Infiltrating monocytes augment alternative complement activation and exacerbate inherited retinal degeneration in a mouse model

In retinal degenerative disease, microglia and macrophages accumulate at sites of pathology and strongly influence disease progression, yet their distinct contributions remain unclear. To define the fate and function of infiltrating monocyte-derived macrophages (MDM) in retinal degeneration, we generated a CCR2-CreER mouse line on the rd10 background to enable precise monocyte-specific tracking and ablation. Infiltrating monocytes rapidly downregulated CCR2 and LY6C upon entering the retina and acquired de novo TMEM119 and P2RY12 expression, together with a ramified, microglia-like morphology. Immunohistochemistry and transcriptomic profiling showed that a subset of these cells was cleared by resident microglia. Microglia–monocyte interactions enhanced Müller cell C3 production, whereas activated microglia increased CFB and decreased CFH expression, thereby promoting complement alternative pathway activation. Selective ablation of infiltrating monocytes reduced microglial activation and phagocytosis, suppressed Müller cell C3 expression and complement deposition, lowered proinflammatory cytokine levels, and ultimately ameliorated photoreceptor degeneration. These findings identify infiltrating monocytes as key drivers of immune dysregulation and proinflammation, highlighting them as potential targets for neuroprotective therapy.