In vitro expansion of cord blood does not prevent engraftment of severe combined immunodeficient repopulating cells

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Abstract
Summary. This study aimed to assess the potential of human cord blood (CB) cells to engraft in the xenogenic non‐obese diabetic/severe combined immunodeficient (NOD/SCID) mouse model after in vitro expansion culture. We also studied the quality of human haemopoiesis arising from the transplantation of fresh or expanded cells in this model. Cord blood CD34+ cells were cultured for 3, 7 or 10 d with stem cell factor, Flt3, thrombopoietin, interleukin 3 (IL‐3), IL‐6 and granulocyte colony‐stimulating factor, all at 10 ng/ml in serum‐replete conditions. Transplantation of mice with fresh CB containing 3 × 104 CD34+ cells and 1–2 SCID repopulating cells (SRC) resulted in a median of 7·4% (0·4%‐76·8%) human engraftment. When mice received the expanded product of 1–2 SRC, the ability to repopulate NOD/SCID mice was maintained even after 10 d of in vitro culture. Serial dilution of the expanded cells suggested that in vitro expansion had increased SRC numbers two‐ to fourfold. Expanded SRC produced long‐term culture‐initiating cells, clonogenic cells and CD34+ cells in the same proportions as fresh cells after successful engraftment. Therefore, expanded SRC were able to differentiate in the same way as fresh SRC. There was a trend towards lower levels of engraftment when d 7 cultured cells were transplanted (median engraftment 0·8%, range 0·0–24·0%) compared with 1–2 fresh SRC. Our data suggest that this is owing to reduced proliferation of cultured cells in vivo. By utilizing limiting numbers of CB SRC, we confirmed that the engraftment potential of SRC in the NOD/SCID model was preserved after in vitro expansion. Furthermore, dilution experiments strongly suggest two‐ to fourfold expansion of SRC in vitro. These studies are relevant for developing clinical stem cell expansion strategies.