American Journal of Kidney Diseases
Volume 55, Issue 5 , Pages 820-823, May 2010

First Identification of an Antigen in Autoimmune Idiopathic Membranous Nephropathy: Toward Targeted Therapy?

  • Pierre Ronco, MD, PhD

      Affiliations

    • Corresponding Author InformationAddress correspondence to Pierre Ronco, MD, PhD, Service de Néphrologie et Dialyses, Hôpital Tenon, 4 rue de la Chine, F-75020, Paris, France
    • ,
  • Hanna Debiec, PhD

UPMC University Paris 06, INSERM, UMR_S 702, Hôpital Tenon, Paris, France

published online 18 November 2009.

Article Outline

 

Commentary on Beck LH Jr, Bonegio RG, Lambeau G, et al. M-Type phospholipase A2 receptor as target antigen in idiopathic membranous nephropathy. N Engl J Med 2009;361(1):11-21.

Membranous nephropathy (MN) is the most common cause of idiopathic nephrotic syndrome in white adults, accounting for ∼20% of cases. Although spontaneous remission occurs in about one-third of patients, ∼40% of patients develop kidney failure after 10 years.1, 2 Eighty percent of cases are classified as “idiopathic,” to conceal our ignorance about causes, whereas ∼20% present with associated clinical conditions, including infections, autoimmune diseases, and cancers, and are classified as having secondary disease. It generally is considered that idiopathic MN is an autoimmune disease, whereas secondary forms involve exogenous antigens, such as viral and tumoral antigens.

MN is characterized by the accumulation of immune deposits on the outer aspect of the glomerular basement membrane, which causes a membrane-like thickening. The immune deposits consist of immunoglobulin G (IgG), with a predominance of IgG43, 4; other currently unidentified antigens; and the membrane attack complex of complement C5b-9. The formation of subepithelial immune deposits and complement activation are responsible for functional impairment of the glomerular capillary wall, which leads to proteinuria.

Treatment of MN often is disappointing,5, 6 and this is caused in part by heterogeneity of the disease and lack of reliable biomarkers because of ignorance of the target antigen(s) and nephritogenic antibodies. Strategies to target B lymphocytes with anti-CD20 antibody7 and inhibit complement8 are steps in the right direction, but more specific concept-driven therapies are needed. The key to a specific hypothesis-driven therapy is the understanding of the development of immune deposits, which first requires identification of the pathogenic antigen(s) and the ensuing events mediated by C5b-9.

We have learned a great deal about idiopathic MN from Heymann nephritis, the rat model of autoimmune MN induced by immunization of the Lewis rat strain with brush-border proteins.9 The autoantigenic target was identified in the early 1980s10, 11 as the podocyte membrane protein now called megalin. This polyspecific receptor, a member of the low-density lipoprotein receptor superfamily, is expressed with clathrin at the sole of podocyte foot processes (where immune complexes are formed).

However, megalin cannot be proved responsible for human MN because it has not been found in human podocytes and has not been detected in glomerular deposits in patients with MN. A first antigen, neutral endopeptidase (NEP), was identified in a small subset of patients with antenatal MN.12 The disease developed because of alloimmunization during pregnancy of NEP-deficient women13: maternal anti-NEP antibodies crossed the placenta and reached their target antigen at the podocyte surface in the fetus' glomeruli (Fig 1). These findings provided the proof of principle that in both experimental and human MN, the podocyte could serve as a source of target antigens for the formation of subepithelial deposits. They paved the way for future discoveries in the autoimmune form of the disease.

  • View full-size image.
  • Figure 1. 

    Proposed scheme of in situ formation of immune deposits in membranous nephropathy (MN). The in situ formation of immune complexes involves circulating antibodies binding to a native podocyte protein. Two antigenic targets have been identified: neutral endopeptidase (in alloimmune MN) and phospholipase A2 receptor (in idiopathic MN). In both cases, the pathogenic antigens are integral glycoproteins of podocytes. The formation of subepithelial immune deposits and complement activation are responsible for functional impairment of the podocytes and proteinuria.

Now, a recent article by Beck et al14 published in 2009 in the New England Journal of Medicine identifies the first antigen involved in idiopathic MN.

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What Does This Important Study Show? 

The antigen identified by Beck et al14 is PLA2R, the M-type phospholipase A2 receptor. Antibodies to PLA2R were detected in 26 of 37 patients (70%) with idiopathic MN, whereas they were not found in patients with other kidney diseases and secondary forms of MN. PLA2R seems to be normally expressed on the podocyte cell membrane and colocalizes with IgG in immune deposits in patients with idiopathic MN. IgG eluted from such deposits recognized PLA2R, whereas those obtained from deposits of lupus membranous or IgA nephropathy did not. Circulating and deposited anti-PLA2R autoantibodies were mostly of the IgG4 subclass. Taken together, these groundbreaking data suggest that in human idiopathic MN, subepithelial deposits are formed in situ through binding of circulating anti-PLA2R autoantibodies to the PLA2R antigen expressed on the podocyte surface (Fig 1). This potentially initiates complement activation and a cascade of events leading to nephrotic syndrome. However, definitive demonstration of the participation of anti-PLA2R in the pathogenesis of idiopathic MN would require transfer of the disease to nonhuman primates (because anti-PLA2R autoantibodies do not detect PLA2R in rodent or rabbit glomerular extract14) or the grafted kidney in patients with recurrent MN.

In addition, Beck et al14 present preliminary data suggesting an association of the presence and titer of anti-PLA2R autoantibodies with disease activity. If confirmed, these observations may be the first step toward better monitoring of disease activity and treatment efficacy.

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How Does This Study Compare With Prior Studies? 

For a long time, antigens involved in the pathogenesis of human MN have eluded identification. Key factors have been the low titer of circulating antibodies to these antigens and insensitive tests to detect them. Alloimmune MN is an exception because of high levels of maternal anti-NEP antibodies.12 We recently developed a sensitive enzyme-linked immunosorbent assay using recombinant human NEP, which showed low, but significant, levels of anti-NEP antibodies in a substantial proportion of adult patients with MN, whereas sera from patients with other nephropathies or healthy controls were negative. In addition, NEP could be identified in the subepithelial immune deposits using confocal microscopy (H.D. and P.R., unpublished observations, 2009).

The presence of PLA2R and NEP in immune deposits does not rule out a role for other antigens. One can speculate that after podocyte injury by C5b-9, intracellular proteins and cryptic epitopes are exposed, which may induce a second wave of immunization. Patient sera analyzed using Western blot with human podocyte lysates showed a variety of antibody profiles that were not observed with control sera. We have identified several proteins for which reactivity with patient sera was confirmed with the recombinant protein (H.D. and P.R., unpublished observations, 2009). Whether the circulating antibodies raised against those proteins are nephritogenic and involved in the perpetuation of the disease remain to be established.

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What Should Clinicians and Researchers Do? 

The observations made by Beck et al14 represent a major breakthrough that will undoubtedly boost investigation into human MN. Several important questions remain to be addressed. The first concerns the variety of antigens potentially involved in the disease. Circulating anti-PLA2R antibodies could not be detected in ∼30% of patients with idiopathic MN, which probably means the disease is heterogeneous, unless current assays used for the detection of anti-PLA2R antibodies lack sensitivity. The autoimmune process may target more than one antigen in the initial or later phases of the disease. The next step for clinicians and researchers will be to analyze serial serum samples from patients with idiopathic MN to establish antibody profiles, which may lead to identification of subsets of patients with different outcomes and response to treatment.

A second question relates to the trigger and development of the immune response. Anti-PLA2R from unrelated patients reacts with a similar reduction-sensitive epitope as in those with Goodpasture syndrome.15, 16 Future studies should examine the conditions that lead to exposure of the PLA2R epitope and also investigate potential cross-reactivity of that epitope with exogenous antigens, including bacterial antigens.17 Another point of importance is B-cell epitope spreading. The primary immune response against the dominant initiating epitope may further extend to other epitopes within the same molecule or among different molecules. This phenomenon may be relevant to the pathogenesis of the disease because in active Heymann nephritis, the onset of proteinuria correlates with intramolecular spreading.18 B-cell epitope spreading has been implicated in many models of antibody-mediated autoimmune disease.19, 20 Finally, as observed in Heymann nephritis,21 autoimmunity against a glomerular antigen could combine with anti-IgG response, frequently of IgG4 subclass in humans.22

A third issue relates to the mechanisms of glomerular lesions and proteinuria. Alloimmune MN is transmitted by maternal IgG1 and IgG4, not by IgG4 alone, and both IgG1 and IgG4 are deposited equally in glomeruli.13 In contrast, IgG4 seems to have a major role in autoimmune idiopathic MN,3, 4, 14 although it does not activate complement. Furthermore, the implication of IgG4 in the formation of subepithelial immune deposits is unclear because polyclonal IgG4 antibodies are functionally monovalent.23 Low amounts of IgG1 and IgG3 present in immune deposits in idiopathic MN could synergize with IgG4 to induce antigen clustering and complement activation. The fine mechanisms of proteinuria also remain to be unraveled. It is likely that the antibody may be directly toxic to endothelial cells and podocytes, for example, by neutralizing enzymatic activity or stimulating receptor-related pathways. One should not neglect a role for CD8 T-cells.24

The fourth question deals with new therapeutic approaches based on epitope-targeted therapy. Identification of one or several pathogenic epitopes recognized by helper T cells makes it possible to envision the induction of specific tolerance, as in experimental Goodpasture syndrome.25, 26

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Acknowledgements 

Support: The research of the authors is supported by grants from INSERM, Agence Nationale pour la Recherche (ANR-07-Physio-016-01), Fondation pour la Recherche Médicale, AURA (Association pour l'Utilisation du Rein Artificiel), AMGEN France, and Coordination Theme 1 (Health) of the European Community's 7th Framework Program, grant agreement no. HEALTH-F2-2007-201590. Drs Ronco and Debiec receive scientific grants from Amgen and Genzyme.

Financial Disclosure: None.

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References 

  1. Wasserstein AG. Membranous glomerulonephritis. J Am Soc Nephrol. 1997;8(4):664–674
  2. Glassock RJ. Diagnosis and natural course of MN. Semin Nephrol. 2003;23(4):324–332
  3. Imai H, Hamai K, Komatsuda A, Ohtani H, Miura AB. IgG subclasses in patients with membranoproliferative glomerulonephritis, MN, and lupus nephritis. Kidney Int. 1997;51(1):270–276
  4. Kuroki A, Shibata T, Honda H, Totsuka D, Kobayashi K, Sugisaki T. Glomerular and serum IgG subclasses in diffuse proliferative lupus nephritis, membranous lupus nephritis, and idiopathic membranous nephropathy. Int Med. 2002;41(11):936–942
  5. Glassock RJ. The treatment of idiopathic membranous nephropathy: a dilemma or a conundrum?. Am J Kidney Dis. 2004;44(3):562–566
  6. Perna A, Schieppati A, Zamora J, et al. Immunosuppressive treatment for idiopathic membranous nephropathy: a systematic review. Am J Kidney Dis. 2004;44(3):385–401
  7. Ruggenenti P, Chiurchiu C, Brusegan V, et al. Rituximab in idiopathic membranous nephropathy: a one-year prospective study. J Am Soc Nephrol. 2003;14(7):1851–1857
  8. Cunningham PN, Quigg RJ. Contrasting roles of complement activation and its regulation in membranous nephropathy. J Am Soc Nephrol. 2005;16(5):1214–1222
  9. Heymann W, Hackel DB, Harwood S, Wilson SGF, Hunter JL. Production of nephrotic syndrome in rats by Freund's adjuvants and rat kidney suspension. Proc Soc Exp Biol Med. 1959;100(4):660–664
  10. Kerjaschki D, Farquhar MG. The pathogenic antigen of Heymann nephritis is a membrane glycoprotein of the renal proximal tubule brush border. Proc Natl Acad Sci U S A. 1982;79(18):5557–5561
  11. Kerjaschki D, Farquhar MG. Immunocytochemical localization of the Heymann nephritis antigen (gp330) in glomerular epithelial cells of normal Lewis rats. J Exp Med. 1983;157(2):667–686
  12. Debiec H, Guigonis V, Mougenot B, et al. Antenatal membranous glomerulonephritis due to anti-neutral endopeptidase antibodies. N Engl J Med. 2002;346(26):2053–2060
  13. Debiec H, Nauta J, Coulet F, et al. Role of truncating mutations in MME gene in fetomaternal alloimmunization and antenatal glomerulopathies. Lancet. 2004;364(9441):1252–1259
  14. Beck LH, Bonegio RG, Lambeau G, et al. M-Type phospholipase A2 receptor as target antigen in idiopathic membranous nephropathy. N Engl J Med. 2009;361(1):11–21
  15. Hellmark T, Burkhardt H, Wieslander J. Goodpasture disease. Characterization of a single conformational epitope as the target of pathogenic autoantibodies. J Biol Chem. 1999;274(36):25862–25868
  16. Hudson BG. The molecular basis of Goodpasture and Alport syndromes: beacons for the discovery of the collagen IV family. J Am Soc Nephrol. 2004;15(10):2514–2527
  17. Kain R, Exner M, Brandes R, et al. Molecular mimicry in pauci-immune focal necrotizing glomerulonephritis. Nat Med. 2008;14(10):1088–1096
  18. Shah P, Tramontano A, Makker SP. Intramolecular epitope spreading in Heymann nephritis. J Am Soc Nephrol. 2007;18(12):3060–3066
  19. Lundberg K, Venables PJ. Epitope spreading in animal models: array of hope in rheumatoid arthritis and multiple sclerosis. Arthritis Res Ther. 2008;10(6):122–123
  20. Chen L, Hellmark T, Pedchenko V, et al. A nephritogenic peptide induces intermolecular epitope spreading on collagen IV in experimental autoimmune glomerulonephritis. J Am Soc Nephrol. 2006;17(11):3076–3081
  21. Zanetti M, Druet P. Passive Heymann's nephritis as a model of immune glomerulonephritis mediated by antibodies to immunoglobulins. Clin Exp Immunol. 1980;41(2):189–195
  22. Hennig C, Rink L, Kirchner H. Evidence for presence of IgG4 anti-immunoglobulin autoantibodies in all human beings. Lancet. 2000;355(9215):1617–1618
  23. Aalberse RC, Schuurman J. IgG4 breaking the rules. Immunology. 2002;105:9–19
  24. Penny MJ, Boyd RA, Hall BM. Permanent CD8(+) T cell depletion prevents proteinuria in active Heymann nephritis. J Exp Med. 1998;188(10):1775–1784
  25. Reynolds J, Prodromidi EI, Juggapah JK, et al. Nasal administration of recombinant rat alpha3(IV)NC1 prevents the development of experimental autoimmune glomerulonephritis in the WKY rat. J Am Soc Nephrol. 2005;16(5):1350–1359
  26. Reynolds J, Abbott DS, Karegli J, Evans DJ, Pusey CD. Mucosal tolerance induced by an immunodominant peptide from rat alpha3(IV)NC1 in established experimental autoimmune glomerulonephritis. Am J Pathol. 2009;174(6):2202–2210

 Originally published online as doi:10.1053/j.ajkd.2009.09.017 on November 18, 2009.

PII: S0272-6386(09)01267-0

doi:10.1053/j.ajkd.2009.09.017

American Journal of Kidney Diseases
Volume 55, Issue 5 , Pages 820-823, May 2010

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