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The prognostic role of activation of the complement pathways in the progression of advanced IgA nephropathy to end-stage renal disease

BMC Nephrology volume 25, Article number: 387 (2024) Cite this article

Abstract

Introduction

The role of complement system in late stage of IgA nephropathy (IgAN) remains unknown. We therefore investigated the effects of complement system on worsening kidney function in advanced (stage 4 CKD) IgAN.

Methods

Renal specimens of 69 IgAN patients who underwent renal biopsy during stage 4 CKD between 2010 and 2021, were stained using immunofluorescence (IF) and immunohistochemistry (IHC) for glomerular complement components. The primary outcome was progression to end-stage renal disease (ESRD). Associations of complement components with baseline clinicopathological characteristics and outcomes were assessed using multivariable Cox regression and Spearman analyses.

Results

During a median follow-up of 18.0 months, 26 (37.7%) patients progressed to ESRD and none died. C1q and C3 deposition were detected in 12 and 66 patients, respectively. Higher eGFR [hazards ratio (HR), 0.852, 95% confidence interval (CI), 0.756–0.959; P = 0.008], higher C3 intensity (HR, 2.955, 95%CI, 1.063–8.220; P = 0.038) and T1-2 score (HR, 2.576, 95%CI, 1.205–5.576, P = 0.015) were predictive of time to ESRD in CKD 4 stage IgAN. Significant expressions of C1q (P = 0.005), C4d (P < 0.001), factor B (P < 0.001), C3 (P = 0.042) and C5b-9 (P = 0.004) were identified in ESRD group than in non-ESRD group by IHC, while MBL expression was low. Although they were not associated with baseline 24 h-UP, higher factor B and C1q expressions were both correlated with a lower baseline eGFR (P < 0.001 and = 0.04, respectively) and the deterioration of kidney function during follow-up (P = 0.046 and 0.015, respectively).

Conclusion

Complement deposition in IgAN patients with stage 4 CKD portends a faster deterioration of kidney function. Activation of classical and alternative complement pathways plays a major role in this stage.

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Introduction

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and remains a leading cause of end-stage kidney disease (ESRD) [1]. Heterogeneous risk of progressive kidney function decline has made it a challenge for clinicians to identify these patients in time. Consequently, the availability of effective treatments and accurate biomarkers of disease severity and progression are limited. Most commonly, IgAN is asymptomatic at the early stage and when diagnosed, some patients have already suffered severe renal dysfunction. In clinic, patients with advanced (stage 4) chronic kidney disease (CKD) are seldom biopsied for a pathological diagnosis because of a high risk of post-biopsy complications such as bleeding, resulting in a limited understanding in clinicopathological characteristics of these patients.

Now the widely accepted framework for understanding of steps involved in pathogenesis of IgAN is the “multi-hit hypothesis” [2], and complement (C) activation is one of these pathogenic steps. The complement system is one of the main defense mechanisms of the innate immune system [3] and can be triggered through classic (CP), alternative (AP) and lectin pathways (LP). A growing body of evidence suggests that activation of complement system may correlate with glomerular inflammation and renal injury [4]. Immunohistochemical findings of markers of AP and LP in mesangium of biopsy samples confirm the activation of the two pathways in IgAN [5]. About 75–90% and 17–25% IgAN patients have shown the activation of AP and LP, respectively [5,6,7]. Furthermore, complement activation in serum or urine have been documented and associated with IgAN severity and prognosis [8, 9]. C1q is rare in the renal tissue of patients with IgAN [10], and its presence and clinical significance in IgAN are still inconclusive. Although evidence of glomerular C3 deposits is common in IgAN, the pathogenic and prognostic relevance of complement activation remains poorly defined in advanced IgAN, especially in those with stage 4 CKD.

Suggesting the importance of complement in IgAN, we therefore conducted this study to clarify the three pathways of complement activation and its prognostic relevance in IgAN patients with stage 4 CKD.

Methods

Study design and participants

This was a retrospective study. We included patients with IgAN as the only glomerular disease diagnosis who underwent a renal biopsy at China-Japan Friendship Hospital from January 1, 2010 to December 31, 2021. The renal biopsy was operated by clinicians with many years of relevant experience in our department. Inclusion criteria were as follows: (1) patients with stage 4 CKD at the time of biopsy; (2) age from 18 to 75 years. The main exclusion criteria were: patients (1) with secondary IgA deposits including systemic IgA-vasculitis or coexistence with other glomerular diseases; (2) who had a biopsy specimen with < 8 total glomeruli; (3) those without follow-up data (Fig. 1). Additionally, another 10 biopsy-proven primary IgAN patients of stage 1 CKD, who had normal renal function with mild mesangial proliferative glomerulonephritis on biopsy, were included as a control group for immunohistochemistry (IHC).

Fig. 1
figure 1

Flowchart of study participants. IgAN, IgA nephropathy; eGFR, estimated glomerular filtration rate; CKD, chronic kidney disease

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This study was approved by the ethics committee of the China-Japan Friendship Hospital (2021-113-K71). As this was a retrospective observational study of deidentified data, patient informed consent was not required. All the procedures that included human participants adhered to the Declaration of Helsinki.

Parameters

The following clinical parameters were collected from the electronic medical system: gender, age, body mass index (BMI), mean arterial pressure (MAP), 24 h urinary protein excretion (24 h-UP), urinary red blood cell count (URBC, the most before renal biopsy), hemoglobin (Hb), total cholesterol (TC), triglyceride (TG), parathyroid hormone (PTH), uric acid (UA) and serum IgA to C3 ratio. Estimated glomerular filtration rate (eGFR) was calculated using the creatinine-based Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation [11]. Post-biopsy use of renin-angiotensin system (RAS) blockade or immunosuppressive (IS) therapy were also retrospectively reviewed.

Endpoints and follow-up

The primary endpoint was a composite of ESRD. ESRD was defined as eGFR < 15 ml/min/1.73m2 or the initiation of maintenance dialysis or renal transplantation. The ESRD group was defined as those with the occurrence of the endpoint events, and the others was defined as the non-ESRD group. A maximum 3-year follow-up [18.0 (12.0–36.0) months] was conducted. The progression rate of kidney disease was assessed in terms of the mean change in eGFR per month during the follow-up time [△eGFR/M = (final eGFR - baseline eGFR) / follow-up time (months)].

Renal biopsy and pathological classification

All the renal biopsy specimens were preserved in the China-Japan Friendship Hospital. For each biopsy specimen, light microscopy, immunofluorescence, and electron microscopy were routinely performed. Renal biopsy specimens were reviewed by investigators blinded to the clinical outcomes.The intensity of direct immunofluorescence staining of C1q and C3 deposits in kidney tissue was graded using a semiquantitative method on a scale of 0–4+ (-, no fluorescence at either low magnification (×100) or high magnification (×400); ±/trace, no fluorescence at low magnification but somewhat visible at high magnification; +, somewhat visible at low magnification but clearly visible at high magnification; ++, clearly visible at either low or high magnification; +++, clearly visible at low magnification but dazzling at high magnification; and ++++, dazzling at low magnification and even more dazzling at high magnification) [12]. The prevalence of the desposits were in most glomeruli and predominantly in the mesangium of the glomerulus. The median number of glomeruli for immunomorphology studies was 16 (10–25).

All patients were performed the standardized MEST-C (mesangial [M] and endocapillary [E] hypercellularity, segmental sclerosis [S], interstitial fibrosis/tubular atrophy [T], and crescents[C]) scores according to the revised Oxford Classification [13]. All the specimens were scored by the same renal pathologist who was blinded to the study at the time of biopsy.

Immunohistochemistry (IHC) of complement pathways

IHC was performed according to previous methods [14, 15]. 3 μm paraffin sections were dewaxed and hydrated, and heat-mediated antigen retrieval was performed for C1q, C4d and C5b-9. Pepsin (Zhongshan Golden Bridge Biotechnology, Beijing, China, ZLI-9013) was used to retrieve C3 antigen at 37℃ for 30 min. The sections for staining factor B (FB) and mannose-binding lectin (MBL) were treated with 0.5 mg/ml of proteinase K (Magen, PK-100) for antigen retrieval at 37℃ for 10 min. After blocked with 3% peroxide-methanol at room temperature for endogenous peroxidase ablation, the sections were then incubated with goat serum at room temperature for 30 min. The antibodies used in this study were as follows: anti-C1q antibody (Abcam, ab268120, 1:500), anti-C3 antibody (Abcam, ab200999, 1:500), anti-factor B (FB) antibody (Abcam, ab192577, 1:500), anti-C4d antibody (Abcam, ab183311, 1:100), anti-mannose-binding lectin (MBL) antibody (Abcam, ab23457, 1:100) and anti-C5b-9 antibody (Abcam, ab55811, 1:500). After immunostaining, all sections were counterstained with hematoxylin. Immunohistochemical images were obtained using a Moticam 2506 instrument (Motic, Fujian, China) from sections observed microscopically at × 400 magnification (Nikon, Tokyo, Japan). The images were acquired using an integrated digital camera system (Nikon). Immunoreactivity was semiquantitatively evaluated in a blinded manner. Image Pro-plus (IPP) computer image analysis software (Media Cybernetics, Bethesda, MD, USA) was used to analyze the pixel density of the stained areas, and to quantify protein expression levels [14].

Statistical analysis

Continuous data were regarded as nonparametric and were presented as median and interquartile range (IQR). Differences were analyzed using the Mann-Whitney U non-parametric test. Categorical variables were reported as percentages and analyzed using the chi-square or Fisher’s exact test. Univariate and multivariate Cox proportional hazard model was used to calculate the hazard ratios (HRs) and 95% confidence intervals (CIs) for variables related to the composite outcome. Variables with P values < 0.05 in univariate analysis were included in multivariate Cox analysis. Spearman correlation analysis was performed to evaluate the relation of baseline eGFR, 24 h-UP and mean change of renal function with markers of alternative pathway (FB) and classical pathway (C1q) in those patients with positive expressions detected by IHC. Statistical analysis was performed using SPSS software (version 24.0; IBM Corp, Armonk, NY) and GraphPad Prism 8.0. Two-sided p values < 0.05 were considered to indicate statistical significance.

Results

Baseline and pathological characteristics

We included a total 69 IgAN patients with stage 4 CKD at the time of biopsy aged 18–75 years. The clinical characteristics of the patients are presented in Table 1. By the end of the follow-up, 26 (37.7%) of the 69 patients progressed to ESRD, and 10 of the 26 patients started renal replacement therapy or underwent renal transplantation during the follow-up time. At baseline, there was no significant difference in main laboratory parameters between the ESRD group and non-ESRD group. C3 and C1q deposits were both observed predominantly in the mesangium. As shown in Table 1, strong intensity of glomerular C3 deposition or C1q positivity were more frequent in ESRD group (P = 0.004 and < 0.001, respectively). The Oxford classification is an important pathological score in IgAN. It could be found that most patients had higher scores of M, E, S and T(1/2), while about half of the patients had C1/2 score. This might indicated an active ongoing glomerular inflammation in many patients, which was regarded as a concurrent cause for kidney dysfunction. Although without a statistical significance, in ESRD group, there were higher percentages of patients with T2 or C2 than those in non-ESRD group.

Table 1 Baseline information of IgAN patients with stage 4 CKD
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Association of C3 and C1q deposition and clinical outcomes

According to our results, the median renal survival time in ESRD group was 12.5 (6.8–27.5) months. No one died in the 3-year follow-up. All ESRD occurred in patients with glomerular C3 deposition, 4 (15.4%) and 22 (84.6%) patients with 1 + ~ 2 + and ≥ 3 + C3 stain, respectively. Although C1q deposition was identified in only 12 patients, 10 (83.3%) of these 12 patients progressed to ESRD. Figure 2A-B and D-E illustrate the degrees of C1q (1+) and C3 (3+) staining in glomeruli. Figure 2C and F show the Kaplan-Meier survival curves for the composite outcome stratified by glomerular C1q and C3 deposits. Figure 2C indicated a lower median renal survival time in patients with glomerular C1q deposition, about 22 months. Figure 2F suggested that ≥ 3 + glomerular C3 deposition was also associated with a higher risk of an adverse kidney outcome (P = 0.031). Multivariate Cox analysis also indicated C3 deposits as an independent risk factor of renal progression in IgAN [hazards ratio (HR), 2.955, 95% confidence interval (CI), 1.063–8.220; P = 0.038]. Although there were no significant differences in MEST-C scores at baseline, Cox analysis suggested a predictive role of T score in these patients [hazards ratio (HR), 2.576, 95% confidence interval (CI), 1.205–5.576; P = 0.015]. The results of the univariate and multivariate Cox proportional hazards analysis are displayed in Table 2.

Table 2 Univariate and multivariate Cox regression analysis between candidate predictors and survival of IgAN patients with stage 4 CKD
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Fig. 2
figure 2

Representative immunofluorescent staining for C1q and C3 at low magnification (×100) and at high magnification (×400) in IgAN patients and Kaplan-Meier survival curves (C, F) for primary outcomes according to glomerular C1q and C3 deposits respectively. (A) C1q 1+: seemingly visible at low magnification (×100), (B) C1q 1+: clearly visible at high magnification (×400), (C) Kaplan-Meier survival curve according to C1q deposits, (D) C3 3+: clearly visible at low magnification (×100), (E) C3 3+: dazzling at high magnification (×400), (F) Kaplan-Meier survival curve according to C3 deposits. Note: There were only 3 patients with no C3 immunofluorescent staining, so we combined those with C3 negative and with 1 + ~ 2 + C3 staining as one group

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Expression of glomerular complement components in advanced IgAN

The expressions of complement components detected by IHC were generally consistent with the results of IF (Supplementary Fig. 1). Our IHC results showed that compared to IgAN patients with stage 1 CKD who had normal renal function (details were displayed in Supplementary Table 1) with mild mesangial proliferative glomerulonephritis on biopsy, C1q was increased in IgAN patients with stage 4 CKD, with a significant difference between the ESRD and non-ESRD groups (P = 0.005). A very weak expression of MBL was found in only 4 patients in our study, while a significant increase of C4d was observed in patients with stage 4 CKD, compared to those with stage 1 CKD. A much higher expression of C4d was detected in ESRD group than in non-ESRD group (P < 0.001, Figs. 3 and 4C4d). FB is a marker of AP. In IgAN patients with stage 4 CKD, a significant expression of FB was identified by IHC compared to the control group (with stage 1 CKD who had normal renal function). Meanwhile, a notably higher expression of FB could be noticed in ESRD group compared with non-ESRD group (P < 0.001). Consequently, the expressions of C3 and C5b-9 were also increased in patients with stage 4 CKD than those with stage 1 CKD, especially in those of ESRD group (P = 0.042 and 0.004, compared to non-ESRD group respectively, Figs. 3 and 4C3 and C5b-9). The IHC staining results of complement components are shown in Figs. 3 and 4.

Fig. 3
figure 3

Immunohistochemical staining suggests increased expression of C1q, C3, C4d, FB, C5b-9 and MBL in advanced IgAN (×400), with significant differences in C1q, C3, C4d, FB and C5b-9 between ESRD and non-ESRD groups

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Fig. 4
figure 4

Significant differences were observed in the expression levels of C1q, C4d, FB, C3 and C5b-9 in renal biopsy specimens between the ESRD and non-ESRD groups. AOD, average optical density. *P < 0.05, **P < 0.01, ***P < 0.001

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Correlation of glomerular complement deposits with renal function and pathological features

To further investigate the correlation of complement activation with renal function and pathological features, the mean change in eGFR during follow-up period (△eGFR/M) was calculated according to glomerular C3 and C1q deposits respectively. There were no significant differences in baseline eGFR between groups stratified by whether C3 or C1q deposits. Table 3 indicated that C3 (P = 0.015) and C1q (P = 0.006) staining were both critical determinants of the slope of △eGFR/M. Spearman correlation analysis was conducted to explore the association of C1q and FB expression and baseline eGFR, 24 h-UP and △eGFR/M. As shown in Fig. 5A-B, higher expression of FB in glomeruli closely correlated with a lower baseline eGFR (r = -0.48, P < 0.001), but not with baseline 24 h-UP (P = 0.66). Similar to FB, glomerular C1q deposition was negative related to eGFR (Fig. 5D, r = -0.60, P = 0.04), rather than 24 h-UP at baseline (Fig. 5E, P = 0.63), but Fig. 5F showed a negative correlation of C1q expression with △eGFR/M (r = -0.68, P = 0.015). The same result could be found in the correlation of FB expression with △eGFR/M (Fig. 5C, r = -0.25, P = 0.046). This might suggest that activation of complement CP or AP made a contribution to the worsening renal function in these advanced (stage 4 CKD) IgAN patients. However, there were no significant differences in MEST-C scores in our study population when stratified by either glomerular C3 or C1q deposits (Table 4).

Table 3 The decline of estimated glomerular filtration rate over time in IgAN patients with stage 4 CKD stratified by glomerular C3 and C1q deposits
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Table 4 Pathologic features of advanced IgAN patients stratified by glomerular C3 and C1q deposits
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Fig. 5
figure 5

Spearman correlation showed a negative correlation of higher expression of FB and C1q with baseline eGFR (A, r = -0.48, P < 0.001; D, r = -0.60, P = 0.04) but not with 24 h-UP (B, P = 0.66; E, P = 0.63). Both FB and C1q expression were related to the mean eGFR decline (C, F, P = 0.046 and 0.015 respectively). AOD, average optical density; △eGFR/M, the mean change in eGFR over the number of follow-up months. Note: FB and C1q were detected in 67 and 12 patients, respectively

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Discussion

IgAN patients may suffer various degrees of renal dysfunction at the time of diagnosis. With a severe renal injury, patients with stage 4 CKD have been seldom biopsied for a definite diagnosis, leading to a limited clinical and pathological data. Activation of the complement system is one of the key participants in the injuries related to IgAN [4]. To our knowledge, this is the first study demonstrating that in advanced (stage 4 CKD at the time of biopsy) IgAN patients, a strong complement activation still can be identified, predominantly AP and partly CP, and both may be correlated with the progression to ESRD in these patients. High C1q and FB expression in glomeruli are closely related to the slope of eGFR decline.

The complement system is a vital effector of human innate immunity and a modulator of adaptive immunity. Abnormality of complement system leads to a variety of diseases, and kidneys are especially vulnerable for complement-mediated attacks [16]. It is acknowledged that complement activation plays a role in a variety of kidney diseases, including atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathy (C3G) and membranous nephropathy (MN) [17, 18]. The important role of complement activation in IgAN has long been recognized [19]. The abundance of mesangial C3 deposition, which can be quantified from immunofuorescence studies, correlates with IgAN severity and progression [20, 21]. In our study, advanced (stage 4 CKD) IgAN patients, with either glomerular C3 or C1q deposits, are more likely to reach ESRD. Further Kaplan-Meier survival curves show higher intensity of C3 (≥ 3+) or C1q deposits may lead to a shorter renal survival, indicating an effect of complement system on the disease prognosis. However, it should be noted that in our study, the abundance of mesangial C3 deposition showed no significant correlation with the Oxford classification scores, which might be inconsistent with other studies [22, 23]. This may be due to our limited samples and our study population. Besides, Our patients have suffered a worse clinical indices and pathological lesions at baseline, for complement activation might interact as both cause and effect. Thus, more studies of larger scales are needed.

Previous studies have regarded AP as a mainly activated pathway in IgAN [17, 24]. Aberrant AP is responsible for the pathogenesis of IgAN [25]. Chiu et al. [24] have proposed that AP was activated in IgAN and factor B was also detected in renal tissue from IgAN patients but not FSGS patients. Besides, the level of plasma factor Ba is positively correlated with clinical disease activity including urinary protein to creatinine ratio (UPCR) and renal function, and associated with galactose-deficient IgA1 (Gd-IgA1) antibody [24]. Even the deposition of regulatory proteins of AP has been related to this disease. An immunostaining approach recently confirmed deposition of complement factor H-related protein 1 (CFHR1) and 5 (CFHR5), with frequency of the finding dependent on disease severity [7]. Exome chip analyses have shown a significantly higher factor B transcripts, an important component of AP, in glomeruli of patients with IgAN, at least 1.5 folds compared to healthy controls [26]. In parallel, our IHC staining of factor B was detected in about 97.1% (67/69) of IgAN patients with stage 4 CKD, suggesting a predominant activation of AP. The level of glomerular factor B expression is negatively correlated with baseline eGFR as well as the mean change rate of eGFR (△eGFR/M), but not with baseline 24 h-UP. However, it still needs further validation in larger cohort studies.

C1q is considered as a critical protein of CP. The presence of C1q deposition shows an involvement of classic pathway, which might be seen in other kidney diseases such as lupus nephritis, thus patients with lupus nephritis were excluded in our study. CP is not generally considered to play a key role in native IgAN, contrast to other glomerular diseases [6]. Some prior studies showed that no association between glomerular mesangial C1q deposition and clinical characteristics in patients with IgAN while others thought it was associated with poor renal prognosis and serious pathological features [27, 28]. Another multi-central, prospective study enrolling 1071 IgAN patients proposed C1q was a predictor of poor renal survival during an average follow-up time of 41.89 months [29]. In our cohort, consistently, glomerular C1q deposition was detected in 12 (17.4%) patients and Kaplan-Meier survival curve indicated a shorter mean renal survival time of C1q positive patients. Among the 12 patients, glomerular C1q expression was negatively associated with baseline eGFR and the slope of eGFR decline, but not proteinuria at baseline. Thus, in advanced (stage 4 CKD) IgAN, according to our data, there is an activation of CP in some patients. Although a small proportion, mesangial C1q deposition might be another predictor of poor renal prognosis. The study of Tan et al. [29] also suggested higher rates of severe pathological changes (E1, T1 or T2, C1 or C2 of Oxford classification) in the C1q positive patients compared with the C1q negative patients, but this association was not observed in IgAN with stage 4 CKD according to our results (Supplementary Table 2). This may be due to the relatively limited sample size and the more severe renal injuries (clinical and pathological changes) of patients in our study at the time of biopsy.

In addition, Tan et al. also found that patients with positive C1q deposition had a high level of proteinuria and a Japanese retrospective cohort study revealed that C1q deposition was closely associated with the clinical remission of both proteinuria and hematuria in IgAN patients [30]. Another study has also reported that C1q deposition in the mesangial area and glomerular capillary loops was related to adverse renal outcomes [31]. These might indicate a predictive role of C1q in IgAN, but the possible mechanisms remain unclear. There is a hypothesis suggesting that deposition of IgA in mesangial areas is usually accompanied by IgG or IgM, which would activate C1q-mediated classical complement pathway to augment the inflammatory cascade and potentiate tissue injury in IgAN [28]. In line with the hypothesis, in our study, IgG or IgM, with C1q co-deposits also seemed to be more frequent in patients of ESRD group (Supplementary Table 3). However, this still needs more evidence-based studies to explore the possible mechanisms.

MBL is reported to be identified in about 25% patients with IgAN [6]. Although C4d is a crossover of both CP and LP activation, in IgAN, it can be still found in those with absent C1q, and considered as a product mainly from LP activity. Clinical observations have proposed that glomerular deposits of MBL, C4d and even arteriolar C4d deposition may be closely related to the disease severity and worse prognosis [32,33,34,35,36,37]. Zhang H et al. also found the involvement of excess complement activation in formation of crescents in IgAN, and urinary C4d as a potential biomarker for disease monitoring in crescentic IgAN [9]. Yet, our IHC results indicated just a very weak expression of MBL in only 4 (5.8%) patients, maybe because the patients enrolled in previous studies about MBL expression in IgAN were most under relatively early stages CKD [6, 33], while our study was set the special emphasis on those with stage 4 CKD at the time of biopsy. C4d expression was significantly higher in advanced IgAN, especially those of ESRD group, in accordance with the correlation of C4d deposition and poor renal survival. However, on the basis of our results, C4d in IgAN with stage 4 CKD might be regarded as a downstream product mainly from the classic pathway, but not lectin pathway. C3 and C5b-9 could be detected in nearly all the patients as the evidence of terminal pathway activation.

The Oxford classification is one of the acknowledged predictive factors for IgAN progression. MEST-C classes all have something to do with the disease prognosis, and dynamic Oxford-based histological evaluation offered by a repeat biopsy, especially T and C changes, improves the prediction of ESRD in patients with IgAN [38]. Since updated in 2016, there have been a series of studies focusing on the link between MEST-C scores and complement activation, either in the serum or kidney. As classical pathway was not known to play a major role in IgAN, most studies were based on the alternative and lectin pathways [39]. C3, the most common complement protein, was associated with different MEST-C classes in different studies [23, 36, 40, 41], while Itami et al. [42] and Pan et al. [43] found there were no relationship between C3 and MEST-C scores, in kidney and serum, respectively. In our results, patients with T2 were more frequent in the ESRD group and Cox analysis proposed the predictive role of T score. However, when stratified by C3 or C1q staining intensity, no significant relationship was found between C3 or C1q and Oxford MEST-C classes, perhaps because of an active glomerular inflammation at the beginning of many patients, which was responsible for the renal dysfunction, and a small size of our samples.

Generally, though immunosuppressive therapy can significantly reduce proteinuria and ESRD risk in patients with IgAN, it may bring a concomitant increase in adverse reactions [44]. Therefore, most mild cases of IgAN do not require immunosuppression, and conservative management (ACEIs/ARBs) is deemed sufficient [45]. Despite the suspected (auto)immune nature of IgAN, the use of immunosuppressants remains controversial [46]. A series of studies on pathogenic mechanisms of IgAN recently has moved into the focus of drug development efforts in the last decade, including the complement system, since its inappropriate or uncontrolled activation has been recognized in many diseases [16]. This has sparked the initiation of multiple clinical trials evaluating the benefits of various inhibitors of the complement cascade in IgAN [47]. Several complement inhibitors, like C3 inhibitor (APL-2), C5a receptor antagonist (CCX168), and selective factor B inhibitor (LNP023), are currently being developed for IgAN treatment, mostly in phase 2 or 3 clinical trials [48]. However, the included population in almost all trials were those with only mild or without renal dysfunction, lacking the data about patients with a eGFR of 15–30 ml/min/1.73m2 or significant renal inflammation at the beginning. Thus, our data may provide some some background for targeted complement inhibition in IgAN, but for those with ongoing inflammation and complement deposits and with severe renal dysfunction. IgAN is a highly complex and heterogeneous nephropathy, with complement being just one of the many contributing factors. Actually, the goal of complement inhibition is to shift the complement balance towards enhanced regulation, and it is also crucial to weigh the anticipated benefits of prolonged complement inhibition against the potential risks of infection and, possibly, other undiscovered long-term side effects, so the safety needs more clinical evidence-based studies [19].

Nevertheless, there are several limitations of our study. First, the number of patients with stage 4 CKD undergoing renal biopsy is still small. Therefore, our findings still need to be confirmed in a multi-center study with a larger sample size. Beside the small number of subjects, the short follow-up time and the lacking of remission data following immunosuppression therapies also should be mentioned, for the immunosuppressants could limit complement-induced inflammation and delay renal function progression. Moreover, this is a retrospective study and methodologic limitations must be taken into account, so causality is difficult to infer. Some of the data, for example, the pre-biopsy treatments might be failed to be collected. It was possible that there might be some patients had received some treatments even like traditional Chinese medication before the biopsy, which could definitely have an influence on their clinical or pathological lesions. Another limitation is that in our study complement component deposits were observed and investigated only on the basis of the glomerular compartment, without considering tubular and vascular complement deposits, which might also mediate the kidney injury. Based on the above facts, our findings might be only related to our study population, who had a relatively active glomerular inflammation, but not generlisable to other IgAN cases, like those with mild inflammation. Last but not least, as mentioned above, the probable correlation between complement deposits and pathological characteristics like MEST-C scores in IgAN should be further investigated in more large-scale and multi-center researches.

Conclusion

In conclusion, our study displays the activation of different complement pathways in IgAN with stage 4 CKD, and classical and alternative pathways, both may accelerate the progression to ESRD in these patients.

Data availability

The data in the current study are all available and displayed in our article.

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Acknowledgements

We thank the participants of the study. We also thank the Department of Nephrology, China-Japan Friends Hospital, for providing the technical assistance.

Funding

This research was funded by the China Postdoctoral Science Foundation (Certificate number 2023M733986 and 2023T160741), the cross-sectional project of China-Japan Friendship Hospital (Certificate number 2023-HX-JC-10) and International Association of Chinese Nephrologists Research Grant (No. IACNRG-01).

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Authors and Affiliations

  1. Department of Nephrology, China-Japan Friendship Hospital, No. 2 East Yinghuayuan Street, Chaoyang District, Beijing, 100029, China

    Ying Wang, Shimin Jiang, Dingxin Di, Guming Zou, Hongmei Gao, Shunlai Shang & Wenge Li

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  1. Ying Wang
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  2. Shimin Jiang
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  3. Dingxin Di
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  4. Guming Zou
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  7. Wenge Li
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Contributions

YW contributed to study design, manuscript writing, experimental implementation, and data analysis. DD, GZ, and HG were involved in the data collection and experimental implementation. SJ participated in the manuscript revision. SL and WL coordinated the funding for the project.

Corresponding authors

Correspondence to Shimin Jiang or Wenge Li.

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Ethics approval

This study was approved by the ethics committee of the China-Japan Friendship Hospital (2021-113-K71).

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As this was a retrospective observational study of deidentified data, patient informed consent was not required. All the procedures that included human participants adhered to the Declaration of Helsinki.

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All authors declare no financial competing interests.

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Wang, Y., Jiang, S., Di, D. et al. The prognostic role of activation of the complement pathways in the progression of advanced IgA nephropathy to end-stage renal disease. BMC Nephrol 25, 387 (2024). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12882-024-03832-3

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Keywords

BMC Nephrology

ISSN: 1471-2369

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