Ex vivo expansion marginally amplifies repopulating cells from baboon peripheral blood mobilized CD34 + cells

Summary.  The ability of ex vivo expansion to increase the long‐term repopulating capacity of a graft is still unknown. One problem is the most reliable way to quantify transplantable cells. We addressed this point in a baboon model based on autologous transplantation of serial limiting doses of non‐manipulated or ex vivo ‐expanded mobilized CD34 + cells and determined the threshold doses of non‐manipulated and expanded cells which supported long‐term multilineage engraftment. In the expansion group, CD34 + cells were cultured for 6 d with a combination of early acting cytokines (Flt3‐ligand, stem cell factor, thrombopoietin and interleukin 3). Grafted cells were characterized by their surface antigens and biological properties [semisolid assays, long‐term culture‐initiating cells (LTC‐IC) and non‐obese diabetic severe combined immunodeficient reconstituting cells (SRC)]. Animals were followed for at least 12 months post transplantation. The expansion protocol yielded 12·3‐fold, 16·9‐fold, 3·7‐fold, 3·5‐fold and 2·2‐fold increases in CD34 + cells, granulocyte‐‐macrophage colony‐forming units (CFU‐GM), megakaryocyte CFU (CFU‐MK), LTC‐IC and SRC respectively. It induced a modest increase in the long term reconstitutive ability of the graft; the threshold value for long‐term engraftment was 0·5 × 10 6 /kg CD34 + cells in the control group and 0·3 × 10 6 /kg CD34 + cells in the expansion group, although one animal in this latter group remained hypoplastic. Frequencies of SRC had a high predictive value of long‐term engraftment ( r > 0·80). The main advantage of the protocol was the acceleration of granulocyte recovery, achieved at the different doses tested. In conclusion, these experiments suggest that this ex vivo expansion protocol marginally amplifies long‐term reconstituting cells.