Complement (C3)‐Receptor‐Mediated Phagocytosis of Agarose Beads by Mouse Macrophages

Abstract

The phagocytosis by macrophages of C3bi‐coated agarose heads reached a plateau after 15 min, compared with 30 min for C3b‐coated beads. By using 125 I‐Iabelled C3bi or C3b coupled to the agarose beads, we found that 70% and 95% of total radioactivity were removed from the heads after 12 h and 36 h of intracellular digestion, respectively. Intracellular degradation of C3bi linked to agarose beads was also demonstrated by testing binding of monoclonal antibodies against human C3c, C3g and C3d to beads extracted from the cells after phagocytosis. Such extracted beads also showed reduced attachment to new macrophages compared with non‐ingested beads. Treatment of the cells with leupeptin, an inhibitor of the lysosomal enzyme cathepsin B, or with dextran sulphate to inhibit phagosome‐lysosome fusion greatly reduced the release of labelled protein from the agarose during the first 12 h. These findings show that C3bi and C3b on agarose is destroyed intracellularly by lysosomal enzymes.