Purification and identification of Haemophilus ducreyi cytotoxin by use of a neutralizing monoclonal antibody.

Haemophilus ducreyi produces a cytotoxin responsible for the killing of cultured human epithelial cells. Cytotoxin-neutralizing antibodies were detected in the majority of sera from patients with culture-proven chancroid, and a significantly higher level of such antibodies in patients than in blood donors was noted both in areas where the disease is endemic and those where it is not. We produced neutralizing monoclonal antibodies (MAbs) in mice with a crude osmotic preparation of the cytotoxin. These antibodies, with high capacity to neutralize cytotoxicity, were used for purification and identification of the cytotoxin. Purification was performed by a two-step procedure which included Sephacryl S-200 filtration followed by immunoaffinity chromatography. The purification resulted in poor cytotoxin protein recovery and contamination with MAbs from the affinity column. The results of the gel filtration experiments and immunoblotting indicate that the active cytotoxin consists of a single, small protein with an approximate molecular mass of 20 kDa. Cytotoxins from different strains seem to have the same or similar epitopes. The cytotoxin protein was not detected in preparations from nontoxic strains. The N-terminal amino acid sequence of the 20-kDa band was E-S-N-P-D-P-T-T-Y-P-D-V-E-L-S-P-P-P. This sequence does not resemble that of any currently known bacterial protein.