Metabolic Challenges in Anticancer CD8 T Cell Functions

Abstract

Cancer immunotherapies continue to face numerous obstacles in the successful treatment of solid malignancies. While immunotherapy has emerged as an extremely effective treatment option for hematologic malignancies, it is largely ineffective against solid tumors due in part to metabolic challenges present in the tumor microenvironment (TME). Tumor-infiltrating CD8⁺ T cells face fierce competition with cancer cells for limited nutrients. The strong metabolic suppression in the TME often leads to impaired T-cell recruitment to the tumor site and hyporesponsive effector functions via T-cell exhaustion. Growing evidence suggests that mitochondria play a key role in CD8⁺ T-cell activation, migration, effector functions, and persistence in tumors. Therefore, targeting the mitochondrial metabolism of adoptively transferred T cells has the potential to greatly improve the effectiveness of cancer immunotherapies in treating solid malignancies.

INTRODUCTION

Despite continued advancements in adoptive T-cell therapies, success in treating cancer patients with solid malignancies remains limited (1). The immunosuppressive tumor microenvironment (TME) of solid tumors presents many obstacles that inhibit the effector functions of CD8⁺ T cells (2). A number of these challenges can be attributed to the high metabolic flux of cancer cells (3). The augmented proliferation rate of cancer cells requires elevated metabolism to produce enough energy and metabolic intermediates (4). Therefore, nutrients in the TME that are available to CD8⁺ T cells are often scarce and metabolic waste products accumulate, thus inhibiting the activity of adoptively transferred T cells (5,6).

One possible way to overcome the metabolic challenges of the TME is to selectively boost the metabolism of only CD8⁺ T cells. Specifically, mitochondrial metabolism has been shown by a number of groups to be essential for various T-cell effector functions (7,8,9,10,11). In this Review, we summarize the metabolic dynamics and plasticity of CD8⁺ T cells, describe the main types of adoptive T-cell therapies, and discuss the metabolic challenges faced by adoptively transferred T cells. Finally, we justify why targeting the mitochondrial metabolism of CD8⁺ T cells is a probable approach to improve the outcomes of adoptive T-cell therapies.

METABOLIC DYNAMICS OF CD8⁺ T CELLS

Immune cells utilize numerous forms of metabolism to obtain sufficient levels of energy and metabolites from available nutrients to support their survival, differentiation, proliferation, and other important cellular functions (12,13). Upon infection or chronic inflammation, such as in cancer, immune cells undergo dramatic metabolic reprogramming to respond optimally and execute their respective effector functions (14).

CD8⁺ T cells can use various forms of metabolism, including glycolysis, the tricarboxylic acid (TCA) cycle, oxidative phosphorylation (OxPhos), glutaminolysis, and fatty acid oxidation (FAO) (Fig. 1) (15). CD8⁺ T cells at each differentiation state have unique metabolic profiles to produce energy and metabolic precursors that are important for specific T-cell functions (12). In addition, activated CD8⁺ T cells are metabolically flexible; therefore, they can adjust their reliance on different metabolic pathways depending on current metabolic needs, as well as what nutrients are available (12,16). Metabolic reprogramming in CD8⁺ T cells is essential for the acquisition of effector functions in activated T cells and for the development of memory T cells (16).

Figure 1. CD8⁺ T cell metabolism. Metabolic pathways used by CD8⁺ T cells include glycolysis, OxPhos, glutaminolysis, and FAO. Glycolysis occurs in the cytoplasm and produces 2 molecules of ATP, 2 NADH, and 2 pyruvate for every molecule of glucose. Pyruvate can be transported into mitochondria and converted into acetyl-CoA to be used in the TCA cycle to make electron carriers NADH and FADH₂. When activated T cells use aerobic glycolysis, pyruvate is converted to lactate and released by the cell, even when oxygen is present, like the Warburg effect in cancer cells. OxPhos requires the electron carriers NADH and FADH₂, with oxygen as the final electron acceptor to produce up to 36 molecules of ATP. When glucose is limited, activated T cells can also use glutaminolysis to support the TCA cycle and OxPhos. Naïve and memory CD8⁺ T cells utilize β-oxidation of fatty acids to sustain the TCA cycle. I–IV indicate complexes of the electron transport chain. Created with BioRender.com.

CoQ, coenzyme Q; α-KG, alpha-ketoglutarate; Cyt c, cytochrome c; OAA, oxaloacetate; SucCoA, succinyl-coenzyme A; Glucose-6P, glucose 6-phosphate; 3PG, 3-phosphoglycerate; PEP, phosphoenolpyruvate; GLUT, glucose transporter.

Naïve CD8⁺ T cells

Naïve CD8⁺ T cells are relatively quiescent and therefore do not have a high demand for energy as they remain in a steady state until they recognize their target antigen (12). Catabolic metabolism utilized by naïve T cells primarily involves the complete oxidation of glucose-derived pyruvate by OxPhos to produce ATP. Given that naïve CD8⁺ T cells are not actively proliferating, they have low levels of nutrient uptake and relatively low rates of glycolysis.

For a cell to completely oxidize glucose-derived pyruvate and produce more ATP than that produced from glycolysis alone, OxPhos needs to occur in the mitochondria. OxPhos requires the electron carriers NADH and FADH₂, which are produced during either the TCA cycle or glycolysis and donate their electrons to complex I or II of the electron transport chain (ETC) in the inner mitochondrial membrane (Fig. 1). While electrons travel through complexes of the ETC, protons are simultaneously pumped from the mitochondrial matrix to the intermembrane space. The proton gradient that is formed across the inner mitochondrial membrane contributes to the mitochondrial membrane potential (Dy_m), which is then used by complex V, also known as ATP synthase of the ETC to phosphorylate ADP to ATP. The production of ATP in mitochondria through OxPhos requires oxygen as the final electron acceptor and can produce up to 36 molecules of ATP for each glucose molecule. In addition to making ATP, OxPhos produces ROS, water, and the oxidized electron carriers NAD⁺ and FAD (17). OxPhos is an efficient way for a cell to produce ATP by completely oxidizing glucose; however, it takes more time than glycolysis alone and requires oxygen. The low demand for energy in naïve CD8⁺ T cells makes OxPhos a suitable form of metabolism.

Effector CD8⁺ T cells

During T cell activation, metabolic reprogramming dependent on CD28 co-stimulation occurs to support anabolic cell growth in proliferating effector CD8⁺ T cells (18). Glycolysis and glucose consumption increase immensely to allow for the rapid production of ATP, metabolic intermediates, and effector cytokines (19). Similarly to the Warburg effect in cancer cells, the glycolysis end-product in effector CD8⁺ T cells is lactate, even in the presence of oxygen (12,20). Glycolysis yields a net of 2 ATP, 2 pyruvate, and 2 NADH molecules for each molecule of glucose (Fig. 1). Additionally, metabolic intermediates that can be utilized for fatty acid, nucleotide and amino acid biosynthesis to support cell growth and proliferation are formed at various steps in glycolysis (12). The pentose phosphate pathway, which uses the first intermediate of glycolysis, is important for nucleotide production in proliferating cells. When effector CD8⁺ T cells have elevated levels of glycolysis even when oxygen is present, pyruvate is converted into lactate and then is released by the cell. However, if pyruvate is to be further oxidized, it enters mitochondria and is made into acetyl-coenzyme A (CoA), a substrate of the TCA cycle, which can also produce metabolic intermediates for protein and fatty acid biosynthesis (12) (Fig. 1).

In addition to glucose, activated CD8⁺ T cells consume high levels of glutamine, which can be processed in the TCA cycle and is important for T cell proliferation and cytokine production (21). When glucose is in short supply and therefore glycolytic-derived pyruvate is minimal, glutamine can enter the TCA cycle after being converted into a-ketoglutarate by glutaminolysis (Fig. 1). Similar to glycolysis, glutaminolysis is also important for producing metabolites that are necessary for biosynthesis (22). In addition to entering the TCA cycle, glutamine is also a precursor for amino acid and nucleotide biosynthesis. Furthermore, a-ketoglutarate in the TCA cycle that is derived from glutamine, or pyruvate from glycolysis, can be made into citrate to be further used in lipid synthesis.

Interestingly, mitochondria in effector CD8⁺ T cells become highly fragmented and contain loose cristae, which can lead to inefficient OxPhos (23). Consequently, activated CD8⁺ T cells rely more on glycolysis and glutaminolysis to support effector functions. However, mitochondrial activity is still important for CD8⁺ T-cell activation and effector function (7,24,25,26). For example, mitochondrial ROS is required for T-cell activation to induce signaling pathways that are important for T-cell proliferation and cytokine production (11). Additionally, mitochondria activity has been shown to be crucial for effector CD8⁺ T cell migration (9,26).

Memory CD8⁺ T cells

Upon resolution of an immune response, any surviving memory CD8⁺ T cells return to a metabolically quiescent state. Memory CD8⁺ T cells, similar to naïve CD8⁺ T cells, utilize catabolic metabolism with a strong reliance on OxPhos and FAO (16,27). When glycolysis produces pyruvate in memory CD8⁺ T cells, pyruvate is oxidized into acetyl-CoA in mitochondria and can then enter the TCA cycle to contribute to OxPhos or produce citrate for fatty acid synthesis. Free fatty acids can undergo β-oxidation to be converted into acetyl-CoA, enter the TCA cycle, and produce ATP through the ETC, which is known as FAO. Additionally, instead of storing long-chain fatty acids like effector CD8⁺ T cells, memory CD8⁺ T cells utilize extracellular glucose to eventually produce fatty acids for FAO (28).

To maximize the efficiency of OxPhos, memory CD8⁺ T cells have an abundance of elongated mitochondria with tight cristae to bring complexes of the ETC in close proximity to each other (23). Another unique characteristic of memory CD8⁺ T cells is that their mitochondrial spare respiratory capacity (SRC) is greater than that of both naïve and effector CD8⁺ T cells (29). SRC is the ability of mitochondria to increase their oxygen consumption rate above the basal oxygen consumption rate in times of increased stress (30). When a memory CD8⁺ T-cell becomes activated, metabolic reprogramming occurs once again, like when naïve CD8⁺ T cells are activated. The increased mitochondrial mass, SRC, and FAO of memory CD8⁺ T cells provides them the ability to rapidly respond upon TCR activation (31).

CD4⁺ vs. CD8⁺ T cell metabolism

The metabolic pathways employed at each differentiation state are largely overlapped in CD4⁺ and CD8⁺ T cells (32). However, CD4⁺ Tregs mainly rely on oxidative metabolism like naïve T cells, whereas both effector CD4⁺ and CD8⁺ T cells require elevated levels of glycolysis (33). In addition, polyamine metabolism has been shown to be essential in the polarization of CD4⁺ helper T cells (34). Furthermore, one unique characteristic of memory CD4⁺ T cells is that they have an increased spare glycolytic capacity in addition to an elevated mitochondrial SRC (35).

CD8⁺ T-cell exhaustion

The immunosuppressive TME comprises many elements that drive T-cell exhaustion (5,8,36). Exhaustion of CD8⁺ T cells severely impedes overall effector functions by suppressing cell proliferation and cytokine production while overexpressing inhibitory receptors (37). Chronic antigen stimulation, as seen in cancer and chronic infections, is one of the main contributors to T-cell exhaustion (38). Importantly, examination of the gene expression profiles of effector, memory, and exhausted CD8⁺ T cells revealed transcriptional downregulation of many genes involved in metabolism in exhausted CD8⁺ T cells. These include genes involved in the TCA cycle and FAO, indicating that exhausted CD8⁺ T cells have a metabolic defect (38). Furthermore, PD-1, a T-cell exhaustion marker, has been shown to suppress both glycolysis and mitochondrial respiration, in addition to mitochondrial biogenesis (39,40). Therefore, many current strategies to improve effector T-cell functions at the tumor site include reagents that boost T-cell metabolism (Table 1) (41).

Table 1. Current strategies to improve CD8⁺ T cell effector function in the TME.

Treatment	Mechanism	Outcome
Deletion or inhibition of prolyl-hydroxylase	Prevents degradation of HIF & increases glycolysis	Improved tumor clearance (42)
Constitutively active Akt/mTORC1 pathway	Increased glucose uptake and glycolysis	Improved T cell effector functions and decreased immune checkpoint expression (43)
FCCP in combination with PD-1 blockade	Increased ROS production leads to activation of mTOR and increases PGC1a expression	Increased T cell accumulation at tumor site and improved anti-tumor activity (44)
Enforced PGC1a expression	Increased mitochondrial function	Improved anti-tumor activity (5)

HIF, hypoxia inducible factor; FCCP, carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone.

CHALLENGES OF T-CELL IMMUNOTHERAPIES

Defining adoptive T-cell immunotherapies

Over the past decade, there has been tremendous progress in the advancement of adoptive T-cell therapies for the treatment of cancer, with higher success rates in hematological malignancies than solid malignancies. The promising outcomes of T-cell immunotherapy for cancer patients with late-stage or refractory disease have led to a rapid increase in immunotherapy research (45). There are 3 main types of adoptive T-cell immunotherapy (46). The first type of adoptive T-cell therapy requires the isolation of tumor-infiltrated lymphocytes (TILs) from a patient’s tumor. TILs that display antitumor activity are selected for and expanded with cytokine-supplemented medium before being infused back into the patient (47). Utilizing tumor-resident TILs for adoptive T-cell therapy showed initial success in the treatment of metastatic melanoma, with an even more pronounced effect in patients who also underwent lymphodepletion and received high doses of IL-2 (48). The other 2 forms of adoptive T-cell immunotherapy require the genetic modification of a cancer patient’s peripheral blood T cells to express tumor-reactive receptors (46). Circulating T cells are isolated by leukapheresis, activated with anti-CD3 and anti-CD28 antibodies, and then transduced with a viral vector to either express a tumor-specific TCR or chimeric antigen receptor (CAR). The genetically modified T cells are further expanded and cryopreserved to be administered at a later date (49). TCR-transduced T cells express the TCR a and b chains, which can recognize tumor-associated antigens in a MHC-dependent manner (50). Unlike TCR T cells, CAR-T cells recognize antigens independent of MHC. The CAR is composed of an extracellular single-chain Fv domain of a monoclonal antibody that can recognize the target antigen and an intracellular CD3z chain that acts as the TCR signaling domain. The CAR also contains a costimulatory domain, either CD28 or 4-1BB, that leads to downstream signaling cascades, stimulating the T-cell to kill the target cell (51,52). Since the first generation of CARs, there have been many improvements to the composition of the CAR to elicit stronger T-cell cytotoxicity, as well as better T-cell survival and proliferation (53).

While CAR-T-cell therapies have showed tremendous success in the treatment of hematological malignancies, there are many challenges that need to be overcome to improve the success of adoptive T-cell therapies for the treatment of solid tumors (54,55). These include improved T-cell trafficking to the tumor, persistent or increased memory T-cell formation, decreased T-cell exhaustion, and surpassed inhibitory signals in the immunosuppressive TME (Fig. 2) (1). The TME of solid cancers contains many factors that inhibit the function of TILs, such as immune checkpoint proteins, immunosuppressive cell types, accumulated immunosuppressive metabolites, and low levels of essential nutrients (6,56,57,58).

Figure 2. Challenges of adoptive T cell therapies. There are numerous obstacles to overcome to improve the efficacy of adoptive T cell therapies, some of which are depicted here. Created with BioRender.com.

T-cell trafficking

The low success rates of adoptive T-cell immunotherapies for the treatment of solid tumors can be in part attributed to impaired T-cell trafficking to the tumor (59). Patients with a higher number of TILs have a much better prognosis than patients with few infiltrated T cells (60,61). After infusion of TCR-modified or CAR-T cells, a large percentage of the transferred cells home to other nontumor-bearing organs, which may result in severe side effects through interactions of the TCR or CAR with cognate antigen expressed on healthy tissue (62).

The TME of solid tumors can prevent the infiltration of T cells into the tumor bed in numerous ways. Many tumor and stromal cells release cytokines that recruit immunosuppressive cells while also decreasing proinflammatory chemokines and cytokines (63). Tumor cells can also block T-cell extravasation into a tumor by downregulating the expression of intercellular adhesion molecule-1 (ICAM-1) while upregulating Fas ligand and inhibitory receptors on surrounding tumor endothelial cells (64). The interaction between ICAM-1 on the endothelium and LFA-1 on the surface of activated T cells is one of the main mechanisms of T-cell extravasation out of a blood vessel and into a solid tumor or the surrounding inflamed tissue (65). Consequently, when cancer cells decrease ICAM-1 expression on the endothelium and decrease proinflammatory chemokine release, it is difficult for tumor-specific CD8⁺ T cells to locate and infiltrate a tumor (64).

The physical barrier created by the tumor vasculature, stromal cells, and extracellular matrix also makes it extremely challenging for tumor-reactive CD8⁺ T cells to enter the tumor bed (66). For example, metastatic breast cancer tumors are highly fibrotic and prevent the infiltration of effector CD8⁺ T cells (67). Furthermore, any CD8⁺ T cells that enter the tumor site face even more challenges. For instance, actively migrating T cells rely on ATP and localize mitochondria to uropods to fuel cytoskeletal rearrangements; however, ATP production is significantly suppressed in the nutrient-depleted TME (9,68,69,70). Therefore, any T cells that can traffic to the tumor still have many barriers to overcome and may not be able to produce sufficient ATP to sustain cell migration within the tumor.

T-cell persistence

Patients with long-term persistence of adoptively transferred T cells are more likely to have a better prognosis (71,72). During the expansion of CAR- or TCR-modified T cells, many of these T cells become terminally differentiated effector T cells, which has a negative impact on T-cell persistence after infusion (73). Therefore, the generation of memory-like T cells to improve persistence after adoptive transfer may be very promising for future therapeutic approaches (27,74). Interestingly, activated CD8⁺ T cells with a low Dy_m have a memory-like T-cell phenotype, increased antitumor function, and increased in vivo self-renewal (10). Overall, the unique metabolic aspects of memory CD8⁺ T cells include decreased glycolysis, a low Dy_m, increased reliance on FAO, and an enhanced mitochondrial SRC (27,31,74). Currently, however, there are no clinically approved techniques to selectively manipulate the metabolism of genetically modified T cells that do not also influence other cell types.

Other potential ways to improve the persistence of adoptively transferred T cells include inhibiting Akt or glycolysis and promoting FAO (27,74,75). Inhibition of Akt was shown to result in an increase in FAO, mitochondrial SRC, and memory-like T cells with enhanced persistence (75). Reducing glycolytic metabolism led to an increase in the formation of memory CD8⁺ T cells with improved antitumor function (74). Additionally, using 4-1BB as the costimulation signal in CAR-T cells strengthened their preference for mitochondrial metabolism, promoting a memory T-cell phenotype and boosting T-cell persistence (76,77). Further strategies that have been identified to improve T-cell persistence in the setting of an infection or cancer are listed in Table 2 (41). Overall, there are numerous studies that suggest manipulating the metabolism of genetically modified T cells is a promising approach to improve the persistence of adoptively transferred T cells.

Table 2. Current strategies to increase CD8⁺ T cell persistence.

Treatment	Mechanism	Outcome
Rapamycin	mTORC1 inhibitor	Increased T cell persistence and memory T cell development (27,78)
Mdivi	Increased mitochondrial fusion in vitro	Increased T cell anti-tumor activity in vivo (23)
Akt inhibitors	Decreased glycolysis & increased mitochondrial metabolism	Increased T cell anti-tumor activity in vivo (75)
JQ1	Bromodomain inhibitor	Increased T cell anti-tumor activity and persistence in vivo (79)
Metformin	Activates AMPK signaling	Promotes T cell persistence in vivo (27)
2-DG	Inhibits glycolysis	Increased T cell lifespan and memory cell induction (74)
L-arginine	Decreases glycolysis and promotes oxidative metabolism	Increased T cell persistence and anti-tumor activity in vivo (80)
IL-7/IL-15	Promotes mitochondrial metabolism and biogenesis	Promotes T cell longevity and memory T cell formation (29,76,81)
Deletion of VHL	Stabilizes HIF1a & increases glycolysis	Increased long-lived effector memory CD8+ T cells (82)
Triiodothyronine (T3)	Improves dendritic cell mediated T cell activation	Increased number of tumor infiltrating CD8+ T cells and IFNy production (83)

AMPK, AMP-activated protein kinase; 2-DG, 2-deoxyglucose; VHL, von Hippel-Lindau; HIF1a, hypoxia inducible factor 1a.

TARGETING T-CELL MITOCHONDRIAL METABOLISM TO IMPROVE CANCER IMMUNOTHERAPY

Activated CD8⁺ T cells and cancer cells utilize overlapping metabolic pathways, including OxPhos, glycolysis and glutaminolysis, which puts T cells and cancer cells in direct competition with each other for limited nutrients (6,84). The battle for glucose in the TME between proliferating TILs and cancer cells makes it difficult for T cells to sustain high levels of glycolysis to support proliferation and biosynthesis (85,86). Additionally, most solid tumors are hypoxic, preventing the use of OxPhos to produce ATP in TILs (36). The harsh, nutrient-depleted conditions of the TME prevent adequate metabolism in TILs, consequently decreasing effector functions and increasing T-cell exhaustion. Furthermore, the metabolic status of immune cells, including CD8⁺ T cells in the TME are likely to be different between cancer types in separate tissues, and even between patients with the same type of cancer due to diverse temporal and spatial coordination of cancer metabolism and extracellular metabolite levels within the tumor (87,88,89,90). Altogether, there is increasing interest in developing a technique to specifically boost TIL metabolism in the TME without benefitting cancer cells (91). Enhancing mitochondrial mass and overall mitochondrial health, thus improving mitochondrial respiration, has the capacity to overcome many challenges that TILs face, as well as advancing the outcome of adoptive T-cell therapies in solid malignancies (Fig. 3).

Figure 3. The role of mitochondria in CAR-T cell function. (A) CAR-T cells cultured in nutrient rich media, such as during CAR-T cell production, have healthy mitochondria that can produce ATP and metabolic intermediates that are important for T cell proliferation. Additionally, functional mitochondria can support cytokine production and effector functions. (B) CAR-T cells that infiltrate a nutrient deprived TME show decreased mitophagy and mitochondrial biogenesis, resulting in the accumulation of dysfunctional mitochondria. Reduced mitochondrial function contributes to T cell exhaustion, poor effector functions, and decreased persistence. Created with BioRender.com.

Mitochondrial health of TILs

The immunosuppressive TME causes a decrease in overall mitochondrial health in TILs, leading to a reduction in OxPhos and increased T-cell exhaustion (5). Recent studies have shown that the hypoxic TME, in addition to chronic antigen stimulation, inhibits mitochondrial ATP production as well as T-cell proliferation and effector functions, thus reducing therapy efficacy (7,36,92,93). Depolarized mitochondria accumulate in TILs due to decreased mitophagy, further contributing to T-cell exhaustion (8). T-cell exhaustion induced by dysfunctional mitochondria can be reversed by decreasing ROS levels, boosting mitophagy, or enforcing mitochondrial biogenesis in TILs (5,7,8,36). Additionally, there is evidence that chronic lymphocytic leukemia patients who receive CD19 CAR-T cells with increased mitochondrial mass are more likely to have a complete response (94). Therefore, developing a strategy to enhance mitochondrial health in adoptive T-cell therapies has tremendous promise.

PD-1 and dysfunctional T-cell mitochondria

Blocking PD-1 signaling with antibodies has shown success in treating a subset of patients with solid malignancies; however, anti-PD-1 treatment does not work well when T cells are terminally exhausted (95,96). Tumor-reactive TILs that are PD-1 positive are typically exhausted and have low antitumor function (97). Previous studies have found that TILs with high PD-1 expression accumulate dysfunctional mitochondria and have a low Dy_m (8,40). Many factors in the immunosuppressive TME, including PD-1 signaling, can cause a decrease in TIL metabolism, eliciting mitochondrial dysfunction. Blocking only the PD-1 pathway could partially remove the metabolic restrictions of exhausted T cells in a viral infection model, but this was not the case for intratumoral T cells (5,39). Studies that have combined anti-PD-1 therapy with techniques to increase mitochondrial biogenesis or overall mitochondrial quality have shown a synergistic effect on antitumor activity (44,77).

Mitochondrial biogenesis and mitophagy

Another way to improve the mitochondrial metabolism of adoptively transferred T cells is by boosting mitochondrial biogenesis and mitophagy (5,8,94). Treatment of CD8⁺ T cells with NAD precursors was shown to improve mitophagy and decrease mitochondrial dysfunction, while in vivo treatment had even more pronounced effects with increased TIL antitumor activity (8,98). Additionally, enforced expression of PPAR-g coactivator 1a (PGC1a), which controls mitochondrial biogenesis, restored mitochondrial function and effector functions in TILs (5,99). Finally, treatment of activated CD8⁺ T cells with 4-1BB agonist antibodies increased mitochondrial fusion and biogenesis, improving the outcome of adoptive T-cell therapy due to increased PGC1a expression (77).

CONCLUDING REMARKS

Cancer immunotherapies continue to evolve and improve; however, extensive advancements are necessary to improve treatment outcomes for a large majority of cancer patients with solid malignancies. Over the last few years, many studies have suggested that targeting CD8⁺ T-cell metabolism could be an ideal approach to strengthen immunotherapy efficacy. Increasing evidence has shown the importance of functional mitochondria in various avenues of T-cell activity, including their activation, proliferation, and differentiation into memory T cells. However, it is difficult to specifically target T-cell metabolism without simultaneously benefiting cancer cells or inducing systemic side effects. Current strategies utilize compounds that can affect many cell types or may need additional genetic manipulation to selectively manipulate T-cell metabolism. Therefore, novel techniques need to be further developed and tested in the clinic to control CD8⁺ T-cell metabolism and to improve the outcomes of adoptive T-cell therapies.

Abbreviations

CAR

chimeric antigen receptor

CoA

coenzyme A

Dy_m

mitochondrial membrane potential

ETC

electron transport chain

FAO

fatty acid oxidation

ICAM-1

intercellular adhesion molecule-1

OxPhos

oxidative phosphorylation

PGC1a

PPAR-g coactivator 1a

SRC

spare respiratory capacity

TCA

tricarboxylic acid

TIL

tumor-infiltrated lymphocyte

TME

tumor microenvironment