Minimizing transgene clearance

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A recent study proved the concept that rAAV-mediated intramuscular delivery of scFv immunoadhesins to rhesus macaques can generate sustained high serum levels of these inhibitors, and protect study animals from a high-dose SIV challenge. However, some of the treated animals in that study developed anti-immunoadhesin antibody responses, resulting in clearance of antiviral inhibitors. Immune clearance of this sort has been well documented in the rAAV gene-therapy literature: expression of foreign proteins from rAAVs in large animal models can in general elicit strong trangene-directed immunity, leading to host clearance of the expressed transgene. Indeed immune clearence remains a key challenge to further translational development of rAAV-delivered therapies. It is therefore necessary to better define immune processes contributing to anti-inhibitor antibody responses, and to develop novel strategies to prevent clearance of expressed transgenes. These are the main objectives of Project 2. We proposed the following studies to accomplish these objectives: 1). To limit transduction and antigen presentation of rAAV-immunoadhesin in unintended target cells and tissues by evaluating a panel of muscle specific promoters for promoter strength, tissue specificity and feasibility for use in the packaging size-limited rAAV genome. 2). To detarget rAAV transduction from antigen presenting cells by harnessing endogenous miRNAs for dendritic cell specific post-transcriptional transgene silencing. 3). To induce sustained systemic tolerance to immunoadhesin expression by hepatotropic rAAV8-mediated and liver-specific transduction. These studies will generate an AAV vector genome optimized to limit clearence of expressed transgenes, an objective critical to the therapeutic use of AAV vectors in many contexts.