Evaluation of Recombinant Forms of the Shiga Toxin Variant Stx2eB Subunit and Non-Toxic Mutant Stx2e as Vaccine Candidates against Porcine Edema Disease

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Porcine edema disease (ED) is a communicable disease of shoats caused by infection with Shiga toxin (Stx)-producing Escherichia coli. Stx2e is classified as a 1A5B-type toxin and is a decisive virulence determinant of ED. The single A subunit of Stx2e possesses enzymatic activity and is accompanied by a pentamer of B subunits, which binds to the host receptor and delivers the A subunit into the cell. In the present study, we used a mouse model to evaluate the immunogenicity of 3 ED vaccine candidates: a non-toxic mutant holotoxin mStx2e and 2 Stx2eB-based fusion proteins, Stx2eA₂B-His and Stx2eB-His. Systemic inoculation of mice with mStx2e- and the Stx2eB-derived antigens induced anti-Stx2e IgG responses that were fully and partially capable of neutralizing Stx2e cellular cytotoxicity, respectively. Intranasal immunization with mStx2e protected the mice from subsequent intraperitoneal challenge with a lethal dose of Stx2e, whereas immunization with Stx2eA₂B-His and Stx2eB-His afforded partial protection. Analysis of serum cytokines revealed that mStx2e, but not the Stx2eB-based antigens, was capable of inducing a Th2-type immune response. These results suggest that although the Stx2eB-based antigens elicited an immune response to Stx2e, they did so through a different mechanism to the Th2-type response induced by mStx2e.