AAV-mediated gene transfer offers great promise for gene therapy. However, the success of early liver based gene therapy trials has been hampered due to pre-existing humoral immunity and transgene expression is short-lived because of an immune response directed against the capsid containing hepatocytes. The goal of this project is to develop a novel class of human gene transfer vectors based on adeno-associated virus (AAV) that combines high efficiency in vivo transduction of target tissues, with the capability to circumvent or overcome host immunologic responses in humans. We have shown proof-of-principle studies using 8 diverse naturally isolated capsids in DNA shuffling (or molecular evolution) reactions to make a library of capsid genes. Each capsid in the library contains a unique sequence derived from a combination of different pieces of the starting capsids. We have selected for capsids that promote AAV infection in human hepatocytes and that are resistant to pooled human immune sera. The first capsid that was vectorized and extensively studied has important biological properties that have resulted in some improvemenet in gene transfer paradigms as well as provided new information related to capsid biology. However, these first studies have led us to believe that in vivo selection of capsids may be more relevant to our ultimate goal related to human liver-based gene therapy. To test this premise, we plan to make additional capsid libraries that will increase the diversity of input capsids prior to selection. Novel AAV capsids will then be selected for on primary cells in culture (as before) as well as in vivo in a new mouse model containing a chimeric liver that is made up of at least 50% human hepatocytes. Selection pressure for isolating AAV capsids resistant to the human immune response will be carried out using a battery of antibody strategies both in vitro and in vivo. AAV vectors with the novel AAV capsids will be tested both in mouse and canine hemophilia B animal models. Importantly, we can also determine the value of these animal models in predicting AAV transduction of human cells in vivo by using these vectors to measure human hepatocyte transduction in the human- chimeric mouse model. Vectors showing promise will be tested for their immunogenic potential for human T cell responses. It is hoped that this form of selective pressure will enable us to isolate AAV capsid sequences allowing efficient in vivo transduction while evading both neutralization from pre-existing immunity, and possibly cell-mediated immune responses that plagued our previous AAV-2 liver clinical trial for hemophilia.
The goal of our proposal is to use molecular evolution to create new AAV gene transfer vectors that have superior properties and overcome the limitations of the current vectors based on viruses found in nature. Once the new AAV vectors are created, they will be tested in animal models of human disease.
