Properties of cross-linked toxoid vaccines made with hyperantigenic forms of synthetic Escherichia coli heat-stable toxin

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Abstract
The ability of hyperantigenic preparations of synthetically produced Escherichia coli heat-stable toxin (ST) to provide an immunogenically more potent vaccine when cross-linked by the glutaraldehyde reaction to the heat-labile toxin B subunit was assessed. Three synthetic ST preparations were evaluated: ST(S) had the same antigenicity and toxicity (secretory potency in the suckling mouse assay) as native ST, ST 1056 had 3.5-fold more antigenicity and 1% toxicity, and ST(C) had 15-fold greater antigenicity and 31% toxicity. Vaccines that contained equal antigenic proportions of ST and B subunit, as determined by enzyme-linked immunosorbent assays, consisted by weight of 52% ST(S), 25% ST 1056, and 9% ST(C). The initially lower toxicity and smaller proportions by weight of hyperantigenic ST preparations yielded vaccines that had nearly 10-fold less residual ST toxicity than the ST(S) vaccine. Immunization of rats with graded dosages of vaccines containing 9% ST(C) and 51% ST(S) by weight, but equal amounts of ST(S) antigenicity, raised to the same degree dose-dependent increases in mucosal immunoglobulin A antitoxin titers to ST(S) which correlated with the amount of protection against challenge with a viable LT-/ST+ strain. These observations indicate that hyperantigenic synthetic ST preparations provide immunologically more potent vaccines than those obtained with the previously used synthetic ST(S) preparation, which has the same biological properties as native ST.