Natural killer cell activation primes macrophages to clear bacterial infection.

Natural killer (NK) cells are major cytokine producers during bacterial sepsis, but their precise role is undefined. This study investigates the effect of NK cell depletion with and without prior activation on macrophage function and bacterial clearance during cecal ligation and puncture. Two different NK cell-depleting antibodies were used: anti-asialo-GM1 (GM1), a nonactivating antibody, and anti-NK1.1 (NK1.1), an NK cell-activating antibody. C57BL/6 mice were NK depleted with either GM1 or NK1.1 by intraperitoneal injection 7 and 3 days before experimentation. Control animals received isotype immunoglobulin G. Depletion was confirmed by flow cytometry. Bacterial levels in peritoneal washout, blood, and liver were determined 4 hours after cecal ligation and puncture. Macrophage activation was measured by phagocytosis ability and by production of nitric oxide and interleukin-6. Depletion with GM1 resulted in significantly higher bacterial levels at 4 hours, whereas depletion with NK1.1 had the opposite effect of significantly decreasing bacterial levels. Macrophage phagocytosis ability was significantly increased in mice depleted with NK1.1 compared with those mice depleted with GM1. We conclude that activation of NK cells improves bacterial clearance by priming macrophages to help clear a subsequent bacterial challenge. Macrophages are less able to clear bacteria when NK cells are depleted without activation. NK cells are therefore important in bacterial clearance through interactions with macrophages.