Localization of collagen XVIII and the endostatin portion of collagen XVIII in aged human control eyes and eyes with age-related macular degeneration.

PURPOSE Endostatin, a C-terminal fragment of collagen XVIII (coll XVIII) formed by proteolysis, specifically inhibits endothelial cell migration and proliferation in vitro and potently inhibits angiogenesis and tumor growth in vivo. The purpose of this study was to examine the immunolocalization of endostatin and coll XVIII in the retina and choroid of human donor tissue sections from aged control donor eyes and to determine whether the localization or relative levels are changed in age-related macular degeneration (AMD). METHODS Ocular tissues were obtained from six aged control donors (age range, 75-86 years; mean age, 80.5 years) without evidence or history of chorioretinal disease and from nine donors with AMD (age range, 74-105 years; mean age, 88.6 years). Tissues were cryopreserved, and streptavidin alkaline phosphatase immunohistochemistry was performed with goat anti-human and mouse anti-human endostatin antibodies and rabbit anti-mouse coll XVIII. Blood vessels were identified with mouse anti-human CD-34 antibody in adjacent sections. Pigment in RPE and choroidal melanocytes was bleached. Three independent observers scored the immunohistochemical reaction product. RESULTS In aged control eyes, coll XVIII and endostatin (the endostatin portion of coll XVIII) immunoreactivity was observed in large retinal blood vessels and in capillaries in some individuals, but the internal limiting membrane (ILM) had the most intense retinal immunostaining. There was no significant difference in immunoreactivity to both antibodies in retinal blood vessels in aged control eyes. In the choroid, endostatin and coll XVIII were localized to blood vessels, Bruch's membrane, and RPE basal lamina. AMD retina and choroid had a similar pattern and intensity of coll XVIII immunostaining, as observed in control eyes but reaction product was more diffuse in the choroid. Endostatin immunoreactivity was significantly higher in ILM (P = 0.037) in AMD retina and significantly lower in the choriocapillaris, Bruch's membrane, and RPE basal lamina of AMD choroids (P < 0.05) and completely negative in some areas of AMD choroids. CONCLUSIONS These data suggest that reduced levels of the endostatin portion of coll XVIII in Bruch's membrane, RPE basal lamina, intercapillary septa, and choriocapillaris in eyes with AMD may be permissive for choroidal neovascularization.