Figure 3. HESC-RPE cells require MERTK for the internalization of POS. HESC-RPE cells express the phagocytic proteins (A) MERTK and (B) αVβ5 integrin. To determine the importance of MERTK in the phagocytosis of POS, we treated HESC-RPE cells with control IgG (C and F) or MERTK antibody (D and G) for 1 h before exposure to Alexa Fluor® 488-labeled porcine POS (C and D) or fluorescent polystyrene beads (F and G) for 5 h. Cells were treated with trypan blue, washed, and fixed. The number of internalized POS (E) and fluorescent beads (H) was then quantified per field view (150 μm × 150 μm). Data shown are mean±SEM (n=6). Pre-incubation with MERTK antibody had a significant effect on the number of outer segments ingested by HESC-RPE cells (p<0.01, Student t-test, n=6). There was no effect of MERTK antibody on the ingestion of polystyrene beads by the cells (p>0.05, Student t-test, n=6). Photomicrographs are single confocal optical slices merged with Nomarski image (<0.8 μm). Scale bars equal 50 µm. MERTK is labeled with Cy5 and counterstained with DAPI, αVβ5 is labeled with FITC.