A mouse model ofHaemophilus ducreyiinfection (chancroid)

Attenuated Salmonella typhimurium SL3261 as a vaccine vector for recombinant antigen in rabbits

Oral live Salmonella vaccine vectors expressing recombinant guest antigens help stimulate systemic, mucosal, humoral, and cell-mediated immune responses against Salmonella and recombinant antigens. It may be possible to use them effectively against Haemophilus ducreyi, the bacterium that causes chancroid, a sexually transmitted genital ulcer disease. This study aimed to test the feasibility of using oral Salmonella vaccine vectors for the evaluation of chancroid vaccine candidates in the temperature-dependent rabbit model of H. ducreyi infection, an in vivo quantitative virulence assay of inducible immunity. We identified 10(8) to 10(9) CFU to be a safe and immunogenic oral dose range of S. typhimurium SL3261, by monitoring post-administration onset and course of illness and antibody titre by enzyme immunoassay (EIA). We successfully transduced plasmid pTETnir15 into the strain to produce recombinant S. typhimurium SL3261(pTETnir15), successfully expressed tetanus toxin fragment C (TetC) in it, and elicited serum anti-TetC titres of 1:6400 by EIA, 4 weeks after inoculation. The course of experimentally induced H. ducreyi skin lesions in rabbits treated with SL3261(pTETnir15) was similar to that in saline-treated controls. We describe a framework that successfully uses Salmonella as a vector for recombinant control antigen in the rabbit model of H. ducreyi infection, and is suitable for pre-clinical evaluation of Salmonella vector-based H. ducreyi vaccine antigen candidates.

Iron and virulence of Haemophilus ducreyi in a primate model

BACKGROUND AND OBJECTIVES

The pathogenesis of chancroid is currently unclear. There are discrepancies between the virulence of Haemophilus ducreyi in the human and animal experiments. Iron availability influences the virulence of many pathogens. The objective of this study was to investigate the role of iron in virulence of H. ducreyi.

STUDY DESIGN

Intradermal inoculation of the primate Macaca mulatta was studied as an animal model for pathogenesis of chancroid. This model then was used to study the influence of iron availability on virulence of H. ducreyi. Eleven strains of H. ducreyi with proven avirulence in the rabbit intradermal test and one virulent control strain were used. Two of the strains had been shown to cause ulcers in a human inoculation experiment. Strains were suspended in fluid with different iron compounds before intradermal injection. Other monkeys were treated with injectable iron before inoculation with bacteria suspended.

RESULTS

All but one of the test strains produced a chancroid-like ulcer from which H. ducreyi could be isolated. The minimal ulcerative dose was 10(5) cfu. Iron in the injection fluid decreased this dose 10-fold. In animals pretreated with intramuscular iron, the minimal ulcerative dose decreased to 10(4) cfu and all eleven strains became ulcerative.

CONCLUSIONS

The availability of iron increases virulence of H. ducreyi. The iron pretreated primate model is useful for the study of virulence factors of H. ducreyi because of its ability to produce lesions with strains that avirulent in other animal models but pathogenic to humans.

Chancroid and Haemophilus ducreyi: an update

Haemophilus ducreyi is a fastidious gram-negative bacillus that causes the sexually transmitted infection chancroid. Chancroid is a major genital ulcerative disease in Africa, Southeast Asia, the Caribbean, and Latin America and is of increasing concern in the United States. Genital ulcerative disease and chancroid in particular have been associated with facilitating the transmission of human immunodeficiency virus. The diagnosis of chancroid based on the clinical appearance of the genital lesion or on the isolation of H. ducreyi on selective medium is relatively insensitive. However, recent advances in nonculture diagnostic tests have enhanced our ability to diagnose chancroid. There has been renewed interest in understanding the pathogenesis of H. ducreyi. In vitro and in vivo models have been developed to help identify important virulence determinants. Through the use of biochemical and molecular techniques, macromolecular components that may be important in virulence have been identified.

The role of Haemophilus ducreyi bacteria, cytotoxin, endotoxin and antibodies in animal models for study of chancroid

Haemophilus ducreyi cytotoxin-positive and -negative strains as well as bacterial sonicates and lipooligosaccharides (LOS) from such strains were evaluated for the capacity to produce dermonecrotic lesions, especially ulcers, after intradermal injections to rabbits and to different mouse strains, including nude mice. Dermonecrotic lesions of the ulcerous type were observed within 4 days and they were developed in both rabbits and mice with about 10(7) colony forming units (cfu) of H. ducreyi. Viable bacteria were isolated from the lesions up to 9 days after inoculation. All lesions healed spontaneously within 2-3 weeks. Bacterial sonicate (heated and unheated) and LOS preparations caused mainly abscess formation in rabbits, while in mice, a superficial, haemorrhagic ulceration was observed. To obtain ulceration at all injection sites, about 200 micrograms of LOS was required. Histological examination of acute, dermonecrotic lesions caused by viable bacteria showed deep necrosis, infiltrate of inflammatory cells, especially granulocytes and dilatation of blood vessels. The same type of inflammatory cells as seen in lesions caused by bacteria, were involved in the mouse lesions caused by bacterial sonicate and LOS preparations. The results indicate that LOS/endotoxin, probably in combination with other bacterial polysaccharides, can play a role in ulceration caused by H. ducreyi in animals; however, a relatively high amount of LOS preparation was necessary to cause dermal ulceration at all injection sites in the mouse model. The development of ulcers correlated with the endotoxin activity in bacterial sonicate and in LOS preparations. The model may therefore be useful to study the role of LOS components in development of ulceration. There was no significant difference in lesions caused by cytotoxin producing, respectively, non-producing H. ducreyi strains and cell-free preparations from such strains. The bacterial sonicates, cytotoxic for human cell lines, failed to kill animal cell lines, indicating that animal models do not adequately reflect the cytotoxin activity in experimental H. ducreyi infection. Antibodies to H. ducreyi sonicate and LOS, tested by means of ELISA, were found in pre-immune sera from both rabbits and mice. There was a significant antibody response to homologous cell sonicate and LOS, after primary and secondary infections with bacteria. Still, there was no clear difference between primary and secondary lesions in animals. Since animal lesions are mainly due to endotoxin activity, this may indicate that antibodies are of minor importance for protection in animal models.

Pathophysiological concept of Haemophilus ducreyi infection (chancroid)

Our knowledge concerning the pathogenesis of infection due to Haemophilus ducreyi is incomplete. In order to produce disease, H. ducreyi must presumably penetrate the skin of the external genitalia, colonize subcutaneous tissues, then produce tissue damage which results in ulcer formation. Penetration of the normal skin most likely occurs via minor abrasions. Adherence of H. ducreyi to different cell lines in vitro has been described, and might be mediated by adhesions such as pili or haemagglutinins. In addition, binding to extracellular matrix proteins has also been reported. Extracellular tissue-degrading enzymes were absent from broth culture supernatants of H. ducreyi. Such supernatants also failed to produce cytopathic effects with established or primary cell lines. Both live and heat-killed H. ducreyi organisms were able to produce lesions in a rabbit or a mouse model, although ulcer formation was dependent on viable H. ducreyi organisms in a recently introduced temperature-dependent rabbit model. With an excessive supply of iron, a more prolonged localized inflammatory disease effect was observed. Results derived from a subcutaneous chamber model demonstrated considerable changes in the expression of outer membrane proteins combined with antibody modulation during in vivo growth of H. ducreyi. These might be important factors for maintenance of infection in the human host particularly as these changes also occur in humans. Despite an increased knowledge of the pathogenesis of chancroid, important questions such as growth requirements, bubo-formation, role of cell-mediated immunity and ulcer formation are still unanswered. The application of molecular biological techniques in order to study these problems will be helpful.

Binding of Haemophilus ducreyi to extracellular matrix proteins

A collection of Haemophilus ducreyi isolates were screened for the ability to bind to fibrinogen, fibronectin, collagen, gelatin and laminin by a particle agglutination test using latex beads coated with the individual proteins. Thirteen of 21 isolates reacted with all five extracellular matrix proteins. Binding of organisms to protein-coated latex beads was inhibited by pretreatment of the bacteria with detergent, trypsin or boiling. Two isolates did not bind to collagen and gelatin with one of these not reacting with laminin either. Seven strains which failed to react with laminin did not express pili when examined by electron microscopy. This observation suggests a specific interaction with the pili of H. ducreyi.

Mouse subcutaneous chamber model for in vivo growth of Haemophilus ducreyi

The ability of Haemophilus ducreyi, the causative agent of chancroid, to grow in subcutaneous chambers implanted in mice was studied. All seven H. ducreyi strains tested were able to maintain a long-term infection in ICR mice; one mouse remained infected with strain Hd175 for more than 4 months. Growth curves obtained following the removal of chamber fluid at various time points from infected mice demonstrated that H. ducreyi was growing during the course of the infection. In addition, all three mouse strains tested (ICR, CBA and BALB/c) were able to maintain long-term H. ducreyi infections. This model will be valuable in studying the effects of in vivo growth on the antigenic composition of H. ducreyi as well as for the identification of virulence factors.

Role of lipooligosaccharides in experimental dermal lesions caused by Haemophilus ducreyi

The mouse and rabbit intradermal injection models have been used to define factors that may be important in Haemophilus ducreyi pathogenesis. We used H. ducreyi strains with diverse geographic origins and phenotypic characteristics to evaluate the experimental models. Injection of live and heat-killed bacteria caused skin abscesses in both models. Semiquantitative cultures of skin injected with live bacteria showed that H. ducreyi failed to replicate in animal tissue. These data suggested that the experimental lesions were caused by a heat-stable substance such as lipooligosaccharide (LOS). In mice, injection of H. ducreyi and Haemophilus influenzae LOS and Escherichia coli lipopolysaccharide caused mild to moderate inflammation. In rabbits, injection of H. ducreyi LOS caused intradermal abscesses that were histologically similar to those caused by live and heat-killed bacteria. H. ducreyi and Neisseria gonorrhoeae LOS caused significantly larger lesions than equivalent amounts of H. influenzae LOS and E. coli lipopolysaccharide in the rabbit model. We conclude that the intradermal injection models are not valid models to study the growth of H. ducreyi in vivo. However, these data indicate that H. ducreyi LOS may play an important role in the pathogenesis of chancroid and that the rabbit model should be useful in studying H. ducreyi LOS toxicity at the cellular level.

Rapid detection of Haemophilus ducreyi in clinical and experimental infections using monoclonal antibody: a preliminary evaluation

A monoclonal antibody raised against Haemophilus ducreyi was tested for its sensitivity and specificity as an immunofluorescence (IF) reagent using simulated vaginal smears containing H. ducreyi, smears taken from skin lesions of mice infected with H. ducreyi and patients from South Africa, Thailand and Malaysia with clinically diagnosed chancroid. The IF test was more sensitive than culture or Gram staining in the simulated smears, theoretically detecting less than 4 organisms/sample. It detected H. ducreyi in 95% of the animal lesions compared with 14% detected by culture. Immunofluorescence testing identified over 90% of culture-positive cases of chancroid but also detected organisms in some culture-negative cases where clinical evidence for the diagnosis was strong. These results suggest that this antibody may provide a simple, rapid and sensitive means of detecting H. ducreyi in cases of chancroid.