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Natural killer (NK) cells are innate immune cells that are unique due to their constitutive expression of an activating receptor for the Fc portion of IgG (FcγRIIIa). FcR-activated NK cells have been shown to secrete interferon (IFN)-γ and other immune modulatory cytokines, and are also able to mediate antibody-dependant cellular cytotoxicity (ADCC) against antibody (Ab)-coated targets. Interleukin (IL)-21 is a recently discovered cytokine that has been shown to enhance NK cell IFN-γ secretion and lytic activity. The present study investigated NK cell activity in response to dual stimulation by IL-21 and Ab-coated targets. Purified human NK cells cultured with IL-21 and immobilized IgG or human breast cancer cells coated with a therapeutic monoclonal Ab (trastuzumab) secreted > 5-fold higher levels of IFN-γ as compared to NK cells stimulated with either mAb-coated tumor cells or IL-21 alone. Enhanced secretion of TNF-α and the T cell-recruiting chemokines IL-8, MIP-1α, IP-10, MIG, and RANTES was also observed in response to co-stimulation. Supernatants derived from NK cells that had been stimulated with IL-21 and monoclonal Ab (mAb)-coated breast cancer cells were able to drive the migration of both naïve and activated T cells in an in vitro chemotaxis assay. NK cell production of IFN-γ in response to mAb-coated tumor cells and IL-21 was dependent on activation of the signal transducer and activator of transcription (STAT)-1 and the p44/p42 mitogen-activated protein kinase (MAPK), as IFN-γ production was not observed from NK cells from STAT-1 knockout mice or NK cells that had been pre-treated with a specific chemical inhibitor of the p44/p42 MAPK. In contrast, IFN-γ production by co-stimulated NK cells was not dependent on activation of Akt or the MAPK family members p38 and Jnk. In addition, the p44/p42 MAPK was activated following FcγRIIIa ligation in STAT1-knockout NK cells, and STAT-1 was activated in response to IL-21 in NK cells pre-treated with the p44/p42 MAPK inhibitor. These results suggested that NK cell IFN-γ production in response to IL-21 and mAb-coated tumor cells was dependent on distinct and non-interacting signals mediated by the IL-21 receptor (STAT-1) and the FcR (p44/p42 MAPK) on the surface of NK cells. IL-21 also enhanced NK cell lytic activity against trastuzumab-coated human breast cancer cells and rituxan-coated B cell lymphomas. Co-administration of IL-21 and mAb-coated tumor cells to immunocompetent mice led to enhanced production of IFN-γ by NK cells. Furthermore, the administration of IL-21 significantly enhanced the effects of an anti-HER2/neu mAb in a murine solid tumor model. These findings suggest that administration of IL-21 could enhance the innate immune response to anti-tumor mAbs in patients with cancer.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]