Functional Studies of Ex Vivo Expanded Hematopoietic Stem Cells in Mice and Monkeys

Hematopoietic stem cell (HSC) transplantation has been widely applied for the treatment of malignant blood diseases. However, obtaining sufficient HLA-matched stem/progenitors for cell transplantation is an obstacle for clinical applications.

We reported here that an optimal cytokine cocktail in a modified IMDM basal medium was developed that contained stem cell factor, Flt-3 ligand, thrombopoietin, interleukin 3, G-CSF and GM-CSF. Up to 7.3 folds of expanded CD34⁺ cells with 66.3% CD34⁺ of whole cells were obtained after 4 days' culture from human umbilical cord blood. Colony-forming unit (CFU) assays showed that expanded CD34⁺ cells retained the same renewal ability as the pre-expanded counterparts. To test the repopulating ability of the expanded CD34⁺  in vivo, sixteen NOD/SCID mice were divided to four groups and injected with saline (group 1), 0.4 million pre-expanded CD34⁺ cells (group 2), 0.4 million 4-day expanded CD34⁺ cells (group 3), and 2.9 million expanded CD34⁺ cells (group 4), respectively. Multi-lineage differentiations in the peripheral blood were assessed by flow cytometry with antibodies against a panel of human cell surface markers. In week 3, human CD34⁺ cells were decreased below 1% in groups 2 and 3, and 1.717%±0.65% in group 4. Whereas, human CD45⁺ was increased up to 3.831%±1.54%, 3.108%±1.18% and 10.408%±3.27% for groups 2, 3 and 4, respectively. The other human CD41⁺, CD71⁺ and CD15⁺ were also increased in groups 2-4. No expression of any human cell lineage markers was detected in group 1, indicating that expanded human CD34⁺ cells possessed the repopulating viability of HSCs in vivo. Furthermore, in week 12, the human CD34⁺ cells were re-isolated from the bone morrow of the mice (one mouse from each group). The isolated human CD34⁺ cells were again transfused into new NOD/SCID mice for the secondary transplantation. In week 6, human CD45⁺, CD15⁺ and CD19⁺ were observed from the bone morrow cells of sacrificed mice. On the other hand, human CD45⁺, CD15⁺ and CD19⁺ were also detectable in bone morrow cells for all remaining mice in week 24, suggesting that the expanded CD34⁺ cells could be successfully engrafted into mice in a long term.

In addition, the cytokine cocktail was further evaluated for its safety and efficacy in primates. The CD34⁺ cells were isolated from the peripheral blood of cynomolgus monkeys and expanded for about 8 folds were obtained on day 9. Harvested CD34⁺ cells were transducted with the gene of green fluorescent protein (GFP). These cynomolgus monkeys (n=11) were administered with cyclophosphamide via intravenous injection at a dose of 50 mg/kg/day for two days. The myelo-suppressed monkeys were randomly divided into three groups as follows: a control group treated with saline (n=3), a group with autologous CD34^- cells (n=3), and a group treated with GFP-labeled, expanded autologous CD34⁺ cells (n=5), respectively. After autologous transplantation, routine blood tests and flow cytometry analysis were performed to determine the proportion of GFP⁺ cells in the peripheral blood.

The flow cytometry analysis revealed that the white blood cells (WBC), neutrophil (NEU) and platelets (PLT) in peripheral blood of cynomolgus monkeys were completely recovered to the normal levels on days 12, 11 and 10 post autologous transplantation of expended CD34⁺ cells, respectively. For the control groups, WBC, NEU and PLT returned to the normal on days 22, 22 and 12 for the saline treatment and on days 20, 20 and 12 for the CD34^- group, respectively. Similarly, the lymphocytes of cynomolgus monkeys were recovered completely on day 20 post autologous CD34⁺ cell transplantation compared with the saline control (day 25) and the CD34^- group (day 22). On day 30 after the autologous transplantation, the GFP⁺ ratio in CD45⁺ populations was around 2% in the peripheral blood. GFP⁺ cells (ranging from 1.8% to 4.1%) were also detected in bone morrow of cynomolgus monkeys. All primates transplanted with the expanded autologous CD34⁺ cells have survived for 18 months without any noticeable abnormalities.

In conclusion, our results indicate that expanded CD34⁺ cells can be safely and efficiently used for repopulating stem cell compartment in mice and primates, underscoring the potential applications in the clinic. Furthermore, the results from successful autologous transplantation of cynomolgus CD34⁺ cells strongly suggest a possible application for personalized treatment of blood diseases.