Fast But Durable Megakaryocyte Repopulation and Platelet Production in NOD/SCID Mice Transplanted with Ex‐Vivo Expanded Human Cord Blood CD34 + Cells

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Abstract
We have previously established a stroma‐free culture with Flt‐3 ligand (FL), stem cell factor (SCF), and thrombopoietin (TPO) that allows the maintenance and the expansion for several weeks of a cord blood (CB) CD34+ cell population capable of multilineage and long‐lasting hematopoietic repopulation in non‐obese diabetic/ severe combined immunodeficient (NOD/SCID) mice. In this work the kinetics of megakarocyte (Mk)‐engraftment that is often poor and delayed in CB transplantation, and human platelet (HuPlt) generation in NOD/SCID mice of baseline CD34+ cells (b34+), and of CD34+ cells reisolated after a 4‐week expansion with FL+SCF+TPO (4w34+) were compared. With b34+ cells Mk‐engraftment was first seen at week 3 (CD41+: 0.4%); 4w34+ cells allowed a more rapid Mk‐engraftment (at weeks 2 and 3 the CD41+ cells were 0.3% and 0.8%). Circulating HuPlts were first seen at weeks 2 and 1, respectively. Mk‐engraftment levels of b34+ and 4w34+ cells 6–8 weeks after transplantation were similar (12 ± 3.5 versus 15 ± 5% CD45+; 1.3 ± 0.5 versus 1.8 ± 0.5% CD41+ cells). Also serial transplant experiments were performed with expanded and reselected CB cells. In secondary and tertiary recipients the Mk population was detected with bone marrow fluorescence‐activated cell sorter analysis; these experiments indicate the effective long‐term repopulation of expanded cells. Selected CD34+ cells after a 4‐week expansion with FL+SCF+TPO are more efficient in Mk engraftment than the same number of unmanipulated cells.