ABSTRACT
Objective:To explore the expression of type 2 complement receptor (CR2) in mesangial cells of the renal tissue in IgA nephropathy (IgAN) and its possible mechanism involved in complement C3 deposition.Methods:The demographic data, samples of plasma and renal tissues of primary IgAN patients diagnosed by renal biopsy in the Guangdong Provincial People's Hospital from August 2021 to May 2022 were collected. According to the fluorescent intensity of mesangial complement C3 deposition, the patients were divided into complement C3 deposition ≥2+ group and complement C3 deposition <2+ group. The circulating IgA and complement C3 levels were detected by enzyme linked immunosorbent assay (ELISA). The influencing factors of kidney prognosis, plasma IgA and complement C3 levels were compared between the two groups. Immunofluorescence was used to detect the expression of IgA, complement C3 and CR2 in the renal mesangial cells of IgAN patients and normal renal tissues around renal carcinoma. Human mesangial cells were cultured in vitro and randomly divided into control group and experimental group. The experimental group was incubated with IgA protein (2 g/L) for 8 hours. The expressions of CR2 protein and mRNA were measured by Western blotting and real-time fluorescence quantitative PCR. The biological function of differential genes was analyzed by gene ontology (GO) and Kyto encyclopedia of genes and genomes (KEGG) enrichment analysis. Results:A total of 75 patients with IgAN were included in this study, including 50 patients in the complement C3 deposition ≥2+ group and 25 patients in the complement C3 deposition <2+ group. The proportions of patients with urine red blood cell count negative, 1+, 2+ and 3+-4+ in the complement C3 deposition ≥2+ group were 2.0%, 8.0%, 18.0% 72.0%, respectively, which were more serious than those in the complement C3 deposition <2+ group (4.0%, 4.0%, 52.0%, 40.0%) ( Z=-2.320, P=0.020). Meanwhile, the proportion of S1 in Oxford pathological classification in the complement C3 deposition ≥2+ group was higher than that in the complement C3 deposition <2+ group (68.0% vs. 40.0%, χ2=5.389, P=0.020), and there were no statistically significant differences in gender, age, 24-hour urinary protein, serum creatinine, other indicators of Oxford pathological classification between the two groups. ELISA results showed that plasma IgA concentration in the complement C3 deposition ≥2+ group was higher than that in the complement C3 deposition <2+ group [3.62 (2.95, 5.53) g/L vs. 2.72 (2.15, 4.24) g/L, Z=2.405, P=0.016], and the plasma complement C3 concentration was lower than that in the complement C3 deposition <2+ group [199.6 (116.0, 328.0) mg/L vs. 319.2 (158.3, 454.5) mg/L, Z=-2.383, P=0.017]. Spearman correlation analysis showed that the complement C3 deposition intensity was positively correlated with IgA deposition intensity in mesangial area ( rs=0.441, P<0.001). Immunofluorescence results showed that there was colocalization of IgA and complement C3 in the glomeruli of IgAN patients. The expression of CR2 in the kidney was consistent with complement C3 deposition, and CR2 was colocalization with complement C3. In vitro experiments, the expression of CR2 in IgA protein group was higher than that in the control group ( P<0.05). GO and KEGG enrichment analysis found that IgA protein induced active changes in various pathways of mesangial cells. Conclusion:IgA protein induces mesangial cells to express CR2 and participates in complement C3 deposition, which may be an important mechanism of complement C3 activation in IgAN.