Strategies to overcome immunity in gene therapy of DMD

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Duchenne muscular dystrophy (DMD) is characterized by progressive muscle degeneration due to mutations in dystrophin, the largest known human gene. In many cases the dystrophin protein is not functional because mutational deletions shift translation to an incorrect reading frame. Gene replacement using recombinant adeno-associated virus vectors (rAAV) is one approach to treatment of DMD. Functional miniaturized dystrophin genes have been developed for delivery by recombinant AAV vectors. Host immune responses to the AAV capsid and the minidystrophin transgene product represent potentially important risks to the success of gene therapy. On one hand neutralizing antibodies and T cells directed against the capsid might prevent or reduce sustained transgene expression. There is also a possibility that T cells directed against miniaturized dystrophin will eliminate transduced myocj/tes. Indeed, we recently detected CD4+ helper and CD8-I- cytotoxic T cell responses to mini-dystrophin in four or six subjects after transduction of muscle with a recombinat AAV vector. In at least one human subject the transgene-encoded epitopes were located in sequences deleted from dystrophin gene. We have developed a non-human primate model of foreign transgene expression to better understand the influence of host immunity on outcomes after rAAV vector transduction of muscle. Rhesus macaques resemble humans in that they often harbor neutralizing capsid-specific antibodies and T cells that could interfere with successful therapy. Moreover expression of a non-self transgene in skeletal muscle is quickly terminated by a cellular immune response that could be directed against the encoded foreign protein or the rAAV capsid protein. In Project 2 we propose to use AAV-immune rhesus macaques to (i) enhance muscle transduction by transient depletion of serum neutralizing antibodies against the rAAV capsid and (ii) assess the impact of transduction on recall of memory T cell responses directed against the capsid. In addition, whether memory T cells against the AAV capsid or transgene product play a dominant role in destruction of transduced myocytes will be assessed. Methods to temper these responses in monkeys will be evaluated.