2.3 VISTA (V-domain Ig Suppressor of T Cell Activation)
V-domain Ig. suppressor of T Cell Activation (VISTA; gene Vsir ; also known as Dies-1, Gi24, BH-75, D1α, and PD-1H) is an emerging immune checkpoint receptor that has garnered attention for its role in regulating immune responses, particularly within the TME [72, 73, 74].. It is a type I transmembrane protein consisting of 279 amino acids (AAs). It includes an extracellular IgV domain comprising 162 AAs, a transmembrane domain spanning 21 AAs, and a cytoplasmic domain containing 96 AAs [75]. VISTA exhibits the highest sequence homology with PD-1 [73]. However, unlike PD-1, VISTA lacks a conventional immunoreceptor tyrosine-based inhibition motif (ITIM) or immunoreceptor tyrosine-based switch motif (ITSM) within its cytoplasmic domain. Instead, the intracellular tail of VISTA contains two putative binding sites for protein kinase C (PKC) and a proline-rich motif that likely serves as docking sites for signaling molecules. These features suggest that VISTA has the potential to function both as a receptor and a ligand [73]. VISTA is predominantly expressed on myeloid cells, including monocytes, macrophages, dendritic cells, microglia, neutrophils, and tumoral cells as well as on certain subsets of tumor-infiltrating lymphocytes (TILs) [72, 76]. Human VISTA has two confirmed binding partners (ligands) with immunosuppressive functions: P-selectin glycoprotein ligand-1 (PSGL-1), V-Set and Immunoglobulin domain containing 3 (VSIG3) and galectin-9 . Additionally, there are less well-confirmed ligands: VSIG8 and FOXD3 [75, 77, 78].
The main role of VISTA is to uphold an immunosuppressive environment, which is essential for preventing excessive immune responses, thereby maintaining the body’s homeostasis and protecting against autoimmune tissue damage [73, 74]. However, in the context of cancer, this immunosuppressive function can be hijacked by tumors to evade immune detection and destruction. VISTA achieves this by delivering inhibitory signals to T cells, dampening their activation, proliferation, and effector functions [79]. Studies have demonstrated that tumor-infiltrating immune cells, such as CD11b+Gr1+ myeloid cells and FoxP3+ Tregs, can exhibit increased expression of VISTA, thereby dampening anti-tumoral immune responses. Additionally, the hypoxic conditions within the TME can induce overexpression of VISTA, facilitating immune evasion by tumor cells [74, 80]
VISTA is structurally similar to other immune checkpoint molecules but operates through distinct mechanisms [81]. It can act both as a receptor and as a ligand, engaging in interactions that suppress T cell activity [78]. When VISTA is expressed on antigen-presenting cells (APCs), it interacts with counter-receptors on T cells, leading to the inhibition of T cell activation. This interaction effectively suppresses the immune response by preventing the T cells from mounting a robust attack against antigens, including those presented by tumor cells [78, 79]. Conversely, when VISTA is expressed on T cells, it can receive inhibitory signals from its ligands present on APCs. This bidirectional inhibitory signaling further enhances immune suppression by dampening the activation and function of T cells [78, 79].
VISTA is often significantly upregulated in tumor-infiltrating immune cells across various cancers, including gastric cancer [82], colorectal cancer [83], melanoma [84], prostate cancer [85], TNBC [86], and acute myeloid leukemia [87]. However, the significance of VISTA expression in the TME for patient survival remains controversial, and further research is required to assess VISTA expression and function within the TME . Recent studies on non-small cell lung cancer(NSCLC) [88], esophageal adenocarcinoma [89], endometrial carcinoma [90], and breast cancer [86] have confirmed that high levels of VISTA ecpression in immune cells are associated with a better prognosis. However, a study by Kuklinski et al. on cutaneous melanoma found a negative correlation between VISTA expression in immune cells and prognosis [84]. Similarly in Ovarian Cancers, Liao et al., observed that high expression of VISTA on immune cell was significantly associated with poor prognosis [91]. In renal cell carcinoma (RCC), the presence of VISTA-positive immune cells in the venous tumor thrombus, but not in the primary RCC, was associated with a poor prognosis [92]. Additionally, recent observations have revealed that VISTA can also be expressed on tumor cells in various cancers, including ovarian [93], endometrial [93], gastric cancer [94], hepatocellular carcinoma [95] and small cell lung cancer [88].
Preclinical studies have demonstrated that blocking VISTA can reinvigorate exhausted T cells and promote their proliferation and cytotoxic activity against tumor cells. Blocking VISTA by anti-VISTA mAb can also inhibit the recruitment of MDSCs and increases dendritic cells [96]. In a study by Le Mercier et al., VISTA monoclonal antibody (mAb) treatment enhanced the infiltration, proliferation, and effector function of tumor-reactive T cells within the TME. VISTA blockade also altered the suppressive nature of the TME by decreasing the presence of monocytic myeloid-derived suppressor cells and increasing the presence of activated dendritic cells within the TME. Additionally, VISTA blockade impaired the suppressive function and reduced the emergence of tumor-specific Foxp3+CD4+ Tregs [80]. In the CT26 colorectal cancer model, treatment of small tumors with anti-VISTA alone led to a reduction in tumor growth. When used in combination therapy for large CT26 tumors, anti-PD-1/CTLA-4 alone was met with complete adaptive resistance. However, the addition of anti-VISTA to the regimen resulted in the rejection of half the tumors. They observed that single-cell RNA sequencing (scRNA-seq) of tumor-specific CD8+ T cells demonstrated that anti-VISTA therapy activated T-cell pathways. This was achieved by promoting co-stimulatory genes and reducing regulators of T-cell quiescence [74]. CA-170, a small molecule inhibitor targeting PD-L1 and VISTA, demonstrated robust restoration of T cell proliferation and effector functions inhibited by PD-L1/L2 and VISTA. It also showed significant anti-tumor efficacy in several immunocompetent mouse tumor models [97]. This has led to increased interest in developing VISTA-targeted therapies as a novel approach in cancer immunotherapy. The first anti-VISTA antibody to undergo human testing is CI-8993. CI-8993 is designed with putative binding sites at four residues within the C-C’ loop of VISTA and has been demonstrated to block interactions with both PSGL-1 and VSIG-3. It features an active IgG1 Fc domain that enhances interactions with Fc gamma receptors and myeloid cells, supporting antibody-dependent cell cytotoxicity (ADCC) [98]. Further anti-VISTA antibodies BMS767) and HMBD-002 are in preclinical development [99]. Mehta et al., used yeast surface display to engineer an anti-VISTA antibody, SG7, which binds with high affinity to VISTA in mice, humans, and cynomolgus monkeys. SG7, as a monotherapy and even more effectively in combination with anti-PD1, slows tumor growth in multiple syngeneic mouse models [99].
In particular, combining VISTA inhibitors with other immunotherapies, such as PD-1/PD-L1/ CTLA-4 inhibitors, holds significant promise [78]. In a murine model of colon cancer, Dr. Lines and her team documented that anti-VISTA treatment enhanced immune cell density within the TME, with increased post-treatment infiltration of NK cells, along with CD45+, CD8+, and CD4+ T cells. Significant synergy was noted when anti-PD-1 and anti-CTLA-4 treatments were combined, compared to monotherapy. This combination resulted in further tumor growth reduction and diminished the suppressive nature of myeloid cells within the TME [74]. The rationale for combination therapy is that simultaneously targeting multiple immune checkpoints can produce synergistic effects, leading to a more robust and sustained anti-tumor immune response [100]. For instance, while PD-1/PD-L1 inhibitors work by lifting the ”brakes” on T cells, VISTA inhibitors can further enhance T cell activation by disrupting additional inhibitory pathways. This multi-faceted approach can potentially overcome resistance mechanisms that limit the effectiveness of single-agent therapies. Clinical trials are currently underway to evaluate the safety and efficacy of VISTA inhibitors, both as monotherapies and in combination with other checkpoint inhibitors [101]. Several VISTA-targeting inhibitors are being tested in phase I and II trials in patients with advanced, metastatic or unresectable solid tumors (NCT04475523) (NCT05082610) (NCT04564417). A phase 1 dose-escalation study (NCT04475523) is currently being conducted in patients with advanced, treatment-resistant solid tumors to evaluate CI-8993 (anti-VISTA antibody). So far, the safety data suggests that the treatment is manageable, with no dose-limiting side effects noted up to a dose of 0.6 mg/kg [102]. In a phase II TRIAL, oral dual inhibitor targeting both VISTA and PD-L1, known as CA-170, exhibited a clinical benefit rate of 75% and achieved a median progression-free survival (PFS) of 19.5 weeks in a cohort of eight previously treated non-squamous NSCLC patients [103].