Development of a glycoconjugate vaccine to prevent invasive Salmonella Typhimurium infections in sub-Saharan Africa

Scott M Baliban,Jin Y Wang,Surekha Shridhar,Myron M Levine,James E Galen,Nicolas Hegerle,Ellen E Higginson,Girish Ramachandran,Mingjun Yang,Brittany Curtis,Andrew Lees,Alexander D Mackerell,Kristen M Varney,Rachel S Laufer,John Van Druff,Raphael Simon,Sharon M Tennant,Mathieu Picardeau

doi:10.1371/journal.pntd.0005493

Abstract

Invasive infections associated with non-typhoidal Salmonella (NTS) serovars Enteritidis (SE), Typhimurium (STm) and monophasic variant 1,4,[5],12:i:- are a major health problem in infants and young children in sub-Saharan Africa, and currently, there are no approved human NTS vaccines. NTS O-polysaccharides and flagellin proteins are protective antigens in animal models of invasive NTS infection. Conjugates of SE core and O-polysaccharide (COPS) chemically linked to SE flagellin have enhanced the anti-COPS immune response and protected mice against fatal challenge with a Malian SE blood isolate. We report herein the development of a STm glycoconjugate vaccine comprised of STm COPS conjugated to the homologous serovar phase 1 flagellin protein (FliC) with assessment of the role of COPS O-acetyls for functional immunity. Sun-type COPS conjugates linked through the polysaccharide reducing end to FliC were more immunogenic and protective in mice challenged with a Malian STm blood isolate than multipoint lattice conjugates (>95% vaccine efficacy [VE] versus 30–43% VE). Immunization with de-O-acetylated STm-COPS conjugated to CRM197 provided significant but reduced protection against STm challenge compared to mice immunized with native STm-COPS:CRM197 (63–74% VE versus 100% VE). Although OPS O-acetyls were highly immunogenic, post-vaccination sera that contained various O-acetyl epitope-specific antibody profiles displayed similar in vitro bactericidal activity when equivalent titers of anti-COPS IgG were assayed. In-silico molecular modeling further indicated that STm OPS forms a single dominant conformation, irrespective of O-acetylation, in which O-acetyls extend outward and are highly solvent exposed. These preclinical results establish important quality attributes for an STm vaccine that could be co-formulated with an SE-COPS:FliC glycoconjugate as a bivalent NTS vaccine for use in sub-Saharan Africa.

Highlights

Non-typhoidal Salmonella (NTS) are important human pathogens worldwide where infection in healthy adults normally results in self-limiting gastroenteritis
We previously reported that conjugates of serovars Enteritidis (SE) lipopolysaccharide-derived core and O-polysaccharide (COPS) and the homologous serovar flagellin protein were effective vaccines against fatal infection in mice with a Malian SE invasive clinical isolate
We document the development of a promising S. Typhimurium (STm) vaccine candidate consisting of STm COPS conjugated to STm phase 1 flagellin (FliC)

Summary

Introduction

Non-typhoidal Salmonella (NTS) are important human pathogens worldwide where infection in healthy adults normally results in self-limiting gastroenteritis. They can cause fulminant invasive disease (e.g., bacteremia, septicemia, meningitis), in hosts with immunological immaturity, immunosenescence, or immunosuppression [1]. In sub-Saharan Africa, hospital-based surveillance of pediatric patients admitted with fever or suspect focal bacterial infections (e.g., meningitis) revealed invasive NTS (iNTS) to be the major pathogen in infants and toddlers following Haemophilus influenzae type b (Hib) and Streptococcus pneumoniae (prior to implementation of Hib and pneumococcal conjugate vaccines) [2, 3]. Genomic sequencing of African STm isolates revealed the emergence and spread of a dominant rare multi-locus sequence type (MLST) 313 (rather than MLST 19 prevalent elsewhere globally). Paratyphi A [4, 5]

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