Studies in rats have shown that intravitreal injections of angiotensin II (20 μg/eye) cause increases in retinal levels of VEGF expression and leukocyte adhesion to the walls of the retinal vasculature, suggesting a mechanism involving production of inflammatory mediators.
18 To see whether upregulation of IL-6 could be involved in this process, we determined the effects of intravitreal injections of angiotensin II (20 μg/5 μL, 24 hours) on IL-6 expression in the rat retina. Immunolocalization studies showed that numerous large cells in the inner retina were intensely reactive for IL-6 in the angiotensin II–injected animals. IL-6 immunoreactivity was also observed in a population of small, microglia-like cells with stellate processes in both the inner and outer retina. By contrast, in the retinas from the vehicle control animals, IL-6 expression was restricted to small, microglia-like cells in the outer nuclear layer (
Fig. 1A). The other retinal layers were negative for IL-6. Double-labeling studies with the microglial/macrophage marker CD11b showed that both the small and the large IL-6–positive cells were also positive for CD11b (
Fig. 1B), suggesting that the IL-6–positive cells were microglia and macrophages. Quantitative real-time RT-PCR analysis of the contralateral retinas from the same animals used for immunolocalization studies showed that intravitreal injection of angiotensin II resulted in a ∼9-fold increase in IL-6 mRNA and a ∼12-fold increase in MCP-1 mRNA compared with the saline-injected controls (
Fig. 1B). IL-6 protein was also determined in angiotensin II–treated mice. The level of IL-6 in both the vehicle-treated wild-type mice and angiotensin II–treated IL-6ko mice was very low and under the detection limit of the kit (1.5 pg/mL). However, the average level of IL-6 in the retina lysates from the angiotensin II–treated wild-type mice was markedly increased (3.1 pg/mL,
Fig. 1C).