Abstract
Membranous nephropathy (MN) is the most common cause of nephrotic syndrome in white adults. It is defined by the presence of subepithelial immune deposits localized between the podocyte and the glomerular basement membrane (GBM) on electron microscopy (EM) examination. Clinical course is variable: Although up to 30% of patients are said to undergo spontaneous remission, greater than 40% of patients will eventually develop end-stage renal disease (ESRD). The discovery of two major podocytes antigens, the M-type phospholipase A2 receptor 1 (PLA2R1) and the thrombospondin type-1 domain-containing 7A (THSD7A) protein, have revolutionized our understanding of the pathogenesis of human MN. Approximately 75% of patients with active disease have circulating anti-PLA2R autoantibodies, and up to 10% of the patients with MN that are anti-PLA2R negative have antibodies against THSD7A. Mounting evidence suggests that quantification and follow-up of anti-PLA2R levels can help in accessing prognosis and evaluating response to treatment. Genetic studies are elucidating predisposing factors for development of the disease. Further research into the antigen-autoantibody systems and the role of antibody spreading and identification of additional antigenic targets is likely to contribute to improving understanding of the pathophysiology and treatment of patients with MN.
Keywords
membranous nephropathy, M-type phospholipase A2 receptor 1, PLA2R1, thrombospondin type-1 domain-containing 7A protein, THSD7A, pathology, serology, tissue staining, prognosis, treatment
Membranous nephropathy (MN) is the most common cause of adult-onset nephrotic syndrome in the white population. It is characterized by deposition of immune complexes and complement components in the glomerular capillary wall and attendant new basement membrane synthesis. This histologic pattern is more properly called nephropathy than nephritis, because there is rarely any inflammatory response in the glomeruli or interstitium.
Previously, most cases were termed idiopathic; however, we now know that antibodies to the M-type phospholipase A2 receptor (PLA2R) are present in approximately 70% of patients with MN. More recently, antibodies against another podocyte antigen, the thrombospondin type-1 domain containing 7A (THSD7A), have been described in a minority of patients with MN that are negative for anti-PLA2R antibodies. Taken together, autoantibodies to podocyte-specific antigens account for approximately 80% of the patients with MN, and these cases should be called primary MN. In the remaining 20%, the disease is secondary to a variety of disorders ( Table 19.1 ). The idiopathic designation is made by exclusion and should be reserved for patients who are anti-PLA2R/THSD7A negative and for whom a causative agent cannot be determined. The list of known secondary causes of MN in Table 19.1 is not complete but provides an indication of the wide array of conditions associated with this histologic pattern. In some, such as hepatitis B or thyroiditis, the specific antigen has been identified as part of the immune complex within the deposits in the glomeruli. In others, the association is less well defined, but the designation remains, because treatment of the underlying condition or removal of the putative agent results in resolution of the clinical and histologic features of the disease. In older patients, neoplasms are the most common cause of secondary MN. Recent studies show an association between the presence of anti-THSD7A antibodies and malignancy-associated MN. In the largest cohort of 25 patients with positive anti-THSD7A antibodies, 7 were found to have a malignant tumor. As such, patients with anti-THSD7A–associated MN should be carefully screened for malignancy.
| Etiology | Examples |
|---|---|
| Neoplasm | Carcinomas, especially solid organ (tumors of the lung, colon, breast, and kidney), leukemia, and non-Hodgkin lymphoma |
| Infections | Malaria, hepatitis B and C, secondary or congenital syphilis, leprosy |
| Drugs | Penicillamine, gold |
| Immunologic | Systemic lupus erythematosus, mixed connective tissue disease, thyroiditis, dermatitis herpetiformis, sarcoidosis |
| Post kidney transplant | Recurrent disease, de novo membranous nephropathy |
| Miscellaneous | Sickle cell anemia |
| Bovine serum albumin | In children |
The clinical manifestations of both primary and secondary MN are similar. Hence a careful history, laboratory evaluation, and review of histologic features must be pursued to rule out potential secondary causes. Ongoing vigilance is also necessary, because the causative agent may not be obvious for months or even years after presentation. For example, in about 45% of malignancy-associated MN, kidney disease antedates the diagnosis of malignancy; in 40%, there is a simultaneous presentation; and in the remaining 10%, MN appears after the diagnosis of malignancy.
Primary MN is rare in children. In pediatric MN cases, a careful screening for other types of immunologically mediated disorders, especially systemic lupus erythematosus (SLE), is necessary. In very young children, the diagnosis of MN should raise the possibility of bovine serum albumin (BSA)–induced MN.
There are also marked geographic differences in etiology. The prevalence of anti-PLA2R antibodies in primary MN is lower in Japanese patients (<50%). Other factors impacting geographic variation include malaria in Africa and hepatitis B in East Asia. Universal hepatitis B vaccination has greatly reduced childhood MN associated with hepatitis B.
Clinical Features
MN presents in 60% to 70% of cases with features associated with the nephrotic syndrome, such as edema, proteinuria greater than 3.5 g/day, hypoalbuminemia, and hyperlipidemia. The other 30% to 40% of cases present with asymptomatic proteinuria, usually in the subnephrotic range (≤3.5 g/day). The majority of patients present with normal glomerular filtration rate (GFR), but about 10% have diminished kidney function. The urine sediment is often bland, although microscopic hematuria is common. Hypertension is uncommon at presentation, occurring in only 10% to 20% of cases. The clinical features associated with nephrotic range proteinuria in MN can be severe; patients with MN almost always have ankle swelling, and ascites, pleural, and rarely pericardial effusions may also be present. This pattern is particularly common in the elderly, and, unless a urinalysis is performed, these symptoms may be incorrectly labeled as signs of primary cardiac failure. Complications of MN include thromboembolic events and cardiovascular events. A recent study showed that clinically apparent venous thromboembolic events affect about 8% of MN patients, with renal vein thrombosis accounting for 30% of the thromboembolic events. This frequency is substantially lower than that previously reported in studies that used systematic screening for thromboembolic events. Secondary hyperlipidemia is common and characterized by an increase in total and low-density lipoprotein (LDL) cholesterol and often a decrease in high-density lipoproteins (HDLs), a profile associated with increased atherogenic risk.
Pathology
In early MN, glomeruli appear normal by light microscopy. Increasing the size and number of immune complexes in the subepithelial space produces a thickening as well as a rigid appearance of the normally lacy-looking glomerular basement membrane (GBM) on light microscopy ( Fig. 19.1 ). Over time, new basement membrane is formed around the immune complexes (deposits do not stain), producing the spikes along the epithelial side of the basement membrane, which are particularly well visualized when using the silver methenamine stains ( Fig. 19.2 ). In contrast, on immunofluorescence microscopy, these immune complexes do stain, most commonly with antihuman immunoglobulin G (IgG) and complement C3 ( Fig. 19.3 ). This produces a beaded appearance along the GBM (capillary wall), a pattern that is pathognomonic of MN on immunofluorescence. In the most extreme cases, this beading can become so dense that careful examination is required to distinguish it from a linear pattern. On electron microscopy, the majority of cases show extensive podocyte foot process effacement, even at the early stages ( Fig. 19.4 ).
Immune complex deposits are initially formed in the subepithelial space, which explains why a proliferative response, as in a glomerulonephritis, does not occur in MN. A classification system has been developed based on their specific location on electron microscopic examination. In stage I, deposits are located only on the surface of the GBM in the subepithelial location, without evidence of new basement membrane formation; in stage II, deposits are partially surrounded by new basement membranes; in stage III, they are surrounded and incorporated into the basement membrane; and in stage IV, the capillary walls are diffusely thickened, but rarefaction (lucent) zones are seen in intramembranous areas previously occupied by the deposits. Unfortunately, the clinical and laboratory correlations with these stages are poor. In some individual cases of MN, the electron microscopic pattern appears as if there had been waves of complex deposition, with all of the preceding stages present in the same glomerulus. In others cases, deposits appear as if there had been a continuous production of complexes with growth in size over time, producing lesions that are all at a similar stage and that can extend from the surface of the subepithelial space and penetrate all the way through the basement membrane.
Features that favor a secondary cause of MN, in particular an autoimmune disease, include (1) proliferative features (mesangial or endocapillary); (2) full-house pattern of Ig staining (G, M, and A), including staining for C1q on immunofluorescence microscopy; (3) electron dense deposits in the subendothelial location of the capillary wall and mesangium or along the tubular basement membrane and vessel walls; and (4) endothelial tubuloreticular inclusions on electron microscopy. Electron microscopy showing only few superficial scattered subepithelial deposits may suggest a drug-induced secondary MN. Additional diagnostic value may be obtained by staining kidney biopsies for IgG subclasses. IgG1, IgG2, and IgG3 tend to be expressed in lupus MN. IgG4 tends to be more commonly expressed in primary MN and absent in MN secondary to malignancy.
Pathogenesis
Over the past decade, major advances have occurred in our understanding of the autoimmune processes involved in the development of human MN.
Antineutral Endopeptidase Antibodies
The first breakthrough involved a case report of a patient with neonatal MN caused by transplacental transfer of circulating antineutral endopeptidase antibodies. Neutral endopeptidase (NEP) is a membrane-bound enzyme that is able to digest biologically active peptides and is expressed on the surface of human podocytes, syncytiotrophoblastic cells, lymphoid progenitors, and many other epithelial cells, as well as polymorphonuclear leukocytes. Mothers with truncating mutations of the metallomembrane endopeptidase (MME) gene fail to express NEP on cell membranes. NEP-deficient mothers, who were immunized during pregnancy, were able to transplacentally transfer nephritogenic antibodies against NEP to their children, causing MN in the newborns. Rabbits injected with the maternal IgG from these mothers also developed MN, providing additional proof that the disease is related to circulating anti-NEP antibodies.
Antibovine Serum Albumin Antibodies
High levels of circulating anti-BSA antibodies of both IgG1 and IgG4 subclasses have been reported as a cause of secondary MN in children and adults. BSA immunopurified from the serum of children migrated in the basic range of pH, whereas the BSA from adult patients migrated in the neutral region as native BSA. BSA staining colocalized with IgG immune deposits only in four children with circulating cationic BSA but in none of the adults with MN for whom biopsy specimens were available, implying that only cationic BSA can induce MN.
Anti–M-Type Phospholipase A2 Receptor Antibodies
The M (muscle)-type phospholipase A2 receptor (PLA2R) is a member of the mannose receptor family and is composed of an N-terminal cysteine-rich (ricin B) domain, a fibronectin-like II domain, eight C-type lectin-like domains (CTLD), a transmembrane region, and a short cytoplasmic tail containing motifs used in endocytic recycling. Anti-PLA2R antibodies are present in 50% to 80% of adult patients with primary MN and a lesser proportion of affected children. These antibodies are not present in the serum of healthy controls or in patients with other kidney or systemic diseases, yielding a 100% specificity for the lesion of MN. In some studies, small numbers of patients with various forms of secondary MN tested positive for anti-PLA2R antibodies. Whether these cases represent true secondary MN, or rather PLA2R-associated MN with coincident secondary disease, requires further investigation.
A growing body of evidence has documented that PLA2R Ab titer tightly correlates with disease activity in MN, although the majority of the studies to date are retrospective. In one study, 75% of patients with active disease were positive for anti-PLA2R antibodies. This contrasted with positivity in only 37% of patients in partial remission and 10% of patients in complete remission. Remission is reported to occur in 50% of patients with low titers but in only 30% of patients with high titers of anti-PLA2R antibodies at the time of diagnosis. High titers of anti-PLA2R antibody have also been associated with lower response rates and longer time to remission. Antibody titers at the time of clinical remission also correlate with the rate of relapse: patients who become anti-PLA2R negative after immunosuppressive treatment have lower relapse rates than patients who remain positive at the end of treatment. Anti-PLA2R levels may indicate which patients presenting with subnephrotic range proteinuria are likely to progress to full nephrotic syndrome. Moreover, high anti-PLA2R antibodies levels are associated with a high risk of kidney failure over time. In one study, more than 50% of patients with high anti-PLA2R levels had a doubling of serum creatinine over 5-year follow-up. Stratification of patients by antibody levels as low (20 to 86 RU/mL by a commercial enzyme-linked immunosorbent assay [ELISA]), medium (87 to 201 RU/mL), or high (≥202 RU/mL) revealed that after a median follow-up time of 27 months, the clinical end point, defined as an increase of serum creatinine by ≥25% and serum creatinine ≥1.3 mg/dL, was reached in 69% of patients in the high anti-PLA2R antibody levels group, versus in 25% of patients with low antibody levels. Patients with high antibody levels also reached this study end point faster (17.7 months) than patients with low titers (30.9 months). The evolution of PLA2R antibody levels in response to immunosuppression reliably predicts outcome. A decline in levels consistently precedes a decline in proteinuria. Generally, antibody levels decrease rapidly in the first 3 months of treatment and disappear over 6 to 9 months, followed by a remission of proteinuria over 12 to 24 months (or longer, as discussed later), independently of the type of immunosuppressive agent used.
Taken together, these observations strongly suggest that serial quantification of anti-PLA2R antibody levels can help in monitoring disease activity and response to immunosuppressive therapy. When taken in concert with follow-up of proteinuria, antibody testing may allow early intervention and earlier stopping of potent immunosuppressive agents.
Immunofluorescence staining for PLA2R can now be performed on kidney biopsy material. Positive staining mirroring the distribution of immune deposits that are detected on electron microscopy (EM) examination strongly favors primary MN over a secondary form of disease. Some patients may be negative in terms of circulating anti-PLA2R autoantibody at the time of kidney biopsy but still exhibit positive glomerular PLA2R staining. This state could represent completely different phases of the disease, either indicating that PLA2R-associated MN has gone into an immune remission while leaving footprints of the previous immunologic activity or indicating that early disease is present, with the very high affinity between anti-PLA2R autoantibodies and the podocyte antigen, leading to such a rapid depletion of antibodies from the circulation and deposition in the kidney that antibody is not measurable despite active disease.
Genetic Associations
Single-nucleotide polymorphisms (SNPs) in the genes encoding M-type PLA2R and HLA complex class II HLA-DQ alpha chain 1 (HLA-DQA1) have been reported in white and Asian populations with MN. The risk for primary MN was significantly higher when both the HLA-DQ1 allele and the PLA2R1 allele were present. Patients carrying one or two alleles for HLA DQA1*05:01 or for DQB1*02:01 have higher anti-PLA2R titers than those with neither of these HLA alleles. A theory has been proposed that the rare confluence of several relatively common factors triggers the development of MN: a particular isoform of HLA-DQA1 that confers increased susceptibility to autoimmunity, polymorphisms in PLA2R1 that alter expression and/or create a unique conformation identified by HLA class II on antigen-presenting cells, and other environmental factors.
Antithrombospondin Type-1 Domain-Containing 7A Antibodies
THSD7A is a transmembrane protein initially described in human endothelial vein cells but also expressed in many organs, including the kidney. The function of THSD7A may relate to binding to extracellular matrix, especially to glycosaminoglycan chains present on matrix proteoglycans. Immunogold EM has localized the protein within podocytes to the foot process near the slit diaphragm and in endosomal structures. Antibodies against THSD7A, predominantly of the IgG4 subclass, are reported in ~10% of patients with MN that are negative for PLA2R antibodies. So far, anti-THSD7A antibodies have not been detected in healthy controls or in patients with other kidney and systemic diseases, yielding 100% specificity for MN. Limited evidence suggests that, as in the case of PLA2R antibodies, circulating anti-THSD7A antibodies correlate with disease activity and may be used to monitor disease activity in patients with THSD7A-associated disease in the future. A genetic link to the THSD7A locus has not yet been established, possibly because of the small number of cases identified so far. Initially it appeared that anti-PLA2R and anti-THSD7A autoantibodies were mutually exclusive, but several cases of dual antibody positivity have been described. Tissue staining for the THSD7A antigen in kidney biopsies is technically more difficult than the “on or off” pattern described for the PLA2R antigen because a linear staining pattern for THSD7A is seen in normal biopsies and other types of glomerular disease.
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