A systematic investigation on the involvement of complement pathway in diabetic retinopathy

Background Complement system play a crucial role in retinal homeostasis. Several proteomic studies have shown deposition of complement protein in ocular tissues from diabetic retinopathy, however, their exact involvement in pathogenesis of DR remains unclear.

Methods We evaluated major complement pathway proteins in the classical and alternative pathway including C1q, C4b, C3, CFB and CFH in vitreous humor and serum samples from PDR patients and controls by western blotting. Quantitative real time (QRT) PCR was done for PDR, NPDR and no-DM controls for correlating the expression of several key pro and anti -angiogenic genes with their correspondingprotein levels. Inflammation in the vitreous humor samples was assessed by ELISA and metalloproteinase activity measured by gelatin zymography. Glial activation and its association with complement activation in diabetic eyes was assessed by immunohistochemistry.

Results A significant increase in C3 proteins, its activated fragment C3bα’ (110kDa) along with a concurrent up regulation of CFH was observed for PDR vitreous. QRT identified a significant upregulation of angiogenic genes and downregulation of antiangiogenic genes in PDR and NPDR cases. PDR vitreous had increased MMP9 activity and upregulation of inflammatory markers IL8, sPECAM and down regulation of anti-inflammatory marker IL-10. Increased C3 deposition and CFH upregulation were observed in DM retina. CFH was found co-localizing with CD11b+ve activated microglial cells in inner nuclear layer of DM retina.

Conclusions The present study confirms increased activation of alternative complement pathway in PDR. The co-localization of CFH in CD11b +ve cells further suggests microglia as a source of CFH in diabetic retina. Increased CFH levels could be a feedback mechanism for arresting excessive complement activation DR eyes.