Safety and immunogenicity in humans of an attenuated Salmonella typhi vaccine vector strain expressing plasmid-encoded hepatitis B antigens stabilized by the Asd-balanced lethal vector system

Abstract

Salmonella typhi organisms which express genes encoding protective antigens of other pathogens have been developed for use as experimental oral vaccines. A ⌬asd S. typhi strain attenuated by deletions in cya, crp, and cdt which contains hepatitis B core (HBc) and pre-S genes encoded on an Asd ؉ pBR-based plasmid vector was constructed. Healthy adult volunteers ingested a single dose of 5 ؋ 10 5 to 5 ؋ 10 8 CFU of strain 4073 (⌬cya ⌬crp ⌬cdt S. typhi Ty2), 6 ؋ 10 7 or 1 ؋ 10 9 CFU of strain 4632(pYA3149), a further derivative of 4073 deleted in asd and containing the Asd ؉ vector without the HBc-pre-S fusion, or 3 ؋ 10 7 or 7 ؋ 10 8 CFU of strain 4632(pYA3167), a derivative containing the vector with the HBc-pre-S fusion. 4073 was generally well tolerated by 22 volunteers. No volunteer had fever or positive blood cultures; 4 of 22 volunteers shed vaccine organisms in the stool in the first 48 h only. Two of 18 volunteers who received one of the plasmid-containing derivatives of 4073 developed low-grade fevers on day 10 or 12 after ingestion. One of these volunteers had positive blood cultures on days 7 and 8. Seven of these 18 volunteers had vaccine organisms detected in their stools in the first 48 h only. Most volunteers developed S. typhi-specific serum responses and developed S. typhi-specific antibody-secreting cells. However, no volunteer developed serum antibody to hepatitis pre-S or pre-S-specific antibody-secreting cells. Although the parent strain 4073 was well tolerated, induced immunoglobulin G seroconversion to S. typhi lipopolysaccharide in 80 to 100% of vaccinees and stimulated specific IgA-secreting lymphocytes in 80 to 100% of vaccinees given a single oral dose of 2 ؋ 10 7 and 5 ؋ 10 8 CFU, 4073 derivatives containing the Asd ؉ vector with and without sequences encoding the HBc-pre-S fusion caused occasional febrile reactions at high doses and did not stimulate detectable immune responses to hepatitis B antigens.

Evidence-Based Biosafety: a Review of the Principles and Effectiveness of Microbiological Containment Measures

“…When developing vector vaccines, the challenge is to develop strains that are well tolerated by the recipient host and no longer persist in the environment, yet still induce protective immune responses. This is not always achieved, however (32,141). Elucidation of biosynthetic pathways has led to the development of Salmonella vectors that were attenuated by disruption of genes encoding metabolic functions or genes located in a pathogenicity island (Salmonella pathogenicity island 2, which encodes a type III secretion system).…”

“…It is evident that strains carrying different mutations differ in properties such as invasiveness and survival. To reduce the possibility of reversion to virulence, strains carrying at least two attenuating distantly located mutations have been produced (67,141). Salmonella enterica serovar Typhimurium VNP20009 was developed to deliver potential therapeutic proteins to tumor sites.…”

Functional analysis of Salmonella Typhi adaptation to survival in water

“…Bacteria and mutant construction S. Typhi BRD948 is a derivative of strain Ty2 (Tacket et al, 1997). The isolate was routinely cultured in Luria Bertani (LB) Broth supplemented with 0.004% phenylalanine, 0.004% tryptophan, 0.001% para-aminobenzoic acid, 0.001% dihydrobenzoic acid and 0.004% tyrosine (aro-mix).…”

Antigen Delivery System II

“…Vaccine strains with this modification were highly immunogenic. These considerations are likely important for S. Typhi and S. Paratyphi A vectored vaccines in humans since mild diarrhea has been observed in some subjects in some trials (Tacket et al, 1997).…”