Dysregulated Interactions Between Gut Mucosal Immunity and Gut Microbiota in IgAN

Background: The origin of nephritogenic galactose-deficient IgA1 (Gd-IgA1), a key pathogenic driver of IgA nephropathy (IgAN), remains unclear. Methods: IgAN patients (n = 35), non-IgAN CKD patients (n = 9), and healthy controls (n = 25) were recruited in Queen Mary Hospital, Hong Kong, China. Combining bacterial flow cytometry and 16S rRNA sequencing, we quantified gut microbial load and characterized gut bacteria preferentially coated with IgA and Gd-IgA1. Shotgun metagenomic sequencing was performed to profile gut microbial pathways. Additionally, we isolated specific gut bacterial strains from IgAN patients to investigate their capacity to directly deglycosylate human IgA into Gd-IgA1. Results: Compared to healthy and non-IgAN CKD controls, IgAN patients exhibited a higher gut microbial load and an increased relative abundance of Gd-IgA1-coated gut bacteria. Principal component analysis (PCA) revealed significant differences in IgA-coated gut bacterial composition between IgAN patients and healthy controls, suggesting a distinct IgA repertoire in IgAN. The stool bacteria most highly coated with IgA and/or Gd-IgA1 in IgAN patients included known pathobionts (e.g. Escherichia-Shigella and Klebsiella pneumoniae) and mucin-degrading species (e.g. Akkermansia muciniphila, Bifidobacterium bifidum, Ruminococcus torques, and Ruminococcus gnavus). Furthermore, the bacterial isolate from an IgAN patient with high IgA/Gd-IgA1-coating was able to de-glycosylate human IgA1 into Gd-IgA1 in-vitro. Immunoblotting further confirmed that these microbiota-derived Gd-IgA1 reacted with serum autoantibodies from IgAN patients. In addition, metagenomic analysis demonstrated that serum Gd-IgA1 levels significantly correlate with gut microbial carbohydrate utilization pathways, including the D-galactose degradation pathway. Conclusion: Using integrated approaches combining bacterial flow cytometry and 16s-RNA sequencing, we define which gut bacteria induce the production of the nephritogenic Gd-IgA1 in IgAN patients. Our findings highlight that gut microbiota may represent a significant exogenous source of serum Gd-IgA1, providing mechanistic insights into disease pathogenesis and potential avenues for therapeutic intervention. Funding: Government Support – Non-U.S.