DNA vaccines designed to inhibit tumor growth by suppression of angiogenesis

Bacteria-Based Microdevices for the Oral Delivery of Macromolecules

The oral delivery of macromolecules is quite challenging due to environmental insults and biological barriers encountered along the gastrointestinal (GI) tract. Benefiting from their living characteristics, diverse bacterial species have been engineered as intelligent platforms to deliver various therapeutics. To tackle difficulties in oral delivery, innovative bacteria-based microdevices have been developed by virtue of advancements in synthetic biology and nanotechnology, with aims to overcome the instability and short half-life of macromolecules in the GI tract. In this review, we summarize the main classes of macromolecules that are produced and delivered through the oral ingestion of bacteria and bacterial derivatives. Furtherly, we discuss the engineering strategies and biomedical applications of these living microdevices in disease diagnosis, bioimaging, and treatment. Finally, we highlight the advantages as well as the limitations of these engineered bacteria used as platforms for the oral delivery of macromolecules and also propose their potential for clinical translation. The results summarized in this review article would contribute to the invention of next-generation bacteria-based systems for the oral delivery of macromolecules.

A phase 1 trial extension to assess immunologic efficacy and safety of prime-boost vaccination with VXM01, an oral T cell vaccine against VEGFR2, in patients with advanced pancreatic cancer

VXM01 is a first-in-kind orally applied tumor vaccine based on live attenuated Salmonella typhi carrying an expression plasmid encoding VEGFR2, an antigen expressed on tumor vasculature and a stable and accessible target for anti-angiogenic intervention. A recent randomized, placebo-controlled, phase I dose-escalation trial in advanced pancreatic cancer patients demonstrated safety, immunogenicity and transient, T-cell response-related anti-angiogenic activity of four priming vaccinations applied within one week. We here evaluated whether monthly boost vaccinations are safe and can sustain increased frequencies of vaccine-specific T cells. Patients with advanced pancreatic cancer were randomly assigned at a ratio of 2:1 to priming with VXM01 followed by up to six monthly boost vaccinations, or placebo treatment. Vaccinations were applied orally at two alternative doses of either 10⁶ colony-forming units (CFU) or 10⁷ CFU, and concomitant treatment with standard-of-care gemcitabine during the priming phase, and any treatment thereafter, was allowed in the study. Immunomonitoring involved interferon-gamma (IFNγ) ELIspot analysis with long overlapping peptides spanning the entire VEGFR2 sequence. A total of 26 patients were treated. Treatment-related adverse events preferentially associated with VXM01 were decreases in lymphocyte numbers in the blood, increased frequencies of neutrophils and diarrhea. Eight out of 16 patients who received at least one boosting vaccination responded with pronounced, i.e. at least 3-fold, increase in VEGFR2-specific T cell response over baseline levels. In the VXM01 vaccination group, VEGFR2-specific T cells peaked preferentially during the boosting phase with an average 4-fold increase over baseline levels. In conclusion, prime/boost vaccination with VXM01 was safe and immunogenic and increased vaccine specific T cell responses compared with placebo treatment.

Use of attenuated bacteria as delivery vectors for DNA vaccines

Live, attenuated bacterial vaccines (LBV) are promising candidates for the induction of a broad-based immune response directed at recombinant heterologous antigens and the corresponding pathogen. LBVs allow vaccination through the mucosal surfaces and specific targeting of professional antigen-presenting cells located at the inductive sites of the immune system. A novel approach exploits attenuated intracellular bacteria as delivery vectors for eukaryotic antigen-expression plasmids (so-called DNA vaccines). Candidate carrier bacteria include attenuated strains of Gram-positive and Gram-negative bacteria. These bacteria have been shown to deliver DNA vaccines to human cells in vitro and have also proven their in vivo efficacy in several experimental animal models of infectious diseases and different cancers. The clinical assessment of the safety, immunogenicity and efficacy of these candidate strains will be the next challenging step towards live bacterial DNA vaccines.

Vaccines targeting the neovasculature of tumors

Angiogenesis has a critical role in physiologic and disease processes. For the growth of tumors, angiogenesis must occur to carry sufficient nutrients to the tumor. In addition to growth, development of new blood vessels is necessary for invasion and metastases of the tumor. A number of strategies have been developed to inhibit tumor angiogenesis and further understanding of the interplay between tumors and angiogenesis should allow new approaches and advances in angiogenic therapy. One such promising angiogenic approach is to target and inhibit angiogenesis with vaccines. This review will discuss recent advances and future prospects in vaccines targeting aberrant angiogenesis of tumors. The strategies utilized by investigators have included whole endothelial cell vaccines as well as vaccines with defined targets on endothelial cells and pericytes of the developing tumor endothelium. To date, several promising anti-angiogenic vaccine strategies have demonstrated marked inhibition of tumor growth in pre-clinical trials with some showing no observed interference with physiologic angiogenic processes such as wound healing and fertility.

Orally administered DNA vaccine delivery by attenuated Salmonella typhimurium targeting fetal liver kinase 1 inhibits murine Lewis lung carcinoma growth and metastasis

The vascular endothelial growth factor (VEGF) receptor 2 (VEGFR-2), also called fetal liver kinase 1 (FLK1) in mice and kinase insert domain receptor (KDR) in humans, is an endothelial cell specific receptor tyrosine kinase that mediates lung cancer angiogenesis. We hypothesized that an active immunotherapy approach targeting FLK1 may inhibit lung cancer growth and metastasis. To test this hypothesis, we evaluated whether immune responses to FLK1 could be elicited in mice by immunization with an orally administered DNA vaccine encoding the extracellular domain (ECD) of FLK1 (pcDNA3.1-FLK1(ECD)) carried by attenuated Salmonella typhimurium. We found that the vaccine was effective at protective antitumor immunity in Lewis lung carcinoma models in mice by breaking immune tolerance to FLK1 self-antigen. Both FLK1-specific humoral and cellular immune responses against endothelial cells can be induced in mice by immunization with pcDNA3.1-FLK1(ECD). Immunization with pcDNA3.1-FLK1(ECD) resulted in tumor suppression and prolonged survival in mice challenged with Lewis lung carcinomas cells. Experimental pulmonary metastases were strongly inhibited in pcDNA3.1-FLK1(ECD) immunized mice challenged with Lewis lung carcinoma cells. Thus, we conclude that the plasmid DNA vaccine encoding the extracellular domain of FLK1 could be an important component of FLK1 DNA vaccine to prevent lung carcinoma recurrence and metastasis after surgery.

Construction of a DNA vaccine encoding Flk-1 extracellular domain and C3d fusion gene and investigation of its suppressing effect on tumor growth

Although the critical role of complement component C3d as a molecular adjuvant in preventing virus infection is well established, its role in cancer prophylaxis and treatment is unclear. In this study, we constructed a recombinant plasmid encoding Flk-1 and C3d3 fusion proteins and investigated its transient expression in vitro in transfected eukaryotic cells and its antibody response in immunized mice. Subsequently, we investigated the vaccine's ability to elicit an immune response leading to suppression of angiogenesis and tumor growth in mice bearing bladder transitional cell carcinoma. Using Western blotting, immunocytochemistry, and flow cytometry, we detected the expression of Flk-1 and C3d3 fusion proteins in COS-7 cells transfected with these recombinant plasmids. Further binding experiment using CR2 (C3d receptor) positive Raji cells that were incubated with transfected COS-7 supernatant indicated that C3d was successfully fused to Flk-1. Although both vaccines elicited peak antibody levels at 5 weeks, Flk-1-specific antibody titer in pSG.SS.Flk-1(ECD).C3d3.YL-immunized mice was significantly higher when compared to pSG.SS.Flk-1(ECD).YL-immunized mice. The results of experiments with bladder tumor-bearing mice showed that the vaccine inhibited tumor growth significantly. These results suggest that C3d plays a critical role in tumor immunotherapy by promoting antibody response in Flk-1-based DNA vaccines. This approach may provide a new strategy for the rational design of anti-angiogenic therapies for the treatment of solid tumors and provide a basis for the further exploitation and application of the anti-angiogenesis DNA vaccines.

Anti-angiogenic active immunotherapy: a new approach to cancer treatment

Tumor angiogenesis plays an important role in tumor growth, aggression and metastasis. Many molecules have been demonstrated as positive regulators of angiogenesis, including vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), epidermal growth factor (EGF), and others. In recent years, significant progress has been made in the research on anti-angiogenic strategies for tumor therapies. In this review, anti-angiogenic active immunotherapies for tumors based on vaccination with xenogeneic homologous molecules and non-xenogeneic homologous molecules are discussed.

Evaluation of a xenogeneic VEGF vaccine in dogs with soft tissue sarcoma

Active immunization against pro-angiogenic growth factors or their receptors is an emerging strategy for controlling tumor growth and angiogenesis. Previous studies in rodent tumor models have indicated that immunization against xenogeneic growth factors is more likely to induce effective anti-tumor responses than immunization against the autologous growth factor. However, the effectiveness or safety of the xenogeneic vaccination approach has not been previously assessed in a clinically relevant outbred, spontaneous tumor model. Therefore, we investigated the safety and anti-tumor and anti-angiogenic effects of a xenogeneic vascular endothelial cell growth factor (VEGF) vaccine in pet dogs with spontaneous cancer. Nine dogs with soft tissue sarcoma were immunized with a recombinant human VEGF vaccine over a 16-week period. The effects of immunization on antibodies to human and canine VEGF, circulating VEGF concentrations, tumor microvessel density (MVD), and tumor growth were assessed. The xenogeneic VEGF vaccine was well-tolerated by all dogs and resulted in induction of humoral responses against both human and canine VEGF in animals that remained in the study long enough to receive multiple immunizations. Three of five multiply immunized dogs also experienced sustained decreases in circulating plasma VEGF concentrations and two dogs had a significant decrease in tumor MVD. The overall tumor response rate was 30% for all treated dogs in the study. We conclude therefore that a xenogeneic VEGF vaccine may be a safe and effective alternative means of controlling tumor growth and angiogenesis.

Construction of recombinant attenuated Salmonella typhimurium DNA vaccine expressing H pylori ureB and IL-2

AIM: To construct a recombinant live attenuated Salm-onella typhimurium DNA vaccine encoding H pylori ureB gene and mouse IL-2 gene and to detect its immunogenicity in vitro and in vivo.

METHODS: H pylori ureB and mouse IL-2 gene fragments were amplified by polymerase chain reaction (PCR) and cloned into pUCmT vector. DNA sequence of the amplified ureB and IL-2 genes was assayed, then cloned into the eukaryotic expression vector pIRES through enzyme digestion and ligation reactions resulting in pIRES-ureB and pIRES-ureB-IL-2. The recombinant plasmids were used to transform competent E. coli DH5α, and the positive clones were screened by PCR and restriction enzyme digestion. Then, the recombinant pIRES-ureB and pIRES-ureB-IL-2 were used to transform LB5000 and the recombinant plasmids extracted from LB5000 were finally introduced into the final host SL7207. After that, recombinant strains were grown in vitro repeatedly. In order to detect the immunogenicity of the vaccine in vitro, pIRES-ureB and pIRES-ureB-IL-2 were transfected to COS-7 cells using Lipofectamine^TM2000, the immunogenicity of expressed UreB and IL-2 proteins was assayed with SDS-PAGE and Western blot. C57BL/6 mice were orally immunized with 1 × 10⁸ recombinant attenuated Salmonella typhimurium DNA vaccine. Four weeks after vaccination, mice were challenged with 1 × 10⁷ CFU of live H pylori SS1. Mice were sacrificed and the stomach was isolated for examination of H pylori 4 wk post-challenge.

RESULTS: The 1700 base pair ureB gene fragment amplified from the genomic DNA was consistent with the sequence of H pylori ureB by sequence analysis. The amplified 510 base pair fragment was consistent with the sequence of mouse IL-2 in gene bank. It was confirmed by PCR and restriction enzyme digestion that H pylori ureB and mouse IL-2 genes were inserted into the eukaryotic expression vector pIRES. The experiments in vitro showed that stable recombinant live attenuated Salmonella typhimurium DNA vaccine carrying ureB and IL-2 genes was successfully constructed and the specific strips of UreB and IL-2 expressed by recombinant plasmids were detected through Western blot. Study in vivo showed that the positive rate of rapid urease test of the immunized group including ureB and ureB-IL-2 was 37.5% and 12.5% respectively, and was significantly lower than that (100%) in the control group (P < 0.01).

CONCLUSION: Recombinant attenuated Salmonella typhimurium DNA vaccine expressing UreB protein and IL-2 protein with immunogenicity can be constructed. It can protect mice against H pylori infection, which may help the development of a human-use H pylori DNA vaccine.

Prophylactic naked DNA vaccination with the human vascular endothelial growth factor induces an anti-tumor response in C57Bl/6 mice

Passive immunotherapy against soluble pro-angiogenic factors and/or their receptors in endothelial cells has become a promising approach in cancer therapeutics. There is also experimental evidence indicating that an active immunotherapy strategy directed towards these target molecules could also be effective. In this paper we show that it is possible to reduce tumor growth or increase the survival of tumor-bearing C57Bl/6 mice when animals are vaccinated with the human vascular endothelial growth factor (VEGF) isoform 121 gene (hVEGF(121)), and later challenged with melanoma or lung carcinoma tumor cells. Immunization was done with 10 microg DNA doses of the hVEGF121 gene, which is highly homologous to its mouse counterpart, administered on a weekly basis using a plasmid bearing 5 CpG bacterial motifs. Histopathology analyses of tumors of hVEGF(121) immunized animals showed a decrease in tumor cell density around vessels and in mitotic figures, as well as an increase in apoptotic tumor cells. A statistically significant cell cytotoxic response was found when spleen cells of immunized mice were co-cultured in vitro with mouse tumor VEGF-producing cells. Vaccination with an hVEGF121 gene mutated to make it deficient for VEGF receptor binding, produced similar in vitro and in vivo results, and significantly reduced the number of spontaneous metastases produced by the mouse Lewis lung carcinoma. Our results indicate that human VEGF DNA can be employed for anti-angiogenic active immunotherapy in mice, and that direct cell cytotoxicity is a contributor mechanism to the overall anti-tumor effects seen in immunized animals.

Identification of H-2Db-Specific CD8+ T-Cell Epitopes From Mouse VEGFR2 That Can Inhibit Angiogenesis and Tumor Growth

Vascular endothelial growth factor receptor 2 (VEGFR2/KDR) plays a crucial role in tumor-associated angiogenesis and vascularization. It has been established that monoclonal antibodies against VEGFR2 can inhibit angiogenesis. In this study, two naturally processed CD8 T-cell epitopes (VILTNPISM and FSNSTNDILI) were identified from murine KDR. Cytotoxic T lymphocytes targeting endothelial cells could be directly monitored by KDR2 and KDR3 Elispots or major histocompatibility complex class I tetramer staining. Immunization with these two peptides effectively reduced angiogenesis and inhibited tumor growth in mouse models. Thus, vaccination with KDR peptides alone or in combination with other anti-angiogenesis agents may afford a novel immunotherapy for inhibition of tumor growth.

A Comparison of Cyclohexanone and Tetrahydro-4H-thiopyran-4-one 1,1-Dioxide as Pharmacophores for the Design of Peptide-Based Inhibitors of the Serine Protease Plasmin

The plasminogen system is important in the proteolytic cascade that facilitates angiogenesis, a process that is essential for tumor growth and metastasis. The serine protease plasmin has a central role in the plasminogen system. This protease acts by degrading several components of the basement membrane and by activating other proteases. Therefore, inhibition of plasmin may be an effective method for blocking angiogenesis and, as a result, inhibiting the growth of primary tumors and secondary metastases. Three pairs of plasmin inhibitors were synthesized to compare the relative potency of inhibitors that are based upon a cyclohexanone or a tetrahydro-4H-thiopyran-4-one 1,1-dioxide nucleus. Compounds 1, 3, and 5 were cyclohexanone-based inhibitors, whereas compounds 2, 4, and 6 were tetrahydro-4H-thiopyran-4-one 1,1-dioxide-based inhibitors. Compounds 5 and 6 are reasonable inhibitors with IC50 values of 25 and 5.5 microM, respectively. Comparisons of the IC50 values of the three pairs show that the electron-withdrawing sulfone functional group is a beneficial element for the design of plasmin inhibitors. The presence of the sulfone increases inhibitor potency by a factor of 3-5 when compared to inhibitors that are based upon a simple cyclohexanone core.

Detection of hypothetical proteins in 10 individual human tumor cell lines

The search for new structures in tumors by genomics and proteomics methods is a major goal in tumor biology and may lead to the detection of markers or antigens for the generation of tumor vaccines. The aim of this study was to identify proteins that have been predicted so far based upon their nucleic acid sequence only or show poor identity to known proteins in tumor cell lines. Cell lines of neuroblastoma, colorectal, cervix carcinoma, adenocarcinoma of the ovary, lung and breast cancer, promyelocytic leukaemia, rhabdomyosarcoma, osteosarcoma and malignant melanoma were used. Cell lysates were run on 2D gel electrophoresis with subsequent in-gel digestion and MALDI-TOF-TOF analysis. A series of 10 hypothetical proteins (HPs) were observed and three of these proteins, hypothetical protein (Q9BTE6), CGI-83 protein (Q9Y392) and similar to CG11334 (Q9BV20), were so far described in tumors exclusively. The other seven proteins were already detected at the transcriptional level in normal and tumor cell lines or tissues. In conclusion, the three HPs observed in lung cancer and malignant melanoma may be candidates for development of tumor markers and generation of tumor vaccines.

Multidrug resistance-1 (MDR-1): a new target for T cell-based immunotherapy

Acquired multidrug resistance (MDR) remains a major challenge in the treatment of cancer with chemotherapeutic drugs. It can be mediated by the up-regulated expression of different proteins within the tumor cell membrane. Here, we used murine multidrug resistance-1 (MDR-1) as a target-antigen for the immunotherapy of cancer. We successfully demonstrated that peripheral T cell tolerance can be broken by oral administration of a DNA vaccine encoding MDR-1 and carried by attenuated Salmonella typhimurium to secondary lymphoid organs. Thus, mice, immunized orally three times at 2-wk intervals and challenged 2 wk thereafter with either MDR-1 expressing CT-26 colon carcinoma cells or MDR-1 expressing Lewis lung carcinoma cells, revealed a significant increase in life span. This was evident, when compared with animals either vaccinated with the empty control vector or challenged with the parental cell lines lacking overexpression of MDR-1. The immune response induced was antigen-specific and CD8+ T cell-mediated. The presence of the target antigen led to up-regulation of activation markers on CD8+ T cells and resulted in a strong cytotoxic T cell response as well as lysis of tumor target cells in vitro. We furthermore established the vaccine to be an effective treatment for established multi-drug-resistant tumor metastases, resulting in a significantly increased life span of experimental animals. Absence of CD8+ T cells due to in vivo depletion led to abrogation of effectiveness. Taken together, our results demonstrate that T cell tolerance against the MDR-1 self-antigen can be broken. It is anticipated that the combination of such an approach with chemotherapy could lead to more effective treatments of cancer.