Immunofluorescence deposits in the mesangial area and glomerular capillary loops did not affect the prognosis of immunoglobulin a nephropathy except C1q:a single-center retrospective study

Immunoglobulin A (IgA) nephropathy (IgAN)—the most prevalent primary glomerular disease worldwide—can progress to kidney failure. Over the past 20–30 years, 10–30% of patients have developed end-stage kidney disease (ESKD) [1, 2]. Therefore, it is of paramount importance to obtain personalized risk predictions. Previous observations have suggested some specific risk predictors for the prognosis of IgAN [2]. Of these observations, proteinuria, abnormal blood pressure, and renal insufficiency at baseline are now widely recognized [3, 4]. Although the newest version of the Oxford classification system for IgAN has been used increasingly in clinical settings [5], immunofluorescent features were unfortunately excluded.

We have previously detected IgG, IgM, C3, and fibrinogen (Fib) (and rarely C1q) deposits in the mesangial region and/or glomerular capillary loops (GCLs) in addition to mesangial IgA deposition. A higher intensity of immunofluorescence deposits was associated with worsened clinical characteristics and poor prognoses of IgAN [6]. However, the relationships between the site of immunofluorescence and the clinicopathologic features and prognoses of IgAN remain unelucidated.

Therefore, we conducted a retrospective study to investigate the roles played by the types and patterns of immunofluorescence deposits in IgAN.

In this retrospective study, we comprehensively described the clinical and histopathologic findings with respect to the pattern of immunofluorescence deposits in IgAN patients. Compared to patients without C1q deposition, the prognosis in patients with C1q deposition in the mesangial+GCLs group was worsened. However, this prognosis was no different vis-à-vis immunofluorescence deposition in either the mesangial-only group or the mesangial+GCLs in patients with IgAN.

It is conventional medical wisdom that IgA deposits are typically detected in the mesangial area by immunofluorescence, and that IgA deposits extending to the GCLs are observed in 15–50% of IgAN patients [9]. Investigators have reported that IgA deposits localized to the GCLs were closely related to heavier persistent proteinuria and more frequent crescents [10, 11]; these results are partially consistent with our findings. Kobayashi et al. showed that deposits of IgA in the GCLs were observed more commonly in patients with unfavorable outcomes [12]. Similarly, D’Amico et al. reported that IgA deposits extending to the GCLs constituted one of the risk factors for progression [13]. Deposition of immune complexes that consisted of glycan-specific IgG antibodies and galactose-deficient IgA1 in the kidney resulted in kidney injury, perhaps explaining why IgA deposits extended to GCLs are associated with worsened clinicopathologic damage and prognoses. Our results also showed that IgA deposits extending to the GCLs were more likely to be accompanied by IgG deposits. In addition, Katafuchi and colleagues reported that prognoses were better in patients with a higher intensity of IgA deposits compared with those with lower intensity in patients treated with corticosteroids [6]. In a recent study, IgA deposition along the capillary walls was associated with greater histologic severity, including higher mesangial and endocapillary hypercellularity; however, these authors reported that renal survival was comparable regardless of presence or absence of IgA staining in the capillary walls [14]. Our results likewise revealed that the prognosis was not significantly different among groups, but treatment such as with corticosteroids cannot be ignored as a confounding factor. Based on the above reports, we suggest that IgA deposition extending to the GCLs is associated with worsened clinicopathologic findings, although its prognostic significance remains unclear.

The IgG immune complexes bound to aberrantly glycosylated IgA1—and deposited in the glomeruli—result in renal injury [15]; however, the impact of IgG deposition on IgAN is controversial. Christina et al. reported that mesangial staining for IgA and IgG (AG) was associated with hypertension and decreased renal survival [16]; while Wada et al. remarked that the proteinuria was more severe, and the presence of IgA deposition on capillary walls was more frequent in patients with AG deposition than without IgG deposition [17]. In our study, patients with IgG deposition in the mesangial area had lower CHOL levels and milder proteinuria compared to those with IgG in the mesangial area and GCLs, and a lower frequency of T(1 + 2) compared to patients without IgG deposition. The subjects in our study also presented with rather mild baseline features. These results indicated that mesangial IgG deposition might occur more frequently in the early phases of IgA nephropathy, even though the patients with IgG deposition in the mesangial area and GCLs presented with greater clinicopathologic damage. We speculate that IgG deposition extended to the GCLs aggravates the disease process. A study from the UK revealed glomerular IgG deposition was correlated with severe mesangial and endocapillary cellularity [14], and our results confirmed that the pattern of IgG deposits was associated with the M1. Korean investigators stated that moderate and marked glomerular IgG deposition was associated with adverse outcomes; notably, they demonstrated that the pattern of IgG staining (mesangial-only, mesangial + capillary wall, and capillary wall-only) was not connected with either blood pressure or eGFR at biopsy, nor with renal outcomes at follow-up [18]. We also herein showed that the location of IgG deposition exhibited no relationship with blood pressure or GFR at diagnosis. We initially demonstrated that patients with IgG deposition in the mesangial+GCLs group presented with higher levels of CHOL and heavier proteinuria compared with the mesangial-only group. A recent study indicated that the co-deposition of IgG did not affect the renal outcome in IgAN; however, more importantly, these authors suggested that the immunofluorescence deposits extending to the GCLs did influence renal outcome [19]. In our study, the pattern of IgG deposition was not significantly correlated with renal outcomes, which is congruent with the aforementioned Korean study [18]. Over 2/3 (68.8%) of the patients with IgG deposition in our mesangial-only group presented with a higher intensity of IgA deposits (≥3+), while the intensity of IgA deposits was evenly distributed in those individuals with IgG deposition in the mesangial+GCLs group. Compared with patients without IgG deposition, patients with IgG deposition in the mesangial area and GCLs manifested conditions that were more often accompanied by Fib and C1q deposition. However, the difference was not statistically significant compared with patients whose deposits were solely in the mesangial area. Over half (53.1%) of the patients with IgG deposition in the mesangial-only group had a higher intensity of C3 deposition (≥2 +), but approximately half of the patients with IgG deposition in the mesangial+GCLs group exhibited no C3 deposition. These results likely explained why the patients with IgG deposition in the mesangial+GCLs group had prognoses similar to those patients with IgG deposition in the mesangial-only group.

The precise mechanism subserving IgM deposition in IgAN remains unclear, although some investigators postulate that IgM deposition is non-specific in patients with IgAN. As is currently understood, IgM is the first antibody synthesized and secreted in response to infection, and can be converted to IgG by T-cells. The switch of IgM to IgG, then, might be influenced by a dysfunction of T-cells, resulting in IgM deposition [20, 21]. The depletion of B cells was shown to attenuate the deposition of IgM, slowing the rate of progression and relieving the severity of the disease [22]. Our study confirmed that IgM deposition either in the mesangial-only group or in the mesangial+GCLs group was associated with higher serum IgM levels compared to individuals without IgM deposition. Extant kidney damage might be further aggravated due to the deposition of IgM via complement activation [23]. In our study, the intensity of C3 deposition was different depending upon the localization of IgM deposition, and IgM deposition in the mesangial+GCLs group was often accompanied by C1q deposition. Based upon the above observations, we suspected that IgM deposition played an important role in the progression of IgAN. Cihan and colleagues reported that mesangial IgM deposition was associated with greater pathology and an unfavorable renal outcome [24]; however, the significance of IgM deposition in GCLs is arcane. We indicated that an over-release of cytokines would lead to an increase in capillary permeability, resulting in the extension of IgM deposits to the GCLs. The pattern of IgM deposition we observed was then associated with the intensity of IgA deposits and the co-deposition of Fib and C1q, which provides the possibility that the immune deposits also consisted of IgM. However, no difference was displayed between the mesangial+GCLs group and the mesangial-only group regarding IgM in our study. More importantly, the pattern of IgM did not affect the prognosis of IgAN patients.

It is well known that immune-complex deposition caused complement activation [25]. For example, Yasar et al. regarded moderate and marked mesangial C3 deposition as a good predictor of IgAN progression [26], and Korean clinicians observed that mesangial C3 deposition was involved in renal impairment, heavier proteinuria, greater glomerular sclerosis, and interstitial fibrosis [27]. Nasri et al. indicated an association between C3 deposits and serum creatinine, endocapillary proliferation, and segmental glomerulosclerosis [28]. Mesangial C3 deposition combined with the Oxford classification also improved the predictive utility of the IgAN prognosis [29]. Lai and colleagues hypothesized that C3 deposition in the mesangial region and GCLs causes podocyte injury and results in the formation of proteinuria—and proteinuria appeared to be more severe in patients with C3 deposition extending to the GCLs in our study [2]. In a Japanese cohort, researchers showed that the renal outcome was adverse in IgAN patients with extraglomerular C3 deposits [30]. In our study, we failed to find an association between prognosis and the pattern of C3 deposition. Their differing results from our study may be due to the spatial arrangement of immunofluorescence deposits. MUDA et al. showed that patients with severely pathologic damage were more likely to exhibit larger C3 deposits or C3 deposits without an outer coat of IgA [31]. Glomerular C3 deposition was also more frequently present with a higher intensity of IgA deposits. As C3 deposition in the mesangial+GCLs group was more frequently associated with IgG, IgM, Fib, and C1q deposits; we suggest that the spatial arrangement of immunofluorescence deposits be taken into consideration.

Although C1q deposition was found in 2.4–17% of IgAN patients, the significance of C1q deposition remains obscure [6]. The presence of C1q staining associated with autoimmune diseases such as lupus nephritis should therefore receive more attention in IgAN. Thus, we excluded the patients whose condition was combined with other autoimmune diseases. Hiroki et al. reported that C1q deposition was closely related to remission of both proteinuria and hematuria [32]. Ritsuko et al. provided a comprehensive evaluation of C1q deposition in IgAN, revealing that C1q deposition was correlated with the presence of crescent formation, and segmental and global sclerosis [6]. Lee et al. also reported that the presence of mesangial C1q staining was connected to an adverse prognosis. Most importantly, the more marked the IgA deposition, the more frequent the IgG and IgM co-depositions were found in patients with mesangial C1q deposition [33]. In our study, we initially showed clinicopathologic features of those patients with C1q deposition in the mesangial region and GCLs. We observed that the frequency of IgA deposition ≥3+ was more common in patients with C1q deposition in the mesangial region and GCLs, with the frequencies of IgG, IgM, and Fib co-deposition at 57.1, 78.6, and 35.7%, respectively. Liu et al. also observed that C1q deposition was more frequent in patients with severe renal impairment [34]. In our study, Kaplan–Meier and Cox regression analyses illustrated that C1q deposition in the mesangial area and GCLs predicted a poor renal prognosis. However, we could not draw a clear conclusion regarding the significance of C1q deposition in IgAN patients due to the small number of patients with this characteristic.

Fib deposits are rarely seen in the glomeruli under normal physiologic conditions. Many scholars posit that extravascular tissue factors activate coagulation factors under inflammatory conditions, leading to Fib deposition [35]. Although Haas has suggested that Fib deposits in the mesangial region possess a granular appearance, staining in the GCLs was nonspecific [36]. In our study, we showed that the pattern of Fib deposits was related to the intensity of IgA deposits and the presence of IgG, IgM, and C1q deposits. These results highlighted the importance of intra-mesangial coagulation in IgAN, and it is therefore of great importance to investigate the mechanism(s) underlying Fib deposition in IgAN.

There were some limitations to our study. First, the number of samples was relatively small and the follow-up time was brief. Second, our conclusions cannot be generalized to other racial or ethnic groups due to this being a single-center study. Additionally, we did not perform IgG subtyping at biopsy. Third, the patients with deposition only in the GCLs were excluded due to their limited number. Finally, some factors that may affect a prognosis were not taken into consideration, including the spatial arrangement of immunofluorescence deposits and treatments that might change during follow-up.

In this study, we explored the different roles for patterns of immunofluorescence deposits in IgAN by conducting a retrospective analysis. The location of glomerular IgA, IgG, IgM, C3, and Fib deposits did not affect the prognosis of IgAN patients—except for C1q deposits. This study strengthened the significance of immunofluorescence deposits, and provides a more comprehensive prognostic evaluation system for IgAN.