Effects of Legionella pneumophila sonicate on human neutrophil granulocyte and monocyte chemotaxis

The interaction of L. pneumophila serogroup 1 sonic extract with human polymorphonuclear neutrophils and monocytes was studied in an in vitro chemotaxis assay. The sonicate showed heat-stable chemotactic activity towards neutrophils and monocytes. It also exhibited cytotoxicity for neutrophils but not for monocytes. Incubation and pretreatment with the sonicate at apparently non-toxic concentration resulted in inhibition of the chemotactic response of neutrophils to various chemoattractants while monocyte chemotaxis was unaffected. The inhibitory activity was reduced to one half by heat treatment of the sonicate. High endotoxic activity was demonstrated in both heated and non-heated extract. Only two antigens (nos 1 and 61) were identified in the sonicate exposed to 121 degrees C for 1 h. It is therefore suggested that the lipopolysaccharide (no. 61) of L. pneumophila could contribute to the chemotactic activity. The adverse effects exerted by L. pneumophila on neutrophils may be one of the mechanisms by which Legionella bacteria resist the normal phagocytic host defenses.