Comparative studies on the biology of Actinobacillus actinomycetemcomitans leukotoxin in primates

Toxin-Triggered Liposomes for the Controlled Release of Antibiotics to Treat Infections Associated with Gram-Negative Bacteria

Antibiotic resistance has become an urgent threat to health care in recent years. The use of drug delivery systems provides advantages over conventional administration of antibiotics and can slow the development of antibiotic resistance. In the current study, we developed a toxin-triggered liposomal antibiotic delivery system, in which the drug release is enabled by the leukotoxin (LtxA) produced by the Gram-negative pathogen, Aggregatibacter actinomycetemcomitans. LtxA has previously been shown to mediate membrane disruption by promoting a lipid phase change in nonlamellar lipids, such as 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-methyl (N-methyl-DOPE). In addition, LtxA has been observed to bind strongly and nearly irreversibly to membranes containing large amounts of cholesterol. Here, we designed a liposomal delivery system composed of N-methyl-DOPE and cholesterol to take advantage of these interactions. Specifically, we hypothesized that liposomes composed of N-methyl-DOPE and cholesterol, encapsulating antibiotics, would be sensitive to LtxA, enabling controlled antibiotic release. We observed that liposomes composed of N-methyl-DOPE were sensitive to the presence of low concentrations of LtxA, and cholesterol increased the extent and kinetics of content release. The liposomes were stable under various storage conditions for at least 7 days. Finally, we showed that antibiotic release occurs selectively in the presence of an LtxA-producing strain of A. actinomycetemcomitans but not in the presence of a non-LtxA-expressing strain. Together, these results demonstrate that the designed liposomal vehicle enables toxin-triggered delivery of antibiotics to LtxA-producing strains of A. actinomycetemcomitans.

Kingella kingae RtxA toxin interacts with sialylated gangliosides

The membrane-damaging RTX family cytotoxin RtxA is a key virulence factor of the emerging pediatric pathogen Kingella kingae, but little is known about the mechanism of RtxA binding to host cells. While we have previously shown that RtxA binds cell surface glycoproteins, here we demonstrate that the toxin also binds different types of gangliosides. The recognition of gangliosides by RtxA depended on sialic acid side groups of ganglioside glycans. Moreover, binding of RtxA to epithelial cells was significantly decreased in the presence of free sialylated gangliosides, which inhibited cytotoxic activity of the toxin. These results suggest that RtxA utilizes sialylated gangliosides as ubiquitous cell membrane receptor molecules on host cells to exert its cytotoxic action and support K. kingae infection.

Kingella kingae RtxA Cytotoxin in the Context of Other RTX Toxins

The Gram-negative bacterium Kingella kingae is part of the commensal oropharyngeal flora of young children. As detection methods have improved, K. kingae has been increasingly recognized as an emerging invasive pathogen that frequently causes skeletal system infections, bacteremia, and severe forms of infective endocarditis. K. kingae secretes an RtxA cytotoxin, which is involved in the development of clinical infection and belongs to an ever-growing family of cytolytic RTX (Repeats in ToXin) toxins secreted by Gram-negative pathogens. All RTX cytolysins share several characteristic structural features: (i) a hydrophobic pore-forming domain in the N-terminal part of the molecule; (ii) an acylated segment where the activation of the inactive protoxin to the toxin occurs by a co-expressed toxin-activating acyltransferase; (iii) a typical calcium-binding RTX domain in the C-terminal portion of the molecule with the characteristic glycine- and aspartate-rich nonapeptide repeats; and (iv) a C-proximal secretion signal recognized by the type I secretion system. RTX toxins, including RtxA from K. kingae, have been shown to act as highly efficient ‘contact weapons’ that penetrate and permeabilize host cell membranes and thus contribute to the pathogenesis of bacterial infections. RtxA was discovered relatively recently and the knowledge of its biological role remains limited. This review describes the structure and function of RtxA in the context of the most studied RTX toxins, the knowledge of which may contribute to a better understanding of the action of RtxA in the pathogenesis of K. kingae infections.

Adaptation by Ancient Horizontal Acquisition of Butyrate Metabolism Genes in Aggregatibacter actinomycetemcomitans

There has been considerable interest in the impact of short-chain fatty acids (SCFAs) on inflammatory effects related to the microbiome. Here, we present evidence that SCFAs may also be important in disease by providing an energy source or disease-associated cue for colonizing pathogens.

Like the bacterial residents of the human gut, it is likely that many of the species in the human oral microbiota have evolved to better occupy and persist in their niche. Aggregatibacter actinomycetemcomitans (Aa) is both a common colonizer of the oral cavity and has been implicated in the pathogenesis of periodontal disease. Here, we present a whole-genome phylogenetic analysis of Aa isolates from humans and nonhuman primates that revealed an ancient origin for this species and a long history of association with the Catarrhini, the lineage that includes Old World monkeys (OWM) and humans. Further genomic analysis showed a strong association with the presence of a short-chain fatty acid (SCFA) catabolism locus (atoRDAEB) in many human isolates that was absent in almost all nonhuman OWM isolates. We show that this locus was likely acquired through horizontal gene transfer. When grown under conditions that are similar to those at the subgingival site of periodontitis (anaerobic, SCFA replete), Aa strains with atoRDAEB formed robust biofilms and showed upregulation of genes involved in virulence, colonization, and immune evasion. Both an isogenic deletion mutant and nonhuman primate isolates lacking the ato locus failed to grow in a robust biofilm under these conditions, but grew well under the carbohydrate-rich conditions similar to those found above the gumline. We propose that the acquisition of the ato locus was a key evolutionary step allowing Aa to utilize SCFAs, adapt, and modulate subgingival disease.

Etiology of aggressive periodontitis in individuals of African descent

Aggressive periodontitis (AgP) is a form of periodontitis that affects adolescents and has a significantly higher prevalence in individuals of African descent. AgP typically shows familial aggregation, suggesting a genetic predisposition. Young age, good health status, rapid attachment loss, and familial aggregation are the primary features of this disease. AgP has been closely linked to specific bacterial strains of Aggregatibacter actinomycetemcomitans. A. actinomycetemcomitans strains isolated from patients with AgP produce leukotoxin (LtxA), which specifically affects polymorphonuclear leukocytes in primates, especially humans. High-throughput 16S rRNA gene sequencing and bioinformatics analyses revealed differences in the subgingival microbiota between patients with AgP and those with chronic periodontitis (ChP). The genera Atopobium and Prevotella show increased prevalences in AgP than in ChP. According to AgP susceptibility, several single nucleotide polymorphisms have been detected in different genes in individuals of African descent. Interleukin (IL)-1α and IL-1β genetic polymorphisms may be associated with the severity of both ChP and AgP. An elevated serum level of IL-17 produced by Th17 cells may be a characteristic of AgP. Analyses of the relationships among bacteria, host defenses, genetic predisposition, and numerous other factors are required to understand the progression of this disease.

Aggregatibacter actinomycetemcomitans leukotoxin: From mechanism to targeted anti-toxin therapeutics

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium associated with localized aggressive periodontitis, as well as other systemic diseases. This organism produces a number of virulence factors, all of which provide some advantage to the bacterium. Several studies have demonstrated that clinical isolates from diseased patients, particularly those of African descent, frequently belong to specific clones of A. actinomycetemcomitans that produce significantly higher amounts of a protein exotoxin belonging to the repeats-in-toxin (RTX) family, leukotoxin (LtxA), whereas isolates from healthy patients harbor minimally leukotoxic strains. This finding suggests that LtxA might play a key role in A. actinomycetemcomitans pathogenicity. Because of this correlation, much work over the past 30 years has been focused on understanding the mechanisms by which LtxA interacts with and kills host cells. In this article, we review those findings, highlight the remaining open questions, and demonstrate how knowledge of these mechanisms, particularly the toxin's interactions with lymphocyte function-associated antigen-1 (LFA-1) and cholesterol, enables the design of targeted anti-LtxA strategies to prevent/treat disease.

Aggregatibacter actinomycetemcomitans leukotoxin causes activation of lymphocyte function-associated antigen 1

Repeats-in-toxin leukotoxin (LtxA) produced by the oral bacterium Aggregatibacter actinomycetemcomitans kills human leukocytes in a lymphocyte function-associated antigen 1 (LFA-1, integrin α_L /β₂ )-dependent manner, although the mechanism for this interaction has not been identified. The LtxA internalisation by LFA-1-expressing cells was explored with florescence resonance energy transfer (FRET) microscopy using a cell line that expresses LFA-1 with a cyan fluorescent protein-tagged cytosolic α_L domain and a yellow fluorescent protein-tagged β₂ domain. Phorbol 12-myristate 13-acetate activation of LFA-1 caused transient cytosolic domain separation. However, addition of LtxA resulted in an increase in FRET, indicating that LtxA brings the cytosolic domains closer together, compared with the inactive state. Unlike activation, this effect was not transient, lasting more than 30 min. Equilibrium constants of LtxA binding to the cytoplasmic domains of both α_L and β₂ were determined using surface plasmon resonance. LtxA has a strong affinity for the cytosolic domains of both the α_L and β₂ subunits (K_d  = 15 and 4.2 nM, respectively) and a significantly lower affinity for the cytoplasmic domains of other integrin α_M , α_X , and β₃ subunits (K_d  = 400, 180, and 230 nM, respectively), used as controls. Peptide fragments of α_L and β₂ show that LtxA binds membrane-proximal domain of α_L and intermediate domain of β₂ .

Membrane localization of the Repeats-in-Toxin (RTX) Leukotoxin (LtxA) produced by Aggregatibacter actinomycetemcomitans

The oral bacterium, Aggregatibacter actinomycetemcomitans, which is associated with localized aggressive periodontitis, as well as systemic infections including endocarditis, produces numerous virulence factors, including a repeats-in-toxin (RTX) protein called leukotoxin (LtxA), which kills human immune cells. The strains of A. actinomycetemcomitans most closely associated with disease have been shown to produce the most LtxA, suggesting that LtxA plays a significant role in the virulence of this organism. LtxA, like many of the RTX toxins, can be divided into four functional domains: an N-terminal hydrophobic domain, which contains a significant fraction of hydrophobic residues and has been proposed to play a role in the membrane interaction of the toxin; the central domain, which contains two lysine residues that are the sites of post-translational acylation; the repeat domain that is characteristic of the RTX toxins, and a C-terminal domain thought to be involved in secretion. In its initial interaction with the host cell, LtxA must bind to both cholesterol and an integrin receptor, lymphocyte function-associated antigen-1 (LFA-1). While both interactions are essential for toxicity, the domains of LtxA involved remain unknown. We therefore undertook a series of experiments, including tryptophan quenching and trypsin digestion, to characterize the structure of LtxA upon interaction with membranes of various lipid compositions. Our results demonstrate that LtxA adopts a U-shaped conformation in the membrane, with the N- and C-terminal domains residing outside of the membrane.

Generation of a recombinant Aggregatibacter actinomycetemcomitans RTX toxin in Escherichia coli

A leukotoxin (LtxA) that is produced by Aggregatibacter actinomycetemcomitans (Aa) is an important virulence determinant in an aggressive form of periodontitis in adolescents. Understanding the function of this protein at the molecular level is critical to elucidating its role in the disease process. To accomplish genetic analysis of the protein structure and relating these observations to toxin function, we have developed an E. coli expression system for the generation and rapid purification of LtxA. Cloning the structural toxin gene, ltxA, from Aa strain JP2 under control of T7 promoter-1 of pCDFDuet-1 vector resulted in expression of a 114 KDa protein which could be easily purified by the presence of a carboxy-terminal engineered double hexahistidine (double-His₆) tag and was immunologically reactive with an anti-LtxA monoclonal antibody, but was not cytotoxic. Cloning a second gene, ltxC, an acyltransferase gene, into the vector under control of T7 promoter-2, resulted in expression of the biologically active LtxA. The toxin was extracted from E. coli inclusion bodies, purified by immobilized metal affinity chromatography, and refolded by dialysis. When compared by circular dichroism (CD) spectroscopy analysis, acylated recombinant LtxA has a secondary structure consistent with wt LtxA, while variations in α-helical structure of nonacylated LtxA were observed. No modifications in α-helix were found upon the toxin's binding with liposome-incorporated cholesterol. Our results suggest that pure, biologically active recombinant LtxA can be isolated by a one-step affinity chromatography from E. coli. The toxic and structural properties of the recombinant LtxA are similar to its wt counterpart.

Inhibition of P2X Receptors Protects Human Monocytes against Damage by Leukotoxin from Aggregatibacter actinomycetemcomitans and α-Hemolysin from Escherichia coli

α-Hemolysin (HlyA) from Escherichia coli and leukotoxin A (LtxA) from Aggregatibacter actinomycetemcomitans are important virulence factors in ascending urinary tract infections and aggressive periodontitis, respectively. The extracellular signaling molecule ATP is released immediately after insertion of the toxins into plasma membranes and, via P2X receptors, is essential for the erythrocyte damage inflicted by these toxins. Moreover, ATP signaling is required for the ensuing recognition and phagocytosis of damaged erythrocytes by the monocytic cell line THP-1. Here, we investigate how these toxins affect THP-1 monocyte function. We demonstrate that both toxins trigger early ATP release and a following increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in THP-1 monocytes. The HlyA- and LtxA-induced [Ca²⁺]_i response is diminished by the P2 receptor antagonist in a pattern that fits the functional P2 receptor expression in these cells. Both toxins are capable of lysing THP-1 cells, with LtxA being more aggressive. Either desensitization or blockage of P2X₁, P2X₄, or P2X₇ receptors markedly reduces toxin-induced cytolysis. This pattern is paralleled in freshly isolated human monocytes from healthy volunteers. Interestingly, only a minor fraction of the toxin-damaged THP-1 monocytes eventually lyse. P2X₇ receptor inhibition generally prevents cell damage, except from a distinct cell shrinkage that prevails in response to the toxins. Moreover, we find that preexposure to HlyA preserves the capacity of THP-1 monocytes to phagocytose damaged erythrocytes and may induce readiness to discriminate between damaged and healthy erythrocytes. These findings suggest a new pharmacological target for protecting monocytes during exposure to pore-forming cytolysins during infection or injury.

Use of a Cholesterol Recognition Amino Acid Consensus Peptide To Inhibit Binding of a Bacterial Toxin to Cholesterol

Recognition of and binding to cholesterol on the host cell membrane is an initial step in the mechanism of numerous pathogens, including viruses, bacteria, and bacterial toxins; however, a viable method of inhibiting this interaction has not yet been uncovered. Here, we describe the mechanism by which a cholesterol recognition amino acid consensus peptide interacts with cholesterol and inhibits the activity of a cholesterol-binding bacterial leukotoxin (LtxA). Using a series of biophysical techniques, we have shown that the peptide recognizes the hydroxyl group of cholesterol with nanomolar affinity and does not disrupt membrane packing, suggesting that it sits primarily near the membrane surface. As a result, LtxA is unable to bind to cholesterol or subsequently become internalized in host cells. Additionally, because cholesterol is not being removed from the cell membrane, the peptide-treated target cells remain viable over extended periods of time. We have demonstrated the use of this peptide in the inhibition of toxin activity for an antivirulence approach to the treatment of bacterial disease, and we anticipate that this approach might have broad utility in the inhibition of viral and bacterial pathogenesis.

Inhibition of Bacterial Toxin Activity by the Nuclear Stain, DRAQ5™

The repeats-in-toxin family of toxins includes proteins produced by Gram negative bacteria such as Escherichia coli (α-hemolysin), Bordetella pertussis (adenylate cyclase toxin), and Aggregatibacter actinomycetemcomitans (LtxA), which contribute to the pathogenesis of these organisms by killing host cells. In the case of LtxA produced by A. actinomycetemcomitans, white blood cells are targeted, allowing the bacteria to avoid clearance by the host immune system. In its association with target cells, LtxA binds to a receptor, lymphocyte function-associated antigen-1, as well as membrane lipids and cholesterol, before being internalized via a lysosomal-mediated pathway. The motivation for this project comes from our discovery that DRAQ5™, a membrane-permeable nuclear stain, prevents the internalization of LtxA in a Jurkat T cell line. We hypothesized that DRAQ5™, in crossing the plasma membrane, alters the properties of the membrane to inhibit LtxA internalization. To investigate how DRAQ5™ interacts with the lipid membrane to prevent LtxA internalization, we used studied DRAQ5™-mediated membrane changes in model membranes using a variety of techniques, including differential scanning calorimetry and fluorescence spectroscopy. Our results suggest that DRAQ5™ inhibits the activity of LtxA by decreasing the fluidity of the cellular lipid membrane, which decreases LtxA binding. These results present an interesting possible anti-virulence strategy; by altering bacterial toxin activity by modifying membrane fluidity, it may be possible to inhibit the pathogenicity of A. actinomycetemcomitans.

Inhibition of LtxA toxicity by blocking cholesterol binding with peptides

The leukotoxin (LtxA) produced by Aggregatibacter actinomycetemcomitans kills host immune cells, allowing the bacterium to establish an ecological niche in the upper aerodigestive tract of its human host. The interaction of LtxA with human immune cells is both complex and multifaceted, involving membrane lipids as well as cell-surface proteins. In the initial encounter with the host cell, LtxA associates with lymphocyte function-associated antigen-1, a cell surface adhesion glycoprotein. However, we have also demonstrated that the toxin associates strongly with the plasma membrane lipids, specifically cholesterol. This association with cholesterol is regulated by a cholesterol recognition amino acid consensus (CRAC) motif, with a sequence of (334) LEEYSKR(340), in the N-terminal region of the toxin. Here, we have demonstrated that removal of cholesterol from the plasma membrane or mutation of the LtxA CRAC motif inhibits the activity of the toxin in THP-1 cells. To inhibit LtxA activity, we designed a short peptide corresponding to the CRAC(336) motif of LtxA (CRAC(336WT)). This peptide binds to cholesterol and thereby inhibits the toxicity of LtxA in THP-1 cells. Previously, we showed that this peptide inhibits LtxA toxicity against Jn.9 (Jurkat) cells, indicating that peptides derived from the cholesterol-binding site of LtxA may have a potential clinical applicability in controlling infections of repeats-in-toxin-producing organisms.

LFA-1-targeting Leukotoxin (LtxA; Leukothera®) causes lymphoma tumor regression in a humanized mouse model and requires caspase-8 and Fas to kill malignant lymphocytes

Leukotoxin (LtxA) is a protein secreted from the oral bacterium Aggregatibacter actinomycetemcomitans. LtxA binds to the β2 integrin lymphocyte-associated function antigen-1 (LFA-1) on human white blood cells (WBCs), resulting in cell death. LtxA is currently under investigation as a novel therapy (Leukothera(®)) for treating hematologic malignancies and autoimmune diseases. We show here that LtxA has potent in vivo anti-lymphoma activity in mice. LtxA caused complete regression of B-cell tumors and promoted long-term survival of mice. The mechanism of LtxA-mediated killing of malignant lymphocytes was further examined. We found that LtxA kills malignant lymphocytes by a novel mechanism requiring the death receptor Fas and caspase-8, but not Fas ligand (FasL) or caspase-9. We also determined that LFA-1 and Fas are closely associated on the cell surface and this proximity of LFA-1 and Fas could explain how signaling through an integrin can lead to cell death. In addition to LFA-1, this work reveals a second surface protein, Fas, that is critical for LtxA-mediated cell death. Knowledge of the mechanism of cell death induced by LtxA will facilitate the development and understanding of this potent experimental therapeutic agent.

Colonization and Persistence of Labeled and "Foreign" Strains of Aggregatibacter actinomycetemcomitans Inoculated into the Mouths of Rhesus Monkeys

Aggregatibacter actinomycetemcomitans (Aa) is a pathobiont and part of a consortium of bacteria that can lead to periodontitis in humans. Our aim was to develop a model for oral inoculation of labeled Aa into a suitable host in order to study Aa traits and ecological factors that either enhance or repress its persistence. Primate species were screened for Aa to select a host for colonization studies. Macaca mulatta (Rhesus/Rh) was selected. Rh Aa strains were isolated, subjected to sequencing and functional analysis for comparison to human strains. "Best" methods for microbial decontamination prior to inoculation were assessed. Three groups were studied; Group 1 (N=5) was inoculated with Aa Spectinomycin resistant (SpecR) Rh strain 4.35, Group 2 (N=5) inoculated with Aa SpecR human strain IDH 781, and Group 3 (N=5) the un-inoculated control. Repeated feeding with pancakes spiked with SpecRAa followed high dose oral inoculation. Cheek, tongue, and plaque samples collected at baseline 1, 2, 3, and 4 weeks after inoculation were plated on agar; 1) selective for Aa, 2) enriched for total counts, and 3) containing 50 µg/ml of Spec. Aa was identified by colonial morphology and DNA analysis. Rh and human Aa had > 93-98 % genome identity. Rh Aa attached to tissues better than IDH 781 in vitro (p < 0.05). SpecR IDH 781 was not recovered from any tissue at any time; whereas, RhSpecR 4.35 was detected in plaque, but never tongue or cheek, in all monkeys at all times (> 1 × 10⁵ colonies/ml; p < 0.001). In conclusion, the primate model provides a useful platform for studying integration of Aa strains into a reduced but established oral habitat. Primate derived SpecRAa was consistently detected in plaque at all collection periods; however, human derived Aa was never detected. The model demonstrated both microbial as well as tissue specificity.

RTX toxin plays a key role in Kingella kingae virulence in an infant rat model

Kingella kingae is a human oral bacterium that can cause diseases of the skeletal system in children and infective endocarditis in children and adults. K. kingae produces a toxin of the RTX group, RtxA. To investigate the role of RtxA in disease pathogenesis in vivo, K. kingae strain PYKK081 and its isogenic RtxA-deficient strain KKNB100 were tested for their virulence and pathological consequences upon intraperitoneal injections in 7-day-postnatal (PN 7) rats. At the doses above 8.0 × 10(6) cells/animal, PYKK081 was able to cause a fatal illness, resulting in rapid weight loss, bacteremia, and abdominal necrotic lesion formation. Significant histopathology was observed in thymus, spleen, and bone marrow. Strain KKNB100 was less toxic to animals. Neither weight loss, bacteremia, nor histopathological changes were evident. Animals injected with KKNB100 exhibited a significantly elevated circulating white blood cell (WBC) count, whereas animals injected with PYKK081 had a WBC count that resembled that of the uninfected control. This observation parallels the subtleties associated with clinical presentation of K. kingae disease in humans and suggests that the toxin contributes to WBC depletion. Thus, our results demonstrate that RtxA is a key K. kingae virulence factor. Furthermore, our findings suggest that the PN 7 rat can serve as a useful model for understanding disease caused by K. kingae and for elucidating diagnostic parameters in human patients.

Sialic acid residues are essential for cell lysis mediated by leukotoxin from Aggregatibacter actinomycetemcomitans

Leukotoxin (LtxA) from Aggregatibacter actinomycetemcomitans is known to target and lyse β2-integrin-expressing cells such as polymorphonuclear leukocytes and macrophages. LtxA is an important virulence factor that facilitates chronic inflammation and is strongly associated with a fast-progressing form of periodontitis caused by the JP2 clone of the bacterium. Here, we show that sialic acid residues are important for LtxA-induced cell lysis, regardless of whether the cell express β2-integrin or not. Clearly, removal of sialic acid groups significantly reduces a β2-integrin-specific LtxA-induced lysis. Moreover, sialic acid presented on alternative proteins, such as, for instance, on erythrocytes that do not express β2-integrin, also makes the cells more sensitive to LtxA. The data also illustrate the importance of the negative charge in order for the sialic acid to associate LtxA with the membrane. Removal of sialic acid is in itself sufficient to significantly reduce the negative charge on the erythrocytes. Moreover, we found that on human erythrocytes there is a positive association between the sensitivity to LtxA and the amount of negative charge caused by sialic acid. Interestingly, these features are not shared by all RTX toxins, since α-hemolysin from Escherichia coli induced cell lysis of both β2-integrin-expressing and nonexpressing cells and this lysis is independent of the presence of sialic acid residues. In conclusion, LtxA not only is cytotoxic to β2-integrin-expressing cells but can potentially initiate cell lysis in all cells that present a sufficient density of sialic acid groups on their plasma membrane.

A comparison of Aggregatibacter actinomycetemcomitans (Aa) virulence traits in a rat model for periodontal disease

Our aim was to explore the effects of Cytolethal Distending toxin (Cdt) in a well established rat model of periodontal disease where leukotoxin (LtxA) was thought to have no known effect. In vitro studies, were used to assess CdtB activity using Aa Leukotoxin as a negative control. These studies showed that both CdtB and LtxA (unexpectedly) exerted significant effects on CD4⁺ T cells. As a result we decided to compare the effects of these two prominent Aa virulence factors on bone loss using our rat model of Aa-induced periodontitis. In this model, Aa strains, mutant in cdtB and ltxA, were compared to their parent non-mutant strains and evaluated for colonization, antibody response to Aa, bone loss and disease. We found that bone loss/disease caused by the ltxA mutant strain, in which cdtB was expressed, was significantly less (p<0.05) than that due to the wild type strain. On the other hand, the disease caused by cdtB mutant strain, in which ltxA was expressed, was not significantly different from the wild type strain. This data indicates that Aa LtxA exerts a greater effect on bone loss than Cdt in this rat model of periodontal disease and supports the utility of this model to dissect specific virulence factors as they relate to immunopathology in studies of Aa-induced disease.

Aggregatibacter actinomycetemcomitans leukotoxin utilizes a cholesterol recognition/amino acid consensus site for membrane association

Aggregatibacter actinomycetemcomitans produces a repeats-in-toxin (RTX) leukotoxin (LtxA) that selectively kills human immune cells. Binding of LtxA to its β2 integrin receptor (lymphocyte function-associated antigen-1 (LFA-1)) results in the clustering of the toxin·receptor complex in lipid rafts. Clustering occurs only in the presence of LFA-1 and cholesterol, and LtxA is unable to kill cells lacking either LFA-1 or cholesterol. Here, the interaction of LtxA with cholesterol was measured using surface plasmon resonance and differential scanning calorimetry. The binding of LtxA to phospholipid bilayers increased by 4 orders of magnitude in the presence of 40% cholesterol relative to the absence of cholesterol. The affinity was specific to cholesterol and required an intact secondary structure. LtxA contains two cholesterol recognition/amino acid consensus (CRAC) sites; CRAC(336) ((333)LEEYSKR(339)) is highly conserved among RTX toxins, whereas CRAC(503) ((501)VDYLK(505)) is unique to LtxA. A peptide corresponding to CRAC(336) inhibited the ability of LtxA to kill Jurkat (Jn.9) cells. Although peptides corresponding to both CRAC(336) and CRAC(503) bind cholesterol, only CRAC(336) competitively inhibited LtxA binding to this sterol. A panel of full-length LtxA CRAC mutants demonstrated that an intact CRAC(336) site was essential for LtxA cytotoxicity. The conservation of CRAC(336) among RTX toxins suggests that this mechanism may be conserved among RTX toxins.

Low-cost periodontal therapy

Periodontitis is a complex infectious disease that affects low-income individuals disproportionately. Periodontitis is associated with specific bacterial species and herpesviruses, and successful prevention and treatment of the disease is contingent upon effective control of these pathogens. This article presents an efficacious, highly safe, minimally invasive, practical and low-cost periodontal therapy that involves professional and patient-administered mechanical therapy and antimicrobial agents. The major components are scaling for calculus removal, periodontal pocket irrigation with potent antiseptics, and treatment with systemic antibiotics for advanced disease. Povidone-iodine and sodium hypochlorite have all the characteristics for becoming the first-choice antiseptics in the management of periodontal diseases. Both agents show excellent antibacterial and antiviral properties, are readily available throughout the world, have been safely used in periodontal therapy for decades, offer significant benefits for individuals with very limited financial resources, and are well accepted by most dental professionals and patients. Four per cent chlorhexidine applied with a toothbrush to the most posterior part to the tongue dorsum can markedly reduce or eliminate halitosis in most individuals. Systemic antibiotics are used to treat periodontopathic bacteria that are not readily reached by topical therapy, such as pathogens within gingival tissue, within furcation defects, at the base of periodontal pockets, and on the tongue, tonsils and buccal mucosae. Valuable antibiotic therapies are amoxicillin-metronidazole (250 mg of amoxicillin and 250 mg of metronidazole, three times daily for 8 days) for young and middle-aged patients, and ciprofloxacin-metronidazole (500 mg of each, twice daily for 8 days) for elderly patients and for patients in developing countries who frequently harbor enteric rods subgingivally. Scaling to remove dental calculus and the prudent use of inexpensive antimicrobial agents can significantly retard or arrest progressive periodontitis in the great majority of patients.

Leukotoxin from Aggregatibacter actinomycetemcomitans causes shrinkage and P2X receptor-dependent lysis of human erythrocytes

Leukotoxin (LtxA) is a virulence factor secreted by the bacterium Aggregatibacter actinomycetemcomitans, which can cause localized aggressive periodontitis and endocarditis. LtxA belongs to the repeat-in-toxin (RTX) family of exotoxins of which other members inflict lysis by formation of membrane pores. Recently, we documented that the haemolytic process induced by another RTX toxin [α-haemolysin (HlyA) from Escherichia coli] requires P2X receptor activation and consists of sequential cell shrinkage and swelling. In contrast, the cellular and molecular mechanisms of LtxA-mediated haemolysis are not fully understood. Here, we investigate the effect of LtxA on erythrocyte volume and whether P2 receptors also play a part in LtxA-mediated haemolysis. We observed that LtxA initially decreases the cell size, followed by a gradual rise in volume until the cell finally lyses. Moreover, LtxA triggers phosphatidylserine (PS) exposure in the erythrocyte membrane and both the shrinkage and the PS-exposure is preceded by increments in the intracellular Ca(2+) concentration ([Ca(2+)](i)). Interestingly, LtxA-mediated haemolysis is significantly potentiated by ATP release and P2X receptor activation in human erythrocytes. Furthermore, the LtxA-induced [Ca(2+)](i) increase and following volume changes partially depend on P2 receptor activation. Theseobservations imply that intervention against local P2-mediated auto- and paracrine signalling may prevent LtxA-mediated cell damage.

The repeat-in-toxin family member TosA mediates adherence of uropathogenic Escherichia coli and survival during bacteremia

Uropathogenic Escherichia coli (UPEC) is responsible for the majority of uncomplicated urinary tract infections (UTI) and represents the most common bacterial infection in adults. UPEC utilizes a wide range of virulence factors to colonize the host, including the novel repeat-in-toxin (RTX) protein TosA, which is specifically expressed in the host urinary tract and contributes significantly to the virulence and survival of UPEC. tosA, found in strains within the B2 phylogenetic subgroup of E. coli, serves as a marker for strains that also contain a large number of well-characterized UPEC virulence factors. The presence of tosA in an E. coli isolate predicts successful colonization of the murine model of ascending UTI, regardless of the source of the isolate. Here, a detailed analysis of the function of tosA revealed that this gene is transcriptionally linked to genes encoding a conserved type 1 secretion system similar to other RTX family members. TosA localized to the cell surface and was found to mediate (i) adherence to host cells derived from the upper urinary tract and (ii) survival in disseminated infections and (iii) to enhance lethality during sepsis (as assessed in two different animal models of infection). An experimental vaccine, using purified TosA, protected vaccinated animals against urosepsis. From this work, it was concluded that TosA belongs to a novel group of RTX proteins that mediate adherence and host damage during UTI and urosepsis and could be a novel target for the development of therapeutics to treat ascending UTIs.

Phagocytic activity of peripheral blood and crevicular phagocytes in health and periodontal disease

Background:

Neutrophils constitute the main phagocytic cell system in mammalian host defense against an infecting agent. Abnormalities in leukocyte number and function are associated with increased susceptibility to periodontal diseases. The purpose of this study is to evaluate the in vitro phagocytic properties of crevicular and peripheral blood neutrophils in healthy and periodontitis subjects.

Patients and Methods:

A total of 30 subjects, that is, 10 patients in each of the following three groups: healthy controls, chronic periodontitis (CP), and localized aggressive periodontitis (LAP), were included in the study. The neutrophils were isolated from the peripheral blood and gingival crevice and tested for phagocytosis of Candida albicans. The percentage of leukocytes with ingested C. albicans was determined by light microscopy.

Results:

A significant reduction in the phagocytic activity of crevicular fluid polymorphonuclear neutrophils (CF-PMN) of LAP subjects (mean: 54.3±7)(P<0.001) was observed, compared to healthy controls (mean: 74.2±9) and chronic periodontitis subjects (mean: 69±9)(P=0.352). The mean percentage of peripheral blood polymorphonuclear neutrophils (PMNs) with phagocytosis of opsonized C. albicans in LAP patients was significantly reduced (mean: 74.9±5)(P<0.0068) compared to the phagocytic activity of neutrophils from controls (mean:82.1±3) and chronic periodontitis subjects (mean: 82.0±5)(P=0.970). There was no significant reduction in the phagocytic activity of CF PMNs (mean: 69±9) (P=0.35) and peripheral blood PMNs (mean: 82.5)(P=0.97) in the chronic periodontitis group when compared to the control group.

Conclusion:

The phagocytic activity of both crevicular and peripheral neutrophils in subjects with periodontitis is altered, increasing the susceptibility to periodontitis. Thus individual susceptibility may be an additional and important modifying factor in the pathogenesis of periodontal disease.

Neutrophil-derived resistin release induced by Aggregatibacter actinomycetemcomitans

Resistin is an adipokine that induces insulin resistance in mice. In humans, resistin is not produced in adipocytes, but in various leukocytes instead, and it acts as a proinflammatory molecule. The present investigation demonstrated high levels of resistin in culture supernatants of neutrophils that are stimulated by a highly leukotoxic strain of Aggregatibacter actinomycetemcomitans. In contrast, the level of resistin was remarkably low when neutrophils were exposed to two other strains that produce minimal levels of leukotoxin and a further isogenic mutant strain incapable of producing leukotoxin. Pretreatment of neutrophils with a monoclonal antibody to CD18, β chain of lymphocyte function-associated molecule 1 (LFA-1), or an Src family tyrosine kinase inhibitor before incubation with the highly leukotoxic strain inhibited the release of resistin. These results show that A. actinomycetemcomitans-expressed leukotoxin induces extracellular release of human neutrophil-derived resistin by interacting with LFA-1 on the surface of neutrophils and, consequently, activating Src family tyrosine kinases.

RTX proteins: a highly diverse family secreted by a common mechanism

Repeats-in-toxin (RTX) exoproteins of Gram-negative bacteria form a steadily growing family of proteins with diverse biological functions. Their common feature is the unique mode of export across the bacterial envelope via the type I secretion system and the characteristic, typically nonapeptide, glycine- and aspartate-rich repeats binding Ca²⁺ ions. In this review, we summarize the current state of knowledge on the organization of rtx loci and on the biological and biochemical activities of therein encoded proteins. Applying several types of bioinformatic screens on the steadily growing set of sequenced bacterial genomes, over 1000 RTX family members were detected, with the biological functions of most of them remaining to be characterized. Activities of the so far characterized RTX family members are then discussed and classified according to functional categories, ranging from the historically first characterized pore-forming RTX leukotoxins, through the large multifunctional enzymatic toxins, bacteriocins, nodulation proteins, surface layer proteins, up to secreted hydrolytic enzymes exhibiting metalloprotease or lipase activities of industrial interest.

Clarithromycin accumulation by phagocytes and its effect on killing of Aggregatibacter actinomycetemcomitans

BACKGROUND

Clarithromycin inhibits several periodontal pathogens and is concentrated inside gingival fibroblasts and epithelial cells by an active transporter. We hypothesized that polymorphonuclear leukocytes (PMNs) and less mature myeloid cells possess a similar transporter for clarithromycin. It is feasible that clarithromycin accumulation inside PMNs could enhance their ability to kill Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans).

METHODS

To test the first hypothesis, purified PMNs and cultured HL-60 cells were incubated with [(3)H]-clarithromycin. Clarithromycin transport was assayed by measuring changes in cell-associated radioactivity over time. The second hypothesis was examined with PMNs loaded by incubation with clarithromycin (5 μg/ml). Opsonized bacteria were incubated at 37°C with control and clarithromycin-loaded PMNs.

RESULTS

Mature human PMNs, HL-60 cells differentiated into granulocytes, and undifferentiated HL-60 cells all took up clarithromycin in a saturable manner. The kinetics of uptake by all yielded linear Lineweaver-Burk plots. HL-60 granulocytes transported clarithromycin with a K(m) of ≈250 μg/ml and a V(max) of 473 ng/min/10(6) cells, which were not significantly different from the values obtained with PMNs. At steady state, clarithromycin levels inside HL-60 granulocytes and PMNs were 28- to 71-fold higher than extracellular levels. Clarithromycin-loaded PMNs killed significantly more A. actinomycetemcomitans and achieved shorter half-times for killing than control PMNs when assayed at a bacteria-to-PMN ratio of 100:1 (P <0.04). At a ratio of 30:1, these differences were not consistently significant.

CONCLUSIONS

PMNs and less mature myeloid cells possess a transporter that takes up and concentrates clarithromycin. This system could help PMNs cope with an overwhelming infection by A. actinomycetemcomitans.

Aggregatibacter actinomycetemcomitans leukotoxin: from threat to therapy

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium that colonizes the human oral cavity and is the causative agent for localized aggressive periodontitis (LAP), an aggressive form of periodontal disease that occurs in adolescents. A. actinomycetemcomitans secretes a protein toxin, leukotoxin (LtxA), which helps the bacterium evade the host immune response during infection. LtxA is a membrane-active toxin that specifically targets white blood cells (WBCs). In this review, we discuss recent developments in this field, including the identification and characterization of genes and proteins involved in secretion, regulation of LtxA, biosynthesis, newly described activities of LtxA, and how LtxA may be used as a therapy for the treatment of diseases.

Bacterial penetration through antibiotic-loaded guided tissue regeneration membranes

BACKGROUND

This study compared bacterial penetration through guided tissue regeneration (GTR) membranes impregnated with antibiotics.

METHODS

Three barrier membranes, expanded polytetrafluoroethylene (ePTFE) membrane, collagen membrane, and glycolide fiber composite membrane, were loaded with amoxicillin or tetracycline. The penetration of Streptococcus mutans and Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans) through the GTR membranes was achieved using a device consisting of an inner tube and an outer bottle filled with culture media.

RESULTS

The penetration of S. mutans or A. actinomycetemcomitans into the inner tubes significantly decreased with all of the antibiotic-loaded membranes compared to membranes without antibiotics. However, differences were found in the behavior of the three membranes. The antibiotic-loaded ePTFE membranes showed the best barrier effect. Moreover, the inhibitory effect of tetracycline on S. mutans was greater than that of amoxicillin for all GTR membranes. Furthermore, the inhibitory effect of tetracycline on A. actinomycetemcomitans was lower than that of amoxicillin with the glycolide fiber membrane.

CONCLUSIONS

The results showed that penetration of S. mutans and A. actinomycetemcomitans through amoxicillin- or tetracycline-loaded ePTFE membrane, glycolide fiber membrane, and collagen membrane was delayed and/or reduced. Thus, incorporation of an antibiotic into the membrane may be of value when controlling membrane-associated infection during GTR therapy.

Anti-leukemia activity of a bacterial toxin with natural specificity for LFA-1 on white blood cells

The oral bacterium, Aggregatibacter actinomycetemcomitans, produces a leukotoxin (LtxA) that is specific for white blood cells (WBCs) from humans and Old World primates by interacting with lymphocyte function antigen-1 (LFA-1) on susceptible cells. To determine if LtxA could be used as a therapeutic agent for the treatment of WBC diseases, we tested the in vitro and in vivo anti-leukemia activity of the toxin. LtxA kills human malignant WBC lines and primary leukemia cells from acute myeloid leukemia patients, but healthy peripheral blood mononuclear cells (PBMCs) are relatively resistant to LtxA-mediated cytotoxicity. Levels of LFA-1 on cell lines correlated with killing by LtxA and the toxin preferentially killed cells expressing the activated form of LFA-1. In a SCID mouse model for human leukemia, LtxA had potent therapeutic value resulting in long-term survival in LtxA-treated mice. Intravenous infusion of LtxA into a rhesus macaque resulted in a drop in WBC counts at early times post-infusion; however, red blood cells, platelets, hemoglobin and blood chemistry values remained unaffected. Thus, LtxA may be an effective and safe novel therapeutic agent for the treatment of hematologic malignancies.

Aggregatibacter actinomycetemcomitans LtxC is required for leukotoxin activity and initial interaction between toxin and host cells

Aggregatibacter actinomycetemcomitans is a human pathogen that produces the RTX toxin (repeats in toxin), leukotoxin (LtxA). Based on other RTX toxin systems, the product of ltxC, the first gene of the ltx operon, is predicted to be involved in fatty acid modification of LtxA. To determine the function of ltxC in A. actinomycetemcomitans, we generated an ltxC mutation in the highly leukotoxic strain JP2N using random mutagenesis. The toxin from the ltxC mutant (LtxA(ltxC)) was expressed and secreted into the cell culture supernatant but could not lyse human leukocytes or erythrocytes. Mass spectrometric analysis of LtxA(ltxC) and LtxA from strain JP2N (LtxA(wt)) revealed two peptides that differed and this data suggests that two internal lysine residues of LtxA from the wild-type strain are modified. In blocking experiments, pre-treatment of cells with LtxA(ltxC) was unable to prevent LtxA(wt) from killing cells. Furthermore, in contrast to LtxA(wt), LtxA(ltxC) did not cause an increase in intracellular calcium levels in human leukocytes. Taken together, our data show that ltxC is required for full activity and modification of LtxA in A. actinomycetemcomitans and that modification is important for initial binding of toxin to host cells, as defined by an increase in intracellular calcium levels.

Membrane morphology and leukotoxin secretion are associated with a novel membrane protein of Aggregatibacter actinomycetemcomitans

Gram-negative bacteria display either a flat or an irregular outer membrane. The periodontal pathogen Aggregatibacter (Actinobacillus) actinomycetemcomitans has an irregular outer membrane. We have identified a gene that is associated with the biogenesis of this morphology. The gene is part of a three-gene operon and codes for a 141-kDa protein designated morphogenesis protein C (MorC), which is conserved in several gram-negative bacteria including Haemophilus influenzae and Pasteurella multocida. Insertional inactivation of this gene resulted in the conversion of an irregularly shaped membrane to a flat membrane. Associated with this morphological change were the autoaggregation of the bacteria during planktonic growth and a concomitant increase in the surface hydrophobicity of the bacterium. The absence of MorC also resulted in the loss of the secretion of leukotoxin but not the ltxA transcription. Our findings suggest that MorC is critical for membrane morphology and leukotoxin secretion in A. actinomycetemcomitans.

Screen for leukotoxin mutants in Aggregatibacter actinomycetemcomitans: genes of the phosphotransferase system are required for leukotoxin biosynthesis

Aggregatibacter (formerly Actinobacillus) actinomycetemcomitans is a pathogen that causes localized aggressive periodontitis and extraoral infections including infective endocarditis. Recently, we reported that A. actinomycetemcomitans is beta-hemolytic on certain growth media due to the production of leukotoxin (LtxA). Based on this observation and our ability to generate random transposon insertions in A. actinomycetemcomitans, we developed and carried out a rapid screen for LtxA mutants. Using PCR, we mapped several of the mutations to genes that are known or predicted to be required for LtxA production, including ltxA, ltxB, ltxD, and tdeA. In addition, we identified an insertion in a gene previously not recognized to be involved in LtxA biosynthesis, ptsH. ptsH encodes the protein HPr, a phosphocarrier protein that is part of the sugar phosphotransferase system. HPr results in the phosphorylation of other proteins and ultimately in the activation of adenylate cyclase and cyclic AMP (cAMP) production. The ptsH mutant showed only partial hemolysis on blood agar and did not produce LtxA. The phenotype was complemented by supplying wild-type ptsH in trans, and real-time PCR analysis showed that the ptsH mutant produced approximately 10-fold less ltxA mRNA than the wild-type strain. The levels of cAMP in the ptsH mutant were significantly lower than in the wild-type strain, and LtxA production could be restored by adding exogenous cAMP to the culture.

Both leukotoxin and poly-N-acetylglucosamine surface polysaccharide protect Aggregatibacter actinomycetemcomitans cells from macrophage killing

Two virulence factors produced by the periodontopathogen Aggregatibacter actinomycetemcomitans are leukotoxin, a secreted lipoprotein that kills human polymorphonuclear leukocytes and macrophages, and poly-N-acetylglucosamine (PGA), a surface polysaccharide that mediates intercellular adhesion, biofilm formation and detergent resistance. In this study we examined the roles of leukotoxin and PGA in protecting A. actinomycetemcomitans cells from killing by the human macrophage cell line THP-1. Monolayers of THP-1 cells were infected with single-cell suspensions of a wild-type A. actinomycetemcomitans strain, or of isogenic leukotoxin or PGA mutant strains. After 48h, viable bacteria were enumerated by dilution plating, macrophage morphology was evaluated microscopically, and macrophage viability was measured by a Trypan blue dye exclusion assay. The number of A. actinomycetemcomitans CFUs increased approximately twofold in wells infected with the wild-type strain, but decreased by approximately 70-90% in wells infected with the leukotoxin and PGA mutant strains. Infection with the wild-type or leukotoxin mutant strain caused a significant decrease in THP-1 cell viability, whereas infection with the PGA mutant strain did not result in any detectable changes in THP-1 viability. Pre-treatment of wild-type A. actinomycetemcomitans cells with the PGA-hydrolyzing enzyme dispersin B rendered them sensitive to killing by THP-1 cells. We concluded that both leukotoxin and PGA are necessary for evasion of macrophage killing by A. actinomycetemcomitans.

Interaction between leukotoxin and Cu,Zn superoxide dismutase in Aggregatibacter actinomycetemcomitans

Aggregatibacter (Actinobacillus) actinomycetemcomitans is a gram-negative oral pathogen that is the etiologic agent of localized aggressive periodontitis and systemic infections. A. actinomycetemcomitans produces leukotoxin (LtxA), which is a member of the RTX (repeats in toxin) family of secreted bacterial toxins and is known to target human leukocytes and erythrocytes. To better understand how LtxA functions as a virulence factor, we sought to detect and study potential A. actinomycetemcomitans proteins that interact with LtxA. We found that Cu,Zn superoxide dismutase (SOD) interacts specifically with LtxA. Cu,Zn SOD was purified from A. actinomycetemcomitans to homogeneity and remained enzymatically active. Purified Cu,Zn SOD allowed us to isolate highly specific anti-Cu,Zn SOD antibody and this antibody was used to further confirm protein interaction. Cu,Zn SOD-deficient mutants displayed decreased survival in the presence of reactive oxygen and nitrogen species and could be complemented with wild-type Cu,Zn SOD in trans. We suggest that A. actinomycetemcomitans Cu,Zn SOD may protect both bacteria and LtxA from reactive species produced by host inflammatory cells during disease. This is the first example of a protein-protein interaction involving a bacterial Cu,Zn SOD.

Human CD18 is the functional receptor for Aggregatibacter actinomycetemcomitans leukotoxin

Aggregatibacter (Actinobacillus) actinomycetemcomitans is the causative organism of localized aggressive periodontitis, a rapidly progressing degenerative disease of the gingival and periodontal ligaments, and is also implicated in causing subacute infective endocarditis in humans. The bacterium produces a variety of virulence factors, including an exotoxic leukotoxin (LtxA) that is a member of the repeats-in-toxin (RTX) family of bacterial cytolysins. LtxA exhibits a unique specificity to macrophages and polymorphonuclear cells of humans and other primates. Human lymphocyte function-associated antigen 1 (LFA-1) has been implicated as the putative receptor for LtxA. Human LFA-1 comprises the CD11a and CD18 subunits. It is not clear, however, which of its subunits serves as the functional receptor that confers species-specific susceptibility to LtxA. Here we demonstrate that the human CD18 is the receptor for LtxA based on experiments performed with chimeric beta2-integrins recombinantly expressed in a cell line that is resistant to LtxA effects. In addition, we show that the cysteine-rich tandem repeats encompassing integrin-epidermal growth factor-like domains 2, 3, and 4 of the extracellular region of human CD18 are critical for conferring susceptibility to LtxA-induced biological effects.

A second Aggregatibacter actinomycetemcomitans autotransporter adhesin exhibits specificity for buccal epithelial cells in humans and Old World primates

Previous work showed that the Aggregatibacter actinomycetemcomitans adhesin Aae demonstrated species specificity and tissue tropism to buccal epithelial cells (BECs) derived from humans and Old World primates, but a second, lower-affinity adhesin was noted. This study was designed to determine if Omp100 (also known as ApiA), a surface-expressed A. actinomycetemcomitans adhesin, is that second adhesin and if so to investigate its tissue tropism and species specificity. A targeted mutagenesis protocol was used to construct an isogenic apiA mutant and an aae apiA double mutant with wild-type A. actinomycetemcomitans. In addition, Escherichia coli strain DH5alpha was used to express apiA to further assess binding parameters. Results indicated that the apiA mutant strain showed significantly less binding to BECs than its parent strain (P < or = 0.05). Further, binding mediated by ApiA was specific to BECs from humans and Old World primates, as seen in both wild-type A. actinomycetemcomitans and E. coli expressing ApiA (P < or = 0.05). Pretreatment of wild-type A. actinomycetemcomitans cells with anti-ApiA antiserum reduced binding in a dose-dependent manner. The aae apiA double mutant completely abrogated A. actinomycetemcomitans binding to both human and Old World primate BECs. Taken together, these studies indicate that ApiA and Aae, in concert, modulate binding of A. actinomycetemcomitans to human BECs. Since the BEC is a prominent reservoir for A. actinomycetemcomitans, identification of this second adhesin could lead to important therapeutic strategies.

Aggregatibacter actinomycetemcomitans leukotoxin requires beta-sheets 1 and 2 of the human CD11a beta-propeller for cytotoxicity

Aggregatibacter actinomycetemcomitans leukotoxin (Ltx) is a repeats-in-toxin (RTX) cytolysin that kills human leukocyte function-associated antigen-1 (LFA-1; alpha(L)/beta(2))-bearing cells. In order to determine whether the alpha(L) portion of the heterodimer is involved in Ltx recognition, we transfected human, mouse and bovine alpha(L) cDNAs into J-beta(2).7, an alpha(L)-deficient cell line, and looked for restoration of Ltx susceptibility. Cells expressing either bovine or human alpha(L) in conjunction with human beta(2) were efficiently killed by Ltx, an indication that bovine alpha(L) could substitute for its human counterpart in critical regions used by Ltx for attachment to LFA-1. On the other hand, cells expressing murine alpha(L) and human beta(2) were not susceptible to the lethal effects of Ltx indicating that the toxin recognition sites are not present in the corresponding mouse sequence. To further identify the region(s) of alpha(L) recognized by Ltx, we constructed and evaluated a panel of chimeric human/murine alpha(L) genes in J-beta(2).7 cells. Analysis of the alpha(L) mutant panel showed that the presence of human N-terminal 128 amino acids on a mouse CD11a background, a region that includes beta-sheets 1 and 2 of the beta-propeller of the human alpha(L) chain, was sufficient for Ltx cytolysis.

TdeA, a TolC-like protein required for toxin and drug export in Aggregatibacter (Actinobacillus) actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is an oral bacterium that causes localized aggressive periodontitis (LAP) and extra-oral infections such as sub-acute infective endocarditis. As part of its array of virulence factors, A. actinomycetemcomitans produces leukotoxin (LtxA), a member of the RTX family of toxins. LtxA kills human leukocytes and we have recently shown that the toxin is required for beta-hemolysis by A. actinomycetemcomitans on solid medium. In other RTX toxin-producing bacteria, an outer membrane channel-forming protein, TolC, is required for toxin secretion and drug export. We have identified an ORF in A. actinomycetemcomitans that encodes a putative protein having predicted structural properties similar to TolC. Inactivation of this ORF resulted in a mutant that was no longer beta-hemolytic and did not secrete LtxA. This mutant was significantly more sensitive to antimicrobial agents compared to the wild type strain and was unable to export the antimicrobial agent berberine. Thus, this ORF was named tdeA for "toxin and drug export". Examination of the DNA sequence surrounding tdeA revealed two upstream ORFs that encode proteins similar to the drug efflux proteins, MacA and MacB. Inactivation of macB in A. actinomycetemcomitans did not alter the drug sensitivity profile or the hemolytic activity of the mutant. The genes macA, macB and tdeA are organized as an operon and are constitutively expressed as a single transcript. These results show that A. actinomycetemcomitans indeed requires a TolC-like protein for LtxA secretion and that this protein, TdeA, also functions as part of a drug efflux system.

Regulation of Aggregatibacter (Actinobacillus) actinomycetemcomitans leukotoxin secretion by iron

The gram-negative oral and systemic pathogen Aggregatibacter (Actinobacillus) actinomycetemcomitans produces a leukotoxin (LtxA) that is a member of the RTX (repeats in toxin) family of secreted bacterial toxins. We have recently shown that LtxA has the ability to lyse erythrocytes, which results in a beta-hemolytic phenotype on Columbia blood agar. To determine if LtxA is regulated by iron, we examined beta-hemolysis under iron-rich and iron-limiting conditions. Beta-hemolysis was suppressed in the presence of FeCl3. In contrast, strong beta-hemolysis occurred in the presence of the iron chelator deferoxamine. We found that secretion of LtxA was completely inhibited by free iron, but expression of ltxA was not regulated by iron. Free chromium, cobalt, and magnesium did not affect LtxA secretion. Other LtxA-associated genes were not regulated by iron. Thus, iron appears to play an important role in the regulation of LtxA secretion in A. actinomycetemcomitans in a manner independent of gene regulation.

Aggressive periodontitis in a 16-year-old Ghanaian adolescent, the original source of Actinobacillus actinomycetemcomitans strain HK1651 - a 10-year follow up

The highly leukotoxic JP2 clone of Actinobacillus actinomycetemcomitans is strongly associated with periodontitis in adolescents. Availability of the DNA sequence of the complete genome of A. actinomycetemcomitans strain HK1651, a representative strain of the JP2 clone (http://www.genome.ou.edu/act.html), has provided new possibilities in basic research regarding the understanding of the pathogenesis of A. actinomycetemcomitans in periodontitis. This case report describes the periodontal treatment of the original source of A. actinomycetemcomitans HK1651, a 16-year-old Ghanaian adolescent girl with aggressive periodontitis. The bacterial examination involved polymerase chain reaction analysis for presence of JP2 and non-JP2 types of A. actinomycetemcomitans. The treatment, including periodontal surgery supplemented by antibiotics, arrested the progression of periodontitis for more than 10 years. Initially, infection by A. actinomycetemcomitans, including the JP2 clone, was detected at various locations in the oral cavity and was not limited to the periodontal pockets. Post-therapy, the JP2 clone of A. actinomycetemcomitans disappeared, while the non-JP2 types of A. actinomycetemcomitans remained a part of the oral microflora.

Actinobacillus actinomycetemcomitans leukotoxin requires lipid microdomains for target cell cytotoxicity

Actinobacillus actinomycetemcomitans produces a leukotoxin (Ltx) that kills leukocyte function-associated antigen-1 (LFA-1)-bearing cells from man, the Great Apes and Old World monkeys. The unique specificity of Ltx for the beta2 integrin, LFA-1, suggests it is capable of providing insight into the pathogenic mechanisms of Ltx and other RTX toxins. Using the Jurkat T cell line and an LFA-1-deficient Jurkat mutant (Jbeta2.7) as models, we found the initial effect of Ltx is to elevate cytosolic Ca2+ [Ca2+]c, an event that is independent of the Ltx/LFA-1 interaction. [Ca2+]c increases initiate a series of events that involve the activation of calpain, talin cleavage, mobilization to, and subsequent clustering of, LFA-1 in cholesterol and sphingolipid-rich regions of the plasma membrane known as lipid rafts. The association of Ltx and LFA-1 within lipid rafts is essential for cell lysis. Jbeta2.7 cells fail to accumulate Ltx in their raft fractions and are not killed, while cholesterol depletion experiments demonstrate the necessity of raft integrity for Ltx function. We propose that toxin-induced Ca2+ fluxes mobilize LFA-1 to lipid rafts where it associates with Ltx. These findings suggest that Ltx utilizes the raft to stimulate an integrin signalling pathway that leads to apoptosis of target cells.

Leukotoxin confers beta-hemolytic activity to Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is the etiologic agent of localized aggressive periodontitis, a rapidly progressing oral disease that occurs in adolescents. A. actinomycetemcomitans can also cause systemic disease, including infective endocarditis. In early work on A. actinomycetemcomitans workers concluded that this bacterium is not beta-hemolytic. More recent reports have suggested that A. actinomycetemcomitans does have the potential to be beta-hemolytic. While growing A. actinomycetemcomitans on several types of growth media, we noticed a beta-hemolytic reaction on media from one manufacturer. Beta-hemolysis occurred on Columbia agar from Accumedia with either sheep or horse blood, but not on similar media from other manufacturers. A surprising result was that mutants of A. actinomycetemcomitans defective for production of leukotoxin, a toxin that is reportedly highly specific for only human and primate white blood cells, are not beta-hemolytic. Purified leukotoxin was able to lyse sheep and human erythrocytes in vitro. This work showed that in contrast to the accepted view, A. actinomycetemcomitans leukotoxin can indeed destroy erythrocytes and that the production of this toxin results in beta-hemolytic colonies on solid medium. In light of these results, the diagnostic criteria for clinical identification of A. actinomycetemcomitans and potentially related bacteria should be reevaluated. Furthermore, in studies on A. actinomycetemcomitans leukotoxin workers should now consider this toxin's ability to destroy red blood cells.

Characterization of leukotoxin from a clinical strain of Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is a Gram negative pathogen that is the etiologic agent of localized aggressive periodontitis (LAP), a rapidly progressing and severe disease of the oral cavity that affects predominantly adolescents. A. actinomycetemcomitans is also found in extraoral infections including infective endocarditis. As one of its many virulence determinants, A. actinomycetemcomitans produces the RTX (repeats in toxin) exotoxin, leukotoxin (LtxA). LtxA specifically kills leukocytes of humans and Old World Monkeys. All of our current knowledge of A. actinomycetemcomitans LtxA is based on the protein from strain JP2, a nonadherent laboratory isolate. Because laboratory isolates can lose virulence properties, we wished to examine LtxA from a clinical isolate, NJ4500. We show that localization patterns of LtxA do not differ between the strains. Subcellular localization studies with NJ4500 revealed that LtxA localizes to the outer membrane and that the interaction between LtxA and the surface of cells is specific. Surface localized LtxA was not removed with NaCl treatment and protease protection experiments revealed that approximately 10 kDa of LtxA is exposed. We purified secreted LtxA from NJ4500 and found that the specific activity of this toxin was greater than that of secreted LtxA from JP2. For other RTX toxins, fatty acid modification affects toxin activity, and A. actinomycetemcomitans LtxA is predicted to be modified. We show by two-dimensional gel electrophoresis that NJ4500 LtxA is more highly modified than JP2 LtxA, suggesting that the difference in activities could be due to differential modification. Studies of A. actinomycetemcomitans pathogenesis should therefore consider LtxA from clinical isolates.

Transmission of periodontal bacteria and models of infection

BACKGROUND

Bacteria play an essential role in the aetiology of periodontitis. Most bacterial species isolated from subgingival plaque are indigenous to the oral cavity. Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis are detected infrequently in periodontal health, which makes these species prime candidates to study person-to-person transmission. The aim of the present study was to review the literature on transmission of these periodontal bacterial species.

METHOD

We review the literature on bacterial typing techniques and summarize the information on clonal distribution of A. actinomycetemcomitans and P. gingivalis in family units based on different typing techniques in order to establish the likelihood for person-to-person transmission of these periodontal pathogens.

RESULTS

Vertical transmission of A. actinomycetemcomitans is estimated to be between 30% and 60%, whereas vertical transmission of P. gingivalis has rarely been observed. Horizontal transmission between spouses ranges between 14% and 60% for A. actinomycetemcomitans and between 30% and 75% for P. gingivalis. There is some evidence to show that cohabitation with a periodontitis patient influences the periodontal status of the spouse; however, substantially more information is needed to prove this hypothesis.

CONCLUSIONS

Transmission of putative periodontal pathogens between family members has been shown. The clinical consequences of these events have been poorly documented. Based on the current knowledge, screening for and prevention of transmission of specific virulent clones of A. actinomycetemcomitans may be feasible and effective in preventing some forms of periodontal disease. P. gingivalis is usually recovered from diseased adult subjects, and transmission of this pathogens seems largely restricted to adult individuals. Horizontal transmission of P. gingivalis may therefore be controlled by periodontal treatment involving elimination or significant suppression of the pathogen in diseased individuals and by a high standard of oral hygiene.

The Actinobacillus actinomycetemcomitans autotransporter adhesin Aae exhibits specificity for buccal epithelial cells from humans and old world primates

Cells of the gram-negative periodontopathogen Actinobacillus actinomycetemcomitans express a surface-exposed, outer membrane autotransporter protein, designated Aae, which has been implicated in epithelial cell binding. We constructed a mutant strain of A. actinomycetemcomitans that contained a transposon insertion in the Aae structural gene (aae) and tested the mutant to determine its ability to bind to buccal epithelial cells (BECs) isolated from healthy volunteers. Significantly fewer mutant cells than wild-type cells bound to BECs. A broad-host-range plasmid that contained an intact aae gene driven by a heterologous tac promoter restored the ability of the mutant strain to bind to BECs at wild-type levels. This plasmid also conferred upon Escherichia coli the ability to express the Aae protein on its surface and to bind to human BECs. Aae-expressing E. coli also bound to BECs isolated from six Old World primates but not to BECs isolated from four New World primates or nine other nonprimate mammals, as well as to human gingival epithelial cells but not to human pharyngeal, palatal, tongue, bronchial, or cervical epithelial cells. Our findings indicate that Aae mediates binding of A. actinomycetemcomitans to BECs from humans and Old World primates and that this process may contribute to the host range specificity and tissue tropism exhibited by this bacterium.

Antiproliferative activity of Actinobacillus (Haemophilus) actinomycetemcomitans and Fusobacterium nucleatum in peripheral blood mononuclear cells

Several studies indicate Actinobacillus (Haemophilus) actinomycetemcomitans and Fusobacterium nucleatum as etiologic agents of periodontal disease. Immunosuppressive factors produced by microorganisms probably contribute to the initiation and evolution of this disease. This study evaluated the antiproliferative activity of ammonium precipitate fractions of A. (H.) actinomycetemcomitans and F. nucleatum isolates from humans and marmosets both with and without periodontal disease. All A. (H.) actinomycetemcomitans and most F. nucleatum strains inhibited PBMC proliferation in a dose-dependent manner. The degree of cell proliferative inhibition of each bacterial species differed among the strains and was independent of host clinical status. The in vitro inhibition of stimulated lymphocyte proliferation induced by different A. (H.) actinomycetemcomitans and F. nucleatum isolates demonstrated the importance of this phenomenon in bacterial virulence, playing a possible suppressor role in host defense mechanisms in vivo. Moreover, our findings pointed out a marked difference between A. (H.) actinomycetemcomitans and F. nucleatum cytoplasmic extracts in their antiproliferative activity, regarding the antigen concentration required for maximum inhibition and their vulnerability to heating and proteolytic treatment.