Immune checkpoint inhibitors: breakthroughs in cancer treatment

Abstract

Over the past two decades, immunotherapies have increasingly been considered as first-line treatments for most cancers. One such treatment is immune checkpoint blockade (ICB), which has demonstrated promising results against various solid tumors in clinical trials. Monoclonal antibodies (mAbs) are currently available as immune checkpoint inhibitors (ICIs). These ICIs target specific immune checkpoints, including cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) and programmed cell death protein 1 (PD-1). Clinical trial results strongly support the feasibility of this immunotherapeutic approach. However, a substantial proportion of patients with cancer develop resistance or tolerance to treatment, owing to tumor immune evasion mechanisms that counteract the host immune response. Consequently, substantial research focus has been aimed at identifying additional ICIs or synergistic inhibitory receptors to enhance the effectiveness of anti-PD-1, anti-programmed cell death ligand 1 (anti-PD-L1), and anti-CTLA-4 treatments. Recently, several immune checkpoint molecular targets have been identified, such as T cell immunoreceptor with Ig and ITIM domains (TIGIT), mucin domain containing-3 (TIM-3), lymphocyte activation gene-3 (LAG-3), V-domain immunoglobulin suppressor of T cell activation (VISTA), B and T lymphocyte attenuator (BTLA), and signal-regulatory protein α (SIRPα). Functional mAbs targeting these molecules are under development. CTLA-4, PD-1/PD-L1, and other recently discovered immune checkpoint proteins with distinct structures are at the forefront of research. This review discusses these structures, as well as clinical progress in mAbs targeting these immune checkpoint molecules and their potential applications.

Keywords: CTLA-4; ICIs; Immunotherapy; PD-1; cancer.

Figure 1

Inhibition of T cell activation by the PD-L1/PD-1 signaling pathway: (A) When PD-L1/PD-L2 binds PD-1, ITSM is phosphorylated by the Src family of kinases and recruits SHP2. SHP2 regulates the PD-1 signaling pathway by inhibiting key molecules such as ZAP70 and PLCγ1 through its phosphatase activity; it also modulates downstream signaling activity and inhibits the function of Lck and phosphorylation of its downstream molecule, ZAP70. This process directly inhibits the TCR-activated signaling pathway and decreases T cell activation. Furthermore, SHP2 inhibits the RAS/MEK/ERK signaling pathways by blocking the activity of PLCγ1. Additionally, SHP2 inhibits T cell activation by suppressing the activity of CK2, which in turn prevents the phosphorylation of PTEN and blocks downstream PI3K signaling. Furthermore, STAT3, a common transcription factor in cancer, is overactivated by IL-6 through phosphorylation, thus promoting tumor growth. These intricate regulatory mechanisms collectively impede the activation of T cells. (B) Targeting the PD-L1/PD-1 pathway through the specific binding of PD-1 mAb or PD-L1 to PD-1 or PD-L1, respectively, has become an effective cancer treatment strategy. This approach blocks the interaction between these two proteins and restores the immune function of T cells, thereby treating cancer by disrupting the PD-L1/PD-1 signaling pathway. PD-1, programmed cell death protein; PD-L1, programmed cell death ligand 1; PD-L2, programmed cell death ligand 2; ITSM, immunoreceptor tyrosine-based switch motif; SHP2, Src homology region 2-containing protein tyrosine phosphatase 2; ZAP70, CD3ζ-chain-associated protein of 70 kDa; PLCγ1, phospholipase Cγ1; CK2, casein kinase; PTEN, phosphatase and tensin homolog; Lck, lymphocyte-specific protein tyrosine kinase; TCR, T cell receptor; STAT3, signal transducer and activator of transcription 3.

Figure 2

CTLA-4 and CD28 with their ligand-binding activities: On the surfaces of T cells, CTLA-4 and CD28 are co-inhibitory and co-stimulatory receptors, respectively. CD80 and CD86 are both ligands for CTLA-4 and CD28, but CD80 has a higher affinity for both receptors. Both ligands have high affinity for CTLA-4, which sends inhibitory signals to T cells and leads to shutdown of the T cell pathway. CTLA-4, cytotoxic T-lymphocyte-associated antigen-4.

Figure 3

Functions of LAG-3 and ligands: FGL-1, Gal-3, LSECtin, and MHC II are all LAG-3 ligands. The Kieele structure of LAG-3 triggers downstream pathways and inhibits T cells. CD4, the homologue of LAG-3, competes with LAG-3 for MHC II binding. The binding of LAG-3 to the ligand MHC II downregulates CD4⁺ T cell activity and decreases cytokine secretion. Additionally, LAG-3 inhibits the activity of CD8⁺ T cells. The activity of CD8⁺ T cells is inhibited by FGL-1, Gal-3, and LSECtin. LAG-3, lymphocyte activation gene-3; MHC II, major histocompatibility complex II; FGL-1, fibrinogen-like protein-1; Gal-3, galectin-3; LSECtin, liver and lymph node sinusoidal endothelial cell C-type lectin.

Figure 4

Mechanisms of TIM-3-mediated T cell activation and suppression: (A) In the absence of the TIM-3 ligand, Bat-3 interacts with the Y256/Y263 residues located in the cytoplasmic tail of TIM-3, promoting the activity of Lck. Subsequently, this process promotes the recruitment of ZAP70 and facilitates T cell activation while suppressing the negative regulation of TIM-3. (B) After binding of TIM-3 to its ligand, phosphorylation of Y256/Y263 triggers the dissociation of Bat-3, thus enabling the binding of another Src kinase, Fyn, to TIM-3. Subsequently, inactivation of Lck and downregulation of ZAP70 function ultimately induce T cell exhaustion. TIM-3, mucin domain containing-3; Gal-9, galectin-9; CEACAM-1, carcinoembryonic antigen-related cell adhesion molecule-1; PtdSer, phosphatidylserine; HMGB-1, high-mobility group box-1; Bat-3, HLA-B-associated transcript 3; Lck, lymphocyte-specific protein tyrosine kinase.

Figure 5

TIM-3 mAbs and CD137 mAbs for treatment in a mouse ID8 ovarian cancer model: In a mouse model of ovarian cancer, mice were treated with TIM-3/CD137 alone or TIM-3 in combination with CD137. By day 3, monotherapy effectively regressed the tumors, but by day 10, the tumors had become larger. In contrast, the combination treatment regressed tumors in 60% of the mice by day 90.

Figure 6

CD226, TIGIT, and CD96 with their ligand-binding activities: CD155 and CD112 are the ligands of TIGIT and CD226. CD96 also binds CD155. TIGIT and CD96 are co-inhibitory receptors that promote the infiltration ability of Tregs after binding ligands. They also transmit inhibitory signals to NK cells and T cells. CD226 is a co-stimulatory receptor responsible for activating NK cells and T cells. TIGIT mAbs bind TIGIT on the surfaces of NK cells and T cells, thus causing TIGIT to bind CD155 and CD112, and restoring the activity of immune cells. TIGIT, T cell immunoreceptor with Ig and ITIM domains; APC, antigen-presenting cell; NK cells, natural killer cells.

Figure 7

Types of immunotherapeutic treatments. Tumor immunotherapy approaches can be broadly categorized as (A) cell therapies (CAR-T), (B) immune checkpoint inhibitor therapies, (C) drug nano-delivery, and (D) oncolytic virus therapies. (A) In CAR-T, for example, T cells are isolated from the human body and genetically engineered in vitro to express CAR and form CAR-T cells, which are then massively expanded in vitro and reinfused into the patient’s body. These CAR-T cells specifically recognize target antigens, proliferate rapidly, and exert anti-tumor effects in vivo. (B) CTLA-4 mAbs preferentially bind CTLA-4, and the ligand CD80/86, which has a stronger affinity for CTLA-4, binds CD80, thus restoring the normal function of T cells and leading to a transition from suppression of anti-tumor immunity to promotion of tumor immunity. (C) Nano-delivered drugs are degraded after being injected into the body and subsequently reach specific target sites, thereby stimulating the secretion of inflammatory factors and cytokines. This nano-delivery system improves tumor immunity efficacy. (D) After infection of tumor cells and normal cells with natural or genetically engineered oncolytic viruses (OVs), normal cells are not affected, whereas OV specifically targets tumor cells and proliferates in them, thus causing tumor cells to undergo lysis, apoptosis, and activating DC cells, NK cells, and cytotoxic T lymphocytes (CTLs) for further attack on tumor cells.