Characterization of a Haemophilus ducreyi mutant deficient in expression of cytolethal distending toxin

Haemophilus ducreyi expresses a soluble cytolethal distending toxin (CDT) t hat kills HeLa, HEp-2, and other human epithelial cells in vitro. H. ducrey i CDT activity is encoded by a three-gene cluster (cdtABC), and antibody to the cdtC gene product can neutralize CDT activity in vitro (L. D. Cope, S. R. Lumbley, J. L. Latimer, J. Klesney-Tait, M. K. Stevens, L. S, Johnson, M, Purven, R. S. Munson, Jr., T. Lagergard, J. D. Radolf, and E. J. Hansen, Proc. Natl. Acad. Sci. USA 94:4056-4061, 1997). Culture supernatant fluid from a recombinant Escherichia coli strain containing the H. ducreyi cdtABC gene cluster readily killed both HeLa cells and HaCaT keratinocytes and ha d a modest inhibitory effect on the growth of human foreskin fibroblasts. I nsertional inactivation of the cdtC gene in this recombinant E. cell strain eliminated the ability of this strain to kill HeLa cells and HaCaT keratin ocytes. This mutated H, ducreyi cdtABC gene cluster was used to construct a n isogenic H. ducreyi cdtC mutant, Monoclonal antibodies against the H. duc reyi CdtA, CdtB, and CdtC proteins were used to characterize protein expres sion by this cdtC mutant. Culture supernatant fluid from this H, ducreyi cd tC mutant did not detectably affect any of the human cells used in this stu dy. The presence of the wild-type H. ducreyi cdtC gene in trans in this H. ducreyi mutant restored its ability to express a CDT that killed both HeLa cells and HaCaT keratinocytes. The isogenic H. ducreyi cdtC mutant was show n to be as virulent as its wild-type parent strain in the temperature-depen dent rabbit model for experimental chancroid. Lack of expression of the H. ducreyi CdtC protein also did mt affect the ability of this H. ducreyi muta nt to survive in the skin of rabbits.