not-yet-known not-yet-known not-yet-known unknown 2.5 BTLA (B and T Lymphocyte Attenuator) BTLA (also known as CD272) is an inhibitory receptor that plays a crucial role in the regulation of immune responses [125]. It was discovered in 2003 as a transcript, highly specific to Th1 cells [126]. The BTLA gene, is situated on chromosome 3 at position q13.2 in reverse orientation, with a total length of 870 bp and consisting of 5 exons [127]. It encodes a type 1 transmembrane glycoprotein comprising 289 AAs and weighing 33 kD. As a member of the CD28 immunoglobulin superfamily (IgSF), the BTLA protein shares structural and functional resemblances with PD-1 and CTLA-4. It includes a signal peptide, an IgC-like extracellular domain, a transmembrane domain, and a cytoplasmic domain [125, 128]. The cytoplasmic region of BTLA contains three highly conserved tyrosine-based motifs: a binding site for growth-factor receptor-bound protein 2 (Grb2), an immunoreceptor tyrosine inhibitory motif (ITIM), and an immunoreceptor tyrosine-based switch motif (ITSM) [129, 130]. It is expressed on various immune cells, including T cells, B cells, dendritic cells, and natural killer (NK) cells [125]. BTLA expression fluctuates during T cell differentiation and activation. It is present in naive T cells, increases transiently upon activation, but decreases in activated T cells. High levels of BTLA expression is distinctive of anergic T-cells and T follicular helper (Tfh) cells [125]. In various human B cell subsets, mature peripheral B cells exhibit the highest BTLA expression. Naive, transitional, and memory peripheral B cells show consistent BTLA levels, while bone marrow-derived precursor B cells have the lowest expression [131]. The primary function of BTLA is to maintain immune homeostasis and prevent overactivation of the immune system, which can lead to autoimmune diseases [130]. BTLA interacts with its ligand, HVEM (herpesvirus entry mediator), which is a member of the tumor necrosis factor receptor (TNFR) superfamily. The N-terminal cysteine-rich domain CRD1 of HVEM binds to single IgC domain of BTLA on the cell surface in a one-to-one ratio [132]. BTLA-HVEM interaction leads to inhibition of both CD28 and CD3ζ phosphorylation resulting in the inhibition of T-cell activation [133, 134]. Hence, the binding of BTLA to HVEM delivers inhibitory signals to immune cells, leading to the suppression of their activation and function. This interaction is critical for maintaining immune tolerance and preventing excessive inflammatory responses. UL144 viral protein was identified as the second BTLA ligand, although it has five fold lower affinity to BTLA, however it activates similar inhibitory signaling pathways [135]. Although UL144 has a lower binding affinity for BTLA compared to HVEM, it more effectively restricts T-cell proliferation [136]. Interestingly, recent research also indicates that BTLA expressed on antigen-presenting cells (APCs) can function as a co-stimulatory ligand for HVEM present on CD8+ T-cells [137]. HVEM is broadly expressed on many cell types, including T cells, B cells, and antigen-presenting cells (APCs) [132, 137]. In the context of cancer, the inhibitory signals mediated by BTLA can contribute to the immune evasion mechanisms of tumors. BTLA is observed to be expressed in tumor-infiltrating lymphocytes (TILs) and is often associated with a diminished anti-tumor immune response. In a study by Oguro et al., it was found that a higher density ratio of BTLA+ cells to CD8+ T cells in TME serves as an independent indicator of poor prognosis in GBC patients. Additionally, the upregulation of BTLA in cancer tissues is implicated in the suppression of antitumor immunity [138]. T cells from lung cancer patients also exhibit elevated BTLA expression [139]. Studies have also found increased BTLA expression in T cells from melanoma patients [140, 141]. Chen et al. observed that BTLA expression in cancerous tissues can serve as a predictor of poor outcomes in patients with epithelial ovarian cancer (EOC) [142]. Sekar et al. noted that type I natural killer T cells highly express BTLA in murine autochthonous mammary tumors [143]. In an intriguing study, it was discovered that BTLA+ T cells exhibit elevated levels of additional checkpoint molecules, including PD-1, TIM-3 and LAG-3. Additionally, these cells display a reduced cytolytic activity, poorly differentiated phenotype, and increased proliferation potential in patients with diffuse large B-cell lymphoma (DLBCL). Furthermore, higher BTLA levels are associated with more advanced disease stages in DLBCL patients [144]. However, some studies have yielded contradictory findings. For example, research by Song et al. indicated that BTLA levels were lower in colorectal cancer tissues compared to matched non-carcinoma tissues [145]. Interestingly, BTLA appears to be capable of both inhibitory and survival signaling, suggesting it may have context-specific roles in TILs [130, 146]. Preclinical studies have demonstrated that blocking BTLA can enhance the activation and effector functions of T cells and other immune cells. By inhibiting the interaction between BTLA and HVEM, BTLA blockade can relieve the suppression of immune responses, leading to increased proliferation, cytokine production, and cytotoxic activity of T cells against tumor cells. This enhanced immune activity can result in improved tumor control and regression [125, 142]. A study on murine autochthonous mammary tumors found that BTLA-neutralizing antibodies inhibit tumor growth and reduce pulmonary metastasis [143]. In a preclinical study by Lasaro et al., it was observed that blocking the BTLA/CD160 pathway along led to the regression of large, established tumor masses in a genetically-engineered murine thyroid adenocarcinoma model [147]. Additionally, blocking BTLA is being investigated in combination with other immunotherapies, particularly immune checkpoint inhibitors like PD-1/PD-L1 inhibitors. A study by Sun et al. found that, dual inhibition of BTLA and PD-1 enhances the therapeutic efficacy of paclitaxel on intraperitoneally disseminated tumors [148]. Chen et al. observed that BTLA blockade enhances cancer therapy by inhibiting IL-6/IL-10-induced CD19high B lymphocytes, both in animal models and in vitro studies [142]. In 2019, the FDA approved Icatolimab (TAB004/JS004), the world’s first-in-class anti-BTLA humanized IgG4 monoclonal antibody, for clinical trials [125]. Currently, BTLA inhibitors are undergoing clinical trials to evaluate the safety and efficacy, both as mono-therapies and in combination with other checkpoint inhibitors [NCT04773951, tifcemalimab (icatolimab, TAB004/JS004; BTLA inhibitors) +   toripalimab (anti-PD-1)] [NCT05000684, tifcemalimab+ toripalimab] [NCT04929080, tifcemalimab + JS001 (anti-PD-1)] [NCT04137900 Tifcemalimab +  toripalimab (anti-PD-1)] [NCT05891080, Tifcemalimab +  toripalimab + chemotherapy + surgery] [NCT05789069, HFB200603 (anti-BTLA) alone or in combination with tislelizumab (anti-PD-1)] These trials aim to determine the optimal dosing, administration, and combination strategies for BTLA blockade in various types of cancer, including melanoma, non-small cell lung cancer, and hematologic malignancies.