EFFECT OF LEGIONELLA-PNEUMOPHILA SONICATE ON KILLING OF LISTERIA-MONOCYTOGENES BY HUMAN POLYMORPHONUCLEAR NEUTROPHILS AND MONOCYTES

Legionella pneumophila shares with other intracellular pathogens the a bility to resist intracellular killing within phagocytes. An increasin g number of cellular components of L. pneumophila are proposed as path ogenic factors of the organism. At the site of infection, the phagocyt ic cells will be exposed to bacterial components, either expressed on the surface of the organisms or released in the environment upon cell lysis. In this study, we have investigated the effect of water-soluble bacterial components present in L. pneumophila sonicate on the phagoc ytosis and bactericidal activity of human polymorphonuclear neutrophil s and monocytes. Preincubation of neutrophils with L. pneumophila soni cate did not affect phagocytosis of L. monocytogenes, whereas Listeria killing was significantly inhibited at sonicate concentrations of 1 a nd 2 mg/ml. The phenol phase of a phenol-water extraction, containing most of the lipopolysaccharide (LPS), had no inhibitory effect on the listericidal activity of neutrophils. Killing of Listeria by monocytes was inhibited in a similar manner. The inhibitory activity was mainly recovered in the sonicate fraction above 100 kDa, suggesting that com ponents organized in larger molecular complexes are most likely to rep resent the inhibitory factors. The 'inhibitory activity of L. pneumoph ila sonic extract appears to be related to inhibition of killing mecha nisms since uptake of Listeria was not affected by the sonicate. Our o bservations indicate that as Legionella infection progresses, bacteria l components liberated by cell lysis could exert a detrimental effect on the antimicrobial function of phagocytes, stressing the importance of early treatment of Legionnaires' disease to reduce bacterial number s in the infected tissues.