Abstract
NK cells, which are composed of phenotypically and functionally heterogeneous subpopulations, play critical roles in immunity against cancer. The mechanism of generation of distinct subsets such as the effector and regulatory subtypes is unclear. Here, we show that this process comprises several steps, including generation of proliferating, highly cytotoxic cells activated by IL-15/IL-18 and differentiation into distinct cell populations induced with IL-12. Freshly prepared murine splenic NK cells expressed IL-15Rs and IL-18Rs and rapidly began to proliferate following stimulation with IL-15/IL-18. The proliferating NK cells highly expressed various activation markers such as B220, CD49b (DX5), lysosome-associated membrane glycoprotein 1 (LAMP-1), DNAX accessory molecule 1, perforin, and granzyme B and showed reduced expression of natural killer cell p46-related protein (NKp46) and IL-18Rα. These cells exerted strong cytotoxicity against YAC-1 cells, but did not secrete cytokines. IL-12 rapidly activated STAT4 in these cells, induced IFN-γ production, and then upregulated p21 and p27, leading to withdrawal from the cell cycle. In parallel, IL-12-stimulated cells gradually reduced cytotoxicity, decreased expression of activation markers, and instead increased expression of Sca-1, CD25, CD49a, and NKp46. Some IL-15/IL-18-induced cells strongly expressed PD-1, whereas NK cells induced with IL-15/IL-18 and IL-12 expressed high levels of T cell immunoglobulin mucin-3, LAG-3, and natural killer group 2 A. Furthermore, these cells spontaneously secreted IL-10 and TGF-β following prolonged incubation. Thus, IL-12 regulates expansion of NK cells activated with IL-15/IL-18, influences the population size of highly cytotoxic cells, and induces differentiation to unique cells sharing some phenotypes of ILCs.