A Class I Haemophilus ducreyi Strain Containing a Class II hgbA Allele Is Partially Attenuated in Humans: Implications for HGBA vaccine efficacy trials

Isabelle Leduc, Kate R. Fortney, Diane M. Janowicz, Beth Zwickl, Sheila Ellinger, Barry P. Katz, Huaiying Lin, Qunfeng Dong, Stanley M. Spinola

Research output: Contribution to journalArticle


Haemophilus ducreyi causes chancroid and is a major cause of cutaneous ulcers in children. Due to environmental reservoirs, both class I and class II H. ducreyi strains persist in cutaneous ulcer regions of endemicity following mass drug administration of azithromycin, suggesting the need for a vaccine. The hemoglobin receptor (HgbA) is a leading vaccine candidate, but its efficacy in animal models is class specific. Controlled human infection models can be used to evaluate vaccines, but only a class I strain (35000HP) has been characterized in this model. As a prelude to evaluating HgbA vaccines in the human model, we tested here whether a derivative of 35000HP containing a class II hgbA allele (FX548) is as virulent as 35000HP in humans. In eight volunteers infected at three sites with each strain, the papule formation rate was 95.8% for 35000HP versus 62.5% for FX548 (P 0.021). Excluding doses of FX548 that were 2-fold higher than those of 35000HP, the pustule formation rate was 25% for 35000HP versus 11.7% for FX548 (P 0.0053). By Western blot analysis, FX548 and 35000HP expressed equivalent amounts of HgbA in whole-cell lysates and outer membranes. The growth of FX548 and 35000HP was similar in media containing hemoglobin or hemin. By whole-genome sequencing and single-nucleotide polymorphism analysis, FX548 contained no mutations in open reading frames other than hgbA. We conclude that by an unknown mechanism, FX548 is partially attenuated in humans and is not a suitable strain for HgbA vaccine efficacy trials in the model.

Original languageEnglish (US)
Article numbere00112
JournalInfection and immunity
Issue number7
StatePublished - Jan 1 2019



  • Haemophilus ducreyi
  • Hemoglobin
  • Vaccines

ASJC Scopus subject areas

  • Parasitology
  • Microbiology
  • Immunology
  • Infectious Diseases

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