T cells are the key players in eliminating malignant tumors. Adoptive transfer of tumor antigen-specific T cells and immune checkpoint blockade has yielded durable antitumor responses in the clinic, but not all patients respond initially and some that do respond eventually have tumor progression. Thus, new approaches to enhance the utility of immunotherapy are needed. T-cell activation and differentiation status are tightly controlled at the transcriptional, epigenetic, and metabolic levels. Amino acids are involved in multiple steps of T-cell antitumor immunity, including T-cell activation, proliferation, effector function, memory formation as well as functional exhaustion. In this review, we briefly discuss how amino acid metabolism is linked to T-cell fate decisions and summarize how amino acid deprivation or accumulation of certain amino acid metabolites within the tumor microenvironment diminishes T-cell functionality. Furthermore, we discuss potential strategies for immunotherapy via modulating amino acid metabolism either in T cells intrinsically or extrinsically to achieve therapeutic efficacy.
Great progress is being made harnessing the immune system to treat cancer in the clinic. On the basis of their strong cytotoxic capacity, CD8+ T cells are recognized as the key players of antitumor immune responses (1). CD4+ T cells can also suppress tumor development directly or indirectly by supporting CD8+ T-cell differentiation and functionality (2–5). Naïve T cells are quiescent and have low metabolic demands; they rely mainly on mitochondrial oxidative phosphorylation (OXPHOS) to generate ATP, which is used primarily for their survival. Upon antigen stimulation, T cells increase nutrient uptake (glucose, amino acids, etc.) and anabolism (OXHPOS, glycolysis, glutaminolysis, lipogenesis, etc.). This metabolic switch is essential to initiate T-cell activation and generate building blocks for subsequent proliferation bursts and effector functions. Amino acids are involved in multiple steps of T-cell activation and differentiation, including activation of key signaling cascades, energy generation, biosynthesis, redox homeostasis as well as posttranscriptional modifications (Fig. 1).
Most effector T cells exert immediate effector functions and die via apoptosis, but a small fraction of effector T cells undergo further differentiation into memory T cells, which persist and provide long-lasting protective responses. Compared with effector T cells, memory T cells preferentially engage OXPHOS and use fatty acid oxidation (FAO) to support mitochondrial spare respiratory capacity (SRC; refs. 6, 7). Specific amino acids play important roles in dictating T-cell effector differentiation or memory formation (8, 9).
Within the tumor microenvironment (TME), continuous antigen stimulation, along with other hostile factors that are present, lead to T-cell dysfunction or exhaustion, which is characterized by largely impaired proliferative capacity and effector functions. Of note, exhausted T cells harbor dysregulated metabolic activity, that is, downregulated glycolysis, glutaminolysis, lipogenesis, and defective mitochondria with reactive oxygen species (ROS) accumulation (1, 10). More importantly, competition for nutrients, including amino acids between tumor cells and immune cells shapes the functionality of diverse immune subsets and greatly impairs antitumor immune responses, often as a result of the accumulation of multiple-immunosuppressive metabolites within the TME (11). Therefore, there is substantial interest in enhancing T-cell–based immunotherapy through metabolic interventions, which aim to promote memory formation, prevent exhaustion, and improve metabolic fitness within the TME.
Herein, we will briefly introduce how amino acid metabolism is linked to T-cell activation and fate decisions, and discuss the key factors in the TME resulting in amino acid deprivation and limited T-cell functionality. In addition, we summarize potential immunotherapeutic approaches to improve T-cell antitumor functionality through the modulation of amino acid metabolism.
Amino Acid Acquisition and Sensing Initiate T-cell Activation
During T-cell activation, amino acids serve not only as substrates for protein synthesis, but also as signals to initiate metabolic reprogramming. To fulfill the increasing demand of amino acid consumption, numerous amino acid transporters belonging to the solute carrier (Slc) superfamily are rapidly upregulated upon T-cell activation to import extracellular amino acids (reviewed in ref. 9), among which Slc7a5 is one of the most significantly upregulated transporters (12). Slc7a5 is a single System L (“leucine-preferring system”) transporter that supplies several essential amino acids such as leucine, methionine, and tryptophan for T-cell activation and differentiation in vitro and in vivo (13–15).
Upon T-cell receptor (TCR) stimulation, leucine is transported mainly by Slc7a5 to activate the leucine-sensing kinase mTORC1, which is a central metabolic regulator that coordinates the environmental and intracellular signals for T-cell activation (16, 17). Notably, Slc7a5 deficiency results in severely impaired activity of the mTORC1/c-Myc signaling cascade and thus it leads to decreased T-cell clonal expansion and effector function (13, 15), suggesting that proper expression of amino acid transporters is critical for T-cell activation and metabolic reprogramming. One downstream effect of mTORC1/c-Myc signaling is the upregulation of several other amino acid transporters such as Slc1a5, Slc7a1, Slc7a5, Slc38a1, and Slc38a2, which are required to sustain T-cell proliferation and differentiation (15).
In addition to transporting leucine, Slc7a5 is the key transporter of methionine. Catabolism of methionine yields the metabolite S-adenosylmethionine (SAM), which binds to SAMTOR, an S-adenosylmethionine sensor, triggering further mTORC1 activation (14). Tryptophan also is mainly transported by Slc7a5 in activated T cells. In the absence of tryptophan, cell-cycle arrest at a mid-G1 point occurs (18).
Altogether, these observations suggest that uptake of leucine, methionine, and tryptophan via Slc7a5 is essential for T-cell growth and clonal expansion via supporting key metabolic activities. In addition, the glutamine transporter Slc1a5 represents another key activator of the mTORC1 pathway because specific deficiency of Slc1a5 in T cells results in decreased glutamine uptake and severely impaired T-cell inflammatory responses (19). In this regard, mTORC1 activation is tightly coupled to amino acid metabolism to support T-cell proliferation and differentiation.
Amino Acids Support Biosynthesis and Energy Supply
Upon TCR activation, T cells switch their energy supply from OXPHOS and FAO to glycolysis, glutaminolysis, and the pentose phosphate pathway for rapid synthesis of nucleotides, proteins, and other biomolecules (16, 17). This metabolic shift requires a substantial increase in amino acid availability to engage anabolic metabolism for T-cell expansion and effector functions. Importantly, several amino acids are needed to enable proper RNA and DNA synthesis for T-cell division. For instance, serine is an essential metabolite for purine synthesis. Upon activation, T cells upregulate key enzymes of the one-carbon metabolic network to rapidly catabolize serine into glycine and one carbon units for de novo nucleotide biosynthesis (20). In this regard, deprivation of serine or inhibition of one-carbon metabolism impairs T-cell proliferation due to impaired production of glycine and one-carbon units for purine biosynthesis, whereas adding glycine and formate rescues T-cell proliferative defects arising from serine deprivation (20, 21). Attenuated T-cell activation due to glutamine deprivation can be partially rescued by adding nucleotides, suggesting that glutamine is also an important precursor for nucleotide synthesis (16).
Polyamines are essential for T-cell growth, and inhibition of polyamine synthesis leads to defective T-cell proliferation and inflammatory responses (16, 22). Polyamines are derived from ornithine through the urea cycle. In this process, arginine is the major substrate to form ornithine for polyamine generation, although a small fraction of ornithine can also be synthesized from glutamine and glutamate (22, 23). A Myc-dependent metabolic pathway coupling glutaminolysis to polyamine biosynthesis has also been reported (16).
To meet the increasing demand of biosynthesis in proliferating T cells, ATP is rapidly generated through the tricarboxylic acid (TCA) cycle and OXPHOS (24). Of note, several amino acids serve as the substrates to fuel the TCA cycle. For instance, glutamine can be metabolized into α-ketoglutarate (25), an important substrate to fuel the TCA cycle and generate energy. In addition, branched-chain amino acids (BCAA), including leucine, isoleucine, and valine can be metabolized into acetyl-CoA and succinyl-CoA as substrates of the TCA cycle to generate ATP (26). Moreover, addition of the serine metabolites formate and glycine improves mitochondrial respiration in murine aged T cells, which provides compelling evidence that serine improves T-cell mitochondrial function (27). Therefore, amino acids are indispensable to support energy production required for T-cell activation and proliferation.
Amino Acids Control Redox Homostasis
Energy production by mitochondria is accompanied by the generation of ROS (28). Physiologic levels of ROS act as a signal to stimulate T-cell differentiation and proliferation, whereas excessive ROS accumulation leads to catastrophic oxidative damage to T-cell (29). To buffer ROS, T cells generate various of antioxidants. Glutathione (GSH) is the most abundant antioxidant. It is derived from glycine, glutamate, and cysteine. GSH de novo synthesis, rather than recycling via GSH reductase, is reported to promote ROS clearance and Th17 differentiation (30), indicating that the level of intracellular amino acids associated with GSH production may dictate T-cell fate. Cysteine is the rate-limiting substrate of GSH production because its abundance is much lower than that of glycine and glutamate (31). In addition, the concentration of cysteine (10 to 25 μmol/L) is significantly lower than the concentration of cystine (100 to 200 μmol/L) in the plasma (32). Naive T cells lack the cystine/glutamate exchange transporter system Xc-. They therefore rely on adjacent macrophages and dendritic cells (DC) for cysteine, which they obtain via an alanine–serine–cysteine transporter during the early phase of activation (33, 34). Antigen-presenting cells can also secrete thioredoxin, which converts extracellular cystine to cysteine, facilitating T-cell uptake (33). In addition, methionine can be converted to cysteine through the transsulfuration pathway. Under oxidative stress, the transsulfuration pathway is upregulated to resist peroxide-induced cell death in naïve T cells (34). Furthermore, serine can generate glycine as well as NADPH via one-carbon metabolism, which is the precursor of GSH and the reduced donor molecules against ROS, respectively (27). Glutamine also participates in GSH de novo synthesis by providing the essential precursor glutamate (30).
Cross-talk between Amino Acid Metabolism and Posttranslational Modifications
CD8+ T-cell differentiation and functionality are tightly controlled at the posttranslational level. TCR signal strength and duration, costimulation as well as cytokine signaling are coordinated to program naive T-cell differentiation either into effector T cells or memory T cells, which is coupled to drastic epigenetic remodeling (35). Posttranslational modifications of histone tails by methylation, acetylation, or phosphorylation can alter the nucleosome conformation or transcription.
Alterations to the DNA methylation landscape represent one key regulatory step to turn on or off genes associated with T-cell differentiation and functionality. DNA and histone methylation rely on the essential amino acid methionine, as its catabolite SAM is the universal methyl donor (36, 37). Trimethylation of histone H3 at lysine 4 (H3K4me3) is indicative of active gene transcription, whereas trimethylation of histone H3 at lysine 27 (H3K27me3) is associated with gene repression (38, 39). Several studies indicate that naïve T cells, T memory stem cells, central memory T cells, and effector memory T cells display distinct epigenetic features, including distinct patterns of histone methylation (38, 39). Further studies suggest that DNA methylation patterns greatly impact effector or memory CD8+ T-cell differentiation, partly by modulating DNA methylation programs at genes associated with the naïve T-cell phenotype and maintaining demethylation at the loci of classically defined effector signature genes (40, 41). In response to lymphocytic choriomeningitis virus infection, the promoter region of Sell (which encodes CD62L) is rapidly methylated and thus, CD62 L expression is drastically decreased compared with naive T cells. Intriguingly, this methylation pattern is erased and CD62 L expression is gradually restored in T cells with memory fate. This process is dependent on the de novo DNA methyltransferase Dnmt3a, and Dnmt3a deficiency can promote differentiation of memory precursors into memory T cells (40). There also is increased H3K27me3 deposition at several prosurvival and promemory genes in effector T cells, leading to cell differentiation and lifespan being restricted. Of note, memory precursor T cells exhibit less chromatin methylation at both proeffector and promemory genes compared with terminal differentiated effector T cells (42). Consistent with these data, trimethylation of histone H3 at lysine 9 (H3K9me3) by histone methyltransferase Suv39h1 silences the expression of memory signature genes during effector CD8+ T-cell differentiation (43).
Histone acetylation is another crucial epigenetic modification linked to transcriptional activation. Histone acetyltransferases use acetyl-CoA to provide acetyl groups as their primary source for histone acetylation. Glutamine and BCAAs can be metabolized to produce acetyl-CoA (44), which thus facilitates lysine acetyltransferase 6A to acetylate histones of the Cd8a and Cd8b1 coreceptor gene (Cd8) locus to enable robust effector responses during viral infection (45). Another study suggests that the transcription factor NR4A1 is preferentially recruited to AP-1–binding sites, where it inhibits AP-1 activity and promotes acetylation of H3K27, which leads to tolerance-related genes and T-cell dysfunction (46).
Glycosylation is another posttranslational modification that is rapidly upregulated in activated T cells to promote T-cell activation and function. TCRs with glycosylation modifications have enhanced ability to bind with galectin, which reduces the distance between the TCR and CD8 coreceptor, thereby enhancing the T-cell response (47). Moreover, o-linked β-N-acetylglucosamine (O-GlcNAc) modifications are prominently increased after activation of lymphoid cells and have been suggested to be required for nuclear translocation of nuclear factor of activated T cells (48). Moreover, IL2 production and proliferation are compromised in the absence of O-GlcNAc (49). Uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) is the substrate for glycosylation of proteins. Glutamine is converted to UDP-GlcNAc through the hexosamine pathway, indicating that glutamine levels are a key factor in determining the level of glycosylation, which subsequently affects T-cell fate determination. Signaling through Notch, the TCR and Myc increase glutamine uptake and thereby upregulate posttranslational modification of proteins by glycosylation, which enhances the self-renewal ability of T cells, which in turn regulates c-Myc expression levels (50). T cells from human ovarian cancer ascites have N-linked protein glycosylation defects, and increasing glutamine uptake by these cells recovers glycosylation and mitochondrial respiration defects to promote their antitumor abilities (51).
In addition, protein phosphorylation is of great importance to control the activity of multiple enzymes and receptors in T cells. In particular, the Src family tyrosine kinase LCK is required for initiating TCR signaling. Asparagine can promote phosphorylation of LCK, thereby leading to CD8+ T-cell activation and differentiation into memory-like cells (52). The key role of amino acids in maintaining appropriate posttranslational modifications highlights the great significance of maintaining the proper amounts of these amino acids for T-cell differentiation.
T-cell Metabolism in the TME
The TME is characterized by hypoxia, low pH, limited levels of glucose and amino acids, and accumulation of metabolic derivatives (11, 53, 54). Cancer cells and multiple immune cell subsets, including CD8+ effector T cells, have similar metabolic requirements and thus compete for glucose and amino acids in the TME. To meet the metabolic demand, the malignant cells in many cancers, including bladder, breast, cervical, and skin cancer, drastically elevate their expression of Slc1a5, Slc7a5, Slc7a2, and Slc6a14 to enhance uptake of glutamine, arginine, tryptophan, and leucine (55–57). Excessive nutrient consumption by tumor cells, stromal cells, and certain immunosuppressive immune cells may lead to deprivation of amino acids for biosynthesis of macromolecules, ATP production and maintenance of redox homeostasis in the TME, which impairs the ability of T cells to eradicate tumors.
T cells in the TME can be deprived of amino acids essential for energy metabolism and biosynthesis, and thereby antitumor function, for many different reasons (Fig. 2). For example, most tumor cells preferentially depend on glutamine metabolism for proliferation and biological function, which results in glutamine deprivation for T cells (58). In estrogen receptor–expressing breast cancer, the cancer cells highly express Slc7a5, which promotes leucine uptake and tumor cell proliferation and a corresponding deprivation of this amino acid for T cells (57). Several studies indicate that tumors like melanoma and hepatocellular carcinoma (HCC) lack expression of argininosuccinate synthetase 1 and thus rely largely on exogenous arginine (59). Arginine depletion in the TME can also occur because metabolites (lactic acid and HIF-1α–inducing molecules) and some cytokines (IL4, IL6, IL13, M-CSF, and GM-CSF) can induce arginase (ARG) or nitric oxide (NO) synthase (NOS) overexpression in tumor-associated macrophages (TAM) and bone marrow–derived suppressor cells (MDSC; refs. 60, 61). Arginine depletion can impair T-cell antitumor immunity because it is required for CD3ζ chain internalization and reexpression (62). It can also induce MDSC accumulation, further blunting antitumor T-cell responses (63). Tryptophan deprivation is also common in the TME. Indoleamine 2,3-dioxygenase 1 (IDO1), which is abundantly expressed in multiple tumor cell types, stromal cells, DCs and macrophages, and IL4 induced 1 (IL4I1), whose expression is enhanced in tumor cells (64), can catabolize tryptophan to generate the immunosuppressive metabolite kynurenine (Kyn). This leads to tryptophan depletion and Kyn accumulation in the TME.
In addition to insufficient fuel supply, certain derivatives from amino acid metabolism play immunosuppressive roles in the TME. Activation of the tryptophan–IDO–Kyn pathway not only consumes tryptophan, impairing T-cell protein synthesis, but also causes the accumulation of Kyn, which can activate the aryl hydrocarbon receptor (AhR) pathway to drive PD-1 upregulation in CD8+ T cells through a transcellular Kyn–AhR pathway (65). In addition, AhR activation induced by Kyn leads to AHR-dependent immunosuppressive regulatory T-cell accumulation, which further inhibits effector T cell functions (66). Furthermore, Kyn competes with amino acids, including leucine and methionine for uptake by Slc7a5 into T cells, further impairing immune responses (67). NO, a metabolite of arginine, is a proinflammatory mediator that can have pro- and antitumor effects. One effect it can have on T cells is that NO-derived peroxynitrite can cause nitration of tyrosine residues, which blocks protein tyrosine phosphorylation, resulting in decreased T-cell proliferation and activation.
High OXPHOS activity drives ATP production and contributes substantially to cellular ROS generation. As mentioned previously, GSH is mainly synthesized from glutamate, glycine, and cysteine, although homocysteine from the transsulfuration pathway can also serve as a precursor for GSH production, which can buffer ROS to maintain the redox balance under physiologic settings. Tumor cells like KRAS-mutant lung adenocarcinoma and pancreatic tumor and p53 mutant tumors usually overexpress system Xc- for cystine uptake, which leads to cystine deprivation for T cells (68–70). In addition, MDSCs express system Xc- and lack the alanine–serine–cysteine transporter to export cysteine, which further consumes cystine and limits the availability of cysteine for T cells, resulting in further GSH limitation and ROS accumulation in T cells (Fig. 2; ref. 71). In addition, high methionine consumption by tumor-initiating cells leads to methionine deprivation in the TME (72), which may reduce production of homocysteine for T-cell GSH synthesis to buffer ROS. Accumulation of ROS can induce DNA damage and trigger cell death. Consistent with these data, several studies indicate that ROS accumulation, mitochondrial dysregulation, and glycolytic insufficiency exacerbate T-cell exhaustion or dysfunction (73–75).
In the TME, as well as during chronic viral infection, CD8+ T cells differentiate into a dysfunctional state termed exhaustion. Exhaustion is characterized by increased immune checkpoint expression, dysregulated mitochondrial activity, and reduced bioenergetics and cytokine production (76–78). During T-cell transition to the exhaustion state, dynamic transcriptional and posttranslational changes occur (79). For example, several studies indicate that memory and exhausted CD8+ T cells have different accessible chromatin landscapes and that exhaustion is accompanied by a broad remodeling of the enhancer landscape and transcription factor binding (78, 80–84). As mentioned above, tumors highly express Slc7a5 to consume methionine in the TME, which leads to insufficient methionine for SAM production in T cells (14). In addition, certain types of tumor cells express high levels of the methionine transporter Slc43a2, which consumes methionine in the TME, leading to decreased T-cell histone H3K79me2 methylation, impaired activity of the downstream mediator signal transducer and activator of transcription 5, and eventually T-cell apoptosis and dysfunction (85). Moreover, it has been shown that methionine restriction reduces histone H3K4me3 at the promoter regions of key genes involved in Th17 cell proliferation and cytokine production (86). Interestingly, SAM and 5-methylthioadenosine in HCC tumor tissues possibly drive effector T cells to exhausted state by reducing global chromatin accessibilities in CD8+ T cells (87). Further characterization of how amino acid metabolism affects T-cell differentiation and function via modulation of DNA or protein methylation and other forms of posttranslational modifications are urgently needed.
Targeting Amino Acid Metabolism to Boost T-cell Antitumor Immunity
Interventions that target amino acid metabolism represent an attractive strategy against cancer. Amino acid starvation of cancer cells exerts effective antitumor effects with minimal collateral damage and has been suggested as having potential for combination with other anticancer therapies (88). Given that effector T-cell functions can be impaired by severe limitations of amino acids in the TME, programming or reprogramming of amino acid metabolism in T cells appears to be a compelling approach with less toxicity and more compatibility with combination treatment. Here, we discuss promising strategies for modulating amino acid availability and metabolism to promote T-cell fitness and antitumor immunity (Fig. 3).
Modulating amino acid availability for T-cell memory formation
During the in vitro activation and expansion phases of T-cell manufacturing for adoptive cell therapy, it is reasonable to modulate the nutritional composition of the culture medium to favor the generation of long-lived memory T cells. In this regard, adding or limiting the levels of amino acids in the activation/expansion medium may allow the generation of memory T cells with enhanced metabolic fitness, which would lead to more durable antitumor responses in vivo. For instance, arginine supplementation during T-cell activation in vitro promotes the switch from glycolysis to OXPHOS and increases the SRC of mitochondria, which consequently leads to differentiation of memory T cells that exhibit superior viability and antitumor ability (89). Overexpressing cationic amino acid transporter 1, the main arginine transporter, via genetic manipulation may enhance arginine uptake both in vitro and in vivo and maintain the memory phenotype of T cells upon transfer. Interestingly, T cells cultured with glutamine restriction or inhibitors of glutamine metabolism show characteristics of memory formation and reduced exhaustion (90). Indeed, administration of a glutamine antagonist to tumor-bearing mice inhibits metabolism of cancer cells while enhancing T-cell oxidative metabolism and memory formation (91). The dual antitumor effects are likely due to distinct cell-intrinsic programs of preferential acquisition of glutamine and glucose by cancer cells and T cells, and inhibiting glutamine uptake promotes glucose uptake in various tumor resident cells, including T cells (58). Hence, transient glutamine starvation or moderate inhibition of glutamine metabolism via glutamine antagonists in T cells is a promising approach to promote T-cell immunity and tumor cell death. However, manipulation of multiple amino acid composition of the culture medium may provide an effective way to generate long-lived memory T cells.
As described previously, increased mTORC1 activity promotes T-cell effector differentiation rather than memory formation (17, 92, 93). Of note, mTORC1 is the pivotal intracellular sensor of the amino acid pool. Thus, uptake of specific amino acids may directly impact mTORC1 activity and influence T-cell fate in that way. For instance, loss of amino acid transporters such as Slc7a1 or Slc38a2 lead to attenuation of mTORC1 activity and increased memory precursor cell differentiation, which preserves the survival fitness to become memory T cells (94). To this end, decreasing mTORC1 activation by regulating levels of amino acid transporters or the corresponding amino acids (glutamine, leucine, methionine, etc.) in vitro may represent a valuable strategy to generate memory T cells for adoptive cell therapy.
Indeed, naïve, effector and memory T cells display drastically distinct DNA methylation landscapes. In effector T cells, methyltransferases such as Dnmt3a and Suv39h1 promote the expression of genes associated with the naïve T-cell phenotype by decreasing DNA and histone methylation, respectively, thus conferring on the cells a long-lived memory phenotype (40, 43). These data imply that chromatin methylation patterns determine T-cell differentiation, and demethylation of chromatin is linked to memory formation. It remains to be determined whether decreasing methionine concentration in expansion medium may be a feasible way to modulate the chromatin methylation landscape and enhance memory formation. Of note, severe and continuous methionine deficiency is reported to impair T-cell immunity (85). Therefore, during T-cell manufacturing in vitro for adoptive cell therapy, the intensity and duration of methionine restriction should be carefully considered.
Modulating amino acid metabolism to maintain T-cell redox balance
T cells from solid tumors reside within a harsh environment and eventually acquire the exhaustion phenotype and fail to control tumor growth. Amino acid metabolism is linked to redox homeostasis and T-cell exhaustion, and interventions of amino acid metabolism may provide a promising means to rejuvenate T-cell antitumor immunity. Notably, exhausted T cells within the TME often harbor fragmented mitochondria with excessive ROS generation. To this end, clearance of the accumulated ROS in the TME is a promising strategy to prevent T-cell dysfunction and exhaustion. The antioxidant N-acetylcysteine (NAC), the precursor of GSH biosynthesis, could partially reverse T-cell dysfunction by limiting oxidative stress. Importantly, when adoptively transferred to B16 tumor–bearing mice, T cells cultured in the presence of NAC show superior tumor control and improve survival compared with T cells cultured without the antioxidant (95). In addition, NAC promotes the expansion of CD8+ stem cell memory T cells, which possess survival advantages and self-renewal capacity to exert potent antitumor effects (96). Moreover, NAC combined with formate can restore the functionality of effector T cells with defective one-carbon metabolism (21). It will be important to establish in future studies whether genetic manipulation of T cells to overexpress cysteine transporters such as Slc7a11 improves the efficiency of T cells in competing for key amino acids to synthesize GSH, which would allow for better buffering of ROS and improved antitumor immunity. Taken together, these findings imply that boosting GSH-associated amino acids may be an effective way to maintain metabolic fitness for preventing or rejuvenating T-cell exhaustion.
Epigenetic remodeling to improve antitumor immunity
T cells gradually acquire the terminal exhaustion state at the later stages of tumor development. This state is imprinted at the epigenetic level and is largely irreversible. Indeed, it has been demonstrated that inhibition of de novo DNA methylation can promote anti–PD-1 immunotherapy (97, 98). Consistent with this, administration of a low-dose of the DNA methyltransferase inhibitor decitabine largely improves the therapeutic efficacy of adoptively transferred CAR T cells, which exhibit stronger antitumor effects and cytokine production capacities (99). Furthermore, dietary restriction of methionine has yielded improved therapeutic responses in chemoresistant RAS-driven patient with colorectal cancer–derived xenografts and autochthonous KRAS-driven soft tissue sarcomas resistant to radiotherapy (100). Along the same line, it would be interesting to test whether limiting the intracellular level of the methylation substrate SAM via dietary restriction of methionine intake promotes T-cell antitumor immunity. The question arising is how to specifically target SAM generation or the methylation landscape within T cells with a systemic intervention strategy.
Modulating key derivatives of amino acid metabolism to improve antitumor immunity
Increasing the amount of certain amino acid derivatives in vivo has significant antitumor effects. With the murine Listeria monocytogenes infection model, acetate-induced acetyl-CoA drives acetylation of GAPDH and subsequently enhances glycolysis for rapid recall responses by memory T cells (101). Moreover, acetate promotes chromatin accessibility by histone acetylation, which enhances cytokine production in glucose-limited CD8+ T cells in vivo (102). These observations indicate that the quantity of acetyl-CoA may determine T-cell antitumor function by modulating acetate levels. Given that acetyl-CoA can also be derived from leucine, isoleucine, and glutamine, it remains to be determined whether dietary supplementation of BCAAs and glutamine, or overexpression of key enzymes regulating metabolism of these amino acids, can upregulate acetyl-CoA levels for enhanced antitumor ability. In addition, inhibition of creatine uptake severely impairs T-cell immunity because creatine is critical for buffering ATP levels (103). Notably, creatine supplementation in vivo synergizes with PD-1/PD-L1 antibody treatment for superior tumor control (103). As precursors of creatine, it deserves further investigation whether arginine and glycine could boost T-cell antitumor immunity by increasing creatine production. Another arginine metabolite, spermidine, can induce autophagy and enhance the formation of memory CD8+ T cells in aged mice (104). In this regard, elevating levels of arginine in T cells via dietary supplementation or boosting key enzymes that catabolize arginine into creatine and spermidine may be an efficient strategy to enhance the therapeutic efficacy of T cells.
Small molecules mitigate amino acid competition in the TME
Tumor cells and certain stromal/immune cells within the TME often have abnormal expression of key enzymes involved in amino acid metabolism, including many that have been proven as potential therapeutic targets. ARG or NOS overexpression by TAMs and MDSCs leads to l-arginine depletion and NO generation in the TME, which induces T-cell apoptosis. The ARG inhibitor CD-1158 effectively blocks MDSC-mediated immunosuppression, greatly reducing tumor growth by restoring the arginine supply for T-cell proliferation (105). In addition, tumor cells and MDSCs express excessive IDO, which consumes tryptophan, severely impairing T-cell functionality. Conversely, IDO inhibition improves tryptophan accessibility, promoting the expansion and function of central memory T cells and preventing their apoptosis (106). So far, much attention has been focused on developing highly specific and effective small-molecule inhibitors of IDO worldwide, and the antitumor efficacy of BM-986205, epacadostat and indoximod are currently undergoing assessment either as single treatment or in combination with immune checkpoint blockade (ICB) in multiple clinical trials across multiple tumor types (107). Moreover, kynureninase increases tumor infiltration and proliferation of polyfunctional CD8+ T cells and synergizes with ICB in melanoma, breast carcinoma, and colon carcinoma models (108). In this regard, inhibitors of amino acid catabolizing enzymes, which are often highly expressed in tumors, may be effective therapeutics that prevent immune evasion in the TME. Further investigation of approaches to target these enzymes is needed.
Conclusions and Perspectives
It is becoming increasingly clear that the composition and quantity of the intracellular and extracellular amino acid pool may determine T-cell fate. Although valuable metabolic intervention strategies have been proposed, there are still many challenges to be addressed. For instance, how to determine or optimize the precise window of intervention for such strategies in vitro or in vivo to induce durable T-cell antitumor immunity? How can we boost the recall response of memory T cells or endow T cells with the ability to outcompete other cells for key amino acids in the TME? In addition, the complex regulation of amino acid uptake and competition between diverse types of cells within the TME greatly shapes cell differentiation status and functionality. Future studies that provide an in-depth understanding of the key features of amino acid metabolism in the TME will provide important insights into designing effective T-cell–based therapeutic strategies against cancer.
P.-C. Ho reports grants and personal fees from Elixiron Immunotherapeutics, grants from Roche, and personal fees from Acepodia outside the submitted work. No disclosures were reported by the other authors.
L. Zhang is in part supported by the Natural Science Foundation of China (NSFC 81971466) and Innovation Fund from the Chinese Academy of Medical Sciences (2016-I2M-1-005). P.-C. Ho is in part supported by the European Research Council Starting grant (802773-MitoGuide), the Swiss National Science Foundation (SNSF) project grants (31003A_182470), the Cancer Research Institute Lloyd J. Old STAR award, the University of Lausanne, and Ludwig Cancer Research. The authors apologize to the scientists whose work was not cited because of space limitations.