Development of a serological test for Haemophilus ducreyi for seroprevalence studies

Epidemiology, clinical features, diagnosis and treatment of Haemophilus ducreyi - a disappearing pathogen?

Chancroid, caused by Haemophilus ducreyi, has declined in importance as a sexually transmitted pathogen in most countries where it was previously endemic. The global prevalence of chancroid is unknown as most countries lack the required laboratory diagnostic capacity and surveillance systems to determine this. H. ducreyi has recently emerged as a cause of chronic skin ulceration in some South Pacific islands. Although no antimicrobial susceptibility data for H. ducreyi have been published for two decades, it is still assumed that the infection will respond successfully to treatment with recommended cephalosporin, macrolide or fluoroquinolone-based regimens. HIV-1-infected patients require careful follow-up due to reports of treatment failure with single dose regimens. Buboes may need additional treatment with either aspiration or excision and drainage.

The laboratory diagnosis of Haemophilus ducreyi

Chancroid is a sexually transmitted infection caused by Haemophilus ducreyi. This fastidious, Gram-negative coccobacilli dies rapidly outside the human host, making diagnostic testing using culture methods difficult. This genital ulcer infection is not common in Canada and, therefore, can often be misdiagnosed. The objective of the present paper is to provide practical approaches for the diagnosis of chancroid in Canadian patients where the prevalence of this infection is low. Issues related to sample collection, sample transport and available diagnostic tests are reviewed, and several alternative approaches are outlined. Although antigen detection, serology and genetic amplification methods have all been reported for H ducreyi, none are commercially available. Culture is still the primary method available to most laboratories. However, the special media necessary for direct bedside inoculation is often not available; therefore, communication with the diagnostic laboratory and rapid specimen transport are essential when chancroid is suspected.

Immunization with the Haemophilus ducreyi hemoglobin receptor HgbA with adjuvant monophosphoryl lipid A protects swine from a homologous but not a heterologous challenge

Haemophilus ducreyi, the etiological agent of chancroid, has a strict requirement for heme, which it acquires from its only natural host, humans. Previously, we showed that a vaccine preparation containing the native hemoglobin receptor HgbA purified from H. ducreyi class I strain 35000HP (nHgbAI) and administered with Freund's adjuvant provided complete protection against a homologous challenge. In the current study, we investigated whether nHgbAI dispensed with monophosphoryl lipid A (MPL), an adjuvant approved for use in humans, offered protection against a challenge with H. ducreyi strain 35000HP expressing either class I or class II HgbA (35000HPhgbAI and 35000HPhgbAII, respectively). Pigs immunized with the nHgbAI/MPL vaccine were protected against a challenge from homologous H. ducreyi strain 35000HPhgbAI but not heterologous strain 35000HPhgbAII, as evidenced by the isolation of only strain 35000HPhgbAII from nHgbAI-immunized pigs. Furthermore, histological analysis of the lesions showed striking differences between mock-immunized and nHgbAI-immunized animals challenged with strains 35000HPhgbAI but not those challenged with strain 35000HPhgbAII. Mock-immunized pigs were not protected from a challenge by either strain. The enzyme-linked immunosorbent assay (ELISA) activity of the nHgbAI/MPL antiserum was lower than the activity of antiserum from animals immunized with the nHgbAI/Freund's vaccine; however, anti-nHgbAI from both studies bound whole cells of 35000HPhgbAI better than 35000HPhgbAII and partially blocked hemoglobin binding to nHgbAI. In conclusion, despite eliciting lower antibody ELISA activity than the nHgbAI/Freund's, the nHgbAI/MPL vaccine provided protection against a challenge with homologous but not heterologous H. ducreyi, suggesting that a bivalent HgbA vaccine may be needed.

An immunogenic, surface-exposed domain of Haemophilus ducreyi outer membrane protein HgbA is involved in hemoglobin binding

HgbA is the sole TonB-dependent receptor for hemoglobin (Hb) acquisition of Haemophilus ducreyi. Binding of Hb to HgbA is the initial step in heme acquisition from Hb. To better understand this step, we mutagenized hgbA by deletion of each of the 11 putative surface-exposed loops and expressed each of the mutant proteins in trans in host strain H. ducreyi FX547 hgbA. All mutant proteins were expressed, exported, and detected on the surface by anti-HgbA immunoglobulin G (IgG). Deletion of sequences in loops 5 and 7 of HgbA abolished Hb binding in two different formats. In contrast, HgbA proteins containing deletions in the other nine loops retained the ability to bind Hb. None of the clones expressing mutant proteins were able to grow on plates containing low concentrations of Hb. Previously we demonstrated in a swine model of chancroid infection that an HgbA vaccine conferred complete protection from a challenge infection. Using anti-HgbA IgG from this study and the above deletion mutants, we show that loops 4, 5, and 7 of HgbA were immunogenic and surface exposed and that IgG directed against loops 4 and 5 blocked Hb binding. Furthermore, loop 6 was cleaved by protease on intact H. ducreyi, suggesting surface exposure. These data implicate a central domain of HgbA (in respect to the primary amino acid sequence) as important in Hb binding and suggest that this region of the molecule might have potential as a subunit vaccine.

Evaluation of the repertoire of the TonB-dependent receptors of Haemophilus ducreyi for their role in virulence in humans

Haemophilus ducreyi contains 3 TonB-dependent receptors: the hemoglobin receptor HgbA, which is required for virulence in humans; the heme receptor TdhA; and an uncharacterized conserved hypothetical protein TdX (HD0646). A double tdX/tdhA mutant (FX527) was constructed on the background of a human-passaged variant of strain 35000 (35000HP). Six volunteers were infected with 35000HP at 3 sites on one arm and with FX527 at 3 sites on the other. The pustule formation rate was 55.6% (95% confidence interval [CI], 35.7%-75.4%) at 18 parent-strain sites and 44.4% (95% CI, 15.0%-73.9%) at 18 mutant-strain sites (P = .51). Similar amounts of 35000HP and FX527 were recovered from pustules in semiquantitative culture. Thus, TdX and TdhA are not necessary for virulence, whereas HgbA is both necessary and sufficient for virulence in humans. The data suggest that hemoglobin is the sole source of heme/iron used by H. ducreyi in vivo and has implications for the potential of HgbA as a vaccine.

Immunization with the Haemophilus ducreyi hemoglobin receptor HgbA protects against infection in the swine model of chancroid

The etiologic agent of chancroid is Haemophilus ducreyi. To fulfill its obligate requirement for heme, H. ducreyi uses two TonB-dependent receptors: the hemoglobin receptor (HgbA) and a receptor for free heme (TdhA). Expression of HgbA is necessary for H. ducreyi to survive and initiate disease in a human model of chancroid. In this study, we used a swine model of H. ducreyi infection to demonstrate that an experimental HgbA vaccine efficiently prevents chancroid, as determined by several parameters. Histological sections of immunized animals lacked typical microscopic features of chancroid. All inoculated sites from mock-immunized pigs yielded viable H. ducreyi cells, whereas no viable H. ducreyi cells were recovered from inoculated sites of HgbA-immunized pigs. Antibodies from sera of HgbA-immunized animals bound to and initiated antibody-dependent bactericidal activity against homologous H. ducreyi strain 35000HP and heterologous strain CIP542 ATCC; however, an isogenic hgbA mutant of 35000HP was not killed, proving specificity. Anti-HgbA immunoglobulin G blocked hemoglobin binding to the HgbA receptor, suggesting a novel mechanism of protection through the limitation of heme/iron acquisition by H. ducreyi. Such a vaccine strategy might be applied to other bacterial pathogens with strict heme/iron requirements. Taken together, these data suggest continuing the development of an HgbA subunit vaccine to prevent chancroid.

Haemophilus ducreyi Outer membrane determinants, including DsrA, define two clonal populations

The Haemophilus ducreyi outer membrane component DsrA (for ducreyi serum resistance A) is necessary for complete resistance to normal human serum (NHS). When DsrA expression in 19 temporally and geographically diverse clinical isolates of H. ducreyi was examined by Western blotting, 5 of the strains expressed a different immunotype of the DsrA protein (DsrA(II)) than the well-characterized prototypical strain 35000HP (DsrA(I)). The predicted DsrA proteins expressed by the DsrA(II) strains were 100% identical to each other but only 48% identical to that of strain 35000HP. In addition to the DsrA(II) protein, class II strains also expressed variant forms of other outer membrane proteins (OMPs) including NcaA (necessary for collagen adhesion A), DltA (ducreyi lectin A), Hlp (H. ducreyi lipoprotein), major OMP, and/or OmpA2 (for OMP A2) and synthesized a distinct, faster-migrating lipooligosaccharide. Based on these data, strains expressing DsrA(I) were termed class I, and those expressing DsrA(II) were termed class II. Expression of dsrA(II) from strain CIP 542 ATCC in the class I dsrA(I) mutant FX517 (35000HP background), which does not express a DsrA protein, rendered this strain resistant to 50% NHS. This demonstrates that DsrA(II) protein is also critical to serum resistance. Taken together, these results indicate that there are two clonal populations of H. ducreyi. The implications of two classes of H. ducreyi strains differing in important antigenic outer membrane components are discussed.

A novel lectin, DltA, is required for expression of a full serum resistance phenotype in Haemophilus ducreyi

Haemophilus ducreyi, the causative agent of chancroid, is highly resistant to the complement-mediated bactericidal activity of normal human serum (NHS). Previously, we identified DsrA (for ducreyi serum resistance A), a major factor required for expression of the serum resistance phenotype in H. ducreyi. We describe here a second outer membrane protein, DltA (for ducreyi lectin A), which also contributes to serum resistance in H. ducreyi. Isogenic dltA mutants, constructed in 35000HP wild-type and FX517 dsrA backgrounds, were more susceptible to the bactericidal effects of NHS than each respective parent, demonstrating the additive effect of the mutations. Furthermore, expression of dltA in H. influenzae strain Rd rendered this highly susceptible strain partially resistant to 5% NHS compared to a vector-control strain. Although primary basic local alignment search tool analysis of the dltA open reading frame revealed no close bacterial homologue, similarity to the beta-chain of the eukaryotic lectin ricin was noted. DltA shares highly conserved structural motifs with the ricin beta chain, such as cysteines and lectin-binding domains. To determine whether dltA was a lectin, ligand blots and affinity chromatography experiments were performed. DltA was affinity purified on immobilized lactose and N-acetylgalactosamine, and N-glycosylated but not glycosidase-treated model glycoproteins bound DltA. These data indicate that DltA is a lectin with specificity for lactose-related carbohydrates (CHO) and is important for H. ducreyi serum resistance.

A humoral immune response confers protection against Haemophilus ducreyi infection

Haemophilus ducreyi is the etiologic agent of the sexually transmitted genital ulcer disease chancroid. Neither naturally occurring chancroid nor experimental infection with H. ducreyi results in protective immunity. Likewise, a single inoculation of H. ducreyi does not protect pigs against subsequent infection. Accordingly, we used the swine model of chancroid infection to examine the impact of multiple inoculations on a host's immune response. After three successive inoculations with H. ducreyi, pigs developed a modestly protective immune response evidenced by the decreased recovery of viable bacteria from lesions. All lesions biopsied 2 days after the first and second inoculations contained viable H. ducreyi cells, yet only 55% of the lesions biopsied 2 days after the third inoculation did. Nearly 90% of the lesions biopsied 7 days after the first inoculation contained viable H. ducreyi cells, but this percentage dropped to only 16% after the third inoculation. Between the first and third inoculations, the average recovery of CFU from lesions decreased approximately 100-fold. The reduced recovery of bacteria corresponded directly with a fivefold increase in H. ducreyi-specific antibody titers and the emergence of bactericidal activity. These immune sera were protective when administered to naïve pigs prior to challenge with H. ducreyi. These data suggest that pigs mount an effective humoral immune response to H. ducreyi after multiple exposures to the organism.

Porphyromonas gingivalis infection during pregnancy increases maternal tumor necrosis factor alpha, suppresses maternal interleukin-10, and enhances fetal growth restriction and resorption in mice

Epidemiological studies have shown a potential association between maternal periodontitis and pregnancy complications. We used a pregnant murine model to study the effect of infection with the periodontal pathogen Porphyromonas gingivalis on pregnancy outcomes. Female BALB/c mice were inoculated with heat-killed P. gingivalis (10(9) CFU) in a subcutaneous chamber and mated 2 weeks later. At gestation day (GD) 7.5, mice were challenged with live P. gingivalis (10(7) CFU) (n = 20) or broth (control; n = 8) and sacrificed at GD 16.5. Fetal growth restriction (FGR, <0.46 g) was defined as fetuses with weights 2 standard deviations (SD) smaller than controls (0.56 +/- 0.05 g [mean +/- SD]). Among the 20 challenged mice, 8 had both normal-weight (0.51 +/- 0.11 g) and FGR (0.34 +/- 0.1 g) fetuses within the same litter. All other challenged dams had normal-weight fetuses (0.57 +/- 0.04 g). Maternal liver, uterus, and spleen samples were examined for P. gingivalis DNA using a PCR technique. Of the eight challenged mice with FGR fetuses, three had PCR signals for P. gingivalis in liver and uterus, but not in the spleen. Liver, uterus, and spleen were negative for P. gingivalis DNA among all other challenged and control mice. In serum of dams with FGR fetuses, tumor necrosis factor alpha levels were elevated significantly, while interleukin-10 levels were significantly reduced compared to levels in dams with normal fetuses. P. gingivalis-specific serum immunoglobulin G levels were significantly elevated in dams with FGR fetuses compared to dams without any FGR fetuses. These data demonstrate that P. gingivalis-induced murine FGR is associated with systemic dissemination of the organism and activated maternal immune and inflammatory responses.

The Haemophilus ducreyi serum resistance antigen DsrA confers attachment to human keratinocytes

Haemophilus ducreyi is the etiologic agent of the sexually transmitted genital ulcer disease chancroid. H. ducreyi serum resistance protein A (DsrA) is a member of a family of multifunctional outer membrane proteins that are involved in resistance to killing by human serum complement. The members of this family include YadA of Yersinia species, the UspA proteins of Moraxella catarrhalis, and the Eib proteins of Escherichia coli. The role of YadA, UspA1, and UspA2H as eukaryotic cell adhesins and the function of UspA2 as a vitronectin binder led to our investigation of the cell adhesion and vitronectin binding properties of DsrA. We found that DsrA was a keratinocyte-specific adhesin as it was necessary and sufficient for attachment to HaCaT cells, a keratinocyte cell line, but was not required for attachment to HS27 cells, a fibroblast cell line. We also found that DsrA was specifically responsible for the ability of H. ducreyi to bind vitronectin. We then theorized that DsrA might use vitronectin as a bridge to bind to human cells, but this hypothesis proved to be untrue as eliminating HaCaT cell binding of vitronectin with a monoclonal antibody specific to integrin alpha(v)beta(5) did not affect the attachment of H. ducreyi to HaCaT cells. Finally, we wanted to examine the importance of keratinocyte adhesion in chancroid pathogenesis so we tested the wild-type and dsrA mutant strains of H. ducreyi in our swine models of chancroid pathogenesis. The dsrA mutant was less virulent than the wild type in both the normal and immune cell-depleted swine models of chancroid infection.

Development of a rapid immunodiagnostic test for Haemophilus ducreyi

Haemophilus ducreyi is the etiologic agent of chancroid, a sexually transmitted disease that increases the rate of transmission of human immunodeficiency virus. Chancroid ulcerations are difficult to distinguish from those produced by syphilis and herpes. Diagnosis based solely on clinical grounds is inaccurate, and culture is insensitive. Highly sensitive PCR has largely superseded culture as the preferred method of laboratory diagnosis; however, neither culture nor PCR is feasible where chancroid is endemic. We developed a rapid (15-min) diagnostic test based on monoclonal antibodies (MAbs) to the hemoglobin receptor of H. ducreyi, HgbA. This outer membrane protein is conserved in all strains of H. ducreyi tested and is required for the establishment of experimental human infection. MAbs to HgbA were generated and tested for cross-reactivity against a panel of geographically diverse strains. Three MAbs were found to be unique and noncompetitive and bound to all strains of H. ducreyi tested. Using an immunochromatography format, we evaluated the sensitivity and specificity of the test using geographically diverse strains of H. ducreyi, other Haemophilus strains, and other bacteria known to superinfect genital ulcers. All H. ducreyi strains were positive, and all other bacteria were negative, resulting in a specificity of 100%. The minimum number of CFU of H. ducreyi detected was 2 x 10(6) CFU, and the minimum amount of purified HgbA protein detected was 8.5 ng. Although this level of sensitivity may not be sufficient to detect H. ducreyi in all clinical specimens, further work to increase the sensitivity could potentially make this a valuable bedside tool in areas where chancroid is endemic.

Multiserotype enzyme-linked immunosorbent assay as a diagnostic aid for periodontitis in large-scale studies

Periodontitis is a common chronic oral infection caused by gram-negative bacteria, including Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis. Periodontitis evokes inflammatory host response locally in the periodontium but also systemically. The systemic humoral antibody response against oral pathogens can conveniently be measured by an immunoassay. The aim of the study was to measure serum immunoglobulin G class antibodies against A. actinomycetemcomitans and P. gingivalis by an enzyme-linked immunosorbent assay (ELISA) in which mixtures of several serotypes of the pathogens were used as antigens to avoid biasing of the results in favor of a particular strain. For A. actinomycetemcomitans the antigen consisted of six strains representing serotypes a, b, c, d, and e and one nonserotypeable strain. In the P. gingivalis ELISA, antigens representing serotypes a, b, and c were used. Serum samples from 90 subjects, including 35 samples from patients with diagnosed periodontitis, 10 samples from periodontally healthy controls, and 45 samples from randomly selected apparently healthy volunteers (referred to as "healthy subjects"), were tested. For both pathogens the antibody levels (means +/- standard deviations) of the patients--xpressed as area under the dilution curve--were significantly higher than those for healthy controls or healthy subjects, with values for A. actinomycetemcomitans and P. gingivalis, respectively, as follows: patients, 22.60 +/- 9.94 mm(2) and 26.72 +/- 11.13 mm(2); healthy controls, 9.99 +/- 3.92 mm(2) and 6.90 +/- 3.38 mm(2); and healthy subjects, 16.85 +/- 6.67 mm(2) and 8.51 +/- 4.23 mm(2). The serotype mixture ELISA is suitable for measuring antibodies against periodontal pathogens in large epidemiological studies in order to evaluate the role of periodontitis as a risk factor for other diseases.

Haemophilus ducreyi: clinical disease and pathogenesis

Haemophilus ducreyi causes the sexually transmitted disease chancroid, which facilitates the transmission of HIV infection. This review focuses on recent advances in the epidemiology, diagnosis, treatment and pathogenesis of this disease.

Cloning, overexpression, purification, and immunobiology of an 85-kilodalton outer membrane protein from Haemophilus ducreyi

We have identified an 85-kDa outer membrane protein that is expressed by all tested strains of Haemophilus ducreyi. Studies of related proteins from other pathogenic bacteria, including Haemophilus influenzae, Pasteurella multocida, Neisseria gonorrhoeae, Neisseria meningitidis, and Shigella dysenteriae, suggested a role for these proteins in pathogenesis and immunity. In keeping with the first such described protein from Haemophilus influenzae type B, we termed the H. ducreyi protein D15. The gene encoding the H. ducreyi D15 protein was cloned and sequenced, and the deduced amino acid sequence was found to be most similar to sequences of the D15-related proteins from other Pasteurella spp. The arrangement of the flanking genes was similar to that of H. influenzae Rd and suggested that D15 was part of a multigene operon. Attempts to make a null mutation of the D15 gene were unsuccessful, paralleling results in other D15 gene studies. Overexpression of H. ducreyi D15 in Escherichia coli resulted in a source of recombinant D15 (rD15) from which it was readily purified. rD15 was immunogenic, and it was found that immunization of rabbits with an rD15 vaccine preparation conferred partial protection against a virulent challenge infection. Antisera to an N-terminal peptide recognized all tested strains of H. ducreyi.

Transcription of candidate virulence genes of Haemophilus ducreyi during infection of human volunteers

Haemophilus ducreyi expresses several putative virulence factors in vitro. Isogenic mutant-to-parent comparisons have been performed in a human model of experimental infection to examine whether specific gene products are involved in pathogenesis. Several mutants (momp, ftpA, losB, lst, cdtC, and hhdB) were as virulent as the parent in the human model, suggesting that their gene products did not play a major role in pustule formation. However, we could not exclude the possibility that the gene of interest was not expressed during the initial stages of infection. Biopsies of pustules obtained from volunteers infected with H. ducreyi were subjected to reverse transcription-PCR. Transcripts corresponding to momp, ftpA, losB, lst, cdtB, and hhdA were expressed in vivo. In addition, transcripts for other putative virulence determinants such as ompA2, tdhA, lspA1, and lspA2 were detected in the biopsies. These results indicate that although several candidate virulence determinants are expressed during experimental infection, they do not have a major role in the initial stages of pathogenesis.

New tests for bacterial sexually transmitted diseases

Recent advances in diagnostic tests for sexually transmitted diseases include the development of a synthetic Venereal Disease Research Laboratory reagent that will improve the sensitivity and stability of nontreponemal serologic tests for syphilis. A second generation user friendly and high throughput nucleic acid amplification test for Chlamydia trachomatis and Neisseria gonorrhoeae has also been developed.

Checkerboard assessments of serum antibodies to oral microbiota as surrogate markers of clinical periodontal status

BACKGROUND

This study addresses whether checkerboard assessments of serum IgG antibodies to oral bacteria may serve as surrogate markers of clinical periodontal status in epidemiologic studies.

METHOD

The analysis involved data from 205 subjects, 132 periodontitis patients and 73 periodontally-intact controls, from whom full-mouth clinical periodontal data and serum IgG titers against 19 periodontal bacterial species were available.

RESULTS

A logistic regression model involving titers against 6 species (Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, Eubacterium nodatum, Eikenella corrodens, Capnocytophaga ochracea and Actinomyces naeslundii genospecies 2) classified correctly 74.5% of the subjects examined, with 84% sensitivity, 57.5% specificity, 78% positive predictive value and 66.7% negative predictive value.

CONCLUSIONS

Checkerboard serology may be useful in providing surrogate markers for clinical periodontal status when such data are not readily available and, thus may serve as a valuable complement in the armamentarium of epidemiologic tools suited for the study of periodontal diseases.