Gene Delivery by Lentivirus Vectors: An Overview

For more than two decades, retroviral biology has been the most intensely studied field in virology. The retroviral genome is encoded by a 7–11 kb positivesense single-stranded RNA molecule, two of which homodimerize and package in lipid-enveloped viral particles. Following attachment and receptor-mediated entry into host cells, viral reverse transcriptase and integrase enzymes mediate reverse transcription and integration of the virus genome into the host-cell chromatin. The ability of a replication competent retrovirus to incorporate a herpes simplex virus thymidine kinase (tk ) gene into the genome of a mouse cell and to convert NIH-3T3 TK^- cells into TK⁺ transformants was first described in 1981 (1 ,2 ). These studies established the basis of using retroviruses as vehicles for efficient therapeutic gene delivery into mammalian cells. Twenty years of extensive research of retrovirus-vector biology resulted in major improvements in vector design and retrovirus-vector production. High-titer concentrated retrovirus vectors (>10⁹ infectious units [IU]/mL) can be generated by several retrovirus-vector stable producer lines. The ability to pseudotype retrovirus vectors with a variety of envelope proteins, including the vesicular stomatitis virus G glycoprotein (VSV-G), significantly broadens the tropism of replication-defective retrovirus vectors. In addition, combinations of synthetic and tissue-specific promoters, which were incorporated into retrovirus vectors, allowed long-term and regulated gene expression in vector transduced cells.