CD31⁺ cells represent highly angiogenic and vasculogenic cells in bone marrow: Novel role of nonendothelial CD31⁺ cells in neovascularization and their therapeutic effects on ischemic vascular disease

Rationale: Bone marrow (BM) cells play an important role in physiological and therapeutic neovascularization. However, it remains unclear whether any specific uncultured BM cell populations have higher angiogenic and vasculogenic activities. Moreover, there has been controversy regarding the vasculogenic ability of BM cells. Objective: Preliminary flow cytometric analysis showed that CD31, traditionally a marker for endothelial cells, is expressed in certain nonendothelial BM mononuclear cells in both human and mouse. Based on the conserved CD31⁺ expression in the axis of hematopoietic stem/progenitor cells (HSC/HPCs) to endothelial cells, we further sought to determine the comprehensive vasculogenic and angiogenic characteristics of human and mouse BM-derived CD31⁺ cells. Methods and Results: Flow cytometric analysis demonstrated that all CD31⁺ cells derived from BM were CD45 and expressed markers for both HSC/HPCs and endothelial cells. Comprehensive gene expression analyses revealed that BM-CD31⁺ cells expressed higher levels of angiogenic genes than CD31⁺ cells. Endothelial progenitor cells, as well as HSC/HPCs, were almost exclusively confined to the CD31⁺ cell fraction, and culture of CD31⁺ cells under defined conditions gave rise to endothelial cells. Finally, injection of CD31⁺ cells into ischemic hindlimb repaired ischemia, increased expression of angiogenic and chemoattractive factors, and, in part, directly contributed to vasculogenesis, as demonstrated by both 3D confocal microscopy and flow cytometry. Conclusions: These data indicate that BM-CD31⁺ cells represent highly angiogenic and vasculogenic cells and can be a novel and highly promising source of cells for cell therapy to treat ischemic cardiovascular diseases.