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.
D000328:Adult; D016976:Aggregatibacter actinomycetemcomitans / Q000276:immunology*; D000911:Antibodies, Monoclonal; D000954:Antigens, Surface / Q000276:immunology*; D017209:Apoptosis / Q000276:immunology*; D053148:Caspase 3; D020169:Caspases / Q000276:immunology; D017077:Culture Media, Conditioned; D015536:Down-Regulation; D004792:Enzyme Precursors / Q000276:immunology; D053222:Fas Ligand Protein; D005260:Female; D005434:Flow Cytometry; D006801:Humans; D008024:Ligands; D008213:Lymphocyte Activation / Q000276:immunology; D008297:Male; D016555:Matched-Pair Analysis; D008562:Membrane Glycoproteins / Q000276:immunology*; D009000:Monocytes / Q000276:immunology; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000276:immunology; D013223:Statistics as Topic; D013601:T-Lymphocytes / Q000276:immunology*; D013997:Time Factors; D015854:Up-Regulation / Q000276:immunology; D019014:fas Receptor / Q000276:immunology*
D000911:Antibodies, Monoclonal; D000954:Antigens, Surface; D017077:Culture Media, Conditioned; D004792:Enzyme Precursors; C505661:FASLG protein, human; D053222:Fas Ligand Protein; D008024:Ligands; D008562:Membrane Glycoproteins; D019253:Proto-Oncogene Proteins c-bcl-2; D019014:fas Receptor; C505407:CASP3 protein, human; D053148:Caspase 3; D020169:Caspases