Kinetics of phagocytosis and bacterial killing by human polymorphonuclear leukocytes and monocytes

The kinetics of phagocytosis and bacterial killing by normal human polymorphonuclear leukocytes (PMNLs) and by monocytes (MNs) were compared by use of [3H]thymidine-labeled Staphylococcus aureus, Escherichia coli, and Listeria monocytogenes. The rate of phagocytosis by PMNLs was approximately twice that by MNs for all three bacterial species. Although a marked difference was found in opsonic requirements for phagocytosis of S. aureus, E. coli, and L. monocytogenes, phagocytosis by PMNLs and MNs was mediated via the same serum factors. All three species were killed rapidly once they were associated with leukocytes; however, the rate of killing by MNs was slower than that of PMNLs. The slower rate of killing appeared to be secondary to slower ingestion of attached bacteria by MNs. Thus, PMNLs and MNs appear to possess receptors with specificity for the same bacterial opsonins; however, PMNLs are capable of more efficienct bacterial phagocytosis (attachment and ingestion) than are MNs.