Adapted focal experimental autoimmune encephalomyelitis to allow MRI exploration of multiple sclerosis features
Research highlights
► Focal EAE can be adapted in such a way that it can be monitored with MRI. ► The MR protocol can be the same one as the standard protocol used for MS patients. ► Adaptation of the model requires balancing the lesion with high pre-sensitization. ► The MR signal of the adapted model reflects the spectrum of MS-like lesion.
Introduction
Magnetic resonance imaging (MRI) is playing an increasing role in the investigation and clinical management of multiple sclerosis (MS) (Bakshi et al., 2008); MRI has helped in diagnosis, provided in vivo parameters for better understanding of the pathophysiology and has allowed monitoring of treatment efficacy in clinical trials (Bar-Zohar et al., 2008).
Animal models of MS are needed to further explore mechanisms of disease initiation and progression and to test therapeutic and restorative approaches (Gold et al., 2006, Steinman and Zamvil, 2006). Experimental autoimmune encephalomyelitis (EAE) is the most widely used animal model of MS because it shares several clinical and histological similarities with MS (Denic et al., 2011). However, the multifocal and disseminated nature of the lesions in EAE models makes it challenging to investigate a lesion at an early stage before it becomes clinically significant. The nature of these EAE lesions also makes it difficult to characterize the evolution and resolution of a given lesion. One solution is to use a focal lesion model, targeting a single EAE lesion that reflects a prototypic MS lesion (Kerschensteiner et al., 2004b). This model consists of a subthreshold sensitization of the Lewis rat with myelin oligodendrocyte protein (MOG) followed 20 days later by the induction of focal inflammation by microinjection of pro-inflammatory cytokines (tumor necrosis factor α [TNFα] and interferon γ [INFγ]). Focal lesions share histological similarities with those seen in MS (Kerschensteiner et al., 2004b, Merkler et al., 2006) and this methodology could provide several advantages, including knowledge of the exact onset and location of the lesion.
As with humans, MR endpoints could be of great interest in rodent studies, allowing in vivo longitudinal monitoring that could be better than the five step clinical scale, which is relatively crude and does not correctly quantify and correlate with structural damage (Wujek et al., 2002). The following two conditions are required when using MRI for rodent inflammatory lesion exploration: (i) the MR signal should reflect the features of a MS lesion, with development of the entire lesion spectrum (including edema, blood brain barrier [BBB] damage and demyelination); (ii) the lesion should be large enough to fall within MRI resolution. MR exploration of EAE lesions is challenging because of their small size and selective location within small anatomic structures such as the brainstem, optic nerve or spinal cord (Sakuma et al., 2004). The focal model, induced into a well-defined anatomic structure within the brain, can facilitate MRI detection. However, the initial protocol has to be more severe to ensure that it is detectable with the lower resolution of MRI. Cytokine doses can be increased (Rausch et al., 2009, Serres et al., 2009a, Serres et al., 2009b) but come with the risk that lesions may appear more cytokine-like than MS-like. This distinction is a key point because the combination of MRI and focal lesion induction could be used as a platform for therapeutic tests, implying that this is a relevant model.
Consequently, considering the focal EAE model as a “two-hit” model (first sensitization and then cytokine injection), we compared two strategies to increase lesion severity for better MR application. In one strategy, we balanced toward a higher sensitization step; in another strategy, we balanced toward a higher cytokine step.
Section snippets
Materials and methods
To compare the high sensitization and high cytokine strategies, we followed two experimental groups over time: MOG50 (balanced toward sensitization) and MOG25 (balanced toward cytokines). We also followed two control groups: complete Freund's adjuvant (CFA) and incomplete Freund's adjuvant (IFA; Fig. 1).
Biological evaluation
MOG50 rats exhibited signs of generalized disease after a mean delay of 16 days post sensitization (mean score = 2.5) in relation with histologically-detected lesions in the brainstem and spinal cord. Cytokine injection did not exacerbate the symptoms in any animals. In contrast, MOG25 rats and control rats (CFA and IFA) did not show any signs of disseminated EAE either before or after cytokine injection.
No anti-MOG antibodies were detected at baseline (the day of sensitization). On the day of
Discussion
In this paper we compared different strategies to increase a focal, “two-hit” (1: pre-sensitization and 2: cytokine injection) inflammatory model to facilitate MR exploration while still ensuring an EAE effect. Because MR resolution and sensitivity is different from that of histological data, a strong pre-sensitization step was necessary to significantly balance the lesion towards an EAE-like focal lesion instead of reflecting the local effect of cytokines only. With this model the lesion could
Acknowledgments
We thank Dr. Nora Abrous (INSERM U862, Bordeaux, France) for surgery supervision.
Grant support:
TT is the recipient of a research fellowship from the French Society of Radiology (Société Française de Radiologie) and CNRS-CHU assistant. Experimental work was supported by Conseil Régional d'Aquitaine (KGP and VD).
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