Introduction
Membranoproliferative glomerulonephritis (MPGN) is a rare type of glomerular injury characterized by mesangial hypercellularity and endocapillary proliferation [1]. While the overall incidence is low (approximately 0.14–0.93 per 100,000 people per year) [2], MPGN had the worst renal outcome in patients with primary glomerulonephritis (GN) [3]. Over half of patients with MPGN progress to end-stage kidney disease (ESKD) within 10 years [4,5].
Traditionally, MPGN was classified into three types (1, 2, and 3) based on electron microscopy findings, particularly the location of immune complex deposition [1,6]. However, in 2011, a new classification system based on immunofluorescence (IF) was introduced, dividing MPGN into two categories: complement 3 glomerulopathy (C3G) and immune complex-mediated MPGN (IC-MPGN) [7,8]. C3G is characterized by dominant deposition of C3 in the glomeruli with minimal to no immunoglobulin (Ig) deposition, while IC-MPGN shows significant deposition of both Ig and complement [7,8].
This shift towards an etiology-based classification reflects the understanding that C3G likely results from dysfunction in the alternative complement pathway, while IC-MPGN originates from immune complex deposition that activates the classical complement pathway [1,9,10]. This new system is expected to improve our understanding of the underlying causes of MPGN and guide treatment approaches [11,12]. Additionally, it has the potential to provide prognostic information. However, current data on prognosis based on this classification is inconsistent. Many studies have focused on children and adolescents or have had small sample sizes [4,13–16].
We investigated the clinical characteristics and prognoses of adult patients with MPGN according to the new classification, reflecting the real-world prevalence of comorbidities like cardiovascular diseases, malignancies, and autoimmune or viral diseases.
Methods
Study population
This multicenter, observational cohort study was conducted in 18 Korean hospitals. Kidney biopsies were performed on patients between 1979 and 2018. Among the 21,697 patients enrolled, we excluded 241 due to missing IF data. This resulted in a final study population of 620 patients diagnosed with MPGN (Fig. 1).
Definition
The estimated glomerular filtration rate (eGFR) was calculated using the Modification of Diet in Renal Disease Study equation based on serum creatinine levels [17]. Hypertension is defined as systolic blood pressure (SBP) ≥140 mmHg, diastolic blood pressure (DBP) ≥90 mmHg, or current use of antihypertensive medication. Diabetes mellitus (DM) is defined as fasting blood glucose ≥126 mg/dL, the use of oral hypoglycemic agents or insulin, or a documented history of DM according to electronic medical records. Treatment was defined as the use of steroids within 30 days before or after the renal biopsy. Coronary artery disease was defined as a history of angina pectoris or myocardial infarction (MI). Cardiovascular disease is defined as angina, MI, or stroke. Remission of proteinuria was defined as a urine protein-to-creatinine ratio (UPCR) <0.15 g/g creatinine at 6 months after the biopsy. Data were retrieved from the Korean Society of Nephrology registry (April 2018), Statistics Korea databases, and electronic medical records.
Renal pathology
The renal pathology evaluation methods used in this study have been previously described [18]. All biopsies underwent light microscopy with hematoxylin and eosin, periodic acid-Schiff, Masson’s trichrome, and periodic acid-methenamine silver stains. Additionally, IF staining was performed using antibodies against IgA, IgG, IgM, C3, C1q, and kappa and lambda light chains. Electron microscopy was employed.
For MPGN classification, biopsies were categorized based on IF staining intensity. To consistently apply the diagnostic criteria updated in 2011, the classification of IC-MPGN and C3G was conducted according to the criteria defined in several established papers [9,11,19,20]. C3G is defined by the presence of MPGN in renal biopsy samples, where glomerular IF staining shows C3 presence that is at least two orders of magnitude more intense than any other immune-reactant, including Ig heavy chains (IgG, IgM, and IgA), Ig light chains (κ and λ), and the complement component C1q. Mesangial hypercellularity, interstitial fibrosis and inflammation, tubular atrophy, and glomerular cellularity were categorized as mild or moderate to severe. The presence of a crescent was defined as any detectible glomerular crescent, while global sclerosis was defined as the involvement of 10% or more of the glomeruli. Segmental sclerosis was defined as any detectible segmental sclerosis.
Statistical analysis
All analyses were conducted using IBM SPSS version 25.0 (IBM Corp.). Data were represented as mean ± standard deviation for continuous variables and as percentages for categorical variables. Differences between groups were analyzed using a chi-square test for categorical variables and an analysis of variance for continuous variables. The Kaplan-Meier method was used to estimate the survival curve, and the log-rank test was used to assess statistical significance. Variables for multivariate logistic regression analysis and Cox proportional hazards analyses were selected based on a p-value of <0.05 in univariate analysis, along with age and sex for adjustment. A p-value of <0.05 was considered statistically significant.
Ethics statement
This study was approved by the Institutional Review Boards (IRBs) of Korea University Anam Hospital (No. 2023AN0492), Kyungpook National University Hospital (No. 2017-08-013), Kyung Hee University Hospital at Gandong (No. 2012-01-130), Kangdong Sacred Heart Hospital (No. KANGDONG 2016-06-008), Yonsei University Health System Clinical Trial Center (No. 3-2018-0031), Korea University Guro Hospital (No. 2017GR0082), Eulji University Hospital (No. EMCS 2018-06-001), SMG-SNU Medical Center (No. 06-2011-50/106), Seoul National University Bundang Hospital (No. B-1707/408-106), Seoul National University Hospital (No. 1802-102-924), Yonsei University Health System Clinical Trial Center (No. 4-2017-0646), Pusan National University Yangsan Hospital (No. 05-2017-171), The Catholic University of Korea, Eunpyeong St. Mary’s Hospital (No. 3-116287-AB-N-01), Ewha Womans University Mokdong Hospital (No. 2017-09-054-004), National Health Insurance Service Ilsan Hospital Clinical Trial Center (No. NHIMC 2017-12-017), Chonnam National University Hospital (No. CNUH-2018-038), Chonbuk National University Hospital (No. 2017-11-021), and Hallym University Sacred Heart Hospital (No. 2017-I136). The IRBs waived the requirement for informed consent because of the retrospective nature of the study.
Results
Baseline patient characteristics
Among 21,456 patients with IF staining, 620 were diagnosed with MPGN. Reclassification identified 50 patients (8.1%) with C3G and 570 patients (91.9%) with IC-MPGN (Fig. 1).
Table 1 describes the baseline characteristics of the two groups. These characteristics were measured at the time of the kidney biopsy. The mean patient age was 47.9 years. A significantly higher proportion of patients in the C3G group were younger than 20 years old compared to that in the IC-MPGN group (10.9% vs. 4.9%, p = 0.083). The sex distribution was similar, with 433 patients (69.8%) being male. The C3G group had significantly lower eGFR compared to that of the IC-MPGN group (49.9 ± 40.7 mL/min/1.73 m2 vs. 62.7 ± 36.5 mL/min/1.73 m2, p = 0.02). However, proteinuria levels were significantly higher in the IC-MPGN group () compared to that in the C3G group (4.7 ± 4.2 g/g creatinine vs. 2.9 ± 2.6 g/g creatinine, p < 0.001). Albuminuria followed a similar trend, with higher levels in the IC-MPGN group (2.9 ± 2.8 g/L vs. 1.5 ± 1.8 g/L, p = 0.048). Furthermore, over half (50.7%) of the patients with IC-MPGN had nephrotic-range proteinuria (≥3.5 g/g creatinine) compared to only 28.6% in the C3G group (p = 0.022). DBP was significantly higher in the IC-MPGN group (p = 0.023). The IC-MPGN group had significantly lower serum protein and albumin levels but higher cholesterol levels compared to those in the C3G group (all p ≤ 0.002). The prevalence of cancer was significantly higher in the IC-MPGN group compared to that in the C3G group (19.0% vs. 2.6%, p = 0.011). No significant differences were observed between the groups in age, sex, weight, SBP, or the prevalence of hypertension, DM, or coronary artery disease (Table 1).
Serologic findings and treatment of patients with membranoproliferative glomerulonephritis
Serum C3 levels were significantly lower in the C3G group compared to that in the IC-MPGN group (p = 0.04). Conversely, the serum IgG level was significantly lower in the IC-MPGN group, while the IgM level was lower in the C3G group (all p ≤ 0.035). Hepatitis B surface antigens were significantly more frequent in the IC-MPGN group (35.3% vs. 7.0%, p < 0.001). Antinuclear antibody, antineutrophil cytoplasmic antibody, hepatitis C virus antibody, and human immunodeficiency virus antibody levels did not differ significantly between the groups (Table 2).
Corticosteroids, mycophenolate mofetil (MMF), cyclophosphamide, azathioprine, calcineurin inhibitors (CNIs), and rituximab were administered to treat MPGN. Overall, 38.3% of patients with MPGN received immunosuppressant therapy. Corticosteroids were the most frequently used medication, with 125 patients (36.5%) in the IC-MPGN group and seven patients (33.3%) in the C3G group receiving them. This was followed by cyclophosphamide, CNIs, and MMF. No significant differences were observed in treatment regimens between the groups (Table 2).
Histologic features of patients with membranoproliferative glomerulonephritis
On light microscopy, mesangial hypercellularity was observed in over 70% of both groups (77.2% in IC-MPGN and 70.8% in C3G). Interstitial fibrosis and tubular atrophy, indicative of chronic changes, were more prevalent in the IC-MPGN group (p ≤ 0.001). Glomerular cellularity was more frequent in the IC-MPGN group compared to that in the C3G group (p = 0.002). Segmental sclerosis was more prevalent in the IC-MPGN group, while the prevalence of global sclerosis and crescents did not differ between the groups (Table 3).
Renal function and proteinuria of patients with membranoproliferative glomerulonephritis after the kidney biopsy
Baseline eGFR was lower in the C3G group than that in the IC-MPGN group. At the time of biopsy, the prevalence of eGFR <30 mL/min/1.73 m2 was significantly higher in the C3G group compared to that in the IC-MPGN group (42.0% vs. 18.6%, p < 0.001). This trend continued at 6 months, with a higher proportion of patients in the C3G group (36.7%) exhibiting reduced renal function (eGFR, <30 mL/min/1.73 m2) compared to that in the IC-MPGN group (20.6%), although this difference was not statistically significant (p = 0.11).
The amount of proteinuria was significantly higher in the IC-MPGN group (4.7 ± 4.2 g/g creatinine vs. 2.9 ± 2.6 g/g creatinine, p < 0.001). However, the prevalence of nephrotic proteinuria (UPCR, >3.5 g/g creatinine) did not differ significantly between the MPGN groups after 6 months (p = 0.18). By 12 months, 21.3% of patients in the IC-MPGN group and 5.6% of patients in the C3G group had nephrotic-range proteinuria (p = 0.11).
During a follow-up period of 71.4 ± 81.6 months, 128 patients (30.3%) in the IC-MPGN group and four patients (10.3%) in the C3G group experienced an eGFR reduction of ≥40% from baseline. When adjusted for age and sex, the risk of a ≥40% decline in eGFR was not significantly different between the IC-MPGN and C3G groups (relative risk [RR], 0.70; 95% confidence interval [CI], 0.26–1.90). Similarly, after adjusting for multiple factors, C3G did not show a significant risk reduction for a ≥40% decline in eGFR (RR, 0.32; 95% CI, 0.09–1.12) (Table 4).
Progression to end-stage kidney disease and mortality in patients with membranoproliferative glomerulonephritis
A total of 159 patients (27.7%) progressed to ESKD during a follow-up period of 122.4 ± 124.2 months. This included 151 patients (28.4%) in the IC-MPGN group and eight patients (19.0%) in the C3G group. The progression to ESKD did not differ significantly between the MPGN groups (p = 0.28) (Fig. 2A).
During the follow-up period of 143.8 ± 129.2 months, 37 patients (6.1%) died. The mortality rate was 32 in the IC-MPGN group (5.7%) and five in the C3G group (10.2%). No statistically significant difference was observed in mortality between the groups (p = 0.18) (Fig. 2B).
Discussion
This study classified MPGN into two groups based on new IF diagnostic criteria. In the Korean population, the prevalence of C3G identified by this new IF-based classification was 8%. At the time of the kidney biopsy, the C3G group exhibited lower eGFR and proteinuria compared to those in the IC-MPGN group. Additionally, serum C3 levels were significantly lower in the C3G group, while serum IgG levels were significantly lower in the IC-MPGN group. The IC-MPGN group had a higher prevalence of malignancy and hepatitis B infection. Histologically, interstitial fibrosis, tubular atrophy, and glomerular cellularity were more prevalent in the IC-MPGN group.
While worse renal function was observed in the C3G group initially, there was no statistically significant difference between the groups in the risk of a 40% eGFR decline at the final follow-up. A total of 159 patients (27.7%) with MPGN progressed to ESKD during a follow-up period of 122.4 ± 124.2 months. The incidence of ESKD and mortality did not differ significantly between the groups. Despite exhibiting fewer chronic histological findings, a significant proportion of patients in the C3G group progressed to ESKD. This highlights the importance of adequate treatment and close monitoring for patients diagnosed with C3G.
C3G is defined as a type of proliferative GN with C3 deposition at least two orders of magnitude greater than any other immune deposition on IF microscopy [9]. Moreover, it is traditionally thought to be primarily diagnosed in children and young adults. Classically, C3G is associated with autoantibodies or mutations in key complement pathway proteins. These factors disrupt the regulation of the alternative complement pathway, leading to its dysregulation [21].
This study identified a significant number of older adults previously diagnosed with MPGN who were reclassified as having C3G. The prevalence of C3G in children and adolescents (10.9%) was higher than that in the IC-MPGN group (4.9%). Conversely, in individuals aged ≥65 years, a higher proportion (28.6%) were classified as C3G. Recent studies report an increasing prevalence of monoclonal gammopathy in patients with C3G >50 years old [22]. While we investigated the presence of multiple myeloma in our study, only one patient in the IC-MPGN group was diagnosed. The reason for the higher proportion of older adults with C3G in our study remains unclear.
However, patients in the C3G group had significantly lower serum C3 levels compared to those in the IC-MPGN group. This finding suggests a higher level of activation in the alternative complement pathway for patients with C3G. Smaller studies involving pediatric patients have shown C3G to have a poorer renal prognosis and be more resistant to immunosuppressant therapy compared to that in IC-MPGN [1,2]. Similarly, a study with 156 patients (30% of whom were 17 years old or younger) had faster progression in C3G than that in IC-MPGN [4]. However, a French multicenter study of 134 patients with MPGN (including 52 with early onset before age 16 years) reported similar renal survival rates between IC-MPGN and C3G subtypes (C3GN and dense deposition disease [DDD]) [15]. In contrast, a Japanese study analyzing 81 patients from 15 hospitals observed a more favorable overall prognosis for patients with C3G [16]. These findings highlight the inconsistent results from previous studies comparing renal outcomes between IC-MPGN and C3G.
This study examined the long-term renal outcomes of patients with immunologically classified IC-MPGN and C3G. Both groups showed unfavorable outcomes after a mean follow-up period of 10.2 years. In the IC-MPGN group, 28.4% of patients progressed to ESKD, while 19.0% of patients in the C3G group did the same. The worsening of renal function observed in the C3G group persisted for up to 6 months after diagnosis. However, there was no significant difference in ESKD development between the two groups during long-term follow-up.
IC-MPGN is typically caused by the deposition of immune complexes resulting from infections, autoimmune diseases, monoclonal gammopathy, and idiopathic causes. In this study, the IC-MPGN group exhibited a significantly higher prevalence of underlying conditions such as hepatitis B or cancer, which might be causative or contribute to glomerular disease. Among the 97 patients diagnosed with cancer, there were 96 patients in the IC-MPGN group (98.9%) and one patient in the C3G group (1.0%). Although information on the types of cancer was available for only a limited number of cases, the following cancers were identified in the IC-MPGN group: thyroid cancer in three patients, breast cancer in one patient, duodenal cancer in one patient, hepatocellular cancer in one patient, rectal cancer in one patient, lung cancer in one patient, malignant lymphoma in one patient, multiple myeloma in one patient, RCC in one patient, and uterine cancer in one patient. Additionally, one patient in the C3G group was diagnosed with early gastric cancer. Additionally, histological findings such as interstitial fibrosis and tubular atrophy were significantly more prevalent in patients with IC-MPGN, suggesting a more chronic disease process.
In contrast, C3G displayed less extensive chronic features on pathology but presented severe renal dysfunction at the initial evaluation. It remains unclear why C3G, despite initially poor renal function, exhibits fewer chronic histologic features. Patients in the IC-MPGN group may have experienced prolonged antigen-antibody responses and subsequent complement activation due to conditions such as viral hepatitis, cancer, or autoimmune diseases prior to diagnosis. In contrast, for C3G, we hypothesize that the occurrence of a second hit (e.g., surgery, infection) might lead to spontaneous complement dysregulation, resulting in a more acute form of kidney injury [19,23]. Our study did not reveal any significant difference in treatment regimens between the groups, and the long-term renal outcomes for C3G remained as unfavorable as those for IC-MPGN. This suggests that medical interventions could potentially benefit patients with C3G; however, further evidence is required to substantiate this potential benefit.
This retrospective study encompasses several limitations as the retrospective nature of the study of limited treatment data and availability. Firstly, this study is a retrospective analysis conducted between 1979 and 2018, involving diagnoses made by different pathologists across 18 hospitals, and reevaluation of the enrolled samples was not performed. Secondly, the number of confirmed cases in the C3G group was smaller compared to the IC-MPGN group. In addition, data on the DDD type within the C3G group were available only for a very limited subset of cases. Because the total number of C3G patients was relatively small, some details of those who progressed to ESKD may be missing in the C3G group. Therefore, we analyzed the baseline characteristics of the C3G group by categorizing patients into those who progressed to ESKD and those who did not. Patients who were older, had worse renal function, and higher levels of albuminuria at the time of biopsy demonstrated a significantly higher rate of progression to ESKD (Supplementary Table 2, available online). Finally, genetic studies related to C3G were not conducted. However, a major strength of this study is its large scale, utilizing data on pathological findings and the new classification system for MPGN from 18 tertiary hospitals to analyze long-term outcomes.
In conclusion, this study reclassified patients previously diagnosed with MPGN based on IF and compared their prognoses. C3G exhibited worse initial renal dysfunction, with significantly lower serum C3 levels. Despite exhibiting fewer chronic histological features in patients with, the long-term renal outcome for C3G remained unfavorable and comparable to IC-MPGN.
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