AbstractThe intestinal immune system is highly adapted to maintaining tolerance to the commensal microbiota and self-antigens while defending against invading pathogens^1,2. Recognizing how the diverse network of local cells establish homeostasis and maintains it in the complex immune environment of the gut is critical to understanding how tolerance can be re-established following dysfunction, such as in inflammatory disorders. Although cell and molecular interactions that control T regulatory (T_reg) cell development and function have been identified^3,4, less is known about the cellular neighbourhoods and spatial compartmentalization that shapes microorganism-reactive T_reg cell function. Here we used in vivo live imaging, photo-activation-guided single-cell RNA sequencing^5–7 and spatial transcriptomics to follow the natural history of T cells that are reactive towards Helicobacter hepaticus through space and time in the settings of tolerance and inflammation. Although antigen stimulation can occur anywhere in the tissue, the lamina propria—but not embedded lymphoid aggregates—is the key microniche that supports effector T_reg (eT_reg) cell function. eT_reg cells are stable once their niche is established; however, unleashing inflammation breaks down compartmentalization, leading to dominance of CD103⁺SIRPα⁺ dendritic cells in the lamina propria. We identify and validate the putative tolerogenic interaction between CD206⁺ macrophages and eT_reg cells in the lamina propria and identify receptor–ligand pairs that are likely to govern the interaction. Our results reveal a spatial mechanism of tolerance in the lamina propria and demonstrate how knowledge of local interactions may contribute to the next generation of tolerance-inducing therapies.