Transient TCR-based T cell therapy in a patient with advanced treatment-resistant MSI-high colorectal cancer.

We previously demonstrated the antitumor effectiveness of transiently T cell receptor (TCR)-redirected T cells recognizing a frameshift mutation in transforming growth factor beta receptor 2. We here describe a clinical protocol using mRNA TCR-modified T cells to treat a patient with progressive, treatment-resistant metastatic microsatellite instability-high (MSI-H) colorectal cancer. Following 12 escalating doses of autologous T cells electroporated with in-vitro-transcribed Radium-1 TCR mRNA, we assessed T cell cytotoxicity, phenotype, and cytokine production. Tumor markers and growth on computed tomography scans were evaluated and immune cell tumor infiltrate at diagnosis assessed. At diagnosis, tumor-infiltrating CD8+ T cells had minimal expression of exhaustion markers, except for PD-1. Injected Radium-1 T cells were mainly naive and effector memory T cells with low expression of exhaustion markers, except for TIGIT. We confirmed cytotoxicity of transfected Radium-1 T cells against target cells and found key cytokines involved in tumor metastasis, growth, and angiogenesis to fluctuate during treatment. The treatment was well tolerated, and despite his advanced cancer, the patient obtained a stable disease with 6 months survival post-treatment. We conclude that treatment of metastatic MSI-H colorectal cancer with autologous T cells electroporated with Radium-1 TCR mRNA is feasible, safe, and well tolerated and that it warrants further investigation in a phase 1/2 study.

A phase I/II escalation trial design T-RAD: Treatment of metastatic lung cancer with mRNA-engineered T cells expressing a T cell receptor targeting human telomerase reverse transcriptase (hTERT).

Background: Adoptive cellular therapy (ACT) with genetically modified T cells aims to redirect T cells against resistant cancers through introduction of a T cell receptor (TCR). The Radium-4 TCR was isolated from a responding patient in a cancer vaccination study and recognizes the enzymatic component of human Telomerase Reverse Transcriptase (hTERT) presented on MHC class II (HLA-DP04). hTERT is a constitutively overexpressed tumor-associated antigen present in most human cancers, including non-small-cell lung cancer (NSCLC), which is the second most common type of cancer worldwide. Treatment alternatives for relapsing NSCLC are limited and survival is poor. To improve patient outcome we designed a TCR-based ACT study targeting hTERT. Methods: T-RAD is a phase I/II study to evaluate the safety and efficacy of Radium-4 mRNA electroporated autologous T cells in the treatment of metastatic NSCLC with no other treatment option. Transient TCR expression is applied for safety considerations. Participants receive two intravenous injections with escalating doses of redirected T cells weekly for 6 consecutive weeks. Primary objectives are safety and tolerability. Secondary objectives include progression-free survival, time to progression, overall survival, patient reported outcomes and overall radiological response. Discussion: Treatment for metastatic NSCLC is scarce and new personalized treatment options are in high demand. hTERT is a tumor target applicable to numerous cancer types. This proof-of-concept study will explore for the first time the safety and efficacy of TCR mRNA electroporated autologous T cells targeting hTERT. The T-RAD study will thus evaluate an attractive candidate for future immunotherapy of solid tumors.

Long-term first-in-man Phase I/II study of an adjuvant dendritic cell vaccine in patients with high-risk prostate cancer after radical prostatectomy.

BACKGROUND: Patients with high-risk prostate cancer (PC) can experience biochemical relapse (BCR), despite surgery, and develop noncurative disease. The present study aimed to reduce the risk of BCR with a personalized dendritic cell (DC) vaccine, given as adjuvant therapy, after robot-assisted laparoscopic prostatectomy (RALP). METHODS: Twelve weeks after RALP, 20 patients with high-risk PC and undetectable PSA received DC vaccinations for 3 years or until BCR. The primary endpoint was the time to BCR. The immune response was assessed 7 weeks after surgery (baseline) and at one-time point during the vaccination period. RESULTS: Among 20 patients, 11 were BCR-free over a median of 96 months (range: 84-99). The median time from the end of vaccinations to the last follow-up was 57 months (range: 45-60). Nine patients developed BCR, either during (n = 4) or after (n = 5) the vaccination period. Among five patients diagnosed with intraductal carcinoma, three experienced early BCR during the vaccination period. All patients that developed BCR remained in stable disease within a median of 99 months (range: 74-99). The baseline immune response was significantly associated with the immune response during the vaccination period (p = 0.015). For patients diagnosed with extraprostatic extension (EPE), time to BCR was longer in vaccine responders than in non-responders (p = 0.09). Among 12 patients with the International Society of Urological Pathology (ISUP) grade 5 PC, five achieved remission after 84 months, and all mounted immune responses. CONCLUSION: Patients diagnosed with EPE and ISUP grade 5 PC were at particularly high risk of developing postsurgical BCR. In this subgroup, the vaccine response was related to a reduced BCR incidence. The vaccine was safe, without side effects. This adjuvant first-in-man Phase I/II DC vaccine study showed promising results. DC vaccines after curative surgery should be investigated further in a larger cohort of patients with high-risk PC.

Improving Dendritic Cell Cancer Vaccine Potency Using RNA Interference.

Dendritic cell cancer vaccines have already become a treatment modality for patients with various cancer types. However, the curative potential of this immunotherapy is limited by the existence of negative feedback mechanisms that control dendritic cells (DCs) and T-cell function. By inhibiting the expression of inhibitory factors using RNA interference technology, a new generation of DC vaccines was developed. Vaccine-stimulated T cells showed antitumor effects both in vitro and in cancer patients. Here, we describe the development and validation of a fully GMP-compliant production process of ex vivo DC cancer vaccines combined with the blockade of immunosuppressive pathways using small interfering RNAs. The protocol can be used for DC-based therapy for all cancer types.

Diversification of Antitumour Immunity in a Patient with Metastatic Melanoma Treated with Ipilimumab and an IDO-Silenced Dendritic Cell Vaccine.

Indoleamine 2,3-dioxygenase (IDO) expression in dendritic cells (DCs) inhibits T-cell activation and promotes T-cell differentiation into regulatory T-cells. Moreover, IDO expression promotes resistance to immunotherapies targeting immune checkpoints such as the cytotoxic T lymphocyte antigen-4 (CTLA-4). Here, a patient with metastatic melanoma pretreated with ipilimumab, an anti-CTLA-4 blocking antibody, was vaccinated with IDO-silenced DCs cotransfected with mRNA for survivin or hTERT tumour antigens. During vaccination, T-cell responses to survivin and hTERT tumour antigens were generated, and a certain degree of clinical benefit was achieved, with a significant reduction in lung, liver, and skin metastases, along with a better performance status. T-cell responses against MART-1 and NY-ESO-1 tumour antigens were also detected in the peripheral blood. The patient also mounted an antibody response to several melanoma proteins, indicating diversification of the antitumour immunity in this patient. The identification of such serum antibody-reacting proteins could facilitate the discovery of tumour neoantigens.

Immunosuppressive factor blockade in dendritic cells via siRNAs results in objective clinical responses.

Over the past decade, immunotherapy has emerged as a promising new form of cancer treatment with the potential to eradicate tumor metastasis. However, its curative potential is in general limited by the existence of negative feedback mechanisms that control dendritic cells (DCs) and T-cell activation. For clinically effective immunity, there is a need of inhibiting the expression of these immune suppressors. This could enhance the activation of DCs, T cells, and natural killer cells, and might be beneficial for cancer immunotherapy. Among the immune inhibitory molecules expressed by DCs is indoleamine 2,3-dioxygenase (IDO), an enzyme that conveys immunosuppressive effects by degrading tryptophan, an essential amino acid required for T-cell proliferation and survival. Depletion of tryptophan by IDO-positive DCs induces T-cell apoptosis and the conversion of naïve CD4+ T cells into regulatory T cells that further suppress antitumor immunity. Herein, we describe a protocol for in vitro synthesis of small interfering RNA against IDO and other immunosuppressive factors such as interleukin-10 and programmed cell death-1 ligands in order to reverse immune suppression mediated by DCs. Vaccination with IDO-silenced DC vaccines enhanced immune responses and antitumor immunity in cancer patients.

Therapeutic vaccination against autologous cancer stem cells with mRNA-transfected dendritic cells in patients with glioblastoma.

BACKGROUND: The growth and recurrence of several cancers appear to be driven by a population of cancer stem cells (CSCs). Glioblastoma, the most common primary brain tumor, is invariably fatal, with a median survival of approximately 1 year. Although experimental data have suggested the importance of CSCs, few data exist regarding the potential relevance and importance of these cells in a clinical setting. METHODS: We here present the first seven patients treated with a dendritic cell (DC)-based vaccine targeting CSCs in a solid tumor. Brain tumor biopsies were dissociated into single-cell suspensions, and autologous CSCs were expanded in vitro as tumorspheres. From these, CSC-mRNA was amplified and transfected into monocyte-derived autologous DCs. The DCs were aliquoted to 9-18 vaccines containing 10(7) cells each. These vaccines were injected intradermally at specified intervals after the patients had received a standard 6-week course of post-operative radio-chemotherapy. The study was registered with the ClinicalTrials.gov identifier NCT00846456. RESULTS: Autologous CSC cultures were established from ten out of eleven tumors. High-quality RNA was isolated, and mRNA was amplified in all cases. Seven patients were able to be weaned from corticosteroids to receive DC immunotherapy. An immune response induced by vaccination was identified in all seven patients. No patients developed adverse autoimmune events or other side effects. Compared to matched controls, progression-free survival was 2.9 times longer in vaccinated patients (median 694 vs. 236 days, p = 0.0018, log-rank test). CONCLUSION: These findings suggest that vaccination against glioblastoma stem cells is safe, well-tolerated, and may prolong progression-free survival.

A novel peptide carrier for efficient targeting of antigens and nucleic acids to dendritic cells.

Dendritic cells (DCs) initiate host immune responses by presenting captured antigens to naive T cells. Hence, DC-binding peptides may be used for antigen targeting to boost naive and memory immune responses. By biopanning peptide phage libraries on human monocyte-derived DCs, we identified novel DC-binding peptides. One of the selected phages, displaying the NW peptide (NWYLPWLGTNDW), bound DCs with high affinity, and its binding was inhibited by the corresponding synthetic peptide. Antigenic peptides or proteins conjugated to the NW peptide bound to DCs and were internalized without negative effects on DC phenotype and function. Ex vivo targeted delivery of CMV-pp65 peptides to DCs via the NW peptide increased T-cell responses in HLA-A2(+)/CMV(+) donors compared to untargeted peptides (P<0.001). Stimulation of CD45RO-depleted peripheral blood mononuclear cells from CMV(-) donors with the NW-pp65 fusion peptides expanded pp65-specific precursor T cells. Moreover, the NW peptide mediated small interfering RNA delivery to DCs, and a significant gene silencing was obtained. Collectively, the data reveal that proteins and nucleic acids can be directed to DCs through the NW peptide, enabling effective uptake and functional effects such as T-cell activation in the context of MHC class I and II molecules.

Silencing of indoleamine 2,3-dioxygenase enhances dendritic cell immunogenicity and antitumour immunity in cancer patients.

Dendritic cells (DCs) are being explored as a therapeutic vaccine for cancers. However, their immunogenic potential is limited by the presence of immunosuppressive factors. Among these factors is the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO). In this study, we have investigated the safety, immunogenicity and clinical response of IDO-silenced DC vaccine in four patients with gynecological cancers. DCs were transfected with IDO small interfering RNA and mRNA encoding human telomerase reverse transcriptase (hTERT) or survivin, two universal tumour antigens. Silencing of IDO in DCs did not affect the expression of the co-stimulatory molecules CD80 and CD86, but enhanced the expression of the CCR7 and CD40 molecules. IDO-silenced DCs showed superior potency to activate allogeneic T cells compared to their IDO-positive counterparts. The immunisation with this novel DC cancer vaccine was well tolerated and all patients developed delayed-type hypersensitivity skin reaction and specific T-cell response against hTERT and survivin tumour antigens. Perhaps most importantly, the immune response seen in the patients was related to objective clinical response. Thus, IDO silencing can enhance the immunogenic function of DCs in vitro and in vivo. Overall, the data provide proof-of-principle that immunisation with IDO-silenced DC vaccine is safe and effective in inducing antitumour immunity.

Light-controlled modulation of gene expression using polyamidoamine formulations.

A promising method that offers both time- and site-specific delivery of macromolecules is photochemical internalization technology (PCI). Here, we have characterized various polyamidoamine (PAMAM) carriers [generation (G) 0-7], for light-directed delivery of nucleic acids in vitro by the use of PCI technology. A number of parameters for optimal delivery of nucleic acids into human cancer cells, that is, various light-doses, carrier-doses, and small interfering RNA (siRNA)/messenger RNA (mRNA) doses were investigated for either up- or down-regulation of enhanced green fluorescent protein (EGFP) gene expression. In summary, our results showed in an osteosarcoma cell line (OHS) [EGFP] model system the possibility for efficient light-directed siRNA silencing (>80% silencing) when using PAMAM G3 to G7 as carriers. Surprisingly, no EGFP mRNA up-regulation was detected either with or without PCI after EGFP mRNA/PAMAM (G0-G7) transfection in standard OHS cells. We have here identified properties for PAMAM formulations enabling light-directed siRNA delivery with the aim of developing a site-specific strategy for delivery of nucleic acids in vivo.

Light-induced gene expression using messenger RNA molecules.

The exploration of messenger RNA (mRNA) as a potential therapeutic regulator of gene expression has been significantly reduced by the inability of polyplexes to escape the endocytic pathway, combined with the lack of specific targeting. In the present study, we have developed a site-specific delivery strategy for mRNA molecules through the use of photochemical internalization (PCI) technology. When using EGFP mRNA as a model system, a 10- to 40-fold increase in EGFP-positive cells was obtainable in PCI-treated samples, compared to untreated PCI samples in a human osteosarcoma cell line. The amount of EGFP-positive cells in both PCI and non PCI-treated samples were highly dependent on the nitrogen/phosphate (N/P) ratio. Potent delivery of mRNA molecules through the endocytic pathway by the use of polyplexes and PCI was achievable without any loss of cell viability. The main benefit of the strategy proposed is the possibility for protein production from the delivered mRNA in a way that is controllable in a time- and site-specific manner.

A full scale comparative study of methods for generation of functional Dendritic cells for use as cancer vaccines.

BACKGROUND: Dendritic cells (DCs) are professional antigen-presenting cells with the ability to induce primary T-cell responses and are commonly produced by culturing monocytes in the presence of IL-4 and GM-CSF for 5-7 days (Standard DC). Recently, Dauer and co-workers presented a modified protocol for differentiation of human monocytes into mature DCs within 48 hours (Fast DC). Here we report a functional comparison of the two strategies for generation of DCs from human monocytes with adaptions for large-scale clinical use. METHODS: The Elutra Cell Selection System was used to isolate monocytes after collection of leukapheresis product. The enriched monocytes were cultured in gas permeable Teflon bags with IL-4 and GM-CSF for 24 hours (Fast DC) or 5 days (Standard DC) to obtain immature DCs. The cells were then transfected with mRNA from the leukemia cell line Jurkat E6 by electroporation and incubated for additional 24 h or 2 days in the presence of pro-inflammatory cytokines (TNFalpha, IL-1beta, IL-6 and PGE2) to obtain mature DCs. RESULTS: Mature Fast DC and Standard DC displayed comparable levels of many markers expressed on DC, including HLA-DR, CD83, CD86, CD208 and CCR7. However, compared to Standard DC, mature Fast DC was CD14high CD209low. Fast DC and Standard DC transfected with Jurkat E6-cell mRNA were equally able to elicit T cell specifically recognizing transfected DCs in vitro. IFNgamma-secreting T cells were observed in both the CD4+ and CD8+ subsets. CONCLUSION: Our results indicate that mature Fast DC are functional antigen presenting cells (APCs) capable of inducing primary T-cell responses, and suggest that these cells may be valuable for generation of anti-tumor vaccines.

Characterization of novel alternative splicing sites in human telomerase reverse transcriptase (hTERT): analysis of expression and mutual correlation in mRNA isoforms from normal and tumour tissues.

BACKGROUND: Human telomerase reverse transcriptase (hTERT) is a key component for synthesis and maintenance of telomeres on chromosome ends and is required for the continued proliferation of cells. Estimation of hTERT expression therefore has broad relevance in oncology and stem cell research. Several splicing variants of hTERT have been described whose regulated expression contributes to the control of telomerase activity. Knowledge of the different hTERT mRNA isoforms and the ability to distinguish between them is an important issue when evaluating telomerase expression. RESULTS: By establishing cDNA-clone panels from lung and colon tissues, we could map hTERT clones individually for differences in DNA sequence. This made possible the identification of novel alternatively spliced sites as well as analysis of their frequency and mutual correlation in mRNA isoforms. Ten different alternatively spliced sites were detected, of which six were novel sites resulting from alternative splicing of intron 2 or 14. The majority of hTERT cDNA clones from normal and tumour lung and colon tissues encoded truncated proteins ending close after exon 2 or 6. CONCLUSION: The increased complexity in telomerase expression revealed here has implications for our understanding of telomerase regulation and for the choice of suitable methods for addressing hTERT expression.

Preclinical full-scale evaluation of dendritic cells transfected with autologous tumor-mRNA for melanoma vaccination.

Most cancer vaccines to date have made use of common tumor antigens or allogenic cancer cell lines. The majority of tumor antigens may, however, be unique patient-specific antigens. Dendritic cells (DCs) are the most potent antigen-presenting cells known. The present report is a full-scale preclinical evaluation of autologous DCs transfected with autologous tumor-mRNA (tDCs) for vaccination in malignant melanoma. By using autologous tumor-mRNA, we intend to make the DCs present a broad spectrum of tumor-associated antigens relevant to each individual patient. Previously, we have described effective methods for mRNA-transfection into DCs by square-wave electroporation and for generating large numbers of DCs. Here, we demonstrate the ability of tDCs, made under full-scale vaccine conditions, to generate in vitro T-cell responses specific for antigens encoded by the transfected tumor-mRNA. T-cell proliferation assays demonstrated tDC-specific responses for all six patients tested. Responses were further studied by IFNgamma ELISPOT and Bioplex cytokine assays (two patients) and by experiments on isolated CD4(+) and CD8(+) T cells, including HLA-blockage (one patient). Moreover, we describe the results of extensive tumor-RNA analysis using Agilent Bioanalyser, a method that we have implemented in the clinical protocol. Based on this preclinical evaluation, a vaccine trial has been started.

Preclinical evaluation of autologous dendritic cells transfected with mRNA or loaded with apoptotic cells for immunotherapy of high-risk neuroblastoma.

Children with high-risk neuroblastoma (NB) have a poor clinical outcome. The purpose of the present study was to evaluate different strategies for immunotherapy of high-risk NB based on vaccination with antigen-loaded dendritic cells (DCs). DCs are professional antigen-presenting cells with the ability to induce antitumor T-cell responses. We have compared DCs either loaded with apoptotic tumor cells or transfected with mRNA from the NB cell line HTB11 SK-N-SH, for their capacity to induce T-cell responses in vitro. Monocyte-derived DCs from healthy donors were loaded with tumor-derived antigens in the form of apoptotic cells or mRNA, matured and used to prime autologous T cells in vitro. After 1 week, T-cell responses against antigen-loaded DCs were measured by ELISPOT assay. DCs loaded with apoptotic NB cells or transfected with NB-cell mRNA were both able to efficiently activate autologous T cells. Both T cells of the CD8+ and CD4+ subset were activated. T cells activated by NB mRNA transfected DCs extensively crossreacted with DCs loaded with apoptotic NB cells and vice versa. The results indicate that loading of DCs with apoptotic NB cells or transfection with tumor mRNA represent promising strategies for development of individualized cancer vaccines/cancer gene therapy in treatment of NB.

Evaluation of dendritic cells loaded with apoptotic cancer cells or expressing tumour mRNA as potential cancer vaccines against leukemia.

BACKGROUND: Leukemia is a clonal disorder characterized by uncontrolled proliferation of haematopoietic cells, and represents the most common form of cancer in children. Advances in therapy for childhood leukemia have relied increasingly on the use of high-dose chemotherapy often combined with stem-cell transplantation. Despite a high success rate and intensification of therapy, children still suffer from relapse and progressive disease resistant to further therapy. Thus, novel forms of therapy are required. METHODS: This study focuses on dendritic cell (DC) vaccination of childhood leukemia and evaluates the in vitro efficacy of different strategies for antigen loading of professional antigen-presenting cells. We have compared DCs either loaded with apoptotic leukemia cells or transfected with mRNA from the same leukemia cell line, Jurkat E6, for their capacity to induce specific CD4+ and CD8+ T-cell responses. Monocyte-derived DCs from healthy donors were loaded with tumor antigen, matured and co-cultured with autologous T cells. After one week, T-cell responses against antigen-loaded DCs were measured by enzyme-linked immunosorbent spot (ELISPOT) assay. RESULTS: DCs loaded with apoptotic Jurkat E6 cells or transfected with Jurkat E6-cell mRNA were both able to elicit specific T-cell responses in vitro. IFNgamma-secreting T cells were observed in both the CD4+ and CD8+ subsets. CONCLUSION: The results indicate that loading of DCs with apoptotic leukemia cells or transfection with tumour mRNA represent promising strategies for development of cancer vaccines for treatment of childhood leukemia.

HLA-A3 restricted mutant ras specific cytotoxic T-lymphocytes induced by vaccination with T-helper epitopes.

Cytotoxic T-lymphocytes are one of the most important elements of the antitumor defense. Stimulation of cytotoxic T-lymphocytes outgrowth after immunization with mutant ras peptides is a desired goal since these cells may kill tumor cells in vivo. In this study we tested responding peripheral mononuclear cells from a patient with pancreatic adenocarcinoma who had received intradermal peptide vaccination with a mixture of 17-mer mutant ras peptides and granulocyte-macrophage colony-stimulating factor as an adjuvant. Responding peripheral T-cells were cloned by limiting dilution and several CD8(+) cytotoxic T-lymphocytes, specific for the K- RAS 12-Cys mutation were obtained. By using a panel of nonamer peptides containing the 12-Cys mutation and covering position 4-21 in the ras molecule, the 9-mer peptide which was actually recognized by the cytotoxic T-lymphocytes could be identified. HLA-A*0302 could be identified as the antigen-presenting molecule, and the amino acid sequence of the T-cell epitope carries the previously identified HLA-A*0302 binding motif. The nonamer peptide was contained within the vaccine peptide originally used for intradermal immunization of the patient. The cytotoxic T-lymphocytes were capable of killing target cells expressing HLA-A*0302 that coexpressed the K- RAS 12-Cys mutation after transfection. These data demonstrate that the peptide used for vaccination (17-mer) is processed and presented in vivo, and that generation of cytotoxic T-lymphocytes by vaccination with T-helper epitopes may be important for further development of specific immunotherapy of cancer patients.

Nonviral and viral gene transfer into different subsets of human dendritic cells yield comparable efficiency of transfection.

Among the many promising cancer immunotherapeutic strategies, dendritic cells (DC) have become of particular interest. This study aims to optimize a clinical grade protocol for culture and transfection of human DC. Monocytes and CD34(+) hematopoietic stem cells (HSC) from same donor were differentiated under serum-free conditions and analyzed for their susceptibility to several recently described nonviral transfection methods as compared with established virally mediated gene transfer. Nonviral gene transfer methods studied were square-wave electroporation, lipofection, and particle-mediated transfer of plasmid DNA or in vitro transcribed mRNA. We conclude that DNA is not suitable for transduction of DC using nonviral methods. In contrast, mRNA and square-wave electroporation reproducibly yields 60% and 50% transfected monocyte- and CD34(+)-derived DC, respectively, measured at protein level, without affecting the cell viability. Thus, the transfection efficiency of this method is comparable with the 40-90% transgene expression obtained using retroviral (RV) or adenoviral (AdV) vectors in CD34(+)- and monocyte-derived DC, respectively. In monocyte-derived DC, however, the amount of protein expressed per-cell basis was higher after AdV (MOI = 1000) compared with mRNA electroporation-mediated transfer. This is the first study directly demonstrating side-by-side that mRNA electroporation into DC of different origin indeed results in a comparable number of transduced cells as when using virus-mediated gene transfer.

mRNA-based electrotransfection of human dendritic cells and induction of cytotoxic T lymphocyte responses against the telomerase catalytic subunit (hTERT).

Dendritic cells (DCs) are recognised as the most potent antigen-presenting cells for induction of cellular immune responses, and vaccination with DCs pulsed with antigens has emerged as a promising strategy for generating protective immunity in mammals. We have developed a transfection method that uses in vitro synthesised mRNA and square-wave electroporation for transient expression in DCs and other cell types. The method is highly efficient and produces almost complete transfection of cells in culture. When using mRNA encoding the enhanced green fluorescence protein (EGFP), highest expression in DCs occurred on the second day after transfection and produced a 76-fold increase in mean fluorescence above background. High levels of expression were maintained for at least 5 days post-transfection. In comparison, square-wave electroporation of DCs with EGFP plasmid DNA yielded 15% transfected cells and a 28-fold increase of mean fluorescence. DCs transfected with mRNA encoding the telomerase catalytic subunit (hTERT) acquired strong telomerase activity and were capable of eliciting a hTERT-specific cytotoxic T lymphocyte (CTL) response in vitro.