MRI of Monocyte Infiltration in an Animal Model of Neuroinflammation Using SPIO-Labeled Monocytes or Free USPIO

Abstract

Magnetic resonance imaging (MRI) has been applied to visualize monocyte infiltration with the use of intravenously injected ultrasmall superparamagnetic iron oxide (USPIO). However, USPIO uptake in vivo remains elusive, and the heterogeneous enhancement patterns observed by MRI point to multiple pathophysiological events. This study focused on specific imaging of monocyte infiltration into the brain by transfusion of superparamagnetic iron oxide (SPIO)-labeled monocytes in a rat model of neuroinflammation, experimentally induced photothrombosis (PT). At day 5 after lesion induction, animals were transfused with SPIO-labeled monocytes (5¾10 6 cells) or free USPIO (17 mg Fe/kg). MRI was performed 24, 72 and, 120 h later. To investigate temporal changes directly after intravenous USPIO administration, MRI was performed repeatedly up to 8 h. Relaxation measurements showed that rat monocytes were efficiently labeled in vitro using SPIO (R 2 = 12 ± 0.9 s À1 ). After transfusion of SPIO-labeled monocytes, a significant increase in contrast enhanced area (340%±106%) in the PT lesion was observed not before 72 h. Contrast enhancement after USPIO injection increased up to 407% ± 39% at a much earlier point of time (24 h) and diminished thereafter. Repetitive MRI directly after USPIO injection showed significant contrast enhancement in the lesion within 2 h. Our study shows that MRI enables in vivo tracking of SPIOlabeled monocytes longitudinally. Moreover, our data suggest that contrast enhancement after injection of free USPIO does not primarily represent signals from peripherally labeled monocytes that migrated toward the inflammatory lesion. The use of SPIO-labeled monocytes provides a better tool to specifically assess the time window of monocyte infiltration.

Superparamagnetic Iron Oxide Nanoparticles in Biomedicine: Applications and Developments in Diagnostics and Therapy

“…In small animal models of MS (experimental autoimmune encephalomyelitis (EAE)), SPIO can be used to image macrophage activity [177] and infiltration [178], lymphocyte infiltration (CD3 + cells [179]) and the labeling of T-cells [180]. Also in the case of heart attack, numerous studies show that SPIO can image the accompanying neuroinflammation or the macrophage activity [181] and macrophage migration [182] similarly to the changes in MS. Moreover, it was able to be shown that early stages of post-radiogenic brain damage can be imaged via targeted SPIO-AB against ICAM-1 [183].…”

MRI Assessment of Post-Ischemic Neuroinflammation in Stroke: Experimental and Clinical Studies

“…To date, the majority of studies using ex vivo labeling in stroke models addressed cellular therapy, using subventricular zone progenitor cells (Athiraman et al, 2009;Cicchetti et al, 2007;Jiang et al, 2005;Zhang et al, 2003), embryonic neural stem cells (Hoehn et al, 2002;Modo et al, 2004) mesenchymal stem cells (Lee et al, 2009;Walczak et al, 2008), spleenderived mononuclear cells (Stroh et al, 2006) or adipose-derived stem cells (Rice et al, 2007). To our knowledge, only one study was published using ex vivo labeling of macrophages in a rat stroke model (Engberink et al, 2008): 4 rats were intravenously injected with 5 million SPIO-labeled monocytes after induction of cerebral ischemia by photothrombosis. MRI was performed at 4.7T pre-and 24h, 72h and 120h post-injection.…”

“…At later time-points, these hyposignals were detected in the lesion core (Kleinschnitz et al, 2003). The hypothesis of passive diffusion of iron oxide nanoparticles through a disrupted blood brain barrier (BBB) was rejected, because post-gadolinium and post-(U)SPIO MR signal changes did not superimpose, suggesting an active mechanism of brain entry, via (U)SPIO-laden infiltrating macrophages (Engberink et al, 2008;Kleinschnitz et al, 2003). Table 1.…”

“…Whereas the accumulation of (U)SPIOs in cells is well explained by phagocytosis activity of macrophages, their route of transport to brain macrophages is not yet well described. Three hypotheses have been put forward to explain the MR signal changes observed after (U)SPIO injection: (i) intravascular trapping of iron particles (Bendszus et al, 2007;Kleinschnitz et al, 2005); (ii) (U)SPIO uptake by phagocytes, on the assumption that (U)SPIOs are primarily taken up by circulating phagocytes (Kleinschnitz et al, 2003); and (iii) interstitial iron particle diffusion into damaged tissue after nonspecific leakage through a disrupted bloodbrain barrier (Engberink et al, 2008). To assess the early brain distribution of iron particles, MRI signal changes after intravenous USPIO injection were then compared with the histological iron and macrophage distribution from 6h to 24h, using the same experimental set-up as before (Desestret et al, 2009).…”

“…T2/T2*-weighted imaging was used in all studies to detect MR signal changes following (U)SPIO injection, typically showing decreasing signal intensity. The most relevant protocol in terms of T2/T2* effects involved injection at day 5 post-injury with a dose of 300 µmol Fe/kg, and follow-up at day 6, whether with USPIO (Ferumoxtran-10) or with SPIO (Ferucarbotran) (Engberink et al, 2008;Kleinschnitz et al, 2003;Saleh et al, 2004b;Schroeter et al, 2004). This 24-hour interval between injection and imaging is necessary in order for the iron oxide particles to wash out from the vascular compartment (the half-life of Ferumoxtran 10 in plasma is 5 hours in rat).…”