Folate Receptor-β Imaging Using ^99mTc-Folate to Explore Distribution of Polarized Macrophage Populations in Human Atherosclerotic Plaque

Abstract

In atherosclerotic plaques, the risk of rupture is increased at sites of macrophage accumulation. Activated macrophages express folate receptor-β (FR-β), which can be targeted by folate coupled to radioactive ligands to visualize vulnerability. The aim of this study was to explore the presence of activated macrophages in human atherosclerotic plaques by 99m Tc-folate imaging and to evaluate whether this technique can discriminate between an M1-like and M2-like macrophage phenotype. Methods: Carotid endarterectomy specimens of 20 patients were incubated with 99m Tc-folate, imaged using micro-SPECT, and divided into 3-mm slices. The mean accumulation was calculated per slice, and the distribution of M1-like and M2-like macrophages per slice was quantified by immunohistochemical staining for CD86 as well as inducible nitric oxide synthase (iNOS) for M1 and CD163 and FR-β for M2 macrophages. Monocytes from healthy donors were differentiated toward M1-like or M2-like phenotype by in vitro culturing. Messenger RNA levels of specific M1 and M2 markers were measured by reversetranscription polymerase chain reaction and expression of FR-β, CD86, and CD163 by flow cytometry. Results: There was a heterogeneous accumulation of 99m Tc-folate in plaques (median, 2.45 [0.77-6.40] MBq/g). Slices with the highest 99m Tc-folate accumulation of each plaque showed significantly more expression of FR-β and CD163, compared with slices with the lowest 99m Tc-folate accumulation, which showed significantly more expression of iNOS. In in vitro polarized macrophages, messenger RNA expression of FR-β, mannose receptor, IL-10, and matrix metalloproteinase-9 was significantly increased in M2-like macrophages, compared with M1-like macrophages. On a receptor level, CD86 was shown to be overexpressed on M1-like macrophages whereas FR-β and CD163 were overexpressed on M2-like macrophages measured by flow cytometry. Conclusion: Higher numbers of M2-like macrophages were present in areas of high 99m Tc-folate accumulation than areas with low accumulation. It is anticipated that 99m Tcfolate imaging using SPECT as a marker for M2-like macrophages in atherosclerosis might be a good indicator for plaque vulnerability.

Comparison of Nanoparticle Penetration into Solid Tumors and Sites of Inflammation: Studies Using Targeted and Nontargeted Liposomes

“…Thus, folate receptor positive macrophages have been shown to accumulate in inflamed lesions of patients with rheumatoid arthritis, Crohn's disease, atherosclerosis, osteoarthritis, Sjogren's syndrome, ischemia reperfusion injury, vasculitis, idiopathic pulmonary fibrosis, asthma, psoriasis and multiple sclerosis [31,[36][37][38][39][40][41][42], and these same sites of inflammation are characterized by increased vascular permeability due to the prominent release of such regulators of endothelial function as TNF-α [43], VEGF [44], histamine [45] and IL-2 [46]. By contrast, while solid tumors are often characterized by a leaky vasculature, their dense extracellular matrices and high cellular densities can strongly suppress NP permeation future science group Comparison of nanoparticle penetration into solid tumors & sites of inflammation Research Article beyond the capillary wall.…”

Direct in vivo evidence of activated macrophages in human osteoarthritis

“…Activated macrophages associated with tumors [40], asthma [42] and atherosclerosis [43] all express FR-β and regulatory M2 macrophage phenotypes [44]. M2 macrophages can be sub-divided into several subsets [45].…”

Activated Microglia in the Brain: Mitochondrial and Cell Membrane-Associated Targets for Positron Emission Tomography

“…To date, most efforts for the development of ligands at FR2 receptors suitable for radio-imaging have been reported on SPECT tracers [126]. One report potentially interesting also for the analysis of cerebral microglia has been provided by Jager et al [126] Overexpression of FR2 in the prefrontal cortex of rats was demonstrated, recently, in a model of acute and chronic stress [48] in parallel with other M2 microglial markers.…”