Interaction of inflammatorily activated retinal pigment epithelium with retinal microglia and neuronal cells

In age-related macular degeneration, inflammatory events are presumed to contribute to disease development. A primary suspect of this contribution is the microglia, the innate immune cell of the retina. In addition, retinal pigment epithelium (RPE) cells can be inflammatorily activated. In this study, we investigate the effect of activated RPE cells on retinal microglia and on neuronal cells. RPE cells and microglia were harvested from porcine eyes. In addition, a neuronal cell line (SHSY-5Y) of human origin was used. For inflammatory activation, agonists of toll-like receptors in different concentrations were used: Pam2CSK4 (Pam; TLR-2), Polyinosinic:polycytidylic acid (Poly I:C; TLR-3) and lipopolysaccharid (LPS; TLR-4). Cell viability was investigated with an MTT assay. The secretion of cytokines was assessed in an ELISA and their expression in real-time PCR. There was no effect of the agonists on cell viability in RPE cells. All agonists induced the secretion of IL-6 and IL-8 in RPE cells with the strongest effect induced by LPS. In microglia, pro-inflammatory stimulation increased the metabolic activity. All agonists induced the secretion of IL-1ß, IL-8, and TNFα in microglia cells while in real-time PCR, LPS and Pam induced the expression of IL-6, IL-1ß and iNOS. Direct stimulation of SHSY-5Y with the agonists induced only minor alterations of viability. Stimulated RPE cell supernatant reduced the secretion of TNFα and IL-8 irrespective of the inducing agent in microglia cells. Additionally a slight induction of IL-1ß was found in microglia treated with supernatant of RPE cells treated with Pam. In real time PCR, the supernatant of RPE cells stimulated with LPS significantly reduced the expression of iNOS and IL-6, but not of IL-1ß. Of note, the expression of iNOS was also reduced by naive RPE cells. The treatment of the SHSY-5Y with supernatant of microglia previously treated with RPE conditioned medium significantly decreased SHSY-5Y viability with and without pro-inflammatory treatment. In conclusion, inflammatory activated RPE cells have a regulatory effect on the pro-inflammatory activation of microglia, stressing the importance of the interaction between these two retinal cell types. Microglia treated with RPE supernatant reduced viability of a neuronal cell line, indicating a neurotoxic effect.