Current Approaches for Glioma Gene Therapy and Virotherapy

Abstract

Glioblastoma (GBM) is the most common and aggressive primary brain tumor in the adult population and it carries a dismal prognosis. Inefficient drug delivery across the blood brain barrier (BBB), an immunosuppressive tumor microenvironment (TME) and development of drug resistance are key barriers to successful glioma treatment. Since gliomas occur through sequential acquisition of genetic alterations, gene therapy, which enables to modification of the genetic make-up of target cells, appears to be a promising approach to overcome the obstacles encountered by current therapeutic strategies. Gene therapy is a rapidly evolving field with the ultimate goal of achieving specific delivery of therapeutic molecules using either viral or non-viral delivery vehicles. Gene therapy can also be used to enhance immune responses to tumor antigens, reprogram the TME aiming at blocking glioma-mediated immunosuppression and normalize angiogenesis. Nano-particles-mediated gene therapy is currently being developed to overcome the BBB for glioma treatment. Another approach to enhance the anti-glioma efficacy is the implementation of viro-immunotherapy using oncolytic viruses, which are immunogenic. Oncolytic viruses kill tumor cells due to cancer cell-specific viral replication, and can also initiate an anti-tumor immunity. However, concerns still remain related to off target effects, and therapeutic and transduction efficiency. In this review, we describe the rationale and strategies as well as advantages and disadvantages of current gene therapy approaches against gliomas in clinical and preclinical studies. This includes different delivery systems comprising of viral, and non-viral delivery platforms along with suicide/prodrug, oncolytic, cytokine, and tumor suppressor-mediated gene therapy approaches. In addition, advances in glioma treatment through BBB-disruptive gene therapy and anti-EGFRvIII/VEGFR gene therapy are also discussed. Finally, we discuss the results of gene therapy-mediated human clinical trials for gliomas. In summary, we highlight the progress, prospects and remaining challenges of gene therapies aiming at broadening our understanding and highlighting the therapeutic arsenal for GBM.

Keywords: FMS-like tyrosine kinase 3 ligand; HSV1-TK; gene therapy; glioma; immunotherapy; mutant IDH1 3; non-viral vectors; viral vectors.

Figure 1

Antitumor mechanisms mediated by oncolytic virus-mediated therapy. Oncolytic viruses (OVs) induce glioma cell death by infecting cells and replicating within them. In addition, OVs trigger immunogenic cell death (ICD) which leads to anti-glioma immunity. Direct virus-mediated cell lysis induces the release of additional virus particles which can infect neighboring glioma cells and continue their replicative cycle. ICD produces immune stimulatory molecules such as tumor cells-derived damage-associated molecular patterns (DAMPs), chemokines and type I interferons (Type I IFN) and they also induce the release of tumor-associated antigens (TAAs). These molecules recruit antigen-presenting cells (APCs) to the site of viral infection, where they get activated, they engulf TAAs and recognize DAMPs which interact with their pattern recognition receptors (PRRs). Mature APCs migrate to the regional lymph node where they prime anti-tumor cytotoxic CD8⁺ T lymphocytes (CTLs) which leads to anti-glioma immunity. Viral-mediated release of type I IFN and chemokine elicits the recruitment of tumor-specific CTLs to the tumor site. As glioma cells express TAAs, presented by their major histocompatibility complex (MHC) class I, they are recognized, and therefore killed, by CD8⁺ T cells.

Figure 2

Mechanism underlying the anti-glioma immune response following TK/Flt3L gene therapy. First generation adenoviral vectors (Ad) encoding HSV1-Thymidine Kinase (TK) and HSV1- FMS-like tyrosine kinase 3 ligand (Flt3L) are injected into the tumor cavity following surgical resection. (1) Dendritic Cell Recruitment to Tumor Microenvironment (TME): Tumor cells infected with Ad-Flt3L express Flt3L (pink circles) releasing it into the circulation. Flt3L in the bone marrow (BM) to induces dendritic cells (DCs) expansion, migration, and accumulation within the TME. (2) Immunogenic Glioblastoma (GBM) Cell Death: The prodrug ganciclovir (GCV) is administered systemically. Tumor cells infected with Ad-TK express TK protein which is capable of converting GCV to GCV-monophosphate (GCVp). This intermediate is further phosphorylated by cellular kinases: guanylate kinase (GK) and nucleoside diphosphokinase (NDK). GCV triphosphate (GCVppp) is a purine analog that selectively inhibits DNA replication in proliferating tumor cells leading to DNA breaks and apoptosis. The expression of TK in the presence of GCV mediates the release of damage associated molecular patterns (DAMPs), i.e., HMBG1, Calreticulin, and ATP from dying tumor cells. Expression of Flt3L recruits DCs into the tumor milieu where they take up brain tumor antigens released from the dying glioma cells. These DAMPs bind their corresponding receptors expressed on DCs. HMGB1 binds to TLR2/4, which promotes the production of cytokines and tumor antigen cross-presentation. The binding of extracellular ATP to purigenic receptor P2X7R further promotes the recruitment of DCs. Calreticulin binds to the CD91 receptor, which plays a major role in immunosurvillence. (3) Tumor Antigen Presentation: The DCs loaded with tumor antigens migrate to the cervical draining lymph node (DLN) where they present tumor antigens (Ag) to naïve T cells on MHC, priming tumor specific anti-glioma effector T cells. (4) Trafficking of Activated T cells: Primed CD8⁺ effector T cells enter circulation from DLN and migrate toward the TME. (5) Cytotoxic Glioma Killing T Cells: The tumor specific effector T cells enter the TME and kill residual glioma cells via the production of granzyme B, perforin and effector cytokine IFN-y. (6) Anti-GBM Immunological Memory: Continual exposure of T cells to tumor antigens promotes immunological memory. Memory T cells (CD103 and CD69) facilitate an anti-tumor response resulting in inhibition of tumor recurrence.