Individual Microbes Shape Various Parts of the Immune System

The gastrointestinal tract, home to a vast number of bacteria, requires finely-tuned regulatory and effector immune mechanisms to maintain homeostasis and tolerance. In a large-scale screen, we studied the impacts of single microbes on major immune populations, whole intestinal tissue homeostasis and metabolism. Bacteria interacted with the host at multiple levels including cytokine responses, accumulation of various T cells, alterations in composition of mononuclear phagocytes and induction of epithelial cell genes as measured by transcriptome analysis of whole intestinal tissue. Interestingly, taxonomically unrelated bacteria elicited similar immune phenotypes and metabolic effects. A more focused analysis of the induction of regulatory mechanisms revealed a microbiota-dependent, context-specific transcriptional control of Foxp3+ regulatory T cells and of IL17 producing T cells. These facets were both regulated by Rorγ, a transcription factor known for its antagonistic effects on Foxp3. Paradoxically, Rorγ expression induced by bacteria in colonic Foxp3+ regulatory T cells was necessary for function of these cells especially in the context of IL17 and IFNγ-mediated colitis. Overall, this large-scale screen provides a comprehensive study of how individual bacterial species shape many aspects of the host immunity and metabolism, and exemplifies a microbiota-dependent, context-specific mechanism that potentiates function in Foxp3+ regulatory T cells.Medical Science