A Novel <i>Shigella</i> O-Polysaccharide–IpaB Conjugate Vaccine Elicits Robust Antibody Responses and Confers Protection against Multiple <i>Shigella</i> Serotypes

Abstract

<i>Shigella</i> is responsible for high burdens of diarrhea and dysentery globally. Children living in areas of endemicity are the most affected, and currently, there are no licensed vaccines to prevent shigellosis. Vaccine approaches have traditionally targeted the bacterial lipopolysaccharide as a protective antigen. <i>Shigella</i> O-polysaccharide (OPS) conjugated to recombinant Pseudomonas aeruginosa exotoxin A (rEPA) or tetanus toxoid (TT) is advanced in clinical evaluation. Adequate efficacy of these vaccines, particularly in the infant target group, remains to be demonstrated. A major limitation of the OPS-glycoconjugate concept is its limited coverage, since immunity to the O antigen is serotype specific, and there are multiple disease-causing serotypes. Another concern is the use of protein carriers already included in multiple other childhood vaccines. This study reports a novel <i>Shigella</i> OPS conjugate vaccine that uses the <i>Shigella</i> invasion plasmid antigen B (IpaB) as the carrier protein. IpaB is a virulence factor component of the <i>Shigella</i> type III secretion system and highly conserved among <i>Shigella</i> serotypes. It is robustly immunogenic and a protective antigen. IpaB and IpaB containing nonnative amino acids (nnAA) were produced at large scale using cell-free protein synthesis. Incorporation of nnAA enabled site-specific conjugation of IpaB to Shigella flexneri 2a OPS using click chemistry, yielding OPS-IpaB glycoconjugate. Parenteral immunization of mice with the OPS-IpaB vaccine resulted in high levels of OPS- and IpaB-specific serum IgG and robust protection against lethal S. flexneri 2a or Shigella sonnei challenge. The OPS-IpaB vaccine is a promising new vaccine candidate with the capacity to confer broad protection against clinically relevant <i>Shigella</i> serotypes. <b>IMPORTANCE</b> Diarrhea caused by <i>Shigella</i> species results in long-term disability and mortality globally, disproportionally affecting younger children living in poor countries. Although it is treatable by antibiotics, the rapid and widespread emergence of resistant strains and the highly contagious nature of the disease compel the development of preventive tools. Currently, several <i>Shigella</i> OPS conjugate vaccines are being evaluated in clinical studies, but these rely exclusively on immunity against the bacterial O antigen, which limits their coverage to only the immunizing serotype; multivalent vaccines are needed to protect against the most prevalent serotypes. This is the first report of a novel <i>Shigella</i> OPS-conjugate vaccine that uses <i>Shigella</i> IpaB as a carrier and protective antigen. This vaccine, administered parenterally, elicited robust immunity and protected mice against lethal infection by S. flexneri 2a or S. sonnei. The OPS-IpaB vaccine is a promising candidate for evaluation in vulnerable populations.