Metrics, distribution and dynamics of inflammatory cells in in vivo retina during multiple sclerosis

Neuroinflammation is a critical factor in multiple sclerosis (MS), but in vivo monitoring of its cellular dynamics remains a significant challenge. The retina, derived from CNS tissue, provides a unique, non-invasive window into these processes. High-resolution retinal imaging techniques, such as adaptive optics scanning laser ophthalmoscopy (AOSLO), enable the observation of cellular dynamics in the retina, advancing our understanding of neuroinflammation in MS. In this prospective cohort study, we used custom-modified AOSLO to examine the retinal ganglion cell layer in 51 MS patients with different phenotypes: relapsing-remitting MS (RMS) with recent optic neuritis (ON) (N=31), RMS without recent ON (N=12), and progressive MS (N=8), alongside nine healthy controls. We identified immune infiltrates for the first time, likely lymphocytes and microglial cells, near the retinal vascular plexus, with the highest densities observed in ON-RMS patients (93.6%). These cells were sometimes detected weeks before clinical ON onset, and their density declined post-ON, though at varying rates. Infiltrates were more frequently found in MS patients than in controls, even outside acute ON episodes. The cells showed minimal movement and often interacted with vessels, suggesting migratory behavior. Our results suggest that AOSLO imaging can detect subtle retinal inflammatory changes not captured by conventional clinical systems, offering a promising tool for monitoring neuroinflammation in MS and other neurodegenerative diseases. These findings support the potential of high-resolution retinal imaging as a non-invasive biomarker for tracking neuroinflammation and therapeutic responses in patients with MS.