607. Organotypic Culture of Adult Porcine Retina as an In Vitro Screening Model for AAV-Mediated Gene Therapy in Ophthalmology

Abstract

Introduction: Gene therapy studies in vivo using animal models provide critical information regarding vector specificity, therapeutic efficacy, and safety. However, long durations to observe transgene expression, high degree of variability between animals, and reduced control of experimental conditions often leads to large sample sizes and expense. Organotypic cultures of the retina can provide an efficient in vitro model with a high degree of throughput and control that can be used for pharmacological screening. These explants preserve complex intercellular processes and communications among the neural retinal layers with minimal manipulation of tissue architecture and molecular pathways, making it a useful model in target-tissue validation of adeno-associated virus (AAV) vector variants. Porcine retina has similar anatomical and physiologic features to the human retina and is therefore a suitable surrogate for preclinical testing in ophthalmology. Methods: We have developed a porcine explant culture system from retinas isolated from adult pigs as a model to screen novel rAAV vectors for transduction efficiency and cellular tropism. Pig eyes were enucleated and the neural retina dissected away from the pigment epithelium within 1-2 hours postmortem. Tissue was cut into 4 mm pieces and placed photoreceptor layer down on polycarbonate transwells, and cultured in Neurobasal-A medium supplemented with B-27. Viability of the explant cultures was examined over time by gross morphology, quantification of glucose-6-phosphate dehydrogenase release from dying cells, and TUNEL assay. Transductions with AAV vectors were performed on retinal explants on the day the cultures were started, and transgene expression was analyzed by live cell imaging and immunofluorescence (IF) staining, or ELISA from tissue supernatants. Results: Retinal explants were viable for up to 4 weeks and IF staining of retinal protein markers including β-III-tubulin, Chx10, glutamine synthetase, rhodopsin and L/M opsin suggested that most retinal cell types were preserved in vitro, with tissue architecture closely resembling that of intact in vivo retina. Transduction with different rAAV serotypes encoding GFP displayed protein expression at 3 days, with gradually rising levels during the first 2 weeks following transduction. Furthermore, IF staining using retinal cell markers revealed differential cellular transgene expression from the AAV vectors encoding various cell-specific or ubiquitous promoters under investigation. Finally, high levels of secreted soluble VEGFR-1 protein expressed from rAAV were detected from explant culture media samples with expression increasing over a period of 3 weeks. Conclusions: Our results demonstrate viability of retinal explants for up to 4 weeks and efficient rAAV transduction with various serotypes. Our organotypic porcine retinal explant model facilitates the evaluation of efficacy, cellular tropism, and promoter selectivity of rAAV vectors in a fast, reproducible, and economical manner, contributing to a reduction in the use of animals for the development of therapies in ophthalmology.