A large number of natural killer (NK) cells with high function are expected to generate especially in tumor adoptive immunotherapy. Here K562 cells were genetically modified to co-express major histocompatibility complex class I chain-related protein A (MICA), 4-1BB ligand, and IL-15, called K562-MICA-4-1BBL-IL-15. The modified K562 cells not only promoted activation, proliferation, and survival of NK cells, but also enhanced NK cell cytotoxicity. In long-term culture tests, K562-MICA-4-1BBL-IL-15 cells stimulated NK cell to expand mean 550 folds in 24-day culture and to cover from 14.8% of total peripheral blood monoclonal lymphocytes on day 1 to 86.7% on day 24. Prevalent NK cells after expansion enhanced the ability of killing targets and producing interferon gamma (IFN-γ), and kept high expression of activating receptors. The results indicated that K562-MICA-4-1BBL-IL-15 cells would be developed for expansion of NK cells ex vivo and may have important implications for clinical immunotherapy.

Interleukin (IL)-15 plays an important role in natural killer (NK) and CD8+ T-cell proliferation and function and is more effective than IL-2 for tumor immunotherapy. The trans-presentation of IL-15 by neighboring cells is more effective for NK cell activation than its soluble IL-15. In this study, the fusion protein dsNKG2D-IL-15, which consisted of two identical extracellular domains of human NKG2D coupled to human IL-15 via a linker, was engineered in Escherichia coli. DsNKG2D-IL-15 could efficiently bind to major histocompatibility complex class I chain-related protein A (MICA) of human tumor cells with the two NKG2D domains and trans-present IL-15 to NK or CD8+ T cells. We transplanted human gastric cancer (SGC-7901) cells into nude mice and mouse melanoma cells with ectopic expression of MICA (B16BL6-MICA) into C57BL/6 mice. Then, we studied the anti-tumor effects mediated by dsNKG2D-IL-15 in the two xenografted tumor models. Human dsNKG2D-IL-15 exhibited higher efficiency than IL-15 in suppressing gastric cancer growth. Exogenous human dsNKG2D-IL-15 was centrally distributed in the mouse tumor tissues based on in vivo live imaging. The frequencies of human CD56+ cells infiltrated into the tumor tissues following the injection of peripheral blood mononuclear cells into nude mice bearing human gastric cancer were significantly increased by human dsNKG2D-IL-15 treatment. Human dsNKG2D-IL-15 also delayed the growth of transplanted melanoma (B16BL6-MICA) by activating and recruiting mouse NK and CD8+ T cells. The anti-melanoma effect of human dsNKG2D-IL-15 in C57BL/6 mice was mostly decreased by the in vivo depletion of mouse NK cells. These data highlight the potential use of human dsNKG2D-IL-15 for tumor therapy.Cellular& Molecular Immunology advance online publication, 14 September 2015; doi:10.1038/cmi.2015.81.

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Tumor-targeted cytokines are a new class of pharmaceutical anticancer agents often considered superior to the corresponding unconjugated cytokines for therapeutic purposes. We generated a new fusion protein, dsNKG2D-IL-15, in which double NKG2D extracellular domains were fused to IL-15, in Escherichia coli. This fusion protein promoted the activation, proliferation, and cytotoxicity of NK cells, and bound to NKG2D ligand-positive tumor cells. These tumor cells were also more susceptible to NK-cell attack when decorated with dsNKG2D-IL-15. The administration of mouse dsNKG2D-IL-15 protein in vivo significantly retarded the growth of transplanted colon cancers and prolonged the survival of tumor-bearing mice. Treatment with dsNKG2D-IL-15 increased the frequencies of NK and CD8 T cells in spleen and tumor tissues. The antitumor effect mediated by dsNKG2D-IL-15 was significantly decreased with in vivo depletion of NK cells or CD8 T cells. Recombinant dsNKG2D-IL-15 thus inhibited NKG2D ligand-positive tumor growth effectively by activating lymphocytes. This new biological fusion protein could potentially be used to elicit immunity in tumor-targeting treatments.

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