Monoclonal Antibodies of a Diverse Isotype Induced by an O-Antigen Glycoconjugate Vaccine MediateIn VitroandIn VivoKilling of African Invasive Nontyphoidal Salmonella

NontyphoidalSalmonella(NTS), particularlySalmonella entericaserovars Typhimurium and Enteritidis, is responsible for a major global burden of invasive disease with high associated case-fatality rates. We recently reported the development of a candidate O-antigen–CRM197glycoconjugate vaccine againstS.Typhimurium. Here, using a panel of mouse monoclonal antibodies generated by the vaccine, we examined the relative efficiency of different antibody isotypes specific for the O:4 antigen ofS. Typhimurium to effectin vitroandin vivokilling of the invasive AfricanS. Typhimurium strain D23580. All O:4-specific antibody isotypes could mediate cell-free killing and phagocytosis ofS. Typhimurium by mouse blood cells. Opsonization ofSalmonellawith O:4-specific IgA, IgG1, IgG2a, and IgG2b, but not IgM, resulted in cell-dependent bacterial killing. At high concentrations, O:4-specific antibodies inhibited both cell-free complement-mediated and cell-dependent opsonophagocytic killing ofS. Typhimuriumin vitro. Using passive immunization in mice, the O:4-specific antibodies providedin vivofunctional activity by decreasing the bacterial load in the blood and tissues, with IgG2a and IgG2b being the most effective isotypes. In conclusion, an O-antigen–CRM197glycoconjugate vaccine can induce O-antigen-specific antibodies of different isotypes that exertin vitroandin vivokilling ofS. Typhimurium.