Quiescent tissue stem cells evade immune surveillance

Stem cells are critical for the maintenance of many tissues, and thus their integrity is crucial, but whether this integrity is maintained in the face of an immune response is not well defined. Here we set out to determine the outcome of T cell interactions with adult stem cells in their physiological niche. We find that fast cycling epithelial stem cells, including Lgr5+ intestinal stem cells, as well as ovary and mammary stem cells, are eliminated by activated T-cells, but slow cycling stem cells, specifically hair follicle and muscle stem cells, are resistant to T-cell killing. This is an intrinsic property of the quiescent stem cells resulting from systemic downregulation of the antigen presentation machinery, including MHC class I, Tap proteins, and the transactivator Nlrc5, which is reversed upon stem cell entry into the cell cycle. These findings identify a link between tissue stem cell quiescence, antigen presentation, and immune evasion, and may help explain why mutations in long-lived stem cells would not lead to immune editing. They also suggest how cancer-initiating cells may evade immune surveillance Lgr5-positive cells were flow-cytometry sorted from the back epidermis of mice that were either 45 days old (Telogen) or 35 days old (Anagen)