Gene Transfer to Repopulating Human CD34+ Cells Using Amphotropic, GALV or RD114 Pseudotyped HIV-1 Based Vectors from Stable Producer Cells.

A novel, stable human immunodeficiency virus type 1 (HIV-1) vector packaging system, STAR, was tested for its ability to transduce human cord blood CD34+ progenitor cells assayed both in vitro and after transplantation to NOD/SCID mice. Vectors pseudotyped with three different gammaretrovirus envelopes were used; the amphotropic MLV envelope (MLV-A), a modified gibbon ape leukemia virus envelope (GALV+) and a modified feline endogenous virus RD114 envelope (RDpro). Titration of vectors harvested from the stable packaging cells gave the following results: MLV-A: 1–3x10⁶ inf.u/ml, GALV+: 1x10⁶ inf.u/ml and RDpro: 3–10x10⁶ inf.u/ml CB CD34+ cells were transduced on Retronectin® preloaded with vector either in fresh medium (PL alone) or in vector containing medium (PL+VCM). Gene transfer to freshly thawed CD34+ in the absence of cytokines was very low. Addition of cytokines (CK; SCF, TPO and FL) increased gene transfer efficiency significantly and this was further augmented if the cells were prestimulated for 24 hours wit the same cytokines. Concentration of the vectors (15-fold) by low-speed ultracentrifugation increased gene transfer to CD34+ cells in vitro even further. More than 90% of cells were transduced with a single exposure to the RDpro vector as determined by GFP expression using flow cytometry. The two other pseudotypes transduced approximately 70% of the cells under the same conditions. Detailed in vitro transduction data is shown in table as percentage GFP positive cells:

Transplantation of CD34+ cells prestimulated for 24 hours and then transduced with a single hit of concentrated vector to irradiated NOD/SCID mice revealed that the RDpro vector transduced 55.1% of NOD/SCID repopulating human cells which was significantly higher than the MLV-A (12.6%) or GALV+ (25.1%) pseudotyped vectors. Thus, the STAR packaging system, especially with the modified RD114 envelope, appears to be a new valuable tool for the genetic modification of primitive human progenitor cells.