Magnetic resonance imaging of monocyte infiltration in an animal model of multiple sclerosis

The aim of this research was to study neuroinflammatory processes and more specifically the infiltration of monocytes in the central nervous system (CNS) in an animal model of multiple sclerosis (MS) using magnetic resonance imaging (MRI). Monocytes play a key role in MS pathology, and the study of their infiltration kinetics in vivo would benefit from a non-invasive imaging strategy. Initially, to visualize monocytes with MRI, we investigated different labeling strategies in human monocytes using super paramagnetic particles of iron oxide (SPIO, diameter > 50nm) and ultra small SPIO (USPIO, diameter 20 – 50nm). Histological analysis and MR validation showed that monocytes are labeled more efficiently using the larger SPIO compared to USPIO. Importantly, our optimal labeling condition allowed detection of monocytes by MRI without compromising important physiological cell functions like cell viability, migratory capacity and cytokine production. Subsequently, we used SPIO to label freshly isolated rat monocytes. Cell tracking experiments were performed in a rat model of neuroinflammation and we directly compared two main approaches of in vivo monocyte imaging: (1) Intravenous injection of monocytes that were labeled ex vivo with SPIO versus (2) the administration of free USPIO. In theory, free USPIO are taken up in the blood stream by circulating monocytes. Using method 1, we demonstrated longitudinal MR detection of SPIO-labeled monocytes that migrate towards an inflammatory site in the brain. Interestingly, method 2 resulted in an early decrease of signal intensity in the lesion (2 hours) and pointed to the detection of blood-brain barrier (BBB) leakage instead of monocyte migration. To monitor monocyte migration in the experimental model of MS in rats (experimental autoimmune encephalomyelitis; EAE), we had to introduce a novel labeling strategy: Magneto-Electroporation (MEP). Our results showed that an intravenous injection of monocytes labeled using MEP resulted in multiple foci of signal loss, predominantly in the white matter of the cerebellum in EAE rats. Therefore, MEP is essential to visualize the dynamic process of monocyte infiltration in pathologies with multiple small lesions in the CNS, as in case of MS. Complementary to our previous studies, we also focused on the administration of free USPIO in EAE rats. We found that USPIO enter the brain parenchyma of EAE rats within 1 hour whereas at 72 hours after injection, MR abnormalities were no longer present in the CNS. Subsequent imaging of the cervical lymph nodes revealed USPIO accumulation. Our data strongly suggest that the process of leakage over the BBB has to be considered next to cellular infiltration when interpreting post-USPIO MR images. Moreover, in this study we identified the potential use of USPIO-enhanced MRI to study non-invasively drainage in the inflammatory brain. In a clinical trial, we administered a novel USPIO to MS patients and compared enhancement patterns in the brain with enhancement obtained after conventional Gd-DTPA injections. We showed that USPIO enhancements were more frequently observed and several types of enhancement patterns could be distinguished and correlated to certain stages of lesion development. In conclusion, cellular MRI using iron oxide particles is a valuable tool to study the multiple aspects of inflammation in MS and may lead to effective treatment protocols for drugs that limit monocyte entry during neuroinflammation.