As important vectors for ectopic protein expression, gene silencing, and progenitor cell barcoding, lentiviruses continue to emerge as versatile research and clinical tools. For studies employing cell types that are relatively resistant to transduction, high-titer lentivirus preparations with low cytotoxicity are required. During lentivirus production, carryover plasmid DNA endotoxins, transfection reagents, damaged packaging cells, and virus concentration procedures are potential sources of cytotoxicity. As an often unevaluated property of lentivirus preparations, cytotoxicity can unwittingly skew estimates of functional titers and complicate interpretations of transduced cell phenotypes. By employing hematopoietic UT7epo cells cultured in erythropoietin (EPO) below maximal dosing, we first define a sensitive flow cytometric bioassay for critically assessing the cytotoxicity (and titers) of lentivirus preparations. Bioassay of custom preparations of research-grade lentiviruses from six commercial sources unexpectedly revealed substantial cytotoxicity (with certain preparations additionally registering titers several log below designated values). To overcome such limiting properties, we further report on unique, efficient workflows for reproducibly preparing and processing high-titer, low-cytotoxicity (HTLC) lentiviruses at research scale. These HTLC lentiviruses reliably transduce peripheral blood hematopoietic stem/progenitor cells (PB-HSPCs) at frequencies ≥40%, with low cytotoxicity. In addition, by employing cyclosporin H (to inhibit IFITM3), PB-HSPCs can be transduced at heightened efficiency with nominal cytotoxicity. Overall, this work provides straightforward approaches to (1) critical assessment of the cytotoxicity of lentivirus preparations; (2) reproducible generation (and concentration) of high-quality lentiviruses via a streamlined workflow; and (3) transduction of PB-HSPCs at benchmark levels with nominal cytotoxicity.
