Analysis of apoptosis in alveolar macrophages induced by periodontopathic bacteria and development of a method of protection from pneumonia

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Infection of murine macrophages in vitro with periodontopathic bacterium Actinobacillus actinomycetemcomitans induces apoptotic cell death. In this study, we investigated the involvement of caspases in apoptotic cell death of A.actinomycetemcomitans-infected macrophages. Two peptide inhibitors of caspases, benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethyl ketone(Z-VAD-FMK)and benzyloxycarbonyl-Asp-Glu-Val-Asp(OMe)-fluoromethyl katone(Z-DEVD-FMK), inhibited apoptotic cell death of mouse macrophage cell line J774.1 infected with A.actinomycetemcomitans. During the process of apoptosis, interleukin-1β(IL-1β) was detected in the culture supernatants of J774.1 cells. IL-1β secretion was blocked by the caspase-1 inhibitor, Z-VAD-FMK, indicating that caspase-1 is involved in not only the induction of apoptosis but also the IL-1β secretion from A.actinomycetemcomintas-infected J774.1 cells. Immunoblot analysis revealed that the infection of A.actinomycetemcomitans to J774.1 cells induced the cleavage of retinoblastoma protein(Rb), suggesting that caspase-3 was activated by A.actinomycetemcomitans-infection. The cytosol from A.actinomycetemcomitans-infected J774.1 cells induced Rb proteollysis in vitro, which was inhibited by the caspase-3 inhibitor, Z-DEVD-FMK.Furthermore, caspase-3-like activity was markedly increased in J774.1 cells infected with A.actinomycetemcomitans between 12h and 24 h, which was subsequently inhibited by the addition of caspase-3 inhibitor, Z-DEVD-FMK.These findings indings indicate that caspase-3 induces apoptosis in J774.1 cells infected with A.actinomycetemcomitans. Taken together, these results suggest that caspase-1 and caspase-3 are involved in the induction of apoptosis in A.actinomycetemcomitans-infected macrophages.