Immunology of Gene and Cell Therapy
Authors: David M. Markusic, Ashley T. Martino, Christopher D. Porada, Thierry VandenDriessche
Journal: Molecular Therapy (2020)

The immune system in humans performs many functions, including recognition and elimination of invading pathogens and surveillance and elimination of tumors. This activity is balanced by inhibitory responses, such as regulatory T cells (Tregs) and checkpoint inhibitory receptors, which mitigate damage to healthy tissues from prolonged inflammation. Immune responses are a major determinant for the outcome of gene and cell therapies. For instance, the innate and adaptive arms of the immune system may respond to components of the vector or its gene products or eliminate cells producing the therapeutic protein, thus causing loss of therapy and/or immunotoxicities. Conversely, gene- and cell-based immunotherapies can be designed to promote immune tolerance or induce a desired response against tumor or vaccine antigens. Through a collection of review and original research articles, the joint special issues of Molecular Therapy and Molecular Therapy - Methods & Clinical Development provide a comprehensive overview of immune obstacles to gene and cell therapies, different cell products used in immunotherapy, and viral- and cell-based oncolytic therapies. Two reviews highlight immune responses following adeno-associated virus (AAV)-based gene therapy and discuss the development of strategies to overcome the immune limitations.1Martino A.T. Markusic D.M. Immune response mechanisms against AAV vectors in animal models.Mol. Ther. Methods Clin. Dev. 2019; 17: 198-208Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar,2Verdera H.C. Kuranad K. Mingozzi F. AAV Vector Immunogenicity in Humans: A Long Journey to Successful Gene Transfer.Mol. Ther. 2019; 28 (this issue): 723-746Abstract Full Text Full Text PDF Scopus (199) Google Scholar First, Martino and Markusic1Martino A.T. Markusic D.M. Immune response mechanisms against AAV vectors in animal models.Mol. Ther. Methods Clin. Dev. 2019; 17: 198-208Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar provide an overview of preclinical studies of innate and adaptive immune mechanisms in response to AAV-based gene therapy. Preclinical studies on innate immunity, particularly the involvement of Toll-like receptor (TLR) signaling, on adaptive immune responses to the AAV vector and the transgene product have provided new and unexpected mechanistic insights into the complex interactions between the innate and adaptive immune response. These mechanistic insights are important to ultimately develop clinically relevant strategies that could curb unwanted immune responses following gene therapy. For example, recent data presented at the annual American Society of Hematology meeting showed negative outcomes in hemophilia AAV gene therapy trials correlated with vector genomes with high CpG content, which is a ligand for TLR9.3Nathwani A.C. Gene therapy for hemophilia.Hematology Am. Soc. Hematol. Educ. Program. 2019; 2019: 1-8Crossref PubMed Scopus (64) Google Scholar Subsequently, Verdera et al.2Verdera H.C. Kuranad K. Mingozzi F. AAV Vector Immunogenicity in Humans: A Long Journey to Successful Gene Transfer.Mol. Ther. 2019; 28 (this issue): 723-746Abstract Full Text Full Text PDF Scopus (199) Google Scholar address the immunogenicity of AAV-based gene therapy in clinical trials, discussing the clinical experience with immunity in the liver, muscle, central nervous system, and eye. Both vector-specific factors (such as production system, vector design and dose, target tissue, and route of administration) and host-specific factors (including the underlying mutation, disease-specific changes in the target tissue, and previous exposure to recombinant protein) are highlighted for consideration to ensure durable therapeutic transgene product expression in patients. The ability to induce immune tolerance to the therapeutic protein after gene therapy offers many intriguing therapeutic perspectives that may ultimately overcome some of the limitations of conventional protein substitution therapy. This occurs most frequently following liver targeting, but it also occurs in other tissues. The mechanisms contributing to induction of immune tolerance after gene therapy are the subject of intense study, but preclinical studies have shown that Tregs are critical. Shirley et al.4Shirley J.L. de Jong Y.P. Terhorst C. Herzog R.W. Immune Responses to Viral Gene Therapy Vectors.Mol. Ther. 2019; 28 (this issue): 709-722Abstract Full Text Full Text PDF Scopus (223) Google Scholar more broadly review immune responses to AAV, adenoviral, and lentiviral vectors. Alternatively, ex vivo genetically modified hematopoietic stem cells (HSCs) are being used to treat a variety of disorders, including primary immune deficiencies, lysosomal storage diseases, and hemoglobinopathies. Many of these therapies are expected to receive marketing authorization following completion of phase III studies. However, Drysdale et al.5Drysdale C.M. Tisdale J.F. Uchida N. Immunoresponse to Gene-Modified Hematopoietic Stem Cells.Mol. Ther. Methods Clin. Dev. 2019; 16: 42-49Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar present potential immunological challenges with an overview of pre-clinical and clinical studies with gene-modified HSCs for treating inherited disorders with an intact immune system. They discuss the need for adjunct immune suppression with less severe conditioning protocols, xenographic (i.e., gene editing nucleases) or congenitally absent proteins, and in diseases with inherent inflammatory responses. To date, chimeric antigen receptor (CAR) T cells are one of the most promising cancer immunotherapies available to patients, as evidenced by two approved CD19-CAR T cell products. Tyagarajan et al.6Tyagarajan S. Spencer T. Smith J. Optimizing CAR-T Cell Manufacturing Processes during Pivotal Clinical Trials.Mol. Ther. Methods Clin. Dev. 2019; 16: 136-144Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar provide an overview of the process involved in generating and clinically validating a CD19-CAR T cell therapy product and subsequent generation of a commercial product. This study highlights the challenges of developing a centralized process to generate a consistent CD19-CAR T cell product from diverse donors and expanding from early academic phase I/II studies into commercial production. Next-generation CARs with improved persistence, potency, and resistance to exhaustion are presently being engineered to improve efficacy against other cancers. Alternatively, oncolytic virus (OV) therapy leverages the lytic and inflammatory properties of OVs for targeted cytotoxicity of malignant cells and enhanced cell-mediated tumor immunity. Lemos de Matos et al.7Lemos de Matos A. Franco L.S. McFadden G. Oncolytic viruses and the immune system: The dynamic duo.Mol. Ther. Methods Clin. Dev. 2020; (Published online January 15, 2020)https://doi.org/10.1016/j.omtm.2020.01.001Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar give a comprehensive overview of immunomodulatory aspects of OVs to optimize viral oncotherapy and list criteria for selecting effective OVs. To overcome the immunosuppressive tumor microenvironment, viral modifications can be made that increase immunogenicity, and "armed" viruses expressing transgenes to promote pro-inflammatory and Th1 cellular responses can be employed. Adenovirus (Ad) vectors, members of the OV family, are immunogenic, which has limited their application in the treatment of genetic disorders. However, oncolytic Ads (OAds) have a long history as a therapy for malignancies, culminating in the first-ever approved gene therapy product for head and neck cancers in China, ONYX-015. Shaw and Suzuki8Shaw A.R. Suzuki M. Immunology of Adenoviral Vectors in Cancer Therapy.Mol. Ther. Methods Clin. Dev. 2019; 15: 418-429Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar summarize development of OAds, describe the biological mechanisms central to Ad immunity, identify methods to enhance the anti-viral immune response in viral infected tumor cells, and discuss the clinical benefits and limitations of OAds. Some cell-based immunotherapies have an inherent plasticity and can be either proinflammatory or immune suppressive, depending on the disease target. Mesenchymal stromal cells (MSCs) have long been known to play a critical role within the hematopoietic microenvironment of the bone marrow, but they are now recognized to be present within all major tissues of the body as pericytes. These cells possess many unique characteristics that make them ideally suited as both a platform for drug and gene delivery and as immunotherapeutics, as they can be alternately primed to either inhibit or promote an immune response. In the special issue of Molecular Therapy - Methods & Clinical Development, Almeida-Porada et al.9Almeida-Porada G. Atala A.J. Porada C.D. Therapeutic Mesenchymal Stromal Cells for Immunotherapy and for Gene and Drug Delivery.Mol. Ther. Methods Clin. Dev. 2020; (Published online January 21, 2020)https://doi.org/10.1016/j.omtm.2020.01.005Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar provide insights into these remarkable cells, discussing their initial discovery, methods for their identification and isolation, their use as gene delivery vehicles, the functionality of these cells upon infusion, and their marked promise in specific targeting of tumors. MSCs can either induce an anti-tumor immune response or function as Trojan horses to deliver prodrugs to tumors, where they can then release their potent chemotherapeutic payload, thereby enhancing anti-tumor effects while simultaneously reducing systemic toxicity. Dendritic cells (DCs) play a critical role in the body's ability to detect and respond appropriately to both foreign invading organisms and tumor cells. Given the ease with which these cells can be derived from monocytes in whole blood or apheresis products, and subsequently loaded with an antigen of choice, DCs have enjoyed widespread clinical use as immunotherapeutics for over 25 years. In the special issue of Molecular Therapy - Methods & Clinical Development, Hopewell and Cox10Hopewell E.L. Cox C. Manufacturing Dendritic Cells for Immunotherapy: Monocyte Enrichment.Mol. Ther. Methods Clin. Dev. 2020; (Published online January 15, 2020)https://doi.org/10.1016/j.omtm.2019.12.017Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar give an introduction to the biology of DCs and their value as immunotherapeutics and then perform a "deep-dive" into the specifics of the various leading methods for isolating and enriching DC and subsequently expanding these cells for clinical applications. They also provide a rigorous comparison of the properties and functionality of DCs isolated and enriched with each of these methods, highlighting the need for the establishment and universal adoption of methods that yield reproducible and robust DC-based products for clinical use. Finally, Scott11Scott D.W. Genetic engineering of T cells for immune tolerance.Mol. Ther. Methods Clin. Dev. 2019; 16: 103-107Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar provides an overview of engineered Tregs expressing either T cell receptors (TCRs) or CARs that recognize a given antigen to suppress or potentially reverse unwanted immune responses (e.g., in the context of auto-immunity or anti-drug antibodies). Alternatively, Tregs or cytotoxic CD8+ T cells can be armed with a specific antigen that is recognized by the B cell antibody receptor (BAR), resulting in suppression and elimination of the B cell response.11Scott D.W. Genetic engineering of T cells for immune tolerance.Mol. Ther. Methods Clin. Dev. 2019; 16: 103-107Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar These emerging concepts are likely to have broad implications in the immunotherapy field.