Quantification of Leukocyte Trafficking in a Mouse Model of Multiple Sclerosis through In Vivo Imaging.

Purpose: Optic neuritis occurring in multiple sclerosis (MS) is a disease characterized by chronic inflammation and demyelination in the optic nerve. Although it has been well appreciated that leukocyte infiltration into the optic nerve is an early event during the course of the disease, there has been no study on visualizing and quantifying leukocyte trafficking in the retina during the progression of MS.

Methods: In this study, we generated green fluorescent protein (GFP)⁺ bone marrow chimeric mice, in which GFP-labeled leukocytes facilitate the visualization of their trafficking in the retina. This reporter was then integrated with a well-established rodent model for MS-experimental autoimmune encephalomyelitis (EAE), allowing high resolution in vivo scanning laser ophthalmoscopy (SLO) to track leukocyte movement in the retina in real time. Quantification of leukocyte trafficking was accomplished through Imaris software.

Results: Through SLO, we were able to localize the GFP signal, allowing us to clearly identify leukocytes within the vascular space. We observed more intense leukocyte migration in the retina of EAE mice, exhibiting three distinct movement behaviors: flowing, rolling/crawling and adherent. There was a marked increase in leukocyte rolling and adhesion in retinal vasculature, particularly in the veins and capillaries after induction of EAE. The velocity of rolling leukocytes ranged from 12.0 to 1065.0 μm/sec in the veins as compared to 14.1 to 942.0 in the capillaries. Furthermore, focal areas of recurrent leukocyte adhesion to endothelial surfaces were observed in EAE retinas.

Conclusion: We generated a novel model that makes it possible to non-invasively track leukocyte trafficking in the retina of EAE mice. Our study demonstrates that leukocyte migration in an MS model is distinctly different from the control, suggesting that leukocytes may play a key role in the development of retinal vascular inflammation and optic neuritis during MS, warranting further investigation of the pathological roles of leukocytes in the disease onset and progression.