The Molecular Mechanism of Natural Killer Cells Function and Its Importance in Cancer Immunotherapy

Abstract

Natural killer (NK) cells are innate immune cells that show strong cytolytic function against physiologically stressed cells such as tumor cells and virus-infected cells. NK cells show a broad array of tissue distribution and phenotypic variability. NK cells express several activating and inhibitory receptors that recognize the altered expression of proteins on target cells and control the cytolytic function. NK cells have been used in several clinical trials to control tumor growth. However, the results are encouraging only in hematological malignancies but not very promising in solid tumors. Increasing evidence suggests that tumor microenvironment regulate the phenotype and function of NK cells. In this review, we discussed the NK cell phenotypes and its effector function and impact of the tumor microenvironment on effector and cytolytic function of NK cells. We also summarized various NK cell-based immunotherapeutic strategies used in the past and the possibilities to improve the function of NK cell for the better clinical outcome.

Keywords: cancer immunotherapy; inflammation; innate immune cells; natural killer cells; tumor microenvironment.

Figure 1

Missing-self recognition of target cells. The activating and inhibitory receptor signaling regulates the natural killer (NK) cells activation. Cells undergoing stress such as tumor cells lose their MHC class I molecules, a ligand for inhibitory receptors on NK cells. At the same time, they acquire stress-associated molecules which act as ligands for activating receptors. Thus, the lack of inhibitory signaling coupled with induction of activating signaling shifts the balance toward NK cell activation, leading to secretion of cytokines and killing of tumor cells.

Figure 2

Schematic representation of natural killer (NK) cell inhibitory receptor signaling. The interaction of NK cell inhibitory receptors natural-killer group 2, member A (NKG2A) and Ly49A with its cognate ligand leads to phosphorylation of immunoreceptor tyrosine-based inhibitory motif (ITIM) in their cytoplasmic tails. Phosphorylated ITIM recruits phosphatases such as Src homology domain-containing tyrosine phosphatase (SHP) and SH2 domain-containing inositol-5-phosphatase (SHIP) that dephosphorylate signaling molecules such as Lck, Fyn, Syk, Zap70, and Vav1, thereby terminating activating receptor signaling in NK cells.

Figure 3

Schematic representation of natural killer (NK) cell activating receptor signaling. Interaction of activating receptor NKG2D and Ly49D with their cognate ligand leads to phosphorylation of YINM motif or immunoreceptor tyrosine-based activation motifs (ITAMs) present in the cytoplasmic tails of associated adapter protein such as DAP10 and DAP12. Phosphorylated ITAM or YINM motif recruits Syk/Zap70, PI3K, and Grb2/Vav1/SLP-76 complex. Grb2/Vav1/SLP-76 pathway activation leads to downstream activation of MEK/extracellular signal-regulated kinase (ERK) pathway. Phosphorylated Syk recruits PLC-γ which in turn activates inducible protein-3 (IP-3) and DAG pathway leading to activation of transcription factors NF-κB and NFAT. The net result of this signaling is the release of cytokines and chemokines as well as cytotoxic molecules by the NK cells (24).

Figure 4

The cytotoxic response of natural killer (NK) cells. The NK cell cytotoxic response is tightly regulated in four discrete stages. (A) Step 1: Recognition of target cells by NK cell results in the reorganization of actin cytoskeleton and the formation of immunological synapse, and clustering of cell adhesion molecules such as lymphocyte function-associated antigen 1 (LFA-1) and CD2. (B) Step 2: microtubule organizing center (MTOC) and secretory lysosome polarize toward the immunological synapse. (C) Step 3: docking which involves moves close to the plasma membrane of NK cell at the synapse. (D) Step 4: secretory lysosome fuse with the target cell plasma membrane and releases the cytotoxic granules into the target cell.

Figure 5

Various natural killer (NK) cell-based immunotherapy approaches. (A) In adoptive cellular therapy, NK cells are freshly isolated from peripheral blood mononuclear cells (PBMCs) from the patient. These NK cells are ex vivo activated and expanded and then transfused back into the patient. (B) In the second strategy, patients are treated with recombinant cytokines such as IL-2, IL-15, IL-12, or IL-18. These cytokines promote activation and proliferation of NK cells in the body leading to better antitumor immunity. (C) In the third strategy, NK cells are engineered to express chimeric antigen receptor (CAR), followed by expansion of these CAR NK cells, and transfused back into the patient. (D) In monoclonal antibody (mAb)-based treatment, patients are treated with IPH2101, a blocking mAb against inhibitory receptors and these mAb can promote NK cells activity in the tumor microenvironment and restrict tumor growth.