Regulatory T cells (T) are indispensable for the control of immune homeostasis and have clinical potential as a cell therapy for treating autoimmunity. T can lose expression of the lineage-defining Foxp3 transcription factor and acquire effector T cell (T) characteristics, a process referred to as T plasticity. The extent and reversibility of such plasticity during immune responses remain unknown. Here, using a murine genetic fate-mapping system, we show that T stability is maintained even during exposure to a complex microbial/antigenic environment. Furthermore, we demonstrate that the observed plasticity of T after adoptive transfer into a lymphopenic environment is a property limited to only a subset of the T population, with the nonconverting majority of T being resistant to plasticity upon secondary stability challenge. The unstable T fraction is a complex mixture of phenotypically distinct T, enriched for naïve and neuropilin-1-negative T, and includes peripherally induced T and recent thymic emigrant T These results suggest that a "purging" process can be used to purify stable T that are capable of robust fate retention, with potential implications for improving cell transfer therapy. Regulatory T cells (T) are indispensable for the control of immune homeostasis and have clinical potential as a cell therapy for treating autoimmunity. T can lose expression of the lineage-defining Foxp3 transcription factor and acquire effector T cell (T) characteristics, a process referred to as T plasticity. The extent and reversibility of such plasticity during immune responses remain unknown. Here, using a murine genetic fate-mapping system, we show that T stability is maintained even during exposure to a complex microbial/antigenic environment. Furthermore, we demonstrate that the observed plasticity of T after adoptive transfer into a lymphopenic environment is a property limited to only a subset of the T population, with the nonconverting majority of T being resistant to plasticity upon secondary stability challenge. The unstable T fraction is a complex mixture of phenotypically distinct T, enriched for naïve and neuropilin-1-negative T, and includes peripherally induced T and recent thymic emigrant T These results suggest that a "purging" process can be used to purify stable T that are capable of robust fate retention, with potential implications for improving cell transfer therapy. Junius S, Mavrogiannis AV, Lemaitre P, Gerbaux M, Staels F, Malviya V, Burton O, Gergelits V, Singh K, Tito Tadeo RY, Raes J, Humblet-Baron S, Liston A, Schlenner SM Science immunology 6 61 34301799 0 Immunology 23 Jul 2021