Protection against human and porcine enterotoxigenic strains of Escherichia coli in rats immunized with a cross-linked toxoid vaccine.

To compare their relative immunogenicities, we used synthetically produced Escherichia coli heat-stable toxin coupled to a protein carrier and the B subunit of porcine heat-labile toxin separately in graded dosages to immunize rats. Equivalent antigen unit dosages of each toxin raised approximately the same level of mucosal immunoglobulin A (IgA) antitoxin response and degree of protection against a challenge with respective heat-stable- or heat-labile-toxin-producing viable bacteria. Conjugation conditions were identified, therefore, which yielded a vaccine of these toxins, cross-linked by the carbodiimide reaction, that consisted of equal antigenic proportions of each toxin component as determined by enzyme-linked immunosorbent assay and expressed in antigen units. The dose-related response to immunization with this vaccine was the same as the response to its components given separately. The toxicity of the heat-stable toxin component was reduced greater than 600-fold. Immunization with optimal antigen unit dosages of the vaccine gave greater than or equal to sixfold increases in mucosal IgA antitoxin titers and provided significant (P less than 0.001) protection against challenge with heterologous serotypes of viable strains, of either human or porcine origin, that produce heat-stable or heat-labile toxin or both.