Concurrent Measures of Fusion and Transduction Efficiency of Primary CD34+ Cells with Human Immunodeficiency Virus 1-Based Lentiviral Vectors Reveal Different Effects of Transduction Enhancers

Lentiviral vectors (LVs) are used for various gene transfer applications, notably for hematopoietic gene therapy, but methods are lacking for precisely evaluating parameters that control the efficiency of transduction in relation to the entry of vectors into target cells. We adapted a fluorescence resonance energy transfer-based human immunodeficiency virus-1 fusion assay to measure the entry of nonreplicative recombinant LVs in various cell types, including primary human hematopoietic stem progenitor cells (HSPCs), and to quantify the level of transduction of the same initially infected cells. The assay utilizes recombinant LVs containing β-lactamase (BLAM)-Vpr chimeric proteins (BLAM-LVs) and encoding a truncated form of the low-affinity nerve growth factor receptor (ΔNGFR). After infection of target cells with BLAM-LVs, the vector entry rapidly leads to BLAM-Vpr release into the cytoplasm, which is measured by cleavage of a fluorescent substrate using flow cytometry. Parallel cultures of the same infected cells show transduction efficiency resulting from ΔNGFR expression. This LV-based fusion/transduction assay is a dynamic and versatile tool, revealing, for instance, the postentry restrictions of LVs known to occur in cells of hematopoietic origin, especially human HSPCs. Furthermore, this BLAM-LV assay allowed us to evaluate the effect of cytokine prestimulation of HSPCs on the entry step of LVs. The assay also shows that transduction enhancers such as Vectofusin-1 or Retronectin can partially relieve the postentry block, but their effects differ in how they promote LV entry. In conclusion, one such assay should be useful to study hematopoietic postentry restrictions directed against LVs and therefore should allow improvements in various LV-based gene therapy protocols.