Natural Killer Cell Immune Checkpoints and Their Therapeutic Targeting in Cancer Treatment

Abstract

Natural killer (NK) cells, serving as pivotal mediators of innate immunity, play an important role in antitumor immunity. Immune checkpoint can be expressed on the surface of NK cells and meticulously regulates their activation states and effector functions through complex signaling networks. In recent years, tumor immunotherapy strategies focusing on NK cell immune checkpoints have demonstrated remarkable advancements. This review systematically elucidates the expression profiles, signaling pathways, and the immune checkpoint molecule regulatory mechanisms localized on the NK cell membrane (e.g., NKG2A, KIRs, and TIGIT) or intracellularly (e.g., BIM, Cbl-b, and EZH2) during tumor immune evasion. Particular attention is devoted to dissecting the regulatory mechanisms through which these immune checkpoint molecules influence NK cell-mediated cytotoxicity, cytokine secretion, proliferative capacity, and tunable modulation of NK cell immune checkpoint expression by diverse factors within the tumor microenvironment. Furthermore, this review comprehensively summarizes preclinical advancements in NK cell immune checkpoint blockade strategies, including single checkpoint blockade, combinatorial checkpoint approaches, and their integration with conventional therapeutic modalities. Additionally, emerging therapeutic advancements, such as gene-editing technologies and chimeric antigen receptor-NK (CAR-NK) cell therapy, are evaluated for their prospective applications in immunotherapy based on NK cells. By thoroughly elucidating the molecular regulatory networks underlying NK cell immune checkpoints and their mechanisms of action within the complex tumor microenvironment, this review aims to provide critical theoretical insights and translational foundations to foster the development of innovative tumor immunotherapy strategies, improvement of combination therapies, and realization of personalized precision medicine.

Fig. 1.

ICs expressed by NK cells. NK cells express a series of IC molecules both on their surface and within their interiors, which play essential roles in regulating their functions. Inhibitory receptors located on the NK cell membrane interact with ligands presented on the surface of tumor cells, thereby initiating biological processes that suppress NK cell activity, promote immune tolerance, and diminish antitumor immunity. Conversely, activating receptors have the ability to enhance NK cell activity. IC molecules residing within NK cells regulate intracellular signaling pathways, thereby affecting processes such as proliferation, metabolism, and apoptosis, which collectively influence their antitumor capabilities. The figure was drawn by Figdraw (www.figdraw.com). NK, natural killer; MHC, major histocompatibility complex; PD-L1/2, programmed cell death ligand 1/2; CD, cluster of differentiation; HLA, major histocompatibility complex; GAL, galectin; PD-1, programmed cell death protein 1; KIR, killer immunoglobulin-like receptors; CTLA-4, cytotoxic T lymphocyte-associated antigen-4; NKG2A, natural killer group 2 member A; TIM-3, T cell immunoglobulin domain and mucin domain-3; LAG-3, lymphocyte activation gene-3; TIGIT, T cell immune receptor with Ig and ITIM domains; PVRIG, poliovirus receptor-related immunoglobulin domain-containing protein; BIM, Bcl-2 interacting mediator of cell death; Cbl-b, Casitas B lineage lymphoma proto-oncogene b; CIS, cytokine-inducible SH2-containing protein; FBP1, fructose-1,6-bisphosphatase; EZH2, enhancer of zeste homolog 2; TIPE2, TNF-α-induced protein 8-like 2; HIF-1α, hypoxia-inducible factor-1α; IL, interleukin; TGF, transforming growth factor; ROS, reactive oxygen species; TNF, tumor necrosis factor.

Fig. 2.

The balance between activating and inhibitory receptors in NK cells. NK cells coexpress both activating and inhibitory receptors, and the balance between these receptors is critical for modulating the antitumor functions of NK cells. These receptors interact with tumor cell surface molecules to either activate or suppress associated signaling pathways, thereby influencing NK cell differentiation, maturation, proliferation, and effector functions. Thus, comprehending the regulatory mechanisms governing these activating and inhibitory receptors, along with their heterogeneity across different tumor types, is pivotal for countering NK cell exhaustion, improving their antitumor potential, and developing innovative cancer therapies. The figure was drawn by Figdraw. ITIM, immunoreceptor tyrosine-based inhibitory motif; SHP, Src homology-2 domain-containing protein tyrosine phosphatase; YINM, tyrosine isoleucine asparagine methionine motif; SFKs, Src family kinases; PI3K, phosphatidylinositol 3-kinase; Akt, protein kinase B; MAPK, mitogen-activated protein kinase; MICA, major histocompatibility complex class I chain-related protein A; MICB, major histocompatibility complex class I chain-related protein B; NKG2D, natural killer group 2 member D.

Fig. 3.

Impact of ICs on the functional activities of NK cells. ICs can modulate NK cell functionality through multiple mechanisms. Inhibitory receptors, such as KIR and NKG2D, down-regulate the NF-κB pathway upon ligand binding, thereby suppressing NK cell-mediated cytotoxicity, cytokine secretion, and proliferation. Consequently, this leads to reduced NK cell activity within the TME, impairing antitumor immune responses and promoting tumor progression. In contrast, activating receptors, such as NKp30, can transmit stimulatory signals to NK cells upon cytokine stimulation, thereby promoting their activation. Tumors evade immune surveillance primarily by up-regulating inhibitory IC molecules. These molecules engage with receptors on the NK cell surface, thereby suppressing their antitumor activity. The figure was drawn by Figdraw. TRAIL, TNF-related apoptosis-inducing ligand; TRAIL-R1/2, TNF-related apoptosis-inducing ligand receptor 1/2; EGFR, epidermal growth factor receptor; Nectin-4, nectin cell adhesion molecule 4; PVR, poliovirus receptor; NKp30/46, natural killer p30/46; KIR, killer immunoglobulin-like receptors; IFN, interferon; IgG, immunoglobulin G.

Fig. 4.

NK cell-based immunotherapies. Currently, numerous immunotherapeutic strategies have been developed with a focus on targeting NK cells. ICIs, whether used as monotherapies or in combination with other immunotherapeutic approaches, have shown promising therapeutic potential in clinical trials. Moreover, the identification and establishment of predictive biomarkers offer critical insights into therapeutic success rates, patient survival outcomes, and the functional evaluation of NK cell-based therapies. CAR-NK cell therapy has also demonstrated significant potential in treating solid tumors. Compared to CAR-T cell therapy, CAR-NK therapy provides distinctive advantages concerning safety and production efficiency. NK cell-based immunotherapies could expand treatment options for tumor-related diseases and are anticipated to play an increasingly pivotal role in the future of cancer therapy. The figure was drawn by Figdraw. PD-1, Programmed cell death protein 1; TIM-3, T cell immunoglobulin domain and mucin domain-3; TIGIT, T cell immunoreceptor with Ig and ITIM domain; LAG-3, Lymphocyte-activation gene 3; PVRIG, Poliovirus receptor-related immunoglobulin domain-containing protein; ICI, IC inhibitors; CAR, Chimeric antigen receptor; GVHD, Graft-versus-host disease.