Gene therapy: efficient targeting of hematopoietic stem cells

Comment on Verhoeyen et al, page 3386

Lentiviral vectors containing recombinant envelope proteins that target hematopoietic stem cells have now been engineered to increase the efficiency of gene transfer.

Efficient gene transfer to human hematopoietic stem cells (HSCs) to develop clinical gene therapy is still a major challenge. HSCs tend to be quiescent and therefore difficult to transduce, and envelope proteins that are commonly used for transduction of mammalian cells do not specifically target HSCs. Therefore, attempts have been made to design transduction protocols that activate HSCs or stimulate their division. Similarly, several attempts have been made to design recombinant envelope proteins that will target specific target cells, HSCs, tumor cells, and others.FIG1 

Engineering recombinant envelope proteins to improve the natural ones has generally met with limited success, although there are some exceptions.¹  For example, Cosset's group (Maurice et al² ) has in recent years successfully targeted lentiviral vectors to T cells for efficient transduction. Resting T cells are quite resistant to lentiviral transduction but by engineering lentiviral vector virions displaying T-cell–activating single-chain antibody polypeptide, resting peripheral blood lymphocytes could be efficiently transduced.²  An improvement of this method was recently reported where HIV-1 vector virions displayed interleukin-7 to increase transduction of resting T cells without inducing a naive-to-memory phenotypic switch.³ 

In this issue of Blood, Verhoeyen and colleagues have used an analogous approach. They have engineered recombinant membrane proteins and incorporated them into lentiviral vector virions displaying cytokines on the surface that can bind to receptors on primitive hematopoietic progenitor and stem cells. A recombinant membrane protein consisting of the transmembrane influenza hemagglutinin glycoprotein was fused to truncated forms of thrombopoietin (TPO), such that the thrombopoietin is displayed on the outside of the producer cell membrane after transfection and thereby ends up on the exterior of the vector virion. Similarly, the stem cell factor (SCF) cDNA was fused to the amphotrophic Moloney murine leukemia virus env glycoprotein, transfected into the packaging cells, and displayed on the lentiviral vector virions. As with standard lentiviral vectors, the vesicular stomatitis virus G (VSV-G) envelope protein was also used. Since the receptors for TPO and SCF, c-mpl and c-kit, respectively, are expressed on HSCs, virions displaying SCF and TPO can bind these receptors on HSCs in addition to the VSV-G binding to phospolipids in the HSC membrane (Figure 1).

Findings in the paper by Verhoeyen et al clearly show that the transduction efficiency of human CD34⁺ cells and hematopoietic progenitor cells is increased several fold using these engineered vectors, and increased transduction efficiency is also seen in candidate human hematopoietic cells that can repopulate immunocompromised mice. In addition, HSCs treated with these vectors do not need recombinant hematopoietic growth factors to survive since these are supplied by the virions.

In order to take these observations to the clinic, it would be preferable to make permanent packaging cells expressing these recombinant cytokine molecules in addition to the other viral genes (including VSV-G) required for proper packaging of the vectors. It will be worth developing this system further since efficient uptake of vectors by stem cells may allow high-level gene transfer at low multiplicity of infection (MOI). ▪