MR immunoimaging evaluating USPIOs for myeloid cell tracing in acute and chronic cardiovascular disease mouse models

Background

USPIOs have been used for quantification of myocardial inflammation with MRI. They have not been tested to image the immune reaction of hematopoietic organs and cell migration in cardiovascular diseases yet.

Methods

We injected the clinically available MRI contrast agent ferumoxytol (USPIOs) in C57Bl/6 mice to track myeloid cells after cardiovascular (CV) events. One day after injection an abdominal MRI was conducted, followed by an acute CV event - LAD occlusion (MI) or pulmonary embolism (PE). Naïve mice were used as controls. Follow-up MRIs were performed one and two days after the acute CV event. MI mice were subjected to cardiac MRI on day two after the event. As a chronic CV disease model atherosclerotic ApoE-/- mice on Western diet (WD) with and without statins were followed up with MRI for 6 weeks after USPO injection. Flow cytometry, histology and atomic absorption spectroscopy (AAS) served as ex vivo work-up.

Results

After USPIO-injection, we observed decreased T2 times of the spleen corresponding to accumulations seen in histology and AAS. In immunohistochemistry, we could identify cells containing iron deposits as CD11b, CCR2 and Ly6G positive, confirming that myeloid cells take up the iron nanoparticles. Comparing different dosages (25 mg/kg and 45 mg/kg), we concluded to operate with 45 mkg/kg BW.

Mice with MI and PE showed an increase of splenic T2 times compared to controls one day after the CV event and two days after USPIO injection, respectively (p<0.05). Flow cytometry confirmed a significant increase of CD11b cells in the blood after MI (p<0.05). Cardiac MRI revealed a signal void and significantly decreased target-to-background ratio of the infarction two days after MI and three days after USPIO injection (p<0.001) accompanied by significantly increased iron in the infarction measured by AAS (p<0.05). Pulmonary embolism mice also showed increased iron in the lung quantified by AAS (p<0.05).

ApoE-/- mice fed with WD diet for 7 weeks showed significant decreased T2 times of the spleen by MRI after USPIO injection. The subgroup receiving atorvastatin therapy after USPIO injection had consistently low T2 times for additional six weeks, whereas the subgroup without statin therapy showed an increase in T2 times after additional six weeks (p<0.01). Histology confirmed smaller CD68+ areas in aortic root plaques of the statin group (p<0.01) and flow cytometry showed increased inflammatory cells in the blood of atherosclerotic mice without atorvastatin treatment (p<0.05). We additionally found a moderate correlation of DT2 of the spleen and CD68+ area of plaques in histology (R2=0.37, p<0.05).

Conclusion

Magnetic resonance myeloid cell migration imaging with USPIOs is feasible in MI, PE and atherosclerosic mice. Immunomodulation with atorvastatin can be monitored by USPIOs in atherosclerosis. USPIO MRI has the potential to track myeloid cell migration in patients with acute and chronic CV diseases.