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Protein and Polysaccharide Conjugate Vaccines to Enteric Diseases

Szu, Shousun
DHHS/NIH - National Institute of Child Health and Human Development
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The capsular polysaccharide of S. typhi (Vi) is a licensed vaccine but with limited efficacy in children less than 5 years old. To provide a vaccine for younger children, Vi was conjugated to recombinant exoprotein A of Pseudomonas aeruginosa (rEPA). An efficacy of 89% at 47 months was shown in 2-to-5-year olds injected with Vi-rEPA. When administered concurrently with routine vaccines in the Expanded Program of Immunization (EPI) at 2, 4, 6 and 12 months, Vi-rEPA showed to be safe, compatible with EPI immunization and 95% infants had higher than the estimated protective antibody level after the last injection; thus Vi-rEPA is suitable for routine immunization of infants.

In collaboration with the International Vaccine Institute, Korea and the Novartis Institute for Global Health, Italy, Vi was conjugated to licensed vaccines such as diphtheria toxoid or its mutant CRM197. In mice, these conjugates elicited similar IgG anti-Vi levels to those induced by Vi-rEPA. In collaboration with the Lanzhou Institute of Biologics, China, a high tittered human plasma pool from volunteers immunized with Vi-rEPA was purified and the level of IgG anti-Vi determined in weight units. It will be available for distribution globally as a human IgG anti-Vi reference standard.

In collaboration with scientists at Children's Hospital, Harvard University (Drs. Lu, Malley and Anderson), we explore the potential of a bivalent pediatric vaccine composed of Vi and a pneumococcal protein.In contrast to S. typhi, there is no licensed vaccine for non-typhoidal Salmonella. Salmonella group D and B are the most common enteric infections in developed countries. In African both sero groups are also the most common Salmonella causing bacterimia. Salmonella paratyphi A infection is more common than S. typhi in China after the routine Vi vaccination.

We demonstrated in Vietnam that LPS based S. paratyphi A conjugate was safe and immunogenic in children 2-5 years old. Similar constructs for group D and B Salmonella also showed to be immunogenic in mice. These projects are under investigation in our laboratory. Vibrio cholerae O1 remains a major health problem in developing countries; especially in the Indian subcontinent and in Africa. A cholera outbreak started in Haiti in October 2010. Up to July 2012, there were more than 600,000 cases and 8,000 deaths. No vaccine is available to control the spread.

Field studies showed that serum vibriocidal activity is directed toward its LPS. In a phase 1 trial, V. cholerae O-SP conjugates elicited IgG anti-LPS with vibriocidal activity. A hexamer, octamer and decamer corresponding to the O-SP were chemically synthesized and conjugated to tetanus toxoid (TT). In mice these conjugates elicited higher vibriocidal activities than the native O-SP conjugate used in our Phase I study. Conjugates formed by decamer elicited highest antibody levels than hexamer or octamer conjugates. Various linkers were studied. A conjugate synthesized with a heptadecamer linker was more immunogenic than the one with a nonamer linker. Linkers with various hydrocarbon or hydroxyl chains were compared for efficiency, stability, and solubility. In collaboration with Novartis Foundation for Global Health, the synthetic hexa and deca saccharide will be linked to clinical lot TT for a phase I study.Rotavirus is the most common cause of infantile diarrhea worldwide.

Two licensed oral rotavirus vaccines conferred limited protection, continued showing cases of the serious adverse reaction of intussusception, and some lots were contaminated with porcine circovirus 1 and 2 DNA. There was also reports on in vivo genetic reversion to virulent strains in infants after vaccination. In collaboration with Drs. Kapikian, Hoshino and Wen at NIAID, we developed a parenteral vaccine based on capsid proteins. Recombinant capsid proteins with truncated C or N termini were expressed in E. coli and elicited neutralizing antibodies in mice and guinea pigs. Conjugation of the recombinant proteins to a polysaccharide vaccine improved protein solubility. The double truncated (both C and N terminals) core region of the capsid protein provided a high yield antigen that elicited neutralizing antibody to the P4, P6 and P8 serotypes in the plaque-forming assay. To improve immunogenicity, conjugates of capsid proteins and VP7 based peptides will be prepared. Enterohemorrahagic E. coli (EHEC) infections are the leading cause of E. coli-related deaths in developed countries. In the US, the prevalent serotype is O157H:7. An outbreak in Germany and neighboring countries in 2011 was caused by E. coli O104.

Both of these EHEC strains contain the Shiga toxin (Stx) gene and the released toxin can cause serious manifestations; from diarrhea, hemorrhagic colitis, to hemolytic uremic syndrome (HUS) and thrombotic thrombocytopenic purpura. HUS is a major cause of acute and chronic kidney damage in young children and could lead to death. In a phase II study in 2-5 years old children in the US, an O157 LPS based conjugate vaccine elicited a >10 fold rise of antibodies with bactericidal activity in 98% of children. Most EHEC infections are caused by strains secreting Stx2. The non-toxic B-subunit of Stx2 was purified in high yield by an improved recombinant clone. Stx2B could serve as a carrier protein for the E. coli O157 O-SP and provide a broader coverage of non-O157 EHEC infections.

Funding Source
Nat'l. Inst. of Child Health and Human Devel.
Project source
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Bacterial Pathogens
Escherichia coli
Meat, Poultry, Game