Synergistic antitumor efficacy of combined DNA vaccines targeting tumor cells and angiogenesis

Tumor vaccines: Toward multidimensional anti-tumor therapies

For decades, immunotherapies have offered hope for patients with advanced cancer. However, they show distinct benefits and limited clinical effects. Tumor vaccines have the potential to prime tumor-antigen-specific T cells and induce broad subsets of immune responses, ultimately eradicating tumor cells. Here, we classify tumor vaccines by their anti-tumor mechanisms, which include boosting the immune system, overcoming tumor immunosuppression, and modulating tumor angiogenesis. We focus on multidimensional tumor vaccine strategies using combinations of two or three of the above mechanisms, as these are superior to single-dimensional treatments. This review offers a perspective on tumor vaccine strategies and the future role of vaccine therapies in cancer treatment.

Alphaviruses in cancer immunotherapy

Alphaviruses have frequently been engineered for cancer therapy, cancer immunotherapy, and cancer vaccine development. As members of self-replicating RNA viruses, alphaviruses provide high levels of transgene expression through efficient self-amplifying of their RNA genome in host cells. Alphavirus vectors can be used as recombinant viral particles or oncolytic viruses. Alternatively, either naked or nanoparticle-encapsulated RNA and DNA replicons can be utilized. In the context of cancer prevention and treatment, antitumor, cytotoxic and suicide genes have been expressed from alphavirus vectors to provide tumor regression and tumor eradication. Moreover, immunostimulatory genes such as cytokines and chemokines have been used for cancer immunotherapy approaches. Expression of tumor antigens has been applied for cancer vaccine development. Alphavirus vectors has demonstrated tumor regression and even cure in various preclinical animal models. Immunization has elicited strong immune responses and showed protection against challenges with tumor cells in animal models. Several clinical trials have confirmed good safety and tolerability of alphaviruses in cancer patients although therapeutic efficacy will still require optimization.

Application of DNA Replicons in Gene Therapy and Vaccine Development

DNA-based gene therapy and vaccine development has received plenty of attention lately. DNA replicons based on self-replicating RNA viruses such as alphaviruses and flaviviruses have been of particular interest due to the amplification of RNA transcripts leading to enhanced transgene expression in transfected host cells. Moreover, significantly reduced doses of DNA replicons compared to conventional DNA plasmids can elicit equivalent immune responses. DNA replicons have been evaluated in preclinical animal models for cancer immunotherapy and for vaccines against infectious diseases and various cancers. Strong immune responses and tumor regression have been obtained in rodent tumor models. Immunization with DNA replicons has provided robust immune responses and protection against challenges with pathogens and tumor cells. DNA replicon-based COVID-19 vaccines have shown positive results in preclinical animal models.

Alphaviruses in Immunotherapy and Anticancer Therapy

Alphaviruses have been engineered as expression vectors for vaccine development and gene therapy. Due to the feature of RNA self-replication, alphaviruses can provide exceptional direct cytoplasmic expression of transgenes based on the delivery of recombinant particles, naked or nanoparticle-encapsulated RNA or plasmid-based DNA replicons. Alphavirus vectors have been utilized for the expression of various antigens targeting different types of cancers, and cytotoxic and antitumor genes. The most common alphavirus vectors are based on the Semliki Forest virus, Sindbis virus and Venezuelan equine encephalitis virus, but the oncolytic M1 alphavirus has also been used. Delivery of immunostimulatory cytokine genes has been the basis for immunotherapy demonstrating efficacy in different animal tumor models for brain, breast, cervical, colon, lung, ovarian, pancreatic, prostate and skin cancers. Typically, therapeutic effects including tumor regression, tumor eradication and complete cure as well as protection against tumor challenges have been observed. Alphavirus vectors have also been subjected to clinical evaluations. For example, therapeutic responses in all cervical cancer patients treated with an alphavirus vector expressing the human papilloma virus E6 and E7 envelope proteins have been achieved.

Applications of self-replicating RNA

Self-replicating RNA viral vectors have been engineered for both prophylactic and therapeutic applications. Mainly the areas of infectious diseases and cancer have been targeted. Both positive and negative strand RNA viruses have been utilized including alphaviruses, flaviviruses, measles viruses and rhabdoviruses. The high-level of RNA amplification has provided efficient expression of viral surface proteins and tumor antigens. Immunization studies in animal models have elicit robust neutralizing antibody responses. In the context of infectious diseases, immunization with self-replicating RNA viral vectors has provided protection against challenges with lethal doses of pathogens in animal models. Similarly, immunization with vectors expressing tumor antigens has resulted in tumor regression and eradication and protection against tumor challenges in animal models. The transient nature and non-integration of viral RNA into the host genome are ideal features for vaccine development. Moreover, self-replicating RNA viral vectors show great flexibility as they can be applied as recombinant viral particles, RNA replicons or DNA replicon plasmids. Several clinical trials have been conducted especially in the area of cancer immunotherapy.

Alphaviruses in Cancer Therapy

Alphaviruses have been engineered as expression vectors for different strategies of cancer therapy including immunotherapy and cancer vaccine development. Administration of recombinant virus particles, RNA replicons and plasmid DNA-based replicons provide great flexibility for alphavirus applications. Immunization and delivery studies have demonstrated therapeutic efficacy in the form of reduced tumor growth, tumor regression and eradication of established tumors in different animal models for cancers such as brain, breast, colon, cervical, lung, ovarian, pancreas, prostate cancers, and melanoma. Furthermore, vaccinated animals have showed protection against challenges with tumor cells. A limited number of clinical trials in the area of brain, breast, cervical, colon prostate cancers and melanoma vaccines has been conducted. Particularly, immunization of cervical cancer patients elicited immune responses and therapeutic activity in all patients included in a phase I clinical trial. Moreover, stable disease and partial responses were observed in breast cancer patients and prolonged survival was achieved in colon cancer patients.

Vaccines targeting angiogenesis in melanoma

Angiogenesis has a significant role in metastasis and progression of melanoma. Even small tumors may be susceptible to metastasis and hence lead to a worse outcome in patients with melanoma. One of the anti-angiogenic treatment approaches that is undergoing comprehensive study is specific immunotherapy. While tumor cells are challenging targets for immunotherapy due to their genetic instability and heterogeneity, endothelial cells (ECs) are genetically stable. Therefore, vaccines targeting angiogenesis in melanoma are appropriate choices that target both tumor cells and ECs while capable of inducing strong, anti-tumor immune responses with limited toxicity. The main targets of angiogenesis are VEGFs and their receptors but other potential targets have also been investigated, especially in preclinical studies. Various types of vaccines that target angiogenesis in melanoma have been studied including DNA, peptide, protein, dendritic cell-based, and endothelial cell vaccines. This review outlines a number of target antigens that are important for potential progress in developing vaccines for targeting angiogenesis in melanoma. We also discuss different types of vaccines that have been investigated, delivery mechanisms and popular adjuvants, and suggest ways to improve future clinical outcomes.

Middle East respiratory syndrome coronavirus vaccine based on a propagation-defective RNA replicon elicited sterilizing immunity in mice

Self-amplifying RNA replicons are promising platforms for vaccine generation. Their defects in one or more essential functions for viral replication, particle assembly, or dissemination make them highly safe as vaccines. We previously showed that the deletion of the envelope (E) gene from the Middle East respiratory syndrome coronavirus (MERS-CoV) produces a replication-competent propagation-defective RNA replicon (MERS-CoV-ΔE). Evaluation of this replicon in mice expressing human dipeptidyl peptidase 4, the virus receptor, showed that the single deletion of the E gene generated an attenuated mutant. The combined deletion of the E gene with accessory open reading frames (ORFs) 3, 4a, 4b, and 5 resulted in a highly attenuated propagation-defective RNA replicon (MERS-CoV-Δ[3,4a,4b,5,E]). This RNA replicon induced sterilizing immunity in mice after challenge with a lethal dose of a virulent MERS-CoV, as no histopathological damage or infectious virus was detected in the lungs of challenged mice. The four mutants lacking the E gene were genetically stable, did not recombine with the E gene provided in trans during their passage in cell culture, and showed a propagation-defective phenotype in vivo. In addition, immunization with MERS-CoV-Δ[3,4a,4b,5,E] induced significant levels of neutralizing antibodies, indicating that MERS-CoV RNA replicons are highly safe and promising vaccine candidates.

Self-Replicating RNA Viruses for Vaccine Development against Infectious Diseases and Cancer

Alphaviruses, flaviviruses, measles viruses and rhabdoviruses are enveloped single-stranded RNA viruses, which have been engineered for recombinant protein expression and vaccine development. Due to the presence of RNA-dependent RNA polymerase activity, subgenomic RNA can replicate close to 106 copies per cell for translation in the cytoplasm providing extreme transgene expression levels, which is why they are named self-replicating RNA viruses. Expression of surface proteins of pathogens causing infectious disease and tumor antigens provide the basis for vaccine development against infectious diseases and cancer. Self-replicating RNA viral vectors can be administered as replicon RNA at significantly lower doses than conventional mRNA, recombinant particles, or DNA plasmids. Self-replicating RNA viral vectors have been applied for vaccine development against influenza virus, HIV, hepatitis B virus, human papilloma virus, Ebola virus, etc., showing robust immune response and protection in animal models. Recently, paramyxovirus and rhabdovirus vector-based SARS-CoV-2 vaccines as well as RNA vaccines based on self-amplifying alphaviruses have been evaluated in clinical settings. Vaccines against various cancers such as brain, breast, lung, ovarian, prostate cancer and melanoma have also been developed. Clinical trials have shown good safety and target-specific immune responses. Ervebo, the VSV-based vaccine against Ebola virus disease has been approved for human use.

Strategies in DNA vaccine for melanoma cancer

According to reports of the international agency for cancer on research, although malignant melanoma shows less prevalence than nonmelanoma skin cancers, it is the major cause of skin cancer mortality. Given that, the production of effective vaccines to control melanoma is eminently required. In this regard, DNA-based vaccines have been extensively investigated for melanoma therapy. DNA vaccines are capable of inducing both cellular and humoral branches of immune responses. These vaccines possess some valuable advantages such as lack of severe side effects and high stability compared to conventional vaccination methods. The ongoing studies are focused on novel strategies in the development of DNA vaccines encoding artificial polyepitope immunogens based on the multiple melanoma antigens, the inclusion of molecular adjuvants to increase the level of immune responses, and the improvement of delivery approaches. In this review, we have outlined the recent advances in the field of melanoma DNA vaccines and described their implications in clinical trials as a strong strategy in the prevention and control of melanoma.

Impact of a Plasmid DNA-Based Alphavirus Vaccine on Immunization Efficiency

Alphavirus vectors have been engineered for high-level gene expression relying originally on replication-deficient recombinant particles, more recently designed for plasmid DNA-based administration. As alphavirus-based DNA vectors encode the alphavirus RNA replicon genes, enhanced transgene expression in comparison to conventional DNA plasmids is achieved. Immunization studies with alphavirus-based DNA plasmids have elicited specific antibody production, have generated tumor regression and protection against challenges with infectious agents and tumor cells in various animal models. A limited number of clinical trials have been conducted with alphavirus DNA vectors. Compared to conventional plasmid DNA-based immunization, alphavirus DNA vectors required 1000-fold less DNA to elicit similar immune responses in rodents.

Application of Viral Vectors for Vaccine Development with a Special Emphasis on COVID-19

Viral vectors can generate high levels of recombinant protein expression providing the basis for modern vaccine development. A large number of different viral vector expression systems have been utilized for targeting viral surface proteins and tumor-associated antigens. Immunization studies in preclinical animal models have evaluated the elicited humoral and cellular responses and the possible protection against challenges with lethal doses of infectious pathogens or tumor cells. Several vaccine candidates for both infectious diseases and various cancers have been subjected to a number of clinical trials. Human immunization trials have confirmed safe application of viral vectors, generation of neutralizing antibodies and protection against challenges with lethal doses. A special emphasis is placed on COVID-19 vaccines based on viral vectors. Likewise, the flexibility and advantages of applying viral particles, RNA replicons and DNA replicon vectors of self-replicating RNA viruses for vaccine development are presented.

Self-Amplifying RNA Viruses as RNA Vaccines

Single-stranded RNA viruses such as alphaviruses, flaviviruses, measles viruses and rhabdoviruses are characterized by their capacity of highly efficient self-amplification of RNA in host cells, which make them attractive vehicles for vaccine development. Particularly, alphaviruses and flaviviruses can be administered as recombinant particles, layered DNA/RNA plasmid vectors carrying the RNA replicon and even RNA replicon molecules. Self-amplifying RNA viral vectors have been used for high level expression of viral and tumor antigens, which in immunization studies have elicited strong cellular and humoral immune responses in animal models. Vaccination has provided protection against challenges with lethal doses of viral pathogens and tumor cells. Moreover, clinical trials have demonstrated safe application of RNA viral vectors and even promising results in rhabdovirus-based phase III trials on an Ebola virus vaccine. Preclinical and clinical applications of self-amplifying RNA viral vectors have proven efficient for vaccine development and due to the presence of RNA replicons, amplification of RNA in host cells will generate superior immune responses with significantly reduced amounts of RNA delivered. The need for novel and efficient vaccines has become even more evident due to the global COVID-19 pandemic, which has further highlighted the urgency in challenging emerging diseases.

Semliki Forest virus-based immunotherapy for cancer

Introduction: Immunotherapy has been introduced as a modern alternative for the treatment of various cancers, including the stimulation of the immune system by introduction of immunostimulatory molecules. Application of viral and non-viral vectors have provided a substantial contribution to improved delivery and expression of these immunostimulators.Areas covered: Alphavirus vectors, based on Semliki Forest virus, have allowed immunization with self-replicating RNA, recombinant virus particles, and layered DNA/RNA vectors. The attractive features of alphaviruses comprise their broad host range and extreme RNA replication in infected cells resulting in very high recombinant protein expression levels providing enhanced immune responses and an excellent basis for immunotherapy.Expert opinion: Immunization studies in animal tumor models have elicited strong humoral and cellular immune response, have provided prophylactic protection against tumor challenges, and have generated therapeutic efficacy in tumor-bearing animals. Clinical trials have indicated safe use of alphavirus vectors, making them attractive for cancer immunotherapy.

Cancer DNA vaccines: current preclinical and clinical developments and future perspectives

The recent developments in immuno-oncology have opened an unprecedented avenue for the emergence of vaccine strategies. Therapeutic DNA cancer vaccines are now considered a very promising strategy to activate the immune system against cancer. In the past, several clinical trials using plasmid DNA vaccines demonstrated a good safety profile and the activation of a broad and specific immune response. However, these vaccines often demonstrated only modest therapeutic effects in clinical trials due to the immunosuppressive mechanisms developed by the tumor. To enhance the vaccine-induced immune response and the treatment efficacy, DNA vaccines could be improved by using two different strategies. The first is to increase their immunogenicity by selecting and optimizing the best antigen(s) to be inserted into the plasmid DNA. The second strategy is to combine DNA vaccines with other complementary therapies that could improve their activity by attenuating immunosuppression in the tumor microenvironment or by increasing the activity/number of immune cells. A growing number of preclinical and clinical studies are adopting these two strategies to better exploit the potential of DNA vaccination. In this review, we analyze the last 5-year preclinical studies and 10-year clinical trials using plasmid DNA vaccines for cancer therapy. We also investigate the strategies that are being developed to overcome the limitations in cancer DNA vaccination, revisiting the rationale for different combinations of therapy and the different possibilities in antigen choice. Finally, we highlight the most promising developments and critical points that need to be addressed to move towards the approval of therapeutic cancer DNA vaccines as part of the standard of cancer care in the future.

Plasmid DNA-based Alphavirus Vaccines

Alphaviruses have been engineered as vectors for high-level transgene expression. Originally, alphavirus-based vectors were applied as recombinant replication-deficient particles, subjected to expression studies in mammalian and non-mammalian cell lines, primary cell cultures, and in vivo. However, vector engineering has expanded the application range to plasmid DNA-based delivery and expression. Immunization studies with DNA-based alphavirus vectors have demonstrated tumor regression and protection against challenges with infectious agents and tumor cells in animal tumor models. The presence of the RNA replicon genes responsible for extensive RNA replication in the RNA/DNA layered alphavirus vectors provides superior transgene expression in comparison to conventional plasmid DNA-based expression. Immunization with alphavirus DNA vectors revealed that 1000-fold less DNA was required to elicit similar immune responses compared to conventional plasmid DNA. In addition to DNA-based delivery, immunization with recombinant alphavirus particles and RNA replicons has demonstrated efficacy in providing protection against lethal challenges by infectious agents and tumor cells.

Self-Replicating RNA Viruses for RNA Therapeutics

Self-replicating single-stranded RNA viruses such as alphaviruses, flaviviruses, measles viruses, and rhabdoviruses provide efficient delivery and high-level expression of therapeutic genes due to their high capacity of RNA replication. This has contributed to novel approaches for therapeutic applications including vaccine development and gene therapy-based immunotherapy. Numerous studies in animal tumor models have demonstrated that self-replicating RNA viral vectors can generate antibody responses against infectious agents and tumor cells. Moreover, protection against challenges with pathogenic Ebola virus was obtained in primates immunized with alphaviruses and flaviviruses. Similarly, vaccinated animals have been demonstrated to withstand challenges with lethal doses of tumor cells. Furthermore, clinical trials have been conducted for several indications with self-amplifying RNA viruses. In this context, alphaviruses have been subjected to phase I clinical trials for a cytomegalovirus vaccine generating neutralizing antibodies in healthy volunteers, and for antigen delivery to dendritic cells providing clinically relevant antibody responses in cancer patients, respectively. Likewise, rhabdovirus particles have been subjected to phase I/II clinical trials showing good safety and immunogenicity against Ebola virus. Rhabdoviruses have generated promising results in phase III trials against Ebola virus. The purpose of this review is to summarize the achievements of using self-replicating RNA viruses for RNA therapy based on preclinical animal studies and clinical trials in humans.

Induction and characterization of anti-tumor endothelium immunity elicited by ValloVax therapeutic cancer vaccine

ValloVax is a placental endothelium derived vaccine which induces tissue-nonspecific antitumor immunity by blocking tumor angiogesis. To elucidate mechanisms of action, we showed that production of ValloVax, which involves treating placental endothelial cells with IFN-gamma, results in upregulation of HLA and costimulatory molecules. It was shown that in mixed lymphocyte reaction, ValloVax induces Type I cytokines and allo-proliferative responses. Plasma from ValloVax immunized mice was capable of killing in vitro tumor-like endothelium but not control endothelium. Using defined antigens associated with tumor endothelial cells, specific molecular entities were identified as being targeted by ValloVax induced antibodies. Binding of predominantly IgG antibodies to ValloVax cells was confirmed by flow cytometry. Further suggesting direct killing of tumor endothelial cells was expression of TUNEL positive cells, as well as, reduction in tumor oxygenation. Supporting a role for antibody mediated responses, cell depletion experiments suggested a predominant role of B cells in maintaining an intact anti-tumor endothelial response. Adoptive transfer experiments suggested that infusion of CD3+ T cells from immunized mice was sufficient to transfer tumor protection. Generation of memory T cells selective to tumor endothelial specific markers was observed. Functional confirmation of memory responses was observed in tumor rechallenge experiments. Furthermore, we observed that both PD-1 or CTLA-4 blockade augmented antitumor effects of ValloVax. These data suggest a T cell induced B cell mediated anti-tumor endothelial response and set the framework clinical trials through elucidation of mechanism of action.

Latest trends in cancer therapy applying viral vectors

Gene therapy based on viral vectors has demonstrated steady progress recently, not only in the area of cancers. A multitude of viral vectors has been engineered for both preventive and therapeutic applications. Two main approaches comprise of viral vector-based delivery of toxic or anticancer genes or immunization with anticancer antigens. Tumor growth inhibition and tumor regression have been observed, providing improved survival rates in animal tumor models. Furthermore, vaccine-based cancer immunotherapy has demonstrated both tumor regression and protection against challenges with lethal doses of tumor cells. Several clinical trials with viral vectors have also been conducted. Additionally, viral vector-based cancer drugs have been approved. This review gives an overview of different viral vector systems and their applications in cancer gene therapy.

Replicon RNA Viral Vectors as Vaccines

Single-stranded RNA viruses of both positive and negative polarity have been used as vectors for vaccine development. In this context, alphaviruses, flaviviruses, measles virus and rhabdoviruses have been engineered for expression of surface protein genes and antigens. Administration of replicon RNA vectors has resulted in strong immune responses and generation of neutralizing antibodies in various animal models. Immunization of mice, chicken, pigs and primates with virus-like particles, naked RNA or layered DNA/RNA plasmids has provided protection against challenges with lethal doses of infectious agents and administered tumor cells. Both prophylactic and therapeutic efficacy has been achieved in cancer immunotherapy. Moreover, recombinant particles and replicon RNAs have been encapsulated by liposomes to improve delivery and targeting. Replicon RNA vectors have also been subjected to clinical trials. Overall, immunization with self-replicating RNA viruses provides high transient expression levels of antigens resulting in generation of neutralizing antibody responses and protection against lethal challenges under safe conditions.

Self-replicating RNA viral vectors in vaccine development and gene therapy

RNA viruses are characterized by their efficient capacity to replicate at high levels in mammalian cells leading to high expression of foreign genes and making them attractive candidates for vectors engineered for vaccine development and gene therapy. Particularly, alphaviruses, flaviviruses, rhabdoviruses and measles viruses have been applied for immunization against infectious agents and tumors. Application of replicon RNA, DNA/RNA-layered vectors and replication-deficient viral particles have provided strong immune responses and protection against challenges with lethal doses of viral pathogens or tumor cells. Moreover, tumor regression has been obtained when RNA replicons have been administered in the form of RNA, DNA and viral particles, including replication-proficient oncolytic particles.