Tissue-resident memory T cells (T RM ) persist locally in non-lymphoid tissues where they provide front-line defense against recurring insults. T RM at barrier surfaces express the markers CD103 and/or CD69 which function to retain them in epithelial tissues. In humans, neither the long-term migratory behavior of T RM nor their ability to re-enter the circulation and potentially migrate to distant tissue sites have been investigated. Using tissue explant cultures, we found that CD4 + CD69 + CD103 + T RM in human skin can downregulate CD69 and exit the tissue. Additionally, we identified a skin-tropic CD4 + CD69 − CD103 + population in human lymph and blood that is transcriptionally, functionally and clonally related to the CD4 + CD69 + CD103 + T RM population in the skin. Using a skin xenograft model, we confirmed that a fraction of the human cutaneous CD4 + CD103 + T RM population can re-enter circulation, and migrate to secondary human skin sites where they re-assume a T RM phenotype. Thus, our data challenge current concepts regarding the strict tissue compartmentalization of CD4 + T cell memory in humans. h4>One Sentence Summary/h4> Human CD4 + CD103 + cutaneous resident memory T cells are found in the circulation of healthy subjects, and these cells can seed distant skin sites.

Extracellular vesicles (EVs) secreted by cells present an attractive strategy for developing new therapies, but progress in the field is limited by several issues: The quality of the EVs varies with the type and physiological status of the producer cells; protocols used to isolate the EVs are difficult to scale up; and assays for efficacy are difficult to develop. In the present report, we have addressed these issues by using human mesenchymal stem/stromal cells (MSCs) that produce EVs when incubated in a protein-free medium, preselecting the preparations of MSCs with a biomarker for their potency in modulating inflammation, incubating the cells in a chemically defined protein-free medium that provided a stable environment, isolating the EVs with a scalable chromatographic procedure, and developing an in vivo assay for efficacy of the cells in suppressing neuroinflammation after traumatic brain injury (TBI) in mice. In addition, we demonstrate that i.v. infusion of the isolated EVs shortly after induction of TBI rescued pattern separation and spatial learning impairments 1 mo later.