Monoclonal antibodies against Escherichia coli heat-stable toxin (STa) and their use in a diagnostic ST ganglioside GM1-enzyme-linked immunosorbent assay

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Abstract
Seven monoclonal antibodies (MAbs) against heat-stable enterotoxin (ST) from a human Escherichia coli isolate were prepared and evaluated for their usefulness in an ST immunodetection assay, the ST ganglioside GM1-enzyme-linked immunosorbent assay (ELISA). This assay is based on the ability of STa, as present in, for example, culture filtrates from ST-producing E. coli, to inhibit specific anti-ST antibody from binding to solid-phase-bound ST ganglioside (GM1-bound ST-cholera B subunit). Four of the MAbs were of immunoglobulin G1 (IgG1), one was of IgG2b, and two were of IgM isotype. All the IgG1 MAbs could be completely inhibited by addition of free ST; 0.2 to 0.4 ng of purified ST inhibited binding of these MAbs by 50%. The non-IgG1 MAbs were, in contrast, not inhibited by 200-fold-higher amounts of purified ST, probably because they were directed against linkage epitopes or were of low affinity or both. When the IgG1 MAbs were tested in the ST GM1-ELISA, ST could be detected in culture filtrates from stock human E. coli isolates with 100% sensitivity and specificity. ST in filtrates from fresh stool cultures was demonstrated with higher sensitivity with the MAbs ST GM1-ELISA than with the conventional infant mouse test. Both subtypes of STa, STaI and STaII, could be detected by the ST GM1-ELISA by using either IgG1 MAb in the immunodetection step, whereas infant-mouse-active ST from Yersinia enterocolitica failed to react.