NKG2A-mediated immune modulation of natural killer cells by Staphylococcus aureus
Natural killer (NK) cells are specialized lymphocytes that help protect against viruses and cancer. However, in the context of bacterial infections, NK cells can be harmful, rather than protective. Such immune pathogenesis by NK cells has been linked to the over-production of pro-inflammatory cytokines like interferon-{gamma} (IFN-{gamma}). In this context, IFN-{gamma}-deficient mice display increased survival rates in response to Staphylococcus aureus (S. aureus), which causes life-threatening, invasive systemic infections with high mortality rates in humans. However, little is known about how NK cells respond to S. aureus in humans. In this study, we found that the peripheral blood of patients with bloodstream S. aureus infection was enriched for NKG2A+ NK cells with greater cytokine producing capacity, compared to those hospitalized with Escherichia coli bloodstream infections. As a possible mechanistic cause, superantigens from S. aureus promoted the expansion of CD57- NKG2A+ NK cells which produced IFN-{gamma} through an IL-12-independent mechanism and exhibited reduced levels of CD16 compared to unstimulated NK cells. These data suggest that S. aureus bloodstream infection in humans promotes a phenotypic shift towards NKG2A+ NK cells with greater IFN-{gamma} producing capacity, providing a plausible way to promote inflammation-driven disease pathogenesis. AUTHOR SUMMARYNatural Killer (NK) cells are specialized immune cells that provide crucial defence against viruses but can also respond to bacterial infections, especially in humans. During bloodstream infection by Staphylococcus aureus, a Gram-positive bacterial pathogen that causes life-threatening infections in humans, NK cells may actually be harmful, rather than protective, to the host. However, very little is known about the NK cell response to invasive Staphylococcus aureus infections in humans. Here, we show that human patients with bloodstream Staphylococcus aureus, but not Escherichia coli, infections have an increased frequency of NK cells with increased pro-inflammatory capacity. Furthermore, we show that toxins produced by Staphylococcus aureus, which help the bacteria evade the protective effects of T cells the immune system ("superantigens"), promoted the expansion of these pro-inflammatory NK cells, providing a possible mechanistic cause for their increased presence in patients with bloodstream Staphylococcus aureus infections. Collectively, these results suggest that Staphylococcus aureus infection triggers phenotypic and functional changes in NK cells that provide a plausible way to promote inflammation-driven disease pathogenesis