Evidence for apoptosis of the majority of T cells activated in vitro with Actinobacillus actinomycetemcomitans

Aggregatibacter actinomycetemcomitans - a tooth killer?

Strong evidence is available on Aggregatibacter actinomycetemcomitans (A.a) on its role as the causative agent of localised juvenile periodontitis (LJP), a disease characterised by rapid destruction of the tooth-supporting tissues. This organism possesses a large number of virulence factors with a wide range of activities which enable it to colonise the oral cavity, invade periodontal tissues, evade host defences, initiate connective tissue destruction and interfere with tissue repair. Adhesion to epithelial and tooth surfaces is dependent on the presence of surface proteins and structures such as microvesicles and fimbriae. Invasion has been demonstrated in vivo and in vitro. The organism has a number of means of evading host defences which include: (i) production of leukotoxin; (ii) producing immunosuppressive factors; (iv) secreting proteases capable of cleaving IgG; and (v) producing Fc-binding.

High incidence of actinobacillus actinomycetemcomitans infection in acute coronary syndrome

Recent epidemiological studies suggest that periodontitis is an important risk factor for coronary heart disease (CHD). The aim of this study was to evaluate the association between periodontitis and CHD, particularly acute coronary syndrome (ACS), focusing on microbiological and immunological features. Twenty-eight CHD patients, 15 with ACS and 13 with chronic CHD, were included in this study. Coronary angiography, periodontal examination, and dental radiography were performed in all patients. Subgingival plaque, saliva, and blood samples were analyzed for the periodontopathogens Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, Tannerella forsythensis, Treponema denticola, and Prevotella intermedia using polymerase chain reaction. Specific serum antibody titers to the 5 periodontal pathogens were determined by enzyme-linked immunosorbent assay. It was found that 33% of the ACS patients (5/15) harbored A. actinomycetemcomitans in oral samples, whereas no A. actinomycetemcomitans (0/13) was found in the chronic CHD patients (P < 0.05). Furthermore, ACS patients showed significantly higher serum IgG titers to A. actinomycetemcomitans (P < 0.05) compared with chronic CHD. More tooth loss and alveolar bone loss were noted in ACS patients than in chronic CHD patients, although the differences were not statistically significant. Periodontal pathogens, particularly A. actinomycetemcomitans, may play a role in the development of ACS.

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.

Expression cloning of a periodontitis-associated apoptotic effector, cagE homologue, in Actinobacillus actinomycetemcomitans

To study anti-bacterial immunity and to identify critical bacterial antigens associated with specific periodontal infection, we screened the genomic library of Actinobacillus actinomycetemcomitans, a major Gram(-) anaerobe causing human periodontitis, by expression cloning using disease-associated periodontal CD4(+)T cells derived from HuPBL-engrafted NOD/SCID mice. Here, we report one of the novel genes identified and designated, cagE homologue (in short: cagE) of A. actinomycetemcomitans, which encodes a putative bacterial type IV secretion system with significant homology to Helicobacter pylori CagE and Agrobacterium tumefaciens VirB4. All serum samples from A. actinomycetemcomitans-infected periodontitis patients, but not from the healthy controls, readily recognized CagE by ELISA and Western blot, suggesting its biological and clinical significance. The CagE protein, upon secretion, elicited significant apoptosis on primary human epithelia, endothelia, osteoblasts, and T cells by 4-12h in vitro. Importantly, both cagE(-) mutant strain and N-terminus truncated CagE protein drastically reduced (p<0.001) the induction of apoptosis on human epithelia in vitro. These data strongly suggest that a novel effector protein, CagE in A. actinomycetemcomitans, induces apoptosis of human cells and destructive immunity, thereby it may play an important role in the pathogenesis of A. actinomycetemcomitans-mediated infections.

Actinobacillus actinomycetemcomitans induces apoptosis of T lymphocytes by the Fas and Fas ligand pathway

Actinobacillus actinomycetemcomitans expresses a number of toxins capable of inducing apoptotic cell death of T lymphocytes. However, the exact mechanism(s) has not been elucidated. The present study investigated the involvement of the Fas (CD95)-mediated apoptotic pathway in A. actinomycetemcomitans-induced T-cell apoptosis. To that end, peripheral blood mononuclear cells (PBMC) were cultured with or without A. actinomycetemcomitans cell-free culture supernatant (CFCS) for 0-96 h. The cells were then labeled with specific monoclonal antibodies and flow cytometry was performed. Results demonstrated up-regulation of Fas and activation of caspase-3 in T cells in response to A. actinomycetemcomitans CFCS. Monocytes were the only cells analyzed to express Fas ligand (FasL) constitutively, and this was further up-regulated in response to A. actinomycetemcomitans CFCS, while T cells expressed FasL only after this stimulation. Depletion of monocytes prior to stimulation with A. actinomycetemcomitans CFCS led to a marked decline in apoptosis. Blocking of Fas-FasL interactions with anti-Fas monoclonal antibody or Fas:Fc fusion protein lead to a significant decline, but not abolition, of T-cell apoptosis. Nearly all T cells expressed Bcl-2 at the outset of culture, and Bcl-2 expression declined in T cells stimulated with A. actinomycetemcomitans CFCS. Collectively, these data provide evidence for the induction of T-cell apoptosis by A. actinomycetemcomitans via the Fas-mediated pathway, involving caspase-3 and Bcl-2. Moreover, this apoptotic response was dependent on the presence of monocytes.

Large-scale early in vitro response to actinobacillus actinomycetemcomitans suggests superantigenic activation of T-cells

The mode of T-cell response to Actinobacillus actinomycetemcomitans is largely unknown. The present study sought to investigate the hypothesis that A. actinomycetemcomitans expresses superantigens, capable of antigen-non-specific T-cell activation. To that end, peripheral blood mononuclear cells were stimulated with A. actinomycetemcomitans, and T-cell expression of the early activation marker, CD69, was determined by flow cytometry. Results showed that A. actinomycetemcomitans activated a large number of T-cells with magnitude similar to that of staphylococcal enterotoxin superantigens. A. actinomycetemcomitans sonicate preferentially activated T-cells expressing Vbeta5.1 and Vbeta8, while the extracellular preparation activated Vbeta5.1+, Vbeta8+, and Vbeta12+ T-cells. T-cell response to A. actinomycetemcomitans was observed in the presence of autologous, as well as heterologous, antigen-presenting cells, suggesting a MHC-non-restricted response. Thus, the in vitro response to A. actinomycetemcomitans is characterized by large-scale T-cell activation in a Vbeta-specific and MHC-non-restricted manner, consistent with the involvement of superantigens.