SFB (green) are members of the commensal microbiota that colonize wildtype mice at the time of weaning. SFB are endemic for the terminal ileum where they form stable interactions with the villous epithelium (pink). SFB form long filaments that often span the length of several villi. When present, SFB specifically induce the differentiation of effector Th17 cells in the lamina propria. Scanning electron micrograph taken by Alice Liang, NYU and Doug Wei, Carl Zeiss Inc.; artificial coloring by Eric Roth, NYU.

(Left panel) Signals from different components of the microbiota (different color arrows) regulate different branches of the mucosal T cell response in the lamina propria (corresponding color immune cells). (Right panel) Changes in the composition of commensal bacteria, e.g. introduction of segmented filamentous bacteria (SFB), shift the immune homeostasis in a different direction, in this case increase in the signals mediating induction of Th17 cells (green arrows). This changes the immunological fitness of the individual. In the case of SFB, the increased production of Th17 cell effector cytokines, e.g. IL-17 and IL-22, and the consecutive increase in antimicrobial peptide production from epithelial cells (blue circles) augments the ability of the host to fight off intestinal infections. At the same time, this increase in pro-inflammatory cytokines may render the host more susceptible to chronic autoimmune inflammation. In this way, differences in the repertoire of commensal bacteria between individuals of the same species may account for differences in the nature and robustness of their response in the face of similar environmental challenges.