Human CD4CD103 cutaneous resident memory T cells are found in the circulation of healthy individuals.

Tissue-resident memory T cells (T) persist locally in nonlymphoid tissues where they provide frontline defense against recurring insults. T 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 nor their ability to reenter the circulation and potentially migrate to distant tissue sites has been investigated. Using tissue explant cultures, we found that CD4CD69CD103 T in human skin can down-regulate CD69 and exit the tissue. In addition, we identified a skin-tropic CD4CD69CD103 population in human lymph and blood that is transcriptionally, functionally, and clonally related to the CD4CD69CD103 T population in the skin. Using a skin xenograft model, we confirmed that a fraction of the human cutaneous CD4CD103 T population can reenter circulation and migrate to secondary human skin sites where they reassume a T phenotype. Thus, our data challenge current concepts regarding the strict tissue compartmentalization of CD4 T cell memory in humans.