Andrew F. Trofa

Safety, immunogenicity, and efficacy of live attenuated Vibrio cholerae 0139 vaccine prototype

New vaccines are needed to prevent cholera caused by Vibrio cholerae O139. Attenuated V cholerae O139 vaccines were made by deleting multiple copies of the cholera-toxin genetic element from two virulent strains of the organism, MO10 and AI4456. The deletion mutants were further modified by insertion of a construct that encoded the B subunit of cholera toxin, thus generating strains Bengal-3 and VRI-16. A stable spontaneous non-motile derivative of Bengal-3 was isolated and designated Bengal-15; VRI-16 is naturally non-motile. Bengal-3, Bengal-15, and VRI-16 were evaluated as oral single-dose cholera vaccine candidates in 4 volunteers each, and MO10 was given to 3 volunteers. 1 of 4 volunteers who received Bengal-3 and all 3 who received MO10 had diarrhoea. VRI-16 caused no significant symptoms but was not immunogenic. Bengal-15 produced few symptoms and was nearly as immunogenic as MO10. Subsequently, Bengal-15 was given to 10 volunteers at a dose of 10(8) colony-forming units. No volunteers had diarrhoea, and other subjective symptoms were as common in vaccinees as in 3 buffer recipients. 1 month after vaccination, 7 vaccinees, the 3 buffer recipients, and 3 unimmunised subjects were challenged with 5 x 10(6) colony-forming units of V cholerae O139. 5 of 6 controls had cholera-like diarrhoea. By contrast, 1 of 7 vaccinees had diarrhoea, which was mild and had a long incubation period. Vaccine protective efficacy was 83%. Our results indicate the Bengal-15 is a safe live attenuated vaccine candidate for cholera caused by the O139 serogroup.

Outpatient studies of the safety and immunogenicity of an auxotrophic Escherichia coli K-12-Shigella flexneri 2a hybrid vaccine candidate, EcSf2a-2

A phase II study was conducted in 244 volunteers at Fort Ord, CA, to determine the safety and immunogenicity of EcSf2a-2, a live, oral Shigella vaccine constructed by transfer of genes from Shigella flexneri to Escherichia coli K-12. In this placebo-controlled study, four doses of vaccine ranging from 2.3 to 9.0 x 10(8) colony-forming units were given on days 0, 3, 14 and 17. Vaccine shedding occurred from 1 to 3 days after each dose. The vaccine was well tolerated at every dose tested. Significant levels of IgA, IgG or IgM antibody-secreting cells (ASC) recognizing S. flexneri 2a lipopolysaccharide (LPS) were found in 94% of a volunteer subset tested 7 days after the first dose of EcSf2a-2. Seven days after the third dose, ASC were detected less often (57%), and were mainly IgA. Significant rises in serum antibody to LPS were detected in 37% of vaccine recipients.

A randomized, placebo-controlled study of oral cimetidine as an immunopotentiator of parenteral immunization with a group B meningococcal vaccine

Cimetidine (CIM) is an H2-receptor antagonist with a long history of clinical use in peptic ulcer disease. In addition to its inhibitory effect upon gastric acid secretion, CIM can also block histamine-mediated immunosuppression by inhibiting H2 receptors on suppressor T cells. CIM results in immunoaugmentation of both cellular and humoral immunity by this mechanism and has been used clinically in the treatment of chronic infectious and neoplastic diseases. We postulated that orally administered CIM, like an adjuvant, could augment the immunologic response to a parenteral vaccine. To test this hypothesis, a randomized placebo (PLB)-controlled, double-blinded study in 14 healthy volunteers was performed using a Group B meningococcal outer membrane protein (OMP) vaccine administered twice, 6 weeks apart. Volunteers were randomized within pairs defined by their screening OMP antibody titers to receive either CIM or PLB which was administered for 5 days, beginning 2 days before each of the two immunizations. All 14 volunteers completed the study with excellent compliance. Sera were tested for anti-OMP and bactericidal antibodies. The groups were comparable in terms of gender distribution, age and baseline anti-OMP titers. Reactogenicity to the vaccine was mild and comparable between groups. There was little effect of CIM (over PLB) on anti-OMP or functional bactericidal antibody levels over time. Geometric means of maximum OMP antibody increase over baseline was 3.3-fold (95% CI: 1.8-6.3) for CIM and 2.4 for PLB (CI: 1.6-3.7). CIM had a corresponding 3.9-fold increase (CI: 1.9-8.3) in bactericidal antibody level compared to 2.2 for PLB (CI: 1.4-3.4). We conclude that oral CIM was not effective as an immunopotentiator of immunization with this group B meningococcal vaccine.

Vaccines for Prevention of Enteric Bacterial Infections Caused by Salmonellae

Two clinical trials of the Vi vaccine provided evidence that serum antibodies to the capsular polysaccharide of Salmonella typhi confer protective immunity to typhoid fever. The immunogenicity of the Vi has been improved by binding this polysaccharide to a protein. in laboratory animals, this conjugate had increased immunogenicity compared to the Vi alone and elicited booster responses. Because there are as yet no vaccines for non-typhoidal Salmonellae, we synthesized a non-toxic conjugate of the O-SP of Salmonella typhimurium O:4,12 which conferred protective immunity in the murine model at doses and routes of immunization which are clinically acceptable. The rationale for developing conjugate vaccines for prevention of other enteric bacterial infections is discussed.