Immunization of rats with heat-labile enterotoxin provides uniform protection against heterologous serotypes of enterotoxigenic Escherichia coli

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Abstract
Rats immunized with Escherichia coli heat-labile (LT) enterotoxin, either in the form of the holotoxin derived from a transformed K-12 strain or the polymyxin-release form obtained from human strains which produce LT toxin alone (LT+/ST- [ST is heat-stable toxin)] or together with ST toxin (LT+/ST+), were challenged with viable organisms of 10 different serotypes, 5 LT+/ST- and 5 LT+/ST+. The serum antitoxin response was monitored by enzyme-linked immunosorbent assay, and the degree of protection was determined by challenge in ligated ileal loops. Immunization with the holotoxin provided a strong antitoxin response and protection against all 10 challenge strains. Immunization with toxin from the LT+/ST+ strain provided equally strong protection against all strains, but immunization with toxin from the LT+/ST- strain yielded only a weak antitoxin response, moderate protection against challenge with LT+/ST- strains, and no protection against LT+/ST- strains, increasing by fivefold the immunization dosage of the LT+/ST- toxin failed to enhance protection. These observations (i) establish the fact that immunization with the LT holotoxin provides uniformly strong protection against heterologous serotypes and (ii) indicate that, for reasons which remain to be determined, the immunogenicity of the polymyxin-release LT from an LT+/ST+ strain differs from that of an LT+/ST- strain.