Targeting the alternative complement pathway as a neuroprotective therapy in glaucoma and optic nerve injury

Abstract

Purpose : Accumulating evidence from both human and animal models of glaucoma implicates the classical complement pathway as an important mediator of neuroinflammation and glaucomatous neurodegeneration. However, the alternative complement pathway mediates amplification of the complement pathway and has recently been implicated in driving inflammation and propagating injury in experimental models of spinal cord injury and brain injury, where specifically inhibiting the alternative pathway significantly reduced the extent of neurodegeneration. Herein we examine whether specifically inhibiting the alternative pathway is neuroprotective in experimental models of glaucoma and optic nerve injury.

Methods : Intracameral injection of magnetic microbeads (control: saline) was used to elevate the intraocular pressure (IOP) in complement factor B knockout mice (Fb^-/-, deficient in the alternative complement pathway), complement component 3 knockout mice (C3^-/-, deficient in all three complement pathways), and wild type (WT) mice. The optic nerve was crushed (ONC) approximately 2mm behind the globe with jeweler’s forceps for 10 seconds. Visual acuity was measured by optomotor reflex (OMR). RGC density was quantified in retina whole mounts stained with a RGC-specific anti-Brn3a antibody. The nerve fiber layer (NFL) thickness was analyzed by spectral domain coherence tomography.

Results : IOP was increased equally in microbead injected Fb^-/-, C3^-/- and WT mice as compared to saline controls. At day 35 post-microbead injection, visual acuity was significantly reduced in WT mice (0.37 ± 0.03 cyc/deg to 0.29 ± 0.04 cyc/deg, p<0.001) while no significant loss was observed in Fb^-/- mice (0.39 ± 0.03 cyc/deg to 0.36 ± 0.03 cyc/deg, p>0.05) or C3^-/- mice (0.35 ± 0.03 cyc/deg to 0.34 ± 0.04 cyc/deg, p>0.05). A significant thinning of the NFL and significant reduction in RGC density was also detected in microbead injected WT mice, but not Fb^-/- mice. Similar results were observed following ONC, with Fb^-/- mice displaying a significant increase in RGC survival at two weeks post ONC when compared to WT mice.

Conclusions : Our data shows that specifically inhibiting the alternative complement pathway is neuroprotective in mouse models of glaucoma and ONC, revealing the importance of the alternative pathway in the pathogenesis of glaucoma and optic nerve injury and introducing a potential new neuroprotective treatment.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.