Recombinant TAT-HOXB4 Protein Promotes Ex Vivo Expansion of Primitive Human Hematopoietic Cells.

The clinical application of therapeutic protocols depending on hematopoietic stem cell (HSC) transplantation for long term reconstitution with donor-derived HSCs, particularly in patients previously exposed to intensive radiation or chemo-therapy, or when grafts are purged of infiltrating malignant or alloreactive T cells, can be severely hampered by limited numbers of HSCs in the graft. In mouse bone marrow transplantation models, engineered overexpression of HOXB4 has been one of the most potent stimulator of HSC expansion identified to date. The simple addition of soluble recombinant TAT-HOXB4 protein was also recently reported to enable rapid in vitro expansion of mouse HSCs that retain their in vivo proliferation and differentiation capacity. To test the feasibility of using TAT-HOXB4 as a stimulator of human HSC expansion, we performed a series of experiments using CD34+ populations isolated from healthy volunteers. The CD34⁺ cell populations were cultured in X-Vivo medium supplemented with Stem Cell Factor (300 ng/mL) and G-CSF (50 ng/mL) in the presence or absence of TAT-HOXB4 protein (50 nmol/L) for 4 days. In response to TAT-HOXB4, total numbers of mononuclear cells demonstrated a modest but distinct 2-fold increase compared to controls. TAT-HOXB4 treatment had the largest proliferation enhancing effect on more primitive cell populations such as CFU-GEMM, BFU-E and BFU-Meg, whose numbers increased 26.5 ± 1.4 fold (mean±S.D.), 2.2 ± 0.7 fold and 2.1 ± 0.2 fold, respectively, over their input values, and 19.1 ± 1.3 fold, 2.7 ± 0.7 and 31 ± 3.4 fold, respectively, compared to growth factor only controls. In response to TAT-HOXB4, the total numbers of CD34+CD38-Lin- cells increased 2.1 ± 0.7 fold above their starting numbers compared to a 1.5 ± 0.5 fold loss of this population in control cultures. HSC numbers were enumerated at the beginning, and after a 4-day TAT-HOXB4 treatment period using a NOD/SCID repopulation assay. In response to 50 nM TAT-HOXB4, NOD/SCID repopulating cell (SRC) numbers increased ~2-fold over their input values, compared to a 9-fold loss in control cultures without TAT-HOXB4. These results show that recombinant TAT-HOXB4 protein has the capacity to rapidly induce ex vivo expansion of primitive human bone marrow populations, and suggest that optimization of treatment conditions will rapidly lead to clinically useful expansion of human HSCs.