Complement C3- and CR3-dependent microglial clearance protects photoreceptors in retinitis pigmentosa

Abstract

Complement activation has been implicated as an inflammatory driver of neurodegeneration in retinal and brain pathologies. However, its involvement and influence of photoreceptor degeneration in retinitis pigmentosa (RP), an inherited, largely incurable blinding disease, is unclear. We discover that markedly upregulated retinal expression of multiple complement components coincided spatiotemporally with photoreceptor degeneration in both the rd10 mouse model and in human specimens of RP, with increased complement C3 expression and activation localizing to infiltrating microglia near photoreceptors. Genetic ablation of C3 in the rd10 background resulted in accelerated structural and functional photoreceptor degeneration and altered retinal expression of inflammatory genes. These effects were phenocopied by the genetic deletion of CR3, a microglia-expressed receptor for the C3 activation product C3b, implicating an adaptive microglial-mediation mechanism involving C3-CR3 interaction. Deficiency of either C3 or CR3 resulted in deficient microglial phagocytosis of apoptotic photoreceptors in vivo, as well as increased microglial neurotoxicity to photoreceptors in vitro. These findings demonstrate a novel adaptive role for complement activation in RP that facilitates microglial clearance of apoptotic photoreceptors, without which increased proinflammatory microglial neurotoxicity ensues. These positive contributions of complement via microglial-mediated mechanisms are important in the design of immunomodulatory therapeutic approaches to neurodegeneration.\n\nOne Sentence SummaryComplement activation mediates adaptive neuroprotection for photoreceptors by facilitating C3-CR3 dependent microglial clearance of apoptotic cells.