Both inflammation and angiogenesis contribute to the progression of abdominal aortic aneurysm (AAA) disease. RGD is a peptide binder of the αvβ3 integrin, which is expressed highly on activated macrophages and angiogenic endothelial cells. Human ferritin (HFn) is a nanoscale protein cage with 12nm diameter and 8nm interior cavity, which we have utilized as a platform for molecular/cellular imaging. We can genetically introduce RGD peptide to HFn. The purpose of this study is to evaluate RGD-conjugated HFn iron oxide nanoparticles for enhanced in vivo MRI of murine AAAs.
1) Mice - Murine AAAs were induced in Apo-E deficient mice by angiotensin II infusion (1µg/kg/min), followed by monitoring of aortic diameter by ultrasound. Control mice were created by saline infusion.
2) RGD-conjugated HFn-iron oxide nanoparticles - HFn was genetically engineered to display 24 copies of an RGD peptide on the exterior surface of the protein cage. Magnetite (Fe3O4) was encapsulated in the interior cavity of RGD-conjugated HFn (RGD+) and non-targeted HFn (RGD-) at loading factors of 5000Fe per cage, giving R2 values of 93 mM-1s-1 (magnetite diameter: 5-7nm). The injected dose was adjusted to 25mgFe/kg in each animal.
3) MRI - All mice were imaged on a whole-body 3T MRI scanner (Signa HDx, GE Healthcare) with a phased array mouse coil (RAPID MR International), using a gradient echo sequence (TR/TE=100ms/10ms, slice thickness=1.0mm, FOV=3cm, matrix=256x256, FA=60, NEX=10). Mice were then injected with either RGD+ or RGD- (6 AAA and 4 control mice for each), followed by MRI at 24 and 48 hours post injection. The nanoparticle accumulation was assessed by measuring the reduction in the T2*-weighted signal intensity of the AAA (or suprarenal aorta in control mice) relative to adjacent normal-size aorta (expressed as % SI loss).
4) Histology - The aortic wall was stained with Perl’s iron (for nanoparticle accumulation), CD-11b (for macrophages), and CD-31 (for endothelial cells).
MRI showed greater T2* signal loss in the AAA with RDG+ than with RDG- (Fig 1(A)), confirmed by quantitative analysis of % SI loss (Fig 1(A) graph, p=0.01). Abdominal aortic diameter on ultrasound correlated more strongly with % SI loss with RDG+ than with RDG- (Fig 2(B)). Perl’s iron staining confirmed greater accumulation of RDG+ in the AAA compared to RGD- (474±51 vs. 277±29 stained cells/cross-sectional area, p=0.01), with colocalization to both macrophages (CD-11b) and endothelial cells (CD-31) within the AAA wall (Fig 2(C)).
HFn iron-oxide nanoparticles with RGD targeting provide a promising MRI approach for comprehensive in vivo detection of inflammation and angiogenesis in high-risk AAAs.
Dr. McConnell receives research support from GE Healthcare and he is on a scientific advisory board for Kowa, Inc.