As recently as the last decade, a novel mechanism of serum resistance by Gram-negative bacteria such as Salmonella enterica, Pseudomonas aeruginosa and Escherichia coli has been discovered in humans1,2,3. This serum resistance is paradoxically mediated by specific antibodies, that instead of promoting killing, inhibit normal complement-mediated bacterial lysis. The antibodies implicated are of the IgG2 subtype and specific for the O-antigen of lipopolysaccharide where they are hypothesised to bind and block access of complement proteins to the cell surface2. The presence of these ‘inhibitory’ antibodies have been implicated with chronic septicaemia and worsened disease outcomes. Despite their prevalence in a broad range of Gram-negative infections, the isotype, target, and mechanism of these antibodies remain poorly understood.
To overcome these lapses in understanding, here we test a preliminary mouse model that attempts to induce inhibitory antibodies by immunisation of purified O-antigen. Mice immunised with purified P. aeruginosa serotypes O1, O3, O11, E. coli serotype O16 and S. Typhimurium serotype O4 antigens induced broad IgM, IgG, IgG1, IgG2b and IgG2c responses. Further, purified IgG targeting P. aeruginosa O11 and E. coli O16 antigens were able to inhibit complement-mediated killing in vitro in a titre- and affinity-dependent manner consistent with human inhibitory antibodies. Subsequently, the antibodies induced from S. Typhimurium O4 immunisation were characterised using two models of acute and chronic Salmonella infection. Despite the presence of these antibodies being counter-intuitive for serum-mediated protection in humans, their presence was found to enhance bacterial clearance in acute yet not chronic infections of S. Typhimurium.