485. Long-Term In Vivo Engraftment and Transgene Expression from CD34+ Human Hematopoietic Stem Cells Transduced with Pseudotyped rAAV
Authors: Sarah Hardy, Margaret Heerwagen, Lane Bilbrey, Kamehameha K. Wong, Saswati Chatterjee
Journal: Molecular Therapy (2006)

The search for the ideal vector for stem cell gene therapy continues as recognized problems remain with all existing viral vectors. The recent identification of over 100 new serotypes of AAV significantly expands the repertoire of available gene transfer vectors. Additionally, the capacity to pseudotype rAAV2 genomes in novel capsids facilitates evaluation. We and others have analyzed rAAV2-mediated gene transfer into hematopoietic stem cells (HSC). However, despite efficient gene-marking of primitive HSCs, there are identified limitations to transgene expression especially at early time points. The use of novel capsids has been shown to overcome these limitations in other tissues and expand the host range of AAV. We tested transduction of CD34+ cells from cytokine-primed peripheral blood, CD34+CD38- cells from cord blood (CB) and the CD34+CD38- cell line TF1a with AAV2-dsRed pseudotyped in capsids of AAV1, 2, 5, 7, 8 and 9. Transduced cells were analyzed by flow cytometry 24 and 48 hours post transduction. Results revealed that the highest level of transgene expression was observed with AAV7 in HSCs from all sources while AAV 2 and 9 showed the lowest. The hierarchy of transgene expression in vitro was: AAV7>AAV8≥AAV1>AAV5>AAV2=AAV9 regardless of the source of CD34+cells. These results indicate that the use of certain novel capsids allows circumvention of limitations to expression from AAV2. We next evaluated in vivo engraftment of human HSCs transduced with pseudotyped rAAV in a NOD/SCID xenotransplant model. Transduced CD34+ cells purified from either CB or cytokine primed peripheral blood were transplanted into sublethally irradiated mice. The pseudotyped rAAV2 genome encoded the firefly luciferase gene under the control of the CBA promoter. Serial live imaging of recipients revealed transgene expression from each of the 6 serotypes tested. However the levels and trends for long term expression variedby serotype. At early time points, the highest initial expression was observed with AAV9 followed by AAV7 and 8. AAV2 showed the lowest level of initial expression. However differences emerged over time. AAV7, 9 and 8 capsids emerged as the best serotypes for long- term expression in vivo, followed by AAV2. AAV5 and 1 showed a rapid decline in expression after initial moderately high levels of expression. These differences suggest that the processing of pseudotyped rAAV in vivo varies with the capsid. AAV serotypes also appear to have different transduction properties in vivo as compared with in vitro. The mechanisms underlying these differences are being investigated. The rapid and robust expression seen in vivo with certain serotypes is highly promising and underscores the need for further evaluation of pseudotyped rAAV for stem cell gene transfer. The search for the ideal vector for stem cell gene therapy continues as recognized problems remain with all existing viral vectors. The recent identification of over 100 new serotypes of AAV significantly expands the repertoire of available gene transfer vectors. Additionally, the capacity to pseudotype rAAV2 genomes in novel capsids facilitates evaluation. We and others have analyzed rAAV2-mediated gene transfer into hematopoietic stem cells (HSC). However, despite efficient gene-marking of primitive HSCs, there are identified limitations to transgene expression especially at early time points. The use of novel capsids has been shown to overcome these limitations in other tissues and expand the host range of AAV. We tested transduction of CD34+ cells from cytokine-primed peripheral blood, CD34+CD38- cells from cord blood (CB) and the CD34+CD38- cell line TF1a with AAV2-dsRed pseudotyped in capsids of AAV1, 2, 5, 7, 8 and 9. Transduced cells were analyzed by flow cytometry 24 and 48 hours post transduction. Results revealed that the highest level of transgene expression was observed with AAV7 in HSCs from all sources while AAV 2 and 9 showed the lowest. The hierarchy of transgene expression in vitro was: AAV7>AAV8≥AAV1>AAV5>AAV2=AAV9 regardless of the source of CD34+cells. These results indicate that the use of certain novel capsids allows circumvention of limitations to expression from AAV2. We next evaluated in vivo engraftment of human HSCs transduced with pseudotyped rAAV in a NOD/SCID xenotransplant model. Transduced CD34+ cells purified from either CB or cytokine primed peripheral blood were transplanted into sublethally irradiated mice. The pseudotyped rAAV2 genome encoded the firefly luciferase gene under the control of the CBA promoter. Serial live imaging of recipients revealed transgene expression from each of the 6 serotypes tested. However the levels and trends for long term expression variedby serotype. At early time points, the highest initial expression was observed with AAV9 followed by AAV7 and 8. AAV2 showed the lowest level of initial expression. However differences emerged over time. AAV7, 9 and 8 capsids emerged as the best serotypes for long- term expression in vivo, followed by AAV2. AAV5 and 1 showed a rapid decline in expression after initial moderately high levels of expression. These differences suggest that the processing of pseudotyped rAAV in vivo varies with the capsid. AAV serotypes also appear to have different transduction properties in vivo as compared with in vitro. The mechanisms underlying these differences are being investigated. The rapid and robust expression seen in vivo with certain serotypes is highly promising and underscores the need for further evaluation of pseudotyped rAAV for stem cell gene transfer.