Repeated PR1 and WT1 peptide vaccination in Montanide-adjuvant fails to induce sustained high-avidity, epitope-specific CD8+ T cells in myeloid malignancies

Update on antigen-specific immunotherapy of acute myeloid leukemia

Among the few drugs that have shown a benefit for patients with acute myeloid leukemia (AML) in randomized clinical trials over the last several decades is the CD33 antibody-drug conjugate, gemtuzumab ozogamicin (GO). Undoubtedly, this experience has highlighted the value of antigen-specific immunotherapy in AML. A wide variety of therapeutics directed against several different antigens on AML cells are currently explored in preclinical and early clinical studies. On the one hand, these include passive strategies such as unconjugated antibodies targeting one or more antigens, antibodies armed with drugs, toxic proteins, or radionuclides, or adoptive immunotherapies, in particular utilizing T cells engineered to express chimeric antigen receptors (CARs) or modified T cell receptor (TCR) genes; on the other hand, these include active strategies such as vaccinations. With the documented benefit for GO and the emerging data with several classes of therapeutics in other leukemias, in particular small bispecific antibodies and CAR T cells, the future is bright. Nevertheless, a number of important questions related to the choice of target antigen(s), patient population, exact treatment modality, and supportive care needs remain open. Addressing such questions in upcoming studies will ultimately be required to optimize the clinical use of antigen-specific immunotherapies in AML and ensure that such treatments become an effective, versatile tool for this disease for which the outcomes have remained unsatisfactory in many patients.

Cancer Vaccines: A Brief Overview

Vaccine approaches for cancer differ from traditional vaccine approaches for infectious disease in tending to focus on clearing active disease rather than preventing disease. In this review, we provide a brief overview of different types of vaccines and adjuvants that have been investigated for the purpose of controlling cancer burdens in patients, some of which are approved for clinical use or in late-stage clinical trials, such as the personalized dendritic cell vaccine sipuleucel-T (Provenge) and the recombinant viral prostate cancer vaccine PSA-TRICOM (Prostvac-VF). Vaccines against human viruses implicated in the development and progression of certain cancers, such as human papillomavirus in cervical cancer, are not considered here. Cancers express "altered self" antigens that tend to induce weaker responses than the "foreign" antigens expressed by infectious agents. Thus, immune stimulants and adjuvant approaches have been explored widely. Vaccine types considered include autologous patient-derived immune cell vaccines, tumor antigen-expressing recombinant virus vaccines, peptide vaccines, DNA vaccines, and heterologous whole-cell vaccines derived from established human tumor cell lines. Opportunities to develop effective cancer vaccines may benefit from seminal recent advances in understanding how immunosuppressive barricades are erected by tumors to mediate immune escape. In particular, targeted ablation of these barricades with novel agents, such as the immune checkpoint drug ipilimumab (anti-CTLA-4) approved recently for clinical use, may offer significant leverage to vaccinologists seeking to control and prevent malignancy.

Immunotherapy Targeting WT1: Designing a Protocol for WT1 Peptide-Based Cancer Vaccine

There is much current excitement about the potential of cancer immunotherapy. WT1 is high on the National Cancer Institute's list of priority antigens for immune therapy. In this chapter we describe a protocol for a clinical trial using a WT1 peptide-based cancer vaccine.

WT1 vaccination in AML and MDS: A pilot trial with synthetic analog peptides

Peptide vaccines are capable of eliciting immune responses targeting tumor-associated antigens such as the Wilms' Tumor 1 (WT1) antigen, often overexpressed in myeloid malignancies. Here, we assessed the safety, tolerability, and immunogenicity of a polyvalent WT1 peptide vaccine. Individuals with WT1-positive acute myeloid leukemia (AML) in first (CR1) or second (CR2) remission or with higher-risk myelodysplastic syndrome (MDS) following at least 1 prior line of therapy were vaccinated with a mixture of peptides derived from the WT1 protein, with sargramostim injections before vaccination to amplify immunogenicity. Six vaccinations were delivered biweekly, continuing then monthly until patients received 12 vaccinations or showed disease relapse or progression. Therapeutic efficacy was evaluated by progression-free and overall survival. Immune responses were evaluated by delayed-type hypersensitivity testing and T-cell IFNγ ELISPOT at specified intervals. In 16 patients who received at least one vaccination, 10 completed the planned course of six vaccinations and six continued for up to six additional monthly vaccinations. Vaccinations were well tolerated, with no patients discontinuing due to toxicity. One of two patients with high-risk MDS experienced a prolonged decrease in transfusion dependence. Two of 14 AML patients demonstrated relapse-free survival >1 year. Both patients were in CR2 at time of vaccination, with duration of their remission exceeding duration of their first remission, suggesting a potential benefit. Our WT1 vaccine was well-tolerated. The clinical benefit that we observed in several patients suggests engagement of a protective immune response, indicating a need for further trials.

Analysis of the functional WT1-specific T-cell repertoire in healthy donors reveals a discrepancy between CD4(+) and CD8(+) memory formation

The Wilms' tumour-1 (WT1) protein is considered a prime target for cancer immunotherapy based on its presumptive immunogenicity and widespread expression across a variety of malignancies. However, little is known about the naturally occurring WT1-specific T-cell repertoire because self-derived antigens typically elicit low frequency responses that challenge the sensitivity limits of current detection techniques. In this study, we used highly efficient cell enrichment procedures based on CD137, CD154, and pHLA class I tetramer staining to conduct a detailed analysis of WT1-specific T cells from the peripheral blood. Remarkably, we detected WT1-specific CD4(+) and CD8(+) T-cell populations in the vast majority of healthy individuals. Memory responses specific for WT1 were commonly present in the CD4(+) T-cell compartment, whereas WT1-specific CD8(+) T cells almost universally displayed a naive phenotype. Moreover, memory CD4(+) and naive CD8(+) T cells with specificity for WT1 were found to coexist in some individuals. Collectively, these findings suggest a natural discrepancy between the CD4(+) and CD8(+) T-cell lineages with respect to memory formation in response to a self-derived antigen. Nonetheless, WT1-specific T cells from both lineages were readily activated ex vivo and expanded in vitro, supporting the use of strategies designed to exploit this expansive reservoir of self-reactive T cells for immunotherapeutic purposes.

Peptide Immunotherapy in Vaccine Development: From Epitope to Adjuvant

Vaccines are designed to educate the host immune system to prevent infectious disease or to fight against various diseases such as cancers. Peptides were first employed to provide specific immune responses while minimizing unintended allergenic or reactogenic adverse effects. Discoveries of virus or cancer-specific antigens and the advanced knowledge of immunology accelerate the peptide vaccine development. Despite the overwhelming research pipelines, a very few of them reached to market approvals or phase III clinical trials, because of the lack of efficacy. Several strategies for the next generation peptide vaccines are devised to overcome the weak immunogenicity and the poor delivery. In this review, we discuss the new promising strategies of peptide vaccine development which are recently developed in preclinical and/or clinical stage focusing the roles of peptides in the vaccine formulation from epitope to adjuvant. Additionally, we discuss the future perspectives of peptide vaccine and immunotherapy.

Haematological malignancies: at the forefront of immunotherapeutic innovation

The recent successes of cancer immunotherapies have stimulated interest in the potential widespread application of these approaches; haematological malignancies have provided both initial proofs of concept and an informative testing ground for various immune-based therapeutics. The immune-cell origin of many of the blood malignancies provides a unique opportunity both to understand the mechanisms of cancer immune responsiveness and immune evasion, and to exploit these mechanisms for therapeutic purposes.

Review of the Results of WT1 Peptide Vaccination Strategies for Myelodysplastic Syndromes and Acute Myeloid Leukemia from Nine Different Studies

We performed a systematic review of data from nine clinical trials of WT1 peptide vaccination in patients with myelodysplastic syndromes and/or acute myeloid leukemia (MDS/AML), published between 2004 and 2012. A total of 51 patients were eligible for analysis. Vaccination with WT1 peptides proved safe and feasible in patients with MDS/AML, in studies from different institutions. Additionally, clinical responses and clinical benefit were observed, with some patients achieving and maintaining remission long-term (more than 8 years). A significant correlation between induction of WT1-specific T cells and normalization/reduction of WT1 mRNA levels and progression-free survival was noted in a number of studies. However, larger studies are warranted to confirm these results. Interestingly, the majority of trials reported the presence of WT1-specific T cells with limited or absent functionality prior to vaccination, which increased in frequency and function after vaccination. In conclusion, WT1 peptide vaccination strategies were safe in this heterogeneous group of patient with MDS/AML. Larger and more homogeneous studies or randomized clinical trials are needed to quantify the contribution of WT1 peptide vaccines to clinical responses and long-term survival.

Differential ion mobility spectrometry coupled to tandem mass spectrometry enables targeted leukemia antigen detection

Differential ion mobility spectrometry (DIMS) can be used as a filter to remove undesired background ions from reaching the mass spectrometer. The ability to use DIMS as a filter for known analytes makes DIMS coupled to tandem mass spectrometry (DIMS-MS/MS) a promising technique for the detection of cancer antigens that can be predicted by computational algorithms. In experiments using DIMS-MS/MS that were performed without the use of high-performance liquid chromatography (HPLC), a predicted model antigen, GLR (FLSSANEHL), was detected at a concentration of 10 pM (20 amol) in a mixture containing 94 competing model peptide antigens, each at a concentration of 1 μM. Without DIMS filtering, the GLR peptide was undetectable in the mixture even at 100 nM. Again, without using HPLC, DIMS-MS/MS was used to detect 2 of 3 previously characterized antigens produced by the leukemia cell line U937.A2. Because of its sensitivity, a targeted DIMS-MS/MS methodology can likely be used to probe for predicted cancer antigens from cancer cell lines as well as human tumor samples.

Wilms' tumor gene 1 immunotherapy in pelvic gynecological malignancies

Pelvic gynecological malignancies account for 6% of all cancers. In the relapsed state, classical treatments are limited. There is an urgent need for new and personalized treatment. Wilms' tumor gene 1 (WT1) is the most important tumor-associated antigen. Although highly present in gynecological tumors, active immunotherapy against it is still underexplored. This review gives an insight into the importance of WT1 in pelvic gynecological malignancies and the first taken steps into the world of WT1 immunotherapy.

Cancer vaccines: Trafficking of tumor-specific T cells to tumor after therapeutic vaccination

Cancer vaccines can induce robust activation of tumor-specific CD8(+) T cells that can destroy tumors. Understanding the mechanism by which cancer vaccines work is essential in designing next-generation vaccines with more potent therapeutic activity. We recently reported that short peptides emulsified in poorly biodegradable, Incomplete Freund's Adjuvant (IFA) primed CD8(+) T cells that did not localize to the tumor site but accumulated at the persisting, antigen-rich vaccination site. The vaccination site eventually became a T cell graveyard where T cells responded to chronically released gp100 peptide by releasing cytokines, including interferon-γ (IFN-γ), which in turn upregulated Fas ligand (FasL) on host cells, causing apoptosis of Fas(+) T cells. T cells that escaped apoptosis rapidly became exhausted, memory formation was poor, and therapeutic impact was minimal. Replacing the non-biodegradable IFA-based formulation with water-based, short-lived formulation in the presence of immunostimulatory molecules allowed T cells to traffic to tumors, causing their regression. In this review, we discuss recent advances in immunotherapeutic approaches that could enhance vaccine-primed immune cells fitness and render the tumor microenvironment more accessible for immune cell infiltration.

Peptide vaccines for hematological malignancies: a missed promise?

Despite the crucial aid that newly developed target therapies are providing to chemotherapy and stem cell transplant, the cure for many hematological malignancies is still an unmet need. Although available therapies are able to induce an effective debulking of the tumor, most of the time, an insidious minimal residual disease survives current treatments and it is responsible for an immediate or delayed relapse. Peptide-derived antitumor vaccines have been developed with the idea that an artificially "educated" immune system may exert an active specific antitumor response able to control and ultimately eradicate underlying post-treatment residual disease. This review will summarize current knowledge of peptide vaccines for hematological malignancies, trying to analyze promises and pitfalls of a safe and intelligent tool that after many years from its first appearance has not yet established its potential role as alternative immune mediated therapeutic approach for hematopoietic tumors.

Wilms' Tumour 1 (WT1) peptide vaccination in patients with acute myeloid leukaemia induces short-lived WT1-specific immune responses

Wilms’ Tumour 1 (WT1) is a zinc finger transcription factor that is over-expressed in acute myeloid leukaemia (AML). Its restricted expression in normal tissues makes it a promising target for novel immunotherapies aiming to accentuate the cytotoxic T lymphocyte (CTL) response against AML. Here we report a phase I/II clinical trial of subcutaneous peptide vaccination with two separate HLA-A2-binding peptide epitopes derived from WT1, together with a pan-DR binding peptide epitope (PADRE), in Montanide adjuvant. Eight HLA-A2-positive patients with poor risk AML received five vaccination cycles at 3-weekly intervals. The three cohorts received 0·3, 0·6 and 1 mg of each peptide, respectively. In six patients, WT1-specific CTL responses were detected using enzyme-linked immunosorbent spot assays and pWT126/HLA-A*0201 tetramer staining, after ex vivo stimulation with the relevant WT1 peptides. However, re-stimulation of these WT1-specific T cells failed to elicit secondary expansion in all four patients tested, suggesting that the WT1-specific CD8⁺ T cells generated following vaccination may be functionally impaired. No correlation was observed between peptide dose, cellular immune response, reduction in WT1mRNA expression and clinical response. Larger studies are indicated to confirm these findings.

Strategies to reduce relapse after allogeneic hematopoietic cell transplantation in acute myeloid leukemia

The incidence of acute myeloid leukemia (AML) is expected to increase in conjunction with our ageing population. Although it is proving to be a heterogeneous disease process, the only treatment with proven survival benefit for poor risk AML remains allogeneic hematopoietic cell transplant. Although this is presumed to be a curative strategy, many patients relapse after transplant, prompting us to examine various ways that we can improve outcomes. These efforts involve every step of AML diagnostics and therapy, including the intricate processes of conditioning, graft manipulation and immunomodulation. The hope is that improvement in these steps will ultimately improve survival and decrease relapse rates for AML patients after transplant.

Lentivirus-induced dendritic cells for immunization against high-risk WT1(+) acute myeloid leukemia

Wilms' tumor 1 antigen (WT1) is overexpressed in acute myeloid leukemia (AML), a high-risk neoplasm warranting development of novel immunotherapeutic approaches. Unfortunately, clinical immunotherapeutic use of WT1 peptides against AML has been inconclusive. With the rationale of stimulating multiantigenic responses against WT1, we genetically programmed long-lasting dendritic cells capable of producing and processing endogenous WT1 epitopes. A tricistronic lentiviral vector co-expressing a truncated form of WT1 (lacking the DNA-binding domain), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-4 (IL-4) was used to transduce human monocytes ex vivo. Overnight transduction induced self-differentiation of monocytes into immunophenotypically stable "SmartDC/tWT1" (GM-CSF(+), IL-4(+), tWT1(+), IL-6(+), IL-8(+), TNF-α(+), MCP-1(+), HLA-DR(+), CD86(+), CCR2(+), CCR5(+)) that were viable for 3 weeks in vitro. SmartDC/tWT1 were produced with peripheral blood mononuclear cells (PBMC) obtained from an FLT3-ITD(+) AML patient and surplus material from a donor lymphocyte infusion (DLI) and used to expand CD8(+) T cells in vitro. Expanded cytotoxic T lymphocytes (CTLs) showed antigen-specific reactivity against WT1 and against WT1(+) leukemia cells. SmartDC/tWT1 injected s.c. into Nod.Rag1(-/-).IL2rγc(-/-) mice were viable in vivo for more than three weeks. Migration of human T cells (huCTLs) to the immunization site was demonstrated following adoptive transfer of huCTLs into mice immunized with SmartDC/tWT1. Furthermore, SmartDC/tWT1 immunization plus adoptive transfer of T cells reactive against WT1 into mice resulted in growth arrest of a WT1(+) tumor. Gene array analyses of SmartDC/tWT1 demonstrated upregulation of several genes related to innate immunity. Thus, SmartDC/tWT1 can be produced in a single day of ex vivo gene transfer, are highly viable in vivo, and have great potential for use as immunotherapy against malignant transformation overexpressing WT1.

Induction of high-titer IgG antibodies against multiple leukemia-associated antigens in CML patients with clinical responses to K562/GVAX immunotherapy

The ability to target myeloid leukemia with immunotherapy would represent a significant therapeutic advance. We report here immunological analysis of clinical trials of primary and secondary vaccination with K562/GM-CSF immunotherapy in adult chronic phase chronic myeloid leukemia patients (CML-CP) with suboptimal responses to imatinib mesylate. Using serological analysis of recombinant cDNA expression libraries of K562 with autologous vaccinated patient serum, we have identified 12 novel chronic myeloid leukemia-associated antigens (LAAs). We show that clinical responses following K562/GM-CSF vaccination are associated with induction of high-titer antibody responses to multiple LAAs. We observe markedly discordant patterns of baseline and induced antibody responses in these identically vaccinated patients. No single antigen was recognized in all responses to vaccination. We demonstrate that an additional 'booster' vaccination series can be given safely to those with inadequate responses to initial vaccination, and is associated with more frequent induction of IgG responses to antigens overexpressed in K562 vaccine compared with primary CML-CP. Finally, those with induced immune responses to the same LAAs often shared HLA subtypes and patients with clinical responses following vaccination recognized a partially shared but non-identical spectrum of antigens; both findings have potentially significant implications for cancer vaccine immunotherapy.

The trifunctional antibody catumaxomab amplifies and shapes tumor-specific immunity when applied to gastric cancer patients in the adjuvant setting

BACKGROUND

Patients with gastric cancer benefit from perioperative chemotherapy, however, treatment is toxic and many patients will relapse. The trifunctional antibody catumaxomab targets EpCAM on tumor cells, CD3 on T cells, and the Fcγ-receptor of antigen-presenting cells. While in Europe catumaxomab is approved for treating malignant ascites, it has not been investigated in the perioperative setting and its exact immunological mode of action is unclear.

METHODS

In our study, gastric cancer patients received neoadjuvant platinum-based chemotherapy, one intraoperative application of catumaxomab, and 4 postoperative doses of intraperitoneal catumaxomab. Immunomonitoring was performed in 6 patients before surgery, after completion of catumaxomab treatment, and one month later.

RESULTS

Intraperitoneal application of catumaxomab caused an increased expression of activation markers on the patients' T cells. This was accompanied by a transient decrease in numbers of CXCR3(+) effector T cells with a T-helper (Th)-1 phenotype in the peripheral blood. All patients evidenced pre-existing EpCAM-specific CD4(+) and/or CD8(+) T cells. While these cells transiently disappeared from the blood stream after intraperitoneal application of catumaxomab, we detected increased numbers of peripheral EpCAM-specific cells and a modified EpCAM-specific T-cell repertoire 4 weeks after completion of treatment. Finally, catumaxomab also amplified humoral immunity to tumor antigens other than EpCAM.

CONCLUSIONS

Our findings suggest that catumaxomab exerts its clinical effects by (1) activating peripheral T cells, (2) redistributing effector T cells from the blood into peripheral tissues, (3) expanding and shaping of the pre-existing EpCAM-specific T-cell repertoire, and (4) spreading of anti-tumor immunity to different tumor antigens.

Developing strategies in the immunotherapy of leukemias

BACKGROUND

In the current treatment paradigms for leukemias, hematopoietic stem cell transplant (HSCT) is considered the best option with a curative potential although more often than not it simply delays disease progression. Advances are needed, both in current therapies and in the development of new strategies. Partly from studying the nuances of the curative potential of stem cell transplant, we have come to appreciate the relevance of the immune response and the potential of immunotherapy.

METHODS

This review article summarizes the recent advances in the field of immunology and immunotherapy for leukemia.

RESULTS

In passive immunotherapy, recent progress in chimeric T-cell antigen receptor technology has been encouraging. In active immunotherapy, a cancer vaccine may potentially enhance HSCT. An overview of various clinical studies of peptide vaccination strategies focusing on molecular targets such as the Wilms' tumor gene 1 (WT1), proteinase 3 (PR3), and receptor for hyaluronan acid-mediated motility (RHAMM) is provided. Cell-based vaccination strategies are also briefly explored.

CONCLUSIONS

The immune system clearly has the capacity to recognize and react to leukemic cells, and recent evidence directs our attention to the importance of mounting inflammatory and CD4 T-cell responses to complement and support the cytotoxic activity elicited by peptide vaccines.

Current strategies in immunotherapy for acute myeloid leukemia

The prognosis of acute myeloid leukemia, particularly when associated with adverse chromosomal or molecular aberrations, is poor due to a high relapse rate after induction chemotherapy. Postremission therapy for elimination of minimal residual disease remains a major challenge. Allogeneic hematopoietic stem cell transplantation has proven to provide a potent antileukemic effect. Novel strategies are needed for patients ineligible for this treatment. Here current immunotherapeutic concepts in acute myeloid leukemia in a nonallogeneic hematopoietic stem cell transplantation setting are reviewed. Data gathered with different monoclonal antibodies are discussed. Adoptive transfer of NK and T cells is reviewed, including evolving data on T-cell engineering. Results of systemic cytokine administration and of therapeutic vaccinations with peptides, modified leukemic cells and dendritic cells are presented. One particular focus of this review is the integration of currently running clinical trials. Recent immunotherapeutic studies have been encouraging and further interesting results are to be expected.

Dendritic cell vaccination in pediatric gliomas: lessons learnt and future perspectives

Immunotherapy of malignant gliomas with autologous dendritic cells (DCs) in addition to surgery and radiochemotherapy has been a focus of intense research during the past decade. Since both children and adults are affected by this highly aggressive brain tumor, 10-15% of the several hundred vaccinated patients represent children, making pediatric glioma patients the largest uniform pediatric vaccination cohort so far. In general, DC vaccination in malignant gliomas has been shown to be safe and several studies with a non-vaccinated control group could clearly demonstrate a survival benefit for the vaccinated patients. Interestingly, children and adolescents below 21 years of age seem to benefit even more than adult patients. This review summarizes the findings of the 25 clinical trials published so far and gives a perspective how DC vaccination could be implemented as part of multimodal therapeutic strategies in the near future.

A BCR/ABL-hIL-2 DNA vaccine enhances the immune responses in BALB/c mice

The use of a DNA vaccine encoding the BCR/ABL fusion gene is thought to be a promising approach for patients with chronic myeloid leukemia (CML) to eradicate minimal residual disease after treatment with chemotherapy or targeted therapy. In this study, our strategy employs genetic technology to create a DNA vaccine encoding the BCR/ABL fusion and human interleukin-2 (hIL-2) genes. The successfully constructed plasmids BCR/ABL-pIRES-hIL-2, BCR/ABL-pIRES, and pIRES-hIL-2 were delivered intramuscularly to BALB/c mice at 14-day intervals for three cycles. The transcription and expression of the BCR/ABL and hIL-2 genes were found in the injected muscle tissues. The interferon-γ (IFN-γ) serum levels were increased, and the splenic CD4⁺/CD8⁺ T cell ratio was significantly decreased in the BCR/ABL-pIRES-hIL-2-injected mice. Furthermore, specific antibodies against K562 cells could be detected by indirect immunofluorescence. These results indicate that a DNA vaccine containing BCR/ABL and hIL-2 together may elicit increased in vivo humoral and cellular immune responses in BALB/c mice.

Persistent antigen at vaccination sites induces tumor-specific CD8⁺ T cell sequestration, dysfunction and deletion

To understand why cancer vaccine-induced T cells often fail to eradicate tumors, we studied immune responses in mice vaccinated with gp100 melanoma peptide in incomplete Freund’s adjuvant (IFA), commonly used in clinical cancer vaccine trials. Peptide/IFA vaccination primed tumor-specific CD8⁺ T cells, which accumulated not in tumors but at the persisting, antigen-rich vaccination site. Once there, primed T cells became dysfunctional and underwent antigen-driven, Interferon-γ (IFN-γ) and Fas ligand (FasL)-mediated apoptosis, resulting in hyporesponsiveness to subsequent vaccination. Provision of anti-CD40 antibody, Toll-like receptor 7 (TLR7) agonist and interleukin-2 (IL-2) reduced T cell apoptosis but did not prevent vaccination site sequestration. A non-persisting vaccine formulation shifted T cell localization towards tumors, inducing superior anti-tumor activity while reducing systemic T cell dysfunction and promoting memory formation. Persisting peptide/IFA vaccine depots can induce specific T cell sequestration, dysfunction and deletion at vaccination sites; short-lived formulations may overcome these limitations and result in greater therapeutic efficacy of peptide-based cancer vaccines.

The role of peptide and DNA vaccines in myeloid leukemia immunotherapy

While chemotherapy and targeted therapy are successful in inducing the remission of myeloid leukemia as acute myeloid leukemia (AML) and chronic myeloid leukemia (CML), the disease remains largely incurable. This observation is likely due to the drug resistance of leukemic cells, which are responsible for disease relapse. Myeloid leukemia vaccines may most likely be beneficial for eradicating minimal residual disease after treatment with chemotherapy or targeted therapy. Several targeted immunotherapies using leukemia vaccines have been heavily investigated in clinical and preclinical trials. This review will focus on peptides and DNA vaccines in the context of myeloid leukemias, and optimal strategies for enhancing the efficacy of vaccines based on myeloid leukemia immunization are also summarized.

Alternative approaches to eradicating the malignant clone in chronic myeloid leukemia: tyrosine-kinase inhibitor combinations and beyond

In patients with chronic myeloid leukemia (CML) in chronic phase who have achieved complete molecular remission on imatinib therapy, clinical trials from France and Australia have demonstrated that the majority experience prompt molecular relapse of their leukemia upon discontinuation of the drug, showing that long-term monotherapy with tyrosine kinase inhibitors is not curative in the majority of patients with CML. This has focused attention on strategies to eradicate residual disease in CML that is presumed to arise from malignant Ph+ stem cells, which should result in permanent cure and long-term leukemia-free survival. Here, we review the evidence that targeting CML stem cells will be of clinical benefit and discuss pharmacological and immunological approaches to accomplish this goal. Where possible, we link preclinical studies of CML stem cell biology to emerging results from clinical trials of agents that may target these cells.

Myelodysplastic syndromes: who and when in the course of disease to transplant

The myelodysplastic syndromes are clonal hematopoietic disorders for which hematopoietic stem cell transplantation remains the only curative therapy. The timing of transplantation, methods of disease risk stratification, patient selection, pretransplantation therapies, and preparative regimens have evolved over the years, resulting in increasing disease-free survival. In recent years, alternative donor sources have been demonstrated to be a viable alternative to traditional sibling and matched unrelated donor stem cell sources. Efforts at transplantation regimen development continue with the aim of maximizing the chances of cure with minimal toxicity and improved quality of life. Integrating new knowledge regarding disease biology will be critical to continue to improve the success of hematopoietic stem cell transplantation. Exciting areas of ongoing research that may lead to reductions in posttransplantation relapse rate include posttransplantation therapies such as DNA methyltransferase inhibitors, vaccine strategies, and donor lymphocyte infusions to enhance the GVL effect.

Immunotherapeutic approaches to improve graft-versus-tumor effect and reduce graft-versus-host disease

The therapeutic efficacy of allogeneic stem cell transplantation is mainly based on the alloreactive immune response of the graft against the host. However, the graft-versus-host process can be viewed as a double-edged sword since it is responsible for both the beneficial graft-versus-tumor effect and the deleterious graft-versus-host disease. During the last two decades, intensive research has been focused on the development of novel immunotherapeutic methods aimed to dissociate graft-versus-host disease from graft-versus-tumor effect. A brief description of these efforts is discussed in this review.

Mapping of novel peptides of WT-1 and presenting HLA alleles that induce epitope-specific HLA-restricted T cells with cytotoxic activity against WT-1(+) leukemias

The Wilms tumor protein (WT-1) is widely recognized as a tumor antigen that is expressed differentially by several malignancies. However, WT-1 peptides known to induce tumoricidal T cells are few. In the present study, we evaluated T-cell responses of 56 healthy donors to in vitro sensitization with autologous APCs loaded with a pool of overlapping 15-mer peptides spanning the sequence of WT-1. Thereafter, we mapped the WT-1 peptides eliciting responses in each individual, defined the immunogenic peptides, and identified their presenting HLA alleles. We report 41 previously unreported epitopes of WT-1: 5 presented by class II and 36 by class I alleles, including 10 that could be presented by more than 1 class I allele. IFNγ(+) T cells responding to 98% of the class I and 60% of the class II epitopes exhibited HLA-restricted cytotoxicity against peptide-loaded targets. T cells specific for 36 WT-1 peptides were evaluable for leukemocidal activity, of which 27 (75%) lysed WT-1(+) leukemic targets sharing their restricting HLA allele. Each epitope identified induced T-cell responses in most donors sharing the epitopes' presenting allele; these responses often exceeded responses to flanking peptides predicted to be more immunogenic. This series of immunogenic epitopes of WT-1 should prove useful for immunotherapies targeting WT-1(+) malignancies.

DNA fusion-gene vaccination in patients with prostate cancer induces high-frequency CD8(+) T-cell responses and increases PSA doubling time

We report on the immunogenicity and clinical effects in a phase I/II dose escalation trial of a DNA fusion vaccine in patients with prostate cancer. The vaccine encodes a domain (DOM) from fragment C of tetanus toxin linked to an HLA-A2-binding epitope from prostate-specific membrane antigen (PSMA), PSMA_27–35. We evaluated the effect of intramuscular vaccination without or with electroporation (EP) on vaccine potency. Thirty-two HLA-A2⁺ patients were vaccinated and monitored for immune and clinical responses for a follow-up period of 72 weeks. At week 24, cross-over to the immunologically more effective delivery modality was permitted; this was shown to be with EP based on early antibody data, and subsequently, 13/15 patients crossed to the +EP arm. Thirty-two HLA-A2⁻ control patients were assessed for time to next treatment and overall survival. Vaccination was safe and well tolerated. The vaccine induced DOM-specific CD4⁺ and PSMA₂₇-specific CD8⁺ T cells, which were detectable at significant levels above baseline at the end of the study (p = 0.0223 and p = 0.00248, respectively). Of 30 patients, 29 had a measurable CD4⁺ T-cell response and PSMA₂₇-specific CD8⁺ T cells were detected in 16/30 patients, with or without EP. At week 24, before cross-over, both delivery methods led to increased CD4⁺ and CD8⁺ vaccine-specific T cells with a trend to a greater effect with EP. PSA doubling time increased significantly from 11.97 months pre-treatment to 16.82 months over the 72-week follow-up (p = 0.0417), with no clear differential effect of EP. The high frequency of immunological responses to DOM-PSMA₂₇ vaccination and the clinical effects are sufficiently promising to warrant further, randomized testing.

Active specific immunotherapy targeting the Wilms' tumor protein 1 (WT1) for patients with hematological malignancies and solid tumors: lessons from early clinical trials

There is a growing body of evidence that Wilms' tumor protein 1 (WT1) is a promising tumor antigen for the development of a novel class of universal cancer vaccines. Recently, in a National Cancer Institute prioritization project, WT1 was ranked first in a list of 75 cancer antigens. In this light, we exhaustively reviewed all published cancer vaccine trials reporting on WT1-targeted active specific immunotherapy in patients with hematological malignancies and solid tumors. In all clinical trials, vaccine-induced immunological responses could be detected. Importantly, objective clinical responses (including stable disease) were observed in 46% and 64% of evaluable vaccinated patients with solid tumors and hematological malignancies, respectively. Immunogenicity of WT1-based cancer vaccines was demonstrated by the detection of a specific immunological response in 35% and 68% of evaluable patients with solid tumors and hematological malignancies, respectively. In order to become part of the armamentarium of the modern oncologist, it will be important to design WT1-based immunotherapies applicable to a large patient population, to standardize vaccination protocols enabling systematic review, and to further optimize the immunostimulatory capacity of the vaccine components. Moreover, improved immunomonitoring tools that reveal clinically relevant T-cell responses will further shape the ideal WT1 immunotherapy strategy. In conclusion, the clinical results obtained so far in WT1-targeted cancer vaccine trials reveal an untapped potential for inducing cancer immunity with minimal side effects and hold promise for a new adjuvant treatment against residual disease and against cancer relapse.

Biased usage of T cell receptor β-chain variable region genes of Wilms' tumor gene (WT1)-specific CD8+ T cells in patients with solid tumors and healthy donors

Wilms' tumor gene 1 (WT1) protein is a promising tumor-associated antigen. In patients with WT1-expressing malignancies, WT1-specific CTLs are spontaneously induced as a result of an immune response to the WT1 protein. In the present study, we performed single cell-level comparative analysis of T cell receptor β-chain variable region (TCR-BV) gene families of a total of 750 spontaneously induced WT1(126) peptide (amino acids 126-134, WT1(126))-specific CTLs in both HLA-A*0201(+) patients with solid tumors and healthy donors (HDs). This is the first report of direct usage analysis of 24 kinds of TCR-BV gene families of WT1(126)-specific CTLs at the single cell level. Usage analysis with single-cell RT-PCR of TCR-BV gene families of individual FACS-sorted WT1(126) tetramer(+) CD8(+) T cells showed, for the first time, that: (i) BVs 3, 6, 7, 20, 27, and 28 were commonly biased in patients and HDs; (ii) BVs 2, 11, and 15 were biased only in patients; and (iii) BVs 4, 5, 9, and 19 were biased only in HDs. However, statistical analysis of similarity of individual usage frequencies of 24 kinds of TCR-BV gene families between patients and HDs indicated that the usage frequencies of TCR-BV gene families in patients reflected those in HDs. These results should provide us with a novel insight for a better understanding of WT1-specific immune responses.

Lymphodepletion is permissive to the development of spontaneous T-cell responses to the self-antigen PR1 early after allogeneic stem cell transplantation and in patients with acute myeloid leukemia undergoing WT1 peptide vaccination following chemotherapy

PR1, an HLA-A*0201 epitope shared by proteinase-3 (PR3) and elastase (ELA2) proteins, is expressed in normal neutrophils and overexpressed in myeloid leukemias. PR1-specific T cells have been linked to graft-versus-leukemia (GVL) effect. We hypothesized that lymphopenia induced by chemo-radiotherapy can enhance weak autoimmune responses to self-antigens such as PR1. We measured PR1-specific responses in 27 patients 30-120 days following allogeneic stem cell transplant (SCT) and correlated these with ELA2 and PR3 expression and minimal residual disease (MRD). Post-SCT 10/13 CML, 6/9 ALL, and 4/5 solid tumor patients had PR1 responses correlating with PR3 and ELA2 expression. At day 180 post-SCT, 8/8 CML patients with PR1 responses were BCR-ABL-negative compared with 2/5 BCR-ABL-positive patients (P = 0.025). In contrast, PR1 responses were detected in 2/4 MRD-negative compared with 4/5 MRD-positive ALL patients (P = 0.76). To assess whether the lymphopenic milieu also exaggerates weak T-cell responses in the autologous setting, we measured spontaneous induction of PR1 responses in 3 AML patients vaccinated with WT1-126 peptide following lymphodepletion. In addition to WT1-specific T cells, we detected PR1-specific T cells in 2 patients during hematopoietic recovery. Our findings suggest that lymphopenia induced by chemo-radiotherapy enhances weak autoimmune responses to self-antigens, which may result in GVL if the leukemia expresses the relevant self-antigen.

Vaccination strategies in lymphomas and leukaemias: recent progress

The successful identification of a range of leukaemia-specific and lymphoma-specific antigens in recent years has stimulated efforts to develop therapeutic vaccination strategies. A number of clinical trials have established the safety and immunogenicity of vaccination against tumour antigens, although there are limited data on the clinical efficacy of this approach in haematological malignancies. After encouraging results of phase I/II trials using idiotype vaccines in lymphoma, the outcome of the three phase III trials has been somewhat disappointing. Several other promising strategies are currently being developed to improve these results, including optimization of antigen delivery. In myeloid leukaemias, clinical trials of vaccination with peptides derived from a number of leukaemia antigens, including WT1, PR1, RHAMM and BCR-ABL, have shown evidence of immunogenicity, but limited data are available on the clinical efficacy of this approach. In this review, we focus on the results of clinical trials of vaccination in leukaemia and lymphoma, and discuss potential strategies to enhance the efficacy of immunotherapy in the future.

Effects of TLR agonists on maturation and function of 3-day dendritic cells from AML patients in complete remission

BACKGROUND

Active dendritic cell (DC) immunization protocols are rapidly gaining interest as therapeutic options in patients with acute myeloid leukemia (AML). Here we present for the first time a GMP-compliant 3-day protocol for generation of monocyte-derived DCs using different synthetic Toll-like receptor (TLR) agonists in intensively pretreated patients with AML.

METHODS

Four different maturation cocktails were compared for their impact on cell recovery, phenotype, cytokine secretion, migration, and lymphocyte activation in 20 AML patients and 25 healthy controls.

RESULTS

Maturation cocktails containing the TLR7/8 agonists R848 or CL075, with and without the addition of the TLR3 agonist poly(I:C), induced DCs that had a positive costimulatory profile, secreted high levels of IL-12(p70), showed chemotaxis to CCR7 ligands, had the ability to activate NK cells, and efficiently stimulated antigen-specific CD8+ T cells.

CONCLUSIONS

Our results demonstrate that this approach translates into biologically improved DCs, not only in healthy controls but also in AML patients. This data supports the clinical application of TLR-matured DCs in patients with AML for activation of innate and adaptive immune responses.

Evaluation of current cancer immunotherapy: hemato-oncology

Hematologic malignancies were the first diseases in clinical oncology for which the potential of harnessing the immune system as targeted therapy was unequivocally demonstrated. Unfortunately, the use of this highly efficacious modality has been limited to only a subset of patients and diseases because of immune-mediated toxicities resulting from incomplete specificity, and disease-specific determinants of sensitivity versus resistance to immune effector mechanisms. Recent studies, however, have begun to elucidate the molecular basis of the observed clinical effects allowing the rational development of next generation of immunotherapeutic combinations. We discuss here cancer antigen targets in hematologic malignancies and the specific approaches to induce immunity being pursued, the importance of modulating the host immunoregulatory environment, and the special features of immunological monitoring in clinical investigation. The hematologic malignancies represent an ideal setting for the development of immunotherapy due to logistical, clinical monitoring, and disease biology factors and may represent an exemplar for immune-based treatment in other cancer types.

DNA fusion vaccines enter the clinic

Induction of effective immune attack on cancer cells in patients requires conversion of weak tumor antigens into strong immunogens. Our strategy employs genetic technology to create DNA vaccines containing tumor antigen sequences fused to microbial genes. The fused microbial protein engages local CD4+ T cells to provide help for anti-tumor immunity, and to reverse potential regulation. In this review, we focus on induction of CD8+ T cells able to kill target tumor cells. The DNA vaccines incorporate tumor-derived peptide sequences fused to an engineered domain of tetanus toxin. In multiple models, this design induces strong CD8+ T-cell responses, able to suppress tumor growth. For clinical relevance, we have used "humanized" mice expressing HLA-A2, successfully inducing cytolytic T-cell responses against a range of candidate human peptides. To overcome physical restriction in translating to patients, we have used electroporation. Clinical trials of patients with cancer are showing induction of responses, with preliminary indications of suppression of tumor growth and evidence for clinically manageable concomitant autoimmunity.

Posttransplantation vaccination: concepts today and on the horizon

Allogeneic hematopoietic stem cell transplantation (allogeneic HSCT) remains a curative treatment for hematological malignancies resistant to other treatment approaches through the unique GVL effect. However, relapse remains a major cause of treatment failure after allogeneic HSCT for patients with high-risk hematological malignancies. Further improvements in exploiting the GVL effect to prevent relapse in high-risk leukemias while minimizing toxicity have focused on the use of targeted antileukemic immunotherapy. These strategies include methods to boost the GVL effect with leukemia vaccines or the adoptive transfer of leukemia-specific lymphocytes. Vaccines can be classified as those against defined antigens such as minor histocompatibility antigens (mHags) or leukemia-associated antigens (PR1, WT1, and BCR-ABL) and those that have broad "antileukemic" activity such as engineered irradiated leukemia cells or leukemia-derived dendritic cells (DCs). The unique posttransplantation milieu, which is characterized by lymphopenia, regulatory T-cell depletion, and the release of growth factors, provides a unique opportunity for effective antitumor immunotherapy and augmenting specific GVL responses. This review focuses on approaches to enhancimg the GVL response by combining allogeneic HSCT with vaccination.