Optic Neuritis: Inflammatory Demyelination of the Visual Pathway in Preclinical Models

EAE is a model of relapsing-remitting and secondary-progressive MS, and optic neuritis within EAE follows a time course that determines the therapeutic window for acute versus chronic interventions. However, most preclinical optic neuritis studies use terminal histological endpoints – RGC counts, optic nerve cross-section myelin quantification – that capture only a single time point and provide no information about the dynamics of functional loss and recovery. Electrophysiological methods such as pattern electroretinography (PERG) and visual evoked potentials (VEP) require anaesthesia or head-fixation and do not easily support the dense longitudinal sampling needed to resolve the relapse-remission cycle. Flash ERG has even lower specificity for the inner retina and optic nerve. The result is that the temporal relationship between inflammatory infiltration, demyelination, axon injury, remyelination, and functional recovery is poorly resolved in most EAE-optic neuritis studies, limiting the ability to select optimal treatment windows.

OptoDrum resolves this problem by measuring spatial visual acuity (cycles per degree, photopic) and contrast sensitivity threshold in awake, freely moving mice in under 5 minutes per animal, with no training, no restraint, and no anaesthesia. Because the test is non-invasive and non-terminal, it can be applied at any density of time points across the EAE course – weekly, twice-weekly, or daily during acute phases – to resolve the full functional time course.

For the complementary broader EAE measurement strategy, see Neuroinflammation and Autoimmune CNS Disease.

MS-associated optic neuritis (typically MOG_35-55-seronegative) and MOGAD optic neuritis (MOG-IgG seropositive) have overlapping clinical presentations but distinct immunopathological mechanisms and differential responses to treatment. MS optic neuritis is predominantly T cell-mediated; MOGAD optic neuritis involves pathogenic MOG-IgG that activates complement at the optic nerve axolemma, producing a more severe inflammatory response and a greater risk of incomplete acuity recovery. AQP4-IgG-positive NMOSD optic neuritis represents a third mechanistic class, targeting astrocytic AQP4 and producing astrocytopathy-driven secondary demyelination. These three mechanistic classes have distinct translational implications: therapies that work in T cell-driven EAE may fail in MOG-IgG or AQP4-IgG models, and vice versa. Selecting the right model for the target mechanism is critical for preclinical drug development.

The development of B cell-dependent EAE models that incorporate MOG-specific antibody responses (Joly et al., 2022, J Neuroinflammation) and MOGAD-specific FcRn blockade models (Remlinger et al., 2022, Neurol Neuroimmunol Neuroinflamm.) now enables direct comparison of T cell vs. humoral immune mechanisms with a shared functional endpoint.

Also see: Ocular Inflammation and Immune-Mediated Eye Disease and MOG-Antibody-Associated Disorder.

The dominant therapeutic paradigm for MS and MOGAD optic neuritis focuses on suppressing adaptive immune cell activation (corticosteroids for acute attack; B cell depletion, S1P modulators, and anti-CD20 agents for prevention). However, within established demyelinating lesions, metabolic stress – especially hypoxia-driven activation of the HIF-1 transcription factor in infiltrating immune cells – amplifies the neuroinflammatory response independently of the adaptive immune trigger. Similarly, cholesterol is the principal lipid component of CNS myelin; when cholesterol homeostasis is disrupted by inflammation, remyelination is impaired and optic nerve axons remain exposed to chronic inflammatory stress. Dietary saturated fat intake influences systemic and CNS immune status via multiple axes, including gut microbiome composition, macrophage polarisation, and lipid availability for remyelination. These three metabolic axes are underexplored as therapeutic targets but represent potentially modifiable contributors to optic neuritis severity.

All three of the studies below used OptoDrum as the primary functional efficacy endpoint in EAE-optic neuritis models, demonstrating that metabolic-inflammatory interventions produce measurable changes in the OMR-assessed visual functional outcome – not merely histological changes. This convergence of evidence positions visual function measurement as the shared endpoint linking the metabolic-inflammatory therapeutic hypothesis to a translational outcome.

For the relationship between neuroinflammation and RGC death in these models, see Neuroinflammation and Retinal Ganglion Cell Pathology.

Microglia are the principal innate immune sentinels of the CNS, and their activation state – surveying, reactive/pro-inflammatory, or engaged in myelin clearance – determines whether their actions are protective or destructive in the context of optic nerve demyelination. CX3CR1 (the fractalkine receptor) is the homeostatic checkpoint regulating the transition of microglia from surveillance to activation; its downregulation is a hallmark of microglial priming in aging and neuroinflammatory disease. In optic neuritis, microglial activation amplifies inflammatory demyelination, but emerging evidence indicates that microglial myelin debris clearance is also a prerequisite for successful remyelination.

The dual nature of microglial involvement creates a challenge for therapeutic development: non-selective microglial suppression may impair remyelination even as it reduces acute inflammation. Resolving the productive (clearance-promoting) from the pathological (demyelination-amplifying) microglial functions requires models in which each function can be measured separately, with visual function as a quantitative readout of net outcome on the visual pathway.

Also see: Neuroinflammation and Optic Nerve Damage and Rare and Inherited CNS and Eye Disorders.

Optic neuritis produces two separable phases of visual pathway damage: (1) acute, reversible conduction block due to demyelination and oedema, which resolves as inflammation clears; and (2) permanent RGC death due to axon damage, glutamate excitotoxicity, mitochondrial dysfunction, and secondary degeneration. The acute functional loss may recover fully even with significant RGC death if the surviving RGC population is sufficient for the tested task; the chronic functional deficit reflects the degree of irreversible RGC loss and incomplete remyelination. Neuroprotective strategies targeting the acute phase (anti-apoptotic agents, neurotrophins, anti-inflammatory cytokines such as EPO) can preserve the RGC population and thereby improve the chronic functional outcome, but demonstrating this requires functional endpoints sensitive enough to distinguish partial from complete recovery.

OptoDrum's contrast sensitivity measurement is particularly well suited to this challenge: contrast sensitivity reflects the functional range of the surviving RGC population across spatial frequencies and is more sensitive to sub-maximal RGC loss than binary acuity thresholds. AcuiSee provides a complementary cortical discrimination endpoint that captures the perceptual resolution of complex visual stimuli, which may be reduced even when OMR-based acuity has recovered following remyelination.

For the structural correlates of RGC neuroprotection in these models, see Retinal Ganglion Cell Pathology. For neuroprotection strategies in acquired optic nerve injury, see Trauma and Acute Injury. For therapeutic approaches targeting visual pathway regeneration, see Glaucoma and Optic Nerve Neurodegeneration.