Sustained Long Term Engraftment and Transgene Expression of Peripheral Blood CD34+ Cells Transduced with Advanced Lentiviral Vectors in the Preence of ABC Inhibitors.

Because mobilized peripheral blood (MPB) represents an attractive source of cells for gene therapy, we investigated the ability of advanced-generation lentiviral vectors (LV) to transfer the Enhanced Green Fluorescent Protein (EGFP) gene into CD34+ cells isolated from MPB in culture conditions that allowed expansion of transplantable human HSC. So far, a few studies reported transduction of MPB cells with LV pseudotyped with VSV-G. However, the critical issue remains whether primitive, hematopoietic repopulating cells have indeed been transduced.

In vitro and in vivo experiments provided here show that MPB CD34+ cells can be effectively transduced by advanced generation LV: they do not lose their proliferation potential, self renewal and multilineage differentiation ability, moreover transgene expression is maintained. Following 48 hour-exposure to FL+TPO+SCF+IL6 and overnight incubation with vector particles, MPB CD34+ cells were further cultured for up to six weeks. During this period of culture the CD34+ cells population, the CD34+/GFP+ fraction and the CFC output were maintained.

Transduction efficiency of NOD/SCID repopulating cells (SRC) was assessed by serial transplants into NOD/SCID mice. Primary mice transplanted with transduced cells showed high level of engraftment (11,5 % CD45+ cells): within the CD45+ cells 11,4% expressed GFP (n=15). Moreover, mice transplanted with transduced, week-1 expanded cells showed higher levels of human engraftment than those transplanted with transduced, non-expanded cells (range from 9.25 % to 10.5 % for transduced basal cells; range from 14, 3 % to 38,19 % for transduced 1-week expanded cells). Engraftment was multilineage, with GFP+/lineage+ cells. Serial transplants were performed with transduced, week 1-expanded cells. Secondary engraftment levels were 1.6 %; 19.3 % of human cells were GFP+ (n=3). Moreover, levels of human engraftment were higher in mice injected with transduced, 1- week expanded cells (3.5 %) rather than in those injected with transduced, non expanded cells (1.2 %). Engraftment was multilineage with GFP+/lINEAGE+ cells.

These results show that LV efficiently transduced HSC, that maintain proliferation and self-renewal ability.