Program death-1 signaling and regulatory T cells collaborate to resist the function of adoptively transferred cytotoxic T lymphocytes in advanced acute myeloid leukemia

Modulating T regulatory cells in cancer: how close are we?

Regulatory T cells (Tregs) are a specialized subset of CD4 T cells that have an indispensable role in maintaining immune homeostasis and tolerance. Although studies in mice and humans have clearly highlighted that the absence of these cells results in severe autoimmunity and inflammation, increased Treg numbers and/or function is not always beneficial. This is best exemplified in certain cancers where increased Tregs promote cancer progression by interfering with immune surveillance. Conversely, in other types of cancers that have an inflammatory component, Tregs can inhibit cancer progression by dampening inflammation. In this review article, we provide a historical perspective of the discovery of Tregs, followed by a summary of the existing literature on the role of Tregs in malignancy.

Restoring immune function of tumor-specific CD4+ T cells during recurrence of melanoma

Recurrent solid malignancies are often refractory to standard therapies. Although adoptive T cell transfer may benefit select individuals, the majority of patients succumb to their disease. To address this important clinical dilemma, we developed a mouse melanoma model in which initial regression of advanced disease was followed by tumor recurrence. During recurrence, Foxp3(+) tumor-specific CD4(+) T cells became PD-1(+) and represented >60% of the tumor-specific CD4(+) T cells in the host. Concomitantly, tumor-specific CD4(+) T effector cells showed traits of chronic exhaustion, as evidenced by their high expression of the PD-1, TIM-3, 2B4, TIGIT, and LAG-3 inhibitory molecules. Although blockade of the PD-1/PD-L1 pathway with anti-PD-L1 Abs or depletion of tumor-specific regulatory T cells (Tregs) alone failed to reverse tumor recurrence, the combination of PD-L1 blockade with tumor-specific Treg depletion effectively mediated disease regression. Furthermore, blockade with a combination of anti-PD-L1 and anti-LAG-3 Abs overcame the requirement to deplete tumor-specific Tregs. In contrast, successful treatment of primary melanoma with adoptive cell therapy required only Treg depletion or Ab therapy, underscoring the differences in the characteristics of treatment between primary and relapsing cancer. These data highlight the need for preclinical development of combined immunotherapy approaches specifically targeting recurrent disease.

Antagonist antibodies to PD-1 and B7-H1 (PD-L1) in the treatment of advanced human cancer

The immune suppressive molecule programmed death-1 (PD-1) is upregulated in activated T lymphocytes and inhibits T-cell function upon binding to its ligands B7-H1 (PD-L1, CD274) and B7-DC (PD-L2, CD273). Substantial experimental data from in vitro cell culture systems and animal models, and more recently from clinical trials, indicate that PD-1/PD-1-ligand interactions are a major mechanism of immune suppression within the tumor microenvironment. Initial clinical studies of antibodies directed against PD-1 and B7-H1 showed both an encouraging safety profile and remarkable antitumor activity in subsets of patients with metastatic disease, including malignancies--such as lung cancer--which were previously thought to be unresponsive to immunotherapy. Preliminary data have suggested a correlation between tumor membrane B7-H1 expression and clinical response to anti-PD-1 antibodies. Several key challenges remain to optimize development of PD-1/B7-H1 pathway blockade, including defining the biologic significance of all potential ligand-receptor interactions in the tumor microenvironment, developing more accurate predictive biomarkers of response, determining the breadth of activity in human malignancies, and developing rational combinations of therapy that address key mechanisms involved in positive and negative regulation of antitumor immune responses.

The role of B7 family molecules in hematologic malignancy

The B7 family consists of structurally related, cell-surface proteins that regulate immune responses by delivering costimulatory or coinhibitory signals through their ligands. Eight family members have been identified to date including CD80 (B7-1), CD86 (B7-2), CD274 (programmed cell death-1 ligand [PD-L1]), CD273 (programmed cell death-2 ligand [PD-L2]), CD275 (inducible costimulator ligand [ICOS-L]), CD276 (B7-H3), B7-H4, and B7-H6. B7 ligands are expressed on both lymphoid and nonlymphoid tissues. The importance of the B7 family in regulating immune responses is clear from their demonstrated role in the development of immunodeficiency and autoimmune diseases. Manipulation of the signals delivered by B7 ligands shows great potential in the treatment of cancers including leukemias and lymphomas and in regulating allogeneic T-cell responses after stem cell transplantation.

Induction of antitumor immunity ex vivo using dendritic cells transduced with fowl pox vector expressing MUC1, CEA, and a triad of costimulatory molecules (rF-PANVAC)

The fowl pox vector expressing the tumor-associated antigens, mucin-1 and carcinoembryonic antigen in the context of costimulatory molecules (rF-PANVAC) has shown promise as a tumor vaccine. However, vaccine-mediated expansion of suppressor T-cell populations may blunt clinical efficacy. We characterized the cellular immune response induced by ex vivo dendritic cells (DCs) transduced with (rF)-PANVAC. Consistent with the functional characteristics of potent antigen-presenting cells, rF-PANVAC-DCs demonstrated strong expression of mucin-1 and carcinoembryonic antigen and costimulatory molecules, CD80, CD86, and CD83; decreased levels of phosphorylated STAT3, and increased levels of Tyk2, Janus kinase 2, and STAT1. rF-PANVAC-DCs stimulated expansion of tumor antigen-specific T cells with potent cytolytic capacity. However, rF-PANVAC-transduced DCs also induced the concurrent expansion of FOXP3 expressing CD4CD25 regulatory T cells (Tregs) that inhibited T-cell activation. Moreover, Tregs expressed high levels of Th2 cytokines [interleukin (IL)-10, IL-4, IL-5, and IL-13] together with phosphorylated STAT3 and STAT6. In contrast, the vaccine-expanded Treg population expressed high levels of Th1 cytokines IL-2 and interferon-γ and the proinflammatory receptor-related orphan receptor γt (RORγt) and IL-17A suggesting that these cells may share effector functions with conventional TH17 T cells. These data suggest that Tregs expanded by rF-PANVAC-DCs, exhibit immunosuppressive properties potentially mediated by Th2 cytokines, but simultaneous expression of Th1 and Th17-associated factors suggests a high degree of plasticity.

Combined blockade of programmed death-1 and activation of CD137 increase responses of human liver T cells against HBV, but not HCV

BACKGROUND & AIMS

In patients with chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infections, antiviral functions of T cells are impaired; these might be increased by blocking T-cell co-inhibitory pathways, such as preventing interaction between the receptor programmed death (PD)-1 and its ligand, PD-L1. We attempted to optimize the restoration of T-cell functions in patients with chronic HBV or HCV infection with a combination of reagents that block PD-1 interaction with PD-L1 and stimulate T-cell signaling via CD137, a member of the tumor necrosis factor-receptor family.

METHODS

We assessed the effects of CD137 stimulation (via CD137L), alone or in combination with antibodies that block PD-1 interaction with PD-L1 (anti-PD-L1), on proliferation and production of interferon-γ and interleukin-2 by intrahepatic and peripheral T cells from patients with chronic HBV or HCV infection. We also analyzed expression of different co-stimulatory molecules on virus-specific CD8+ and forkhead box P3+CD4+ cells by flow cytometry.

RESULTS

Incubation of intrahepatic T cells with CD137L and anti-PD-L1 increased their responses to HBV, but not HCV. However, HCV-specific T cells isolated from peripheral blood were sensitive to these reagents. Virus-specific T cells from some, but not all patients, had increased responses to anti-PD-L1 when CD137L was added because in some cases the combination of anti-PD-L1 and CD137L overstimulated T cells, leading to their inhibition. Intrahepatic HBV- and HCV-specific CD8+ T cells had different costimulatory profiles; liver cells from patients with chronic HBV infection had a higher proportion of forkhead box P3+ regulatory T cells, with higher levels of PD-1, compared with liver cells from patients with chronic HCV infection.

CONCLUSIONS

A combination of reagents that prevent interaction between PD-1 and its ligand and activate CD137 signaling increase responses of intrahepatic HBV-specific T cells and circulating HCV-specific T cells. This strategy might be developed to increase T-cell responses to these viruses in patients with chronic hepatitis B or C, and tailoring the dose of CD137L administered will help optimize results.

PD-L1/PD-1 Co-Stimulation, a Brake for T cell Activation and a T cell Differentiation Signal

For T cell activation, three signals have to be provided from the antigen presenting cell; Signal 1 (antigen recognition), signal 2 (co-stimulation) and signal 3 (cytokine priming). Blocking negative co-stimulation during antigen presentation to T cells is becoming a promising therapeutic strategy to enhance cancer immunotherapy. Here we will focus on interference with PD-1/PD-L1 negative co-stimulation during antigen presentation to T cells as a therapeutic approach. We will discuss the potential mechanisms and the therapeutic consequences by which interference/inhibition with this interaction results in anti-tumour immunity. Particularly, we will comment on whether blocking negative co-stimulation provides differentiation signals to T cells undergoing antigen presentation. A major dogma in immunology states that T cell differentiation signals are given by cytokines and chemokines (signal 3) rather than co-stimulation (signal 2). We will discuss whether this is the case when blocking PD-L1/PD-1 negative co-stimulation.

Dendritic cells tolerized with adenosine A₂AR agonist attenuate acute kidney injury

DC-mediated NKT cell activation is critical in initiating the immune response following kidney ischemia/reperfusion injury (IRI), which mimics human acute kidney injury (AKI). Adenosine is an important antiinflammatory molecule in tissue inflammation, and adenosine 2A receptor (A₂AR) agonists protect kidneys from IRI through their actions on leukocytes. In this study, we showed that mice with A₂AR-deficient DCs are more susceptible to kidney IRI and are not protected from injury by A₂AR agonists. In addition, administration of DCs treated ex vivo with an A₂AR agonist protected the kidneys of WT mice from IRI by suppressing NKT production of IFN-γ and by regulating DC costimulatory molecules that are important for NKT cell activation. A₂AR agonists had no effect on DC antigen presentation or on Tregs. We conclude that ex vivo A₂AR-induced tolerized DCs suppress NKT cell activation in vivo and provide a unique and potent cell-based strategy to attenuate organ IRI.

Role of the PD-1 pathway in the immune response

Understanding immunoregulatory mechanisms is essential for the development of novel interventions to improve long-term allograft survival. Programmed death 1 (PD-1) and its ligands, PD-L1 and PD-L2, have emerged as critical inhibitory signaling pathways that regulate T cell response and maintain peripheral tolerance. PD-1 signaling inhibits alloreactive T cell activation, and can promote induced regulatory T cell development. Furthermore, the upregulation of PD-L1 on nonhematopoietic cells of the allograft may actively participate in the inhibition of immune responses and provide tissue-specific protection. In murine transplant models, this pathway has been shown to be critical for the induction and maintenance of graft tolerance. In this review, we discuss the current knowledge of the immunoregulatory functions of PD-1 and its ligands and their therapeutic potential in transplantation.

Autocrine adenosine signaling promotes regulatory T cell-mediated renal protection

Regulatory T cells (Tregs) suppress the innate inflammation associated with kidney ischemia-reperfusion injury (IRI), but the mechanism is not well understood. Tregs express CD73, the final enzyme involved in the production of extracellular adenosine, and activation of the adenosine 2A receptor (A(2A)R) on immune cells suppresses inflammation and preserves kidney function after IRI. We hypothesized that Treg-generated adenosine is required to block innate immune responses in kidney IRI and that the Treg-generated adenosine would signal through A(2A)Rs on inflammatory cells and, in an autocrine manner, on Tregs themselves. We found that adoptively transferred wild-type Tregs protected wild-type mice from kidney IRI, but the absence of adenosine generation (CD73-deficient Tregs) or adenosine responsiveness (A(2A)R-deficient Tregs) led to inhibition of Treg function. Pharmacologic stimulation of A(2A)R before adoptive transfer augmented the ability of wild-type and CD73-deficient Tregs to suppress kidney IRI. Microarray analysis and flow cytometry revealed that A(2A)R activation enhanced surface PD-1 expression on Tregs in the absence of any other activation signal. Treatment of Tregs with a PD-1 blocking antibody before adoptive transfer reversed their protective effects, even if pretreated with an A(2A)R agonist. Taken together, these results demonstrate that the simultaneous ability to generate and respond to adenosine is required for Tregs to suppress innate immune responses in IRI through a PD-1-dependent mechanism.

Coinhibitory molecules in hematologic malignancies: targets for therapeutic intervention

The adaptive immune system can be a potent defense mechanism against cancer; however, it is often hampered by immune suppressive mechanisms in the tumor microenvironment. Coinhibitory molecules expressed by tumor cells, immune cells, and stromal cells in the tumor milieu can dominantly attenuate T-cell responses against cancer cells. Today, a variety of coinhibitory molecules, including cytotoxic T lymphocyte–associated antigen-4, programmed death-1, B and T lymphocyte attenuator, LAG3, T-cell immunoglobulin and mucin domain 3, and CD200 receptor, have been implicated in immune escape of cancer cells. Sustained signaling via these coinhibitory molecules results in functional exhaustion of T cells, during which the ability to proliferate, secrete cytokines, and mediate lysis of tumor cells is sequentially lost. In this review, we discuss the influence of coinhibitory pathways in suppressing autologous and allogeneic T cell–mediated immunity against hematologic malignancies. In addition, promising preclinical and clinical data of immunotherapeutic approaches interfering with negative cosignaling, either as monotherapy or in conjunction with vaccination strategies, are reviewed. Numerous studies indicate that coinhibitory signaling hampers the clinical benefit of current immunotherapies. Therefore, manipulation of coinhibitory networks is an attractive adjuvant immunotherapeutic intervention for hematologic cancers after standard treatment with chemotherapy and hematopoietic stem cell transplantation.

The B7 homologues and their receptors in hematologic malignancies

The B7 homologues and their receptors regulate both peripheral tolerance and adaptive immunity. This field is rapidly evolving as new ligands and receptors are being identified. Much of the work supporting their role in the regulation of host anti-tumor immunity has been derived from experimental models and clinical trials in solid malignancies. However, a growing body of evidence demonstrates that the B7-H family has important immunologic and non-immunologic functions in a variety of hematologic malignancies. Herein, we will review recent evidence that supports the therapeutic targeting of the B7 homologues in hematologic malignancies.

Immune suppression in premalignant respiratory papillomas: enriched functional CD4+Foxp3+ regulatory T cells and PD-1/PD-L1/L2 expression

PURPOSE

Respiratory papillomas, caused by human papillomaviruses types 6 and 11 (HPV6/11), are premalignant lesions with potential for malignant conversion. The cytokine and chemokine micromilieu of papillomas is T(H)2-like with a marked absence of IFN-γ expression. To illuminate why patients with recurrent respiratory papillomatosis (RRP) fail to effectively control their disease, we further investigated the suppressive cellular microenvironment in papillomas.

EXPERIMENTAL DESIGN

CD4(+)CD25(+)CD127(low/-)Foxp3(+) regulatory T cells (Treg) and CD4(+)CD25(-)CD127(low/-)Foxp3(-) T cells within papillomas were characterized and isolated. Their suppressor function was measured by inhibition of peripheral blood mononuclear cell (PBMC) proliferation. Expression of PD-1, CD69, and Helios was identified on these T cells. PD-L1, PD-L2, CCL17, and CCL22 mRNA was also identified in papillomas by quantitative PCR.

RESULTS

Functional Tregs were markedly enriched in papillomas and strongly inhibited anti-CD3 and anti-CD28 antibody activated PBMC proliferation. The natural Treg marker Helios was reduced on Tregs from papillomas, indicating that the majority of Tregs in papillomas are adaptive. The majority of the papilloma-derived CD4(+) T cells expressed the CD4(+)CD25(-)CD127(low/-)Foxp3(-)PD1(+)CD69(+) phenotype and failed to suppress PBMC proliferation, suggesting that they are chronically activated and exhausted. The Treg-attracting chemokine CCL22 was equally expressed by all laryngeal tissues examined. However, CCL17 was robustly expressed by papillomas compared with unaffected laryngeal tissues from RRP patients and individuals without RRP. PD-L1 was elevated in papillomas compared with control laryngeal tissues.

CONCLUSIONS

Papilloma CD4(+) T cells are enriched with functional Tregs, and the adaptive Helios(-) Treg fraction was increased within the T(H)2-like papilloma micromilieu. CD4(+)CD25(-)CD127(low/-)Foxp3(-) T-cells failed to suppress PBMC proliferation and may be exhausted. The PD-1/PDL-1 pathway may represent an additional immunosuppressive mechanism that contributes to defective HPV6/11 clearance in RRP.

Cytotoxic chemotherapy and CD4+ effector T cells: an emerging alliance for durable antitumor effects

Standard cytotoxic chemotherapy can initially achieve high response rates, but relapses often occur in patients and represent a severe clinical problem. As increasing numbers of chemotherapeutic agents are found to have immunostimulatory effects, there is a growing interest to combine chemotherapy and immunotherapy for synergistic antitumor effects and improved clinical benefits. Findings from recent studies suggest that highly activated, polyfunctional CD4+ effector T cells have tremendous potential in strengthening and sustaining the overall host antitumor immunity in the postchemotherapy window. This review focuses on the latest progresses regarding the impact of chemotherapy on CD4+ T-cell phenotype and function and discusses the prospect of exploiting CD4+ T cells to control tumor progression and prevent relapse after chemotherapy.

Delineation of antigen-specific and antigen-nonspecific CD8(+) memory T-cell responses after cytokine-based cancer immunotherapy

Memory T cells exhibit tremendous antigen specificity within the immune system and accumulate with age. Our studies reveal an antigen-independent expansion of memory, but not naive, CD8(+) T cells after several immunotherapeutic regimens for cancer resulting in a distinctive phenotype. Signaling through T-cell receptors (TCRs) or CD3 in both mouse and human memory CD8(+) T cells markedly up-regulated programmed death-1 (PD-1) and CD25 (IL-2 receptor α chain), and led to antigen-specific tumor cell killing. In contrast, exposure to cytokine alone in vitro or with immunotherapy in vivo did not up-regulate these markers but resulted in expanded memory CD8(+) T cells expressing NKG2D, granzyme B, and possessing broadly lytic capabilities. Blockade of NKG2D in mice also resulted in significantly diminished antitumor effects after immunotherapy. Treatment of TCR-transgenic mice bearing nonantigen expressing tumors with immunotherapy still resulted in significant antitumor effects. Human melanoma tissue biopsies obtained from patients after topically applied immunodulatory treatment resulted in increased numbers of these CD8(+) CD25(-) cells within the tumor site. These findings demonstrate that memory CD8(+) T cells can express differential phenotypes indicative of adaptive or innate effectors based on the nature of the stimuli in a process conserved across species.

Th17 cells are long lived and retain a stem cell-like molecular signature

Th17 cells have been described as short lived, but this view is at odds with their capacity to trigger protracted damage to normal and transformed tissues. We report that Th17 cells, despite displaying low expression of CD27 and other phenotypic markers of terminal differentiation, efficiently eradicated tumors and caused autoimmunity, were long lived, and maintained a core molecular signature resembling early memory CD8(+) cells with stem cell-like properties. In addition, we found that Th17 cells had high expression of Tcf7, a direct target of the Wnt and β-catenin signaling axis, and accumulated β-catenin, a feature observed in stem cells. In vivo, Th17 cells gave rise to Th1-like effector cell progeny and also self-renewed and persisted as IL-17A-secreting cells. Multipotency was required for Th17 cell-mediated tumor eradication because effector cells deficient in IFN-γ or IL-17A had impaired activity. Thus, Th17 cells are not always short lived and are a less-differentiated subset capable of superior persistence and functionality.

TNFR2 on non-haematopoietic cells is required for Foxp3+ Treg-cell function and disease suppression in EAE

The TNF/TNFR system exerts multiple proinflammatory and immunosuppressive functions in the pathogenesis of chronic inflammation and autoimmunity. In EAE, the experimental model of Multiple Sclerosis (MS), genetic ablation of TNFR2, results in exacerbated immune reactivity and chronic disease course. The underlying mechanism driving this immunosuppressive function of TNFR2 remains unclear. We show here that chronic exacerbated EAE in TNFR2 KO mice is associated with increased Th17-cell responses and reduced numbers of Foxp3(+) Treg cells both in the spinal cord and peripheral lymphoid organs. Treg cells from TNFR2-deficient animals developing EAE show decreased proliferative and suppressive functions, both ex vivo and in vivo, and appear responsible for the exacerbated non-remitting disease, as evidenced by phenotypic rescue following adoptive transfer of Treg cells from WT but not TNFR2(-/-) donors. Reciprocal BM transplantation experiments between WT and TNFR2-deficient mice demonstrated that the capacity of TNFR2 to support Treg-cell expansion and function during EAE is non-intrinsic to Treg or other haematopoietic cells but requires expression of TNFR2 in radiation-resistant cells of the host. These results reveal a previously unsuspected role for non-haematopoietic TNFR2 in modulating Treg-cell expansion and immune suppression during development of autoimmunity and suggest that a similar mechanism may affect chronicity and relapses characterizing human autoimmune disease, including MS.

Interactions between NKT cells and Tregs are required for tolerance to combined bone marrow and organ transplants

We used a model of combined bone marrow and heart transplantation, in which tolerance and stable chimerism is induced after conditioning with fractionated irradiation of the lymphoid tissues and anti-T-cell antibodies. Graft acceptance and chimerism required host CD4(+)CD25(+) Treg production of IL-10 that was in-turn enhanced by host invariant natural killer (NK) T-cell production of IL-4. Up-regulation of PD-1 on host Tregs, CD4(+)CD25(-) conventional T (Tcon) cells, and CD8(+) T cells was also enhanced by NKT cell production of IL-4. Up-regulated PD-1 expression on Tregs was linked to IL-10 secretion, on CD8(+) T cells was linked to Tim-3 expression, and on CD4(+) Tcon cells was associated with reduced IFNγ secretion. Changes in the expression of PD-1 were induced by the conditioning regimen, and declined after bone marrow transplantation. In conclusion, NKT cells in this model promoted changes in expression of negative costimulatory receptors and anti-inflammatory cytokines by Tregs and other T-cell subsets in an IL-4-dependent manner that resulted in tolerance to the bone marrow and organ grafts.

Targeted immunotherapy for acute myeloid leukemia

Allogeneic hematopoietic stem cell transplantation (HSCT) demonstrated convincingly the potential of allogeneic T cells in causing sustained remissions in high-risk hematologic malignancies. However toxicity of allogeneic HSCT limits its application to a broader group of patients. An improved understanding of NK biology along with mechanisms of natural killer (NK) cell and T-cell-mediated alloreactivity against leukemia has led to several clinical immunotherapeutic strategies that preserve graft versus leukemia (GVL) while minimizing the toxicity of HSCT. Here we review strategies being explored both in HSCT and non-HSCT settings that include an emphasis on two key aspects: (a) Maximizing cytotoxicity of alloreactive cells, ie, NK cells and T cells, and (b) Targeted manipulation of critical pathways in T and NK cells contributing to sustained anti-leukemia effects.

Regulatory T cells in acute myelogenous leukemia: is it time for immunomodulation?

The microenviroment of acute myelogenous leukemia (AML) is suppressive for immune effector cells. Regulatory T cells (Tregs) have been recognized as a contributor factor and may be recruited and exploited by leukemic cells to evade immunesurveillance. Studies have shown that the frequencies of marrow and blood Tregs are greater in patients with AML than in control patients. Although increased Tregs have been associated with a decreased risk of GVHD after allogeneic HCT and hence may impede the graft-versus-tumor effect, recent findings indicate that that this may not be the case. Because there is a need to improve outcomes of standard treatment (chemotherapy with or without allogeneic HCT) in AML, targeting Tregs present an outstanding opportunity in AML because discoveries may apply throughout its treatment. Here, we review data on the roles of Tregs in mediating immune system-AML interactions. We focused on in vitro, animal, and observational human studies of Tregs in AML biology, development, prognosis, and therapy in different settings (eg, vaccination and HCT). Manipulation of Tregs or other types of immunomodulation may become a part of AML treatment in the future.

PD-1/PD-L1 interactions contribute to functional T-cell impairment in patients who relapse with cancer after allogeneic stem cell transplantation

Tumor relapses remain a serious problem after allogeneic stem cell transplantation (alloSCT), despite the long-term persistence of minor histocompatibility antigen (MiHA)-specific memory CD8(+) T cells specific for the tumor. We hypothesized that these memory T cells may lose their function over time in transplanted patients. Here, we offer functional and mechanistic support for this hypothesis, based on immune inhibition by programmed death-1 (PD-1) expressed on MiHA-specific CD8(+) T cells and the associated role of the PD-1 ligand PD-L1 on myeloid leukemia cells, especially under inflammatory conditions. PD-L1 was highly upregulated on immature human leukemic progenitor cells, whereas costimulatory molecules such as CD80 and CD86 were not expressed. Thus, immature leukemic progenitor cells seemed to evade the immune system by inhibiting T-cell function via the PD-1/PD-L1 pathway. Blocking PD-1 signaling using human antibodies led to elevated proliferation and IFN-γ production of MiHA-specific T cells cocultured with PD-L1-expressing leukemia cells. Moreover, patients with relapsed leukemia after initial MiHA-specific T-cell responses displayed high PD-L1 expression on CD34(+) leukemia cells and increased PD-1 levels on MiHA-specific CD8(+) T cells. Importantly, blocking PD-1/PD-L1 interactions augment proliferation of MiHA-specific CD8(+) memory T cells from relapsed patients. Taken together, our findings indicate that the PD-1/PD-L pathway can be hijacked as an immune escape mechanism in hematological malignancies. Furthermore, they suggest that blocking the PD-1 immune checkpoint offers an appealing immunotherapeutic strategy following alloSCT in patients with recurrent or relapsed disease.

FOXO3 programs tumor-associated DCs to become tolerogenic in human and murine prostate cancer

The limited success of cancer immunotherapy is often attributed to the loss of antigen-specific T cell function in situ. However, the mechanism for this loss of function is unknown. In this study, we describe a population of tumor-associated DCs (TADCs) in both human and mouse prostate cancer that tolerizes and induces suppressive activity in tumor-specific T cells. In tumors from human prostate cancer patients and transgenic adenocarcinoma of the mouse prostate (TRAMP) mice, TADCs expressed elevated levels of FOXO3 and Foxo3, respectively, which correlated with expression of suppressive genes that negatively regulate T cell function. Silencing FOXO3 and Foxo3 with siRNAs abrogated the ability of human and mouse TADCs, respectively, to tolerize and induce suppressive activity by T cells. Silencing Foxo3 in mouse TADCs was also associated with diminished expression of tolerogenic mediators, such as indoleamine-2,3-dioxygenase, arginase, and TGF-β, and upregulated expression of costimulatory molecules and proinflammatory cytokines. Importantly, transfer of tumor-specific CD4+ Th cells into TRAMP mice abrogated TADC tolerogenicity, which was associated with reduced Foxo3 expression. These findings demonstrate that FOXO3 may play a critical role in mediating TADC-induced immune suppression. Moreover, our results identify what we believe to be a novel target for preventing CTL tolerance and enhancing immune responses to cancer by modulating the immunosuppressive activity of TADCs found in the tumor microenvironment.

Coexpression of Tim-3 and PD-1 identifies a CD8+ T-cell exhaustion phenotype in mice with disseminated acute myelogenous leukemia

Tumor-associated immune suppression can lead to defective T cell-mediated antitumor immunity. Here, we identified a unique phenotype of exhausted T cells in mice with advanced acute myelogenous leukemia (AML). This phenotype is characterized by the coexpression of Tim-3 and PD-1 on CD8(+) T cells in the liver, the major first site of AML metastases. PD-1 and Tim-3 coexpression increased during AML progression. PD-1(+)Tim-3(+) CD8(+) T cells were deficient in their ability to produce IFN-γ, TNF-α, and IL-2 in response to PD-1 ligand (PDL1) and Tim-3 ligand (galectin-9) expressing AML cells. PD-1 knockout (KO), which were partially resistant to AML challenge, up-regulated Tim-3 during AML progression and such Tim-3(+)PD-1- KO CD8(+) T cells had reduced cytokine production. Galectin-9 KO mice were more resistant to AML, which was associated with reduced T-regulatory cell accumulation and a modest induction of PD-1 and Tim-3 expression on CD8(+) T cells. Whereas blocking the PD-1/PDL1 or Tim-3/galectin-9 pathway alone was insufficient to rescue mice from AML lethality, an additive effect was seen in reducing-albeit not eliminating-both tumor burden and lethality when both pathways were blocked. Therefore, combined PD-1/PDL1 and Tim-3/galectin-9 blockade may be beneficial in preventing CD8(+) T-cell exhaustion in patients with hematologic malignancies such as advanced AML.

On the Mechanism of T cell receptor down-modulation and its physiological significance

Effective, long-lasting immune responses largely depend upon T cell reponses. Antigen-specific T lymphocytes are activated and differentiate into effector T cells after antigen presentation by professional antigen presenting cells (APCs). However, T cell responses are tightly regulated to prevent T cell hyperactivation which may end up in autoimmune pathology. One of these regulatory mechanisms is ligand-induced TCR down-modulation, a process by which TCRs are removed from the T cell surface shortly after engagement with their cognate antigenic peptide associated to MHC molecules on the APC. TCR down-modulation is a complicated process. Here we briefly describe the three main models that attempt to clarify this mechanism in the context of T cell activation and function.

Immune checkpoint proteins: a new therapeutic paradigm for cancer--preclinical background: CTLA-4 and PD-1 blockade

Much of the recent excitement in the translational field of tumor immunology and immunotherapy has been generated by the recognition that immune checkpoint proteins can be blocked by human antibodies with profound effects in vitro, in animal tumor systems, and in patients. Promising clinical data have already been generated in melanoma and other tumor types with human antibodies directed against cytotoxic T lymphocyte antigen-4 (CTLA-4) and programmed death-1 (PD-1). The preclinical data that supported the clinical development of these two antibodies will be discussed in detail in this review, showing that many of the therapeutic effects of these two agents were predicted by the animal models, as were the immune-related side effects noted with these drugs. In contrast, much of the early work with anti-CTLA-4 antibodies indicated that it had a potent therapeutic effect only when combined with granulocyte-macrophage colony-stimulating factor (GM-CSF)-transduced tumor vaccines, and that the antibody alone was effective only in the most immunogenic tumor models in mice. Intriguingly, in patients, the drug alone clearly has had important therapeutic effects, but the addition of vaccines has not added to its clinical benefit. Murine experiments also suggested that CTLA-4 abrogation might function via important effects on natural T-regulatory cells that were CD4(+), CD25(+high), and FOXp3(+), but this has not been borne out in experiments using peripheral blood mononuclear cells from patients treated with anti-CTLA-4 antibodies, and unlike in animals, in humans the exact mechanism(s) by which CTLA-4 abrogation induced an anti-tumor effect is still unclear. Abrogation of PD-1 functions via different immune signaling pathways than CTLA-4 and is likely to have a different spectrum of effects than blocking CTLA-4. For PD-1 blockade, murine experiments have suggested that the antibody alone and combined with adoptive cell transfer or vaccine approaches would be therapeutically beneficial, and that clear effects on T-cell proliferation and activation, as well as T-regulatory cell function would be observed in patients. The clinical development of anti-PD-1 antibody so far has shown that it has a potent effect when administered alone, and trials of vaccines with anti-PD-1 are just being initiated to test the idea that the predicted effects of that antibody observed in animal systems also would be seen in patients. These observations support the idea that animal preclinical therapeutic experiments are an important guide to the conduct of trials employing abrogation of immune checkpoint proteins in T cells in patients. Nonetheless, clinical investigators must be flexible and prepared to find that the biology of those systems may be very different in humans compared to mice.

PD-L1 blockade effectively restores strong graft-versus-leukemia effects without graft-versus-host disease after delayed adoptive transfer of T-cell receptor gene-engineered allogeneic CD8+ T cells

Adoptive transfer (AT) of T cells forced to express tumor-reactive T-cell receptor (TCR) genes is an attractive strategy to direct autologous T-cell immunity against tumor-associated antigens. However, clinical effectiveness has been hampered by limited in vivo persistence. We investigated whether the use of major histocompatibility complex-mismatched T cells would prolong the in vivo persistence of tumor-reactive TCR gene expressing T cells by continuous antigen-driven proliferation via the endogenous potentially alloreactive receptor. Donor-derived CD8(+) T cells engineered to express a TCR against a leukemia-associated antigen mediated strong graft-versus-leukemia (GVL) effects with reduced graft-versus-host disease (GVHD) severity when given early after transplantation. AT later after transplantation resulted in a complete loss of GVL. Loss of function was associated with reduced expansion of TCR-transduced T cells as assessed by CDR3 spectratyping analysis and PD-1 up-regulation on T cells in leukemia-bearing recipients. PD-L1 blockade in allogeneic transplant recipients largely restored the GVL efficacy without triggering GVHD, whereas no significant antileukemia effects of PD-L1 blockade were observed in syngeneic controls. These data suggest a clinical approach in which the AT of gene-modified allogeneic T cells early after transplantation can provide a potent GVL effect without GVHD, whereas later AT is effective only with concurrent PD-L1 blockade.