Peptoanaerobacter stomatis Primes Human Neutrophils and Induces Granule Exocytosis

Peptoanaerobacter stomatis is a newly appreciated taxon associated with periodontal diseases; however, little is known about the organism's pathogenic potential or its interaction with the host immune response. Neutrophils are the most abundant innate immune cell present in the gingival tissue and function to constrain the oral microbial challenge. However, some periodontal pathogens have developed strategies to evade phagocytosis and killing by neutrophils. Therefore, to begin to understand the role of P. stomatis in periodontitis, we studied its interactions with human neutrophils. Our data showed that after 30 min of incubation, neutrophils failed to engulf P. stomatis efficiently; however, when P. stomatis was internalized, it was promptly eradicated. P. stomatis challenge induced a robust intracellular respiratory burst; however, this response did not contribute to bacterial killing. Minimal superoxide release was observed by direct bacterial challenge; however, P. stomatis significantly increased N-formyl-methionyl-leucyl phenylalanine (fMLF)-stimulated superoxide release to an extent similar to that of cells primed with tumor necrosis factor alpha (TNF-α). When neutrophils were challenged with P. stomatis, 52% of the bacterium-containing phagosomes were enriched for the specific granule marker lactoferrin and 82% with the azurophil granule marker elastase. P. stomatis challenge stimulated exocytosis of the four neutrophil granule subtypes. Moreover, P. stomatis susceptibility to extracellular killing could be attributed to the exocytosis of antimicrobial components present in neutrophil granules. Priming neutrophils for an enhanced respiratory burst together with promoting granule content release could contribute to the chronic inflammation and tissue destruction that characterize periodontal diseases.