Oral immunization of rabbits with enterotoxigenic Escherichia coli protects against intraintestinal challenge

The development of a successful oral vaccine against enterotoxigenic Escherichia coli depends upon the identification of appropriate protective antigens which can be delivered effectively to intestinal mucosa. We have determined in a modified RITARD model the relative protection against intraintestinal challenge afforded by oral immunization with live enterotoxigenic E. coli carrying different candidate antigens. Studies were done with both wild-type strains and genetically manipulated strains of enterotoxigenic E. coli (parent strain E1392/75 2A) which carried plasmids containing intact heat-labile toxin (LT) gene sequences or various mutations of the LT genes. Immunizations were done by orogastric tube inoculation on days 0, 7, and 14; challenges were done on day 33. Protection against diarrhea with a homologous challenge was found to be 84 to 100% (P < 0.01). Protection against diarrhea with challenges in which specific antigens could be tested included the following: (i) O and H antigens (O6:H16), 87 to 100% protection with different E. coli strains with identical O and H antigens (P < 0.01) but no protection against a heterologous challenges; (ii) LT or the B subunit of LT only, approximately 50% protection (P < 0.02). These findings suggest that O antigens are highly protective in this model but afford only serotype-specific protection and that the B subunit (with or without the A subunit) affords protection but confers cross-protection against heterologous strains producing LT. This model should be useful in further defining appropriate protective antigens for candidate enterotoxigenic E. coli vaccine strains.