Neutralizing antibodies to Haemophilus ducreyi cytotoxin

The Cytolethal Distending Toxin Contributes to Microbial Virulence and Disease Pathogenesis by Acting As a Tri-Perditious Toxin

This review summarizes the current status and recent advances in our understanding of the role that the cytolethal distending toxin (Cdt) plays as a virulence factor in promoting disease by toxin-producing pathogens. A major focus of this review is on the relationship between structure and function of the individual subunits that comprise the AB₂ Cdt holotoxin. In particular, we concentrate on the molecular mechanisms that characterize this toxin and which account for the ability of Cdt to intoxicate multiple cell types by utilizing a ubiquitous binding partner on the cell membrane. Furthermore, we propose a paradigm shift for the molecular mode of action by which the active Cdt subunit, CdtB, is able to block a key signaling cascade and thereby lead to outcomes based upon programming and the role of the phosphatidylinositol 3-kinase (PI-3K) in a variety of cells. Based upon the collective Cdt literature, we now propose that Cdt is a unique and potent virulence factor capable of acting as a tri-perditious toxin that impairs host defenses by: (1) disrupting epithelial barriers; (2) suppressing acquired immunity; (3) promoting pro-inflammatory responses. Thus, Cdt plays a key role in facilitating the early stages of infection and the later stages of disease progression by contributing to persistence and impairing host elimination.

Detoxified Haemophilus ducreyi cytolethal distending toxin and induction of toxin specific antibodies in the genital tract

Haemophilus ducreyi causes genital ulceration (chancroid), a sexually transmitted infection and still an important factor which contributes to the spread of HIV in developing countries. The bacterium produces a cytolethal distending toxin (HdCDT) causing cell cycle arrest and apoptosis/necrosis of human cells and contributes to the aggravation of ulcers. The aim of the study was to induce toxin-neutralizing antibodies in the genital tract of mice. Repeated subcutaneous (sc) immunisations with 5-10microg active HdCDT induced low levels of serum anti-HdCDT IgG without neutralizing capacity. High levels of specific IgG1 antibodies in serum and genital tract were generated after sc immunisations with 10microg formaldehyde detoxified HdCDT toxoid alone and the addition of aluminium salts or RIBI (based on the lipid A moiety) as adjuvant further increased the level of serum antibodies. A high correlation was found between elevated levels of anti-HdCDT IgG in sera, the level of neutralizing activity and the antibody level in genital tract (r=0.8). Thus, induction of high antibody levels specific to HdCDT in the genital tissue can be achieved by parenteral immunisation with the toxoid. The HdCDT toxoid can be considered as a candidate component in vaccine against chancroid.

Immune response to cytolethal distending toxin of Aggregatibacter actinomycetemcomitans in periodontitis patients

BACKGROUND AND OBJECTIVE

Cytolethal distending toxin (CDT) is a genotoxin produced by Aggregatibacter actinomycetemcomitans. In spite of its association with pathogenesis, little is known about the humoral immune response against the CDT. This study aimed to test whether subgingival colonization and humoral response to A. actinomycetemcomitans would lead to a response against CDT.

MATERIAL AND METHODS

Sera from periodontally healthy, localized and generalized aggressive periodontitis and chronic periodontitis subjects (n = 80) were assessed for immunoglobulin G titers to A. actinomycetemcomitans serotypes a/b/c and to each CDT subunit (CdtA, CdtB and CdtC) by ELISA. A. actinomycetemcomitans subgingival levels and neutralization of CDT activity were also analyzed.

RESULTS

Sera from 75.0% localized and 81.8% generalized aggressive periodontitis patients reacted to A. actinomycetemcomitans. A response to serotype b was detected in localized (66.7%) and generalized aggressive periodontitis (54.5%). Reactivity to A. actinomycetemcomitans correlated with subgingival colonization (R = 0.75, p < 0.05). There was no correlation between A. actinomycetemcomitans colonization or response to serotypes and the immunoglobulin G response to CDT subunits. Titers of immunoglobulin G to CdtA and CdtB did not differ among groups; however, sera of all generalized aggressive periodontitis patients reacted to CdtC. Neutralization of CDT was not correlated with levels of antibodies to CDT subunits.

CONCLUSION

Response to CdtA and CdtB did not correlate with the periodontal status of the subject in the context of an A. actinomycetemcomitans infection. However, a response to CdtC was found in sera of generalized but not of localized aggressive periodontitis subjects. Differences in response to CdtC between generalized and localized aggressive periodontitis subjects indicate that CDT could be expressed differently by the infecting strains. Alternatively, the antibody response to CdtC could require the colonization of multiple sites.

Formaldehyde treatment increases the immunogenicity and decreases the toxicity of Haemophilus ducreyi cytolethal distending toxin

Haemophilus ducreyi cytolethal distending toxin (HdCDT) is a tripartite AB toxin, which causes DNA damage in affected cells. We investigated the effects of formaldehyde on the chemical, biological, and immunological properties of the HdCDT complex, which was purified by immobilizing the glutathione S-transferase (GST)-CdtB fusion protein, followed by binding of the CdtA and CdtC recombinant proteins. The HdCDT was treated with increasing concentrations of formaldehyde in the presence of lysine. The treatment of HdCDT at 1 and 0.1 mg protein/ml with 320 and 80 mM of formaldehyde, respectively, resulted in the complete abrogation of cytotoxic activity, loss of DNase activity, and loss of binding capacity to HeLa cells. The toxoid showed protein bands of 75-150 kDa in SDS-PAGE, composed of the three cross-linked CDT components detected by immunoblotting. Three doses of 10 microg protein/mouse of the formaldehyde-treated HdCDT elicited toxin-neutralizing antibodies at titers about 200 times higher than those elicited by the native toxin. The described methodology may be applied to produce immunogenic toxoids from other CDTs, which might be used as candidate components in vaccines against CDT-producing bacteria, including H. ducreyi.

Specificity of antibodies directed against the cytolethal distending toxin of Haemophilus ducreyi in patients with chancroid

Antibodies specific for the cytolethal-distending toxin of Haemophilus ducreyi (HdCDT) complex and for the CdtA, CdtB, and CdtC components were measured by ELISA in the sera of 50 patients with culture and/or PCR proven chancroid, 42 patients with periodontitis, 50 blood donors from Tanzania, 50 blood donors from Sweden. In addition, the biological activity e.g. neutralization capacity of the sera were tested. Our results demonstrate that majority of chancroid patients and healthy individuals had detectable levels of serum antibodies to HdCDT complex and to separate toxin components. However, high levels (> or =100 units) of antibodies to HdCDT complex were significantly more prevalent in the sera of patients with both chancroid and periodontitis than in the sera of the corresponding controls (P=0.001 and P=0.04, respectively). In the sera of the 50 patients with chancroid, antibodies to CdtA, CdtB, and CdtC were detected in 50, 35, and 34 individuals, respectively. Antibodies to CdtC, being less frequently detected than the antibodies to other components, show a good correlation with the neutralizing capacity of sera. High levels of neutralizing antibodies (> or =160) were detected in only 22 and 2% of the patients with chancroid and periodontitis, respectively. The data suggest that the low levels of anti-HdCDT antibodies, which include neutralizing antibodies, may contribute to limited protection in chancroid and since anti-HdCDT antibodies, may be detected in healthy individuals and in patients with certain disease conditions (e.g. periodontitis), they may not be specific markers for chancroid infection.

Toxicity and immunogenicity of purified Haemophilus ducreyi cytolethal distending toxin in a rabbit model

The cytolethal distending toxin of Haemophilus ducreyi (HdCDT) is a three-component toxin that induces the arrest of the mammalian cell cycle in the G2 phase. All of the individual gene products, CdtA, CdtB and CdtC, are required for toxic activity on cultured mammalian cells. The CdtB component alone exerts nuclease activity. The individual HdCDT components were purified by affinity chromatography or ion-exchange chromatography followed by gel-filtration. HdCDT was reconstituted and purified by the immobilization of a GST-CdtB fusion on a GSTrap column and the subsequent addition of cell sonicates from Escherichia coli recombinants that produced CdtA and CdtC. The purified HdCDT preparation contained all three CDT proteins, as detected by immuno-blotting, and had high cytotoxic activity (10(6)CPU/ml). Immunization of rabbits with the HdCDT complex and with the individual CdtA, CdtB and CdtC proteins elicited high titres of antibodies, as detected by ELISA. All of the immune sera had toxin-neutralizing activities. The pathological effects of the HdCDT complex were investigated in rabbits, since the proliferation of two rabbit cell lines, SIRC and RK-13, was inhibited by HdCDT. Intradermal injection of HdCDT (1, 10, 50 and 100microg protein) into naive rabbits resulted in dose-dependent skin reactions (erythema) about 24h after injection. Similar effects were not observed when the individual HdCDT proteins were injected. HdCDT injection into immune rabbits resulted in dose-dependent skin responses that were characterized by both erythema and oedema. Histological evaluation of the 24-h lesions in naive rabbits that were injected with HdCDT, revealed moderate levels of inflammatory cells, which were mainly granulocytes and macrophages, and dilatation of blood vessels. The skin reactions in HdCDT-injected immunized rabbits showed pronounced vascular changes and extensive infiltration of inflammatory cells, including eosinophils. All of the pathological changes healed after 3 days. In conclusion, purified HdCDT holotoxin is a complex of all three CDT proteins and all three components induce neutralizing antibodies when injected in rabbits. HdCDT causes dose-dependent pathologic skin reactions in both naive and immune rabbits, which is characterized by increased inflammatory responsiveness after each immunization.

Induction of apoptosis in human T cells by Actinobacillus actinomycetemcomitans cytolethal distending toxin is a consequence of G2 arrest of the cell cycle

We have previously shown that Actinobacillus actinomycetemcomitans produces an immunosuppressive factor that is encoded by the cdtB gene, which is homologous to a family of cytolethal distending toxins (Cdt) expressed by several Gram-negative bacteria. Moreover, we have shown that CdtB impairs lymphocyte function by inducing G(2) arrest of the cell cycle. We now report that both CdtB as well as an extract prepared from an Escherichia coli strain that expresses all three of the A. actinomycetemcomitans cdt genes (rCdtABC) induce apoptosis. Pretreatment of lymphocytes with either CdtB or rCdtABC leads to DNA fragmentation in activated lymphocytes at 72 and 96 h. No DNA fragmentation was induced in nonactivated cells. Flow cytometric analysis of the Cdt-treated lymphocytes demonstrates a reduction in cell size and an increase in nuclear condensation. Mitochondrial function was also perturbed in cells pretreated with either CdtB or rCdtABC. An increase in the expression of the mitochondria Ag, Apo 2.7, was observed along with evidence of the development of a mitochondrial permeability transition state; this includes a decrease in the transmembrane potential and elevated generation of reactive oxygen species. Activation of the caspase cascade, which is an important biochemical feature of the apoptotic process, was also observed in Cdt-treated lymphocytes. Overexpression of the bcl-2 gene in the human B lymphoblastoid cell line, JY, led to a decrease in Cdt-induced apoptosis. Interestingly, Bcl-2 overexpression did not block Cdt-induced G(2) arrest. The implications of our results with respect to the immunosuppressive functions of Cdt proteins are discussed.

The role of different protein components from the Haemophilus ducreyi cytolethal distending toxin in the generation of cell toxicity

Cytolethal distending toxin of Haemophilus ducreyi (HdCDT) is a multicomponent toxin, encoded by an operon consisting of three genes, cdtABC. To investigate the role of the individual products in generation of toxicity, recombinant plasmids were constructed allowing expression of each of the genes individually or in different combinations in Escherichia coli and Vibrio cholerae. Expression of all three genes (cdtABC) was necessary to generate toxicity on cells, and no activity was obtained using combinations in which only one or two of the genes were expressed. Of the individual gene products, the CdtA was shown to exist in two forms with an MW of 23 and 17 kDa, respectively. The CdtB protein alone resulted in DNase activity. CdtC purified from both toxic and non-toxic extracts (from strains expressing cdtCAB and cdtC, respectively) had a molecular weight of about 20 kDa and reacted with a CdtC-specific monoclonal antibody. However, the protein isoelectric point (pI) of CdtC from toxic preparations was about 1.5 pH units more basic than from non-toxic ones. Both forms were immunogenic giving rise to toxin-neutralizing antibodies. Toxicity was reconstructed by combining non-toxic cell sonicates from E. coli, expressing CdtA, CdtB and CdtC proteins individually. Only combinations including all three products gave toxicity, indicating that all are actively involved in the generation of toxic activity on cells. The reconstruction resulted in a 1.5 pH unit shift in the PI of CdtC, making it identical to that of the protein isolated from bacteria expressing cdtABC. The results showed that the CdtB component produces DNase activity, but cell toxicity depends on the involvement of the other two components of CDT and is associated with absorption of all three proteins by HEp-2 cells.

The Haemophilus ducreyi cytolethal distending toxin induces cell cycle arrest and apoptosis via the DNA damage checkpoint pathways

The cytolethal distending toxins (CDTs) induce cell cycle arrest by a mechanism still not well characterized. We demonstrate that the effect of the Haemophilus ducreyi CDT (HdCDT) is cell type-specific: B cell lines underwent apoptosis, epithelial cells and keratinocytes arrested exclusively in G(2), whereas normal fibroblasts arrested both in G(1) and G(2). We studied normal keratinocytes and fibroblasts, which are relevant for understanding the pathogenicity of H. ducreyi. The response to HdCDT resembles the checkpoint response activated by ionizing radiation. Both responses were characterized by an early induction of the p53 gene and the cyclin-dependent kinase inhibitor p21 in fibroblasts, and activation of the chk2 kinase in epithelial cells. In the Ataxia Telangiectasia-mutated gene (ATM)-deficient lymphoblastoid cell lines, intoxication was significantly delayed compared with ATM wild type cells, and was associated with a slower kinetic of p53 stabilization, suggesting that the early response to HdCDT is ATM-dependent. Activation of ATM-dependent pathways was further confirmed by the ability of caffeine to partially override the HdCDT-mediated cell cycle arrest. Our data shed new light on the mechanism of action of this novel family of bacterial toxins, limiting the target candidates to DNA or molecules directly involved in activation of checkpoint responses.

Cytolethal distending toxin of Haemophilus ducreyi induces apoptotic death of Jurkat T cells

The immune response to Haemophilus ducreyi is mediated in part by T cells infiltrating the site of infection. In this study, we show that H. ducreyi antigen preparations inhibited the proliferation of peripheral blood mononuclear cells and primary human T-cell lines. H. ducreyi also inhibited Jurkat T-cell proliferation and induced apoptosis of Jurkat T cells, confirmed through the detection of DNA degradation and membrane unpacking. The cytotoxic product(s) was present in cell-free culture supernatant and whole-cell preparations of H. ducreyi and was heat labile. H. ducreyi produces two known heat-labile toxins, a hemolysin and a cytolethal distending toxin (CDT). Whole cells and supernatants prepared from a hemolysin-deficient mutant had the same inhibitory and apoptotic effects on Jurkat T cells as did its isogenic parent. Preparations made from an H. ducreyi cdtC mutant were less toxic and induced less apoptosis than the parent. The toxic activity of the cdtC mutant was restored by complementation in trans. CdtC-neutralizing antibodies also inhibited H. ducreyi-induced toxicity and apoptosis. The data suggest that CDT may interfere with T-cell responses to H. ducreyi by induction of apoptosis.

Examination of early interactions between Haemophilus ducreyi and host cells by using cocultured HaCaT keratinocytes and foreskin fibroblasts

Haemophilus ducreyi is the etiologic agent of chancroid, a sexually transmitted genital ulcer disease. Keratinocytes are likely the first cell type encountered by H. ducreyi upon infection of human skin; thus, the interaction between H. ducreyi and keratinocytes is probably important for the ability of H. ducreyi to establish infection. We have used the HaCaT keratinocyte cell line grown in monolayers and in cocultures with HS27 fibroblasts to investigate H. ducreyi interactions with keratinocytes and the host-cell response to H. ducreyi infection. Using quantitative adherence and gentamicin protection assays, we determined that approximately 13% of H. ducreyi adhered to HaCaT cell monolayers, while only a small proportion (0.0052%) was intracellular. By transmission electron microscopy, we observed numerous H. ducreyi organisms adherent to but rarely within HaCaT cells cocultured with fibroblasts. Both live H. ducreyi and purified H. ducreyi lipooligosaccharide (LOS) induced significant interleukin 8 (IL-8) expression from HaCaT cell-HS27 cell cocultures. However, the level of IL-8 expression in response to LOS alone was not as pronounced. H. ducreyi LOS was a more potent inducer of IL-8 from cocultures than Escherichia coli lipopolysaccharide (LPS) at the same concentration, suggesting a unique effect of H. ducreyi LOS on cocultures. Neither live H. ducreyi nor purified H. ducreyi LOS or E. coli LPS induced tumor necrosis factor alpha expression from cocultures. H. ducreyi induced drastically different cytokine profiles from cocultures than from HS27 or HaCaT cells cultured separately. IL-8 expression by skin cells in response to H. ducreyi infection in vivo may be responsible for the massive influx of polymorphonuclear leukocytes and other inflammatory cells to the site of infection. This influx of inflammatory cells may be partly responsible for the tissue destruction characteristic of chancroid.

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

Haemophilus ducreyi expresses a soluble cytolethal distending toxin (CDT) that kills HeLa, HEp-2, and other human epithelial cells in vitro. H. ducreyi 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 had a modest inhibitory effect on the growth of human foreskin fibroblasts. Insertional inactivation of the cdtC gene in this recombinant E. coli strain eliminated the ability of this strain to kill HeLa cells and HaCaT keratinocytes. This mutated H. ducreyi cdtABC gene cluster was used to construct an isogenic H. ducreyi cdtC mutant. Monoclonal antibodies against the H. ducreyi CdtA, CdtB, and CdtC proteins were used to characterize protein expression by this cdtC mutant. Culture supernatant fluid from this H. ducreyi cdtC mutant did not detectably affect any of the human cells used in this study. 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 shown to be as virulent as its wild-type parent strain in the temperature-dependent rabbit model for experimental chancroid. Lack of expression of the H. ducreyi CdtC protein also did not affect the ability of this H. ducreyi mutant to survive in the skin of rabbits.

The cytolethal distending toxin from the chancroid bacterium Haemophilus ducreyi induces cell-cycle arrest in the G2 phase

The potent cytolethal distending toxin produced by Haemophilus ducreyi is a putative virulence factor in the pathogenesis of chancroid. We studied its action on eukaryotic cells, with the long-term goal of understanding the pathophysiology of the disease. Intoxication of cultured human epithelial-like cells, human keratinocytes, and hamster fibroblasts was irreversible, and appeared as a gradual distention of three- to fivefold the size of control cells. Organized actin assemblies appeared concomitantly with cell enlargement, promoted by a mechanism that probably does not involve small GTPases of the Rho protein family. Intoxicated cells did not proliferate. Similar to cells treated with other cytolethal distending toxins, these cells accumulated in the G2 phase of the cell cycle, demonstrating an increased level of the tyrosine phosphorylated (inactive) form of the cyclin-dependent kinase p34(cdc2). DNA synthesis was not affected until several hours after this increase, suggesting that the toxin acts directly on some kinase/phosphatase in the signaling network controlling the p34(cdc2) activity. We propose that this toxin has an important role both in the generation of chancroid ulcers and in their slow healing. The toxin may also be an interesting new tool for molecular studies of the eukaryotic cell- cycle machinery.

Cloning and characterization of tdhA, a locus encoding a TonB-dependent heme receptor from Haemophilus ducreyi

Haemophilus ducreyi is unable to synthesize heme and must acquire it from its only known host, humans. We cloned and sequenced a gene encoding an outer membrane receptor for heme. It was designated tdhA (for TonB-dependent heme receptor A) since it was related by sequence homology to the family of TonB-dependent receptors. TdhA was strikingly similar to open reading frame HI0113 from the genome of Haemophilus influenzae Rd and also shared homology with five other heme receptors, including HxuC, HemR, HmuR, ChuA, and ShuA, from gram-negative bacteria. An Escherichia coli hemA tonB mutant strongly expressing H. ducreyi tdhA grew on low levels of heme as a source of heme only when an intact H. ducreyi Ton system plasmid was present, formally demonstrating functional TonB dependence. tdhA was expressed poorly in vitro by H. ducreyi and only under conditions of heme limitation. A survey of H. ducreyi revealed that all tested strains but one synthesized small amounts of TdhA in vitro under heme-limiting conditions. Surprisingly, an isogenic mutant of tdhA as well as its parent, 35000, both required the same high levels of heme for growth (50 microgram/ml [77 microM] on agar medium). This result, together with previous findings, suggests that in vitro, the uptake of heme by H. ducreyi is mediated by a TonB- and TdhA-independent mechanism, possibly diffusion.

Role of the Haemophilus ducreyi Ton system in internalization of heme from hemoglobin

By cloning into Escherichia coli and construction of isogenic mutants of Haemophilus ducreyi, we showed that the hemoglobin receptor (HgbA) is TonB dependent. An E. coli hemA tonB mutant expressing H. ducreyi hgbA grew on low levels of hemoglobin as a source of heme only when an intact H. ducreyi Ton system plasmid was present. In contrast, growth on heme by the E. coli hemA tonB mutant expressing hgbA was observed only at high concentrations of heme, was TonB independent, and demonstrated that H. ducreyi HgbA was not sufficient to function as a typical TonB-dependent heme receptor in E. coli. Allelic replacement of the wild-type H. ducreyi exbB, exbD, and tonB loci with the exbB, exbD, and tonB deletion resulted in an H. ducreyi isogenic mutant unable to utilize hemoglobin but able to utilize hemin at the same levels as the parent strain to fulfill its heme requirement. This finding confirms the TonB dependence of HgbA-mediated hemoglobin utilization and suggests that uptake of hemin in H. ducreyi is TonB independent. Additionally, the H. ducreyi Ton system mutant synthesized increased amounts of HgbA and other heme-regulated outer membrane proteins, consistent with derepression of these proteins due to lower intracellular heme and/or iron concentrations in the mutant. Sequencing of the Ton system genes revealed that the arrangement of the genes was exbB exbD tonB. The proximity and structure of these genes suggested that they are transcribed as an operon. This arrangement, as well as the DNA and deduced amino acid sequences of these H. ducreyi genes, was most similar to those from other pasteurellae.

Purification and identification of Haemophilus ducreyi cytotoxin by use of a neutralizing monoclonal antibody

Haemophilus ducreyi produces a cytotoxin responsible for the killing of cultured human epithelial cells. Cytotoxin-neutralizing antibodies were detected in the majority of sera from patients with culture-proven chancroid, and a significantly higher level of such antibodies in patients than in blood donors was noted both in areas where the disease is endemic and those where it is not. We produced neutralizing monoclonal antibodies (MAbs) in mice with a crude osmotic preparation of the cytotoxin. These antibodies, with high capacity to neutralize cytotoxicity, were used for purification and identification of the cytotoxin. Purification was performed by a two-step procedure which included Sephacryl S-200 filtration followed by immunoaffinity chromatography. The purification resulted in poor cytotoxin protein recovery and contamination with MAbs from the affinity column. The results of the gel filtration experiments and immunoblotting indicate that the active cytotoxin consists of a single, small protein with an approximate molecular mass of 20 kDa. Cytotoxins from different strains seem to have the same or similar epitopes. The cytotoxin protein was not detected in preparations from nontoxic strains. The N-terminal amino acid sequence of the 20-kDa band was E-S-N-P-D-P-T-T-Y-P-D-V-E-L-S-P-P-P. This sequence does not resemble that of any currently known bacterial protein.

A diffusible cytotoxin of Haemophilus ducreyi

Little is known about the virulence mechanisms employed by Haemophilus ducreyi in the production of genital ulcers. This Gram-negative bacterium previously has been shown to produce a soluble cytotoxic activity that kills HeLa and HEp-2 cells. We have now identified a cluster of three H. ducreyi genes that encode this cytotoxic activity. The predicted proteins encoded by these genes are most similar to the products of the Escherichia coli cdtABC genes that comprise the cytolethal distending toxin (CDT) of this enteric pathogen. Eleven of 12 H. ducreyi strains were shown to possess this gene cluster and culture supernatants from these strains readily killed HeLa cells. The culture supernatant from a single strain of H. ducreyi that lacked these genes was unable to kill HeLa cells. When the H. ducreyi cdtABC gene cluster was cloned into E. coli, culture supernatant from the recombinant E. coli clone killed HeLa cells. A monoclonal antibody that neutralized this soluble cytotoxic activity of H. ducreyi was shown to bind to the H. ducreyi cdtC gene product. This soluble H. ducreyi cytotoxin may play a role in the development or persistence of the ulcerative lesions characteristic of chancroid.

Swine model of Haemophilus ducreyi infection

Haemophilus ducreyi is a strict human pathogen that causes sexually transmitted genital ulcer disease. We infected domestic swine with H. ducreyi 35000, resulting in the development of cutaneous ulcers histologically resembling human chancroid lesions. Intraepidermal lesions progressed from pustules to ulcers containing polymorphonuclear leukocytes and were accompanied by a dermal inflammatory infiltrate containing T cells and macrophages. H. ducreyi was recovered from lesions up to 17 days after inoculation, and pigs did not develop immunity to reinfection with the challenge strain. Features of the model include inoculation through abrasions in the epidermis, ambient housing temperatures for infected pigs, the ability to deliver multiple different inocula to a single host, and the availability of monoclonal antibodies against porcine immune cells permitting immunohistochemical characterization of the host immune response to H. ducreyi infection.

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.

Cytotoxin production in 100 strains of Haemophilus ducreyi from different geographic locations

One-hundred strains of Haemophilus ducreyi, representing isolates from different parts of the world, including the reference strains, were obtained from different collections and characterized with special reference to cytotoxin production in vitro. The cytotoxic activity on cultured epithelial cells (HEp-2) was examined with two methods. The activity in bacterial sonicates was tested on freshly trypsinated cells and strains manifesting little or no cytotoxic activity in sonicates were investigated using attached living bacteria on HEp-2 cell-monolayers. Sonicates from the majority of the H. ducreyi strains (89%) produced significant cytotoxic effects on HEp-2 cells. The reciprocal cytotoxic titers of the sonicates ranged from 2.4 x 10(2) to 5.3 x 10(5). Sonicates of 11 strains had low cytotoxic titers (< or = 1:3 to 1:81), eight of those originating from Asia and three from Africa. These 11 strains caused no damage to the cell monolayer, indicating that the 11 strains produce little or no cytotoxic activity in vitro. In summary, the majority of H. ducreyi isolates produce cytotoxic activity, which support the hypothesis that the cytotoxin may be an important virulence factor of this species.

Vaccines for bacterial sexually transmitted infections: a realistic goal?

Bacterial infections of the genital tract (gonorrhea, chlamydia, chancroid, syphilis) are common and cause significant morbidity. Their importance is heightened by recent appreciation of their roles in facilitation of transmission of the human immunodeficiency virus (HIV). Each is capable of causing repeated infections, suggesting lack of permanent broadly effective immunity. An effective vaccine has yet to be developed for any of these diseases. Rapid progress in understanding the molecular basis for pathogenesis of infection, including mechanisms for escape from otherwise effective immune surveillance and mechanisms for causing injury to host cells, has stimulated renewed efforts to make vaccines for some of these infections. Progress has been greatest for Neisseria gonorrhoeae and Chlamydia trachomatis. Present emphasis is on the major or principal outer membrane proteins of N. gonorrhoeae and C. trachomatis, based on evidence for neutralizing antibodies directed against surface-exposed variable domains of each of these proteins. Other surface-exposed proteins, including the iron-repressible transferrin receptor in gonococci and certain heat-shock proteins in chlamydia, also may be targets for vaccines. Although much remains to be learned, cautious optimism is warranted.