An efficient selection system for packaging herpes simplex virus amplicons

Due to their simplicity and flexibility of genomic construction, herpes simplex virus (HSV) amplicon-based vectors are attractive vehicles for gene delivery. However, a significant problem faced in the generation of amplicon stocks is the low amplicon to helper virus (A/H) ratio. In order to improve the proportion of amplicons generated, a selection system for amplicon production was developed in which the HSV thymidine kinase (TK) gene is inserted into an amplicon plasmid and an HSV mutant with both TK and glycoprotein H (gH) genes deleted is used as a helper virus. Using a protocol in which amplicon stocks are passaged 2-3 times in BHK cells of TK- and gH+ genotype in the presence of selection medium containing methotrexate, stock preparations with high A/H ratio (up to 5:1) and high amplicon titre (>1 x 10(9) infectious units/ml) were generated. In vitro characterization demonstrated that a high level of biologically functional products can be efficiently produced from these amplicon constructs.