Evaluation of translocator protein quantification as a tool for characterising macrophage burden in human carotid atherosclerosis

Abstract

Macrophage presence within atherosclerotic plaque is a feature of instability and a risk factor for plaque rupture and clinical events. Activated macrophages express high levels of the translocator protein/peripheral benzodiazepine receptor (TSPO/PBR). In this study, we investigated the potential for quantifying plaque inflammation by targeting this receptor. TSPO expression and distribution in the plaque were quantified using radioligand binding assays and autoradiography. We show that cultured human macrophages expressed 20 times more TSPO than cultured human vascular smooth muscle cells (VSMCs), the other abundant cell type in plaque. The TSPO ligands [3H](R)-1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoquinoline carboxamide ([3H](R)-PK11195) and [3H]N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide ([3H]-DAA1106) bound to the same sites in human carotid atherosclerotic plaques in vitro, and demonstrated significant correlation with macrophage-rich regions. In conclusion, our data indicate that radioisotope-labelled DAA1106 has the potential to quantify the macrophage content of atherosclerotic plaque.

Vascular imaging with positron emission tomography

“…The translocator protein ⁄ peripheral benzodiazepine receptor (TSPO) is expressed on macrophages at 20 times greater density than vascular smooth muscle cells. In vitro, the TSPO ligands PK11195 and DAA1106 will bind to macrophage-rich regions in human carotid plaque [97]. However, it has become clear that for in vivo PET imaging of their distribution, there appears to be a wide spectrum of binding affinity of the receptors for these ligands in patients [98,99].…”

Targeting activated macrophages to identify the vulnerable atherosclerotic plaque

“…16 In human atherosclerotic tissue, TSPO was observed to be 209 greater than in vascular smooth muscle cells. 12 The author comments that inability to differentiate plaque from normal aorta may also be due to ''spill-over'' from residual blood activity into the arterial walls which is also related to limited spatial resolution for PET imaging in rodents. While the physics of positron annihilation and design of PET camera favors a higher resolution than SPECT in human subjects, it actually favors a lower resolution in mice.…”

“…11,12 One of the earliest ligands developed for the TSPO was isoquinoline carboxamide PK11195 shown by 3 H autoradiography to be taken up in macrophage-rich regions of human carotid atherosclerotic vessel tissue preparations. 11 Based on these reports, two subsequent pilot clinical imaging studies were performed in small groups of patients with arterial vasculitis and then in patients with carotid atherosclerosis using 11 C labeled PK11195 and imaging on a 16-slice PET/CT scanner.…”

The 18 kDa Translocator Protein and Atherosclerosis in Mice Lacking Apolipoprotein E

“…According to these models, the inner surface of the channel formed by the TSPO molecule would present a hydrophilic but uncharged pathway, allowing amphiphilic cholesterol molecules to cross the outer mitochondrial membrane (Papadopoulos et al, 1997(Papadopoulos et al, , 2006Veenman et al, 2007). At cellular levels TSPO is present in virtually all of the cells of the cardiovascular system, where they appear to take part in the responses to various challenges that an organism and its cardiovascular system face (Veenman & Gavish, 2006), including atherosclerosis and accompanying symptoms (Onyimba et al, 2011;Bird et al, 2010;Dimitrova-Shumkovska et al, 2010a,b,c, 2012.…”