FIG. 2.

CD36 and protease-sensitive ligands mediate the uptake of RPEs by murine macrophages. (A) CD36 wild-type (WT) (▪) or CD36-null (KO [knockout]) (□) murine macrophages were incubated in the presence of a panel of MAbs (as indicated) prior to phagocytosis assays with RPEs. Fc receptor-blocked macrophages were also treated with 3 μg of mannan per ml at room temperature for 30 min to block the mannose receptor. Phagocytosis of RPEs by CD36-null murine macrophages was reduced by ∼90% compared to phagocytosis by CD36 wild-type murine macrophages (*, P < 0.01; n = ≥4). (B) Treating RPEs with trypsin reduces their phagocytosis by murine macrophages. Cultures containing nonopsonized RPEs were incubated with different concentrations of trypsin (25, 50, and 100 μg/ml), washed repeatedly, and then incubated with macrophages. Phagocytosis of trypsin-treated RPEs was reduced by ∼60% ± 23%, 77% ± 18%, and 92% ± 20% (mean ± standard deviation), respectively, by treatment with 25, 50, and 100 μg of trypsin per ml compared to phagocytosis of control nontreated RPEs. Treatment with 25 μg of trypsin per ml decreased phagocytosis of RPEs to a degree similar to that of receptor blockade by treatment with 10 μg of anti-CD36 per ml (*, P < 0.05). When the concentration of trypsin used for treatment was increased, RPE uptake decreased proportionally compared to control uptake (**, P < 0.01; n = 3).