Natural Killer Cell-Mediated Antitumor Immunity: Molecular Mechanisms and Clinical Applications

Abstract

Natural killer (NK) cells are pivotal effectors in innate antitumor immunity by mediating cytotoxicity, secreting cytokines, or expressing cell membrane receptors, which facilitate interactions with other immune cells. The cytotoxic activity and immune function of NK cells are governed by dynamic receptor-ligand interactions, cytokine networks, and metabolic-epigenetic crosstalk within the tumor microenvironment (TME). Recent years, NK cell-based therapies are emerging as a promising clinical approach for antitumor treatment, owing to their rapid response, unique recognition mechanisms, potent cytotoxic capabilities, and memory-like characteristics, along with their low risk of posttreatment adverse effects and cost effectiveness. However, immunosuppression and metabolic reprogramming driven by TME subvert NK cell surveillance, impairing its antitumor function. This review comprehensively details molecular mechanisms underpinning NK cell dysfunction, including dysregulated activating/inhibitory receptor signaling, metabolic reprogramming, and epigenetic silencing of effector genes. We further synthesize advances in clinical strategies to restore NK cytotoxicity including ex vivo expansion for adoptive transfer, chimeric antigen receptor-NK engineering, TME-remodeling agents, immune checkpoint blockade, cytokine-based therapies, and NK cell engagers targeting tumor antigens. By bridging mechanistic insights with translational applications, this work provides a framework for rationally designed NK cell-based immunotherapies to overcome resistance across solid and hematologic malignancies.

Keywords: NK cells; TME; clinical strategies; metabolic reprogramming; molecular crosstalk.

FIGURE 1

Cytokine‐ and signaling pathway‐mediated regulation of NK cell function within the TME. Multiple cytokines and checkpoint molecules—including IL‐2, IL‐12, IL‐15, and TGF‐β‐dynamically regulate NK cell activation, cytotoxicity, and survival through modulation of intracellular signaling cascades and cell–cell interactions within the TME. Engagement of IL‐2/IL‐15 receptors activates JAK1/2/3, leading to phosphorylation of STAT1/3/4/5. STAT1/4/5 promote NK cell proliferation, differentiation, and cytotoxicity, whereas STAT3 acts as a negative feedback regulator, suppressing NK cell tumor‐killing activity. Tumor‐derived TGF‐β activates Smad2/3 in NK cells, driving transcriptional programs—partly via BATF—that induce exhaustion, reduce IFN‐γ production, and promote epigenetic–metabolic reprogramming. NK cell‐derived IFN‐γ and IFN‐α act in autocrine/paracrine fashion to enhance NK cell differentiation into IFN‐γ‐producing subsets, amplifying antitumor immune responses despite proliferative suppression.

FIGURE 2

TME shapes NK cell metabolisms. As shown in panel (A), in the resting state, NK cells primarily rely on low‐level glycolysis and mitochondrial OXPHOS to sustain their basic physiological functions. Panel a: Upon stimulation with cytokine IL‐2 or the combination of IL‐12 and IL‐15, the levels of OXPHOS and glycolysis in NK cells are markedly increased, facilitating enhanced glucose uptake and supporting their normal function. Stimulation by IL‐2 activates the mTORC1 signaling pathway in NK cells, which promotes glycolysis and OXPHOS through cMyc and ultimately contributes to the regulation of IFN‐γ production. Panel b: Simultaneously, the expression of amino acid transporters SLC1A5 and CD98 on NK cell surfaces is significantly upregulated, indicating enhanced amino acid uptake to meet the demands of increased amino acid metabolism following activation. Additionally, the stability of cMyc translation requires amino acids, highlighting their role in this process. Panel c: To ensure an adequate energy supply, lipid metabolism plays a crucial role in the growth, differentiation, maturation, and function of NK cells. Lipid levels in NK cells are primarily regulated by the PPAR. Alpha‐ketoglutaric acid (αKG), an intermediate of the TCA cycle, is generated from the catabolism of fatty acids and glutamine, thereby supplying energy to NK cells. Panel (B) demonstrates that in the TME, three types of NK cell metabolism undergo alterations. Panel a: The glucose metabolism of NK cells, encompassing glycolysis and OXPHOS, is significantly reduced. On one hand, the superior metabolic capacity of tumor cells restricts the availability of glucose for NK cells within the tumor microenvironment. Conversely, the accumulation of lactate, along with the activation of signaling pathways such as TGFβ and STAT3, impairs the mitochondrial function of NK cells. Panel b: Similarly, amino acid metabolism in NK cells is constrained within the TME. As tumor cells preferentially utilize arginine, tryptophan, and glutamine, the metabolic levels of these amino acids in NK cells significantly decrease, thereby impairing their proliferation, cytotoxicity, and IFN‐γ production. Reduced tryptophan metabolism increases the susceptibility of NK cells to ROS‐induced damage. A reduction in glutamine levels within NK cells leads to a corresponding decrease in cMyc levels. Additionally, the accumulation of lipids results in reduced expression of acetylase P300, thereby hindering the acetylation of cMyc and ultimately impairing NK cell proliferation and antitumor function. Panel c: The buildup of ROS leads to oxidative stress, causing cellular damage. NK cells enhance lipid metabolism by increasing the absorption and storage of lipids within the lipid‐rich tumor microenvironment to counteract oxidative stress. However, this increase in lipid metabolism correlates with a decrease in IFN‐γ secretion.

FIGURE 3

Signaling pathways involved in metabolism reprogramming of NK in TME. The green section of this figure illustrates byproducts of the elevated metabolism in tumor cells, including hypoxia, lactic acid accumulation, and the activation of signaling pathways like TGFβ and STAT3, can impair mitochondrial function in NK cells, leading to a significant reduction in their glucose metabolism. On one hand, the impaired glycolytic function of NK cells is attributed to the reduced expression of Glut1, the glucose transporter on the NK cell membrane, and the restricted availability of glucose in the TME. Conversely, the accumulation of TGF‐β in the TME induces an aberrant increase in FBP1, thereby inhibiting glycolysis. Meanwhile, the increased expression of GARP and LAP activates the TGFβ signaling pathway while inhibiting the mTOR pathway, thereby reducing NK cell metabolism. The substantial accumulation of lactic acid produced by tumor cell glycolysis creates a low pH environment around NK cells, leading to lactate buildup within these cells. This accumulation induces mitochondrial stress, disrupting the OXPHOS process and potentially leading to cell death. The yellow section of this figure illustrates that during tumor development and progression, tumor cells modulate lipid metabolism to meet the demands of new cell membrane synthesis and signal transduction. This regulation includes increased uptake and storage of peripheral lipids via overexpression of fatty acid transport molecules (CD36, MSR1, and CD68), enhanced FAs synthesis, and increased FAO utilizing acetyl‐CoA from other metabolic pathways. Conversely, fatty acids absorbed and stored in lipid droplets (LDs) can directly neutralize the ROS response within tumor cells and also interact with antioxidant enzymes in the mitochondria to generate NADPH, thereby inhibiting ROS production. To counteract the damaging effects of oxidative stress, NK cells similarly upregulate lipid metabolism by increasing lipid absorption and storage. Within the TME, NK cells enhance their metabolic flexibility by increasing fatty acid uptake and upregulating the expression of carnitine palmitoyl transferase I (CPT1A) to promote fatty acid oxidation. Accumulated Fe²⁺ in NK cells react with FAs through lipid peroxidation, which induces ferroptosis. Exposure to NO and H₂O₂ impairs the cytotoxicity of NK cells and inhibits their activity. Following cell death in the TME, significant amounts of ATP are released. Tumor cells overutilize this ATP via the exonuclides CD39 and CD73, promoting its conversion to ADO. Subsequently, ADO accumulated in the TME binds to G protein‐coupled adenosine receptors (A1, A2A, A2B, and A3) on NK cells. This binding directly inhibits NK cell proliferation and maturation, while also downregulating the expression of activating receptors NKG2D and NKp30, thereby limiting NK cell activation. In addition, ADO can also reduce the glycolysis and OXPHOS rate of NK cells by inhibiting the IL‐12 and IL‐15 activated STAT5 and mTOR pathways, thereby impelling the cytotoxicity of NK cells. Hypoxia within the TME induces the expression of the HIF family, particularly HIF‐1α, which subsequently activates STAT3 and mTORC1, thereby upregulating glycolytic pathways in response to hypoxic conditions. The impact of enhanced HIF‐1α expression on NK cell function within the TME remains controversial. The purple section of this figure illustrates that tumor growth necessitates the absorption of a significant number of amino acids for protein synthesis, leading to a tumor microenvironment marked by amino acid deficiency, which in turn constrains the normal amino acid metabolism of NK cells. The excessive consumption of arginine within the TME impairs NK cell proliferation, cytotoxicity, and IFN‐γ production. NK cells utilize nicotinamide adenine dinucleotide (NAD+) generated by the tryptophan/NAD metabolic pathway in conjunction with HIF‐1α to counteract damage caused by accumulated ROS in the TME. The high amino acid metabolism of tumor cells intensifies tryptophan consumption within the TME. Furthermore, the enhanced metabolism of tryptophan by tumor cells results in the accumulation of l‐kynurenine in the TME. The elevated concentration of l‐kynurenine is transported to NK cells through the SLC7A5 amino acid transporter, which further inhibits NK cell proliferation, cytotoxic activity, and cytokine secretion. Tumor cells and NK cells exhibit differing requirements and absorption capacities for glutamine in the TME, leading to the preferential consumption of glutamine by tumor cells. The equilibrium between cMyc synthesis and degradation in NK cells is dependent on the availability of glutamine. Reduced glutamine levels consequently decrease cMyc levels, impairing NK cell proliferation and antitumor function. The acetylation of cMyc initiates the transcription and expression of IFN‐γ, PFN, and GZMB genes, thus facilitating the immune function of NK cells. The acetylase P300, which acetylates cMyc, is influenced by lipid metabolism. Enhanced lipid metabolism in NK cells induces chromatin remodeling due to lipid accumulation, subsequently reducing the expression of histone acetyltransferase P300.