Tissue-resident macrophages maintain choroidal homeostasis by complement dependent and independent mechanisms

Age-related macular degeneration (AMD) is characterized by disruption of the choriocapillaris (CC) and retinal pigment epithelium (RPE) dysfunction, leading to drusen accumulation. The CC and RPE form a tightly interdependent unit that maintains homeostasis where the CC supplies oxygen and nutrients to the RPE, while the RPE produces vascular endothelial growth factor (VEGF) to maintain the CC. Genetic studies link alternative complement pathway variants to AMD, and complement deposition on the CC increases during both aging and AMD. Macrophages express complement protein, receptors, and inhibitors, suggesting that they may be a missing link in understanding the role of complement in AMD. In support, previous groups have shown that macrophage depletion disrupts RPE-CC homeostasis, leading to AMD-like pathology, but the mechanism remains unclear.

To investigate the role of macrophages in CC-RPE homeostasis, we generated Cx3cr1^CreERCsf1r^i-DTR and Ms4a3^CreRosa26^DTR mice. In Cx3cr1^CreERCsf1r^i-DTR mice, tamoxifen administration induced diphtheria toxin receptor (DTR) expression, allowing ablation of all macrophages (many tamoxifen injections) or long-lived, tissue-resident macrophages (tamoxifen followed by a 3-4 week wash out period). Ms4a3^CreRosa26^DTR mice were used to deplete monocyte-derived macrophages. Ablation of all macrophages caused decreased CC density, increased CC apoptosis, RPE disorganization, and membrane attack complex (MAC) accumulation. Tissue-resident macrophage ablation phenocopied this result while monocyte-derived macrophage ablation had no phenotype. Additionally, long-term depletion of tissue-resident macrophages led to formation of drusen-like sub-RPE deposits and retinal thinning, mimicking AMD pathology. Finally, pharmacologic depletion of all macrophages similarly reduced CC density to genetic ablation, but C3^-/- mice showed an attenuated phenotype.

These data demonstrate that long-lived, tissue-resident macrophages are essential for maintaining CC-RPE homeostasis while monocyte-derived macrophages are dispensable in this context. Further, ocular macrophage ablation led to MAC accumulation, while C3^-/- mice were resistant to CC regression. Together, these findings suggest that tissue-resident choroidal macrophages maintain CC-RPE homeostasis partially by complement dependent mechanisms. Further, the loss of tissue-resident choroidal macrophages over time is a potential mechanism of AMD pathogenesis.