Metabolic pathways encoded within the gut microbiome utilize dietary nutrients to produce bio-active molecules that interact with host gene products. For example, bacterial-derived secondary bile acids and short chain fatty acids, and dietary antigens support the expansion of gut resident Rorγt+ Treg cells. Rorγt+ Treg cells are crucial for maintaining host-microbiota homeostasis, however, the molecular network governing Treg cell maintenance in the gastro-intestinal tract still remains poorly understood. The essential amino acid Tryptophan (Trp) is metabolized by commensal microbiota to produce bioactive indoles and indole-based metabolites. These bacterially-derived Trp metabolites behave as Aryl-hydrocarbon receptor (AhR) ligands, which activate AhR mediated transcription in many cell types. AhR is an important regulator of barrier integrity, metabolism and immunity.
Here we have utilized a model of acute dietary Trp deficiency to investigate the effect of Trp metabolism in regulating immune-microbiota crosstalk., We first wanted to understand how acute dietary Trp deficiency impacted the microbiota, metabolite profile and transcriptome of the small intestine by using “multi-omics” approach. We placed mice on control or Trp-deficient diets and compared their microbial diversity by 16S, metabolomics profile and the transcriptional landscape of the intestinal ileum. As to be expected, Trp depletion significantly altered the fecal metabolite profile and the alpha and beta diversity of the microbiota. More specifically, a significant reduction in L.reuteri was observed, an important Trp metabolizing probiotic. Trp deficiency further resulted in robust transcriptional changes of key immune processes in a microbiota dependent manor.
As intestinal Treg cells are acutely sensitive to the intestinal microbiota and its metabolites, we next investigated whether microbial and metabolic changes resulting from a Trp deficient diet were impacting Treg homeostasis. Interestingly, acute dietary Trp depletion resulted in the expansion and proliferation of Rorγt+ Treg cells in a microbiota dependent manor. Furthermore, introduction of the AhR ligand Indole-3-Carbinol to our Trp deficient diet was sufficient to revert the effects of Trp-deficiency on Rorγt+ Treg cells. Therefore, our findings bring to light the importance of Trp metabolism in the maintenance of Rorγt+ Treg and suggests that AhR ligands negatively regulate the expansion of Rorγt+ Treg cells.