Phagocytosis of rod outer segments by retinal pigment epithelial cells requires αvβ5 integrin for binding but not for internalization

Phagocytosis of shed photoreceptor rod outer segments (ROS) by the retinal pigment epithelium (RPE) is essential for retinal function. Here, we demonstrate that this process requires αvβ5 integrin, rather than αvβ3 integrin utilized by systemic macrophages. Although adult rat RPE expressed both αvβ3 and αvβ5 integrins, only αvβ3 was expressed at birth, when the retina is immature and phagocytosis is absent. Expression of αvβ5 was first detected in RPE at PN7 and reached adult levels at PN11, just before onset of phagocytic activity. Interestingly, αvβ5 localized in vivo to the apical plasma membrane, facing the photoreceptors, and to intracellular vesicles, whereas αvβ3 was expressed basolaterally. Using quantitative fluorimaging to assess in vitro uptake of fluorescent particles by human (ARPE-19) and rat (RPE-J) cell lines, αvβ5 function-blocking antibodies were shown to reduce phagocytosis by drastically decreasing (85%) binding of ROS but not of latex beads. In agreement with a role for αvβ5 in phagocytosis, immunofluorescence experiments demonstrated codistribution of αvβ5 integrin with internalized ROS. Control experiments showed that blocking αvβ3 function with antibodies did not inhibit ROS phagocytosis and that αvβ3 did not colocalize with phagocytosed ROS. Taken together, our results indicate that the RPE requires the integrin receptor αvβ5 specifically for the binding of ROS and that phagocytosis involves internalization of a ROS-αvβ5 complex. αvβ5 integrin does not participate in phagocytosis by other phagocytic cells and is the first of the RPE receptors involved in ROS phagocytosis that may be specific for this process.