Immunometabolism: crosstalk with tumor metabolism and implications for cancer immunotherapy

Glycolysis

Glycolysis is a central metabolic pathway that critically influences immune cell function and fate (Fig. 1). Naive T cells primarily meet their energy demands through oxidative phosphorylation (OXPHOS). Upon activation of CD8⁺ T cells, a metabolic shift toward glycolysis occurs, which is further amplified by the synergistic co-stimulatory effects of CD28 [13–15]. This enhanced glycolytic activity supports the subsequent expansion and differentiation of CD8⁺ T cells into short-lived effector cells [16]. However, glycolysis hinders the longevity and establishment of immune memory function in memory CD8⁺ T cells [17]. In CD4⁺ T cells, increased glycolysis promotes their differentiation towards Th17/Th1 subtypes while concurrently impeding their transition into regulatory T cells (Tregs) [18, 19]. Decreased glycolysis in CD4⁺ T cells enhances the immunosuppressive capacity of Tregs and boosts the immune memory function of CD4⁺ memory T cells [20]. Natural killer (NK) cells also exhibit dynamic metabolic regulation during their development and activation. Generally, quiescent or steady NK cells exhibit low metabolic demand and depend on OXPHOS for homeostasts. Following activation, these cells significantly increase glycolysis and OXPHOS flux to satisfy the high-energy requirements and molecular synthesis necessary for their functional activities [21, 22]. Inhibition of glycolysis compromises NK cell function, reducing the secretion of interferon (IFN)-γ and granzyme B [23]. The mammalian target of rapamycin complex (mTORC) 1 serves as a central metabolic regulator maintaining NK cell function. Restricting mTORC1 signaling impairs glucose uptake in activated NK cells, ultimately compromising NK cell effector function [23]. Dendritic cells (DCs) also greatly depend on glycolysis for their maturation, activation, and functional execution. Toll-like receptor (TLR) signaling induces glycolytic reprogramming, which equips DCs with the necessary energy and molecular building blocks for activation [24]. Suppression of glycolysis at the onset of activation negatively impacts the viability and functional capabilities of these activated DCs [25, 26], highlighting the indispensable regulatory function of glycolysis in DC immunocompetence. Myeloid-derived suppressor cells (MDSCs) also exhibit marked upregulation of glycolytic genes. Inhibition of mTOR by rapamycin can lower the glycolytic activity in MDSCs and decrease their immunosuppressive effects [27]. Phosphoenolpyruvate, a key intermediate in glycolysis, has been reported to exert antioxidant effects by preventing excessive reactive oxygen species (ROS), thereby promoting MDSC survival [28]. Collectively, these findings emphasize the pivotal role of glycolytic pathways in governing immune cell fate and function. Modulating glycolysis in immune cells represents a promising strategy for controlling tumor immunity and warrants further investigation as a potential therapeutic target.

The tricarboxylic acid (TCA) cycle

Pyruvate derived from glycolysis is transformed into acetyl-CoA and enters the TCA cycle in mitochondria, where it supports OXPHOS through the generation of reducing equivalents. Beyond energy production, the TCA cycle profoundly influences immune cell function through metabolic intermediates and epigenetic regulators (Fig. 2). For example, succinate dehydrogenase (SDH) enhances Th1 cell terminal effector function, while inhibiting their proliferation and histone acetylation [29]. On the other hand, the malate-aspartate shuttle and mitochondrial citrate export sustain aspartate production, supporting Th1 cell expansion and histone acetylation, while selectively regulating gene expression related to T cell activation [29]. Alpha-ketoglutarate (α-KG) can suppress the differentiation of naive CD4⁺ T cells into FoxP3⁺ Tregs by promoting OXPHOS, fatty acid synthesis (FAS), and triglyceride storage, while simultaneously enhancing Th1 cell maturation and cytokine secretion [30]. Furthermore, α-KG, as a co-substrate of α-KG dependent dioxygenases, is crucial for the function of histone and DNA demethylases, whereas succinate and fumarate can suppress the activity of these enzymes, thereby affecting gene expression [31, 32]. The ratio of succinate to α-KG enhances pro-inflammatory gene expression and promotes the differentiation of Th1 and Th17 cells [32]. The migratory competence of CD8⁺ T cells relies on TCA cycle activity, with ATP and ROS generated within the mitochondria serving as indispensable mediators for T cell migration and infiltration into tumor tissues [33]. Therefore, targeting mitochondrial metabolism to improve CAR-T cell penetration into tumors may offer an encouraging therapeutic strategy. Additionally, ketone bodies, particularly β-hydroxybutyrate, serve as the main substrate for acetyl-CoA production in the TCA cycle of CD8⁺ T cells, and can also optimize T cell effector responses by modulating histone acetylation [34].

Macrophage function and polarization are closely associated with the TCA cycle. Typically, M1 macrophages predominantly utilize aerobic glycolysis, accompanied by compromised TCA cycle and OXPHOS, whereas M2 macrophages rely on the TCA cycle and OXPHOS [35, 36]. Moreover, αKG promotes the demethylation of histone H3K27me3 on the promoters of M2 macrophage-specific genes in a JMJD3-dependent manner by acting as a cofactor for JMJD3, which is a histone demethylase [37]. This demethylation process is necessary for the expression of M2 genes and the subsequent activation of M2 macrophages. Furthermore, succinate acts as a competitive inhibitor of JMJD3. An elevated αKG/succinate ratio in macrophages alleviates succinate-mediated inhibition of JMJD3, thereby enhancing H3K27me3 demethylation and driving M2 polarization [37]. In addition to macrophages, other immune cells also depend on the TCA cycle for their metabolic demands. NK cells require exogenous pyruvate to support TCA cycle, which is essential for maintaining their effector functions, signaling, and cellular activity [38]. In MDSCs, the β2-adrenergic receptor-mediated activation of transcription 3 (STAT3) signaling enhances glutamine consumption and itaconate production. This process suppresses mitochondrial ROS generation, consequently increasing MDSC resistance to doxorubicin [39]. Overall, the evidence underscores the critical influence of the TCA cycle in shaping immunometabolism, epigenetic regulation, and functional outcomes, highlighting its potential as a strategic intervention point in cancer therapy.

OXPHOS

OXPHOS is the final common pathway of cellular metabolism, driven by electron transfer from NADH and FADH₂ to oxygen, resulting in ATP production and water formation. It primarily occurs in mitochondria and works in coordination with glycolysis and the TCA cycle to satisfy cellular energy needs. OXPHOS critically regulates immune cell function, proliferation, activation, and antitumor immunity (Fig. 1). Naive T cells exhibit low energy demands, predominantly depending on TCA cycle and OXPHOS for their metabolic needs [40]. Dysfunction of cytochrome c oxidase reduces the availability of acetyl-CoA, thereby impairing both OXPHOS and TCA cycle, and limiting the proliferation and activation of naive T cells [41]. Compared to naive CD4⁺ T cells, memory CD4⁺ T cells and Th17 cells exhibit elevated OXPHOS activity to support survival and effector function [42–44]. Besides, Tregs primarily depend on fatty acid oxidation (FAO) and OXPHOS as their major metabolic pathways [45], which facilitates the maintenance of their immunoregulatory activity and functional integrity in nutrient-limited environments [46]. Notably, enhancing OXPHOS to reprogram cellular metabolism has become a potential approach for revitalizing terminally exhausted T cells, thus enhancing the efficacy of cancer treatment [47]. Similar to T cells, B cells exhibit diverse metabolic features based on their activation status and specific functional needs. Naive B cells exhibit low metabolic activity, which is rapidly enhanced through increased glycolysis and OXPHOS upon antigen activation [48]. Long-lived memory B cells display a metabolically quiescent state with moderate OXPHOS activity, whereas plasma cells, due to their requirement for antibody production and secretion, typically consume more glucose and engage in glycolysis and OXPHOS to meet their energy demands [49, 50].

M1 macrophages exhibit impaired OXPHOS, partly due to NO-mediated mitochondrial inhibition [51]. In contrast, M2 macrophages depend on OXPHOS. Modulating macrophage metabolism can shift their phenotype. Research suggests that delivery of mitochondria from M1 macrophages coated with mannosylated polyethyleneimine to M2 macrophages reprograms their metabolism from OXPHOS to glycolysis, shifting them to an antitumor M1 state [52]. In comparison to tumor-associated macrophages (TAMs) that have not phagocytosed tumor cells, phagocytic TAMs exhibit increased transcription of OXPHOS-related genes, elevated mitochondrial content, and enhanced oxygen utilization [53]. Similarly, phagocytic DCs show a transition from classical DC1s (cDC1s) to mature DCs enriched in immunoregulatory molecules (mregDCs), accompanied by enhanced OXPHOS and the development of an immunosuppressive phenotype [53, 54]. These characteristics are closely associated with their tumor-promoting functions. Finally, OXPHOS is essential for the functionality of NK cells [55, 56]. Docosahexaenoic acid, IL-2, and IL-15 promote OXPHOS, thereby augmenting the functional responses of NK cells [57, 58]. In contrast, transforming growth factor (TGF)-β impairs mitochondrial metabolism and OXPHOS, leading to compromised NK cell function [59]. In summary, OXPHOS critically influences the metabolic landscapes of various immune cells. Manipulating OXPHOS and related metabolic pathways represents a promising approach for boosting antitumor immunity and modulating immune responses in cancer.

FAO and FAS

Fatty acid metabolism is crucial for lipid homeostasis and significantly impacts tumor development and immune cell function (Figs. 1 and 2). FAO and FAS are two key metabolic pathways that regulate cellular activity within the TME. FAO is an aerobic mitochondrial process that generates energy through the breakdown of fatty acids [60], whereas FAS encompasses a set of enzymatic reactions that convert acetyl-CoA into long-chain fatty acids [61]. Research indicates that FAS provides necessary energy for the expansion and differentiation of effector T cells following antigen stimulation, while FAO actively contributes to Tregs differentiation and memory T cell production [62, 63]. Memory CD8⁺ T cells store lipids, particularly triglycerides, to fuel FAO, which sustains mitochondrial function and enhances the immediate reactivity to antigen stimulation [64, 65]. Under low-glucose and hypoxic TME, CD8⁺ T cells enhance the catabolic metabolism of fatty acids, which allows them to sustain effector functions within the challenging tumor milieu [66]. However, in obesity-related TME, an increase in FAO driven by STAT3 inhibits CD8⁺ T effector cell function, thus accelerating tumor growth, especially in breast cancer [67]. This illustrates that fatty acid metabolism may exert diametrically opposing effects in different metabolic TMEs. Chronic tumor antigen exposure drives T cell exhaustion—marked by upregulated inhibitory receptors that profoundly reshape T cell metabolism [68]. PD-1 activation impairs glycolysis and amino acid metabolism in T cells while promoting FAO through upregulation of the key enzyme carnitine palmitoyltransferase (CPT) and enhanced lipolysis [69]. This metabolic shift promotes the survival of T cells as long-lived cells but may also limit their effector function. In CD8⁺ T cells exhibiting elevated PD-1 levels, there is a significant increase in lipid uptake and content, along with increased expression of the scavenger receptor CD36 [70]. CD36 not only mediates the uptake of fatty acids by CD8⁺ T cells but also induces lipoperoxidation and ferroptosis, resulting in decreased expression of cytotoxic cytokines and compromised antitumor capabilities [71, 72].

The phenotypic transition of monocytes and macrophages is closely linked to fatty acid metabolism. Monocytes with elevated intracellular fatty acid levels, especially those expressing higher levels of fatty acid synthetic enzymes, are more prone to differentiate into macrophages [73]. Conversely, inhibiting FAS in macrophages results in more monocyte-like characteristics [74]. Within tumors, MDSCs exhibit increased FAO activity, accompanied by enhanced mitochondrial biogenesis, activation of pivotal FAO enzymes, and a higher rate of oxygen utilization [75]. NK cells also show elevated fatty acid uptake and high levels of CPT1, a key enzyme in long-chain fatty acid oxidation. By enhancing FAO, the metabolic adaptability of NK cells can be improved, thereby providing potential for improving NK cell-based immunotherapies [76]. These findings demonstrate the complex role of fatty acid metabolism within tumors. While it supports the survival and functionality of immune cells, under specific conditions, it can also facilitate tumor progression. A comprehensive understanding of how fatty acid metabolism shapes the TME furnishes a robust scientific basis for the evolution of innovative metabolic-targeted therapeutic strategies, presenting encouraging avenues for enhancing cancer immunotherapy.

Core enzymes in glycolysis

Hexokinase 2 (HK2) is the first rate-limiting enzyme in glycolysis, primarily responsible for catalyzing the phosphorylation of glucose to glucose-6-phosphate (Fig. 2). In T cell biology, the absence of HK2 impairs effector T cell function, consequently dampening anticancer immunity [77, 78]. Notably, NF-κB-inducing kinase regulates T cell metabolism by preventing lysosomal degradation of HK2, which plays a role in avoiding CD8⁺ T cell exhaustion and optimizing adoptive cell therapeutic outcomes [78]. The purinergic receptor P2Y12 stabilizes HK2 by modulating PI3K/Akt signaling and suppressing lysosomal degradation, thereby promoting T cell activation and immune responses [79]. HK2 also regulates NLRP3 inflammasome activation, thereby promoting IL-1β production through a caspase-1-dependent pathway, which contributes to modulating immune cell inflammatory responses [80, 81]. Furthermore, immune checkpoints can also regulate the expression of HK2. Tim-3 suppresses the expression of HK2 in macrophages via the STAT1 pathway, leading to reduced production of TNF-α and IL-1β [82]. In non-small cell lung cancer, PD-L1-induced HK2 upregulation is inversely correlated with T cell effector gene expression [83]. Besides its impact on immune cells, HK2 also exerts a vital influence on tumor cells, influencing their metabolic rewiring and immune evasion mechanisms. Enhanced HK2 expression in tumor cells promotes phosphorylation and degradation of the NF-κB inhibitor IκBα, leading to NF-κB pathway activation and increased PD-L1 expression, thereby enhancing tumor immune evasion [84]. These findings suggest the promise of combining HK2 inhibitors with PD-1 monoclonal antibodies for cancer treatment. Furthermore, targeting HK2 in tumor cells suppresses aerobic glycolysis, reduces the accumulation of immunosuppressive metabolites such as lactate, alleviates the suppression of immune cell functions, and offers a novel strategy for cancer immunotherapy [85].

Phosphofructokinase 1, a pivotal rate-limiting enzyme in the glycolytic pathway, is frequently overexpressed in various cancers and correlates with poor patient prognosis [86]. Specifically, phosphofructokinase 1 platelet isoform (PFKP) promotes vascular endothelial growth factor (VEGF) expression through epidermal growth factor receptor (EGFR) activation. In turn, VEGF enhances PFKP activity, collectively driving tumor angiogenesis and growth [87]. Additionally, PFKP upregulates PD-L1 expression in glioblastoma cells by augmenting EGFR activation, thereby facilitating tumor immune evasion [88]. Ganetespib has been reported to suppress both PFKP and pyruvate kinase M2 (PKM2), leading to reduced tumor glycolysis. This reduction promotes cytotoxic T cell infiltration via the suppression of glycolysis-dependent IL-8 signaling, thereby enhancing antitumor immunity [89]. Preclinical models have demonstrated that targeting PFKP significantly inhibits tumor progression, suggesting its promise as a therapeutic candidate [90]. Notably, genetically engineering T cells to express critical metabolic enzymes such as phosphofructokinase and glucose transporter 3 (GLUT3) enhances cytokine secretion, upregulates T cell activation markers, increases glycolytic capacity, and demonstrates superior therapeutic efficacy in human tumor xenograft models [91].

PKM2 is a splice isoform of pyruvate kinase that catalyzes the final step of glycolysis, in which phosphoenolpyruvate is converted into pyruvate, accompanied by ATP generation. Within tumor cells, PKM2 predominantly exists as a low-activity dimeric form, which promotes metabolic reprogramming characterized by enhanced aerobic glycolysis and OXPHOS, thereby supplying both biosynthetic precursors and energy to support tumor cell proliferation and invasion [92, 93]. PKM2 serves as a key regulator of immunometabolism within the TME. Tumor-derived PKM2 interacts with integrin α_vβ₃ on macrophages to activate the integrin-focal adhesion kinase-PI3K signaling axis, thereby suppressing phosphatase and tensin homolog expression and upregulating arginase 1 expression, which promotes M2 macrophage polarization [94, 95]. Targeting PKM2 disrupts this interaction, shifts M2 macrophages toward an M1 phenotype, and enhances antitumor immune responses [94]. Moreover, PKM2 enhances PD-L1 expression through activation of the GATA3 pathway in cancer cells while simultaneously suppressing PD-L2 expression, representing a dual regulatory mechanism contributing to immune escape [96]. Interestingly, PKM2 also exerts immunostimulatory effects under certain contexts. Its activation in CD8⁺ T cells and CAR-T cells enhances mitochondrial function via modulation of one-carbon metabolism and DNA methylation, resulting in improved antitumor activity and memory responses [97]. PKM2 agonists have been shown to potentiate both adoptive cell therapy and immune checkpoint blockade (ICB), while reducing intratumoral Treg infiltration, highlighting their therapeutic potential in cancer immunotherapy [97].

Core enzymes in amino acid metabolism

Glutaminase catalyzes the hydrolysis of glutamine to glutamate, representing the pivotal step underlying the “glutamine addiction” of tumor cells. By competitively depleting glutamine, tumor cells inhibit the activation and expansion of CD8⁺ T cells, establishing a characteristic “metabolic tug-of-war” [98]. In mouse tumor models, tumor cell-specific deletion of glutaminase significantly restores CD8⁺ T cell function and delays tumor progression [99]. Glutaminase also regulates CD4⁺ T cell differentiation: inhibition of glutaminase promotes Th1 differentiation and upregulates IFN-γ secretion, thus promoting antitumor immunity [100]. Preclinical studies have further revealed that glutaminase inhibitor CB-839 enhances the cytotoxic function of tumor-infiltrating lymphocytes against cancer cells in vitro [101]. In vivo, CB-839 in combination with ICB markedly increases effector T cell infiltration and boosts the response to immunotherapy [101]. Based on these findings, CB-839 has advanced into multiple clinical trials. In combination with nivolumab, early clinical evidence reveals encouraging synergistic antitumor activity [98, 102, 103]. Moreover, glutaminase inhibitors target glutamine metabolism, while mTOR inhibitors (such as everolimus and sapanisertib) block glucose metabolism. The combination of these two approaches simultaneously disrupts both the energy supply and biosynthetic pathways in tumor cells, resulting in synergistic antitumor effects. This intervention has been evaluated in multiple clinical trials [102, 104, 105].

Arginase is a metalloenzyme that catalyzes the hydrolysis of L-arginine to produce ornithine and urea. In the TME, the expression of arginase can be significantly upregulated, particularly in TAMs, MDSCs and polymorphonuclear neutrophil granulocytes, and this upregulation is key to shaping an immunosuppressive TME [106–108]. The depletion of local L-arginine by arginase limits the proliferation and functional activity of T and NK cells [109]. Arginase has been found to reduce the expression of CD3ζ and ε chains, essential components of the TCR signaling, consequently compromising T cell functionality [110]. In addition, elevated levels of arginase1 correlate with the polarization of TAMs towards an M2-like state, which exhibits stronger immunosuppressive properties and exacerbates immune escape [111]. Deletion of arginase 1 in myeloid cells reshapes the TME, improves CD8⁺ T cell function, and slows tumor progression [112]. In multiple murine tumor models, targeting arginase 1 not only enhances CD8⁺ T cell infiltration but also improves the response to PD-1 ICB [112, 113]. The arginase-1-based peptide vaccine represents a novel immunotherapeutic strategy that restores L-arginine levels in the TME by targeting and eliminating arginase-1-expressing immunosuppressive cells, thereby promoting T cell activation and proliferation [114]. Early-phase clinical trial results demonstrate that this vaccine is well tolerated, capable of inducing antigen-specific immune responses, and shows preliminary clinical efficacy in a subset of patients [115].

Glutamate-cysteine ligase (GCL), the rate-limiting enzyme in glutathione (GSH) biosynthesis, consists of a catalytic subunit GCLC and a regulatory subunit GCLM. Together with glutathione synthetase, GCL regulates GSH synthesis and cellular redox homeostasis, contributing to tumor cell survival, metastasis, and chemoresistance. Additionally, GCL profoundly influences the functional state of immune cells. In the TME, tumor cells competitively uptake cystine by overexpressing solute carrier family 7 member 11, leading to cystine deprivation in T cells. This disrupts the GSH synthesis pathway in T cells, resulting in oxidative stress and lipid peroxidation accumulation, ultimately inducing T cell exhaustion [116]. Overexpression of GCLC can restore GSH synthesis within T cells, inhibit CD36-mediated lipid uptake, and thereby enhance antitumor immune [116]. Moreover, tumor cells maintain high GCL activity to support GSH synthesis, alleviating ROS-mediated damage and promoting metastasis [117]. In contrast, inhibition of GCL can reduce circulating tumor cells and diminish the metastatic potential of tumors [118]. In colorectal cancer, the loss of the GCLM competitively inhibits NF-κB signaling, thereby enhancing tumor cell sensitivity to platinum-based chemotherapy [119]. However, in glioblastoma, inhibition of GCLC prompts tumor cells to upregulate glutaminase and other glutamine-utilizing enzymes, thereby activating de novo pyrimidine nucleotide synthesis pathways [120]. This adaptive compensatory mechanism suggests that combined targeting of GCLC and glutamine metabolism could offer a promising therapeutic strategy for glioblastoma.

Core enzymes in fatty acid metabolism

CPTs are pivotal enzymes of FAO, comprising four distinct members: CPT1A, CPT1B, CPT1C, and CPT2. These isoforms exhibit significant differences in tissue distribution and catalytic activity. Members of the CPT1 family are primarily localized to the outer mitochondrial membrane, where they catalyze the formation of acylcarnitines; in contrast, CPT2 is located on the inner mitochondrial membrane, facilitating the conversion of acylcarnitines back into acyl-CoA, thereby promoting mitochondrial FAO [121]. All CPT isoforms are involved in tumor metabolic reprogramming and exert essential functions in cancer progression. Research has revealed that CPT2 functions as a tumor suppressor, with its downregulation being correlated with the development of multiple cancers [122–125]. Conversely, CPT1 promotes apoptotic resistance in tumor cells, supports metastasis and chemoresistance, and is implicated in the metabolic crosstalk between tumor and immune cells [126, 127]. Specifically, CPT1A is highly expressed in cancer stem cells, where it catalyzes the succinylation of mitochondrial fission factor via its lysine succinyltransferase activity, thereby activating sterol regulatory element-binding protein 1 and driving lipid desaturation to maintain stemness [128]. Moreover, CPT1A collaborates with L-carnitine sourced from TAMs to establish a CPT1A/c-Myc positive feedback loop within cancer stem cells, activating the nuclear factor erythroid 2-related factor 2/glutathione peroxidase 4 axis and reducing lipid peroxidation, thereby suppressing ferroptosis and impairing CD8⁺ T cell function [129]. IFN-γ secreted by T cells induces CPT1A-mediated FAO in tumor cells through an AMPK-dependent pathway, generating pro-survival signals that enable resistance to immune attack [130]. Targeting CPT1A restores ferroptosis sensitivity, enhances antitumor immunity, and improves immunotherapy outcomes. Particularly in ErbB2⁺ breast cancer models, CPT1A suppression not only curbs tumor progression but also fosters an immunostimulatory microenvironment that potentiates anti-HER2 therapy [131].

Acetyl-CoA carboxylase (ACC) is a key metabolic enzyme that catalyzes the conversion of acetyl-CoA to malonyl-CoA, a rate-limiting step in FAS. ACC exists as two major isoforms: ACC1, which predominantly participates in cytosolic lipid biosynthesis, and ACC2, which is situated at the outer mitochondrial membrane and inhibits FAO by blocking CPT1 activity [132, 133]. It has been established that tumor cells depend on ACC1-mediated FAS to support growth and survival [134, 135]. However, ACC1 demonstrates a multifaceted role in T cell biology and functions in a context-dependent manner. In a T cell-specific ACC1 knockout mouse model, CD8⁺ T cells exhibit impaired homeostatic proliferation and reduced expansion upon in vitro stimulation. These defects can be rescued by exogenous fatty acid supplementation, highlighting an essential role for ACC1-driven FAS in CD8⁺ T cell expansion [136]. Conversely, ACC1 deletion induces memory cell fate in CD4⁺ T cells, a phenomenon associated with enhanced FAO [137]. In the TME, CD8⁺ T cells display elevated ACC1 expression, leading to lipid accumulation and metabolic dysfunction [138]. Inhibition of ACC1 restores FAO capacity in CD8⁺ T cells, enhances ATP production, and improves both stemness and persistence [139]. These metabolic interventions significantly boost antitumor immune responses, suggesting that targeting ACC1 could be an effective means to potentiate cancer immunotherapy.

SCD-1 is an integral protein embedded in the endoplasmic reticulum membrane and facilitates the transformation of saturated fatty acids to monounsaturated fatty acids [140]. Specifically, SCD-1 catalyzes the transformation of palmitoyl-CoA and stearoyl-CoA into palmitoleoyl-CoA and oleoyl-CoA, respectively [141]. These products act as essential precursors for the biosynthesis of various lipids. SCD-1 is upregulated in human cancers, where it contributes to tumor initiation, progression, stemness, and chemoresistance by modulating lipid metabolism and signaling pathways [142]. Inactivation of SCD-1 enhances the differentiation of naive CD8⁺ T cells toward effector phenotypes, promoting IFN-γ production and cytotoxic activity [143]. The mechanisms by which SCD-1 inhibition boosts the anticancer capabilities of CD8⁺ T cells can be understood from both direct and indirect perspectives. From a direct perspective, targeting SCD-1 augments the function of CD8⁺ T cells by lowering cellular oleic acid levels and esterified cholesterol levels [144]. Indirectly, SCD-1 inhibition promotes CCL4 production by suppressing Wnt/β-catenin signaling in tumor cells and reducing endoplasmic reticulum stress in CD8⁺ T cells, thereby enhancing DC recruitment and antitumor T cell responses [145]. Furthermore, SCD-1 upregulation in CD4⁺ T cells upregulates Th1-associated markers like TBX21, IL-2, and IFN-γ, whereas its inhibition results in elevated levels of Treg-specific markers, including FOXP3 and TGF-β [146]. Notably, SCD-1 upregulation in CD4⁺ T cells promotes CXCL11 secretion, and subsequent engagement of CXCR3 on CD8⁺ T cells amplifies their effector function and inhibits tumor growth [146]. In preclinical models, SCD-1 inhibitor has been reported to synergistically improve the antitumor effects of immune checkpoint inhibitors and targeted treatments [145, 147]. Consequently, SCD-1 emerges as an attractive target to enable the creation of innovative therapeutic approaches designed to enhance current cancer therapies.

Core enzymes in other metabolic pathways

Malic enzymes (MEs) function as NAD(P)H-generating biocatalysts mediating the oxidative decarboxylation of malate to pyruvate. MEs exist as multiple isoenzyme forms across diverse cell types, exhibiting distinct subcellular localization (cytosol or mitochondria) and coenzyme specificity (NAD⁺ or NADP⁺ utilization) to fulfill compartmentalized metabolic requirements [148]. Emerging evidence highlights an essential role of ME2 in CD8⁺ T cells. ME2 depletion disrupts the TCA cycle in these cells, resulting in elevated intracellular levels of the metabolic intermediate fumarate. Fumarate impedes tuberous sclerosis complex 2/mTORC1 signaling pathway by inhibiting death associated protein kinase 1, thereby suppressing the activation and cytotoxic activity of CD8⁺ T cells [149]. Furthermore, overexpression of ME1 promotes the activation and expansion of CD8⁺ T cells, partly through the type I IFN pathway. It further enhances the metabolic adaptation of CD8⁺ T cells through enhanced mitochondrial respiration and ATP production, while maintaining redox homeostasis to prevent the accumulation of excess ROS [150]. Compared to memory, effector, and progenitor exhausted T cells, exhausted T cells exhibit mitochondrial dysfunction and reduced OXPHOS metabolism, leading to a decreased NAD/NADH ratio and ammonia accumulation within the exhausted T cells [151]. The malate shuttle in T cells helps maintain the NAD⁺/NADH balance, detoxifies ammonia, and provides longevity to exhausted T cells induced by chronic infections, thereby resisting ammonia-induced cell death [152]. Furthermore, MEs critically regulate cancer cell growth, cell cycle progression, and redox homeostasis. ME2 not only maintains the protein stability of mutant p53 by sustaining the levels of 2-hydroxyglutarate (2-HG), thereby promoting tumor growth [153], but also accelerates cell cycle dynamics and tumor cell expansion through α-KG-mediated transcription of cyclin D1 [154]. Additionally, Akt1 induces a metabolic shift of the mitochondrial TCA cycle to glycolysis through phosphorylation of cytoplasmic ME2 at serine 9, which leads to tumorigenesis [155]. Moreover, NADPH generated by ME metabolism not only serves as a key component in cellular antioxidant systems to support rapid tumor proliferation, but also directly interacts with histone deacetylase 3, disrupting its association with the co-activator nuclear receptor corepressor and thereby impairing histone deacetylase 3 activation [156]. This leads to increased histone acetylation, a modification that typically leads to a more open chromatin structure, enabling transcription factor recruitment and the activation of genes such as GLUT4. Enhanced expression of GLUT4 boosts glucose uptake and glycolytic capacity, further supporting tumor cell proliferation [156]. Moreover, NADPH produced by MEs metabolism can directly interact with and suppress HDAC3 activity, thereby regulating the epigenetic state of cells [156]. In summary, MEs exert diverse influences on tumor immune responses, highlighting their potential as therapeutic targets for cancer treatment and immunomodulation.

Located in the mitochondrial matrix, the pyruvate dehydrogenase complex (PDHC) catalyzes the irreversible oxidative decarboxylation of pyruvate, resulting in the production of acetyl-CoA. This reaction bridges glycolysis and the TCA cycle, and is vital for sustaining cellular bioenergetics [157]. Pyruvate dehydrogenase kinase 1 (PDHK1) is an atypical Ser/Thr kinase that inhibits PDHC activity by phosphorylating the E1α subunit of PDHC [158]. This inhibition impairs pyruvate metabolism to acetyl-CoA, consequently decreasing acetyl-CoA flux into the the TCA cycle and downregulating OXPHOS [159]. PDHK1 serves as a key regulator of energy metabolism. In cancer cells, overexpression of PDHK1 redirects cellular metabolism toward glycolysis and increases lactate production [160]. The buildup of lactate and the resulting acidic environment enhance the expression of arginase1 in macrophages, subsequently restraining the expansion and activation of T cells [161]. Additionally, in T cells, TCR activation enhances PDHK1 activity, redirecting pyruvate metabolism from the TCA cycle to glycolysis, which is crucial for T cells to rapidly respond to external stimuli and synthesize cytokines [162]. Aberrant activation or overexpression of PDHK1 has been observed in a range of malignancies [163–165]. PDHK1 inhibitors, such as dichloroacetate (DCA), can redirect pyruvate metabolism from glycolysis back to the TCA cycle. This metabolic shift reduces lactate production, decreases ARG1 expression in macrophages, and restores the proliferative capacity of CD8⁺ T cells [161]. Moreover, DCA is capable of improving the activity of exhausted NK cells in tumors [166]. Combined treatment with DCA and NK cell therapy exhibits significant antitumor effects in a hepatocellular carcinoma mouse model [166]. In summary, PDHK1 is an important molecule in regulating cellular metabolism, and the application of its inhibitors may hold potential value for immune responses and antitumor therapy.

Phosphoribosyl pyrophosphate synthetase (PRPS) catalyzes the reaction between 5-phosphoribosyl and ATP to produce 5-phosphoribosyl-1-pyrophosphate, a critical intermediate in the synthesis of purine nucleotides, pyrimidine nucleotides, and cofactors such as NAD⁺. PRPS is a key regulator of cellular metabolism, with its activity being essential for cell proliferation and DNA synthesis. PRPS1 has been found to be aberrantly overexpressed in tumors, where it facilitates malignant progression by enhancing oncogenic traits such as proliferation and migration, and by evading apoptosis [167]. Additionally, O-GlcNAc modification of PRPS1 promotes tumorigenesis and contributes to resistance against radiotherapy and chemotherapy [168]. PRPS2 bypasses ADP/GDP feedback inhibition to sustain ATP overproduction, directly stabilizes MAT2A to accelerate S-adenosylmethionine synthesis and m6A RNA methylation, and thereby drives lung tumorigenesis [169]. The absence of PRPS2 leads to elevated NADPH levels in lymphoma cells, triggering reductive stress-mediated cell death, suggesting its potential role in modulating oxidative balance and cell survival [170]. Phosphoribosylpyrophosphate amidotransferase catalyzes the conversion from 5-phosphoribosyl-1-pyrophosphate to phosphoribosylamine, which is the first step in purine ring formation. In tumors, phosphoribosylpyrophosphate amidotransferase expression is significantly upregulated, enhancing de novo purine biosynthesis and promoting cell proliferation and migration [171, 172]. The role of key enzymes in purine metabolism as drivers of carcinogenesis is well-established; however, the intricate mechanisms by which they sculpt the function of immune cells, and the metabolic-immune crosstalk implicated in immune surveillance and antitumor responses, remain elusive and warrant profound investigation. Deciphering this interaction is poised to lay the foundation for a deeper understanding of tumor immunoregulation and to create opportunities for the identification of novel therapeutic targets in future immunotherapeutic approaches.

Dihydroorotate dehydrogenase (DHODH), a key enzyme located on the inner mitochondrial membrane, catalyzes the oxidation of dihydroorotate to orotate in the de novo pyrimidine synthesis. The dependence of cancer cells on de novo pyrimidine biosynthesis for rapid growth underscores the essential role of DHODH in tumor progression [173]. Research has shown that inhibition of DHODH induces ferroptosis in tumor cells by disrupting CDP-choline-dependent lipid metabolism and promotes CD8⁺ T cell infiltration, enhancing the efficacy of PD-1 blockade in mouse models [174]. Additionally, DHODH inhibition increases cancer cell antigen presentation, thereby enhancing the effectiveness of ICB [175]. Uridine-cytidine kinase 2 (UCK2) is a key enzyme in the salvage pathway for pyrimidine nucleotides, primarily responsible for converting uridine and cytidine into their corresponding monophosphate forms: UMP and CMP. UCK2 has been identified as a key player in multiple cancers, driving tumor growth and resistance to therapy [176]. UCK2 has also been recognized as a potential biomarker of the immunosuppressive microenvironment, and its downregulation not only induces cell cycle arrest but also enhances tumor cell susceptibility to T cell-mediated cytotoxicity [177]. Cytidine deaminase is another crucial enzyme involved in pyrimidine metabolism, catalyzing the hydrolytic deamination of cytidine and deoxycytidine to produce uridine and deoxyuridine, respectively. In immunotherapy-resistant tumors, high expression of cytidine deaminase increases extracellular UDP levels, which further bind to P2Y6 receptors on TAMs, promoting the formation of an immunosuppressive environment and affecting the effectiveness of antitumor immune responses [178].

In summary, metabolic enzymes within the TME play pivotal roles by mediating metabolic competition and crosstalk between tumor and immune cells, as well as regulating the differentiation and function of immune cells. Targeting these key enzymes holds promise for amplifying antitumor immune responses, with potential synergistic effects when combined with ICB. Table 1 lists the unique roles of more metabolic enzymes in immune regulation and compares their distinct functions in tumor and immune cells. This comprehensive analysis furnishes a robust theoretical foundation, laying the groundwork for the advancement of novel immunometabolic therapies.

Glucose

Glucose, as a central source of cellular energy metabolism, plays multifaceted roles in the TME. First, it is involved in the competition for limited nutrients between tumor cells and immune cells, as well as among immune cells themselves [179, 180]. Second, the accumulation of intermediate and end products derived from glucose metabolism may exert suppressive effects on immune cells [181, 182]. Furthermore, the expression and functional regulation of glucose transporters exhibit significant differences among various immune cells, which in turn influence their metabolic activities and effector functions. Lastly, abnormal glucose metabolism may also promote tumor immune evasion through multiple mechanisms, thereby weakening antitumor immune responses.

The uptake of glucose is primarily mediated by two families of transporters: glucose transporter/solute carrier 2 A (GLUT/SLC2A) and sodium-dependent glucose transporter/solute carrier 5 A (SGLT/SLC5A) [183]. These transporters exhibit distinct expression patterns and functional roles across various immune cell populations. GLUT1 is recognized as the principal glucose transporter that facilitates glucose uptake and metabolism in CD4⁺ T cells, critically contributing to their activation, proliferation, and effector functions [184]. CD8⁺ T cells rely primarily on GLUT10 for their activation and antitumor immune capabilities [185]. However, lactate accumulation within the TME compromises GLUT10 functionality, thereby impeding glucose uptake in CD8⁺ T cells and consequently dampening their cytotoxic function [185]. Restoring GLUT10 function or mitigating lactate accumulation may represent potential approaches to enhance CD8⁺ T cell-mediated antitumor immunity. DCs also utilize GLUT1 for extracellular glucose uptake, with GLUT1 expression being upregulated during DC activation [186]. Recent studies have highlighted GLUT1 as a reliable target for antigen delivery to DCs. For instance, J. Liu et al. developed a novel glycosylated nanovaccine that binds to GLUT1 on DCs, facilitating antigen delivery and presentation, DC maturation, and antitumor immune responses [186]. Furthermore, macrophage polarization is influenced by glucose metabolism, particularly through the expression of SLC2A3. Elevated SLC2A3 levels contribute to the transition of macrophages toward the M2 subtype, a phenotype correlated with immune suppression [187]. Additionally, NK cells benefit from increased expression of SLC2A1, which enhances their capacity to utilize glucose for ATP and pyruvate production [188]. This metabolic adaptation promotes glycolysis and OXPHOS, bolstering NK cell-mediated immunity through increased cytokine secretion and cytotoxic activity [189]. While both tumor and immune cells depend on glucose, they exhibit differential preferences for glucose transporter isoforms. Tumor cells predominantly express GLUT1, whereas immune cells favor GLUT3 [190, 191]. In Tregs, GLUT3-mediated glucose metabolism enhances the O-GlcNAc modification of cellular proteins, which induces a tumor-associated gene expression profile and thereby maintains immune tolerance [192]. Inhibiting GLUT1 in cancer cells redirects their metabolic pathways from glycolysis to OXPHOS, leading to elevated ROS levels. Elevated ROS levels may increase tumor cell vulnerability to cytotoxic T lymphocyte (CTL)-mediated bystander killing by reducing c-FLIP, a principal suppressor of TNF-α-induced apoptosis [193]. Exploiting this metabolic vulnerability offers a potential therapeutic avenue for augmenting antitumor immunity.

Epidemiological and clinical studies have shown a strong link between malignant tumors and hyperglycemia as well as diabetes. On one hand, patients with hyperglycemia and diabetes are at significantly higher risk for developing various cancers, which is closely connected to unfavorable outcomes. On the other hand, certain tumors can directly cause hyperglycemia and diabetes, a phenomenon referred to as tumor-associated hyperglycemia. Hyperglycemia exerts multifaceted impacts on the TME, not only promoting tumor cell growth, migration, and invasion but also enhancing tumor immune evasion through multiple mechanisms [194–197]. Research indicates that hyperglycemia can suppress CD8⁺ T cell function by upregulating PD-L1 expression on tumor cells. The underlying mechanisms include the following: high glucose concentrations downregulate PTRH1 via EGFR-RAS signaling activation, thereby enhancing PD-L1 mRNA stability [198]. Additionally, high glucose stimulates HK2 release from mitochondria, facilitates IκBα phosphorylation at T291, and enhances its interaction with µ-calpain, resulting in IκBα degradation and NF-κB-mediated upregulation of PD-L1 expression [84]. Although anti-PD-1 therapy shows efficacy in some cancers, the overall response rate remains suboptimal. Studies have shown that IFNα reduces glucose consumption in hepatocellular carcinoma cells by inhibiting FosB transcription and hypoxia-inducible factor (HIF)−1α signaling pathways, creating a high-glucose environment and expanding the pool of effector CD27⁺CD8⁺ T cells, which subsequently improves the response to anti-PD-1 immunotherapy [199]. Furthermore, hyperglycemia can also induce a Warburg effect in Vγ9Vδ2 T cells, causing lactate accumulation that impairs cytolytic machinery trafficking to the Vγ9Vδ2 T cell-tumor synapse, thereby inhibiting cytotoxic molecule secretion and antitumor activity [200].

Diabetic mouse tumor models have revealed differences in the immune landscape of the TME under normoglycemic and hyperglycemic conditions. Compared with non-diabetic mice, tumors in diabetic breast cancer mice are larger and exhibit distinct characteristics, including collagen remodeling, enhanced M2 TAM infiltration, and rapid metastatic dissemination [201]. Mechanistically, hyperglycemia promotes extracellular matrix glycation, leading to increased TME stiffness, which in turn activates mechanotransduction signaling to enhance tumor growth and epithelial-mesenchymal transition [202]. Notably, M2 TAMs possess the highest intratumoral glucose uptake capacity among immune cells [203]. Enhanced glucose metabolism in these cells elevates Cathepsin B O-GlcNAcylation, thereby promoting the generation of mature Cathepsin B [204]. Cathepsin B facilitates tumor progression and metastasis by degrading extracellular matrix and basement membrane proteins, and contributes to chemoresistance by protecting cancer cells from apoptosis and stimulating the generation of inflammatory IL-1β [205]. Moreover, glucose also enhances the immunosuppressive role of monocyte-derived macrophages through histone lactylation of lysine residues, leading to increased IL-10 production and suppressed T cell function [206]. To address these challenges, researchers have created an innovative material system (glucose-PEO-b-PLLA-TEMPO), encapsulating the clinical drug CUDC-101 and photosensitizer IR780. This system re-educates TAMs, successfully converting immunosuppressive “cold” tumors into immunoresponsive “hot” tumors, providing an innovative strategy for efficient tumor treatment [207]. Additionally, metformin, a diabetes medication showing promising anticancer activity, normalizes blood glucose levels and reshapes the TME, boosting the therapeutic outcomes of cancer immunotherapy [208].

Amino acid

As critical constituents of cellular architecture and essential energy sources, amino acids are crucial for supporting cell growth, differentiation, and function. Beyond their role in protein synthesis, amino acids have a pivotal influence on regulating immune response through multiple mechanisms (Fig. 3). These processes provide both energy and biomass, facilitating metabolic reprogramming upon immune cell activation to facilitate proliferation, differentiation and functional activity [209]. There are nine essential amino acids that must be acquired through the diet. Under conditions of high proliferation and metabolism, certain non-essential amino acids can also become conditionally essential, requiring their uptake from the extracellular environment. Consequently, the transport of amino acids mediated by specific amino acid transporters is crucial for regulating immune cell function. This subsection highlights how amino acid transporters and amino acids modulate metabolic remodeling and immune cell functions.

Immune cells dynamically regulate the expression of amino acid transporters in response to antigen stimulation, thereby remodeling amino acid metabolism and activating their functional programs. For example, proteomic analysis of CD8⁺ T cells reveals marked upregulation of the transporters SLC1A5 and SLC7A5 in antigen-activated CTLs compared to naive cells [210]. The absence of glutamine transporter ASCT2 hinders the development of Th1 and Th17 cells and reduces inflammatory T cell responses [211]. Macrophages in a resting state rely on arginine supplied by the amino acid transporter SLC7A1/CAT1 to meet cellular metabolic demands. However, during activation, arginine uptake via SLC7A2/CAT2 becomes essential [212]. Additionally, SLC38A2-mediated glutamine transport is essential for cDC1s activation [213], while the cystine transporter SLC7A11 negatively regulates efferocytosis in DCs [214].

Glutamine functions as a precursor for a multitude of metabolic pathways. For instance, it supplies α-KG to the TCA cycle through glutamate, thereby supporting cellular biosynthesis and functional activities. Glutamine is essential for T cell activation and the production of IL-2 or IFN-γ [215, 216]. Glutamine deprivation inhibits the synthesis of cytokines and hampers the expansion of Th1 and Th17 cells, while promoting the development of Tregs [217, 218]. The molecular mechanisms underlying glutamine-induced differentiation of T cells into different subtypes vary. Along the glutamine-glutamate-αKG-TCA metabolic axis, glutamate promotes Th17 differentiation by inhibiting ROS, whereas α-KG drives Th1 and CTL differentiation through chromatin modifications and subsequent inhibition of IL-2-mediated mTORC1 signaling [219]. Therefore, Th17 cells respond more rapidly to glutamine and glutamate metabolic signals, being affected earlier, while Th1 and CTL responses occur at a later stage. Additionally, glutamine is indispensable for the metabolism and functional capacity of NK cells; however, its mode of action is independent of glutaminolysis. Instead, glutamine predominantly serves as a signaling molecule that sustains cMyc expression, thereby controlling NK cell metabolic programming and effector function [56]. Within the TME, tumor cells exhibit high glutamine consumption, which restricts its supply to infiltrating T cells and cDC1s, which in turn hinders their anticancer effects [99, 213]. Intratumoral glutamine supplementation can enhance cDC1-mediated antitumor immune responses and overcome resistance to immunotherapy [213]. Additionally, studies have shown that inhibition of glutamine metabolism suppresses tumor proliferation while impairing MDSC expansion and recruitment, which correlates with reduced expression of tumor-derived colony-stimulating factor 3 [220]. Moreover, targeting glutamine metabolism within MDSCs induces activation-dependent apoptosis and their reprogramming into inflammatory macrophages, thereby enhancing antitumor immune responses [220]. The findings reveal that glutamine metabolism exerts intricate regulatory effects on immunity, indicating a delicate balance. On one hand, supplementing glutamine can enhance the functionality of immune cells, thus inhibiting tumor progression. On the other hand, inhibiting glutamine metabolism can reduce tumor cell metabolic demands while decreasing the recruitment of immunosuppressive cells, ultimately enhancing antitumor immunity. This dual mechanism suggests that in cancer treatment, it is necessary to precisely modulate glutamine metabolism based on the specific features of the tumor to achieve optimal therapeutic outcomes.

Serine, a non-essential amino acid, can be synthesized in the body from glycine and 3-phosphoglycerate. It serves not only as a key substrate for the production of various essential biomolecules, including glycine, cysteine, purine and pyrimidine nucleotides, neurotransmitters, and phospholipids, but also plays a pivotal role in methylation reactions, thereby influencing the synthesis of proteins and DNA. These metabolic activities are crucial for maintaining the function, expansion, differentiation, and antioxidant defense mechanisms of immune cells. Studies have shown that activated T cells significantly upregulate enzymes involved in the serine, glycine, and one-carbon metabolic network, rapidly converting serine into one-carbon units to support rapid T cell proliferation. Notably, despite adequate glucose levels capable of supporting T cell stimulation and energy production, extracellular serine is still an indispensable factor for optimal T cell proliferation [221]. Moreover, serine metabolism is also of significant importance to Tregs. High concentrations of serine within the TME enhance the accumulation of Tregs via sphinganine-mediated regulation of c-Fos [222]. Glutathione restricts the availability of serine to maintain the immunosuppressive properties of Tregs. Glutathione deficiency enhances serine metabolism in Tregs, increases its uptake and synthesis, thereby strengthening one-carbon metabolism, activating the mTOR signaling pathway, and downregulating FOXP3 expression, which weakens the immunosuppressive capacity of Tregs [223]. Serine deprivation in cancer cells results in mitochondrial dysfunction and cytosolic accumulation of mitochondrial DNA. This subsequently activates the cGAS-STING pathway, induces a type I IFN response, and enhances immune cell infiltration in tumors, suggesting that serine may function as an inhibitor of anticancer immunity [224, 225]. Furthermore, serine metabolism coordinates macrophage polarization. Deficiency in serine metabolism increases insulin-like growth factor 1 expression by decreasing the promoter abundance of S-adenosyl methionine-dependent histone H3 lysine 27 trimethylation. Insulin-like growth factor 1 then activates the p38-dependent JAK-STAT1 signaling to promote M1 polarization and suppress STAT6-mediated M2 activation [226]. Additionally, serine deficiency reduces IL-1β synthesis in M1 macrophages through inhibition of mTOR signaling [227]. This mechanism highlights the critical role of serine in regulating immune cell function.

Methionine, a sulfur-containing amino acid, fulfills a vital function in protein synthesis and serves as a precursor for S-adenosylmethionine, a central methyl donor involved in the methylation of DNA, RNA, proteins, and phospholipids. Tumor cells have been shown to elevate methionine transporter expression, promoting increased methionine uptake and consequently lowering its levels in the TME [228, 229]. Methionine scarcity can result in decreased dimethylation at lysine 79 of histone H3 in CD8⁺ T cells, subsequently suppressing STAT5 expression and weakening T cell immune function [230, 231]. However, some research indicates that methionine restriction may promote anticancer immune responses by inhibiting the activity of the methyltransferase SUV39H1 in cancer cells, boosting the activation of cGAS and type I IFN signaling pathways [232, 233]. Additionally, S-adenosylmethionine promotes N6-methyladenosine modifications in immune checkpoint molecules in tumor cells, thereby increasing the expression of immune checkpoints like PD-L1 and VISTA, leading to immune evasion [234]. Notably, dietary methionine restriction both restores CD8⁺ T cell infiltration in the TME and disrupts tumor cell one-carbon metabolism, affecting redox balance and nucleotide synthesis, making tumor cells more sensitive to antimetabolites or radiotherapy [234, 235]. Methionine can be metabolized into homocysteine within the body, which can further be converted into cysteine. For T cells, the uptake of cystine (the oxidized form of cysteine, where two cysteine molecules are linked by a disulfide bond) or cysteine is crucial for their activation, DNA synthesis, and cell proliferation [236, 237]. However, T cells lack cystathionase and cannot synthesize cysteine from methionine, nor can they import and reduce cystine due to an incomplete xc − transport system. Instead, they rely on antigen-presenting cells to supply cysteine [238]. MDSCs, which lack the ASC transporter, compete with antigen-presenting cells for cystine uptake but cannot release cysteine [238], resulting in cysteine depletion within the TME and subsequent suppression of T cell activity.

Arginine is a conditionally essential amino acid that is critically involved in various metabolic pathways, including the urea cycle, polyamine synthesis, NO production, and immune regulation. The presence of L-arginine critically influences the potency of antitumor T cell responses [239]. Increased intracellular L-arginine levels promote a metabolic shift in activated T cells from glycolysis dependence to mitochondrial OXPHOS, and maintain T cells in a central memory-like state rather than further differentiation [240]. This metabolic adaptability and memory-like phenotype confer greater persistence and antitumor efficacy to T cells. Low arginine levels (20 µM) can induce metabolic and transcriptional reprogramming in T cells through the ATF4-SLC7A11-GSH axis, leading activated T cells to acquire immunosuppressive properties similar to Tregs [241]. However, tumor cells often deplete arginine to inhibit T cell proliferation, thereby weakening antitumor immune responses. Additionally, myeloid cells within the TME competitively take up arginine via SLC7A1/SLC7A2 and SLC7A6/SLC7A7 transporters while secreting elevated ornithine, establishing a metabolic environment characterized by high ornithine and low arginine [242, 243]. This metabolic imbalance suppresses T cell growth, signaling, and antitumor activity, ultimately diminishing the response to ICB therapy [112]. Furthermore, different macrophage subtypes play distinct roles in arginine metabolism. M1 macrophages rely on inducible nitric oxide synthase to metabolize arginine into NO, which inhibits tumor progression and enhances T cell immune activity [244]. In contrast, M2 macrophages express arginase 1, fostering an anti-inflammatory microenvironment, inhibiting T cell function, and promoting tumor growth through arginine metabolites such as ornithine and polyamines [244]. Given the pivotal function of arginine in tumor immunity, arginine supplementation has been investigated as a potential therapeutic approach to enhance anticancer immune responses. In this context, a genetically engineered probiotic strain of Escherichia coli Nissle 1917 has been designed to colonize tumors and transform metabolic byproduct in the TME into L-arginine [239]. The increased L-arginine levels enhance T cell infiltration and activation, improving immune conditions and synergizing strongly with PD-L1 blockade therapy [239]. However, the increase in arginine levels in the TME carries the potential risk of promoting tumor cell proliferation. To address this issue, strategies to enhance arginine metabolism specifically in T cells can be employed to optimize their antitumor capabilities. For example, CAR-T cells can be genetically engineered to express argininosuccinate synthetase 1 or ornithine transcarbamylase, thereby increasing their ability to resynthesize L-arginine [245]. This adaptation enables CAR-T cells to function effectively in low-arginine environments and enhances their antitumor potency.

Fatty acid

Fatty acids occupy a key position in immunometabolism, serving not only as crucial energy sources and structural components of cells, but also as active participants in signal transduction, gene expression regulation, and tumor immunity modulation. CD36, a fatty acid translocase ubiquitously present on cellular membranes, is pivotal in enhancing the uptake of exogenous fatty acids. Research has shown that CD36 significantly influences immune cell differentiation and activation, thereby shaping cell fate. Specifically, CD36-mediated ferroptosis inhibits the functionality of CD8⁺ T cells and suppresses antitumor immunity [71]. Additionally, the uptake of oxidized lipids via CD36 promotes lipid peroxidation and dysfunction in CD8⁺ T cells [72]. Further studies have revealed that CD36 is specifically elevated in Tregs, acting as a principal metabolic regulator. Through peroxisome proliferator-activated receptor (PPAR) β signaling, CD36 fine-tunes mitochondrial fitness, enabling Tregs to adjust to a lactate-enriched TME [246]. Targeted deletion of CD36 in Tregs reduces the infiltration of intratumoral Tregs, enhances antitumor immune responses, and inhibits tumor progression [246]. Moreover, CD36 is involved in autophagy within B cells, and its deficiency leads to impaired humoral immunity, characterized by reduced plasma cell generation, proliferation, mitochondrial mobilization, and OXPHOS levels [247]. In TAMs, the upregulation of CD36 inhibits IFN-I signaling via p38 activation downstream of oxidized lipid signaling, closely linked to its immunosuppressive characteristics [248]. Myeloid-specific deletion of CD36 markedly impedes cancer progression [248]. IL-34 triggers an increase in FAO through CD36, leading to the M2-like polarization of foam-like TAMs, which suppress CD8⁺ T cell antitumor responses and facilitate immune evasion [249]. Additionally, CD36-mediated lipid uptake enhances FAO in MDSCs, thereby promoting the formation of a pro-tumorigenic microenvironment [250]. This series of studies reveals the multifunctionality of CD36 across various immune cells and its significant effect on the TME, offering new avenues for the development of effective cancer treatments.

Fatty acids can be categorized based on their chain length and degree of saturation into various types, including short-chain, medium-chain, and long-chain fatty acids, as well as saturated and unsaturated fatty acids. Different types of fatty acids exert significantly varied effects on immune cells. Short-chain fatty acids, specifically valerate and butyrate, enhance anticancer activity of CTLs and CAR-T cells by activating the mTOR pathway and suppressing histone deacetylases, thereby increasing the production of effector molecules such as CD25, IFN-γ, and TNF-α [251]. Long-chain fatty acid palmitic acid disrupts mitochondrial activity through Akt/mTOR-mediated Ca²⁺ flux, leading to T cell exhaustion and an increase in Tregs within the TME, thereby accelerating tumor growth [252]. However, oleic acid can reverse the T cell exhaustion induced by palmitic acid, improving antitumor immune responses [252]. Oleic acid significantly contributes to the development of Tregs. In an environment lacking oleic acid, thymocytes express more ATP2A2, which enhances the calcium-NFAT1-FOXP3 axis, promoting the differentiation of naive CD4⁺ T cells into Tregs [253]. Moreover, oleic acid also induces M2 polarization of myeloid cells via the mTOR pathway, further promoting tumor development [254, 255]. Linoleic acid, another unsaturated fatty acid, exhibits complex and controversial effects on immune cell metabolism. On one hand, studies have shown that linoleic acid promotes mitochondrial ROS generation and lipid peroxidation, leading to apoptosis of naive T cells and decreased TNF-α expression, thus impairing antitumor T cell responses [256]. On the other hand, research indicates that linoleic acid enhances the formation of endoplasmic reticulum-mitochondrial contacts, promoting calcium signaling and mitochondrial energetics, thereby enhancing effector functions, preventing exhaustion, and stimulating a memory-like phenotype in T cells [257]. These seemingly contradictory findings highlight the complexity of linoleic acid’s impact on immune cells and underscore the need for further investigation into the specific contexts and conditions under which linoleic acid exerts its diverse effects. Understanding these nuances could provide valuable insights for developing more targeted and effective immunotherapeutic strategies.

Dietary trans-vaccenic acid, a long-chain fatty acid, is primarily found in the meat and dairy products of ruminant animals. Trans-vaccenic acid deactivates the immune-regulatory G protein-coupled receptor GPR43, thereby antagonizing its activation by short-chain fatty acids. This activates the cAMP-PKA-CREB signaling pathway to enhance the function of CD8⁺ T cells [258]. Dietary elaidic acid boosts MHC-I expression and antigen presentation in tumor cells through metabolism mediated by Acyl-CoA Synthetase Long Chain Family Member 5, thereby amplifying the cytotoxic activity of CD8⁺ T cells and offering potential to enhance cancer immunotherapy [259]. These findings emphasize the significance of considering dietary fatty acid metabolism as a promising approach for improving cancer clinical treatments. In addition, fatty acid binding protein 5 (FABP5) has a vital function in coordinating fatty acid transport and regulating cellular metabolism. FABP5 interacts with the cytoskeletal organizer transgelin 2 to promote lipid uptake, mitochondrial respiration, and cytotoxic capabilities in CD8⁺ T cells [260]. Long-chain unsaturated fatty acids secreted by tumor cells activate PPARγ via FABP5, conferring immunosuppressive properties to TAMs [261]. Inhibition of FABP5 suppresses the production of inhibitory molecules in TAMs, which in turn enhances the effector function of CD8⁺ T cells [262]. In summary, fatty acids influence immune cell functions through complex mechanisms, emphasizing their promise as therapeutic targets in tumor immunotherapy. Additional studies are required to explore these mechanisms in depth to optimize therapeutic strategies based on fatty acids.

Lactate-mediated metabolic regulation

The Warburg effect, a metabolic phenomenon prevalent in various cancers, involves tumor cells preferentially undergoing glycolysis even in the presence of ample oxygen to rapidly generate ATP, fulfilling their energy demands [276, 277]. This metabolic preference leads to lactate accumulation within tumor cells, which is then released extracellularly through monocarboxylate transporters, leading to acidification of the TME [278, 279]. Immune cells sense extracellular lactate, triggering intracellular signaling pathways that influence their functions [280]. Tregs capitalize on lactate metabolism to maintain their immunosuppressive functions and proliferative capacity [281]. The transcription factor FOXP3 reconfigures the metabolic profile of Tregs to thrive in glucose-poor, lactate-rich conditions by enhancing OXPHOS, increasing NAD⁺ oxidation, and inhibiting Myc and glycolysis [46]. Lactate has demonstrated its ability to promote tumorigenesis through regulation of MOESIN lactylation and enhancement of TGF-β signaling in Tregs [182]. Moreover, lactate increases the expression of CTLA-4 through RNA splicing, bolstering the immunosuppressive activity of Tregs and contributing to the efficacy of CTLA-4 blockade therapy [5]. Lactate has also been reported to inhibit the expansion and cytokine secretion of CTLs, diminishing their cytotoxic activity [181, 282]. Furthermore, lactate induces the expression of VEGF and arginase1 in macrophages through HIF-1α, driving their differentiation into immunosuppressive M2-like macrophages [283, 284]. The findings also suggest that lactate activates GPR81 in DCs, impairing tumor antigen presentation [285], and inhibits the functions of invariant NKT cells through a phosphodiesterase-5 dependent pathway [286]. It also promotes the differentiation of M-MDSCs into G-MDSCs, thereby suppressing immune responses and facilitating tumorigenesis [287]. In colorectal cancer liver metastasis, lactate-induced acidification impairs liver-resident NK cells’ ability to control internal pH levels, triggering mitochondrial stress and apoptosis, thus allowing tumors to evade immune surveillance [288]. Interestingly, the latest studies have uncovered an unconventional immunoprotective role for lactate in tumor immunity. The research suggests that lactate exerts differential effects on CD8⁺ T cells depending on the environmental pH. In an acidic milieu, lactate undermines the survival of CD8⁺ T cells, whereas in a neutral environment, it enhances the stemness of CD8⁺ T cells, thus promoting antitumor immunity [289]. This finding highlights the significance of considering pH as a critical factor in the immunosupportive or immunosuppressive effects of lactate.

Lactate dehydrogenase A (LDHA) and monocarboxylate transporters (MCTs) are key regulators of lactate production and transport. Their coordinated activity facilitates efficient lactate metabolism that underpins tumor growth and progression. As such, targeting LDHA and MCTs represents a compelling therapeutic approach in cancer therapy. Oxamate, a classical inhibitor of LDHA, has demonstrated potent antitumor effects across multiple malignancies [290–294]. In addition to its direct antitumor activity, oxamate also enhances tumor sensitivity to standard therapeutic modalities, including chemotherapy, radiotherapy, and immunotherapy [290, 295–297]. In parallel, inhibition of lactate transport via MCTs represents another compelling therapeutic approach. AZD3965, a selective MCT1 inhibitor, has shown promising efficacy in multiple tumor types, including urothelial bladder cancer [298], triple-negative breast cancer [299], and retinoblastoma [300], while also enhancing the efficacy of ICB [301, 302]. Recent studies have explored alternative strategies to inhibit MCT activity. For instance, Zhang et al. developed a Basigin-targeting antibody capable of suppressing MCT function, leading to reduced lactate transport and enhanced antitumor immune responses [303]. Likewise, Liu et al. designed a tumor-selective nanocarrier system for delivering the MCT4 inhibitor Syro, which effectively blocks lactate efflux in tumor cells and restores T cell function within the TME [304]. Together, these findings underscore the therapeutic promise of targeting both lactate production and transport as a means to reprogram tumor metabolism and amplify the effectiveness of conventional and immunotherapeutic treatments.

Prostaglandin E2 (PGE2)-mediated metabolic regulation

PGE2 exerts a crucial and intricate influence within the TME by binding to its receptors EP2 and EP4, significantly influencing immune cell function, promoting immune evasion and facilitating tumor development. Typically, PGE2 suppresses immune responses by hindering T cell activation, NK cell cytotoxicity and B cell-mediated immunity [305–307]. In cancer immune responses, cDC1s have a pivotal function in the capture and transport of tumor antigens to draining lymph nodes, where they prime antitumor CD8⁺ T cells. However, this antitumor function can be compromised by PGE2. Specifically, PGE2 induces cDC1 dysfunction within tumors by activating the cAMP signaling pathway downstream of EP2 and EP4 receptors, thereby impairing their capacity to coordinate effective antitumor CD8⁺ T cell responses [7]. Unlike cDC1s, which are responsible for cross-presenting antigens, inflammatory monocytes can acquire and present peptide-MHC class I complexes to T cells through a “cross-dressing” mechanism, playing a key role in intratumoral T cell stimulation [308]. Nonetheless, hyperactivation of the MAPK signaling in tumor cells induces the release of PGE2 while simultaneously suppressing the generation of type I IFN, thus impeding this process and compromising the inflammatory monocyte state as well as intratumoral T cell activation [308]. It is noteworthy that in the TME, active inflammation coexists with immunosuppression, both contributing to tumor progression. PGE2-EP2/EP4 signaling not only enhances active inflammation by activating NF-κB in myeloid cells but also induces immunosuppression by promoting the mregDCs-Tregs axis, which facilitates Treg recruitment and activation [309]. Stem-like CD8⁺ T cells have a significant part in driving protective anticancer immunity by sustaining self-renewal and facilitating the differentiation of effector cells. While PGE2 does not influence the priming of stem-like CD8⁺ T cells in lymph nodes, it limits the intratumoral generation of both early and late effector T cells derived from these precursors by inhibiting the IL-2 signaling [310]. Moreover, PGE2 suppresses IL-2 signaling in T cells by downregulating the IL-2Rγc chain, which disrupts the formation of IL-2Rβ-IL2Rγc membrane dimers and consequently leads to impaired IL-2-mTOR signaling, repression of PGC1α transcription, and ultimately induces oxidative stress and ferroptosis in T cells [311]. Furthermore, PGE2 can also induce exhaustion of CD8⁺ T cells by upregulating PD-1 expression, contributing to an immunotolerant state [312, 313]. After anti-PD-1 therapy, PGE2 further promotes TAMs to acquire a CX3CR1⁺ phenotype, and these CX3CR1⁺ TAMs induce T cell exhaustion by secreting IL-27, thereby contributing to resistance against immunotherapy [314]. Taken together, PGE2 orchestrates a multidimensional immune-suppressive network within the TME.

Emerging evidence indicates that targeting the PGE2-EP2/EP4 signaling axis can reinvigorate local antitumor immune responses, potentiate ICB, and overcome resistance to immunotherapy [315]. Several antagonists targeting the EP receptors have been identified, among which EP4 inhibitors are considered particularly promising candidates due to their synergistic effects when combined with other immunotherapies or targeted agents. In a clinical trial involving patients with colorectal cancer, the EP4 antagonist vorbipiprant combined with anti-PD-1 antibody balstilimab significantly improved response rates, particularly in tumors exhibiting immunogenic features [316]. This enhancement is thought to result, at least in part, from restored tumor-specific CD8⁺ T cell proliferation and effector differentiation following inhibition of EP2/EP4 signaling [310]. These findings indicate the therapeutic potential of EP2/EP4 blockade, both as a monotherapy and as part of rational combination regimens.

2-HG-mediated metabolic regulation

2-HG is critically involved in metabolic reprogramming across various malignancies and exists in two enantiomeric forms, D-2HG and L-2HG. Mutations in isocitrate dehydrogenase (IDH) lead to the loss of its native oxidative decarboxylation function and result in the abnormal conversion of α-KG into D-2HG. In contrast, L-2HG is primarily generated under hypoxic conditions by promiscuous activity of enzymes such as malate dehydrogenase 1 and 2 and LDHA [317]. D-2HG appears to exert oncogenic effects by promoting breast cancer cell metastasis and pro-tumorigenic macrophage polarization [318]. However, under glucose-limited conditions, 2-HG suppresses ATP synthase, thereby triggering metabolic collapse and subsequent death of glioblastoma cells [319]. These findings reveal novel mechanisms underlying the complex roles of 2-HG in tumor biology. Tumor-derived 2-HG can be internalized by immune cells and interfere with their antitumor responses. In T cells, D-2HG uptake impairs cytotoxic activity and IFN-γ signaling, resulting in compromised metabolism and diminished antitumor function [320]. In macrophages, following the uptake of D-2HG, the aryl hydrocarbon receptor (AHR) translocates to the nucleus and drives the production of anti-inflammatory cytokines such as IL-10 and TGF-β, thereby promoting an immunosuppressive phenotype that hinders T cell proliferation and effector function [321]. Furthermore, D-2HG impairs the differentiation of DCs, downregulates MHC II expression, and reprograms their metabolism toward increased lactate production, collectively leading to a tolerogenic phenotype [322]. In contrast to the immunosuppressive effects of D-2HG, L-2HG primarily enhances antitumor immune responses. Culturing T cells in the presence of L-2HG improves mitochondrial function and significantly enhances in vivo proliferation and cytotoxic activity in adoptive cell therapy models [323, 324]. Likewise, L-2HG-treated macrophages exhibit increased chromatin accessibility and adopt a pro-inflammatory, antitumor phenotype, thereby sensitizing tumors to anti-PD-1 blockade [325]. Targeting D-2HG represents a potential therapeutic approach to enhance the efficacy of tumor immunotherapy. Ivosidenib, a selective inhibitor of mutant IDH1 that is approved for relapsed or refractory acute myeloid leukemia, effectively reduces intratumoral D-2HG levels [326]. Moreover, CD44 promotes NADPH production via the pentose phosphate pathway, which supports D-2HG synthesis by mutant IDH. Thus, combining IDH inhibitors with CD44 blockade has been shown to enhance tumor clearance [327]. Currently, several clinical trials targeting 2-HG are underway across various tumor types, including cholangiocarcinoma [328], low grade glioma [329], chondrosarcoma [330], glioblastoma and others [331].

Succinate-mediated metabolic regulation

Succinate is a critical intermediate of the TCA cycle, formed from succinyl-CoA via the catalytic activity of succinate synthase and subsequently oxidized to fumarate by SDH [332]. In recent years, succinate has increasingly been recognized for its role in mediating metabolic interactions between cancer cells and immune cells. T cells can readily absorb extracellular succinate, and elevated levels of succinate have been shown to inhibit glucose flux through the TCA cycle. In conjunction with activation of the succinate receptor SUCNR1, this results in compromised effector functions of CD4⁺ and CD8⁺ T cells, including diminished IFN-γ production [333]. In innate immune cells, succinate exerts diverse immunoregulatory effects. It promotes M1 macrophage polarization by inducing succinylation of IDH2 and histone H3K122 at the LDHA promoter [334]. Moreover, succinate influences thymic stromal lymphopoietin signaling to activate macrophages, enhancing type I IFN responses while inhibiting the TGF-β pathway to sustain M1 macrophage functionality [335]. In DCs, succinate enhances antigen presentation by upregulating thymic stromal lymphopoietin and EGFR signaling, while concurrently suppressing the TGF-β pathway [336]. However, recent evidence also implicates succinate in promoting tumor-supportive immune phenotypes. Tumor cells can release succinate into the TME, which activates the PI3K-HIF-1α signaling axis through SUCNR1, leading to M2 macrophage polarization and thereby promoting tumor progression and metastasis [337, 338]. SDH is frequently deficient in tumor cells and represents a hallmark of several cancer types. Loss of SDH activity leads to succinate accumulation, which contributes to oncogenesis [339]. Succinate inhibits prolyl hydroxylase, thereby stabilizing HIF-1α and activating hypoxia-related signaling pathways, even under normoxic conditions. This pseudohypoxia state drives tumor angiogenesis and proliferation [340, 341]. Additionally, succinate facilitates metastasis via multiple mechanisms. It activates SUCNR1/PI3K/HIF-1α signaling to promote cancer cell migration while simultaneously suppressing ten-eleven translocation 2 activity, which leads to reduced 5-hydroxymethylcytosine levels, global DNA hypermethylation, and enhanced tumor aggressiveness [337, 342]. Targeting succinate metabolism represents a potential therapeutic strategy. Inhibition of succinate accumulation disrupts cancer cell stemness and impairs tumor growth [343]. Given that most of the oncogenic effects of succinate are mediated by SUCNR1, targeting this receptor provides a potential intervention to suppress tumor progression and metastasis. Notably, SUCNR1 inhibition has demonstrated therapeutic efficacy in preclinical models of lung cancer and other malignancies [337].

Fumarate-mediated metabolic regulation

Fumarate is a central metabolite involved in multiple biological processes. Within the TCA cycle, it is produced by SDH and subsequently converted to malate via fumarate hydratase. Under normal metabolic conditions, this conversion supports mitochondrial energy production and maintains cellular metabolic homeostasis. In the context of antitumor immunity, elevated intracellular fumarate levels have been shown to induce early mitochondrial morphological alterations and promote the release of mitochondrial DNA into the cytosol via mitochondria-derived vesicles. This release activates the cGAS-STING signaling axis, leading to innate immune activation [344]. Regarding adaptive immunity, fumarate hydratase-deficient tumors accumulate fumarate, which directly modifies and inactivates ZAP70a critical kinase in TCR signalingby succinating cysteine residues (C96 and C102), thereby suppressing CD8⁺ T cell activation, expansion, and effector function [345]. Similarly, in B cells, LYN is succinated by fumarate, theraby inhibiting BCR signaling and B cell activation [346]. These findings collectively indicate that fumarate acts as a metabolite that negatively regulates adaptive antitumor immune responses. Fumarate also exerts multifaceted effects on oncogenic signaling. Its buildup in tumor cells is frequently associated with inactivating mutations or downregulated expression of fumarate hydratase [347]. Elevated fumarate levels inhibit Tet-mediated demethylation of the antimetastatic miR-200ba429 cluster, thereby upregulating transcription factors associated with epithelial-mesenchymal transition [348]. Furthermore, fumarate-induced succinylation of prolyl 4-hydroxylase subunit beta stabilizes the enzyme, enhances type I collagen production, and promotes tumor cell migration and invasion [349]. In addition, fumarate generated by adenylosuccinate lyase under hypoxia not only suppresses STING-mediated IFN signaling to promote immune evasion, but also inhibits Beclin1 demethylation, thereby enhancing autophagy and tumor cell survival [350, 351]. Moreover, fumarate induces succinylation of HIF-1α at cysteine 800, promoting its nuclear translocation, recruitment of the coactivator p300 and PKM2, and subsequent transcriptional activation of PD-L1 [352]. This mechanism suggests that fumarate modulation could sensitize tumors to anti-PD-1 therapy, providing a rationale for combining metabolic interventions with ICB.