<i>Salmonella</i>Paratyphi A Outer Membrane Vesicles Displaying Vi Polysaccharide as a Multivalent Vaccine against Enteric Fever

Abstract

Typhoid and paratyphoid fevers have a high incidence worldwide and coexist in many geographical areas, especially in low-middle-income countries (LMIC) in South and Southeast Asia. There is extensive consensus on the urgent need for better and affordable vaccines against systemic <i>Salmonella</i> infections. Generalized modules for membrane antigens (GMMA), outer membrane exosomes shed by <i>Salmonella</i> bacteria genetically manipulated to increase blebbing, resemble the bacterial surface where protective antigens are displayed in their native environment. Here, we engineered <i>S</i> Paratyphi A using the pDC5-<i>viaB</i> plasmid to generate GMMA displaying the heterologous <i>S</i> Typhi Vi antigen together with the homologous O:2 O antigen. The presence of both Vi and O:2 was confirmed by flow cytometry on bacterial cells, and their amount was quantified on the resulting vesicles through a panel of analytical methods. When tested in mice, such GMMA induced a strong antibody response against both Vi and O:2, and these antibodies were functional in a serum bactericidal assay. Our approach yielded a bivalent vaccine candidate able to induce immune responses against different <i>Salmonella</i> serovars, which could benefit LMIC residents and travelers.