Role of specific antibody in interaction of leptospires with human monocytes and monocyte-derived macrophages.

It has previously been shown that human neutrophils ingest and kill nonpathogenic Leptospira biflexa in the absence of serum but that pathogenic Leptospira interrogans is not ingested by neutrophils even in the presence of normal serum. We extended this study by examining the interactions of human monocytes and monocyte-derived macrophages with pathogenic L. interrogans (serovar icterohaemorrhagiae) and evaluating the opsonizing effect of serotype-specific immune serum on the phagocytosis of pathogenic leptospires by monocytes and neutrophils. Leptospires were incubated with monocytes in pellets at 37 degrees C in 5% CO2. No ingestion or killing of pathogenic leptospires occurred when 10% normal serum was used. However, when the pathogenic leptospires were pretreated with serotype-specific immune serum, monocytes or neutrophils in pellets ingested 96% of the organisms and killed 94% of those ingested. Microscopic observations of the interaction confirmed that phagocytosis of the opsonized pathogenic leptospires by monocytes, monocyte-derived macrophages, and neutrophils had occurred. The opsonizing effect of specific antibody may play an important role in the mechanism of host defense against leptospirosis.