AAV-Vectored Fms-Related Tyrosine Kinase 3 Ligand Inhibits CD34 + Progenitor Cell Engraftment in Humanized Mice.

Humanized mice have become useful animal models for HIV/AIDS. Since NOD.Cg-Prkdc scid Il2rgtm1Wjl/SzJ (NSG) mice allow the engraftment of primary human immune cells, we aim to determine the role of human Fms-related tyrosine kinase 3 ligand (hFlt3L), a major growth factor for dendritic cells (DCs), in regulating the differentiation of cord blood-derived CD34+ progenitor cells in this murine species. Soluble recombinant hFlt3L protein and AAV-vectored hFlt3L were administrated before or after human CD34+ progenitor cell transplantation, respectively. We then measured the peripheral levels of hFlt3L by ELISA. Meantime, reconstituted human immune cells were analyzed by flow cytometry over time. We found that without hFlt3L there were significantly increased types of human immune cells in NSG-huCD34 compared with NSG-huPBL mice but the frequency of human DCs remains low. Transient treatment with recombinant hFlt3L expanded human conventional CD1c+ and CD141+ DCs as well as plasmacytoid DCs in humanized NSG-huCD34 mice. Surprisingly, however, the prolonged in vivo expression of AAV-vectored hFlt3L resulted in significant suppression of total human CD34+ cell engraftment and differentiation. The suppression occurred within 2 weeks when AAV-vectored hFlt3L was administered either before or after the transplantation of CD34+ progenitor cells, which was likely associated with the induction of murine myeloid-derived immune suppressive cells and reactive oxygen species in NSG-huCD34 mice. Since chronic  HIV-1 patients displayed significantly high levels of hFlt3L expression, our findings may have implication to explore the role of prolonged hFlt3L in regulating  the differentiation of human CD34+ progenitor cells in both NSG-huCD34 mice and infected people. Graphical Abstract ᅟ.