Hiding in Plain Sight: Thymic CD8+FOXP3+Tregs sequester CD25 and are enriched in human tissues

Abstract

AbstractFor decades regulatory T cell (Treg) research has focussed on CD4+FOXP3+ Tregs, whilst characterisation of CD8+FOXP3+ Tregs has been limited by their paucity in blood. Here, by studying 98 tissues from 27 deceased transplant organ donors we demonstrate that despite representing less than 5% of circulating Tregs, fully functional, thymically derived CD8+FOXP3+ Tregs are highly enriched in human tissues particularly in non-lymphoid tissues and bone marrow, where they reside as CD25lo/-CD8+CD69+CD103+TLR9+HELIOS+FOXP3+ Tregs. Despite lacking surface CD25 expression, CD8+ Tregs in tissue are demethylated at the FOXP3 TSDR and express CD25 intracellularly. Surface CD25 expression is quickly regainedin vitro, allowing cell sorting for therapeutic expansion and confirmation of their suppressive function. We suggest that these elusive cells likely play an essential but previously unappreciated role in maintaining peripheral tolerance within human tissues.One Sentence SummaryFOXP3+CD8+Tregs, expressing tissue residency markers and CD25 intracellularly, are enriched in human non-lymphoid tissues.