**5. Conclusions**

Oxidative stress and activation of the complement system cause retinal degeneration, but the mechanism behind this is still a matter of investigation. We showed for the first time that oxidative stress can increase endogenous ARPE-19 cell complement components and receptors and that the process was associated with the release of proinflammatory and proangiogenic factors.

Our data o ffer a steppingstone for numerous further investigations regarding the function of a cell-associated complement system in primary human RPE. Many questions were raised during this project: How are the complement components activated? Independent of external complement sources, what is (are) the signaling pathway(s) of the complement receptors? How are inflammasome regulation and FOXP3 activity modulated by endogenous complement components in RPE cells? Can endogenous complement factors be targeted to a ffect cell-associated signaling pathways? These new perspectives will hopefully help to decipher the function of intracellular complement components in retinal health and disease and o ffer new strategies for the treatment of retinal degeneration.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3921/8/11/548/s1, Figure S1: ARPE-19 cells showed a stable, confluent monolayer, and H2O2 treatment increased the expression of epithelial–mesenchymal transition markers; Figure S2: H2O2 treatment did not influence the transcription levels of several genes in ARPE-19 cells; Figure S3: The secretome of ARPE-19 cells was influenced by cell passages and H2O2 addition; Figure S4: The stable expression of complement components and related genes after Olaparib and oxidative stress treatment in ARPE-19 cells; Figure S5: Full immunoblots for Figures 2H and 4C; Figure S6: Time-dependent changes of H2O2 treatment in ARPE-19 cells; Table S1: Primary and secondary antibodies; Table S2: QuantiTec PrimerAssays; Table S3: In-house-designed RT-qPCR primers.

**Author Contributions:** Conceptualization, T.-O.T., J.W., V.E., and D.P.; methodology, T.-O.T., N.S., M.R., K.K., R.J.M.G.E.B., E.J.M.T., F.U., and D.P.; validation, J.W., D.P.; investigation, T.-O.T, N.S., M.R., K.K., R.J.M.G.E.B., E.J.M.T., F.U., and D.P.; data curation, T.-O.T., F.U., J.W., V.E., and D.P.; writing—original draft preparation, D.P.; writing—review and editing, T.-O.T., N.S., K.K., R.J.M.G.E.B., E.J.M.T., F.U., J.W., and V.E.; visualization, F.U. and D.P.; supervision, J.W., V.E., and D.P.; project administration, V.E. and D.P.; funding acquisition, V.E. and D.P.

**Funding:** This research was funded by the Velux Foundation, gran<sup>t</sup> #1103, to V.E. and D.P.

**Acknowledgments:** We thank Renate Foeckler, Andrea Dannullis, and Elfriede Eckert for excellent technical support.

**Conflicts of Interest:** The authors declare no competing interests.
