Identification of a New Cell Population Constitutively Circulating in Healthy Conditions and Endowed with a Homing Ability Toward Injured Sites

Stem and progenitor cells are the critical units for tissue maintenance, regeneration, and repair. The activation of regenerative events in response to tissue injury has been correlated with mobilization of tissue-resident progenitor cells, which is functional to the wound healing process. However, until now there has been no evidence for the presence of cells with a healing capacity circulating in healthy conditions. We identified a rare cell population present in the peripheral blood of healthy mice that actively participates in tissue repair. These Circulating cells, with a Homing ability and involved in the Healing process (CH cells), were identified by an innovative flowcytometry strategy as small cells not expressing CD45 and lineage markers. Their transcriptome profile revealed that CH cells are unique and present a high expression of key pluripotency- and epiblast-associated genes. More importantly, CH-labeled cells derived from healthy Red Fluorescent Protein (RFP)-transgenic mice and systemically injected into syngeneic fractured wild-type mice migrated and engrafted in wounded tissues, ultimately differentiating into tissue-specific cells. Accordingly, the number of CH cells in the peripheral blood rapidly decreased following femoral fracture. These findings uncover the existence of constitutively circulating cells that may represent novel, accessible, and versatile effectors of therapeutic tissue regeneration.