Group IIA secreted phospholipase A 2 (sPLA2) is known to display potent Gram-positive bactericidal activity in vitro and in vivo. We have analyzed the bactericidal activity of the full set of recombinant murine and human groups I, II, V, X, and XII sPLA2s on Listeria monocytogenes, Staphylococcus aureus, and Escherichia coli. The rank order potency among human sPLA2s against Gram-positive bacteria is group IIA > X > V > XII > IIE > IB, IIF (for murine sPLA2s: IIA > IID > V > IIE > IIC, X > IB, IIF), and only human group XII displays detectable bactericidal activity against the Gramnegative bacterium E. coli. These studies show that highly basic sPLA2s display potent bactericidal activity with the exception of the ability of the acidic human group X sPLA2 to kill Gram-positive bacteria. By studying the Bacillus subtilis and S. aureus bactericidal potencies of a large panel of human group IIA mutants in which basic residues were mutated to acidic residues, it was found that: 1) the overall positive charge of the sPLA2 is the dominant factor in dictating bactericidal potency; 2) basic residues on the putative membrane binding surface of the sPLA2 are modestly more important for bactericidal activity than are other basic residues; 3) relative bactericidal potency tracks well with the ability of these mutants to degrade phospholipids in the bacterial membrane; and 4) exposure of the bacterial membrane of Gram-positive bacteria by disruption of the cell wall dramatically reduces the negative effect of charge reversal mutagenesis on bactericidal potency.The secreted phospholipases A 2 (sPLA2s) 1 comprise a large family of water-soluble enzymes that have the ability to bind to membrane surfaces as a prelude to the hydrolysis of the sn-2 ester of membrane phospholipids (1). Pancreatic sPLA2 (group IB sPLA2) was the first of these enzymes to be purified and extensively studied as a paradigm for interfacial enzymology.

Group V phospholipase A2 (PLA2) is a recently characterized 14-kDa secretory PLA2 of mammalian heart and macrophage-derived cells. Group IIA PLA2, which is structurally close to group V PLA2, has been shown to kill Gram-positive bacteria in vitro and to prevent symptoms of Gram-positive infection in vivo. We studied the antibacterial properties of fully active recombinant rat group IIA and V PLA2s. Both group IIA and V PLA2s were highly bactericidal against Gram-positive bacteria, including methicillin-resistant staphylococci and vancomycin-resistant enterococci. Only high concentrations of group IIA PLA2 showed some bactericidal effect against the Gram-negative bacterium Escherichia coli. Our results confirm that group IIA PLA2 is a potent antibacterial enzyme against Gram-positive bacteria. Moreover, we show here that group V PLA2 is a novel antibacterial mammalian protein, but is less potent than group IIA PLA2. Both enzymes may be considered as future therapeutic agents against bacterial infections.

Group IIA phospholipase A2 (PLA2-IIA) is a newly recognized antibacterial acute phase protein. The concentration of PLA2-IIA increases up to 500-fold in the blood plasma of patients with severe acute diseases, compared with healthy persons. Despite numerous studies, the exact roles of this enzyme in human diseases are unknown. This study investigated the antibacterial properties of PLA2-IIA in human acute phase serum. PLA2-IIA in serum samples of patients with bacterial infections was capable of killing 90% of Staphylococcus aureus and 99% of Listeria monocytogenes in vitro after incubation for 2 h. At concentrations found in normal human serum, PLA2-IIA killed 90% of L. monocytogenes but did not kill S. aureus or Escherichia coli. The bactericidal effects of acute phase and normal human serum were abolished after depletion of PLA2-IIA by immunoadsorption.

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 A 2 (hGIIA PLA 2 ) is an effective antibacterial enzyme in serum of patients with severe bacterial infections. Our aim was to investigate the significance of complement and hGIIA PLA 2 in acute phase serum. Serum samples were collected from patients with acute bacterial infections and from healthy control subjects. We prepared hGIIA PLA 2 -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 PLA 2 significantly reduced the antibacterial effect. Complement had a weak bactericidal effect against L. monocytogenes. We conclude that hGIIA PLA 2 is the major antibacterial factor in human acute phase serum against the gram-positive bacteria S. aureus and L. monocytogenes, exceeding complement in efficiency.