FP189
HIGH LEVELS OF INTESTINAL-ACTIVATED IGA+ B LYMPHOCYTES SUPPORT THE PATHOGENIC ROLE OF INTESTINAL MUCOSAL HYPERRESPONSIVENESS IN IGA NEPHROPATHY PATIENTS

INTRODUCTION: 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, due to the activity of secretory IgA in the intestinal mucosa. However, to date, even if hypothesized, the relationship between commensal bacteria, elevated BAFF levels, perturbed homeostasis of intestinal-activated B cells and intestinal IgA class switch has not been demonstrated in IgAN patients. Here we studied the intestinal-renal axis connections analyzing levels of BAFF, April and intestinal-activated B cells in patients and healthy subjects (HS).

METHODS: Serum and fecal samples were collected from 44 IgAN patients and 23 HS with similar clinical features. Baff and APRIL serum levels were measured by ELISA assay. Metabolomic analysis of fecal microbiome was performed by mass spectrometry. B cell subsets were identified by FACS. We also used anti-IgA, anti-Integrin β7 and anti-CCR9 antibodies to identify B cell subsets producing IgA and B cells from intestinal mucosa.

RESULTS: IgAN patients had increased serum levels of Baff cytokine correlating to higher amounts of 5 specific microbiota metabolites (mean ± SEM, 907 ± 54  and 689 ± 54.6  pg/ml in IgAN and HS, respectively; p=0.012). Moreover, we found also high April cytokine serum levels in IgAN patients. 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. In addition, we found that subjects with IgAN have a higher proportion of circulating Breg activated at the intestinal level (CCR9+INTB7+) compared to HS (% mean  ± SEM = 24.6 ± 5.8  and 8.5 ± 2.9 in IgAN and HS, respectively; p = 0.02).

Moreover, IgAN patients had also high levels of CCR9+/INTB7+ memory B cells (% mean  ± SEM = 13.4 ± 2.8  and 5.51 ± 1.3 in IgAN and HS, respectively; p = 0.006) and of intestinal IgA-producing memory B cells (CCR9 +/ INTB7 +/IgA+; % mean  ± SEM = 22.2 ± 3.5  and 10.3 ± 3.3 in IgAN and HS, respectively; p = 0.03). Interestingly, they were significantly increased in IgAN patients but not in patients with non-IgA glomerulonephritis.

Finally, we found that IgAN patients had high levels both of total plasmablasts (% mean  ± SEM = 4.1 ± 0.9  and 1.2 ± 0.6 in IgAN and HS, respectively; p = 0.001) and of intestinal-activated plasmablasts (% mean  ± SEM = 23.6 ± 3.8 and 11.2 ± 2.1 in IgAN and HS, respectively; p = 0.01).

CONCLUSIONS: The results of our study showed for the first time an important difference in the amount of intestinal-activated B lymphocytes among patients with IgAN and HS, 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. Therefore, this represents an important area of research for new targeted therapies aiming to stop the evolution towards end stage renal disease.