Chancroid is a sexually-transmitted infection (STI) prevalent in impoverished regions of the developing world. Caused by the gram-negative bacterium Haemophilus ducreyi (Hd), an obligate human pathogen, chancroid is characterized by purulent genital skin ulceration and increased risk of acquiring other STI including HIV. Chancroid is treatable with antibiotic therapy but does not elicit protective immunity to subsequent Hd infection. To further explore Hd-host interactions, we examined two targets: an Hd surface protein targeted by the host's modest anti-Hd humoral response, and a secreted Hd toxin implicated in pathogenesis and immune modulation. We identified Hd outer-membrane protein NcaA as an Oca-family bacterial adhesin. We demonstrated that NcaA mediated Hd binding to the host skin protein collagen. Furthermore, we found that NcaA was essential for virulence in human and swine models of chancroid, suggesting an important link between skin colonization and survival in the host. Most Hd isolates secrete cytolethal distending toxin (CDT), a trimeric complex that intoxicates many eukaryotic cell types, including some found in skin (fibroblasts, keratinocytes) and others relevant to immunity (macrophages, lymphocytes). We therefore investigated CDT as a contributor to Hd pathogenesis and host immune modulation. Two independent Hd CDT-deletion ([delta]CDT) mutants were attenuated in the swine chancroid model, but neither could be complemented, showing that CDT deletion selected for cryptic attenuating mutations. These results support but cannot confirm the importance of CDT to Hd pathogenesis. Additionally, pigs inoculated twice with [delta]CDT Hd were as susceptible to wild type Hd challenge as pigs previously inoculated twice with CDT+ Hd. Though this finding did not illustrate CDT-mediated immune alteration, [delta]CDT strains may not persist sufficiently in vivo to elicit immunity regardless of any CDT effect. Following published findings attributing CDT toxicity to nuclease activity of subunit CdtB, we generated an Hd mutant with minimal amino acid variations mapping to the CdtB active site, attempting to circumvent compensatory mutation. This mutant retained CDT toxicity; furthermore, purified mutant CdtB protein retained in vitro nuclease activity. Further studies are warranted to address the importance of CdtB nuclease activity and CDT toxicity to Hd host-pathogen interaction
