Apoptotic blebs from leukemic cells as a preferred source of tumor-associated antigen for dendritic cell-based vaccines

Advances in biological functions and applications of apoptotic vesicles

BACKGROUND

Apoptotic vesicles are extracellular vesicles generated by apoptotic cells that were previously regarded as containing waste or harmful substances but are now thought to play an important role in signal transduction and homeostasis regulation.

METHODS

In the present review, we reviewed many articles published over the past decades on the subtypes and formation of apoptotic vesicles and the existing applications of these vesicles.

RESULTS

Apoptotic bodies were once regarded as vesicles released by apoptotic cells, however, apoptotic vesicles are now regarded to include apoptotic bodies, apoptotic microvesicles and apoptotic exosomes, which exhibit variation in terms of biogenesis, sizes and properties. Applications of apoptotic vesicles were first reported long ago, but such reports have been rarer than those of other extracellular vesicles. At present, apoptotic vesicles have been utilized mainly in four aspects, including in direct therapeutic applications, in their engineering as carriers, in their construction as vaccines and in their utilization in diagnosis.

CONCLUSION

Building on a deeper understanding of their composition and characteristics, some studies have utilized apoptotic vesicles to treat diseases in more novel ways. However, their limitations for clinical translation, such as heterogeneity, have also emerged. In general, apoptotic vesicles have great application potential, but there are still many barriers to overcome in their investigation. Video Abstract.

Apoptotic bodies: bioactive treasure left behind by the dying cells with robust diagnostic and therapeutic application potentials

Apoptosis, a form of programmed cell death, is essential for growth and tissue homeostasis. Apoptotic bodies (ApoBDs) are a form of extracellular vesicles (EVs) released by dying cells in the last stage of apoptosis and were previously regarded as debris of dead cells. Recent studies unraveled that ApoBDs are not cell debris but the bioactive treasure left behind by the dying cells with an important role in intercellular communications related to human health and various diseases. Defective clearance of ApoBDs and infected-cells-derived ApoBDs are possible etiology of some diseases. Therefore, it is necessary to explore the function and mechanism of the action of ApoBDs in different physiological and pathological conditions. Recent advances in ApoBDs have elucidated the immunomodulatory, virus removal, vascular protection, tissue regenerative, and disease diagnostic potential of ApoBDs. Moreover, ApoBDs can be used as drug carriers enhancing drug stability, cellular uptake, and targeted therapy efficacy. These reports from the literature indicate that ApoBDs hold promising potential for diagnosis, prognosis, and treatment of various diseases, including cancer, systemic inflammatory diseases, cardiovascular diseases, and tissue regeneration. This review summarizes the recent advances in ApoBDs-related research and discusses the role of ApoBDs in health and diseases as well as the challenges and prospects of ApoBDs-based diagnostic and therapeutic applications.

Gold Compounds and the Anticancer Immune Response

Gold compounds are not only well-explored for cytotoxic effects on tumors, but are also known to interact with the cancer immune system. The immune system deploys innate and adaptive mechanisms to protect against pathogens and prevent malignant transformation. The combined action of gold compounds with the activated immune system has shown promising results in cancer therapy through in vivo and in vitro experiments. Gold compounds are known to induce innate immune responses; however, these responses may contribute to adaptive immune responses. Gold compounds play the role of a major hapten that acts synergistically in innate immunity. Gold compounds support cancer cell antigenicity and promote anti-tumor immune response by inducing the release of CRT, ATP, HMGB1, HSP, and NKG2D to enhance immunogenicity. Gold compounds affect various immune cells (including suppressor regulatory T cells), inhibit myeloid derived suppressor cells, and enhance the function and number of dendritic cells. Gold nanoparticles (AuNPs) have potential for improving the effect of immunotherapy and reducing the toxicity and side effects of the treatment process. Thus, AuNPs provide an ideal opportunity for exploring the combination of anticancer gold compounds and immunotherapeutic interventions.

Palmitoylated antigens for the induction of anti-tumor CD8+ T cells and enhanced tumor recognition

Induction of tumor-specific cytotoxic CD8⁺ T cells (CTLs) via immunization relies on the presentation of tumor-associated peptides in major histocompatibility complex (MHC) class I molecules by dendritic cells (DCs). To achieve presentation of exogenous peptides into MHC class I, cytosolic processing and cross-presentation are required. Vaccination strategies aiming to induce tumor-specific CD8⁺ T cells via this exogenous route therefore pose a challenge. In this study, we describe improved CD8⁺ T cell induction and in vivo tumor suppression of mono-palmitic acid-modified (C16:0) antigenic peptides, which can be attributed to their unique processing route, efficient receptor-independent integration within lipid bilayers, and continuous intracellular accumulation and presentation through MHC class I. We propose that this membrane-integrating feature of palmitoylated peptides can be exploited as a tool for quick and efficient antigen enrichment and MHC class I loading. Importantly, both DCs and non-professional antigen-presenting cells (APCs), similar to tumor cells, facilitate anti-tumor immunity by efficient CTL priming via DCs and effective recognition of tumors through enhanced presentation of antigens.

The Roles of Stroma-Derived Chemokine in Different Stages of Cancer Metastases

The intricate interplay between malignant cells and host cellular and non-cellular components play crucial role in different stages of tumor development, progression, and metastases. Tumor and stromal cells communicate to each other through receptors such as integrins and secretion of signaling molecules like growth factors, cytokines, chemokines and inflammatory mediators. Chemokines mediated signaling pathways have emerged as major mechanisms underlying multifaceted roles played by host cells during tumor progression. In response to tumor stimuli, host cells-derived chemokines further activates signaling cascades that support the ability of tumor cells to invade surrounding basement membrane and extra-cellular matrix. The host-derived chemokines act on endothelial cells to increase their permeability and facilitate tumor cells intravasation and extravasation. The tumor cells-host neutrophils interaction within the vasculature initiates chemokines driven recruitment of inflammatory cells that protects circulatory tumor cells from immune attack. Chemokines secreted by tumor cells and stromal immune and non-immune cells within the tumor microenvironment enter the circulation and are responsible for formation of a "pre-metastatic niche" like a "soil" in distant organs whereby circulating tumor cells "seed' and colonize, leading to formation of metastatic foci. Given the importance of host derived chemokines in cancer progression and metastases several drugs like Mogamulizumab, Plerixafor, Repertaxin among others are part of ongoing clinical trial which target chemokines and their receptors against cancer pathogenesis. In this review, we focus on recent advances in understanding the complexity of chemokines network in tumor microenvironment, with an emphasis on chemokines secreted from host cells. We especially summarize the role of host-derived chemokines in different stages of metastases, including invasion, dissemination, migration into the vasculature, and seeding into the pre-metastatic niche. We finally provide a brief description of prospective drugs that target chemokines in different clinical trials against cancer.

Cell Death in the Tumor Microenvironment: Implications for Cancer Immunotherapy

The physiological fate of cells that die by apoptosis is their prompt and efficient removal by efferocytosis. During these processes, apoptotic cells release intracellular constituents that include purine nucleotides, lysophosphatidylcholine (LPC), and Sphingosine-1-phosphate (S1P) that induce migration and chemo-attraction of phagocytes as well as mitogens and extracellular membrane-bound vesicles that contribute to apoptosis-induced compensatory proliferation and alteration of the extracellular matrix and the vascular network. Additionally, during efferocytosis, phagocytic cells produce a number of anti-inflammatory and resolving factors, and, together with apoptotic cells, efferocytic events have a homeostatic function that regulates tissue repair. These homeostatic functions are dysregulated in cancers, where, aforementioned events, if not properly controlled, can lead to cancer progression and immune escape. Here, we summarize evidence that apoptosis and efferocytosis are exploited in cancer, as well as discuss current translation and clinical efforts to harness signals from dying cells into therapeutic strategies.

Dendritic cells loaded with CD44+ CT-26 colon cell lysate evoke potent antitumor immune responses

Increasing evidence supports the concept that cancer stem cells (CSCs) are responsible for cancer progression and metastasis, therapy resistance and relapse. In addition to conventional therapies for colon cancer, the development of immunotherapies targeting cancer stem cells appears to be a promising strategy to suppress tumor recurrence and metastasis. In the present study, dendritic cells (DCs) were pulsed with whole-tumor cell lysates or total RNA of CD44⁺ colon cancer stem cells (CCSCs) isolated from mouse colon adenocarcinoma CT-26 cell cultures and investigated for their antitumor immunity against CCSCs in vivo and in vitro. In a model of colon adenocarcinoma using BALB/c mice, a sequential reduction in tumor volume and weight was associated with an extended survival in tumor-bearing mice vaccinated with DCs pulsed with RNA or CCSC lysate. In addition, a lactate dehydrogenase assay indicated that cytotoxic T-cells derived from the treated mice exhibited strong cytotoxic activity. Additionally, an enzyme-linked immunosorbent assay revealed that the cytotoxic T-cells of the treated mice released higher levels of interferon-γ against CCSCs compared with those of the control group. In all experiments, the antitumor efficacy of the lysate-pulsed DC-treated and RNA-pulsed DC-treated groups were significantly higher compared with that of the DC-treated and control groups. The results of the present study indicated the potential use of DCs pulsed with cancer stem cell lysates as a potent therapeutic antigen to target CSCs in colon cancer. Additionally, the results provided a rationale for using lysate-pulsed DCs in vivo to eliminate residual tumor deposits in post-operative patients.

Glycan-Modified Apoptotic Melanoma-Derived Extracellular Vesicles as Antigen Source for Anti-Tumor Vaccination

Tumors that lack T cell infiltration are less likely to respond to immune checkpoint inhibition and could benefit from cancer vaccination for the initiation of anti-tumor T cell responses. An attractive vaccine strategy is in vivo targeting of dendritic cells (DCs), key initiators of antigen-specific T cell responses. In this study we generated tumor-derived apoptotic extracellular vesicles (ApoEVs), which are potentially an abundant source of tumor-specific neo-antigens and other tumor-associated antigens (TAAs), and which can be manipulated to express DC-targeting ligands for efficient antigen delivery. Our data demonstrates that by specifically modifying the glycocalyx of tumor cells, high-mannose glycans can be expressed on their cell surface and on extracellular vesicles derived after the induction of apoptosis. High-mannose glycans are the natural ligands of dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), a dendritic cell associated C-type lectin receptor (CLR), which has the ability to efficiently internalize its cargo and direct it to both major histocompatibility complex (MHC)-I and MHC-II pathways for the induction of CD8⁺ and CD4⁺ T cell responses, respectively. Compared to unmodified ApoEVs, ApoEVs carrying DC-SIGN ligands are internalized to a higher extent, resulting in enhanced priming of tumor-specific CD8⁺ T cells. This approach thus presents a promising vaccination strategy in support of T cell-based immunotherapy of cancer.

Microvesicles and chemokines in tumor microenvironment: mediators of intercellular communications in tumor progression

Increasing evidence indicates that the ability of cancer cells to convey biological information to recipient cells within the tumor microenvironment (TME) is crucial for tumor progression. Microvesicles (MVs) are heterogenous vesicles formed by budding of the cellular membrane, which are secreted in larger amounts by cancer cells than normal cells. Recently, several reports have also disclosed that MVs function as important mediators of intercellular communication between cancerous and stromal cells within the TME, orchestrating complex pathophysiological processes. Chemokines are a family of small inflammatory cytokines that are able to induce chemotaxis in responsive cells. MVs which selective incorporate chemokines as their molecular cargos may play important regulatory roles in oncogenic processes including tumor proliferation, apoptosis, angiogenesis, metastasis, chemoresistance and immunomodulation, et al. Therefore, it is important to explore the association of MVs and chemokines in TME, identify the potential prognostic marker of tumor, and develop more effective treatment strategies. Here we review the relevant literature regarding the role of MVs and chemokines in TME.

Human Bone Marrow-Derived Myeloid Dendritic Cells Show an Immature Transcriptional and Functional Profile Compared to Their Peripheral Blood Counterparts and Separate from Slan+ Non-Classical Monocytes

The human bone marrow (BM) gives rise to all distinct blood cell lineages, including CD1c+ (cDC2) and CD141+ (cDC1) myeloid dendritic cells (DC) and monocytes. These cell subsets are also present in peripheral blood (PB) and lymphoid tissues. However, the difference between the BM and PB compartment in terms of differentiation state and immunological role of DC is not yet known. The BM may represent both a site for development as well as a possible effector site and so far, little is known in this light with respect to different DC subsets. Using genome-wide transcriptional profiling we found clear differences between the BM and PB compartment and a location-dependent clustering for cDC2 and cDC1 was demonstrated. DC subsets from BM clustered together and separate from the corresponding subsets from PB, which similarly formed a cluster. In BM, a common proliferating and immature differentiating state was observed for the two DC subsets, whereas DC from the PB showed a more immune-activated mature profile. In contrast, BM-derived slan+ non-classical monocytes were closely related to their PB counterparts and not to DC subsets, implying a homogenous prolife irrespective of anatomical localization. Additional functional tests confirmed these transcriptional findings. DC-like functions were prominently exhibited by PB DC. They surpassed BM DC in maturation capacity, cytokine production, and induction of CD4+ and CD8+ T cell proliferation. This first study on myeloid DC in healthy human BM offers new information on steady state DC biology and could potentially serve as a starting point for further research on these immune cells in healthy conditions as well as in diseases.

Vaccine therapy in hematologic malignancies

Immune-based therapy has emerged as a paradigm shift in cancer therapy with dramatic responses observed in previously incurable disease. Cancer vaccines are being developed to disrupt tumor-associated tolerance and activate and selectively expand tumor-specific lymphocytes within the native effector cell repertoire while maintaining immune-regulatory protection against autoimmunity. Although individual antigen approaches result in immune response with a suggestion of clinical effect in some settings, broader efficacy may be dependent on presentation of multiple antigens that capture clonal diversity presented in the context of functionally potent antigen-presenting cells. The use of whole cell–based strategies such as dendritic cell/tumor fusions have yielded provocative results in single-arm studies and are currently being explored in multicenter randomized trials. The posttransplant setting is a potentially promising platform for vaccination due to cytoreduction and relative depletion of inhibitory accessory cells fostering greater immune responsiveness. Integration of these efforts with other immunotherapeutic strategies and agents that target the tumor microenvironment is being studied in an effort to generate durable immunologic responses with clinically meaningful impact on disease.

Tumor-Derived Apoptotic Vesicles: With Death They Do Part

Tumor cells release lipid particles known as extracellular vesicles (EV) that contribute to cancer metastasis, to the immune response, and to thrombosis. When tumors are exposed to radiation or chemotherapy, apoptotic vesicles (ApoVs) are released in abundance as the plasma membrane delaminates from the cytoskeleton. Recent studies have suggested that ApoVs are distinct from the EVs released from living cells, such as exosomes or microvesicles. Depending on their treatment conditions, tumor-released ApoV have been suggested to either enhance or suppress anti-cancer immunity. In addition, tumor-derived ApoV possess procoagulant activity that could increase the thrombotic state in cancer patients undergoing chemotherapy or radiotherapy. Since ApoVs are one of the least appreciated type of EVs, we focus in this review on the distinctive characterization of tumor ApoVs and their proposed mechanistic effects on cancer immunity, coagulation, and metastasis.

Novel Therapies for Acute Myeloid Leukemia: Are We Finally Breaking the Deadlock?

Acute myeloid leukemia (AML) is one of the best studied malignancies, and significant progress has been made in understanding the clinical implications of its disease biology. Unfortunately, drug development has not kept pace, as the '7+3' induction regimen remains the standard of care for patients fit for intensive therapy 40 years after its first use. Temporal improvements in overall survival were mostly confined to younger patients and driven by improvements in supportive care and use of hematopoietic stem cell transplantation. Multiple forms of novel therapy are currently in clinical trials and are attempting to bring bench discoveries to the bedside to benefit patients. These novel therapies include improved chemotherapeutic agents, targeted molecular inhibitors, cell cycle regulators, pro-apoptotic agents, epigenetic modifiers, and metabolic therapies. Immunotherapies in the form of vaccines; naked, conjugated and bispecific monoclonal antibodies; cell-based therapy; and immune checkpoint inhibitors are also being evaluated in an effort to replicate the success seen in other malignancies. Herein, we review the scientific basis of these novel therapeutic approaches, summarize the currently available evidence, and look into the future of AML therapy by highlighting key clinical studies and the challenges the field continues to face.

Dendritic cells that phagocytose apoptotic macrophages loaded with mycobacterial antigens activate CD8 T cells via cross-presentation

While homeostatic apoptosis is immunologically silent, macrophage apoptosis during Mycobacterium tuberculosis infection can potentially induce an immune response against the mycobacteria. To examine the role of dendritic cells in this response, macrophage apoptosis was induced by incubating the macrophage with cell wall extracts of mycobacteria expressing LpqH. The apoptogenic proteins of the cell wall extracts were engulfed by the macrophage and then were translocated from the cytosol to the nuclei of the dying cells. Dendritic cells that engulfed the apoptotic macrophages acquired an immunogenic phenotype that included upregulation of MHC-I, increased expression of the costimulatory molecules, CD40, CD80, and CD86, and increased production of IL-12, IL-10, TNF-α, and TGF-β. In addition, the dendritic cells triggered a proliferative response of CD8+ T cells with IFN-γ production via cross-presentation. Taken together, these findings support a model in which phagocytosis of whole apoptotic cells carrying mycobacterial antigens promotes a potentially protective immune response.

Transcriptional profiling reveals functional dichotomy between human slan+ non-classical monocytes and myeloid dendritic cells

Human 6-sulfo LacNac-positive (slan⁺) cells have been subject to a paradigm debate. They have previously been classified as a distinct dendritic cell (DC) subset. However, evidence has emerged that they may be more related to monocytes than to DCs. To gain deeper insight into the functional specialization of slan⁺ cells, we have compared them with both conventional myeloid DC subsets (CD1c⁺ and CD141⁺) in human peripheral blood (PB). With the use of genome-wide transcriptional profiling, as well as functional tests, we clearly show that slan⁺ cells form a distinct, non-DC-like population. They cluster away from both DC subsets, and their gene-expression profile evidently suggests involvement in distinct inflammatory processes. An extensive transcriptional meta-analysis confirmed the relationship of slan⁺ cells with the monocytic compartment rather than with DCs. From a functional perspective, their ability to prime CD4⁺ and CD8⁺ T cells is relatively low. Combined with the finding that "antigen presentation by MHC class II" is at the top of under-represented pathways in slan⁺ cells, this points to a minimal role in directing adaptive T cell immunity. Rather, the higher expression levels of complement receptors on their cell surface, together with their high secretion of IL-1β and IL-6, imply a specific role in innate inflammatory processes, which is consistent with their recent identification as non-classical monocytes. This study extends our knowledge on DC/monocyte subset biology under steady-state conditions and contributes to our understanding of their role in immune-mediated diseases and their potential use in immunotherapeutic strategies.

Next-generation dendritic cell-based vaccines for leukemia patients

Up to today treatment of leukemia patients remains challenging and different therapies have been developed, among them the generation of dendritic cell (DC) vaccines. DCs, highly specific for immunogenic cancer antigens, are generated either ex vivo or in vivo and boost the immune response against leukemic cells. Nevertheless, response rates are still heterogeneous and DC vaccines need improvement. New methods for generating DC vaccines have been summed up under the term 'next-generation DC vaccines'. They range from the analysis of human leukocyte antigen-ligandomes to immunogenic cell death inducers, from the production of viral vectors to mRNA transfection and finally from delivering peptides to DCs in vivo through either antibodies or cell-penetrating peptides. This review gives an overview of the latest developments in this still evolving field.

Generation of populations of antigen-specific cytotoxic T cells using DCs transfected with DNA construct encoding HER2/neu tumor antigen epitopes

BACKGROUND

Recent fundamental and clinical studies have confirmed the effectiveness of utilizing the potential of the immune system to remove tumor cells disseminated in a patient's body. Cytotoxic T lymphocytes (CTLs) are considered the main effectors in cell-mediated antitumor immunity. Approaches based on antigen presentation to CTLs by dendritic cells (DCs) are currently being intensively studied, because DCs are more efficient in tumor antigen presentation to T cells through their initiation of strong specific antitumor immune responses than other types of antigen-presenting cells. Today, it has become possible to isolate CTLs specific for certain antigenic determinants from heterogeneous populations of mononuclear cells. This enables direct and specific cell-mediated immune responses against cells carrying certain antigens. The aim of the present study was to develop an optimized protocol for generating CTL populations specific for epitopes of tumor-associated antigen HER2/neu, and to assess their cytotoxic effects against the HER2/neu-expressing MCF-7 tumor cell line.

METHODS

The developed protocol included sequential stages of obtaining mature DCs from PBMCs from HLA A*02-positive healthy donors, magnet-assisted transfection of mature DCs with the pMax plasmid encoding immunogenic peptides HER2 p369-377 (E75 peptide) and HER2 p689-697 (E88 peptide), coculture of antigen-activated DCs with autologous lymphocytes, magnetic-activated sorting of CTLs specific to HER2 epitopes, and stimulation of isolated CTLs with cytokines (IL-2, IL-7, and IL-15).

RESULTS

The resulting CTL populations were characterized by high contents of CD8⁺ cells (71.5% in cultures of E88-specific T cells and 90.2% in cultures of E75-specific T cells) and displayed strong cytotoxic effects against the MCF-7 cell line (percentages of damaged tumor cells in samples under investigation were 60.2 and 65.7% for E88- and E75-specific T cells, respectively; level of spontaneous death of target cells was 17.9%).

CONCLUSIONS

The developed protocol improves the efficiency of obtaining HER2/neu-specific CTLs and can be further used to obtain cell-based vaccines for eradicating targeted tumor cells to prevent tumor recurrence after the major tumor burden has been eliminated and preventing metastasis in patients with HER2-overexpressing tumors.

Adjuvant-Loaded Subcellular Vesicles Derived From Disrupted Cancer Cells for Cancer Vaccination

Targeted subunit vaccines for cancer immunotherapy do not capture tumor antigenic complexity, and approaches employing tumor lysate are often limited by inefficient antigen uptake and presentation, and low immunogenicity. Here, whole cancer cells are processed to generate antigen-rich, membrane-enclosed subcellular particles, termed "reduced cancer cells", that reflect the diversity and breadth of the parent cancer cell antigen repertoire, and can be loaded with disparate adjuvant payloads. These vesicular particles enhance the uptake of the adjuvant payload, and potentiate the activation of primary dendritic cells in vitro. Similarly, reduced cancer cell-associated antigens are more efficiently presented by primary dendritic cells in vitro than their soluble counterparts or lysate control. In mice, vaccination using adjuvant-loaded reduced cancer cells facilitates the induction of antigen-specific cellular and humoral immune responses. Taken together, these observations demonstrate that adjuvant-loaded reduced cancer cells could be utilized in cancer vaccines as an alternative to lysate.

Involvement of Membrane Blebbing in Immunological Disorders and Cancer

Cellular blebbing is a unique form of dynamic protrusion emanating from the plasma membrane which can be either apoptotic or nonapoptotic in nature. Blebs have been observed in a wide variety of cell types and in response to multiple mechanical and chemical stimuli. They have been linked to various physiological and pathological processes including tumor motility and invasion, as well as to various immunological disorders. They can form and retract extremely rapidly in seconds or minutes, or slowly over hours or days. This review focuses on recent evidence regarding the role of blebbing in cell locomotion with particular emphasis on its role in tumor metastasis, indicating the role of specific causative molecules. The phenomenon of blebbing has been observed in endocrine-resistant breast cancer cells in response to brief exposure to extracellular alkaline pH, which leads to enhanced invasive capacity. Genetic or pharmacological targeting of cellular blebs could serve as a potential therapeutic option to control tumor metastasis.

A Human Cell Line Model for Interferon-α Driven Dendritic Cell Differentiation

The CD34⁺ MUTZ-3 acute myeloid leukemia cell line has been used as a dendritic cell (DC) differentiation model. This cell line can be cultured into Langerhans cell (LC) or interstitial DC-like cells using the same cytokine cocktails used for the differentiation of their primary counterparts. Currently, there is an increasing interest in the study and clinical application of DC generated in the presence of IFNα, as these IFNα-DC produce high levels of inflammatory cytokines and have been suggested to be more potent in their ability to cross-present protein antigens, as compared to the more commonly used IL-4-DC. Here, we report on the generation of IFNα-induced MUTZ-DC. We show that IFNα MUTZ-DC morphologically and phenotypically display characteristic DC features and are functionally equivalent to “classic” IL-4 MUTZ-DC. IFNα MUTZ-DC ingest exogenous antigens and can subsequently cross-present HLA class-I restricted epitopes to specific CD8⁺ T cells. Importantly, mature IFNα MUTZ-DC express CCR7, migrate in response to CCL21, and are capable of priming naïve antigen-specific CD8⁺ T cells. In conclusion, we show that the MUTZ-3 cell line offers a viable and sustainable model system to study IFNα driven DC development and functionality.

Differential capacity of human interleukin-4 and interferon-α monocyte-derived dendritic cells for cross-presentation of free versus cell-associated antigen

Dendritic cells (DC) vaccination is a potent therapeutic approach for inducing tumor-directed immunity, but challenges remain. One of the particular interest is the induction of an immune response targeting multiple (unknown) tumor-associated antigens (TAA), which requires a polyvalent source of TAA. Previously, we described the preferred use of apoptotic cell-derived blebs over the larger apoptotic cell remnants, as a source of TAA for both in situ loading of skin-resident DC and in vitro loading of monocyte-derived DC (MoDC). Recent reports suggest that MoDC cultured in the presence of GM-CSF supplemented with IFNα (IFNα MoDC), as compared to IL-4 (IL-4 MoDC), have an increased capacity to cross-present antigen to CD8(+) T cells. As culture conditions, maturation methods and antigen sources differ between the conducted studies, we analyzed the functional differences between IL-4 MoDC and IFNα MoDC, loaded with blebs, in a head-to-head comparison using commonly used protocols. Our data show that both MoDC types are potent (cross-) primers of CD8(+) T cells. Whereas IFNα MoDC were more potent in their capacity to cross-present a 25-mer MART-1 synthetic long peptide (SLP) to a MART-1aa26-35 recognizing CD8(+) T cell line, IL-4 MoDC proved more potent cross-primers of antigen-specific CD8(+) T cells when loaded with blebs. The latter is likely due to the observed greater capacity of IL-4 MoDC to ingest apoptotic blebs. In conclusion, our data indicate the use of IFNα MoDC over IL-4 MoDC in the context of DC vaccination with SLP, whereas IL-4 MoDC are preferred for vaccination with bleb-derived antigens.

Immunotherapy for Acute Myeloid Leukemia

Despite longstanding efforts in basic research and clinical studies, the prognosis for patients with acute myeloid leukemia (AML) remains poor. About half of the patients are not medically fit for intensive induction therapy to induce a complete remission and are treated with palliative treatment concepts. The patients medically fit for intensive induction therapy have a high complete remission rate but the majority suffers from relapse due to chemo-refractory leukemic cells. Allogeneic stem cell transplantation as post-remission therapy can significantly reduce the likelihood of relapse, but it is associated with a high rate of morbidity and mortality. Novel therapeutic concepts are therefore urgently sought after. During recent years, the focus has shifted towards the development of novel immunotherapeutic strategies. Some of the most promising are drug-conjugated monoclonal antibodies, T-cell engaging antibody constructs, adoptive transfer with chimeric antigen receptor (CAR) T cells, and dendritic cell vaccination. Here, we review recent progress in these four fields and speculate about the optimal time points during the course of AML treatment for their application.

In situ loading of skin dendritic cells with apoptotic bleb-derived antigens for the induction of tumor-directed immunity

The generation and loading of dendritic cells (DC) ex-vivo for tumor vaccination purposes is laborious and costly. Direct intradermal (i.d.) administration of tumor-associated antigens could be an attractive alternative approach, provided that efficient uptake and cross-presentation by appropriately activated skin DCs can be achieved. Here, we compare the efficiency of i.d. delivery of relatively small apoptotic blebs (diameter ∼0.1-1 μm) derived from MART-1 transduced acute myeloid leukemia (AML) HL60 cells, to that of larger apoptotic cell remnants (ACR; 2-10 μm) in a physiologically highly relevant human skin explant model. Injection of either fluorescently-labelled ACRs or blebs alone did not affect the number or distribution of migrated DC subsets from skin biopsies after 48 hours, but resulted in a general up-regulation of the co-stimulatory molecules CD83 and CD86 on skin DCs that had ingested apoptotic material. We have previously shown that i.d. administration of GM-CSF and IL-4 resulted in preferential migration of a mature and highly T cell-stimulatory CD11^hiCD1a⁺CD14^- dermal DC subset. Here, we found that co-injection of GM-CSF and IL-4 together with either ACRs or blebs resulted in uptake efficiencies within this dermal DC subset of 7.6% (±6.1%) and 19.1% (±15.9%), respectively, thus revealing a significantly higher uptake frequency of blebs (P < 0.02). Intradermal delivery of tumor-derived blebs did not affect the T-cell priming and T_H-skewing abilities of migratory skin DC. Nevertheless, in contrast to i.d. administration of ACR, the injection of blebs lead to effective cross-presentation of MART-1 to specific CD8⁺ effector T cells. We conclude that apoptotic bleb-based vaccines delivered through the skin may offer an attractive, and broadly applicable, cancer immunotherapy.