Vitronectin is responsible for serum-stimulated uptake of rod outer segments by cultured retinal pigment epithelial...

To examine whether the vitronectin (VN) in serum is responsible for the serum stimulation of phagocytosis in the rod outer segment (ROS) by cultured retinal pigment epithelial (RPE) cells.

Vitronectin was removed from fetal bovine serum by heparin-agarose affinity chromatography. Concentrations in normal and depleted serum were determined by enzyme-linked immunosorbent assay, using a polyclonal antibody against bovine VN and commercially prepared human VN as a standard. A monoclonal antibody against human alpha v beta 5 was used in localization and in blocking experiments. Rod outer segment phagocytosis was measured using a flow cytometric assay.

Affinity chromatography removed 95% of the VN from serum as determined by enzyme-linked immunosorbent assay. Vitronectin-depleted serum did not stimulate ROS phagocytosis by RPE cells. Commercially prepared VN added to serum-free medium stimulated ROS phagocytosis in a dose-dependent manner. Pretreatment of RPE cells with an antibody against alpha v beta 5, an integrin receptor for VN, had no effect on phagocytosis in the absence of serum but completely blocked the serum stimulation of ROS phagocytosis. Antibody against alpha v beta 5 demonstrated a variable labeling pattern on the cultured RPE cell surface with morphologically distinct cell clusters exhibiting less labeling. Those cell clusters exhibiting less receptor labeling also showed less uptake of fluorescent-labeled ROS.

Vitronectin is the component responsible for serum stimulation of ROS uptake, and this uptake appears to be mediated by an alpha v beta 5 integrin. Although clearly important in vitro, a role for VN in ROS uptake by RPE cells in situ remains to be determined.