VISTA Regulates the Development of Protective Antitumor Immunity

Basic Overview of Current Immunotherapy Approaches in Cancer

Recent success of immunotherapy strategies such as immune checkpoint blockade in several malignancies has established the role of immunotherapy in the treatment of cancer. Cancers use multiple mechanisms to co-opt the host-tumor immune interactions, leading to immune evasion. Our understanding of the host-tumor interactions has evolved over the past few years and led to various promising new therapeutic strategies. This article will focus on the basic principles of immunotherapy, novel pathways/agents, and combinatorial immunotherapies.

Immunosuppressive cells in tumor immune escape and metastasis

Tumor immune escape and the initiation of metastasis are critical steps in malignant progression of tumors and have been implicated in the failure of some clinical cancer immunotherapy. Tumors develop numerous strategies to escape immune surveillance or metastasize: Tumors not only modulate the recruitment and expansion of immunosuppressive cell populations to develop the tumor microenvironment or pre-metastatic niche but also switch the phenotype and function of normal immune cells from a potentially tumor-reactive state to a tumor-promoting state. Immunosuppressive cells facilitate tumor immune escape by inhibiting antitumor immune responses and furthermore promote tumor metastasis by inducing immunosuppression, promoting tumor cell invasion and intravasation, establishing a pre-metastatic niche, facilitating epithelial-mesenchymal transition, and inducing angiogenesis at primary tumor or metastatic sites. Numerous translational studies indicate that it is possible to inhibit tumor immune escape and prevent tumor metastasis by blocking immunosuppressive cells and eliminating immunosuppressive mechanisms that are induced by either immunosuppressive cells or tumor cells. Furthermore, many clinical trials targeting immunosuppressive cells have also achieved good outcome. In this review, we focus on the underlying mechanisms of immunosuppressive cells in promoting tumor immune escape and metastasis, discuss our current understanding of the interactions between immunosuppressive cells and tumor cells in the tumor microenvironment, and suggest future research directions as well as potential clinical strategies in cancer immunotherapy.

Emerging targets in cancer immunotherapy: beyond CTLA-4 and PD-1

Manipulation of co-stimulatory or co-inhibitory checkpoint proteins allows for the reversal of tumor-induced T-cell anergy observed in cancer. The field has gained credence given success with CTLA-4 and PD-1 inhibitors. These molecules include immunoglobulin family members and the B7 subfamily as well as the TNF receptor family members. PD-L1 inhibitors and LAG-3 inhibitors have progressed through clinical trials. Other B7 family members have shown promise in preclinical models. TNFR superfamily members have shown variable success in preclinical and clinical studies. As clinical investigation in tumor immunology gains momentum, the next stage becomes learning how to combine checkpoint inhibitors and agonists with each other as well as with traditional chemotherapeutic agents.

Negative immune checkpoints on T lymphocytes and their relevance to cancer immunotherapy

The term 'inhibitory checkpoint' refers to the broad spectrum of co-receptors expressed by T cells that negatively regulate T cell activation thus playing a crucial role in maintaining peripheral self-tolerance. Co-inhibitory receptor ligands are highly expressed by a variety of malignancies allowing evasion of anti-tumour immunity. Recent studies demonstrate that manipulation of these co-inhibitory pathways can remove the immunological brakes that impede endogenous immune responses against tumours. Antibodies that block the interactions between co-inhibitory receptors and their ligands have delivered very promising clinical responses, as has been shown by recent successful trials targeting the CTLA-4 and PD-1 pathways. In this review, we discuss the mechanisms of action and expression pattern of co-inhibitory receptors on different T cells subsets, emphasising differences between CD4(+) and CD8(+) T cells. We also summarise recent clinical findings utilising immune checkpoint blockade.

Combination cancer immunotherapy and new immunomodulatory targets

Targeting immune checkpoints such as programmed cell death protein 1 (PD1), programmed cell death 1 ligand 1 (PDL1) and cytotoxic T lymphocyte antigen 4 (CTLA4) has achieved noteworthy benefit in multiple cancers by blocking immunoinhibitory signals and enabling patients to produce an effective antitumour response. Inhibitors of CTLA4, PD1 or PDL1 administered as single agents have resulted in durable tumour regression in some patients, and combinations of PD1 and CTLA4 inhibitors may enhance antitumour benefit. Numerous additional immunomodulatory pathways as well as inhibitory factors expressed or secreted by myeloid and stromal cells in the tumour microenvironment are potential targets for synergizing with immune checkpoint blockade. Given the breadth of potential targets in the immune system, critical questions to address include which combinations should move forward in development and which patients will benefit from these treatments. This Review discusses the leading drug targets that are expressed on tumour cells and in the tumour microenvironment that allow enhancement of the antitumour immune response.

Melanoma: oncogenic drivers and the immune system

Advances and in-depth understanding of the biology of melanoma over the past 30 years have contributed to a change in the consideration of melanoma as one of the most therapy-resistant malignancies. The finding that oncogenic BRAF mutations drive tumor growth in up to 50% of melanomas led to a molecular therapy revolution for unresectable and metastatic disease. Moving beyond BRAF, inactivation of immune regulatory checkpoints that limit T cell responses to melanoma has provided targets for cancer immunotherapy. In this review, we discuss the molecular biology of melanoma and we focus on the recent advances of molecularly targeted and immunotherapeutic approaches.

Advances in immunotherapy for treatment of lung cancer

Different approaches for treating lung cancer have been developed over time, including chemotherapy, radiotherapy and targeted therapies against activating mutations. Lately, better understanding of the role of the immunological system in tumor control has opened multiple doors to implement different strategies to enhance immune response against cancer cells. It is known that tumor cells elude immune response by several mechanisms. The development of monoclonal antibodies against the checkpoint inhibitor programmed cell death protein 1 (PD-1) and its ligand (PD-L1), on T cells, has led to high activity in cancer patients with long lasting responses. Nivolumab, an anti PD-1 inhibitor, has been recently approved for the treatment of squamous cell lung cancer patients, given the survival advantage demonstrated in a phase III trial. Pembrolizumab, another anti PD-1 antibody, has received FDA breakthrough therapy designation for treatment of non-small cell lung cancer (NSCLC), supported by data from a phase I trial. Clinical trials with anti PD-1/PD-L1 antibodies in NSCLC have demonstrated very good tolerability and activity, with response rates around 20% and a median duration of response of 18 months.

Control of signaling-mediated clearance of apoptotic cells by the tumor suppressor p53

The inefficient clearance of dying cells can lead to abnormal immune responses, such as unresolved inflammation and autoimmune conditions. We show that tumor suppressor p53 controls signaling-mediated phagocytosis of apoptotic cells through its target, Death Domain1α (DD1α), which suggests that p53 promotes both the proapoptotic pathway and postapoptotic events. DD1α appears to function as an engulfment ligand or receptor that engages in homophilic intermolecular interaction at intercellular junctions of apoptotic cells and macrophages, unlike other typical scavenger receptors that recognize phosphatidylserine on the surface of dead cells. DD1α-deficient mice showed in vivo defects in clearing dying cells, which led to multiple organ damage indicative of immune dysfunction. p53-induced expression of DD1α thus prevents persistence of cell corpses and ensures efficient generation of precise immune responses.

Beyond CTLA-4 and PD-1, the Generation Z of Negative Checkpoint Regulators

In the last two years, clinical trials with blocking antibodies to the negative checkpoint regulators CTLA-4 and PD-1 have rekindled the hope for cancer immunotherapy. Multiple negative checkpoint regulators protect the host against autoimmune reactions but also restrict the ability of T cells to effectively attack tumors. Releasing these brakes has emerged as an exciting strategy for cancer treatment. Conversely, these pathways can be manipulated to achieve durable tolerance for treatment of autoimmune diseases and transplantation. In the future, treatment may involve combination therapy to target multiple cell types and stages of the adaptive immune responses. In this review, we describe the current knowledge on the recently discovered negative checkpoint regulators, future targets for immunotherapy.

Engineered Materials for Cancer Immunotherapy

Immunotherapy is a promising treatment modality for cancer as it can promote specific and durable anti-cancer responses. However, limitations to current approaches remain. Therapeutics administered as soluble injections often require high doses and frequent re-dosing, which can result in systemic toxicities. Soluble bolus-based vaccine formulations typically elicit weak cellular immune responses, limiting their use for cancer. Current methods for ex vivo T cell expansion for adoptive T cell therapies are suboptimal, and achieving high T cell persistence and sustained functionality with limited systemic toxicity following transfer remains challenging. Biomaterials can play important roles in addressing some of these limitations. For example, nanomaterials can be employed as vehicles to deliver immune modulating payloads to specific tissues, cells, and cellular compartments with minimal off-target toxicity, or to co-deliver antigen and danger signal in therapeutic vaccine formulations. Alternatively, micro-to macroscale materials can be employed as devices for controlled molecular and cellular delivery, or as engineered microenvironments for recruiting and programming immune cells in situ. Recent work has demonstrated the potential for combining cancer immunotherapy and biomaterials, and the application of biomaterials to cancer immunotherapy is likely to enable the development of effective next-generation platforms. This review discusses the application of engineered materials for the delivery of immune modulating agents to the tumor microenvironment, therapeutic cancer vaccination, and adoptive T cell therapy.

Targeting PD-1/PD-L1 in lung cancer: current perspectives

Increased understanding of tumor immunology has led to the development of effective immunotherapy treatments. One of the most important advances in this field has been due to pharmacological design of antibodies against immune checkpoint inhibitors. Anti-PD-1/PD-L1 antibodies are currently in advanced phases of clinical development for several tumors, including lung cancer. Results from Phase I-III trials with anti-PD-1/PD-L1 antibodies in non-small-cell lung cancer have demonstrated response rates of around 20% (range, 16%-50%). More importantly, responses are long-lasting (median duration of response, 18 months) and fast (50% of responses are detected at time of first tumor evaluation) with very low grade 3-4 toxicity (less than 5%). Recently, the anti-PD-1 antibody pembrolizumab received US Food and Drug Administration (FDA) breakthrough therapy designation for treatment of non-small-cell lung cancer, supported by data from a Phase Ib trial. Another anti-PD-1 antibody, nivolumab, has also been approved for lung cancer based on survival advantage demonstrated in recently released data from a Phase III trial in squamous cell lung cancer.

Selective Involvement of the Checkpoint Regulator VISTA in Suppression of B-Cell, but Not T-Cell, Responsiveness by Monocytic Myeloid-Derived Suppressor Cells from Mice Infected with an Immunodeficiency-Causing Retrovirus

Inhibition of T-cell responses in tumor microenvironments by myeloid-derived suppressor cells (MDSCs) is widely accepted. We demonstrated augmentation of monocytic MDSCs whose suppression of not only T-cell, but also B-cell, responsiveness paralleled the immunodeficiency during LP-BM5 retrovirus infection. MDSCs inhibited T cells by inducible nitric oxide synthase (iNOS)/nitric oxide (NO), but uniquely, inhibition of B cells was ~50% dependent each on iNOS/NO and the MDSC-expressed negative-checkpoint regulator VISTA. Blockade with a combination of iNOS/NO and VISTA caused additive or synergistic abrogation of MDSC-mediated suppression of B-cell responsiveness.

Inhibitory receptors as targets for cancer immunotherapy

Inhibitory receptors expressed on T cells control immune responses while limiting autoimmunity. However, tumors can hijack these "checkpoints" for protection from immune attack. Tumor-specific T cells that exhibit an exhausted, unresponsive phenotype express high levels of inhibitory receptors including CTLA4, PD1, and LAG3, among others. Intratumoral regulatory T cells promote immunosuppression and also express multiple inhibitory receptors. Overcoming this inhibitory receptor-mediated immune tolerance has thus been a major focus of recent cancer immunotherapeutic developments. Here, we review how boosting the host's immune system by blocking inhibitory receptor signaling with antagonistic mAbs restores the capacity of T cells to drive durable antitumor immune responses. Clinical trials targeting the CTLA4 and PD1 pathways have shown durable effects in multiple tumor types. Many combinatorial therapies are currently being investigated with encouraging results that highlight enhanced antitumor immunogenicity and improved patient survival. Finally, we will discuss the ongoing identification and dissection of novel T-cell inhibitory receptor pathways, which could lead to the development of new combinatorial therapeutic approaches.

Immune-checkpoint proteins VISTA and PD-1 nonredundantly regulate murine T-cell responses

V-domain immunoglobulin suppressor of T-cell activation (VISTA) is a negative immune-checkpoint protein that suppresses T-cell responses. To determine whether VISTA synergizes with another immune-checkpoint, programmed death 1 (PD-1), this study characterizes the immune responses in VISTA-deficient, PD-1-deficient (KO) mice and VISTA/PD-1 double KO mice. Chronic inflammation and spontaneous activation of T cells were observed in both single KO mice, demonstrating their nonredundancy. However, the VISTA/PD-1 double KO mice exhibited significantly higher levels of these phenotypes than the single KO mice. When bred onto the 2D2 T-cell receptor transgenic mice, which are predisposed to development of inflammatory autoimmune disease in the CNS, the level of disease penetrance was significantly enhanced in the double KO mice compared with in the single KO mice. Consistently, the magnitude of T-cell response toward foreign antigens was synergistically higher in the VISTA/PD-1 double KO mice. A combinatorial blockade using monoclonal antibodies specific for VISTA and PD-L1 achieved optimal tumor-clearing therapeutic efficacy. In conclusion, our study demonstrates the nonredundant role of VISTA that is distinct from the PD-1/PD-L1 pathway in controlling T-cell activation. These findings provide the rationale to concurrently target VISTA and PD-1 pathways for treating T-cell-regulated diseases such as cancer.

The future of immune checkpoint therapy

Immune checkpoint therapy, which targets regulatory pathways in T cells to enhance antitumor immune responses, has led to important clinical advances and provided a new weapon against cancer. This therapy has elicited durable clinical responses and, in a fraction of patients, long-term remissions where patients exhibit no clinical signs of cancer for many years. The way forward for this class of novel agents lies in our ability to understand human immune responses in the tumor microenvironment. This will provide valuable information regarding the dynamic nature of the immune response and regulation of additional pathways that will need to be targeted through combination therapies to provide survival benefit for greater numbers of patients.

Mechanistic Assessment of PD-1H Coinhibitory Receptor-Induced T Cell Tolerance to Allogeneic Antigens

PD-1H is a recently identified cell surface coinhibitory molecule of the B7/CD28 immune modulatory gene family. We showed previously that single injection of a PD-1H agonistic mAb protected mice from graft-versus-host disease (GVHD). In this study, we report two distinct mechanisms operate in PD-1H-induced T cell tolerance. First, signaling via PD-1H coinhibitory receptor potently arrests alloreactive donor T cells from activation and expansion in the initiation phase. Second, donor regulatory T cells are subsequently expanded to maintain long-term tolerance and GVHD suppression. Our study reveals the crucial function of PD-1H as a coinhibitory receptor on alloreactive T cells and its function in the regulation of T cell tolerance. Therefore, PD-1H may be a target for the modulation of alloreactive T cells in GVHD and transplantation.

Immune checkpoint targeting in cancer therapy: toward combination strategies with curative potential

Research in two fronts has enabled the development of therapies that provide significant benefit to cancer patients. One area stems from a detailed knowledge of mutations that activate or inactivate signaling pathways that drive cancer development. This work triggered the development of targeted therapies that lead to clinical responses in the majority of patients bearing the targeted mutation, although responses are often of limited duration. In the second front are the advances in molecular immunology that unveiled the complexity of the mechanisms regulating cellular immune responses. These developments led to the successful targeting of immune checkpoints to unleash anti-tumor T cell responses, resulting in durable long-lasting responses but only in a fraction of patients. In this Review, we discuss the evolution of research in these two areas and propose that intercrossing them and increasing funding to guide research of combination of agents represent a path forward for the development of curative therapies for the majority of cancer patients.

Immune checkpoint targeting in cancer therapy: toward combination strategies with curative potential

Research in two fronts has enabled the development of therapies that provide significant benefit to cancer patients. One area stems from a detailed knowledge of mutations that activate or inactivate signaling pathways that drive cancer development. This work triggered the development of targeted therapies that lead to clinical responses in the majority of patients bearing the targeted mutation, although responses are often of limited duration. In the second front are the advances in molecular immunology that unveiled the complexity of the mechanisms regulating cellular immune responses. These developments led to the successful targeting of immune checkpoints to unleash anti-tumor T cell responses, resulting in durable long-lasting responses but only in a fraction of patients. In this Review, we discuss the evolution of research in these two areas and propose that intercrossing them and increasing funding to guide research of combination of agents represent a path forward for the development of curative therapies for the majority of cancer patients.

Immune checkpoint protein inhibition for cancer: preclinical justification for CTLA-4 and PD-1 blockade and new combinations

Over the last two decades, our understanding of the molecular basis of immunity has revealed the complexity of regulatory pathways involved in immune responses to cancer. A significant body of data support the critical importance of immune checkpoints in the control of the adaptive immune response to malignancy, and suggest that inhibitors of those checkpoints might have significant utility in treating cancer. This has been borne out by the recent US Food and Drug Administration (FDA) approvals of two different antibodies, one against cytotoxic T-lymphocyte antigen-4 (CTLA-4) and one against programmed death-1 (PD-1). Here, we provide a comprehensive review of the literature regarding the preclinical justification for the use of CTLA-4 and PD-1 blockade as monotherapy, and as combination therapy in the treatment of cancer. The animal data strongly supported the use of these drugs in patients, and in many cases suggested strategies that directly led to successful registration trials. In contrast, many of the toxicities, and some of the unusual response patterns seen in patients with these drugs, were not predicted by the preclinical work that we cite, highlighting the importance of early-phase trials with patients to inform future drug development. In addition, we review herein the preclinical data surrounding emerging immune checkpoint proteins, including BTLA, VISTA, CD160, LAG3, TIM3, and CD244 as potential targets for inhibition. The current comprehensive review of the literature regarding CTLA-4 and PD-1, as well as a number of novel checkpoint proteins demonstrates a strong preclinical basis for the use of these antibodies singly and in combination to overcome checkpoint inhibition in the treatment of cancer. We also suggest that the use of these antibodies may augment the efficacy of other activating immune antibodies, cytokines, radiation, and adoptive cell therapy in human cancer.

The evolution of checkpoint blockade as a cancer therapy: what's here, what's next?

Unleashing the immune system to fight cancer has become one of the main treatment modalities since the anti-CTLA-4 antibody, ipilimumab was approved for patients with advanced melanoma in 2011. Pembrolizumab and nivolumab, two anti-PD-1 antibodies recently approved for the treatment of patients with metastatic melanoma, are being actively investigated for the treatment of multiple caners including lung, breast, bladder and renal cancers along with other anti-PD-1/L1 antibodies. Early results of combining of anti-CTLA-4 antibody and anti-PD-1 antibody treatment for advanced melanoma patients are showing impressive response rates with manageable toxicity profiles. There are several other checkpoint molecules that are likely potential inhibitory targets. The outcome of blocking some of these negative immune regulators, such as LAG-3 or TIM-3, is being pursued in the clinic or about to enter clinical development. Blockade of these molecules is demonstrating promising preclinical activity alone or when combined with anti-PD-1/L1. Future studies will define bio-markers of these therapies and how to target them alone or in combination with other immunotherapies, chemotherapy, radiotherapy and small molecule inhibitors.

The CD28-B7 Family in Anti-Tumor Immunity: Emerging Concepts in Cancer Immunotherapy

The interactions between B7 molecules and CD28-family receptors are crucial in the regulation of adaptive cellular immunity. In cancer, the aberrant expression of co-inhibitory B7 molecules has been attributed to reduced anti-tumor immunity and cancer immune evasion, prompting the development of cancer therapeutics that can restore T cell function. Murine tumor models have provided significant support for the targeting of multiple immune checkpoints involving CTLA-4, PD-1, ICOS, B7-H3 and B7-H4 during tumor growth, and clinical studies investigating the therapeutic effects of CTLA-4 and PD-1 blockade have shown exceptionally promising results in patients with advanced melanoma and other cancers. The expression pattern of co-inhibitory B7 ligands in the tumor microenvironment has also been largely correlated with poor patient prognosis, and recent evidence suggests that the presence of several B7 molecules may predict the responsiveness of immunotherapies that rely on pre-existing tumor-associated immune responses. While monotherapies blocking T cell co-inhibition have beneficial effects in reducing tumor burden, combinatorial immunotherapy targeting multiple immune checkpoints involved in various stages of the anti-tumor response has led to the most substantial impact on tumor reduction. In this review, we will examine the contributions of B7- and CD28-family members in the context of cancer development, and discuss the implications of current human findings in cancer immunotherapy.

Immune escape mechanisms as a guide for cancer immunotherapy

Immunotherapy has demonstrated impressive outcomes for some patients with cancer. However, selecting patients who are most likely to respond to immunotherapy remains a clinical challenge. Here, we discuss immune escape mechanisms exploited by cancer and present strategies for applying this knowledge to improving the efficacy of cancer immunotherapy.

Disruption of the immune-checkpoint VISTA gene imparts a proinflammatory phenotype with predisposition to the development of autoimmunity

V domain-containing Ig suppressor of T-cell activation (VISTA) is a negative checkpoint regulator that suppresses T cell-mediated immune responses. Previous studies using a VISTA-neutralizing monoclonal antibody show that VISTA blockade enhances T-cell activation. The current study describes a comprehensive characterization of mice in which the gene for VISTA has been deleted. Despite the apparent normal hematopoietic development in young mice, VISTA genetic deficiency leads to a gradual accumulation of spontaneously activated T cells, accompanied by the production of a spectrum of inflammatory cytokines and chemokines. Enhanced T-cell responsiveness was also observed upon immunization with neoantigen. Despite the presence of multiorgan chronic inflammation, aged VISTA-deficient mice did not develop systemic or organ-specific autoimmune disease. Interbreeding of the VISTA-deficient mice with 2D2 T-cell receptor transgenic mice, which are predisposed to the development of experimental autoimmune encephalomyelitis, drastically enhanced disease incidence and intensity. Disease development is correlated with the increase in the activation of encephalitogenic T cells in the periphery and enhanced infiltration into the CNS. Taken together, our data suggest that VISTA is a negative checkpoint regulator whose loss of function lowers the threshold for T-cell activation, allowing for an enhanced proinflammatory phenotype and an increase in the frequency and intensity of autoimmunity under susceptible conditions.

Characterization of programmed death-1 homologue-1 (PD-1H) expression and function in normal and HIV infected individuals

Chronic immune activation that persists despite anti-retroviral therapy (ART) is the strongest predictor of disease progression in HIV infection. Monocyte/macrophages in HIV-infected individuals are known to spontaneously secrete cytokines, although neither the mechanism nor the molecules involved are known. Here we show that overexpression of the newly described co-stimulatory molecule, PD1 homologue (PD-1H) in human monocyte/macrophages is sufficient to induce spontaneous secretion of multiple cytokines. The process requires signaling via PD-1H as cytokine secretion could be abrogated by deletion of the cytoplasmic domain. Such overexpression of PD-1H, associated with spontaneous cytokine expression is seen in monocytes from chronically HIV-infected individuals and this correlates with immune activation and CD4 depletion, but not viral load. Moreover, antigen presentation by PD-1H-overexpressing monocytes results in enhanced cytokine secretion by HIV-specific T cells. These results suggest that PD-1H might play a crucial role in modulating immune activation and immune response in HIV infection.

VISTA is a novel broad-spectrum negative checkpoint regulator for cancer immunotherapy

In the past few years, the field of cancer immunotherapy has made great progress and is finally starting to change the way cancer is treated. We are now learning that multiple negative checkpoint regulators (NCR) restrict the ability of T-cell responses to effectively attack tumors. Releasing these brakes through antibody blockade, first with anti-CTLA4 and now followed by anti-PD1 and anti-PDL1, has emerged as an exciting strategy for cancer treatment. More recently, a new NCR has surfaced called V-domain immunoglobulin (Ig)-containing suppressor of T-cell activation (VISTA). This NCR is predominantly expressed on hematopoietic cells, and in multiple murine cancer models is found at particularly high levels on myeloid cells that infiltrated the tumors. Preclinical studies with VISTA blockade have shown promising improvement in antitumor T-cell responses, leading to impeded tumor growth and improved survival. Clinical trials support combined anti-PD1 and anti-CTLA4 as safe and effective against late-stage melanoma. In the future, treatment may involve combination therapy to target the multiple cell types and stages at which NCRs, including VISTA, act during adaptive immune responses.

VISTA is an immune checkpoint molecule for human T cells

V-domain Ig suppressor of T cell activation (VISTA) is a potent negative regulator of T-cell function that is expressed on hematopoietic cells. VISTA levels are heightened within the tumor microenvironment, in which its blockade can enhance antitumor immune responses in mice. In humans, blockade of the related programmed cell death 1 (PD-1) pathway has shown great potential in clinical immunotherapy trials. Here, we report the structure of human VISTA and examine its function in lymphocyte negative regulation in cancer. VISTA is expressed predominantly within the hematopoietic compartment with highest expression within the myeloid lineage. VISTA-Ig suppressed proliferation of T cells but not B cells and blunted the production of T-cell cytokines and activation markers. Our results establish VISTA as a negative checkpoint regulator that suppresses T-cell activation, induces Foxp3 expression, and is highly expressed within the tumor microenvironment. By analogy to PD-1 and PD-L1 blockade, VISTA blockade may offer an immunotherapeutic strategy for human cancer.