Characterization of Haemophilus ducreyi cdtA, cdtB, and cdtC mutants in in vitro and in vivo systems

Studying the Interaction of Neutrophils and Glaesserella Parasuis Indicates a Serotype Independent Benefit from Degradation of NETs

Glaesserella (G.) parasuis is one of the most important porcine pathogens causing Glaesser’s disease. Neutrophil granulocytes are the major counteracting cell type of the innate immune system, which contribute to the host defense by phagocytosis or the formation of neutrophil extracellular traps (NETs). Recently, NET-formation has been shown to facilitate the survival of bacteria from the Pasteurellaceae family. However, the interaction of NETs and G. parasuis is unclear so far. In this study, we investigated the interplay of three G. parasuis serotypes with porcine neutrophils. The production of reactive oxygen species by neutrophils after G. parasuis infection varied slightly among the serotypes but was generally low and not significantly influenced by the serotypes. Interestingly, we detected that independent of the serotype of G. parasuis, NET formation in neutrophils was induced to a small but significant extent. This phenomenon occurred despite the ability of G. parasuis to release nucleases, which can degrade NETs. Furthermore, the growth of Glaesserella was enhanced by external DNases and degraded NETs. This indicates that Glaesserella takes up degraded NET components, supplying them with nicotinamide adenine dinucleotide (NAD), as this benefit was diminished by inhibiting the 5′-nucleotidase, which metabolizes NAD. Our results indicate a serotype-independent interaction of Glaesserella with neutrophils by inducing NET-formation and benefiting from DNA degradation.

Delivery, structure, and function of bacterial genotoxins

Bacterial genotoxins are peptide or protein virulence factors produced by several pathogens, which make single-strand breaks (SSBs) and/or double-strand DNA breaks (DSBs) in the target host cells. If host DNA inflictions are not resolved on time, host cell apoptosis, cell senescence, and/or even bacterial pathogen-related cancer may occur. Two multi-protein AB toxins, cytolethal distending toxin (CDT) produced by over 30 bacterial pathogens and typhoid toxin from Salmonella Typhi, as well as small polyketide-peptides named colibactin that causes the DNA interstrand cross-linking and subsequent DSBs is the most well-characterized bacterial genotoxins. Using these three examples, this review discusses the mechanisms by which these toxins deliver themselves into the nucleus of the target host cells and exert their genotoxic functions at the structural and functional levels.

Prophylactic potential of cytolethal distending toxin B (CdtB) subunit of typhoid toxin against Typhoid fever

Typhoid fever caused by Salmonella enterica serovar Typhi (S.Typhi) continues to be a major problem, especially in developing countries. Due to the rapid emergence of multi-drug-resistant (MDR) strains, which limits the efficacy of conventional antibiotics as well as problems associated with the existing vaccines, efforts are being made to develop effective prophylactic agents. CdtB subunit of typhoid toxin was selected for assessing its vaccine potential due to its high conservation throughout the Typhi strains. In-vitro assessment of DNase activity of cloned and purified CdtB protein showed a significant decrease in the band intensity of DNA. The measure of metabolic activity and morphological alterations assessed using different cell lines in the presence of CdtB protein showed no significant signs of toxicity. These observations were further strengthened by cell cycle analysis, assessed by flow cytometry. Keeping these observations in mind, the immunoprotective potential of CdtB was assessed using S.Typhi induced mouse peritonitis model. A significant titer of IgG antibodies (>128000) against CdtB protein was recorded in the immunized mice by enzyme-linked immunosorbent assay (ELISA), which was also validated by immunoblotting. Active immunization with the protein protected 75% mice against a lethal dose of S.Typhi Ty2. The data indicated a significant (up to 5 log) reduction in the bacterial load in the spleen and liver of immunized-infected mice compared to control (unimmunized-infected) mice which might have resulted in the modulation of histoarchitecture of spleen and liver and the levels of cytokines (IL-6, TNF-α and IL-10) production; thereby indicating the effectiveness of the subunit. The observations deduced from the study give the proof of concept of immunogenic potential of protein. However, further studies involving the immunoreactivity of CdtB with the statistically significant number of sera samples obtained from the human patients would be helpful in establishing the relevance of CdtB protein in humans and for making the strategies to develop it as an effective vaccine candidate.

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.

The Enterobacterial Genotoxins: Cytolethal Distending Toxin and Colibactin

While the DNA damage induced by ionizing radiation and by many chemical compounds and drugs is well characterized, the genotoxic insults inflicted by bacteria are only scarcely documented. However, accumulating evidence indicates that we are exposed to bacterial genotoxins. The prototypes of such bacterial genotoxins are the Cytolethal Distending Toxins (CDTs) produced by Escherichia coli and Salmonella enterica serovar Typhi. CDTs display the DNase structure fold and activity, and induce DNA strand breaks in the intoxicated host cell nuclei. E. coli and certain other Enterobacteriaceae species synthesize another genotoxin, colibactin. Colibactin is a secondary metabolite, a hybrid polyketide/nonribosomal peptide compound synthesized by a complex biosynthetic machinery. In this review, we summarize the current knowledge on CDT and colibactin produced by E. coli and/or Salmonella Typhi. We describe their prevalence, genetic determinants, modes of action, and impact in infectious diseases or gut colonization, and discuss the possible involvement of these genotoxigenic bacteria in cancer.

Distinct Roles for CdtA and CdtC during Intoxication by Cytolethal Distending Toxins

Cytolethal distending toxins (CDTs) are heterotrimeric protein exotoxins produced by a diverse array of Gram-negative pathogens. The enzymatic subunit, CdtB, possesses DNase and phosphatidylinositol 3-4-5 trisphosphate phosphatase activities that induce host cell cycle arrest, cellular distension and apoptosis. To exert cyclomodulatory and cytotoxic effects CDTs must be taken up from the host cell surface and transported intracellularly in a manner that ultimately results in localization of CdtB to the nucleus. However, the molecular details and mechanism by which CDTs bind to host cells and exploit existing uptake and transport pathways to gain access to the nucleus are poorly understood. Here, we report that CdtA and CdtC subunits of CDTs derived from Haemophilus ducreyi (Hd-CDT) and enteropathogenic E. coli (Ec-CDT) are independently sufficient to support intoxication by their respective CdtB subunits. CdtA supported CdtB-mediated killing of T-cells and epithelial cells that was nearly as efficient as that observed with holotoxin. In contrast, the efficiency by which CdtC supported intoxication was dependent on the source of the toxin as well as the target cell type. Further, CdtC was found to alter the subcellular trafficking of Ec-CDT as determined by sensitivity to EGA, an inhibitor of endosomal trafficking, colocalization with markers of early and late endosomes, and the kinetics of DNA damage response. Finally, host cellular cholesterol was found to influence sensitivity to intoxication mediated by Ec-CdtA, revealing a role for cholesterol or cholesterol-rich membrane domains in intoxication mediated by this subunit. In summary, data presented here support a model in which CdtA and CdtC each bind distinct receptors on host cell surfaces that direct alternate intracellular uptake and/or trafficking pathways.

Interplay between copper and zinc homeostasis through the transcriptional regulator Zur in Enterococcus faecalis

By integrating the microarray expression data and a global E. faecalis transcriptional network we identified a sub-network activated by zinc and copper. Our analyses indicated that the transcriptional response of the bacterium to copper and zinc exposure involved the activation of two modules, module I that contains genes implicated in zinc homeostasis, including the Zur transcriptional repressor, and module II containing a set of genes associated with general stress response and basal metabolism. Bacterial exposure to zinc and copper led to the repression of the zinc uptake systems of module I. Upon deletion of Zur, exposure to different zinc and copper conditions induced complementary homeostatic mechanisms (ATPase efflux proteins) to control the intracellular concentrations of zinc. The transcriptional activation of zinc homeostasis genes by zinc and copper reveals a functional interplay between these two metals, in which exposure to copper also impacts on the zinc homeostasis. Finally, we present a new zinc homeostasis model in E. faecalis, positioning this bacterium as one of the most complete systems biology model in metals described to date.

Cytolethal Distending Toxin: A Unique Variation on the AB Toxin Paradigm

Some of the most potent toxins produced by plants and bacteria are members of a large family known as the AB toxins. AB toxins are generally characterized by a heterogenous complex consisting of two protein chains arranged in various monomeric or polymeric configurations. The newest class within this superfamily is the cytolethal distending toxin (Cdt). The Cdt is represented by a subfamily of toxins produced by a group of taxonomically distinct Gram negative bacteria. Members of this subfamily have a related AB-type chain or subunit configuration and properties distinctive to the AB paradigm. In this review, the unique structural and cytotoxic properties of the Cdt subfamily, target cell specificities, intoxication pathway, modes of action, and relationship to the AB toxin superfamily are compared and contrasted.

Contribution of Helicobacter hepaticus cytolethal distending toxin subunits to human epithelial cell cycle arrest and apoptotic death in vitro

BACKGROUND

Cytolethal distending toxin (CDT) is the only known virulence factor found in H. hepaticus, the cause of chronic typhlocolitis and hepatitis leading to colonic and hepatocellular carcinomas in mice. Interaction of the tripartite polypeptide CdtA, CdtB, and CdtC subunits produced by H. hepaticus CDT (HhepCDT) causes cell cycle arrest and apoptotic death of cultured cells; however, the contribution of individual subunit to these processes has not been investigated.

MATERIALS AND METHODS

The temporal relationship between cell cycle and apoptotic death of human epithelial HeLa and INT407 cells intoxicated with HhepCDT holotoxin or reconstituted recombinant HhepCDT was compared by flow cytometry. The genotoxic activity of individual and combinations of recombinant HhepCDT protein subunits or increasing concentrations of individual recombinant HhepCDT protein subunits transfected into HeLa cells was assessed at 72 hours post-treatment by flow cytometry.

RESULTS

Similar time course of HhepCDT-induced G2 /M cell cycle arrest and apoptotic death was found with both cell lines which reached a maximum at 72 hours. The presence of all three HhepCDT subunits was required for maximum cell cycle arrest and apoptosis of both cell lines. Transfection of HeLa cells with HhepCdtB, but not with HhepCdtA or HhepCdtC, resulted in a dose-dependent G2 /M arrest and apoptotic death.

CONCLUSION

All three subunits of HhepCDT are required for maximum epithelial cell cycle arrest and progression to apoptotic death, and HhepCdtB subunit alone is necessary and sufficient for epithelial cell genotoxicity.

A new functional site W115 in CdtA is critical for Aggregatibacter actinomycetemcomitans cytolethal distending toxin

Aggregatibacter actinomycetemcomitans, a specific pathogen of localized aggressive periodontitis, produces a cytolethal distending toxin (CDT) that arrests eukaryotic cells irreversibly in G0/G1 or G2/M phase of the cell cycle. Although structural studies show that the aromatic patch region of CdtA plays an important role in its biological activity, the functional sites of CdtA have not been firmly established. In this study, site-specific mutagenesis strategy was employed for cdtA point mutations construction so as to examine the contributions of individual amino acids to receptor binding and the biological activity of holotoxin. The binding ability was reduced in CdtA^Y181ABC holotoxin and the biological function of CDT was not weaken in CdtA^Y105ABC, CdtA^Y125ABC, CdtA^F109ABC and CdtA^S106NBC holotoxin suggesting that these sites were not critical to CDT. But the binding activity and cell cycle arrest ability of holotoxin complexes were inhibited in CdtA^W115GBC. And this site did not affect the holotoxin assembly by size exclusion chromatography. Therefore, W115 might be a critical site of CdtA binding ability. These findings suggest that the functional sites of CdtA are not only in the aromatic patch region. W115, the new functional site is critical for receptor binding and cell cycle arrest, which provides potential targets for pharmacological disruption of CDT activity.

Bacterial toxin modulation of the eukaryotic cell cycle: are all cytolethal distending toxins created equally?

The cytolethal distending toxins (CDTs) comprise a family of intracellular-acting bacterial protein toxins whose actions upon eukaryotic cells result in several consequences, the most characteristic of which is the induction of G(2)/M cell cycle arrest. Most CDTs are hetero-tripartite assemblies of CdtA, CdtB, and CdtC, with CdtB required for CDT-mediated cell cycle arrest. Several lines of evidence indicate that CdtA and CdtC are required for the optimal intracellular activity of CdtB, although the exact functional roles of CdtA and CdtC remain poorly understood. The genes encoding the CDTs have been identified in a diverse array of Gram-negative pathogenic bacteria. More recently, the genes encoding several CdtB subunits have been associated with alternatively linked subunits resembling the B-subunits of pertussis toxin. Although the CDTs are generally considered to all function as bacterial genotoxins, the extent to which individual members of the CDTs employ similar mechanisms of cell surface binding, uptake, and trafficking within sensitive cells is poorly understood. Recently, data have begun to emerge suggesting differences in the molecular basis by which individual CDTs interact with and enter host cells, suggesting the possibility that CDTs possess properties reflecting the specific niches idiosyncratic to those CDT bacterial pathogens that produce them. The extent to which functional differences between individual CDTs reflect the specific requirements for intoxicating cells and tissues within the diverse range of host microenvironments colonized by CDT-producing pathogenic bacteria remains to be experimentally explored.

Cytolethal distending toxin: a conserved bacterial genotoxin that blocks cell cycle progression, leading to apoptosis of a broad range of mammalian cell lineages

Cytolethal distending toxin (CDT) is a heterotrimeric AB-type genotoxin produced by several clinically important Gram-negative mucocutaneous bacterial pathogens. Irrespective of the bacterial species of origin, CDT causes characteristic and irreversible cell cycle arrest and apoptosis in a broad range of cultured mammalian cell lineages. The active subunit CdtB has structural homology with the phosphodiesterase family of enzymes including mammalian DNase I, and alone is necessary and sufficient to account for cellular toxicity. Indeed, mammalian cells treated with CDT initiate a DNA damage response similar to that elicited by ionizing radiation-induced DNA double strand breaks resulting in cell cycle arrest and apoptosis. The mechanism of CDT-induced apoptosis remains incompletely understood, but appears to involve both p53-dependent and -independent pathways. While epithelial, endothelial and fibroblast cell lines respond to CDT by undergoing arrest of cell cycle progression resulting in nuclear and cytoplasmic distension that precedes apoptotic cell death, cells of haematopoietic origin display rapid apoptosis following a brief period of cell cycle arrest. In this review, the ecology of pathogens producing CDT, the molecular biology of bacterial CDT and the molecular mechanisms of CDT-induced cytotoxicity are critically appraised. Understanding the contribution of a broadly conserved bacterial genotoxin that blocks progression of the mammalian cell cycle, ultimately causing cell death, should assist with elucidating disease mechanisms for these important pathogens.

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.

Cytolethal distending toxin type I and type IV genes are framed with lambdoid prophage genes in extraintestinal pathogenic Escherichia coli

Five types of cytolethal distending toxin (CDT-I to CDT-V) have been identified in Escherichia coli. In the present study we cloned and sequenced the cdt-IV operon and flanking region from a porcine extraintestinal pathogenic E. coli (ExPEC) strain belonging to serogroup O75. We confirmed that similar to other CDTs, CDT-IV induced phosphorylation of host histone H2AX, a sensitive marker of DNA double-strand breaks, and blocked the HeLa cell cycle at the G(2)-M transition. The cdt-IV genes were framed by lambdoid prophage genes. We cloned and sequenced the cdt-I operon and flanking regions from a human ExPEC O18:K1:H7 strain and observed that cdt-I genes were also flanked by lambdoid prophage genes. PCR studies indicated that a gene coding for a putative protease was always associated with the cdtC-IV gene but was not associated with cdtC genes in strains producing CDT-I, CDT-III, and CDT-V. Our results suggest that the cdt-I and cdt-IV genes might have been acquired from a common ancestor by phage transduction and evolved in their bacterial hosts. The lysogenic bacteriophages have the potential to carry nonessential "cargo" genes or "morons" and therefore play a crucial role in the generation of genetic diversity within ExPEC.

In vivo virulence properties of bacterial cytolethal-distending toxin

Multiple pathogenic Gram-negative bacteria produce cytolethal-distending toxins (CDTs). CDT is typically composed of three subunits: the catalytic subunit CdtB has DNase I-like activity, whereas CdtA and CdtC are binding proteins for delivering CdtB into target cells. Translocation of CdtB to the nucleus induces genotoxic effects on host DNA, triggering DNA repair cascades that lead to cell cycle arrest and eventual cell death. Several lines of evidence indicate that this toxin contributes to the pathogenicity of CDT-producing bacteria in vivo. Helicobacter hepaticus and Campylobacter jejuni CDTs are essential for persistent infection of the gastrointestinal tract and increase the severity of mucosal inflammation or liver disease in susceptible mouse strains. Haemophilus ducreyi CDT may contribute to the pathogenesis of chancroid in rabbits. Recently, H. hepaticus CDT has been shown to play a crucial role in promoting the progression of infectious hepatitis to pre-malignant, dysplastic lesions via activation of a pro-inflammatory NF-kappaB pathway and increased proliferation of hepatocytes, providing the first evidence that CDT has carcinogenic potential in vivo. Thus, both in vitro and in vivo data indicate that CDT is a bacterial virulence factor.

Haemophilus ducreyi partially activates human myeloid dendritic cells

Dendritic cells (DC) orchestrate innate and adaptive immune responses to bacteria. How Haemophilus ducreyi, which causes genital ulcers and regional lymphadenitis, interacts with DC is unknown. H. ducreyi evades uptake by polymorphonuclear leukocyte and macrophage-like cell lines by secreting LspA1 and LspA2. Many H. ducreyi strains express cytolethal distending toxin (CDT), and recombinant CDT causes apoptosis of DC in vitro. Here, we examined interactions between DC and H. ducreyi 35000HP, which produces LspA1, LspA2, and CDT. In human volunteers infected with 35000HP, the ratio of myeloid DC to plasmacytoid DC was 2.8:1 in lesions, compared to a ratio of 1:1 in peripheral blood. Using myeloid DC derived from monocytes as surrogates for lesional DC, we found that DC infected with 35000HP remained as viable as uninfected DC for up to 48 h. Gentamicin protection and confocal microscopy assays demonstrated that DC ingested and killed 35000HP, but killing was incomplete at 48 h. The expression of LspA1 and LspA2 did not inhibit the uptake of H. ducreyi, despite inactivating Src kinases. Infection of DC with live 35000HP caused less cell surface marker activation than infection with heat-killed 35000HP and lipopolysaccharide (LPS) and inhibited maturation by LPS. However, infection of DC with live bacteria caused the secretion of significantly higher levels of interleukin-6 and tumor necrosis factor alpha than infection with heat-killed bacteria and LPS. The survival of H. ducreyi in DC may provide a mechanism by which the organism traffics to lymph nodes. Partial activation of DC may abrogate the establishment of a full Th1 response and an environment that promotes phagocytosis.

Bacterial cytolethal distending toxin promotes the development of dysplasia in a model of microbially induced hepatocarcinogenesis

Bacterial cytolethal distending toxins (CDTs) containing DNase I-like activity can induce limited host DNA damage that leads to activation of the DNA-damage repair responses in cultured cell lines. However, in vivo experimental evidence linking CDTs to carcinogenesis is lacking. In this study, infection of A/JCr mice with an isogenic mutant of Helicobacter hepaticus lacking CDT activity (CDT mutant) induced chronic hepatitis comparable to wild-type H. hepaticus (Hh) infection at both 4 and 10 months post inoculation (MPI); however, the CDT mutant-infected mice did not develop hepatic dysplasic nodules at 10 MPI, whereas those infected with Hh did. There was no significant difference in hepatic colonization levels between the CDT mutant and Hh at both time points (P > 0.05). At 4 MPI, mice infected with Hh had significantly enhanced hepatic transcription of proinflammatory TNF-alpha, IFN-gamma and Cox-2, growth mediators IL-6 and TGF-alpha, anti-apoptotic Bcl-2 and Bcl-X(L), and increased hepatocyte proliferation (P < 0.05) compared with the control or the CDT mutant-infected mice. In addition, Hh infected male mice had upregulated hepatic mRNA levels of RelA (p65), p50, GADD45beta and c-IAP1, components of the NF-kappaB pathway compared with the CDT mutant-infected mice. At 10 MPI, Hh infection was associated with significant upregulation of IL-6 mRNA. Activation of the inflammatory NF-kappaB pathway and upregulation of proinflammatory cytokines plus IL-6 in the Hh but not in the CDT mutant-infected mice suggest that Hh CDT plays a key role in promoting the dysplastic changes in Hh-infected mouse livers.

The contribution of cytolethal distending toxin to bacterial pathogenesis

Cytolethal distending toxin (CDT) is a bacterial toxin that initiates a eukaryotic cell cycle block at the G2 stage prior to mitosis. CDT is produced by a number of bacterial pathogens including: Campylobacter species, Escherichia coli, Salmonella enterica serovar Typhi, Shigella dystenteriae, enterohepatic Helicobacter species, Actinobacillus actinomycetemcomitans (the cause of aggressive periodontitis), and Haemophilus ducreyi (the cause of chancroid). The functional toxin is composed of three proteins; CdtB potentiates a cascade leading to cell cycle block, and CdtA and CdtC function as dimeric subunits, which bind CdtB and delivers it to the mammalian cell interior. Once inside the cell, CdtB enters the nucleus and exhibits a DNase I-like activity that results in DNA double-strand breaks. The eukaryotic cell responds to the DNA double-strand breaks by initiating a regulatory cascade that results in cell cycle arrest, cellular distension, and cell death. Mutations in CdtABC that cause any of the three subunits to lose function prevent the bacterial cell from inducing cytotoxicity. The result of CDT activity can differ somewhat depending on the eukaryotic cell types affected. Epithelial cells, endothelial cells, and keratinocytes undergo G2 cell cycle arrest, cellular distension, and death; fibroblasts undergo G1 and G2 arrest, cellular distension, and death; and immune cells undergo G2 arrest followed by apoptosis. CDT contributes to pathogenesis by inhibiting both cellular and humoral immunity via apoptosis of immune response cells, and by generating necrosis of epithelial-type cells and fibroblasts involved in the repair of lesions produced by pathogens resulting in slow healing and production of disease symptoms. Thus, CDT may function as a virulence factor in pathogens that produce the toxin.

Role of intrachain disulfides in the activities of the CdtA and CdtC subunits of the cytolethal distending toxin of Actinobacillus actinomycetemcomitans

The cytolethal distending toxin (Cdt) of Actinobacillus actinomycetemcomitans is an atypical A-B-type toxin consisting of a heterotrimer composed of the cdtA, cdtB, and cdtC gene products. The CdtA and CdtC subunits form two heterogeneous ricin-like lectin domains which bind the holotoxin to the target cell. Point mutations were used to study CdtC structure and function. One (mutC216(F97C)) of eight single-amino-acid replacement mutants identified yielded a gene product that failed to form biologically active holotoxin. Based on the possibility that the F97C mutation destabilized a predicted disulfide, targeted mutagenesis was used to examine the contribution of each of four cysteine residues, in two predicted disulfides (C96/C107 and C135/C149), to CdtC activities. Cysteine replacement mutations in two predicted disulfides (C136/C149 and C178/C197) in CdtA were also characterized. Flow cytometry and CHO cell proliferation assays showed that changing either C96 or C149 in CdtC to alanine abolished the biological activity of holotoxin complexes. However, replacing C107 or C135 in CdtC and any of the four cysteines in CdtA with alanine or serine resulted in only partial or no loss of holotoxin activity. Changes in the biological activities of the mutant holotoxins correlated with altered subunit binding. In contrast to elimination of the B chain of ricin, the elimination of intrachain disulfides in CdtC and CdtA by genetic replacement of cysteines destabilizes these subunit proteins but not to the extent that cytotoxicity is lost. Reduction of the wild-type holotoxin did not affect cytotoxicity, and the reduced form of wild-type CdtA exhibited a statistically significant increase in binding to ligand. A diminished role for intrachain disulfides in stabilizing CdtA and CdtC may have clinical relevance for the A. actinomycetemcomitans Cdt. The cdt gene products secreted by this pathogen assemble and bind to target cells in periodontally involved sites, which are decidedly reduced environments in the human oral cavity.

Modulation of host immune responses by the cytolethal distending toxin of Helicobacter hepaticus

Persistent murine infection with Helicobacter hepaticus leads to chronic gastrointestinal inflammation and neoplasia in susceptible strains. To determine the role of the virulence factor cytolethal distending toxin (CDT) in the pathogenesis of this organism, interleukin-10-deficient (IL-10-/-) mice were experimentally infected with wild-type H. hepaticus and a CDT-deficient isogenic mutant. Both wild-type H. hepaticus and the CDT-deficient mutant successfully colonized IL-10-/- mice, and they reached similar tissue levels by 6 weeks after infection. Only animals infected with wild-type type H. hepaticus developed significant typhlocolitis. However, by 4 months after infection, the CDT-deficient mutant was no longer detectable in IL-10-/- mice, whereas wild-type H. hepaticus persisted for the 8-month duration of the experiment. Animals infected with wild-type H. hepaticus exhibited severe typhlocolitis at 8 months after infection, while animals originally challenged with the CDT-deficient mutant had minimal cecal inflammation at this time point. In follow-up experiments, animals that cleared infection with the CDT-deficient mutant were protected from rechallenge with either mutant or wild-type H. hepaticus. Animals infected with wild-type H. hepaticus developed serum immunoglobulin G1 (IgG1) and IgG2c responses against H. hepaticus, while animals challenged with the CDT-deficient mutant developed significantly lower IgG2c responses and failed to mount IgG1 responses against H. hepaticus. These results suggest that CDT plays a key immunomodulatory role that allows persistence of H. hepaticus and that in IL-10-/- mice this alteration of the host immune response results in the development of colitis.

Characterization of point mutations in the cdtA gene of the cytolethal distending toxin of Actinobacillus actinomycetemcomitans

The Cdt is a family of gram-negative bacterial toxins that typically arrest eukaryotic cells in the G0/G1 or G2/M phase of the cell cycle. The toxin is a heterotrimer composed of the cdtA, cdtB and cdtC gene products. Although it has been shown that the CdtA protein subunit binds to cells in culture and in an enzyme-linked immunosorbent assay (CELISA) the precise mechanisms by which CdtA interacts with CdtB and CdtC has not yet been clarified. In this study we employed a random mutagenesis strategy to construct a library of point mutations in cdtA to assess the contribution of individual amino acids to binding activity and to the ability of the subunit to form biologically active holotoxin. Single unique amino acid substitutions in seven CdtA mutants resulted in reduced binding of the purified recombinant protein to Chinese hamster ovary cells and loss of binding to the fucose-containing glycoprotein, thyroglobulin. These mutations clustered at the 5'- and 3'-ends of the cdtA gene resulting in amino acid substitutions that resided outside of the aromatic patch region and a conserved region in CdtA homologues. Three of the amino acid substitutions, at positions S165N (mutA81), T41A (mutA121) and C178W (mutA221) resulted in gene products that formed holotoxin complexes that exhibited a 60% reduction (mutA81) or loss (mutA121, mutA221) of proliferation inhibition. A similar pattern was observed when these mutant holotoxins were tested for their ability to induce cell cycle arrest and to convert supercoiled DNA to relaxed and linear forms in vitro. The mutations in mutA81 and mutA221 disrupted holotoxin formation. The positions of the amino acid substitutions were mapped in the Haemophilus ducreyi Cdt crystal structure providing some insight into structure and function.

Mechanisms of assembly and cellular interactions for the bacterial genotoxin CDT

Many bacterial pathogens that cause different illnesses employ the cytolethal distending toxin (CDT) to induce host cell DNA damage, leading to cell cycle arrest or apoptosis. CDT is a tripartite holotoxin that consists of a DNase I family nuclease (CdtB) bound to two ricin-like lectin domains (CdtA and CdtC). Through the use of structure-based mutagenesis, biochemical and cellular toxicity assays, we have examined several key structural elements of the CdtA and CdtC subunits for their importance to toxin assembly, cell surface binding, and activity. CdtA and CdtC possess N- and C-terminal nonglobular polypeptides that extensively interact with each other and CdtB, and we have determined the contribution of each to toxin stability and activity. We have also functionally characterized two key binding elements of the holotoxin revealed from its crystal structure. One is an aromatic cluster in CdtA, and the other is a long and deep groove that is formed at the interface of CdtA and CdtC. We demonstrate that mutations of the aromatic patch or groove residues impair toxin binding to HeLa cells and that cell surface binding is tightly correlated with intoxication of cultured cells. These results establish several structure-based hypotheses for the assembly and function of this toxin family.

The cytolethal distending toxin is used by many bacteria to damage the DNA of infected organisms. This DNA damage prevents cells from dividing and eventually leads to cell death, which raises the possibility that this genomic damage may be a contributing factor to carcinogenesis. The cytolethal distending toxin is composed of three proteins that form a tightly associated complex. After secretion by the bacterium, two proteins in this complex adhere to the cell surface and achieve the delivery of the third protein into the cell, where it causes DNA lesions. This report examines how this toxin is assembled and how it adheres to host cell surfaces. A set of molecular features on the toxin is shown to be critical for this cell adherence and for the ability of the cytolethal distending toxin to inhibit cell division. These results tie together for the first time aspects of the molecular structure of the cytolethal distending toxin and its ability to adhere to host cell surfaces, contributing to mechanistic understanding of the activity of this genotoxin.

The cytolethal distending toxin of Haemophilus ducreyi aggravates dermal lesions in a rabbit model of chancroid

Haemophilus ducreyi, the etiologic agent of the sexually transmitted disease chancroid, produces a cytolethal distending toxin (HdCDT) that inhibits cultured cell proliferation, leading to cell death. A rabbit model of dermal infection was used to investigate the roles of H. ducreyi bacteria and HdCDT in the development, clinical appearance, and persistence of infection. A non-toxin producing H. ducreyi strain, and for comparison purposes a non-capsulated Haemophilus influenzae strain, were inoculated intradermally, with and without co-administration of purified HdCDT. Co-administration of HdCDT resulted in significant aggravation of H. ducreyi-induced inflammatory lesions, and development of ulcers in rabbit skin. Less pronounced inflammatory lesions and lack of epithelial eruption were observed after inoculation with H. influenzae. Histopathological sections of the H. ducreyi-induced lesions, in both the presence and absence of HdCDT, showed dense infiltrates of the same type inflammatory cells, with the exception of a prominent endothelial cell proliferation noted in sections from lesions caused by H. ducreyi and toxin. Signs of chronic inflammation with involvement of T cells, macrophages, eosinophils, and granuloma formation were observed after H. ducreyi inoculation both with and without toxin. In conclusion, H. ducreyi causes a pronounced, chronic inflammation with involvement of T cells and macrophages, and in combination with HdCDT production of ulcers in the rabbit model. These pathogenic mechanisms may promote the development and persistence of chancroid ulcers.

Cytolethal distending toxin: creating a gap in the cell cycle

Cytolethal distending toxin (CDT) is a novel bacterial toxin that is produced by a variety of pathogenic bacteria. The mechanism of cytotoxicity of CDT is unique in that it enters into eukaryotic cells and breaks double-stranded DNA. This initiates the cell's own DNA damage-response mechanisms, resulting in the arrest of the cell cycle at the G2/M boundary. Affected cells enlarge until they finally undergo programmed cell death. This review encompasses recent work on CDT and focuses on the molecular mechanisms used by this toxin to block cell-cycle progression, the benefit to the bacterium of possession of this toxin and the clinical relevance of intoxication.

Differential expression of porins OmpP2A and OmpP2B of Haemophilus ducreyi

Haemophilus ducreyi is a strict human pathogen and the causative agent of the sexually transmitted disease chancroid. The genome of the human-passaged strain of H. ducreyi (35000HP) contains two homologous genes whose protein products have estimated molecular masses of 46 and 43 kDa. A comparative analysis of the deduced amino acid sequences revealed that these proteins share 27 to 33% identity to the outer membrane protein P2 (OmpP2), a major porin of Haemophilus influenzae. Therefore, these proteins have been designated OmpP2A and OmpP2B, respectively. The detection of ompP2A and ompP2B transcripts by reverse transcriptase PCR indicated that these genes were independently transcribed in H. ducreyi 35000HP. Western blot analysis of outer membrane proteins isolated from a geographically diverse collection of H. ducreyi clinical isolates revealed that OmpP2A and OmpP2B were differentially expressed among these strains. Although PCR analysis suggested that ompP2A and ompP2B were conserved among the strains tested, the differential expression observed was due to nucleotide additions and partial gene deletions. Purified OmpP2A and OmpP2B were isolated under nondenaturing conditions, and subsequent analysis demonstrated that these two proteins exhibited porin activity. OmpP2A and OmpP2B are the first porins described for H. ducreyi.

Cytolethal distending toxins

The cytolethal distending toxins (CDTs) constitute the most recently discovered family of bacterial protein toxins. CDTs are unique among bacterial toxins as they have the ability to induce DNA double strand breaks (DSBs) in both proliferating and nonproliferating cells, thereby causing irreversible cell cycle arrest or death of the target cells. CDTs are encoded by three linked genes ( cdtA, cdtB and cdtC) which have been identified among a variety of Gram-negative pathogenic bacteria. All three of these gene products are required to constitute the fully active holotoxin, and this is in agreement with the recently determined crystal structure of CDT. The CdtB component has functional homology with mammalian deoxyribonuclease I (DNase I). Mutation of the conserved sites necessary for this catalytic activity prevents the induction of DSBs as well as all subsequent intoxication responses of target cells. CDT is endocytosed via clathrin-coated pits and requires an intact Golgi complex to exert the cytotoxic activity. Several issues remain to be elucidated regarding CDT biology, such as the detailed function(s) of the CdtA and CdtC subunits, the identity of the cell surface receptor(s) for CDT, the final steps in the cellular internalization pathway, and a molecular understanding of how CDT interacts with DNA. Moreover, the role of CDTs in the pathogenesis of diseases still remains unclear.

Assembly and function of a bacterial genotoxin

The tripartite cytolethal distending toxin (CDT) induces cell cycle arrest and apoptosis in eukaryotic cells. The subunits CdtA and CdtC associate with the nuclease CdtB to form a holotoxin that translocates CdtB into the host cell, where it acts as a genotoxin by creating DNA lesions. Here we show that the crystal structure of the holotoxin from Haemophilus ducreyi reveals that CDT consists of an enzyme of the DNase-I family, bound to two ricin-like lectin domains. CdtA, CdtB and CdtC form a ternary complex with three interdependent molecular interfaces, characterized by globular, as well as extensive non-globular, interactions. The lectin subunits form a deeply grooved, highly aromatic surface that we show to be critical for toxicity. The holotoxin possesses a steric block of the CdtB active site by means of a non-globular extension of the CdtC subunit, and we identify putative DNA binding residues in CdtB that are essential for toxin activity.

In vitro and in vivo characterization of Helicobacter hepaticus cytolethal distending toxin mutants

Helicobacter hepaticus expresses a member of the cytolethal distending toxin (CDT) family of bacterial cytotoxins. To investigate the role of CDT in the pathogenesis of H. hepaticus, transposon mutagenesis was used to generate a series of isogenic mutants in and around the cdtABC gene cluster. An H. hepaticus transposon mutant with a disrupted cdtABC coding region no longer produced CDT activity. Conversely, a transposon insertion outside of the cluster did not affect the CDT activity. An examination of these mutants demonstrated that CDT represents the previously described granulating cytotoxin in H. hepaticus. Challenge of C57BL/6 interleukin 10(-/-) mice with isogenic H. hepaticus mutants revealed that CDT expression is not required for colonization of the murine gut. However, a CDT-negative H. hepaticus mutant had a significantly diminished capacity to induce lesions in this murine model of inflammatory bowel disease.

The LspB protein is involved in the secretion of the LspA1 and LspA2 proteins by Haemophilus ducreyi

The LspA1 and LspA2 proteins of Haemophilus ducreyi 35000 are two very large macromolecules that can be detected in concentrated culture supernatant fluid. Both of these proteins exhibit homology with the N-terminal region of the Bordetella pertussis filamentous hemagglutinin (FHA), which is involved in secretion of the latter macromolecule. The lspA2 open reading frame is flanked upstream by a gene, lspB, that encodes a predicted protein with homology to the B. pertussis FhaC outer membrane protein that is involved in secretion of FHA across the outer membrane. The H. ducreyi lspB gene encodes a protein with a predicted molecular mass of 66,573 Da. Reverse transcription-PCR analysis suggested that the lspB gene was transcribed together with the lspA2 gene on a single mRNA transcript. Polyclonal H. ducreyi LspB antiserum reacted with a 64-kDa antigen present in the Sarkosyl-insoluble cell envelope fraction of H. ducreyi 35000, which indicated that the LspB protein is likely an outer membrane protein. Concentrated culture supernatant fluids from H. ducreyi lspB and lspA1 lspB mutants did not contain detectable LspA1 and detectable LspA2, respectively. However, complementation of the lspB mutant with the wild-type lspB gene on a plasmid restored LspB protein expression and resulted in release of detectable amounts of the LspA1 protein into culture supernatant fluid. When evaluated in the temperature-dependent rabbit model of infection, the lspB mutant was attenuated in the ability to cause lesions and was never recovered in a viable form from lesions. These results indicated that the H. ducreyi LspB protein is involved in secretion of the LspA1 and LspA2 proteins across the outer membrane.

Caspase-2 and caspase-7 are involved in cytolethal distending toxin-induced apoptosis in Jurkat and MOLT-4 T-cell lines

Cytolethal distending toxin (CDT) from Actinobacillus actinomycetemcomitans is a G2/M cell-cycle-specific growth-inhibitory toxin that leads to target cell distension followed by cell death. To determine the mechanisms by which A. actinomycetemcomitans CDT acts as an immunosuppressive factor, we examined the effects of highly purified CDT holotoxin on human T lymphocytes. Purified CDT was cytolethal toward normal peripheral T lymphocytes that were activated by in vitro stimulation with phytohemagglutinin. In addition, purified CDT showed cytolethal activity against Jurkat and MOLT-4 cells, which are known to be sensitive and resistant, respectively, to Fas-mediated apoptosis. Death in these cell lines was accompanied by the biochemical features of apoptosis, including membrane conformational changes, intranucleosomal DNA cleavage, and an increase in caspase activity in the cells. Pretreatment of Jurkat cells with the general caspase inhibitor z-VAD-fmk mostly suppressed CDT-induced apoptosis. Furthermore, specific inhibitors of caspase-2 and -7 showed significant inhibitory effects on CDT-induced apoptosis in Jurkat cells, and these inhibitory effects were fully associated with reduced activity of caspase-2 or -7 in the CDT-treated Jurkat cells. These results strongly suggest that CDT possesses the ability to induce human T-cell apoptosis through activation of caspase-2 and -7.

Induction of T-cell apoptosis by Actinobacillus actinomycetemcomitans mutants with deletion of ltxA and cdtABC genes: possible activity of GroEL-like molecule

The pathogenic bacterium Actinobacillus actinomycetemcomitans expresses a leukotoxin (Ltx) and cytolethal distending toxin (CDT) with cytolytic properties. CDT also has cytostatic properties, inducing a G2 cell cycle block. The extent of the contribution of these, as well as other toxins, to the cytolytic and cytostatic activities of this microorganism have not been defined and the aim of this study was to determine their contribution. To that end, a naturally transformable A. actinomycetemcomitans clinical strain (D7S-smooth) was used to construct a series of deletion mutants (DeltacdtA, DeltacdtB, DeltacdtC, DeltacdtABC, DeltaltxA, DeltaltxA/DeltacdtABC). Human peripheral blood mononuclear cells were incubated with cell-associated and extracellular bacterial preparations. The ability of wild type and isogenic mutants to induce T-cell apoptosis and cell cycle arrest was compared. The expression of ltxA and each of the cdt gene loci partially contributed to A. actinomycetemcomitans apoptosis, since each of the isogenic mutants exhibited reduced ability to induce T-cell apoptosis. Conversely, the ability to induce cell cycle block was abolished in each of the cdt isogenic mutants. A mutant with simultaneous deletion of ltxA and cdtABC genes retained potent ability to induce apoptosis in its cell-associated, but not extracellular, preparation. Neutralization with Escherichia coli anti-GroEL monoclonal antibody, lead to significant diminution of apoptosis-inducing activity of the DeltaltxA/DeltacdtABC cell-associated preparation. These data provide evidence for the expression of other A. actinomycetemcomitans cytolytic molecule(s) distinct from CDT and leukotoxin, with a possible role for GroEL-like molecule in T-cell apoptosis.

[Bacterial cyclostatin, or how do bacteria manipulate the eukaryotic cell cycle]

Several bacterial proteins have been recently described that share the ability to inhibit the proliferation of cells in culture without causing early signs of cytotoxicity. Such observations suggest the existence of bacterial mechanisms of control of the eukaryotic cell cycle contributing to pathogenicity or adaptation to the host. This emerging concept of cellular microbiology is critically analyzed considering as a model the cytolethal distending toxins (CDT), a family of toxins whose mode of action on the cell cycle has been thoroughly studied over the last few years. CDTs activate a physiological G2 checkpoint in exposed cells, probably from an initial DNA alteration whose precise molecular nature has not yet been determined. Experimental data are lacking to extrapolate in vivo the antiproliferative effect of these bacterial proteins that we tentatively propose to call cyclostatins.

A CdtA-CdtC complex can block killing of HeLa cells by Haemophilus ducreyi cytolethal distending toxin

The cytolethal distending toxin (CDT) of Haemophilus ducreyi is comprised of the CdtA, CdtB, and CdtC proteins, with the CdtB protein having demonstrated enzymatic (i.e., DNase) activity. Using a single recombinant Escherichia coli strain with two plasmids individually containing the H. ducreyi cdtA and cdtC genes, we purified a noncovalent CdtA-CdtC complex. Incubation of this CdtA-CdtC complex with HeLa cells blocked killing of these cells by CDT holotoxin. Furthermore, the addition of purified recombinant CdtB to HeLa cells preincubated with the CdtA-CdtC complex resulted in the killing of these human epithelial cells.

Expression of the cytolethal distending toxin in a geographically diverse collection of Haemophilus ducreyi clinical isolates

OBJECTIVE

To screen a collection of isolates of Haemophilus ducreyi for expression of the cytolethal distending toxin (CDT).

METHODS

45 clinical isolates of H ducreyi were screened for cytotoxic activity by examining the effect of culture supernatants on Hela cells. Expression was confirmed using immunoblotting with CDT specific monoclonal antibodies and the presence of the cdt genes determined by amplification of the cdt genes in a multiplex polymerase chain assay.

RESULTS

Of the 45 clinical isolates, six isolates from differing geographical origins did not demonstrate cytotoxic activity. Expression of CDT was also not detected in these six isolates using immunoblotting and the genes cdtA, cdtB, and cdtC were not amplified using PCR. The remaining isolates demonstrated cytotoxic activity, expressed the CDT proteins, and the presence of the cdt genes was confirmed.

CONCLUSIONS

CDT is considered a virulence factor of H ducreyi but was found to be absent in 13% of isolates from different geographical origins.

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.

Haemophilus ducreyi requires the flp gene cluster for microcolony formation in vitro

Haemophilus ducreyi, the etiologic agent of chancroid, has been shown to form microcolonies when cultured in the presence of human foreskin fibroblasts. We identified a 15-gene cluster in H. ducreyi that encoded predicted protein products with significant homology to those encoded by the tad (for tight adhesion) locus in Actinobacillus actinomycetemcomitans that is involved in the production of fimbriae by this periodontal pathogen. The first three open reading frames in this H. ducreyi gene cluster encoded predicted proteins with a high degree of identity to the Flp (fimbria-like protein) encoded by the first open reading frame of the tad locus; this 15-gene cluster in H. ducreyi was designated flp. RT-PCR analysis indicated that the H. ducreyi flp gene cluster was likely to be a polycistronic operon. Mutations within the flp gene cluster resulted in an inability to form microcolonies in the presence of human foreskin fibroblasts. In addition, the same mutants were defective in the ability to attach to both plastic and human foreskin fibroblasts in vitro. An H. ducreyi mutant with an inactivated tadA gene exhibited a small decrease in virulence in the temperature-dependent rabbit model for experimental chancroid, whereas another H. ducreyi mutant with inactivated flp-1 and flp-2 genes was as virulent as the wild-type parent strain. These results indicate that the flp gene cluster is essential for microcolony formation by H. ducreyi, whereas this phenotypic trait is not linked to the virulence potential of the pathogen, at least in this animal model of infection.

The cytolethal distending toxin of Haemophilus ducreyi inhibits endothelial cell proliferation

Haemophilus ducreyi, the etiologic agent of the sexually transmitted disease chancroid, produces a cytolethal distending toxin (HdCDT) that inhibits mammalian cell proliferation. We investigated the effects of HdCDT on normal human endothelial cells and on tubule formation in an in vitro model of angiogenesis. Endothelial cells were arrested in the G2 phase of the cell cycle, and tubule formation was inhibited in a dose-dependent manner. The antiproliferative activities of HdCDT on endothelial cells might contribute to the characteristic slow healing and persistence of chancroid ulcers.

Functional studies of the recombinant subunits of a cytolethal distending holotoxin

Cytolethal distending toxin (CDT) is a multicomponent bacterial holotoxin that targets most eukarytotic cells causing distension and cell cycle arrest. A number of diverse pathogenic bacterial species associated with diarrhoea, chancroid, chronic hepatitis and periodontal disease produce a CDT. Synthesis of the holotoxin is directed by the expression of three genes, cdtA, cdtB and cdtC. Although the product of the CdtB gene was previously identified as a type I deoxyribonuclease, the functions of the cdtA and cdtC products have not been characterized. Using the periodontal pathogen, Actinobacillus actinomycetemcomitans, we demonstrate that the recombinant product of the CdtA gene binds to the surface of Chinese hamster ovary (CHO) cells. This protein did not induce distension or cytotoxicity when introduced into the cytosol using a lipid-based protein delivery system. Recombinant CdtB and CdtC proteins failed to bind to CHO cells. However, the delivery of either CdtB or CdtC into the cytosol resulted in the characteristic pattern of distension followed by cell death. Based on these results, it appears that the CdtA protein subunit alone is responsible for anchoring the holotoxin to the cell surface. The CdtC subunit, in concert with CdtB, contributes to the cytotoxic activities of the holotoxin. The specific mechanism of CdtC cytotoxicity is currently unknown.