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R.A. Bush, C.–C. Chan, H. Takase, M. Santos–Muffley, J. Tuo, O.M. Z. Howard, W.A. Kuziel, I. Gery; The Chemokine CCL2/MCP–1 Protects Against Light Damage, But Not by Cell Recruitment . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4811.
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Chemokines are important in inflammatory responses and phagocytic cell recruitment and activation. Inefficient removal of debris in the retina is thought to play a role in age–related macular degeneration (AMD). Chemokines, such as CCL2 (MCP–1) and CX3CL1 (fractalkine), have been linked to AMD and some chemokines show increased expression in retinal light damage (LD). We analyzed the pathological changes following LD in mice deficient in CCL2, its receptor, CCR2, and CX3CR1, the receptor for CX3CL1.
Chemokine and chemokine receptor deficient (–/–) and wild type (CCL2+/+ and C57Bl/6) mice, 3 to 9 months old, were exposed to 5000 lux white fluorescent light continuously for 14 days (LD) under constant temperature (24 ± 1oC), food and water ad lib. Control mice were kept in cyclic colony room light. The eyes were processed for light microscopy immediately following LD and sectioned vertically via the papillary–optic nerve head axis. Outer nuclear (ONL) width in the superior retina was measured at 12 locations in one eye of each animal. Other signs of photoreceptor layer damage were noted qualitatively. TUNEL assay was done on the other retina.
LD CCL2–/– retinas had much more severe photoreceptor loss than LD CCL2+/+ retinas. The mean ONL width was 9.8 ± 6.8µm in CCL2–/– and 33.0 ± 1.7µm in CCL2+/+ retinas (n = 7, 4; p<0.0001). ONL widths in non–LD control CCL2–/– and CCL2+/+ retinas were not different (35.6 ± 3.2µm and 36.7 ± 2.7µm, respectively, n=4). LD CCR2–/– showed no ONL thinning (37.7 ± 4.2µm, n=4) compared to control mice (36.9 ± 2.9µm, n=4). Likewise, ONL widths in LD C57Bl/6 (32.4 ± 2.4µm, n=3) and CX3CR1–/– (30.6 ± µm, n=3) mice were similar to controls (35.0µm and 28.2µm, respectively). No TUNEL positive cells presented in LD retinas, indicating cell death was mostly complete for this exposure duration. The rod outer segment (ROS) layer of LD CCL2–/– mice was almost totally eliminated and was noticeably thinned in the superior retinas of the other strains of LD mice. In the CX3CR1–/– LD retina a few pyknotic condensed nuclei were observed in the ROS layer and vacuoles were seen in the ONL.
CCL2 expression protects against LD, but cell recruitment mediated by its receptor or the fractalkine receptor does not play a role in LD protection. CCL2 may function in the retina as an anti–apoptotic factor, as previously shown in human CNS neurons in vitro.
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