Receptor expression on the apical and basal surfaces of RPE. Cells were plated in transwell hanging cell culture inserts to allow access to both the apical and basal cell surfaces. Cells were treated on either their apical or basal surfaces with primary antibody to nectin-1 (A,B), HVEM (C,D), or PILR-α (E,F). Cells mock treated with PBS containing 3% BSA were used as controls (G-J). All cells were then incubated with FITC-conjugated secondary antibody on the appropriate surface and imaged by confocal microscopy. FITC-conjugated anti-mouse secondary antibody was used for nectin-1 and HVEM and FITC-conjugated anti-rat secondary antibody was used for PILR-α. The presence of green indicates receptor expression. Controls were made for each of the three receptors, but for clarity one set of controls is shown for each type of secondary antibody.