Reconstitution of human haematopoiesis in non-obese diabetic/severe combined immunodeficient mice by clonal cells expanded from single CD34+CD38- cells expressing Flk2/Flt3

High expression of FLT3 is a risk factor in leukemia

Several studies have shown that internal tandem duplication (ITD) of FMS-like tyrosine kinase 3 (FLT3) can result in the failure of leukemia treatment and contribute to a poor prognosis. However, the role of the overexpression of FLT3 in leukemia remains to be fully elucidated. By mining public database, the present study first identified that the expression of FLT3 in leukemia was markedly higher, compared with that in other types of tumor and cell lines, indicating that FLT3 is important in leukemia. In leukemia, FLT3 was found to be significantly upregulated in acute myeloid leukemia and acute lymphoblastic leukemia, and a high expression of FLT3 contributed to reduced survival rates. By analyzing Gene Expression Omnibus and The Cancer Genome Atlas data, it was found that genetic alterations and modification of DNA methylation increased the expression of FLT3 in leukemia. FLT3-ITD and FLT3 tyrosine kinase domain point mutations increased the expression of FLT3 in four independent datasets. In addition, the status of FLT3 gene methylation was negatively correlated with the expression of FLT3, and haploinsufficiency of DNA methyltransferase 1 increased the expression of Flt3 in mouse leukemia cells. By analyzing the enrichment of differentially-expressed genes in chemical and genetic perturbation datasets, it was found that genes, which were upregulated in the FLT3 high expression group had myeloid lymphoid leukemia- and nucleophosmin 1-like signatures, indicating that the overexpression of FLT3 may use the same mechanism to promote leukemia. Collectively, the results of the present study showed that the overexpression of FLT3 is a potential risk factor in leukemia.

Allogeneic CD4+CD25high T cells regulate obliterative bronchiolitis of heterotopic bronchus allografts in both porcinized and humanized mouse models

BACKGROUND

Bronchiolitis obliterans syndrome is caused by a fibroproliferative process in lung allografts resulting in irreversible damage. In this study, we induced obliterative bronchiolitis and studied the contribution of regulatory T cells to its development in immune-deficient mice receiving heterotopic porcine bronchus transplants, and major histocompatibility complex-mismatched porcine peripheral blood mononuclear cell. Furthermore, we aimed to corroborate our findings in a humanized mouse model.

METHODS

Heterotopic bronchus transplantation was performed in 33 NOD.rag(−/−)γc(−/−) mice, using miniature pigs as tissue donors.The recipient mice then either received saline (negative control), unsorted MHC-mismatched PBMC (positive control), PBMC enriched with CD4(+)CD25(high) cells or PBMC depleted of CD4(+)CD25(high) cells for reconstitution. The results were validated in 28 NOD.rag(−/−)γc(−/−) mice undergoing heterotopic human bronchus transplantation and reconstitution with allogeneic human PBMC.

RESULTS

Histological lesions similar to those typical for obliterative bronchiolitis developed in vivo after reconstitution with allogeneic PBMC and were more severe in animals engrafted with PBMC depleted of CD4(+)CD25(high) cells. In contrast, the group reconstituted with PBMC enriched with CD4(+)CD25(high) cells showed well-preserved histology. The results of the humanized model confirmed those obtained in the porcinized model.

CONCLUSIONS

In conclusion, both porcinized and humanized mouse models of heterotopic subcutaneous bronchus transplantation imitate the in vivo development of bronchiolitis obliterans syndrome-like lesions and reveal its sensitivity to T-cell regulation.

FLT3 receptor and ligand are dispensable for maintenance and posttransplantation expansion of mouse hematopoietic stem cells

Originally cloned from hematopoietic stem cell (HSC) populations and its ligand being extensively used to promote ex vivo HSC expansion, the FMS-like tyrosine kinase 3 (FLT3; also called FLK2) receptor and its ligand (FL) were expected to emerge as an important physiologic regulator of HSC maintenance and expansion. However, the role of FLT3 receptor and ligand in HSC regulation remains unclear and disputed. Herein, using Fl-deficient mice, we establish for the first time that HSC expansion in fetal liver and after transplantation is FL independent. Because previous findings in Flk2(-/-) mice were compatible with an important role of FLT3 receptor in HSC regulation and because alternative ligands might potentially interact directly or indirectly with FLT3 receptor, we here also characterized HSCs in Flk2(-/-) mice. Advanced phenotypic as well as functional evaluation of Flk2(-/-) HSCs showed that the FLT3 receptor is dispensable for HSC steady-state maintenance and expansion after transplantation. Taken together, these studies show that the FLT3 receptor and ligand are not critical regulators of mouse HSCs, neither in steady state nor during fetal or posttransplantation expansion.

Investigation of various methods for the cryopreservation of canine bone marrow-derived CD34(+) cells

Optimal condition for the cryopreservation of canine CD34(+) cells was explored. Canine bone marrow CD34(+) cells were isolated from 5 healthy dogs by a magnetic- activated cell-sorting system using a monoclonal antibody specific to canine CD34. These cells were cryopreserved by 4 different methods: 2 different cryoprotectant solutions-solutions A (fetal bovine serum containing 10% dimethylsulfoxide (DMSO) and B (physiological saline containing 5% DMSO, 6% hydroxyethyl starch, and 4% bovine serum albumin)-were used in combination with 2 different freezing procedures-in a rate-controlled programmed freezer (PF) and in an ordinary freezing container. The cell viability, cell recovery rates, and colony-forming unit (CFU) recovery rates were examined following cryopreservation for 1 week, 4 weeks, and 6 months. The values of these parameters were significantly higher for the CD34(+) cells that had been frozen in Solution B than for those that had been frozen in Solution A, regardless of the freezing procedure employed. The highest CFU recovery rate following cryopreservation for 6 months corresponded to the cells that had been cryopreserved with Solution B and frozen in a PF. In conclusion, cryopreservation with Solution B in a PF proved to be the most efficient of the 4 cryopreservation procedures investigated in terms of maintaining the quality of canine bone marrow-derived CD34(+) cells. This method will be useful for clinical applications involving the use of canine bone marrow-derived CD34(+) cells.

Optimized Retroviral Transduction Protocol Which Preserves the Primitive Subpopulation of Human Hematopoietic Cells

Though both low-speed centrifugation and the use of fibronectin (Retronectin) fragments increase gene transduction efficiency, they still do not overcome the adverse effects of the presence of virus-containing medium (VCM). In this study, we improved transduction efficiency of primitive human hematopoietic cells by optimizing the conditions for preadsorbing culture dishes with retrovirus using a centrifugation protocol allowing subsequent infection to be carried out in the absence of VCM. We also demonstrate that preadsorbing tissue culture plates with retrovirus is dependent on the volume of VCM used for preadsorption and the length of centrifugation and the type of plasticware used but not on the temperature of centrifugation (4-33 degrees C). Direct exposure of CD34+ target cells to VCM depletes the primitive CD34+CD38neg subpopulation by more than 30%, whereas the optimized VCM-free infection protocol targets this population with equivalent efficiency but had no detrimental effects on CD34+CD38neg frequency. In summary, we demonstrate a high-frequency transduction protocol which preserves the therapeutically relevant primitive subpopulation of human hematopoietic cells.

Human Flt3 is expressed at the hematopoietic stem cell and the granulocyte/macrophage progenitor stages to maintain cell survival

FLT3/FLK2, a member of the receptor tyrosine kinase family, plays a critical role in maintenance of hematopoietic homeostasis, and the constitutively active form of the FLT3 mutation is one of the most common genetic abnormalities in acute myelogenous leukemia. In murine hematopoiesis, Flt3 is not expressed in self-renewing hematopoietic stem cells, but its expression is restricted to the multipotent and the lymphoid progenitor stages at which cells are incapable of self-renewal. We extensively analyzed the expression of Flt3 in human (h) hematopoiesis. Strikingly, in both the bone marrow and the cord blood, the human hematopoietic stem cell population capable of long-term reconstitution in xenogeneic hosts uniformly expressed Flt3. Furthermore, human Flt3 is expressed not only in early lymphoid progenitors, but also in progenitors continuously along the granulocyte/macrophage pathway, including the common myeloid progenitor and the granulocyte/macrophage progenitor. We further found that human Flt3 signaling prevents stem and progenitors from spontaneous apoptotic cell death at least through up-regulating Mcl-1, an indispensable survival factor for hematopoiesis. Thus, the distribution of Flt3 expression is considerably different in human and mouse hematopoiesis, and human FLT3 signaling might play an important role in cell survival, especially at stem and progenitor cells that are critical cellular targets for acute myelogenous leukemia transformation.

Identification of long-term repopulating potential of human cord blood-derived CD34-flt3- severe combined immunodeficiency-repopulating cells by intra-bone marrow injection

Recently, we have identified human cord blood (CB)-derived CD34-negative (CD34(-)) severe combined immunodeficiency (SCID)-repopulating cells (SRCs) using the intra-bone marrow injection (IBMI) method (Blood 2003;101:2924). In contrast to murine CD34(-) Kit(+)Sca-1(+)Lineage(-) (KSL) cells, human CB-derived Lin(-)CD34(-) cells did not express detectable levels of c-kit by flow cytometry. In this study, we have investigated the function of flt3 in our identified human CB-derived CD34(-) SRCs. Both CD34(+)flt3(+/-) cells showed SRC activity. In the CD34(-) cell fraction, only CD34(-)flt3(-) cells showed distinct SRC activity by IBMI. Although CD34(+)flt3(+) cells showed a rather weak secondary repopulating activity, CD34(+)flt3(-) cells repopulated many more secondary recipient mice. However, CD34(-)flt3(-) cells repopulated all of the secondary recipients, and the repopulating rate was much higher. Next, we cocultured CD34(-)flt3(-) cells with the murine stromal cell line HESS-5. After 1 week, significant numbers of CD34(+)flt3(+/-) cells were generated, and they showed distinct SRC activity. These results indicated that CB-derived CD34(-)flt3(-) cells produced CD34(+)flt3(-) as well as CD34(+)flt3(+) SRCs in vitro. The present study has demonstrated for the first time that CB-derived CD34(-) SRCs, like murine CD34(-) KSL cells, do not express flt3. On the basis of these data, we propose that the immunophenotype of very primitive long-term repopulating human hematopoietic stem cells is Lin(-)CD34(-)c-kit(-)flt3(-). Disclosure of potential conflicts of interest is found at the end of this article.

Isolation and characterization of human myeloid progenitor populations--TpoR as discriminator between common myeloid and megakaryocyte/erythroid progenitors

OBJECTIVE

The common myeloid progenitor (CMP) and its progeny, the granulocyte/monocyte progenitor (GMP) and megakaryocyte/erythrocyte progenitor (MEP), have been isolated based on the surface expression of CD34, IL-3Ralpha, and CD45RA. However, high resolution of IL-3Ralpha- and IL-3Ralphalo cells required to adequately separate the CMP and MEP populations is difficult to achieve. The aim of this study was to find a complementary surface marker to obtain a better separation of these two populations and to further characterize the acquired progenitor populations.

MATERIALS AND METHODS

To evaluate the thrombopoietin receptor (TpoR) as a candidate marker, CD19-/CD34+/IL-3Ralphalo/-/CD45RA-/TpoR- (CMP), CD19-/CD34+/IL-3Ralphalo/CD45RA+/TpoR- (GMP), and CD19-/CD34+/IL-3Ralphalo/-/CD45RA-/TpoR+ (MEP) cells from human bone marrow were sorted to semisolid cultures for colony assays, and in addition analyzed for their surface expression of other growth factor receptors (GFRs) and sorted to real-time RT-PCR for gene expression analysis.

RESULTS

The colony-formation and gene expression assays showed that inclusion of TpoR as a marker gave a distinct and reproducible separation of the myeloid progenitors. Furthermore, most GFR surface expression correlated to gene expression, but there were also striking discrepancies, in particular for the common beta-chain of the IL-3R, GM-CSFR, and IL-5R and for TpoR.

CONCLUSION

Our data establish the TpoR as an important tool for isolation of the myeloid progenitors and demonstrate that the surface expression of GFRs cannot be predicted by their gene expression. Importantly, the refined isolation of CMPs will allow more detailed studies of regulatory mechanisms steering CMPs towards erythropoiesis vs granulopoiesis in steady state and response to peripheral demands.

Converging pathways in leukemogenesis and stem cell self-renewal

Studies over the last 40 years have led to an understanding of the hierarchical organization of the hematopoietic system and the role of the pluripotential hematopoietic stem cell. Earlier recognition of the importance of bone marrow hematopoietic microenvironments has evolved into the recognition of specific niches that regulate stem cell pool size, proliferative status, mobilization, and differentiation. The discovery of the role of multiple hematopoietic growth factors and their receptors in the orchestration of stem cell self-renewal and differentiation has been followed by recognition of the importance of the Notch and Wnt pathways. The homeobox family of transcription factors serve as master regulators of development and are increasingly found to be critical regulators of hematopoiesis. In parallel with this understanding of normal hematopoiesis has come a recognition that stem cell dysregulation at various levels is involved in leukemogenesis. Furthermore, the progression from chronic leukemia or myelodysplasia to acute leukemia involves accumulation of at least two mutational events that lead to enhancement of stem cell proliferation, or acquisition of stem cell behavior by a progenitor cell, coupled with maturation inhibition. Translocations resulting in development of oncogenic fusion genes are found in AML and the transforming potential of two of these, AML1-ETO and NUP98-HOXA9, will be discussed. Secondary, constitutively activating mutations of the Flt3 and c-kit receptors and of K- and N-ras are found with high frequency in AML, and the transforming potential of mutated FLT3 and the role of STAT5A activation in human stem cell transformation will be reviewed.

Methylation status of the p15 and p16 genes in paediatric myelodysplastic syndrome and juvenile myelomonocytic leukaemia

Aberrant DNA methylation is frequently observed in adults with myelodysplastic syndrome (MDS), and is recognized as a critical event in the disease's pathogenesis and progression. This is the first report to investigate the methylation status of p15 and p16, cell cycle regulatory genes, in children with MDS (n = 9) and juvenile myelomonocytic leukaemia (JMML; n = 18) by using a methylation-specific polymerase chain reaction. The frequency of p15 hypermethylation in paediatric MDS was 78% (7/9), which was comparable to that in adult MDS. In contrast, p15 hypermethylation in JMML was a rare event (17%; 3/18). In JMML, clinical and laboratory characteristics including PTPN11 mutations and aberrant colony formation were not different between the three patients with hypermethylated p15 and the others. Aberrant methylation of p16 was not detected in children with either MDS or JMML. Since p15 and p16 genes were unmethylated in two children with JMML, in whom the disease had progressed with an increased number of blasts, a condition referred to as blastic crisis, we infer that the aberrant methylation of these genes is not responsible for the progression of JMML. The results suggest that demethylating agents may be effective in most children with MDS and a few patients with JMML.

A combination of catalase and trehalose as additives to conventional freezing medium results in improved cryoprotection of human hematopoietic cells with reference to in vitro migration and adhesion properties

BACKGROUND

Cryopreservation of hematopoietic cells from cord blood is an essential component in unrelated transplant settings. Cell damage during freezing is caused by multiple factors, of which membrane damage and oxygen free radical generation are two major factors. It was reported earlier that a combination of catalase and trehalose as additives in freezing medium affords better cryoprotection in terms of long-term culture assays.

STUDY DESIGN AND METHODS

Mononuclear and CD34+ cells isolated from cord blood were used as a source of hematopoietic cells. KG1a cell line was used as a model system in adhesion assays. The cells were frozen in a programmable freezer and stored at -196 degrees C. Various homing-related assays were carried out on the frozen cells.

RESULTS

Herein it is reported that these two additives afford better preservation of adhesion- and migration-related properties of the frozen cells. The test cells frozen with additives resulted in improved migration toward stromal-derived factor-1alpha and showed higher expression of its receptor CXCR4. Colony-forming unit assay of migrated test cells showed that these cells are early progenitors having capacity to give rise to all types of myeloid colonies. Test cells also show increased expression of FLT3R and improved responsiveness to FLT3 ligand, the homing-related cytokine. Adhesion to stroma and extracellular matrix was better in test cells as compared to control cells.

CONCLUSION

The present data provide evidence that addition of catalase and trehalose to the conventional freezing medium preserves migration- and adhesion-related properties of the hematopoietic graft.

Isolation and characterization of pediatric canine bone marrow CD34+ cells

Historically, the dog has been a valuable model for bone marrow transplantation studies, with many of the advances achieved in the dog being directly transferable to human clinical bone marrow transplantation protocols. In addition, dogs are also a source of many well-characterized homologues of human genetic diseases, making them an ideal large animal model in which to evaluate gene therapy protocols. It is generally accepted that progenitor cells for many human hematopoietic cell lineages reside in the CD34+ fraction of cells from bone marrow, cord blood, or peripheral blood. In addition, CD34+ cells are the current targets for human gene therapy of diseases involving the hematopoietic system. In this study, we have isolated and characterized highly enriched populations of canine CD34+ cells isolated from dogs 1 week to 3 months of age. Bone marrow isolated from 2- to 3-week-old dogs contained up to 18% CD34+ cells and this high percentage dropped sharply with age. In in vitro 6-day liquid suspension cultures, CD34+ cells harvested from 3-week-old dogs expanded almost two times more than those from 3-month-old dogs and the cells from younger dogs were also more responsive to human Flt-3 ligand (Flt3L). In culture, the percent and number of CD34+ cells from both ages of dogs dropped sharply between 2 and 4 days, although the number of CD34+ cells at day 6 of culture was higher for cells harvested from the younger dogs. CD34+ cells harvested from both ages of dogs had similar enrichment and depletion values in CFU-GM methylcellulose assays. Canine CD34+/Rho123lo cells expressed c-kit mRNA while the CD34+/Rhohi cells did not. When transplanted to a sub-lethally irradiated recipient, CD34+ cells from 1- to 3-week-old dogs gave rise to both myeloid and lymphoid lineages in the periphery. This study demonstrates that canine CD34+ bone marrow cells have similar in vitro and in vivo characteristics as human CD34+ cells. In addition, ontogeny-related functional differences reported for human CD34+ cells appear to exist in the dog as well, suggesting pediatric CD34+ cells may be better targets for gene transfer than adult bone marrow. The demonstration of similarities between canine and human CD34+ cells enhances the dog as a large, preclinical model to evaluate strategies for improving bone marrow transplantation protocols, for gene therapy protocols that target CD34+ cells, and to study the engraftment potential of various cell populations that may contain hematopoietic progenitor cell activity.

Hair follicle dermal cells differentiate into adipogenic and osteogenic lineages

The adult hair follicle dermal papilla (DP) and dermal sheath (DS) cells are developmentally active cell populations with a proven role in adult hair follicle-cycling activity and unique inductive powers. In stem cell biology, the hair follicle epithelium has recently been the subject of a great deal of investigation, but up to now, the follicle dermis has been largely overlooked as a source of stem cells. Following the sporadic appearance of muscle, lipid and bone-type cells in discretely isolated follicle DP and DS cell primary cultures, we demonstrated that cultured papilla and sheath cell lines were capable of being directed to lipid and bone differentiation. Subsequently, for the first time, we produced clonal DP and DS lines that had extended proliferative capabilities. Dye exclusion has been reported to be an identifying feature of stem cells; therefore, clonal papilla and sheath lines with differing capacity to exclude rhodamine 123 were cultured in medium known to induce adipocyte and osteocyte differentiation. Both DS- and DP-derived clones showed the capacity to make lipid and to produce calcified material; however, different clones had varied behaviour and there was no obvious correlation between their stem cell capabilities and dye exclusion or selected gene expression markers. As a highly accessible source, capable of being discretely isolated, the follicle has important potentially as a stem cell source for tissue engineering and cell therapy purposes. It will also be interesting to compare follicle dermal stem cell properties with the broader stem cell capabilities discovered in skin dermis and investigate whether, as we believe, the follicle is a key dermal stem cell niche. Finally, the discovery of stem cells in the dermis may have implications for certain pathologies in which abnormal differentiation occurs in the skin.

Human leukaemic stem cells: a novel target of therapy

Acute myeloid leukaemia (AML) is a life-threatening haematopoietic disease that is characterized by clonal growth and the accumulation of myelopoietic progenitor cells. Although AML cells only have a limited potential to undergo differentiation and maturation, each AML clone is organized in a hierarchical manner similar to normal haematopoiesis. Recent data have shown that each AML clone consists of leukaemic stem cells and their progeny, and that AML stem cells differ from more mature cells in several aspects, including survival and target antigen profiles. Most importantly, AML stem cells, but not their progeny, have the capacity to repopulate haematopoietic tissues with leukaemias in NOD/SCID mice. Furthermore, AML stem cells are thought to be responsible for the infinite growth of leukaemias in patients with AML. The phenotypic properties of AML stem cells have also been described. In most cases, these cells are detectable within the CD34+, CD38-, Lin-, CD123+ subpopulation of AML cells. Because of their AML-initiating and -renewing capacity and their unique phenotype, which includes several molecular targets of drug therapy, AML stem cells have recently been proposed as novel important target cell populations in the context of curative therapies. The present article gives an overview of our knowledge about AML stem cells, their phenotype, and their role as a 'therapy-target' in new concepts to treat and to cure patients with AML.

CD34+CD38- hematopoietic precursors derived from human embryonic stem cells exhibit an embryonic gene expression pattern

Gene expression patterns of CD34(+)CD38(-) cells derived from human embryonic stem cells (ESCs) were compared with those of cells isolated from adult human bone marrow (BM) using microarrays; 1692 and 1494 genes were expressed at levels at least 3-fold above background in cells from BM and ESCs, respectively. Of these, 494 showed similar levels of expression in cells from both sources, 791 genes were overexpressed in cells from BM (BM versus ESCs, at least 2-fold), and 803 genes were preferentially expressed in cells from ESCs (ESCs versus BM, at least 2-fold). The message of the flt-3 gene was markedly decreased in cells from ESCs, whereas there was substantial flt-3 expression in cells from BM. High levels of embryonic epsilon-globin expression were observed-but no adult beta-globin message-in CD34(+)CD38(-) cells from ESCs, whereas high levels of beta-globin expression-but no embryonic epsilon-globin message-could be detected in cells from BM. Furthermore, high levels of major histocompatibility complex (MHC) gene expression were demonstrated in cells from BM but very low levels of MHC message in corresponding cells from ESCs. These observations demonstrate that CD34(+)CD38(-) cells derived from ESCs correspond consistently to an early developmental stage at which the yolk sac and fetal liver are the primary sites of hematopoiesis.

Human CD34+ hematopoietic stem cells capable of multilineage engrafting NOD/SCID mice express flt3: distinct flt3 and c-kit expression and response patterns on mouse and candidate human hematopoietic stem cells

The cytokine tyrosine kinase receptors c-kit and flt3 are expressed and function in early mouse and human hematopoiesis. Through its ability to promote ex vivo expansion and oncoretroviral transduction of primitive human hematopoietic progenitors, the flt3 ligand (FL) has emerged as a key stimulator of candidate human hematopoietic stem cells (HSCs). However, recent studies in the mouse suggest that though it is present on short-term repopulating cells, flt3 is not expressed on bone marrow long-term reconstituting HSCs, the ultimate target for the development of cell replacement and gene therapy. Herein we demonstrate that though only a fraction of human adult bone marrow and cord blood CD34+long-term culture-initiating cells (LTC-ICs) express flt3, most cord blood lymphomyeloid HSCs capable of in vivo reconstituting nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice are flt3+. The striking difference in flt3 and c-kit expression on mouse and candidate human HSCs translated into a corresponding difference in flt3 and c-kit function because FL was more efficient than SCF at supporting the survival of candidate human HSCs. In contrast, SCF is far superior to FL as a viability factor for mouse HSCs. Thus, the present data provide compelling evidence for a contrasting expression and response pattern of flt3 and c-kit on mouse and human HSCs.