Targeting mannose receptor expression on macrophages in atherosclerotic plaques of apolipoprotein E-knockout mice using 111 In-tilmanocept

Abstract

Background

Atherosclerotic plaque phenotypes are classified based on the extent of macrophage infiltration into the lesions, and the presence of certain macrophage subsets might be a sign for plaque vulnerability. The mannose receptor (MR) is over-expressed in activated macrophages. Tilmanocept is a tracer that targets MR and is approved in Europe and the USA for the detection of sentinel lymph nodes. In this study, our aim was to evaluate the potential of ¹¹¹In-labelled tilmanocept for the detection of MR-positive macrophages in atherosclerotic plaques of apolipoprotein E-knockout (ApoE-KO) mouse model.

Methods

Tilmanocept was labelled with ¹¹¹In. The labelling stability and biodistribution of the tracer was first evaluated in control mice (n = 10) 1 h post injection (p.i.). For in vivo imaging studies, ¹¹¹In-tilmanocept was injected into ApoE-KO (n = 8) and control (n = 8) mice intravenously (i.v.). The mice were scanned 90 min p.i. using a dedicated animal SPECT/CT. For testing the specificity of ¹¹¹In-tilmanocept uptake in plaques, a group of ApoE-KO mice was co-injected with excess amount of non-labelled tilmanocept. For ex vivo imaging studies, the whole aortas (n = 9 from ApoE-KO and n = 4 from control mice) were harvested free from adventitial tissue for Sudan IV staining and autoradiography. Cryosections were prepared for immunohistochemistry (IHC).

Results

¹¹¹In radiolabelling of tilmanocept provided a yield of greater than 99%. After i.v. injection, ¹¹¹In-tilmanocept accumulated in vivo in MR-expressing organs (i.e. liver and spleen) and showed only low residual blood signal 1 h p.i. MR-binding specificity in receptor-positive organs was demonstrated by a 1.5- to 3-fold reduced uptake of ¹¹¹In-tilmanocept after co-injection of a blocking dose of non-labelled tilmanocept. Focal signal was detected in atherosclerotic plaques of ApoE-KO mice, whereas no signal was detected in the aortas of control mice. ¹¹¹In-tilmanocept uptake was detected in atherosclerotic plaques on autoradiography correlating well with Sudan IV-positive areas and associating with subendothelial accumulations of MR-positive macrophages as demonstrated by IHC.

Conclusions

After i.v. injection, ¹¹¹In-tilmanocept accumulated in MR-expressing organs and was associated with only low residual blood signal. In addition, ¹¹¹In-tilmanocept uptake was detected in atherosclerotic plaques of mice containing MR-expressing macrophages suggesting that tilmanocept represents a promising tracer for the non-invasive detection of macrophages in atherosclerotic plaques.

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