Neutralizing antibodies to Haemophilus ducreyi cytotoxin

Neutralizing antibodies against cytotoxin produced by Haemophilus ducreyi bacteria were studied in rabbits by an assay employing HEp-2 cells and diluted crude cytotoxin preparations from the organism. Antisera to 12 different H. ducreyi strains were prepared by immunization of rabbits with bacterial sonicates combined with Freund's adjuvant. The antibody response during infection with H. ducreyi was studied in two groups of rabbits which were infected with five live strains by either single or multiple intradermal injections. Neutralizing antibodies in hyperimmune sera to sonicates from 12 H. ducreyi strains tested against their homologous cytotoxin preparations were present with titers ranging from 1:80 to 1:640. Similar antibody titers against heterologous cytotoxin preparations were recorded, indicating immunological similarity or identity between cytotoxins from the various H. ducreyi strains. Three strains did not produce cytotoxin, and these strains did not induce toxin-neutralizing antibodies. Hyperimmune sera to other gram-negative bacteria had no detectable neutralizing capacity, indicating species specificity of the H. ducreyi cytotoxin. Cytotoxin-neutralizing antibodies were not detectable in rabbit sera before infection with H. ducreyi. Repeated single injections with live bacteria resulted in development of low levels of neutralizing antibodies. Multiple primary injection of live bacteria of the cytotoxin-producing strain CCUG 7470 resulted in a low immune response to the cytotoxin preparation from the same strain. A booster infection resulted in the development of neutralizing antibodies in all rabbits infected with cytotoxin-producing strains. The antibody titers determined against the homologous cytotoxin preparation were similar to those recorded for two heterologous cytotoxin preparations.