Complement defects in aged mice compromise phagocytosis of Pseudomonas aeruginosa

Purpose. The role of complement in phagocytosis and killing of P. aeruginosa was examined using serum from aged vs young donor mice. Methods. Phagocytosis, complement hemolytic and microbicidal assays were used. Results. Serum from young donor mice contained a heat-labile factor which significantly enhanced phagocytic activity of cells from young mice compared with similarly treated aged donor serum. Use of cobra venom factor (CVF) to destroy C3 and the terminal complement components in serum from young or aged donor mice also significantly decreased the phagocytic activity of young cells. EGTA treatment of young or aged donor serum, to activate the alternative pathway and selectively inhibit activation of the classical pathway, resulted in a significant decrease in phagocytosis by young cells in the presence of donor serum from either group. Alternative pathway mediated hemolysis also was measured and was significantly reduced in aged vs young donor serum. PMN microbicidal activity was tested using cells from young mice in the presence of aged vs young donor serum, but no significant differences were noted. Conclusion. These data provide evidence that defects in the alternative pathway of complement in the serum of aged animals lead to decreased phagocytic activity of cells from young mice, but not impaired bacterial killing.