IgA nephropathy (IgAN) is the most prevalent type of primary glomerulonephritis globally, defined as prominent IgA deposition in the glomerular mesangial area (1, 2). Nearly 20–30% of patients with IgAN develop end-stage renal disease (ESRD) eventually and need renal replacement therapies (3, 4). Persistent proteinuria, hypertension, impaired renal function, and the Oxford classification of MEST-C scores are all the established risk factors for poor renal outcomes (5–11). Previous studies indicated that tubulointerstitial injury was a major contributor to the loss of renal function even in primary glomerular diseases (12, 13). Interstitial fibrosis (IF) and tubular atrophy (TA) are common pathological changes and predict renal progression in IgAN (14, 15). Renal pathology is the gold standard for the evaluation of tubulointerstitial lesions. However, the biopsy is an invasive procedure and cannot be used as a routine method. Meanwhile, due to the variability of clinical manifestations of IgAN, patients in the absence of apparent symptoms may never be aware or undergo a renal biopsy. Therefore, it is clinically valuable to find non-invasive biomarkers that predict tubulointerstitial lesions and renal disease progression. Both the β2-microglobulin (β2-MG) and retinol-binding protein (RBP) are low-molecular-weight proteins (11.8 and 21 kD, respectively) that are present in low concentrations in the plasma of healthy people and are freely filtered by the normal glomerulus and then almost completely reabsorbed and catabolized by cells of the proximal tubules (16, 17). Increased levels of urine β2-MG and RBP were found in patients with renal tubulointerstitial damage (18–20). However, their roles in the long-term outcome of IgAN have not been well-assessed. In this study, we aimed to identify the effects of baseline urinary β2-MG and RBP levels on renal disease progression in patients with IgAN.

Of 4,985 patients screened, 2,153 patients with IgAN met the selection criteria and were included in this study (Figure 1). The baseline clinical and pathological characteristics of the study population are shown in Table 1. There were 1,022 (47.47%) men and the mean age at the time of renal biopsy was 38.91 ± 12.47 years. The baseline urinary β2-MG and RBP levels were 56.0 mg/mol Cr (IQR, 30.0–127.5) and 783.0 mg/mol Cr (IQR, 267.0–1774.5), respectively. On biopsy, the 24-h proteinuria level was 0.87 g/day (IQR, 0.45–1.69) and the average eGFR was 82.93 ± 29.28 ml/min/1.73 m². In 1,741 patients whose renal biopsies were scored by the Oxford classification, the distributions of M1, E1, S1, T1-T2, and C1-C2 were 11.32, 5.69, 70.13, 11.60, and 40.44%, respectively. After a median follow-up of 20.40 months (IQR, 9.13–43.60), the composite renal outcomes occurred in 140 (6.50%) patients including 83 (3.86%) patients with ESRD.

IgA nephropathy is a highly heterogeneous disease characterized by variable clinical courses and pathologic findings ranging from asymptomatic microhematuria with mild mesangial proliferation to rapidly progressive glomerulonephritis with extensive crescents formation (22). Several clinical and pathologic parameters have been identified in associations with IgAN progression including blood pressure, proteinuria, baseline renal function, and the Oxford classification of the MEST-C scores (5–11, 23, 24). However, the individual outcomes of IgAN remain difficult to predict. Tubulointerstitial damage is common in IgAN. A study of 609 Chinese patients with IgAN found that nearly 38.3% of the included patients had moderate or severe tubulointerstitial lesions at the time of renal biopsy (15). In-vitro studies have suggested that a mesangial-podocytic-tubular cross-talk with mediators released from the mesangium along with the filtered proteins from the impaired podocyte, contributes to the pathogenesis of tubulointerstitial damage in IgAN (13, 25). Clinically, a subgroup of IgAN with proximal tubular epithelial cells and tubulointerstitial damage often is associated with rapid progression to ESRD (14, 25). Thus, detection of tubulointerstitial lesions in early stage, especially by non-invasive biomarkers, is of clinical significance. Tubulointerstitial damage leads to the dysfunctional proximal reabsorption of physiologically filtered low-molecular-weight proteins and increases urine concentrations of these substances. Urine β2-MG and urine RBP are thought as typical markers for estimation of tubulointerstitial malfunction (19, 26, 27). This study suggested that baseline urine β2-MG and RBP can be used as additional variables in prediction of progression in IgAN.

β2-microglobulin is a low-molecular-weight protein present in low levels in normal human plasma and body fluid. Elevated serum β2-MG levels may result from decreased glomerular filtration function or increased filtration loads such as chronic inflammatory diseases and hematological malignancies (16, 28). Though serum levels of β2-MG were not measured in this study, patients accompanied by conditions that may result in highly increased serum β-MG were excluded. β2-MG is excreted in large amounts in the urine of patients with tubulointerstitial diseases and it is a sensitive marker for early detection of tubular dysfunction (29, 30). In 1986, Portman et al. (18) diagnosed proximal tubular dysfunction by testing urine β2-MG and found an association between higher urinary levels of β2-MG and progression to ESRD in children. Previous studies found that in patients with IgAN, urine β2-MG was positively correlated with SCr and the severity of tubulointerstitial lesions (19, 31). However, few studies investigated the relationship between urine β2-MG and renal disease progression of IgAN and the results remain controversial due to the limited number of patients included (31, 32). Therefore, larger studies with more participants are needed to identify urine β2-MG as a marker of the renal disease progression events in clinical practice. This study found that elevated urine level of β2-MG was independently associated with increased risk of renal disease progression in IgAN. Among cases in this study, participants with the highest vs. the lowest tertile demonstrated a 2.921-fold greater risk of renal progression.

Retinol-binding protein is also a low-molecular mass protein and increased urine RBP excretion also indicates tubular dysfunction (33, 34). Previous studies demonstrated that urine RBP was as sensitive as β2-MG in screening renal tubular function and was a marker of the extent of IF/TA in chronic kidney disease (20, 35, 36). Numerous studies have reported an increased urinary excretion of RBP in diabetic nephropathy and idiopathic membranous nephropathy and recognized the correlations of urine RBP with tubulointerstitial injury and disease progression (36–39). However, the role of urine RBP as a marker of tubulointerstitial injury and renal disease progression in IgAN has not been elucidated. This study found that an increased level of urine RBP was independently associated with the renal disease progression events. Patients with the highest vs. the lowest tertile showed a 2.859-fold greater risk of renal progression.

Consistent with previous studies, this study showed that both the baseline urinary β2-MG and RBP levels were significantly correlated with traditional risk factors of renal disease progression, such as proteinuria, renal dysfunction, and the IF/TA T score in patients with IgAN. Notably, we also found that baseline urine β2-MG and RBP were strongly associated with renal disease progression events in IgAN even after adjustment for sex, age, MAP, SCr, proteinuria, and the Oxford classification of the MEST-C scores, especially in those with eGFR <90 ml/min/1.73 m². However, only 20 patients in eGFR ≥90 ml/min/1.73 m² group reached the composite renal outcomes and the differences may result from the relatively short follow-up period for these patients, with fewer endpoints observed. Further studies with extensive follow-up duration are needed for patients with baseline eGFR ≥90 ml/min/1.73 m².

This is a large sample size study with a relatively large number of renal disease progression events. To the best of our knowledge, this is the first study to evaluate the role of urine RBP in renal disease progression in IgAN and also the largest study to evaluate the role of urine β2-MG in IgAN. However, there are some limitations of this study. First, it was a retrospective cohort study performed in a single center and the follow-up time was relatively short. Also, the baseline Oxford classification of renal biopsy was not obtained in 19.14% of all the participants. Second, information about 24-h urinary β2-MG and RBP excretion was not available, so we used a spot urine sample for testing urinary β2-MG and RBP and corrected the results by urine creatinine. Third, due to data limitations, treatments such as the use of renin-angiotensin-aldosterone system inhibitors (RAASi), steroids, and other immunosuppressants were not examined during follow-up and our analyses were not adjusted for these factors. In addition, urine β2-MG and RBP were highly skewed to the right in this cohort of patients; therefore, log-transformation and tertiles of urinary β2-MG and RBP levels were used in the Cox proportional hazards models. Finally, we recognize that our results may not be applicable to all the populations, as patients with IgAN in our cohort showed milder pathological lesions than other IgAN cohorts (15); though we have included the Oxford classification of the MEST-C scores as confounders in the multivariate adjusted Cox models, further studies are needed to confirm the findings.

In conclusion, in this study, we demonstrated that baseline levels of urine β2-MG and RBP correlated with proteinuria, eGFR, and the Oxford classification of IF/TA T score. Higher levels of urine β2-MG and RBP were associated with renal disease progression in patients with IgAN. Our findings suggest that baseline levels of urine β2-MG and RBP may have prognostic utility in IgAN.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

JCheng and XS conceived the study. XS, GY, XL, and HL performed data collection and analyses. XS prepared the first draft of the manuscript. JChen and JCheng reviewed and provided revisions and comments to the manuscript. All the authors contributed to the article and approved the submitted version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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