Most T lymphocytes, including regulatory T cells (Treg cells), differentiate in the thymus. The age-dependent involution of this organ leads to decreasing production of T cells. Here we found that the output of new Treg cells from the thymus decreased substantially more than that of conventional T cells. Peripheral mouse and human Treg cells recirculated back to the thymus, where they constituted a large proportion of the pool of Treg cells and displayed an activated and differentiated phenotype. In the thymus, the recirculating cells exerted their regulatory function by inhibiting interleukin 2 (IL-2)-dependent de novo differentiation of Treg cells. Thus, Treg cell development is controlled by a negative feedback loop in which mature progeny cells return to the thymus and restrain development of precursors of Treg cells.

The gut microbiota plays a crucial role in the maturation of the intestinal mucosal immune system of its host1,2. Within the thousand bacterial species present in the intestine, the symbiont Segmented Filamentous Bacterium (SFB) is unique in its ability to potently stimulate the postnatal maturation of the B and T cell compartments and induce a striking increase in the small intestinal Th17 responses3-5. Unlike other commensals, SFB intimately attaches to absorptive epithelial cells in the ileum and cells overlying Peyer's patches6,7. This colonization does not result in pathology but rather protects the host from pathogens4. Yet, little is known about the host-SFB interaction that underlies the important immunostimulatory properties of SFB as SFB has resisted in vitro culturing for over 50 years. Here we succeeded in growing mouse SFB outside of its host in an SFB-host cell co-culturing system. Single-celled SFB isolated from monocolonized mice undergo filamentation, segmentation, and differentiation to release viable infectious particles, the intracellular offsprings or IOs, that can colonize mice to induce signature immune responses. In vitro, IOs can attach to mouse and human host cells and recruit actin. In Author Contributions: NCB, VGR, PSa. and PSc conceived the project and discussed experiments. PSc. designed and performed all in vitro experiments. VGR and MG performed the in vivo challenge experiments, VGR maintained SFB mice and MG analysed the TC7 and RF the mICl2 host response. MMN processed SEM samples and took images with PS. GN assisted in vitro experiments. PSc wrote the paper and PSa, NCB and VGR edited the manuscript.