Serological responses to the B subunit of Shiga-like toxin 1 and its peptide fragments indicate that the B subunit is a vaccine candidate to counter action of the toxin

The B subunit of Shiga toxin and Shiga-like toxin (SLT-1) and its fragments are potentially immunogenic and may generate protective humoral responses against the action of these toxins. We have analyzed the antibody response of rabbits immunized with pure B subunit of SLT-1 or synthetic fragments of the subunit. The immune response to the native B subunit was found to be largely directed at conformational epitopes. More importantly, rabbits immunized with the B subunit were protected from a lethal challenge with SLT-1, indicating that the B subunit represents an excellent vaccine candidate to counter the effects of Shiga toxin and SLT-1 in humans. Polyclonal antibodies against a synthetic peptide corresponding to residues 28 to 40 of the B subunit neutralized the cytotoxicity of SLT-1 towards Vero cells. This region is thus exposed in the native state of the B subunit. The sequence specificity of other antipeptide antisera also provides clues to the state of folding and assembly of the B subunit. Antisera to synthetic peptides representing the N- and C-terminal regions of the SLT-1 B subunit did not cross-react with native B subunit but strongly recognized denatured forms of the protein. Finally, the monoclonal antibody 13C4 was shown to bind to a discontinuous epitope expressed only on the native form of the protein. These immunological reagents can be used to probe the conformational state of the B subunit and the holotoxin as it relates to their functional properties.