Dramatic efficacy improvement of a DC-based vaccine against AML by CD25 T cell depletion allowing the induction of a long-lasting T cell response

Immune therapy: a new therapy for acute myeloid leukemia

Although complete remission could be achieved in about 60%-70% of acute myeloid leukemia (AML) patients after conventional chemotherapy, relapse and the state of being refractory to treatment remain the main cause of death. In addition, there is a great need for less intensive regimens for all medically frail patients (both due to age/comorbidity and treatment-related). Immune therapy anticipates improved prognosis and reduced toxicities, which may offer novel therapeutic rationales. However, one of the major difficulties in developing immune therapies against AML is that the target antigens are also significantly expressed on healthy hematopoietic stem cells; B-cell malignancies are different because CD20/CD19/healthy B-cells are readily replaceable. Only the anti-CD33 antibody-drug conjugate gemtuzumab-ozogamicin is approved by the FDA for AML. Thus, drug development remains extremely active, although it is still in its infancy. This review summarizes the clinical results of immune therapeutic agents for AML, such as antibody-based drugs, chimeric antigen receptor therapy, checkpoint inhibitors, and vaccines.

Immunosuppressive Cell Subsets and Factors in Myeloid Leukemias

Simple Summary

Effector immune system cells have the ability to kill tumor cells. However, as a cancer (such as leukemia) develops, it inhibits and evades the effector immune response. Such a state of immunosuppression can be driven by several factors – receptors, soluble cytokines, as well as by suppressive immune cells. In this review, we describe factors and cells that constitute immunosuppressive microenvironment of myeloid leukemias. We characterize factors of direct leukemic origin, such as inhibitory receptors, enzymes and extracellular vesicles. Furthermore, we describe suppressive immune cells, such as myeloid derived suppressor cells and regulatory T cells. Finally, we sum up changes in these drivers of immune evasion in myeloid leukemias during therapy.

Abstract

Both chronic myeloid leukemia and acute myeloid leukemia evade the immune response during their development and disease progression. As myeloid leukemia cells modify their bone marrow microenvironment, they lead to dysfunction of cytotoxic cells, such as CD8+ T cells or NK cells, simultaneously promoting development of immunosuppressive regulatory T cells and suppressive myeloid cells. This facilitates disease progression, spreading of leukemic blasts outside the bone marrow niche and therapy resistance. The following review focuses on main immunosuppressive features of myeloid leukemias. Firstly, factors derived directly from leukemic cells – inhibitory receptors, soluble factors and extracellular vesicles, are described. Further, we outline function, properties and origin of main immunosuppressive cells - regulatory T cells, myeloid derived suppressor cells and macrophages. Finally, we analyze interplay between recovery of effector immunity and therapeutic modalities, such as tyrosine kinase inhibitors and chemotherapy.

A biomaterial-based vaccine eliciting durable tumour-specific responses against acute myeloid leukaemia

Acute myeloid leukaemia (AML) is a malignancy of haematopoietic origin that has limited therapeutic options. The standard-of-care cytoreductive chemotherapy depletes AML cells to induce remission, but is infrequently curative. An immunosuppressive AML microenvironment in the bone marrow and the paucity of suitable immunotherapy targets limit the induction of effective immune responses. Here, in mouse models of AML, we show that a macroporous-biomaterial vaccine that delivers the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF), the Toll-like-receptor-9 agonist cytosine-guanosine oligodeoxynucleotide and one or multiple leukaemia antigens (in the form of a defined peptide antigen, cell lysates or antigens sourced from AML cells recruited in vivo) induces local immune-cell infiltration and activated dendritic cells, evoking a potent anti-AML response. The biomaterial-based vaccine prevented the engraftment of AML cells when administered as a prophylactic and when combined with chemotherapy, and eradicated established AML even in the absence of a defined vaccine antigen. Biomaterial-based AML vaccination can induce potent immune responses, deplete AML cells and prevent disease relapse.

Phenotypic profile of dendritic and T cells in the lymph node of Balb/C mice with breast cancer submitted to dendritic cells immunotherapy

Breast cancer (BC) is the most common malignant neoplasm and the cause of death by cancer among women worldwide. Its development influenced by various mutations that occur in the tumor cell and by the immune system's status, which has a direct influence on the tumor microenvironment and, consequently, on interactions with non-tumor cells involved in the immunological response. Strategies using dendritic cells (DCs) or antigen-presenting cells (APCs), therapeutic mode, in cancer have been developed for some time. The proper interaction between DCs and T cells upon antigen presentation is of greatest importance for an antitumor immune response activation. Thus, various receptors on the surface of T cells must be able to recognize ligands that are located on the surface of APCs. However, little is known about the real behavior and interaction forms of CDs and T cells after vaccination. Due to the crucial importance of DCs in an effective anti-tumor immune response activation and the search for compliant results in inducing this response by immunotherapies with DCs, the phenotypic profile of DCs and T cells in lymph nodes obtained from female Balb/C mice with breast cancer induced by 4T1 cells and DCs treated with vaccines was investigated. We evaluated through flow cytometry based on the surface and intracellular molecules marking; as well as the presence of cytokines and chemokines, IL-2, IL-4, IL-10, IL-12, IFN-γ, TNF-α and TGF-β in the supernatant of the culture of Balb/C lymph nodes by ELISA. The results show that the vaccination with DCs, in the maturation parameters used in this study, was able to stimulate the secretion of cytokines such as IFN-γ and IL-12 and inhibit the secretion of TGF-β and IL-10 in nodal lymph infiltrates, as well as co-stimulatory activating (CD86) and adhesion molecules in DCs and T cells LFA-1/ICAM-1 and inhibit the secretion of CTLA-4 present in lymph nodes. Facts that led to aTh1 profile polarization, immuno competent in relation to breast cancer. We indirectly evaluated the interaction between DCs and T cells dependent on the vaccination with DCs in tumor draining lymph nodes, in breast cancer in Balb/C mice and we believe that, this way, we will be able to achieve a model vaccine protocol in the future, based on the correct interaction between cells that enable the induction of anti-tumor effective response. Breast cancer (BC) is the most common malignant neoplasm and the cause of death by cancer among women worldwide. Its development influenced by various mutations that occur in the tumor cell and by the immune system's status, which has a direct influence on the tumor microenvironment and, consequently, on interactions with non-tumor cells involved in the immunological response. Strategies using dendritic cells (DCs) or antigen-presenting cells (APCs), therapeutic mode, in cancer have been developed for some time. The proper interaction between DCs and T cells upon antigen presentation is of greatest importance for an antitumor immune response activation. Thus, various receptors on the surface of T cells must be able to recognize ligands that are located on the surface of APCs. However, little is known about the real behavior and interaction forms of DCs and T cells after vaccination. Due to the crucial importance of DCs in an effective anti-tumor immune response activation and the search for compliant results in inducing this response by immunotherapies with DCs, the phenotypic profile of DCs and T cells in lymph nodes obtained from female Balb/C mice with breast cancer induced by 4T1 cells and DCs treated with vaccines was investigated. We evaluated through flow cytometry based on the surface and intracellular molecules marking; as well as the presence of cytokines and chemokines, IL-2, IL-4, IL-10, IL-12, IFN-γ, TNF-α and TGF-β in the supernatant of the culture of Balb/C lymph nodes by ELISA. The results show that the vaccination with DCs, in the maturation parameters used in this study, was able to stimulate the secretion of cytokines such as IFN-γ and IL-12 and inhibit the secretion of TGF-β and IL-10 in nodal lymph infiltrates, as well as co-stimulatory activating (CD86) and adhesion molecules in DCs and T cells LFA-1/ICAM-1 and inhibit the secretion of CTLA-4 present in lymph nodes. Facts that led to aTh1 profile polarization, immuno competent in relation to breast cancer. We indirectly evaluated the interaction between DCs and T cells dependent on the vaccination with DCs in tumor draining lymph nodes, in breast cancer in Balb/C mice and we believe that, this way, we will be able to achieve a model vaccine protocol in the future, based on the correct interaction between cells that enable the induction of anti-tumor effective response.

PGE2-induced IDO1 inhibits the capacity of fully mature DCs to elicit an in vitro antileukemic immune response

In the last years, dendritic cells (DC) have been evaluated for antitumor vaccination. Although DC-based vaccines have raised great expectations, their clinical translation has been largely disappointing. For these results, several explanations have been proposed. In particular, the concomitant expression by DCs of tolerogenic pathways, such as the immunosuppressive agent indoleamine 2,3-dioxygenase-1 (IDO1), has been demonstrated. The aim of this study is to evaluate both the stimulatory and the tolerogenic feature of monocyte-derived DCs (Mo-DCs) after maturation with PGE₂. In particular, the role of IDO1 expression in PGE₂-matured Mo-DCs has been addressed. Here we show that PGE₂, which is required for full maturation of DCs, is one mediator of DC tolerance by enhancing IDO1. PGE₂-mediated expression of IDO1 results in the production of kynurenine, in the generation of T_regs, and in the inhibition of either the allogeneic or the autologous antigen-specific stimulatory capacity of DCs. When pulsed with leukemic lysates and matured with PGE₂, DCs are impaired in the induction of IFN-γ secreting CD4⁺ and CD8⁺ T cells due to IDO1 upregulation. Moreover, the inhibition of IDO1 enhances the antileukemic response. Overall, these results point toward the use of IDO1 inhibitors to enhance the vaccination capacity of DCs, matured with PGE₂.

Inhibition of CD4+CD25+ regulatory T cell function and conversion into Th1-like effectors by a Toll-like receptor-activated dendritic cell vaccine

Despite the success of vaccines against some microbial pathogens, their utility in the prevention and treatment of cancer has thus far been limited. We have previously demonstrated that vaccination with dendritic cells activated with the TLR-4 ligand LPS and IFN-γ promotes an antigen-specific anti-tumor response that prevents tumor recurrence. To evaluate this mechanistically, we here studied the effects of this TLR-activated DC on regulatory T cell activity. Dendritic cells activated with LPS and IFN- γ negated the effects of regulatory T cells on responder cell proliferation. Restoration of responder cell proliferation was noted when TLR-activated dendritic cells were separated from both regulators and responders by a semi-permeable membrane. The effect is therefore mediated by a soluble factor but was independent of both IL-6 and IL-12. Furthermore, the soluble mediator appeared to act at least in part on the regulators themselves rather than responder cells exclusively. Because recent studies have demonstrated conversion of T regulatory cells into IL-17-producing effectors, we further questioned whether the TLR-activated dendritic cell would induce cytokine production and effector function in our system. We found that regulators produced a substantial amount of IFN- γ in the presence of TLR-activated dendritic cells but not immature dendritic cells. IFN-γ production was associated with upregulation of the Th1 transcriptional regulator T-bet, and a significant fraction of IFN-γ-producing regulators coexpressed T-bet and FoxP3. While the effects of the LPS-activated dendritic cell on responder cell proliferation were IL-12 independent, upregulation of T-bet was inhibited by a neutralizing anti-IL-12 antibody. Collectively, these and prior data suggest that varying innate immune signals may direct the phenotype of the immune response in part by inhibiting suppressor T cells and promoting differentiation of these regulators into particular subsets of effectors.

NK cells: key to success of DC-based cancer vaccines?

The cytotoxic and regulatory antitumor functions of natural killer (NK) cells have become attractive targets for immunotherapy. Manipulation of specific NK cell functions and their reciprocal interactions with dendritic cells (DCs) might hold therapeutic promise. In this review, we focus on the engagement of NK cells in DC-based cancer vaccination strategies, providing a comprehensive overview of current in vivo experimental and clinical DC vaccination studies encompassing the monitoring of NK cells. From these studies, it is clear that NK cells play a key regulatory role in the generation of DC-induced antitumor immunity, favoring the concept that targeting both innate and adaptive immune mechanisms may synergistically promote clinical outcome. However, to date, DC vaccination trials are only infrequently accompanied by NK cell monitoring. Here, we discuss different strategies to improve DC vaccine preparations via exploitation of NK cells and provide a summary of relevant NK cell parameters for immune monitoring. We underscore that the design of DC-based cancer vaccines should include the evaluation of their NK cell stimulating potency both in the preclinical phase and in clinical trials.

Dendritic cells the tumor microenvironment and the challenges for an effective antitumor vaccination

Many clinical trials have been carried out or are in progress to assess the therapeutic potential of dendritic-cell- (DC-) based vaccines on cancer patients, and recently the first DC-based vaccine for human cancer was approved by the FDA. Herewith, we describe the general characteristics of DCs and different strategies to generate effective antitumor DC vaccines. In recent years, the relevance of the tumor microenvironment in the progression of cancer has been highlighted. It has been shown that the tumor microenvironment is capable of inactivating various components of the immune system responsible for tumor clearance. In particular, the effect of the tumor microenvironment on antigen-presenting cells, such as DCs, does not only render these immune cells unable to induce specific immune responses, but also turns them into promoters of tumor growth. We also describe strategies likely to increase the efficacy of DC vaccines by reprogramming the immunosuppressive nature of the tumor microenvironment.

Nanoliposome-mediated FL/TRAIL double-gene therapy for colon cancer: in vitro and in vivo evaluation

OBJECTIVE

To investigate the therapeutic effects of cationic nanoliposome-mediated gene therapy combined with immunotherapy for colon cancer treatment.

METHODS

Recombinant plasmids containing green and red fluorescent protein reporter genes were constructed using gene cloning methods. Gene-carrying cationic nanoliposomes were prepared based on the electrostatic adherence principle and then transfected into dendritic cells (DC), which were transplanted into colon cancer cells.

RESULTS

Recombinant plasmids containing green or red fluorescent protein reporter genes were successfully constructed by gene cloning and confirmed by restriction enzyme digestion and sequencing. Gene-carrying cationic nanoliposomes were transfected into colon cancer cells, and good gene expression was detected. A better level of apoptosis was observed in the combined group of tyrosine kinase receptor 3 ligand (FL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), while the lowest level was detected in the control group. The parameters in the FL and TRAIL groups were between the above-mentioned combined group.

CONCLUSION

Cationic nanoliposomes have the advantage of being gene carriers. The joint therapeutic effects of the two genes are superior to those of a single gene. Gene therapy combined with immunotherapy has significant implications for cancer treatment.

Murine rhabdomyosarcoma is immunogenic and responsive to T-cell-based immunotherapy

BACKGROUND

Immunotherapies targeting cellular immunity are currently approved for treatment of melanoma, renal cell carcinoma, and prostate cancer. Studies on the immunogenicity and immune responsiveness of pediatric tumors are limited, therefore, it remains unclear to what extent T-cell-based immunotherapy holds promise for pediatric solid tumors.

PROCEDURE

A new rhabdomyosarcoma cell line (M3-9-M) was derived from an embryonal rhabdomyosarcoma (ERMS) occurring in a C57BL/6 mouse transgenic for hepatocyte growth factor and heterozygous for mutated p53. Primary tumors and metastases derived from M3-9-M were studied for similarities to human ERMS, and for immunogenicity and immune responsiveness.

RESULTS

Primary and metastatic tumors develop after orthotopic injection of M3-9-M into immunocompetent C57BL/6 mice, which mirror human ERMS with regard to histology, gene expression, and metastatic behavior. Whole cell vaccination using irradiated M3-9-M cells or M3-9-M-pulsed dendritic cells (DC)-induced tumor-specific T-cell responses that prevent tumor growth following low-dose tumor injection, and slow tumor growth following higher doses. Administration of anti-CD25 moAbs to deplete CD4(+)CD25(+)FOXP3(+) regulatory T cells prior to tumor vaccination enhanced the potency of the ERMS tumor vaccine. Adoptive immunotherapy with M3-9-M primed T cells plus DC-based vaccination resulted in complete eradication of day 10 M3-9-M derived tumors.

CONCLUSIONS

M3-9-M derived murine ERMS is immunogenic and immunoresponsive; regulatory T cells contribute to immune evasion by murine rhabdomyosarcoma. Adoptive immunotherapy with DC vaccination can eradicate low tumor burdens. Future work will seek to identify the tumor-associated antigens that mediate protective and therapeutic immunity in this model.

Pulsing with blast cell lysate or blast-derived total RNA reverses the dendritic cell-mediated cytotoxic activity of cytokine-induced killer cells against allogeneic acute myelogenous leukemia cells

Immunotherapeutic strategies may be a treatment option in patients with refractory acute myelogenous leukemia (AML) or, in cases of complete remission after conventional therapy regimens, may help to reduce disease recurrence or delay time to progression. Evidence suggests a key role of dendritic cells (DCs) in cancer immunotherapy due to their capacity to present tumour antigens to effector cells. We generated cytokine-induced killer (CIK) cells from healthy donors and examined their responses in vitro in an LDH release assay against three cell lines and allogeneic HLA non-matched blasts from three patients with de novo AML after coincubation with autologous peripheral blood monocyte-derived DCs. Although DCs were unable to enhance CIK cell effects against all three cell lines tested, the cytotoxic activity against the patients’ AML cells increased after coculture with mature DCs, which was significant in two of three patients. However, neither prior pulsing of the DCs with blast cell lysates nor with leukemic cell-derived total RNA further enhanced the lytic capacity of the CIK cells. On the contrary, pulsing reduced or even reversed the cytotoxic activity of the effector cells. This decrease of allogeneic cytotoxicity led us to conclude that monocyte-derived DCs may be useful in autologous or allogeneic vaccine strategies for the treatment of AML or in priming donor lymphocytes in vitro, but unfractionated antigens as pulsing agents may have inhibitory effects on T cell efficiency and their employment in immunotherapeutic strategies for AML seems questionable.

Role of regulatory T-cells in immunization strategies involving a recombinant alphavirus vector system

BACKGROUND

Regulatory T-cells (Treg) hamper immune responses elicited by cancer vaccines. Therefore, depletion of Treg is being used to improve the outcome of vaccinations.

METHODS

We studied whether an alphavirus vector-based immunotherapeutic vaccine changes the number and/or activity of Treg and if Treg depletion improves the efficacy of this vaccine against tumours. The vaccine is based on a Semliki Forest virus (SFV). The recombinant SFV replicon particles encode a fusion protein of E6 and E7 from human papillomavirus (HPV) type 16 (SFVeE6,7).

RESULTS

We demonstrated that SFVeE6,7 immunization did not change Treg levels and their suppressive activity. Depletion of Treg in mice, using the novel anti-folate receptor 4 antibody, did not enhance the immune response induced by SFVeE6,7 immunization. Both the priming and the proliferation phases of the HPV-specific response elicited with SFVeE6,7 were not affected by the immune-suppressive activity of Treg. Moreover, Treg depletion did not improve the therapeutic antitumour response of SFVeE6,7 in a murine tumour model.

CONCLUSIONS

The efficacy of the SFVeE6,7 vaccine was not hampered by Treg. Therefore, SFVeE6,7 seems a very promising candidate for the treatment of HPV-induced disease, as it may not require additional immune interventions to modulate Treg activity.

Anti-tumor immune responses in immune-reconstituted mice injected with a tumor vaccine

Homeostasis-driven proliferation of T cells is an important means of reconstituting T-cell-dependent immunity after lymphodepletion regimens, such as chemotherapy or radiotherapy. Immune-reconstituted mice that receive a tumor vaccine mount more efficient anti-tumor immune responses compared with control mice. In the present study, we evaluated the anti-tumor immune responses in immune-reconstituted mice vaccinated with inactivated leukemia cells and explored the mechanisms underlying these immune responses. Test C57BL/6 mice were lymphodepleted by irradiation and immune-reconstituted with naïve mouse spleen lymphocytes. Mice were then injected with an inactivated FBL-3 tumor cell vaccine and challenged with FBL-3 tumor cells. Anti-tumor responses were evaluated by determining the rate of tumor formation, latency, tumor size, interferon gamma levels, and macrophage and CTL cytotoxicities. When challenged with tumor cells, immune-reconstituted, vaccinated mice exhibited a significantly lower mortality, smaller average tumor volume, and a significantly longer mean survival time. They had more robust cellular immunity, reflected by higher levels of INF-γ production and higher macrophage- and CTL-mediated cytotoxicities. Our results suggest that immune reconstitution enhanced the anti-tumor immune responses in mice injected with a tumor vaccine via generation of CTLs. These results have important implications for immunotherapy used for leukemia.

Genetic background and lymphocyte populations after total-body exposure to iron ion radiation

PURPOSE

Particle radiations could significantly impact astronaut health during space missions. This study quantified the effects of iron ion radiation on lymphocytes in two strains of mice differing in susceptibility to radiation-induced acute myeloid leukemia (AML) and thymic lymphoma (TL): C57BL/6 (AML resistant, TL sensitive) and CBA/Ca (AML sensitive, TL resistant).

MATERIALS AND METHODS

The animals (n = 60/strain) were irradiated with ⁵⁶Fe(26+) (1 GeV) to total doses of 0, 0.5, 2 and 3 Gray (Gy) at an average dose rate of 1 Gy/min and euthanised on days 4 and 30 thereafter; blood, spleen, and bone marrow were collected for flow cytometry analyses. Cells expressing the following molecules were quantified: Cluster of differentiation (CD) 4, CD8, CD25, CD34, CD71, B220 (isoform of CD45 on B cells), NK1.1 (marker on natural killer or NK cells, C57B mice), panNK (marker on NK cells, CBA mice), and Sca1 (stem cell antigen 1).

RESULTS

Exposure to radiation resulted in different distribution patterns in lymphocyte populations and leukocytes expressing activation and progenitor markers in the two mouse strains. Significant main effects were dependent upon strain, as well as radiation dose, body compartment, and time of assessment. Especially striking differences were noted on day 4 after 3 Gy irradiation, including in the CD4:CD8 ratio [blood, C57 (2.83 ± 0.25) vs. CBA (6.19 ± 0.24); spleen, C57 (2.29 ± 0.12) vs. CBA (4.98 ± 0.22)], %CD25(+) mononuclear cells in bone marrow [C57 (5.62 ± 1.19) vs. CBA (12.45 ± 0.93)] and %CD34(+)Sca1(+) cells in bone marrow [CD45¹° gate, C57 (2.72 ± 0.74) vs. CBA (21.44 ± 0.73)].

CONCLUSION

The results show that genetic background, as well as radiation dose and time post-exposure, had a profound impact on lymphocyte populations, as well as other leukocytes, after exposure to iron ion radiation.

Recent advances in antigen-loaded dendritic cell-based strategies for treatment of minimal residual disease in acute myeloid leukemia

Therapeutic vaccination with dendritic cells (DCs) is recognized as an important experimental therapy for the treatment of minimal residual disease in acute myeloid leukemia. Many sources of leukemia-associated antigens and different methods for antigen loading of DCs have been used in an attempt to optimize anti-tumor responses. For instance, monocyte-derived DCs have been loaded with apoptotic whole-cell suspensions, necrotic cell lysates, tumor-associated peptides, eluted peptides and cellular DNA or RNA. Furthermore, monocyte-derived DCs can be chemically or electrically fused with leukemic blasts, and DCs have been cultured out of leukemic blasts. However, it remains a challenge in cancer immunotherapy to identify which of these methods is the most optimal for antigen loading and activation of DCs. This review discusses recent advances in DC research and the application of this knowledge towards new strategies for antigen loading of DCs in the treatment of minimal residual disease in acute myeloid leukemia.

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.

Combination of CTL-associated antigen-4 blockade and depletion of CD25 regulatory T cells enhance tumour immunity of dendritic cell-based vaccine in a mouse model of colon cancer

Immune regulation has been shown to be involved in the progressive growth of some murine tumours. Interruption of immune regulatory pathways via CTL-associated antigen-4 (CTLA-4) blockade or removal of CD4(+) CD25(+) regulatory T (Treg) cells appears to be a promising strategy for cancer immunotherapy. In this study, we tested the hypothesis that the combination of CTLA-4 blockade and depletion of Treg cells would improve the potency of dendritic cell (DC)-based vaccine in a clinically relevant mouse model, which is transgenic for both carcinoembryonic antigen (CEA) and HLA-A2 for the treatment of colon carcinoma in a therapeutic setting. We found that administration of anti-CD25 antibody prior to vaccination or systemic administration of anti-CTLA-4 antibody with the vaccine improved tumour-free survival against CEA-expressing tumours compared with mice immunized with DC-based vaccine alone. However, the efficacy of the vaccine proved to be most effective when anti-CTLA-4 antibody was combined with Treg inhibition. This vaccination strategy dramatically improved the tumour-free survival and allowed the development of long-lasting immune responses. The combined vaccination strategy resulted in increased secretion of IFN-gamma and enhanced HLA-A2-restricted CEA-specific CTL responses. Furthermore, coadministration of anti-CD25 and anti-CTLA-4 antibodies along with the vaccine was effective against more advanced tumours. These results provide evidence that simultaneous blockade of T-cell regulatory pathways is a promising approach for the induction of therapeutic antitumour immunity against CEA(+) colon carcinoma.