Vi capsular polysaccharide-protein conjugates for prevention of typhoid fever. Preparation, characterization, and immunogenicity in laboratory animals.

Abstract

The Vi has proven to be a protective antigen in two double masked, controlled clinical trials in areas with high rates of typhoid fever (approximately 1% per annum). In both studies the protective efficacy of the Vi was approximately 70%. Approximately 75% of subjects in these areas responded with a fourfold or greater rise of serum Vi antibodies. In contrast, the Vi elicited a fourfold or greater rise in 95-100% of young adults in France and the United States. Methods were devised, therefore, to synthesize Vi-protein conjugates in order to both enhance the antibody response and confer T-dependent properties to the Vi (and theoretically increase its protective action in populations at high risk for typhoid fever). We settled on a method that used the heterobifunctional crosslinking reagent, N-succinimidyl-3-(2-pyridyldithio)-propionate (SPDP), to bind thiol derivatives of the Vi to proteins. This synthetic scheme was reproducible, provided high yields of Vi-protein conjugates, and was applicable to several medically relevant proteins such as diphtheria and tetanus toxoids. The resultant conjugates were more immunogenic in mice and juvenile Rhesus monkeys than the Vi alone. In contrast to the T-independent properties of the Vi, conjugates of this polysaccharide with several medically relevant proteins induced booster responses in mice and in juvenile Rhesus monkeys. Clinical studies with Vi-protein conjugates are planned. This scheme is also applicable to synthesize protein conjugates with other polysaccharides that have carboxyl functions.

Immune response to Vi polysaccharide, heat-killed whole cells, and outer membrane protein of Salmonella typhi

“…However, the response to these doses was nearly similar without significant difference, in agreement with the findings of Szu et al [29], who reported a non-significant difference in responses using different dose levels of Vi antigen. In general, the responses to 4 µg ViCPS, injected via SC or IP, were relatively higher when compared to the lower doses justified by immunogenicity studies which linked the Oacetylation to the antibody response, as O-acetyls are important in the binding of Vi to antibodies [30].…”

Vaccines based on the cell surface carbohydrates of pathogenic bacteria

“…The polysaccharides from Salmonella typhi and Staphylococcus aureus, in contrast, have few free hydroxyl groups and are extremely resistant to hydrolysis. These polysaccharides have been activated by the addition of a bifunctional reagent to the uronic acid carboxyl group, which allows conjugation to a suitably activated carrier protein (Szu et al 1987, Fattom et al 1988. Treatment of a polysaccharide with cyanogen bromide, or a cyanogen bromide analogue such as CDAP (Shafer et al 2000), results in random activation of hydroxyl groups to which a bifunctional linker, such as adipic acid dihydrazide can be attached (Shafer et al 2000).…”

Physical and Chemical Characterization and Immunologic Properties ofSalmonella entericaSerovar Typhi Capsular Polysaccharide-Diphtheria Toxoid Conjugates

“…The concentration of serum IgG anti-Vi is correlated with immunity to the pathogen (22,25,26,28,36,38,49). However, Vi is not suitable for routine immunization of infants and young children because of its age-related immunogenicity and T-cell independence.…”

“…There was no statistical difference among conjugates after the 1st and 2nd injections (P Ͼ 0.083 and P Ͼ 0.69, respectively). (7,8,23,(48)(49)(50). Within the range of dosages studied here, the higher the dosage, the higher the level of anti-Vi elicited.…”