Myeloid leukemic progenitor cells can be specifically targeted by minor histocompatibility antigen LRH-1–reactive cytotoxic T cells

Tissue- and temporal-specific roles of extracellular ATP on T cell metabolism and function

The activation and function of T cells is fundamental for the control of infectious diseases and cancer, and conversely can mediate several autoimmune diseases. Among the signaling pathways leading to T cell activation and function, the sensing of extracellular adenosine triphosphate (eATP) has been recently appreciated as an important component. Through a plethora of purinergic receptors, most prominently P2RX7, eATP sensing can induce a wide variety of processes in T cells, such as proliferation, subset differentiation, survival, or cell death. The downstream roles of eATP sensing can vary according to (a) the T cell subset, (b) the tissue where T cells are, and (c) the time after antigen exposure. In this mini-review, we revisit the recent findings on how eATP signaling pathways regulate T-cell immune responses and posit important unanswered questions on this field.

P2X receptors in cancer growth and progression

It is increasingly appreciated that ion channels have a crucial role in tumors, either as promoters of cancer cell growth, or modulators of immune cell functions, or both. Among ion channels, P2X receptors have a special status because they are gated by ATP, a common and abundant component of the tumor microenvironment. Furthermore, one P2X receptor, i.e. P2X7, may also function as a conduit for ATP release, thus fuelling the increased extracellular ATP level in the tumor interstitium. These findings show that P2X receptors and extracellular ATP are indissoluble partners and key regulators of tumor growth, and suggest the exploitation of the extracellular ATP-P2X partnership to develop innovative therapeutic approaches to cancer.

HA-1H T-Cell Receptor Gene Transfer to Redirect Virus-Specific T Cells for Treatment of Hematological Malignancies After Allogeneic Stem Cell Transplantation: A Phase 1 Clinical Study

Graft-vs.-leukemia (GVL) reactivity after HLA-matched allogeneic stem cell transplantation (alloSCT) is mainly mediated by donor T cells recognizing minor histocompatibility antigens (MiHA). If MiHA are targeted that are exclusively expressed on hematopoietic cells of recipient origin, selective GVL reactivity without severe graft-vs.-host-disease (GVHD) may occur. In this phase I study we explored HA-1H TCR gene transfer into T cells harvested from the HA-1H negative stem-cell donor to treat HA-1H positive HLA-A^*02:01 positive patients with high-risk leukemia after alloSCT. HA-1H is a hematopoiesis-restricted MiHA presented in HLA-A^*02:01. Since we previously demonstrated that donor-derived virus-specific T-cell infusions did not result in GVHD, we used donor-derived EBV and/or CMV-specific T-cells to be redirected by HA-1H TCR. EBV and/or CMV-specific T-cells were purified, retrovirally transduced with HA-1H TCR, and expanded. Validation experiments illustrated dual recognition of viral antigens and HA-1H by HA-1H TCR-engineered virus-specific T-cells. Release criteria included products containing more than 60% antigen-specific T-cells. Patients with high risk leukemia following T-cell depleted alloSCT in complete or partial remission were eligible. HA-1H TCR T-cells were infused 8 and 14 weeks after alloSCT without additional pre-conditioning chemotherapy. For 4/9 included patients no appropriate products could be made. Their donors were all CMV-negative, thereby restricting the production process to EBV-specific T-cells. For 5 patients a total of 10 products could be made meeting the release criteria containing 3–280 × 10⁶ virus and/or HA-1H TCR T-cells. No infusion-related toxicity, delayed toxicity or GVHD occurred. One patient with relapsed AML at time of infusions died due to rapidly progressing disease. Four patients were in remission at time of infusion. Two patients died of infections during follow-up, not likely related to the infusion. Two patients are alive and well without GVHD. In 2 patients persistence of HA-1H TCR T-cells could be illustrated correlating with viral reactivation, but no overt in-vivo expansion of infused T-cells was observed. In conclusion, HA-1H TCR-redirected virus-specific T-cells could be made and safely infused in 5 patients with high-risk AML, but overall feasibility and efficacy was too low to warrant further clinical development using this strategy. New strategies will be explored using patient-derived donor T-cells isolated after transplantation transduced with HA-1H-specific TCR to be infused following immune conditioning.

Mice Lacking the Purinergic Receptor P2X5 Exhibit Defective Inflammasome Activation and Early Susceptibility to Listeria monocytogenes

P2X5 is a member of the P2X purinergic receptor family of ligand-gated cation channels and has recently been shown to regulate inflammatory bone loss. In this study, we report that P2X5 is a protective immune regulator during Listeria monocytogenes infection, as P2X5-deficient mice exhibit increased bacterial loads in the spleen and liver, increased tissue damage, and early (within 3-6 d) susceptibility to systemic L. monocytogenes infection. Whereas P2X5-deficient mice experience normal monocyte recruitment in response to L. monocytogenes, P2X5-deficient bone marrow-derived macrophages (BMMs) exhibit defective cytosolic killing of L. monocytogenes We further showed that P2X5 is required for L. monocytogenes-induced inflammasome activation and IL-1β production and that defective L. monocytogenes killing in P2X5-deficient BMMs is substantially rescued by exogenous IL-1β or IL-18. Finally, in vitro BMM killing and in vivo L. monocytogenes infection experiments employing either P2X7 deficiency or extracellular ATP depletion suggest that P2X5-dependent anti-L. monocytogenes immunity is independent of the ATP-P2X7 inflammasome activation pathway. Together, our findings elucidate a novel and specific role for P2X5 as a critical mediator of protective immunity.

PD-L1 siRNA-mediated silencing in acute myeloid leukemia enhances anti-leukemic T cell reactivity

Acute myeloid leukemia (AML) is an immune-susceptible malignancy, as demonstrated by its responsiveness to allogeneic stem cell transplantation (alloSCT). However, by employing inhibitory signaling pathways, including PD-1/PD-L1, leukemia cells suppress T cell-mediated immune attack. Notably, impressive clinical efficacy has been obtained with PD-1/PD-L1 blocking antibodies in cancer patients. Yet, these systemic treatments are often accompanied by severe toxicity, especially after alloSCT. Here, we investigated RNA interference technology as an alternative strategy to locally interfere with PD-1/PD-L1 signaling in AML. We demonstrated efficient siRNA-mediated PD-L1 silencing in HL-60 and patients' AML cells. Importantly, WT1-antigen T cell receptor⁺ PD-1⁺ 2D3 cells showed increased activation toward PD-L1 silenced WT1⁺ AML. Moreover, PD-L1 silenced AML cells significantly enhanced the activation, degranulation, and IFN-γ production of minor histocompatibility antigen-specific CD8⁺ T cells. Notably, PD-L1 silencing was equally effective as PD-1 antibody blockade. Together, our study demonstrates that PD-L1 silencing may be an effective strategy to augment AML immune-susceptibility. This provides rationale for further development of targeted approaches to locally interfere with immune escape mechanisms in AML, thereby minimizing severe toxicity. In combination with alloSCT and/or adoptive T cell transfer, this strategy could be very appealing to boost graft-versus-leukemia immunity and improve outcome in AML patients.

Minor Histocompatibility Antigen-Specific T Cells

Minor Histocompatibility (H) antigens are major histocompatibility complex (MHC)/Human Leukocyte Antigen (HLA)-bound peptides that differ between allogeneic hematopoietic stem cell transplantation (HCT) recipients and their donors as a result of genetic polymorphisms. Some minor H antigens can be used as therapeutic T cell targets to augment the graft-vs.-leukemia (GVL) effect in order to prevent or manage leukemia relapse after HCT. Graft engineering and post-HCT immunotherapies are being developed to optimize delivery of T cells specific for selected minor H antigens. These strategies have the potential to reduce relapse risk and thereby permit implementation of HCT approaches that are associated with less toxicity and fewer late effects, which is particularly important in the growing and developing pediatric patient. Most minor H antigens are expressed ubiquitously, including on epithelial tissues, and can be recognized by donor T cells following HCT, leading to graft-vs.-host disease (GVHD) as well as GVL. However, those minor H antigens that are expressed predominantly on hematopoietic cells can be targeted for selective GVL. Once full donor hematopoietic chimerism is achieved after HCT, hematopoietic-restricted minor H antigens are present only on residual recipient malignant hematopoietic cells, and these minor H antigens serve as tumor-specific antigens for donor T cells. Minor H antigen-specific T cells that are delivered as part of the donor hematopoietic stem cell graft at the time of HCT contribute to relapse prevention. However, in some cases the minor H antigen-specific T cells delivered with the graft may be quantitatively insufficient or become functionally impaired over time, leading to leukemia relapse. Following HCT, adoptive T cell immunotherapy can be used to treat or prevent relapse by delivering large numbers of donor T cells targeting hematopoietic-restricted minor H antigens. In this review, we discuss minor H antigens as T cell targets for augmenting the GVL effect in engineered HCT grafts and for post-HCT immunotherapy. We will highlight the importance of these developments for pediatric HCT.

Immunotherapy for acute myeloid leukemia: from allogeneic stem cell transplant to novel therapeutics

Immunotherapy in the form of allogeneic stem cell transplantation (SCT) plays an instrumental role in the treatment of acute myeloid leukemia (AML), with non-transplant modalities of immunotherapy including checkpoint blockade now being actively explored. Here, we provide an overview of the graft versus leukemia (GVL) effect in AML as a window into understanding the prospects of AML immunotherapy. We explore the roles of various cell types in orchestrating anti-leukemic immunity, as well as those contributing to the unique immune suppressive state of myeloid diseases. We discuss specific approaches to engage the immune system, while noting the challenges of the AML antigen landscape and the barriers to immune modulation. We review the potential for immunomodulatory agents in combination with cellular therapies, donor lymphocyte infusion, and following SCT. Finally, to address the challenge of minimal residual disease (MRD) following chemotherapy, we propose combination epigenetic and immunotherapy for the eradication of MRD.

CD4 Donor Lymphocyte Infusion Can Cause Conversion of Chimerism Without GVHD by Inducing Immune Responses Targeting Minor Histocompatibility Antigens in HLA Class II

Under non-inflammatory conditions HLA class II is predominantly expressed on hematopoietic cells. Therefore, donor CD4 T-cells after allogeneic stem cell transplantation (alloSCT) may mediate graft-vs.-leukemia reactivity without graft-vs.-host disease (GVHD). We analyzed immune responses in four patients converting from mixed to full donor chimerism without developing GVHD upon purified CD4 donor lymphocyte infusion (DLI) from their HLA-identical sibling donor after T-cell depleted alloSCT. In vivo activated T-cells were clonally isolated after CD4 DLI. Of the alloreactive T-cell clones, 96% were CD4 positive, illustrating the dominant role of CD4 T-cells in the immune responses. We identified 9 minor histocompatibility antigens (MiHA) as targets for alloreactivity, of which 8 were novel HLA class II restricted MiHA. In all patients, MiHA specific CD4 T-cells were found that were capable to lyse hematopoietic cells and to recognize normal and malignant cells. No GVHD was induced in these patients. Skin fibroblasts forced to express HLA class II, were recognized by only two MiHA specific CD4 T-cell clones. Of the 7 clones that failed to recognize fibroblasts, two targeted MiHA were encoded by genes not expressed in fibroblasts, presentation of one MiHA was dependent on HLA-DO, which is absent in fibroblasts, and T-cells recognizing the remaining 4 MiHA had an avidity that was apparently too low to recognize fibroblasts, despite clear recognition of hematopoietic cells. In conclusion, purified CD4 DLI from HLA-identical sibling donors can induce conversion from mixed to full donor chimerism with graft-vs.-malignancy reactivity, but without GVHD, by targeting HLA class II restricted MiHA.

Increased Coexpression of PD-1, TIGIT, and KLRG-1 on Tumor-Reactive CD8+ T Cells During Relapse after Allogeneic Stem Cell Transplantation

Allogeneic stem cell transplantation (allo-SCT) can be a curative treatment for patients with a hematologic malignancy due to alloreactive T cell responses recognizing minor histocompatibility antigens (MiHA). Yet tumor immune escape mechanisms can cause failure of T cell immunity, leading to relapse. Tumor cells display low expression of costimulatory molecules and can up-regulate coinhibitory molecules that inhibit T cell functionality on ligation with their counter-receptors on the tumor-reactive T cells. The aim of this explorative study was to evaluate immune checkpoint expression profiles on T cell subsets and on cytomegalovirus (CMV)- and/or MiHA-reactive CD8⁺ T cells of allo-SCT recipients using a 13-color flow cytometry panel, and to correlate these expression patterns to clinical outcomes. MiHA-reactive CD8⁺ T cells exhibited an early differentiated CD27⁺⁺/CD28⁺⁺ phenotype with low KLRG-1 and CD57 expression. These T cells also displayed increased expression of PD-1, TIM-3, and TIGIT compared with total effector memory T cells and CMV-specific CD8⁺ T cells in healthy donors and allo-SCT recipients. Remarkably, high coexpression of PD-1, TIGIT, and KLRG-1 on MiHA-reactive CD8⁺ T cells was associated with relapse after allo-SCT. Taken together, these findings indicate that MiHA-specific CD8⁺ T cells of relapsed patients have a distinctive coinhibitory expression signature compared with patients who stay in remission. This phenotype may serve as a potential monitoring tool in patients. Moreover, these findings suggest that PD-1 and TIGIT play important roles in regulating T cell-mediated tumor control, providing a rationale for immunotherapy with blocking antibodies to treat relapse after allo-SCT.

Graft-Versus-Leukemia Effect of Allogeneic Stem-Cell Transplantation and Minimal Residual Disease in Patients With Acute Myeloid Leukemia in First Complete Remission

Purpose

The detection of minimal residual disease (MRD) in patients with acute myeloid leukemia (AML) may improve future risk-adapted treatment strategies. We assessed whether MRD-positive and MRD-negative patients with AML benefit differently from the graft-versus-leukemia effect of allogeneic hematopoietic stem-cell transplantation (alloHSCT).

Methods

A total of 1,511 patients were treated in subsequent Dutch-Belgian Hemato-Oncology Cooperative Group and the Swiss Group for Clinical Cancer Research AML trials, of whom 547 obtained a first complete remission, received postremission treatment (PRT), and had available flow cytometric MRD before PRT. MRD positivity was defined as more than 0.1% cells with a leukemia-associated immunophenotype within the WBC compartment. PRT consisted of alloHSCT (n = 282), conventional PRT by a third cycle of chemotherapy (n = 160), or autologous hematopoietic stem-cell transplantation (n = 105).

Results

MRD was positive in 129 patients (24%) after induction chemotherapy before proceeding to PRT. Overall survival and relapse-free survival were significantly better in patients without MRD before PRT compared with MRD-positive patients (65% ± 2% v 50% ± 5% at 4 years; P = .002; and 58% ± 3% v 38% ± 4%; P < .001, respectively), which was mainly because of a lower cumulative incidence of relapse (32% ± 2% compared with 54% ± 4%; P < .001, respectively). Multivariable analysis with adjustment for covariables showed that the incidence of relapse was significantly reduced after alloHSCT compared with chemotherapy or autologous hematopoietic stem cell transplantation (hazard ratio [HR], 0.36; P < .001), which was similarly exerted in both MRD-negative and MRD-positive patients (HR, 0.38; P < .001; and HR, 0.35; P < .001, respectively).

Conclusion

The graft-versus-leukemia effect of alloHSCT is equally present in MRD-positive and MRD-negative patients, which advocates a personalized application of alloHSCT, taking into account the risk of relapse determined by AML risk group and MRD status, as well as the counterbalancing risk of nonrelapse mortality.

The P2X7 Receptor in Infection and Inflammation

Adenosine triphosphate (ATP) accumulates at sites of tissue injury and inflammation. Effects of extracellular ATP are mediated by plasma membrane receptors named P2 receptors (P2Rs). The P2R most involved in inflammation and immunity is the P2X7 receptor (P2X7R), expressed by virtually all cells of innate and adaptive immunity. P2X7R mediates NLRP3 inflammasome activation, cytokine and chemokine release, T lymphocyte survival and differentiation, transcription factor activation, and cell death. Ten human P2RX7 gene splice variants and several SNPs that produce complex haplotypes are known. The P2X7R is a potent stimulant of inflammation and immunity and a promoter of cancer cell growth. This makes P2X7R an appealing target for anti-inflammatory and anti-cancer therapy. However, an in-depth knowledge of its structure and of the associated signal transduction mechanisms is needed for an effective therapeutic development.

Hematopoietic stem cell-derived myeloid and plasmacytoid DC-based vaccines are highly potent inducers of tumor-reactive T cell and NK cell responses ex vivo

Because of the potent graft-versus-tumor (GVT) effect, allogeneic stem cell transplantation (alloSCT) can be a curative therapy for hematological malignancies. However, relapse remains the most frequent cause of treatment failure, illustrating the necessity for development of adjuvant post-transplant therapies to boost GVT immunity. Dendritic cell (DC) vaccination is a promising strategy in this respect, in particular, where distinct biologic functions of naturally occurring DC subsets, i.e. myeloid DCs (mDCs) and plasmacytoid DCs (pDCs), are harnessed. However, it is challenging to obtain high enough numbers of primary DC subsets from blood for immunotherapy due to their low frequencies. Therefore, we present here an ex vivo GMP-compliant cell culture protocol for generating different DC subsets from CD34⁺ hematopoietic stem and progenitor cells (HSPCs) of alloSCT donor origin. High numbers of BDCA1⁺ mDCs and pDCs could be generated, sufficient for multiple vaccination cycles. These HSPC-derived DC subsets were highly potent in inducing antitumor immune responses in vitro. Notably, HSPC-derived BDCA1⁺ mDCs were superior in eliciting T cell responses. They efficiently primed naïve T cells and robustly expanded patient-derived minor histocompatibility antigen (MiHA)-specific T cells. Though the HSPC-pDCs also efficiently induced T cell responses, they exhibited superior capacity in activating NK cells. pDC-primed NK cells highly upregulated TRAIL and possessed strong cytolytic capacity against tumor cells. Collectively, these findings indicate that HSPC-derived DC vaccines, comprising both mDCs and pDCs, may possess superior potential to boost antitumor immunity post alloSCT, due to their exceptional T cell and NK cell stimulatory capacity.

Extracellular purines, purinergic receptors and tumor growth

Virtually, all tumor cells as well as all immune cells express plasma membrane receptors for extracellular nucleosides (adenosine) and nucleotides (ATP, ADP, UTP, UDP and sugar UDP). The tumor microenvironment is characterized by an unusually high concentration of ATP and adenosine. Adenosine is a major determinant of the immunosuppressive tumor milieu. Sequential hydrolysis of extracellular ATP catalyzed by CD39 and CD73 is the main pathway for the generation of adenosine in the tumor interstitium. Extracellular ATP and adenosine mold both host and tumor responses. Depending on the specific receptor activated, extracellular purines mediate immunosuppression or immunostimulation on the host side, and growth stimulation or cytotoxicity on the tumor side. Recent progress in this field is providing the key to decode this complex scenario and to lay the basis to harness the potential benefits for therapy. Preclinical data show that targeting the adenosine-generating pathway (that is, CD73) or adenosinergic receptors (that is, A2A) relieves immunosuppresion and potently inhibits tumor growth. On the other hand, growth of experimental tumors is strongly inhibited by targeting the P2X7 ATP-selective receptor of cancer and immune cells. This review summarizes the recent data on the role played by extracellular purines (purinergic signaling) in host-tumor interaction and highlights novel therapeutic options stemming from recent advances in this field.

Acute megakaryoblastic leukemia, unlike acute erythroid leukemia, predicts an unfavorable outcome after allogeneic HSCT

Acute erythroid leukemia (FAB-M6) and acute megakaryoblastic leukemia (FAB-M7) exhibit closely related properties in cells regarding morphology and the gene expression profile. Although allogeneic hematopoietic stem cell transplantation (allo-HSCT) is considered the mainstay of the treatment for both subtypes of leukemia due to their refractoriness to chemotherapy and high rates of relapse, it remains unclear whether allo-HSCT is curative in such cases due to their scarcity. We retrospectively examined the impact of allo-HSCT in 382 patients with M6 and 108 patients with M7 using nationwide HSCT data and found the overall survival (OS) and relapse rates of the M6 patients to be significantly better than those of the M7 patients after adjusting for confounding factors and statistically comparable with those of the patients with M0/M1/M2/M4/M5 disease. Consequently, the factors of age, gender, performance status, karyotype, disease status at HSCT and development of graft-vs.-host disease predicted the OS for the M6 patients, while the performance status and disease status at HSCT were predictive of the OS for the M7 patients. These findings substantiate the importance of distinguishing between M6 and M7 in the HSCT setting and suggest that unknown mechanisms influence the HSCT outcomes of these closely related subtypes of leukemia.

The Aryl Hydrocarbon Receptor Antagonist StemRegenin1 Improves In Vitro Generation of Highly Functional Natural Killer Cells from CD34(+) Hematopoietic Stem and Progenitor Cells

Early natural killer (NK)-cell repopulation after allogeneic stem cell transplantation (allo-SCT) has been associated with reduced relapse rates without an increased risk of graft-versus-host disease, indicating that donor NK cells have specific antileukemic activity. Therefore, adoptive transfer of donor NK cells is an attractive strategy to reduce relapse rates after allo-SCT. Since NK cells of donor origin will not be rejected, multiple NK-cell infusions could be administered in this setting. However, isolation of high numbers of functional NK cells from transplant donors is challenging. Hence, we developed a cytokine-based ex vivo culture protocol to generate high numbers of functional NK cells from granulocyte colony-stimulating factor (G-CSF)-mobilized CD34(+) hematopoietic stem and progenitor cells (HSPCs). In this study, we demonstrate that addition of aryl hydrocarbon receptor antagonist StemRegenin1 (SR1) to our culture protocol potently enhances expansion of CD34(+) HSPCs and induces expression of NK-cell-associated transcription factors promoting NK-cell differentiation. As a result, high numbers of NK cells with an active phenotype can be generated using this culture protocol. These SR1-generated NK cells exert efficient cytolytic activity and interferon-γ production toward acute myeloid leukemia and multiple myeloma cells. Importantly, we observed that NK-cell proliferation and function are not inhibited by cyclosporin A, an immunosuppressive drug often used after allo-SCT. These findings demonstrate that SR1 can be exploited to generate high numbers of functional NK cells from G-CSF-mobilized CD34(+) HSPCs, providing great promise for effective NK-cell-based immunotherapy after allo-SCT.

Immunology of chronic myeloid leukemia: current concepts and future goals

Although chronic myeloid leukemia is a rare malignancy, it has developed into a model system for the study of a variety of aspects of cancer biology and immunology. The introduction of tyrosine kinase inhibitors has resulted in a significant prolongation of the survival rates of chronic myeloid leukemia patients but has not resulted in a cure. There is a growing conviction that this aim can be achieved through immunotherapy. For this concept to be successful, a considerable increase in the present understanding of chronic myeloid leukemia immunology is required. The authors attempt to review and evaluate the current findings that demonstrate a number of immunological aberrations in patients prior to the start of any therapy and their normalization after achieving remission. They also discuss the recent clinical trials with experimental therapeutic vaccines and then present their own strategy on how to address the problem.

Shifting paradigms in the treatment of chronic lymphocytic leukemia

The treatment of chronic lymphocytic leukemia (CLL) is evolving rapidly. Insight into the genetics and biology of the disease, including the importance of intracellular signaling pathways and interactions with the microenvironment has led to the development of rational targeted therapies which are having a major impact on the survival of patients with relapsed and high-risk disease. In addition, an exciting array of cellular therapies and immunotherapy options are in various stages of development. We review the current understanding of CLL genetics and biology, current treatment strategies in specific patient groups and opportunities for future treatment combinations which will bring the goal of cure or long-term disease control with minimal toxicity within reach for the majority of patients.

Detecting T-cell reactivity to whole cell vaccines: Proof of concept analysis of T-cell response to K562 cell antigens in CML patients

BCR-ABL⁺ K562 cells hold clinical promise as a component of cancer vaccines, either as bystander cells genetically modified to express immunostimulatory molecules, or as a source of leukemia antigens. To develop a method for detecting T-cell reactivity against K562 cell-derived antigens in patients, we exploited the dendritic cell (DC)-mediated cross-presentation of proteins generated from apoptotic cells. We used UVB irradiation to consistently induce apoptosis of K562 cells, which were then fed to autologous DCs. These DCs were used to both stimulate and detect antigen-specific CD8⁺ T-cell reactivity. As proof-of-concept, we used cross-presented apoptotic influenza matrix protein-expressing K562 cells to elicit reactivity from matrix protein-reactive T cells. Likewise, we used this assay to detect increased anti-CML antigen T-cell reactivity in CML patients that attained long-lasting clinical remissions following immunotherapy (donor lymphocyte infusion), as well as in 2 of 3 CML patients vaccinated with lethally irradiated K562 cells that were modified to secrete high levels of granulocyte macrophage colony-stimulating factor (GM-CSF). This methodology can be readily adapted to examine the effects of other whole tumor cell-based vaccines, a scenario in which the precise tumor antigens that stimulate immune responses are unknown.

Immunotherapy in acute myeloid leukemia

Treatment of acute myeloid leukemia (AML) with current chemotherapy regimens is still disappointing, with overall survival rates of ≤40% at 5 years. It is now well established that AML cells can evade the immune system through multiple mechanisms, including the expression of the enzyme indoleamine 2,3 dioxygenase. Immunotherapeutic strategies, including both active, such as vaccination with leukemia-associated antigens, and passive, such as adoptive transfer of allogeneic natural killer cells, may overcome leukemia escape and lead to improved cure. Allogeneic hemopoeitic stem cell transplantation, the most effective treatment of AML, is the best known model of immunotherapy. Following transplant, recipient AML cells are eradicated by donor immune cells through the graft-versus-leukemia (GVL) effect. However, GVL is clinically associated with graft-versus-host disease, the major cause of mortality after transplant. GVL is mediated by donor T cells recognizing either leukemia-associated antigens or minor as well as major histocompatibility antigens. Several innovative strategies have been devised to generate leukemia reactive T cells so as to increase GVL responses with no or little graft-versus-host disease.

Minor histocompatibility antigens and the maternal immune response to the fetus during pregnancy

The tolerance of the semiallogeneic fetus by the maternal immune system is an important area of research for understanding how the maternal and fetal systems interact during pregnancy to ensure a successful outcome. Several lines of research reveal that the maternal immune system can recognize and respond to fetal minor histocompatibility antigens during pregnancy. Reactions to these antigens arise because of allelic differences between the mother and fetus and have been shown more broadly to play an important role in mediating transplantation outcomes. This review outlines the discovery of minor histocompatibility antigens and their importance in solid organ and hematopoietic stem cell transplantations, maternal T-cell responses to minor histocompatibility antigens during pregnancy, expression of minor histocompatibility antigens in the human placenta, and the potential involvement of minor histocompatibility antigens in the development and manifestation of pregnancy complications.

Leukemia-associated antigens and their relevance to the immunotherapy of acute myeloid leukemia

The graft-versus-leukemia effect of allogeneic hematopoietic stem cell transplantation (HSCT) has shown that the immune system is capable of eradicating acute myeloid leukemia (AML). This knowledge, along with the identification of the target antigens against which antileukemia immune responses are directed, has provided a strong impetus for the development of antigen-targeted immunotherapy of AML. The success of any antigen-specific immunotherapeutic strategy depends critically on the choice of target antigen. Ideal molecules for immune targeting in AML are those that are: (1) leukemia-specific; (2) expressed in most leukemic blasts including leukemic stem cells; (3) important for the leukemic phenotype; (4) immunogenic; and (5) clinically effective. In this review, we provide a comprehensive overview on AML-related tumor antigens and assess their applicability for immunotherapy against the five criteria outlined above. In this way, we aim to facilitate the selection of appropriate target antigens, a task that has become increasingly challenging given the large number of antigens identified and the rapid pace at which new targets are being discovered. The information provided in this review is intended to guide the rational design of future antigen-specific immunotherapy trials, which will hopefully lead to new antileukemia therapies with more selectivity and higher efficacy.

B and T lymphocyte attenuator mediates inhibition of tumor-reactive CD8+ T cells in patients after allogeneic stem cell transplantation

Allogeneic stem cell transplantation (allo-SCT) can cure hematological malignancies by inducing alloreactive T cell responses targeting minor histocompatibility antigens (MiHA) expressed on malignant cells. Despite induction of robust MiHA-specific T cell responses and long-term persistence of alloreactive memory T cells specific for the tumor, often these T cells fail to respond efficiently to tumor relapse. Previously, we demonstrated the involvement of the coinhibitory receptor programmed death-1 (PD-1) in suppressing MiHA-specific CD8(+) T cell immunity. In this study, we investigated whether B and T lymphocyte attenuator (BTLA) plays a similar role in functional impairment of MiHA-specific T cells after allo-SCT. In addition to PD-1, we observed higher BTLA expression on MiHA-specific CD8(+) T cells compared with that of the total population of CD8(+) effector-memory T cells. In addition, BTLA's ligand, herpes virus entry mediator (HVEM), was found constitutively expressed by myeloid leukemia, B cell lymphoma, and multiple myeloma cells. Interference with the BTLA-HVEM pathway, using a BTLA blocking Ab, augmented proliferation of BTLA(+)PD-1(+) MiHA-specific CD8(+) T cells by HVEM-expressing dendritic cells. Notably, we demonstrated that blocking of BTLA or PD-1 enhanced ex vivo proliferation of MiHA-specific CD8(+) T cells in respectively 7 and 9 of 11 allo-SCT patients. Notably, in 3 of 11 patients, the effect of BTLA blockade was more prominent than that of PD-1 blockade. Furthermore, these expanded MiHA-specific CD8(+) T cells competently produced effector cytokines and degranulated upon Ag reencounter. Together, these results demonstrate that BTLA-HVEM interactions impair MiHA-specific T cell functionality, providing a rationale for interfering with BTLA signaling in post-stem cell transplantation therapies.

Clinical options after failure of allogeneic hematopoietic stem cell transplantation in patients with hematologic malignancies

Disease recurrence is the single most common cause of death after allogeneic or autologous hematopoietic stem cell transplantation (HSCT). Disease status and chemosensitivity at the time of transplantation, as well as the development of graft-versus-host disease (GVHD), are factors known to influence the risk of relapse post-HSCT. Both acute and chronic GVHD have been associated with decreased relapse rates; however, owing to toxicity, overall survival is not consistently improved in these patients. Furthermore, there is a transient period of immunodeficiency after HSCT, which may permit residual malignant cells to proliferate early in the post-transplant course, before the donor immune system can establish a graft-versus-tumor response. Patients who fail an initial HSCT have an extremely poor outcome; therefore, maneuvers to prevent, identify and treat recurrent disease as early as possible in these situations are necessary. Strategies to distinguish graft-versus-tumor from GVHD, to enhance both general and disease-specific immune reconstitution after transplantation, and to increase donor-mediated anti-host immune reactions are being investigated in clinical trials. Single agent nontoxic post-HSCT chemotherapy, cellular therapies and second allogeneic HSCT using reduced intensity regimens are among the modalities under investigation.

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.

Exploiting T cells specific for human minor histocompatibility antigens for therapy of leukemia

Minor histocompatibility (H) antigens are major targets of a graft-versus-leukemia (GVL) effect mediated by donor CD8(+) and CD4(+) T cells following allogeneic hematopoietic cell transplantation (HCT) between human leukocyte antigen identical individuals. In the 15 years since the first molecular characterization of human minor H antigens, significant strides in minor H antigen discovery have been made as a consequence of advances in cellular, genetic and molecular techniques. Much has been learned about the mechanisms of minor H antigen immunogenicity, their expression on normal and malignant cells, and their role in GVL responses. T cells specific for minor H antigens expressed on leukemic cells, including leukemic stem cells, can be isolated and expanded in vitro and infused into allogeneic HCT recipients to augment the GVL effect to prevent and treat relapse. The first report of the adoptive transfer of minor H antigen-specific T-cell clones to patients with leukemic relapse in 2010 illustrates the potential for the manipulation of alloreactivity for therapeutic benefit. This review describes the recent developments in T-cell recognition of human minor H antigens, and efforts to translate these discoveries to reduce leukemia relapse after allogeneic HCT.

Exploiting T cells specific for human minor histocompatibility antigens for therapy of leukemia

Minor histocompatibility (H) antigens are major targets of a graft-versus-leukemia (GVL) effect mediated by donor CD8(+) and CD4(+) T cells following allogeneic hematopoietic cell transplantation (HCT) between human leukocyte antigen identical individuals. In the 15 years since the first molecular characterization of human minor H antigens, significant strides in minor H antigen discovery have been made as a consequence of advances in cellular, genetic and molecular techniques. Much has been learned about the mechanisms of minor H antigen immunogenicity, their expression on normal and malignant cells, and their role in GVL responses. T cells specific for minor H antigens expressed on leukemic cells, including leukemic stem cells, can be isolated and expanded in vitro and infused into allogeneic HCT recipients to augment the GVL effect to prevent and treat relapse. The first report of the adoptive transfer of minor H antigen-specific T-cell clones to patients with leukemic relapse in 2010 illustrates the potential for the manipulation of alloreactivity for therapeutic benefit. This review describes the recent developments in T-cell recognition of human minor H antigens, and efforts to translate these discoveries to reduce leukemia relapse after allogeneic HCT.

Immune response against tumor antigens expressed on human cancer stem-like cells/tumor-initiating cells

Cancer stem-like cells (CSCs)/tumor-initiating cells (TICs) are a small population of cancer cells that have the properties of tumor-initiating ability, self-renewal and differentiation. These properties suggest that CSCs/TICs are essential for tumor maintenance, recurrence and distant metastasis. Thus, elimination of CSCs/TICs is essential to cure malignant diseases. However, there are several studies reporting that CSCs/TICs are more resistant to standard cancer therapies, including chemotherapy and radiotherapy, than non-CSC/TIC populations. How then, can we eliminate CSCs/TICs? Immunotherapy might be the possible answer. In recent analysis, innate immunity (natural killer cells and gammadeltaT cells) and also adaptive immunity (cytotoxic T lymphocyte-based cellular immunity and antibody-based humoral immunity) can recognize CSCs/TICs in vitro efficiently. Furthermore, CSC/TIC-specific monoclonal antibody therapies are also efficient in vivo. In this article, we describe the potency, possibilities and problems of CSC/TIC-targeting immunotherapy.

Immunotherapy of chronic myeloid leukemia: present state and future prospects

In spite of the considerable successes that have been achieved in the treatment of chronic myeloid leukemia (CML), cure for the disease can only be obtained by the present means in a rather small minority of patients. During the past decade, considerable progress has been made in the understanding of the immunology of CML, which has raised hopes that this disease may be curable by supplementing the current targeted chemotherapy with immunotherapeutic approaches. More than ten small-scale clinical trials have been carried out with experimental vaccines predominantly based on the p210bcr-abl fusion protein. Their results suggested beneficial effects in some patients. Recent data obtained in human patients as well as in animal models indicate that the p210bcr-abl protein does not carry the immunodominant epitope(s). These observations, combined with the recognition of an ever increasing number of other immunogenic proteins in CML cells, strongly support the concept that gene-modified, cell-based vaccines containing the full spectrum of tumor antigens might be the most effective immunotherapeutic approach. Recently created mathematical models have provided important leads for the timing of the combination of targeted drug therapy with vaccine administration. A strategy of how targeted drug therapy might be combined with vaccination is outlined.

siRNA silencing of PD-L1 and PD-L2 on dendritic cells augments expansion and function of minor histocompatibility antigen-specific CD8+ T cells

Tumor relapse after human leukocyte antigen-matched allogeneic stem cell transplantation (SCT) remains a serious problem, despite the long-term presence of minor histocompatibility antigen (MiHA)-specific memory T cells. Dendritic cell (DC)-based vaccination boosting MiHA-specific T-cell immunity is an appealing strategy to prevent or counteract tumor recurrence, but improvement is necessary to increase the clinical benefit. Here, we investigated whether knockdown of programmed death ligand 1 (PD-L1) and PD-L2 on monocyte-derived DCs results in improved T-cell activation. Electroporation of single siRNA sequences into immature DCs resulted in efficient, specific, and long-lasting knockdown of PD-L1 and PD-L2 expression. PD-L knockdown DCs strongly augmented interferon-γ and interleukin-2 production by stimulated T cells in an allogeneic mixed lymphocyte reaction, whereas no effect was observed on T-cell proliferation. Moreover, we demonstrated that PD-L gene silencing, especially combined PD-L1 and PD-L2 knockdown, resulted in improved proliferation and cytokine production of keyhole limpet hemocyanin-specific CD4(+) T cells. Most importantly, PD-L knockdown DCs showed superior potential to expand MiHA-specific CD8(+) effector and memory T cells from leukemia patients early after donor lymphocyte infusion and later during relapse. These data demonstrate that PD-L siRNA electroporated DCs are highly effective in enhancing T-cell proliferation and cytokine production, and are therefore attractive cells for improving the efficacy of DC vaccines in cancer patients.

Immunotherapy prospects for acute myeloid leukaemia

While chemotherapy is successful at inducing remission of acute myeloid leukaemia (AML), the disease has a high probability of relapse. Strategies to prevent relapse involve consolidation chemotherapy, stem cell transplantation and immunotherapy. Evidence for immunosurveillance of AML and susceptibility of leukaemia cells to both T cell and natural killer (NK) cell attack and justifies the application of immune strategies to control residual AML persisting after remission induction. Immune therapy for AML includes allogeneic stem cell transplantation, adoptive transfer of allogeneic or autologous T cells or NK cells, vaccination with leukaemia cells, dendritic cells, cell lysates, peptides and DNA vaccines and treatment with cytokines, antibodies and immunomodulatory agents. Here we describe what is known about the immunological features of AML at presentation and in remission, the current status of immunotherapy and strategies combining treatment approaches with a view to achieving leukaemia cure.

Immune reconstitution after allogeneic transplantation and expanding options for immunomodulation: an update

Allogeneic hematopoietic stem cell transplantation (HSCT) has advanced to a common procedure for treating also older patients with malignancies and immunodeficiency disorders by redirecting the immune system. Unfortunately, cure is often hampered by relapse of the underlying disease, graft-versus-host disease, or severe opportunistic infections, which account for the majority of deaths after HSCT. Enhancing immune reconstitution is therefore an area of intensive research. An increasing variety of approaches has been explored preclinically and clinically: the application of cytokines, keratinocyte growth factor, growth hormone, cytotoxic lymphocytes, and mesenchymal stem cells or the blockade of sex hormones. New developments of allogeneic HSCT, for example, umbilical cord blood or haploidentical graft preparations leading to prolonged immunodeficiency, have further increased the need to improve immune reconstitution. Although a slow T-cell reconstitution is regarded as primarily responsible for deleterious infections with viruses and fungi, graft-versus-host disease, and relapse, the importance of innate immune cells for disease and infection control is currently being reevaluated. The groundwork has been prepared for the creation of individualized therapy partially based on genetic features of the underlying disease. We provide an update on selected issues of development in this fast evolving field; however, we do not claim completeness.

Diverse patterns of T-cell response against multiple newly identified human Y chromosome-encoded minor histocompatibility epitopes

PURPOSE

Donor T cells respond to minor histocompatibility antigens (mHA), resulting in both graft-versus-host disease and graft versus leukemia after allogeneic hematopoietic stem cell transplantation. Because relatively few mHAs are known, we developed a new approach to predict and subsequently validate candidate mHA.

EXPERIMENTAL DESIGN

We developed an algorithm based on genetic disparities between Y chromosome-encoded and X chromosome-encoded proteins and known requirements for binding to HLA class I molecules to predict Y chromosome-derived, HLA A*0201-restricted peptides (HY) and ranked peptides based on potential immunogenicity. We evaluated T-cell responses to 41 candidate peptides in 28 male recipients with female donors (FM), 22 male recipients with male donors (MM), and 26 normal individuals. All patients and donors were HLA A*0201 positive.

RESULTS

Thirteen peptides derived from five proteins elicited significantly greater T-cell responses in FM patients compared with MM patients and in normal females compared with normal males. Six peptides were more immunogenic than the only previously known HLA A*0201-restricted Y-encoded mHA. Twenty-seven of 28 FM patients responded to at least one HY peptide, but despite a common Y chromosome mismatch and expression of HLA A*0201, each patient responded to a unique set of peptides.

CONCLUSIONS

Novel HLA A*0201-restricted HY epitopes can be predicted and validated in patients after allogeneic hematopoietic stem cell transplantation. Highly diverse patterns of T-cell response against these epitopes have been identified. Prospective monitoring of responses to large panels of immunogenic peptides can facilitate the identification of clinically relevant targets of graft-versus-host disease and graft versus leukemia.

Antibodies specifically target AML antigen NuSAP1 after allogeneic bone marrow transplantation

Identifying the targets of immune response after allogeneic hematopoietic cell transplantation (HCT) promises to provide relevant immune therapy candidate proteins. We used protein microarrays to serologically identify nucleolar and spindle-associated protein 1 (NuSAP1) and chromatin assembly factor 1, subunit B (p60; CHAF1b) as targets of new antibody responses that developed after allogeneic HCT. Western blots and enzyme-linked immunosorbent assays (ELISA) validated their post-HCT recognition and enabled ELISA testing of 120 other patients with various malignancies who underwent allo-HCT. CHAF1b-specific antibodies were predominantly detected in patients with acute myeloid leukemia (AML), whereas NuSAP1-specific antibodies were exclusively detected in patients with AML 1 year after transplantation (P < .001). Complete genomic exon sequencing failed to identify a nonsynonymous single nucleotide polymorphism (SNP) for NuSAP1 and CHAF1b between the donor and recipient cells. Expression profiles and reverse transcriptase-polymerase chain reaction (RT-PCR) showed NuSAP1 was predominately expressed in the bone marrow CD34(+)CD90(+) hematopoietic stem cells, leukemic cell lines, and B lymphoblasts compared with other tissues or cells. Thus, NuSAP1 is recognized as an immunogenic antigen in 65% of patients with AML following allogeneic HCT and suggests a tumor antigen role.

Partial T cell-depleted allogeneic stem cell transplantation following reduced-intensity conditioning creates a platform for immunotherapy with donor lymphocyte infusion and recipient dendritic cell vaccination in multiple myeloma

Allogeneic stem cell transplantation (SCT) in multiple myeloma (MM) may induce a curative graft-versus-myeloma (GVM) effect. Major drawback in unmanipulated reduced-intensity conditioning (RIC) SCT is the risk of severe and longstanding graft-versus-host-disease (GVHD). This study demonstrates that transplantation with a partial T cell-depleted graft creates a platform for boosting GVM immunity by preemptive donor lymphocyte infusion (DLI) and recipient dendritic cell (DC) vaccination, with limited GVHD. All 20MM patients engrafted successfully. Chimerism analysis in 19 patients evaluable at 3 months revealed that 7 patients were complete donor, whereas 12 patients were mixed chimeric. Grade II acute GVHD (aGVHD) occurred in 7 patients (35%) and only 4 patients (21%) developed chronic GVHD (cGVHD). Fourteen patients received posttransplantation immunotherapy, 8 preemptive DLI, 5 patients both DLI and DC vaccination, and 1 patient DC vaccination only. DC vaccination was associated with limited toxicity, and none of these patients developed GVHD. Importantly, overall treatment-related mortality (TRM) at 1 year was low (10%). Moreover, the overall survival (OS) is 84% with median follow-up of 27 months, and none of the patients died from progressive disease. These findings illustrate that this novel approach is associated with limited GVHD and mortality, thus creating an ideal platform for adjuvant immunotherapy.