Antibodies against linear epitopes on the Goodpasture autoantigen and kidney injury

Three Diseases Mediated by Different Immunopathologic Mechanisms-ANCA-Associated Vasculitis, Anti-Glomerular Basement Membrane Disease, and Immune Complex-Mediated Glomerulonephritis-A Common Clinical and Histopathologic Picture: Rapidly Progressive Crescentic Glomerulonephritis

Immune mechanisms play an important role in the pathogenesis of glomerulonephritis (GN), with autoimmunity being the main underlying pathogenetic process of both primary and secondary GN. We present three autoimmune diseases mediated by different autoimmune mechanisms: glomerulonephritis in vasculitis mediated by anti-neutrophil cytoplasmic antibodies (ANCAs), glomerulonephritis mediated by anti-glomerular basement membrane antibodies (anti-GBM antibodies), and immune complex-mediated glomerulonephritis. Some of these diseases represent a common clinical and histopathologic scenario, namely rapidly progressive crescentic glomerulonephritis. This is a severe illness requiring complex therapy, with the main role being played by therapy aimed at targeting immune mechanisms. In the absence of immune therapy, the crescents, the characteristic histopathologic lesions of this common presentation, progress toward fibrosis, which is accompanied by end-stage renal disease (ESRD). The fact that three diseases mediated by different immunopathologic mechanisms have a common clinical and histopathologic picture reveals the complexity of the relationship between immunopathologic mechanisms and their clinical expression. Whereas most glomerular diseases progress by a slow process of sclerosis and fibrosis, the glomerular diseases accompanied by glomerular crescent formation can progress, if untreated, in a couple of months into whole-nephron glomerulosclerosis and fibrosis. The outcome of different immune processes in a common clinical and histopathologic phenotype reveals the complexity of the relationship of the kidney with the immune system. The aim of this review is to present different immune processes that lead to a common clinical and histopathologic phenotype, such as rapidly progressive crescentic glomerulonephritis.

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.

Anti-glomerular basement membrane disease (Goodpasture disease): From pathogenesis to plasma exchange to IdeS

Anti-glomerular basement membrane (GBM) disease (Goodpasture disease) often presents with severe kidney failure and pulmonary hemorrhage. Anti-GBM antibodies are pathogenic, and other autoantibodies such as laminin-521 have been identified recently, potentially indicating a subset with a more severe disease phenotype and poor prognosis. Around 30%-40% of patients are also anti-neutrophil cytoplasmatic antibody (ANCA)-positive and this subset combines features of anti-GBM disease and ANCA-associated vasculitis, with particular impact on long-term treatment. A combination of therapeutic plasma exchange (or immunoadsorption), cyclophosphamide, and glucocorticoids is considered standard of care management, but despite early initiation, patients with poor prognostic factors often remain dialysis dependent. Imlifidase (IdeS), capable to cleave IgG within hours, has been tested in a phase II trial. Among 15 patients, 10 with poor prognosis at baseline (eGFR <15 ml/min/1.73 m² ) were dialysis independent at 6 months. Further developments are needed to refine treatment approaches in anti-GBM disease.

Atypical anti-glomerular basement membrane disease complicated by methicillin-susceptible Staphylococcus aureus infection-related rapidly progressive glomerulonephritis: a case report and literature review

Atypical anti-glomerular basement membrane (GBM) disease, which is characterized by low levels of or negativity for anti-GBM antibodies in circulation but positivity in the kidney, has been recognized in this decade. However, a therapeutic strategy has not been established to date because its outcome is better than that of classic anti-GBM disease. This case report and literature review highlight atypical anti-GBM disease in infection-related rapidly progressive glomerulonephritis. A 72-year-old Japanese man diagnosed with methicillin-susceptible Staphylococcus aureus (MSSA)-induced vertebral osteomyelitis experienced for 2 months was referred to our hospital because of renal insufficiency. He developed rapidly progressive glomerulonephritis with a serum creatinine level of 6.8 mg/dL, C-reactive protein level of 9.7 mg/dL, urinary protein-to-creatinine ratio of 3.37 g/gCr, and gross hematuria. The serum anti-GBM antibody concentration was 3.5 U/mL, which was slightly above the normal range (< 3.0 U/mL). Conservative treatment, mainly with antibiotics, improved the symptoms and renal function. The serum anti-GBM antibody concentration peaked at 4.0 U/mL on day 7 and decreased to an undetectable range at the end of eight-week antibiotic therapy. This is the first case report describing the presentation and disappearance of serum anti-GBM antibody in a patient with MSSA infection. Conservative treatment may be effective for patients with atypical anti-GBM disease complicated by infectious diseases.

Experimental Antiglomerular Basement Membrane GN Induced by a Peptide from Actinomyces

BACKGROUND

Antiglomerular basement membrane (anti-GBM) disease is associated with HLA-DRB1*1501 (the major predisposing genetic factor in the disease), with α3_127-148 as a nephritogenic T and B cell epitope. Although the cause of disease remains unclear, the association of infections with anti-GBM disease has been long suspected.

METHODS

To investigate whether microbes might activate autoreactive T and B lymphocytes via molecular mimicry in anti-GBM disease, we used bioinformatic tools, including BLAST, SYFPEITHI, and ABCpred, for peptide searching and epitope prediction. We used sera from patients with anti-GBM disease to assess peptides recognized by antibodies, and immunized WKY rats and a humanized mouse model (HLA-DR15 transgenic mice) with each of the peptide candidates to assess pathogenicity.

RESULTS

On the basis of the critical motif, the bioinformatic approach identified 36 microbial peptides that mimic human α3_127-148. Circulating antibodies in sera from patients with anti-GBM recognized nine of them. One peptide, B7, derived from Actinomyces species, induced proteinuria, linear IgG deposition on the GBM, and crescent formation when injected into WKY rats. The antibodies to B7 also targeted human and rat α3_127-148. B7 induced T cell activation from human α3_127-148-immunized rats. T cell responses to B7 were detected in rats immunized by Actinomyces lysate proteins or recombinant proteins. We confirmed B7's pathogenicity in HLA-DR15 transgenic mice that developed kidney injury similar to that observed in α3_135-145-immunized mice.

CONCLUSIONS

Sera from patients with anti-GBM disease recognized microbial peptides identified through a bioinformatic approach, and a peptide from Actinomyces induced experimental anti-GBM GN by T and B cell crossreactivity. These studies demonstrate that anti-GBM disease may be initiated by immunization with a microbial peptide.

Clinicopathological features and outcome of antibody-negative anti-glomerular basement membrane disease

AIM

To explore the clinicopathological characteristics of patients with anti-GBM antibody-negative anti-GBM disease.

METHODS

The clinical and renal pathological findings were retrospectively studied in 19 patients. All patients met the following inclusion criteria: linear GBM IgG deposition on immunofluorescence(IF); and lack of serum anti-GBM antibodies by ELISA and indirect immunofluorescence assay.

RESULTS

There were 17 male and two female patients, with a median age of 36 years (range 15-61 years). Hypertension was present in 68% of cases, nephrotic-range proteinuria (> 3.5 g/24 hours) in 42%, nephrotic syndrome in 37%, microhaematuria in 95%, renal insufficiency in 63% and lung involvement in 16%. On biopsy all patients had linear GBM staining for polyclonal IgG by IF. The dominant IgG subtype was IgG4 or IgG1. By light microscopy, mesangial proliferative GN without crescents was seen in four patients; proliferative GN (mesangial proliferative GN in eight; endocapillary proliferative GN in two; and membranoproliferative GN in two) with crescents (focal in 11; diffuse in one) in 12 patients; and crescentic GN without mesangial or endocapillary proliferative or membranoproliferative changes in three patients. By electron microscopy, six patients showed scarce electron dense deposits in glomeruli and 11 patients had global podocyte effacement. Totally, 10 (53%) patients received immunosuppressive therapy. The median follow-up was 15 months and six (32%) patients progressed to end-stage renal disease.

CONCLUSIONS

Anti-GBM antibody-negative anti-GBM disease was different from classic anti-GBM disease clinically and pathologically. The pathogenesis of the renal injury in these patients has not been elucidated until now and it should be studied and identified further.

Fever and prodromal infections in anti-glomerular basement membrane disease

AIM

Anti-glomerular basement membrane (GBM) disease is an autoimmune disorder with rapidly progressive glomerulonephritis and alveolar haemorrhage. Fever symptoms and prodromal infections have been reported in many cases, but still not been elucidated.

METHODS

Our study enrolled 140 consecutive patients with anti-GBM disease and retrospectively analyzed the characteristics of fever symptoms and the possible reasons.

RESULTS

Among the 140 patients, 94 (67.1%) patients presented with fever (over 37.5°C) prior to admission or within 48 h of hospitalization. Among those with fever, 74 (78.7%) patients had infections, 15 (16.0%) patients had positive serum anti-neutrophil cytoplasmic antibodies, all towards myeloperoxidase, which was comparable to the patients without fever (17.4%, P = 0.830). There were 93/140 patients suffered from infections, with 47.3% in lungs and 31.2% on upper respiratory tract. In some cases, we identified the microbes of infections, including Candida albicans, Escherichia coli, Acinetobacter baumannii, Enterococcus faecalis, Klebsiella pneumoniae, Hemolytic staphylococci, Pseudomonas aeruginosa and Citrobacter braakii. Patients with fever had higher levels of serum anti-GBM antibodies (154.9 ± 58.4 vs. 106.0 ± 63.2 IU/mL, P < 0.001), higher serum creatinine (733.4 ± 402.5 vs. 580.6 ± 368.1 μmol/L, P = 0.032), higher percentage of crescents (87.0 ± 15.6 vs. 67.4 ± 37.6%, P = 0.021), and higher frequency of progression to end stage renal disease (ESRD) (80.9% vs. 60.9%, P = 0.011).

CONCLUSION

We concluded that fever is a common symptom in anti-GBM disease and associates with more severe glomerulonephritis. The majority of patients at presentation had fever with respiratory tract infections, which needs further investigation to reveal their role in the pathogenesis of anti-GBM disease.

T cell responses to peptides of Goodpasture autoantigen in patients with anti-glomerular basement membrane disease

AIM

Cell-mediated autoimmunity, especially autoreactive T cells, is crucial in the initiation of anti-glomerular membrane (GBM) disease. Epitopes for T cells on Goodpasture autoantigen are not fully defined. This study investigated T cell epitopes in anti-GBM patients, aiming to identify the epitopes and their clinical significance.

METHODS

Peripheral blood mononuclear cells (PBMC) were collected from 13 patients with anti-GBM disease. Twenty-four overlapping linear peptides were synthesized covering the whole sequence of human α3(IV)NC1. PBMC response to each peptide was detected by proliferation assay. Their associations with clinical features were further analyzed.

RESULTS

Peripheral blood mononuclear cells proliferative responses to linear peptides on α3(IV)NC1 could be detected in all patients. Five major epitopes were identified as stimulatory in over half of the patients: α3(IV)NC1_127-148 (P14) (69.2%), α3(IV)NC1_159-178 (77.8%), α3(IV)NC1_179-198 (55.6%), α3(IV)NC1_189-208 (P19) (75.0%) and α3(IV)NC1_141-154 (57.1%). P14 and P19 were highly recognized in patients comparing with healthy controls (69.2% vs. 0.0%, P = 0.011; 75.0% vs. 0.0%, P = 0.021, respectively).

CONCLUSION

T cell proliferation to linear epitopes was detected in human anti-GBM disease. α3_127-148 was a mutual T and B cell epitope, implying its initial role in epitope spreading process.

Goodpasture's autoimmune disease - A collagen IV disorder

Goodpasture's (GP) disease is an autoimmune disorder characterized by the deposition of pathogenic autoantibodies in basement membranes of kidney and lung eliciting rapidly progressive glomerulonephritis and pulmonary hemorrhage. The principal autoantigen is the α345 network of collagen IV, which expression is restricted to target tissues. Recent discoveries include a key role of chloride and bromide for network assembly, a novel posttranslational modification of the antigen, a sulfilimine bond that crosslinks the antigen, and the mechanistic role of HLA in genetic susceptibility and resistance to GP disease. These advances provide further insights into molecular mechanisms of initiation and progression of GP disease and serve as a basis for developing of novel diagnostic tools and therapies for treatment of Goodpasture's disease.

A New Classification System for IgG4 Autoantibodies

IgG4 autoimmune diseases are characterized by the presence of antigen-specific autoantibodies of the IgG4 subclass and contain well-characterized diseases such as muscle-specific kinase myasthenia gravis, pemphigus, and thrombotic thrombocytopenic purpura. In recent years, several new diseases were identified, and by now 14 antigens targeted by IgG4 autoantibodies have been described. The IgG4 subclass is considered immunologically inert and functionally monovalent due to structural differences compared to other IgG subclasses. IgG4 usually arises after chronic exposure to antigen and competes with other antibody species, thus "blocking" their pathogenic effector mechanisms. Accordingly, in the context of IgG4 autoimmunity, the pathogenicity of IgG4 is associated with blocking of enzymatic activity or protein-protein interactions of the target antigen. Pathogenicity of IgG4 autoantibodies has not yet been systematically analyzed in IgG4 autoimmune diseases. Here, we establish a modified classification system based on Witebsky's postulates to determine IgG4 pathogenicity in IgG4 autoimmune diseases, review characteristics and pathogenic mechanisms of IgG4 in these disorders, and also investigate the contribution of other antibody entities to pathophysiology by additional mechanisms. As a result, three classes of IgG4 autoimmune diseases emerge: class I where IgG4 pathogenicity is validated by the use of subclass-specific autoantibodies in animal models and/or in vitro models of pathogenicity; class II where IgG4 pathogenicity is highly suspected but lack validation by the use of subclass specific antibodies in in vitro models of pathogenicity or animal models; and class III with insufficient data or a pathogenic mechanism associated with multivalent antigen binding. Five out of the 14 IgG4 antigens were validated as class I, five as class II, and four as class III. Antibodies of other IgG subclasses or immunoglobulin classes were present in several diseases and could contribute additional pathogenic mechanisms.

The critical amino acids of a nephritogenic epitope on human Goodpasture autoantigen for binding to HLA-DRB1*1501

BACKGROUND

Anti-GBM disease is caused by autoimmunity to Goodpasture antigen on α3(IV)NC1 and had strong associations with HLA-DRB1*1501. Previous studies identified α3_127-148 (P14: TDIPPCPHGWISLWKGFSFIMF) as a T cell epitope. The present study was aimed to investigate the binding capacity of P14 to HLA-DRB1*1501 and the critical amino acids for this binding.

METHODS

A line of EBV-transformed human B cells homozygous for HLA-DRB1*1501 was used to detect the binding capacity of peptides to HLA-DRB1*1501 using flow cytometry analysis. P14 was sequentially truncated into 8 peptides with 15 amino acids to identify the core binding motif. A set of alanine substituted peptides of P14-2 was then synthesized to identify its critical residues for binding to HLA-DRB1*1501. The structure of HLA-DR2b-Peptide-TCR complex was constructed by modeling to analyze the interaction of each amino acids of P14-2 with the HLA-DR2b molecule.

RESULTS

P14 could bind to HLA-DRB1*1501 expressed on B cell surface. The N-terminus of P14 was the core binding motif and the truncated peptide P14-2 (DIPPCPHGWISLWKG) _128-142 had the strongest binding capacity. After sequential amino acid substitution, we found the binding capacity of P14-2 was completely lost by the substitution of cysteine (C) ₁₃₂ and significantly decreased by the substitution of tryptophan (W) ₁₃₆, lysine (K) ₁₄₁, or glycine (G) ₁₄₂, but still at a high level. The modeling showed that (C) ₁₃₂ had a strong interaction with pocket 4 on the β chain of DR2b. Thus, C₁₃₂, W ₁₃₆, K₁₄₁, and G₁₄₂ were defined as the critical amino acid residues for the binding capacity of P14 to HLA-DRB1*1501.

CONCLUSION

We identified α3_128-142 (DIPPCPHGWISLWKG) as the core binding motif of P14 to HLA-DRB1*1501 molecule. And the critical amino acid residues for this binding were further defined as C₁₃₂, W ₁₃₆, K ₁₄₁, and G ₁₄₂.

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 susceptible HLA class II alleles and their presenting epitope(s) in Goodpasture's disease

Goodpasture's disease is closely associated with HLA, particularly DRB1*1501. Other susceptible or protective HLA alleles are not clearly elucidated. The presentation models of epitopes by susceptible HLA alleles are also unclear. We genotyped 140 Chinese patients and 599 controls for four-digit HLA II genes, and extracted the encoding sequences from the IMGT/HLA database. T-cell epitopes of α3(IV)NC1 were predicted and the structures of DR molecule-peptide-T-cell receptor were constructed. We confirmed DRB1*1501 (OR = 4·6, P = 5·7 × 10^-28 ) to be a risk allele for Goodpasture's disease. Arginine at position 13 (ARG13) (OR = 4·0, P = 1·0 × 10^-17 ) and proline at position 11 (PRO11) (OR = 4·0, P = 2·0 × 10^-17 ) on DRβ1, encoded by DRB1*1501, were associated with disease susceptibility. α_134-148 (HGWISLWKGFSFIMF) was predicted as a T-cell epitope presented by DRB1*1501. Isoleucine₁₃₇ , tryptophan₁₄₀ , glycine₁₄₂ , phenylalanine₁₄₃ and phenylalanine₁₄₅ , were presented in peptide-binding pockets 1, 4, 6, 7 and 9 of DR2b, respectively. ARG13 in pocket 4 interacts with tryptophan₁₄₀ and forms a hydrogen bond. In conclusion, we propose a mechanism for DRB1*1501 susceptibility for Goodpasture's disease through encoding ARG13 and PRO11 on MHC-DRβ1 chain and presenting T-cell epitope, α_134-148 , with five critical residues.

Plasma from patients with anti-glomerular basement membrane disease could recognize microbial peptides

Infection has long been suspected as a trigger of autoimmune diseases, and molecular mimicry mechanism was hypothesized in this study. Microbe originated peptides were searched from the Uniprot database based on a previous defined critical amino acid motif within α3_129−150, isoleucine137, tryptophan140, glycine142, phenylalanine 143 and phenylalanine 145. 23826 microbial peptides were identified using our searching strategy, among which seven were related with human infections. Circulating IgG and IgM antibodies against the seven microbial peptides were detected using ELISA in 76 patients with anti-GBM disease. Four peptides were recognized by both IgG and IgM antibodies, and one peptide was recognized by IgG antibodies only. Peptides from Bacteroides, Saccharomyces cerevisiae, and Bifidobacterium thermophilum possessed the highest recognition frequency with the prevalence of 73.7%, 61.8% and 67.1% for IgG, 56.6%, 44.7% and 67.1% for IgM in anti-GBM patients. Patients with antibodies against these microbial peptides showed more severe kidney injury, including higher serum creatinine and higher percentage of crescent formation. In conclusion, antibodies against microbial peptides were identified in the circulation of anti-GBM patients, implying its etiological role in eliciting autoimmune response against α3(IV)NC1 through molecular mimicry.

Clinicopathological characteristics of typical and atypical anti-glomerular basement membrane nephritis

Anti-glomerular basement membrane (GBM) antibody disease is a rare pathological condition that mainly involves renal and/or pulmonary parenchyma. It is characterized by the presence of circulating anti-GBM antibodies accompanied by a linear deposition of immunoglobulins (Ig) detected through immunofluorescence (IF) technique and typical signs and symptoms of organ dysfunction, such as rapidly progressive glomerulonephritis (RPGN) and pulmonary hemorrhage (PH). However, recently atypical forms of anti-GBM disease have been described and the presence of overlapping diseases contributed to make its diagnosis challenging. In this review will be discussed the entire spectrum of renal anti-GBM related conditions, focusing the attention on the differences in terms of pathogenesis, diagnosis and therapy of these disparate entities.

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.

The clinicopathologic characteristics and outcome of atypical anti-glomerular basement membrane nephritis

Classic anti-glomerular basement membrane (GBM) disease presents with rapidly progressive glomerulonephritis (GN) with or without pulmonary hemorrhage. On biopsy typical disease displays bright polytypic linear GBM staining for IgG by immunofluorescence and diffuse crescentic/necrotizing GN on light microscopy. Here, we studied 20 patients with atypical anti-GBM nephritis typified by bright linear GBM staining for immunoglobulins but without a diffuse crescentic phenotype. Patients had hematuria, proteinuria, and mild renal insufficiency, without pulmonary hemorrhage. Light microscopy showed endocapillary proliferative GN in 9 patients, mesangial proliferative GN in 6, membranoproliferative GN in 3, and focal segmental glomerulosclerosis with mesangial hypercellularity in 2. Eight of the 20 showed features of microangiopathy. Crescents/necrosis were absent in 12 and were focal in 8 patients. Bright linear GBM staining for IgG was seen in 17 patients, IgM in 2, and IgA in 1 patient, which was polytypic in 10 patients and monotypic in 10 patients. No circulating α3NC1 antibodies were detected by commercial ELISA. The 1-year patient and renal survival rates were 93% and 85%, respectively. Thus, atypical anti-GBM nephritis is a rare variant of anti-GBM disease characterized clinically by an indolent course, no pulmonary involvement, and undetectable circulating α3NC1 antibodies. Further studies are needed to characterize the molecular architecture of GBM autoantigens in these patients and establish optimal therapy.

Autoantibodies against Linear Epitopes of Myeloperoxidase in Anti-Glomerular Basement Membrane Disease

BACKGROUND AND OBJECTIVES

Approximately 20%-30% of patients with anti-glomerular basement membrane disease present coexisting anti-myeloperoxidase (MPO) autoantibodies. We previously showed the recognition of a linear fragment of the MPO heavy chain N-terminus ((1)H, MPO279-409) in plasma from most double-positive patients. Herein, we investigated the frequency of autoantibodies against overlapping (1)H-derived linear peptides in plasma from patients with anti-glomerular basement membrane disease.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We synthesized 13 overlapping linear peptides ((1)H-1 to (1)H-13) covering MPO279-409. We retrospectively collected plasma samples from 67 patients with anti-glomerular basement membrane disease from 1996 to 2012, and we screened them for IgG autoantibodies by ELISA using intact human MPO and the overlapping peptides as antigens, and we further investigated the clinical significance. Autoantibody binding to the linear MPO structure was confirmed by Western blotting.

RESULTS

We followed up the 67 patients until 2015, with a median follow-up time of 10.0 (2.3-36.0) months, and 56 ESRD events occurred among the 67 patients with follow-up data. Plasma from 23.9% (16) of the patients recognized intact human MPO, whereas 62.7% (42) plasma samples recognized MPO279-409 linear peptides. Of the 13 linear peptides, (1)H-4 (44.8%, 30 patients) and (1)H-12 (40.3%, 27 patients) exhibited the highest recognition frequencies. Patients with autoantibodies against (1)H-11 or (1)H-12 (MPO371-400) were older (46.1±18.8 versus 34.1±16.6 years; P<0.01), had higher serum creatinine upon diagnosis (median 7.8 mg/dl, interquartile range 4.9-12.6 mg/dl versus median 5.4 mg/dl, interquartile range 2.4-7.3 mg/dl; P=0.02), and had a higher probability of progressing to ESRD; however, multivariate Cox regression analysis showed that (1)H-11 or 12 reaction was not an independent risk factor for renal failure (hazard ratio, 1.2; 95% confidence interval, 0.8 to 2.8; P=0.19).

CONCLUSIONS

Autoantibodies against linear peptides of MPO can be detected in the majority of patients with anti-glomerular basement membrane disease, and several are associated with disease severity. The potential common pathogenic mechanism between anti-glomerular basement membrane antibodies and anti-MPO autoantibodies in anti-glomerular basement membrane disease requires further investigation.

Coexistence of Anti-Glomerular Basement Membrane Antibodies and Anti-Neutrophil Cytoplasmic Antibodies in a Child With Human Leukocyte Antigen Susceptibility and Detailed Antibody Description: A Case Report

Anti-glomerular basement membrane (anti-GBM) disease and anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis both could cause rapidly progressive glomerulonephritis. The coexistence of ANCAs and anti-GBM antibodies was known as "double positive," which was extremely rare in children. We report a pediatric case with coexistence of ANCAs and anti-GBM antibodies. A 6-year-old girl presented with acute renal failure, hematuria, proteinuria, and oliguria. She was double positive of ANCAs specific to myeloperoxidase, and anti-GBM antibodies. Kidney biopsy confirmed linear immunoglobulin (Ig)G deposit along GBM and 100% of crescent formation in glomeruli; among them 83.3% were cellular crescents. Human leukocyte antigen (HLA) gene typing showed DRB1*1501, an allele strongly associated with anti-GBM disease, and DRB1*0405, an independent risk factor for renal failure in patients with ANCA-associated vasculitis. The titer of anti-GBM antibodies was 1:800, and the predominant IgG subclass was IgG1, which was closely related with severe kidney injury and worse outcome. The target antigen of anti-GBM antibodies was restricted on the noncollagen domain 1 of the α3 chain of type IV collagen (α3[IV]NC1), with recognitions to both epitopes, EA (α317-31) and EB (α3127-141). This is the first reported pediatric case with coexistence of ANCAs and anti-GBM antibodies, in which the HLA typing and immunologic characters of autoantibodies were identified. The findings on this early-onset patient are meaningful for understanding the mechanisms of both anti-GBM disease and ANCA-associated vasculitis.

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.

Vasculitis: determinants of disease patterns

The vasculitides are a large group of heterogeneous diseases for which it has been assumed that pathogenesis is largely autoimmune. As clinicians, we distinguish one form of vasculitis from another on the basis of observed patterns of organ injury, the size of the vessels affected and histopathological findings. The terms 'small-vessel', 'medium-vessel' and 'large-vessel' vasculitis are useful clinical descriptors, but fail to inform us about why vessels of a certain calibre are favoured by one disease and not another. Classification based on vessel size also fails to consider that vessels of a specific calibre are not equally prone to injury. Distinct vulnerabilities undoubtedly relate to the fact that same-size vessels in different tissues may not be identical conduits. In fact, vessels become specialized, from the earliest stages of embryonic development, to suit the needs of different anatomical locations. Vessels of the same calibre in different locations and organs are as different as the organ parenchymal cells through which they travel. The dialogue between developing vessels and the tissues they perfuse is designed to meet special local needs. Added to the story of vascular diversity and vulnerability are changes that occur during growth, development and ageing. An improved understanding of the unique territorial vulnerabilities of vessels could form the basis of new hypotheses for the aetiopathogenesis of the vasculitides. This Review considers how certain antigens, including infectious agents, might become disease-relevant and how vascular diversity could influence disease phenotypes and the spectrum of vascular inflammatory diseases.

Principles of separation: indications and therapeutic targets for plasma exchange

Extracorporeal "blood purification," mainly in the form of hemodialysis has been a major portion of the clinical activity of many nephrologists for the past 5 decades. A possibly older procedure, therapeutic plasma exchange, separates and then removes plasma as a method of removing pathogenic material from the patient. In contrast to hemodialysis, therapeutic plasma exchange preferentially removes biologic substances of high molecular weight such as autoantibodies or alloantibodies, antigen-antibody complexes, and Ig paraproteins. These molecular targets may be cleared through two alternative procedures: centrifugal separation and membrane separation. This review presents operational features of each procedure, with relevance to the nephrologist. Kinetics of removal of these plasma constituents are based on the principles of separation by the apheresis technique and by features specific to each molecular target, including their production and compartmentalization in the body. Molecular targets for common renal conditions requiring therapeutic plasma exchange are also discussed in detail.