Antibodies against Listerial Protein 60 Act as an Opsonin for Phagocytosis ofListeria monocytogenesby Human Dendritic Cells

Human-monocyte-derived dendritic cells (MoDC) are very efficient in the uptake ofListeria monocytogenes, a gram-positive bacterium which is an important pathogen in humans and animals causing systemic infections with symptoms such as septicemia and meningitis. In this work, we analyzed the influence of blood plasma on the internalization ofL. monocytogenesinto human MoDC and compared the uptake ofL. monocytogeneswith that ofSalmonella entericaserovar Typhimurium andYersinia enterocolitica. While human plasma did not significantly influence the uptake of serovar Typhimurium andY. enterocoliticaby human MoDC, the efficiency of the uptake ofL. monocytogenesby these phagocytes was strongly enhanced by human plasma. In plasma-free medium the internalization ofL. monocytogeneswas very low, whereas the addition of pooled human immunoglobulins resulted in the internalization of these bacteria to a degree comparable to the highly efficient uptake observed with human plasma. All human plasma tested contained antibodies against the 60-kDa extracellular protein ofL. monocytogenes(p60), and anti-p60 antibodies were also found in the commercially available pooled immunoglobulins. Strikingly, in contrast toL. monocytogeneswild type, aniapdeletion mutant (totally deficient in p60) showed only a minor difference in the uptake by human MoDC in the presence or the absence of human plasma. These results support the assumption that antibodies against the listerial p60 protein may play an important role in Fc-receptor-mediated uptake ofL. monocytogenesby human MoDC via opsonization of the bacteria. This process may have a major impact in preventing systemic infection inL. monocytogenesin immunocompetent humans.