Anticancer therapy based on the inhibition of immune checkpoints (ICs) is an actively developing field of study, and it has been widely used. Antibodies blocking immune checkpoints are used as therapeutics. The targeted checkpoints are mainly the PD-L1 (programmed death-ligand 1), expressed by the tumor, and the PD-1 (programmed cell death protein 1) and CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) immune cell receptors. To increase the effectiveness of therapy by blocking ICs, additional receptors and ligands are being investigated as targets of immunotherapy.
1. TIM-3
TIM-3 (T-cell immunoglobulin and mucin domain-3) is a transmembrane protein, expressed by T-cells, IFNγ-secreting T-regulatory cells (Treg), natural killer cells (NK cells), dendritic cells (DCs), macrophages, and mast cells
[1]. TIM-3 is a receptor, an immune response regulator that ensures the formation of immunological tolerance and prevents the occurrence of autoimmune diseases by regulating the homeostasis of T-helper type 1
[2]. A decreased expression level of TIM-3 is associated with the development of diabetes and multiple sclerosis
[3]. At the same time, the overexpression of Tim3 can contribute to the depletion of T-cells by limiting the pool of memory T-cells while enhancing the initial activation of T-cells and the generation of short-lived effector cells in acute and chronic infections
[4]. In addition, the participation of TIM-3 in the activation of mast cells was revealed
[5]. Increased TIM-3 expression by tumor-infiltrating lymphocytes (TILs) is indicated in many malignant neoplasms and is characteristic of effector lymphocytes with a depleted phenotype
[6][7]. On the other hand, TIM-3 expression is characteristic of activated regulatory T-cells with immunosuppressive activity
[8]. A significant role of TIM-3, expressed in antigen-presenting cell (APC) and T-cells, in the regulation of CD8+ TILs trogocytosis in tumors has been shown. The use of mAb to TIM-3 is able to counteract the fratricidal process undergone by trogocytosed CD8+ T-cells
[9].
2. LAG-3
The LAG-3 (lymphocyte-activation gene 3) gene (CD223) encodes a protein that negatively regulates the activation, proliferation, effector functions, and homeostasis of T-cells
[10][11] and dendritic cells participating in preventing the development of autoimmune reactions in normal tissues
[12] and regulating the immune response in chronic infections
[13]. Due to the partial similarity of extracellular domains, LAG-3 and CD4 were presumably developed by gene duplication. However, differences in their intracellular domains result in their opposite functions
[14]. The LAG-3 protein is presented in a transmembrane and soluble form (sLAG-3) formed by alternative splicing. It has been shown that under the action of ADAM10 and ADAM17 metalloproteases, the extracellular part of the receptor also passes into a soluble form
[15]. LAG-3 is constitutively expressed by natural T-regulatory cells (Tr1), DCs, NK cells, and B-cells and is not found on naive T-cells; however, its expression is strongly increased after the activation of CD4+ and CD8+ lymphocytes, including TILs
[16]. The modulating functions of LAG-3 correlate with the level of receptor expression
[17]. The activation of LAG-3 reduces the production of various immunostimulatory interleukins (IL) and increases sensitivity to Treg signaling, thereby increasing T-cell tolerance and accelerating their depletion
[16].
3. TIGIT
TIGIT (T-cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif (ITIM) domains) is a co-inhibitory receptor, expressed by all types of T-lymphocytes, as well as NK cells
[18]. The receptor is involved in maintaining self-tolerance. The positive effect of TIGIT in regenerative hyperplasia was revealed: the absence of the receptor impairs liver regeneration in vivo
[19].
Several immunoregulatory mechanisms involving TIGIT have been described to date. The interaction of TIGIT with the ligand causes the phosphorylation of its cytoplasmic domain, which triggers processes that block the transmission of intracellular signals along the PI3K and MAPK pathways and the activation of NF-κB, which, in turn, leads to the suppression of the cytotoxic functions of NK cells
[20]. In addition, the interaction of this receptor with the ligand leads to the phosphorylation of the latter and the triggering of modulating signals in DCs
[21]. TIGIT has been reported to directly inhibit T-cell proliferation and effector functions by downregulating T-cell receptor (TCR) and activating CD28 signaling
[22].
4. VISTA
VISTA (V-domain Ig suppressor of T-cell activation) or PD-1H (programmed death-1 homolog) is predominantly expressed by myeloid cells, as well as by CD4+ and Foxp3+ T-regulatory cells
[23]. Studies of VISTA expression in cancer diseases have shown the presence of protein on TILs and macrophages and its absence on cells of most types of tumors
[24]. However, in a number of studies, the expression of VISTA by tumor cells was detected in different proportions of samples in non-small cell lung cancer (NSCLC),
[25], hepatocellular carcinoma
[26], ovarian and endometrial cancer
[27], melanoma, stomach cancer, and breast cancer
[28]. VISTA negatively regulates T-cell activation, proliferation, and cytokine production
[29] and specifically suppresses the immune response mediated by CD4+ T-cells
[30]. However, in a study by Mercier et al., the suppression of lymphocyte functions was mediated by the activation of cell receptors by a fusion protein (VISTA-Ig) acting as a ligand
[31]. On the other hand, the increased proliferation and production of VISTA−/− cytokines by CD4+ T-cells indicates VISTA receptor function
[30]. In addition, VISTA directly regulates the effector functions of myeloid cells
[32]. Thus, understanding the complex functioning of VISTA requires a detailed study of the associated immune regulatory mechanisms.
5. BTLA
BTLA (B- and T-lymphocyte attenuator) or CD272 is a transmembrane receptor expressed by naive T-lymphocytes, B-cells, macrophages, DCs, and natural killer T-cells (NKT)
[33][34]. BTLA is involved in the regulation of immune cell homeostasis by inhibiting proliferation, the activation of B- and T-cells, and the production of cytokines
[35]. In particular, BTLA negatively regulates the expansion and function of γδ T-cells
[36], various subtypes of which both contribute to the progression of cancer and have antitumor activity
[37]. A soluble form of the BTLA protein (sBTLA) is described as a potential prognostic and predictive marker in patients with clear cell renal cell carcinoma, pancreatic adenocarcinoma, and prostate cancer
[38][39][40].
A recent study in patients treated with immune checkpoint (ICT) inhibitors for solid tumors found an association between serum levels of soluble BTLA (sBTLA) and median overall survival
[41].
Data on the clinical significance of the molecules considered in the table, as well as the results of preclinical studies, are presented in Table 1.
Table 1. Clinical significance and results of preclinical studies of immune checkpoints (ICs) and their ligands.
Data on current clinical trials utilizing the considered immune checkpoints are presented in Table 2.
Table 2. Summary of ongoing clinical trials of receptor inhibitors.