Characteristics of the hematopoietic stem cell compartment in adult mice

Distinct hematopoietic stem cell subtypes are differentially regulated by TGF-beta1

The traditional view of hematopoiesis has been that all the cells of the peripheral blood are the progeny of a unitary homogeneous pool of hematopoietic stem cells (HSCs). Recent evidence suggests that the hematopoietic system is actually maintained by a consortium of HSC subtypes with distinct functional characteristics. We show here that myeloid-biased HSCs (My-HSCs) and lymphoid-biased HSCs (Ly-HSCs) can be purified according to their capacity for Hoechst dye efflux in combination with canonical HSC markers. These phenotypes are stable under natural (aging) or artificial (serial transplantation) stress and are exacerbated in the presence of competing HSCs. My- and Ly-HSCs respond differently to TGF-beta1, presenting a possible mechanism for differential regulation of HSC subtype activation. This study demonstrates definitive isolation of lineage-biased HSC subtypes and contributes to the fundamental change in view that the hematopoietic system is maintained by a continuum of HSC subtypes, rather than a functionally uniform pool.

Long-term propagation of distinct hematopoietic differentiation programs in vivo

Heterogeneity in the differentiation behavior of hematopoietic stem cells is well documented but poorly understood. To investigate this question at a clonal level, we isolated a subpopulation of adult mouse bone marrow that is highly enriched for multilineage in vivo repopulating cells and transplanted these as single cells, or their short-term clonal progeny generated in vitro, into 352 recipients. Of the mice, 93 showed a donor-derived contribution to the circulating white blood cells for at least 4 months in one of four distinct patterns. Serial transplantation experiments indicated that two of the patterns were associated with extensive self-renewal of the original cell transplanted. However, within 4 days in vitro, the repopulation patterns subsequently obtained in vivo shifted in a clone-specific fashion to those with less myeloid contribution. Thus, primitive hematopoietic cells can maintain distinct repopulation properties upon serial transplantation in vivo, although these properties can also alter rapidly in vitro.

Long-term repopulating ability of telomerase-deficient murine hematopoietic stem cells

Telomere length must be tightly regulated in highly proliferative tissues, such as the lymphohematopoietic system. Under steady-state conditions, the levels and functionality of hematopoietic-committed or multipotent progenitors were not affected in late-generation telomerase-deficient mice (mTerc(-/-)) with critically short telomeres. Evaluation of self-renewal potential of mTerc(-/-) day-12 spleen colony-forming units demonstrated no alteration as compared with wildtype progenitors. However, the replating ability of mTerc(-/-) granulocyte-macrophage CFUs (CFU-GMs) was greatly reduced as compared with wildtype CFU-GMs, indicating a diminished capacity of late-generation mTerc(-/-) committed progenitors when forced to proliferate. Long-term bone marrow cultures of mTerc(-/-) bone marrow (BM) cells show a reduction in proliferative capacity; this defect can be mainly attributed to the hematopoietic, not to the stromal, mTerc(-/-) cells. In serial and competitive transplantations, mTerc(-/-) BM stem cells show reduced long-term repopulating capacity, concomitant with an increase in genetic instability compared with wildtype cells. Nevertheless, in competitive transplantations late-generation mTerc(-/-) precursors can occasionally overcome this proliferative impairment and reconstitute irradiated recipients. In summary, our results demonstrate that late-generation mTerc(-/-) BM cells with short telomeres, although exhibiting reduced proliferation ability and reduced long-term repopulating capacity, can still reconstitute myeloablated animals maintaining stem cell function.

FLRF, a novel evolutionarily conserved RING finger gene, is differentially expressed in mouse fetal and adult hematopoietic stem cells and progenitors

Through differential screening of mouse hematopoietic stem cell (HSC) and progenitor subtracted cDNA libraries we have identified a HSC-specific transcript that represents a novel RING finger gene, named FLRF (fetal liver ring finger). FLRF represent a novel evolutionarily highly conserved RING finger gene, present in Drosophila, zebrafish, Xenopus, mouse, and humans. Full-length cDNA clones for mouse and human gene encode an identical protein of 317 amino acids with a C3HC4 RING finger domain at the amino terminus. During embryonic hematopoiesis FLRF is abundantly transcribed in mouse fetal liver HSC (Sca-1+c-kit+AA4.1+Lin- cells), but is not expressed in progenitors (AA4.1-). In adult mice FLRF is not transcribed in a highly enriched population of bone marrow HSC (Rh-123lowSca-1+c-kit+Lin- cells). Its expression is upregulated in a more heterogeneous population of bone marrow HSC (Lin-Sca-1+ cells), downregulated as they differentiate into progenitors (Lin-Sca-1- cells), and upregulated as progenitors differentiate into mature lymphoid and myeloid cell types. The human FLRF gene that spans a region of at least 12 kb and consists of eight exons was localized to chromosome 12q13, a region with frequent chromosome aberrations associated with multiple cases of acute myeloid leukemia and non-Hodgkin's lymphoma. The analysis of the genomic sequence upstream of the first exon in the mouse and human FLRF gene has revealed that both putative promoters contain multiple putative binding sites for several hematopoietic (GATA-1, GATA-2, GATA-3, Ikaros, SCL/Tal-1, AML1, MZF-1, and Lmo2) and other transcription factors, suggesting that mouse and human FLRF expression could be regulated in a developmental and cell-specific manner during hematopoiesis. Evolutionary conservation and differential expression in fetal and adult HSC and progenitors suggest that the FLRF gene could play an important role in HSC/progenitor cell lineage commitment and differentiation and could be involved in the etiology of hematological malignancies.

The fleet feet of haematopoietic stem cells: rapid motility, interaction and proteopodia

Haematopoietic stem cells (HSCs) have been extensively characterized regarding in vivo engraftment, surface epitopes and genetic regulation. However, little is known about the homing of these rare cells, and their intrinsic motility and membrane deformation capacity. We used high-speed optical-sectioning microscopy and inverted fluorescent videomicroscopy to study highly purified murine lineage-negative, rhodamine-low, Hoechst-low HSCs over time under various in vitro conditions. We discovered extremely rapid motility, directed migration to stromal cells and marked membrane modulation. High resolution images with three-dimensional reconstruction showed the general presence of microspikes. Further, pseudopodia (proteopodia) were observed that were induced by stromal-derived factor-1 and steel factor. Proteopodia were directed towards and were quenched by stromal cells, at times bridged HSCs, and could rapidly retract or detach from cells. Proteopodia were also observed in vivo with homed HSCs in frozen sections of murine spleen, lung and heart. This is the first demonstration that HSCs are both fast and highly malleable in phenotype.

Residual damage of lymphohematopoietic repopulating cells after irradiation of mice at different stages of development

OBJECTIVE

The purpose of this study was to evaluate the repopulating properties of bone marrow (BM) from mice irradiated during embryonic and adult stages of development.

MATERIALS AND METHODS

Four-day-old embryos, 17-day-old fetuses, and 12-week-old mice were irradiated with of 1 or 3 Gy of x-rays. At 3 and 9 months postirradiation, the effects generated within the different compartments of repopulating cells (RCs) were evaluated by determining, in a BM competition assay, the contribution of the irradiated precursors to the lymphohematopoiesis of recipients at different times posttransplantation (3, 9, and 15 months).

RESULTS

The irradiation of 4-day-old embryos with either 1 or 3 Gy did not produce residual repopulation or differentiation effects within the different RCs assayed. However, significant impairments in RC functionality were observed in mice irradiated on the 17th day postconception or at the 12th week of age. Whereas irradiation of these animals with 1 Gy did not impair the long-term functionality of the very primitive 15-month-old RCs, irradiation with 3 Gy generated sustained impairment in all tested types of hematopoietic progenitors and RCs. Moreover, repopulation data derived from the analysis of recipient BM and thymus strongly suggested that the observed effects were produced within the multipotent pool of lymphohematopoietic RCs.

CONCLUSIONS

Our data show the generation of long-term effects in the multipotent RCs of mice irradiated at fetal and adult stages of growth and reveals the normal functionality of the RCs from animals irradiated during the early stages of embryonic development.

Myelodysplasia and the leukemias

The armistice after World War II marked the beginning of an era that was to last to the end of the present century. It was an era in which many changes in medicine and nursing combined to alter the entire philosophy of managing malignant disease. More specifically, the fluid-phase tumors, which comprise myelodysplasia and the leukemias, were singled out for special attention. First there was the ease with which blood and bone marrow could be sampled, making serial investigations simple and practical. Second, cytotoxic drugs became available ranging from nitrogen mustard through cytosine arabinoside, the anthracycline antibiotics, and the epi-podophyllotoxins. Although cytomorphology of the hematopoietic tissue had been exquisitely defined with the use of Romanowsky stains coupled with electron microscopy, the diagnosis of leukemia was, before 1945, a death sentence for want of effective therapy. This changed dramatically with the introduction of the folate antagonists, and progress was unremitting as the range of new products expanded. Suddenly responses could be obtained with single agents, and fairly rapidly combinations were developed for cumulative antitumor effect. Many agents had undesirable toxicity among different organs. Although slightly different for myeloblastic or lymphoblastic variants, this approach produced apparent disease eradication. The concept of complete remission, both clinical and hematologic, was born. Some of our early enthusiasm has had to be tempered with the somber appreciation that not all patients can improve and many others experience relapses. Where then do we stand? Leukemic cells themselves seldom kill. It is the relentless and uncontrolled expansion of a neoplastic clone that leads to bone marrow failure, albeit at different rates in the various subtypes. In the acute forms, the common presentation remains symptomatic anemia, neutropenic sepsis, and thrombocytopenic bleeding. Differentiation from marrow aplasia may not be possible at first on clinical grounds, although bone tenderness, gingival hypertrophy, and skin infiltration are among the general useful differential signs. Findings in the circulation and the marrow are of cardinal importance in diagnosis; they provide the basis for classification. Improved accuracy has followed the introduction of cytochemical stains, and a widening range of monoclonal antibodies, and greater recourse to karyotyping, have enhanced diagnostic acumen. Treatment decisions rest on many variables or prognostic factors that include age, performance status, comorbidity, and disease category, with an ever increasing regard for the part played by cellular and molecular genetics. Despite skillful utilization of this wealth of information for optimal management, outcome often leaves much to be desired. Myelodysplasia encompasses a number of different syndromes in which the refractory anemias are indolent, whereas those with excess blasts progress toward overt leukemia. Considerable judgment is necessary in selecting patients for whom supportive therapy alone is appropriate and recognizing others, up to one third of patients for whom use growth factors that include erythropoietin, granulocyte or granulocyte monocyte-colony stimulating factors, and thrombopoietin can be justified. The often unfavorable result has been a stimulus to current investigations that examine the value of intensive chemotherapy or the more innovative bone marrow transplantation and its peripheral blood equivalent. Autografting is a newer alternative that does not have proved potential. Acute leukemia, whether myeloblastic or lymphoblastic, has been managed with mixed success. Remission rates have steadily increased and, notably among children, moved toward 100% in certain groupings. The downside of nonspecific drug regimens is that some patients simply may not respond, whereas others experience remissions and then relapses. (ABSTRACT TRUNCATED)

Rescue from lethal irradiation correlates with transplantation of 10-20 CFU-S-day 12

Repopulation by donor cells of a bone marrow ablated by irradiation is now recognized to proceed in two phases: initial repopulation that may be temporary followed by permanent engraftment of longterm repopulating cells (LTRC). While a single LTRC has been shown to be capable of restoring the entire lymph-hemopoietic system of an irradiated animal, the identity of the temporary repopulating cells has not been established unequivocally. We used the results of transplantation of subpopulations successively enriched for LTRC and containing varying numbers of CFU-S-12 (colony-forming units in the spleen at day 12 post transplantation) and progenitors to determine the likely cell type and number of cells needed for initial survival after radiation. Subpopulations from untreated and 5-fluorouracil-treated mice were discriminated on the basis of antibody reactivity, Hoechst 33342 and rhodamine 123 fluorescence intensity and light scattering properties. The minimum rescue inocula varied greatly in CFU-GEMM, BFU-E and CFU-GM content. One to two CFU-S-12 were uniformly present in all isolated suspensions that rescued 50% of lethally irradiated animals. In view of the known average seeding efficiency of CFU-S, our studies suggest that transfusion of 10-20 CFU-S day 12/13 is responsible for radioprotection. Evidence that multiple CFU-S day 12/13 are needed for initial repopulation is also supported by quantitative estimates of the number of mature cells that can be produced by CFU-S. Transfusion of a single CFU-S day 12/13 can be shown to be grossly inadequate to provide the number of peripheral blood cells needed to ameliorate the severe pancytopenia following lethal irradiation by day 12-14. Our data also indicate that 5-fluorouracil-treated marrow subpopulations appear inferior to untreated subpopulations in their ability to contribute to initial repopulation when transfused at low cell doses into lethally irradiated recipients.

Biological and clinical aspects of hematopoietic stem cells

Recent advances in cell isolation techniques have greatly enhanced our understanding of the phenotype and function of hematopoietic stem cells in mice and humans. Many clinical studies have established the efficacy of using peripheral blood stem cells to supplement or replace bone marrow transplantation as a therapeutic modality for several types of malignancies. This new approach to malignant disease management, perhaps in combination with posttransplantation cytokine therapy, promises to completely alter the clinical course of bone marrow transplantation.