Membranoproliferative Glomerulonephritis — A New Look at an Old Entity

Sanjeev Sethi, M.D., Ph.D., and Fernando C. Fervenza, M.D., Ph.D.

N Engl J Med 2012; 366:1119-1131March 22, 2012

Article
References: Membranoproliferative glomerulonephritis (MPGN), also termed mesangiocapillary glomerulonephritis, is diagnosed on the basis of a glomerular-injury pattern that is common to a heterogeneous group of diseases. MPGN accounts for approximately 7 to 10% of all cases of biopsy-confirmed glomerulonephritis1-4 and ranks as the third or fourth leading cause of end-stage renal disease among the primary glomerulonephritides.2,5 Although some diseases associated with MPGN are well known, recent advances have identified additional MPGN-associated conditions.

Clinical Presentation
MPGN most commonly presents in childhood but can occur at any age. The clinical presentation and course are extremely variable — from benign and slowly progressive to rapidly progressive. Thus, patients can present with asymptomatic hematuria and proteinuria, the acute nephritic syndrome, the nephrotic syndrome, chronic kidney disease, or even a rapidly progressive glomerulonephritis. The varied clinical presentation is caused by differences in the pathogenesis of the disorder and in the timing of the diagnostic biopsy relative to the clinical course. The degree of kidney impairment also varies, and hypertension may or may not be present. Patients who present early in the disease process, when the kidney biopsy shows proliferative lesions, are more likely to have a nephritic phenotype, and those with crescentic MPGN may present with a rapidly progressive glomerulonephritis. In contrast, patients with biopsies showing advanced changes that include both repair and sclerosis are more likely to have a nephrotic phenotype. Patients with classic MPGN often have features of both the acute nephritic syndrome and the nephrotic syndrome — termed the nephritic–nephrotic phenotype.

Classification and Pathophysiology
The typical features of MPGN on light microscopy include mesangial hypercellularity, endocapillary proliferation, and capillary-wall remodeling (with the formation of double contours) — all of which result in lobular accentuation of the glomerular tufts. These changes result from the deposition of immunoglobulins, complement factors, or both in the glomerular mesangium and along the glomerular capillary walls. On the basis of the electron-microscopical findings, MPGN is traditionally classified as primary (idiopathic) MPGN type I (MPGN I), type II (MPGN II), or type III (MPGN III) or secondary MPGN. MPGN I, the most common form, is characterized by subendothelial deposits, and MPGN III has both subepithelial and subendothelial deposits.6,7 MPGN II is characterized by dense deposits in the glomerular basement membrane (“dense-deposit disease”). Secondary MPGN, described by Rennke,8 is most often due to hepatitis C and other infections.
As currently classified, MPGN I and MPGN III are likely to include cases of both immune-complex–mediated and complement-mediated MPGN. Given recent advances in our understanding of the role of the alternative pathway of complement in MPGN, a practical approach is to view MPGN as immune-complex–mediated or complement-mediated.9 Thus, immune-complex–mediated MPGN may occur when there are increased levels of circulating immune complexes, and complement-mediated MPGN may occur because of disorders associated with dysregulation of the alternative pathway of complement.

Immune-Complex–Mediated MPGN
Immune-complex–mediated MPGN results from the deposition of immune complexes in the glomeruli owing to persistent antigenemia, with antigen-antibody immune complexes forming as a result of chronic infections, elevated levels of circulating immune complexes due to autoimmune diseases, or paraproteinemias due to monoclonal gammopathies. The immune complexes trigger the activation of the classical pathway of complement and the deposition of complement factors of the classical pathway and terminal complement pathway in the mesangium and along the capillary walls (Figure 1Figure 1Normal Glomerular Capillary Wall and Immune-Complex–Mediated Membranoproliferative Glomerulonephritis (MPGN).; and Fig. 1 in the Supplementary Appendix, available with the full text of this article at NEJM.org). A kidney-biopsy specimen typically shows immunoglobulin and complement on immunofluorescence microscopy.

Hepatitis C and Other Infections
Chronic viral infections such as hepatitis C and hepatitis B, with or without circulating cryoglobulins, are an important cause of MPGN. Hepatitis C, which was recognized as a common cause of immune-complex–mediated MPGN in the 1990s, is now considered to be the main viral infection causing MPGN. 8,10-13 In addition to viral infections, chronic bacterial infections (e.g., endocarditis, shunt nephritis, and abscesses), fungal infections, and parasitic infections are associated with MPGN, particularly in the developing world.14-18 Bacteria associated with MPGN include staphylococcus, Mycobacterium tuberculosis, streptococci, Propionibacterium acnes, Mycoplasma pneumoniae, brucella, Coxiella burnetii, nocardia, and meningococcus. 19-30

Autoimmune Diseases
MPGN occurs in a number of autoimmune diseases. These include systemic lupus erythematosus and, occasionally, Sjögren's syndrome, rheumatoid arthritis, and mixed connective-tissue disorders. 31-35

Monoclonal Gammopathy
Recent studies indicate that glomerular deposition of monoclonal immunoglobulin as a result of monoclonal gammopathy (also called dysproteinemia or plasma-cell dyscrasia), with or without cryoglobulins, is associated with MPGN.36-39– Monoclonal gammopathy is characterized by the proliferation of a single clone of immunoglobulin-producing lymphocytes or plasma cells, resulting in the circulation of monoclonal immunoglobulin. In one single-center study, 41% of the patients who had MPGN without an autoimmune process or chronic infection had evidence of monoclonal gammopathy, as assessed by means of serum electrophoresis, urine electrophoresis, or both.36 Bone marrow biopsies on such patients revealed a variety of conditions: monoclonal gammopathy of undetermined significance (MGUS) (the most common condition), low-grade B-cell lymphoma, lymphoplasmacytic lymphoma, chronic lymphocytic leukemia, and multiple myeloma. The authors suggested that patients with monoclonal gammopathy and MPGN should be classified as having “monoclonal gammopathy–associated MPGN” rather than MGUS.36

Complement-Mediated MPGN
The complement cascade plays an important role in innate immunity. Complement factors can induce a potent inflammatory response that results in phagocyte chemotaxis, with opsonization and lysis of cells, including microorganisms. Complement activation occurs through the classical, lectin, or alternative pathways, all of which converge to form C3 convertase, which cleaves C3 into C3a and C3b. C3b, in the presence of factor B and factor D, associates with C3 convertase, generating even more C3 convertase and resulting in a potent amplification loop (Figure 2Figure 2Complement-Mediated MPGN., and Fig. 1 in the Supplementary Appendix). Thus, C3 convertase is a nodal point in the complement cascade. The association of C3b and C3 convertase also results in the formation of C5 convertase, which activates the terminal complement complex pathway and the formation of the membrane-attack complex (C5b–C9) on cell surfaces, thereby resulting in cell lysis.40,41
The alternative pathway is continually active at low levels in the circulation (fluid phase) through spontaneous hydrolysis of the thioester bond of C3 (“tick over” mechanism), which generates C3b; C3b then binds to host cell membranes and extracellular membranes such as the glomerular basement membrane (surface phase) as well as membranes of pathogenic microorganisms.42,43 To prevent self-damage, activation of the alternative pathway occurs in a tightly regulated, sequential manner. Multiple complement-regulating and complement-inhibiting proteins operate at different levels of the cascade, particularly at the C3 and C5 convertase level.41 Such plasma or fluid-phase regulators include factors H and I and factor H–related proteins 1 through 5 and cell-bound and surface regulators such as decay-accelerating factor (CD55), complement receptor 1, CD59, membrane cofactor protein (CD46), and complement receptor of the immunoglobulin superfamily.40-42 Factor H accelerates the breakdown of C3 convertase and is a cofactor for factor I–mediated cleavage and inactivation of C3b,41,42 thereby controlling the alternative pathway in the fluid phase. Fluid-phase regulators of the terminal complement complex include vitronectin and clusterin. Some of the fluid-phase regulators, including factor H and factor H–related protein 1, also attach to cell surfaces and extracellular membranes, adding an extra protective mechanism to prevent the formation of active complement products.41 Surface regulators control C3 convertase through the inactivation of C3b deposited on cell surfaces and basement membranes.44
Dysregulation of the alternative pathway can occur because of mutations in or autoantibodies to complement-regulating proteins (Figure 3Figure 3Acquired and Genetic Abnormalities Associated with Complement-Mediated MPGN.). For example, mutations in proteins that regulate the assembly and activity of C3 convertase and degradation of C3b, such as factors H, I, and B and factor H–related protein 5, result in dysregulation of the alternative pathway.45-53 Heterozygous mutations in C3 itself cause fluid-phase dysregulation of the alternative pathway, because the mutant C3 is resistant to cleavage by C3 convertase. In addition, the generation through the tick-over mechanism of an abnormal C3 convertase, which contains the C3 mutation, renders it resistant to inactivation by factor H. The abnormal C3 convertase then cleaves C3 produced by the normal C3 allele, resulting in increased levels of C3 breakdown products.54 Similarly, antibodies to the complement-regulating proteins (such as factors H and B) and to C3 convertase itself can result in overactivity of the alternative pathway.49,55 Antibodies to C3 convertase (called C3 nephritic factor) stabilize the convertase and prolong its half-life by preventing its inactivation and degradation, thereby activating the alternative pathway.40,45
Certain genetic polymorphisms in factors H and B, membrane cofactor protein, and C3 are also associated with MPGN.56,57 Polymorphisms in the gene encoding factor H, notably Tyr402His allele variants, are the polymorphisms that have been studied most often. As compared with Tyr402, His402 is overrepresented in patients with MPGN and alternative-pathway abnormalities, and functional studies show that His402 impairs factor H–mediated regulation of C3 convertase on cell surfaces.49,58,59
Whatever the mechanism may be, dysregulation of the alternative pathway results in activated complement products, including C3b and terminal complement factors, which are delivered indiscriminately to endothelial surfaces, including glomeruli.41,60 The deposition of these complement products and debris in the mesangium and subendothelial region triggers glomerular inflammation and leads to MPGN. Immunoglobulins are not directly involved; thus, complement-mediated MPGN is typically immunoglobulin-negative but complement-positive on immunofluorescence studies.
Despite multiple genetic risk factors, MPGN due to complement abnormalities often develops relatively late in life, suggesting that additional insults or environmental factors are required. Furthermore, MPGN does not develop in all genetically similar members of high-risk families, again indicating that additional disease-inducing factors are required. We speculate that when MPGN does not occur despite the presence of a mutation or of an allele variant that confers a predisposition to the disease, redundant control mechanisms may be present.43 However, when an additional insult such as a complement-activating infection occurs, it may overwhelm the compensatory regulatory mechanism, triggering glomerular deposition of complement factors.41 This scenario may explain the recurrent episodes of macroscopic hematuria associated with infections (synpharyngitic hematuria) that are noted in many patients with MPGN. Similarly, an additional insult, such as the production of monoclonal proteins that act as autoantibodies to complement-regulating proteins in patients with MGUS, could result in dysregulation of the alternative pathway and the development of MPGN.61,62

Pathological Features
The deposition of immunoglobulin, complement, or both in the mesangium and subendothelial region of the capillary wall triggers an acute injury, which is often followed by an inflammatory (cellular or proliferative) phase, with an influx of inflammatory cells. A subsequent reparative phase occurs, during which new mesangial matrix results in mesangial expansion, along with the generation of new glomerular basement membrane, which looks like a duplicated basement membrane (so-called tram tracks or double contours) (Figure 1 and Figure 2).
Immunofluorescence findings are used to distinguish immune-complex–mediated MPGN from complement-mediated MPGN and can often point to a specific cause. For example, MPGN associated with monoclonal gammopathy shows monotypic immunoglobulin with kappa or lambda light-chain restriction (Figure 4Figure 4Representative Findings on Light, Immunofluorescence, and Electron Microscopy in MPGN.). MPGN associated with hepatitis C infection typically shows IgM, IgG, C3, and kappa and lambda light chains. An MPGN pattern in association with autoimmune diseases often includes multiple immunoglobulins and complement proteins — IgG, IgM, IgA, C1q, C3, and kappa and lambda light chains. MPGN associated with alternative-pathway dysfunction is characterized by bright C3 immunostaining in the mesangium and along the capillary walls (Figure 4). The absence of marked immunoglobulin staining on immunofluorescence microscopy distinguishes MPGN due to alternative-pathway dysfunction from immune-complex–mediated MPGN.
Electron microscopy typically reveals mesangial and subendothelial deposits and, in some cases, intramembranous and subepithelial deposits. During the reparative phase, new basement membrane forms, entrapping capillary-wall deposits, along with cellular elements derived from inflammatory, mesangial, and endothelial cells, within the new basement-membrane material; the result is a thickening of the capillary walls and the formation of double contours along the capillary walls. With the exception of dense-deposit disease, electron microscopy cannot distinguish between immune-complex–mediated MPGN and complement-mediated MPGN.
MPGN due to alternative-pathway dysregulation may be subdivided into dense-deposit disease and C3 glomerulonephritis (C3GN), on the basis of electron-microscopical findings. Dense-deposit disease is characterized by osmiophilic, sausage-shaped, wavy, dense deposits that replace the glomerular basement membrane and also occur in the mesangium, whereas C3GN has mesangial, subendothelial, and sometimes subepithelial and intramembranous deposits (Figure 4). On the basis of the morphologic characteristics of C3GN on electron microscopy, C3GN is most likely to be termed MPGN I or MPGN III according to the older classification. Data from laser microdissection and mass spectrometric analysis of glomeruli obtained from patients with C3GN are consistent with unrestricted activation of the alternative pathway, and the proteomic profile in such patients is similar to that in patients with dense-deposit disease.49,60 The hypothesis that dense-deposit disease and C3GN are part of a continuum is further supported by cases that show features that are intermediate between dense-deposit disease and C3GN, with some capillary loops showing the sausage-shaped intramembranous deposits of dense-deposit disease and other loops showing the subendothelial and subepithelial deposits of C3GN on electron microscopy.
Five cases of immune-complex–mediated or complement-mediated MPGN with a clearly identifiable cause are discussed in the Supplementary Appendix.

Alternative-Pathway Dysregulation and Disease Subtype
Dysregulation of the alternative pathway results in dense-deposit disease in some patients and C3GN in others, most likely because of differences in the degree or site (or both) of the dysregulation. In addition, certain allele variations of complement-regulating proteins may be associated with dense-deposit disease, and others may be associated with C3GN.57

Other Patterns of Immune-Complex–Mediated and Complement-Mediated Glomerular Injury
Other patterns of glomerular injury besides MPGN may result from the deposition of immunoglobulin, complement, or both. For example, mesangial proliferative glomerulonephritis, diffuse proliferative glomerulonephritis, crescentic glomerulonephritis, and a sclerosing glomerulopathy can be present in both C3GN and dense-deposit disease.63,64 The umbrella term “C3 glomerulopathy” describes the various patterns of injury,65 which probably depend on multiple factors, including the severity of injury and the phase of the disease process (acute or chronic) at the time the biopsy is performed. Prior treatment may also affect the biopsy findings.

MPGN without Immune Complexes or Complement
A pattern of injury consistent with MPGN is also noted in thrombotic microangiopathies resulting from injury to the endothelial cells. In the acute phase, mesangiolysis, endothelial swelling, and fibrin thrombi are present in the glomerular capillaries. As the process evolves into a reparative and chronic phase, mesangial expansion and remodeling of the glomerular capillary walls, including double-contour formation, take place. Thus, the healing phase of thrombotic thrombocytopenic purpura or hemolytic–uremic syndrome, atypical hemolytic–uremic syndrome associated with complement abnormalities, the antiphospholipid antibody syndrome, drug-induced thrombotic microangiopathies, nephropathy associated with bone marrow transplantation, radiation nephritis, malignant hypertension, and connective-tissue disorders can all present with an MPGN pattern of injury on biopsy.66,67 In thrombotic microangiopathies, immunoglobulin and complement are typically absent on immunofluorescence, and electron-dense deposits are not present in the mesangium or along the capillary walls on electron microscopy.

Evaluation
Persistently decreased serum levels of complement C3, C4, or both are commonly seen in patients with MPGN. Low C3 and low C4 complement levels are more common in immune-complex–mediated MPGN, whereas low C3 and normal C4 levels are more common in alternative-pathway dysfunction, particularly in the acute phase. A normal C3 level does not rule out alternative-pathway dysfunction.
When a kidney-biopsy specimen from a patient with MPGN shows immunoglobulins, an evaluation for infections, autoimmune diseases, and monoclonal gammopathies is indicated (Figure 5Figure 5Pathophysiology of MPGN.). Relevant tests for the detection of infections include blood cultures and polymerase-chain-reaction and serologic tests for viral, bacterial, and fungal infections. Cryoglobulins may be present. Tests for the detection of monoclonal gammopathy include serum and urine electrophoresis, immunofixation studies, and free light-chain assays; positive results necessitate bone marrow studies for a more precise diagnosis. Positive screening tests for an autoimmune disease should be followed by specific tests for the autoimmune disease.
If the biopsy specimen from a patient with MPGN shows bright C3 immunostaining (with minimal or no immunoglobulin staining), an evaluation to detect abnormalities of the alternative pathway is indicated regardless of whether an electron microscopic examination shows dense-deposit disease or C3GN (Figure 5). The initial evaluation of the alternative pathway should include measurement of serum complement levels and serum levels of the membrane-attack complex, an alternative pathway functional assay, and hemolytic complement assays, followed by genetic analysis for mutations and allele variants of complement factors and assays for the presence of autoantibodies to complement-regulating proteins, including tests for the detection of C3 nephritic factor (Fig. 7 in the Supplementary Appendix).
Even after extensive evaluation, the cause may remain enigmatic in a few cases of immune-complex–mediated or complement-mediated MPGN. In some patients, immune-complex–mediated MPGN may be initiated by immunoglobulin deposition, but the disease may be accelerated by alternative-pathway abnormalities.68 Over time, new methods will probably be developed that can further separate specific causes of MPGN from idiopathic cases.

Therapy
Early reports on the treatment of “idiopathic” MPGN should be interpreted with caution. In many instances, historical controls were used, the statistical significance was marginal, or the power to detect substantial differences was small.69,70 Most early studies antedated the use of angiotensin-converting–enzyme (ACE) inhibitors and angiotensin II–receptor blockers, and the protean pathogenic processes that lead to MPGN were as yet unknown. Thus, most studies on MPGN conflated various types of MPGN in unknown proportions.71
The benefit of long-term alternate-day glucocorticoid therapy for idiopathic MPGN in children was suggested by a few uncontrolled studies72-74 and one randomized, controlled trial.75 However, these studies included a mix of patients with MPGN I, MPGN III, and dense-deposit disease, limiting the conclusions that could be reached. There has been no systematic evaluation of glucocorticoid therapy for idiopathic MPGN in adults. Retrospective studies showed no clear benefit from glucocorticoid therapy, but treatment was not as prolonged in adults as it was in children.70,76
Early claims of the beneficial effects of anticoagulants (i.e., heparin and warfarin), frequently combined with glucocorticoids and cytotoxic agents, have not been confirmed in a prospective study.77 Similarly, an early randomized, controlled trial showed that the combination of aspirin and dipyridamole slowed the decline in the glomerular filtration rate in adults with idiopathic MPGN,78 but there was no long-term benefit, suggesting that prolonged antiplatelet therapy is required for a sustained benefit.70 Limited uncontrolled data suggest that calcineurin inhibitors may reduce proteinuria in some patients with MPGN.79-82 In patients with a rapidly progressive course and crescents on renal biopsy, a few small, uncontrolled studies have suggested a benefit with high-dose “pulse” glucocorticoids, either as monotherapy83-85 or in combination with azathioprine,86 cyclophosphamide,87 or mycophenolate mofetil.88-90
The lack of randomized, controlled trials and the current understanding that multiple pathogenic processes lead to MPGN make it impossible to give strong treatment recommendations in this patient population. Pragmatic considerations would suggest that patients with MPGN due to chronic infections should undergo treatment of the infection, and those with MPGN due to an autoimmune disease should undergo treatment of the autoimmune disease. Similarly, patients with MPGN due to a monoclonal gammopathy should undergo treatment aimed at attaining remission of the hematologic dyscrasia. A recent study involving patients with MPGN associated with monoclonal immunoglobulin deposits and no overt hematologic cancer showed that patients had a good response to rituximab.91 Patients with normal kidney function, no active urinary sediment, and non–nephrotic-range proteinuria can be treated conservatively with angiotensin II blockade to control blood pressure and reduce proteinuria, since the long-term outcome is relatively benign in this context.92,93, Follow-up is required to detect early deterioration in kidney function.
A better understanding of the causes and pathogenesis of complement-mediated MPGN would logically set the stage for the possible use of newer drugs, including anticomplement drugs. However, current recommendations are based on theory, not studies. For example, patients with MPGN due to autoantibodies to complement-regulating proteins may benefit from immunosuppressive therapy (e.g., glucocorticoids and rituximab), whereas those with MPGN due to a genetic mutation in complement-regulating proteins may benefit from treatment with drugs that inhibit formation of the membrane-attack complex (e.g., eculizumab).
Eculizumab, an anti-C5 monoclonal antibody that inhibits C5 activation, has been used successfully in patients with atypical hemolytic–uremic syndrome due to complement abnormalities in the alternative pathway.94-96 The role of such anticomplement agents in MPGN is not delineated but offers exciting possibilities for the future. It is also conceivable that patients with elevated serum levels of the membrane-attack complex may be more likely than those with normal levels to have a response to treatment with eculizumab. Patients with MPGN due to a deficiency of factor H might benefit from plasma infusion97 or infusion of factor H. Patients who present with advanced renal insufficiency and severe tubulointerstitial fibrosis on renal biopsy are unlikely to benefit from immunosuppressive therapy.

Recurrence after Kidney Transplantation
MPGN often recurs in kidney-transplant recipients. Recurrence rates range from 27 to 65%, depending on the study.98-100 One recent study, which excluded from the analysis patients with dense-deposit disease, showed a recurrence rate of 41%; of patients with a recurrence, 36% had a monoclonal gammopathy. The study showed that recurrent MPGN due to deposition of monoclonal immunoglobulin was associated with early recurrence and a more aggressive course.100 Low complement levels were shown to be an early marker for recurrent MPGN.100 Few data exist on the recurrence of C3GN. In patients with dense-deposit disease, there is almost universal recurrence of disease, with a 5-year rate of allograft failure of 50%.40,45

Conclusions
Two major pathophysiological factors — the deposition of immunoglobulin and the deposition of complement in the glomerular mesangium and capillary walls — may lead to MPGN. The presence of immune-complex–mediated MPGN necessitates evaluation for infections, autoimmune diseases, and monoclonal gammopathy. Complement-mediated MPGN is further subdivided into dense-deposit disease and C3GN, depending on the electron-microscopical findings; the presence of complement-mediated MPGN necessitates evaluation of the alternative pathway. Evaluation of MPGN according to the underlying pathophysiological processes may facilitate proper treatment.