Abstract
Background and objectives
Glomerular IgM deposition is commonly shown in primary FSGS and sometimes accompanied by C3 deposition. Clinical presentation and treatment outcomes of these patients are not investigated in detail.
Design, setting, participants, &measurements
One hundred six consecutive patients with biopsy–proven primary FSGS from 2004 to 2014 were enrolled retrospectively. Clinical features and treatment outcomes were compared between patients with and without IgM/C3 deposition.
Results
Fifty-eight (54.7%) patients presented with IgM glomerular deposition on sclerotic segments. C3 and C1q depositions were shown exclusively in patients with IgM deposition (34.5% versus 0.0%; P<0.001 and 8.6% versus 0.0%; P=0.04, respectively). Patients with IgM deposition were younger (median; range: 24.5; 18.8–39.0 versus 46.5; 26.0–64.0 years old; P=0.001), had higher level of serum IgM (142.5; 96.3–206.0 versus 107.0; 71.0–140.0 mg/dl; P=0.01), and had higher level of eGFR (median; range 97.7; 48.0–135.8 versus 62.1; 33.7–93.9 ml/min per 1.73 m2; P=0.01) at the time of kidney biopsy. The percentage of sclerosis lesions was significantly higher in patients with C3 deposition (median; range: 21.7%; 15.3%–31.1% versus 9.2%; 6.6%–20.0%; P=0.002). Although patients received comparable immunosuppressive treatments during 58.9 (29.5–81.1) months of follow-up, a significantly higher prevalence of refractory cases (no response or steroid dependent) occurred in patients with combined IgM and C3 deposition compared with patients with IgM deposition alone or without IgM deposition (58.8% versus 22.2% versus 15.6%, respectively; P=0.004). Multivariate analysis identified combined IgM and C3 deposition (odds ratio, 11.32; 95% confidence interval, 2.26 to 56.65; P=0.003) as an independent risk factor for refractory patients; 19 of 98 patients developed renal dysfunction when their serum creatinine levels increased >30% from baseline and reached >1.5 mg/dl. Combined IgM and C3 deposition (hazard ratio, 5.67; 95% confidence interval, 1.34 to 23.84; P=0.02) was identified as an independent risk factor for renal dysfunction.
Conclusions
Patients with primary FSGS and IgM and C3 deposition showed unfavorable therapeutic responses and worse renal outcomes, which indicate that IgM and C3 deposition might involve disease progression via complement activation.
Keywords: focal segmental glomerulosclerosis, complement, outcomes, Biopsy, Complement Activation, creatinine, Disease Progression, Follow-Up Studies, Humans, Immunoglobulin M, Prevalence, risk factors
Introduction
FSGS is one of the most common primary glomerulopathies that lead to ESRD in adults, and it is characterized by focal and segmental glomerular sclerosis by light microscope, deposition of IgM with or without C3 on the sclerotic segment by immunofluorescence microscopy, and diffuse effacement of podocyte foot processes by electron microscopy (1).
Although irregular IgM staining is commonly shown in the sclerotic segments of FSGS (2), its pathogenic role for contributing to glomerular disease progression is unclear. Circulating IgM and C3 might be passively trapped in the sclerotic glomeruli. However, Strassheim et al. (3) recently showed that IgM activated the complement system within the glomerulus and mediated glomerular injury in adriamycin-induced FSGS. The depletion of B cells reduced glomerular IgM deposition and corresponded with an attenuated degree of albuminuria and glomerulosclerosis. In a model of nonimmune complex glomerular disease, IgM was also identified as binding to glomerular epitopes and contributing to glomerular damage progression (4).
In patients with primary FSGS, the clinical and pathologic significance of IgM and C3 deposition is not elucidated. The effect of IgM and C3 deposition on renal outcomes and its indication for optimal treatment approaches are also unclear. To our knowledge, this is the first study designed to investigate the significance of IgM and C3 glomerular deposition on clinical and pathologic features and outcomes of patients with primary FSGS in a large consecutive cohort. Our findings are helpful for understanding the mechanism of IgM and C3 deposition and its indication for clinical practice in patients with primary FSGS.
Materials and Methods
Patients
According to the definition of primary FSGS in the Columbia classification (5), 106 consecutive patients with renal biopsy–proven primary FSGS diagnosed at Peking University First Hospital from 2004 to 2014 were enrolled in this study. Their pathologic variants included 47 not otherwise specified (NOS) variants, 38 tip variants, and 21 cellular variants. Patients with familial, genetic, or known secondary FSGS, such as virus-associated FSGS, drug-induced FSGS, obesity-induced FSGS, or FSGS secondary to other glomerular diseases, were excluded. The latest clinical data before renal biopsy and during treatments and follow-up were collected from the medical records. The research was in compliance with the Declaration of Helsinki and approved by the ethics committee of our hospital.
The eGFR was calculated with the Modification of Diet in Renal Disease Study equation adjusted for the Chinese population: eGFR=175×(serum creatinine)−1.234× age−0.179×0.79 (if a woman) (6).
Renal Histopathology
Renal biopsy was performed for all patients at the time of diagnosis. Renal specimens were evaluated with direct immunofluorescence, light microscopy, and electron microscopy as described previously (7). The minimum requirement is 10 glomeruli for light microscopy and three glomeruli for immunofluorescence microscopy. Two pathologists reviewed the results separately, being blinded to each other as well as the patients’ clinical data. Discrepancy in diagnosis between the two pathologists was resolved by rereviewing the biopsies and coming to a consensus.
The fluorescence intensity was determined using a semiquantitative scale from zero to four: zero, negative; one, weak staining; two, moderate staining; three, strong staining; and four, glaring staining. The kidney pathologic variants of FSGS were defined according to the Columbia classification (5). The chronic tubulointerstitial injury included tubular atrophy and interstitial fibrosis, whereas the acute tubulointerstitial injury included the fall off of tubular brush border and interstitial infiltration. The scoring was graded semiquantitatively from zero to four: zero, normal; one, 5.0%–25.0% of interstitia affected; two, 25.0%–50.0% of interstitia affected; three, 50.0%–75.0% of interstitia affected; and four, >75.0% of interstitia affected.
Treatment and Response
Patients with nephrotic syndrome were treated with corticosteroid combined with immunosuppressive agents, including cyclophosphamide, cyclosporin A, and tacrolimus. Oral prednisone commenced at 1 mg/kg per day for up to 12–16 weeks with subsequent tapering, oral cyclophosphamide was at 1.5–2 mg/kg per day for 3 months, and cyclosporin A was at 2–3 mg/kg per day with a trough concentration around 100–150 μg/ml for 6–12 months. All patients were treated with angiotensin–converting enzyme inhibitors or angiotensin receptor blockers. No patient received rituximab therapy. Patients admitted after 2012 received treatments according to the Kidney Disease Improving Global Outcomes (KDIGO) guidelines for GN (8).
For evaluation of therapeutic responses, complete remission was defined as urinary protein excretion <0.3 g/24 h with normal renal function. Partial remission was defined as urinary protein excretion <3.5 g/24 h but >0.3 g/24 h and ≥50% reduction from peak values with stable serum creatinine (≤25% increase from baseline). Treatment failure was defined as not reaching the criteria of partial remission. Relapse was defined as urinary protein excretion >3.5 g/24 h after remission. Steroid dependent was defined as two relapses during or within 2 weeks of completing steroid therapy. Persistence of proteinuria despite steroid treatments for >4 months was defined as steroid resistant. The refractory FSGS was defined as no response to treatments or steroid dependent.
For evaluation of renal outcomes, the primary end point was ESRD, which was defined as dependence on RRT; the secondary end point was renal dysfunction, which was defined as serum creatinine increasing >30% from the baseline and reaching >1.5 mg/dl. Renal survival time was the time point when patients reached the end point or the last visit if the patients did not reach end point during the entire follow-up period.
Statistical Analyses
Statistical analysis was performed using statistical software SPSS 13.0 (IBM SPSS, Chicago, IL). Differences in quantitative parameters were assessed using t tests for data that were normally distributed and nonparametric tests for data that were not normally distributed. Differences in semiquantitative data were tested using the Mann–Whitney U test. Differences in qualitative data were compared using the chi-squared or Fisher exact test. Risk factors for refractory nephrotic syndrome in patients with primary FSGS were analyzed using the logistic regression model. Results were expressed as odds ratios (ORs) and 95% confidence intervals (95% CIs). Kaplan–Meier curves were used to analyze the outcomes of patients. If the P value of the candidate predictor in univariate survival analysis was <0.05, this predictor was included in multivariable Cox regression models. Results were expressed as hazard ratios (HRs) and 95% CIs. All statistical analyses were two tailed, and a P value <0.05 was considered as significant.
Results
Demographic and Clinical Parameters of Patients
In total, 106 consecutive patients with renal biopsy–proven primary FSGS were enrolled; 58 (54.7%) patients had IgM deposition in glomeruli, especially on the sclerosis lesions, whereas 48 patients did not have IgM deposition. The demographic and clinical data of patients in these two groups are shown in Table 1.
Table 1.
The clinical and pathologic parameters of patients with primary FSGS with and without IgM deposition
| Parameter | IgM+, n=58 | IgM−, n=48 | P Value |
|---|---|---|---|
| Sex, men/women | 40/18 | 30/18 | 0.48 |
| Age, yr | 24.5, 18.8–39.0 | 46.5, 26.0–64.0 | 0.001 |
| Urinary protein, g/24 h | 7.4, 4.9–14.6 | 7.6, 5.6–12.4 | 0.96 |
| Time from onset to biopsy, mo | 6.0, 0.7–24.0 | 4.5, 1.0–24.0 | 0.86 |
| Serum albumin, g/dl | 2.2±0.8 | 2.0±0.5 | 0.18 |
| Nephrotic syndrome, n (%) | 53 (91.4) | 47 (97.9) | 0.30 |
| Serum cholesterol, mg/dl | 359.1±158.3 | 351.4±150.6 | 0.79 |
| Hematuria, n (%) | 28 (48.3) | 30 (62.5) | 0.14 |
| Hypertension, n (%) | 20 (34.5) | 25 (52.1) | 0.07 |
| Serum creatinine, mg/dl | 0.9, 0.7–1.6 | 1.2, 0.9–2.0 | 0.04 |
| eGFR, ml/min per 1.73 m2 | 97.7, 48.0–135.8 | 62.1, 33.7–93.9 | 0.01 |
| Declined eGFR, <60 ml/min per 1.73 m2, n (%) | 21 (36.2) | 26 (54.2) | 0.06 |
| AKI, n (%) | 11 (19.0) | 19 (39.6) | 0.02 |
| Serum IgG, mg/dl | 406.0, 227.0–670.0 | 365.5, 270.0–478.0 | 0.62 |
| Serum IgA, mg/dl | 186.0, 135.0–265.0 | 176.0, 124.0–235.0 | 0.50 |
| Serum IgM, mg/dl | 142.5, 96.3–206.0 | 107.0, 71.0–140.0 | 0.01 |
| Serum C3, mg/dl | 104.8±27.1 | 103.0±37.1 | 0.80 |
| Serum C4, mg/dl | 27.4±8.9 | 30.1±13.0 | 0.27 |
| Glomerular variants, n (%) | 0.61 | ||
| NOS | 25 (43.1) | 22 (45.8) | |
| Cellular | 10 (17.2) | 11 (22.9) | |
| Tip | 23 (39.7) | 15 (31.3) | |
| Glomeruli with sclerosis, % | 12.4, 7.4–24.4 | 13.6, 8.3–27.1 | 0.30 |
| Chronic tubulointerstitial injury, +, n (%) | 1.1±0.8 | 0.8±0.7 | 0.07 |
| Grade 0 | 12 (20.7) | 16 (33.3) | |
| Grade 1 | 35 (60.3) | 28 (58.3) | |
| Grade 2 | 7 (12.1) | 1 (2.1) | |
| Grade 3 | 3 (5.2) | 3 (6.3) | |
| Grade 4 | 1 (1.7) | 0 (0.0) | |
| Acute tubulointerstitial injury, n (%) | 11 (19.0) | 22 (45.8) | 0.003 |
| IgG deposition, n (%) | 0 (0.0) | 0 (0.0) | >0.99 |
| IgA deposition, n (%) | 8 (13.8) | 4 (8.3) | 0.38 |
| C3 deposition, n (%) | 20 (34.5) | 0 (0.0) | <0.001 |
| C1q deposition, n (%) | 5 (8.6) | 0 (0.0) | 0.04 |
| Steroids and immunosuppressive therapy, n (%) | 48/53 (90.6) | 39/45 (84.8) | 0.38 |
| Response to treatment, n (%) | 0.43 | ||
| Complete remission | 29/53 (54.7) | 30/45 (66.7) | |
| Partial remission | 14/53 (26.4) | 10/45 (22.2) | |
| No response | 10/53 (18.9) | 5/45 (11.1) | |
| Relapse after remission, n (%) | 27/43 (62.8) | 23/40 (57.5) | 0.62 |
| Refractory nephrotic syndrome, n (%) | 18/53 (34.0) | 7/45 (15.6) | 0.04 |
| Renal dysfunction, n (%) | 15/53 (28.3) | 4/45 (8.9) | 0.02 |
| Renal survival time, mo | 47.4, 20.7–78.1 | 46.9, 22.8–79.0 | 0.66 |
| Follow-up time, mo | 66.8, 28.8–82.0 | 48.8, 30.6–80.2 | 0.64 |
NOS, not otherwise specified.
Among the 58 patients with IgM deposition, 40 patients were men, and 18 were women, with a median age of 24.5 (18.8–39.0) years old. All patients presented with proteinuria (median =7.4; range =4.9–14.6 g/24 h), with 53 (91.4%) patients having nephrotic syndrome and 28 (48.3%) patients having hematuria. Serum albumin level was 2.2±0.8 g/dl, and serum creatinine concentration was 0.9 (0.7–1.6) mg/dl at the time of renal biopsy.
Compared with the patients lacking IgM deposition, patients with FSGS and IgM deposition were younger (median; range: 24.5; 18.8–39.0 versus 46.5; 26.0–64.0 years old; P=0.001), presented with a higher level of serum IgM (median; range: 142.5; 96.3–206.0 versus 107.0; 71.0–140.0 mg/dl; P=0.01), had lower prevalence of AKI (19.0% versus 39.6%; P=0.02) during hospitalization, and had better renal function, presenting with a significantly higher level of eGFR (median; range: 97.7; 48.0–135.8 versus 62.1; 33.7–93.9 ml/min per 1.73 m2; P=0.01) and a lower level of serum creatinine (median; range: 0.9, 0.7–1.6 versus 1.2, 0.9–2.0 mg/dl; P=0.04) at the time of kidney biopsy (Table 1). The level of serum IgM was correlated with the intensity of glomerular IgM staining (r=0.27; P=0.01).
Pathologic Parameters
The 58 patients with IgM deposition had 25 NOS variants, 23 tip variants, and 10 cellular variants. The patients without IgM had 22 NOS variants, 15 tip variants, and 11 cellular variants. There was no difference between the two groups (P=0.61). The percentage of sclerosis lesions in glomeruli was also comparable between the patients with and without IgM deposition (median; range: 12.4%; 7.4%–24.4% versus 13.6%; 8.3%–27.1%; P=0.30) (Table 1). Acute tubulointerstitial injury was less common in the patients with IgM deposition (19.0% versus 45.8%; P=0.003) (Table 1).
On direct immunofluorescence, C3 and C1q depositions on glomeruli were detected in 20 of 58 (34.5%) and five of 58 (8.6%) patients with IgM deposition, respectively, whereas C3 and C1q depositions on glomeruli were shown in none (0.0%; P<0.001 and 0.0%, P=0.04, respectively) of the patients without IgM deposition (Table 1). The intensity of IgM deposition was correlated with the intensity of C3 deposition (r=0.43; P<0.001). The percentage of sclerosis lesions was significantly higher in patients with C3 deposition compared with in those without C3 deposition (median; range: 21.7%; 15.3%–31.1% versus 9.2%; 6.6%–20.0%; P=0.002). In patients with C3 deposition, there were five of 20 patients with C1q deposition, whereas in patients without C3 deposition, no patients had C1q deposition (25.0% versus 0.0%; P=0.01) (Table 2).
Table 2.
The parameters of patients with IgM deposition with and without combined C3 deposition
| Parameter | C3+, n=20 | C3−, n=38 | P Value |
|---|---|---|---|
| Sex, men/women | 13/7 | 27/11 | 0.64 |
| Age, yr | 27.0, 18.3–38.0 | 23.5, 18.8–40.5 | 0.93 |
| Urinary protein, g/24 h | 13.4, 4.8–16.6 | 7.1, 4.9–11.3 | 0.31 |
| Serum albumin, g/dl | 2.1±0.9 | 2.2±0.8 | 0.50 |
| Hematuria, n (%) | 10 (50.0) | 18 (47.4) | 0.85 |
| eGFR, ml/min per 1.73 m2 | 78.9, 49.4–126.0 | 105.6, 47.8–142.5 | 0.21 |
| Serum creatinine, mg/dl | 1.0, 0.8–1.7 | 0.9, 0.7–1.4 | 0.31 |
| Intensity of IgM+ deposition | 1.6±0.7 | 1.5±0.6 | 0.74 |
| C1q deposition, n (%) | 5 (25.0) | 0 (0.0) | 0.01 |
| Glomeruli with sclerosis, % | 21.7, 15.3–31.1 | 9.2, 6.6–20.0 | 0.002 |
| Glomerular variants, n (%) | 0.21 | ||
| NOS | 10 (50.0) | 15 (39.5) | |
| Cellular | 5 (25.0) | 5 (13.2) | |
| Tip | 5 (25.0) | 18 (47.4) | |
| Immunosuppressive therapy, n (%) | 16/17 (94.1) | 32/36 (88.9) | 0.92 |
| Response to treatment, n (%) | 0.02 | ||
| Complete remission | 6/17 (35.3) | 23/36 (63.9) | |
| Partial remission | 4/17 (23.5) | 10/36 (27.8) | |
| No response | 7/17 (41.2) | 3/36 (8.3) | |
| Relapse after remission, n (%) | 5/10 (50.0) | 22/33 (66.7) | 0.56 |
| Refractory nephrotic syndrome, n (%) | 10/17 (58.8) | 8/36 (22.2) | 0.01 |
| ESRD, n (%) | 5/17 (29.4) | 3/36 (8.3) | 0.11 |
| Renal survival time, mo | 29.9, 12.7–62.0 | 64.5, 30.8–82.0 | 0.02 |
| Follow-up time, mo | 54.4, 23.8–82.5 | 67.8, 30.8–82.0 | 0.39 |
NOS, not otherwise specified.
Treatment Responses
During follow-up, eight patients were lost, including five (8.6%) patients with IgM deposition and three (6.3%) patients without IgM. The percentages of missing patients were comparable between the two groups (P=0.93). The follow-up time was also comparable between patients with and without IgM deposition (median; range: 66.8; 28.8–82.0 versus 48.8; 30.6–80.2 months; P=0.64).
Of the 53 patients with IgM deposition, 48 (90.6%) patients received immunosuppressive treatments combined with prednisone, which was similar to that of patients without IgM deposition (84.8%; P=0.38) (Table 1). After treatments, 43 of 53 (81.1%) patients obtained remission, including 29 (54.7%) with complete remission and 14 (26.4%) with partial remission. However, 27 of 43 (62.8%) patients underwent relapse during follow-up. There were no differences in the remission and relapse rates compared with those in the patients without IgM deposition (Table 1).
Of the 17 patients with combined IgM and C3 deposition, ten (58.8%) patients obtained remission, including six (35.3%) patients with complete remission and four (23.5%) patients with partial remission; this was significantly lower than that of patients without C3 deposition (33 of 36; 91.7%; P=0.02). The relapse rates were comparable between the patients with and without C3 deposition (Table 2).
The percentage of refractory patients was significantly higher in the patients with combined IgM and C3 deposition compared with in patients with IgM deposition alone and patients without IgM deposition (58.8% versus 22.2% versus 15.6%, respectively; P=0.004) (Figure 1). Univariate logistic regression analysis showed that the combined IgM and C3 deposition was a risk factor for refractory nephrotic syndrome in patients with primary FSGS (OR, 7.76; 95% CI, 2.20 to 27.29; P=0.001). Higher intensity of C3 staining in glomeruli was also a predictor of refractory patients (OR, 3.32; 95% CI, 1.69 to 6.54; P=0.001). Multiple logistic regression analysis further identified the combined IgM and C3 deposition as an independent risk factor for refractory nephrotic syndrome (OR, 11.32; 95% CI, 2.26 to 56.65; P=0.003), whereas IgM deposition alone was not. Higher intensity of C3 staining also independently predicted refractory nephrotic syndrome (OR, 2.74; 95% CI, 1.25 to 5.98; P=0.01). The baseline eGFR was also an independent risk factor for refractory nephrotic syndrome (OR, 0.85; 95% CI, 0.74 to 0.97; P=0.02) (Table 3).
Figure 1.
Comparison of the percentage of refractory nephrotic syndrome among patients without IgM deposition, patients with IgM deposition alone, and patients with combined IgM and C3 deposition. Patients with combined IgM and C3 deposition had a higher percentage of refractory nephrotic syndrome (58.8% versus 22.2% versus 15.6%, respectively; P=0.004).
Table 3.
Risk factors for refractory nephrotic syndrome in patients with primary FSGS
| Parameter | Univariate Analysis | Multivariate Analysis | ||
|---|---|---|---|---|
| OR (95% CI) | P Value | OR (95% CI) | P Value | |
| Sex, men | 0.77 (0.29 to 2.00) | 0.59 | — | — |
| Age, per 10 yr | 0.91 (0.72 to 1.16) | 0.45 | — | — |
| Urinary protein, per 1 g/24 h | 1.05 (0.98 to 1.13) | 0.18 | — | — |
| eGFR, per 10 ml/min per 1.73 m2 | 0.88 (0.79 to 0.98) | 0.02 | 0.85 (0.74 to 0.97) | 0.02 |
| IgM deposition | ||||
| IgM− | Reference | |||
| IgM+, C3− | 1.55 (0.50 to 4.78) | 0.45 | 3.97 (0.93 to 17.00) | 0.06 |
| IgM+, C3+ | 7.76 (2.20 to 27.29) | 0.001 | 11.32(2.26 to 56.65) | 0.003 |
| Intensity of IgM staining, per 1+ | 1.60 (0.98 to 2.62) | 0.06 | — | — |
| Intensity of C3 deposition, per 1+ | 3.32 (1.69 to 6.54) | 0.001 | 2.74 (1.25 to 5.98) | 0.01 |
| Percentage of sclerosis, per 10% | 1.85 (1.23 to 2.77) | 0.003 | 1.63 (0.99 to 2.68) | 0.06 |
| Pathology variants | ||||
| NOS variant | Reference | |||
| Cell variant | 1.24 (0.38 to 4.06) | 0.72 | 1.12 (0.25 to 4.86) | 0.90 |
| Tip variant | 0.26 (0.08 to 0.87) | 0.03 | 0.32 (0.08 to 1.37) | 0.13 |
| Chronic tubulointerstitial injury, per 1+ | 2.26 (1.25 to 4.09) | 0.01 | 1.18 (0.58 to 2.41) | 0.65 |
| Acute tubulointerstitial injury | 1.90 (0.73 to 4.93) | 0.19 | — | — |
OR, odds ratio; 95% CI, 95% confidence interval; —, not included in the multiple analysis; NOS, not otherwise specified.
Renal Outcomes
During follow-up, eight of 53 (15.1%) patients with IgM deposition had ESRD, whereas only three of 45 (6.7%) patients without IgM deposition had ESRD, although this was not statistically significant (P=0.19). Five of 17 (29.4%) patients with C3 deposition had ESRD, whereas only three of 36 (8.3%) patients without C3 deposition had ESRD (P=0.11). The time for renal survival was significantly shorter in the patients with C3 deposition compared with those without C3 deposition (median; range: 29.9; 12.7–62.0 versus 64.5; 30.8–82.0 months; P=0.02) (Table 2).
Compared with those without IgM deposition, patients with IgM deposition presented with a higher percentage of renal dysfunction (28.3% versus 8.9%; P=0.02) (Table 1). Kaplan–Meier curve revealed that renal outcome of the patients with combined IgM and C3 deposition was significantly worse than that of patients with IgM deposition alone and patients with negative IgM deposition(P=0.03) (Figure 2).
Figure 2.
Kaplan–Meier analysis of renal dysfunction in patients with primary FSGS: A comparison among patients without IgM deposition, patients with IgM deposition alone, and patients with combined IgM and C3 deposition. Patients with FSGS and combined IgM and C3 glomerular deposition had worse renal outcome during follow-up (P=0.03).
Using univariate Cox regression analysis, combined IgM and C3 glomerular deposition was identified as one of the risk factors for renal dysfunction in patients with primary FSGS (HR, 4.95; 95% CI, 1.39 to 17.58; P=0.01). Higher intensity of IgM staining predicted renal dysfunction (HR, 1.81; 95% CI, 1.13 to 2.89; P=0.01) as well as higher intensity of C3 staining (HR, 1.82; 95% CI, 1.22 to 2.72; P=0.003). Multivariate analysis identified the combined IgM and C3 deposition (HR, 5.67; 95% CI, 1.34 to 23.84; P=0.02) and IgM deposition alone (HR, 4.75; 95% CI, 1.32 to 17.05; P=0.02) both as independent risk factors for renal dysfunction. The baseline eGFR was also an independent risk factor for renal dysfunction (HR, 0.82; 95% CI, 0.71 to 0.94; P=0.01) (Table 4).
Table 4.
Risk factors for renal dysfunction of patients with primary FSGS.
| Parameter | Univariate Analysis | Multivariate Analysis | ||
|---|---|---|---|---|
| HR (95% CI) | P Value | HR (95% CI) | P Value | |
| Sex, men | 0.46 (0.19 to 1.14) | 0.10 | — | — |
| Age, per 10 yr | 0.93 (0.73 to 1.18) | 0.55 | — | — |
| Urinary protein, per 1 g/24 h | 1.04 (0.97 to 1.12) | 0.26 | — | — |
| eGFR, per 10 ml/min per 1.73 m2 | 0.85 (0.76 to 0.96) | 0.01 | 0.82 (0.71 to 0.94) | 0.01 |
| IgM deposition | ||||
| IgM− | Reference | |||
| IgM+, C3− | 2.65 (0.82 to 8.62) | 0.10 | 4.75 (1.32 to 17.05) | 0.02 |
| IgM+, C3+ | 4.95(1.39 to 17.58) | 0.01 | 5.67(1.34 to 23.84) | 0.02 |
| Intensity of IgM staining, per 1+ | 1.81 (1.13 to 2.89) | 0.01 | 1.62 (0.94 to 2.78) | 0.08 |
| Intensity of C3 deposition, per 1+ | 1.82 (1.22 to 2.72) | 0.003 | 1.50 (0.94 to 2.41) | 0.09 |
| Percentage of sclerosis, per 10% | 1.55 (1.08 to 2.24) | 0.02 | 1.19 (0.77 to 1.84) | 0.43 |
| Pathology variants | ||||
| NOS variant | Reference | |||
| Cell variant | 1.07 (0.38 to 3.06) | 0.89 | 1.07 (0.33 to 3.50) | 0.91 |
| Tip variant | 0.19 (0.04 to 0.85) | 0.03 | 0.24 (0.05 to 1.23) | 0.09 |
| Chronic tubulointerstitial injury, per 1+ | 2.76 (1.76 to 5.31) | <0.001 | 1.48 (0.89 to 2.47) | 0.13 |
| Acute tubulointerstitial injury | 1.98 (0.80 to 4.93) | 0.14 | — | — |
HR, hazard ratio; 95% CI, 95% confidence interval; —, not included in the multiple analysis; NOS, not otherwise specified.
Discussion
IgM irregular staining is commonly shown on the sclerotic segments of glomeruli in patients with primary FSGS, but its significance has not been elucidated yet. In this large consecutive cohort of patients with biopsy–proven primary FSGS, we revealed a prevalence of 54.7% of patients with IgM glomerular deposition, mostly on the sclerotic segments with an average intensity of 2+ (0–4+ scoring) and a minority in mesangial area with low intensity. IgG deposition was not seen in all patients. IgA weak staining was observed in only 11.3% of patients in mesangial areas. Electron-dense deposits were not found on nonsclerotic glomeruli and segments, which confirmed the finding of direct immunofluorescence. C3 and C1q depositions on sclerotic segments were detected in 18.9% and 4.7% of all patients, respectively. Although they were not as common as IgM deposit, their staining was exclusively shown in the patients with IgM deposition. The high prevalence and specific staining of IgM on sclerotic segment and its exclusive combination with C3 indicate that IgM glomerular deposits may involve in the kidney injury of primary FSGS rather than a nonspecific trap from circulation.
Patients with IgM deposition presented with heavy proteinuria and low levels of serum albumin, with high prevalence of nephrotic syndrome. These features were similar to those of the patients without IgM deposition. The pathologic variants and percentage of sclerotic glomeruli were also comparable between the two groups. These findings indicated that IgM deposition is not likely the initiation of FSGS lesion, which has been identified as several genes mutations or variants (9–13), circulating permeability factors (14–16), and others (17–23). However, IgM and complement-mediated injury may be secondary phenomena that occur after the primary renal insult. In a mouse model of nonsclerotic and nonimmune complex glomerular diseases, Panzer et al. (4) identified specific IgM antibodies binding to glomerular epitopes and contributing to glomerular damage progression. Strassheim et al. (3) found that the depletion of peritoneal B cells by hypotonic shock could prevent glomerular IgM deposition in an adriamycin–induced FSGS animal model without affecting the total IgM in serum. We found that the patients with IgM deposition presented with higher levels of serum IgM. This is consistent with the observations in IgM nephrology (23,24). These data implied that the glomerular IgM deposits are composed of specific clones of IgM, possibly from B-1a B cells (3,25,26), which are produced in circulation and bind to some epitopes on sclerotic segments. Epitope mapping of these neoepitopes is of importance for understanding the mechanism of IgM production and deposition on FSGS lesions.
At the time of kidney biopsy, the baseline level of eGFR was better and AKI was unlikely in patients with IgM deposition. All patients received the same treatments, irrespective of IgM or C3 deposition. However, over a comparable duration of long–term (58.9 months) follow-up, patients with IgM deposition presented with a higher prevalence of refractory nephrotic syndrome and higher tendency for renal dysfunction. Previous studies on minimal change disease found that the presence of mesangial IgM with dense deposits identified by electron microscopy worsens disease prognosis and thus, justifies altering the diagnosis to IgM nephropathy (27). In primary FSGS, this is the first report that combined IgM and C3 glomerular deposition is an independent risk factor for both treatment responses and renal outcomes. These findings indicated that IgM deposition may contribute to disease progression through activation of complement system.
IgM is a classic pathway activator. Dual staining of FSGS kidney tissue showed colocalization of IgM and C3d when they were both present (3). In an adriamycin–induced FSGS animal model, complement activation was shown in both glomeruli and tubule, which could lead to tissue damage (28,29). Recently, IgM was shown as the activator for complements and to contribute to kidney injury progression (3,4). Measurement of complement activation fragments in plasma further showed that the complement system is activated in this disease (30). However, both classic pathway and alternative pathway amplification were identified in this disease (J. Huang et al., unpublished data). It is possible that glomerular IgM, which binds to injury-associated epitopes, activates the complement system through the classic pathway and further amplifies it through the alternative pathway.
Our patients with primary FSGS and IgM deposition were treated according to the 2012 KDIGO guideline (8), irrespective of the glomerular deposition. Most (84.7%) patients did respond well and got complete or partial remission. However, 41.2% of patients with C3 deposition did not obtain remission, which occurred in only 8.3% of patients without C3 deposition. Combined IgM and C3 staining was identified as an independent risk factors for refractory nephrotic syndrome after treatments. These findings suggested that, for those patients with IgM and C3 deposition, the conventional immunosuppressive treatments, such as corticosteroids and calcineurin inhibitors, had limited efficacy in reducing antibody formation and inhibiting complement activation (31–33). Treatment using rituximab has shown efficacy in an adriamycin–induced FSGS animal model by reducing the deposition of IgM in glomeruli and attenuating the development of albuminuria (3). Successful clinical attempts using rituximab were also reported in several patients with primary FSGS (34,35). Treatments targeting complement, such as eculizumab, might also be beneficial for this disease. However, all of these suggestions need to be investigated in well controlled clinical trials. Another indication from these findings is in the requirement for identification of glomerular IgM and C3 deposition when randomizing patients into different groups or analyzing treatment responses in clinical trials for patients with FSGS, because these patients may have unfavorable responses to conventional immunosuppressive treatments.
The limitation of this study was that it was a retrospectively observational study; thus, a cause-effect relationship could not be established. This is the first report revealing clinical significance of IgM and C3 deposition in patients with primary FSGS; findings from this single-center study require validation from multicenter studies with larger cohorts. Although IgM was found in one half of patients with FSGS, C3 deposition was less common; thus, the mechanism for IgM and C3 involved in kidney injury progression was not fully interpreted. Findings from this study were all from Chinese adult patients and may not generalize to white or black patients or children with FSGS.
In conclusion, this study shows that glomerular combined IgM and C3 deposition of patients with primary FSGS was an independent risk factor for inferior treatment responses and worse renal outcomes. This finding indicates that IgM might bind to injury-associated epitopes and be involved in disease progression by activating complement system.
Disclosures
None.
Acknowledgments
The technical support of Lu Bai was greatly appreciated.
This work was supported by National Natural Science Foundation of China grants 81321064 (to the Innovation Research Group), 81330020 (to other programs), 81370801 (to other programs), and 81500542 (to other programs).
Footnotes
Published online ahead of print. Publication date available at www.cjasn.org.
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