Roles of group IIA phospholipase A2 and complement in killing of bacteria by acute phase serum

The complement system is regarded as an important component of the innate defence system against invading bacteria. However, synergistic actions between the complement and the other components of innate immunity are incompletely known. Human group IIA phospholipase A2 (hGIIA PLA2) is an effective antibacterial enzyme in serum of patients with severe bacterial infections. Our aim was to investigate the significance of complement and hGIIA PLA2 in acute phase serum. Serum samples were collected from patients with acute bacterial infections and from healthy control subjects. We prepared hGIIA PLA2-depleted serum by immunoadsorption and inhibited the activity of complement by a specific inhibitor, compstatin. The bactericidal effects of treated and untreated serum were compared by incubating Staphylococcus aureus and Listeria monocytogenes in the presence of serum. Acute phase serum effectively killed S. aureus and L. monocytogenes, and depletion of hGIIA PLA2 significantly reduced the antibacterial effect. Complement had a weak bactericidal effect against L. monocytogenes. We conclude that hGIIA PLA2 is the major antibacterial factor in human acute phase serum against the gram-positive bacteria S. aureus and L. monocytogenes, exceeding complement in efficiency.