Development of a Novel Shigella Quadrivalent Conjugate Vaccine Using Shigella O-Polysaccharide and IpaB Carrier Protein

<b>Background/Objectives:</b> <i>Shigella</i> is the leading bacterial cause of diarrheal disease worldwide. Although multiple vaccine candidates are under development and in clinical trials, no <i>Shigella</i> vaccine is currently available on the market. <i>Shigella</i> comprises four species: <i>S. dysenteriae</i>, <i>S. flexneri</i>, <i>S. boydii</i>, and <i>S. sonnei</i>. <i>S. flexneri</i> has been recognized as the most prevalent species, particularly in low- and middle-income countries (LMICs), and the top serotypes are <i>S. flexneri 2a</i>, <i>3a</i> and <i>6</i>. Conversely, <i>S. sonnei</i> has a single serotype and predominates in high-income countries (HICs). Invasion plasmid antigen B (IpaB) is a critical virulence factor of <i>Shigella</i> type III secretion system (T3SS) that is highly conserved across <i>Shigella</i> serotypes. Here, we report the development of a <i>Shigella</i> quadrivalent O-polysaccharide-IpaB conjugate vaccine candidate (IVT Shigella-04). <b>Methods:</b> IVT Shigella-04 contains O-polysaccharides (O-PS) from <i>S. flexneri</i> 2a, 3a, 6, and <i>S. sonnei</i>, each individually conjugated to recombinantly expressed IpaB as the carrier protein using 1-cyano-4-dimethylaminopyridinium tetrafluoroborate (CDAP) chemistry. The immunogenicity of IVT Shigella-04 was evaluated in a rabbit immunization model. <b>Results:</b> Baseline (day 0) IgG concentrations were low for all four <i>Shigella</i> serotypes (<0.5 µg/mL). Following two doses on day 0 and day 28 (2.5 µg of each conjugate per dose; total 10 µg), IgG geometric mean concentrations increased significantly (<i>p</i> < 0.001) by day 42, reaching 67.96 µg/mL (2a), 91.56 µg/mL (3a), 371.31 µg/mL (6), and 11.00 µg/mL (<i>sonnei</i>). Consistently, serum bactericidal activity (SBA) at day 42 increased 13-fold (2a), 34-fold (3a), 63-fold (6), and 224-fold (<i>sonnei</i>) relative to baseline (day 0). <b>Conclusions:</b> IVT Shigella-04 elicited robust serotype-specific humoral and functional immune responses in preclinical models, supporting its further development toward clinical evaluation.