Marrow-repopulating Ability Research Articles

Retroviral vector integration occurs in preferred genomic targets of human bone marrow–repopulating cells

Increasing use of hematopoietic stem cells for retroviral vector-mediated gene therapy and recent reports on insertional mutagenesis in mice and humans have created intense interest to characterize vector integrations on a genomic level. We studied retrovirally transduced human peripheral blood progenitor cells with bone marrow-repopulating ability in immune-deficient mice. By using a highly sensitive and specific ligation-mediated polymerase chain reaction (PCR) followed by sequencing of vector integration sites, we found a multitude of simultaneously active human stem cell clones 8 weeks after transplantation. Vector integrations occurred with significantly increased frequency into chromosomes 17 and 19 and into specific regions of chromosomes 6, 13, and 16, although most of the chromosomes were targeted. Preferred genomic target sites have previously only been reported for wild-type retroviruses. Our findings reveal for the first time that retroviral vector integration into human marrow-repopulating cells can be nonrandom (P =.000 37).

Delta-1 enhances marrow and thymus repopulating ability of human CD34(+)CD38(-) cord blood cells.

We investigated the effect of Notch signaling, a known regulator of cell fate in numerous developmental systems, on human hematopoietic precursors. We show that activation of endogenous Notch signaling in human CD34(+)CD38(-) cord blood precursors with immobilized Delta-1 in serum-free cultures containing fibronectin and hematopoietic growth factors inhibited myeloid differentiation and induced a 100-fold increase in the number of CD34(+) cells compared with control cultures. Immobilized Delta-1 also induced a multifold expansion of cells with the phenotype of common lymphoid precursors (CD34(+)CD7(+)CD45RA(+)) and promoted the development of cytoplasmic CD3(+) T/NK cell precursors. IL-7 enhanced the promotion of T/NK cell differentiation by immobilized Delta-1, but granulocytic differentiation occurred when G-CSF was added. Transplantation into immunodeficient mice showed a substantial increase in myeloid and B cell engraftment in the marrow and also revealed thymic repopulation by CD3(+) T cells due to cells being cultured for a longer period with immobilized Delta-1. These data suggest that Delta-1 can enhance myeloid and lymphoid marrow-repopulating ability and promote the generation of thymus-repopulating T cell precursors.

Delta-1 enhances marrow and thymus repopulating ability of human CD34+CD38– cord blood cells

We investigated the effect of Notch signaling, a known regulator of cell fate in numerous developmental systems, on human hematopoietic precursors. We show that activation of endogenous Notch signaling in human CD34+CD38– cord blood precursors with immobilized Delta-1 in serum-free cultures containing fibronectin and hematopoietic growth factors inhibited myeloid differentiation and induced a 100-fold increase in the number of CD34+ cells compared with control cultures. Immobilized Delta-1 also induced a multifold expansion of cells with the phenotype of common lymphoid precursors (CD34+CD7+CD45RA+) and promoted the development of cytoplasmic CD3+ T/NK cell precursors. IL-7 enhanced the promotion of T/NK cell differentiation by immobilized Delta-1, but granulocytic differentiation occurred when G-CSF was added. Transplantation into immunodeficient mice showed a substantial increase in myeloid and B cell engraftment in the marrow and also revealed thymic repopulation by CD3+ T cells due to cells being cultured for a longer period with immobilized Delta-1. These data suggest that Delta-1 can enhance myeloid and lymphoid marrow-repopulating ability and promote the generation of thymus-repopulating T cell precursors.

Hypoxia maintains and interleukin-3 reduces the pre-colony-forming cell potential of dividing CD34(+) murine bone marrow cells.

The aim of this study was to determine whether the combination of a sizable generation of colony-forming cells (CFC) with the maintenance of their progenitors (pre-CFC) ensured by incubation in hypoxia is associated with a certain degree of cell cycling, ultimately responsible for "self-renewal" of pre-CFC. The effects of interleukin-3 (IL-3) on the cycling and CFC-generation potential of pre-CFC also was investigated. In severely hypoxic (0.9% O(2)) murine bone marrow cell cultures containing stem cell factor, interleukin-6, and granulocyte colony-stimulating factor, pre-CFC maintenance was characterized by the culture-repopulating ability assay, an in vitro analogue of the marrow-repopulating ability assay. The proliferative history of CD34(+) cells in primary cultures was determined by PKH2 staining and related to their CFC-generation potential. In some experiments, subsets of CD34(+) cells sorted on the basis of the number of cell divisions (0, 1, >1) were independently characterized. In hypoxia, the numbers of CFC and CD34(+) cells were markedly reduced, whereas pre-CFC were maintained better than in air. Addition of 5-fluorouracil to hypoxic cultures for 2 days suppressed their CFC-generation potential. The CFC-generation potential of divided CD34(+) cells was maintained or increased with respect to that of undivided cells in hypoxia but not in air. IL-3 decreased CFC-generation potential at both oxygen concentrations. IL-3 also increased the number of CD34(+) cells that divided more than once in hypoxia, decreasing their CFC-generation potential. Maintenance of CFC-generation potential in hypoxia occurs mainly in a subset of cycling progenitors, despite their proliferation (self-renewal). IL-3 decreased CFC-generation potential by increasing the rate of pre-CFC proliferation beyond the first cycle, which probably results in their differentiation and loss of CFC-generation potential.

Importance of CD44v7 isoforms for homing and seeding of hematopoietic progenitor cells

The adhesion molecule CD44 consists of many isoforms of which particularly CD44v7 is of major importance in hematopoietic progenitor cell homing. An increase of progenitor cells in the periphery was observed after treating mice with a CD44v7-specific antibody, concomitant with a substantially augmented marrow-repopulating ability (MRA). Because CD44v7 is expressed on a fraction of bone marrow cells (BMC) as well as on long-term bone marrow culture-derived stromal cells, we aimed to differentiate between the functional relevance of CD44v7 on either cell type by transferring CD44v7+/+ BMC into CD44v7-/- mice and vice versa. CD44v7+/+ BMC homed poorly in the bone marrow of CD44v7-/- mice and their MRA was severely impaired. CD44v7-/- BMC, instead, exhibited an improved MRA when transferred into the CD44v7+/+ host, although their MRA remained below that of CD44v7+/+ BMC. Thus, it is predominantly, but not exclusively, expression of CD44v7 on stromal cells which supports progenitor cell homing.

The expansion of murine bone marrow cells preincubated in hypoxia as an in vitro indicator of their marrow-repopulating ability.

In liquid cultures of murine bone marrow cells stimulated with interleukin-3 and granulocyte/macrophage colony-stimulating factor, hypoxia (1% oxygen) induced a reversible block of hematopoiesis, maintaining the progenitors' expansion potential unreduced. Progenitors repopulating day-14 hypoxic cultures with cells or granulocyte/macrophage colony-forming units (CFU-GM) were found, on the basis of their maintenance in hypoxia (12% and 76%, respectively), to belong to different subsets, the latter being much more efficiently maintained. The maintenance in hypoxic cultures of progenitors detectable by marrow-repopulating ability (MRA) assay was 18% for MRAcell progenitors and 69% for MRACFU progenitors. Thus, the repopulation of hypoxic cultures with cells or CFU-GM closely reflected the presence of progenitors capable of repopulating, with cells or CFU-GM, the bone marrow of lethally irradiated syngeneic animals. Progenitors repopulating hypoxic cultures were, like MRA progenitors, significantly resistant to 5-fluorouracil, progenitors repopulating cultures with CFU-GM being two-fold more resistant than those repopulating cultures with cells. We concluded that the repopulation of day-14 hypoxic cultures occurring after their transfer to air is to be considered an indicator of the maintenance of MRA progenitors in hypoxia. The relevance of these results to stem cell biology and their potential practical applications are discussed.

Incubation of murine bone marrow cells in hypoxia ensures the maintenance of marrow-repopulating ability together with the expansion of committed progenitors.

We developed previously a hypoxic culture system in which progenitors endowed with marrow-repopulating ability (MRA), unlike committed progenitors, were selected and maintained better than in air. We report here an improvement to this system targeted at combining the maintenance of progenitors sustaining MRA with the numerical expansion of multipotent and committed progenitors. Murine bone marrow cells were incubated at 1% oxygen in liquid medium supplemented with stem cell factor, granulocyte colony-stimulating factor, interleukin-6 and interleukin-3. In day 8 hypoxic cultures, the numbers of high proliferative potential and granulocyte/macrophage colony-forming cells (HPP-CFC and CFU-GM) were increased with respect to time zero. Colonies generated by HPP-CFC derived from hypoxic cultures exhibited a high replating ability, whereas colonies generated by HPP-CFC derived from control cultures exhibited a low replating ability. MRA was fully maintained in hypoxia and markedly reduced in air. Thus, severe hypoxia is able to ensure a full maintenance of progenitors sustaining MRA, together with a significant expansion of in vitro-detectable clonogenic progenitors, including those endowed with replating ability. This system could contribute to the improvement of current techniques for the in vitro treatment of human haematopoietic cell populations before transplantation.

A Single Dose of Thrombopoietin Shortly After Myelosuppressive Total Body Irradiation Prevents Pancytopenia in Mice by Promoting Short-Term Multilineage Spleen-Repopulating Cells at the Transient Expense of Bone Marrow–Repopulating Cells

Thrombopoietin (TPO) has been used in preclinical myelosuppression models to evaluate the effect on hematopoietic reconstitution. Here we report the importance of dose and dose scheduling for multilineage reconstitution after myelosuppressive total body irradiation (TBI) in mice. After 6 Gy TBI, a dose of 0.3 μg TPO/mouse (12 μg/kg) intraperitoneally (IP), 0 to 4 hours after TBI, prevented the severe thrombopenia observed in control mice, and in addition stimulated red and white blood cell regeneration. Time course studies showed a gradual decline in efficacy after an optimum within the first hours after TBI, accompanied by a replacement of the multilineage effects by lineage dominant thrombopoietic stimulation. Pharmacokinetic data showed that IP injection resulted in maximum plasma levels 2 hours after administration. On the basis of the data, we inferred that a substantial level of TPO was required at a critical time interval after TBI to induce multilineage stimulation of residual bone marrow cells. A more precise estimate of the effect of dose and dose timing was provided by intravenous administration of TPO, which showed an optimum immediately after TBI and a sharp decline in efficacy between a dose of 0.1 μg/mouse (4 μg/kg; plasma level 60 ng/mL), which was fully effective, and a dose of 0.03 μg/mouse (1.2 μg/kg; plasma level 20 ng/mL), which was largely ineffective. This is consistent with a threshold level of TPO required to overcome initial c-mpl–mediated clearance and to reach sufficient plasma levels for a maximum hematopoietic response. In mice exposed to fractionated TBI (3 × 3 Gy, 24 hours apart), IP administration of 0.3 μg TPO 2 hours after each fraction completely prevented the severe thrombopenia and anemia that occurred in control mice. Using short-term transplantation assays, ie, colony-forming unit–spleen (CFU-S) day 13 (CFU-S-13) and the more immature cells with marrow repopulating ability (MRA), it could be shown that TPO promoted CFU-S-13 and transiently depleted MRA. The initial depletion of MRA in response to TPO was replenished during long-term reconstitution followed for a period of 3 months. Apart from demonstrating again that MRA cells and CFU-S-13 are separate functional entities, the data thus showed that TPO promotes short-term multilineage repopulating cells at the expense of more immature ancestral cells, thereby preventing pancytopenia. The short time interval available after TBI to exert these effects shows that TPO is able to intervene in mechanisms that result in functional depletion of its multilineage target cells shortly after TBI and emphasizes the requirement of dose scheduling of TPO in keeping with these mechanisms to obtain optimal clinical efficacy.© 1998 by The American Society of Hematology.

A Single Dose of Thrombopoietin Shortly After Myelosuppressive Total Body Irradiation Prevents Pancytopenia in Mice by Promoting Short-Term Multilineage Spleen-Repopulating Cells at the Transient Expense of Bone Marrow–Repopulating Cells

Thrombopoietin (TPO) has been used in preclinical myelosuppression models to evaluate the effect on hematopoietic reconstitution. Here we report the importance of dose and dose scheduling for multilineage reconstitution after myelosuppressive total body irradiation (TBI) in mice. After 6 Gy TBI, a dose of 0.3 μg TPO/mouse (12 μg/kg) intraperitoneally (IP), 0 to 4 hours after TBI, prevented the severe thrombopenia observed in control mice, and in addition stimulated red and white blood cell regeneration. Time course studies showed a gradual decline in efficacy after an optimum within the first hours after TBI, accompanied by a replacement of the multilineage effects by lineage dominant thrombopoietic stimulation. Pharmacokinetic data showed that IP injection resulted in maximum plasma levels 2 hours after administration. On the basis of the data, we inferred that a substantial level of TPO was required at a critical time interval after TBI to induce multilineage stimulation of residual bone marrow cells. A more precise estimate of the effect of dose and dose timing was provided by intravenous administration of TPO, which showed an optimum immediately after TBI and a sharp decline in efficacy between a dose of 0.1 μg/mouse (4 μg/kg; plasma level 60 ng/mL), which was fully effective, and a dose of 0.03 μg/mouse (1.2 μg/kg; plasma level 20 ng/mL), which was largely ineffective. This is consistent with a threshold level of TPO required to overcome initial c-mpl–mediated clearance and to reach sufficient plasma levels for a maximum hematopoietic response. In mice exposed to fractionated TBI (3 × 3 Gy, 24 hours apart), IP administration of 0.3 μg TPO 2 hours after each fraction completely prevented the severe thrombopenia and anemia that occurred in control mice. Using short-term transplantation assays, ie, colony-forming unit–spleen (CFU-S) day 13 (CFU-S-13) and the more immature cells with marrow repopulating ability (MRA), it could be shown that TPO promoted CFU-S-13 and transiently depleted MRA. The initial depletion of MRA in response to TPO was replenished during long-term reconstitution followed for a period of 3 months. Apart from demonstrating again that MRA cells and CFU-S-13 are separate functional entities, the data thus showed that TPO promotes short-term multilineage repopulating cells at the expense of more immature ancestral cells, thereby preventing pancytopenia. The short time interval available after TBI to exert these effects shows that TPO is able to intervene in mechanisms that result in functional depletion of its multilineage target cells shortly after TBI and emphasizes the requirement of dose scheduling of TPO in keeping with these mechanisms to obtain optimal clinical efficacy.© 1998 by The American Society of Hematology.

Continuous infusion of macrophage inflammatory protein MIP-1alpha enhances leucocyte recovery and haemopoietic progenitor cell mobilization after cyclophosphamide

Macrophage inflammatory protein 1alpha (MIP-1alpha) inhibits haemopoietic stem cell proliferation. This property has been exploited in a murine chemotherapy model and has been shown to ameliorate cytotoxic-induced myelosuppression after S-phase-specific cytotoxic therapy. We have now shown that BB-10010, a stable mutant of MIP-1alpha, (a) is more effective when administered as a continuous infusion than when bolus injected and (b), when administered via a 7-day infusion during and after cyclophosphamide treatment, results in an earlier recovery of leucocyte numbers. This effect was accompanied by progenitor cell mobilization into the peripheral blood and included primitive cells with marrow-repopulating ability (MRA). Maximal mobilization and recovery of leucocytes occurred when MIP-1alpha was combined with granulocyte colony-stimulating factor (G-CSF) therapy. The findings suggest that MIP1-alpha used alone or in combination with G-CSF may allow delivery of a greater chemotherapy dose intensity as a consequence of both accelerated leucocyte recovery and maintenance of high-quality mobilized progenitor cells for harvesting and peripheral blood stem cell transplantation.

BB-10010/MIP-1 alpha in vivo maintains haemopoietic recovery following repeated cycles of sublethal irradiation.

Macrophage inflammatory protein-1 alpha (MIP-1 alpha) is an inhibitor of stem cell proliferation affording protection against damage from agents that express their cytotoxicity specifically in the DNA synthesis phase of the cell cycle. Its ability also to modify the self-renewal capacity of the regenerating cells is now shown to improve and maintain haemopoietic recovery following therapy (sublethal irradiation) whose cytotoxic damage is not limited solely to the DNA-S phase of this cycle. Such non-cell cycle-active cytotoxic agents are used clinically in repeated treatment regimens, which are often limited or terminated because of accumulating haemopoietic damage. BB-10010, a non-aggregating variant of MIP-1 alpha, was administered as a continuous dose (1600 micrograms kg-1 24 h-1) via a subcutaneously implanted pump over a period of 7 days. A dose of 4.5 Gy total-body gamma-rays was given 3-4 h after implantation. Day 8 and 12 spleen colony-forming units (CFU-S) were assayed on days 1, 7 and 14 after irradiation. This cycle of treatment was repeated four times (total 56 days), and on day 14 of the last two cycles the marrow-repopulating ability (MRA) was also measured. In the control bone marrow (no BB-10010) CFU-S fell to < 1% of normal within 1 day of irradiation and recovered to 40% at 14 days. Repeated treatments increased the level of damage, and after four cycles CFU-S recovered to only 10% of normal. BB-10010 afforded little benefit in the first treatment cycle, but by the end of the fourth cycle CFU-S still recovered to 35% of normal. MRA was reduced to 7% of normal by the irradiation protocol-about half that maintained by BB-10010 protection. We conclude that BB-10010 (MIP-1 alpha) reduces the degree of accumulated haemopoietic stem cell damage following repeated non-cell cycle-specific cytotoxic insults-a principle which should be valuable in repeated clinical cytotoxic therapy regimens.

Characterization of Peripheral Blood Stem Cells in Mice

Peripheral blood stem cells (PBSCs) were mobilized in mice by treatment with cytosine-arabinoside on day 0, followed by the administration by injection of granulocyte colony-stimulating factor for 4 days. There were remarkable increases in the numbers of cells with lineage-negative (Lin-) c-kit+ markers, cells with colony-forming unit-cell (CFU-C) and colony-forming unit-spleen (CFU-S) activities, and cells with marrow-repopulating ability (MRA) in the extramedullary sites (the spleen, peripheral blood, and liver) on day 5, whereas the number of these immature hematopoietic cells decreased in the bone marrow (BM) on day 5. This finding suggests the mobilization of immature hematopoietic cells from the BM to the extramedullary sites. Three-color flow cytometric analyses showed that CD4 antigen was not expressed on the Lin-Sca-1+ cells in the mobilized PB cells (PBCs), although CD4lo cells were found in those of normal BM cells. Lin-c-kit+ cells in the mobilized PBCs contained more cells with immature phenotypes (Sca-1+, Thy1.2lo, CD71-, and Rh123dull) than in normal BMCs, indicating an alteration of the hierarchical composition of the Lin-c-kit+ cells. The Lin-c-kit+Sca-1+ cells in the mobilized PBCs had similar CFU-C and CFU-S activities to those in normal BMCs. Electron microscopic studies of these cells in the mobilized PBCs showed that only 10% to 20% of these cells had a thin rim of cytoplasm with poorly developed organelles. Allogeneic transplantation [B6 --> C3H] of PBSCs showed long-term reconstituting activity across the major histocompatibility complex barrier 24 weeks after transplantation, although longer observation is necessary.

In vivo self-renewal of c-Kit+ Sca-1+ Lin(low/-) hemopoietic stem cells.

The long term marrow-repopulating ability of mouse bone marrow c-Kit+ Sca-1+ Lin(low/-) cells was studied. Injection of as few as 100 c-Kit+ Sca-1+ Lin(low/-) cells could rescue a lethally irradiated recipient mouse and reconstitute both myeloid and lymphoid cells in their peripheral blood for at least 10 mo. When limiting dilution analysis was performed in the presence of host-derived rescue cells, as low as five c-Kit+ Sca-1+ Lin(low/-) cells were shown to be capable of repopulating lymphohemopoietic cells. Subsequently, we examined whether c-Kit+ Sca-1+ Lin(low/-) cells had a capacity for self-renewal in vivo. After transplantation of 500 c-Kit+ Sca-1+ Lin(low/-) Ly-5.1+ cells into lethally irradiated Ly-5 congenic mice, the expansion of cells with the same phenotypes as the injected cells was monitored. By day 28 post-transplantation, more than 50,000 donor type c-Kit+ Sca-1+ Lin(low/-) Ly-5.1+ cells were found in the spleen and over 18,000 cells were found in bone marrow. The expansion in spleen, however, was transient in that by day 60 cells of the donor phenotype were found only in bone marrow. The c-Kit+ Sca-1+ Lin(low/-) LY-5.1+ cells expanded in spleen or bone marrow contained as many high proliferative potential colony-forming cells as the originally injected cells. They also contained cells with LTRA, but the frequency appeared to be less compared with naive c-Kit+ Sca-1+ Lin(low/-) cells. These data provide evidence that c-Kit+ Sca-1+ Lin(low/-) cells in bone marrow are capable of multilineage differentiation as well as self-renewal and that spleen is a primary site of stem cell expansion after transplantation.

Modification of rhodamine staining allows identification of hematopoietic stem cells with preferential short-term or long-term bone marrow-repopulating ability.

We have developed a modified rhodamine (Rho) staining procedure to study uptake and efflux in murine hematopoietic stem cells. Distinct populations of Rho++ (bright), Rho+ (dull), and Rho- (negative) cells could be discriminated. Sorted Rho- cells were subjected to a second Rho staining procedure with the P-glycoprotein blocking agent verapamil (VP). Most cells became Rho positive [Rho-/Rho(VP)+ cells] and some remained Rho negative [Rho-/Rho(VP)- cells]. These cell fractions were characterized by their marrow-repopulating ability in a syngeneic, sex-mismatch transplantation model. Short-term repopulating ability was determined by recipient survival for at least 6 weeks after lethal irradiation and transplantation--i.e., radioprotection. Long-term repopulating ability at 6 months after transplantation was measured by fluorescence in situ hybridization with a Y-chromosome-specific probe, by graft function and recipient survival. Marrow-repopulating cells were mainly present in the small Rho- cell fraction. Transplantation of 30 Rho- cells resulted in 50% radioprotection and > 80% donor repopulation in marrow, spleen, and thymus 6 months after transplantation. Cotransplantation of cells from both fractions in individual mice directly showed that within this Rho- cell fraction, the Rho-/Rho(VP)+ cells exhibited mainly short-term and the Rho-/Rho(VP)- cells exhibited mainly long-term repopulating ability. Our results indicate that hematopoietic stem cells have relatively high P-glycoprotein expression and that the cells responsible for long-term repopulating ability can be separated from cells exhibiting short-term repopulating ability, probably by a reduced mitochondrial Rho-binding capacity.

Mobilization of Early Hematopoietic Progenitor Cells With BB-10010: A Genetically Engineered Variant of Human Macrophage Inflammatory Protein-la

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.

The use of the APAAP technique as a rapid indicator of peripheral blood progenitor cell levels

Rapid and sustained engraftment following autotransplantation with peripheral blood stem cells (PBSC) depends on adequate numbers of stem cells and progenitor cells. In this study we have compared the number of myeloid progenitor cells quantitated using the colony forming units-granulocyte macrophage (CFU-GM) clonogenic assay with the number of CD34+ cells estimated both by flow cytometry and by the alkaline phosphatase anti-alkaline phosphatase (APAAP) technique. We have analysed 15 peripheral blood mononuclear cells (PBMNC) samples from 13 normal subjects and 179 PBMNC from 32 patients undergoing PBSC harvests during the recovery phase of high dose cyclophosphamide chemotheraphy. The number of CD34+ cells measured by the APAAP technique correlated well with the number of CD34+ cells measured by flow cytometry (r = 0.727, p = 0.0001), and also with the number of CFU-GM measured in the clonogenic assay (r = 0.721, p = 0.0001). The APAAP method provides a rapid, reliable measure of progenitor cell levels that can be used to monitor the optimal time to harvest peripheral blood stem cells (PBSC), and to estimate the marrow repopulating ability (MRA) of stem cell preparations used for transplantation.

Proliferation of spleen colony forming units (CFU-S8, CFU-S13) and cells with marrow repopulating ability.

The practicalities of gene transfer therapy using retroviral vector systems require both that host cells be as primitive as possible and that those cells be proliferating. Here, the kinetics of hemopoietic stem cells with marrow repopulating ability (MRA) have been studied with a view to defining the timescale over which these normally quiescent cells can be triggered into cell cycle. Mice were injected with hydroxyurea (1 g/kg) four times over a period of 26 h and assayed at intervals up to eight days for 8-day and 13-day spleen colony-forming units (CFU-S) and for generation of 12-day CFU-S in the bone marrow (MRA assay). The proliferative activity of these cell populations was measured by in vitro tritiated thymidine assays. CFU-S were reduced rapidly to 11% of normal and induced into cycle. Their number and proliferative quiescence recovered by four to five days. Cells with MRA reached their nadir after four days and only then started to proliferate. For each of these progenitor cell subclasses, the proliferative activity inversely reflects their numbers and indicates regulation by negative feedback processes.

Hematopoietic stem cell depletion by restorative growth factor regimens during repeated high-dose cyclophosphamide therapy [see comments

We studied the effects of six cycles of repeated cyclophosphamide (CTX) therapy followed by restorative therapy with either granulocyte- macrophage colony-stimulating factor (GM-CSF) or G-CSF on the hematopoietic stem cell compartment. Stem cell function was assessed by serially transferring bone marrow cells from CTX-CSF-treated mice into lethally irradiated recipient mice. Bone marrow cells from mice that initially received either G-CSF or GM-CSF after CTX therapy more rapidly lost the ability to repopulate the recipient lymphoid organs, showed a dramatic loss of hematopoietic progenitors, a more rapid loss of CFU-S capacity, and a 40% to 50% reduction in marrow repopulating ability (MRA). Interleukin-1 (IL-1) appeared to have little effect on the CTX-treated mice when used alone. However, when administered before the CTX-CSF regimen, IL-1 prevented the stem cell depletion as determined by CFU-C, CFU-S, and MRA through the serial transplantation procedures. These results support the hypothesis that repeated treatments with myelosuppressive drugs followed by stimulation with the CSFs may induce damage to the host stem cell compartment, and further suggest that pretreatment with IL-1 before CTX therapy may prevent this stem cell damage.

Hematopoietic Stem Cell Depletion by Restorative Growth Factor Regimens During Repeated High-Dose Cyclophosphamide Therapy

We studied the effects of six cycles of repeated cyclophosphamide (CTX) therapy followed by restorative therapy with either granulocyte-macrophage colony-stimulating factor (GM-CSF) or G-CSF on the hematopoietic stem cell compartment. Stem cell function was assessed by serially transferring bone marrow cells from CTX-CSF-treated mice into lethally irradiated recipient mice. Bone marrow cells from mice that initially received either G-CSF or GM-CSF after CTX therapy more rapidly lost the ability to repopulate the recipient lymphoid organs, showed a dramatic loss of hematopoietic progenitors, a more rapid loss of CFU-S capacity, and a 40% to 50% reduction in marrow repopulating ability (MRA). Interleukin-1 (IL-1) appeared to have little effect on the CTX-treated mice when used alone. However, when administered before the CTX-CSF regimen, IL-1 prevented the stem cell depletion as determined by CFU-C, CFU-S, and MRA through the serial transplantation procedures. These results support the hypothesis that repeated treatments with myelosuppressive drugs followed by stimulation with the CSFs may induce damage to the host stem cell compartment, and further suggest that pretreatment with IL-1 before CTX therapy may prevent this stem cell damage.

In vitro frequency analysis of spleen colony-forming and marrow-repopulating hemopoietic stem cells in the mouse

An assay is described for Day-12 spleen colony-forming cells (CFU-S-12) and hemopoietic stem cells with marrow-repopulating ability (MRA) in the mouse using a miniaturized stroma-dependent bone marrow culture assay in vitro. Bone marrow cells are grown in liquid culture in microtiter wells, and the resulting adherent stromal layers are depleted of all hemopoietic activity by 20 Gy gamma irradiation. Subsequently, single cell suspensions containing stem cells are overlaid in a range of concentrations, and the presence of one or more emerging phase nonrefractive cell clones (cobblestone areas) in a single well scored as positive. The frequencies of cobblestone area-forming cells (CAFC) are then calculated by employing Poisson statistics. It is shown that the CAFC Day-10 and CAFC Day-28 frequencies closely correlate with those of CFU-S-12 and MRA cells, respectively.