K. M. Brawner1, W. Van Der Pol2, L. Duck3, V. Yeramilli1, L. Smythies3, C. Morrow4, C. Elson3, C. Martin1 1University Of Alabama at Birmingham,Surgery,Birmingham, Alabama, USA 2University Of Alabama at Birmingham,Center For Clinical And Translational Science,Birmingham, Alabama, USA 3University Of Alabama at Birmingham,Medicine,Birmingham, Alabama, USA 4University Of Alabama at Birmingham,Cell, Developmental, And Integrative Biology,Birmingham, Alabama, USA
Introduction: Proper immune responses to gut microbes are mediated through host-cell receptor signaling. The aryl hydrocarbon receptor (Ahr) is engaged by microbial-derived tryptophan (Trp) derivatives to contribute to the development of a functional intestinal barrier. We have shown that removal of dietary sources of Ahr ligands results in decreased fecal immunoglobulin (Ig) A levels. The effect of dietary Ahr ligand manipulation on intestinal microbiome composition and function is largely unknown.
Methods: We randomly assigned mouse littermates to a conventional diet or an Ahr ligand-free diet. DNA from fecal samples was isolated for 16S rRNA gene sequencing or fecal samples were cultured anaerobically. Faecalibaculum rodentium was grown anaerobically in broth with or without carbohydrates and supplemented or not with Trp. Ahr ligands in F. rodentium –conditioned supernatants were measured by luciferase assay. An in vitro model of IgA transcytosis was used to determine if fecal microbiota from Ahr ligand-free diet mice and F. rodentium-conditioned supernatants are capable of degrading IgA. For gene expression analysis, RNA was harvested from primary epithelial cells used in IgA transcytosis assays.
Results: We observed a global alteration of fecal microbiota upon dietary Ahr ligand deprivation. Mice on the Ahr ligand-free diet harbored a microbiota that was less diverse compared to mice on the conventional diet. The most common DNA sequence from the combined dataset of both groups of mice belonged to the genus Allobaculum. This genus was enriched in the feces of mice on the Ahr ligand-free diet. The Allobaculum sequences in our dataset were most closely related to F. rodentium. Compared with non-conditioned broth, F. rodentium-conditioned supernatant contained lower amounts of Ahr ligands. Cultured fecal bacteria from mice on the Ahr ligand-free diet, but not the conventional diet, were able to degrade IgA. Finally, F. rodentium-conditioned supernatant was also capable of IgA degradation without affecting IgA transcytosis.
Conclusion: Here we show that removal of dietary Ahr ligands alters the composition and function of intestinal microbiota. The ability of F. rodentium-conditioned supernatants to degrade Ahr ligands suggests Allobaculum thrives in an environment in which Ahr is inactive. Our IgA transcytosis experiments indicate a cause and effect relationship between altered microbiota and reduced fecal IgA on the Ahr ligand-free diet. We also show that F. rodentium is capable of IgA degradation, recapitulating our results from cultures of whole fecal samples. To our knowledge, this is the first time a specific commensal species has been implicated in Ahr ligand and IgA degradation.
