Messenger RNA electroporation of human monocytes, followed by rapid in vitro differentiation, leads to highly stimulatory antigen-loaded mature dendritic cells

Cell-Based Drug Delivery Systems with Innate Homing Capability as a Novel Nanocarrier Platform

Nanoparticle-based drug delivery systems have been designed to treat various diseases. However, many problems remain, such as inadequate tumor targeting and poor therapeutic outcomes. To overcome these obstacles, cell-based drug delivery systems have been developed. Candidates for cell-mediated drug delivery include blood cells, immune cells, and stem cells with innate tumor tropism and low immunogenicity; they act as a disguise to deliver the therapeutic payload. In drug delivery systems, therapeutic agents are encapsulated intracellularly or attached to the surface of the plasma membrane and transported to the desired site. Here, we review the pros and cons of cell-based therapies and discuss their homing mechanisms in the tumor microenvironment. In addition, different strategies to load therapeutic agents inside or on the surface of circulating cells and the current applications for a wide range of disease treatments are summarized.

Characterization of Interleukin-15-Transpresenting Dendritic Cells for Clinical Use

Personalized dendritic cell- (DC-) based vaccination has proven to be safe and effective as second-line therapy against various cancer types. In terms of overall survival, there is still room for improvement of DC-based therapies, including the development of more immunostimulatory DC vaccines. In this context, we redesigned our currently clinically used DC vaccine generation protocol to enable transpresentation of interleukin- (IL-) 15 to IL-15Rβγ-expressing cells aiming at boosting the antitumor immune response. In this study, we demonstrate that upon electroporation with both IL-15 and IL-15Rα-encoding messenger RNA, mature DC become highly positive for surface IL-15, without influencing the expression of prototypic mature DC markers and with preservation of their cytokine-producing capacity and their migratory profile. Functionally, we show that IL-15-transpresenting DC are equal if not better inducers of T-cell proliferation and are superior in tumor antigen-specific T-cell activation compared with DC without IL-15 conditioning. In view of the clinical use of DC vaccines, we evidence with a time- and cost-effective manner that clinical grade DC can be safely engineered to transpresent IL-15, hereby gaining the ability to transfer the immune-stimulating IL-15 signal towards antitumor immune effector cells.

Electroporation of dendritic cells with autologous total RNA from tumor material

Dendritic cells (DC) are unique antigen-presenting cells that initiate and orchestrate adaptive immunity. Theoretically, cancer cells that express tumor-specific antigens can be destroyed by cytotoxic T cells. However, inherent antitumor responses are often not efficient, since tumor cells can mask their antigens and do not activate DC, an event required for the development of tumor antigen-specific cytotoxic T cell responses. Over a decade ago, the ex vivo preparation of autologous tumor antigen-loaded monocyte-derived DC vaccines as a novel potent anticancer strategy was launched. Phase I and II trials have been performed employing this strategy to treat several malignancies, such as B cell lymphoma, myeloma, melanoma, prostate, colon, ovarian, pancreatic, breast cancer, and renal cell carcinoma. So far, DC immunotherapy is well tolerated with little side or toxic effects. An issue of concern is the way DC are loaded with tumor antigens. An effective strategy is the loading of DC with tumor antigen through electroporation with tumor RNA. In this chapter, a comprehensive description of a protocol for loading of ex vivo-derived DC with total tumor RNA through electroporation is provided.

mRNA-based dendritic cell vaccines

Cancer immunotherapy has been proposed as a powerful treatment modality. Active immunotherapy aspires to stimulate the patient's immune system, particularly T cells. These cells can recognize and kill cancer cells and can form an immunological memory. Dendritic cells (DCs) are the professional antigen-presenting cells of our immune system. They take up and process antigens to present them to T cells. Consequently, DCs have been investigated as a means to stimulate cancer-specific T-cell responses. An efficient strategy to program DCs is the use of mRNA, a well-defined and safe molecule that can be easily generated at high purity. Importantly, vaccines consisting of mRNA-modified DCs showed promising results in clinical trials. Therefore, we will introduce cancer immunotherapy and DCs and give a detailed overview on the application of mRNA to generate cancer-fighting DC vaccines.

Antigen-specific mRNA transfection of autologous dendritic cells

Several reports have described the use of tumor-extracted RNA as source of tumor antigen for the preparation of vaccines based on dendritic cells (DC) and its potential use for antigen-specific or polyvalent tumor vaccination. Upon transfection, RNA is transcribed into proteins that enter the cytoplasmic degradation pathway and can be presented by DC through class I major histocompatibility complex (MHC)-I, thus inducing specific T cell cytotoxic responses. In this chapter, we present a protocol to transfect murine dendritic cells with tumor mRNA by means of electroporation.

Antitumor vaccination with synthetic mRNA: strategies for in vitro and in vivo preclinical studies

Synthetic antigen-encoding mRNA is increasingly exploited as a tool for delivery of genetic information of complete antigens into professional antigen presenting dendritic cells for HLA haplotype-independent antigen-specific vaccination against cancer. Two strategies for mRNA-based antitumor vaccination have emerged into the clinical setting. One is transfection of autologous dendritic cells with synthetic mRNA for adoptive transfer into the patient. The other is direct injection of naked synthetic mRNA. Both methods have proven to be feasible and safe and to elicit antigen-specific immune responses. The design of novel synthetic vaccines employing synthetic mRNA requires further in-depth investigation of its bioavailability and immune pharmacology. This chapter summarizes the state-of-art in this field and describes methods elementary for preclinical studies of mRNA-based antitumor vaccine protocols.

Chemical transfection of dye-conjugated microRNA precursors for microRNA functional analysis of M2 macrophages

MicroRNAs (miRNAs) are short noncoding ribonucleic acids known to affect gene expression at the translational level and there is mounting evidence that miRNAs play a role in the function of tumor-associated macrophages (TAMs). To aid the functional analyses of miRNAs in an in-vitro model of TAMs known as M2 macrophages, a transfection method to introduce artificial miRNA constructs or miRNA molecules into primary human monocytes is needed. Unlike differentiated macrophages or dendritic cells, undifferentiated primary human monocytes have been known to show resistance to lentiviral transduction. To circumvent this challenge, other techniques such as electroporation and chemical transfection have been used in other applications to deliver small gene constructs into human monocytes. To date, no studies have compared these two methods objectively to evaluate their suitability in the miRNA functional analysis of M2 macrophages. Of the methods tested, the electroporation of miRNA-construct containing plasmids and the chemical transfection of miRNA precursor molecules are the most efficient approaches. The use of a silencer siRNA labeling kit (Ambion) to conjugate Cy 3 fluorescence dyes to the precursor molecules allowed the isolation of successfully transfected cells with fluorescence-activated cell sorting. The chemical transfection of these dye-conjugated miRNA precursors yield an efficiency of 37.5 ± 0.6% and a cell viability of 74 ± 1%. RNA purified from the isolated cells demonstrated good quality, and was fit for subsequent mRNA expression qPCR analysis. While electroporation of plasmids containing miRNA constructs yield transfection efficiencies comparable to chemical transfection of miRNA precursors, these electroporated primary monocytes seemed to have lost their potential for differentiation. Among the most common methods of transfection, the chemical transfection of dye-conjugated miRNA precursors was determined to be the best-suited approach for the functional analysis of M2 macrophages.

An in vitro-transcribed-mRNA polyepitope construct encoding 32 distinct HLA class I-restricted epitopes from CMV, EBV, and Influenza for use as a functional control in human immune monitoring studies

Interest and activity in the areas of clinical immunotherapy and therapeutic vaccines are growing dramatically, thus there is a pressing need to develop robust tools for assessment of vaccine-induced immunity. CD8+ T cell immunity against specific antigens is normally measured by either flow cytometry using MHC tetramer reagents or via biological assays such as intracellular cytokine staining or ELISPOT after stimulation with specific peptide epitopes. However, these methodologies depend on precise knowledge of HLA-restricted epitopes combined with HLA typing of subjects. As an alternative approach, electroporation of antigen presenting cells (APC) with in vitro-transcribed mRNA (IVT-mRNA) encoding the antigen of interest bypasses the requirements for HLA typing and knowledge of specific epitopes. A current limitation of the IVT-mRNA technique is the lack of robust positive control RNAs to verify the efficacy of electroporation and to ensure that the electroporated APC retain the ability to stimulate T cells. Herein we describe an IVT-mRNA construct wherein all 32 HLA class I-restricted epitopes of the widely used CEF (Cytomegalovirus, Epstein-Barr Virus and Influenza Virus) positive control peptide pool have been genetically spliced together to generate a single polyepitope construct. Each epitope is flanked by three amino- and three carboxy-terminal amino acids from the original parent protein to facilitate proteolytic processing by the proteasome. Using cells obtained from a panel of normal healthy donors and cancer patients we report that dendritic cells, CD40-activated B cells, PHA blasts, and even tumor cells can be transfected with CEF polyepitope IVT-mRNA and can elicit robust CEF-specific responses from autologous T cells, as measured by IFN-gamma ELISPOT. Moreover, the response elicited by CEF IVT-mRNA-transfected APC was similar in magnitude to the response elicited by the complete pool of CEF minimal peptide epitopes, implying that the polyepitope parent protein encoded by the CEF mRNA was efficiently processed into individual epitopes by the proteolytic machinery of the APC. In summary, the CEF polyepitope IVT-mRNA described herein comprises a robust positive control for immunomonitoring studies requiring IVT-mRNA transfection and potentially provides a unique tool for assessing MHC class I processing regardless of HLA haplotype.

An autologous therapeutic dendritic cell vaccine transfected with total lung carcinoma RNA stimulates cytotoxic T lymphocyte responses against non-small cell lung cancer

OBJECTIVE

The development of immunotherapy for malignancy is greatly limited by the characteristic weak antigenicity of tumors. The primary goal of this study was to circumvent the isolation and purification of tumor-specific antigen determinants by producing a vaccine using lung tumor RNA-loaded dendritic cells (DCs), and to test the response against lung cancer.

METHODS

Total RNA was isolated from 18 lung carcinomas with positive carcinoembryonic antigen (CEA) and mucin-1 (MUC1) staining, as identified by immunohistochemistry. DCs and T-cells from peripheral blood mononuclear cells were generated in vitro, and then DCs in different stages were transfected with RNA using several different methods. The expression of CEA and MUC1 in RNA-transfected DCs was measured using flow cytometry. T-cells stimulated by DCs were harvested as effectors, and primary tumor cells cultured in vitro were used as targets. Cytotoxicity was determined by lactic dehydrogenase detection assay.

RESULTS

Immature RNA-transfected DCs significantly increased the expression of CEA and MUC1, compared to mature transfected DCs. RNA transfection via electroporation resulted in significantly greater CEA and MUC1 expression than did transfection via lipofection or passive pulsing. Lymphocytes stimulated by DCs transfected with lung tumor RNA initiated a cytotoxic T lymphocyte (CTL) tumor-specific response.

CONCLUSION

Immature DCs transfected with total lung carcinoma RNA by electroporation in vitro effectively stimulate antigen-specific CTL responses against tumor cells.

Immunotherapy of cancer with dendritic cells loaded with tumor antigens and activated through mRNA electroporation

Since decades, the main goal of tumor immunologists has been to increase the capacity of the immune system to mediate tumor regression. Considerable progress has been made in enhancing the efficacy of therapeutic anticancer vaccines. First, dendritic cells (DCs) have been identified as the key players in orchestrating primary immune responses. A better understanding of their biology and the development of procedures to generate vast amounts of DCs in vitro have accelerated the development of potent immunotherapeutic strategies for cancer. Second, tumor-associated antigens have been identified which are either selectively or preferentially expressed by tumor cells and can be recognized by the immune system. Finally, several studies have been performed on the genetic modification of DCs with tumor antigens. In this regard, loading the DCs with mRNA, which enables them to produce/process and present the tumor antigens themselves, has emerged as a promising strategy. Here, we will first overview the different aspects that must be taken into account when generating an mRNA-based DC vaccine and the published clinical studies exploiting mRNA-loaded DCs. Second, we will give a detailed description of a novel procedure to generate a vaccine consisting of tumor antigen-expressing dendritic cells with an in vitro superior capacity to induce anti-tumor immune responses. Here, immature DCs are electroporated with mRNAs encoding a tumor antigen, CD40 ligand (CD40L), CD70, and constitutively active (caTLR4) to generate mature antigen-presenting DCs.

Short-term cultured, interleukin-15 differentiated dendritic cells have potent immunostimulatory properties

Background

Optimization of the current dendritic cell (DC) culture protocol in order to promote the therapeutic efficacy of DC-based immunotherapy is warranted. Alternative differentiation of monocyte-derived DCs using granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-15 has been propagated as an attractive strategy in that regard. The applicability of these so-called IL-15 DCs has not yet been firmly established. We therefore developed a novel pre-clinical approach for the generation of IL-15 DCs with potent immunostimulatory properties.

Methods

Human CD14⁺ monocytes were differentiated with GM-CSF and IL-15 into immature DCs. Monocyte-derived DCs, conventionally differentiated in the presence of GM-CSF and IL-4, served as control. Subsequent maturation of IL-15 DCs was induced using two clinical grade maturation protocols: (i) a classic combination of pro-inflammatory cytokines (tumor necrosis factor-α, IL-1β, IL-6, prostaglandin E₂) and (ii) a Toll-like receptor (TLR)7/8 agonist-based cocktail (R-848, interferon-γ, TNF-α and prostaglandin E₂). In addition, both short-term (2-3 days) and long-term (6-7 days) DC culture protocols were compared. The different DC populations were characterized with respect to their phenotypic profile, migratory properties, cytokine production and T cell stimulation capacity.

Results

The use of a TLR7/8 agonist-based cocktail resulted in a more optimal maturation of IL-15 DCs, as reflected by the higher phenotypic expression of CD83 and costimulatory molecules (CD70, CD80, CD86). The functional superiority of TLR7/8-activated IL-15 DCs over conventionally matured IL-15 DCs was evidenced by their (i) higher migratory potential, (ii) advantageous cytokine secretion profile (interferon-γ, IL-12p70) and (iii) superior capacity to stimulate autologous, antigen-specific T cell responses after passive peptide pulsing. Aside from a less pronounced production of bioactive IL-12p70, short-term versus long-term culture of TLR7/8-activated IL-15 DCs resulted in a migratory profile and T cell stimulation capacity that was in favour of short-term DC culture. In addition, we demonstrate that mRNA electroporation serves as an efficient antigen loading strategy of IL-15 DCs.

Conclusions

Here we show that short-term cultured and TLR7/8-activated IL-15 DCs fulfill all pre-clinical prerequisites of immunostimulatory DCs. The results of the present study might pave the way for the implementation of IL-15 DCs in immunotherapy protocols.

Reporter gene-expressing bone marrow-derived stromal cells are immune-tolerated following implantation in the central nervous system of syngeneic immunocompetent mice

Background

Cell transplantation is likely to become an important therapeutic tool for the treatment of various traumatic and ischemic injuries to the central nervous system (CNS). However, in many pre-clinical cell therapy studies, reporter gene-assisted imaging of cellular implants in the CNS and potential reporter gene and/or cell-based immunogenicity, still remain challenging research topics.

Results

In this study, we performed cell implantation experiments in the CNS of immunocompetent mice using autologous (syngeneic) luciferase-expressing bone marrow-derived stromal cells (BMSC-Luc) cultured from ROSA26-L-S-L-Luciferase transgenic mice, and BMSC-Luc genetically modified using a lentivirus encoding the enhanced green fluorescence protein (eGFP) and the puromycin resistance gene (Pac) (BMSC-Luc/eGFP/Pac). Both reporter gene-modified BMSC populations displayed high engraftment capacity in the CNS of immunocompetent mice, despite potential immunogenicity of introduced reporter proteins, as demonstrated by real-time bioluminescence imaging (BLI) and histological analysis at different time-points post-implantation. In contrast, both BMSC-Luc and BMSC-Luc/eGFP/Pac did not survive upon intramuscular cell implantation, as demonstrated by real-time BLI at different time-points post-implantation. In addition, ELISPOT analysis demonstrated the induction of IFN-γ-producing CD8+ T-cells upon intramuscular cell implantation, but not upon intracerebral cell implantation, indicating that BMSC-Luc and BMSC-Luc/eGFP/Pac are immune-tolerated in the CNS. However, in our experimental transplantation model, results also indicated that reporter gene-specific immune-reactive T-cell responses were not the main contributors to the immunological rejection of BMSC-Luc or BMSC-Luc/eGFP/Pac upon intramuscular cell implantation.

Conclusion

We here demonstrate that reporter gene-modified BMSC derived from ROSA26-L-S-L-Luciferase transgenic mice are immune-tolerated upon implantation in the CNS of syngeneic immunocompetent mice, providing a research model for studying survival and localisation of autologous BMSC implants in the CNS by real-time BLI and/or histological analysis in the absence of immunosuppressive therapy.

Immunosuppression induced by immature dendritic cells is mediated by TGF-beta/IL-10 double-positive CD4+ regulatory T cells

Dendritic cells (DC) have important functions in T cell immunity and T cell tolerance. Previously, it was believed that T cell unresponsiveness induced by immature DC (iDC) is caused by the absence of inflammatory signals in steady-state in vivo conditions and by the low expression levels of costimulatory molecules on iDC. However, a growing body of evidence now indicates that iDC can also actively maintain peripheral T cell tolerance by the induction and/or stimulation of regulatory T cell populations. In this study, we investigated the in vitro T cell stimulatory capacity of iDC and mature DC (mDC) and found that both DC types induced a significant increase in the number of transforming growth factor (TGF)-beta and interleukin (IL)-10 double-positive CD4(+) T cells within 1 week of autologous DC/T cell co-cultures. In iDC/T cell cultures, where antigen-specific T cell priming was significantly reduced as compared to mDC/T cell cultures, we demonstrated that the tolerogenic effect of iDC was mediated by soluble TGF-beta and IL-10 secreted by CD4(+)CD25(-)FOXP3(-) T cells. In addition, the suppressive capacity of CD4(+) T cells conditioned by iDC was transferable to already primed antigen-specific CD8(+) T cell cultures. In contrast, addition of CD4(+) T cells conditioned by mDC to primed antigen-specific CD8(+) T cells resulted in enhanced CD8(+) T cell responses, notwithstanding the presence of TGF-beta(+)/IL-10(+) T cells in the transferred fraction. In summary, we hypothesize that DC have an active role in inducing immunosuppressive cytokine-secreting regulatory T cells. We show that iDC-conditioned CD4(+) T cells are globally immunosuppressive, while mDC induce globally immunostimulatory CD4(+) T cells. Furthermore, TGF-beta(+)/IL-10(+) T cells are expanded by DC independent of their maturation status, but their suppressive function is dependent on immaturity of DC.

Whole tumor antigen vaccination using dendritic cells: comparison of RNA electroporation and pulsing with UV-irradiated tumor cells

Because of the lack of full characterization of tumor associated antigens for solid tumors, whole antigen use is a convenient approach to tumor vaccination. Tumor RNA and apoptotic tumor cells have been used as a source of whole tumor antigen to prepare dendritic cell (DC) based tumor vaccines, but their efficacy has not been directly compared. Here we compare directly RNA electroporation and pulsing of DCs with whole tumor cells killed by ultraviolet (UV) B radiation using a convenient tumor model expressing human papilloma virus (HPV) E6 and E7 oncogenes. Although both approaches led to DCs presenting tumor antigen, electroporation with tumor cell total RNA induced a significantly higher frequency of tumor-reactive IFN-gamma secreting T cells, and E7-specific CD8+ lymphocytes compared to pulsing with UV-irradiated tumor cells. DCs electroporated with tumor cell RNA induced a larger tumor infiltration by T cells and produced a significantly stronger delay in tumor growth compared to DCs pulsed with UV-irradiated tumor cells. We conclude that electroporation with whole tumor cell RNA and pulsing with UV-irradiated tumor cells are both effective in eliciting antitumor immune response, but RNA electroporation results in more potent tumor vaccination under the examined experimental conditions.

A simple immunomonitoring procedure for mRNA-loaded dendritic cell therapy

To develop a simple immunomonitoring method for dendritic cell therapy using messenger RNA (mRNA) as antigen, we evaluated whether Mycobacterium tuberculosis antigen 85A (Ag85A) mRNA-transfected peripheral blood mononuclear cells (PBMCs) could be used to stimulate the induction of interferon (IFN)-gamma-producing T cells. PBMCs from 10 healthy donors were cocultured with autologous PBMCs transfected with mRNA overnight, and the number of IFN-gamma-producing T cells was measured by flow cytometry. IFN-gamma-producing CD4+ and CD8+ T cells were detected in 4 and 5 donors, respectively. PBMCs from 3 donors with negative results were then cocultured with Ag85A mRNA-transfected autologous PBMCs for 1 week to achieve in vitro primary induction of Ag85A-specific T cells. After restimulation with freshly prepared stimulator cells, a small but significant number of IFN-gamma-producing CD8+ T cells was detected. The induction of IFN-gamma-producing CD8+ T cells by overnight coculture was completely abolished by anti-class II or anti-interleukin-12 antibodies, whereas it was partially inhibited by anti-class I antibody. These data suggest that Ag85A mRNA-transfected PBMCs induce specific IFN-gamma-producing T cells and might be applicable for immunomonitoring of mRNA-loaded dendritic cell therapy.

Efficient in vitro expansion of human immunodeficiency virus (HIV)-specific T-cell responses by gag mRNA-electroporated dendritic cells from treated and untreated HIV type 1-infected individuals

Developing an immunotherapy to keep human immunodeficiency virus type 1 (HIV-1) replication suppressed while discontinuing highly active antiretroviral therapy (HAART) is an important challenge. In the present work, we evaluated in vitro whether dendritic cells (DC) electroporated with gag mRNA can induce HIV-specific responses in T cells from chronically infected subjects. Monocyte-derived DC, from therapy-naïve and HAART-treated HIV-1-seropositive subjects, that were electroporated with consensus codon-optimized HxB2 gag mRNA efficiently expanded T cells, secreting gamma interferon (IFN-gamma) and interleukin 2 (IL-2), as well as other cytokines and perforin, upon restimulation with a pool of overlapping Gag peptides. The functional expansion levels after 1 week of stimulation were comparable in T cells from HAART-treated and treatment-naïve patients and involved both CD4(+) and CD8(+) T cells, with evidence of bifunctionality in T cells. Epitope mapping of p24 showed that stimulated T cells had a broadened response toward previously nondescribed epitopes. DC, from HAART-treated subjects, that were electroporated with autologous proviral gag mRNA equally efficiently expanded HIV-specific T cells. Regulatory T cells did not prevent the induction of effector T cells in this system, whereas the blocking of PD-L1 slightly increased the induction of T-cell responses. This paper shows that DC, loaded with consensus or autologous gag mRNA, expand HIV-specific T-cell responses in vitro.

Enhanced protein expression by internal ribosomal entry site-driven mRNA translation as a novel approach for in vitro loading of dendritic cells with antigens

Transfection of dendritic cells (DCs) with messenger RNAs (mRNAs) of tumor-associated antigens (TAAs) is a promising strategy for cancer vaccines. TAA mRNA can be generated by in vitro transcription using DNA encoding the TAA gene as a template. A cap analog is usually added upon in vitro transcription to stabilize mRNA and enhance the efficiency of mRNA translation. However, the inclusion of the cap analog correlates with significantly lower-yield mRNA transcription, potentially leading to an expensive vaccine manufacturing process. To solve this problem, we present a novel approach in which DNA templates are modified with an internal ribosomal entry site (IRES) sequence inserted upstream of the gene of interest to replace the use of the cap analog. The presence of IRES greatly enhanced transcription for the mRNA in vitro compared with the cap analog. Also, higher transgene expression was achieved using luciferase (Luc) mRNA with IRES than using capped Luc mRNA to transfect DCs. Immunization of mice with DCs transfected with IRES-containing mRNA encoding chicken ovalbumin (OVA) induced significant levels of antigen-specific interferon gamma-producing CD8(+) T cells and in vivo killing of antigen-bearing cells. Consistently, mice immunized with IRES-containing OVA mRNA-transfected DCs were protected from pulmonary metastasis of melanoma cells injected intravenously. We suggest that IRES can be used for the production of larger quantities of mRNA and that such IRES-containing mRNAs may be useful for DC-based antitumor immunotherapy.

Current approaches in dendritic cell generation and future implications for cancer immunotherapy

The discovery of tumor-associated antigens, which are either selectively or preferentially expressed by tumors, together with an improved insight in dendritic cell biology illustrating their key function in the immune system, have provided a rationale to initiate dendritic cell-based cancer immunotherapy trials. Nevertheless, dendritic cell vaccination is in an early stage, as methods for preparing tumor antigen presenting dendritic cells and improving their immunostimulatory function are continuously being optimized. In addition, recent improvements in immunomonitoring have emphasized the need for careful design of this part of the trials. Still, valuable proofs-of-principle have been obtained, which favor the use of dendritic cells in subsequent, more standardized clinical trials. Here, we review the recent developments in clinical DC generation, antigen loading methods and immunomonitoring approaches for DC-based trials.

Simultaneous ex vivo quantification of antigen-specific CD4+ and CD8+ T cell responses using in vitro transcribed RNA

Assessment of antigen-specific T-cell responses has been greatly facilitated by development of ELISPOT and intracellular cytokine flow cytometry (CFC) assays. The use of autologous antigen presenting cells transfected with in vitro transcribed RNA as stimulators allows in principle quantification of antigen-specific T-cells independent of the knowledge of the epitopes. We describe here a cytokine secretion assay that enables simultaneous assessment of both antigen-specific CD4+ as well as CD8+ T-cells directly from clinical samples without the need for generation of dendritic cells. To this aim, bulk PBMCs were electroporated with RNA encoding the antigen fused to trafficking signal sequences derived from a MHC class I molecule and used as stimulators. With human cytomegalovirus (HCMV) phosphoprotein 65 (pp65) as antigen we show that for measuring ex vivo T-cell responses in ELISPOT and CFC such stimulators are superior or at least equivalent to a pool of overlapping peptides representing the entire pp65 sequence as well as to untagged pp65 encoding RNA. This approach avoids the time consuming generation of dendritic cells as immune stimulators and, in particular when used in the context of the CFC, is robust, broadly applicable and fast.

Potential of an altered peptide ligand of lipocalin allergen Bos d 2 for peptide immunotherapy

BACKGROUND

Peptide immunotherapy is a promising alternative for treating allergic diseases. One way to enhance the efficacy of peptide immunotherapy is to use altered peptide ligands (APLs) that contain amino acid substitutions compared with the natural peptide.

OBJECTIVE

To evaluate the potential of an APL of the immunodominant epitope of lipocalin allergen Bos d 2 for peptide immunotherapy.

METHODS

Peripheral blood CD4(+) T-cell responses of 8 HLA-DR4-positive subjects to the natural ligand of Bos d 2 (p127-142) or to an APL (pN135D) were analyzed by MHC class II tetramer staining after in vitro expansion with the peptides. Long-term T-cell lines (TCLs) were induced with the peptides, and the cytokine production, cross-reactivity, and T-cell receptor Vbeta subtype expression of the TCLs were analyzed.

RESULTS

CD4(+) T cells specific for both p127-142 and pN135D were readily detected in peripheral blood after a single in vitro stimulation. Whereas the TCLs induced with p127-142 were T(H)2/T(H)0-deviated, those induced with pN135D were T(H)1/T(H)0-deviated and highly cross-reactive with p127-142. Moreover, the pN135D-induced TCLs appeared to use a broader repertoire of T-cell receptor Vbeta subtypes than those induced with p127-142.

CONCLUSION

An APL of an immunodominant allergen epitope was able to induce a novel T(H)1-deviated T-cell population cross-reactive with the natural epitope in vitro. This cell population could have a therapeutic immunomodulatory function in vivo through bystander suppression.

CLINICAL IMPLICATIONS

These results support the idea that altered peptide ligands may be used to enhance the efficacy of peptide immunotherapy.

An improved protocol for generation of immuno-potent dendritic cells through direct electroporation of CD14+ monocytes

In this study we demonstrate a novel protocol showing that electroporation of CD14+ monocytes directly isolated from blood with green fluorescent protein (GFP) RNA results in a 3-fold higher yield of antigen presenting dendritic cells (DCs) when compared to conventional methods employing immature DCs for electroporation. We further show a stable electroporation efficacy resulting in 60% of GFP positive cells. Expression of co-stimulatory molecules and maturation markers such as CD80, CD86, CD83 as well of the chemokine receptor 7 (CCR7) was found in 90% of the mature DCs. Importantly, production of IL-12p70 was 10 times higher in cells electroporated at the monocyte stage compared to cells electroporated at the immature DC stage. Stimulation of autologous naïve lymphocytes by DCs electroporated at monocytes stage elicited proliferation of CD8+ T-cell with 7-fold increase in IFN-gamma release. Blocking of the MHC-Class I molecules significantly inhibited the IFN-gamma release, indicating that antigen presentation was MHC-Class I mediated. In summary, electroporation of CD14+ monocytes with RNA results in a high yield of antigen presenting DCs with high immuno-stimulatory capacity and antigen presentation on MHC-Class I molecules. This improved method may represent an attractive approach for RNA-based DC immunotherapy.

RNA loading of leukemic antigens into cord blood-derived dendritic cells for immunotherapy

The manipulation of dendritic cells (DCs) ex vivo to present tumor-associated antigens for the activation and expansion of tumor-specific cytotoxic T lymphocytes (CTLs) attempts to exploit these cells' pivotal role in immunity. However, significant improvements are needed if this approach is to have wider clinical application. We optimized a gene delivery protocol via electroporation for cord blood (CB) CD34(+) DCs using in vitro-transcribed (IVT) mRNA. We achieved > 90% transfection of DCs with IVT-enhanced green fluorescent protein mRNA with > 90% viability. Electroporation of IVT-mRNA up-regulated DC costimulatory molecules. DC processing and presentation of mRNA-encoded proteins, as major histocompatibility complex/peptide complexes, was established by CTL assays using transfected DCs as targets. Along with this, we also generated specific antileukemic CTLs using DCs electroporated with total RNA from the Nalm-6 leukemic cell line and an acute lymphocytic leukemia xenograft. This significant improvement in DC transfection represents an important step forward in the development of immunotherapy protocols for the treatment of malignancy.

Simultaneous activation of viral antigen-specific memory CD4+ and CD8+ T-cells using mRNA-electroporated CD40-activated autologous B-cells

Recently, it has become obvious that not only CD8 T-cells, but also CD4 T-helper cells are required for the induction of an effective, long-lasting cellular immune response. In view of the clinical importance of cytomegalovirus (CMV) and human immunodeficiency virus (HIV) infection, we developed 2 strategies to simultaneously reactivate viral antigen-specific memory CD4 and CD8 T-cells of CMV-seropositive and HIV-seropositive subjects using mRNA-electroporated autologous CD40-activated B cells. In the setting of HIV, we provide evidence that CD40-activated B cells can be cultured from HAART-naive HIV-1 seropositive patients. These cells not only express and secrete the HIV p24 antigen after electroporation with codon-optimized HIV-1 gag mRNA, but can also be used to in vitro reactivate Gag antigen-specific interferon-gamma-producing CD4 and CD8 autologous T-cells. For the CMV-specific approach, we applied mRNA coding for the pp65 protein coupled to the lysosomal-associated membrane protein-1 to transfect CD40-activated B cells to induce CMV antigen-specific CD4 and CD8 T-cells. More detailed analysis of the activated interferon-gamma-producing CMV pp65 tetramer positive CD8 T-cells revealed an effector memory phenotype with the capacity to produce interleukin-2. Our findings clearly show that the concomitant activation of both CD4 and CD8 (memory) T-cells using mRNA-electroporated CD40-B cells is feasible in CMV and HIV-1-seropositive persons, which indicates the potential value of this approach for application in cellular immunotherapy of infectious diseases.

Characterization of the ribonuclease activity on the skin surface

The rapid degradation of ribonucleic acids (RNA) by ubiquitous ribonucleases limits the efficacy of new therapies based on RNA molecules. Therefore, our aim was to characterize the natural ribonuclease activities on the skin and in blood plasma i.e. at sites where many drugs in development are applied. On the skin surfaces of Homo sapiens and Mus musculus we observed dominant pyrimidine-specific ribonuclease activity. This activity is not prevented by a cap structure at the 5'-end of messenger RNA (mRNA) and is not primarily of a 5'- or 3'-exonuclease type. Moreover, the ribonuclease activity on the skin or in blood plasma is not inhibited by chemical modifications introduced at the 2'OH group of cytidine or uridine residues. It is, however, inhibited by the ribonuclease inhibitor RNasin^® although not by the ribonuclease inhibitor SUPERase· In™. The application of our findings in the field of medical science may result in an improved efficiency of RNA-based therapies that are currently in development.

Induction of antigen-specific CD8+ cytotoxic T cells by dendritic cells co-electroporated with a dsRNA analogue and tumor antigen mRNA

The maturation state of dendritic cells (DCs) is an important determinant for the initiation and regulation of adaptive immune responses. In this study, we wanted to assess whether functional activation of human monocyte-derived DCs can be achieved by electroporation of an activation signal in the form of double-stranded (ds) RNA and whether simultaneous electroporation of the dsRNA with tumor antigen encoding mRNA can lead to the induction of a cytotoxic T-lymphocyte (CTL) response. Electroporation of immature DCs with poly(I:C(12)U), a dsRNA analogue, resulted in phenotypic as well as functional changes, indicative of DC maturation. Co-electroporation of DCs with both poly(I:C(12)U) and Melan-A/MART-1 encoding mRNA induced strong anti-Melan-A/MART-1 CD8(+) T-cell responses in vitro. Higher numbers of Melan-A/MART-1-specific CTLs were consistently obtained with poly(I:C(12)U)-activated DCs compared to DCs matured in the presence of an inflammatory cytokine cocktail. These results indicate that DC co-electroporation with both dsRNA and tumor antigen encoding mRNA induces fully activated and antigen-loaded DCs that promote antigen-specific CTL responses and may provide the basis for future immunotherapeutic strategies.

Trypanosoma cruzi modulates the profile of memory CD8+ T cells in chronic Chagas' disease patients

We present a cross-sectional analysis of the maturation and migratory properties of the memory CD8(+) T cell compartment, in relation to the severity of heart disease in individuals with chronic Trypanosoma cruzi infection removed from endemic areas for longer than 20 years. Subjects with none or mild heart involvement were more likely to mount T. cruzi-specific memory IFN-gamma responses than subjects with more advanced cardiac disease, and the T. cruzi-specific CD8(+) T cell population was enriched in early-differentiated (CD27(+)CD28(+)) cells in responding individuals. In contrast, the frequency of CD27(+)CD28(+)CD8(+) T cells in the total memory CD8(+) T cell population decreases, as disease becomes more severe, while the proportion of fully differentiated memory (CD27(-)CD28(-)) CD8(+) T cells increases. The analysis of CCR7 expression revealed a significant increase in total effector/memory CD8(+) T cells (CD45RA(-)CCR7(-)) in subjects with mild heart disease as compared with uninfected controls. Altogether, these results are consistent with the hypothesis of a gradual clonal exhaustion in the CD8(+) T cell population, perhaps as a result of continuous antigenic stimulation by persistent parasites.

An improved RNA amplification procedure results in increased yield of autologous RNA transfected dendritic cell-based vaccine

Use of antigen encoding RNA transfected Dendritic cells in the field of cancer immunotherapy has been well established. The use of RNA overcomes limitations inherent to other autologous DC-based vaccines as it does not require specific HLA haplotypes, identification and characterization of antigens, and captures the broadest antigen repertoire. RNA offers yet another advantage-it could be amplified minimizing the requirement of tumor mass for autologous vaccine production, and will afford the opportunity to treat patients with minimal tumor burden. The original procedure described for RNA amplification resulted in a proportion of RNA transcribed in the antisense orientation. This study also demonstrates that the presence of double-stranded RNA correlates with the presence of antisense RNA. Alternative design of oligonucleotides that removes sequence redundancy eliminates the formation of both antisense and double-stranded RNA species. We provide further evidence that amplified RNA containing antisense and double-stranded RNA species results in lower recovery of DCs post-transfection and maturation, presumably through sequence-specific gene silencing. The removal of the double-stranded species from amplified RNA results in higher recovery of mature autologous amplified RNA transfected dendritic cells. Higher DC yield will allow for reduction of cost of vaccine manufacturing and prolonged treatment of a patient.

Cancer immunotherapy using RNA-loaded dendritic cells

Dendritic cells (DC) are the most professional antigen-presenting cells of the immune system and are capable of initiating immune responses in vitro and in vivo. One of the great challenges in immunotherapy protocols is to introduce relevant antigens into DC for stimulation of major histocompatibility complex (MHC) class I- and class II-restricted anti-tumour or anti-viral immunity. This review will focus on the development of mRNA-loaded DC-based immunotherapy vaccines. First, several published results concerning mRNA transfection efficiency in DC are compared. Next, an overview is given for several published studies describing CD8+ and CD4+ T-cell clone activation using RNA-loaded DC. These data show that RNA-loaded DC efficiently process and present antigenic epitopes. Next, published data from in vitro T-cell activation studies using RNA-loaded DC are summarized and provide evidence that RNA-loaded DC can efficiently stimulate in vitro primary and secondary immune responses. Finally, the summarized data provide evidence that RNA-loaded DC are a promising strategy for the development of future cancer vaccination strategies.

HuBMSC-MCP, a novel member of mitochondrial carrier superfamily, enhances dendritic cell endocytosis

A novel member of mitochondrial carrier superfamily has been identified from human bone marrow stromal cells (BMSC) and designated as human BMSC-derived mitochondrial carrier protein (HuBMSC-MCP). It encodes a 321 amino-acid protein with three tandem related domains of about 100 amino acids. Each domain contains two hydrophobic stretches, which are thought to span the membrane as alpha-helices. Distant relationship analysis indicates that the protein is highly conserved between species from Caenorhabditis elegans to human. HuBMSC-MCP gene is mapped to chromosome 11p11. HuBMSC-MCP mRNA expression is detectable in various human tissues and cell lines. By confocal imaging, HuBMSC-MCP is localized to mitochondria and also detected in the pseudopodial protrusion of human breast adenocarcinoma MCF-7 cells. When transfected into dendritic cells (DC), HuBMSC-MCP could enhance DCs endocytotic capacity. Thus, HuBMSC-MCP is a phylogenetically conserved and widely expressed mitochondrial carrier protein which perhaps associates with mitochondrial oxidative phosphorylation.

Minor histocompatibility antigens on canine hemopoietic progenitor cells

Adoptive immunotherapy with CTL against minor histocompatibility Ags (mHA) provides a promising way to treat leukemia relapse in allogeneic chimeras. Here we describe the in vitro generation of CTL against mHA in the dog. We tested their inhibitory effect on the growth of hemopoietic progenitor cells stimulated by hemopoietic growth factors in a 4-day suspension culture. CTL were produced by coculture of donor PBMC with bone marrow-derived dendritic cells (DCs). These DCs were characterized by morphology, high expression of MHC class II and CD1a, and the absence of the monocyte-specific marker CD14. Characteristically these cells stimulated allogeneic lymphocytes (MLR) and, after pulsing with a foreign Ag (keyhole limpet hemocyanin), autologous T cells. CTL were generated either ex vivo by coculture with DCs of DLA-identical littermates or in vivo by immunization of the responder with DCs obtained from a DLA-identical littermate. In suspension culture assays the growth of hemopoietic progenitor cells was inhibited in 53% of DLA-identical littermate combinations. In canine families mHA segregated with DLA as restriction elements. One-way reactivity against mHA was found in five littermate combinations. In two cases mHA might be Y chromosome associated, in three cases autosomally inherited alleles were detected. We conclude that CTL can be produced in vitro and in vivo against mHA on canine hemopoietic progenitor cells using bone marrow-derived DCs.

Efficient removal of LoxP-flanked genes by electroporation of Cre-recombinase mRNA

Introduction of Cre-recombinase in target cells is currently achieved by transfection of plasmid DNA or by viral-mediated transduction. However, efficiency of non-viral DNA transfection is often low in many cell types, and the use of viral vectors for transduction implies a more complex and laborious manipulation associated with safety issues. We have developed a non-viral non-DNA technique for rapid and highly efficient excision of LoxP-flanked DNA sequences based on electroporation of in vitro transcribed mRNA encoding Cre-recombinase. A K562-DSRed[EGFP] cell line was developed in order to measure Cre-mediated recombination by flow cytometric analysis. These cells have a stable integrated DSRed reporter gene flanked by two LoxP sites, and an EGFP reporter gene, which could only be transcribed when the coding sequence for DSRed was removed. The presented data show recombination efficiencies, as measured by appearance of EGFP-fluorescence, of up to 85% in Cre-recombinase mRNA-electroporated K562-DSRed[EGFP] cells. In conclusion, mRNA electroporation of Cre-recombinase is a powerful, safe, and clinically applicable alternative to current technologies used for excision of stably integrated LoxP-flanked DNA sequences.

Prospects for immunotherapy of malignant disease

The majority of T cell-recognized tumour antigens in humans are encoded by genes that are also present in normal tissues. Low levels of gene expression in normal cells can lead to the inactivation of high-avidity T cells by immunological tolerance mechanisms. As a consequence, low-avidity T cell responses in patients are often inadequate in providing tumour protection. Recently, several technologies have been developed to overcome tolerance, allowing the isolation of high-affinity, HLA-restricted receptors specific for tumour-associated peptide epitopes. Furthermore, transfer of HLA-restricted antigen receptors provides an opportunity to empower patient T cells with new tumour-reactive specificities that cannot be retrieved from the autologous T cell repertoire.

RNA-transfected dendritic cells

Based on their unique ability to stimulate primary immune responses, dendritic cells are the most potent antigen-presenting cells known. This ability stems from the fact that they are very efficient at the uptake and processing of antigen and they express high levels of major histocompatibility complex class I and class II, as well as costimulatory molecules, which are required to prime naive cytotoxic T-cells. Many groups of investigators have tried to take advantage of these features by developing dendritic cell-based vaccines against tumors and infectious diseases. While the basic principle in these studies is the same--dendritic cells pulsed with antigen are used to elicit cytotoxic T-cell responses--the methods used are varied. This is particularly true with respect to the nature of the antigen used and the method of antigen delivery. In this article, we will focus on the use of RNA as a form of antigen with which to load dendritic cells. We will discuss the rationale behind using RNA as an antigen source and will review recent studies in both murine and human settings that use RNA-pulsed dendritic cells as vaccines.