Purpose: Natural killer (NK) cells express an activating Fc receptor (FcgRIIIa) that mediates antibody-dependent cellular cytotoxicity (ADCC) and production of immune modulatory cytokines in response to antibody-coated targets. Cetuximab is a therapeutic monoclonal antibody directed against the HER1 antigen. We hypothesized that the NK cell response to cetuximab-coated tumor cells could be enhanced by the administration of NK cell^stimulatory cytokines. Experimental Design: Human NK cells stimulated with cetuximab-coated tumor cells and interleukin-2 (IL-2), IL-12, or IL-21 were assessed for ADCC and secretion of IFN-g and T cell^recruiting chemokines. IL-21 and cetuximab were given to nude mice bearing HER1-positive xenografts. Results: Stimulation of human NK cells with cetuximab-coated tumor cells and IL-2, IL-12, or IL-21 resulted in 3-fold to 10-fold higher IFN-g production than was observed with either agent alone. NK cell^derived IFN-g significantly enhanced monocyte ADCC against cetuximab-coated tumor cells. Costimulated NK cells also secreted elevated levels of chemokines (IL-8, macrophage inflammatory protein-1a, and RANTES) that could direct the migration of naive and activated Tcells. IL-2, IL-12, and IL-21enhanced NK cell ADCC against tumor cells treated with cetuximab. The combination of cetuximab, trastuzumab (an anti-HER2 monoclonal antibody), and IL-21 mediated greater NK cell cytokine secretion and ADCC than any agent alone. Furthermore, administration of IL-21enhanced the effects of cetuximab in a murine tumor model. Conclusions: These results show that cetuximab-mediated NK cell activity can be significantly enhanced in the presence of NK cell^stimulatory cytokines. These factors, therefore, may be effective adjuvants to administer, in combination with cetuximab, to patients with HER1-positive malignancies.

Interleukin-21 (IL-21

In the current report, we have examined the ability of natural killer (NK) cells to produce T cell-recruiting chemokines following dual stimulation with interleukin (IL)-2 or IL-12 and human breast cancer cells coated with an antitumor antibody (trastuzumab). NK cells stimulated in this manner secreted an array of T cell-recruiting chemotactic factors, including IL-8, macrophage-derived chemokine, macrophage inflammatory protein 1A (MIP-1A), monocyte chemoattractant protein 1, and regulated on activation, normal T-cell expressed and secreted (RANTES), whereas stimulation of NK cells with either agent alone had minimal effect. Furthermore, these factors were functional for T-cell chemotaxis as culture supernatants derived from costimulated NK cells induced migration of both naïve and activated T cells in an in vitro chemotaxis assay. T-cell migration was significantly reduced when neutralizing antibodies to IL-8, MIP-1A, or RANTES were added to culture supernatants before their use in the chemotaxis assay. In addition, coadministration of trastuzumab-coated tumor cells and IL-12 to mice led to enhanced serum MIP-1A. As a clinical correlate, we examined the chemokine content of serum samples from breast cancer patients enrolled on a phase I trial of trastuzumab and IL-12, and found elevated levels of IL-8, RANTES, IFN-; inducible protein 10, monokine induced by IFN-;, and MIP-1A, specifically in those patients that experienced a clinical benefit. Sera from these patients exhibited the ability to direct T-cell migration in a chemotaxis assay, and neutralization of chemokines abrogated this effect. These data are the first to show chemokine production by NK cells, specifically in response to stimulation with antibody-coated tumor cells, and suggest a potential role for NK cell-derived chemokines in patients receiving therapeutic monoclonal antibodies. (Cancer Res 2006; 66(1): 517-26)

NK cells express an activating FcR (FcγRIIIa) that mediates Ab-dependent cellular cytotoxicity and the production of immune modulatory cytokines in response to Ab-coated targets. IL-21 has antitumor activity in murine models that depends in part on its ability to promote NK cell cytotoxicity and IFN-γ secretion. We hypothesized that the NK cell response to FcR stimulation would be enhanced by the administration of IL-21. Human NK cells cultured with IL-21 and immobilized IgG or human breast cancer cells coated with a therapeutic mAb (trastuzumab) secreted large amounts of IFN-γ. Increased secretion of TNF-α and the chemokines IL-8, MIP-1α, and RANTES was also observed under these conditions. NK cell IFN-γ production was dependent on distinct signals mediated by the IL-21R and the FcR and was abrogated in STAT1-deficient NK cells. Supernatants derived from NK cells that had been stimulated with IL-21 and mAb-coated breast cancer cells were able to drive the migration of naive and activated T cells in an in vitro chemotaxis assay. IL-21 also enhanced NK cell lytic activity against Ab-coated tumor cells. Coadministration of IL-21 and Ab-coated tumor cells to immunocompetent mice led to synergistic production of IFN-γ by NK cells. Furthermore, the administration of IL-21 augmented the effects of an anti-HER2/neu mAb in a murine tumor model, an effect that required IFN-γ. These findings demonstrate that IL-21 significantly enhances the NK cell response to Ab-coated targets and suggest that IL-21 would be an effective adjuvant to administer in combination with therapeutic mAbs.

Bacterial DNA contains a high frequency of unmethylated CpG motifs that stimulate immune cells via TLR9. NK cells express a low-affinity activating receptor for the Fc portion of IgG (FcγRIIIa), but were not thought to express TLR9 protein. The direct response of NK cells to CpG oligodeoxynucleotides (ODN) in the presence of FcR stimulation was investigated. Human NK cells cultured in the presence of CpG ODN plus immobilized IgG or Ab-coated tumor cells secreted large amounts of IFN-γ (>2000 pg/ml), whereas cells stimulated with Ab alone, CpG ODN alone, or Ab and control ODN produced negligible amounts. Enhanced secretion of IL-8, macrophage-derived chemokine, and MIP-1α was also observed after costimulation. NK cell cytokine production was not the result of interactions with APCs or their cytokine products. Flow cytometric analysis revealed that 36 ± 3.5% of human NK cells expressed basal levels of TLR9. TLR9 expression in human NK cells was confirmed by immunoblot analysis. Only TLR9-expressing NK cells responded to CpG ODN and Ab, because cytokine production was not observed in NK cells from TLR9-deficient mice. Mice receiving CpG ODN and HER2/neu-positive tumor cells treated with an anti-HER2 Ab exhibited enhanced systemic levels of IFN-γ compared with mice receiving either agent alone. TLR9−/− animals reconstituted with TLR9+/+ NK cells secreted IFN-γ in response to CpG ODN and Ab-coated tumor cells. These findings indicate that CpG ODN can directly enhance the NK cell cytokine response to Ab-coated targets via activation of TLR9.