Platelet interaction with progenitor cells: potential implications for regenerative medicine.

Circulating endothelial progenitor cells have been shown to instigate new vessel formation via angiogenesis and neovascularisation and to induce ongoing vascular and tissue repair by domiciliation to sites of vascular or tissue damage. However, the mechanisms that recruit circulating endothelial progenitor cells towards vascular lesions and regulate repair mechanisms of ischemic peripheral organs are poorly described. Domiciliation of endothelial progenitor cells in peripheral tissue is a multi-step cascade including initial adhesion to subendothelial matrix or endothelium, transmigration and invasion of the target tissue. Platelets are the first circulating blood cells that interact with the injured vessel wall. They contain a number of growth factors, chemokines, cytokines and adhesive proteins that are released or surface-expressed upon platelet activation including adhesion. Recent studies suggest that platelet interaction with endothelial progenitor cells influences chemotaxis, adhesion, activation and differentiation of progenitor cells. Release of the chemokine SDF-1 from platelets enhances neovascularization through mobilization of progenitor cells. Adherent platelets recruit bone marrow-derived progenitor cells to arterial thrombi in vitro and in vivo and induce their subsequent differentiation towards an endothelial phenotype. Moreover, platelet accumulation in a co-culture system with CD34(+) progenitor cells results in the differentiation of the latter to macrophages in vitro. Although further studies are needed to elucidate the mechanisms that platelets determine the fate of endothelial progenitor cells into vascular lesions, platelet interaction with progenitor cells seems to play a decisive role in vascular and tissue regeneration.