In vivo imaging of inflammatory cells trafficking to atherosclerotic lesions
January 7, 2008 By Benjamin A. Olenchock, M.D. Ph.D. 
Boston Researchers have developed a way to image the trafficking of monocytes into atherosclerotic plaques using single photon emission computed tomography (SPECT/CT). The technique involves labeling purified monocytes with 111-In-oxine, an FDA-approved radiolabel, and transferring the labeled cells into a live recipient animal. Cells can then be visualized over time with sub-millimeter accuracy. This advance will help research into the pathogenesis and treatment of atherosclerosis, and in the future might be used in humans to characterize inflammatory cell infiltration into atherosclerotic plaques.
Monocytes were isolated to >90% purity from peripheral blood using negative selection. 111-In-oxine labeling did not affect monocyte viability or ability to traffic as assessed in a thioglycolate-induced peritonitis model. Three-million labeled monocytes were transferred into intact, anesthetized apoE-deficient mice, which are a commonly used genetic strain predisposed to atherosclerosis. Mice were then imaged at various time points by SPECT/CT. During the first 24-hours, the radiotracer was seen to localize in the heart and lungs, and after 36-hours in the liver and spleen. At the 5-day time point, a focal concentration of radiotracer was visible in the ascending aorta. Imaging results were verified by histology after transfer of GFP-labeled monocytes. The specificity to monocytes was verified by transfer of labeled peripheral blood mononuclear cell fractions that had been depleted of monocytes, which did not localize in a similar fashion. The intensity of the SPECT/CT image was directly related to the absolute number of monocytes recruited (R^2 of 0.87).
In the first application of this model, the investigators asked whether statins might block monocyte recruitment to atherosclerotic plaques. Pre-treatment of recipient animals for 3 days with either atorvastatin, simvastatin, or mevastatin significantly impaired the relative SPECT/CT image intensity in the aorta at day 5 (~5-fold reduction, p<0.001). The statins appear to affect the recipient blood vessels and not the monocytes. Ex vivo quantification of extravasated monocytes demonstrated a similar reduction when recipients, but not donors, were pre-treated with statins.
The authors explain that the advantage of the SPECT/CT technique over other techniques such as MRI, optical coherence tomography or CT is the ability to evalulate monocyte residence in atherosclerotic lesions over time in the same animal. The resolution is slightly lower than that of MRI, however. This technique will be incredibly useful for asking how various therapeutics affect vascular inflammation over time. The large number of monocytes needed for transfer experiments might add a note of caution regarding the physiologic relevance of findings, and more pre-clinical studies will have to be done to determine the whether there is utility to this technique for disease characterization, treatment response, or prognosis in humans. The finding that statins modulate vascular inflammation over such a short time period without affecting lipoprotein levels adds more support to the anti-inflammatory effects of statins.
- Kircher MF, Grimm J, Swirski FK, Libby P, Gerszten RE, Allport JR, Weissleder R. Noninvasive In Vivo Imaging of Monocyte Trafficking to Atherosclerotic Lesions. Circulation. 2008 Jan 2; [Epub ahead of print] PMID: 18172031