Binding to and opsonophagocytic activity of O-antigen-specific monoclonal antibodies against encapsulated and nonencapsulated Klebsiella pneumoniae serotype O1 strains

The high mortality of nosocomial infections caused by Klebsiella spp. has a cted as a stimulus to develop immunotherapeutic approaches targeted against surface molecules of these bacteria. Since O-antigen-specific antibodies m ay add to the protective effect of K antisera, we tested the functional and binding capacity of O-antigen-specific monoclonal antibodies (MAbs) raised against different Klebsiella O antigens. The MAbs tested mere specific for the O-polysaccharide partial antigens D-galactan II (MAb Ru-Ol), D-galacta n I (MAb IV/4-5), or core oligosaccharide (MAb V/9-5) of the Klebsiella ser ogroup O1 antigen. In enzyme-linked immunosorbent assay binding experiments , me found that all MAbs recognized their epitopes on intact capsule-free b acteria,, however, binding to encapsulated wild-type strains belonging to d ifferent K-antigen serotypes was significantly reduced. The K2 antigen acte d as the strongest penetration barrier, while the K7 and K21 antigens allow ed some. though diminished, antibody binding. In vitro phagocytic killing e xperiments showed that MAb Ru-O1 possessed significant opsonizing activity for nonencapsulated O1 serogroup strains and also, to a much lesser extent, for encapsulated strains belonging to the O1:K7 and O1:K21 serotypes. MAbs or antisera specific for the D-galactan II antigen may thus be the most pr omising agents for further efforts to develop a second-generation Klebsiell a hyperimmune globulin comprising both K- and O-antigen specificities.