99. A Novel Mechanism of Transduction of Human Cells By Recombinant AAV Vectors

Abstract

The human embryonic kidney cell line, HEK293, is readily transduced by the most commonly used AAV2 serotype vectors because these adherent cells abundantly express the cell surface receptor, heparan sulfate proteoglycan (HSPG), and one of the co-receptors, human fibroblast growth factor receptor 1 (FGFR1) for AAV2. However, a human erythroleukemia cell line, K562, commonly used as a model for human hematopoietic cell transduction, is not transduced as efficiently, although these cells grown in suspension, express both HSPG and FGFR1 only at modestly lower levels. We reasoned that the lack of proximity of HSPG and FGFR1 on K562 cells might account for the suboptimal transduction of these cells, and hypothesized that if the transduction was performed at high cell density, presumably allowing for HSPG on one cell to come in close proximity to FGFR1 on the neighboring cell, then AAV2 bound to HSPG on one cell could utilize FGFR1 on the neighboring cell to gain entry in the latter, and vice versa, thus leading to increased transduction. To test this hypothesis, HEK293 and K562 cells were transduced with 3×103 vgs/cell of scAAV2-EGFP vectors either at low-density (60,000 cells in 50 ml) or at high-density (480,000 cells in 50 ml). Transgene expression was evaluated by flow cytometry 48 hrs post-transduction. Whereas ~90% of HEK293 cells expressed the transgene at low-density, the transduction efficiency at high-density increased up to ~95%. On the other hand, only ~35% of K562 cells were transduced at low-density, but interestingly, the transduction efficiency at high-density increased to ~65%, thus corroborating that close cell-cell contact facilitated vector entry into these cells. That the initial cell-cell contact was critical in achieving improved transduction, was further corroborated by experiments in which cells were transduced at low-density, and subsequently pooled together to reach high-density, and conversely, cells were transduced at high-density, and soon after transduction, were diluted to low-density. The increased transduction was observed only under the latter condition. Similar results were obtained with two additional human cell lines, M07e and Raji, which express low to extremely low levels of HSPG and FGFR1, respectively, and consequently, are transduced extremely poorly by AAV2 vectors. These studies were also extended to include primary human CD34+ hematopoietic stem/progenitor cells (HSPCs), also grown in suspension, and known to be transduced sub-optimally by AAV2 vectors. In these studies also, whereas only~5% of these cells were transduced at low-density, the transduction efficiency increased up to ~20% at high-density. Taken together, our studies have revealed a novel mechanism, which we have termed “cross-transduction”, which AAV vectors exploit to gain entry into target cells, and which may have implications in the optimal use of AAV vectors in human gene therapy applications.