The effect of enhancers on the lentiviral transduction efficiency in the human RPE cells: Insights for advancing retinal gene therapies

Background: Viral vectors including lentiviruses (LV), adenoviruses (AV) and adeno-associated viruses (AAV) have been used as common vehicles for gene transfer in gene therapy of various human diseases. The efficacy of gene transfer, however, still remains unsatisfying and thus, a number of biologic and chemical substances are used for enhancing the transduction efficiency. In this article, we aim to evaluate the cytotoxicity and impact of individual and combinational treatment of two polycationic agents hexadimethrine bromide (polybrene; Pb) and protamine sulfate (PS) on the transduction efficiency of lentiviral particles in the primary human retinal pigment epithelial (RPE) cells. Methods: Cytotoxicity of Pb and PS at individual and combinational concentrations was evaluated using MTT cell viability assay in RPE cells. Lentiviral particles were produced using a set of second-generation vectors and different combinations of two enhancers, Pb and PS, were added to the transduction medium. The transduction efficiency of lentiviral particles in RPE cells was evaluated using flow cytometry and calculating the mean fluorescence intensity (MFI), as well as the percentage of green fluorescent protein (GFP)-positive cells. All the treatments were performed in three replicates. Results: Cell viability assay revealed that individual treatment of Pb at all concentrations by up to 25 μg/ml was safe to RPE cells with no visible toxicity and its combination with PS did not significantly improve its effect on the cell viability. Interestingly, Pb at all concentrations significantly improved the transduction efficiency compared to control virus with the best MFI result seen at 10 μg/ml concentration. The mean population of GFP-positive cells was also most enhanced at that concentration (p-value: 0.006). At a combinational concentration of 10 μg/ml of Pb and 2 μg/ml of PS, the highest level of transduction efficiency was reported (MFI: 801, GFP+: 65.4 %); however, the value was not significant when compared to enhancers used in individual treatments or relative to other combinations. Conclusion: Pb enhanced the transduction efficiency of lentiviral particles in RPE cells and in combination with PS achieved the highest level of MFI and GFP-positive percentage. Although, the efficiency of the combination was not significant compared to that of individual treatments, this study may suggest the potential of combinational enhancers for applications in gene therapy.