This study examined the accumulation of superparamagnetic iron oxide (SPIO) nanoparticles in vulnerable atherosclerotic plaques. In vitro experiments demonstrated that human macrophages actively take up fluorescent-labeled SPIO particles over 24 hours in a time- and concentration-dependent manner. In vivo studies in mice and rabbit models of atherosclerosis found SPIO particles in macrophage-rich areas of plaques, with a strong correlation between iron staining and foam cell density. Cytokine treatment increased SPIO uptake in plaques. The results suggest MRI using SPIO has potential for noninvasively assessing monocyte recruitment in vulnerable plaques.

1. Histologic evidence ofHistologic evidence of
SPIO accumulation inSPIO accumulation in
vulnerable plaquesvulnerable plaques
Silvio Litovsky, MDSilvio Litovsky, MD
June 7, 2002June 7, 2002

2. In-vitro Study of Macrophage
SPIO Uptake
 In a series of in-vitro studies we have tested
the rate of SPIO uptake by human activated
monocytes in different conditions regarding
incubation time and concentration of SPIO.
SPIO particles were labeled by a fluorescent
dye (DCFA).

3. FL-labeled SPIO Incubated Macrophages after 24hr

4. Double DAPI Staining with Fluorescence-labeled SPIO Macrophages
after 24hr Incubation

5. Macrophages labeled with
fluorescent microspheres adhere
to atherosclerotic plaques.

6. In-vivo distribution of SPIO in ApoE
deficient and wild type mice:
•For the initial study, we use the mouse model of
atherosclerosis.
•ApoE deficient mouse has similar atherosclerotic
lesions to human and the lesions are more common in
the aortic arch and thoracic aorta.
• We used ApoE deficient mice and normal variant
(C57BL mice) as control.
•Animals were sacrificed on day 3 and 5 after injection.

7. •Tissues from different organs including liver,
kidneys, lung, heart, spleen and aorta
(including valve region, ascending, descending
and abdominal) were obtained and stained.
• We used Hematoxyline and Eosin (H&E), Iron,
CD68 and Movat staining.
• After Iron staining, aortic walls of the ApoE
deficient mice and normal variant were
compared based on the number of Iron
particles.

8. ApoE k/o Mouse 5 Days AfterApoE k/o Mouse 5 Days After
Injection- LiverInjection- Liver

9. Histopathologic Study of the Mice Injected
with SPIO (Aortic Root)
ApoE K/O mouse, Movat staining, aortic root
Coronary
Cross section
Atherosclerotic
plaque

10. Histopathologic study of ApoE K/O Mouse injected
With SPIO (Aortic Root)
CD68 staining
(coronary plaque)
Iron Staining (aortic plaque) Iron Staining (coronary section)
Iron particles Iron particles

11. Histopathologic Study of Wild Type Mouse
Injected with SPIO (Thoracic Aorta)
H&E staining
CD68 stainingIron staining

12. Comparison of the Number of the Iron Particles (per
HPF) in ApoE K/O Mice Plaque vs. Normal Wall
0
5
10
15
Atherosclerotic
Aorta
Average
number of iron
particles per
sample
P <0.001

13. Histopathologic studies of Thoracic aorta in Watanabe
Hereditary Hypercholesterolemic rabbit after SPIO injection
H&E staining
Macrophage staining Iron staining
Iron particles

14. Hypercholesterolemic Rabbit, Aorta, 4 days after
SPIO injection
Perls’ Staining H&E Staining
X40 X40

15. Electron Microscopy Evidence of Intracellular
SPIO in the Rabbit Aorta
Endothelial cell, x7500 Foamy cell, x4000

16. 0
10
20
30
40
50
60
0 10 20 30 40 50 60 70
macrophage (foam cell) density
SPIOpositivecell-Iron
staining
Series1
R=0.956
Correlation Between Iron Positive Cells in Iron
Staining and Foam Cell Density in H&E Staining in Rabbit
Atherosclerotic Aorta.

17. Plaque Iron in ApoE k/o mousePlaque Iron in ApoE k/o mouse
after SPIOafter SPIO
H&E IRON STAIN

18. Plaque Iron in Cytokine-TreatedPlaque Iron in Cytokine-Treated
Mice after SPIOMice after SPIO
H&E stain Iron stain

19. Plaque Iron in Cytokine–Treated MicePlaque Iron in Cytokine–Treated Mice
after SPIOafter SPIO
Overlying plaques Plaque-free area

20. Conclusion:
1) SPIO nanoparticles actively accumulate in
superficial
areas of atherosclerotic lesions in rabbits and mice.
2) There is a strong correlation between areas of foamy
cells infiltration and SPIO uptake in mice and rabbit
plaques.
3) Cytokines markedly increase the SPIO uptake by
atherosclerotic plaques, thus providing a method for
quantifying recruitment of monocytes.
4) MRI has the potential for non-invasively assessing
monocyte homing into atherosclerotic plaque.

21. Center for Vulnerable Plaque ResearchCenter for Vulnerable Plaque Research
Houston, TexasHouston, Texas