Renal Inflammation: Targeted Iron Oxide Nanoparticles for Molecular MR Imaging in Mice

Abstract

Purpose:To determine the feasibility of T2-weighted magnetic resonance (MR) imaging in the noninvasive quantifi cation of renal infl ammation by using superparamagnetic iron oxide (SPIO) nanoparticles targeted to tissue-bound C3 activation fragments in a mouse model of lupus nephritis. Materials and Methods:All animal procedures were approved by the University of Colorado-Denver animal care and use committee. SPIO nanoparticles were encapsulated by using aminefunctionalized phospholipids. A recombinant protein containing the C3d-binding region of complement receptor type 2 (CR2) was then conjugated to the surface of the SPIO nanoparticle. Five MRL/lpr mice (a model of lupus nephritis) and six C57BL/6 wild-type mice were assessed with T2-weighted MR imaging at baseline and after SPIO injection. The same fi ve MRL/lpr mice and three C57BL/6 mice also underwent MR imaging after injection of CR2-targeted SPIO. A series of T2-weighted pulses with 16 echo times was used to enable precise T2 mapping and calculation of T2 relaxation times in the cortex and outer and inner medulla of the kidneys, as well as in the spleen, muscle, and fat. The effects of treatment and animal genotype on T2 relaxation times were analyzed with repeatedmeasures analysis of variance. Results:At baseline, the T2-weighted signal intensity in the kidneys of MRL/lpr mice was higher than that in the kidneys of wild-type mice. Injection of untargeted SPIO did not alter the T2-weighted signal in the kidneys in either strain of mice. Injection of CR2-targeted SPIO in MRL/lpr mice, however, caused a signifi cant accumulation of targeted iron oxide with a subsequent decrease in T2 relaxation times in the cortex and outer and inner medulla of the kidneys. No changes in T2 relaxation time were observed in the wild-type mice after injection of targeted SPIO. Conclusion:Injection of CR2-conjugated SPIO caused a signifi cant reduction in T2-weighted MR imaging signal and T2 relaxation time in nephritic kidneys.q RSNA, 2010Supplemental material: http://radiology.rsna.org/lookup /suppl

Detection of complement activation using monoclonal antibodies against C3d

“…Because tissue-bound C3 fragments are associated with local inflammation, they have also been exploited as addressable binding ligands for targeted therapeutics and diagnostic agents in several tissues, including the kidneys, the heart, the brain, and the eyes (8)(9)(10)(11)(12). These targeted agents have employed recombinant forms of complement receptor 2 (CR2), a protein that can discriminate between intact C3 in the plasma and tissue-bound C3 fragments.…”

Non-invasive imaging to monitor lupus nephritis and neuropsychiatric systemic lupus erythematosus

“…We performed T2-weighted MRI before and after injection with the nanoparticles and analyzed the signal in the kidneys. In MRL/lpr mice, the injection of CR2-targeted SPIONs caused a significant decrease in the T2 signal within the kidneys 8 . The T2 signal did not decrease in age-matched MRL/lpr mice injected with untargeted SPIONs, however, nor in healthy control mice injected with CR2-targeted SPIONs.…”

“…CR2 binds the C3d cleavage fragment of C3 with a K D of approximately 0.5 μM 77 . Because CR2 does not bind intact plasma C3 it can be used to target therapeutic and diagnostic agents to sites of complement activation 8, 78– 80 . We have also developed several monoclonal antibodies to C3d that do not bind to intact C3 or to C3b 81 .…”

“…To test whether tissue C3 deposits can be detected by MRI, we conjugated recombinant CR2 to the surface of superparamagnetic iron-oxide nanoparticles (SPIONs) 8 . Superparamagnetic iron-oxide causes rapid dephasing of nearby protons and, as a result, accelerates the spin-spin relaxation rate (R2) 82 .…”

“…However, antibodies can deposit in tissues prior to infiltration with granulocytes, causing inflammatory tissue injury without high 18 FDG uptake 7 . Hence, physiological and molecular imaging methods are being developed to detect organ dysfunctional and locate specific molecular markers in affected tissues in autoimmune diseases 8– 11 . These methods could potentially allow clinicians to non-invasively monitor lupus disease activity.…”