Tissue regulatory T cells: regulatory chameleons.

The FOXP3+CD4+ regulatory T (Treg) cells located in non-lymphoid tissues differ in phenotype and function from their lymphoid organ counterparts. Tissue Treg cells have distinct transcriptomes, T cell receptor repertoires and growth and survival factor dependencies that arm them to survive and operate in their home tissue. Their functions extend beyond immune surveillance to tissue homeostasis, including regulation of local and systemic metabolism, promotion of tissue repair and regeneration, and control of the proliferation, differentiation and fate of non-lymphoid cell progenitors. Treg cells in diverse tissues share a common FOXP3+CD4+ precursor located within lymphoid organs. This precursor undergoes definitive specialization once in the home tissue, following a multilayered array of common and tissue-distinct transcriptional programmes. Our deepening knowledge of tissue Treg cell biology will inform ongoing attempts to harness Treg cells for precision immunotherapeutics.