Interaction of leptospires with human polymorphonuclear neutrophils

The role of the polymorphonuclear neutrophils (PMN) in defense against leptospires has not been adequately studied, in part, because of difficulty in quantitating pathogenic leptospires. By using pour plates to quantitate nonpathogenic leptospires and the most-probable-number procedure to quantitate the pathogenic leptospires, we examined the interactions of nonpathogenic Leptospira biflexa and pathogenic Leptospira interrogans serovar icterohemorrhagiae with human neutrophils. Phase-contrast, scanning electron, and transmission electron microscopic observations were made. Leptospires were incubated with PMN at 37 degrees C and tumbled together. There was no ingestion or killing of nonpathogenic leptospires (with no serum) or of pathogenic organisms (with 10% normal serum). However, when leptospires were incubated with PMN (without serum) in a pellet and then resuspended, 91 +/- 6% of nonpathogenic leptospires were removed from the supernatant, and 93 +/- 4% of these organisms were killed. The pathogenic leptospires became cell associated in a pellet, but were not killed by PMN even in the presence of 10% normal serum. Observations of morphological interactions indicated that PMN phagocytized the nonpathogenic leptospires in the absence of serum and that the pathogenic leptospires attached to but were not ingested by neutrophils in the presence of 10% normal serum. PMN do not seem to be an efficient defense factor for pathogenic leptospires in nonimmune hosts. The virulence of leptospires appears to be related to their ability to resist killing by serum and to resist ingestion and killing by neutrophils.