Novel strategy to block Nabs for AAV gene delivery

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Adeno-associated virus (AAV) vectors have been applied in clinical trials in patients with different disorders. Although successful in clinical studies, one of major concerns for broader AAV vector application for patients is high prevalence of neutralizing antibody (Nab). In the general human population, over 95% of individuals have been infected by AAV and, on average, approximate 50% of them have Nabs. Several approaches have been explored to evade AAV Nabs, including epitope masking with PEG or exosome, different serotype of AAV vector, rational design and combinatorial mutagenesis of the capsid in situ, as well as biological depletion of Nabs (empty capsid utilization, B cell depletion, plasma-apheresis and IgG cleavage enzymes). Generally, these approaches have low efficiency or side effects or AAV tropism change. Therefore, it is imperative to develop ideal strategies to evade Nabs, but without a change in tissue tropism from capsid engineering or negative side effects from pharmacological treatment. Recently, we have developed a vector independent protein based strategy to universally block Nabs and demonstrated that this approach is effective against a broad range of pre-existing Nab concentrations by use of a unique mycoplasma derived protein and it’s analogues, termed Protein-M. Protein-M is able to interact with immunoglobulin from any species without antigen dependence. We have found that protein M protected AAV vector neutralization over 100 fold in vitro and 1000 fold in mice with adoptive transfer of Nab positive serum, so far, the most effective strategy to evade AAV Nabs. However, the wild type protein M is structurally unstable at body temperature (37°C), making it challenging for clinical application. The protein unfolding at body temperature may correspond with decreased Nab blockade when using the protein in vivo. In order to improve PM stability, we have rationally designed in silico a library of 150 individual protein mutants engineered for improved thermal stability. At least 10 mutants were identified with enhanced thermal stability and high biological activity. In this proposal, we will further characterize the efficacy of these mutants in vitro and in mice with adoptive transfer of Nab positive serum (Aim 1). Next, we will study the role of protein M in AAV re-dosing in mice with pre-immunization (Aim 2). Then we will move forward to a disease model to test the ability of the best protein M variants to block AAV Nabs in animals for phenotypic correction with AAV vector mediated gene delivery (Aim 3). The long term objective is to develop a more effective strategy to evade Nab activity in future gene therapy with AAV vectors in patients with Nabs.