Clearance of Pseudomonas aeruginosa from the murine gastrointestinal tract is effectively mediated by O-antigen-specific circulating antibodies.

The colonization of mucosal surfaces by Pseudomonas aeruginosa can lead to local or disseminated disease. Secretory immunoglobulin A (IgA) has been assumed to be responsible for preventing mucosal colonization by interfering with the binding of bacterial ligands to epithelial surface receptors. However, the efficacy of this mechanism of immunity derives little actual support from in vivo experiments. In an investigation of the role of local and systemic immunization strategies in reducing colonization of the gastrointestinal tract of mice by P. aeruginosa, the bacterial antigens that were potential targets for immune effectors promoting mucosal clearance were identified. Levels of gastrointestinal colonization were reduced when immunity to homologous O antigens, but not that to pili or flagella, was elicited. Oral vaccination with attenuated Salmonella typhimurium expressing P. aeruginosa serogroup O11 antigen elicited mucosal and serum IgA antibodies and serum IgG antibodies specific for the recombinant antigen. Oral challenge of immunized mice with P. aeruginosa serogroup O11 demonstrated protection against gastrointestinal colonization. Intraperitoneal immunization with a serogroup O11 high-molecular-weight O-polysaccharide antigen elicited only serum IgG and IgM antibodies yet was as effective as oral vaccination in protecting mice against gastrointestinal colonization. This finding was confirmed by the demonstration that intraperitoneal immunization with purified lipopolysaccharide was also protective against mucosal surface colonization. These results call into question the need for local immune effectors, particularly secretory IgA, directed at bacterial ligands for epithelial surface components, in protecting a mucosal surface from bacterial challenge.