Triggers, Bullets and Targets, Puzzle of Membranous Nephropathy

Introduction Idiopathic Membranous Nephropathy (MN) is a common cause of adult nephrotic syndrome. Recently, M-type phospholipase A2 receptor (PLA2-R) has been discovered as the main podocyte antigen in the pathogenesis of idiopathic MN. Materials and Methods In this mini review, the author searched English-language MEDLINE for the terms “membranous nephropathy”, “rituximab”, and “phospholipase A2 receptor” up to October 2011. Results In support of its earlier discovery, reports from China and Europe confirmed the major pathogenic role of non-complement fixing IgG4 antibody against PLA2-R in the pathogenesis of idiopathic MN. Antibodies against aldose reductase (AR) and manganese superoxide dismutase (SOD2 ), and sub-epithelial deposition of cationic bovine serum albumin (BSA) are also reported in rare occasions. It seems that Rituximab is a good therapeutic choice for those patients who need immunosuppressive therapy. Conclusions Great discoveries in the diagnosis and treatment of idiopathic MN have been performed but pathogenic mechanism and triggers for anti-PLA2-R production are still unknown.


Introduction
new findings iMN should no longer be considered as an idiopathic disease. Very recently a new indirect immuno-Idiopathic Membranous Nephropathy (iMN) is a com fluorescence test (IIFT) enabledeasy detection of anti-PLA mon cause of adult nephrotic syndrome. For more than 2 R antibody in the serum of patients with iMNby which 50 years researchers have debated for an autoimmune anti-PLA 2 R antibodies were found in 52% of patients with basis of MN, but the auto-antigen remained elusive (1). In biopsy-proven iMN (2). However, the triggers of autoan 2009, Beck and colleagues discovered that autoantigen tibody production and mechanisms of its action are yet is mainly a M-type trans-membrane phospholipase A2 reunknown and investigations for other likely antigens are ceptor (PLA2-R) located on podocytes, and autoantibody in progress (3,4). is mainly a non-complement fixing-IgG4 (1). With these

Materials and Methods
Inthis mini review, the author searched the MEDLINE as at initiation of ideas about Rituximab treatment and M-type phospholipase A2 receptor autoantibody in iMN up to October 2011. All English-language studies that re ported diagnosis and treatment of idiopathic MN were searched using the terms "membranous nephropathy", "rituximab" and, "phospholipase A2 receptor". We in cluded important studies by cross-referencing. We also included author's definitions of some conditions such as complete remission (CR) and partial remission (PR) in our review.

Anti-Phospholipase A2 -Receptor Antibodies
In its early discovery by Beck and colleagues, using West ern blot assay 26 out of 37 (70%) serum samples of Ameri can patients with iMN showedan antibody (mainly IgG4) against a 185-kD PLA2-R glycoprotein in the glomerular extract (1). In a report from China, by the use of a Western blot assay, 49 out of 60 patients (82%) with idiopathic MN demonstrated detectable anti-PLA2-R auto-antibodies (5).
In an European cohort of iMN patients, they measured anti-PLA-R auto-antibody levels by a Western blot immu noassay in serum samples of patients in nephritic pro teinuria, remission, or relapse periods. Fourteen out of 18 (77.8%) patients showed IgG4 auto-antibody in their ac tive phases; it was decreased significantly during remis sion and increased again during relapse (6). In a recent report, PLA2-R autoantibody was found in5 out of 10 pa tients with recurrent iMN but in none of 9 patients with de novoMN following renal transplantation (7).

Antigen Targets
The super-family of phospholipase A2 (PLA2) consists of distinct types of structurally related enzymes (8). Fam ily of secreted PLA2s (sPLA2) by itself has a widespread distribution in nature and presents in different fluids and tissues as an antibacterial protection (9,10). PLA2 -IIA is found in snake venom and other serpent's maxil lary glands (11). High-molecular weight cytosolic PLA2 (cPLA2) is located on the cell membrane or endoplasmic reticulum and acts as intracellular second messenger. There is an intensive relationship between cPLA2 and sPLA2 . PLA2-R isexpressed not only in human kidney but also in lung, pancreas, placenta, and skeletal muscle (13). Inflammatory cytokines such as IL-6, TNF-α, and IL-1β in duce the synthesis and release of sPLA2 from different cells (13). Mammalian sPLA2 could attach to PLA2-R and induce pro-inflammatory signals by a receptor-mediated mechanism. Heymannnephritis (HN) is an experimental rat model of Nephrotic syndrome (NS), but Megalin, the auto-antigen of HN does not express on human podo cytes. Epithelial cell injury in Heymann nephritis is in duced by complement C5b-9 deposition, and sub-lytic injury to podocytes, activate finally cPLA2, an important mediator of podocyte injury and actin cytoskeleton col lapse, through increasing intracellular calcium and pro tein kinase C (PKC) activation, and extracellular signalregulated kinase (ERK) (3,14). IgG4 is unique among IgG subclasses because it weakly activates the complement. Whether IgG4-PLA2-R interaction in human is similar to sub-lytic C5b-9 induced podocyte injury in Heymann ne phritis and what the triggers are for anti-PLA2-R produc tion still remainedunknown.
PLA2-R has been discussed as the major candidate anti gen of idiopathic MN (Table1 and Figure 1). The nature of antigens involved in remaining cases of idiopathic MN is unclear. In a very rare form of neonatal nephritic syn drome, neutral endopeptidase (NEP) deficient mother may develop antibody against NEP during conception and transmitthe antibody during next pregnancy to sub sequent fetus. Transmitted antibody is mainly IgG1. Nota bly, when the transmitted antibody is non-complement fixing IgG4, membranous nephropathy is not developed in the child (15, 16).
Antibody against aldose reductase (AR) and manga nese superoxide dismutase (SOD2) have been detected in Anti-BSA, mainly lgG4 Anti PLA 2-R antibody, mainly lgG4 Anti-AR and anti-SOD2, lgG4 Anti-NEP amtibody, lgG4/lgG1 sera and in elutes prepared from renal biopsy tissue of a group of patients with idiopathic MN (17). Superoxide dis mutase-2 (SOD2), aldose reductase (AR), and α-enolase are cryptic cytosol proteins. A temporal hierarchy is possible whereby as a consequence of primary podocyte damage by anti-PLA2-R, these cryptic antigens are exposed to im mune system (3).
In a small group of children with iMN, circulating cationic bovine serum albumin (BSA) and glomerular depos its of cationic BSA have been found. Cationic BSA acts as an externally planted antigen. It is pathogenic through binding to the anionic glomerular capillary wall and insitu formation of immune complexes (IgG4 and IgG1) with sub-epithelial deposition of membrane-attack com plex C5b-C9. The amount of intact and undigestedbo vine serum albumin entering the circulation is probably higher during infancy and childhood. In this type of iMN, eliminating the bovine milk from the diet could be ben eficial (18).

Treatment
Spontaneous remission (not induced by immunosup pressive therapy) is a well-known characteristic of iMN;its incidence ranges from 30% to 50% and carries a favorable outcome. Conservative therapy has been recommended for all patients with iMN and ifproteinuria is decreased more than fifty percent (> 50%) of baseline during the first year of follow-up, it predicts the appearance of spon taneous remission. Immunosuppressive therapy should be considered without delay for patients with deteriorat ing renal function and for those without proteinuria re duction during this period (19). Therapeutic approaches to iMN mostly relies on steroids and cytotoxics with or without calcineurin inhibitors (20). Rituximab, a mono clonal antibody to CD20 antigen of B cells offers a new therapeutic approach. It safely and persistently reduced proteinuria in a group of patients with iMN who had pre viously failed to respond to steroids, alkylating agents, or calcineurin inhibitors, or who experienced the relapse after transient remission (21)(22)(23). It is also proposed that rituximab reduces the proteinuria by an immune-inde pendent mechanism by targeting sphingomyelin-phos phodiesterase acid-like3B (SMPDL-3b) protein that pre serves the podocyte cytoskeleton (24). Majority of studies gave rituximab at a dose of 375 mg/(sqare meter) of body syrface once weekly for 4 weeks (22,25,26). Fervenzaet al. gave 1 g rituximab on days 1 and 15; this regimen was re peated at 6 months if B cells were > 15/µL and proteinuria was still in the nephrotic range (21,26). In a recent report at the end of two years of follow-up, CR and PR was ob served in 16 out of 20 (80%) patients with iMN. More than half of these patients had failed previous immunosup pressive therapy (21). It is believedthat the time has come to conduct a prospective randomized controlled trial in multiple centers with diverse patient populations to as sess the efficacy and long-term benefits of rituximab to be compared with traditional immunosuppressive treat ment (27).

Conclusions
Great discoveries have been performed and great ques tions remain to be answered. Up to now pathogenic mechanism and triggers for anti-PLA2-R production ar estill unknown. Idiopathic MN might be exhibited as a heterogeneous entity rather than a uniform disease. Autoimmune process could also target other poorly un derstood glomerular antigens beyond what have been discovered recently. Latest findings about the pathogenic role of cationic BSA raise the possibility that other food antigens might be involved in development of idiopathic membranous nephropathy.