Use of limiting-dilution type long-term marrow cultures in frequency analysis of marrow-repopulating and spleen colony-forming hematopoietic stem cells in the mouse.

We have developed an in vitro clonal assay of murine hematopoietic precursor cells that form spleen colonies (CFU-S day 12) or produce in vitro clonable progenitors in the marrow (MRA cells) of lethally irradiated mice. The assay is essentially a long-term bone marrow culture in microtiter wells containing marrow-derived stromal "feeders" depleted for hematopoietic activity by irradiation. To test the validity of the assay as a quantitative in vitro stem cell assay, a series of unsorted and physically sorted bone marrow cells were simultaneously assayed in vivo and overlaid on the feeders in a range of concentrations, while frequencies of cells forming hematopoietic clones (cobblestone area forming cells, CAFC) were calculated by means of Poisson statistics. Linear regression analysis of the data showed high correlations between the frequency of CFU-S day 12 and CAFC day 10, and between MRA cells and CAFC day 28. A majority of MRA activity and CAFC day 28 was separable from CFU-S day 12 and CAFC day 10. This correlation study validates the CAFC system as a clonal assay facilitation both the quantitative assessment of a series of subsets in the hematopoietic stem cell hierarchy and the study of single long-term repopulating cells in vitro.     

Variations in radiation sensitivity and repair among different hematopoietic stem cell subsets following fractionated irradiation

The radiation dose-survival of various hematopoietic cell subsets in murine bone marrow (BM) was determined in the cobblestone area forming cell (CAFC) assay under conditions of single-, split-, and multiple- dose irradiation. A greater recovery in cell survival with decreasing dose per fraction, or increasing fraction number, was observed for primitive CAFC day-28 and day-35 than for CAFC day-6 and day-12 (colony- forming unit (CFU)-granulocyte macrophage and CFU-spleen day-12 equivalents). Linear quadratic (LQ) model analysis of CAFC survival data provided an estimate of the alpha/beta ratio that is an inverse index of the fractionation effect and is known to be lower for late than for acutely responding tissues. This analysis gave decreasing alpha/beta ratios with increasing primitiveness of the CAFC subset. These values were found to be comparatively low (about 4 Gy) for CAFC day-28 and day-35 and are in general agreement with previous studies on long-term repopulation in vivo. In contrast, alpha/beta ratios of CAFC day-6 and day-12 were relatively high (above 6 Gy) and are consistent with values obtained from acute marrow failure. Delayed harvesting of BM after a single dose of 6 Gy showed little evidence of proliferative repopulation over 1 week and hence the differential dose-sparing effect of fractionation among the CAFC subsets appears to be mostly attributable to the influence of sublethal damage repair. These results require a reevaluation of previous notions of marrow stem cell radiosensitivity and repair based on acute marrow lethality (LD50/30) or spleen colony (CFU-S) data, especially when applied to fractionated total body irradiation effects on long-term repopulating stem cells in a BM transplant setting.     

O6-Benzylguanine potentiates BCNU but not busulfan toxicity in hematopoietic stem cells

OBJECTIVE: Busulfan (BU) is often used in conditioning regimens prior to bone marrow transplantation, but its mechanism of action remains to be resolved. We have examined the possibility that BU may exert part of its toxic effects via DNA alkylation at the O6 position of guanine as this might provide an approach to improving the conditioning regimen. METHODS: Survival of LAMA-84 and RJKO cells was assessed by colony-forming assay and cell counting, respectively. O6-alkylguanine-DNA alkyltransferase (ATase) activity was assayed by transfer of radioactivity from [3H]-methylated DNA. Colony-forming potential of normal human bone marrow cells (BMC) was measured in the presence of appropriate growth factors as the formation of both granulocyte-macrophage colony-forming units (CFU-GM) or burst-forming unit erythroids (BFU-E) within the same assay. Murine hematopoietic precursors were grown under a bone marrow stromal cell line to allow measurement of the frequency of cobblestone area-forming cells (CAFC) that correspond to CFU-GM, spleen colony-forming units (CFU-S), and the primitive stem cells with long-term repopulating ability. RESULTS: Inactivation of ATase by O6-benzylguanine (O6-BeG) sensitized a human erythromegakaryocytic cell line (LAMA-84) and normal human bone marrow progenitors to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) but not to BU toxicity. BCNU, but not BU, inactivated ATase in LAMA-84 cells. Overexpression of human ATase in cDNA transfected Chinese hamster cells attenuated the toxicity of BCNU but not BU. Finally, the in vivo treatment of mice showed that the depletion of primitive stem cells by BU as measured in the CAFC assay was not affected by addition of O6-BeG. O6-BeG did, however, dramatically potentiate BCNU toxicity in all CAFC subsets, leading to depletion of more than 99% stem cells. CONCLUSION: These data suggest that BU does not elicit toxicity via alkylation at the O6 position of guanine in DNA in a way that can be influenced by ATase modulation.     

Sca+CD34- murine side population cells are highly enriched for primitive stem cells

OBJECTIVE: The aim of this study was to characterize murine side population (SP) stem cells and SP cell subpopulations for primitive stem cell capacity. MATERIALS AND METHODS: SP cells, characterized by a specific Hoechst dye efflux pattern, were isolated by flow cytometric analysis and sorting from murine adult whole bone marrow (WBM). Different subpopulations of SP cells were isolated by staining with anti-Sca and anti-CD34 antibodies. Primitive stem cell content of SP cells and SP subsets were determined by cobblestone area-forming cell (CAFC) frequencies. RESULTS: Measurement of CAFC frequencies revealed that SP cells are greatly enriched for both primitive stem cells (day-28-35 CAFC) and somewhat more mature hematopoietic cells (day-14-21 CAFC) compared to WBM. The day-28 and day-35 CAFC enrichments in SP cells vs WBM cells were 1065 and 471, respectively. Analysis of the subpopulations of SP cells revealed that SP(+)Sca(-)CD34(+) cells contained almost exclusively day-7 CAFC and had little day-28-35 CAFC activity. SP(+)Sca(+)CD34(+) cells had high day-7-14 CAFC frequencies, but lower day-35 CAFC frequencies compared to SP(+)Sca(+)CD34(-) cells. SP(+)Sca(+)CD34(-) cells contained very low day-7 CAFC activity, but nearly 2200 times the day-28-35 CAFC activity as normal bone marrow. To evaluate the influence of Hoechst dye efflux capacity, we divided the SP tail into four groups of cells. The SP cells with lowest efflux of Hoechst dye contained the highest progenitor activity (day-7-14 CAFC). The highest day-35 CAFC frequencies, nearly 6000 times those of normal marrow, were seen in the SP cells with the greatest efflux of the Hoechst dye. CONCLUSIONS: Murine SP cells contain both progenitor and primitive populations of hematopoietic stem cells. The most primitive stem cells measured in the in vitro CAFC assay mark for Sca(+) and CD34(-) and have a high ability to efflux Hoechst dye. Isolation of these cells may provide the means to directly study mechanisms of primitive stem cell damage.     

Quantitative analysis of cobblestone area-forming cells in bone marrow of patients with aplastic anemia by limiting dilution assay

In the past, the analysis of primitive human hematopoietic progenitor cells with repopulating activity was limited by lack of appropriate in vitro assay systems. It was recently shown that cobblestone area- forming cells (CAFC) giving rise to cobblestone areas after 5 weeks in long-term marrow cultures (LTMC) represent a population of pluripotent progenitor cells with long-term marrow-repopulating activity. We have used a microtiter limiting dilution-type human LTMC system to quantitate the frequency of CAFC (week 5) in aplastic anemia (AA). In bone marrow mononuclear cells (BM-MNC) of healthy donors (n = 36) we observed a mean frequency of 84.4 CAFC per 10(5) BM-MNC (95% confidence interval limits, 66.4 to 102.4). The mean frequency of CAFC in BM of 31 AA patients was 6.6 per 10(5) BM-MNC (95% confidence interval limits, 5.3 to 7.9; n = 47). This frequency is significantly lower as compared with controls (P < .0001). The frequency of CAFC was reduced not only in pancytopenic AA patients (6.2 per 10(5) BM-MNC; P < .0001 v control), but also in patients in remission after immunosuppression (7.6; P < .0001 v control; P = .1 v pancytopenic AA patients). The CAFC frequency did not correlate with the severity or duration of the disease and did not predict response to immunosuppressive treatment. In summary, the frequency of primitive hematopoietic progenitor cells, as measured by the CAFC assay, is significantly reduced in AA. CAFC remain severely reduced even after hematologic recovery after immunosuppressive treatment. The low frequency of CAFC in remission patients is in keeping with other data pointing to a persisting defect of hematopoiesis in patients in remission after immunosuppressive treatment.     

Separation of CFU-S from primitive cells responsible for reconstitution of the bone marrow hemopoietic stem cell compartment following irradiation: evidence for a pre-CFU-S cell

We have studied the in vivo spleen colony-forming ability and marrow repopulating ability of murine bone marrow cells differing in mitochondrial activity. Following centrifugal elutriation the cells were sorted on the basis of rhodamine-123 fluorescence intensity within a predetermined light scatter window. It is shown that a class of hemopoietic stem cells exists that differs from the majority of day-12 spleen colony-forming units (CFU-S) in that it has low mitochondrial activity and a high capacity to generate in time new day-12 CFU-S and cells that rescue recipients from radiation-inflicted death. These data add direct evidence for the identity of pre-CFU-S and CFU-S by physical separation.     

Isolation of hemopoietic stem cell subsets from murine bone marrow: I. Radioprotective ability of purified cell suspensions differing in the proportion of day-7 and day-12 CFU-S

We have studied the ability of bone marrow cell suspensions greatly differing in the relative proportion of day-7 and day-12 spleen colony-forming units (CFU-S) to rescue mice from radiation-inflicted death, and to repopulate the irradiated bone marrow and spleen with nucleated cells. Counterflow centrifugal elutriation in combination with removal of adherent cells and fluorescence-activated cell sorting on differences in wheat germ agglutinin (WGA)-fluorescein isothiocyanate (FITC) affinity and light scatter properties were used consecutively to enrich large numbers of hemopoietic stem cells from mouse bone marrow. Enrichments of 50- to 200-fold have been achieved for day-12 CFU-S and radioprotective ability (RPA), permitting 50% of lethally irradiated mice to survive over a period of 30 days with as few as 50-80 donor cells. The ratio of day-7 and day-12 CFU-S in the various suspensions could be significantly modulated on the basis of their WGA binding and perpendicular light scatter characteristics. This finding enabled us to investigate the properties of day-7 and day-12 CFU-S with respect to their RPA. We found a highly significant log/log relationship between enrichment factors for (1) RPA, (2) the number of day-12 CFU-S, and (3) spleen cellularity as measured on day 13. In addition, similar numbers of sorted and unfractionated day-12 CFU-S were required to obtain the same level of protection. Enrichment for RPA was significantly less related to either the number of day-7 CFU-S injected, or the bone marrow cellularity of the irradiated mice on day 13.     

Identification of hematopoietic stem cell subsets on the basis of their primitiveness using antibody ER-MP12

Monoclonal antibody ER-MP12 defines a novel antigen on murine hematopoietic stem cells. The antigen is differentially expressed by different subsets in the hematopoietic stem cell compartment and enables a physical separation of primitive long-term repopulating stem cells from more mature multilineage progenitors. When used in two-color immunofluorescence with ER-MP20 (anti-Ly-6C), six subpopulations of bone marrow (BM) cells could be identified. These subsets were isolated using magnetic and fluorescence-activated cell sorting, phenotypically analyzed, and tested in vitro for cobblestone area-forming cells (CAFC) and colony-forming units in culture (CFU-C; M/G/E/Meg/Mast). In addition, they were tested in vivo for day-12 spleen colony-forming units (CFU-S-12), and for cells with long-term repopulating ability using a recently developed alpha-thalassemic chimeric mouse model. Cells with long-term repopulation ability (LTRA) and day-12 spleen colony-forming ability appeared to be exclusively present in the two subpopulations that expressed the ER-MP12 cell surface antigen at either an intermediate or high level, but lacked the expression of Ly- 6C. The ER-MP12med20- subpopulation (comprising 30% of the BM cells, including all lymphocytes) contained 90% to 95% of the LTRA cells and immature day-28 CAFC (CAFC-28), 75% of the CFU-S-12, and very low numbers of CFU-C. In contrast, the ER-MP12hi20- population (comprising 1% to 2% of the BM cells, containing no mature cells) included 80% of the early and less primitive CAFC (CAFC-5), 25% of the CFU-S-12, and only 10% of the LTRA cells and immature CAFC-28. The ER-MP12hi cells, irrespective of the ER-MP20 antigen expression, included 80% to 90% of the CFU-C (day 4 through day 14), of which 70% were ER-MP20- and 10% to 20% ER-MP20med/hi. In addition, erythroblasts, granulocytes, lymphocytes, and monocytes could almost be fully separated on the basis of ER-MP12 and ER-MP20 antigen expression. Functionally, the presence of ER-MP12 in a long-term BM culture did not affect hematopoiesis, as was measured in the CAFC assay. Our data demonstrate that the ER-MP12 antigen is intermediately expressed on the long-term repopulating hematopoietic stem cell. Its level of expression increases on maturation towards CFU-C, to disappear from mature hematopoietic cells, except from B and T lymphocytes.     

Marrow- and spleen-seeding efficiencies of all murine hematopoietic stem cell subsets are decreased by preincubation with hematopoietic growth factors

The cobblestone-area forming cell (CAFC) assay permits a direct measurement of the seeding of primitive and more mature murine hematopoietic stem cell subsets by comparing the number of CAFC in the original transplant with the number of CAFC retrieved from bone marrow (BM) and spleen after transplantation. We found no differences in seeding efficiency between the more mature and primitive CAFC subsets, nor between seeding efficiencies of stem cells from low-density (LD) fractions of normal and day-6 post-5-fluorouracil BM. The data show that 18% to 20% of all intravenously transplanted stem cell subsets seed to the BM, whereas 8% to 10% seed to the spleen. In addition, similar seeding efficiencies were found for day-12 spleen colony- forming unit (CFU-S-12) as was determined by retransplantation. Previously, it has been reported that a 2- to 3-hour preincubation of BM with interleukin-3 (IL-3) enhances the in vivo repopulating ability of a graft. To test whether hematopoietic growth factors affected this increased engraftment by enhancing the seeding of the transplanted marrow, we assessed the 16- to 18-hour seeding efficiency of short- and long-term in vivo repopulating stem cell subsets to BM and spleen using the CAFC assay, after preincubation with or without hematopoietic growth factors. A 2- to 3-hour preincubation with IL-3, or a combination of IL-3, IL-12, and steel factor, at 37 degrees C, led to a substantial decrease in seeding compared with control (which was kept on ice) of all hematopoietic subsets measured, both in spleen and BM. In concert with these data, the long-term in vivo repopulating ability of growth-factor incubated BM was also decreased when compared with control. In conclusion, we have been unable to observe a beneficial effect of growth factor preincubation on the repopulating ability of a graft.     

Pluripotent hematopoietic stem cells of low and high density can repopulate W/Wv mice.

We have studied several features of pluripotent hematopoietic stem cells (PHSC) and day-12 spleen colony-forming units (CFU-S12) in murine bone marrow. C57BL/6J marrow cell suspensions were separated by elutriation and fractions were obtained at flow rates (FR) of 25 ml/min, 29/30 ml/min, 35 ml/min, and with the rotor off. All four fractions contained PHSC that could repopulate W/Wv mice, but significant numbers of CFU-S12 were found only in the three higher FR fractions. Cells in the FR29/30 fraction were shown to have almost three-fold more repopulating activity than fresh marrow in a competitive repopulation assay. The PHSC in fractions separated by elutriation were enriched by depleting cells expressing specific lineage markers with monoclonal antibodies and magnetic immunobeads. As few as 10(4) lineage negative (lin-) cells from FR35 or 10(5) lin--cells from FR25 conferred long-term multilineage repopulation in W/Wv mice, as demonstrated by Southern blot analysis of DNA from recipient thymus and bone marrow. We conclude that PHSC are heterogeneous for cell size and density and that the highest concentration of PHSC resides in the subset of intermediate density present in the FR29/30 fraction.     

Individual stem cell quality in leukapheresis products is related to the number of mobilized stem cells

Peripheral blood stem cells (PBSC) are used for stem cell support in patients after intensive chemotherapy and generally permit faster hematopoietic recovery than bone marrow. The development of different protocols for chemotherapy conditioning, mobilization, and ex vivo manipulation of PBSC may potentially lead to loss of primitive hematopoietic stem cells or reduction of their quality. To characterize the frequency of different stem cell subsets and their quality per mobilized PBSC, we have studied 47 leukapheresis products (LPs) of 21 cancer patients using stroma-dependent long-term culture (LTC) and limiting dilution-type cobblestone area forming cell (CAFC) assays. A large variation in CAFC week-type frequencies between the LPs was observed. The frequencies of CAFC week 2 as a tentative indicator of progenitor cells and transiently repopulating hematopoietic stem cells ranged from 0.89 to 205 per 10(5) mobilized nucleated cells and the frequencies of more primitive CAFC week 6 varied between 0.37 and 48. The average total colony-forming cell (CFC) production per CAFC at week 6 varied between 1.2 and 730, as determined in parallel LTC. In contrast to LPs, bone marrow samples generated 4.2 to 48 CFC per CAFC at week 6. Notably, a poor stem cell quality was consistently found in LPs that contained less than 5,000 CAFC week 6 per kilogram of body weight. Frequency analyses of CFCs, CAFC subtypes, and immunophenotypic subsets showed a good level of mutual correlation, suggesting identical mobilization kinetics of different stem cell subsets. The premobilization chemotherapy intensity was directly correlated with both decreasing frequency and quality of the CAFC week 6 in LPs. The frequency of CFCs, immunophenotypic subsets, and CAFC subsets transplanted and the transplant quality as determined in LTC assays was related to the neutrophil and platelet recovery time after PBSC transplantation. Although the number of progenitor cells transplanted and the in vitro transplant quality showed the best correlation with early hematopoietic recovery, the data did not permit determination of which stem cell subsets are predominantly responsible for early posttransplantation recovery. As a result, frequency and quality analysis of stem cell subsets may be a useful tool to monitor and calibrate the efficacy of novel mobilization regimens and ex vivo manipulation of PBSC.     

Transplantation of hematopoietic stem cells obtained by a combined dye method fractionation of murine bone marrow

Hematopoietic stem cells were purified from murine bone marrow cells (BMC). Their characteristic density, size, internal complexity, Hoechst 33342 dye uptake, and wheat germ agglutinin (WGA) affinity were used to distinguish them from other cells in the bone marrow. BMC suspensions were centrifuged over Ficoll Lymphocyte Separation Media (Organon Teknika, Durham, NC; density 1.077 to 1.08). The lower-density cells were drawn off, stained with Hoechst and labeled with biotinylated WGA bound to streptavidin conjugated to phycoerythrin (WGA-B*A-PE) or with WGA conjugated to Texas Red. These cells were then analyzed and sorted by an Ortho Cytofluorograph 50-H cell sorter. The cells exhibiting medium to high forward light scatter, low to medium right angle light scatter, low Hoechst intensity, and high WGA affinity were selected. Sorted BMC (SBMC) were stained with Romanowsky-type stains for morphologic assay, and were assayed in lethally irradiated (LI) mice for their ability to produce colony-forming units in the spleen (CFU-S) and for their ability to produce survival. The spleen seeding factor for day 8 CFU-S upon retransplantation of the isolated cells was 0.1. The isolated cells were found to have consistent morphology, were enriched up to 135-fold as indicated by day 8 CFU-S assay, 195-fold as indicated by day 14 CFU-S assay, and 150 sorter-selected BMC were able to produce long-term survival in LI mice with retention of donor karyotype. When recipients of this first transplantation were themselves used as BMC donors, their number of day 8 and day 12 CFU-S were found to be reduced. However, 3 X 10(5) of their BMC provided 100% survival among secondary recipients. When the previously SBMC were competed after one transplantation against fresh nonsorted BMC in a mixed donor transplant, they showed the decline in hematopoietic potency normally seen in previously transplanted BMC. We conclude that the use of combinations of vital dyes for fluorescence-activated cell sorting (FACS) selection of survival-promoting murine hematopoietic stem cells provides results comparable with those produced by antibody-selected FACS and has the advantage of a method directly transferable to human BMC.     

Enrichment and characterization of murine hematopoietic stem cells that express c-kit molecule.

The proto-oncogene c-kit encodes a transmembrane tyrosine kinase receptor for stem cell factor (SCF). The c-kit/SCF signal is expected to have an important role in hematopoiesis. A monoclonal antibody (ACK-2) against the murine c-kit molecule was prepared. Flow cytometric analysis showed that the bone marrow cells that expressed the c-kit molecule (approximately 5%) were B220(B)-, TER119(erythroid)-, Thy1negative-low, and WGA+. A small number of Mac-1(macrophage)+ or Gr-1(granulocyte)+ cells were c-kit-low positive. Colony-forming unit in culture (CFU-C) and day-8 and day-12 CFU-spleen (CFU-S) existed exclusively in the c-kit-positive fraction. About 20% of the Lin(lineage)-c-kit+ cells were rhodamine-123low and this fraction contained more day-12 CFU-S than day-8 CFU-S. On the basis of these findings, murine hematopoietic stem cells were enriched with normal bone marrow cells. One of two and one of four Thy-1lowLin-WGA+c-kit+ cells were CFU-C and CFU-S, respectively. Long-term repopulating ability was investigated using B6/Ly5 congenic mice. Eight and 25 weeks after transplantation of Lin-c-kit+ cells, donor-derived cells were found in the bone marrow, spleen, thymus, and peripheral blood. In peripheral blood, T cells, B cells, and granulocyte-macrophages were derived from donor cells. Injection of ACK-2 into the irradiated mice after bone marrow transplantation decreased the numbers of day-8 and day-12 CFU-S in a dose-dependent manner. Day-8 spleen colony formation was completely suppressed by the injection of 100 micrograms ACK-2, but a small number of day-12 colonies were spared. Our data show that the c-kit molecule is expressed in primitive stem cells and plays an essential role in the early stages of hematopoiesis.     

Isolation of hemopoietic stem cell subsets from murine bone marrow: II. Evidence for an early precursor of day-12 CFU-S and cells associated with radioprotective ability

Counterflow centrifugal elutriation (CCE) in combination with plastic adherence and fluorescence-activated cell sorting were used consecutively to enrich functionally different subpopulations of pluripotent hemopoietic stem cells (HSC) from mouse bone marrow. The nonadherent CCE fractions were labeled with wheat germ agglutinin (WGA)-fluorescein isothiocyanate (FITC) and sorted according to differences in fluorescence within various windows on the basis of forward (FLS) and perpendicular (PLS) light scatter. The sorted cells were then assayed for their (1) in vivo colony-forming ability (day-7 and day-12 spleen colony-forming units [CFU-S]), (2) radioprotective ability (RPA; 30-day survival), and (3) their ability to repopulate the bone marrow or spleen over a 13-day period with day-12 CFU-S, granulocyte-macrophage colony-forming units (CFU-GM), nucleated cells, or cells associated with RPA. The highest incidence of day-12 CFU-S and cells with RPA was obtained by sorting the most WGA-positive cells with relatively high PLS (enrichment, 50- to 200-fold), lowering the effective dose (ED 50/30) to an average of 80 cells. The separative procedure enabled hemopoietic stem cells that repopulate both bone marrow and spleen with secondary RPA cells, CFU-S-12, and CFU-GM to be enriched and separated from part of the RPA cells, CFU-S-12, and cells that reconstitute the cellularity of bone marrow and spleen. These data suggest that cells generating both day-12 CFU-S and RPA cells differ from day-12 CFU-S and RPA cells themselves on the basis of PLS characteristics and affinity for WGA. Such early stem cells have also been detected in sorted fractions meeting the FLS/PLS characteristics of lymphocytes.     

Marrow repopulating cells, but not CFU-S, establish long-term in vitro hemopoiesis on a marrow-derived stromal layer

We have studied the ability of subpopulations of hemopoietic stem cells, obtained from murine bone marrow using elutriation and multiparameter sorting, to establish and maintain hemopoiesis following their deposition on irradiated stromal layers of long-term bone marrow cultures. Two fractions were obtained that differed in their mitochondrial activity as indicated by the retention of Rhodamine-123 dye. The Rhodamine-bright cell fraction, containing the majority of day-8 and day-12 spleen colony-forming units (CFU-S) and in vitro clonable progenitors, showed hemopoiesis only in the first weeks. In contrast, the Rhodamine-dull fraction, which was depleted for day-8 CFU-S and which contained the majority of cells with marrow-repopulating ability, maintained hemopoiesis for a prolonged time after an initial week of delay. These data fully support and extend previously published in vivo data, indicating that CFU-S have low capability to generate new CFU-S and granulocyte-macrophage colony-forming units (CFU-GM), and that cells responsible for long-term generation of hemopoietic precursors and for maintenance of hemopoiesis both in vivo and in vitro are the precursors of CFU-S and of in vitro clonable progenitor cells. In addition, the present findings form the basis for an in vitro assay for a primitive precursor of CFU-S, namely the marrow-repopulating cell.     

Stable multilineage hematopoietic chimerism in alpha-thalassemic mice induced by a bone marrow subpopulation that excludes the majority of day-12 spleen colony-forming units

We have investigated the contribution of highly purified day-12 spleen colony-forming units (CFU-S-12) as well as more primitive cells to sustained blood cell production using in vivo and in vitro assays that allow frequency analysis. Normal or day-6 post-5-fluorouracil light- density bone marrow (BM) was sorted on the basis of differences in rhodamine-123 (Rh123) retention or wheat germ agglutinin (WGA) affinity and tested in vivo using a recently developed alpha-thalassemic chimeric mouse model. In addition, short-term and long-term clonal activity was assessed in vitro using a limiting dilution-type long-term BM culture, the cobblestone area forming cell assay. When sublethally irradiated alpha-thalassemic mice were transplanted with as many as 281 purified WGAbright CFU-S-12, derived from a fraction containing 95% of all CFU-S-12 from day-6 post-5-fluorouracil light-density BM of wild- type mice, detectable chimerism was not observed at 6 months posttransplantation. In contrast, only three CFU-S-12 were included in the Rh123dull and WGAdim subpopulations that induced 29% to 58% and 21% to 31% stable multilineage donor-type chimerism of erythrocytes and leukocytes, respectively. The Rh123dull and WGAdim cells were up to 240- fold enriched for long-term repopulating ability (LTRA) as compared with unseparated BM. A comparable level of chimerism was found in the different hematopoietic organs and at the level of BM CFU-S-12. The frequency of the LTRA unit capable of inducing a 10% sustained level of donor-type erythrocytes was calculated to be 1 to 2 per 10(5) BM cells. Several reports have suggested that LTRA and spleen colony formation could be capacities of the same stem cell subset. However, the present results show that the majority of CFU-S-12 have only short-term repopulating ability and are physically separable from more primitive stem cells with long-term multilineage reconstituting capacities.     

Concomitant histamine, interleukin 4, and interleukin 6 production by hematopoietic progenitor subsets in response to interleukin 3.

Murine interleukin 3 (IL-3) induces a strong, concomitant increase in histamine, interleukin 6 (IL-6), and interleukin 4 (IL-4) synthesis by progenitor-enriched bone marrow cell populations, whereas interleukin 2 (IL-2) or interferon-gamma (IFN-gamma) are undetectable. This phenomenon is observed between 4 and 12 h after exposure to the growth factor and attains maximal cytokine and histamine levels within 24 and 48 h, respectively. None of these mediators is produced by lymphoid populations such as lymph node cells or by granulocytes. Splenocytes secrete only low histamine and IL-6 levels, in accordance with the lower incidence of progenitors in the spleen, whereas total bone marrow cells generate substantial amounts of the three mediators even before enrichment. Histamine, IL-4-, and IL-6-producing cells copurify with immature cells and cannot be separated from each other throughout the sorting procedures used herein. They are concentrated in the low-density layers (buoyant density 1.069-1.086 g/cm3) of a discontinuous Ficoll gradient (less than 4% of the total bone marrow) together with the majority of hematopoietic progenitors (marrow-repopulating ability [MRA] cells, spleen colony-forming units [CFU-S] day-8 and day-12, granulocyte-macrophage colony-forming units [CFU-GM], and mast cell precursors). Their lightscatter characteristics are those of relatively large, granular cells. They do not belong to the most primitive stem cell subset (MRA and part of CFU-S day-12), but to a population with high mitochondrial activity identified by their important rhodamine retention (colony-forming unit cells [CFU-C], blast cells). In addition, we provide evidence that histamine, IL-4, and IL-6 do not depend on each other for their respective expression. Taken together, our data are consistent with the notion that in certain conditions, immature hematopoietic cells are a potent source of histamine and cytokines.     

Heterogeneity within the spleen colony-forming cell population in rat bone marrow

The pluripotent hemopoietic stem cell (HSC) of the rat can be enumerated in a spleen colony assay (SCA) in rats as well as mice. After injection of rat bone marrow into lethally irradiated mice, macroscopically visible spleen colonies (CFU-S) are found from day 6 through 14, but the number varies on consecutive days. In normal bone marrow a constant ratio of day-8 to day-12 colony numbers is observed. However, this ratio is changed after in vivo treatment of rats with cyclophosphamide, as well as after in vitro treatment of rat bone marrow with cyclophosphamide derivatives. This indicates that the CFU-S that form colonies on day 8 react differently to this treatment than the CFU-S that form colonies on day 12, and suggests heterogeneity among the CFU-S population. Posttreatment regrowth of day-8 and day-12 CFU-S is characterized by differences in population-doubling times (Td = 0.85 days vs 1.65 days). Another argument in support of the postulate of heterogeneity within the rat CFU-S population is derived from the fact that (in contrast to normal rat spleen) the spleen of leukemic rats contains high numbers of CFU-S that show a ratio of day-8 to day-12 CFU-S of 4.5, which is different than that observed for a CFU-S population in normal bone marrow (a ratio of 2.4). It is concluded that, in rat hemopoiesis, two populations of spleen colony-forming cells can be distinguished using the rat-to-mouse SCA. This indicates that mouse and rat hemopoiesis are comparable in this respect and that heterogeneity in the stem cell compartment is a general phenomenon.     

Mobilization of early hematopoietic progenitor cells with BB-10010: a genetically engineered variant of human macrophage inflammatory protein- 1 alpha

BB-10010 is a genetically engineered variant of human macrophage inflammatory protein-1 alpha with improved solution properties. We show here that it mobilizes stem cells into the peripheral blood. We investigated the mobilizing effects of BB-10010 on the numbers of circulating 8-day spleen colony-forming units (CFU-S8), CFU-S12, and progenitors with marrow repopulating ability (MRA). A single subcutaneous dose of BB-10010 caused a twofold increase in circulating numbers of CFU-S8, CFU-S12, and MRA 30 minutes after dosing. We also investigated the effects of granulocyte colony-stimulating factor (G- CSF) and the combination of G-CSF with BB-10010 on progenitor mobilization. Two days of G-CSF treatment increased circulating CFU-S8, CFU-S12, and MRA progenitors by 25.7-, 19.8-, and 27.7-fold. A single administration of BB-10010 after 2 days of G-CSF treatment increased circulating CFU-S8, CFU-S12, and MRA even further to 38-, 33-, and 100- fold. Splenectomy resulted in increased circulating progenitor numbers but did not change the pattern of mobilization. Two days of treatment with G-CSF then increased circulating CFU-S8, CFU-S12, and MRA by 64-, 69-, and 32-fold. A single BB-10010 administration after G-CSF treatment further increased them to 85-, 117-, and 140-fold, respectively, compared with control. We conclude that BB-10010 causes a rapid increase in the number of circulating hematopoietic progenitors and further enhances the numbers induced by pretreatment with G-CSF. BB- 10010 preferentially mobilized the more primitive progenitors with marrow repopulating activity, releasing four times the number achieved with G-CSF alone. Translated into a clinical setting, this improvement in progenitor cell mobilization may enhance the efficiency of harvest and the quality of grafts for peripheral blood stem cell transplantation.     

Thymus regeneration by bone marrow cell suspensions differing in the potential to form early and late spleen colonies

We have determined the thymus-repopulating capacity of purified hemopoietic stem cells, bone marrow cells from mice injected four days previously with 5-fluorouracil (5-FUBM), and bone marrow cells cultured in the presence of stem-cell-activating factor (SAF; SAFBM). SAF is identical to interleukin 3 (IL-3). Purified stem cells are more enriched in day-12 CFU-S than in day-8 CFU-S. 5-FUBM consists of CFU-S that give rise to late (day-12) spleen colonies. SAFBM contains predominantly CFU-S that give rise to early spleen colonies (days 6-8). There is also a net increase in the number of spleen colony-forming units (CFU-S) in these cultures. Thymus regeneration after transplantation with either purified stem cells or 5-FUBM was delayed approximately three days as compared with that after transplantation with normal bone marrow cells. This delay can be ascribed to the absence of prothymocytes in these preparations. Thymus regeneration by SAFBM was delayed approximately ten days as compared with that after transplantation with normal bone marrow cells. The most likely explanation of these results is as follows. The prothymocytes in normal bone marrow produce a relatively limited offspring in the thymus soon after transplantation. This is rapidly replaced by the offspring of newly formed prothymocytes, the results of differentiation of the pluripotent stem cells. These stem cells also give rise to late spleen colonies. Stem cells that give rise to early spleen colonies appear to have lost the capacity for differentiation into the T-cell lineage.