Bacterial Lipopolysaccharides Induce Suppression of Natural Killer Cell Function

Objective: To determine the effect of F. nucleatum and bacterial lipopolysaccharides (LPS) treated Oral Squamous Cancer Stem Cells (OSCSCs) on the cytotoxic and cytokine secretion of Natural Killer (NK) cells and to establish that bacteria treated OSCSCs inhibit NK cell mediated cytotoxicity due to increased differentiation in OSCSCs during interaction with NK cells.

Methods: Oral Squamous Cancer Stem Cells (OSCSCs) were treated with F. nucleatum bacteria at 250:1 bacteria to OSCSCs, and with LPS at 10mg/ml in the presence and absence of irradiated monocytes for 24 hours, 3 days and 5 days before they were washed extensively and cultured with untreated, anti-CD16 mAb and/or IL-2 treated NK cells. NK cell function was assessed using 51Cr release assay. Cytokine secretion was quantified with ELISA.   

Results: Both F. nucleatum and LPS treated OSCSCs significantly inhibited NK cell mediated cytotoxicity while they were able to augment secretion of cytokines by the NK cells. Secretion of IFN-g and IL-6 increased significantly in the co-cultures of NK cells with F. nucleatum and LPS treated OSCSCs. Addition of monocytes to F. nucleatum and LPS treated OSCSCs further suppressed the cytotoxic function of NK cells and resulted in a greater secretion of cytokines. Decrease in the cytotoxicity of NK cells and augmented cytokine secretion by the NK cells was also observed when NK cells were treated with anti-CD16 mAb which induces split anergy in NK cells.

Conclusions: F. nucleatum and LPS treatment of OSCSCs in the presence of monocytes trigger suppression of NK cell mediated cytotoxicity due to an increased differentiation of OSCSCs because of conditioning of NK cells to lose cytotoxicity and gain in cytokine secretion capability (split anergy). Differentiated OSCSCs secrete higher levels of cytokines and express increased levels of B7H1 inhibitory co-stimulatory molecule expression.