Analysis of Reactivity and Functionality of Serum Antibodies From Monovalent and Multivalent Shigella Bioconjugate Vaccinated Rabbits Against Shigella Isolates From Kenya
ODUNDO , ELIZABETH AWINO
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Shigella species is a major cause of bacterial diarrheal mortalities and morbidities worldwide and second in Kenya, especially in children under five years. Antibiotics are needed in treatment of shigellosis however, increasing resistance necessitates other control measures such as Shigella vaccine but none has widely been approved. An immunogenic novel monovalent and multivalent bioconjugate vaccine (linking Shigella O-antigen (LPS) to Exoprotein A of Pseudomonas aeruginosa) against the four major Shigella serotypes responsible for 80% global morbidity; S. flexneri (Sf) 2a, 3a, 6 and S. sonnei is in development. The monovalent and quadrivalent bioconjugate vaccine constructs have been evaluated for immunogenicity in rabbit models and is planned for clinical trial in Kenya. Therefore, it is necessary to further understand the antibody response (reactivity and functionality (BA)) from the monovalent and quadrivalent (adjuvanted 4V-adj) and without an adjuvant (4V) Shigella bioconjugate vaccinated rabbit serum against Shigella isolates from Kenya. This retrospective laboratory-based study, nested within an ongoing parent diarrheal surveillance protocol in Kenya since 2009, was conducted to test the immunized rabbit serum against 129 archived Shigella strains from Kenyan. Specifically, this study assesses the specific reactivity of serum antibodies against the four major Shigella serotypes targeted; cross reactivity of the serum antibodies against Shigella serotypes (Sf 1b, 2b, 4a, 4b, Shigella spp, S. dysenteriae and S. boydii) not targeted by the formulation and the BA of the serum antibodies against a subset of reactive Shigella isolates. The 129 isolates were serotyped and screened for retention of the virulence factor O-antigen LPS by plating on Congo red (CR) culture plate. The specific and cross reactivity of the immunized rabbit serum to the virulent Shigella isolates was determined by colony blot assay. The BA of the serum antibodies against reactive Shigella isolates was assessed by a serum bactericidal assay (SBA) and analyzed using Nist Integrated Colony Enumerator and Opsititre software. Of the 129 Shigella isolates, 109 (85%) retained their virulence; 67 serotypes were targeted by the vaccine while 42 were not-targeted serotypes. From the virulent isolates, a subset of 22 isolates targeted by vaccine Sf 2a (4), 3a (10), 6 (4), S. sonnei form I (4) and 19 isolates not targeted by the vaccine Sf 1b (5), 2b (5), 4a (2), 4b (3), S boydii (2) and S. dysenteriae (3) were assessed for reactivity. Specific reactivity Serum from rabbits immunized with the 4V-Adj and the respective monovalent vaccines against the 22 target serotypes had 100% specific reactivity. The serum from 4V-Adj vaccinated rabbits cross-reacted with Sf 2b (75%), Sf 4a (100%), one Sf 1b (25%) and Sf 4b (33%). Serum from rabbits immunized with the Sf 2a monovalent vaccine cross-reacted with Shigella serotypes; one Sf 3a and three Sf 2b, while the Sf 3a monovalent serum cross-reacted with one Sf 2a. The surviving colonies were enumerated using excel based Nist Integrated Colony Counter and analyzed using the Opsititre software and presented in tables. There was ≥4-fold increase in BA between the pre and post vaccine rabbit sera with all the Shigella serotypes tested except one cross-reactive Sf 3a indicative that, the rabbit’s serum antibodies had functional activity. Collectively, these results demonstrate that the monovalent/ quadrivalent bioconjugate Shigella vaccine may be more broadly protective than designed offering a promising solution to reduced morbidity and mortalities associated with Shigella spp. These results strengthen the vaccine development strategies and lay a solid foundational basis for Shigella bioconjugate vaccine human challenge studies here in Kenya.
- School of Medicine