Abstract
Background and Aims
The IgA nephropathy (IgAN) is the most frequent primitive glomerulonephritis. In the last years, the role of mucosal immunity in IgAN, together with that of the gut microbiota in the activation of innate and adaptive immune cells, has gained
importance. Particularly interesting is the role of the microbiota and intestinal immunity in IgAN. BAFF and APRIL can be produced by the intestinal epithelium, in response to signals triggered by TLRs once activated by the commensal bacteria present in the intestinal lumen, representing the link between microbiota and intestinal immunity. To date, even if hypothesized, this relationship in IgAN patients has not been investigated.
Here, we studied the intestinal-renal axis connections analyzing levels of BAFF, April and intestinal-activated B cells in IgAN patients.
Method
Serum and fecal samples were collected from 44 IgAN patients, 22 non-IgA glomerulonephritides (controls) and 22 healthy subjects (HS) with similar clinical features. BAFF and APRIL serum levels were measured by ELISA assay. Metabolomic analysis of fecal microbiome was performed using Biochrom 30 series amino acid analyzer and gas-chromatography mass spectrometry/solid-phase microextraction (GC-MS/SPME) analysis. B cell subsets were investigated by FACS.
Results
IgAN patients had increased serum levels of BAFF cytokine compared to the control group of patients with non-IgA glomerulonephritis and compared with HS (p<0.0001and p=0.012, respectively). We found that serum BAFF levels positively correlated with the levels of 24h-proteinuria in IgAN patients (r2 = 0.2269, p <0.001).
We correlated serum BAFF levels with fecal concentration of 5 different metabolites of 30 IgAN patients, which were previously investigated for the fecal microbiota. These organic compounds had been found at significantly higher levels in the feces of IgAN patients compared to HS. Serum BAFF levels positively correlated with the levels of fecal metabolites: 4-(1,1,3,3-tetramethylbutyl) phenol (r2 = 0.2882, p = 0.0027), p-tert-butyl-phenol (r2 = 0.386, p = 0.0003), methyl neopentyl phthalic acid (r2 = 0.3491, p =0.0007), hexadecyl ester benzoic acid (r2 = 0.2832, p =0.003) and furanone A (r2 = 0.1743, p = 0.024).
Serum levels of APRIL were significantly increased in IgAN patients respect to control groups (4.49 ± 0.54 vs 2.27 ± 1 ng/ml, p=0.0014). We found a correlation between APRIL and serum creatinine (r2 = 0.159, p =0.04) and eGFR (r2 = 0.2395, p =0.0082), while no correlation was found between APRIL and fecal metabolite levels in IgAN patients. In addition, we found that subjects with IgAN have a significantly higher proportion of circulating Bregs, Memory B cells and IgA secreting-plasmablasts activated at the intestinal level (CCR9+INTB7+) compared to HS.
Conclusion
The results of our study showed for the first time an important correlation of serum levels of BAFF with intestinal microbiota in patients with IgAN, confirming the hypothesis of the pathogenic role of intestinal mucosal hyperresponsiveness in the IgAN patients. The intestinal-renal axis plays a crucial role in Berger's glomerulonephritis, whose complex pathogenesis may contribute several factors as genetics, pathogens and food.
The role of cGAS/STING in intestinal immunity
