Experimental anti-GBM glomerulonephritis induced in rats by immunization with synthetic peptides based on six alpha chains of human type IV collagen

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.

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.

Genetic mapping found major QTLs for antibody-induced glomerulonephritis in WKY rats

Genetic bases of glomerulonephritis, a major cause of kidney dysfunction in humans and one of the most characteristic complications of autoimmune disorders such as Goodpasture syndrome, are complex. The Wistar-Kyoto (WKY) rat strain is well characterized for its susceptibility to autoantibodies against glomerular basement membrane (GBM), however the molecular mechanisms underlining the phenotype are largely unknown. Here we performed a whole genome scan using a backcross (BC) F(1) (WKY x DA) x WKY population, for which the DA rat is a nonsusceptible control strain. We found two significant QTLs on chromosomes 1 and 12, which were involved in elevated levels of proteinuria and kidney weight index, respectively. The relevance of these QTLs with the genetic factors involved in autoimmunity and renal disease is discussed.

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.

Autoantibodies that bind glomeruli: cross-reactivity with bacterial antigen

OBJECTIVE

Systemic lupus erythematosus (SLE) is characterized by the production of multiple autoantibodies. Anti-DNA antibodies are associated with glomerulonephritis in SLE. It has been shown that anti-DNA antibodies cross-react with bacterial polysaccharide and, thus, might be elicited by microbial exposure. Non-DNA-binding antibodies also contribute significantly to the pathogenesis of lupus nephritis. The goal of this study was to characterize non-DNA-binding, kidney-binding antibodies.

METHODS

We generated a combinatorial library derived from spleen cells of a patient with SLE who had just previously received pneumococcal vaccine. The phage library was used in an in vivo biopanning technique to identify non-DNA-binding, kidney-binding antibodies. Antibodies were then analyzed for binding to bacterial polysaccharide and to renal antigens.

RESULTS

Eight antibodies were characterized that bound glomeruli, but not DNA. All antibodies isolated by this protocol were IgG class, suggesting that there is affinity maturation for glomerular binding. Four of the antibodies cross-reacted with pneumococcal polysaccharide. Six of the antibodies bound to renal antigens that have previously been reported to be cross-reactive with DNA; the other 2 bound to histone.

CONCLUSION

This study suggests that both DNA-binding and non-DNA-binding antibodies in SLE may be elicited by the same bacterial antigens.

High nephritogenicity of monoclonal antibodies belonging to IgG2a and IgG2b subclasses in rat anti-GBM nephritis

BACKGROUND

To examine a subclass-effect relationship and a dose-effect relationship of autoantibodies in the rat antiglomerular basement membrane (GBM) antibody-induced glomerulonephritis (anti-GBM nephritis) model, we injected homologous monoclonal antibodies against the NC1 domains of rat type IV collagen into inbred Wistar-Kyoto (WKY) rats.

METHODS

Eight different autoantibodies from each of the IgG1, IgG2a, and IgG2b subclasses were established and administered to groups of four WKY rats at a dose of 300 microg/rat. To examine the dose-effect relationship, we administered 0 to 300 microg/rat of autoantibodies from each subclass to rats.

RESULTS

All IgG1 antibodies induced mild nephritis, whereas the IgG2a and IgG2b antibodies induced moderate to severe nephritis. Some IgG2a and IgG2b antibodies induced pulmonary hemorrhage as well. These antibodies were reactive with alpha3(IV)NC1, alpha4(IV)NC1, or alpha5(IV)NC1. The minimum dose of antibody required to induce nephritis was 30 microg/rat for IgG1, 3 microg/rat for IgG2a, and 1 microg/rat for IgG2b. At doses of 30 microg/rat or less, antibody deposition was generally restricted to the GBM. At doses of 100 microg/rat or greater, antibody deposition extended to both the GBM and tubular basement membrane (TBM). Pulmonary hemorrhage was observed only when a large amount of pulmonary hemorrhagic antibody was administered.

CONCLUSION

The severity of nephritis was dependent on both subclass and dose of autoantibodies. It becomes clear that pulmonary hemorrhage in anti-GBM nephritis is induced by autoantibodies.

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.

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

The anti-glomerular basement membrane (GBM) Ab has been regarded as a prototypical example of pathogenic autoantibodies. However, the mechanism for elicitation of this Ab remains unknown. In the present paper, we report that the Ab to diverse GBM Ags was induced by a single nephritogenic T cell epitope in a rat model. The T cell epitope pCol(28-40) of noncollagen domain 1 of collagen type IV alpha3 chain not only uniformly induced severe glomerulonephritis but also elicited anti-GBM Ab in 76% of the immunized rats after prominent glomerular injury. Furthermore, we demonstrated that the anti-GBM Ab was not related to the peptidic B cell epitope nested in pCol(28-40); that is, 1) elimination of the B cell epitope, either by substitution of the critical residues of the B cell epitope or by truncation, failed to abrogate anti-GBM Ab production, and 2) the anti-GBM Ab, eluted from the diseased kidneys, reacted only with native GBM, but not with pCol(28-40). Confocal microscopy and immunoprecipitation further demonstrated that the eluted anti-GBM Ab recognized conformational B cell epitope(s) of multiple native GBM proteins. We conclude that autoantibody response to diverse native GBM Ags was induced by a single nephritogenic T cell epitope. Thus, anti-GBM Ab may actually be a consequence of T cell-mediated 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.

T-cell epitope of alpha3 chain of type IV collagen induces severe glomerulonephritis

BACKGROUND

Anti-glomerular basement membrane (GBM) glomerulonephritis is among the earliest recognized human autoimmune diseases. However, the etiology of anti-GBM glomerulonephritis remains unclear. We have previously shown that CD4+ T cells, specific to alpha3 NC1 of type IV collagen (Col4alpha3NC1), were able to induce anti-GBM glomerulonephritis in Wistar-Kyoto (WKY) rats. In the present study, we continued to map the nephritogenic T cell epitope in Col4alpha3NC1.

METHODS

Synthetic peptides, which covered Col4alpha3NC1, were used as immunogens to induce glomerulonephritis in WKY rats. T-cell and B-cell responses to the peptides in the animals were analyzed.

RESULTS

One potent nephritogenic T-cell epitope, pCol(28-40) (SQTTANPSCPEGT), was identified. A single immunization with pCol(28-40) induced extremely severe glomerulonephritis in all 23 rats. Renal pathology revealed nearly 100% of glomeruli with crescentic lesions or tuft necrosis in 21 animals. pCol(28-40) elicited a T-cell response to the peptide; T cells isolated from rats immunized with recombinant Col4alpha3NC1 reacted with pCol(28-40). pCol(28-40) elicited a peptide specific antibody response, which did not react with polypeptide Col4alpha3NC1 or native GBM. An 11-mer peptide, pCol(a30-40) (Ac-TTANPSCPEGT), was further mapped to be the core of the T-cell epitope in pCol(28-40). As expected, immunization with pCol(a30-40) induced severe glomerulonephritis in 10 out of 19 rats.

CONCLUSION

Our study not only demonstrated that a single T-cell epitope of Col4alpha3NC1 is sufficient to induce severe glomerulonephritis, but also provides a unique model for studying T-cell-mediated mechanisms in anti-GBM glomerulonephritis pathogenesis.

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.

CD4(+) T cells specific to a glomerular basement membrane antigen mediate glomerulonephritis

Ab-mediated mechanisms have been considered the major causes of glomerulonephritis (GN). However, recent studies suggest that T cells may be more important in mediating GN. To investigate the effects of antigen-specific CD4(+) T cells, we generated Th1 cell lines specific for this antigen from rats that had been immunized with a recombinant form of the glomerular basement membrane (GBM) antigen, Col4alpha3NC1. Upon the transfer of in vitro-activated T cell lines to pertussis toxin-primed, naive syngeneic rats, the recipients developed severe proteinuria/albuminuria, which plateaued after approximately 35 days. Although no IgG binding to GBM or C3 deposition could be detected by immunofluorescence, five out of eleven rats exhibited severe GN, as judged by the formation of characteristic crescent-shaped lesions in the glomeruli, whereas the others exhibited modest GN. Thus Col4alpha3NC1-specific T cells directly initiated glomerular injury in the recipients. One notable difference from GN induced by active immunization was a T cell infiltration in the renal interstitium, which affected some tubules. We therefore injected fluorescence-labeled Col4alpha3NC1-specific into naive rats, and we found that they were enriched 4.5-fold in the kidney cortex relative to nonspecific control T cells 24 hours later. Many of the T cells were located in the Bowman's space and had a flattened shape, suggesting that the primary target for the T cells was in or adjacent to the Bowman's capsule.

Glomerulonephritis induced by recombinant collagen IV alpha 3 chain noncollagen domain 1 is not associated with glomerular basement membrane antibody: a potential T cell-mediated mechanism

Glomerulonephritis is believed to result commonly from Ab-mediated glomerular injury. However, Ab-associated mechanisms alone cannot explain many cases of human glomerulonephritis. We developed a rat model of human anti-glomerular basement membrane (GBM) disease to investigate T cell and Ab response, and their associations with the disease. A single immunization of highly denatured recombinant mouse collagen IV alpha3 chain noncollagen domain 1 (rCol4alpha3NC1) induced severe glomerulonephritis in 100% of Wistar Kyoto rats, 33% of which died of this disease around day 35 postimmunization. The renal pathology demonstrated widespread glomerular damage and a mononuclear cell infiltration within the interstitial tissue. T cells from immunized rats responded not only to rCol4alpha3NC1, but also to isolated rat GBM. Sera Abs to rCol4alpha3NC1 were detectable in 100% of the rats, but only 20% of the rats had low levels of Ab to isolated rat GBM by Western blot, and none by immunofluorescence. Furthermore, IgG/M binding to or C3 deposition on endogenous GBM in immunized rats were not detected in most of the experimental rats, and showed no statistical correlation with disease severity. Additionally, no electronic dense deposition in the glomeruli was detected in all rats. Those data revealed a disassociation between the disease and anti-GBM Ab. T cell-mediated mechanisms, which are currently under our investigation, may be responsible for the glomerular disease.

Induction of anti-GBM nephritis in rats by recombinant alpha 3(IV)NC1 and alpha 4(IV)NC1 of type IV collagen

The capability of the noncollagenous (NC1) domains of the six alpha chains of human type IV collagen to induce anti-glomerular basement membrane (GBM) nephritis in WKY rats was determined. This was accomplished by using recombinant technology to express the six NC1 domains in mammalian 293 cells and to purify the proteins using an anti-Flag affinity column. All rats injected with alpha 3(IV)NC1 and alpha 4(IV)NC1 developed proteinuria and hematuria. Rats injected with alpha 5(IV)NC1 developed mild hematuria, whereas rats injected with the alpha 1(IV)NC1, alpha 2(IV)NC1 and alpha 6(IV)NC1 domains developed neither proteinuria nor hematuria. The renal lesions induced by alpha 3(IV)NC1 and alpha 4(IV)NC1 domains were characteristic of those in patients with anti-GBM nephritis and Goodpasture syndrome. The experimental nephritis is mediated by anti-basement membrane antibodies that are targeted to alpha 3(IV)NC1 and alpha 4(IV)NC1 domains and which bind to the glomerular basement membrane. The uniqueness of the alpha 3(IV)NC1 and alpha 4(IV)NC1 domains, among the six NC1 domains, to induce severe anti-GBM disease may relate to the accessibility of epitopes in the GBM for binding of antibody. The pathogenicity of the alpha 4(IV)NC1 antibodies establishes a conundrum because the pathogenic antibodies in patients are not targeted to the alpha 4(IV)NC1, but are targeted to the alpha 3(IV)NC1 domain in anti-GBM nephritis and to the alpha 3(IV)NC1 and alpha 5(IV)NC1 domains in Alport post-transplant anti-GBM nephritis.

Purification and characterization of human nephritogenic antigen that induces anti-GBM nephritis in rats

Human nephritogenic antigen induces anti-glomerular basement membrane antibody glomerulonephritis in rats. This antigen was purified from collagenase-solubilized renal basement membrane by means of gel filtration and affinity chromatography using a rabbit antibody. Western blots of the purified nephritogenic antigen using epitope-defined monoclonal antibodies showed that it contains the NC1 domains of the a1 to a6 chains of type IV collagen. Nephritogenicity was thought to be a feature of the NC1 domains of the a3 to a5 chains, because the a6 chain is not located in the glomerular basement membrane, and because an NC1 fraction consisting of the NC1 domains of the a1 and a2 chains was poorly nephritogenic. Autoantibodies in the sera of patients with Goodpasture's syndrome were detected by ELISA using the purified nephritogenic antigen. These results indicate that the nephritogenic antigen contains the Goodpasture antigen, defined as the antigen reactive with sera from patients with Goodpasture's syndrome.