Targeting the TCR signaling checkpoint: a therapeutic strategy to reactivate memory T cells in the tumor microenvironment

Immune checkpoints and cancer development: Therapeutic implications and future directions

Over the past few decades, different inhibitory receptors have been identified, which have played prominent roles in reducing anti-tumor immune responses. The role of immune checkpoint inhibitors in cancer was revealed by critical blockade of the cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and programmed cell death protein-1 (PD-1) checkpoints. Immune checkpoint inhibitors, including anti-PD-1 (nivolumab and pembrolizumab), anti-PD-L1 (Atezolizumab, avelumab, and duravulumab), and anti-CTLA-4 (ipilimumab, tremelimumab), are currently FDA-approved treatment options for a broad range of cancer types. However, regarding immunotherapy advances in recent years, most studies have been focused on finding the antibodies against other inhibitory immune checkpoints in the tumor microenvironment such as lymphocyte activation gene-3 (LAG-3), T cell immunoglobulin, and mucin domain 3 (TIM-3), B7-homolog 3 (B7-H3), V-domain immunoglobulin-containing suppressor of T-cell activation (VISTA), diacylglycerol kinase-α (DGK-α), T cell immunoglobulin and ITIM domain (TIGIT), and B and T lymphocyte attenuator (BTLA). This immune checkpoint exerts differential inhibitory impacts on various types of lymphocytes. The suppression of immune responses demonstrates a surprising synergy with PD-1. Therefore, most antibodies against these immune checkpoints are undertaking clinical trials for cancer immunotherapy of advanced solid tumors and hematologic malignancies. In this review, we will summarize recent findings of immune checkpoint and the role of monoclonal antibodies in cancer immunotherapy targeting these receptors.

Immune checkpoint blockade opens a new way to cancer immunotherapy

Among the main promising systems to triggering therapeutic antitumor immunity is the blockade of immune checkpoints. Immune checkpoint pathways regulate the control and eradication of infections, malignancies, and resistance against a host of autoantigens. Initiation point of the immune response is T cells, which have a critical role in this pathway. As several immune checkpoints are initiated by ligand-receptor interactions, they can be freely blocked by antibodies or modulated by recombinant forms of ligands or receptors. Antibodies against cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) were the first immunotherapeutics that achieved the US Food and Drug Administration approval. Preliminary clinical results with the blockers of additional immune checkpoint proteins, such as programmed cell death protein 1 (PD-1) indicate extensive and different chances to boost antitumor immunity with the objective of conferring permanent clinical effects. This study provides an overview of the immune checkpoint pathways, including CTLA-4, PD-1, lymphocyte activation gene 3, T-cell immunoglobulin and mucin domain 3, B7-H3, and diacylglycerol kinase α and implications of their inhibition in the cancer therapy.

Exosomes Associated with Human Ovarian Tumors Harbor a Reversible Checkpoint of T-cell Responses

Nano-sized membrane-encapsulated extracellular vesicles isolated from the ascites fluids of ovarian cancer patients are identified as exosomes based on their biophysical and compositional characteristics. We report here that T cells pulsed with these tumor-associated exosomes during TCR-dependent activation inhibit various activation endpoints including translocation of NFκB and NFAT into the nucleus, upregulation of CD69 and CD107a, production of cytokines, and cell proliferation. In addition, the activation of virus-specific CD8⁺ T cells that are stimulated with the cognate viral peptides presented in the context of class I MHC is also suppressed by the exosomes. The inhibition occurs without loss of cell viability and coincidentally with the binding and internalization of these exosomes. This exosome-mediated inhibition of T cells was transient and reversible: T cells exposed to exosomes can be reactivated once exosomes are removed. We conclude that tumor-associated exosomes are immunosuppressive and represent a therapeutic target, blockade of which would enhance the antitumor response of quiescent tumor-associated T cells and prevent the functional arrest of adoptively transferred tumor-specific T cells or chimeric antigen receptor T cells. Cancer Immunol Res; 6(2); 236-47. ©2018 AACR.

Extracellular Vesicles Present in Human Ovarian Tumor Microenvironments Induce a Phosphatidylserine-Dependent Arrest in the T-cell Signaling Cascade

The identification of immunosuppressive factors within human tumor microenvironments, and the ability to block these factors, would be expected to enhance patients' antitumor immune responses. We previously established that an unidentified factor, or factors, present in ovarian tumor ascites fluids reversibly inhibited the activation of T cells by arresting the T-cell signaling cascade. Ultracentrifugation of the tumor ascites fluid has now revealed a pellet that contains small extracellular vesicles (EV) with an average diameter of 80 nm. The T-cell arrest was determined to be causally linked to phosphatidylserine (PS) that is present on the outer leaflet of the vesicle bilayer, as a depletion of PS-expressing EV or a blockade of PS with anti-PS antibody significantly inhibits the vesicle-induced signaling arrest. The inhibitory EV were also isolated from solid tumor tissues. The presence of immunosuppressive vesicles in the microenvironments of ovarian tumors and our ability to block their inhibition of T-cell function represent a potential therapeutic target for patients with ovarian cancer.

Humanized mouse model of ovarian cancer recapitulates patient solid tumor progression, ascites formation, and metastasis

Ovarian cancer is the most common cause of death from gynecological cancer. Understanding the biology of this disease, particularly how tumor-associated lymphocytes and fibroblasts contribute to the progression and metastasis of the tumor, has been impeded by the lack of a suitable tumor xenograft model. We report a simple and reproducible system in which the tumor and tumor stroma are successfully engrafted into NOD-scid IL2Rγ^null (NSG) mice. This is achieved by injecting tumor cell aggregates derived from fresh ovarian tumor biopsy tissues (including tumor cells, and tumor-associated lymphocytes and fibroblasts) i.p. into NSG mice. Tumor progression in these mice closely parallels many of the events that are observed in ovarian cancer patients. Tumors establish in the omentum, ovaries, liver, spleen, uterus, and pancreas. Tumor growth is initially very slow and progressive within the peritoneal cavity with an ultimate development of tumor ascites, spontaneous metastasis to the lung, increasing serum and ascites levels of CA125, and the retention of tumor-associated human fibroblasts and lymphocytes that remain functional and responsive to cytokines for prolonged periods. With this model one will be able to determine how fibroblasts and lymphocytes within the tumor microenvironment may contribute to tumor growth and metastasis, and will make it possible to evaluate the efficacy of therapies that are designed to target these cells in the tumor stroma.

Tumor-resident CD8+ T-cell: the critical catalyst in IL-12-mediated reversal of tumor immune suppression

Tumor-resident T cells display a functionally impaired effector/memory (Tem) phenotype. Sustained intratumoral administration of IL-12, on the other hand, can restore cytolytic function to pre-existing CD8+ Tem, resulting in effective tumor kill. Whereas cytotoxic T lymphocytes (CTL) are generally assumed to mediate tumor regression via direct tumor cytotoxicity, recent work revealed that activated CD8+ Tem mobilize a systemic, multi-component effector cascade that includes both innate and adaptive immune mechanisms. Here we summarize these mechanisms, review how tumor-resident CD8+ Tem orchestrate this cascade and discuss the potential clinical implications of these findings.

T cells and stromal fibroblasts in human tumor microenvironments represent potential therapeutic targets

The immune system of cancer patients recognizes tumor-associated antigens expressed on solid tumors and these antigens are able to induce tumor-specific humoral and cellular immune responses. Diverse immunotherapeutic strategies have been used in an attempt to enhance both antibody and T cell responses to tumors. While several tumor vaccination strategies significantly increase the number of tumor-specific lymphocytes in the blood of cancer patients, most vaccinated patients ultimately experience tumor progression. CD4+ and CD8+ T cells with an effector memory phenotype infiltrate human tumor microenvironments, but most are hyporesponsive to stimulation via the T cell receptor (TCR) and CD28 under conditions that activate memory T cells derived from the peripheral blood of the cancer patients or normal donors. Attempts to identify cells and molecules responsible for the TCR signaling arrest of tumor-infiltrating T cells have focused largely upon the immunosuppressive effects of tumor cells, tolerogenic dendritic cells and regulatory T cells. Here we review potential mechanisms by which human T cell function is arrested in the tumor microenvironment with a focus on the immunomodulatory effects of stromal fibroblasts. Determining in vivo which cells and molecules are responsible for the TCR arrest in human tumor-infiltrating T cells will be necessary to formulate and test strategies to prevent or reverse the signaling arrest of the human T cells in situ for a more effective design of tumor vaccines. These questions are now addressable using novel human xenograft models of tumor microenvironments.

IL-12 delivered intratumorally by multilamellar liposomes reactivates memory T cells in human tumor microenvironments

Using a novel loading technique, IL-12 is reported here to be efficiently encapsulated within large multilamellar liposomes. The preclinical efficacy of the cytokine loaded liposomes to deliver IL-12 into human tumors and to reactive tumor-associated T cells in situ is tested using a human tumor xenograft model. IL-12 is released in vivo from these liposomes in a biologically active form when injected into tumor xenografts that are established by the subcutaneous implantation of non-disrupted pieces of human lung, breast or ovarian tumors into immunodeficient mice. The histological architecture of the original tumor tissue, including tumor-associated leukocytes, tumor cells and stromal cells is preserved anatomically and the cells remain functionally responsive to cytokines in these xenografts. The local and sustained release of IL-12 into the tumor microenvironment reactivates tumor-associated quiescent effector memory T cells to proliferate, produce and release IFN-gamma resulting in the killing of tumor cells in situ. Very little IL-12 is detected in the serum of mice for up to 5 days after an intratumoral injection of the IL-12 liposomes. We conclude that IL-12 loaded large multilamellar liposomes provide a safe method for the local and sustained delivery of IL-12 to tumors and a therapeutically effective way of reactivating existing tumor-associated T cells in human solid tumor microenvironments. The potential of this local in situ T cell re-stimulation to induce a systemic anti-tumor immunity is discussed.

Central role of tumor-associated CD8+ T effector/memory cells in restoring systemic antitumor immunity

Sustained delivery of IL-12 and GM-CSF to tumors induces the activation of tumor-resident CD8(+) T effector/memory cells (Tem) followed by cytotoxic CD8(+) T effector cell expansion. To determine whether the secondary effectors expanded from tumor-associated Tem or were primed de novo, activation kinetics of tumor-draining lymph node (TDLN) CD8(+) T cells were analyzed. Treatment promoted a 4-fold increase in the numbers of TDLN CD8(+) T cells displaying a CD69(+)CCR5(+)CD62L(-) periphery-homing effector phenotype by day 4 posttherapy. Pulse labeling of tumor and TDLN T cells with BrdU confirmed that proliferation occurred exclusively within the draining lymph nodes between days 1 and 4 with subsequent migration of primed CD8(+) T effectors to tumors on day 7. Day 4 CD8(+) T effector cells preferentially homed to and lysed experimental, but not control, tumors, establishing tumor specificity. To determine whether the secondary CD8(+) T effector cell response was dependent on activation of tumor-resident CD8(+) Tem, mice that were selectively depleted of tumor-infiltrating CD8(+) T cells were treated and monitored for T effector priming. In the absence of tumor-resident CD8(+) Tem, T effector cell expansion was completely abrogated in the TDLN, revealing that restoration of CD8(+) Tem function was critical to the induction of secondary T effectors. T cell priming failed to occur in IFN-gamma or perforin knockout mice, demonstrating that the requirement for Tem activation was associated with induction of Tem cytotoxicity. These data confirm that intratumoral IL-12 plus GM-CSF induces de novo priming of tumor-specific CD8(+) T effector cells in the TDLN and establish the critical role of preexisting intratumoral CD8(+) Tem in driving this process.