Stimulation of rod outer segment phagocytosis by serum occurs only at the RPE apical surface

The phagocytosis of isolated rod outer segments by cultured rat retinal pigment epithelium (RPE) previously has been shown to be stimulated by serum in the culture medium. In vivo, serum is normally in contact with the basolateral surface of the RPE. Components of serum have also been detected in the interphotoreceptor matrix, raising the possibility that these components may be in contact with the apical RPE surface. However, with conventional culture techniques it is not clear whether the stimulation of phagocytosis by serum occurs at the basolateral or apical surface. To resolve this uncertainty, phagocytosis was studied using RPE cultured on microporous filters, permitting control of which RPE surfaces are in contact with serum. Serum was found to have no influence on phagocytosis when present at the RPE basolateral surface, regardless of the serum concentration (2 or 20%). In contrast, phagocytosis was elevated three- to fivefold when 2% serum was present at the apical RPE surface, irrespective of the presence of serum at the basolateral surface. It is concluded that serum stimulates the phagocytosis of ROS only when the serum is present at the RPE apical surface.