Tolerance and Autoimmunity in Anti-GBM Disease

The anti-PLA2R antibody in membranous nephropathy: what we know and what remains a decade after its discovery

The discovery in 2009 of the M-type phospholipase A2 receptor (PLA2R) as the primary target in membranous nephropathy (MN) greatly advanced basic and clinical research. Primary MN is now considered a renal-limited autoimmune disease, with antibodies against PLA2R (aPLA2Rab) identified in 70-80 % of patients of various ethnic groups. Although the use of aPLA2Rab as a diagnostic and prognostic biomarker is now widely accepted, many questions related to the development of the auto-immune response, the role of IgG subclasses and antigenic epitopes, and the pathways to podocyte injury remain unresolved. PLA2R-associated MN most likely develops governed by factors such as genetic susceptibility, loss of tolerance, alterations in antigen expression with a role for environmental factors like air pollution, smoking, and infections. More detailed knowledge of genetic factors, the relevant B- and T-cell epitopes, and the mechanisms of podocyte injury is needed to identify patients at risk for disease progression and to develop optimized, targeted treatment strategies. In this review we highlight unresolved issues, addressing initiation of antibody formation, the timeline of antibody production, the role of IgG subclass, and the pathogenicity of the antibodies in concert with complement to produce glomerular pathology and proteinuria.

Atypical anti-glomerular basement membrane disease

Background

Anti-glomerular basement membrane (anti-GBM) disease classically presents with aggressive necrotizing and crescentic glomerulonephritis, often with pulmonary hemorrhage. The pathologic hallmark is linear staining of GBMs for deposited immunoglobulin G (IgG), usually accompanied by serum autoantibodies to the collagen IV alpha-3 constituents of GBMs.

Methods

Renal pathology files were searched for cases with linear anti-GBM to identify cases with atypical or indolent course. Histopathology, laboratory studies, treatment and outcome of those cases was reviewed in detail.

Results

Five anti-GBM cases with atypical clinicopathologic features were identified (accounting for ∼8% of anti-GBM cases in our laboratory). Kidney biopsies showed minimal glomerular changes by light microscopy; one patient had monoclonal IgG deposits in an allograft (likely recurrent). Three patients did not have detectable serum anti-GBM by conventional assays. Three patients had indolent clinical courses after immunosuppressive treatment. One patient, untreated after presenting with brief mild hematuria, re-presented after a short interval with necrotizing and crescentic glomerulonephritis.

Conclusions

Thorough clinicopathologic characterization and close follow-up of patients with findings of atypical anti-GBM on renal biopsy are needed. Review of the literature reveals only rare well-documented atypical anti-GBM cases to date, only one of which progressed to end-stage kidney disease.

Performance evaluation of a novel chemiluminescence assay for detection of anti-GBM antibodies: an international multicenter study

BACKGROUND

Autoantibodies to the non-collagen region (NC1) of the alpha-3 subunit of collagen IV represent a serological hallmark in the diagnosis of Goodpasture's syndrome (GPS). The objective of our study was to carefully analyze the performance characteristics of a novel anti-glomerular basement membrane (GBM) chemiluminescence immunoassay (CIA).

METHODS

Sera from patients with GPS (n = 90) were collected from four clinical centers. Samples from different disease groups (n = 397) and healthy individuals (n = 400) were used as controls. All samples were tested for anti-GBM antibodies by a rapid, random access CIA (QUANTA Flash™ GBM). Most of the samples were also tested using other methods including different commercial anti-GBM IgG assays and research assays for anti-GBM IgA and IgM.

RESULTS

The sensitivity and specificity of the novel CIA was 95.6% [95% confidence interval (CI) 89.0-98.8%] and 99.6% (95% CI 98.9-99.9%), respectively. Receiver operating characteristic analysis showed good discrimination between GPS patients and controls. The area under the curve was 0.98 (CI 0.96-1.0). The three anti-GBM antibody-positive samples from the control group were from two healthy individuals and one human immunodeficiency virus (HIV)-infected patient. All three individuals had low levels of anti-GBM antibodies [20, 24 and 25 chemiluminescent unit (CU), cutoff 20 CU]. When the results of the new CIA were compared to other methods, good agreement was observed: 95.8% (kappa = 0.92) versus EliA™ GBM, 97.4% (kappa = 0.95) versus both BINDAZYME™ Anti-GBM and QUANTA Lite® GBM. Anti-GBM IgA was detectable in low concentrations in patients with GPS and was associated with anti-GBM IgG but was less useful in discriminating GPS patients and controls. No discrimination was found for anti-GBM IgM.

CONCLUSION

The novel QUANTA Flash™ GBM CIA demonstrated good sensitivity and specificity and had good agreement with other methods. Our data confirm that ∼5% of patients with GPS do not have detectable levels of anti-GBM antibodies.

An asthmatic adolescent with hematuria and hemoptysis

When alveolar hemorrhage occurs in association with glomerulonephritis, one of the leading causes is Goodpasture syndrome. We report a case of an adolescent who presented to the emergency department in respiratory distress. Subsequent evaluation was consistent with the diagnosis of Goodpasture syndrome.

Natural autoantibodies against glomerular basement membrane exist in normal human sera

Autoimmunity to glomerular basement membrane (GBM) could induce Goodpasture disease, and natural autoantibodies against GBM in the sera of normal individuals were not reported. The aim of the study was to identify and characterize natural autoantibodies against GBM in normal human sera. Natural anti-GBM autoantibodies were purified from the sera of five healthy persons by affinity chromatography, using purified bovine alpha(IV)non-collagenous (NC1) as solid-phase ligands. Antigen specificity, immunoglobulin G (IgG) subclasses, and antibody avidity of the natural autoantibodies were investigated by enzyme-linked immunosorbant assay (ELISA), Western-blot analysis, indirect immunofluorescence, and antigen-inhibition ELISA, and compared with those of 32 patients with anti-GBM disease. Natural anti-GBM autoantibodies could be purified from IgG fractions of all the five persons, with an average amount of 0.5% of total IgG fractions. Antigen specificity of the natural autoantibodies was identified by blotting to human alpha(IV)NC1, reactivity to recombinant alpha3(IV)NC1, and linear staining along the GBM of normal kidney sections. Titers of the natural autoantibodies were much lower than those of patients (1:60.6 vs 1:993.6, P<0.001). IgG subclasses distribution of the natural autoantibodies was restricted to IgG2 (100%) and IgG4 (100%), while for patients it was mainly IgG1 (93.8%) and IgG4 (90.6%). Avidity of the natural autoantibodies was lower than that of patients, the amount of alpha(IV)NC1 used for 50% inhibition was 1.65 and 0.46 microg, respectively (P<0.05). In conclusion, natural anti-GBM autoantibodies exist in normal human sera. Antibody levels, IgG subclasses, and avidity of the natural autoantibodies were different from those of patients. Fine specificity of the natural autoantibodies needed to be elucidated.