Patients With Immunoglobulin A Nephropathy Show Abnormal Frequencies of B Cell Subsets, Unconventional T Cells, and High Levels of Galactose-Deficient IgA1-Coated Gut Bacteria.

Mucosal inflammation is involved in the pathophysiology of immunoglobulin-A nephropathy (IgAN); however, peripheral immune phenotype analyses of patients with IgAN often do not include unconventional T cells, the major subset in mucosal immunity. We measured serum total IgA, galactose-deficient IgA1 (gd-IgA1), secretory IgA (SIgA), B cell-activating factor (BAFF), and A proliferation-inducing ligand (APRIL) in 66 patients with IgAN and 30 healthy controls (HCs). We also quantified the total IgA and gd-IgA1 in stool supernatant along with the same coated on bacteria. In 35 patients and 14 controls, we performed extensive phenotyping using cytometry by Time-of-Flight (CyTOF) of circulating immune cells, including unconventional T cells (mucosal associated invariant T [MAIT] cells, γδ T, and natural killer [NK] T cells). The results were validated using RNAseq data from a larger cohort of 179 patients with IgAN, 140 patients with minimal change disease, and 91 HCs. Patients with IgAN had higher circulating levels of total IgA, gd-IgA1, and APRIL, and higher IgA and gd-IgA1-coated gut bacteria than controls, whereas serum levels of SIgA and BAFF did not differ between groups. Patients with IgAN showed more class-switched memory (CSM) and double negative (DN) B cells than controls. MAIT cells and γδ T cells were significantly lower, and CD4-CD8- NK T cells were significantly higher in patients with IgAN than in HCs. We validated the significant decrease in MAIT cells in an independent cohort of patients with IgAN. The data indicate that patients with IgAN have increased circulating CSM and DN B cells associated with abnormal T cell immunity, involving defects in unconventional T cell frequency. This may suggest putative alterations at mucosal sites because of cell migration leading to altered IgA production.