434 REPLICATION COMPETENT LENTIVIRUS (RCL) ANALYSIS IN RECIPIENT ANIMALS OF TRANSGENIC EMBRYOS PRODUCED BY LENTIVIRAL TRANSFER

Abstract

Lentiviral vectors have become a useful tool for gene therapies and the expression of small hairpin (sh)RNAs to target genes both in vitro and in vivo. This is due primarily to their ability to integrate transgenes into both dividing and nondividing cells, as well as the lack of silencing in the germ cell line. However, the retroviral basis for these recombinant, replication-incompetent viruses has prompted investigation into their safety for use in therapeutics and transgenic animal production. Concerns are that recombination with wild-type viruses or endogenous retroviral elements may allow the integrated provirus genome to become replication competent. In order to investigate this, transgenic embryos produced by lentiviral-mediated gene transfer were transferred into recipient animals. The lentiviral plasmids used in this experiment contained a self-inactivating 3′ untranslated region as well as a green fluorescent protein (GFP) reporter gene (Miyoshi et al. 1998 J.Virol. 72, 8150-8157). Recombinant lentivirus was produced through cotransfection of HEK293T cells with the lentiviral transfer plasmid as well as a packaging plasmid and a plasmid encoding the vesicular stomatitis virus glycoprotein (VSV-G), which was used to pseudotype viral particles. Two methods were used for production of transgenic embryos. The first was lentiviral transduction of bovine fetal fibroblasts followed by somatic cell nuclear transfer. The second was incubation of IVP hatched ovine blastocysts in culture medium containing infectious recombinant lentiviral particles. Recipients were then sacrificed and analyzed for the presence of replication competent lentivirus (RCL). Tissues collected from each recipient included blood, lung, lymph node, kidney, liver, mammary gland, ovary, skeletal muscle, spleen, and uterus. In addition, when available, fetal and placental samples were collected. Analyses for RCL included a serum ELISA test for presence of the p24 HIV antigen as well as real-time quantitative PCR (qRT-PCR) on genomic DNA for the presence of VSV-G. To date, a total of 13 recipients including both sheep and cattle have been analyzed. All animals had p24 titers below the level of detection for the assay (<12.5 pg mL^-1). Additionally, the tissues mentioned above have been analyzed by qRT-PCR for 6 of the 13 recipients so far, and all have been negative for VSV-G as determined by comparison with positive and negative control samples. Additional collections and analysis are ongoing. A lack of detection of RCL in these animals will build confidence in the use of lentiviral vectors in transgenic animal production and will lend support for their safety in both animal and human therapies.