Simple method for Shiga toxin 2e purification by affinity chromatography via binding to the divinyl sulfone group

Shiga toxin type 2 B subunit protects mice against toxin challenge when leashed and bundled by a stable pentameric coiled-coil molecule

Vaccines against Shiga toxin (Stx)-producing Escherichia coli (STEC) have not yet been developed. Two immunologically distinct serotypes of Stx (Stx1 and Stx2) are the main virulence factors of STEC. Thus, blocking their B subunits (StxB) from binding to the cell surface receptor globotriaosylceramide (Gb3) efficiently prevents the action of these toxins. We expressed Stx1B and Stx2B in E. coli inclusion bodies and reassembled them into pentamers by a stepwise dialysis. Stx1B pentamer fully protected mice against Stx1 challenge, but Stx2B pentamer failed to protect mice against Stx2 challenge. To explain those observations, we proposed that the pentamer of Stx2B readily dissociates into its constituent monomers, especially under in vivo conditions, thus being unable to induce pentamer-specific immunity. To increase pentamer stability, we fused the B subunit to a pentameric coiled-coil domain of the cartilage oligomeric matrix protein (COMP). This "five-to-five" fusion hybrid molecule (Stx2B-COMP) was shown to be protective against Stx2 challenge, demonstrating that the Stx2B subunit when leashed and bundled by a rigid pentameric coiled-coil domain mount a pentamer-specific immune response and efficiently neutralize the toxin both in vitro and in vivo. Our data strongly suggest that the Stx2B subunit moiety fluctuates between a pentameric and monomeric state within the fusion protein, which may increase the likelihood of the immune system recognizing the pentameric conformation for toxin neutralization.

Evaluation of immunochromatographic test of Shiga toxin 2e in enrichment cultures of swine edema disease clinical samples

To simplify the diagnosis of swine edema disease, overnight culture supernatants of swine clinical samples were assayed using immunochromatographic test strips we developed previously. Small-intestinal contents, mesenteric lymph nodes, and fecal samples were cultured in casamino acid-yeast extract broth overnight, after which supernatants were loaded onto immunochromatographic test strips to determine whether they could detect Shiga toxin 2e (Stx2e). Among 23 clinical samples in which PCR-identified stx2e-positive E. coli were isolated, samples from seven of ten small-intestinal contents, one of three mesenteric lymph nodes and six of ten fecal samples showed Stx2e-positive reactions in the protein-based immunochromatographic test. Additionally, one small-intestinal content sample, in which stx2e-positive E. coli were not isolated, showed an Stx2e-positive reaction. Furthermore, the immunochromatographic test results of the samples were associated with the toxin concentration determined by sandwich ELISA and cytotoxicity assay results on Vero cells. The toxin concentration range of the samples with positive and negative reactions were 2.1–196.2 ng/ml and 0–12.8 ng/ml, respectively. The sensitivity and specificity of this immunochromatographic test strip calculated from all clinical samples analyzed in this study were 60.9% and 94.4%, respectively. Our immunochromatographic test strip has strong potential for simple and accurate diagnosis for edema disease by detecting toxin expression, complementing the PCR method.

Development of a homogeneous time-resolved FRET (HTRF) assay for the quantification of Shiga toxin 2 produced by E. coli

Shiga toxin-producing Escherichia coli (STEC) is a major intestinal pathogen and causes serious gastrointestinal illness, which includes diarrhea, hemorrhagic colitis, and life-threatening hemolytic uremic syndrome. The major virulence factors of STEC are Shiga toxins (Stx1 and Stx2), which belong to the AB-type toxin family. Among several subtypes of Stx1 and Stx2, the production of Stx2a is thought to be a risk factor for severe STEC infections, but Stx2a production levels vary markedly between STEC strains, even strains with the same serotype. Therefore, quantitative analyses of Stx2 production by STEC strains are important to understand the virulence potential of specific lineages or sublineages. In this study, we developed a novel Stx2 quantification method by utilizing homogeneous time-resolved fluorescence resonance energy transfer (HTRF) technology. To determine suitable “sandwich” assay conditions, we tested 6 combinations of fluorescence-labeled monoclonal antibodies (mAbs) specific to Stx2 and compared the HTRF signal intensities obtained at various incubation times. Through this analysis, we selected the most suitable mAb pair, one recognizing the A subunit and the other recognizing the B subunit, thus together detecting Stx holotoxins. The optimal incubation time was also determined (18 h). Then, we optimized the concentrations of the two mAbs based on the range for linearity. The established HTRF assay detected 0.5 ng/ml of the highly purified recombinant Stx2a and Stx2e proteins and the working range was 1–64 ng/ml for both Stx2a and Stx2e. Through the quantification analysis of Stx proteins in STEC cell lysates, we confirmed that other Stx2 subtypes (Stx2b, Stx2c, Stx2d and Stx2g) can also be quantified at a certain level of accuracy, while this assay system does not detect Stx2f, which is highly divergent in sequence from other Stx2 subtypes, and Stx1. As the HTRF protocol we established is simple, this assay system should prove useful for the quantitative analysis of Stx2 production levels of a large number of STEC strains.

Development of a simple and rapid diagnosis method for swine edema disease to specifically detect Stx2e protein by immunochromatographic test

Edema disease in piglets is caused by Shiga toxin 2e (Stx2e)-producing Escherichia coli. However, there is currently no available Stx2e-specific immunochromatographic test strip to differentiate Stx2e from other types of Shiga toxin 2. In the present study, to develop an Stx2e-specific immunochromatographic test strip, we isolated nine different monoclonal antibody-producing hybridoma clones from Stx2e toxoid-immunized mice and confirmed that six antibodies were A subunit-specific whereas three antibodies were B subunit-specific. Only one A subunit-specific monoclonal antibody (45B2) was cross-reactive with prototype Stx2 (Stx2a) at the same sensitivity, but the remaining eight monoclonal antibodies were not. In immunochromatographic tests using the highly sensitive antibodies, test strips using some combinations of gold colloid-conjugated monoclonal antibody with the B subunit-specific monoclonal antibody on the membrane detected Stx2e, but not other types of Shiga toxin 2. These test strips had the ability to detect Stx2e in the culture supernatant of clinically isolated Stx2e gene-positive strains, but not in those of Stx2e gene-negative strains. These results indicate that our test strip is practical for the specific detection of Stx2e to diagnose swine edema disease.

Evaluation of Shiga toxin 2e-specific chicken egg yolk immunoglobulin: production and neutralization activity

Chicken egg yolk immunoglobulin (IgY) against Shiga toxin 2e (Stx2e), a major cause of swine edema disease, was prepared to evaluate its possible clinical applications. The titer of Stx2e-specific IgY in egg yolk derived from three chickens that had been immunized with an Stx2e toxoid increased 2 weeks after primary immunization and remained high until 90 days after this immunization. Anti-Stx2e IgY was found to neutralize the toxicity of Stx2e by reacting with its A and B subunits, indicating that IgY is a cost-effective agent to develop for prophylactic foods or diagnosis kits for edema disease.