Uptake of nicotinic acetylcholine receptor imaging agent is reduced in the pro-inflammatory macrophage.

Introduction: Macrophages play a vital role in the development of atherosclerotic cardiovascular disease. Macrophages are functionally and phenotypically heterogeneous immune cells and commonly exist in two distinct or polarized subsets: pro-inflammatory M1 and anti-inflammatory M2 phenotypes. Previous reports suggest that stimulation of α7 or α4β2 nicotinic acetylcholine receptors (nAChRs) in macrophages leads to an anti-inflammatory response. However, the biological link between nAChR expression on macrophages and the polarization state is unknown. Therefore, we evaluated the relationship between nAChRs and polarized macrophages in peritoneal macrophages and atherosclerotic plaques of apolipoprotein E knockout (ApoE) mice.

Methods: Peritoneal macrophages isolated from mice were polarized into M1 and M2 macrophages, and the uptake of the nAChR-imaging agents, (R)-2-[C]methylamino-benzoic acid 1-aza-bicyclo[2.2.2]oct-3-yl ester ([C]MeQAA) or 2-[F]fluoro-3-(2(S)-azetidinylmethoxy) pyridine ([F]2FA), and 2-deoxy-2-[F]fluoro-D-glucose ([F]FDG) was assessed. We also evaluated the accumulation of imaging agents in atherosclerotic plaques of ApoE mice by autoradiography. After an autoradiogram was obtained, the same aortic tissue was used for immunohistochemical staining of CD68, inducible nitric oxide synthase (iNOS), and arginine-1.

Results: In an in vitro assay, the uptake of [C]MeQAA or [F]2FA was lower in M1 than in M0 and M2 macrophages. In comparison, the uptake of [F]FDG was higher in M1 macrophages. Ex vivo autoradiography showed that [C]MeQAA was localized to the extensive plaque area. By contrast, the accumulation of [F]2FA and [F]FDG was heterogeneous and found only in some plaques. Moreover, the expression of CD68 and iNOS was higher in [F]2FA non-uptake than [F]2FA uptake plaques.

Conclusion: Macrophage polarization was related to nAChR expression, and α4β2 nAChR expression was suppressed in the M1 macrophage. These findings suggest that nAChR imaging has the potential to identify the inflammatory status of atherosclerotic plaque.