Oxidative photoreceptor cell damage in autoimmune uveitis

Abstract

Uveitis comprises an extensive array of intraocular inflammatory diseases and often results in irreversible visual loss. Experimental autoimmune uveitis (EAU) is an animal model used to study human uveitis. Both innate and adaptive immune responses are known to mediate retinal damage in EAU. The innate immune response occurs first with activation of toll-like receptors which upregulate inflammatory cytokines, leading to oxidative stress; subsequently, the adaptive immune response results in inflammatory cytokine upregulation and mitochondrial oxidative stress. In early EAU, mitochondrial DNA is damaged before inflammatory cellular infiltration and alters mitochondrial protein levels and the functions of mitochondria in AU. Our recent study confirms the importance of TLR4 in the generation of inflammatory cytokines, initiation of oxidative DNA damage, and induction of mitochondrial oxidative stress. Like EAU, sympathetic ophthalmia also results in photoreceptor mitochondrial oxidative damage. Agents that prevent mitochondrial oxidative stress and photoreceptor apoptosis may help prevent retinal damage and preserve vision in uveitis.

Keywords: Complete Freund’s adjuvant; Experimental autoimmune uveitis; Mitochondrial oxidative stress; Sympathetic ophthalmia; TLR4.

Fig. 1

Morphologic changes can be seen throughout experimental autoimmune uveoretinitis (EAU). Photomicrographs showing hematoxylin and eosin-stained sections of mice (B10RIII) retinas reveal that during early EAU (day 5 postimmunization), retinas remain intact; whereas at the peak of inflammation, during the amplification phase of EAU (day 14), macrophage and other inflammatory cell infiltration is prominent and the retinal structure is no longer well-preserved

Fig. 2

During early EAU, inflammatory cytokines are upregulated, specifically tumor necrosis factor-α (TNF-α). TNF-α is a cytokine known to diffuse and upregulate inducible nitric oxide synthase in adjacent cells, resulting in the generation of NO and the subsequent formation of peroxynitrite (ONOO⁻) in the mitochondria which leads to mitochondrial oxidative stress. The nitration process takes place primarily in the photoreceptor inner segments, an area rich in mitochondria. Mitochondrial oxidative stress causes the initiation of lipid peroxidation and the formation of hydroperoxides; hydroperoxides, along with the nitrated mitochondrial proteins, function as chemotactic agents that amplify the inflammatory processes by recruiting macrophages, resulting in photoreceptor apoptosis

Fig. 3

Immunofluorescence localization of 8-OHdG in the retinas of wild-type (WT; C57BL/6) mice and TLR4-deficient (TLR4 ^−/−) mice with EAU day 7 after immunization. Tissues were labeled using the polyclonal antibody against 8-OHdG and Texas Red dye-conjugated donkey anti-goat IgG as the primary and secondary antibodies respectively. No 8-OHdG staining was present in the nonimmunized (control) WT and TLR4^−/− mice. In the WT mice with EAU, intense 8-OHdG staining was detected in the inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), and in the inner segments of the photoreceptor layer; such staining was significantly reduced in the outer plexiform layer and inner segments and was absent in the remainder of the retina in the TLR4^−/− mice during early EAU

Fig. 4

Immunofluorescence colocalization of nitrotyrosine with apoptotic cells in retinas of patients with sympathetic ophthalmia (SO). SO globes were labeled using polyclonal rabbit antinitrotyrosine (primary antibody)and sheep-antirabbit IgG conjugated with flourescein (secondary antibody), followed by TUNEL staining using a TACS TdT Apoptosis Detection Kit (American Qualex). Nitrotyrosine was localized primarily in the photoreceptor inner segments (IS). Apoptotic cells were detected mainly in the outer nuclear layer (ONL), with a few apoptotic cells in the inner nuclear layer (INL) in SO retinas