Clinicopathological and prognostic study of IgA-dominant postinfectious glomerulonephritis

Background The clinicopathological and prognostic features of IgA-dominant postinfectious glomerulonephritis and its difference from the primary IgA nephropathy remains to be investigated. Methods The clinical and pathological data of 6542 patients who underwent renal biopsy from 2009 to 2020 in our hospital were reviewed and 50 patients who met the selection criteria of IgA-dominant postinfectious glomerulonephritis were enrolled to conduct a retrospective and observational single-center study. The selection criteria were: meet the characteristics of IgA dominance or codominance in immunofluorescence, and conform to 3 of the following 5 criteria: 1.Clinical or laboratory evidence show that there is infection before or at the onset of glomerulonephritis; 2.The level of serum complement decreased; 3.Renal pathology is consistent with endocapillary proliferative glomerulonephritis; 4. Glomerular immunofluorescence staining showed complement C3 dominance or codominance; 5. Hump-like subepithelial immune complex deposition was observed under electron microscopy. According to age, sex, renal function (estimated glomerular filtration rate, eGFR) and follow-up time, the control group was constructed with 1:3 matched cases of primary IgA nephropathy. The clinicopathological and prognostic differences between the two groups were analyzed. Results The most common histological pattern of IgA-dominant postinfectious glomerulonephritis was acute endocapillary proliferative glomerulonephritis and exudative glomerulonephritis. Immunofluorescence showed mainly IgA deposition or IgA deposition only, mainly deposited in the mesangial area (deposition rate 100 %), with typical C3 high-intensity staining (intensity++~+++), mainly deposited in the mesangial area (deposition rate 92.0 %). The fluorescence intensity of kappa is usually not weaker than lambda. The probability of the appearance of typical hump-like electron deposition under electron microscopy is low. Compared to primary IgA nephropathy, patients with IgA-dominant postinfectious glomerulonephritis had higher proportion of crescents (p = 0. 005) and endocapillary hypercellularity (p < 0.001) in pathological manifestations. Using serum creatinine level doubled of the baseline or reached end-stage renal disease as the endpoint, the prognosis of IgA-dominant postinfectious glomerulonephritis patients was worse than that of primary IgA nephropathy patients (p = 0.013). Conclusions The clinicopathological features of patients with IgA-dominant postinfectious glomerulonephritis was different from that of primary IgA nephropathy, and the prognosis was worse.


Introduction
In 2003, Nasr et al. firstly reported the IgA-dominant postinfectious glomerulonephritis, mostly secondary to skin infections and often related to staphylococcal infection. Diabetes and advanced age were risk factors. The diagnosis mainly depended on renal pathology, which was characterized by diffuse endocapillary proliferative glomerulonephritis under light microscopy, IgAdominant or codominant immune complex deposits under immunofluorescence and "hump-like" electrondense deposits in the mesangial area and subepithelium under electron microscopy [1][2][3][4]. Reports of the IgAdominant postinfectious glomerulonephritis have gradually increased in recent years, and many cases have an atypical presentation, making it difficult to differentiate the disease from primary IgA nephropathy, where the prognosis differs considerably. Most of these research were case reports, retrospective reviews or case series in a single center, and the sample size was often small, leading it hard to fully explain the clinicopathological characteristics and prognosis of the disease and the differentiation from similar diseases [5][6][7][8][9][10][11][12]. Therefore, indepth research of the clinicopathological features and prognosis of IgA-dominant postinfectious glomerulonephritis has an important clinical significance.
In this study, 50 cases of IgA-dominant postinfectious glomerulonephritis were selected from 2009 to 2020 in our hospital, and their clinical and pathological features were analyzed. In addition, 150 patients with primary IgA nephropathy whose age, sex, renal function at diagnosis (estimated glomerular filtration rate, eGFR) and follow-up time matched were selected as a control group to compare the clinical, pathological and prognostic differences, thus providing a reference for clinical diagnosis and treatment.

Patient profiles
Among a total of 6542 patients who underwent renal biopsy in our hospital from January 2009 to October 2020, 50 patients met the selection criteria of IgA-dominant postinfectious glomerulonephritis and were enrolled to conduct a retrospective and observational single-center study. The diagnosis of IgA-dominant postinfectious glomerulonephritis must meet the characteristics of IgAdominant or codominant IgA in immunofluorescence (IF), and conform to 3 of the following 5 criteria: 1.Clinical or laboratory evidence show that there is infection before or at the onset of glomerulonephritis; 2.The level of serum complement decrease; 3. Renal pathology is consistent with endocapillary proliferative glomerulonephritis; 4. Glomerular immunofluorescence (IF) staining shows complement C3 dominance or codominance; 5. Hump-like subepithelial immune complex deposition is observed under electron microscopy (EM). The patients who met the following situations are excluded: (1) The clinical data are incomplete; (2) There is no definite pathological diagnosis in our hospital. The clinical and pathological data of 50 patients who met the criteria above were collected. In addition, according to age, sex, renal function (estimated glomerular filtration rate, eGFR) and follow-up time, the control group was constructed with 1:3 matched cases of primary IgA nephropathy, and the clinical, pathological and prognostic differences between the two groups were analyzed. The selection criteria of primary IgA nephropathy are as follows: (1) Clinically diagnosed as primary IgA nephropathy; (2) The clinical and pathological data are complete; (3) No other primary or secondary glomerular diseases were found. This study was approved by the Ethics Committee of The First Affiliated Hospital of Wenzhou Medical University.

Histopathological examination
All the specimens obtained from renal biopsy were examined by light microscopy (LM) and immunofluorescence (IF). Some of the specimens were examined by electron microscopy (EM). For LM, all the specimens were stained with hematoxylin-eosin (HE), periodic acid-Schiff (PAS), periodic acid methenamine silver (PAM), and elastic-masson trichrome. For IF, frozen sections were stained with the direct immunofluorescent method. Samples for EM were fixed with 2.5 % glutaraldehyde and processed for regular transmission EM. The pathological diagnosis was based on the Oxford pathological classification (MESTC score) criteria, including mesangial hypercellularity(M0/1), endocapillary hypercellularity(E0/1), segmental glomerulosclerosis(S0/S1), tubular atrophy and interstitial fibrosis(T0/1/2), and crescent (C0/1). According to the results of light microscopy, statistics were made on the number of glomeruli, global sclerosis, mesangial hypercellularity, mesangial matrix expansion, endocapillary hypercellularity, crescent, adhesion and so on. According to the results of immunofluorescence, statistics were made on the deposition sites of immunoglobulin(IgG, IgA, IgM), complement(C3, C4, C1q), fibrinogen, light chain (kappa and lambda), and the staining intensity was classified into -、+、++、+++、++++. According to the results of electron microscopy, statistic were made on the mesangial hypercellularity, mesangial matrix hyperplasia, basement membrane thickening, foot process fusion and the sites of electron dense deposition.

Outcomes
The endpoint was defined as: (1) Serum creatinine increased to (2) 0 times of the baseline, or eGFR decreased by 50 %. 2.Reached end-stage renal disease and received renal replacement therapy; (3) Compound endpoint: serum creatinine level doubled or reached end-stage renal disease; (4) Died. The average follow-up time was 40.16 months (3 months~89 months).

Statistical analysis
Continuous variables with normal distribution were expressed by mean ± standard deviation, and Student's t test was used for inter-group comparison. Non-normal continuous variables were expressed by median [quartile], and Wilcoxon rank sum test was used for intergroup comparison. The classification variables were expressed by the number of cases (%), and χ 2 test was used for inter-group comparison. The IgA-dominant postinfectious glomerulonephritis group and primary IgA nephropathy group were matched according to age, sex, eGFR and follow-up time at 1:3, and the propensity score-matching algorithm was used to compare the classification data between the two groups. Cox regression analysis and χ 2 test were used to analyze the risk factors of patients with IgA-dominant post-infectious glomerulonephritis. p < 0.05 on a two-tailed test was regarded as statistically significant. The cox regression curve was used to display the endpoint. All data were analyzed and processed by R 4.0.3 software [13].

Basic information and medical history of patients
According to the inclusion and exclusion criteria, a total of 50 patients with IgA-dominant postinfectious glomerulonephritis were selected from patients who underwent renal biopsy pathological examination at our centre from January 2009 to October 2020, and their basic information and clinical data were collected, as shown in Table 1. The data were then summarized and the results are shown in Table 2. The average age of the patients at the time of diagnosis was 42.8 years old, the maximum age was 80 years old, and the minimum age was 19 years old. The male-to-female ratio was 1:1. The results are shown in Table 3. In this study, the average number of glomeruli in renal biopsies was 22.18. The most common histological pattern of IgA-dominant postinfectious glomerulonephritis is acute endocapillary proliferative glomerulonephritis and exudative glomerulonephritis, similar to acute post-streptococcal glomerulonephritis, showing acute diffuse endocapillary proliferative glomerulonephritis with significant neutrophil and monocyte endocapillary infiltration and obvious lumen blockage (Fig. 1A-D). This pattern accounted for 80.0 % of the cases. Other histological patterns included mesangial proliferative glomerulonephritis, membranous proliferative glomerulonephritis and crescentic glomerulonephritis, accounting for 12.0 %, 6.0 and 2.0 % of the cases, respectively. Mesangial hypercellularity occurred in 94.0 % of patients, and mesangial matrix expansion occurred in 84.0 % of patients. The average number of global sclerosis is 3.33. Adhesion occurred in 50.0 % of At the same time, there were 3 cases of hypertensive nephropathy and 1 case of diabetic nephropathy. Most patients had renal interstitial inflammation, showing interstitial lymphocyte and plasma cell infiltration, renal tubular atrophy and renal interstitial fibrosis. 12 cases accompanied with acute interstitial inflammation, including 8 cases of mild inflammation, 3 cases of medium inflammation and 1 case of heavy inflammation. 20 cases accompanied with chronic interstitial inflammation, including 5 cases of mild inflammation, 14 cases of medium inflammation and 1 case of heavy inflammation. Most patients had varying degrees of vascular lesions, including mild endarterium hyperplasia in 16 cases, medium endarterium hyperplasia in 12 cases, heavy endarterium hyperplasia in 3 cases, and hyalinosis in 28 cases.

Immunofluorescence
The results are shown in Table 4. The fluorescent tissue contains an average of 4.2 glomeruli (2~9 glomeruli). IgA is the dominant or co-dominant immunoglobulin deposited in glomeruli (intensity +~++++). The mode of deposition is coarse granular or massive deposition in glomerular mesangial area and / or capillary wall. The deposition rate of IgA was 100 % in the mesangial area and 18.0 % in the capillary wall. Typical C3 highintensity staining (intensity ++~+++)was observed. 92 % of the cases showed C3 deposited in the mesangial area and 22.0 % in the capillary wall. The deposition rates of IgG and IgM in the mesangial area and capillary wall were 22.0 %、6.0 and 42.0 %、16.0 %, respectively. C1q and C4 staining were negative in most patients. In addition, fibrinogen deposition was found in 10.0 % of the patients in the capillary wall and 20.0 % of the patients in the mesangial area. 32 cases were positive for kappa staining (intensity+~+++), and 42 cases were positive for lambda staining(intensity+~+++). The staining intensity of kappa was usually not weaker than that of lambda (Fig. 1E, F), which was different from primary IgA nephropathy ( In primary IgA nephropathy, the staining intensity of lambda is usually not weaker than that of kappa).

Electron microscopy
The results are shown in Table 5. 14 of the 50 IgAdominant postinfectious glomerulonephritis cases were examined by electron microscopy. Ultrastructurally, 71.4 % of the cases showed electronic dense deposits in the mesangial area, 50 % in the paramesangial area, 21.4 % in the subepithelial and 7.1 % in the subendothelial. Only one patient showed hump-like deposits under the electron microscopy, accounting for 7.1 % (Fig. 1G). Foot process fusion occurred in all patients.

Comparison of clinical data between IgA-dominant postinfectious glomerulonephritis and primary IgA nephropathy
After matching age, sex, eGFR and follow-up time, there was no significant difference in demographic data between IgA-dominant postinfectious glomerulonephritis group (right column of Table 6) and primary IgA nephropathy group (left column of Table 6). As shown in Table 6, IgA-dominant postinfectious glomerulonephritis group had higher average serum total cholesterol level (p < 0.001), higher average serum high density lipoprotein level (p = 0.039), lower average serum low density lipoprotein level (p = 0.021), higher average 24-hour urinary protein level (p < 0.004), lower average serum albumin level (p < 0.001), higher average fibrinogen level (p = 0. 001), higher proportion of crescent (p = 0. 005), and higher proportion of endocapillary hypercellularity

Prognosis and risk factors
The average follow-up time in this study was 40.16 months. When regarding serum creatinine doubling or reaching end-stage renal disease as the compound endpoint, the prognosis of patients with IgA-dominant postinfectious glomerulonephritis was worse than that of primary IgA glomerulonephritis (p = 0.013), as shown in Fig. 2. Using serum creatinine doubling or entering endstage renal disease as the compound endpoint, univariate cox regression analysis was performed on the related factors of IgA-dominant postinfectious glomerulonephritis. As shown in Table 7, factors that have statistical differences(p < 0.1) were sent to make the selection(Forward-backward stepwise regression method) according to the Akaike information criterion and the factors remained were selected to make multivariate analysis. The results showed that the increase of serum albumin level was an independent protective factor (p < 0.004), and the increased serum creatinine level (p < 0.001), serum uric acid level (p = 0.01) and decreased eGFR level (p < 0.001) were independent risk factors. Renal tubular atrophy or interstitial fibrosis > 50 % (p = 0.20) and the existence of crescents (p = 0.19) were risk factors, but there was no significant statistical difference, as shown in Table 7.

Discussion
IgA-dominant postinfectious glomerulonephritis, as a special form of postinfectious glomerulonephritis, has    of patients with IgA-dominant postinfectious glomerulonephritis is often reported worse [15][16][17]. However, the above studies are mainly case reports, or retrospective reviews, or single-center case series, and the sample size is often small, so it is difficult to fully explain the clinicopathological characteristics, prognosis and differentiation of the disease from other similar diseases. In our study, the selection criteria of IgA-dominant postinfectious glomerulonephritis were established with reference to the criteria of Handa, Nasr and others. A total of 6542 cases of renal biopsy were performed in our center from 2009 to 2020. We selected 50 cases of IgA-dominant postinfectious glomerulonephritis which were clinically and pathologically diagnosed, and studied their clinical and pathological features. In addition, 150 patients with primary IgA nephropathy matched with age, sex, eGFR and follow-up time were selected and matched according to 1:3, and the differences in clinical, pathology and prognosis were analyzed. The results are as follows.

Infection site and pathogen
It has been previously reported that classic IgAdominant postinfectious glomerulonephritis is often secondary to skin infection, and mostly related to staphylococcal infection [1]. However, our study found that upper respiratory tract and urinary tract infections were more often than skin infections. The detectable rate of staphylococci was not high (4 %), and atypical pathogens such as Escherichia coli and Ureaplasma Urealyticum could be detected in a few patients. Most pathogens could not be found (pathogen detectable rate was 20 %). The proportion of patients with Hepatitis B is relatively high (14 %). The reason may be due to the high prevalence rate of Hepatitis B in China, or there may be some correlation between the two diseases which was mentioned by Miquelestorena-Standley, E. et al. in their report and is worthy of further study [12].

Laboratory examination
Compared with primary IgA nephropathy, IgAdominant postinfectious glomerulonephritis had more proteinuria and lower serum albumin level, indicating that it had greater damage on glomerular filtration barrier, which may be one of the reasons for more serious disease progression. On the other hand, patients with  IgA-dominant postinfectious glomerulonephritis had higher serum fibrinogen levels. As an inflammatory factor, fibrinogen suggests that the inflammation in this disease may be more severe, or it may be used as a clinical feature for differentiation from primary IgA nephropathy.

Renal pathological changes
The pathological manifestations of IgA-dominant postinfectious glomerulonephritis have been reported in the past, mainly were endocapillary proliferative glomerulonephritis under light microscopy, IgA-dominant or codominant immune complex deposits under immunofluorescence, and hump-like dense depositions in the mesangial area and subepithelium under electron microscopy [6,14]. Our findings are alomst consistent with the above. Under light microscopy, compared with primary IgA nephropathy, IgA-dominant postinfectious glomerulonephritis was more likely to form crescents (p = 0.05), and endocapillary hypercellularity were more severe (p < 0.001), accompanied by significant neutrophil and monocyte capillary infiltration, and obvious blockage of the lumen, indicating that the disease may have more serious effect and destruction on renal structure. The manifestation of immunofluorescence was very similar to that of IgA nephropathy, which showed IgA dominant or co-dominant deposition in the mesangial area. The main differentiation may depend on the comparison of fluorescence staining intensity of lambda and kappa. In IgA-dominant postinfectious glomerulonephritis, the fluorescence staining intensity of kappa is often not weaker than that of lambda, while the immunofluorescence of primary IgA nephropathy often showed the opposite, which is consistent with our findings. Under electron microscopy, hump-like deposits are considered to be the typical manifestation [2], but our study found that the probability of typical hump-like deposits is low (only 1 out of 14 cases found typical hump-like deposits). Therefore, it may be not feasible to rely on the results of electron microscopy as differential diagnosis.
The retrospective comparative study of TakayaHanda et al. [3] showed that

Prognosis and its influencing factors
Our study found that in patients with IgA-dominant postinfectious glomerulonephritis, the higher level of serum creatinine and uric acid at diagnosis led to the worse prognosis, which is consistent with previous studies, suggesting that patients with severe renal function damage and elevated uric acid should be treated more actively. The pathological results showed that renal tubulointerstitial atrophy and sclerosis > 50 %(T2) and crescent(C1) were also risk factors for poor prognosis, but there was no statistical difference. Since Bogdan Obrişcă et al. [18] have argued that whether MESTC score can be routinely applied to patients with secondary IgAN remained unsure, the result may need more indepth studies. Previous studies have suggested that sex and age are risk factors for the disease, but our investigation found that age and sex may be confounding factors and were not directly related to the prognosis of the disease.
Previous studies showed that IgA-dominant postinfectious glomerulonephritis had a poorer prognosis comparing to primary IgA nephropathy. For example, the retrospective comparative study of Takaya Handa et al. [6] found that the prognosis of IgA-dominant postinfectious glomerulonephritis was worse than that of primary IgA nephropathy, which may be due to the fact that the former has more obvious systemic inflammation and local inflammation of glomerular capillary wall. After an average follow-up of 40.16 months, our study found that the prognosis of patients with IgA-dominant postinfectious glomerulonephritis was worse than that of primary IgA glomerulonephritis, and was not disturbed by other factors, which was consistent with the previous studies.
Some of the results of our study were different from previous studies, indicating that there may be some limitations in the previous understanding of IgA-dominant postinfectious glomerulonephritis. The so-called "typical manifestations" may cover only the minority [19][20][21], and there may be diversity in pathogens, inducements, clinical, pathology and prognosis [22].However, there are still some disadvantages in our study, such as small sample size, few specimens examined by electron microscopy, relatively short time of follow-up, and was only a retrospective and observational single-center study, so the clinicopathological and prognostic characteristics of IgA-dominant postinfectious glomerulonephritis need to be further studied by multicenter with larger samples.

Conclusions
The pathology and clinical manifestations of IgAdominant postinfectious glomerulonephritis have certain characteristics, and the differential diagnosis from other similar diseases depends on clinical and pathological features. The prognosis of IgA-dominant postinfectious glomerulonephritis is worse than that of primary IgA nephropathy, which may be related to stronger inflammatory reaction caused by infection-mediated IgA deposition, but its clinicopathological and prognostic characteristics need to be further studied. Endpoint is the increase of serum creatinine to 2 times the base line or end-stage renal diseasse. Factors that have statistical differences (p < 0.1) were sent to make the selection(Forward-backward stepwise regression method) according to the Akaike information criterion and the factors remained were selected to make multivariate analysis HR Hazard ratio, CI confidence interval, eGFR estimated glomerular filtration rate, MESTC-score the assessment of mesangial hypercellularity/endocapillary hypercelluarrity/segmental sclerosis/renal tubular lesion/crescent according to the Oxford classification, M1 mesangial hypercellularity, E1 endocapillary hypercellularity, S1 segmental glomerulosclerosis, T1/2 tubular atrophy and interstitial fibrosis > 25 %, C1/2 crescents in at least one glomerulus, IgA-DPIGN IgAdominant postinfectious glomerulonephritis All the data above are acquired before the biopsy