Pre-existing antibodies to candidate gene therapy vectors (adeno-associated vector serotypes) in domestic cats

Durable contraception in the female domestic cat using viral-vectored delivery of a feline anti-Müllerian hormone transgene

Eighty percent of the estimated 600 million domestic cats in the world are free-roaming. These cats typically experience suboptimal welfare and inflict high levels of predation on wildlife. Additionally, euthanasia of healthy animals in overpopulated shelters raises ethical considerations. While surgical sterilization is the mainstay of pet population control, there is a need for efficient, safe, and cost-effective permanent contraception alternatives. Herein, we report evidence that a single intramuscular treatment with an adeno-associated viral vector delivering an anti-Müllerian hormone transgene produces long-term contraception in the domestic cat. Treated females are followed for over two years, during which transgene expression, anti-transgene antibodies, and reproductive hormones are monitored. Mating behavior and reproductive success are measured during two mating studies. Here we show that ectopic expression of anti-Müllerian hormone does not impair sex steroids nor estrous cycling, but prevents breeding-induced ovulation, resulting in safe and durable contraception in the female domestic cat.

Preexisting Neutralizing Antibodies against Different Adeno-Associated Virus Serotypes in Humans and Large Animal Models for Gene Therapy

Gene therapy is a potential cure for several inherited retinal dystrophies, and adeno-associated virus (AAV) has emerged as a vector of choice for therapeutic gene delivery to the retina. However, prior exposure to AAVs can cause a humoral immune response resulting in the presence of antibodies in the serum, which can subsequently interfere with the AAV-mediated gene therapy. The antibodies bind specifically to a serotype but often display broad cross-reactivity. A subset of these antibodies called neutralizing antibodies (NABs) can render the AAV inactive, thereby reducing the efficacy of the therapy. The preexisting NAB levels against different serotypes vary by species, and these variations need to be considered while designing studies. Since large animals often serve as preclinical models to test gene therapies, in this review we compile studies reporting preexisting NABs against commonly used AAV serotypes in humans and large animal models and discuss strategies to deal with NABs.

Overcoming the Challenges Imposed by Humoral Immunity to AAV Vectors to Achieve Safe and Efficient Gene Transfer in Seropositive Patients

One of the major goals of in vivo gene transfer is to achieve long-term expression of therapeutic transgenes in terminally differentiated cells. The extensive clinical experience and the recent approval of Luxturna^® (Spark Therapeutics, now Roche) and Zolgensma^® (AveXis, now Novartis) place vectors derived from adeno-associated viruses (AAV) among the best options for gene transfer in multiple tissues. Despite these successes, limitations remain to the application of this therapeutic modality in a wider population. AAV was originally identified as a promising virus to derive gene therapy vectors because, despite infecting humans, it was not associated with any evident disease. Thee large proportion of AAV infections in the human population is now revealing as a limitation because after exposure to wild-type AAV, anti-AAV antibodies develops and may neutralize the vectors derived from the virus. Injection of AAV in humans is generally well-tolerated although the immune system can activate after the recognition of AAV vectors capsid and genome. The formation of high-titer neutralizing antibodies to AAV after the first injection precludes vector re-administration. Thus, both pre-existing and post-treatment humoral responses to AAV vectors greatly limit a wider application of this gene transfer modality. Different methods were suggested to overcome this limitation. The extensive preclinical data available and the large clinical experience in the control of AAV vectors immunogenicity are key to clinical translation and to demonstrate the safety and efficacy of these methods and ultimately bring a curative treatment to patients.

Prevalence of pre-existing neutralizing antibodies against AAV serotypes 1, 2, 5, 6, 8, and 9 in sera of different pig strains

Pre-existing neutralizing antibodies (NAb) to adeno-associated virus (AAV) may diminish the efficacy of AAV-based therapies depending on the titer. To support gene therapy studies in pigs, the seroprevalence of NAb to AAV 1, 2, 5, 6, 8, and 9 serotypes were assessed in the sera of 3 different strains of pigs consisting of 60 Norsvin Topigs-20 strain, 22 Gottingen minipigs, and 40 Yucatan minipigs. Cell-based NAb assays were developed for various AAV serotypes. The sera were tested for NAb in a Lec-2 cell line for AAV9 vector and in a COS-7 cell line for the other AAV serotypes. In the 60 Topigs-20 strain aged 2 to 4 years old, 100% were positive for AAV2 NAb, 45 % positive for AAV6 NAb, and ~20% positive for each of AAV1, 5, 8, and 9 NAb. These data showed that approximately 80% of Norsvin Topigs-20 pigs evaluated were seronegative for pre-existing NAb to the AAV1, 5, 8, and 9 serotypes, respectively. In 22 Gottingen minipigs at 5-6 months of age, serum AAV-serotype specific NAb co-existed with that of various other AAV serotypes at 32 to 46 % between two serotypes. These results suggested that coexisting NAb resulted either from multiple AAV serotype co-infection or from one (or more) serotypes that can cross-react with other AAV serotypes in some minipigs. Among the 40 Yucatan minipigs, 20 of the minipigs were less than 3 months old and were all negative for NAb against AAV5, 8 and 9, and only one of these 20 pigs was positive to AAV1 and 6. We further determined the titers in those positive pigs and found most Gottingen minipigs had low titer at 1:20, whereas some of Topigs-20 pigs had titers between 1:80 to 1: 320, and some of Yucatan pigs had titers between 1: 160 to 1: 640. These results suggested that the majority of the pigs in the three strains would be amenable to gene therapy study using AAV1, AAV5, AAV8, and AAV9 and that prescreening on circulating AAV antibodies could be helpful before inclusion of pigs into studies.

Optimization of AAV6 transduction enhances site-specific genome editing of primary human lymphocytes

Adeno-associated virus serotype 6 (AAV6) is a valuable reagent for genome editing of hematopoietic cells due to its ability to serve as a homology donor template. However, a comprehensive study of AAV6 transduction of hematopoietic cells in culture, with the goal of maximizing ex vivo genome editing, has not been reported. Here, we evaluated how the presence of serum, culture volume, transduction time, and electroporation parameters could influence AAV6 transduction. Based on these results, we identified an optimized protocol for genome editing of human lymphocytes based on a short, highly concentrated AAV6 transduction in the absence of serum, followed by electroporation with a targeted nuclease. In human CD4⁺ T cells and B cells, this protocol improved editing rates up to 7-fold and 21-fold, respectively, when compared to standard AAV6 transduction protocols described in the literature. As a result, editing frequencies could be maintained using 50- to 100-fold less AAV6, which also reduced cellular toxicity. Our results highlight the important contribution of cell culture conditions for ex vivo genome editing with AAV6 vectors and provide a blueprint for improving AAV6-mediated homology-directed editing of human T and B cells.

Adeno-Associated Vector-Delivered CRISPR/SaCas9 System Reduces Feline Leukemia Virus Production In Vitro

Feline leukemia virus (FeLV) is a retrovirus of cats worldwide. High viral loads are associated with progressive infection and the death of the host, due to FeLV-associated disease. In contrast, low viral loads, an effective immune response, and a better clinical outcome can be observed in cats with regressive infection. We hypothesize that by lowering viral loads in progressively infected cats, using CRISPR/SaCas9-assisted gene therapy, the cat’s immune system may be permitted to direct the infection towards a regressive outcome. In a step towards this goal, the present study evaluates different adeno-associated vectors (AAVs) for their competence in delivering a gene editing system into feline cells, followed by investigations of the CRISPR/SaCas9 targeting efficiency for different sites within the FeLV provirus. Nine natural AAV serotypes, two AAV hybrid strains, and Anc80L65, an in silico predicted AAV ancestor, were tested for their potential to infect different feline cell lines and feline primary cells. AAV-DJ revealed superior infection efficiency and was thus employed in subsequent transduction experiments. The introduction of double-strand breaks, using the CRISPR/SaCas9 system targeting 12 selected FeLV provirus sites, was confirmed by T7 endonuclease 1 (T7E1), as well as Tracking of Indels by Decomposition (TIDE) analysis. The highest percentage (up to 80%) of nonhomologous end-joining (NHEJ) was found in the highly conserved gag and pol regions. Subsequent transduction experiments, using AAV-DJ, confirmed indel formation and showed a significant reduction in FeLV p27 antigen for some targets. The targeting of the FeLV provirus was efficient when using the CRISPR/SaCas9 approach in vitro. Whether the observed extent of provirus targeting will be sufficient to provide progressively FeLV-infected cats with the means to overcome the infection needs to be further investigated in vivo.

Adeno-associated virus-binding antibodies detected in cats living in the Northeastern United States lack neutralizing activity

Cats are a critical pre-clinical model for studying adeno-associated virus (AAV) vector-mediated gene therapies. A recent study has described the high prevalence of anti-AAV neutralizing antibodies among domestic cats in Switzerland. However, our knowledge of pre-existing humoral immunity against various AAV serotypes in cats is still limited. Here, we show that, although antibodies binding known AAV serotypes (AAV1 to AAV11) are prevalent in cats living in the Northeastern United States, these antibodies do not necessarily neutralize AAV infectivity. We analyzed sera from 35 client-owned, 20 feral, and 30 specific pathogen-free (SPF) cats for pre-existing AAV-binding antibodies against the 11 serotypes. Antibody prevalence was 7 to 90% with an overall median of 50%. The AAV-binding antibodies showed broad reactivities with other serotypes. Of 44 selected antibodies binding AAV2, AAV6 or AAV9, none exhibited appreciable neutralizing activities. Instead, AAV6 or AAV9-binding antibodies showed a transduction-enhancing effect. AAV6-binding antibodies were highly prevalent in SPF cats (83%), but this was primarily due to cross-reactivity with preventive vaccine-induced anti-feline panleukopenia virus antibodies. These results indicate that prevalent pre-existing immunity in cats is not necessarily inhibitory to AAV and highlight a substantial difference in the nature of AAV-binding antibodies in cats living in geographically different regions.