Retinal pigment epithelial cells reduce vascular leak and proliferation in retinal neovessels

Retinal pigment epithelial (RPE)-cells possess numerous functions and may respond to stress and damage of the neuroretina. In different neurodegenerative diseases, including age-related macular degeneration (AMD), retinitis pigmentosa, and macular telangiectasia type 2 (MacTel), RPE-cells have been shown to proliferate and migrate into the neuroretina, forming intraretinal pigment plaques. Though pigmentary changes are associated with disease progression, it is not known if their presence is protective or detrimental. In this study, we evaluated the impact of pigment plaques on vascular changes and disease progression in patients with macular telangiectasia type 2 (MacTel), an example of a progressive neurodegenerative retinal disease. We then studied underlying pathomechanisms using a mouse model mirroring these changes, the very-low-density lipoprotein receptor mutant (Vldlr-/-) mouse. In a retrospective, longitudinal study, we analyzed multimodal retinal images of patients with MacTel and showed that pigment plaques were associated with a decrease in vascular leakage and stabilized neovascular growth. Using genetic approaches, we analyzed changes in expression levels of relevant genes in the RPE and retinas of Vldlr-/- mice during RPE-proliferation and migration. Our data indicated that RPE-cells transitioned from an epithelial to a mesenchymal state ('epithelial-mesenchymal transition', EMT), proliferated and accumulated along neovessels. Using dextran angiography and immunofluorescence, we demonstrated that the perivascular accumulation of RPE-cells reduced vascular leakage. Pharmacologic inhibition of EMT led to a decrease in pigment coverage and exacerbation of neovascular growth and exudation. Our findings indicate that the proliferation, migration and perivascular accumulation of RPE-cells may stabilize vascular proliferation and exudation, thereby exerting a protective effect on the diseased retina. We conclude that interfering with this 'natural repair mechanism' may have detrimental effects on the course of the disease and should thus be avoided.