| dc.description.abstract | The gut microbiota has been recognized for influencing human health and immune homeostasis, with metabolites being a biochemical mediator of these functions. The immune-modulatory effects of microbially-derived tryptophan metabolites
(MDTMs) have been demonstrated in various immune contexts, we report further characterization of their effects in epithelial cells, macrophages, and T cells. We find the MDTM indole acts as a potent inhibitor of TCDD mediated aryl hydrocarbon receptor
(AhR) signaling in colon epithelial cells and as an inhibitor of LPS-stimulated inflammation in macrophages. The effects of indole on macrophages are AhR independent, suggesting cell-type specific signaling mechanisms. Inhibition of phosphatidylinositide-3’OH kinase (PI3K), c-JUN NH2-terminal protein kinase (Jnk), p38 mitogen-activated protein kinase (MAPK), or Akt kinase pathways failed to ablate the reduction of LPS-stimulated TNF accumulation by indole. Interestingly, we fail to capture an indole-mediated reduction in Tnf mRNA levels. Inhibition of proteasomal and lysosomal degradation pathways also fail to ablate indoles effects. The microRNAs miR-181b and miR-124 may act as repressors of TNF translation in macrophages. However, we find no induction of their expression by indole.
To further probe the underlying signaling mechanisms of indole, we synthesize a novel cell-impermeant molecule with an indole moiety covalently bound to bovine serum albumin (I3B), limiting interactions to cell-surface mediated effects. We observe a 50-fold increase in the potency of I3B compared to indole, with 10 μM I3B having a similar effect on TNF accumulation in macrophages as 500 μM free indole. Transcriptome profiling and subsequent experiments demonstrate I3B functions in part through NRF2.
In T cells, we find indole augments the differentiation of naïve T cells to regulatory T cells (Tregs) and inhibit differentiation to T helper 17 cells (Th17s). Another MDTM, 5-hydroxyindole (5-HI), has opposing effects, inhibiting Treg and augmenting Th17 differentiation. Transcriptome profiling suggests activation of IRF3 and IRF7 in indole-augmented Tregs, while 5-HI augmented Th17s exhibit signatures of increased TNF and LPS mediated signaling pathways with a particularly high level of Il22 induction. | |
