Evaluation of the Safety, Genetic Stability, and Immunogenicity of an Attenuated <i>Shigella</i> Live Vector Expressing Enterotoxigenic <i>Escherichia coli</i> Antigens, Vaccine Strain CVD 1208S-122

BACKGROUND: There are no licensed vaccines to protect against Shigella or enterotoxigenic Escherichia coli (ETEC), yet they are among the most common causes of bacterial diarrhea. We engineered an attenuated S. flexneri 2a expressing CFA/I and LTB from ETEC and conducted a first-in-human randomized-controlled trial of ascending doses of the vaccine (CVD 1208S-122). METHODS: Dose-escalating cohorts received a single oral dose of 108-1010 colony forming units (CFU) of CVD 1208S-122 or placebo. A fourth cohort received one or two oral doses of 108 CFU CVD 1208S-122 or placebo. We measured the safety and clinical acceptability of the vaccine, genetic stability of fecally-shed vaccine organisms, and immune responses elicited. RESULTS: The 108 CFU doses were well-tolerated (100%, 6/6). One recipient of 109 CFU (20%, 1/5) manifested diarrhea and 4 individuals who ingested 1010 CFU had either fever or diarrheal reactogenicity (67%, 4/6). Fecal shed vaccine organisms were genetically stable. Responses were observed to the LPS and IpaB of Shigella and CFA/I fimbriae and LTB of ETEC in serum, among antibody secreting cells, antibody in lymphocyte supernatant, and fecal samples. Serum antibodies exhibited Shigella bactericidal activity and inhibition of ETEC adherence in vitro. CONCLUSIONS: This study demonstrates safety with low-dose but unacceptable reactogenicity following highest-dose ingestion, genetic stability of all recovered fecal isolates, and immunogenicity of a prototype combined monovalent Shigella-ETEC vaccine. These results encourage expansion to a multivalent formulation to elicit broader protection against the most prevalent Shigella serotypes and ETEC CFA types that cause diarrheal disease.