Differential effects of the type of iron chelator on the absolute number of hematopoietic peripheral progenitors in patients with ß-thalassemia major.

Several studies have established an association between iron chelation therapy with deferasirox and hematopoietic improvement in patients with myelodysplastic syndromes. There are no data from patients with ß-thalassemia major. In a cross-sectional study, we evaluated the absolute number of several hematopoietic peripheral progenitors (colony-forming unit-granulocyte/macrophage, erythroid burst-forming units, colony-forming unit-granulocyte/erythrocyte/macrophage/megakaryocyte, and long-term culture-initiating cells) in 30 patients with ß-thalassemia major (median age 29.5 years, 40% males) and 12 age-matched controls. For the ß-thalassemia major patients, data on splenectomy status, the type of iron chelator used, and serum ferritin levels reflecting changes in iron status on the chelator were also retrieved. All patients had to be using the same iron chelator for at least 6 months with >80% compliance. The absolute number of all hematopoietic peripheral progenitors was higher in ß-thalassemia major patients than in controls, and varied between splenectomized and non-splenectomized patients (lower number of erythroid burst-forming units and higher numbers of colony-forming unit-granulocyte/macrophage, colony-forming unit-granulocyte/erythrocyte/macrophage/megakaryocyte, and long-term culture-initiating cells). The number of erythroid burst-forming units was significantly higher in patients taking deferasirox (n=10) than in those taking either deferoxamine (n=10) or deferiprone (n=10) (P<0.05). After adjusting for age, sex, splenectomy status, and serum ferritin changes, the association between a higher absolute number of erythroid burst-forming units in deferasirox-treated patients than in patients taking deferoxamine or deferiprone remained statistically significant (P=0.011). In conclusion, in ß-thalassemia major patients, compared with other iron chelators, deferasirox therapy is associated with higher levels of circulating erythroid burst-forming units. This variation is independent of iron status changes and is more likely to be due to the type of chelator.

Endothelial colony-forming cells from patients with chronic myeloproliferative disorders lack the disease-specific molecular clonality marker.

Two putative types of circulating endothelial progenitor cells have been recently identified in vitro: (1) endothelial colony-forming cell (ECFC) and (2) colony-forming unit-endothelial cell (CFU-EC). Only the former is now recognized to belong to endothelial lineage. We have used the ECFC and CFU-EC assays to readdress the issue of the clonal relation between endothelial progenitor cells and hematopoietic stem cells in patients with Philadelphia-positive and Philadelphia-negative chronic myeloproliferative disorders. Both ECFCs and CFU-ECs were cultured from peripheral blood mononuclear cells, and either BCR-ABL rearrangement or JAK2-V617F mutation were assessed in both types of endothelial colonies. We found that ECFCs lack the disease-specific markers, which are otherwise present in CFU-ECs, thus reinforcing the concept that the latter belongs to the hematopoietic lineage, and showing that in chronic myeloproliferative disorders the cell that gives rise to circulating ECFC has a distinct origin from the cell of the hematopoietic malignant clone.

Direct intrabone transplant of unrelated cord-blood cells in acute leukaemia: a phase I/II study.

BACKGROUND: Cord-blood transplants are associated with delayed or failed engraftment in about 20% of adult patients. The aim of this phase I/II study was to establish the safety and efficacy of a new administration route (intrabone) for cord-blood cells, measured by the donor-derived neutrophil and platelet engraftment. METHODS: Adult patients with acute leukaemia, for whom an unrelated stem-cell transplantation was indicated and no suitable unrelated human leucocyte antigen (HLA)-matched donor had been identified, were included in the study and underwent a cord-blood transplant in San Martino Hospital, Genoa, Italy. Eight patients were in first complete remission, ten in second complete remission, and 14 had advanced-stage, refractory disease. HLA matching was 5/6, 4/6, and 3/6 for 9, 22, and one patient, respectively. Cord-blood cells were concentrated in four 5-mL syringes, and were infused in the superior-posterior iliac crest under rapid general anaesthesia. Median transplanted cell dose was 2.6 x 10(7)/kg (range 1.4-4.2). The primary endpoint was the probability of neutrophil and platelet recovery after intrabone cord-blood transplantantion. Secondary endpoints included the incidence of acute graft-versus-host disease, relapse, and overall survival. This trial is registered on the ClinicalTrials.gov website, number NCT 00696046. FINDINGS: Between March 31, 2006, and Jan 25, 2008, 32 consecutive patients with acute myeloid leukaemia (n=20) or acute lymphoblastic leukaemia (n=12) underwent a cord-blood transplant (median age 36 years [range 18-66]). No complications occurred during or after the intrabone infusion of cells. Four patients with advanced-stage disease died within 12 days of the procedure. Median time to recovery of neutrophils in 28 patients (>/=0.5 x 10(9)/L) was 23 days (range 14-44) and median time to recovery of platelets in 27 patients (>/=20 x 10(9)/L) was 36 days (range 16-64). All patients were fully chimeric from 30 days after transplantation to the last follow-up visit, suggesting an early complete donor engraftment. No patient developed grade III-IV acute graft-versus-host disease. Causes of death were transplant related (n=5), infection (n=7), and relapse (n=4). 16 patients were alive and in haematological remission at a median follow-up of 13 months (range 3-23). INTERPRETATION: Our preliminary data suggest that direct intrabone cord-blood transplantation overcomes the problem of graft failure even when low numbers of HLA-mismatched cord-blood cells are transplanted, thus leading to the possibility of use of this technique in a large number of adult patients.

Donor lymphocyte infusions for the treatment of minimal residual disease in acute leukemia / Infusão de linfócitos do doador para o tratamento da doença residual mínima

Minimal residual disease (MRD) was monitored in 80 patients with acute lymphoid (ALL, n=44) or myeloid (AML, n=36) leukemia, undergoing allogeneic haemopoietic stem cell transplantations. MRD markers were IgH-VDJ and TCR gene re-arrangement for ALL, and Wilm's Tumor (WT1) expression for AML. The overall cumulative incidence (CI) of MRD was positive in 45 percent and the CI of hematologic relapse was 24 percent (36 percent in MRD+ vs. 16 percent in MRD patients, p=0.03). The median interval from transplant to first MRD positivity was 120 days and to hematologic relapse 203 days. Patients were divided in 3 MRD groups: MRD (n=44), MRD+ given donor lymphocyte infusions (DLI) (n=17) and MRD+ not given DLI (n=19): leukemia relapse rates in these 3 groups were 16 percent, 6 percent and 63 percent, respectively (p<0.0001); the actuarial 3-year survival rates were 78 percent, 80 percent and 26 percent (p=0.001). In multivariate COX analysis, the MRD group was predictor of relapse (p<0.0001) and survival (p=0.01), together with disease phase and chronic graft versus host disease. In MRD+ patients, DLI protected against relapse (p=0.003) and improved survival (p=0.01). In conclusion, MRD positivity post-transplant predicts leukemia relapse: however, when MRD+ patients are given DLI, their outcomes are comparable to MRD- patients.

A doença residual mínima (DRM) foi monitorada em 80 pacientes com leucemia linfóide aguda (n=44) e mielóide aguda (n=36) que foram submetidos ao transplante alogênico de célula-tronco. Marcadores da DRM foram a IgH-VDJ e rearranjo do TCR para a LLA e expressão do Tumor de Wills (WT1) para LMA. A incidência acumulada global (IC) para a DRM foi positiva em 45 por cento e a IC para recaída hematológica foi 24 por cento (36 por cento na DRM+ versus 16 por cento na DRM-, p=0.03). O intervalo mediano entre o TMO e a primeira DRM positividade foi dia +120, e para a recaída hematológica, dia +203. Os pacientes puderam ser divididos em três grupos: DRM-(n=44), DRM+ onde foi dada a infusão de linfócitos do doador (ILD) (n=17) e DRM+ não dado ILD (n=19): a recidiva nos três grupos foi de 16 por cento, 6 por cento e 63 por cento, respectivamente (p<0.0001); a sobrevida em três anos foi 78 por cento, 80 por cento e 26 por cento (p=0.001). No modelo de Cox, o grupo de DRM foi preditor de recidiva (p<0.0001) e sobrevida global (p=0.01), juntamente com a fase da doença e a doença do enxerto contra o hospedeiro. Na DRM+, IDL protegeu contra a recidiva (p=0.003) e melhorou a sobrevida (p=0.01). Em conclusão, a positividade para a DRM pós-transplante prediz recidiva da leucemia. Entretanto, quando é dada a ILD ao paciente DRM+, a evolução destes pacientes é comparável aos pacientes DRM-.

Clinical scale ex vivo expansion of cord blood-derived outgrowth endothelial progenitor cells is associated with high incidence of karyotype aberrations.

OBJECTIVE: Endothelial progenitor cells (EPCs) are involved in neovessel formation. So far, therapeutic angiogenesis is hampered by the low frequency and limited proliferative potential of these cells isolated from peripheral blood. Recently, it has been shown that cord blood-derived EPCs (CB EPCs) can be ex vivo expanded on a clinical scale. In this study, we evaluated the expansion potential of CB EPCs together with their phenotypic, functional, and chromosomal stability over time. MATERIALS AND METHODS: Flow cytometry, in vitro tube formation, and proliferation assays were performed to characterize CB EPC-derived cells. Chromosomal stability was evaluated by karyotype analysis. In vitro and in vivo tumorigenicity was evaluated by soft agar assay and injection into nonobese diabetic/severe combined immunodeficient mice, respectively. RESULTS: We showed that CB EPC-derived cells displayed phenotypic and functional features of EPCs, although a process of maturation was observed over time. Although we confirmed that CB EPCs have a greater expansion potential compared to peripheral blood EPCS, we observed a high incidence of cytogenetic alterations (71%) in the expanded endothelial cell population, even at early times of culture. In two cases, spontaneous transformation in vitro was documented, but none of the samples tested showed tumorigenic potential in vivo. Conversely, no karyotype alterations have been observed on peripheral blood EPCs-derived cells. CONCLUSIONS: We confirm that CB represents a good source for clinical ex vivo expansion of EPCs. However, because of high frequency of karyotype alterations, these cells cannot be considered free of risk in clinical application.

AKT/NF-kappaB inhibitor xanthohumol targets cell growth and angiogenesis in hematologic malignancies.

BACKGROUND: Leukemias are dependent on Akt/NF-kappaB activation and angiogenesis. METHODS: The antiangiogenic Akt/NF-kappaB inhibitor xanthohumol (XN) has in vitro activity against acute and chronic myelogenous leukemia cell lines (AML, CML) and fresh samples from patients were investigated. RESULTS: Inhibition of cell proliferation is associated with induction of apoptosis and reduced VEGF secretion. Decreased cell invasion, metalloprotease production, and adhesion to endothelial cells observed in the presence of XN could prevent in vivo life-threatening complications of leukostasis and tissue infiltration. CONCLUSIONS: As endothelial cells and hematopoietic cells are mutually correlated in their development and growth, targeting both tumor cells and endothelial cells with agents possessing cytotoxic and antiangiogenic activities may lead to synergistic antitumor effects interrupting a reciprocal stimulatory loop between leukemia and endothelial cells.

Boost of CD34+-selected peripheral blood cells without further conditioning in patients with poor graft function following allogeneic stem cell transplantation.

BACKGROUND AND OBJECTIVES: A proportion of patients develop poor graft function (PGF) following an allogeneic hemopoietic stem cell transplant (HSCT). It is uncertain whether a boost of donor marrow or blood cells is beneficial in terms of trilineage recovery and non-relapse-related mortality (NRM). DESIGN AND METHODS: The aim of this study was to compare outcomes in patients with PGF and full donor chimerism following an allogeneic HSCT who did or did not receive a boost of donor stem cells. The study included patients with primary PGF--i.e. those failing to achieve sustained graft function- and secondary PGF--i.e. those developing PGF after complete hematologic recovery. We studied 54 patients with PGF: 20 patients received no further donor cell infusion (group A), 14 received a boost of unmanipulated marrow or blood cells from the original donor, without further conditioning (group B), and 20 received donor cells after CD34 selection without conditioning (group C). The three groups were comparable for disease phase, patients' age, donor type, primary or secondary PGF, full donor chimerism and duration of PGF. RESULTS: Trilineage recovery was seen in 40%, 36% and 75% of the patients in, respectively, groups A, B and C (p=0.02). In multivariate Cox analysis trilineage recovery was more frequent in patients with secondary PGF (RR of complete recovery 2.82, p=0.01) and in patients receiving CD34+-selected cells (RR of complete recovery 3.0; p=0.007). There was no effect of donor type on hematologic recovery. The rate of NRM was 55%, 64%, 20% in groups A, B and C, respectively (p=0.06) and was highly correlated with trilineage recovery (RR 0.36, p<0.0001). PGF was the primary cause of death in 30%, 21% and 10% of the patients in the three groups, graft-versus-host disease (GVHD) in 5%, 36%, and 10%. INTERPRETATIONS AND CONCLUSIONS: In patients with poor graft function (a) a boost of CD34+-selected peripheral blood donor cells is associated with a high chance of trilineage recovery and a low risk of acute GVHD; (b) a boost of unmanipulated donor cells does not seem to offer a survival advantage over no infusion of cells; and (c) NRM is lower when using peripheral blood cells for the boost. These data may be useful when discussing second stem cell donations for patients with poor graft function.

Donor multipotent mesenchymal stromal cells may engraft in pediatric patients given either cord blood or bone marrow transplantation.

OBJECTIVE: Multipotent mesenchymal stromal cells (MSCs) are endowed with multilineage differentiative potential and immunomodulatory properties. It is still a matter of debate whether donor MSCs have sustained engraftment potential in host bone marrow (BM) after allogeneic hematopoietic stem cell transplantation (allo-HSCT). The aim of this study was to analyze the donor/recipient origin of MSCs in children receiving allogeneic either BM or cord blood (CB) transplantation. METHODS: Thirty-seven pediatric patients undergoing allo-HSCT for either a malignant or a nonmalignant disorder were enrolled in the study; 19 received CB and 18 BM transplantation. Results were compared with those obtained in 14 adults given BM transplantation for either malignant or nonmalignant disorders. MSCs were grown from BM aspirates obtained 1-17 and 2-192 months after allo-HSCT in pediatric and adult patients, respectively. MSC samples at the third-fourth passage were phenotypically characterized. Donor/recipient origin of MSCs was assessed by amelogenin assay and microsatellite analysis. RESULTS: MSCs could be grown from 30 of 37 children; at the third-fourth passage MSCs resulted positive (> or = 98%) for CD73, CD105, CD106, CD29, CD13, CD44 and negative (< or = 1%) for CD34, CD45, CD14. Mixed chimerism with donor cells was observed in 4 BM and 5 CB transplantation recipients, respectively; full recipient chimerism was detected in the remaining children. Full recipient MSC chimerism was observed also in all assessable (12/14) adult patients. CONCLUSIONS: BM of pediatric patients might be a more favorable milieu than that of adults for sustained engraftment of transplanted MSCs. MSCs able to engraft in the host can be transferred with cryopreserved CB units.

Loss of inhibitory semaphorin 3A (SEMA3A) autocrine loops in bone marrow endothelial cells of patients with multiple myeloma.

Vascular endothelial growth factor165 (VEGF165) and semaphorin3A (SEMA3A) elicit pro- and antiangiogenic signals respectively in endothelial cells (ECs) by binding to their receptors VEGFR-2, neuropilin-1 (NRP1), and plexin-A1. Here we show that the VEGF165-driven angiogenic potential of multiple myeloma (MM) ECs is significantly higher than that of monoclonal gammopathy of undetermined significance (MGUS) ECs (MGECs) and human umbilical vein (HUV) ECs. This is probably due to a constitutive imbalance of endogenous VEGF165/SEMA3A ratio, which leans on VEGF165 in MMECs but on SEMA3A in MGECs and HUVECs. Exogenous VEGF165 induces SEMA3A expression in MGECs and HUVECs, but not in MMECs. Moreover, by counteracting VEGF165 activity as efficiently as an anti-VEGFR-2 antibody, exogenous SEMA3A restrains the over-angiogenic potential of MMECs. Our data indicate that loss of endothelial SEMA3A in favor of VEGF165 could be responsible for the angiogenic switch from MGUS to MM.

T-cell suppression mediated by mesenchymal stem cells is deficient in patients with severe aplastic anemia.

OBJECTIVE: To compare the suppressive effect of mesenchymal stem cells (MSC), derived from normal individuals or severe aplastic anemia patients (SAA), on T-cell activation. PATIENTS AND METHODS: We studied bone marrow MSC from 19 healthy donors and 23 SAA patients in different phases of the disease: at diagnosis (n = 3), following immunosuppressive therapy (IS) (n = 16), or after a bone marrow transplant (BMT) (n = 4). MSC were tested for T-cell suppression in the following assays: mixed lymphocyte reaction (MLR), phytohemaglutinin (PHA)-primed cultures, activation surface markers, gamma-IFN production, hematopoietic colony formation (CFC), production of cyclic ADP-ribose (cADPR). RESULTS: The abnormalities of SAA MSC included: 1) significantly lower suppression of T-cell proliferation induced by alloantigens (p = 0.009) or PHA (p = 0.006); 2) impaired capacity to suppress CD38 expression on PHA-primed T cells (p = 0.001); 3) impaired ability to suppress gamma-IFN production in PHA cultures, resulting in an 11-fold higher gamma-IFN concentration; 4) no preventive effect on T cell-mediated inhibition of CFC; and 5) significantly reduced (p = 0.009) production of cADPR, a universal calcium mobilizer. MSC-mediated suppression of PHA-induced T-cell proliferation was restored to control levels in 3 of 4 patients post-BMT. CONCLUSION: The ability of MSC to downregulate T-cell priming, proliferation, and cytokine release is deficient in patients with SAA, persists indefinitely after immunosuppressive therapy, but seems to be restored after BMT. Whether these abnormalities are relevant to the pathogenesis of aplastic anemia remains to be determined.

Interaction of human mesenchymal stem cells with cells involved in alloantigen-specific immune response favors the differentiation of CD4+ T-cell subsets expressing a regulatory/suppressive phenotype.

BACKGROUND AND OBJECTIVES: Experimental evidence and preliminary clinical studies have demonstrated that human mesenchymal stem cells (MSC) have an important immune modulatory function in the setting of allogeneic hematopoietic stem cell (HSC) transplantation. We extended the evaluation of mechanisms responsible for the immune regulatory effect derived from the interaction of human MSC with cells involved in alloantigen-specific immune response in mixed lymphocyte culture (MLC). DESIGN AND METHODS: Dendritic cell (DC) differentiation, T- and natural killer (NK)-lymphocyte expansion, alloantigen-specific cytotoxic activity and differentiation of CD4+ T-cell subsets co-expressing CD25 and/or CTLA4 molecules were assessed, comparing the effect observed using third-party MSC with that obtained employing MSC autologous to the MLC responder. RESULTS: We found that human MSC strongly inhibit alloantigen-induced DC1 differentiation, down-regulate alloantigen-induced lymphocyte expansion, especially that of CD8+ T cells and of NK lymphocytes, decrease alloantigen-specific cytotoxic capacity mediated by either cytotoxic T lymphocytes or NK cells and favor the differentiation of CD4+ T-cell subsets co-expressing CD25 and/or CTLA4. More effective suppressive activity on MLC-induced T-cell activation was observed when MSC were third-party, rather than autologous, with respect to MLC-responder cells. INTERPRETATION AND CONCLUSIONS: Our results strongly suggest that MSC-mediated inhibition of alloantigen-induced DC1 differentiation and preferential activation of CD4+ CD25+ T-cell subsets with presumed regulatory activity represent important mechanisms contributing to the immunosuppressive activity of MSC. Collectively, these data provide immunological support for the use of MSC to prevent immune complications related to both HSC and solid organ transplantation and to the theory that MSC are universal suppressors of immune reactivity.

Reducing transplant-related mortality after allogeneic hematopoietic stem cell transplantation.

BACKGROUND AND OBJECTIVES: Transplant-related mortality (TRM) following allogeneic hematopoietic stem cell transplantation (HSCT) has been reported to be related to disease stage, duratiion of disease and type of donor. Furthermore, the outcome of transplants performed in the 1990s appears to be better than that of transplants done in the previous decade. The aims of this study were to determine whether these relationships still hold and whether the outcome of transplants is continuing to improve. DESIGN AND METHODS: We analyzed 1180 consecutive patients with leukemia (n=979) or other hematologic malignancies (n=201) undergoing HSCT in 4 time periods: before 1990, 1991-1995, 1996-2000, and 2001-2002. Changes during these eras include increasing patient age, more unrelated transplants, more patients with advanced disease, different graft-versus-host disease (GvHD) prophylaxis, and different management of infections. RESULTS: The actuarial 2-year transplant-related mortality (TRM) differed significantly between the transplant eras (p<0.001) with a significant interaction with disease phase (p=0.018). In patients in first remission (n=585) TRM was 34%, 25%, 21% and 6% in the four transplant eras. The reduction in TRM was less evident in patients in second remission (n=284) (37%, 35%, 30%, 25%) and absent in relapsed patients (n=311) (TRM=45%, 41%, 29%, 51%). This is a consequence of reductions in GvHD, infections and multiorgan failure among patients in remission but not among those who relapse. The actuarial 2-year survival has improved significantly in patients in first remission (54%, 66%, 72%, 78%) but not in those in second remission (38%, 46%, 52%,45%), or relapsed patients (31%, 25%, 36%, 21%). INTERPRETATION AND CONCLUSIONS: In conclusion, TRM has been significantly reduced in first remission patients, suggesting an allograft should be considered in this phase, when appropriate, without delay. There has been no improvement in survival for patients beyond first remission, due to persisting high risk of infections and organ toxicity, a possible consequence of prolonged pre-transplant chemotherapy and neutropenia.

Intra-bone marrow injection of bone marrow and cord blood cells: an alternative way of transplantation associated with a higher seeding efficiency.

OBJECTIVE: Intravenous (IV) injection is currently the normal method for transplanting hematopoietic cells. However, the problem of seeding efficiency and homing is relevant especially when a limited number of stem cells is available. Intra-bone marrow (IBM) injection of bone marrow cells (BMCs) may overcome this problem. MATERIALS AND METHODS: Irradiated (750 cGy) C57BL/6J mice were transplanted with 1 x 10(5) BMCs harvested from transgenic mice expressing an enhanced version of the green fluorescent protein (EGFP+) via IBM or with 1 x 10(6) EGFP+ BMCs via IV. Irradiated (320 cGy) NOD/SCID mice were transplanted with 1 x 10(6) human cord blood (CB) cells via IBM or with 1 x 10(7) human CB cells via IV. RESULTS: In C57BL/6J mice after 90 days, the fraction of EGFP+ cells harvested was 37% and 53% in IV-treated and IBM-treated (contralateral tibia and femur in the IBM) mice, respectively: the expansion folds were 114 and 1760, respectively. In NOD/SCID mice, the percentages of CD45+ cells and CD45+/CD34+ cells were, at 30 days, 3.3% and 0.3% in IV-treated mice, and 4.4% and 1.1% in IBM-treated mice. At 60 days, the percentages of CD45+ cells and CD45+/CD34+ cells were 2.1% and 0.3% in IV-treated mice and 1.4% and 0.4% in IBM-treated mice. At day 90 the percentages of CD45+ cells and CD45+/CD34+ cells were 3% and 0.3% in IV-treated mice and 2.3% and 0.4% in IBM-treated mice. CONCLUSION: Our data demonstrate that IBM transplantation is associated with a seeding efficiency 15 times greater than IV transplantation. IBM transplantation may improve the results of transplant and may be useful in several settings: 1) when a limited number of hematopoietic progenitors are available; and 2) in experiments aiming to place in the bone marrow stem cells of other lineages (CNS, muscle, etc.).

Cord blood transplantation provides better reconstitution of hematopoietic reservoir compared with bone marrow transplantation.

Delayed hematopoietic recovery is the main factor precluding a wider use of cord blood (CB) transplants. We hypothesized that this delayed engraftment might not be related to an insufficient number of stem cells in the graft, but to an intrinsic difficulty of these cells to undergo differentiation. To test our hypothesis, 2 groups of children were compared; 12 received a CB transplant and 12 an adult bone marrow (BM) transplant. We studied neutrophil and platelet recovery and, at a median time of approximately 1 year after transplantation, the frequency of colony-forming cells (CFCs) and long-term culture initiating cells (LTC-ICs) in the BM of the 2 groups. Recipients of BM transplants received 1-log more cells and had significantly faster neutrophil and platelet recovery. Conversely, the frequency of committed and early progenitors was significantly higher in the BM of children given CB cells compared with BM transplant recipients (median count of CFC/2 x 10(4) BM mononuclear cells, 20 versus 11, P =.007; median count of LTC-IC/10(6) BM mononuclear cells, 8.2 versus 0.2 P =.001). CB, but not adult BM stem cells, can better restore the host hematopoietic progenitor cell reservoir; the delayed engraftment after CB transplantation may reflect the difficulty of CB progenitors to reprogram themselves toward differentiation.