CD40 Signaling Restricts Retrograde Viral Spread and Provides Neuroprotection to Retinal Ganglion Cells in a Murine β-Coronavirus Model of Optic Neuritis

CD40, a co-stimulatory receptor of the tumor necrosis factor receptor superfamily expressed on microglia and macrophages, is an upstream regulator of innate antiviral defense in coronavirus-induced neuroinflammation, but its specific role in the visual system remains undefined. Here, we demonstrate that CD40 signaling is essential for restricting retrograde axonal transport of the murine β-coronavirus RSA59 from the brain to the retina and for preventing chronic neurodegeneration in a model of viral optic neuritis. Wild-type (WT) and CD40−/− mice were intracranially inoculated with RSA59, and viral burden, neuroinflammation, and neurodegeneration were assessed at acute (day 5), bridging (day 7), and chronic (day 30) stages. CD40−/− mice exhibited significantly increased clinical severity and ∼30% mortality by day 12 post-infection (p.i.), compared to 100% survival in WT mice. CD40 deficiency resulted in elevated viral loads in the optic nerve and enhanced retrograde viral dissemination across all retinal layers, whereas in WT mice, the virus was largely confined to the ganglion cell layer. CD40−/− mice exhibited impaired early microglial activation and compensatory astrogliosis during the acute and bridging phases. By day 30 p.i., although viral nucleocapsid protein was undetectable by immunohistochemistry in both genotypes, CD40−/− optic nerves retained significantly higher persistent viral RNA and exhibited extensive demyelination, oligodendrocyte loss, axonal depletion, and upregulation of phagocytic markers. Critically, CD40−/− retinas showed persistent astrogliosis, accumulation of phagocytic microglia/macrophages, and a significant loss of Brn3a+ retinal ganglion cells. These findings establish CD40 as a critical molecular node governing coronavirus optic neuritis, linking early innate immune regulation to long-term neuronal survival.