CD34+/AC133+ endothelial progenitor cells as imaging probes for neovascularization of tumors

It was generally considered that blood vessel formation during postnatal life is restricted to angiogenesis only, and for decades tumor vascularization was thought to be the exclusive result of the sprouting of new vessels from the preexisting ones. However, recent studies demonstrated the existence of additional angiogenic and vasculogenic mechanisms associated with tumor growth. Tumor growth and metastasis strongly depends on neovascularization. Endothelial cells that contribute to tumor neovasculatures can originate from sprouting and co-option of neighboring pre-existing vessels. However, there is emerging evidence indicating that bone-marrow derived endothelial progenitor cells (EPCs) also contribute to the vasculogenesis and growth of certain tumors. A subpopulation of CD34+ human hematopoietic stem cells (HSCs) identified by the cell-surface molecule CD133 (AC133), have been shown to be more specific for endothelial differentiation and angiogenesis. We have published the reports showing involvement of peripheral blood, cord blood and bone marrow derived EPCs into tumor neovascularization, and the EPCs’ involvement in tumor neovascularization was determined by in vivo magnetic resonance and nuclear medicine imaging. In this communication different imaging modalities for in vivo tracking of EPCs to the sites of active angiogenesis/vasculogenesis are discussed, where EPCs are used as imaging probes.