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  1. Biologics Research Projects

Immunogenicity of Gene Therapy Products

Ronit Mazor, PhD

Office of Tissues and Advanced Therapies
Division of Cellular and Gene Therapies
Gene Transfer and Immunogenicity Branch



Dr. Mazor earned her undergraduate and MSc degrees from the Sackler Faculty of Medicine in Tel Aviv University, Israel. She performed her graduate research studies in the National Institute of Health International Graduate Partnership Program (GPP), earning a PhD from Tel Aviv University. Dr. Mazor performed her post-doctoral training in the National Cancer Institute in Bethesda, MD studying the immunogenicity of recombinant immunotoxins for cancer therapy. After her post-doctoral training, Dr. Mazor joined the Department of Antibody Discovery and Protein Engineering in Medimmune/AstraZeneca (Gaithersburg, MD) where she established cellular and in silico, pre-clinical immunogenicity prediction platforms.

In 2019, Dr. Mazor became a principal investigator in the Gene Transfer and Immunogenicity Branch in the Office of Tissues and Advanced Therapies of CBER. Her laboratory studies the interaction between the immune system and gene therapy viral vectors.

General Overview

Gene therapy is a technique that modifies the expression of genes or alters biological properties of cells or tissues for therapeutic purposes. This includes replacement, inactivation, or introduction of new genes. Introducing the gene therapy product into human cells requires a vector that will deliver the gene into the cells and incorporate those genes into the gene expression mechanism in those cells.

The most commonly used vectors for gene delivery are viruses. Viruses attach to a host cell and transfer their genetic material into it, forcing the cell to make copies of the virus. In gene therapy, we adapt this mechanism to deliver therapeutic genes into the human body. We modify viruses so they contain only therapeutic genetic material, while retaining their original ability to infect human cells. This enables the viruses to transfer therapeutic DNA into the host cells, which then express the gene products.

Adeno-associated virus (AAV) is a small nonpathogenic virus in nature that can be used as a transduction vector in humans for gene therapy. AAV vectors have been used successfully in several clinical and preclinical studies of several disorders. The first two gene therapy products approved by the FDA used AAV2 and AAV9 vectors for gene delivery.

A major problem of many types of therapeutics, including protein therapeutics, gene therapies, and cellular therapies, is their immunogenicity. This process occurs when a therapeutic agent elicits an immune response that blocks the efficacy of a therapeutic product by immune mediated clearance, neutralization, or a cellular immune response. Immunogenicity of gene therapy products is associated with various arms of the immune system and can cause diverse immune related toxicities.

Our research program is focused on characterizing and understanding the immunological components that play a role in the immune response to AAV vectors. The goal is to develop new methods for sensitively and efficiently monitoring immune responses in treated patients and to predict immune responses in patients prior to treatment, based on their cellular reactivity.

Scientific Overview

Adeno-associated virus (AAV) is a small virus that can be used as a transduction vector in gene therapy.  AAV is among the most widely used vectors for gene delivery and is used in a growing number of FDA applications and clinical trials worldwide.

Due to the viral origin of AAV, the capsids can induce cellular and humoral immune responses that trigger neutralization of the vector with anti-AAV antibodies, which prevents transduction in patients. Over 90 percent of humans have preexisting binding antibodies to some AAV serotypes. Some of these antibodies are neutralizing and may cause loss of efficacy even on the first dose. Therefore, there is a critical need to improve our understanding of risks of AAV immunogenicity and to develop technologies for evaluating and mitigating this response.

Our research program focuses on characterizing and understanding the immunological components that play a role in the immune response against AAV vectors. To study these immunological components in human cells we established a highly sensitive, high-throughput T cell assay to characterize common factors in AAV that impact T cells from various human samples and diverse HLA backgrounds. We use these methods to study the impact of post-translational modifications in recombinant AAV on the immune response.

Using this data, we are developing new methods to sensitively and efficiently monitor cellular and humoral immune responses against AAV capsids. These methods will help to ensure the safety and efficacy of gene therapies during their development and use.


  1. Mazor R and Pastan I. (2020) Immunogenicity of Immunotoxins containing Pseudomonas Exotoxin A: Causes, Consequences and Mitigation. Front Immunol. 11:1261.
  2. Mazor R, King EM, Pastan I. Anti-drug antibodies to LMB-100 are enhanced by mAbs targeting OX40 and CTLA4 but not by mAbs targeting PD1 or PDL-1v. Cellular Immunol. 2018 Dec; 334:38-41.
  3. Leshem Y, King EM, Mazor R, Reiter Y, Pastan I. SS1P Immunotoxin Induces Markers of Immunogenic Cell Death and Enhances the Effect of the CTLA-4 Blockade in AE17M Mouse Mesothelioma Tumors. Toxins. 2018 Nov 14;10(11)
  4. Mazor R, King EM, Pastan I. Strategies to Reduce the Immunogenicity of Recombinant Immunotoxins. Am J Pathol. 2018 Aug;188(8):1736-1743.
  5. Mazor R, King E, Onda M, Kishimoto K, Addissie S, Pastan I. Tolerogenic Nanoparticles Restore the Antitumor Activity of Recombinant Immunotoxins by Mitigating Immunogenicity. Proc Natl Acad Sci U S A. 2018 Jan 23;115(4).
  6. King E*, Mazor R*, Cuburu Nicolas, Pastan I. Low-dose methotrexate for the induction of immune tolerance towards anti-mesothelin immunotoxin LMB-100. J Immunol. 2018 Mar 15;200(6):2038-2045.
  7. Kaplan G*, Mazor R*, Lee F, Jang Y, Pastan I. Improving the In Vivo Efficacy of an Anti-Tac (CD25) Immunotoxin by Pseudomonas Exotoxin A Domain II Engineering. Mol Cancer Ther. 2018 Jul;17(7):1486-1493.
  8. Mazor R, Kaplan G, Jang Y, Lee F, Kreitman R, Pastan A. Rational design of Recombinant immunotoxins targeting CD25+ cells in T cell malignancies by mutating T cell epitopes in domain II and III of PE38. Cellular Immunology. 2017 Jan 5.
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