Killer Dendritic Cells Link Innate and Adaptive Immunity against Established Osteosarcoma in Rats

Advancements in osteosarcoma management: integrating immune microenvironment insights with immunotherapeutic strategies

Osteosarcoma, a malignant bone tumor predominantly affecting children and adolescents, presents significant therapeutic challenges, particularly in metastatic or recurrent cases. Conventional surgical and chemotherapeutic approaches have achieved partial therapeutic efficacy; however, the prognosis for long-term survival remains bleak. Recent studies have highlighted the imperative for a comprehensive exploration of the osteosarcoma immune microenvironment, focusing on the integration of diverse immunotherapeutic strategies-including immune checkpoint inhibitors, tumor microenvironment modulators, cytokine therapies, tumor antigen-specific interventions, cancer vaccines, cellular therapies, and antibody-based treatments-that are directly pertinent to modulating this intricate microenvironment. By targeting tumor cells, modulating the tumor microenvironment, and activating host immune responses, these innovative approaches have demonstrated substantial potential in enhancing the effectiveness of osteosarcoma treatments. Although most of these novel strategies are still in research or clinical trial phases, they have already demonstrated significant potential for individuals with osteosarcoma, suggesting the possibility of developing new, more personalized and effective treatment options. This review aims to provide a comprehensive overview of the current advancements in osteosarcoma immunotherapy, emphasizing the significance of integrating various immunotherapeutic methods to optimize therapeutic outcomes. Additionally, it underscores the imperative for subsequent research to further investigate the intricate interactions between the tumor microenvironment and the immune system, aiming to devise more effective treatment strategies. The present review comprehensively addresses the landscape of osteosarcoma immunotherapy, delineating crucial scientific concerns and clinical challenges, thereby outlining potential research directions.

Effective Cytotoxicity of Dendritic Cells against Established T Cell Lymphomas in Mice

T cell lymphomas arise in mice that constitutively express a single TCR in the absence of NK cells. Upon TCR engagement these lymphomas are able to corrupt tumor surveillance by decreasing NK cell numbers. In this study, we investigate the outcome of interactions between these T cell lymphomas and dendritic cells. Bone marrow–derived dendritic cells mediated effective killing of T cell lymphomas after activation with IFN-γ and TLR ligands in culture. This cytotoxicity was independent of MHC compatibility. Cell lysis was reduced by the presence of the peroxynitrite inhibitors FeTTPS and L-NMMA, whereas inhibitors of apoptosis, death receptors, and degranulation were without effect, suggesting NO metabolites as the main mediators. When injected together with GM-CSF and R848 into lymphoma-bearing mice, in vitro–expanded bone marrow–derived dendritic cells caused significant survival increases. These data show that dendritic cell adaptive immunotherapy can be used as treatment against T cell lymphomas in mice.

Stimulated bone marrow–derived dendritic cells lyse T lymphoma target cells in vitro.

Dendritic cell–mediated cytotoxicity is dependent on peroxynitrite.

Dendritic cell transfers into T lymphoma-bearing mice show antitumor efficacy.

Cytotoxic and antigen presenting functions of T helper-1-activated dendritic cells

Although primarily defined by their cardinal antigen-presenting function, dendritic cells (DCs) are also equipped with cytotoxic properties. We have recently reported that DCs activated by IFNγ-secreting Th-1 lymphocytes can kill cancer cells and subsequently present the acquired tumor-derived antigens to T lymphocytes both in vitro and in vivo.

Identification of potential crucial genes and key pathways in osteosarcoma

Background

The aim of this study is to identify the potential pathogenic and metastasis-related differentially expressed genes (DEGs) in osteosarcoma through bioinformatic analysis of Gene Expression Omnibus (GEO) database.

Results

Gene expression profiles of GSE14359, GSE16088, and GSE33383, in total 112 osteosarcoma tissue samples and 7 osteoblasts, were analyzed. Seventy-four normal-primary DEGs (NPDEGs) and 764 primary-metastatic DEGs (PMDEGs) were screened. VAMP8, A2M, HLA-DRA, SPARCL1, HLA-DQA1, APOC1 and AQP1 were identified continuously upregulating during the oncogenesis and metastasis of osteosarcoma. The enriched functions and pathways of NPDEGs include procession and presentation of antigens, activation of MHC class II receptors and phagocytosis. The enriched functions and pathways of PMDEGs include mitotic nuclear division, cell adhesion molecules (CAMs) and focal adhesion. With protein-protein interaction (PPI) network analyzed by Molecular Complex Detection (MCODE) plug-in of Cytoscape software, one hub NPDEG (HLA-DRA) and 7 hub PMDEGs (CDK1, CDK20, CCNB1, MTIF2, MRPS7, VEGFA and EGF) were eventually selected, and the most significant pathways in NPDEGs module and PMDEGs module were enriched in the procession and presentation of exogenous peptide antigen via MHC class II and the nuclear division, respectively.

Conclusions

By integrated bioinformatic analysis, numerous DEGs related to osteosarcoma were screened, and the hub DEGs identified in this study are possibly part of the potential biomarkers for osteosarcoma. However, further experimental studies are still necessary to elucidate the biological function and mechanism of these genes.

A Review of T-Cell Related Therapy for Osteosarcoma

Osteosarcoma is one of the most common primary malignant tumors of bone. The combination of chemotherapy and surgery makes the prognosis better than before, but therapy has not dramatically improved over the last three decades. This is partially because of the lack of a novel specialized drug for osteosarcoma, which is known as a tumor with heterogeneity. On the other hand, immunotherapy has been one of the most widely used strategies for many cancers over the last ten years. The therapies related to T-cell response, such as immune checkpoint inhibitor and chimeric antigen receptor T-cell therapy, are well-known options for some cancers. In this review, we offer the accumulated knowledge of T-cell-related immunotherapy for osteosarcoma, and discuss the future of the therapy.

Immunotherapy for osteosarcoma: Where do we go from here?

Osteosarcoma is the most common bone tumor in children and young adults, with few advances in survival and treatment, especially for metastatic disease, in the last 30 years. Recently, immunotherapy has begun to show promise in various adult cancers, but the utility of this approach for osteosarcoma remains relatively unexplored. In this review, we outline the mechanisms and status of immunotherapies currently in clinical trials as well as future therapies on the horizon, and discuss their potential application for osteosarcoma.

Re-calculating! Navigating through the osteosarcoma treatment roadblock

The survival rates for patients with osteosarcoma have remained almost static for the past three decades. Current standard of care therapy includes chemotherapies such as doxorubicin, cisplatin, and methotrexate along with complete surgical resection and surgery with or without ifosfamide and etoposide for relapse, though outcomes are hoped to be improved through clinical trials. Additionally, increased understanding of the genetics, signaling pathways and microenvironmental factors driving the disease have led to the identification of promising agents and potential paths towards translation of an exciting array of novel targeted therapies. Here, we review the mechanism of action of these emerging therapies and how, with clinical translation, they can potentially improve the survival rates for osteosarcoma patients in the near future.

Progress and opportunities for immune therapeutics in osteosarcoma

Survival outcomes for osteosarcoma have plateaued since the 1980s, and patients with relapsed or refractory disease have a particularly dismal outcome. Treatment options for these patients are limited primarily due to the paucity of effective therapeutics. Immune therapies such as tumor vaccines and traditional antigen-targeted monoclonal antibodies have had limited success in solid tumors. The recent discovery of novel immune checkpoint blockade strategies and their success in adult cancers has revitalized the use of immunotherapy strategies for the treatment of solid tumors. This paper summarizes existing data supporting the use of immune therapies in osteosarcoma and the progress of this class of drugs in osteosarcoma therapy.

A novel prognostic model for osteosarcoma using circulating CXCL10 and FLT3LG

BACKGROUND

Osteosarcoma (OS) is the most common malignant pediatric bone tumor. The identification of novel biomarkers for early prognostication will facilitate risk-based stratification and therapy. This study investigated the significance of circulating cytokines/chemokines for predicting the prognosis at the initial diagnosis.

METHODS

Luminex assays were used to measure cytokine/chemokine concentrations in blood samples from a discovery cohort of OS patients from Texas Children's Hospital (n = 37) and an independent validation cohort obtained from the Children's Oncology Group (n = 233). After the validation of the biomarkers, a multivariate model was constructed to stratify the patients into risk groups.

RESULTS

The circulating concentrations of C-X-C motif chemokine ligand 10 (CXCL10), Fms-related tyrosine kinase 3 ligand (FLT3LG), interferon γ (IFNG), and C-C motif chemokine ligand 4 (CCL4) were significantly associated with overall survival in both cohorts. Among these candidates, CXCL10 and FLT3LG were independent of the existing prognostic factor, metastasis at diagnosis, and CCL4 further discriminated cancer cases from controls. CXCL10, FLT3LG, and the metastatic status at diagnosis were combined to develop a multivariate model that significantly stratified the patients into 4 distinct risk groups (P = 1.6 × 10^-8 ). The survival analysis showed that the 5-year overall survival rates for the low-, intermediate-, high-, and very high-risk groups were 77%, 54%, 47%, and 10%, respectively, whereas the 5-year event-free survival rates were 64%, 47%, 27%, and 0%, respectively. Neither CXCL10 nor FLT3LG tumor expression was significantly associated with survival.

CONCLUSIONS

High circulating levels of CXCL10 and FLT3LG predicted worse survival for patients with OS. Because both CXCL10 and FL3LG axes are potentially targetable, further study may lead to novel risk-based stratification and therapy for OS. Cancer 2017;144-154. © 2016 American Cancer Society.

Dendritic cells combined with anti-GITR antibody produce antitumor effects in osteosarcoma

We attempted to enhance the antitumor effects of tumor lysate-pulsed dendritic cells by eliminating regulatory T cells. The combinatorial effects of dendritic cells and agonist anti-glucocorticoid-induced tumor necrosis factor receptor (anti-GITR) antibodies were investigated with respect to enhancement of the systemic immune response, elimination of regulatory T cells, and inhibition of tumor growth. To determine whether the combination of dendritic cells and anti‑GITR antibodies could enhance systemic immune responses and inhibit primary tumor growth in a murine osteosarcoma (LM8) model. We established the following 4 groups of C3H mice (20 mice in total): i), control IgG-treated mice; ii), tumor lysate-pulsed dendritic cell‑treated mice; iii), agonist anti-GITR antibody-treated mice; and iv), agonist anti-GITR antibody- and tumor lysate-pulsed dendritic cell‑treated mice.The mice that received the agonist anti-GITR antibodies and tumor lysate-pulsed dendritic cells displayed inhibited primary growth, prolonged life time, reduced numbers of regulatory T lymphocytes in the spleen, elevated serum interferon-γ levels, increased number of CD8+ T lymphocytes. The mice that received combined therapy had reduced level of immunosuppressive cytokines in tumor tissue and serum. Combining agonist anti-GITR antibodies with tumor lysate-pulsed dendritic cells enhanced the systemic immune response. These findings provide further support for the continued development of agonist anti-GITR antibodies as an immunotherapeutic strategy for osteosarcoma. We suggest that our proposed immunotherapy could be developed further to improve osteosarcoma treatment.

Genome-wide gene expression profiling of low-dose, long-term exposure of human osteosarcoma cells to bisphenol A and its analogs bisphenols AF and S

The bisphenols AF (BPAF) and S (BPS) are structural analogs of the endocrine disruptor bisphenol A (BPA), and are used in common products as a replacement for BPA. To elucidate genome-wide gene expression responses, estrogen-dependent osteosarcoma cells were cultured with 10 nM BPA, BPAF, or BPS, for 8 h and 3 months. Genome-wide gene expression was analyzed using the Illumina Expression BeadChip. Three months exposure had significant effects on gene expression, particularly for BPS, followed by BPAF and BPA, according to the number of differentially expressed genes (1980, 778, 60, respectively), the magnitude of changes in gene expression, and the number of enriched biological processes (800, 415, 33, respectively) and pathways (77, 52, 6, respectively). 'Embryonic skeletal system development' was the most enriched bone-related process, which was affected only by BPAF and BPS. Interestingly, all three bisphenols showed highest down-regulation of genes related to the cardiovascular system (e.g., NPPB, NPR3, TXNIP). BPA only and BPA/BPAF/BPS also affected genes related to the immune system and fetal development, respectively. For BPAF and BPS, the 'isoprenoid biosynthetic process' was enriched (up-regulated genes: HMGCS1, PDSS1, ACAT2, RCE1, DHDDS). Compared to BPA, BPAF and BPS had more effects on gene expression after long-term exposure. These findings stress the need for careful toxicological characterization of BPA analogs in the future.

Translational biology of osteosarcoma

For the past 30 years, improvements in the survival of patients with osteosarcoma have been mostly incremental. Despite evidence of genomic instability and a high frequency of chromothripsis and kataegis, osteosarcomas carry few recurrent targetable mutations, and trials of targeted agents have been generally disappointing. Bone has a highly specialized immune environment and many immune signalling pathways are important in bone homeostasis. The success of the innate immune stimulant mifamurtide in the adjuvant treatment of non-metastatic osteosarcoma suggests that newer immune-based treatments, such as immune checkpoint inhibitors, may substantially improve disease outcome.

In vitro antitumor effect of sodium butyrate and zoledronic acid combined with traditional chemotherapeutic drugs: a paradigm of synergistic molecular targeting in the treatment of Ewing sarcoma

Histone deacetylase inhibitors and bisphosphonates have a promising future in the treatment of cancer as targeted anticancer drugs, particularly when used together or in combination with other cytotoxic agents. However, the effects of these combined treatments have not yet been systematically evaluated in Ewing sarcoma. The in vitro effects on cellular proliferation, viability and survival were investigated in two Ewing sarcoma cell lines, SK-ES-1 and RD-ES. The cell lines were treated with sodium butyrate, a histone deacetylase inhibitor and zoledronic acid, a bisphosphonate, alone, together or in combination with chemotherapeutic drugs recommended for clinical treatment of Ewing sarcoma. The data demonstrated that the combination of sodium butyrate and zoledronic acid had a synergistic cytotoxic effect at 72 h following treatment, persisting for 10-14 days post-treatment, in both cell lines tested. All combinations between sodium butyrate or zoledronic acid and the traditional antineoplastic drugs (doxorubicin, etoposide and vincristine) demonstrated a synergistic cytotoxic effect at 72 h in SK-ES-1 and RD-ES cells, except for the combinations of sodium butyrate with vincristine and of zoledronic acid with doxorubicin, which showed only an additive effect in RD-ES cell lines as compared to each agent alone. These acute effects observed in both Ewing sarcoma cell lines were confirmed by the clonogenic assay. The present data suggest that combining histone deacetylase inhibitors and bisphosphonates with traditional chemotherapeutic drugs is a promising therapeutic strategy for the treatment of Ewing sarcoma, and provides a basis for further studies in this field.

Dendritic cell-based immunotherapy for glioma: multiple regimens and implications in clinical trials

High grade glioma is a highly invasive brain tumor and recurrence is almost inevitable, even after radical resection of the tumor mass. Cytotoxic immune responses and immunological memory induced by immunotherapy might prevent tumor recurrence. Dendritic cells (DCs) are professional antigen-presenting cells of the innate immune system with the potential to generate robust antigen-specific T cell immune responses. DC-based immunotherapeutic strategies have been intensively studied in both preclinical and clinical settings. Although advances have been made in the experimental use of DCs, there are still considerable challenges that need to be addressed for clinical translation. In this review, we describe the variability of regimens currently available for DC-based immunotherapy and then review strategies to optimize DC therapeutic efficacy against glioma.

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.

Impact of oncopediatric dosing regimen of zoledronic acid on bone growth: preclinical studies and case report of an osteosarcoma pediatric patient

Osteosarcoma and Ewing sarcoma represent the two most frequent primary bone tumors that arise in the pediatric population. Despite recent improvement in their therapeutic management, no improvement in survival rate has been achieved since early 1980 s. Among new therapeutic approaches, bisphosphonates are promising candidates as potent inhibitors of bone resorption. However, their effects on bone growth must be studied at dosing regimen corresponding to pediatric protocols. To this aim, several protocols using zoledronic acid (ZOL) were developed in growing mice (50 µg/kg every 2 days × 10). Parameters of bone remodeling and bone growth were investigated by radiography, micro-computed tomography, histology, and biologic analyses. Extramedullar hematopoiesis was searched for in spleen tissue. A transient inhibitory effect of ZOL was observed on bone length, with a bone-growth arrest during treatment owing to an impressive increase in bone formation at the growth plate level (8- to 10-fold increase in BV/TV). This sclerotic band then shifted into the diaphysis as soon as endochondral bone formation started again after the end of ZOL treatment, revealing that osteoclasts and osteoblasts are still active at the growth plate. In conclusion, endochondral bone growth is transiently disturbed by high doses of ZOL corresponding to the pediatric treatment of primary bone tumors. These preclinical observations were confirmed by a case report in a pediatric patient treated in the French OS2006 protocol over 10 months who showed a growth arrest during the ZOL treatment period with normal gain in size after the end of treatment.

Cytotoxic dendritic cells generated from cancer patients

Known for years as professional APCs, dendritic cells (DCs) are also endowed with tumoricidal activity. This dual role of DC as killers and messengers may have important implications for tumor immunotherapy. However, the tumoricidal activity of DCs has mainly been investigated in animal models. Cancer cells inhibit antitumor immune responses using numerous mechanisms, including the induction of immunosuppressive/ tolerogenic DCs that have lost their ability to present Ags in an immunogenic manner. In this study, we evaluated the possibility of generating tumor killer DCs from patients with advanced-stage cancers. We demonstrate that human monocyte-derived DCs are endowed with significant cytotoxic activity against tumor cells following activation with LPS. The mechanism of DC-mediated tumor cell killing primarily involves peroxynitrites. This observed cytotoxic activity is restricted to immature DCs. Additionally, after killing, these cytotoxic DCs are able to activate tumor Ag-specific T cells. These observations may open important new perspectives for the use of autologous cytotoxic DCs in cancer immunotherapy strategies.

Alloantigen specific deletion of primary human T cells by Fas ligand (CD95L)-transduced monocyte-derived killer-dendritic cells

Numerous studies have been performed in vitro and in various animal models to modulate the interaction of dendritic cells (DC) and T cells by Fas (CD95/Apo-1) signalling to delete activated T cells via induction of activation-induced cell death (AICD). Previously, we could demonstrate that Fas ligand (FasL/CD95L)-expressing 'killer-antigen-presenting cells' can be generated from human monocyte-derived mature DC (mDC) using adenoviral gene transfer. To evaluate whether these FasL-expressing mDC (mDC-FasL) could eliminate alloreactive primary human T cells in vitro, co-culture experiments were performed. Proliferation of human T cells was markedly reduced in primary co-cultures with allogeneic mDC-FasL, whereas a strong proliferative T-cell response could be observed in co-cultures with enhanced green fluorescent protein-transduced mDC. Inhibition of T-cell proliferation was related to the transduction efficiency, and the numbers of mDC-FasL present in co-cultures. In addition, proliferation of pre-activated alloreactive CD4(+) and CD8(+) T cells could be almost completely inhibited in secondary co-cultures using mDC-FasL as stimulatory cells, which was the result of induction of apoptosis in the majority of preactivated T cells. The specific deletion of alloreactive T cells by mDC-FasL was confirmed by an unaffected proliferative response of surviving T cells towards allogeneic 'third-party' peripheral blood mononuclear cells in a third stimulation, or upon unspecific stimulation with anti-CD3/CD28 beads. The results of this study demonstrate that allospecifically activated T cells are efficiently eliminated by mDC-FasL, supporting further investigations to apply FasL-expressing 'killer-DC' as a novel strategy for the treatment of allograft rejection.

Micro-RNA profiles in osteosarcoma as a predictive tool for ifosfamide response

Micro-RNAs (miRNA) are currently used as cancer biomarkers for hematological cancers and solid tumors. Osteosarcoma is the first primary malignant bone tumor, characterized by a complex genetic and resistance to conventional treatments. For this latter property, the median survival has not been improved since 1990 despite preoperative administration of chemotherapeutic agents. The prediction of tumor response before chemotherapy treatment would constitute a major progress for this pathology. We assessed in this study if miRNA profiling could surpass the current limitations for osteosarcoma diagnosis. We measured the miRNA expression in different osteosarcoma samples: (i) 27 osteosarcoma paraffin-embedded tumors from patients, (ii) human osteosarcoma cell lines, and (iii) tumors from a syngeneic rat osteosarcoma model, recapitulating human osteosarcoma. miRNA profiles were determined using microfluidic cards performing high-throughput TaqMan(®) -based PCR assays, called TaqMan(®) Low Density Arrays. Osteosarcoma of rat and human origins showed a miRNA signature, which could discriminate good from bad responders. In particular, we identified five discriminating miRNAs (miR-92a, miR-99b, miR-132, miR-193a-5p and miR-422a) in patient tumors, which could be easily transferable to diagnosis. These discriminating miRNAs, as well as those identified in rat, targeted the TGFβ, the Wnt and the MAP kinase pathways. These results indicate that our platform constitutes a potent diagnostic tool to predict tumor sensitivity to a drug in attempt to better adapt treatment to tumor biological specificities and also to identify new potential therapeutic strategies.

Identification of a bone marrow-derived CD8αα+ dendritic cell-like population in inflamed autoimmune target tissue with capability of inducing T cell apoptosis

DCs play critical roles in promotion of autoimmunity or immune tolerance as potent APCs. In our anti-GBM GN model, WKY rats develop severe T cell-mediated glomerular inflammation followed by fibrosis. A DC-like cell population (CD8αα(+)CD11c(+)MHC-II(+)ED1(-)) was identified in the inflamed glomeruli. Chimera experiments demonstrated that the CD8αα(+) cells were derived from BM. The CD8αα(+) cells infiltrated glomeruli at a late stage (Days 28-35), coincident with a rapid decline in glomerular inflammation before fibrosis. The CD8αα(+) cells isolated from inflamed glomeruli were able to migrate rapidly from the bloodstream into inflamed glomeruli but not into normal glomeruli, suggesting that the migration was triggered by local inflammation. Despite high-level expression of surface and cellular MHC class II molecules, in vitro experiments showed that this CD8αα(+) DC-like cell induced apoptosis but not proliferation in antigen-specific CD4(+) T cells from T cell lines or freshly isolated from lymph nodes; they were not able to do so in the absence of antigens, suggesting induction of apoptosis was antigen-specific. Furthermore, apoptotic T cells were detected in a large number in the glomeruli at Day 32, coincident with the infiltration of the cells into glomeruli, suggesting that the cells may also induce T cell apoptosis in vivo. A potential role of this CD8αα(+) DC-like population in peripheral immune tolerance and/or termination of autoimmune inflammation was discussed.

Dendritic cells transfected with lentiviral vector-encoding human granulocyte-macrophage colony-stimulating factor augment anti-tumour T-cell response in vitro

Dendritic cells (DC) are professional antigen-presenting cells that can actively taken up and present tumour-derived proteins to induce a tumour-specific immune response. Granulocyte-macrophage colony-stimulating factor (GM-CSF) plays a pivotal role in the generation, sensitization, maturation and survival of DC. We charged the peripheral blood monocyte cell-derived DC with tumour lysate, and then transfected the DC with lentiviral vector-encoding human GM-CSF (hGM-CSF). The antigen-presenting capacity of the hGM-CSF-transfected DC was tested by means of the mixed lymphocyte reaction and cytotoxic T-lymphocyte assay using wild-type DC as the control. The Lenti-hGM-CSF-transfected DC was able to stimulate the proliferation of naive allogeneic T lymphocytes and to generate tumour-specific cytotoxic T lymphocytes more efficiently than the wild-type DC. This data indicates that Lenti-hGM-CSF-transfected DC could potentially be used as an effective clinical approach for cancer immunotherapy.

Killer dendritic cells and their potential for cancer immunotherapy

Known for years as the principal messengers of the immune system, dendritic cells (DC) represent a heterogeneous population of antigen presenting cells critically located at the nexus between innate and adaptive immunity. DC play a central role in the initiation of tumor-specific immune responses as they are endowed with the unique ability to take up, process and present tumor antigens to naïve CD4(+) or CD8(+) effector T lymphocytes. By virtue of the cytokines they produce, DC also regulate the type, strength and duration of T cell immune responses. In addition, they can participate in anti-tumoral NK and NKT cell activation and in the orchestration of humoral immunity. More recent studies have documented that besides their primary role in the induction and regulation of adaptive anti-tumoral immune responses, DC are also endowed with the capacity to directly kill cancer cells. This dual role of DC as killers and messengers may have important implications for tumor immunotherapy. First, the direct killing of malignant cells by DC may foster the release and thereby the immediate availability of specific tumor antigens for presentation to cytotoxic or helper T lymphocytes. Second, DC may participate in the effector phase of the immune response, potentially augmenting the diversity of the killing mechanisms leading to tumor elimination. This review focuses on this non-conventional cytotoxic function of DC as it relates to the promotion of cancer immunity and discusses the potential application of killer DC (KDC) in tumor immunotherapy.