Detection of diphtheria toxin in culture supernates of Corynebacterium diphtheriae and C. ulcerans by immunoassay with monoclonal antibody.

Monoclonal antibody (MAb) to diphtheria toxin was produced in mouse hybridomas, and shown by ELISA to be of sub-class IgG1. Hybridomas were inoculated into mice to produce ascitic fluid from which MAb was purified by caprylic acid. The MAb was shown by immunoblotting to be directed against the A fragment of the toxin and also against the intact toxin molecule. After conjugation with fluorescein isothiocyanate, it was used in an immunoassay to detect toxin in culture supernates of Corynebacterium diphtheriae and C. ulcerans. The assay correlated well with the Elek test and with virulence in guinea-pigs; but it gave occasional false positive results, probably by binding of MAb to defective toxin.