Phase I study of intranodal delivery of a plasmid DNA vaccine for patients with Stage IV melanoma

Analysis of physical and biological delivery systems for DNA cancer vaccines and their translation to clinical development

DNA cancer vaccines as an approach in tumor immunotherapy are still being investigated in preclinical and clinical settings. Nevertheless, only a small number of clinical studies have been published so far and are still active. The investigated vaccines show a relatively stable expression in in-vitro transfected cells and may be favorable for developing an immunologic memory in patients. Therefore, DNA vaccines could be suitable as a prophylactic or therapeutic approach against cancer. Due to the low efficiency of these vaccines, the administration technique plays an important role in the vaccine design and its efficacy. These DNA cancer vaccine delivery systems include physical, biological, and non-biological techniques. Although the pre-clinical studies show promising results in the application of the different delivery systems, further studies in clinical trials have not yet been successfully proven.

Lymph node targeting for immunotherapy

Immunotherapy that aims to boost the body's immune responses against pathogens or diseased cells has achieved significant progress for treating different diseases over the past several decades, especially with the success of checkpoint blockades, chimeric antigen receptor T therapy, and cancer vaccines in clinical cancer treatment. Effective immunotherapy necessitates the generation of potent and persistent humoral and T-cell responses, which lies in the ability of modulating and guiding antigen-presenting cells to prime antigen-specific T and B cells in the lymphoid tissues, notably in the lymph nodes proximal to the disease site. To this end, various types of strategies have been developed to facilitate the delivery of immunomodulatory agents to immune cells (e.g. dendritic cells and T cells) in the lymph nodes. Among them, intranodal injection enables the direct exposure of immunomodulators to immune cells in lymph nodes, but is limited by the technical challenge and intrinsic invasiveness. To address, multiple passive and active lymph node-targeting technologies have been developed. In this review, we will provide an overview of different lymph node-targeting technologies developed to date, as well as the mechanism and merits of each approach.

Nanovaccine-based strategies for lymph node targeted delivery and imaging in tumor immunotherapy

Therapeutic tumor vaccines have attracted considerable attention in the past decade; they can induce tumor regression, eradicate minimal residual disease, establish lasting immune memory and avoid non-specific and adverse side effects. However, the challenge in the field of therapeutic tumor vaccines is ensuring the delivery of immune components to the lymph nodes (LNs) to activate immune cells. The clinical response rate of traditional therapeutic tumor vaccines falls short of expectations due to inadequate lymph node delivery. With the rapid development of nanotechnology, a large number of nanoplatform-based LN-targeting nanovaccines have been exploited for optimizing tumor immunotherapies. In addition, some nanovaccines possess non-invasive visualization performance, which is benefit for understanding the kinetics of nanovaccine exposure in LNs. Herein, we present the parameters of nanoplatforms, such as size, surface modification, shape, and deformability, which affect the LN-targeting functions of nanovaccines. The recent advances in nanoplatforms with different components promoting LN-targeting are also summarized. Furthermore, emerging LNs-targeting nanoplatform-mediated imaging strategies to both improve targeting performance and enhance the quality of LN imaging are discussed. Finally, we summarize the prospects and challenges of nanoplatform-based LN-targeting and /or imaging strategies, which optimize the clinical efficacy of nanovaccines in tumor immunotherapies.

Ex vivo intranodal administration of sirolimus

BACKGROUND

Immune-mediated adverse effects of current systemic immunosuppression therapy compromise long-term survival of liver transplant recipients. Our recently observed results showed that intranodal delivery of sirolimus induced interleukin (IL)-10-driven CD4+ CD25+ Foxp3+ regulatory T cells. The present report investigated the feasibility of intra-nodal delivery of sirolimus ex vivo into a human liver common bile duct lymph node.

METHODS

We used a discarded donor human liver to directly administer sirolimus into a distal common bile duct lymph node. Sirolimus was injected once using an ultrasound-guided method.

RESULTS

The porta hepatis and its lymph node along the distal common bile duct were exposed. A handheld ultrasound probe (L15-7io, Koninklijke Philips N.V.) with a layer of standoff Aquasonic 100 Ultrasound Transmission Gel (Parker Laboratories, Inc) was applied to the exposed lymph node. Using a 1.0-mL 25G hypodermic needle, 0.05 mL of sirolimus solution was injected directly into the exposed lymph node.

CONCLUSIONS

Under sonographic guidance, direct injection of sirolimus into a hepatic draining lymph node along the common bile duct is accomplished precisely and reliably. Direct administration of therapeutic agents into local lymph nodes is a viable approach for effective targeted immunotherapy.

In Silico Model Estimates the Clinical Trial Outcome of Cancer Vaccines

Over 30 years after the first cancer vaccine clinical trial (CT), scientists still search the missing link between immunogenicity and clinical responses. A predictor able to estimate the outcome of cancer vaccine CTs would greatly benefit vaccine development. Published results of 94 CTs with 64 therapeutic vaccines were collected. We found that preselection of CT subjects based on a single matching HLA allele does not increase immune response rates (IRR) compared with non-preselected CTs (median 60% vs. 57%, p = 0.4490). A representative in silico model population (MP) comprising HLA-genotyped subjects was used to retrospectively calculate in silico IRRs of CTs based on the percentage of MP-subjects having epitope(s) predicted to bind ≥ 1–4 autologous HLA allele(s). We found that in vitro measured IRRs correlated with the frequency of predicted multiple autologous allele-binding epitopes (AUC 0.63–0.79). Subgroup analysis of multi-antigen targeting vaccine CTs revealed correlation between clinical response rates (CRRs) and predicted multi-epitope IRRs when HLA threshold was ≥ 3 (r = 0.7463, p = 0.0004) but not for single HLA allele-binding epitopes (r = 0.2865, p = 0.2491). Our results suggest that CRR depends on the induction of broad T-cell responses and both IRR and CRR can be predicted when epitopes binding to multiple autologous HLAs are considered.

Nucleic Acid-Based Approaches for Tumor Therapy

Within the last decade, the introduction of checkpoint inhibitors proposed to boost the patients' anti-tumor immune response has proven the efficacy of immunotherapeutic approaches for tumor therapy. Furthermore, especially in the context of the development of biocompatible, cell type targeting nano-carriers, nucleic acid-based drugs aimed to initiate and to enhance anti-tumor responses have come of age. This review intends to provide a comprehensive overview of the current state of the therapeutic use of nucleic acids for cancer treatment on various levels, comprising (i) mRNA and DNA-based vaccines to be expressed by antigen presenting cells evoking sustained anti-tumor T cell responses, (ii) molecular adjuvants, (iii) strategies to inhibit/reprogram tumor-induced regulatory immune cells e.g., by RNA interference (RNAi), (iv) genetically tailored T cells and natural killer cells to directly recognize tumor antigens, and (v) killing of tumor cells, and reprograming of constituents of the tumor microenvironment by gene transfer and RNAi. Aside from further improvements of individual nucleic acid-based drugs, the major perspective for successful cancer therapy will be combination treatments employing conventional regimens as well as immunotherapeutics like checkpoint inhibitors and nucleic acid-based drugs, each acting on several levels to adequately counter-act tumor immune evasion.

Cell and tissue engineering in lymph nodes for cancer immunotherapy

In cancer, lymph nodes (LNs) coordinate tumor antigen presentation necessary for effective antitumor immunity, both at the levels of local cellular interactions and tissue-level organization. In this review, we examine how LNs may be engineered to improve the therapeutic outcomes of cancer immunotherapy. At the cellular scale, targeting the LNs impacts the potency of cancer vaccines, immune checkpoint blockade, and adoptive cell transfer. On a tissue level, macro-scale biomaterials mimicking LN features can function as immune niches for cell reprogramming or delivery in vivo, or be utilized in vitro to enable preclinical testing of drugs and vaccines. We additionally review strategies to induce ectopic lymphoid sites reminiscent of LNs that may improve antitumor T cell priming.

Immune therapy of melanoma: Overview of therapeutic vaccines

Melanoma is the most serious type of skin cancer which develops from the occurrence of genetic mutations in the melanocytes. Based on the features of melanoma tumors such as location, genetic profile and stage, there are several therapeutic strategies including surgery, chemotherapy, and radiotherapy. However, because of the appearance resistance mechanisms, the efficiency of these treatments strategies may be reduced. It has been demonstrated that therapeutic monoclonal antibodies can improve the efficiency of melanoma therapies. Recently, several mAbs, such as nivolumab, pembrolizumab, and ipilimumab, were approved for the immunotherapy of melanoma. The antibodies inhibit immune checkpoint receptors such as CTL4 and pd-1. Another therapeutic strategy for the treatment of melanoma is cancer vaccines, which improve clinical outcomes in patients. The combination therapy using antibodies and gene vaccine give us a new perspective in the treatment of melanoma patients. Herein, we present the recent progressions in the melanoma immunotherapy, especially dendritic cells mRNA vaccines by reviewing recent literature.

Biomaterials for vaccine-based cancer immunotherapy

The development of therapeutic cancer vaccines as a means to generate immune reactivity against tumors has been explored since the early discovery of tumor-specific antigens by Georg Klein in the 1960s. However, challenges including weak immunogenicity, systemic toxicity, and off-target effects of cancer vaccines remain as barriers to their broad clinical translation. Advances in the design and implementation of biomaterials are now enabling enhanced efficacy and reduced toxicity of cancer vaccines by controlling the presentation and release of vaccine components to immune cells and their microenvironment. Here, we discuss the rational design and clinical status of several classes of cancer vaccines (including DNA, mRNA, peptide/protein, and cell-based vaccines) along with novel biomaterial-based delivery technologies that improve their safety and efficacy. Further, strategies for designing new platforms for personalized cancer vaccines are also considered.

DNA Vaccines-How Far From Clinical Use?

Two decades ago successful transfection of antigen presenting cells (APC) in vivo was demonstrated which resulted in the induction of primary adaptive immune responses. Due to the good biocompatibility of plasmid DNA, their cost-efficient production and long shelf life, many researchers aimed to develop DNA vaccine-based immunotherapeutic strategies for treatment of infections and cancer, but also autoimmune diseases and allergies. This review aims to summarize our current knowledge on the course of action of DNA vaccines, and which factors are responsible for the poor immunogenicity in human so far. Important optimization steps that improve DNA transfection efficiency comprise the introduction of DNA-complexing nano-carriers aimed to prevent extracellular DNA degradation, enabling APC targeting, and enhanced endo/lysosomal escape of DNA. Attachment of virus-derived nuclear localization sequences facilitates nuclear entry of DNA. Improvements in DNA vaccine design include the use of APC-specific promotors for transcriptional targeting, the arrangement of multiple antigen sequences, the co-delivery of molecular adjuvants to prevent tolerance induction, and strategies to circumvent potential inhibitory effects of the vector backbone. Successful clinical use of DNA vaccines may require combined employment of all of these parameters, and combination treatment with additional drugs.

Clinical Evaluations of Toxicity and Efficacy of Nanoparticle-Mediated Gene Therapy

Considerable efforts have been devoted to develop safe and efficient gene therapies for life-threatening or inherited diseases. The choice of gene delivery vehicle plays key roles in enhancing the therapeutic effect of nucleic acid cargo. To date, gene therapy approaches involving both viral vectors and nonviral vectors have been evaluated in clinical trials. With improvements in material science and nanotechnologies, positively charged nanoparticles have emerged as potential gene delivery vehicles. In this review, we highlight clinical trials that examined cationic nanocarrier-mediated gene therapy as well as discuss both the toxicity and efficacy of nanocarrier-based therapeutics.

Trial watch: DNA-based vaccines for oncological indications

DNA-based vaccination is a promising approach to cancer immunotherapy. DNA-based vaccines specific for tumor-associated antigens (TAAs) are indeed relatively simple to produce, cost-efficient and well tolerated. However, the clinical efficacy of DNA-based vaccines for cancer therapy is considerably limited by central and peripheral tolerance. During the past decade, considerable efforts have been devoted to the development and characterization of novel DNA-based vaccines that would circumvent this obstacle. In this setting, particular attention has been dedicated to the route of administration, expression of modified TAAs, co-expression of immunostimulatory molecules, and co-delivery of immune checkpoint blockers. Here, we review preclinical and clinical progress on DNA-based vaccines for cancer therapy.

The progress and current status of immunotherapy in acute myeloid leukemia

Recently, there has been remarkable progress in basic and preclinical studies of acute myeloid leukemia (AML). The improved outcomes of AML can largely be attributed to advances in supportive care and hematopoietic cell transplantation as opposed to conventional chemotherapy. However, as the 5-year survival rate remains low due to a high incidence of relapse, novel and effective treatments are urgently needed. Increasing attention is focusing on identifying suitable immunotherapeutic strategies for AML. Here, we describe the immunological features, mechanisms of immune escape, and recent progress in immunotherapy for AML. Problems encountered in the clinic will also be discussed. Although current outcomes may be limited, ongoing preclinical or clinical efforts are aimed at improving immunotherapy modalities and designing novel therapies, such as vaccines, monoclonal antibody therapy, chimeric antibody receptor-engineered T cells (CAR-T), TCR-engineered T cells (TCR-T), and checkpoint inhibitors, which may provide promising and effective therapies with higher specificity and efficacy for AML.

mRNA vaccine delivery using lipid nanoparticles

mRNA vaccines elicit a potent immune response including antibodies and cytotoxic T cells. mRNA vaccines are currently evaluated in clinical trials for cancer immunotherapy applications, but also have great potential as prophylactic vaccines. Efficient delivery of mRNA vaccines will be key for their success and translation to the clinic. Among potential nonviral vectors, lipid nanoparticles are particularly promising. Indeed, lipid nanoparticles can be synthesized with relative ease in a scalable manner, protect the mRNA against degradation, facilitate endosomal escape, can be targeted to the desired cell type by surface decoration with ligands, and as needed, can be codelivered with adjuvants.

Fusion of the dendritic cell-targeting chemokine MIP3α to melanoma antigen Gp100 in a therapeutic DNA vaccine significantly enhances immunogenicity and survival in a mouse melanoma model

Background

Although therapeutic cancer vaccines have been mostly disappointing in the clinic, the advent of novel immunotherapies and the future promise of neoantigen-based therapies have created the need for new vaccine modalities that can easily adapt to current and future developments in cancer immunotherapy. One such novel platform is a DNA vaccine fusing the chemokine Macrophage Inflammatory Protein-3α (MIP-3α) to an antigen, here melanoma antigen gp100. Previous published work has indicated that MIP-3α targets nascent peptides to immature dendritic cells, leading to processing by class I and II MHC pathways. This platform has shown enhanced efficacy in prophylactic melanoma and therapeutic lymphoma model systems.

Methods

The B16F10 melanoma syngeneic mouse model system was utilized, with a standard therapeutic protocol: challenge with lethal dose of B16F10 cells (5 × 10⁴) on day 0 and then vaccinate by intramuscular electroporation with 50 μg plasmid on days three, 10, and 17. Efficacy was assessed by analysis of tumor burden, tumor growth, and mouse survival, using the statistical tests ANOVA, mixed effects regression, and log-rank, respectively. Immunogenicity was assessed by ELISA and flow cytometric methods, including intracellular cytokine staining to assess vaccine-specific T-cell responses, all tested by ANOVA.

Results

We demonstrate that the addition of MIP3α to gp100 significantly enhances systemic anti-gp100 immunological parameters. Further, chemokine-fusion vaccine therapy significantly reduces tumor burden, slows tumor growth, and enhances mouse overall survival compared to antigen-only, irrelevant-antigen, and mock vaccines, with efficacy mediated by both CD4+ and CD8+ effector T cells. Antigen-only, irrelevant-antigen, and chemokine-fusion vaccines elicit significantly higher and similar CD4+ and CD8+ tumor-infiltrating lymphocyte (TIL) levels compared to mock vaccine. However, vaccine-specific CD8+ TILs are significantly higher in the chemokine-fusion vaccine group, indicating that the critical step induced by the fusion vaccine construct is the enhancement of vaccine-specific T-cell effectors.

Conclusions

The current study shows that fusion of MIP3α to melanoma antigen gp100 enhances the immunogenicity and efficacy of a DNA vaccine in a therapeutic B16F10 mouse melanoma model. This study analyzes an adaptable and easily produced MIP3α-antigen modular vaccine platform that could lend itself to a variety of functionalities, including combination treatments and neoantigen vaccination in the pursuit of personalized cancer therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s40425-016-0189-y) contains supplementary material, which is available to authorized users.

Combinational Immunotherapy with Allo-DRibble Vaccines and Anti-OX40 Co-Stimulation Leads to Generation of Cross-Reactive Effector T Cells and Tumor Regression

It is well-known that vaccines comprising of irradiated whole tumor cells or tumor-derived heat shock proteins can generate tumor-specific immune responses. In contrast, we showed recently that vaccines composed of autophagosomes (DRibbles) derived from syngeneic sarcomas could induce cross-reactive T-cell responses and cross-protection against the tumor. This unusual property of DRibbles was related to the selective recruitment of defective ribosomal products (DRiPs) and other short-lived proteins (SLiPs) into autophagosomes via sequestosome (SQSTM1, p62) mediated association of ubiquitinated SLiPs to the autophagy gene product LC3. Here, we extend our observations to mammary carcinomas from mice of different genetic background. We demonstrated that combined of intranodal administration of autologous or allogeneic DRibbles together with anti-OX40 antibody led to robust proliferation, expansion, and differentiation of memory and effector T cells. We also showed that SLiPs is an excellent source of antigen for cross-priming of CD8⁺ T-cells that recognize shared tumor antigens in the context of host MHC class I molecules. Thus, our results provide a strong basis for novel clinical trials that combine allogeneic “off-the-shelf” DRibble vaccines together with antibodies against co-stimulatory molecules.

The Use of Gene Therapy in Cancer Research and Treatment

Gene therapy involves identifying a gene of interest and then manipulating the expression of this gene through a variety of techniques. Here we specifically address gene therapy's role in cancer research. This paper will encompass thoroughly investigated techniques such as cancer vaccines and suicide gene therapy and the latest advancements in and applications of these techniques. It will also cover newer techniques such as Antisense Oligonucleotides and small interfering RNAs and how these technologies are being developed and used. The use of gene therapy continues to expand in cancer research and has an integral role in the advancement of cancer treatment.

DNA vaccines, electroporation and their applications in cancer treatment

Numerous animal studies and recent clinical studies have shown that electroporation-delivered DNA vaccines can elicit robust Ag-specific CTL responses and reduce disease severity. However, cancer antigens are generally poorly immunogenic, requiring special conditions for immune response induction. To date, many different approaches have been used to elicit Ag-specific CTL and anti-neoplastic responses to DNA vaccines against cancer. In vivo electroporation is one example, whereas others include DNA manipulation, xenogeneic antigen use, immune stimulatory molecule and immune response regulator application, DNA prime-boost immunization strategy use and different DNA delivery methods. These strategies likely increase the immunogenicity of cancer DNA vaccines, thereby contributing to cancer eradication. However, cancer cells are heterogeneous and might become CTL-resistant. Thus, understanding the CTL resistance mechanism(s) employed by cancer cells is critical to develop counter-measures for this immune escape. In this review, the use of electroporation as a DNA delivery method, the strategies used to enhance the immune responses, the cancer antigens that have been tested, and the escape mechanism(s) used by tumor cells are discussed, with a focus on the progress of clinical trials using cancer DNA vaccines.

Intralymphatic immunotherapy

Gold Standard allergen-specific immunotherapy is associated with low efficacy because it requires either many subcutaneous injections of allergen or even more numerous sublingual allergen administrations to achieve amelioration of symptoms. Intralymphatic vaccination can maximize immunogenicity and hence efficacy. We and others have demonstrated that as few as three low dose intralymphatic allergen administrations are sufficient to effectively alleviate symptoms. Results of recent prospective and controlled trials suggest that this strategy may be an effective form of allergen immunotherapy.

Vitiligo-like depigmentation in patients with stage III-IV melanoma receiving immunotherapy and its association with survival: a systematic review and meta-analysis

PURPOSE

Vitiligo-like depigmentation in patients with melanoma may be associated with more favorable clinical outcome. We conducted a systematic review of patients with stage III to IV melanoma treated with immunotherapy to determine the cumulative incidence of vitiligo-like depigmentation and the prognostic value of vitiligo development on survival.

METHODS

We systemically searched and selected all studies on melanoma immunotherapy that reported on autoimmune toxicity and/or vitiligo between 1995 and 2013. Methodologic quality of each study was appraised using adapted criteria for systematic reviews in prognostic studies. Random-effect models were used to calculate summary estimates of the cumulative incidence of vitiligo-like depigmentation across studies. The prognostic value of vitiligo-like depigmentation on survival outcome was assessed using random-effects Cox regression survival analyses.

RESULTS

One hundred thirty-seven studies were identified comprising 139 treatment arms (11 general immune stimulation, 84 vaccine, 28 antibody-based, and 16 adoptive transfer) including a total of 5,737 patients. The overall cumulative incidence of vitiligo was 3.4% (95% CI, 2.5% to 4.5%). In 27 studies reporting individual patient data, vitiligo development was significantly associated with both progression-free-survival (hazard ratio [HR], 0.51; 95% CI, 0.32 to 0.82; P < .005) and overall survival (HR, 0.25; 95% CI, 0.10 to 0.61; P < .003), indicating that these patients have two to four times less risk of disease progression and death, respectively, compared with patients without vitiligo development.

CONCLUSION

Although vitiligo occurs only in a low percentage of patients with melanoma treated with immunotherapy, our findings suggest clear survival benefit in these patients. Awareness of vitiligo induction in patients with melanoma is important as an indicator of robust antimelanoma immunity and associated improved survival.

Trial watch: DNA vaccines for cancer therapy

The foundation of modern vaccinology dates back to the 1790s, when the English physician Edward Jenner uncovered the tremendous medical potential of prophylactic vaccination. Jenner’s work ignited a wave of nationwide vaccination campaigns abating the incidence of multiple life-threatening infectious diseases and culminating with the eradication of natural smallpox virus, which was definitively certified by the WHO in 1980. The possibility of using vaccines against cancer was first proposed at the end of the 19th century by Paul Ehrlich and William Coley. However, it was not until the 1990s that such a hypothesis began to be intensively investigated, following the realization that the immune system is not completely unresponsive to tumors and that neoplastic cells express immunogenic tumor-associated antigens (TAAs). Nowadays, anticancer vaccines are rapidly moving from the bench to the bedside, and a few prophylactic and therapeutic preparations have already been approved by FDA for use in humans. In this setting, one interesting approach is constituted by DNA vaccines, i.e., TAA-encoding circularized DNA constructs, often of bacterial origin, that are delivered to patients as such or by means of specific vectors, including (but not limited to) liposomal preparations, nanoparticles, bacteria and viruses. The administration of DNA vaccines is most often performed via the intramuscular or subcutaneous route and is expected to cause (1) the endogenous synthesis of the TAA by myocytes and/or resident antigen-presenting cells; (2) the presentation of TAA-derived peptides on the cell surface, in association with MHC class I molecules; and (3) the activation of potentially therapeutic tumor-specific immune responses. In this Trial Watch, we will summarize the results of recent clinical trials that have evaluated/are evaluating DNA vaccines as therapeutic interventions against cancer.

The ReNAissanCe of mRNA-based cancer therapy

About 25 years ago, mRNA became a tool of interest in anticancer vaccination approaches. However, due to its rapid degradation in situ, direct application of mRNA was confronted with considerable skepticism during its early use. Consequently, mRNA was for a long time mainly used for the ex vivo transfection of dendritic cells, professional antigen-presenting cells known to stimulate immunity. The interest in direct application of mRNA experienced a revival, as researchers became aware of the many advantages mRNA offers. Today, mRNA is considered to be an ideal vehicle for the induction of strong immune responses against cancer. The growing numbers of preclinical trials and as a consequence the increasing clinical application of mRNA as an off-the-shelf anticancer vaccine signifies a renaissance for transcript-based antitumor therapy. In this review, we highlight this renaissance using a timeline providing all milestones in the application of mRNA for anticancer vaccination.

Plasmid DNA vaccines against cancer: cytotoxic T-lymphocyte induction against tumor antigens

In recent years, a number of tumor vaccination strategies have been developed. Most of these rely on the identification of tumor antigens that can be recognized by the immune system. DNA vaccination represents one such approach for the induction of both humoral and cellular immune responses against tumor antigens. Studies in animal models have demonstrated the feasibility of utilizing DNA vaccination to elicit protective antitumor immune responses. However, most tumor antigens expressed by cancer cells in humans are weakly immunogenic, and therefore require the development of strategies to potentiate DNA vaccine efficacy in the clinical setting. This review focuses on recent advances in understanding of the immunology of DNA vaccines, as well as strategies used to increase DNA vaccine potency with respect to cytotoxic T-lymphocyte activity.

Preclinical and clinical development of DNA vaccines for prostate cancer

Prostate cancer is the most commonly diagnosed cancer in the United States. It is also the second leading cause of cancer-related death in men, making it one of the largest public health concerns today. Prostate cancer is an ideal disease for immunotherapies because of the generally slow progression, the dispensability of the target organ in the patient population, and the availability of several tissue-specific antigens. As such, several therapeutic vaccines have entered clinical trials, with one autologous cellular vaccine (sipuleucel-T) recently gaining Food and Drug Administration approval after demonstrating overall survival benefit in randomized phase III clinical trials. DNA-based vaccines are safe, economical, alternative "off-the-shelf" approaches that have undergone extensive evaluation in preclinical models. In fact, the first vaccine approved in the United States for the treatment of cancer was a DNA vaccine for canine melanoma. Several prostate cancer-specific DNA vaccines have been developed in the last decade and have shown promising results in early phase clinical trials. This review summarizes anticancer human DNA vaccine trials, with a focus on those conducted for prostate cancer. We conclude with an outline of special considerations important for the development and successful translation of DNA vaccines from the laboratory to the clinic.

Is cancer gene therapy an empty suit?

Gene therapy as a treatment for cancer is regarded as high in promise, but low in delivery, a deficiency that has become more obvious with ever-increasing reports of the successful correction of monogenic disorders by this approach. We review the commercial and scientific obstacles that have led to these delays and describe how they are progressively being overcome. Recent and striking successes and correspondingly increased commercial involvement suggest that gene transfer could finally become a powerful method for development of safe and effective cancer therapeutic drugs.

Highly optimized DNA vaccine targeting human telomerase reverse transcriptase stimulates potent antitumor immunity

High levels of human telomerase reverse transcriptase (hTERT) are detected in more than 85% of human cancers. Immunologic analysis supports that hTERT is a widely applicable target recognized by T cells and can be potentially studied as a broad cancer immunotherapeutic, or a unique line of defense against tumor recurrence. There remains an urgent need to develop more potent hTERT vaccines. Here, a synthetic highly optimized full-length hTERT DNA vaccine (phTERT) was designed and the induced immunity was examined in mice and non-human primates (NHP). When delivered by electroporation, phTERT elicited strong, broad hTERT-specific CD8 T-cell responses including induction of T cells expressing CD107a, IFN-γ, and TNF-α in mice. The ability of phTERT to overcome tolerance was evaluated in an NHP model, whose TERT is 96% homologous to that of hTERT. Immunized monkeys exhibited robust [average 1,834 spot forming unit (SFU)/10(6) peripheral blood mononuclear cells (PBMC)], diverse (multiple immunodominant epitopes) IFN-γ responses and antigen-specific perforin release (average 332 SFU/10(6) PBMCs), suggesting that phTERT breaks tolerance and induces potent cytotoxic responses in this human-relevant model. Moreover, in an HPV16-associated tumor model, vaccination of phTERT slows tumor growth and improves survival rate in both prophylactic and therapeutic studies. Finally, in vivo cytotoxicity assay confirmed that phTERT-induced CD8 T cells exhibited specific cytotoxic T lymphocyte (CTL) activity, capable of eliminating hTERT-pulsed target cells. These findings support that this synthetic electroporation-delivered DNA phTERT may have a role as a broad therapeutic cancer vaccine candidate.

Tumor-altered dendritic cell function: implications for anti-tumor immunity

Dendritic cells (DC) are key regulators of both innate and adaptive immunity, and the array of immunoregulatory functions exhibited by these cells is dictated by their differentiation, maturation, and activation status. Although a major role for these cells in the induction of immunity to pathogens has long been appreciated, data accumulated over the last several years has demonstrated that DC are also critical regulators of anti-tumor immune responses. However, despite the potential for stimulation of robust anti-tumor immunity by DC, tumor-altered DC function has been observed in many cancer patients and tumor-bearing animals and is often associated with tumor immune escape. Such dysfunction has significant implications for both the induction of natural anti-tumor immune responses as well as the efficacy of immunotherapeutic strategies that target endogenous DC in situ or that employ exogenous DC as part of anti-cancer immunization maneuvers. In this review, the major types of tumor-altered DC function will be described, with emphasis on recent insights into the mechanistic bases for the inhibition of DC differentiation from hematopoietic precursors, the altered programing of DC precursors to differentiate into myeloid-derived suppressor cells or tumor-associated macrophages, the suppression of DC maturation and activation, and the induction of immunoregulatory DC by tumors, tumor-derived factors, and tumor-associated cells within the milieu of the tumor microenvironment. The impact of these tumor-altered cells on the quality of the overall anti-tumor immune response will also be discussed. Finally, this review will also highlight questions concerning tumor-altered DC function that remain unanswered, and it will address factors that have limited advances in the study of this phenomenon in order to focus future research efforts in the field on identifying strategies for interfering with tumor-associated DC dysfunction and improving DC-mediated anti-tumor immunity.

Immune system targeting by biodegradable nanoparticles for cancer vaccines

The concept of therapeutic cancer vaccines is based on the activation of the immune system against tumor cells after the presentation of tumor antigens. Nanoparticles (NPs) have shown great potential as delivery systems for cancer vaccines as they potentiate the co-delivery of tumor-associated antigens and adjuvants to dendritic cells (DCs), insuring effective activation of the immune system against tumor cells. In this review, the immunological mechanisms behind cancer vaccines, including the role of DCs in the stimulation of T lymphocytes and the use of Toll-like receptor (TLR) ligands as adjuvants will be discussed. An overview of each of the three essential components of a therapeutic cancer vaccine - antigen, adjuvant and delivery system - will be provided with special emphasis on the potential of particulate delivery systems for cancer vaccines, in particular those made of biodegradable aliphatic polyesters, such as poly(lactic-co-glycolic acid) (PLGA) and poly-ε-caprolactone (PCL). Some of the factors that can influence NP uptake by DCs, including size, surface charge, surface functionalization and route of administration, will also be considered.

The role of peptide and DNA vaccines in myeloid leukemia immunotherapy

While chemotherapy and targeted therapy are successful in inducing the remission of myeloid leukemia as acute myeloid leukemia (AML) and chronic myeloid leukemia (CML), the disease remains largely incurable. This observation is likely due to the drug resistance of leukemic cells, which are responsible for disease relapse. Myeloid leukemia vaccines may most likely be beneficial for eradicating minimal residual disease after treatment with chemotherapy or targeted therapy. Several targeted immunotherapies using leukemia vaccines have been heavily investigated in clinical and preclinical trials. This review will focus on peptides and DNA vaccines in the context of myeloid leukemias, and optimal strategies for enhancing the efficacy of vaccines based on myeloid leukemia immunization are also summarized.

The dominant roles of ICAM-1-encoding gene in DNA vaccination against Japanese encephalitis virus are the activation of dendritic cells and enhancement of cellular immunity

We investigated the cellular immune responses elicited by a plasmid DNA vaccine encoding prM-E protein from the Japanese encephalitis (JE) virus (JEV) with or without various forms of intercellular adhesion molecule (ICAM)-1 gene to maximize the immune responses evoked by the JE DNA vaccine. We observed that co-immunization with the construct containing murine ICAM-1 gene (pICAM-1) resulted in a significant increase in the percentage of CD4(+)T cells, high level of JEV-specific cytotoxic T lymphocyte response, and high production of T helper 1 (Th1)-type cytokines in splenic T cells. Furthermore, the co-expression of ICAM-1 and DNA immunogens was found to be more effective in generating T cell-mediated immune responses than those induced by immunization with pJME in combination with pICAM-1. Our results suggested that ICAM-1 enhanced T cell receptor signaling and activated Th1 immune responses in the JEV model system by increasing the induction of CD4(+)Th1 cell subset and activating dendritic cells.

mRNA: From a chemical blueprint for protein production to an off-the-shelf therapeutic

Two decades ago, mRNA became the focus of research in molecular medicine and was proposed as an active pharmaceutical ingredient for the therapy of cancer. In this regard, mRNA has been mainly used for ex vivo modification of antigen-presenting cells (APCs), such as dendritic cells (DCs). This vaccination strategy has proven to be safe, well tolerated and capable of inducing tumor antigen-specific immune responses. Recently, the direct application of mRNA for in situ modification of APCs, hence immunization was shown to be feasible and at least as effective as DC-based immunization in pre-clinical models. It is believed that application of mRNA as an off-the-shelf vaccine represents an important step in the development of future cancer immunotherapeutic strategies. Here, we will discuss the use of ex vivo mRNA-modified DCs and “naked mRNA” for cancer immunotherapy focusing on parameters such as the employed DC subtype, DC activation stimulus and route of immunization. In addition, we will provide an overview on the clinical trials published so far, trying to link their outcome to the aforementioned parameters.

Engineering nano- and microparticles to tune immunity

The immune system can be a cure or cause of disease, fulfilling a protective role in attacking cancer or pathogenic microbes but also causing tissue destruction in autoimmune disorders. Thus, therapies aimed to amplify or suppress immune reactions are of great interest. However, the complex regulation of the immune system, coupled with the potential systemic side effects associated with traditional systemic drug therapies, has presented a major hurdle for the development of successful immunotherapies. Recent progress in the design of synthetic micro- and nano-particles that can target drugs, deliver imaging agents, or stimulate immune cells directly through their physical and chemical properties is leading to new approaches to deliver vaccines, promote immune responses against tumors, and suppress autoimmunity. In addition, novel strategies, such as the use of particle-laden immune cells as living targeting agents for drugs, are providing exciting new approaches for immunotherapy. This progress report describes recent advances in the design of micro- and nano-particles for immunotherapies and diagnostics.

Antigen-specific versus antigen-nonspecific immunotherapeutic approaches for human melanoma: the need for integration for optimal efficacy?

Due to its immunogenecity and evidence of immune responses resulting in tumor regression, metastatic melanoma has been the target for numerous immunotherapeutic approaches. Unfortunately, based on the clinical outcomes, even the successful induction of tumor-specific responses does not correlate with efficacy. Immunotherapies can be divided into antigen-specific approaches, which seek to induce T cells specific to one or several known tumor associated antigens (TAA), or with antigen-nonspecific approaches, which generally activate T cells to become nonspecifically lytic effectors. Here the authors critically review the different immunotherapeutic approaches in melanoma.

A phase 1 study of a vaccine targeting preferentially expressed antigen in melanoma and prostate-specific membrane antigen in patients with advanced solid tumors

Preferentially expressed antigen in melanoma (PRAME) and prostate-specific membrane antigen (PSMA) are tumor-associated antigens implicated in cellular differentiation, genetic stability, and angiogenesis. MKC1106-PP is an immunotherapeutic regimen cotargeting PRAME and PSMA, comprised of a recombinant plasmid (pPRA-PSM encoding fragments derived from both antigens) and 2 peptides (E-PRA and E-PSM derived from PRAME and PSMA, respectively). This multicenter study evaluated MKC1106-PP with a fixed plasmid dose and 2 different peptide doses, administered by intralymph node injection in a prime-boost sequence in human leukocyte antigen-A*0201 and tumor-antigen-positive patients with progressing metastatic solid tumors who had failed standard therapy. Immune monitoring was done by tetramer and enzymatic-linked immune spot analysis. The treatment was well tolerated, with no significant differences in safety, immune response, and clinical outcome relative to peptide doses. Fifteen of 24 evaluable patients showed an immune response, as defined by the expansion of PRAME-specific or PSMA-specific T cells in the blood. There were no partial or complete responses by the Response Evaluation Criteria in Solid Tumors. Seven patients showed stable disease (SD) for 6 months or longer, or prostate specific antigen decline: 4 of 10 with prostate carcinoma, 2 of 2 with renal clear cell carcinoma, and 1 of 10 with metastatic melanoma. In addition, there was an association between the induction and persistence of antigen-specific T cells in blood above baseline levels and disease control, defined as SD for 6 months or longer. These results support further development of MKC1106-PP in specific clinical indications.

DNA vaccines: an historical perspective and view to the future

This review provides a detailed look at the attributes and immunologic mechanisms of plasmid DNA vaccines and their utility as laboratory tools as well as potential human vaccines. The immunogenicity and efficacy of DNA vaccines in a variety of preclinical models is used to illustrate how they differ from traditional vaccines in novel ways due to the in situ antigen production and the ease with which they are constructed. The ability to make new DNA vaccines without needing to handle a virulent pathogen or to adapt the pathogen for manufacturing purposes demonstrates the potential value of this vaccine technology for use against emerging and epidemic pathogens. Similarly, personalized anti-tumor DNA vaccines can also readily be made from a biopsy. Because DNA vaccines bias the T-helper (Th) cell response to a Th1 phenotype, DNA vaccines are also under development for vaccines against allergy and autoimmune diseases. The licensure of four animal health products, including two prophylactic vaccines against infectious diseases, one immunotherapy for cancer, and one gene therapy delivery of a hormone for a food animal, provides evidence of the efficacy of DNA vaccines in multiple species including horses and pigs. The size of these target animals provides evidence that the somewhat disappointing immunogenicity of DNA vaccines in a number of human clinical trials is not due simply to the larger mass of humans compared with most laboratory animals. The insights gained from the mechanisms of protection in the animal vaccines, the advances in the delivery and expression technologies for increasing the potency of DNA vaccines, and encouragingly potent human immune responses in certain clinical trials, provide insights for future efforts to develop DNA vaccines into a broadly useful vaccine and immunotherapy platform with applications for human and animal health.

Intralymphatic immunotherapy: from the rationale to human applications

Allergen specific immunotherapy (SIT) is the only treatment of IgE mediated allergies that is causative and has a long-term effect. Classically, SIT requires numerous subcutaneous injections of the allergen during 3-5 years. Over the last decade sublingual allergen applications have established as an alternative, but treatment duration could not be shortened. This review focuses on direct administration of vaccines in general and of allergens in particular into lymph nodes with the aim to enhance immunotherapy. Several studies have found that direct injection of antigens into lymph nodes enhanced immune responses. Recently we have focused on intralymphatic allergen administration in order to enhance SIT. Data in mouse models and in clinical trials showed that intralymphatic allergen administration strongly enhanced SIT, so that the number of allergen injections could be reduced to three, and the allergen dose could be reduced 10-100 fold. Intralymphatic injections proved easy, practically painless and safe. In mice and men, intralymphatic immunotherapy injecting allergens into a subcutaneous lymph node markedly enhances the protective immune response, so that both the dose and number of allergen injections can be reduced, making SIT safer and faster, which enhances patient convenience and compliance.

DNA vaccine encoding prostatic acid phosphatase (PAP) elicits long-term T-cell responses in patients with recurrent prostate cancer

Prostatic acid phosphatase (PAP) is a tumor antigen in prostate cancer and the target of several anti-tumor vaccines in earlier clinical trials. Ultimately, the goal of anti-tumor vaccines is to elicit a sustainable immune response, able to eradicate a tumor, or at least restrain its growth. We have investigated plasmid DNA vaccines and have previously conducted a phase 1 trial in which patients with recurrent prostate cancer were vaccinated with a DNA vaccine encoding PAP. In this study, we investigated the immunologic efficacy of subsequent booster immunizations, and conducted more detailed longitudinal immune analysis, to answer several questions aimed at guiding optimal schedules of vaccine administration for future clinical trials. We report that antigen-specific cytolytic T-cell responses were amplified after immunization in 7 of 12 human leukocyte antigen-A2-expressing individuals, and that multiple immunizations seemed necessary to elicit PAP-specific interferon-gamma-secreting immune responses detectable by enzyme-linked immunosorbent spot assay. Moreover, among individuals who experienced a >/=200% increase in prostate-specific antigen doubling time, long-term PAP-specific interferon-gamma-secreting T-cell responses were detectable in 6 of 8, but in only 1 of 14 individuals without an observed change in prostate-specific antigen doubling time (P=0.001). Finally, we identified that immune responses elicited could be further amplified by subsequent booster immunizations. These results suggest that future trials using this DNA vaccine, and potentially other anti-tumor DNA vaccines, could investigate ongoing schedules of administration with periodic booster immunizations. Moreover, these results suggest that DNA vaccines targeting PAP could potentially be combined in heterologous immunization strategies with other vaccines to further augment PAP-specific T-cell immunity.

DNA vaccination: using the patient's immune system to overcome cancer

Cancer is one of the most challenging diseases of today. Optimization of standard treatment protocols consisting of the main columns of chemo- and radiotherapy followed or preceded by surgical intervention is often limited by toxic side effects and induction of concomitant malignancies and/or development of resistant mechanisms. This requires the development of therapeutic strategies which are as effective as standard therapies but permit the patients a life without severe negative side effects. Along this line, the development of immunotherapy in general and the innovative concept of DNA vaccination in particular may provide a venue to achieve this goal. Using the patient's own immune system by activation of humoral and cellular immune responses to target the cancer cells has shown first promising results in clinical trials and may allow reduced toxicity standard therapy regimen in the future. The main challenge of this concept is to transfer the plethora of convincing preclinical and early clinical results to an effective treatment of patients.

Multivalent immunity targeting tumor-associated antigens by intra-lymph node DNA-prime, peptide-boost vaccination

Active immunotherapy of cancer has yet to yield effective therapies in the clinic. To evaluate the translatability of DNA-based vaccines we analyzed the profile of T-cell immunity by plasmid vaccination in a murine model, using transcriptome microarray analysis and flow cytometry. DNA vaccination resulted in specific T cells expressing low levels of co-inhibitory molecules (most notably PD-1), strikingly different from the expression profile elicited by peptide immunization. In addition, the T-cell response primed through this dual-antigen-expressing plasmid (MART-1/Melan-A and tyrosinase) translated into a substantial proliferation capacity and functional conversion to antitumor effector cells after tyrosinase and MART-1/Melan-A peptide analog boost. Furthermore, peptide boost rescued the immune response against the subdominant tyrosinase epitope. This immunization approach could be adapted to elicit potent immunity against multiple tumor antigens, resulting in a broader immune response that was more effective in targeting human tumor cells. Finally, this study sheds light on a novel mechanism of immune homeostasis through synchronous regulation of co-inhibitory molecules on T cells, highly relevant to heterologous prime boost approaches involving DNA vaccines as priming agents.

TAA polyepitope DNA-based vaccines: a potential tool for cancer therapy

DNA-based cancer vaccines represent an attractive strategy for inducing immunity to tumor associated antigens (TAAs) in cancer patients. The demonstration that the delivery of a recombinant plasmid encoding epitopes can lead to epitope production, processing, and presentation to CD8+ T-lymphocytes, and the advantage of using a single DNA construct encoding multiple epitopes of one or more TAAs to elicit a broad spectrum of cytotoxic T-lymphocytes has encouraged the development of a variety of strategies aimed at increasing immunogenicity of TAA polyepitope DNA-based vaccines. The polyepitope DNA-based cancer vaccine approach can (a) circumvent the variability of peptide presentation by tumor cells, (b) allow the introduction in the plasmid construct of multiple immunogenic epitopes including heteroclitic epitope versions, and (c) permit to enroll patients with different major histocompatibility complex (MHC) haplotypes. This review will discuss the rationale for using the TAA polyepitope DNA-based vaccination strategy and recent results corroborating the usefulness of DNA encoding polyepitope vaccines as a potential tool for cancer therapy.

HLA-A2-restricted T-cell epitopes specific for prostatic acid phosphatase

Prostatic acid phosphatase (PAP) has been investigated as the target of several antigen-specific anti-prostate tumor vaccines. The goal of antigen-specific active immunotherapies targeting PAP would ideally be to elicit PAP-specific CD8+ effector T cells. The identification of PAP-specific CD8+ T-cell epitopes should provide a means of monitoring the immunological efficacy of vaccines targeting PAP, and these epitopes might themselves be developed as vaccine antigens. In the current report, we hypothesized that PAP-specific epitopes might be identified by direct identification of pre-existing CD8+ T cells specific for HLA-A2-restricted peptides derived from PAP in the blood of HLA-A2-expressing individuals. 11 nonamer peptides derived from the amino acid sequence of PAP were used as stimulator antigens in functional ELISPOT assays with peripheral blood mononuclear cells from 20 HLA-A2+ patients with prostate cancer or ten healthy blood donors. Peptide-specific T cells were frequently identified in both groups for three of the peptides, p18-26, p112-120, and p135-143. CD8+ T-cell clones specific for three peptides, p18-26, p112-120, and p299-307, confirmed that these are HLA-A2-restricted T-cell epitopes. Moreover, HLA-A2 transgenic mice immunized with a DNA vaccine encoding PAP developed epitope-specific responses for one or more of these three peptide epitopes. We propose that this method to first identify epitopes for which there are pre-existing epitope-specific T cells could be used to prioritize MHC class I-specific epitopes for other antigens. In addition, we propose that the epitopes identified here could be used to monitor immune responses in HLA-A2+ patients receiving vaccines targeting PAP to identify potentially therapeutic immune responses.

Intralymphatic immunotherapy

PURPOSE OF REVIEW

IgE-mediated allergy can be treated by subcutaneous allergen-specific immunotherapy (SCIT). However, the percentage of allergic patients undergoing SCIT is low, mainly due to the long duration of the therapy and the risk of severe systemic allergic reactions associated with the allergen administration. Typically, SCIT requires dozens of subcutaneous allergen injections that stretch over 3-5 years. Over the last decade, sublingual immunotherapy has been established as an alternative to SCIT, but treatment duration and dosing frequencies could not be reduced. Recently, immunotherapy by direct administration of the allergen into lymph nodes [intralymphatic immunotherapy (ILIT)] has proven a promising alternative and this method is the focus of the present review.

RECENT FINDINGS

Several studies on animals and on humans have shown that direct injection into lymph nodes enhanced immune responses to protein, peptide, and naked DNA vaccines. Moreover, ILIT strongly improved allergen immunotherapy, so that the number of allergen administrations as well as the allergen dose could be reduced. As ILIT was also well tolerated, practically painless, and easy to perform, patient compliance was improved as compared with SCIT.

SUMMARY

Direct ILIT into a subcutaneous lymph node markedly enhances protective immune responses, so that both the dose and the number of allergen injections can be reduced, making ILIT safer and faster than other forms of immunotherapy, and most importantly, this enhances patient convenience and compliance.

DNA vaccines: developing new strategies against cancer

Due to their rapid and widespread development, DNA vaccines have entered into a variety of human clinical trials for vaccines against various diseases including cancer. Evidence that DNA vaccines are well tolerated and have an excellent safety profile proved to be of advantage as many clinical trials combines the first phase with the second, saving both time and money. It is clear from the results obtained in clinical trials that such DNA vaccines require much improvement in antigen expression and delivery methods to make them sufficiently effective in the clinic. Similarly, it is clear that additional strategies are required to activate effective immunity against poorly immunogenic tumor antigens. Engineering vaccine design for manipulating antigen presentation and processing pathways is one of the most important aspects that can be easily handled in the DNA vaccine technology. Several approaches have been investigated including DNA vaccine engineering, co-delivery of immunomodulatory molecules, safe routes of administration, prime-boost regimen and strategies to break the immunosuppressive networks mechanisms adopted by malignant cells to prevent immune cell function. Combined or single strategies to enhance the efficacy and immunogenicity of DNA vaccines are applied in completed and ongoing clinical trials, where the safety and tolerability of the DNA platform are substantiated. In this review on DNA vaccines, salient aspects on this topic going from basic research to the clinic are evaluated. Some representative DNA cancer vaccine studies are also discussed.

Progress and challenges in the vaccine-based treatment of head and neck cancers

Head and neck (HN) cancer represents one of the most challenging diseases because the mortality remains high despite advances in early diagnosis and treatment. Although vaccine-based approaches for the treatment of advanced squamous cell carcinoma of the head and neck have achieved limited clinical success, advances in cancer immunology provide a strong foundation and powerful new tools to guide current attempts to develop effective cancer vaccines. This article reviews what has to be rather what has been done in the field for the development of future vaccines in HN tumours.

Enhancing DNA vaccination by sequential injection of lymph nodes with plasmid vectors and peptides

DNA vaccines or peptides are capable of inducing specific immunity; however, their translation to the clinic has generally been problematic, primarily due to the reduced magnitude of immune response and poor pharmacokinetics. Herein, we demonstrate that a novel immunization strategy, encompassing sequential exposure of the lymph node milieu to plasmid and peptide in a heterologous prime-boost fashion, results in considerable MHC class I-restricted immunity in mice. Plasmid-primed antigen expression was essential for the generation of a population of central memory T cells, expressing CD62L and low in PD-1, with substantial capability to expand and differentiate to peripheral memory and effector cells, following subsequent exposure to peptide. These vaccine-induced T cells dominated the T cell repertoire, were able to produce large amounts of chemokines and pro-inflammatory cytokines, and recognized tumor cells effectively. In addition to outlining a feasible and effective method to transform plasmid DNA vaccination into a potentially viable immunotherapeutic approach for cancer, this study sheds light on the mechanism of heterologous prime-boost and the considerable heterogeneity of MHC class I-restricted T cell responses.