The role of immune checkpoints in antitumor response: a potential antitumor immunotherapy

Abstract

Immunotherapy aims to stimulate the immune system to inhibit tumor growth or prevent metastases. Tumor cells primarily employ altered expression of human leukocyte antigen (HLA) as a mechanism to avoid immune recognition and antitumor immune response. The antitumor immune response is primarily mediated by CD8+ cytotoxic T cells (CTLs) and natural killer (NK) cells, which plays a key role in the overall anti-tumor immune response. It is crucial to comprehend the molecular events occurring during the activation and subsequent regulation of these cell populations. The interaction between antigenic peptides presented on HLA-I molecules and the T-cell receptor (TCR) constitutes the initial signal required for T cell activation. Once activated, in physiologic circumstances, immune checkpoint expression by T cells suppress T cell effector functions when the antigen is removed, to ensures the maintenance of self-tolerance, immune homeostasis, and prevention of autoimmunity. However, in cancer, the overexpression of these molecules represents a common method through which tumor cells evade immune surveillance. Numerous therapeutic antibodies have been developed to inhibit immune checkpoints, demonstrating antitumor activity with fewer side effects compared to traditional chemotherapy. Nevertheless, it's worth noting that many immune checkpoint expressions occur after T cell activation and consequently, altered HLA expression on tumor cells could diminish the clinical efficacy of these antibodies. This review provides an in-depth exploration of immune checkpoint molecules, their corresponding blocking antibodies, and their clinical applications.

Keywords: HLA antigens; immune checkpoint inhibitors; immune evasion; immunotherapy; neoplasms.

Figure 1

Interaction of immune system cell receptors and ligands in a tumor microenvironment. Immune checkpoint molecules can modulate the response of T cells to self-proteins, chronic infections, or tumor antigens. The pathways used by immune checkpoints are unique and non-redundant, demonstrating their important role in regulating immune homeostasis and highlighting the relevance of conducting research to develop immunotherapies based on multiple checkpoint blockades that enhance antitumor immunity.

Figure 2

CTLA-4 checkpoint. CD28 signaling promotes T cell activation and CTLA-4 upregulation, which inhibits T cell proliferation. (A) T cell activation requires two signals: TCR binding to the HLA of the APC and co-stimulation given by the interaction of CD28 in lymphocytes with CD80/CD86 (B7-1/B7-2) in the APC, which increases IL-2 expression and the proliferation of antigen-specific T cells. During the activation process, conventional T-cells express low levels of CTLA-4, but upon complete activation, their expression increases on the cell surface. (B) CTLA-4 competes for binding to CD80/CD86 with a higher affinity than CD28. Once CTLA-4 binds to CD80/CD86, T cells activation is inhibited by disrupting CD28 signaling with CD80/CD86. Consequently, T cells proliferation is inhibited. (C) The mechanism of action of ipilimumab to block the binding of CTLA-4 to its ligands and prevent T cells inhibition.

Figure 3

PD-1/PD-L1 checkpoint. The action mechanism begins when T cells are activated upon recognition of the antigen presented in HLA-I, triggering a signaling cascade and the release of cytokines that activate their proliferation. (A) In a normal microenvironment, after T cell activation and proliferation, immune checkpoint proteins, such as PD-1 and its ligand PDL-1, are expressed that prevent their excessive activation. (B) In a tumor microenvironment, the expression of immune checkpoints favors escape from immunosurveillance. Once T cells recognize the tumor antigen presented in HLA-I, they are activated and produce IFN-γ that binds to the IFN-γ receptor, inducing PD-L1 overexpression in tumor cells. PD-L1 binds to PD-1, which is overregulated in T cells, and thus inhibits the immune response. (C) PD-L1 can interact with CD80/CD86 on the antigen-presenting cell (APC) and disrupt PD-L1/PD-1 binding. It has been documented that CD80/CD86 are not only expressed on APCs but also on T cells, suggesting that it may be another pathway in T cells that may serve to downregulate responses and prevent T cell signals. (D) Anti-PD-1 antibody or anti-PD-L1 blocks the interaction of PD-1 and PD-L1 and suppresses the inhibition of CD8+ T cells, thereby enhancing antitumor activity.

Figure 4

HLA-G checkpoint. Through the interaction of LILRB1 (ILT-2) and LILRB2 (ILT-4) receptors, HLA-G inhibits cytotoxic T cells, NK cells, and B cells and modulates myeloid cells. Soluble isoforms can be generated by proteolytic cleavage of membrane-bound HLA-G forms and move to other tissues via blood or extracellular vesicles.

Figure 5

LAG-3 checkpoint. LAG-3 expressed primarily on activated T cells binds to HLA-II with higher affinity than CD4 and generates the overexpression of immunoregulatory cytokines, such as IL-10 and TGF-β, which suppress tumor-specific T cells. It acts synergistically with PD-1 to suppress antitumor immunity. The main ligand of LAG-3 is HLA-II, and four others have been discovered: FGL-1, galectin-3, LSECtin, and α-syn.

Figure 6

TIM-3 checkpoint. (A) TIM-3 has five tyrosine residues in its cytoplasmic tail, of which Y265 and Y272 are the most important for signal transduction. It has been shown that the cytoplasmic protein Bat3 is able to modulate cell proliferation: Bat3 binds to TIM-3 and protects T cells from signaling. (B) When TIM-3 binds to galectin-9, Bat3 dissociates from TIM-3, and signaling pathways are activated that lead to the inhibition of T cell proliferation and suppression of IL-2, TNFα, and IFNγ production.

Figure 7

Main ligands of several activating KIRs (2DS and 3DS).

Figure 8

Following interaction with CD137L, CD137 signaling initiates through the recruitment of TRAF1 and TRAF2, and TRAF3 in a hypothetical scenario. TRAF proteins assemble into homo- or hetero-trimers which recruit cIAP1/2. This interaction plays a pivotal role in activating downstream effector signals orchestrating a cascade of events that transmit signals across various pathways to the nucleus, including NF-κB, ERK, p38 MAPK, and JNK pathways. Activation of these pathways results in the upregulation of anti-apoptotic proteins, T cell proliferation, differentiation, effector functions and overall survival, and down regulation of the pro-apoptotic protein Bim, highlighting the role of CD137 signaling in the modulation of key cellular processes with implications for the immune response homeostasis.