Vascular compartment labeling by intravenous rAAV9 results in cell lineage tracing to diverse parenchymal cells in adult mice

A growing number of studies have reported the transduction of a myriad of parenchymal cell types following an intravenous recombinant adeno-associated virus (rAAV) injection. This finding has significant implications for potential routes of minimally invasive human gene therapy. However, to date, an empirically backed rationale for any mechanism underlying this extravascular transgene expression is lacking. What’s more, the literature is fragmented, with studies focusing on a single cell type or tissue that is often within the context of a disease, thereby confounding a broader interpretation of these results. We sought to build upon our previous study, which focused on the notion of endothelial cell contributions via transdifferentiation or extracellular vesicles to transgene expression by parenchymal cells. This previous work employed tamoxifen-induced cell lineage tracing in adult transgenic mice using the vascular endothelial cadherin (VEcad) promoter to drive reporter expression. Since VEcad has been shown to be expressed by very rare (0.3%) non-endothelial cell types, we initiated cell lineage tracing in this study specifically within the vascular compartment by intravenous delivery of rAAV serotype 9 (rAAV9), a variant shown to readily transduce endothelial cells among many other cell types. We also utilized multiple time points that matched our previous study for direct comparison in addition to examining 25 tissues and cell types in a context that did not involve pathogenic induction so that broad interpretations might be more readily possible. We found numerous cell types traced from intravenous rAAV9, all of which have been previously reported. However, importantly, we also observed that five cell types (including skeletal myocytes, pancreatic beta cells, pancreatic acinar cells, duodenal epithelial cells and ileal epithelial cells) traced from both intravenous rAAV9 and the VEcad promoter from our previous study. Thus, these five cell types warrant further investigation as potential downstream targets of endothelial cell transdifferentiation and/or extracellular vesicle recipients in vivo.