Abstract
Peripheral blood (PB) contains several types of stem/progenitor cells, including hematopoietic stem and endothelial progenitor cells. We identified a population positive for both the pluripotent surface marker SSEA-3 and leukocyte common antigen CD45 that comprises 0.04% ± 0.003% of the mononuclear cells in human PB. The average size of the SSEA-3(+)/CD45(+) cells was 10.1 ± 0.3 µm and ∼22% were positive for CD105, a mesenchymal marker; ∼85% were positive for CD19, a B cell marker; and ∼94% were positive for HLA-DR, a major histocompatibility complex class II molecule relevant to antigen presentation. These SSEA-3(+)/CD45(+) cells expressed the pluripotency markers Nanog, Oct3/4, and Sox2, as well as sphingosine-1-phosphate (S1P) receptor 2, and migrated toward S1P, although their adherence and proliferative activities in vitro were low. They expressed NeuN at 7 d, Pax7 and desmin at 7 d, and alpha-fetoprotein and cytokeratin-19 at 3 d when supplied to mouse damaged tissues of the brain, skeletal muscle and liver, respectively, suggesting the ability to spontaneously differentiate into triploblastic lineages compatible to the tissue microenvironment. Multilineage-differentiating stress enduring (Muse) cells, identified as SSEA-3(+) in tissues such as the bone marrow and organ connective tissues, express pluripotency markers, migrate to sites of damage via the S1P-S1P receptor 2 system, and differentiate spontaneously into tissue-compatible cells after homing to the damaged tissue where they participate in tissue repair. After the onset of acute myocardial infarction and stroke, patients are reported to have an increase in the number of SSEA-3(+) cells in the PB. The SSEA-3(+)/CD45(+) cells in the PB showed similarity to tissue-Muse cells, although with difference in surface marker expression and cellular properties. Thus, these findings suggest that human PB contains a subset of cells that are distinct from known stem/progenitor cells, and that CD45(+)-mononuclear cells in the PB comprise a novel subpopulation of cells that express pluripotency markers.