Immunotherapy of Angiogenesis with DNA Vaccines

The understanding of the molecular mechanisms of tumor neovascularization has identified several important molecular targets that are specifically expressed on tumor neovasculature but not on physiological neovasculature. These molecular targets can potentially be used in the development of novel therapeutic approaches against tumor angiogenesis. Immunotherapy targeting angiogenesis has emerged as a potentially promising approach compared to the use of angiogenesis inhibitors due to its ability to afford long-term therapeutic protection. This presents remarkable opportunities for the development of innovative cancer therapies.Immunotherapy using DNA vaccines has gained momentum for antiangiogenesis therapy due to their stability, simplicity and excellent safety profile, and may prove to be a potentially useful strategy for targeting angiogenesis. In the current review, we discuss the various strategies and molecular targets employed in the form of DNA vaccines to target: (1) the endothelial cells within the tumor; (2) biological factors important for angiogenesis; and (3) the extracellular matrix and stromal cells associated with the tumor in order to control tumor angiogenesis in preclinical models. Introduction Alternative Treatment is Necessary for Effective Control of Cancer Cancer is one of the major causes of death in developed countries and is extremely difficult to cure. Current efforts to reduce this high mortality rate, including improvements in early detection and treatment, have been relatively unsuccessful. Thus, there is an urgent need to develop an innovative therapeutic approach to control cancer. Existing therapies, such as chemotherapy and radiation therapy, rarely result in long-term benefits for patients with locally advanced or metastatic disease. This is because of their inability to destroy tumors without harming normal cells. The ideal cancer therapy should have the potency to eradicate systemic tumors at multiple sites in the body, as well as the specificity to discriminate between malignant and normal cells. In both of these respects, anti-angiogenesis therapy is an attractive approach for combating cancer. Antiangiogenesis as a Therapeutic Approach for Blocking Tumor Growth Antiangiogenesis represents a potentially ideal therapeutic approach for the control of cancer. The natural remodeling of the body’s vasculature is referred to as angiogenesis. In the normal adult body, a steady balance between activators and inhibitors of endothelial cell (EC) growth tightly controls the process of neovascularization. This occurs only during wound repair, pregnancy, and in the female reproductive cycle (for review see [1,2]). Antiangiogenic strategies for the control of cancer should aim to destroy the blood vessels that supply cancer cells the nutrients and oxygen necessary for their survival, without interfering with the normal neovascularization. As a result, they would lead to starvation of cancer cells and ultimate shrinkage of the tumor [3]. Since tumor neovasculature is different from the physiological neovasculature, antiangiogenesis strategies 451 Departments of Pathology, Oncology, Molecular Microbiology and Immunology, and Obstetrics and Gynecology, Johns Hopkins School of Medicine, Baltimore, MD, USA *Corresponding Author: T.-C. Wu, Department of Pathology, Johns Hopkins School of Medicine, Cancer Research Building II, Room 309, 1550 Orleans Street, Baltimore, MD, USA E-mail: wutc@jhmi.edu 452 C.-F. Hung et al. have the specificity to discriminate between malignant and normal cells. Antiangiogenic therapy has a number of advantages over conventional cancer therapies. In general, most cancer therapies target the tumor cells, which undergo high rates of mutation. In contrast, antiangiogenic therapy targets endothelial cells and/or the surrounding stroma and matrix, which have a low rate of mutation, thus making drug resistance highly unlikely [4]. Since one endothelial cell supports ca. 10–100 tumor cells, targeting ECs may lead to amplification of tumor cell death. These properties make inhibition of angiogenesis advantageous to chemotherapy or radiotherapy in the longterm treatment of cancer [5]. In addition, antiangiogenic therapy could be applied universally to all kinds of cancers since it targets a process that is essential for the growth of all malignant tumors. Therefore, there is a significant interest in developing strategies to induce an antiangiogenic effect to control tumors. Immunotherapy Represents a Promising Approach to Generate Sustained Anti-Angiogenesis Effect Immunotherapy targeting angiogenesis has emerged as an attractive approach for the generation of sustained antiangiogenic effect to control tumors. Compared to other angiogenesis-based approaches, such as employment of angiogenesis inhibitors, the immunotherapy approach has a number of advantages. This approach overcomes the inherent difficulties of repeated administration of angiogenesis inhibitor drugs over a long course of treatment because immunotherapy can lead to immunological memory, which can provide long-term therapeutic effects. This feature may make immunotherapy more cost-effective compared to conventional treatment. In addition, immunotherapy may also be potentially less toxic than angiogenesis inhibitors derived from synthetic chemical compounds due to the specificity of the immune system. Immunotherapy targeting angiogenesis also has considerable advantages over immunotherapy of cancer, where patients are immunized against antigens presented by tumor cells. Firstly, resistance is less likely to develop in immunotherapy targeting ECs because they are genetically more stable compared to tumor cells. Another major advantage of immunologic targeting of angiogenesis-associated products is that one vaccine can be applicable to many different types of tumors as long as the angiogenic process is similar among different tumors. In contrast, tumor immunotherapy may be specific to each cancer type. Furthermore, ECs are easily accessible in the bloodstream, and effector T cells or antibodies specific for the tumor neovasculature can effectively reach the target tissue. In comparison, immunotherapy targeting tumors may have to cross multiple cell layers in order to reach the tumor cells that are far away from the vasculature (For review, see [6]). Table 39.1 summarizes the comparison of immunotherapy targeting angiogenesis versus immunotherapy targeting tumor cells. DNA Vaccines as an Attractive Approach for Immunotherapy Targeting Angiogenesis There are various forms of vaccines that may be employed in antiangiogenic immunotherapy, such as DNA-based, peptidebased, protein-based, viral and bacterial vector-based, and cellbased vaccines. Each of these vaccines has their advantages and disadvantages. Table 39.2 summarizes the advantages and disadvantages of the various forms of vaccine technology. Among these various forms of vaccines, DNA-based vaccines have emerged as an attractive approach for antiangiogenesis therapy (for review, see [7]). Naked DNA vaccines are advantageous because of their purity, simplicity of preparation, stability and excellent safety profile. Moreover, they are relatively inexpensive and easy to prepare. DNA vaccines allow for long-term sustained expression of antigen on MHC-peptide complexes compared to peptide or protein vaccines. In addition, various DNA vaccines targeting different angiogenesis-associated antigens can be combined together and repeatedly administered. However, a major challenge of DNA vaccines targeting angiogenesis is the breaking of immune tolerance to endogenous angiogenesis-associated molecular targets. Strategies to Break Immune Tolerance to Endogenous Molecular Targets Molecular targets related to tumor angiogenesis are mainly endogenous and are thus tolerated by the host’s immune system. This ‘self’-tolerance may be responsible for the weak immunogenicity of many angiogenesis-associated antigens. It is therefore important to develop methods for successfully breaking immunologic tolerance to endogenous angiogenesisassociated antigens. Several different methods for breaking tolerance have been employed with varying degrees of success. For example, DNA vaccines encoding xenogenic proteins have been found to break tolerance in mouse models by generating autoantibody and/or antigen-specific T cellmediated immune responses against the target endogenous Table 39.1. Comparison between immunotherapy of angiogenesis and immunotherapy targeting tumor cells. Immunotherapy of Immunotherapy targeting angiogenesis tumor cells Targets endothelial cells, Targets tumor cells