CD36 PARTICIPATES IN THE PHAGOCYTOSIS OF ROD OUTER SEGMENTS BY RETINAL-PIGMENT EPITHELIUM

Mechanisms of phagocytosis are complex and incompletely understood. Th e retinal pigment epithelium provides an ideal system to study the spe cific aspects of phagocytosis since an important function of this cell is the ingestion of packets of membranous discs that are normally dis carded at the apical ends of rod and cone cells during outer segment r enewal. Here we provide evidence that rod outer segment phagocytosis b y retinal pigment epithelium is mediated by CD36, a transmembrane glyc oprotein which has been previously characterized on hematopoietic cell s as a receptor for apoptotic neutrophils and oxidized low density lip oprotein. Immunocytochemical staining with monoclonal and polyclonal a ntibodies demonstrated CD36 expression by both human and rat retinal p igment epithelium in transverse cryostat sections of normal retina and in primary cultured cells. By western blot analysis of retinal pigmen t epithelial cell lysates, polyclonal and monoclonal antibodies to CD3 6 recognized an 88 kDa protein which comigrated with platelet CD36. Fu rthermore, the synthesis of CD36 mRNA by retinal pigment epithelium wa s confirmed by reverse transcriptase-PCR using specific CD36 oligonucl eotides. The addition of CD36 antibodies to cultured retinal pigment e pithelial cells reduced the binding and internalization of I-125-label ed rod outer segments by 60%. Immunofluorescence confocal microscopy c onfirmed that outer segment uptake was significantly diminished by an antibody to CD36. Moreover, we found that transfection of a human mela noma cell line with CD36 cDNA enabled these cells to bind and internal ize isolated photoreceptor outer segments as seen by double immunofluo rescent staining for surface bound and total cell-associated rod outer segments, and by measurement of cell-associated I-125-labeled rod out er segments. We conclude that the multifunctional scavenger receptor C D36 participates in the clearance of photoreceptor outer segments by r etinal pigment epithelium and thus, participates in the visual process .