Cross-reactivity of Shigella flexneri serotype 2a O antigen antibodies following immunization or infection.

To study the cross-reactivity pattern of Shigella flexneri 2a O-antigen antibodies, sera from humans and monkeys challenged with S. flexneri 2a, and from humans and guinea pigs immunized with a recombinant vaccine expressing serotype 2a O-antigen, were tested against a panel of lipopolysaccharide extracted from heterologous S. flexneri. Sera from the two groups of humans, who were volunteers in either a clinical challenge or vaccination study, showed similar patterns: cross-reactivity was more often seen with IgA antibodies, and these were mostly cross-reactive with serotype 2b, which shares the type II antigen, and serotypes 1a, 5a, and Y, which share 4 or 3, 4 group antigen, with 2a. The majority of sera from immunized guinea pigs showed both IgG and IgA cross-reactivity with 1a, 5a, and Y, but not 2b. The majority of sera from challenged monkeys showed cross-reactivity with almost all flexneri serotypes tested, with 1a, 2b, and Y being recognized most often, and the cross-reactive antibodies were more often IgG than IgA. These results show that either immunization or challenge with the 2a serotype induces cross-reactive antibodies which recognize similar subsets of heterologous serotypes, and suggest that it may be possible to design multivalent vaccines against S. flexneri.

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.

Local immune response and protection in the guinea pig keratoconjunctivitis model following immunization with Shigella vaccines.

This study used the guinea pig keratoconjunctivitis model to examine the importance of route of administration (mucosal versus parenteral), frequency and timing of immunization (primary versus boosting immunization), and form of antigen given (live attenuated vaccine strain versus O-antigen-protein conjugate) on the production of protective immunity against Shigella infection. Since local immune response to the lipopolysaccharide (LPS) O-antigen of Shigella spp. is thought to be important for protection against disease, O-antigen-specific antibody-secreting cells (ASC) in the spleen and regional lymph nodes of immunized animals were measured by using an ELISPOT assay. Results indicated that protective efficacy was associated with a strong O-antigen-specific ASC response, particularly in the superficial ventral cervical lymph nodes draining the conjunctivae. In naive animals, a strong ASC response in the cervical lymph nodes and protection against challenge were detected only in animals that received a mucosal immunization. Protection in these animals was increased by a boosting mucosal immunization. While parenteral immunization alone with an O-antigen-protein conjugate vaccine did not protect naive animals against challenge, a combined parenteral-mucosal regimen elicited enhanced protection without the addition of a boosting immunization. Although O-antigen-specific serum immunoglobulin A titers were significantly higher in animals receiving a mucosal immunization, there was no apparent correlation between levels of serum antibody and protection against disease.