Molecular imaging of endothelial progenitor cell engraftment using contrast-enhanced ultrasound and targeted microbubbles

Imaging methods to track the fate of progenitor cells after their delivery would be useful in assessing the efficacy of cell-based therapies. We hypothesized that contrast-enhanced ultrasound (CEU) using microbubbles targeted to a genetically engineered cell-surface marker on endothelial progenitor cells (EPCs) would allow the targeted imaging of vascular engraftment. Rodent bone marrow-derived EPCs were isolated, cultured, and transfected to express the marker protein, H-2Kk, on the cell surface. Non-transfected EPCs and EPCs transfected with either null plasmid or Firefly luciferase served as controls. Control microbubbles (MBC) and microbubbles targeted to H-2Kk expressed on EPCs (MBH-2Kk) were constructed. Binding of targeted microbubbles to EPCs was assessed in vitro using a parallel plate flow chamber system. CEU imaging of EPC-targeted microbubbles was assessed in vivo using subcutaneously implanted EPC-supplemented Matrigel plugs in rats. In flow chamber experiments, there was minimal attachment of microbubbles to plated control EPCs. Although numbers of adhered MBC were also low, there was greater and more diffuse attachment of MBH-2Kk to plated H-2Kk-transfected EPCs. Targeted CEU demonstrated marked contrast enhancement at the periphery of the H-2Kk-transfected EPC-supplemented Matrigel plug for MBH-2Kk, whereas contrast enhancement was low for MBC. Contrast enhancement was also low for both microbubbles within control mock-transfected EPC plugs. The signal intensity within the H-2Kk-transfected EPC plug was significantly greater for MBH-2Kk when compared with MBC. Microbubbles targeted to a genetically engineered cell-surface marker on EPCs exhibit specific binding to EPCs in vitro. These targeted microbubbles bind to engrafted EPCs in vivo within Matrigel plugs and can be detected by their enhancement on CEU imaging.