Long-term retinal transgene expression with FIV versus adenoviral vectors

Purpose: Gene therapy for chronic retinal diseases will require long-term expression of therapeutic transgenes. Lentiviral and adenoviral (Ad) vectors are gene delivery systems with markedly different properties. Lentiviral vectors require integration into the host genome, which facilitates long-term expression, while Ad vectors remain episomal. We compared time course, location, and extent of transgene expression from replication-deficient feline immunodeficiency virus (FIV) vectors and Ad vectors in neonatal rat retina. Methods: A dose-response study was conducted to determine the optimal subretinal dose for comparison of FIV and Ad vectors with an internal cassette expressing β-galactosidase under transcriptional control of the CMV immediate-early gene promoter/enhancer. Forty-two five-day old Sprague-Dawley rats received subretinal injections of 2 μl containing 2 × 10³ transducing units (TU, n=14), 2 × 10⁴ TU (n=14) or 2 × 10⁵ TU (n=14) of FIV vector (right eye) and Ad vector (left eye). Expression was evaluated 48 h after transduction. In the subsequent long-term expression study, 60 five-day old rats received a subretinal injection of 2×10⁵ TU FIV vector (right eye) and Ad vector (left eye). Ten pairs of eyes were analyzed at 1 week, 1 month, 3 months, 6 months, 12 months, and the remainder at 16 months. Eye cups were evaluated in a masked manner for extent of β-galactosidase expression (graded 0-5) by whole mount microscopy and by cross sectional histology. Results: In the dose-response study, 2×10⁵ TU resulted in consistent, widespread retinal transduction with both vectors and was selected as the dose for the subsequent study. In the long-term expression study, FIV vector resulted in a higher grade of expression than Ad at multiple single time points and produced higher overall expression when data from all eyes across the entire 16 month study were analyzed (p=0.01). Retinal expression was present at 16 months with both vectors. β-galactosidase expression was limited to the retinal pigment epithelium (RPE) until the first month, but later was also found to a lesser extent in neurosensory retina with each vector. In contrast to FIV, most Ad injected eyes showed signs of focal accumulation of macrophage-like cells with disrupted retinal architecture. Conclusions: Both FIV and Ad vectors result in long-term transgene expression in RPE after subretinal injection. FIV vectors show more promise than Ad as delivery systems for retinal diseases since they transduce greater areas of RPE, result in less cellular infiltrate, and cause less disruption of retinal architecture. The persistent expression at 16 months of follow-up suggests that these lentiviral vectors are useful for gene therapy of chronic retinal diseases.