Research Article: Induction of Immune Tolerance to Foreign Protein via Adeno-Associated Viral Vector Gene Transfer in Mid-Gestation Fetal Sheep

Date Published: January 31, 2017

Publisher: Public Library of Science

Author(s): Marcus G. Davey, John S. Riley, Abigail Andrews, Alec Tyminski, Maria Limberis, Jennifer E. Pogoriler, Emily Partridge, Aliza Olive, Holly L. Hedrick, Alan W. Flake, William H. Peranteau, Eduard Ayuso.


A major limitation to adeno-associated virus (AAV) gene therapy is the generation of host immune responses to viral vector antigens and the transgene product. The ability to induce immune tolerance to foreign protein has the potential to overcome this host immunity. Acquisition and maintenance of tolerance to viral vector antigens and transgene products may also permit repeat administration thereby enhancing therapeutic efficacy. In utero gene transfer (IUGT) takes advantage of the immunologic immaturity of the fetus to induce immune tolerance to foreign antigens. In this large animal study, in utero administration of AAV6.2, AAV8 and AAV9 expressing green fluorescent protein (GFP) to ~60 day fetal sheep (term: ~150 days) was performed. Transgene expression and postnatal immune tolerance to GFP and viral antigens were assessed. We demonstrate 1) hepatic expression of GFP 1 month following in utero administration of AAV6.2.GFP and AAV8.GFP, 2) in utero recipients of either AAV6.2.GFP or AAV8.GFP fail to mount an anti-GFP antibody response following postnatal GFP challenge and lack inflammatory cellular infiltrates at the intramuscular site of immunization, 3) a serotype specific anti-AAV neutralizing antibody response is elicited following postnatal challenge of in utero recipients of AAV6.2 or AAV8 with the corresponding AAV serotype, and 4) durable hepatic GFP expression was observed up to 6 months after birth in recipients of AAV8.GFP but expression was lost between 1 and 6 months of age in recipients of AAV6.2.GFP. The current study demonstrates, in a preclinical large animal model, the potential of IUGT to achieve host immune tolerance to the viral vector transgene product but also suggests that a single exposure to the vector capsid proteins at the time of IUGT is inadequate to induce tolerance to viral vector antigens.

Partial Text

Adeno-associated viral vectors (AAVs) hold considerable promise for the therapeutic management of a spectrum of life-threatening inherited disorders. AAVs are non-pathogenic and can result in durable expression of the transgene product without incorporating into the host genome making them one of the most clinically relevant viral vector systems. A major limitation to successful AAV gene transfer is the generation of host immune responses to vector capsid proteins and the transgene product [1–3]. Experimentally, the generation of anti-AAV neutralizing antibodies following initial vector exposure has been shown to inhibit transduction upon repeat vector delivery [4,5]. Since repeat administration of AAV vector and the corrective transgene will be necessary for the management of many target diseases, a clinical need exists to develop safe strategies to overcome host immune responses to both the transgene product and the vector capsid proteins.

AAV vectors have been investigated in several clinical trials for a spectrum of monogenetic disorders [22–32]. In these trials, a significant barrier to success is the host immune response to the vector capsid proteins and/or the transgene product. This is exemplified in all preclinical and clinical studies of gene therapy for congenital myopathies in which anti-capsid and anti-transgene product related humoral and cellular responses have limited success [23]. As the promise of AAV-based gene therapy becomes increasingly realized, safe strategies to overcome both preexisting and induced host immune responses to vector capsid antigens and the transgene product are required. This challenge is even more significant for genetic diseases which may require repeat administration of the viral vector to boost transgene expression. A number of these genetic diseases such as cystic fibrosis and Duchenne muscular dystrophy can be prenatally diagnosed [33]. The ability to prenatally diagnose a target disease combined with the natural immunologic immaturity of the fetus highlights the possibility of delivering the viral vector and transgene product to a pre-immune fetus with subsequent induction of immune tolerance to vector capsid antigens and the transgene product. In the current study, we demonstrate, in a large animal preclinical model, that postnatal tolerance to a foreign protein (i.e. GFP) can be achieved following intravascular delivery of AAV-IUGT expressing a foreign protein. Despite the ability to induce tolerance to the transgene product, IUGT with AAV.GFP failed to induce tolerance to the AAV vector capsid proteins. Finally, at the termination of the experiment (6 months of age), persistent hepatic transgene expression was seen in the in utero recipient of AAV8.GFP while the recipient of AAV6.2.GFP lost transgene expression between 1 and 6 months of age.