MRI as a Novel Non–Invasive Method for in vivo Tracking of Endothelial Progenitor Cells in a Mouse Model of Choroidal Neovascularization (CNV)

Abstract

Purpose: : We implemented an innovative method to track in vivo migration and engraftment of endothelial progenitor cells (EPCs) (CD34⁺ cells) to areas of injury in a CNV model by magnetic resonance (MR) imaging techniques using monocrystalline iron oxide nanoparticles (MION) as a contrast agent. We also tested the effect of MION on EPC behavior ex vivo.

Methods: : 5,000 EPCs (per condition) were incubated with 1:2000, 1:1000, and 1:500 dilution of MION overnight in 5% CO₂ at 37^oC. Cell migration was tested using a Boyden chamber assay. Cell viability and apoptosis was tested using Annexin V–FITC apoptosis detection kit. To test the ability to detect single labeled cells, EPCs were loaded with MION and dispersed at very low cell density in 2% agar and loaded into 5mm NMR tubes. Samples were imaged using T2* weighted gradient echo imaging techniques (FOV 19x19 mm; matrix 512x400; thickness 180 µm; TE 15 ms). For in vivo studies, CNV was induced in C57BL/6J by laser rupture of Bruch’s membrane. MION–labeled EPCs were injected. Using a 5mm custom built coil and high–resolution 3D gradient echo imaging (TR 0.5 sec; TE 7.5 ms; matrix 200x180x80; FOV 3.8x3.8x3.8 mm; NA 6), the EPCs were tracked.

Results: : MION did not adversely affect cell migration compared to sham–treated cells. MION labeling did not induce apoptosis of EPCs compared to controls. Ex vivo MION–labeled cells were detectable on a 17.6 Tesla magnet at various incubation conditions (dilutions) of MION. The extraordinary resolution (in–plane of 20 µm²) clearly showed at least six of the retinal layers in the eyes and MION–labeled EPCs were detectable in the retina twenty four hours post injection.

Conclusions: : In this study we implemented an innovative method to track in vivo migration and engraftment of EPCs to areas of injury in a mouse model of CNV by MRI techniques. Labeling EPCs with MION does not affect their behavior, while it provides an excellent MR imaging agent. This technique, offering the advantage of non–invasively monitoring of in vivo cell migration, will open a new avenue for understanding ocular repair mechanisms.

Keywords: choroid: neovascularization • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • neovascularization