Understanding of molecular mechanisms in natural killer cell therapy

Abstract

Cancer cells and the immune system are closely related and thus influence each other. Although immune cells can suppress cancer cell growth, cancer cells can evade immune cell attack via immune escape mechanisms. Natural killer (NK) cells kill cancer cells by secreting perforins and granzymes. Upon contact with cancer cells, NK cells form immune synapses to deliver the lethal hit. Mature NK cells are differentiated from hematopoietic stem cells in the bone marrow. They move to lymph nodes, where they are activated through interactions with dendritic cells. Interleukin-15 (IL-15) is a key molecule that activates mature NK cells. The adoptive transfer of NK cells to treat incurable cancer is an attractive approach. A certain number of activated NK cells are required for adoptive NK cell therapy. To prepare these NK cells, mature NK cells can be amplified to obtain sufficient numbers of NK cells. Alternatively, NK cells can be differentiated and amplified from hematopoietic stem cells. In addition, the selection of donors is important to achieve maximal efficacy. In this review, we discuss the overall procedures and strategies of NK cell therapy against cancer.

Figure 1

Natural killer (NK) cell activation and its translation to therapeutic application. The encounter between the NK cell and target cell results in adhesion and conjugation (Immune Synapse). The dynamic balance between inhibitory and activating receptor signaling at the cell–cell interface decides the outcome of the immune synapse. Engagement of NK cell activating receptors induces the phosphorylation of ITAM or kinase and tight actin cytoskeleton rearrangements that, in turn, lead to a more stable conjugation (Activation). NK cells can be primed or activated by cytokines locally secreted by other immune cells, inducing various types of immune-related gene expression including cytokines, NK cell effectors and noncoding microRNAs (miRNAs). Sustained stimulatory signaling causes robust actin polymerization and polarization of the MTOC to the immune synapse. Lytic granules containing effectors (for example, GzmB and Prf1) are transported along microtubule tracks for subsequent release. Prf1 and GzmB can induce target cell apoptosis. Upon activation, cytokines, including pro-inflammatory cytokines (for example, interferon-γ (IFN-γ)), are also secreted for immune regulation (NK cell functions). Major strategies to harness NK cell functions in cancer (Tuning NK cell activity). First, treatment with cytokines promotes NK cell proliferation, differentiation and activation and/or inhibits NK cell suppressors. Second, manipulating the balance between stimulatory and inhibitory signaling can promote NK cell activation through the use of CAR and/or adoptive transfer of allosteric NK cells. In addition, these strategies are focused on NK effector activation by promoting TRAIL (tumor-necrosis factor (TNF)-related apoptosis-inducing ligand) or CD178 expression, as well as ADCC activation via engineered Fc. Third, these strategies may be enhanced by anti-inhibitory antibodies specific for negative regulators such as KIR, NKG2A and PD1. ADCC, antibody-dependent cellular cytotoxicity; CAR, chimeric antigen receptor; GzmB, granzyme B; ITAM, immunoreceptor tyrosine-based activation motif; ITIM, immunoreceptor tyrosine-based inhibitory motif; KIR, killer cell immunoglobulin-like receptor; MHC, major histocompatibility; MTOC, microtubule-organizing center; PD1, programmed cell death 1; Prf1, perforin; −, NK cell inhibition; +, NK cell activation. Substances that act on NK cells are indicated in red text.

Figure 2

Production of natural killer (NK) cells in vitro for immunotherapy. Human NK cells can be obtained from various sources, such as peripheral blood (PB), umbilical cord blood (UCB), bone marrow (BM), mobilized PB (mPB) or embryonic stem cells (ESCs). NK cells can be isolated from PB or UCB and expanded. In addition, NK cells can be generated using CD34⁺ hematopoietic stem cells (HSCs) from UCB, BM, mPB or ESCs. CD34⁺ HSCs are differentiated into CD56⁺ mature NK cells (mNK) via CD122⁺ precursor NK cells (pNK). Finally, mNK cells armed with effector functions are infused into the patient for adoptive immunotherapy.

Figure 3

From hematopoietic stem cells (HSCs) to clinical application. Natural killer (NK) cells are differentiated from HSCs. For efficient NK cell therapy, each step of NK cell preparation is important. NK cell differentiation, activation and genetically modified NK cells (chimeric antigen receptor (CAR)-NK) are major strategies for the generation of potent NK cell therapy.