550. Primary Human CD34+ Hematopoietic Stem/Progenitor Cell (HSPC) Transduction By AAV6 Serotype Vectors: Strategies for Overcoming the Donor-Variation
Authors: Zifei Yin, George V. Aslanidi, Chen Ling, Yuanhui Zhang, Changquan Ling, Mengqun Tan, Mervin C. Yoder, Arun Srivastava
Journal: Molecular Therapy (2015)

We have reported that of the 10 commonly used AAV serotype vectors, AAV6 is the most efficient in transducing primary human hematopoietic stem/progenitor cells (HSPCs), both in vitro and in murine xenograft models in vivo (Cytotherapy, 15: 986-998, 2013; PLoS One, 8(3): e58757, 2013). However, the transduction efficiency of these vectors ranged between ~6-87% in HSPCs obtained from several different donors (n=11). Such a wide range of transduction efficiency of AAV6 vectors is presumably due to different levels of expression of the putative receptors and/or co-receptors on these cells. In our more recent studies, we observed that the transduction efficiency of AAV2 vectors could be augmented both by performing transduction of HSPCs with the wild-type (wt)-AAV2 vectors at high cell density, or by using capsid-modified Y444F+Y500F+Y731F+T491V-mutant AAV2 vectors. In the present studies, we examined whether similar strategies could also be employed to increase the transduction efficiency of HSPCs from donors that are not transduced efficiently by wt-AAV6 vectors. Primary human HSPCs were transduced with either 3×103 or 3×104 vgs/cell of scAAV6-EGFP vectors either at low-density (20,000 cells in 50 ml) or at high-density (200,000 cells in 50 ml). scAAV2-EGFP vectors were also used for comparison. Transgene expression was evaluated by flow cytometry 48 hrs post-transduction. These results showed that whereas only ~14% of the cells transduced at low-density with high moi expressed the transgene, the transduction efficiency at high-density increased up to ~20% and 25%, at low, and high mois, respectively, also with a significant increase in the mean fluorescence intensity, thus corroborating that the initial cell-cell contact was a critical factor in achieving increased transduction. Next, the transduction efficiencies of the wt- and the capsid-modified triple-mutant (Y705F+Y731F+T492V) AAV6 vectors were compared. Again, the wt- and the capsid-modified quadruple-mutant (y444F+Y500f+Y731F+T491V) AAV2 vectors were used for comparison. HSPCs were transduced with 1×104 vgs/cell at high-density (200,000 cells in 50 ml), and transgene expression was evaluated by flow cytometry 48 hrs post-transduction. These results also clearly documented that the ~27% transduction efficiency of the wt-AAV6 vectors was increased by up to ~45% with the capsid-modified AAV6 vectors, with a concomitant increase in the mean fluorescence intensity. Taken together, these studies further corroborate the novel mechanism of "cross-transduction" by recombinant AAV vectors of human cells in general, and HSPCs in particular. Additional human HSPC transplant studies in a murine xenograft model in vivo are currently underway, the successful completion of which is likely to have significant implications of these strategies in the optimal use of AAV6 vectors in the potential gene therapy of human disorders involving the hematopoietic system.