Complement activation on photoreceptor outer segments triggers their phagocytosis by macrophages and results in photoreceptor degeneration

Abstract

Purpose : Polymorphisms in complement genes have been linked to age-related macular degeneration (AMD). We sought to further understand how complement is activated in the murine retina and why complement activation results in photoreceptor degeneration.

Methods : Murine peritoneal macrophages were elicited with thioglycolate, harvested, and cultured for phagocytosis assays. Bovine photoreceptor outer segments (POS) were labeled with Alexa-Fluor 488 fluorescent dye and incubated with C3-sufficient or –deficient serum. POS were added to the macrophage culture in the presence or absence of complement receptor 3 (CR3) blocking antibodies. Phagocytosis was quantified by flow cytometry. Retina pigmented epithelial cell death was induced by systemic injection of sodium iodate (NaIO₃) in complement C3-sufficient and C3-deficient mice. Retina degeneration was measured by SD-OCT (Heidelberg). Retina function was measured by electroretinogram (ERG) (Espion2). Retina cells were counted by flow cytometry. The outer nuclear layer was processed for transmission electron microscopy (TEM).

Results : Photoreceptor outer segments incubated with C3-sufficient serum increased phagocytosis by macrophages by >50% (p<0.001) as compared to POS incubated with C3-deficient serum. Phagocytosis could be fully blocked with a CR3 blocking antibody. In C3 sufficient mice, increased numbers of macrophages were found in the outer segment layer of the retina 3 days following NaIO3 treatment. All macrophages had ingested otherwise intact POS, as shown by TEM. Macrophage numbers in the retina were reduced by >60% and photoreceptor rods were spared by 50% (p<0.0001) in NaIO₃-treated mice lacking C3. ERG a- and b-wave amplitudes were restored (p<0.0001) in C3 deficient compared to C3 sufficient mice 7 days following NaIO3 treatment.

Conclusions : Complement activation on POS may contribute to NaIO₃-induced retina dysfunction by increasing macrophage recruitment to the photoreceptor layer and by enhancing phagocytosis of POS by macrophages.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.