Objectives Alpha-sarcoglycan deficiency is a severe form of muscular dystrophy (LGMD2D) without treatment. Gene replacement represents a strategy for correcting the underlying defect. Questions related to this approach were addressed in this clinical trial, particularly the need for immunotherapy, and persistence of gene expression. Methods A double-blind, randomized controlled trial using rAAV1.tMCK.hSGCA injected into the extensor digitorum brevis (EDB) muscle was conducted. Control sides received saline. A three-day course of methylprednisolone accompanied gene transfer without further immune suppression. Results No adverse events were encountered. SGCA gene expression increased 4-5 fold over control sides when examined at 6 weeks (two subjects) and 3 months (one subject). The full sarcoglycan complex was restored in all subjects and muscle fiber size was increased in the 3-month subject. AAV1 neutralizing antibodies were seen as early as 2 weeks. Neither CD4+ nor CD8+ cells were increased over contralateral sides. Scattered foci of inflammation could be found but showed features of programmed cell death. ELISpot showed no IFN-γ response to α-SG or AAV1 capsid peptide pools with the exception of a minimal capsid response in one subject. Restimulation to detect low frequency capsid specific T cells by ELISpot assays was negative. Results of the first three subjects successfully achieved study aims precluding the need for additional enrollment. Interpretation The finding of this gene replacement study in LGMD2D has important implications for muscular dystrophy. Sustained gene expression was seen, but studies over longer time periods without immunotherapy will be required for design of vascular delivery gene therapy trials

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