• Vaccine Name: S. flexneri 2a strain CVD 1207
• Target Pathogen: Shigella
• Target Disease: Shigellosis
• Vaccine Ontology ID: VO_0000677
• Type: Recombinant vector vaccine
• IcsA/VirG gene engineering:
  • Type: Recombinant protein preparation
  • Description: CVD 1207 was constructed from wild-type S. flexneri 2a strain 2457T by a series of double homologous recombinations using suicide plasmid deletion cassette technology as described in detail elsewhere. In brief, a specific, in-frame deletion mutation in the guaBA operon was first introduced, followed by a second in-frame deletion mutation in the plasmid virulence gene virG. The chromosomal mutation set was accomplished with
    deletion of 85% of subunit A of set. Finally, a sen cassette was constructed by fusing two 700-bp segments that include the N and C termini of sen minus 300 bp corresponding to the putative active site in the N-terminal region. The ars operon, conferring resistance to arsenite, was cloned into the sen locus to allow facile transfer of the double-deletion mutation (virG and sen) virulence plasmid to candidate Shigella vaccine strains and as a marker to distinguish CVD 1207 in the field. As previously described, CVD 1207 does not grow in minimum medium unless supplemented with guanine. The lack of enterotoxic activity has been confirmed in Ussing chambers. CVD 1207 is significantly less invasive for HeLa cells than its wild-type parent strain 2457T (approximately 1 log unit fewer intracellular CFU detected) but does not differ from its single-mutant strain progenitor guaBA CVD 1204 (unpublished observations). CVD 1207 undergoes fewer intracellular generations in HeLa cells than either CVD 204 (10-fold; 4.5 doublings in 4 h) or 2457T (30-fold; 5 doublings in 4 h) (unpublished observations) (Kotloff et al., 2000).
  • Detailed Gene Information: Click Here.
• Preparation: Shigella flexneri strain CVD 1207 carries deletions of the plasmid gene virG (also known as icsA), which encodes a protein responsible for cell-to-cell spread of Shigella in the intestinal epithelium; (ii) the chromosomal gene set encoding Shigella enterotoxin 1 (ShET1), which is present almost exclusively in S. flexneri 2a; (iii) the plasmid gene sen, encoding Shigella enterotoxin 2 (ShET2), which is present in virtually all serotypes of Shigella; and (iv) the guaBA chromosomal operon that regulates synthesis of IMP dehydrogenase (encoded by guaB) and GMP synthetase (encoded by guaA), two enzymes employed in the distal de novo purine biosynthesis pathway. CVD 1207 thus expresses type-specific O-polysaccharide and invades epithelial cells (albeit less competently than the wild type) but undergoes only limited intracellular proliferation and intercellular spread and has no detectable enterotoxic activity (Kotloff et al., 2000).
• Virulence: Protective efficacy against shigellosis following rechallenge was 70% (Kotloff et al., 2000).

Human Response

• Vaccination Protocol: Groups of 3 to 7 outpatient volunteers were assigned, in an incremental fashion, to receive a single oral dose of CVD 1207 at a desired inoculum (the actual inocula administered are in parentheses) of either 10^6,10^7, 10^8, 10^9, or 10^10 CFU. Fasting volunteers swallowed the vaccine suspended in a solution of NaHCO3 buffer, as previously described (Kotloff et al., 2000).
• Persistence: Fecal excretion of the vaccine strain in the volunteer’s stools was measured on days 1, 2, 3, 7, 10, 14, and 21 after ingestion of the vaccine. The duration of excretion was 1 to 3 days except for two recipients of ca. 10^9 CFU, who each had one additional positive stool culture 2 weeks after vaccination (Kotloff et al., 2000).
• Immune Response: A dose-related, immunoglobulin A antibody-secreting cell (ASC) response to S. flexneri 2a O-specific lipopolysaccharide was seen, with geometric mean peak values of 6.1 to 35.2 ASCs/10^6 peripheral blood mononuclear cells (PBMC) among recipients of 10^7 to 10^10 CFU. The cytokine response to Shigella-specific antigens observed in volunteers’ PBMC following vaccination suggested a Th1 pattern with stimulation of gamma interferon and absence of interleukin 4 (IL-4) or interleukin 5(Kotloff et al., 2000).
• Side Effects: Some test subjects experienced diarrhea and vomiting. No subjects experienced fever or dysentery (Kotloff et al., 2000).