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A more efficient killing machine : how CpG-oligodeoxynucleotides enhance natural killer cell cytokine production and cytotoxicity against leukemia initiating cells Guillon, Laura Katharine

Abstract

Natural killer (NK) cells are lymphocytes that comprise part of the innate immune system and play a key role in the early defence against pathogenic organisms and cancer. CpG oligodeoxynucelotides (ODNs) are short synthetic ODN containing unmethylated CpG dinucleotide motifs that have immune-enhancing effects. NK cell-derived IFN-γ is essential for the effects of CpG ODNs, but how NK cells become activated by CpG ODNs remains unclear. We found that CpG ODN-mediated stimulation of NK cells requires IL-12 or IL-18. CpG ODNs did not stimulate IL-12-deficient mouse spleen cells and IL-12 neutralization almost completely inhibited IFN-γ production. Although IL-18 was undetectable in cultures, neutralization significantly dampened the IFN-γ response and addition of exogenous IL-18 greatly enhanced CpG ODN-mediated NK cell stimulation. IL-12 is mainly produced by Gr-1⁺ monocytes and neutrophils, while what cells produce IL-18 remains unknown. We then tested the anti-leukemia effects of CpG ODN-stimulated NK cells. Studies with human acute myeloid leukemia (AML) patients have shown that haploidentical NK cells effectively kill AML blasts, but their ability to lyse leukemia initiating cells (LICs) has not been studied. Therefore, we tested NK cells from haploidentical F1 mice against the mouse AML cell line MN1. F1 mouse NK cells expanded in cultures in the presence of IL-15 and stimulated by CpG ODNs plus IL-18, effectively killed bulk MN1 cells in vitro and reduced the numbers of in vitro colony forming cells. NK cell-treated MN1 cells were also injected into irradiated B6 mice to test whether AML LICs were also killed. F1 mouse NK cells seemed to kill some AML initiating cells since mice receiving NK-treated MN1 cells survived significantly longer than those given untreated MN1 cells, but the frequency of LICs did not significantly differ between MN1 cells incubated with or without NK cells. For NK cells to be used as a treatment for AML, we must find a way to induce a higher cytotoxicity in NK cells or to target them specifically towards LIC.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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