244. Successful AAV Vector-Mediated Gene Transfer into Canine Skeletal Muscle Required Suppression of Excess Immune Responses

Abstract

Duchenne muscular dystrophy (DMD) is an X-linked, lethal muscle disorder caused by a mutation in the dystrophin gene (14 kb cDNA). An adeno-associated virus (AAV) vector-mediated gene transfer is one of attractive approaches to the treatment of DMD, but it has a limitation in insertion size up to 4.9 kb. To find a short but functional dystrophin cDNA, we have previously constructed three micro-dystrophin cDNAs, and generated transgenic (Tg) dystrophin-deficient mdx mice expressing micro-dystrophin. Among them, CS1-Tg mdx mice showed lowest levels of serum creatine kinase, complete amelioration of muscle pathology, and nearly full restoration of contractile force (BBRC. 293:1265, 2002). We also showed that muscle-specific MCK promoter in AAV vector could drive longer expression of the LacZ gene than the CMV promoter in skeletal muscle (Gene Ther. 23:1576, 2002). Furthermore, we constructed the AAV2 vector expressing ΔCS1 micro-dystrophin driven by MCK promoter, and demonstrated that AAV vector-mediated ΔCS1 transfer could ameliorate dystrophic phenotypes in mdx muscles (7th ASGT Annual Meeting 2004, in submission). For the application of this strategy to DMD patients, however, it is necessary to examine therapeutic effects and the safety issue in larger animal models, such as dystrophic dogs. We recently established a colony of beagle-based canine X-linked muscular dystrophy in Japan (Exp. Anim. 52: 93, 2003). When the AAV vector encoding the LacZ gene driven by a CMV promoter was introduced into skeletal muscles of dogs, β-galactosidase (β-gal) was expressed only in few fibers of injected muscle after 2 weeks of injection. No β-gal-positive fiver was detected in canine muscle at 4 and 8 weeks post-injection. Instead, large numbers of mononuclear cells appeared around β-gal-expressing fibers in injected muscle. To clarify mechanisms of low transduction and cellular infiltration in canine muscle after transfer of AAV vector, we examined viral infectivity, cytotoxicity and immune responses. First, we infected AAV vector into canine primary myotubes. This in vitro study showed that AAV vector could allow higher transgene expression in canine myotubes than in murine ones. Second, we tested whether injection of AAV particle elicit cytotoxicity or not. When the AAV vector expressing no transgene was injected into canine muscle, almost no infiltrating cells was observed in injected muscle. Third, we investigated immune responses. A lot of CD4- or CD8-positive cells were detected in clusters of infiltrating cells, together with elevated serum level of anti-β-gal IgG. To confirm low transduction depending on immune response, dogs received daily oral administration of cyclosporine (20 mg/kg/day) from -5 day of the introduction of the AAV vector. Immunosuppression considerably improved transduction efficiency by an AAV vector introduction in canine muscle. These results suggested that AAV vector-mediated gene transfer elicited stronger immune responses in canine muscle, and it was necessary to know the molecular background of excess immune responses and to find the way to minimize and suppress immune responses.