Homing and engraftment defects in ex vivo expanded murine hematopoietic cells are associated with downregulation of beta1 integrin.

OBJECTIVE: Hematopoietic progenitors generated by ex vivo expansion "home" less efficiently to the bone marrow (BM) after intravenous transplantation than fresh cells. To explore the underlying cause of this transplantation defect, we examined the homing and engraftment properties in vivo of fresh and cultured marrow cells differing in beta1 integrin expression. MATERIALS AND METHODS: Fresh murine BM cells, or the expanded progeny of enriched Sca-1(+) c-kit(+)Lin(-) stem cells, were fractionated into beta1(-/lo) and beta1(+) subpopulations by cell sorting. These populations were assayed for their content of in vitro colony-forming cells (CFCs), cells able to provide radioprotection, and early and long-term multilineage hematopoietic reconstitution following transplantation into myeloablated recipients. These endpoints were correlated with the homing properties of beta1(-/lo) and beta1(+) cells that were labeled with 5- (and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE) and tracked to hematopoietic organs 24 hours after injection into lethally irradiated mice. RESULTS: Most normal stem and progenitor cells express high levels of beta1 integrin. In contrast, most clonogenic cells generated in vitro are beta1(-/lo). Consequently, expanded beta1(-/lo) progenitors failed to provide radioprotection or repopulate the hematopoietic system following intravenous transplantation. Defective engraftment of expanded cells was associated with reduced homing of beta1(-/lo) cells to the bone marrow. CONCLUSION: Downregulation of beta1 integrin on primitive hematopoietic cells during ex vivo expansion reduces their homing efficiency and negatively impacts hematopoietic reconstitution in vivo. Strategies directed at preserving beta1 integrin expression during culture may improve the clinical utility of expanded hematopoietic cells.

Differential homing and engraftment properties of hematopoietic progenitor cells from murine bone marrow, mobilized peripheral blood, and fetal liver.

The rate of reconstitution following hematopoietic stem cell (HSC) transplantation differs widely depending on the tissue source of the cells infused. To test the hypothesis that variability in engraftment kinetics is related to differences in the efficiency with which intravenously transplanted HSCs "home" to the bone marrow (BM), the homing properties of murine fetal liver (FL), adult BM, and mobilized peripheral blood (MPB) cells were compared. Lethally irradiated mice transplanted with 2 x 10(6) FL, BM, or MPB cells exhibited sequentially slower recovery of circulating leukocytes and platelets that correlates with the progressively lower frequency of colony-forming cells (CFCs) in these tissues. However, differences in the rate and degree of early and long-term reconstitution were maintained even after infusing equal numbers of CFCs derived from FL, BM, and MPB. To compare the homing of progenitors from these tissues, cells were labeled with fluorescent PKH26 dye and injected into lethally irradiated hosts. Three hours later, PKH26(+) cells were reisolated from the BM and spleen by fluorescence-activated cell sorting and assayed for in vitro CFCs. Despite the higher level of very late antigen (VLA)-2, VLA-4, and VLA-5 on Sca-1(+)c-kit(+) cells from FL compared to BM, 10-fold fewer FL CFCs homed to hematopoietic organs than those from BM. MPB cells homed slightly better, but still less efficiently than BM cells. Therefore, clonogenic cells from different tissues exhibit striking variations in homing efficiency that does not necessarily correlate with engraftment kinetics. Homing is likely counterbalanced by intrinsic differences in proliferative potential that ultimately determine the rate of hematopoietic reconstitution.

The interleukin-3 receptor alpha chain is a unique marker for human acute myelogenous leukemia stem cells.

Recent studies suggest that the population of malignant cells found in human acute myelogenous leukemia (AML) arises from a rare population of leukemic stem cells (LSCs). LSCs have been documented for nearly all AML subtypes and have been phenotypically described as CD34+/CD38- or CD34+/HLA-DR-. Given the potentially critical role of these primitive cells in perpetuating leukemic disease, we sought to further investigate their molecular and cellular characteristics. Flow cytometric studies using primary AML tissue showed that the interleukin-3 receptor alpha chain (IL-3Ralpha or CD123) was strongly expressed in CD34+/CD38- cells (98 +/- 2% positive) from 16 of 18 primary specimens. Conversely, normal bone marrow derived CD34+/CD38- cells showed virtually no detectable expression of the CD123 antigen. To assess the functional role of IL-3Ralpha positive cells, purified CD34+/CD123+ leukemia cells were transplanted into immune deficient NOD/SCID mice. These experiments showed that CD123+ cells were competent to establish and maintain leukemic populations in vivo. To begin to elucidate a biological role for CD123 in leukemia, primary AML samples were analyzed with respect to signal transduction activity in the MAPK, Akt, and Stat5 pathways. Phosphorylation was not detected in response to IL-3 stimulation, thereby suggesting CD123 is not active in conventional IL-3-mediated signaling. Collectively, these data indicate that CD123 represents a unique marker for primitive leukemic stem cells. Given the strong expression of this receptor on LSCs, we propose that targeting of CD123 may be a promising strategy for the preferential ablation of AML cells.

Distinct functional properties of highly purified hematopoietic stem cells from mouse strains differing in stem cell numbers.

We have previously demonstrated that young adult DBA/2 (DBA) mice have more stem cells than C57BL/6 (B6) mice, as measured in a cobblestone area-forming cell (CAFC) assay using unfractionated marrow. To study the nature of this difference, we have now compared the proliferative fate of single, highly enriched Sca-1(+)c-kit(+)Lin(-) stem cells from these strains. Although equal in frequency, functional comparison revealed that Sca-1(+)c-kit(+)Lin(-) cells from DBA mice contained twice as many cells with CAFC activity. DBA clones persisted much longer in vitro, and developed later in time. To assess whether these differences were of any functional relevance in vivo, we compared engraftment of lethally irradiated mice transplanted with 1000 B6 or DBA Sca-1(+)c-kit(+)Lin(-) cells. Recipients of enriched DBA cells recovered much faster than animals transplanted with B6 cells. We also studied endogenous hematopoietic recovery after 5-fluorouracil (5-FU) treatment in vivo. Progenitors and peripheral blood cells recovered twice as fast in DBA mice. Thus, DBA stem cells have superior proliferative potential compared with phenotypically identical stem cells obtained from B6 mice. Such genetically determined quantitative and qualitative differences in stem cell behavior likely contribute to the dramatically different hematopoietic recovery rates observed in human transplant patients. (Blood. 2000;96:1374-1379)

Effects of cell cycle activation on the short-term engraftment properties of ex vivo expanded murine hematopoietic cells.

Loss of long-term hematopoietic stem cell function in vitro is associated with cell cycle progression. To determine whether cytokine-induced proliferation also limits the rate of short-term engraftment and potential clinical utility of ex vivo expanded hematopoietic cells, murine Sca-1(+)c-kit(+)Lin(-) cells were cultured in interleukin-6 (IL-6), IL-11, granulocyte colony-stimulating factor (G-CSF), stem cell factor, flk-2 ligand, and thrombopoietin for 7 days. Cells amplified 2000-fold were then stained with Hoechst 33342, separated into G(0)/G(1) (72% +/- 3%) or S/G(2)/M (27% +/- 3%) fractions by flow sorting, and injected into lethally irradiated mice. Although long-term (more than 6 months) engraftment of lymphoid and myeloid lineages was greater in primary and secondary recipients of expanded cells residing in G(0)/G(1) at the time of transplantation, there were no noted differences in the short-term (less than 6 weeks) recovery kinetics of circulating blood cells. When hematopoietic cells were expanded in cultures containing the tetrapeptide stem cell inhibitor N-Acetyl-Ser-Asp-Lys-Pro (AcSDKP) to reduce progenitor cycling prior to transplantation, again there were no differences observed in short-term reconstitution by inhibited or uninhibited cells. Interestingly, AcSDKP significantly accelerated engraftment by expanded hematopoietic cells when administered in vivo at the time of transplantation. Leukocytes recovered to 20% of normal levels approximately 1 week faster, and thrombocytopenia was largely abrogated in AcSDKP-treated versus untreated mice. Therefore, while AcSDKP can accelerate the engraftment of ex vivo expanded hematopoietic progenitors, which suggests a relatively simple approach to improve their clinical utility, its effects appear unrelated to cell cycle arrest. (Blood. 2000;95:2829-2837)