Entry of human cytomegalovirus into retinal pigment epithelial and endothelial cells by endocytosis

tPURPOSE. Human retinal pigment epithelial (RPE) cells and endothelial cell s (HUVECs) are targets of human cytomegalovirus (HCMV) infection in vivo wi th significantly protracted replication in vitro compared with that in fibr oblasts. This study analyzes the kinetics and mechanisms of HCMV entry into both cell types. METHODs. RPE cells were obtained from donor eyes. HUVECs were isolated from human umbilical cords. HCMV entrance was followed by electron microscopy a nd immunofluorescence in the presence of lysosomotropic agents and cytochal asin B. RESULTs. Human cytomegalovirus entered into RPE cells and HUVECs as early a s 5 minutes after virus-cell contact. Entry was mediated by endocytosis, wh ereas HCMV enters fibroblasts through fusion. Most internalized viral parti cles and dense bodies appeared to be degraded within vacuoles. Viral entry, transport of viral proteins to the nucleus, and onset of viral transcripti on (immediate early [IE] protein expression) were significantly blocked by cytochalasin B. Lysosomotropic agents did not significantly reduce IE expre ssion in RPE cells or HUVECs. CONCLUSIONS. This study shows that HCMV penetrates these highly specialized relevant cells via endocytosis. The low level of infection and the delay i n the onset of HCMV expression seen in these cells compared with fibroblast s map be related to the sequestration and degradation of incoming viral par ticles in endocytic vacuoles.