyond the target tissues if new epitopes are on the molecules shared with other tissues. This study is aimed addressing this question in a rat anti-glomerular basement membrane (GBM) glomerulonephritis model induced by a T cell epitope of glomerulus-specific collagen4α3. We have demonstrated inter-molecular B cell epitope spreading. Four novel epitopes were first identified by screening a phage display random peptide library against autoantibodies isolated from the GBM of immunized rats. All four epitopes were derived from GBM proteins with three from laminins and one from collagen4α4. Three out of four synthetic peptides were nephritogenic. Importantly, two peptides from lamininα1 and lamininβ1, respectively, induced severe inflammation in glomeruli but not in the interstitial tissues, despite the presence of more abundant laminins in the tubular basement membranes. Our study suggests that surrounding tissues may display a lower or altered susceptibility to autoimmune inflammation. Thus, preventing extension of autoimmune inflammation beyond the original target tissue.

Antibodies against linear epitopes on Goodpasture autoantigen in patients with anti-neutrophil cytoplasmic antibody-associated vasculitis

In a substantial number of patients with crescentic glomerulonephritis, both anti-glomerular basement membrane (GBM) antibodies and anti-neutrophil cytoplasmic antibodies (ANCA) are detected simultaneously. ANCA is presumed to be the initial event but the mechanism is unknown. In the present study, we investigated the antibodies against linear epitopes on Goodpasture autoantigen in sera from patients with ANCA-associated vasculitis, aiming to reveal the mechanisms of the coexistence of the two kinds of autoantibodies. Thirty-one patients with ANCA-associated vasculitis were enrolled in this study. Twenty-four overlapping linear peptides were synthesized across the whole sequence of Goodpasture autoantigen. Serum antibodies against linear peptides were detected by ELISA and their associations with clinical features were further analyzed. Twenty-five out of the thirty-one (80.6%) sera from patients with ANCA-associated vasculitis possessed antibodies against linear peptides on Goodpasture autoantigen. These antibodies could be detected in 50% of patients with normal renal function (Scr ≤ 133 μmol/L), 70% of patients with moderate renal dysfunction (133 μmol/L < Scr ≤ 600 μmol/L), and 94% of patients with renal failure (Scr > 600 μmol/L) (P = 0.032). The highest recognition frequencies were found for peptides P4 (51.6%), P14 (54.8%), and P24 (54.8%), which contained the sequences that constitute the conformational epitopes of E_A (P4) and E_B (P14) recognized by anti-GBM antibodies. The level of anti-P4 antibodies was positively correlated with the percentage of crescents in glomeruli (r = 0.764, P = 0.027). Patients with anti-P24 antibodies had a significantly higher prevalence of renal dysfunction on diagnosis (88.2 vs. 42.9%, P = 0.018). Antibodies against linear epitopes on Goodpasture autoantigen could be detected in sera of patients with ANCA-associated vasculitis, which might mediate the production of antibodies towards the conformational epitopes on Goodpasture autoantigen, namely, the anti-GBM antibodies.

The pathogenicity of T cell epitopes on human Goodpasture antigen and its critical amino acid motif

Goodpasture antigen, the non‐collagenous domain of α3 chain of type IV collagen [α3(IV)NC1], is the target antigen of anti‐glomerular basement membrane (GBM) antibodies. The pathogenicity of T cell epitopes is not elucidated clearly. In this study, we aim to define the nephritogenic T cell epitopes and its critical amino acid residues. Twenty‐four overlapping linear peptides were synthesized covering the whole sequence of human α3(IV)NC1. Wistar–Kyoto rats were immunized with linear peptides, and experimental autoimmune glomerulonephritis was evaluated. Critical amino acid was identified by the loss of nephritogenic function after each amino acid substitution by alanine. Of the 24 peptides, P14 (α3_127‐148) could induce 90.5% (19/21) of WKY rats developing anti‐GBM glomerulonephritis with proteinuria, elevated serum urea and creatinine, IgG linear deposit on GBM and substantial (in average 82.4 ± 5.6%) crescent formation in glomeruli. Lymphocytes of immunized rats proliferated in response to α3_127‐148 and α3(IV)NC1 in vitro. Sera of these rats recognized α3_127‐148 and later on together with intact human α3(IV)NC1. Antibodies towards α3_127‐148 and intact α3(IV)NC1 could also be detected from the kidney elutes. These antibodies showed no cross‐reaction with each other, which implies intramolecular epitope spreading during disease progress. After sequential amino acid substitution, the α3_127‐148 with substitution of tryptophan₁₃₆, isoleucine₁₃₇, leucine₁₃₉ or tryptophan₁₄₀ lost its nephritogenicity. Human α3_127‐148 is a nephritogenic T cell epitope in WKY rats, with the critical amino acids as W₁₃₆I₁₃₇xL₁₃₉W₁₄₀. These findings might facilitate future investigation on microbial aetiology and potential specific immunotherapy of anti‐GBM disease.

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.

Identification of critical residues of linear B cell epitope on Goodpasture autoantigen

Background

The autoantigen of anti-glomerular basement membrane (GBM) disease has been identified as the non-collagenous domain 1 of α3 chain of type IV collagen, α3(IV)NC1. Our previous study revealed a peptide on α3(IV)NC1 as a major linear epitope for B cells and potentially nephrogenic, designated as P14 (α3129-150). This peptide has also been proven to be the epitope of auto-reactive T cells in anti-GBM patients. This study was aimed to further characterize the critical motif of P14.

Methods

16 patients with anti-GBM disease and positive anti-P14 antibodies were enrolled. A set of truncated and alanine substituted peptides derived from P14 were synthesized. Circulating antibodies against the peptides were detected by enzyme linked immunosorbent assay (ELISA).

Results

We found that all sera with anti-P14 antibodies reacted with the 13-mer sequence in the C-terminus of P14 (P14c) exclusively. The level of antibodies against P14 was highly correlated with the level of antibodies against P14c (r=0.970, P<0.001). P14c was the core immunogenic region and the amino acid sequence (ISLWKGFSFIMFT) was highly hydrophobic. Each amino acid residue in P14c was sequentially replaced by alanine. Three residues of glycine142, phenylalanine143, and phenylalanine145 were identified crucial for antibody binding based on the remarkable decline (P<0.001) of antibody reaction after each residue replacement.

Conclusions

We defined GFxF (α3142, 143,145) as the critical motif of P14. It may provide some clues for understanding the etiology of anti-GBM disease.

Differentiating Glomerular Inflammation from Fibrosis in a Bone Marrow Chimera for Rat Anti-Glomerular Basement Membrane Glomerulonephritis

BACKGROUND

Many types of glomerulonephritis (GN) undergo tandem connected phases: inflammation and fibrosis. Fibrosis in human GNs leads to irreversible end-stage disease. This study investigated how these 2 phases were controlled.

METHODS

Using a rat anti-glomerular basement membrane GN model, we established bone marrow (BM) chimeras between GN-resistant Lewis (LEW) and GN-susceptible Wistar Kyoto (WKY) rats. Glomerular inflammation and fibrosis were compared between chimeras.

RESULTS

LEW's BM to WKY chimeras with or without co-transfer of host WKY's T cells were GN-resistant. On the other hand, WKY's BM to LEW (LEW(WKY)) chimeras developed glomerular inflammation and albuminuria upon immunization. Quantitative analysis showed that the number and composition of inflammatory cells in glomeruli of immunized LEW(WKY) chimeras were similar to those in immunized WKY rats at their inflammatory peak. Thus, glomerular inflammation was controlled by BM-derived non-T cell populations. However, unlike WKY rats, LEW(WKY) rats did not develop fibrosis until the end of experiments (84 days) in spite of persistent inflammation and albuminuria.

CONCLUSION

Inflammation alone was not sufficient to trigger fibrosis, suggesting a critical role of glomerular cells in the fibrotic process. As LEW(WKY) chimera allows us to separate glomerular inflammation from fibrosis, this model provides a useful tool to study how fibrosis is initiated following inflammation.

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

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

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.

Identification of a bone marrow-derived CD8αα+ dendritic cell-like population in inflamed autoimmune target tissue with capability of inducing T cell apoptosis

DCs play critical roles in promotion of autoimmunity or immune tolerance as potent APCs. In our anti-GBM GN model, WKY rats develop severe T cell-mediated glomerular inflammation followed by fibrosis. A DC-like cell population (CD8αα(+)CD11c(+)MHC-II(+)ED1(-)) was identified in the inflamed glomeruli. Chimera experiments demonstrated that the CD8αα(+) cells were derived from BM. The CD8αα(+) cells infiltrated glomeruli at a late stage (Days 28-35), coincident with a rapid decline in glomerular inflammation before fibrosis. The CD8αα(+) cells isolated from inflamed glomeruli were able to migrate rapidly from the bloodstream into inflamed glomeruli but not into normal glomeruli, suggesting that the migration was triggered by local inflammation. Despite high-level expression of surface and cellular MHC class II molecules, in vitro experiments showed that this CD8αα(+) DC-like cell induced apoptosis but not proliferation in antigen-specific CD4(+) T cells from T cell lines or freshly isolated from lymph nodes; they were not able to do so in the absence of antigens, suggesting induction of apoptosis was antigen-specific. Furthermore, apoptotic T cells were detected in a large number in the glomeruli at Day 32, coincident with the infiltration of the cells into glomeruli, suggesting that the cells may also induce T cell apoptosis in vivo. A potential role of this CD8αα(+) DC-like population in peripheral immune tolerance and/or termination of autoimmune inflammation was discussed.

T cells and dendritic cells in glomerular disease: the new glomerulotubular feedback loop

A newly described glomerulotubular feedback loop may explain the relationship between glomerular damage, epitope spreading, tubulointerstitial nephritis, proteinuria as a progression factor, and the importance of the local milieu in kidney damage. It also opens the horizons for exciting innovative approaches to therapy of both acute and chronic kidney diseases.

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.

Spontaneous recovery from early glomerular inflammation is associated with resistance to anti-GBM glomerulonephritis: tolerance and autoimmune tissue injury

Different susceptibility to anti-GBM glomerulonephritis (GN) among animal strains has been reported. Using our rat model for T cell-mediated anti-GBM GN, this study initiated an investigation on the mechanism related with GN susceptibility. Anti-GBM GN was induced either through immunization with the nephritogenic T cell epitope pCol(28-40) from Col4alpha3NC1 or through the transfer of specific T cells. WKY rats were highly susceptible to GN while immuno-compatible LEW rats were GN-resistant. GN-resistance in LEW rats was not associated to the immune response to pCol(28-40). First, both strains mounted a Th1 T cell response to pCol(28-40) with identical specificities; transfer of T cells from LEW to WKY rats induced glomerular injury. Second, co-transfer of antibody from WKY to LEW failed to induce GN. Time-course studies revealed that LEW rats did develop T cell-mediated inflammation in glomeruli at early stages similar to WKY rats, as evidenced by histopathology, proteinuria, CD4(+) T cell infiltration in glomeruli, and glomerular expression of inflammatory molecules. However, glomerular inflammation in LEW rats was transient followed by a full recovery. Thus, GN-resistance in LEW rats was due to its ability to contain early T cell-mediated autoimmune glomerular damage. Our model may reveal a potential tolerance mechanism after autoimmune tissue damage has been initiated.

Intramolecular epitope spreading in Heymann nephritis

Immunization with megalin induces active Heymann nephritis, which reproduces features of human idiopathic membranous glomerulonephritis. Megalin is a complex immunological target with four discrete ligand-binding domains (LBDs) that may contain epitopes to which pathogenic autoantibodies are directed. Recently, a 236-residue N-terminal fragment, termed "L6," that spans the first LBD was shown to induce autoantibodies and severe disease. We used this model to examine epitope-specific contributions to pathogenesis. Sera obtained from rats 4 weeks after immunization with L6 demonstrated reactivity only with the L6 fragment on Western blot, whereas sera obtained after 8 weeks demonstrated reactivity with all four recombinant fragments of interest (L6 and LBDs II, III, and IV). We demonstrated that the L6 immunogen does not contain the epitopes responsible for the reactivity to the LBD fragments. Therefore, the appearance of antibodies directed at LBD fragments several weeks after the primary immune response suggests intramolecular epitope spreading. In vivo, we observed a temporal association between increased proteinuria and the appearance of antibodies to LBD fragments. These data implicate B cell epitope spreading in antibody-mediated pathogenesis of active Heymann nephritis, a model that should prove valuable for further study of autoimmune dysregulation.

Experimental anti-GBM disease as a tool for studying spontaneous lupus nephritis

Lupus nephritis is an immune-mediated disease, where antibodies and T cells both play pathogenic roles. Since spontaneous lupus nephritis in mouse models takes 6-12 months to manifest, there is an urgent need for a mouse model that can be used to delineate the pathogenic processes that lead to immune nephritis, over a quicker time frame. We propose that the experimental anti-glomerular basement membrane (GBM) disease model might be a suitable tool for uncovering some of the molecular steps underlying lupus nephritis. This article reviews the current evidence that supports the use of the experimental anti-GBM nephritis model for studying spontaneous lupus nephritis. Importantly, out of about 25 different molecules that have been specifically examined in the experimental anti-GBM model and also spontaneous lupus nephritis, all influence both diseases concordantly, suggesting that the experimental model might be a useful tool for unraveling the molecular basis of spontaneous lupus nephritis. This has important clinical implications, both from the perspective of genetic susceptibility as well as clinical therapeutics.

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.

A nephritogenic peptide induces intermolecular epitope spreading on collagen IV in experimental autoimmune glomerulonephritis

This group previously identified a peptide p13 of alpha3(IV)NC1 domain of type IV collagen that induces experimental autoimmune glomerulonephritis (EAG) in rats with generation of antibodies to sites on alpha3(IV)NC1 external to the peptide as a result of intramolecular epitope spreading. It was hypothesized that intermolecular epitope spreading to other collagen IV chains also was induced. Rats were immunized with nephritogenic peptide that was derived from the amino terminal part of rat alpha3(IV)NC1 domain, and serum and kidney eluate were examined for antibodies to both native and recombinant NC1 domains of collagen IV. Peptide induced EAG with proteinuria and decreased renal function and glomerular basement membrane IgG deposits. Sera from these rats were examined by ELISA, which revealed reactivity not only to immunizing peptide but also to human and rat alpha3(IV)NC1 and to human alpha4(IV)NC1 domains. Kidney eluate that was depleted of alpha3(IV)NC1 antibodies still reacted to alpha4(IV)NC1, and alpha3(IV)NC1 column-bound antibody reacted with alpha3(IV)NC1. There was minimal reactivity to other collagen chains. Eluate that was adsorbed to NC1 hexamer from rat glomerular basement membrane lost all reactivity to glomerular constituents, and the eluted antibodies reacted to alpha3(IV)NC1 and alpha4(IV)NC1 domains. These studies show that a T cell epitope of alpha3(IV)NC1 induces EAG, intramolecular epitope spreading along alpha3(IV)NC1, and intermolecular epitope spreading to alpha4(IV)NC1 domain with minimal or no reactivity to other collagen chains or glomerular constituents. This is the first demonstration in EAG of intermolecular epitope spreading and identification of the spread epitopes.

Characterization of the T-cell epitope that causes anti-GBM glomerulonephritis

BACKGROUND

We have demonstrated that a single T-cell epitope pCol(28-40) (SQTTANPSCPEGT) alone, which is derived from NC1 domain of alpha3 chain of type IV collagen (Col4alpha3 NC1), can induce severe glomerulonephritis in Wistar Kyoto rats. This study further characterized this T-cell epitope.

METHODS

A series of synthetic peptides derived from pCol (28-40) were tested in vivo and in vitro for their T-cell epitope activity and nephritogenicity. Major histocompatability complex (MHC) class II molecules in Wistar Kyoto rats were cloned, and MHC restriction of pCol(28-40) was determined.

RESULTS

The T-cell epitope pCol(28-40) was restricted by rat MHC class II RT.1Bl. Ten amino acid residues (29 to 38) were mapped to be the minimum core of the T-cell epitope, which was capable of inducing the T-cell response and severe glomerulonephritis. Only three residues were identified as absolutely critical for the T-cell epitope: position 31 (T) was an anchor residue to the class II molecule, and positions 33 (N) and 34 (P) contributed to the specificity of the T-cell epitope. Thus, only substitution at those positions completely abrogated nephritogenicity of the T-cell epitope. Interestingly, pCol (28-40) also bound to human MHC class II human MHC class II molecule HLA-DRB*1501, which has been linked to human anti-glomerular basement membrane (GBM) disease, suggesting that human homologue of pCol(28-40) could be a potential human T-cell epitope.

CONCLUSION

Our study demonstrated that only few residues in the nephritogenic T-cell epitope pCol(28-40) were critical. Our finding also revealed that pCol(28-40) is a potential nephritogenic T-cell epitope in Goodpasture's syndrome.

T Cell Epitope Mimicry in Antiglomerular Basement Membrane Disease

Antiglomerular basement membrane (GBM) disease or Goodpasture's syndrome is among the earliest recognized human autoimmune diseases. Although collagen 4alpha3 NC1 (Col4alpha3NC1) has been identified as the responsible autoantigen, it remains unknown how autoimmunity to this autoantigen is provoked. We have demonstrated in our rat model that a single nephritogenic T cell epitope pCol28-40 of Col4alpha3NC1 induces glomerulonephritis. We hypothesized that microbial peptides that mimic this T cell epitope could induce the disease. Based on the critical residue motif (xxtTxNPsxx) of pCol28-40, seven peptides derived from human infection-related microbes were chosen through GenBank search and synthesized. All peptides showed cross-reactivity with pCol28-40-specific T cells at various levels. Only four peptides induced transient proteinuria and minor glomerular injury. However, the other three peptides induced severe proteinuria and modest to severe glomerulonephritis in 16-25% of the immunized rats. Unexpectedly, the most nephritogenic peptide, pCB, derived from Clostridium botulinum, also induced modest (25%) to severe (25%) pulmonary hemorrhage, another important feature of anti-GBM disease; this was not correlated with the severity of glomerulonephritis. This finding suggests that subtle variations in T cell epitope specificity may lead to different clinical manifestations of anti-GBM disease. In summary, our study raises the possibility that a single T cell epitope mimicry by microbial Ag may be sufficient to induce the anti-GBM disease.

Zebrafish to humans: evolution of the alpha3-chain of type IV collagen and emergence of the autoimmune epitopes associated with Goodpasture syndrome

Goodpasture syndrome is an autoimmune vascular disease associated with kidney and lung failure, with pathogenic circulating autoantibodies targeted to a set of discontinuous epitope sequences within the noncollagenous domain-1 (NC1) of the alpha3 chain of type IV collagen (alpha3(IV)NC1), the Goodpasture autoantigen. We demonstrate that basement membrane extracted NC1 domain preparations from Caenorhabditis elegans, Drosophila melanogaster, and Danio rerio do not bind Goodpasture autoantibodies, while Xenopus laevis, chicken, mouse and human alpha3(IV)NC1 domains bind autoantibodies. The alpha3(IV) chain is not present in C elegans and Drosophila melanogaster, but is first detected in the Danio rerio. Interestingly, native Danio rerio alpha3(IV)NC1 does not bind Goodpasture autoantibodies. Next, we cloned, sequenced, and generated recombinant Danio rerio alpha3(IV)NC1 domain. In contrast to recombinant human alpha3(IV)NC1 domain, there was complete absence of autoantibody binding to recombinant Danio rerio alpha3(IV)NC1. Three-dimensional molecular modeling from existing x-ray coordinates of human NC1 domain suggest that evolutionary alteration of electrostatic charge and polarity due to the emergence of critical serine, aspartic acid, and lysine residues, accompanied by the loss of asparagine and glutamine, contributes to the emergence of the 2 major Goodpasture epitopes on the human alpha3(IV)NC1 domain, as it evolved from the Danio rerio over 450 million years.

Activation of glomerular basement membrane-specific B cells in the renal draining lymph node after T cell-mediated glomerular injury

Linear binding of IgG to the glomerular basement membrane (GBM) is the hallmark of anti-GBM glomerulonephritis (GN). However, the precise mechanism by which diverse autoantibodies to GBM are induced in GN has not been determined. It was demonstrated previously that a single T cell epitope pCol(28-40) derived from collagen IV alpha3 chain not only induced severe GN in Wistar Kyoto rats but also triggered a diversified anti-GBM antibody response through "B cell epitope spreading." In this study, an expansion of T and B cells in the renal draining lymph node (RDLN) of diseased animals after glomerular injury was observed. RDLN was demonstrated to be the location of GBM-specific B cell activation. First, B cells from RDLN of pCol(28-40)-immunized rats produced in vitro anti-GBM antibodies and antinuclear antibodies. Second, B cells specific to the peptidic B cell epitope in pCol(28-40) were absent among expanding B cells in RDLN. Those findings provided a unique opportunity to track activation of diverse GBM-specific B cells in RDLN. Expression of B lymphocyte-induced maturation protein-1, which is involved in differentiation of plasma cells, in B cells of RDLN was detected and further elevated only after T cell-mediated prominent glomerular injury (day 19). This was supported by the fact that anti-GBM antibodies became detectable only after day 20. Those results suggest that T cell-mediated glomerular injury may trigger de novo internal immunization of autoantigens released from damaged GBM, which further leads to activation of a group of GBM-specific B cells in RDLN.

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.

Nasal Administration of Recombinant Rat α3(IV)NC1 Prevents the Development of Experimental Autoimmune Glomerulonephritis in the WKY Rat

Experimental autoimmune glomerulonephritis (EAG), an animal model of Goodpasture's disease, can be induced in Wistar Kyoto (WKY) rats by immunization with either collagenase-solubilized rat glomerular basement membrane (GBM) or the recombinant NC1 domain of the alpha3 chain of type IV collagen [alpha3(IV)NC1]. EAG is characterized by circulating and deposited anti-glomerular basement membrane antibodies, focal necrotizing glomerulonephritis with crescent formation, and glomerular infiltration by T cells and macrophages. Previous studies have demonstrated that oral administration of collagenase-solubilized GBM to WKY rats prevented the development of EAG. Nasal administration of specific autoantigens has been reported to be more effective than oral administration in other models of autoimmune disease. The main aim of this study was to investigate further the concept of mucosal tolerance in EAG by examining the effect of nasal administration of recombinant rat alpha3(IV)NC1. Groups of WKY rats with EAG, induced by immunization with recombinant rat alpha3(IV)NC1, were given alpha3(IV)NC1 nasally on 3 consecutive days before immunization, at total cumulative doses of 25, 100, or 250 microg per rat. A dose-dependent effect was observed on the development of EAG. A dose of 25 microg had no effect on disease; 100 microg resulted in a moderate reduction in the severity of nephritis; and 250 microg led to a marked reduction in circulating and deposited antibodies, albuminuria, severity of glomerular abnormalities, and numbers of glomerular CD8+ T cells and macrophages. In addition, there was a reduction in the proliferative response of splenocytes from rats in the high dose group (250 microg) to alpha3(IV)NC1 in vitro. The results from this study clearly demonstrate for the first time that mucosal tolerance in EAG can be induced by nasal administration of recombinant rat alpha3(IV)NC1 and that this approach is effective in the prevention of crescentic glomerulonephritis. Further work using new antigen-specific treatment strategies may provide a novel approach to the treatment of patients with anti-glomerular basement membrane disease.

Quality of Recombinant Protein Determines the Amount of Autoreactivity Detected against the Tumor-Associated Epithelial Cell Adhesion Molecule Antigen: Low Frequency of Antibodies against the Natural Protein

The human epithelial cell adhesion molecule (EpCAM) is expressed on normal epithelial cells and is overexpressed in most carcinomas. EpCAM-targeted immunotherapy has been tried in several clinical studies. High titers of autoantibodies against EpCAM have been reported by different authors. We have generated large amounts of purified protein in S2 Drosophila cells (S2-EpCAM) with a purity of >96%. In contrast, the protein produced in baculovirus-dependent systems (baculo-EpCAM) that has been used in previous studies shows a purity of 79%. (1)H nuclear magnetic resonance spectrum of S2-EpCAM is typical of folded protein, whereas the baculo-EpCAM sample shows a spectrum corresponding to a partially unfolded protein. Using S2-EpCAM, denatured S2-EpCAM, and baculo-EpCAM, we measured EpCAM Abs of different isotypes in the serum of healthy controls and cancer patients. We found Ab titers against EpCAM in a much lower percentage of sera as published previously, and support the hypothesis that Ab reactivity in some published studies might be due to reactivity against denatured protein, to contaminating proteins in the baculovirus preparations, and to reactivity with BSA. Tetanus toxoid-reactive IgG Abs are present in 1000-fold higher titers compared with EpCAM-reactive Abs. Only IgA Abs were found in higher proportions and in higher concentrations than tetanus toxoid-specific Abs. Our study shows that EpCAM only rarely induces autoantibodies against native protein and emphasizes the importance of using extremely purified Ag preparations when evaluating Abs against tumor-associated Ags.