Cell engineering for muscle gene therapy:: Extemporaneous production of retroviral vector packaging macrophages using defective herpes simplex virus type 1 vectors harbouring gag, pol, env genes

Gene therapy as a treatment for neuromuscular diseases is an ever-developing concept based on the use of DNA as the therapeutic agent. In the search for appropriate strategies a bottleneck exists, however, concerning the targeting of vectors carrying the therapeutic gene, to all pathologic sites. These diseases are often characterised by multiple widespread lesions spread over a large area, rendering administration by local injection into tissues, clinically irrelevant. With this in mind, we have proposed that circulating cells (monocytes/macrophages), which home naturally to inflammatory lesions, characteristic of degenerating muscle, could be used as shuttles able to track down every damaged site, and deliver there a corrective gene. Our aim is to mobilise a corrective gene from these infiltrating monocyte-macrophages, into muscle cells, a process of in situ cell to cell gene transfer which could be accomplished using a retroviral vector, since the regeneration process involves the proliferation of muscle precursors before they fuse to form replacement fibres. For this, monocyte-macrophages must be engineered into `packaging cells' containing both the replication deficient retrovirus carrying the gene of interest and an helper genome (gag-pol-env) needed for its assembly and secretion. Here, we have transduced a monocyte cell line using a defective murine Moloney leukemia retrovirus carrying the LacZ reporter gene. This provided us with a platform to investigate the possibility of gag-pol-env vector driven packaging of the defective retrovirus by macrophages. We show that an herpes simplex virus type I amplicon harbouring the Moloney gag, pol, env sequences is able to rescue the defective retrovirus vector from macrophages, allowing gene transfer into muscle precursor cells. After fusion, these cells gave rise to genetically modified myotubes in vitro.