Developmental and Functional Control of Natural Killer Cells by Cytokines

Abstract

Natural killer (NK) cells are effective in combating infections and tumors and as such are tempting for adoptive transfer therapy. However, they are not homogeneous but can be divided into three main subsets, including cytotoxic, tolerant, and regulatory NK cells, with disparate phenotypes and functions in diverse tissues. The development and functions of such NK cells are controlled by various cytokines, such as fms-like tyrosine kinase 3 ligand (FL), kit ligand (KL), interleukin (IL)-3, IL-10, IL-12, IL-18, transforming growth factor-β, and common-γ chain family cytokines, which operate at different stages by regulating distinct signaling pathways. Nevertheless, the specific roles of each cytokine that regulates NK cell development or that shapes different NK cell functions remain unclear. In this review, we attempt to describe the characteristics of each cytokine and the existing protocols to expand NK cells using different combinations of cytokines and feeder cells. A comprehensive understanding of the role of cytokines in NK cell development and function will aid the generation of better efficacy for adoptive NK cell treatment.

Keywords: cytokines; cytotoxicity; development; expansion; natural killer cells.

Figure 1

Cytokine requirements for natural killer (NK) cell development and function. NK cell development from HSCs is regulated by multiple cytokines in the fetal liver, bone marrow, and thymus. The sequential expression of receptors for different cytokines implies the functional maturation of NK cells. The proliferation and differentiation of HSCs requires FL, KL, IL-3, and IL-7, which interact with their respective receptors. The acquisition of CD122 expression is indicative of the commitment of NK cells. IL-15 is indispensable for NK cell differentiation from CLPs to mature NK cells. Mature NK cells are shaped by cytokine signals from the diverse tissue environments in which they reside. In the peripheral blood or spleen, the abundance of stimulatory cytokines, such as IL-2, IL-12, IL-15, IL-18, and IL-21, may maintain NK cells in a cytotoxic state to combat infections. Tolerant NK cells that reside in the liver, and regulatory NK cells that reside in the uterus, are primarily regulated by TGF-β and IL-10 or by TGF-β and IL-15, respectively. Abbreviations: FL, fms-like tyrosine kinase 3 ligand; KL, kit ligand; IL, interleukin; HSC, hematological stem cell; CLP, common lymphoid progenitor; TGF-β, transforming growth factor-β.

Figure 2

Cytokine regulation of natural killer (NK) cell expansion and cytotoxicity. Genetically modified K562 cells and IL-2 or IL-2 and OKT-3 without feeder cells, applied for the expansion of primary NK cells, can generate significant amounts of functional NK cells. The differentiation and expansion of NK cells from CD34⁺ HSCs are regulated by early activating cytokines, such as FL, KL, and IL-7, to promote HSC proliferation and differentiation, as well as by cytokines to activate NK cells, such as IL-15, IL-12, IL-21, and IGF-1. To improve NK cell survival or antitumor function, relative signals, such as the expression of mbIL-15 or preactivation with IL-12/15/18, strengthen activating or block inhibitory signals. These are vital for improving NK cell efficacy in adoptive cell therapy. Abbreviations: PBMC, peripheral blood mononuclear cell; CBMC, cord blood mononuclear cell; BMMC, bone marrow mononuclear cell; IL, interleukin; mbIL-15, membrane-bound IL-15; 4-1BBL, 4-1BB ligand; IGF-1, insulin-like growth factor 1; NKG2D, natural killer group 2D; DAP10, DNAX-activating protein 10; KIR, killer cell immunoglobulin-like receptors; NKG2A, natural killer group 2A.