Objectives: The renin-angiotensin system and local phagocytic activity play a major role in atherosclerotic plaque development. Treatment with irbesartan, an antagonist of angiotensin II receptor, can decrease atherosclerotic lesion formation. Iron oxide-enhanced magnetic resonance imaging (MRI) can be successfully used to evaluate the phagocytic activity in the atherosclerotic plaque in mice. In this study, we used 2 iron oxide-enhanced MRI strategies, in vivo labeling by injection of iron oxide particles and injection of in vitro labeled macrophages, to investigate the effect of irbesartan on both atherosclerotic plaque size and macrophage content in apolipoprotein (Apo) E-deficient mice.
Materials and methods: ApoE-/- female mice (C57BL/6 background; Charles-River, France) were divided into 2 groups (irbesartan treated [TG] or not treated [NTG]) and started on a high-fat diet (Harlan TD88137 Western Diet, 21% fat, 0.2% cholesterol). Animals underwent magnetic resonance examinations on a 7-T scanner at baseline and at 14 and 28 weeks of treatment. At each time point, 2 MRI sessions were performed, before and 48 hours after administration of an iron oxide agent (P904; Guerbet, France) or magnetically labeled macrophages (MФΦ). At the end of the follow-up, blood samples were taken for plasma lipid dosing and aorta samples for histology. The study was approved by the animal experimentation ethic committee of our institution.Vessel wall area measurements were performed on high-resolution spin echo transverse images. Multiecho gradient echo images acquired with the same geometry were used to calculate T2* maps of the vessel wall using a pixel-by-pixel monoexponential fit. Irbesartan effect on vessel wall area over time was assessed using a factorial analysis of variance test. T2* values of the vessel wall at pre- and post-ultrasmall superparamagnetic iron oxide (USPIO) administration were analyzed with a 1-way analysis of variance test with Bonferroni post hoc.
Results: Irbesartan treatment resulted in significantly smaller vessel wall areas at 28 weeks of treatment (P = 0.04). Postinjection values varied significantly over time for both the NTG-P904 (P = 0.02) and the TG-P904 (P = 0.01) groups. Furthermore, when comparing the TG-P904 with the NTG-P904 group at 28 weeks of treatment, a significant difference was obtained for both pre- and post-USPIO administration values (P = 0.01). In the labeled-macrophage group, postinjection T2* values were smaller than the preinjection ones for the NTG animals at 14 weeks of treatment. No T2* changes were observed in the TG-MΦ group.The difference between pre- and post-USPIO administration T2* values (ΔT2*) was significantly smaller in the TG-P904 group compared with the NTG-P904 group at 28 weeks of treatment. At this point, a good correlation (R = 0.7, P = 0.03) was found between the ΔT2* values in the P904 imaging group and the macrophage-covered area by immunohistological analysis.
Conclusions: The present study illustrates an MRI follow-up of intraplaque macrophages using in vivo labeling by iron oxide particle injection and macrophage injection after in vitro USPIO labeling in the assessment of a therapeutic effect in a mouse model of atherosclerosis. Even though in vivo labeling is not fully specific of macrophage uptake, it enabled the detection of a treatment-related reduction in the macrophage content of atherosclerotic plaques in ApoE-/- mice.