[Ex vivo expansion of human hematopoietic stem cells by passive transduction of the HOXB4 homeoprotein].

Expansion of human hematopoietic stem cells (HSCs) is a challenge for cellular therapy. It currently relies on either the use of recombinant cytokines or transfer of transcription factor genes. Among these, the HOXB4 homeoprotein is of particular interest since it promotes the expansion of mouse HSCs without inducing leukemia. To prevent potential deleterious side effects associated with stable HOXB4 gene transfer into the cells, we took advantage of the ability of homeoproteins to passively pass through cell membranes. We have shown that, when co-cultured with stromal cells engineered to secrete HOXB4, human stem cells and immature progenitors clearly were expanded. This expansion was associated with enhanced stem cell repopulating capacity in vivo and maintenance of pluripotentiality. The role that HOXB4 plays on stem cell expansion has also been tested on human lymphoid progenitors. We found that our model of protein transfer was also able to induce the expansion of the immature lympho-myeloid and pro-T/NK progenitors as well as of more mature NK progenitors. We then looked for synergistic activities between HOXB4 and other homeoproteins such as HOXC4. We found that HOXC4 was able to promote the expansion of human hematopoietic cells in vitro roughly as HOXB4 did and that the presence of both HOXB4 and HOXC4 molecules induced even higher expansion levels of these cells. Our method provides a basis for developing cell therapy strategies using expanded HSCs that are not genetically modified.