Basic fibroblast growth factor-stimulated ex vivo expansion of haematopoietic progenitor cells from human placental and umbilical cord blood

Protective Effect of the c-mpl Agonist Romiplostim on Megakaryocytopoiesis of Human CD34+ Hematopoietic Progenitor Cells Exposed to Ionizing Radiation

A thrombopoiesis-stimulating protein, the myeloproliferative leukemia virus protooncogene (Mpl) ligand romiplostim (RP), is currently approved as a therapeutic agent for idiopathic thrombocytopenic purpura in many countries. Although the action of the initial MPL ligand thrombopoietin (TPO) on human megakaryocytic regeneration from irradiated human hematopoietic stem cells has been examined, there are few reports on the action of RP. In the present study, freshly prepared nonirradiated and 2-Gy X-irradiated human CD34 positive (CD34⁺) cells from placental umbilical cord blood were cultured with a combination of RP and various cytokines. As a result, the effect of RP on cell proliferation of nonirradiated CD34⁺ cells was found to be comparable to that of TPO. However, the stimulating activity of RP on megakaryocytic progenitor-derived colony formation was markedly lower compared with TPO. Regarding the action of RP with various cytokines, the present results showed that a combination of RP with interleukin-3 (IL-3) or IL-3 plus stem cell factor (SCF) showed a high regenerative effect on cell proliferation, megakaryopoiesis, thrombopoiesis, and megakaryocyte colony formation from X-irradiated CD34⁺ cells. The present study showed that human recombinant RP has potential effects on human megakaryocytic regeneration from X-irradiated human CD34⁺ cells and synergistically acts with IL-3 and IL-3 plus SCF, just as observed with TPO.

Molecular and clinical significance of fibroblast growth factor 2 (FGF2 /bFGF) in malignancies of solid and hematological cancers for personalized therapies

Fibroblast growth factor (FGF) signaling is essential for normal and cancer biology. Mammalian FGF family members participate in multiple signaling pathways by binding to heparan sulfate and FGF receptors (FGFR) with varying affinities. FGF2 is the prototype member of the FGF family and interacts with its receptor to mediate receptor dimerization, phosphorylation, and activation of signaling pathways, such as Ras-MAPK and PI3K pathways. Excessive mitogenic signaling through the FGF/FGFR axis may induce carcinogenic effects by promoting cancer progression and increasing the angiogenic potential, which can lead to metastatic tumor phenotypes. Dysregulated FGF/FGFR signaling is associated with aggressive cancer phenotypes, enhanced chemotherapy resistance and poor clinical outcomes. In vitro experimental settings have indicated that extracellular FGF2 affects proliferation, drug sensitivity, and apoptosis of cancer cells. Therapeutically targeting FGF2 and FGFR has been extensively assessed in multiple preclinical studies and numerous drugs and treatment options have been tested in clinical trials. Diagnostic assays are used to quantify FGF2, FGFRs, and downstream signaling molecules to better select a target patient population for higher efficacy of cancer therapies. This review focuses on the prognostic significance of FGF2 in cancer with emphasis on therapeutic intervention strategies for solid and hematological malignancies.

Characteristics of human CD34+ cells exposed to ionizing radiation under cytokine-free conditions

To clarify the mechanisms underlying radiation-induced hematopoietic stem cell death, we investigated the effects of excessive ionizing radiation on the clonogenic potential of CD34(+) cells obtained from human umbilical cord blood under cytokine-free conditions. The CD34(+) cells were X-ray-irradiated (up to 2 Gy) and were cultured for 0-48 h under cytokine-free conditions. At various time-points, the CD34(+) cells were investigated for survival, clonogenic potential and the generation of mitochondrial superoxide. At 12 h after X-ray irradiation, the number of viable cells had decreased to ∼70-80% compared with the 0-h non-irradiated control, whereas the clonogenic potential in the X-ray-irradiated cells had decreased to ∼50%-60% compared with the 0-h non-irradiated control. Furthermore, significant generation of mitochondrial superoxide was observed at 6 h, and reached a maximum value between 12 and 24 h after X-ray irradiation. However, no significant differences were observed between non-irradiated and X-ray-irradiated cells in terms of the generation of reactive oxygen species or in the intracellular mitochondrial contents. In addition, a cDNA microarray analysis showed that the majority of the altered genes in the CD34(+) cells at 6 h after X-ray irradiation were apoptosis-related genes. These results suggest the possibility that the elimination of the clonogenic potentials of CD34(+) cells involves the generation of mitochondrial superoxide induced by ionizing radiation.

Ponatinib overcomes FGF2-mediated resistance in CML patients without kinase domain mutations

Development of resistance to kinase inhibitors remains a clinical challenge. Kinase domain mutations are a common mechanism of resistance in chronic myeloid leukemia (CML), yet the mechanism of resistance in the absence of mutations remains unclear. We tested proteins from the bone marrow microenvironment and found that FGF2 promotes resistance to imatinib in vitro. Fibroblast growth factor 2 (FGF2) was uniquely capable of promoting growth in both short- and long-term assays through the FGF receptor 3/RAS/c-RAF/mitogen-activated protein kinase pathway. Resistance could be overcome with ponatinib, a multikinase inhibitor that targets BCR-ABL and FGF receptor. Clinically, we identified CML patients without kinase domain mutations who were resistant to multiple ABL kinase inhibitors and responded to ponatinib treatment. In comparison to CML patients with kinase domain mutations, these patients had increased FGF2 in their bone marrow when analyzed by immunohistochemistry. Moreover, FGF2 in the marrow decreased concurrently with response to ponatinib, further suggesting that FGF2-mediated resistance is interrupted by FGF receptor inhibition. These results illustrate the clinical importance of ligand-induced resistance to kinase inhibitors and support an approach of developing rational inhibitor combinations to circumvent resistance.

Human cardiac stem cells isolated from atrial appendages stably express c-kit

The in vivo studies of myocardial infarct using c-kit⁺/Lin⁻ cardiac stem cells (CSCs) are still in the early stage with margin or no beneficial effects for cardiac function. One of the potential reasons may be related to the absence of fully understanding the properties of these cells both in vitro and in vivo. In the present study, we aimed to systematically examine how CSCs adapted to in vitro cell processes and whether there is any cell contamination after long-term culture. Human CSCs were enzymatically isolated from the atrial appendages of patients. The fixed tissue sections, freshly isolated or cultured CSCs were then used for identification of c-kit⁺/Lin⁻ cells, detection of cell contamination, or differentiation of cardiac lineages. By specific antibody staining, we demonstrated that tissue sections from atrial appendages contained less than 0.036% c-kit⁺/Lin⁻ cells. For the first time, we noted that without magnetic activated cell sorting (MACS), the percentages of c-kit⁺/Lin⁻ cells gradually increased up to ∼40% during continuously culture between passage 2 to 8, but could not exceed >80% unless c-kit MACS was carried out. The resulting c-kit⁺/Lin⁻ cells were negative for CD34, CD45, CD133, and Lin markers, but positive for KDR and CD31 in few patients after c-kit MACS. Lin depletion seemed unnecessary for enrichment of c-kit⁺/Lin⁻ cell population. Following induced differentiation, c-kit⁺/Lin⁻ CSCs demonstrated strong differentiation towards cardiomyocytes but less towards smooth and endothelial cells. We concluded that by using an enzymatic dissociation method, a large number, or higher percentage, of relative pure human CSCs with stable expression of c-kit⁺ could be obtained from atrial appendage specimens within ∼4 weeks following c-kit MACS without Lin depletion. This simple but cost-effective approach can be used to obtain enough numbers of stably-expressed c-kit⁺/Lin⁻ cells for clinical trials in repairing myocardial infarction.

Influence of obstetric factors on the yield of mononuclear cells, CD34+ cell count and volume of placental/umbilical cord blood

AIM

Placental/umbilical cord blood (CB) has been used increasingly not only for transplantations, but also in the field of life science research. However, little information is available on the biological characteristics of CB units collected in rural areas because no medical facilities are affiliated with CB banks. Little attention has been paid to the collection of CB units in rural areas compared to CB collected in metropolitan areas. CB is a precious source for life science research due to the recent low birth rate in Japan. Therefore, to efficiently utilize CB units, the purpose of the present study was to investigate the optimum obstetric factors associated with a higher yield of mononuclear/CD34(+) cells per CB unit.

METHODS

CB units were collected at a single hospital (Hirosaki National Hospital). A total of 126 CB units from 105 vaginal deliveries and 21 cesarean section deliveries were available for cell separation within 24 h. Mononuclear low-density (LD) cells were separated using Ficoll-Paque and then processed for CD34(+) cell enrichment using magnetic cell sorting. Associations between the maternal/neonatal factors and the yield of LD/CD34(+) cells were analyzed.

RESULTS

Despite the larger net weight of CB collected from cesarean section deliveries, the total number of LD cells collected from vaginal deliveries was significantly higher than that collected from cesarean section deliveries. The total number of LD cells per CB unit from primigravidae was significantly higher compared with that collected from from multigravidae.

CONCLUSION

CB units from vaginal deliveries of primigravidae may be more favorable because they contain a higher yield of mononuclear cells.

Fibroblast growth factor andex vivoexpansion of hematopoietic progenitor cells

Fibroblast growth factor (FGF) belongs to a family of heparin-binding polypeptides and shows multiple functions including cell proliferation, differentiation, survival and motility. The expression of FGF receptors is widely distributed on different hematopoietic progenitor cells and stromal cells, and FGFs play an important role in hematopoietic stem cell homeostasis. FGFs have been shown to sustain the proliferation of hematopoietic progenitor cells, maintaining their primitive phenotype. Basic FGF (bFGF, FGF-2) stimulates the formation of an adherent stromal cell layer in human long-term bone marrow cultures, and promotes hematopoietic cell development. FGF-2 has also been shown to synergize with other hematopoietic growth factors to enhance in vitro colony formation by several classes of hematopoietic progenitor cells. Results of ex vivo expansion and clinical trials to date suggest that hematopoietic cells cultured under stroma-free cytokine combination conditions may be insufficient to restore hematopoiesis after a myeloablative conditioning regimen, although some recent trials demonstrated an improvement in engraftment and a reduction of the period of pancytopenia, especially neutrophils and platelets, after transplantation. A recent study by our group demonstrated that FGF-2 is effective in supporting the generation of megakaryocytic progenitor cells during ex vivo expansion. These observations could be useful in reducing the long period of severe thrombocytopenia that occurs frequently after umbilical/placental cord blood transplantation. The development of more effective amplifying systems for hematopoietic stem/progenitor cells can be expected since FGFs have multiple functions in hematopoiesis.

Maternal and neonatal factors associated with the high yield of mononuclear low-density/CD34+ cells from placental/umbilical cord blood

Placental/umbilical cord blood (CB) contains nucleated cells and hematopoietic stem/progenitor cells (CD34(+) cells). However it is difficult to predict the number of nucleated/CD34(+) cells in each CB before cell processing. Despite many previous studies from institutes affiliated with CB banks in metropolitan areas, little information is available regarding the characteristics of CB units from other medical facilities. The purpose of the present study was to analyze the maternal/neonatal factors on the yield of cells in CB units. A total of 176 CB units were obtained from single-birth and normal vaginal deliveries. Mononuclear low-density (LD) cells were separated using Ficoll-Paque within 24 hrs after CB collection and then processed for the purification of CD34(+) cells. A multiple linear regression analysis was performed to assess the correlations between the yield of cells and maternal/neonatal factors including maternal age, gravid status, duration of labor, gestational age, neonatal height and weight, cord length, and meconium in the amniotic fluid. The total LD cells per CB unit had a weak positive correlation with the maternal age of primigravidae. The total LD cells per CB unit from the primigravidae aged > or = 25 were significantly higher than those from the primigravidae aged < or = 24. The total CD34(+) cells per CB unit from the 1-gravidae were significantly higher than those from the 2-gravidae and 3-gravidae, respectively among all donors. These results indicate that the CB units from the primigravidae aged > or = 25 are more likely to contain higher yield of LD/CD34(+) cells.

Effect of fibroblast growth factor-2 on equine mesenchymal stem cell monolayer expansion and chondrogenesis

OBJECTIVE

To determine whether fibroblast growth factor-2 (FGF-2) treatment of equine mesenchymal stem cells (MSCs) during monolayer expansion enhances subsequent chondrogenesis in a 3-dimensional culture system.

ANIMALS

6 healthy horses, 6 months to 5 years of age.

PROCEDURES

Bone marrow-derived MSCs were obtained from 6 horses. First-passage MSCs were seeded as monolayers at 10,000 cells/cm(2) and in medium containing 0, 1, 10, or 100 ng of FGF-2/mL. After 6 days, MSCs were transferred to pellet cultures (200,000 cells/pellet) and maintained in chondrogenic medium. Pellets were collected after 15 days. Pellets were analyzed for collagen type II content by use of an ELISA, total glycosaminoglycan content by use of the dimethylmethylene blue dye-binding assay, and DNA content by use of fluorometric quantification. Semiquantitative PCR assay was performed to assess relative concentrations of collagen type II and aggrecan mRNAs.

RESULTS

Use of 100 ng of FGF-2/mL significantly increased pellet DNA and glycosaminoglycan content. Collagen type II content of the pellet was also increased by use of 10 and 100 ng of FGF-2/mL. Collagen type II and aggrecan mRNA transcripts were increased by treatment with FGF-2. Some control samples had minimal evidence of collagen type II and aggrecan transcripts after 35 cycles of amplification.

CONCLUSIONS AND CLINICAL RELEVANCE

FGF-2 treatment of bone marrow-derived MSC monolayers enhanced subsequent chondrogenic differentiation in a 3-dimensional culture. This result is important for tissue engineering strategies dependent on MSC expansion for cartilage repair.

Application of proteoglycan extracted from the nasal cartilage of salmon heads for ex vivo expansion of hematopoietic progenitor cells derived from human umbilical cord blood

Highly purified proteoglycan (PG) extracted from the nasal cartilage of salmon heads was applied to the ex vivo expansion of hematopoietic progenitor cells prepared from human umbilical cord blood in serum-free cultures supplemented with the combination of early-acting cytokines, thrombopoietin (TPO), interleukin-3 (IL-3) and stem cell factor (SCF). PG showed no promoting effects on the cell proliferation rate; however, they promoted the generation of progenitor cells for granulocyte-macrophages, erythrocytes and/or megakaryocytes in culture with TPO alone or SCF plus TPO. However, no promoting effect was observed in a combination of IL-3 plus SCF, which showed the highest cell proliferation rate. PG failed to promote the generation of mixed colony-forming units (i.e. the relatively immature cells in hematopoiesis). These results suggest that PG acts on the relatively mature stem/progenitor cells, and may function as a regulatory factor in the differentiation pathway of hematopoiesis.

Effect of fibroblast growth factor 2 on stromal cell-derived factor 1 production by bone marrow stromal cells and hematopoiesis

BACKGROUND

Reduction of intramedullary hematopoiesis and the development of myelofibrosis and splenic hematopoiesis are frequent complications of clonal myeloid disorders that cause severe morbidity and death and present a therapeutic challenge. However, the pathogenesis of these complications is still unknown. We evaluated the effect of fibroblast growth factor 2 (FGF-2), the level of which is elevated in patients with clonal myeloid disorders, on bone marrow stromal cell expression of stromal cell-derived factor 1 (SDF-1), a chemokine that is essential for normal hematopoiesis.

METHODS

Reverse transcription-polymerase chain reaction analysis, immunoblot analysis, and enzyme-linked immunosorbent assays were used to examine effects of human recombinant FGF-2 exposure on SDF-1 expression in mouse stromal MS-5 and S-17 cells. Cocultures of human CD34-positive peripheral blood stem cells or mouse pre-B DW34 cells with mouse stromal cells were used to characterize the functional relevance of the effects of FGF-2 on SDF-1 expression. The in vivo hematologic effects of FGF-2 were determined by systemic administration to mice (n = 10). All statistical tests were two-sided.

RESULTS

FGF-2 reduced constitutive SDF-1 mRNA expression and secretion in stromal cells (SDF-1 levels in supernatants: MS-5 cells cultured for 3 days in medium only versus in medium with FGF-2, 95.4 ng/mL versus 22.2 ng/mL, difference = 73.2 ng/mL, 95% confidence interval [CI] = 60.52 to 85.87 ng/mL; P = .002, two-sided Student's t test; S-17 cultured in medium only versus in medium with FGF-2, 203.53 ng/mL versus 32.36 ng/mL, difference = 171.17 ng/mL, 95% CI = 161.8 to 180.6 ng/mL; P<.001). These effects of FGF-2 were reversible. FGF-2 compromised stromal cell support of the growth and survival of pre-B DW34 and myeloid lineage cells, and these effects were reversed in part by exogenous recombinant SDF-1alpha (rSDF-1alpha) (DW34 pre-B cells recovery on S-17 stromal cells, expressed as a percentage of DW34 cells recovered from medium only: with FGF-2 versus without FGF-2, 27.6% versus 100%, difference = 72.4%, 95% CI = 45.34% to 99.51%, P = .008; with FGF-2 plus rSDF1 versus with FGF-2 only, 60.3% versus 27.6%, difference = 32.7%, 95% CI = 9.35% to 56.08%, P = .034; fold increase in number of myeloid lineage cells after culture on S-17 stromal cells: with FGF-2 versus without FGF-2, 0.25-fold versus 3.8-fold, difference = 3.55-fold, 95% CI = 2.66- to 4.44-fold, P<.001; recovery of myeloid cells on S-17 stromal cells, expressed as a percentage of myeloid cells recovered from medium only: FGF-2 plus rSDF-1alpha versus FGF-2 only, 76.5% versus 32.4%, difference = 44.1%, 95% CI = 32.58% to 55.68%, P<.001). Administration of FGF-2 to mice reversibly reduced bone marrow levels of SDF-1 and cellularity and induced immature myeloid cell mobilization, extramedullary hematopoiesis, and splenomegaly.

CONCLUSIONS

Systemic administration of FGF-2 in mice disrupts normal bone marrow hematopoiesis in part through reduced expression of SDF-1. Thus, endogenous FGF-2 may represent a potential therapeutic target in clonal myeloid disorders characterized by bone marrow failure.

Regeneration of Megakaryocytopoiesis and ThrombopoiesisIn Vitrofrom X-Irradiated Human Hematopoietic Stem Cells

In the present study, we investigated whether X-irradiated hematopoietic stem cells can be induced to undergo megakaryocytopoiesis and thrombopoiesis in vitro using cytokine combinations that have been demonstrated to be effective for conferring increased survival on irradiated human CD34(+) megakaryocytic progenitor cells (colony-forming unit megakaryocytes; CFU-Meg), such as thrombopoietin (TPO), interleukin 3 (IL3), stem cell factor and FLT3 ligand. Culture of nonirradiated CD34(+) cells in serum-free medium supplemented with multiple cytokine combinations led to an approximately 200- to 600-fold increase in the total cell numbers by day 14 of culture. In contrast, the growth of X-irradiated cells was observed to be one-sixth to one-tenth that of the nonirradiated cultures. Similarly, total megakaryocytes were increased by 50- to 130-fold, while culture of X-irradiated cells yielded one-fourth to one-eighth of the control numbers. At this time, CD41(+) particles, which appeared to be platelets, were produced in the medium harvested from nonirradiated and irradiated cultures. Although radiation suppressed cell growth and megakaryocytopoiesis, there were no significant differences in thrombopoiesis between the two types of culture. These results suggest that X-irradiated CD34(+) cells can be induced to undergo nearly normal terminal maturation through megakaryocytopoiesis and thrombopoiesis by stimulation with appropriate cytokine combinations.

Reversal of ethanol toxicity in embryonic neurons with growth factors and estrogen

Prenatal exposure to ethanol is the cause of fetal alcohol syndrome, which is characterized by brain abnormalities and decreased mental capacity. In the current study, cultured neurons from embryonic rat cortices were used to study the reversal of ethanol toxicity on neuronal survival and neurite outgrowth. Ethanol treatment followed by treatment with estrogen and certain growth factors were used to assess the potential of these growth factors and estrogen to reverse the effects of ethanol damage. Cortical neurons from embryonic day (E) 16 rats were grown in defined medium with a glial plane at a distance of 1mm from the neurons. Ethanol (45 mM) was administered on day in vitro 1 (DIV 1) and DIV 4. Insulin-like growth factor-I (IGF-I, 10 ng/ml), insulin-like growth factor-II (IGF-II, 10 ng/ml), basic fibroblast growth factor (bFGF, 5 ng/ml), nerve growth factor (NGF, 100 ng/ml), and estrogen (Es, 10 ng/ml) were administered on DIV 4 and DIV 5. Cell viability was determined on DIV 6 using the intravital dyes fluorescein diacetate and propidium iodide. IGF-I and bFGF reduced ethanol's toxic effect on neuronal survival. Estrogen, bFGF, and NGF increased total neurite length after ethanol treatment. Although none of the treatments had a statistically significant effect on the mean number of primary neurites, all caused a statistically significant increase in the mean number of secondary neurites per cell (a measure of neuritic branching) relative to the ethanol treatment alone.

A human stromal-based serum-free culture system supports the ex vivo expansion/maintenance of bone marrow and cord blood hematopoietic stem/progenitor cells

OBJECTIVE

We investigated the role of human stromal layers (hu-ST) on the ex vivo expansion/maintenance of human hematopoietic stem/progenitor cells (HSC) from adult bone marrow (BM) and umbilical cord blood (CB).

MATERIALS AND METHODS

BM and CB CD34(+)-enriched cells were cultured in serum-free medium supplemented with SCF, bFGF, LIF, and Flt-3, in the presence or absence of stroma, and analyzed for proliferation, phenotype, and clonogenic potential.

RESULTS

Significant expansion of BM and CB CD34(+) and CD34(+)CD38(-) cells were achieved in the presence of hu-ST. The differentiative potential of both BM and CB CD34(+)-enriched cells cocultured with hu-ST was primarily shifted toward the myeloid lineage, while maintaining/expanding a CD7(+) population. Clonogenic analysis of the expanded cells showed increases in progenitors of the myeloid lineage, including colony-forming unit-granulocyte, macrophage (CFU-GM) and colony-forming unit-granulocyte, erythroid, macrophage, megakaryocyte (CFU-Mix) for both BM (stroma and stroma-free conditions) and CB cells in the presence of stroma.

CONCLUSIONS

These results indicate that adult hu-ST in the presence of appropriate cytokines can be used to efficiently expand/maintain myeloid and lymphoid cell populations from human BM and CB HSC.

FGF-2 controls the differentiation of resident cardiac precursors into functional cardiomyocytes

Recent evidence suggests that the heart possesses a greater regeneration capacity than previously thought. In the present study, we isolated undifferentiated precursors from the cardiac nonmyocyte cell population of neonatal hearts, expanded them in culture, and induced them to differentiate into functional cardiomyocytes. These cardiac precursors appear to express stem cell antigen-1 and demonstrate characteristics of multipotent precursors of mesodermal origin. Following infusion into normal recipients, these cells home to the heart and participate in physiological and pathophysiological cardiac remodeling. Cardiogenic differentiation in vitro and in vivo depends on FGF-2. Interestingly, this factor does not control the number of precursors but regulates the differentiation process. These findings suggest that, besides its angiogenic actions, FGF-2 could be used in vivo to facilitate the mobilization and differentiation of resident cardiac precursors in the treatment of cardiac diseases.

Multipotent neural precursors express neural and hematopoietic factors, and enhance ex vivo expansion of cord blood CD34+ cells, colony forming units and NOD/SCID-repopulating cells in contact and noncontact cultures

In view of the possible crosstalks between hematopoiesis and neuropoiesis, we evaluated two microenvironments, murine neonatal neural cell line C17.2 and primary embryonic aorta-gonad-mesonephros (AGM) stromal cells, on the ex vivo expansion of CD34+ cells from human cord blood. In a contact culture system, C17.2 or AGM cells significantly enhanced the expansion of CD34+ cells to a panel of early and committed hematopoietic progenitor cells. In a noncontact transwell system, pre-established C17.2 cells significantly increased the expansion of total nucleated cells, CD34+ cells and multilineage colony forming cells (P<0.01). Expanded cells were infused into nonobese diabetic/severe-combined immunodeficient mice. The engraftment of human (hu)CD45+ cells in the bone marrow of these mice was consistently higher in all the 10 experiments conducted with the support of C17.2 cells when compared with those in respective control groups (11.9 vs 2.43%, P=0.03). Using RT-PCR and Southern blot analysis, we showed that AGM and C17.2 cells expressed a panel of hematopoietic, bone morphogenetic and neurotrophic factors. Our data provided the first evidence on the promoting effects of a neural progenitor cell line on hematopoiesis at a noncontact condition. The mechanism could be mediated by the expression of multilineage regulatory factors.

High sensitivity of megakaryocytic progenitor cells contained in placental/umbilical cord blood to the stresses during cryopreservation

In placental/umbilical cord blood (PCB) banking and PCB transplantation (PCBT), long-term cryopreservation of hematopoietic stem and progenitor cells is a unique requirement as compared to that for bone marrow transplantation and cytokine-mobilized peripheral blood transplantation. A long period of severe thrombocytopenia is a problem in many patients after PCBT. The object of this study was to define whether megakaryocytic progenitor cells (CFU-Meg), which produce platelets, are more sensitive to cryopreservation than the other myeloid progenitor cells in PCB. The leukocyte concentrates (LCs) obtained from clinical PCB banks were cryopreserved, and progenitor cell recoveries were determined by differential count of colony-forming cells (CFCs). The LCs were exposed to stresses which cells face during freezing, thawing, and washing out cryoprotectants. Most of the myeloid progenitor cells contained in the LCs showed good survival when cryopreserved at slow cooling rates, although cellular injury was observed at higher cooling rates and higher osmolalities. In contrast, the recovery rate of CFU-Meg was significantly lower than other progenitor cells, indicating a higher sensitivity to the various stresses they were exposed to during cryopreservation. Thrombocytopenia observed in patients receiving PCBT may be explained, at least in part, by the disappearance of CFU-Meg during cryopreservation.