Hierarchical structure of human megakaryocyte progenitor cells

Megakaryocytopoiesis is a complex biological process involving a series of cellular events that begins with the pluripotent hematopoietic stem cell and ultimately results in the biogenesis of platelets. A hierarchy of megakaryocyte (MK) progenitor cells has been previously defined based upon studies of in vitro megakaryocytopoiesis. Ontogeny-related changes in MK progenitor cells were analyzed in order to further define this cellular hierarchy. Unifocal colony-forming unit-megakaryocyte (CFU-MK)-derived colonies cloned from fetal bone marrow (FBM) formed after fewer days of in vitro culture and were 2.6-fold larger than those colonies cloned from adult bone marrow (ABM). The frequency of CFU-MK-derived colonies cloned from ABM was significantly greater. MK colonies, however, cloned from FBM morphologically consisted of both pure MK colonies and mixed colonies containing MKs, in which a core of CD41- cells were surrounded by CD41+ MKs. Large colonies resembling the primitive BFU-MK also were assayed from both FBM and ABM. These BFU-MK-derived colonies appeared after fewer days of incubation when FBM was assayed, compared to ABM, but at a significantly lower frequency. In addition, large unifocal MK colonies consisting of >300 cells (300-1000) appeared from cells cloned from fetal, but not adult, marrow. This type of colony represents a unique type of MK progenitor cell, termed the high-proliferative-potential cell-MK. Such colonies represent the progeny of the most primitive human MK progenitor cell identified to date. We also attempted to investigate the process of commitment of stem cells to the MK lineage. We explored the actions of thrombopoietin (TPO) on primitive hematopoietic cells in order to gain an understanding of stem cell commitment. CD34+ Thy-1+ Lin- marrow cells, which are enriched for pluripotent hematopoietic stem cells, were shown to express c-Mpl by the polymerase chain reaction. In addition, TPO alone was capable of inducing CD34+ Thy-1+ Lin- cells after two to three weeks to produce progeny composed entirely of MKs. These studies indicate that TPO has a profound effect on hematopoietic stem cells, and that the hierarchy of MK progenitor cells begins with the pluripotent hematopoietic stem cell.

Thrombopoietin, flt3, and kit ligands together suppress apoptosis of human mobilized CD34+ cells and recruit primitive CD34+ Thy-1+ cells into rapid division

Various combinations of cytokines have profoundly different effects on inhibition of apoptosis and stimulation of self-renewal division of hematopoietic stem cells (HSC) in short-term, ex vivo culture. Our goal was to quantitate expansion of cells with a primitive CD34+ Thy-1+ phenotype, as well as cell cycling, division history, differentiation, and apoptosis of CD34+ cells enriched from normal donor mobilized peripheral blood (MPB) cells. The balance of these parameters determines the net number of transplantable HSC produced in ex vivo cultures. Comparing several different combinations of cytokines added to 90-hour cultures of MPB CD34 cells, thrombopoietin (TPO), flt3 ligand (FL), and c-kit ligand (KL) gave the best result, with the lowest percentage of apoptotic cells and a mean 1.2-fold increase in the number of CD34+ Thy-1+ cells. A combination of interleukin 3 (IL-3), interleukin 6 (IL-6), and leukemia inhibitory factor (LIF) gave the worst outcome, including a decrease of CD34+ Thy-1+ cell number to a mean of 30% of the starting cell number. Cell division history was tracked using the dye 5-(and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE). Division of CD34+ Thy-1+ cells was faster and more synchronous in TPO, FL, and KL than in IL-3, IL-6, and LIF, which left a significant proportion of CD34+ cells undivided. Such detailed analyses of short-term, ex vivo cultures generated "replication scores," which allowed prediction of a sixfold improvement of the efficiency of gene transduction of primitive hematopoietic progenitors from MPB, using TPO, FL, and KL to replace IL-3, IL-6, and LIF. Analysis of retroviral transduction efficiency confirmed the increase of transgene expression from MPB primitive hematopoietic progenitors assayed after stromal culture was fivefold, validating the usefulness of multiparameter analysis of short-term cultures for survival and replication of CD34+ Thy-1+ cells.

Quantitation of the proliferative potential of highly enriched human primitive hematopoietic progenitor cells using a stroma-free limiting dilution assay with automated scoring

BACKGROUND

Increasing use of phenotypically-enriched stem cell populations for clinical hematopoietic transplants has led to an urgent demand for a reliable, rapid and simple functional assay which would provide an estimation of the reconstituting potential of cells prior to transplantation.

METHODS

We have developed a 2-week quantitative, stroma-free assay to measure the frequency of primitive progenitors within hematopoietic cell samples. This relatively short-term assay provides frequency information which correlates with that measured by a 5-week stroma-dependent CAFC assay. Cells with the phenotype CD34+Thy-1+ were purified by fluorescence-activated cell sorting from peripheral blood apheresis products of multiple myeloma patients mobilized with cytoxan and GM-CSF. CD34+Thy-1+ cells were plated at limiting dilution into microtiter wells and cultured in an Iscove's based serum-deprived culture medium, supplemented with the cytokines, interleukin (IL)-3, IL-6, G-CSF, Flk2/Flt3 ligand (FL) and Kit ligand (KL). After 2 weeks, cell proliferation in individual wells was quantified by microscopy and bright-field imaging, or by using a fluorescent nucleic acid-binding dye and fluorimetry. Poisson statistics were used to calculate the frequency of wells containing cells with high proliferative potential (wells containing > or = 500 cells).

RESULTS

Progenitor cell frequencies generated using this assay were compared by linear regression analysis to those generated from 32 parallel CAFC and CFU-C assays performed on the same patient samples. Correlations were r = 0.80, r2 = 0.65, and r = 0.76, r2 = 0.58, respectively; these correlations were highly significant (p < 10(-7)).

DISCUSSION

This limiting dilution assay should more directly quantitate the potential of primitive hematopoietic cells than a CFU-C assay. It also has advantages over both the CAFC and the CFU-C assay, in that scoring has been automated, making it simple, rapid, and objective compared with manual cobblestone area or colony counting. The described limiting dilution assay may provide a useful alternative to assays currently used to evaluate the viability and proliferative potential of purified hematopoietic cells intended for transplant.

Bone marrow repopulation by human marrow stem cells after long-term expansion culture on a porcine endothelial cell line

In vitro exposure of murine hematopoietic stem cells (HSCs) to cell cycle-inducing cytokines has been shown to result in a defect in the ability of these cells to engraft. We used a porcine microvascular endothelial cell (PMVEC) line in conjunction with exogenous interleukin (IL)-3, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), and stem cell factor (SCF) to expand human HSCs that express the CD34 and Thy-1 antigens but lack lineage-associated markers (CD34+Thy-1+Lin- cells). Ex vivo expansion of hematopoietic cells was evaluated in comparison to stromal cell-free, cytokine-supplemented cultures. Cells expressing the CD34+Thy-1+Lin- phenotype were detectable in both culture systems for up to 3 weeks. These cells were reisolated from the cultures and their ability to engraft human fetal bones implanted into SCID mice (SCID-hu bone) was tested. HSCs expanded in PMVEC coculture were consistently capable of competitive marrow repopulation with multilineage (CD19+ B lymphoid, CD33+ myeloid, and CD34+ cells) progeny present 8 weeks postengraftment. In contrast, grafts composed of cells expanded in stroma-free cultures did not lead to multilineage SCID-hu bone repopulation. Proliferation analysis revealed that by 1 week of culture more than 80% of the cells in the PMVEC cocultures expressing the primitive CD34+CD38- phenotype had undergone cell division. Fewer than 1% of the cells that proliferated in the absence of stromal cells remained CD34+CD38-. These data suggest that the proliferation of HSCs in the presence of IL-3, IL-6, GM-CSF, and SCF without stromal cell support may result in impairment of engraftment capacity, which may be overcome by coculture with PMVECs.

Thrombopoietin mobilizes CD34+ cell subsets into peripheral blood and expands multilineage progenitors in bone marrow of cancer patients with normal hematopoiesis

Thrombopoietin (TPO), the primary regulator of megakaryocytopoiesis, also mediates biologic effects in vitro on hematopoietic cells more primitive than those committed to the megakaryocyte (MK) lineage. To assess the spectrum of hematopoietic effects of recombinant human (rh)TPO in vivo, we evaluated its proliferative effect on bone marrow (BM) progenitor cells, its maturation effect on BM MKs, and its mobilizing effect on peripheral blood (PB) progenitor cells during a phase I clinical laboratory investigation in which rhTPO was administered to cancer patients with normal hematopoiesis. Twelve patients received a single dose of rhTPO (0.3, 0.6, 1.2, or 2.4 microg/kg of body weight) prior to chemotherapy. BM and PB samples from these patients were analyzed 1 to 2 days before (baseline) and 7 days after rhTPO administration. At higher doses (1.2-2.4 microg/kg), rhTPO produced increased concentrations of primitive CD34+Thy-1+Lin-cells (mean 2.1-fold), CD34+mpl+ cells (mean 5.2-fold), CD34+CD41+CD14- promegakaryoblasts (mean 2.9-fold), and myeloerythroid colony-forming cells (mean threefold) in BM. No significant increases in the frequency of BM colony-forming unit (CFU)-MK were observed. Elevated numbers of both immature (2N-8N) and more mature (64N and 128N) CD41+ MKs were detected in BM, with modal ploidy remaining at 16N. Higher doses of rhTPO (1.2-2.4 microg/kg) also induced increased concentrations of CD34+ cell subsets in PB, including both primitive CD34+Thy-1+Lin- (mean 8.8-fold) and MK lineage-committed CD34+CD41+CD14- cells (mean 14.6-fold) as well as various myeloerythroid colony-forming cells (mean 3.6- to 5.5-fold). These results demonstrate that rhTPO given as a single dose not only promotes proliferation and maturation of cells of the MK lineage, but also expands the pool of BM primitive hematopoietic cells. In addition, rhTPO induces mobilization of hematopoietic progenitors into peripheral circulation. The extent to which such multilineage effects on human progenitor cells will contribute to clinical efficacy remains to be determined.

Thrombopoietin, kit ligand, and flk2/flt3 ligand together induce increased numbers of primitive hematopoietic progenitors from human CD34+Thy-1+Lin- cells with preserved ability to engraft SCID-hu bone

CD34(+)Thy-1(+)Lin- cells are enriched for primitive hematopoietic progenitor cells (PHP), as defined by the cobblestone area-forming cell (CAFC) assay, and for bone marrow (BM) repopulating hematopoietic stem cells (HSC), as defined by the in vivo SCID-hu bone assay. We evaluated the effects of different cytokine combinations on BM-derived PKH26-labeled CD34(+)Thy-1(+)Lin- cells in 6-day stroma-free cultures. Nearly all (>95%) of the CD34(+)Thy-1(+)Lin- cells divided by day 6 when cultured in thrombopoietin (TPO), c-kit ligand (KL), and flk2/flt3 ligand (FL). The resulting CD34(hi) PKHlo (postdivision) cell population retained a high CAFC frequency, a mean 3.2-fold increase of CAFC numbers, as well as a capacity for in vivo marrow repopulation similar to freshly isolated CD34(+)Thy-1(+)Lin- cells. Initial cell division of the majority of cells occurred between day 2 and day 4, with minimal loss of CD34 and Thy-1 expression. In contrast, cultures containing interleukin-3 (IL-3), IL-6, and leukemia inhibitory factor contained a mean of 75% of undivided cells at day 6. These CD34(hi) PKHhi cells retained a high frequency of CAFC, whereas the small population of CD34(hi) PKHlo postdivision cells contained a decreased frequency of CAFC. These data suggest that use of a combination of TPO, KL, and FL for short-term culture of CD34(+)Thy-1(+)Lin- cells increases the number of postdivision PHP, measured as CAFC, while preserving the capacity for in vivo engraftment.

Thrombopoietin stimulates megakaryocytopoiesis, myelopoiesis, and expansion of CD34+ progenitor cells from single CD34+Thy-1+Lin- primitive progenitor cells

Thrombopoietin (TPO) or MpI ligand is known to stimulate megakaryocyte (MK) proliferation and differentiation. To identify the earliest human hematopoietic cells on which TPO acts, we cultured single CD34+Thy-1+Lin- adult bone marrow cells in the presence of TPO alone, with TPO and interleukin-3 (IL-3), or with TPO and c-kit ligand (KL) in the presence of a murine stromal cell line (Sys1). Two distinct growth morphologies were observed: expansion of up to 200 blast cells with subsequent differentiation to large refractile CD41b+ MKs within 3 weeks or expansion to 200-10,000 blast cells, up to 25% of which expressed CD34. The latter blast cell expansions occurred over a 3- to 6-week period without obvious MK differentiation. Morphological staining, analysis of surface marker expression, and colony formation analysis revealed that these populations consisted predominantly of cells committed to the myelomonocytic lineage. The addition of IL-3 to TPO-containing cultures increased the extent of proliferation of single cells, whereas addition of KL increased the percentage of CD34+ cells among the expanding cell populations. Production of multiple colony-forming unit-MK from single CD34+Thy-1+Lin- cells in the presence of TPO was also demonstrated. In limiting dilution assays of CD34+Lin- cells, TPO was found to increase the size and frequency of cobblestone areas at 4 weeks in stromal cultures in the presence of leukemia inhibitory factor and IL-6. In stroma-free cultures, TPO activated a quiescent CD34+Lin-Rhodamine 123lo subset of primitive hematopoietic progenitor cells into cycle, without loss of CD34 expression. These data demonstrate that TPO acts directly on and supports division of cells more primitive than those committed to the MK lineage.