Inhibition of invariant chain expression in dendritic cells presenting endogenous antigens stimulates CD4+ T-cell responses and tumor immunity

Cell-based and cell-free immunotherapies for glioblastoma: current status and future directions

Glioblastoma (GBM) is among the most fatal and recurring malignant solid tumors. It arises from the GBM stem cell population. Conventional neurosurgical resection, temozolomide (TMZ)-dependent chemotherapy and radiotherapy have rendered the prognosis of patients unsatisfactory. Radiotherapy and chemotherapy can frequently induce non-specific damage to healthy brain and other tissues, which can be extremely hazardous. There is therefore a pressing need for a more effective treatment strategy for GBM to complement or replace existing treatment options. Cell-based and cell-free immunotherapies are currently being investigated to develop new treatment modalities against cancer. These treatments have the potential to be both selective and successful in minimizing off-target collateral harm in the normal brain. In this review, several aspects of cell-based and cell-free immunotherapies related to GBM will be discussed.

Challenges and exploration for immunotherapies targeting cold colorectal cancer

In recent years, immune checkpoint inhibitors (ICIs) have made significant breakthroughs in the treatment of various tumors, greatly improving clinical efficacy. As the fifth most common antitumor treatment strategy for patients with solid tumors after surgery, chemotherapy, radiotherapy and targeted therapy, the therapeutic response to ICIs largely depends on the number and spatial distribution of effector T cells that can effectively identify and kill tumor cells, features that are also important when distinguishing malignant tumors from “cold tumors” or “hot tumors”. At present, only a small proportion of colorectal cancer (CRC) patients with deficient mismatch repair (dMMR) or who are microsatellite instability-high (MSI-H) can benefit from ICI treatments because these patients have the characteristics of a “hot tumor”, with a high tumor mutational burden (TMB) and massive immune cell infiltration, making the tumor more easily recognized by the immune system. In contrast, a majority of CRC patients with proficient MMR (pMMR) or who are microsatellite stable (MSS) have a low TMB, lack immune cell infiltration, and have almost no response to immune monotherapy; thus, these tumors are “cold”. The greatest challenge today is how to improve the immunotherapy response of “cold tumor” patients. With the development of clinical research, immunotherapies combined with other treatment strategies (such as targeted therapy, chemotherapy, and radiotherapy) have now become potentially effective clinical strategies and research hotspots. Therefore, the question of how to promote the transformation of “cold tumors” to “hot tumors” and break through the bottleneck of immunotherapy for cold tumors in CRC patients urgently requires consideration. Only by developing an in-depth understanding of the immunotherapy mechanisms of cold CRCs can we screen out the immunotherapy-dominant groups and explore the most suitable treatment options for individuals to improve therapeutic efficacy.

The Quest for mRNA Vaccines

The success of mRNA vaccines against COVID-19 is nothing short of a medical revolution. Given its chemical lability the use of mRNA as a therapeutic has been counterintuitive and met with skepticism. The development of mRNA-based COVID-19 vaccines was the culmination of long and painstaking efforts by many investigators spanning over 30 years and culminating with the seminal studies of Kariko and Weissman. This review will describe one chapter in this saga, studies that have shown that mRNA can function as a therapeutic. It started with our seminal observation that dendritic cells (DCs) transfected with mRNA in vitro administered to mice inhibits tumor growth, and led to first-in-human clinical trials with mRNA vaccines in cancer patients. The clinical development of this patient-specific DCs-mRNA approach and use on a larger scale was hindered by the challenges associated with personalized cell therapies. Confirmed and extended by many investigators, these studies did serve as impetus and motivation that led scientists to persevere, eventually leading to the development of simple, broadly applicable, and highly effective protocols of directly injecting mRNA into patients, culminating in the COVID-19 mRNA vaccines.

Tumor-targeted silencing of the peptide transporter TAP induces potent antitumor immunity

Neoantigen burden is a major determinant of tumor immunogenicity, underscored by recent clinical experience with checkpoint blockade therapy. Yet the majority of patients do not express, or express too few, neoantigens, and hence are less responsive to immune therapy. Here we describe an approach whereby a common set of new antigens are induced in tumor cells in situ by transient downregulation of the transporter associated with antigen processing (TAP). Administration of TAP siRNA conjugated to a broad-range tumor-targeting nucleolin aptamer inhibited tumor growth in multiple tumor models without measurable toxicity, was comparatively effective to vaccination against prototypic mutation-generated neoantigens, potentiated the antitumor effect of PD-1 antibody or Flt3 ligand, and induced the presentation of a TAP-independent peptide in human tumor cells. Treatment with the chemically-synthesized nucleolin aptamer-TAP siRNA conjugate represents a broadly-applicable approach to increase the antigenicity of tumor lesions and thereby enhance the effectiveness of immune potentiating therapies.

The Emerging Role of In Vitro-Transcribed mRNA in Adoptive T Cell Immunotherapy

Adoptive T cell therapy is a form of cellular therapy that utilizes human immune cells, often empowered by the expression of recombinant proteins, to attack selected targets present on tumor or infected cells. T cell-based immunotherapy has been progressing over the past several decades, and reached a milestone with the recent US Food and Drug Administration (FDA) approval of chimeric antigen receptor T cell therapy for relapsed and refractory leukemia and lymphoma. Although most studies have used viral vectors, a growing number of researchers have come to appreciate in vitro-transcribed (IVT) mRNA for the development, testing, and application of T cell-based immunotherapeutics. IVT mRNA offers inherent safety features, highly efficient recombinant protein translation, and the ability to control pharmacokinetic properties of the therapy. In this review, we discuss the history of IVT mRNA in adoptive T cell therapy, from tumor-infiltrating lymphocytes and T cell receptor-based therapies to chimeric antigen receptor therapy and gene-editing techniques, as well as prior and ongoing clinical trials.

Novel T cells with improved in vivo anti-tumor activity generated by RNA electroporation

The generation of T cells with maximal anti-tumor activities will significantly impact the field of T-cell-based adoptive immunotherapy. In this report, we found that OKT3/IL-2-stimulated T cells were phenotypically more heterogeneous, with enhanced anti-tumor activity in vitro and when locally administered in a solid tumor mouse model. To further improve the OKT3/IL-2-based T cell manufacturing procedure, we developed a novel T cell stimulation and expansion method in which peripheral blood mononuclear cells were electroporated with mRNA encoding a chimeric membrane protein consisting of a single-chain variable fragment against CD3 and the intracellular domains of CD28 and 4-1BB (OKT3-28BB). The expanded T cells were phenotypically and functionally similar to T cells expanded by OKT3/IL-2. Moreover, co-electroporation of CD86 and 4-1BBL could further change the phenotype and enhance the in vivo anti-tumor activity. Although T cells expanded by the co-electroporation of OKT3-28BB with CD86 and 4-1BBL showed an increased central memory phenotype, the T cells still maintained tumor lytic activities as potent as those of OKT3/IL-2 or OKT3-28BB-stimulated T cells. In different tumor mouse models, T cells expanded by OKT3-28BB RNA electroporation showed anti-tumor activities superior to those of OKT3/IL-2 T cells. Hence, T cells with both a less differentiated phenotype and potent tumor killing ability can be generated by RNA electroporation, and this T cell manufacturing procedure can be further optimized by simply co-delivering other splices of RNA, thus providing a simple and cost-effective method for generating high-quality T cells for adoptive immunotherapy.

Improved anti-leukemia activities of adoptively transferred T cells expressing bispecific T-cell engager in mice

Despite the impressive clinical efficacy of T cells engineered to express chimeric antigen receptors (CAR-Ts), the current applications of CAR-T cell therapy are limited by major treatment-related toxicity. Thus, safer yet effective alternative approaches must be developed. In this study, we compared CD19 bispecific T-cell engager (BiTE)-transferred T cells that had been transfected by RNA electroporation with CD19 CAR RNA-transferred T cells both in vitro and in an aggressive Nalm6 leukemia mouse model. BiTEs were secreted from the transferred T cells and enabled both the transferred and bystander T cells to specifically recognize CD19⁺ cell lines, with increased tumor killing ability, prolonged functional persistence, increased cytokine production and potent proliferation compared with the CAR-T cells. More interestingly, in comparison with CD3/CD28 bead-stimulated T cells, T cells that were expanded by a rapid T-cell expansion protocol (REP) showed enhanced anti-tumor activities for both CAR and BiTE RNA-electroporated T cells both in vitro and in a Nalm6 mouse model (P<0.01). Furthermore, the REP T cells with BiTE RNAs showed greater efficacy in the Nalm6 leukemia model compared with REP T cells with CAR RNA (P<0.05) and resulted in complete leukemia remission.

Personalized approaches to active immunotherapy in cancer

Immunotherapy is emerging as a promising anti-cancer curative modality. However, in contrast to recent advances obtained employing checkpoint blockade agents and T cell therapies, clinical efficacy of therapeutic cancer vaccines is still limited. Most vaccination attempts in the clinic represent "off-the shelf" approaches since they target common "self" tumor antigens, shared among different patients. In contrast, personalized approaches of vaccination are tailor-made for each patient and in spite being laborious, hold great potential. Recent technical advancement enabled the first steps in the clinic of personalized vaccines that target patient-specific mutated neo-antigens. Such vaccines could induce enhanced tumor-specific immune response since neo-antigens are mutation-derived antigens that can be recognized by high affinity T cells, not limited by central tolerance. Alternatively, the use of personalized vaccines based on whole autologous tumor cells, overcome the need for the identification of specific tumor antigens. Whole autologous tumor cells could be administered alone, pulsed on dendritic cells as lysate, DNA, RNA or delivered to dendritic cells in-vivo through encapsulation in nanoparticle vehicles. Such vaccines may provide a source for the full repertoire of the patient-specific tumor antigens, including its private neo-antigens. Furthermore, combining next-generation personalized vaccination with other immunotherapy modalities might be the key for achieving significant therapeutic outcome.

Programming human dendritic cells with mRNA

Transfecting with in vitro transcribed, protein-encoding mRNA is a simple yet effective method to express high levels of the desired RNA-encoded proteins in primary cells. Cells can be transfected with antigen-encoding mRNA, which is translated into protein and is processed by the cellular antigen-processing pathway to generate antigen-presenting cells. Another elegant and increasingly popular application is to transfect cells with mRNA that encodes immune modulating molecules (cytokines, chemokines, toll-like receptors (TLRs), immune receptor ligands, immune receptor targeting antibodies) which, when translated into protein, can program cell behavior and/or function. In this chapter we describe an efficient method to deliver mRNA into human dendritic cells (DCs) by electroporation. This is currently the method of choice to deliver mRNA into antigen-presenting cells for generating vaccines for cancer immunotherapy.

Alternative Ii-independent antigen-processing pathway in leukemic blasts involves TAP-dependent peptide loading of HLA class II complexes

During HLA class II synthesis in antigen-presenting cells, the invariant chain (Ii) not only stabilizes HLA class II complexes in the endoplasmic reticulum, but also mediates their transport to specialized lysosomal antigen-loading compartments termed MIICs. This study explores an alternative HLA class II presentation pathway in leukemic blasts that involves proteasome and transporter associated with antigen processing (TAP)-dependent peptide loading. Although HLA-DR did associate with Ii, Ii silencing in the human class II-associated invariant chain peptide (CLIP)-negative KG-1 myeloid leukemic cell line did not affect total and plasma membrane expression levels of HLA-DR, as determined by western blotting and flow cytometry. Since HLA-DR expression does require peptide binding, we examined the role of endogenous antigen-processing machinery in HLA-DR presentation by CLIP(-) leukemic blasts. The suppression of proteasome and TAP function using various inhibitors resulted in decreased HLA-DR levels in both CLIP(-) KG-1 and ME-1 blasts. Simultaneous inhibition of TAP and Ii completely down-modulated the expression of HLA-DR, demonstrating that together these molecules form the key mediators of HLA class II antigen presentation in leukemic blasts. By the use of a proteasome- and TAP-dependent pathway for HLA class II antigen presentation, CLIP(-) leukemic blasts might be able to present a broad range of endogenous leukemia-associated peptides via HLA class II to activate leukemia-specific CD4(+) T cells.

Selective modification of antigen-specific T cells by RNA electroporation

It has been observed that the efficient transfection of T cells by RNA electroporation requires prior activation of T cells with mitogens or by anti-CD3 antibody stimulation. We hypothesized that this requirement for T cell activation could be leveraged to express marker genes within activated T cells responding to antigen-pulsed dendritic cells and allow for the selective enrichment and modification of antigen-specific T cells. Using electroporation of mRNA encoding green fluorescent protein as a marker gene, we demonstrate that RNA electroporation can efficiently allow for the separation of cytomegalovirus-specific CD8+ and CD4+ T cells from bulk culture responding to cytomegalovirus pp65 antigen-pulsed dendritic cells. Furthermore, we demonstrate that cytomegalovirus-specific T cells can be functionally modified by RNA transfection of the C-X-C chemokine receptor, CXCR2, to migrate efficiently toward a variety of CXCR2-specific chemokines in vitro and in vivo. These studies demonstrate the utility of RNA transfection as a simple method by which to purify and selectively modify the function of antigen-specific T cells for use in adoptive immunotherapy, and importantly provide evidence that transient expression of proteins by RNA transfection is an efficient means of modulating the in vivo function of activated T cells.

[mRNA-transfected dendritic cells: a promising strategy in immunotherapy]

Dendritic cells play a central role in the initiation of the immune response as they are the only antigen-presenting cells able to prime naive T cells. This makes the dendritic cells the vector of choice to use as a cell-based vaccine in immunotherapy. Although there are several strategies to deliver antigen to dendritic cells, the ones transfected with mRNA coding for tumor or viral antigens are able to induce potent antigen specific T-cell responses directed against multiple epitopes. In this review, we report several advances made in the field of anti-tumoral and anti-HIV immunotherapy using mRNA-transfected dendritic cells-based approaches.

NK cells negatively regulate antigen presentation and tumor-specific CTLs in a syngeneic lymphoma model

NK cells are known to kill tumor cells and produce proinflammatory cytokines that lead to the generation of tumor-specific CTLs. Many studies have used MHC class I-deficient tumor cells and/or adjuvants that induce NK cell responses. In this study, the focus was on less-immunogenic lymphoma cells that express MHC class I as a model to study NK cell responses to tumors that do not directly stimulate NK cell activation. When RMA tumor cells that expressed a truncated version of OVA, or RMA cells alone, were injected into mice that were depleted of NK cells, the mice developed an increased number of tumor-specific CTLs, increased IFN-gamma responses, and a higher amount of Ag presentation in draining LNs compared with mice with intact NK cells. These data suggest that NK cells can inhibit the development of effective adaptive immunity in the absence of signals that trigger NK cell activation.

Presentation of tumor antigens by dendritic cells genetically modified with viral and nonviral vectors

Genetic modification of dendritic cells (DCs) with recombinant vectors encoding tumor antigens may aid in developing new immunotherapeutic treatments for patients with cancer. Here, we characterized antigen presentation by human DCs genetically modified with plasmid cDNAs, RNAs, adenoviruses, or retroviruses, encoding the melanoma antigen gp100 or the tumor-testis antigen NY-ESO-1. Monocyte-derived DCs were electroporated with cDNAs or RNAs, or transduced with adenoviruses. CD34+ hematopoietic stem cell-derived DCs were used for retroviral transduction. Genetically modified DCs were coincubated with CD8+ and CD4+ T cells that recognized major histocompatibility complex class I- and class II-restricted epitopes from gp100 and NY-ESO-1, and specific recognition was evaluated by interferongamma secretion. Cytokine release by both CD8+ and CD4+ T cells was consistently higher in response to DCs modified with adenoviruses than cDNAs or RNAs, and maturation of DCs after genetic modification did not consistently alter patterns of recognition. Also, retrovirally transduced DCs encoding gp100 were well recognized by both CD8+ and CD4+ T cells. These data suggest that DCs transduced with viral vectors may be more efficient than DCs transfected with cDNAs or RNAs for the induction of tumor reactive CD8+ and CD4+ T cells in vitro and in human vaccination trials.

Suppression of major histocompatibility complex class II-associated invariant chain enhances the potency of an HIV gp120 DNA vaccine

Summary One function of the major histocompatibility complex (MHC) class II-associated invariant chain (Ii) is to prevent MHC class II molecules from binding endogenously generated antigenic epitopes. Ii inhibition leads to MHC class II presentation of endogenous antigens by APC without interrupting MHC class I presentation. We present data that in vivo immunization of BALB/c mice with HIV gp120 cDNA plus an Ii suppressive construct significantly enhances the activation of both gp120-specific T helper (Th) cells and cytotoxic T lymphocytes (CTL). Our results support the concept that MHC class II-positive/Ii-negative (class II(+)/Ii(-)) antigen-presenting cells (APC) present endogenously synthesized vaccine antigens simultaneously by MHC class II and class I molecules, activating both CD4(+) and CD8(+) T cells. Activated CD4(+) T cells locally strengthen the response of CD8(+) CTL, thus enhancing the potency of a DNA vaccine.

Presentation of tumor antigens by dendritic cells genetically modified with viral and nonviral vectors

Genetic modification of dendritic cells (DCs) with recombinant vectors encoding tumor antigens may aid in developing new immunotherapeutic treatments for patients with cancer. Here, we characterized antigen presentation by human DCs genetically modified with plasmid cDNAs, RNAs, adenoviruses, or retroviruses, encoding the melanoma antigen gp100 or the tumor-testis antigen NY-ESO-1. Monocyte-derived DCs were electroporated with cDNAs or RNAs, or transduced with adenoviruses. CD34+ hematopoietic stem cell-derived DCs were used for retroviral transduction. Genetically modified DCs were coincubated with CD8+ and CD4+ T cells that recognized major histocompatibility complex class I- and class II-restricted epitopes from gp100 and NY-ESO-1, and specific recognition was evaluated by interferongamma secretion. Cytokine release by both CD8+ and CD4+ T cells was consistently higher in response to DCs modified with adenoviruses than cDNAs or RNAs, and maturation of DCs after genetic modification did not consistently alter patterns of recognition. Also, retrovirally transduced DCs encoding gp100 were well recognized by both CD8+ and CD4+ T cells. These data suggest that DCs transduced with viral vectors may be more efficient than DCs transfected with cDNAs or RNAs for the induction of tumor reactive CD8+ and CD4+ T cells in vitro and in human vaccination trials.

Persistent antigenic stimulation alters the transcription program in T cells, resulting in antigen-specific tolerance

Repetitive antigen stimulation induces peripheral T cell tolerance in vivo. It is not known, however, whether multiple stimulations merely suppress T cell activation or, alternatively, change the transcriptional program to a distinct, tolerant state. In this study, we have discovered that STAT3 and STAT5 were activated in response to antigen stimulation in vivo, in marked contrast to the suppression of AP-1, NF-kappaB and NFAT. In addition, a number of transcription factors were induced in tolerant T cells following antigen challenge in vivo, including T-bet, Irf-1 and Egr-2. The altered transcription program in tolerant cells associates closely with the suppression of cell cycle progression and IL-2 production, as well as with the induction of IL-10. Studies of T-bet and Egr-2 show that the function of T-bet in peptide treatment-induced regulatory T cells is not associated with Th1 differentiation, but correlates with the suppression of IL-2, whereas expression of Egr-2 led to an up-regulation of the cell cycle inhibitors p21(cip1) and p27(kip). Our results demonstrate a balanced transcription program regulated by different transcription factors for T cell activation and/or tolerance during antigen-induced T cell responses. Persistent antigen stimulation can induce T cell tolerance by changing the balance of transcription factors.

Nonviral delivery of cancer genetic vaccines

The potential use of genetic vaccines to address numerous diseases including cancer is promising, but currently unrealized. Here, we review advances in the nonviral delivery of antigen-encoded plasmid DNA for the purpose of treating cancer through the human immune system, as this disease has drawn the most attention in this field to date. Brief overviews of dendritic cell immunobiology and the mechanism of immune activation through genetic vaccines set the stage for the desirability of delivery technology. Several promising nonviral delivery techniques are discussed along with a mention of targeting strategies aimed at improving the potency of vaccine formulations. Implications for the future of genetic vaccines are also presented.

Tumor immunotherapy targeting fibroblast activation protein, a product expressed in tumor-associated fibroblasts

Murine studies have shown that immunologic targeting of the tumor vasculature, a key element of the tumor stroma, can lead to protective immunity in the absence of significant pathology. In the current study, we expand the scope of stroma-targeted immunotherapy to antigens expressed in tumor-associated fibroblasts, the predominant component of the stroma in most types of cancer. Mice were immunized against fibroblast activation protein (FAP), a product up-regulated in tumor-associated fibroblasts, using dendritic cells transfected with FAP mRNA. Using melanoma, carcinoma, and lymphoma models, we show that tumor growth was inhibited in tumor-bearing mice vaccinated against FAP and that the magnitude of the antitumor response was comparable to that of vaccination against tumor cell-expressed antigens. Both s.c. implanted tumors and lung metastases were susceptible to anti-FAP immunotherapy. The antitumor response could be further enhanced by augmenting the CD4+ T-cell arm of the anti-FAP immune response, achieved by using a lysosomal targeting sequence to redirect the translated FAP product into the class II presentation pathway, or by covaccination against FAP and a tumor cell-expressed antigen, tyrosinase-related protein 2. No morbidity or mortality was associated with anti-FAP vaccination except for a small delay in wound healing. The study suggests that FAP, a product which is preferentially expressed in tumor-associated fibroblasts, could function as a tumor rejection antigen in a broad range of cancers.

Generation of antigen-presenting cells using cultured dendritic cells and amplified autologous tumor mRNA

Novel antigen-presenting cells (APCs) were generated using cultured dendritic cells (DCs) and amplified tumor mRNA, and the potential of tumor antigen-reactive T cell induction by the tumor RNA-introduced DCs (DC/tumor RNA) was analyzed in a patient with melanoma antigen-encoding gene (MAGE3)-positive malignant melanoma of the esophagus. DCs were generated from an adherent fraction of peripheral blood mononuclear cells in the presence of granulocyte macrophage colony-stimulating factor and interleukin-4. Tumor mRNA was purified from tumor tissue, amplified in vitro using a T7 RNA polymerase system, and then introduced into DCs by electroporation (150 V/150 microF or 100 V/200 microF). The gene introduction efficiency was 44-55% as measured by enhanced green fluorescent protein reporter gene expression, and the viability of RNA-introduced DCs was approximately 80%. DC/tumor RNA could induce tumor antigen-reactive cytotoxic T lymphocytes (CTLs) in an mRNA-specific manner, but had no effect on the self-antigen-reactive T cells. DC/tumor RNA could induce the polyspecific antigen-reactive CTL responses mediated by both human leukocyte antigen class I and class II molecules, whereas MAGE3 peptide-pulsed DCs induced only the monospecific MAGE3-reactive CTL responses mediated by human leukocyte antigen class I molecules, showing the superiority of the DC/tumor RNA over the DC/peptide. It is suggested that the use of DC/tumor RNA as antigen-presenting cells may be more effective, convenient and practical for the DC-based anti-cancer immunotherapy.

Loss of major histocompatibility class II expression in non-immune-privileged site diffuse large B-cell lymphoma is highly coordinated and not due to chromosomal deletions

Decreased major histocompatibility class II (MHCII) expression is associated with poor survival in diffuse large B-cell lymphoma (DLBCL). Immune-privileged site DLBCL (IP-DLBCL) patients reportedly have frequent large deletions at the MHCII locus whereas the mechanism of decreased expression in non-IP-DLBCL is unknown. Gene expression profiling data were used for correlation analyses between expression levels of MHCII genes with each other and their transcriptional regulator, CIITA. Comparative genomic hybridization (CGH) assessed chromosomal alterations at MHCII-related loci. Finally, a map was created of expression of genes that are telomeric, within, or centromeric to the MHCII locus. Correlation coefficients among MHCII genes ranged from 0.73 to 0.92, whereas those between adjacent and intervening genes were lower (-0.12 to 0.49). Correlations between MHCII and CIITA expression were higher (0.53 to 0.60) than between CIITA and neighboring genes (-0.05 to 0.22). In 23 MHCII(-) cases, CGH detected 2 losses and 2 gains at MHCII loci. Expression of genes telomeric, within, and centromeric to MHCII loci were near normal in most MHCII(-) cases. Large deletions of the MHCII locus are uncommon in non-IP-DLBCL, implicating altered transcription as the operative mechanism for decreased expression.

High-efficiency transfection of primary human and mouse T lymphocytes using RNA electroporation

The use of nonviral gene transfer methods in primary lymphocytes has been hampered by low gene transfer efficiency and high transfection-related toxicity. In this report, high gene transfection efficiency with low transfection-related toxicity was achieved by electroporation using in vitro-transcribed mRNA. Using these methods, >90% transgene expression with >80% viable cells was observed in stimulated primary human and murine T lymphocytes transfected with GFP or mCD62L. Electroporation of unstimulated human PBMCs or murine splenocytes with GFP RNA yielded 95 and 56% GFP+ cells, respectively. Electroporation of mRNA for NY-ESO-1, MART-1, and p53 antigen-specific TCRs into human T lymphocytes redirected these lymphocytes to recognize melanoma cell lines in an MHC-restricted manner. The onset of gene expression was rapid (within 30 min) and durable (up to 7 days postelectroporation) using both GFP and TCR-mediated recognition of target cells. There was no adverse effect observed on the T lymphocytes subjected to RNA electroporation evaluated by cell growth rate, annexin-V staining of apoptotic cells, BrdU incorporation, tumor antigen-specific recognition or antigen-specific TCR affinity. The results of this study indicate that mRNA electroporation provides a powerful tool to introduce genes into both human and murine primary T lymphocytes.

Direct Cross-Priming by Th Lymphocytes Generates Memory Cytotoxic T Cell Responses

Under optimal Ag stimulation, CTL become functional effector and memory T cells. Professional APCs (pAPC) are considered essential for the activation of CTL, due to their unique capacity to provide costimulation and present exogenous Ags through MHC class I molecules. In this study, we report a novel means by which Th lymphocytes acquire and present MHC class I determinants to naive CTL. Although previous studies have looked at T cell Ag presentation to activated T cells, this study presents the first example of Ag presentation by Th cells to naive CTL. We report that activated Th cells can function as effective pAPC for CTL. Our results show that: 1) In addition to acquisition of cell surface molecules, including MHC class I/peptide complexes, from pAPC, Th cells can acquire and present MHC class I-binding peptides through TCR-MHC class II interactions with pAPC; 2) the acquired Ag can be functionally presented to CTL; and 3) Ag presentation by Th cells induces naive CTL to proliferate and preferentially differentiate into cells that phenotypically and functionally resemble central memory T cells. These findings suggest a novel role of Th cells as pAPC for the development of memory immune responses.

Cancer immunotherapy with mRNA-transfected dendritic cells

Bone marrow-derived dendritic cells (DCs) are the most potent antigen-presenting cells capable of activating naïve T cells. Loading DCs ex vivo with tumor antigens can stimulate potent antitumor immunity in tumor-bearing mice. This review describes the use of mRNA-encoded tumor antigens as a form of antigen loaded onto DCs, including our early experience from clinical trials in urological cancers. Transfection of DCs with mRNA is simple and effective. Comparative studies suggest that mRNA transfection is superior to other antigen-loading techniques in generating immunopotent DCs. The ability to amplify RNA from microscopic amounts of tumor tissue extends the use of DC vaccination to virtually every cancer patient. The striking observation from two phase I clinical trials, in patients with prostate cancer immunized with prostate-specific antigen mRNA-transfected DCs and patients with renal cancer immunized with autologous tumor RNA-transfected DCs, was that the majority of patients exhibited a vaccine-induced T-cell response. Suggestive evidence of clinically related responses was seen in both the trials. Immunization with mRNA-transfected DCs is a promising strategy to stimulate potent antitumor immunity and could serve as a foundation for developing effective treatments for cancer.

Manipulating dendritic cell biology for the active immunotherapy of cancer

Dendritic cells (DCs) are specialized antigen-presenting cells (APCs) that have an unequaled capacity to initiate primary immune responses, including tolerogenic responses. Because of the importance of DCs in the induction and control of immunity, an understanding of their biology is central to the development of potent immunotherapies for cancer, chronic infections, autoimmune disease, and induction of transplantation tolerance. This review discusses recent advances in DC research and the application of this knowledge toward new strategies for the clinical manipulation of DCs for cancer immunotherapy.

Dendritic Cells

A recent workshop on "Dendritic Cells: Biology and Therapeutic Applications," sponsored by the Juan March Foundation, brought together basic and clinical research scientists to discuss the mechanisms underlying the control of immune responses and tolerance by dendritic cells (DCs), as well as recent research in cancer immunotherapy based on DC vaccination. Particular emphasis was placed on antigen processing and presentation by DCs, C-type lectin antigen receptors, DC maturation and polarization of T cell responses, the control of immunity versus tolerance by DCs, the developmental origin of DCs, and the use of DCs in cancer immunotherapy.