LONG-TERM HISTOLOGICAL FOLLOW-UP OF GENETICALLY-MODIFIED MYOBLASTS GRAFTED INTO THE BRAIN

Although primary muscle cells have been used as intracerebral vehicles for transgene expression in the past, data concerning their long-term survival after grafting into the brain, and the reaction of the host tissue to their implantation are lacking. In order to study these aspe cts, we have implanted, into the brain, primary muscle cells infected ex vivo with recombinant retroviruses carrying the E. coli LacZ gene. The muscle cells were delivered stereotaxically into different areas o f the brain of adult rats and the grafts were analyzed up to 105 days after implantation. Intraventricular implantations did not lead to sur viving grafts. In contrast, myoblasts developed when they were grafted into gray or white matter regions. They appeared numerous during the first weeks, but decreased dramatically in number over time. Over mont hs, the grafts appeared to fill up with collagen. Astrocytes elaborate d a continuous glia limitans surrounding the implant. Blood vessels co ming from the host tissue were found within the grafts. The blood-brai n barrier was permanently disrupted within the transplants. beta-Galac tosidase activity was abundant during the first weeks, but decreased t o a very low level subsequently. This decrease paralleled that of the number of muscle cells. In conclusion, myoblasts transplanted into the adult brain survived only temporarily, which implies a transient tran sgene expression. In addition, before being eliminated, muscle cells w ere surrounded by a glia limitans, which may limit exchanges with the host tissue. Altogether, these results suggest that intracerebral tran splantation of myoblasts may possibly provide a relevant vehicle only for shea-term delivery of a gene product.