Phagocytosis of Salmonella montevideo by human neutrophils: immune adherence increases phagocytosis, whereas the bacterial surface determines the route of intracellular processing.

Complement-opsonized particles become immune adherent to complement receptor 1 (CR1 or CD35) on human erythrocytes, allowing particles to be ingested by phagocytes in the liver and the spleen. We investigated the role that immune adherence plays in the uptake and killing of Salmonella montevideo by human neutrophils. Exposure to serum induced loss of flagella and facilitated immune adherence, which was followed by more-efficient phagocytosis and killing, compared with that after exposure to serum-opsonized, free bacteria. One correlate of bacterial killing is the fusion of phagosomes with lysosomes, which can be monitored by Lyso-Tracker or lysosomal-associated membrane protein 2 colocalization with the intracellular bacteria. At 5 min, phagolysosmal fusion was significantly faster for immune-adherent bacteria than for non-immune-adherent bacteria, but, by 35 min, the difference between the 2 groups was minimal. Immune adherence also facilitated the ingestion of antibody complement-opsonized fluorescent microspheres, but, unlike bacteria, most internalized microspheres failed to fuse with lysosomes. However, addition of lipopolysaccharide, a Toll-like receptor ligand, to microspheres directed their intracellular trafficking, resulting in rapid lysosomal fusion. Thus, immune adherence facilitates phagocytosis, but the route of intracellular processing depends on the molecular nature of the target and is independent of host complement and antibody.