Purpose: Vulnerable plaques are key factors for ischemic diseases. Thus, their precise detection is necessary for the diagnosis of such diseases. Immunoglobulin G (IgG)-based imaging probes have been developed for imaging biomolecules related to plaque formation for the diagnosis of atherosclerosis. However, IgG accumulates nonspecifically in atherosclerotic regions, and its accumulation mechanisms have not yet been clarified in detail. Therefore, we explored IgG accumulation mechanisms in atherosclerotic lesions and examined images of radiolabeled IgG for the diagnosis of atherosclerosis.
Procedures: Mouse IgG without specificity to biomolecules was labeled with technetium-99m via 6-hydrazinonicotinate to yield [99mTc]IgG. ApoE-/- or C57BL/6J mice were injected intravenously with [99mTc]IgG, and their aortas were excised 24 h after injection. After radioactivity measurement, serial aortic sections were autoradiographically and histopathologically examined. RAW264.7 macrophages were polarized into M1 or M2 and then treated with [99mTc]IgG. The radioactivities in the cells were measured after 1 h of incubation. [99mTc]IgG uptake in M1 macrophages was also evaluated after the pretreatment with an anti-Fcγ receptor (FcγR) antibody. The expression levels of FcγRs in the cells were measured by western blot analysis.
Results: [99mTc]IgG accumulation levels in the aortas were significantly higher in apoE-/- mice than in C57BL/6J mice (5.1 ± 1.4 vs 2.8 ± 0.5 %ID/g, p < 0.05). Autoradiographic images showed that the accumulation areas highly correlated with the macrophage-infiltrated areas. M1 macrophages showed significantly higher levels of [99mTc]IgG than M2 or M0 (nonpolarized) macrophages [2.2 ± 0.3 (M1) vs 0.5 ± 0.1 (M2), 0.4 ± 0.1 (M0) %dose/mg protein, p < 0.01] and higher expression levels of FcγRI and FcγRII. [99mTc]IgG accumulation in M1 macrophages was suppressed by pretreatment with the anti-FcγR antibody [2.2 ± 0.3 (nonpretreatment) vs 1.2 ± 0.2 (pretreatment) %ID/mg protein, p < 0.01].
Conclusions: IgG accumulated in pro-inflammatory M1 macrophages via FcγRs in atherosclerotic lesions. Thus, the target biomolecule-independent imaging of active inflammation should be taken into account in the diagnosis of atherosclerosis using IgG-based probes.
Keywords: Atherosclerosis; Immunoglobulin G; Macrophage; Nuclear imaging; Polarization.