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

Abstract

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. Importance Coronaviruses can invade the nervous system and cause lasting neurological damage, but the host immune mechanisms that limit viral spread within neural circuits are poorly understood. This study identifies CD40, a receptor on immune cells in the nervous system, as an essential defender against the retrograde spread of murine coronavirus RSA59 from the brain to the retina along nerve cell fibers. When CD40 is absent, the virus travels unchecked into the retina, initial immune responses by microglia are blunted, and a cycle of chronic inflammation ensues, ultimately destroying the retinal ganglion cells responsible for vision. These findings reveal a critical link between early immune activation and long-term neuronal protection against virus-induced damage, providing insight into how coronavirus infections lead to persistent neurological problems and highlighting CD40 as a potential therapeutic target.