In their study published in PLOS Pathogens, David Feehan and Dr. Christopher Mody demonstrate that NK cells facilitate bacterial killing against the extracellular pathogen, Pseudomonas aeruginosa, using a contact-dependent mechanism that requires granzyme-induced ROS production.

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NK cells are primarily known for their cytotoxic functions against tumour and virally infected cells. There is increasing evidence that NK cells may also play an important role in antibacterial immunity, but these mechanisms remain largely unknown. NK cells indirectly facilitate antibacterial immune responses through the recruitment of neutrophils and macrophages via IFN-γ and TNF-α production. Some studies have also observed their ability to directly bind to and kill microbes through induction of apoptosis and release of cytotoxic factors. Although NK cell antimicrobial activity has been studied in the context of intracellular bacterial and extracellular fungal infections, their role in fighting extracellular bacteria, specifically P. aeruginosa, has not been thoroughly investigated.

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To confirm the antimicrobial activity of NK cells in P. aeruginosa, Feehan et al. cocultured P. aeruginosa with peripheral blood NK (pNK) cells isolated from healthy donors, as well as a human NK cell line (YT cells). Both co-cultures led to a reduction in P. aeruginosa CFU. Using a transwell assay system, they determined that contact between NK cells and P. aeruginosa is necessary for killing via enhancement of bacterial membrane permeability. Additionally, CRISPR knockout of perforin and granulysin in the YT NK cell line showed similar bactericidal activity compared to the wild-type YT cells, demonstrating that perforin and granulysin are not necessary for P. aeruginosa killing. The authors further demonstrated that inhibiting serine protease activity (e.g. granzymes) with DCI (3,4-dichloroisocoumarin) reduced YT and pNK cell bacterial killing in a dose-dependent manner. Granzyme expression analysis in both YT and pNK cells revealed that granzymes B and H are both required to kill P. aeruginosa. Finally, they determined that ROS production was necessary for granzyme-mediated bacterial killing, as the addition of a ROS scavenger, Tiron, reduced the killing of P. aeruginosa.

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This research highlights a novel role that NK cells play in defence against P. aeruginosa infections that may be observed in other bacterial infections. Further understanding of the mechanisms of NK cell-mediated antibacterial activity will help guide the development of new treatments for bacterial infections.