The Role of Immune Checkpoints in Cancer Progression

Abstract

Immune checkpoint proteins are like two-faced swords that first act as gatekeepers of the immune system to protect the host from tissue damage. In contrast, these proteins can corroborate cancer progression by inhibiting tumor-specific immune responses. Here, we summarized the regulation and signaling cascade of immune checkpoints molecules (PD-1/PD-L1, CTLA-4, TIM3, TIGIT, LAG3, and BTLA), including their role in providing co-inhibitory signals for regulating T-cell response. The involvement of immune checkpoint molecules to drive cancer growth is elaborated with explanations about various anticancer strategies, such as (1) the overexpression of immune checkpoints in cancer cells, immune cells, or the surrounding environment leading to incapabilities of the tumor-specific immune response, (2) immune checkpoints interference to metabolic pathways then deplete nutrients needed by immune cells, (3) the interaction between immune checkpoints and regulatory T cells. Lastly, future challenges of immune checkpoint inhibitors are discussed briefly to get insight into their applicability in the clinical setting.

1. Introduction

Cancer or tumor cells express neoantigens that the immune system can identify from healthy neighboring cells due to genetic mutations. These changes typically result in a tumor-reactive T cell response, most notably CD8⁺ T cells. However, this mechanism is frequently ineffective at eradicating cancer cells [1]. One cause for this failure is the suppression of invading T cells by a wide range of immunosuppressive mechanisms found in the tumor microenvironment (TME), such as regulatory T cells (Tregs) or immunosuppressive cytokines [2, 3].

Furthermore, binding of the T cell receptor (TCR) to the antigenic peptide bound to the major histocompatibility complex (MHC) of the antigen-presenting cell (APC) is not adequate to yield an immune response, particularly to eradicate cancer cells. Thus, the additional stimulatory co-signal produced by co-receptors is required. These co-receptors play an essential role in modulating T cell responsiveness and balancing co-stimulatory and inhibitory (i.e., immune checkpoint) signals [4]. Extended TCR signals generated from T cells exposure to their cognate antigen result in enhanced and persistent expression of inhibitory co-receptors like cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed death protein 1 (PD-1), or many other immune checkpoints. At this moment, T cells enter a state of dysfunction or exhaustion, allowing cancer cells to grow unchecked [5, 6]. Therefore, blockage to these immune checkpoints can reinvigorate the anti-tumor function of immune cells. This chapter aimed to elaborate on the involvement of immune checkpoints in cancer development. It includes the explanation of the normal trafficking and inhibitory signaling of each checkpoint, followed by discussions about how immune checkpoint contributes to cancer growth.

2. Regulation and signaling of immune checkpoints

Immune checkpoints serve as the immune system’s gatekeepers and are required for sustaining self-tolerance, thus protecting the host from tissue damage. These immunological checkpoint molecules have modulated T cell responses to self-proteins, persistent infections, and tumor antigens. A few of them, including but are not limited to PD-1, CTLA-4, Lymphocyte activation gene 3 (LAG3; or known as cluster of differentiation 223 [CD223]), T cell immunoglobulin and mucin-domain containing-3 (TIM-3), T cell immunoreceptor with immunoglobulin and ITIM—immunoreceptor tyrosine-based inhibitory motif—domain (TIGIT), and B and T lymphocyte attenuator (BTLA; or known as CD272), have been discovered and investigated as targets in cancer immunotherapy. In general, immune checkpoints are membrane proteins expressed in the endoplasmic reticulum (ER) and subsequently transported to the cell surface to perform their inhibitory roles, which requires the protein sorting system to transport them sequentially through the Golgi apparatus secretory vesicles. Glycosylation acts as quality control during surface delivery, ensuring that only mature and functional immunological checkpoints reach the cell surface. Immune checkpoints are internalized and recycled when they reach the cell surface, providing a quick regulatory pathway to control their surface levels. Immune checkpoints can be ubiquitinated and sorted to the proteasome or lysosome for destruction, another critical method for controlling protein levels. The surface level of immunological checkpoints is determined by several biological mechanisms, which affect cell signaling [7]. This section elaborates on the normal regulations and signaling of each immune checkpoints molecules before discussing its involvement in cancer development.

2.1 PD-1/PD-L1 regulations and signaling

PD-1 trafficking in the membrane is regulated by the core fucosyltransferase 8 (fut8) in ER. Upon Tcell activation, PD-1 is internalized, then ubiquitinated by F-box protein 38 (FBXO38) for proteasome degradation or recycled back to the surface with the help of thymocyte selection-associated high mobility group box protein (Tox), thus prolonged PD-1 activity. Additionally, Tox expression induces Tcell depletion in hepatocellular cancer [7, 8]. Besides, another extension of PD-1 activity is caused by FBXO38’s low transcriptional level in the TME. Hence, the FBXO38-mediated PD-1 degradation pathway is defective. TCR signaling was the source of FBXO38 downregulation in the absence of concurrent CD28-CD80/86 signaling. CD28-CD80/86 binding provides critical signals for T cell activation in the presence of TCR stimulation. Persistent tumor antigen binding and low CD80/86 expression on cancerous cells might explain the lower FBXO38 expression in tumor-infiltrating lymphocytes (TILs) [7, 9].

Similar to PD-1, its first functionally identified ligand of PD-L1 (also known as B7 homolog 1 [B7-H1] or CD274) is constantly internalized, recycled, or degraded. Regulation of PD-L1 recycling is managed by CKLF-like MARVEL transmembrane domain containing 6 (CMTM6). Meanwhile, ubiquitination and degradation are regulated by multiple proteins such as cyclin D–CDK4 and the cullin 3–SPOP [10], β-TrCP [11], COP9 signalosome 5 (CSN5] [12], Huntingtin-interacting protein 1-related (HIP1R) [13], and others. Each protein is a drugable target to inhibit PD-L1 accumulation, thereby increasing T cell-mediated cytotoxicity.

Regarding the inhibitory signals following the binding of PD1 to PD-L1 or other ligands, it blocks kinases that play a role in activating T cells through the phosphatase SHP2. Besides, since PD1 inhibition blocks the TCR ‘stop signal’, this pathway can alter the length of T cell–APC or T cell–target cell interaction [14]. In detail, PD-1 is phosphorylated through immune receptor tyrosine-based switch motif (ITSM) and ITIM. Then, PD-1 binds the Src homology 2 (SH2) domains of SH2-containing phosphatase 2 (SHP2) or SHP1, which initiate its inhibitory effect by suppressing both TCR and CD28 co-stimulatory signaling [7, 15, 16, 17]. Moreover, PD-1 signaling also reduces cytokine production (interleukin [IL]-2, interferon [IFN]- α, tumor necrosis factor [TNF]-α), cell cycle progression, and pro-survival Bcl-xL gene expression by interfering with early TCR/CD28 signaling. PD-1/PD-L1 interaction is associated with IL-2-dependent positive feedback and transcription factors involved in effector functions such as GATA-3, T-bet, and Eomes. As signal transduction can only occur during TCR-dependent signaling, PD-1 activity is thus only relevant during simultaneous T cell activation. Mice without the receptor appear healthy at first. Still, they acquire autoimmune disorders such as lupus-like proliferative glomerulonephritis and arthritis, as well as enhanced inflammation after infections at a later age. In humans, genetic variations in the PD-1 region are more likely to suffer autoimmune disorders [18, 19].

2.2 CTLA-4 regulations and signaling

Unlike PD-1/PD-L1, which is constitutively expressed on the membrane, CTLA-4 is primarily stored inside the cytoplasm of resting naïve T lymphocytes. The T cell receptor-interacting molecule (TRIM)/LAX/Rab8 complex and phospholipase D (PLD)/ADP ribosylation factor-1 (ARF1)-dependent exocytosis are required for CTLA-4 trafficking from trans Golgi network (TGN) to the cell surface [20]. Exocytosis of CTLA-4-containing vesicles causes upregulation of CTLA-4 on the cell surface due to stimulatory signals originating from TCR and CD28-B7 interaction. More robust TCR signaling causes more CTLA-4 to be translocated to the cell surface, and this process works in a graded feedback loop. CTLA-4 on the surface is rapidly internalized during normal physiologic conditions, resulting in relatively low expression. The clathrin-associated adaptor complex (AP-2) interaction to the unphosphorylated YVKM motif promotes rapid CTLA-4 internalization, which is then either destroyed in the lysosome or returned to the cell surface through LPS responsive beige-like anchor protein (LRBA). Besides, CTLA-4 in TGN may also be transported to the lysosome for destruction through AP-1 binding [7, 21, 22].

The intrinsic signaling of CTLA-4 that dampens T cell immune response has been widely contested with no agreement [23]. However, both CTLA-4 and CD28 interact with the identical ligands, CD80 (B7–1) and CD86 (B7–2). Because CTLA-4 has a 20-fold higher binding affinity than CD28, the intrinsic inhibitory signal rises once CTLA-4 outcompetes CD28, even if CTLA-4 is activated later [24, 25]. In addition to T cell response intrinsic inhibition, CTLA-4 is hypothesized to decrease extrinsic T cell signaling. For example, CTLA-4 suppresses CD80/86 expression on APCs via trans-endocytosis or by increasing tumor growth factor β (TGFβ), which in turn suppresses CD80/86 expression [26]. CTLA-4 is phosphorylated when it binds to its ligands, activating phosphoinositide 3-kinase (PI3K) pathways leading to dephosphorylation of the CD3 chain, decreasing the TCR’s signaling potential. CTLA-4 also prevents T cells from proliferating by inhibiting IL-2 transcription. Additionally, CTLA-4 stimulates the production of indoleamine 2,3-dioxygenase (IDO) in dendritic cells via CD80/86 ligation, resulting in T cell suppression [27].

2.3 TIM-3 regulations and signaling

TIM-3 is expressed on both T cells and innate immune cells. Four ligands have been identified: carcinoembryonic antigen cell adhesion molecule 1 (Ceacam1), C-type lectin galectin9 (Galectin9), high-mobility group box 1 (HMGB1), and non-protein ligand phosphatidylserine (PtdSer). Ceacam1 is a transmembrane protein that interacts in cis and trans directions. Ceacam1-TIM-3 cis binding induces TIM3 surface expression in T cells while trans binding inhibits the effector T cell causing exhaustion and maintaining T cell tolerance [28]. TIM-3 binding to both Ceacam1 and galectin-9 results in the release of Bat3, a TIM-3 signaling pathway inhibitory regulator, from its binding location on the Tim-3 cytoplasmic tail [29]. The other ligands, HMGB1, primarily modulate innate immunity like dendritic cells (DC). In DCs, HMGB1 is required for nucleic acid trafficking into endosomal vesicles, a fundamental step in sensing tumor-derived stressors or pathogen-associated molecular patterns and initiating host defenses against malignancies or pathogen infections [30].

TIM-3 is more related to co-stimulatory proteins induced in activated T cells than to a dominant inhibitory protein like PD-1; thus, TIM-3 signaling remains a matter of debate. As checkpoint proteins, TIM-3 is a repressor of IFN-γ-secreting CD4⁺ Th1 and CD8⁺ T cells. These findings confirmed that inhibiting TIM-3 might correct the defective phenotype of T cells in vivo. In contrast, TIM-3 lacks a conventional ITIM or ITSM in its intracellular domain and lacks structural features that facilitate the recruitment of inhibitory phosphatases. Rather than that, both murine and human TIM-3 cytoplasmic tails include five conserved tyrosine residues, two of which, Y₂₅₆ and Y₂₆₃ in mice (Y₂₆₅, Y₂₇₂ in humans), have been demonstrated to be crucial for coupling to downstream signaling pathways. Y₂₅₆ and Y₂₆₃ in TIM-3’s C-terminal tail interact with Bat3 in the absence of ligand-mediated TIM-3 signaling. Bat3 binds the catalytically active form of Lck in this state, resulting in the formation of an intracellular molecular complex with TIM-3 that retains and maybe enhances T cell signaling while repressing TIM-3-mediated cell death and exhaustion [31, 32]. TIM-3 activation on exhausted effector T cells is closely attributed to PD-1 expression, confirming the functional relationship between TIM-3 and PD-1 throughout the development of T cell exhaustion [33]. Concomitant therapy of anti-TIM-3 and anti-PD-1 is significantly more successful in these models, resulting in more significant tumor regression than either TIM-3 or PD-1 inhibition alone. TIM-3 inhibition in the setting of adaptive resistance to PD-1 treatment may be a useful way to treat individuals who develop resistance to anti-PD-1 therapy. This therapy regimen may be particularly beneficial for malignancies with resistance and immune escape from PD-1 inhibition [34, 35].

2.4 TIGIT regulations and signaling

Like CTLA-4 and CD28, TIGIT and CD226 can interact with identical ligands, CD112 and CD155. TIGIT is a co-inhibitory receptor, while CD226 is a co-stimulatory receptor. Nevertheless, TIGIT possesses a higher affinity to its ligands than CD226; thus, TIGIT can inhibit co-stimulation signals by outcompeting CD226 ligands binding. TIGIT can bind directly to CD226 in cis, disrupting its homodimer formation and co-stimulatory activity [36, 37].

TIGIT’s signaling is mostly studied in natural killer (NK) cells and activated CD4 and CD8 T cells. The cytoplasmic region of TIGIT comprises an ITIM motif and an immunoglobulin tail tyrosine (ITT)-like motif. Several studies demonstrate that tyrosine (Tyr₂₂₅) phosphorylation in either the ITIM or ITT-like motif is required for TIGIT’s inhibitory action in human NK cells. According to Liu et al. (2013), the ITT-like motif recruits Src homology domain-containing inositol phosphatases (SHIP1) via cytosolic adaptor proteins Grb2. Recruited SHIP1 then suppresses phosphatidyl-inositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signal to abolish NK cell function. Furthermore, TIGIT signaling can modulate the IFN-γ production of NK cells via the NF-κB pathway. In this context, β-arrestin 2, another TIGIT adaptor, is involved in phosphorylating TIGIT and then inhibits TNF receptor-associated factor 6 (TRAF6) autoubiquitination, hence inhibiting NF-κB activation and suppressing IFN-γ production [38, 39, 40].

2.5 LAG3 regulations and signaling

LAG3 inhibits CD4-dependent T cell activity by binding to MHC-II due to being structurally homologous with four extracellular immunoglobulin superfamily-like domains. Other investigations demonstrated that LAG3’s inhibitory activity is not dependent on CD4 competition, but rather LAG-3 inhibited T cells responding to stable peptide-MHC-II by transducing inhibitory signals via its intracellular domain. Thus, LAG-3 may act more selectively, allowing tolerance to dominant autoantigens to persist [41, 42]. Alternatively, LAG3 can interact with another ligand like Galectin3 in TME and mediate the suppression of CD8 T cells [43]. Besides, liver sinusoidal endothelial cell lectin (LSECtin) can bind to LAG3 in human melanoma, causing tumor growth by abolishing IFN-γ production and proliferation of tumor-specific T cells [44]. Lastly, fibrinogen-like protein 1 (FGL1) was recently discovered as a novel LAG3 ligand. FGL1 is typically produced in trace amounts into the bloodstream by the liver. However, overexpression of FGL1 has been observed in some human malignancies. Inhibiting the interaction between FGL1 and LAG3 by monoclonal antibodies improves T cells’ anticancer activity [45].

The signal transduction mechanism of LAG3 is regulated by two transmembranes, a disintegrin and metalloproteinase domain-containing protein 10 and 17 (ADAM10 and ADAM17)-mediated cleavage. TCR signaling enhances ADAM10 and ADAM17 cleavage activity, releasing sLAG3. The function of sLAG3 remains controversial as some studies consider this does not have a biological process, while the others state that sLAG3 allows effective T cell proliferation and function [46]. Besides, sLAG3 affects monocyte differentiation into macrophages and DCs, which have decreased immunostimulatory capacity [47].

2.6 BTLA regulations and signaling

BTLA and CD160 inhibit T cell activity via the same ligand, herpesvirus entry mediator (HVEM). BTLA-HVEM is an example of crosstalk between two superfamilies in which the ligand is a member of the TNF/TNFR superfamily. However, HVEM interaction with members of the TNF superfamily LIGHT (Lymphotoxins, Inducible, competes with herpes simplex virus (HSV) Glycoprotein D for HVEM, expressed by T cells) produces a co-stimulatory signal on B and T cells. Hence, HVEM may be considered as a molecular switch that enables co-signaling between stimulatory and inhibitory T cells. Additionally, signaling between HVEM and its ligands appears to interact bidirectionally. The cis interaction between BTLA and HVEM inhibits the trans-ligation of HVEM by LIGHT and thus inhibits HVEM stimulatory signaling triggered by LIGHT binding [7, 48, 49].

Regarding the inhibitory signaling of BTLA, it follows the mechanism of PD-1/PD-L1 involving ITIM and ITSM to recruit SHP1/SHP2 [50]. In B-chronic lymphocytic leukemia (B-CLL), both HVEM and BTLA are overexpressed. This co-expression of HVEM and BTLA in CLL cells implies that an unsuccessful autocrine inhibitory loop is triggered. In addition, BTLA is typically downregulated during the development of human CD8⁺ T cells to effector cells. However, BTLA expression was more significant in melanoma-specific CD8⁺ T lymphocytes specialized for tumor antigens (TA). Despite effector differentiation, BTLA expression remained persistent, confounding T cell proliferation and IFN-y production. Thus, BTLA may function similarly to PD1 as a T-cell inhibitory receptor in TME [51].

3. Immune checkpoints dysregulation affecting cancer cells

Recent studies have established that immune checkpoint molecules drive cancer growth via various anticancer strategies. The first one is the overexpression of immune checkpoints in cancer cells, immune cells, or the surrounding environment leading to incapabilities of the tumor-specific immune response. Subsequently, immune checkpoints can interfere with metabolic pathways and deplete nutrients needed by immune cells. Lastly, immune checkpoints cripple cancer-specific immune responses by collaborating with regulatory T cells. This section deliberates each strategy thoroughly to get insight into how to combat those actions.

3.1 Overexpression of immune checkpoints favoring tumor growth

Accumulating evidence showed that several immune checkpoint molecules are overexpressed not only on the surface of cancer cells but also in T cells, Tregs, or even in TME. Here, we thoroughly describe how the immune checkpoint is upregulated and then inhibits antitumor activity. PD-1/PD-L1 are overexpressed on the surface of many cancer cells. Several proinflammatory mediators, which are secreted by activated T cells (types I and II IFN-γ, TNF-α, IL-10, and IL-4) or produced in TME (GM-CSF and VEGF), upregulate PD-L1 expression in the cancer cells resulting in suppression of PD1⁺ T cells activity. Moreover, cancer cells commonly carry altered PTEN (phosphatase and tension homolog deleted on chromosome ten)—PD-L1 suppressor gene—which may activate the S6K1 gene, resulting in a significant increase in PD-L1 mRNA to polysomes, which promotes PD-L1 mRNA translation and plasma membrane expression [6]. In pancreatic cancer cells, PTEN gene deletion influences PD-L1 expression at the translational level by activating the PI3K/AKT downstream mTOR-S6K1 signaling pathway, thereby increasing PD-L1 production and T lymphocyte apoptosis [52, 53].

Furthermore, amplification and translocation of CD274 on chromosome 9p24.1 have been associated with elevated expression of PD-L1 in Hodgkin’s lymphoma, small cell lung cancer (SCLC), non-small cell lung cancer (NSCLC), lymphoma, Epstein-Barr virus (EBV)-positive gastric cancer, and oral squamous cell carcinoma (OSCC). In SCLC, chromosomal rearrangements produce CD274 amplification without changing the open reading frame. It is found in various organs, but it is most commonly found in activated T and B lymphocyte cells, dendritic cells, monocytes, and other types of TCs. The CD274/PD-L1 gene is highly conserved, with homologs discovered across the vertebrate lineage (from Danio rerio to Primates), implying its wide range of functions. The CD274/PD-L1 promoter retains CpG methylation sites in the 5′ untranslated region (UTR) and exon 1, but translation begins in exon 2 [54, 55]. JAK2, which encodes Janus kinase 2, an upstream kinase that controls PD-L1 expression, is also present on chromosome 9p, with a high alternation rate. The JAK family has been shown to contribute to PD-L1 upregulation by raising PD-L1 RNA expression through amplification and mutation. Because of the increased activity of the Janus kinase2 (JAK2) signal transducers and activators of transcription (STAT) signaling pathway, PD-L1 expression rises. DNA double-strand breaks (DSBs) consistently activate STAT signaling via the ataxia-telangiectasia mutant (ATM)/ATM- and ataxia-telangiectasia-related (ATR)/checkpoint kinase 1 (Chk1) kinases, leading to PD-L1 expression increase. Moreover, structural changes in CD274’s 30 UTR boost protein production and improve cancer-immune evasion in human malignancies [55].

PD-L1 induction has also been associated with inflammatory stimuli such as IL-1b, IL-4, IL-6, IL-10, IL-12, IL-17, IL-27, tumor necrosis factor-α (TNF-α), and transforming growth factor-β (TGF-β). Among the several soluble inflammatory agents, IFN-ϒ is the most important in promoting PD-L1 expression. IFN-ϒ is a proinflammatory cytokine primarily generated by T and NK cells. IFN-ϒ attaches to its receptor, the interferon-gamma receptor (IFNGR), activating the JAK-STAT signaling pathway via STAT1. As a result, it increases the expression of transcription factors, particularly interferon-responsive factors (IRFs). IRF1 is a critical downstream signaling molecule of STAT1 that causes IFN-induced PD-L1. Other proinflammatory agents, IL-4 and TNF-α, have a synergistic impact on the activation of PD-L1 in renal cell carcinoma (RCC) via activating signaling molecules such as NF-κB, IκB, and STAT6. In dendritic cells and monocytes, blocking PD-L1 was associated with decreased IL-10 levels. Furthermore, IL-10 levels on Tyro3, Axl, and Mer (TAM) were closely connected to PD-L1 expression. In monocyte-derived macrophages, IL-12 upregulates PD-L1 expression, but in THP-1-derived macrophages, it downregulates PD-L1 expression. In monocytes, IL-17 is involved in the induction of PD-L1. IL-17 and TNF-α activate NF-κB signaling in prostate cancer and NF-κB and ERK1/2 in colon cancer, respectively, and upregulate PD-L1 expression. PD-L1 expression in dendritic cells is upregulated by IL-1b and IL-27. Furthermore, IL-27 activates phospho-STAT1 and phospho-STAT3 to enhance PD-L1 expression [54, 56].

Meanwhile, CTLA-4 is often constitutively overexpressed on Tregs and has been demonstrated to alter Tregs-mediated immune control. In multiple myeloma patients, FOXP3 and CTLA-4 genes from bone marrow samples were considerably overexpressed [57]. Another sample from peripheral blood mononuclear cells (PBMC) of breast cancer patients showed significantly higher mRNA expression of FOXP3 and CTLA-4 than healthy individuals [58]. Taken together, these results indicated the pivotal role of CTLA-4 in the accumulation of immunosuppressive Tregs in TME, leading to repression of anti-tumor immunity.

Regarding TIM-3 overexpression, it is induced by cytokine stimulation, especially in NK cells. TIM-3 is also extensively expressed on tumor-infiltrating lymphocytes. Similar to its expression pattern during persistent viral infection, TIM-3 is generally co-expressed with PD-1 and represents the most dysfunctional T cell subgroup. TIM-3 overexpression in human malignancies, particularly on immune cells, might be a predictive biomarker for a range of cancers. TIM-3 expression on CD4+ and CD8+ T lymphocytes was enhanced in individuals with hepatitis B virus-related hepatocellular carcinoma (HCC). TIM-3⁺T cells were replicative senescent and exhibited senescence-related surface and genomic markers. Furthermore, the quantity of tumor-infiltrating cells in TIM-3⁺ was inversely linked with HCC patient survival [59].

Furthermore, LAG3 is mainly expressed in activated T and natural killer (NK) cells, and it has been identified as a marker for CD4+ and CD8+ T cell activation. Increased LAG3 expression on T cells was observed in combination with other inhibitory receptors such as PD-1, TIGIT, TIM-3, CD160, and 2B4 under pathological conditions such as chronic inflammation or in TME, resulting in T cell exhaustion and reduced cytokine release. In melanoma and colon cancer, LAG3 expression was identified in tissue-infiltrating lymphocytes and peripheral Tregs, tumor-involved lymph nodes, and inside the tumor tissue itself. LAG3 was found on tumor-infiltrating Tregs in patients with head and neck squamous cell carcinoma and non-small cell lung cancer [60].

Similar to other checkpoint molecules, TIGIT is also significantly expressed on Tregs taken from PBMC of cancer patients, and it is further elevated in the TME. Increased TIGIT expression in Tregs is coupled with hypomethylation and FOXP3 binding at the TIGIT gene, distinguishing Tregs from activated effector CD4⁺ T cells. Furthermore, the Fap2 protein from Fusobacterium nucleatum, an anaerobic Gram commensal bacteria linked to colorectal cancer, binds directly to TIGIT but not CD226 to suppress NK cells and T cell-mediated tumor response. These findings imply that the gut microbiota modulates innate immune responses via TIGIT [61].

3.2 Immune checkpoints mediating metabolic reprogramming in TME

Due to cancer cells’ resource intake and vascularization defects, TME is typically deficient in nutrients and oxygen. Cancer cells’ increased need for glucose promotes competition in the TME, which has a detrimental effect on surrounding cells, such as immune cells. Immune checkpoint proteins have been shown to modulate the metabolic energetics of tumor cells, TME, and the tumor-specific immune response, resulting in metabolic reprogramming of both cancerous and immune cells. For instance, CD80 (B7–1) activated the mTOR kinase in naïve CD8⁺ T cells via the PI3K and STAT4 pathways in solid tumors. mTOR signaling is required to promote glycolysis via hypoxia-inducible factor-1α (HIF-1α) and protein synthesis for supporting cancer cell growth. This activation shifts nutrition balance, and cancerous cells outcompete the immune cells, then evading immune surveillance [62, 63].

Because amino acids are the building blocks of proteins, their availability is critical for tumor development. At the same time, immune cells need amino acids to differentiate and perform their effector activities, hence regulating tumor formation. Given this, a greater knowledge of how each cell species use amino acids in the TME looks critical for successfully stimulating anti-tumor immunity. Tryptophan deficiency impairs CD8⁺ T cell functions and enhances CD4⁺ Tregs cell functions, resulting in immunosuppression mediated by the CTLA-4 and PD-1/PD-L1 pathways. The effects are achieved mechanistically by activating the stress response kinase GCN2, which inhibits mTORC2 and its downstream target AKT [64, 65]. The other amino acids, such as glutamine and arginine, are also extensively consumed by the tumors and directly impoverish T cells, leading to the development of immunosuppressive TME [66]. Additionally, tumors may produce and accumulate toxic compounds like aerobic glycolysis byproduct (lactate) in TME, leading to local acidification. Lactate acidosis and hypoxia can activate HIF-1α and then upregulate PD-L1, further inhibiting T-cell responses specific to tumors. Besides, an acidic condition in the surrounding tumors environment suppresses cytokine production (IFN-γ) and limits the activity of T cell cytotoxic, NK cells, and dendritic cells [66, 67].

In contrast to the effector T cells, glucose deprivation may exert a negligible effect on intratumoral Tregs and lactic acid found in the TME may offer nourishment, thus supporting the immunosuppressive function of Tregs [68]. In addition, Tregs differentiation and recruitment is also supported by kynurenine, a metabolite produced from tryptophan through indoleamine 2,3 dioxygenase (IDO)-catabolization in TME [69]. Furthermore, hypoxia and fatty acids production may facilitate Tregs accumulation, thereby favoring its suppressive function [70].

3.3 Interaction between immune checkpoints and Treg cells

Another immune checkpoint favoring cancer growth strategy is its interaction with Tregs cells either by the expression on Tregs surface or inducing Tregs population and function. Treg cells function in the immune system to regulate and suppress other effector T cells. These cells are responsible for the homeostatic process of the immune system to maintain its unresponsiveness to self-antigens and protect the body from autoimmune reactions or excessive inflammation [71]. However, in this context, the interaction of two immunosuppressive mechanistics is critical in cancer survival from immunosurveillance and progression.

Almost all of the immune checkpoint molecules discussed in this chapter, except BTLA, are expressed in Tregs [72]. CTLA-4 is expressed constitutively on Tregs and induced on effector T cells when activated. CTLA-4 deficiency in Tregs was shown to affect their suppressive effects in animal models. Upon TCR stimulation, CTLA-4 is constitutively recruited on the Tregs cell surface, allowing continuous transendocytosis signaling. Hence, Tregs (CD4⁺ Foxp3⁺) can outperformed activated conventional T cells (CD4⁺ Foxp3⁻) [73]. Subsequently, downregulation of B7 ligands on APCs leading to diminished CD28 co-stimulation is another way by which Tregs are hypothesized to govern effector T cells [74, 75].

In tumor tissue of non-small cell lung cancer (NSCLC) patients, the PD-L1 expressing CD25⁺ CD4⁺ (PD-L1^hiTregs) population is higher than in blood or normal tissue. Interestingly, PD-L1^hiTregs also correlated with PD-1⁺ CD8 [76]. In another cancer, highly expressed PD-L1 glioblastoma cells can induce Tregs expansion and maintain its immunosuppressives through PD-1/PD-L1 stimulation. Disrupting the PD-L1/PD-1 axis could target two immunosuppressive mechanisms: inhibition of signaling due to PD-1/PD-L1 ligation and stimulatory proliferation of Tregs cells, which indirectly promotes immunoresistance of high PD-L1 cancers. Thus, Tregs abundance may be a predictive biomarker for patients likely to react to anti-PD-1/anti-PD-L1 therapy or monitor treatment response [77].

Multiple immune checkpoints protein can coexpress and accumulate on the T cell surface, thus increasing dysfunctionality. On CD8⁺ TILs, it is found that TIGIT is coexpressed with TIM-3, PD-1, and LAG-3 [78]. Although, further investigation is needed to show whether these pathways synergize and whether coblockade is becoming a more efficient immunotherapeutic approach.

4. Future challenges and applicability of immune checkpoint inhibitors

Immunological tolerance is normally maintained so that the immune system can recognize and distinguish between self and non-self antigens or neoantigens. Although the immune system is expected to protect the host from exposure to non-self antigens, its robust effector mechanism allows to reverse the attack and disrupt the homeostasis of the immune system. Immune checkpoints, which have gained notoriety as possible cancer therapy targets, are essential immunoregulatory processes found throughout the body. Dysregulation of immune checkpoints promotes tumor cell evasion and plays a significant role in cancer pathogenesis. Therefore, several monoclonal antibodies have been made to block the interaction between ligand and receptor of immune checkpoints, enhancing host immunologic competence against tumors. The list of immune checkpoints inhibitors (ICI), which gained Food and Drug Administration (FDA) approval or are in ongoing clinical trials, is comprehensively summarized in [79]. However, only a tiny proportion of patients respond meaningfully to these therapies due to the signaling complexity and overlapped pathways as mentioned above. Thus, new routes and compounds are being investigated to enhance therapeutic responsiveness and applicability. In clinical practice, the difficulties in treating cancer patients revolve on eliminating the tumor and alleviating symptoms such as pain, fatigue, nausea/vomiting, cough, and diarrhea. Then, concomitant use of medications is negligible and generates new threats for drug interaction such as analgesics [80], steroids [81], antibiotics [82], or many others. Moreover, the use of ICI is often associated with immune-related adverse effects (irAEs). A retrospective study reported that among 1091 patients receiving ICI therapy, 487 (44.63%) patients experienced adverse effects. The most common is fatigue (13.9%), then dermatologic irAEs (12%), endocrine-related irAEs (9.89%), gastrointestinal toxicities (8.4%) and hepatotoxicities (4.94%) [83].

Acknowledgments

The authors thank our institutions Akademi Farmasi Surabaya and the University of Surabaya, for the support.

Conflict of interest

The authors declare no conflict of interest.