Epirubicin and docetaxel with or without capecitabine as neoadjuvant treatment for early breast cancer: final results of a randomized phase III study (ABCSG-24).

BACKGROUND: This randomized phase III trial compared pathologic complete response (pCR) rates of early breast cancer (EBC) following neoadjuvant epirubicin-docetaxel (ED)±capecitabine (C), and evaluated the addition of trastuzumab in HER2-positive tumors. PATIENTS AND METHODS: Patients with invasive breast cancer (except T4d) were randomly assigned to receive six 3-weekly cycles of ED (both 75 mg/m2)±C (1000 mg/m2, twice daily, days 1-14). Patients with HER2-positive disease were further randomized to receive trastuzumab (8 mg/kg, then 6 mg/kg every 3 weeks) or not. Primary end point: pCR rate at the time of surgery. RESULTS: Five hundred thirty-six patients were randomized to ED (n=266) or EDC (n=270); 93 patients were further randomized to trastuzumab (n=44) or not (n=49). pCR rate was significantly increased with EDC (23.0% versus 15.4% ED, P=0.027), and nonsignificantly further increased with trastuzumab (38.6% EDC versus 26.5% ED, P=0.212). Rates of axillary node involvement at surgery and breast conservation were improved with EDC versus ED, but not significantly; the addition of trastuzumab had no further impact. Hormone receptor status, tumor size, grade, and C (all P≤0.035) were independent prognostic factors for pCR. Trastuzumab added to ED±C significantly increased the number of serious adverse events (35 versus 18; P=0.020), mainly due to infusion-related reactions. CONCLUSION: These findings show that the integration of C into a neoadjuvant taxane-/anthracycline-based regimen is a feasible, safe, and effective treatment option, with incorporation of trastuzumab in HER2-positive disease. CLINICAL TRIAL NUMBER: NCT00309556, www.clinicaltrials.gov.

Differential cytokine effects on primitive (CD34+CD38-) human hematopoietic cells: novel responses to Flt3-ligand and thrombopoietin.

A high proportion of the CD34+CD38- cells in normal human marrow are defined as long-term culture-initiating cells (LTC-IC) because they can proliferate and differentiate when co-cultured with cytokine-producing stromal feeder layers. In contrast, very few CD34+CD38- cells will divide in cytokine-containing methylcellulose and thus are not classifiable as direct colony-forming cells (CFC), although most can proliferate in serum-free liquid cultures containing certain soluble cytokines. Analysis of the effects of 16 cytokines on CD34+CD38- cells in the latter type of culture showed that Flt3-ligand (FL), Steel factor (SF), and interleukin (IL)-3 were both necessary and sufficient to obtain an approximately 30-fold amplification of the input LTC-IC population within 10 d. As single factors, only FL and thrombopoietin (TPO) stimulated a net increase in LTC-IC within 10 d. Interestingly, a significantly increased proportion of the CFC produced from the TPO-amplified LTC-IC were erythroid. Increases in the number of directly detectable CFC of > 500-fold were also obtainable within 10 d in serum-free cultures of CD34+CD38- cells. However, this required the presence of IL-6 and/or granulocyte/colony-stimulating factor and/or nerve growth factor beta in addition to FL, SF, and IL-3. Also, for this response, the most potent single-acting factor tested was IL-3, not FL. Identification of cytokine combinations that differentially stimulate primitive human hematopoietic cell self-renewal and lineage determination should facilitate analysis of the intracellular pathways that regulate these decisions as well as the development of improved ex vivo expansion and gene transfer protocols.

Different subsets of primary chronic myeloid leukemia stem cells engraft immunodeficient mice and produce a model of the human disease.

Xenograft models of chronic phase human chronic myeloid leukemia (CML) have been difficult to develop because of the persistence of normal hematopoietic stem cells in most chronic phase CML patients and the lack of methods to selectively isolate the rarer CML stem cells. To circumvent this problem, we first identified nine patients' samples in which the long-term culture-initiating cells were predominantly leukemic and then transplanted cells from these samples into sublethally irradiated NOD/SCID and NOD/SCID-beta2microglobulin-/- mice. This resulted in the consistent and durable (>5 months) repopulation of both host genotypes with similar numbers of BCR-ABL+/Ph+ cells. The regenerated leukemic cells included an initial, transient population derived from CD34+CD38+ cells as well as more sustained populations derived from CD34+CD38- progenitors, indicative of a hierarchy of transplantable leukemic cells. Analysis of the phenotypes produced revealed a reduced output of B-lineage cells, enhanced myelopoiesis with excessive production of erythroid and megakaropoietic cells and the generation of primitive (CD34+) leukemic cells displaying an autocrine IL-3 and G-CSF phenotype, all characteristics of primary CML cells. These findings demonstrate the validity of this xenograft model of chronic phase human CML, which should enable future investigation of disease pathogenesis and new approaches to therapy.

Effect of deoxycytidine on 2-chloro-deoxyadenosine-mediated growth inhibition of normal human erythroid and myeloid progenitor cells.

The cytotoxic effect of chlorodeoxyadenosine (CdA) on lymphocytes and monocytes requires phosphorylation by the enzyme deoxycytidine kinase and can be antagonized by coadministration of deoxycytidine (dCyt), a competitive substrate of deoxycytidine kinase. It has also been shown for lymphocytes that coadministration of 3-aminobenzamide (3-ABA), an inhibitor of the enzyme poly-(ADP ribose) synthetase, is activated by CdA-mediated DNA strand breaks, consumes intracellular nicotinamide-dinucleotide (NAD) and can antagonize the lethal effect of CdA. Recent in vitro studies have shown that not only growth of lymphocytes and monocytes, but also colony formation by erythroid and myeloid progenitors derived from normal human bone marrow, is inhibited by CdA in a dose-dependent manner. In this study we examined the effect of various doses of dCyt (10(-6) to 10(-3) M) on CdA-mediated growth inhibition of erythroid and myeloid progenitor cells in vitro. Our results show that colony formation by human bone marrow-derived progenitor cells--CFU-E (colony-forming unit erythroid), BFU-E (burst-forming unit erythroid) and CFU-GM (colony-forming unit granulocyte/macrophage)-in semisolid medium is protected by a high, but clinically achievable and non-toxic, concentration of dCyt (> 10(-4) M) against the inhibitory effects of coadministered high concentrations of CdA. The protective effect of dCyt was markedly different on the various subclasses of progenitor cells, however. Thus, with coadministration of 10(-4) M dCyt, the CFU-E colony formation could be restored to almost 100% despite the presence of high concentrations of CdA (160 nM) compared to control cultures, whereas the colony formation of BFU-E and CFU-GM was restored to only 50%. At a concentration of 10(-3) M dCyt, colony formation of BFU-E and CFU-GM was raised to 80% of control cultures even in the presence of high concentrations of CdA (160 nM). Further experiments in which 3-ABA was coadministered to CdA-treated cultures showed that in all concentrations tested (0.3 to 5 mM) 3-ABA was not able to prevent CdA-mediated cytotoxicity on bone marrow progenitors. Based on these studies, we suggest that the CdA toxicity on CFU-E is mainly mediated by phosphorylation by deoxycytidine kinase, whereas additional mechanisms may be operative in BFU-E and CFU-GM. Considerable biochemical differences seem to exist between hematopoietic stem cells on the one hand and lymphocytes and monocytes from peripheral blood on the other.

Growth-inhibitory effect of 2-CdA on myeloid CFU-GM progenitors in normal human long-term bone marrow cultures and its compensation by G-CSF or IL-3.

As neutropenia is a common side effect of treatment with 2-chlorodeoxyadenosine (2-CdA), we investigated the myelosuppressive action of 2-CdA in Dexter-type human long-term bone marrow cultures (LTBMCs). LTBMCs were incubated with varying doses of 2-CdA (5 to 20 nM/L) during the first week. At 20 and 10 nM/L 2-CdA, we found a marked reduction in colony-forming unit-granulocyte/macrophage (CFU-GM) production throughout the culture period of 7 weeks (maximum reduction to 3.5% of untreated control cultures with 20 nM/L and to 27.2% with 10 nM/L, respectively). Even the lowest 2-CdA dose tested (5 nM/L) strongly reduced the number of CFU-GM progenitors during the first 3 weeks (maximum reduction to 52.4% of untreated controls), but this effect was transient, and values had recovered to normal within in 5 weeks. 2-CdA was also shown to cause a dose-dependent decrease in long-term culture-initiating cell (LTCIC) detections after 5 weeks in culture (49.6% of control cultures with 10 nM/L 2-CdA and 14% with 20 nM/L 2-CdA, respectively). When 2-CdA was added to LTBMCs initiated on preformed irradiated stromal feeder layers, similar results on CFU-GM production were obtained, indicating that the effects observed were not secondary to effects on the formation of a supportive layer. In addition, IL-6-concentrations in the supernatant of LTBMCs measured at various intervals after the addition of fresh medium with or without 2-CdA showed no significant decrease in cultures treated with 2-CdA. As neutropenia has been shown to be associated with a small but significant risk of fatal infection, we subsequently investigated the reversal potential of the 2-CdA effect by addition of recombinant human granulocyte colony-stimulating factor (rhG-CSF) or rh interleukin-3 (rhIL-3). The weekly addition of 100 ng/mL rhG-CSF counteracted the 2-CdA-mediated decrease in CFU-GM numbers during the entire period of 7 weeks, reaching statistical significance from weeks 3 to 7 (p < 0.05). Addition of rhIL-3 (100 ng/mL) showed an enhancement of CFU-GM output in 2-CdA-treated cultures that resulted in their numbers exceeding those in control cultures (without 2-CdA) from weeks 1 to 5 (p < 0.05) with a maximum increase of 5.1-fold over the parallel control value at week 3.(ABSTRACT TRUNCATED AT 400 WORDS)

Evaluation of optimal survival of primitive progenitor cells (LTC-IC) from PBPC apheresis products after overnight storage.

Optimal overnight (ON) storage of PBPC aphereses is becoming an increasingly important issue and different options for storing PBPC products exist. The survival of primitive progenitor cells is of major interest, as recent data suggest that these progenitors are not only important for long-term engraftment but also contribute significantly to the early phase of hematopoietic engraftment after myeloablative therapy. We therefore investigated the survival of primitive progenitor cells (ie long-term culture initiating cells, LTC-IC) before (ie within 2 h after finishing the apheresis procedure) and after ON storage lasting 16 to 20 h. In addition, we compared the % of recovery of LTC-IC with that of mature progenitors (ie colony-forming cells, CFC) and with the % viability of the mononuclear cells in the apheresis product. Aliquots of PBPC aphereses products were tested in collection bags at room temperature (RT), in EDTA tubes both at RT or 4 degrees C +/- the addition of autologous plasma (AP; 2.6-fold the apheresis volume) and +/- the possibility of gas exchange. Mean viable cell counts did not show strong differences between the different storage conditions and were poor predictors for the survival of CFC and LTC-IC. At RT (collection bags, EDTA tubes +/- gas exchange) recoveries (% of input) of both, CFC (18%, 18% and 31%) and LTC-IC (10%, 4%, 17%) were low. The addition of AP at RT improved the survival of CFC and LTC-IC to 66% and 38%, respectively. Optimal recoveries for both types of progenitors (CFC: 99%, LTC-IC: 109%) were obtained at 4 degrees C in the presence of AP. In addition, a good correlation between the survival of CFC and LTC-IC was obtained (r = 0.76) suggesting that the analysis of CFC may also allow some conclusions to be drawn on the survival of LTC-IC. Bone Marrow Transplantation (2000) 25, 197-200.

Hematopoietic stem cells.

The adequate production of blood cells is maintained by a set of immature hematopoietic stem cells (HSC) located in the bone marrow after birth. HSC are able to reconstitute the hematopoietic system in disease-related bone marrow failure and bone marrow aplasia. Nowadays, HSC cells can be mobilized from the bone marrow into the peripheral blood using hematopoietic cytokines, allowing a convenient harvest of these cells for clinical transplantation. This review outlines the development of the hematopoietic system in the embryo and in adults and the characterization, enumeration, purification and ex vivo expansion of HSC for clinical use. Future directions include the genetic manipulation of HSC and the identification/expansion of bone marrow-derived stem cells capable of generating non-hematopoietic tissues.

Impact of natural killer cell dose and donor killer-cell immunoglobulin-like receptor (KIR) genotype on outcome following human leucocyte antigen-identical haematopoietic stem cell transplantation.

To define the role of quantitative graft composition and donor killer-cell immunoglobulin-like receptor (KIR) genotype in clinical outcome following unmanipulated peripheral blood stem cell transplantation (PBSCT) from human leucocyte antigen (HLA)-identical siblings, 43 consecutive transplants for haematological malignancies were analysed retrospectively. Twenty-four patients underwent myeloablative conditioning and 19 received busulphan/fludarabine-based reduced intensity conditioning (RIC). In patients with acute myelogenous leukaemia or myelodysplastic syndrome (AML/MDS; n = 18), no relapse occurred following transplants meeting both a high (above median) natural killer (NK) cell count and missing HLA-ligand(s) to donor's KIR(s), compared to all other AML/MDS patients (0% versus 44%; P = 0.049). Missing HLA-B and/or HLA-C ligand combined with missing HLA-A3/11 (KIR3DL2 unblocked) predicted for reduced relapse incidence regardless of diagnosis or conditioning type (P = 0.028). Moreover, in AML/MDS patients, this constellation predicted superior overall survival (OS) (P = 0.046). Transplants with more than two different activating donor KIRs were associated with an increased risk for non-relapse mortality (NRM), both by univariate and multivariate analysis. Quantitative graft composition had a significant impact exclusively in RIC transplants. Here, a trend towards reduced relapse incidence was found in patients receiving high numbers of NK cells (16% versus 54%; P = 0.09). In patients receiving high versus low T cell numbers, OS was superior (83% versus 37%; P = 0.01), due mainly to reduced NRM (0% versus 33%; P = 0.046). By multivariate analysis, relapse risk was decreased significantly in patients receiving high NK cell numbers (P = 0.039). These data suggest that both the number of transplanted NK cells and the donor KIR genotype play a role in graft-versus-malignancy mechanisms in HLA-identical PBSCT.

Selective expansion of primitive normal hematopoietic cells in cytokine-supplemented cultures of purified cells from patients with chronic myeloid leukemia.

We have previously reported that primitive normal hematopoietic cells detectable as long-term culture-initiating cells (Ph-LTC-IC) are present at high levels in the blood of some patients with chronic myeloid leukemia (CML). We now show that this population can be expanded several-fold when highly purified CD34+CD38- cells isolated from the blood of such patients are cultured for 10 days in a serum-free medium containing 100 ng/mL of Flt3-ligand and Steel factor and 20 ng/mL of interleukin-3 (IL-3) and IL-6, and granulocyte colony-stimulating factor. In similar cultures initiated with CD34+CD38- cells from CML blood samples in which all of the LTC-IC were leukemic (Ph+), Ph+ LTC-IC activity was rapidly lost both in the presence and absence of admixed CD34+CD38- cells isolated from normal marrow. Conversely, the ability of normal LTC-IC to expand their numbers was shown to be independent of the presence of Ph+LTC-IC and later types of Ph+colony-forming cell (CFC) progenitors. In contrast to the LTC-IC, CFC were consistently amplified in cultures initiated with CML-derived CD34+CD38- cells and the additional CFC present after 10 days were, like the starting population of CFC, almost exclusively Ph+ regardless of the genotype(s) of the LTC-IC in the original CML samples. Amplification of the Ph+CFC population in these cultures showed the same factor dependence as previously demonstrated for the in vitro expansion of CFC from normal marrow CD34+CD38- cells. Ph+LTC-IC disappeared regardless of the cytokines present. Taken together these findings support a model of CML in which the leukemic stem cells are characterized by a decreased probability of self-renewal and an increased probability of differentiation. In addition, they suggest new opportunities for improving the treatment of CML using strategies that require autologous stem cell rescue.

Cytokine manipulation of primitive human hematopoietic cell self-renewal.

Previous studies have shown that primitive human hematopoietic cells detectable as long-term culture-initiating cells (LTC-ICs) and colony-forming cells (CFCs) can be amplified when CD34(+) CD38(-) marrow cells are cultured for 10 days in serum-free medium containing flt3 ligand (FL), Steel factor (SF), interleukin (IL)-3, IL-6, and granulocyte colony-stimulating factor. We now show that the generation of these two cell types in such cultures is differentially affected at the single cell level by changes in the concentrations of these cytokines. Thus, maximal expansion of LTC-ICs (60-fold) was obtained in the presence of 30 times more FL, SF, IL-3, IL-6, and granulocyte colony-stimulating factor than could concomitantly stimulate the near-maximal (280-fold) amplification of CFCs. Furthermore, the reduced ability of suboptimal cytokine concentrations to support the production of LTC-ICs could be ascribed to a differential response of the stimulated cells since this was not accompanied by a change in the number of input CD34(+) CD38(-) cells that proliferated. Reduced LTC-IC amplification in the absence of a significant effect on CFC generation also occurred when the concentrations of FL and SF were decreased but the concentration of IL-3 was high (as compared with cultures containing high levels of all three cytokines). To our knowledge, these findings provide the first evidence suggesting that extrinsically acting cytokines can alter the self-renewal behavior of primary human hematopoietic stem cells independent of effects on their viability or proliferation.

Cellular determinants affecting the rate of cytokine in cultures of human hematopoietic cells.

The present study was undertaken to define parameters that may limit the cytokine-mediated expansion of primitive hematopoietic cells in stirred suspension cultures of normal human marrow cells. In a first series of experiments, parallel measurements of the rate and extent of progenitor expansion and cytokine depletion from the medium were made for such cultures in which the cells were exposed to different cytokine concentrations. Supplementation of the medium with 2 ng/mL of interleukin-3 (IL-3), IL-6 and IL-11 plus 10 ng/mL of Flt-3 ligand (FL) and Steel factor (SF) allowed a 45-fold expansion of directly clonogenic cell (CFC) numbers within 2 weeks along with a 2.5-fold expansion of their precursors, detectable as longterm culture-initiating cells (LTC-IC). The addition of 5-fold higher levels of these cytokines enhanced the 2 week output of both CFC and LTC-IC numbers (to 66-fold and 9-fold above input respectively). However, this was also associated with an increase in the individual average rates of depletion of immunoreactive IL-3, SF and FL. As a result, even biweekly addition of fresh medium supplemented with the highest concentrations of cytokines tested failed to prevent a continuing decline in their levels relative to the input medium levels. A similar dependence of the IL-3 depletion rate on its extracellular concentration was demonstrable in suspension cultures of Mo7e cells, an IL-3-dependent human leukemic cell line.Additional experiments with various highly purified marrow cell fractions showed that the rate of cytokine depletion varied according to the type of responding cell as well as the specific cytokine. CD34(+)CD38(-) cells exhibited the greatest average cell-specific cytokine depletion rates (35-fold higher than unseparated bone marrow cells). These findings establish new principles that will be important for the optimization of hematopoietic cell bioreactors. In addition, they suggest that cytokine depletion may provide a novel feedback control mechanism in vivo which would contribute to the control of primitive hematopoietic cell proliferation and differentiation.

Aspergillus osteoarthritis in acute lymphoblastic leukemia.

We report an unusual case of arthritis of the right wrist due to Aspergillus fumigatus without evidence for a generalized infection, following chemotherapy for acute lymphoblastic leukemia. The diagnosis was made by surgical biopsy. Amphotericin-B (Am-B) was not tolerated by the patient. Liposomal preparations of Am-B penetrate poorly into bone and cartilage. Therefore, oral itraconazole was given; the arthritis improved and chemotherapy was continued without infectious complications. Two weeks after complete hematopoietic recovery, an intracranial hemorrhage from a mycotic aneurysm of a brain vessel occurred, although the patient was still receiving itraconazole. We emphasize the importance of prompt and thorough efforts to identify the causative agent in immunocompromised patients with a joint infection. Itraconazole is effective in Aspergillus osteoarthritis but, due to its poor penetration into the brain, the combination with a liposomal formulation of Am-B is recommended.

BCR-ABL expression in different subpopulations of functionally characterized Ph+ CD34+ cells from patients with chronic myeloid leukemia.

In patients with chronic myeloid leukemia (CML), the leukemic (BCR-ABL+/Ph+) clone typically includes cells belonging to all of the myeloid lineages and frequently some B cells. From such observations it has been inferred that the initial BCR-ABL gene rearrangement event occurs in a pluripotent hematopoietic stem cell and that the clone subsequently generated is maintained by a subpopulation of neoplastic, BCR-ABL-expressing cells that retain at least some of the defining properties of normal hematopoietic stem cells. To test this hypothesis directly, we isolated various subpopulations of CD34+ cells from fresh or cryopreserved samples of peripheral blood from 5 CML patients with high white blood cell counts, 4 of which were selected because of their exclusive content of Ph+ progenitors (both colony-forming cells and long-term culture-initiating cells [LTC-IC]). Cells in each of the CD34+ subpopulations isolated were examined for the presence of BCR-ABL mRNA using a reverse transcriptase-polymerase chain reaction technique that reproducibly gave a positive signal from single K562 cells. BCR-ABL mRNA was detected in 117 of 147 samples (80%) in which actin mRNA was demonstrable. This included 60% to 90% of a large number of individually analyzed CD34+ cells including 46 single CD34+CD71-CD38- cells and 27 single CD34+CD71+CD38+ cells from 3 patients. In 2 of these cases, the same populations also contained a very high frequency of Ph+ LTC-IC. Our findings demonstrate BCR-ABL gene expression in neoplastic cells with functional as well as surface marker characteristics of very primitive normal hematopoietic cells. This implicates the BCR-ABL gene product directly in the acquisition by these cells of properties that alter their interactions with the microenvironment and deregulate their proliferation control.

Candida ciferrii, a new fluconazole-resistant yeast causing systemic mycosis in immunocompromised patients.

Systemic infections related to fluconazole-resistant yeasts are increasingly observed in immunocompromised patients receiving fluconazole as a prophylactic antifungal treatment. Here, we report a case of invasive candidiasis caused by Candida ciferrii in a patient with acute myeloid leukemia and who suffered a relapse after autologous peripheral blood progenitor cell transplantation. Erythematous skin papulae and spotted pulmonary infiltrations were present. A skin biopsy led to the diagnosis of invasive candidiasis, emphasizing the diagnostic usefulness of this procedure. The yeast was identified as Candida ciferrii and in vitro susceptibility testing revealed its resistance to fluconazole. Until now, Candida ciferrii has not been known to cause invasive fungal infections in humans. Thus, we add another fungus to the list of flucanozole-resistant yeasts and suggest that in vitro susceptibility testing of isolated fungi should be performed for the selection of appropriate antimycotic drugs.

Optimal timing for the collection and in vitro expansion of cytotoxic CD56(+) lymphocytes from patients undergoing autologous peripheral blood stem cell transplantation.

To identify the optimal time for the collection of CD56(+) cytotoxic lymphocytes for adoptive immunotherapy in patients undergoing high-dose chemotherapy (HDCT) and peripheral blood stem cell (PBSC) transplantation, 18 breast cancer patients receiving either three cycles of epirubicin/paclitaxel (CT x 3) followed by HDCT and PBSC transplantation (n = 12) or CTx6 (n = 6) were studied. Blood samples were obtained before each CT/HDCT cycle, from PBSC collections, and repeatedly after autografting for up to 12 months. The number of CD56(+)3(-) and CD56(+)3(+) lymphocytes, their in vitro expandability with interleukin-2, and their cytotoxicity against MCF-7 and Daudi cells were analyzed. Six healthy females served as controls. CD56(+) cell counts in both treatment groups were subnormal but stable during the observation period. The cytotoxicity of the expanded CD56(+) cells was normal and unaffected by the treatment. The in vitro CD56(+) cell expandability (controls, 100 +/- 31-fold, mean +/- SEM) was normal before CT1 and CT2, but reduced in PBSC harvests performed after CT2 and application of G-CSF (21 +/- 6-fold; p < 0.01). After PBSC harvesting, the CD56(+) cell expandability increased to 185 +/- 74-fold and 170 +/- 69-fold (before CT3 and HDCT). This increase was not observed in those patients who did not undergo PBSC mobilization. Two weeks after autografting, the CD56(+) cell expandability was minimal (6 +/- 1-fold), and recovered to 34 +/- 6-fold. Thus, CT, HDCT and autografting do not alter the frequency and inducible cytotoxicity of CD56(+) cells in breast cancer patients. However, the proliferative capacity of CD56(+) cells obtained from PBSC harvests and after autografting is impaired. Therefore, instead of the PBSC graft, maximally expandable CD56(+) cells obtained at least 1 week after PBSC collection should be considered for adoptive immunotherapy after PBSC autografting.

Self-renewal of primitive human hematopoietic cells (long-term-culture-initiating cells) in vitro and their expansion in defined medium.

A major goal of experimental and clinical hematology is the identification of mechanisms and conditions that support the expansion of transplantable hematopoietic stem cells. In normal marrow, such cells appear to be identical to (or represent a subset of) a population referred to as long-term-culture-initiating cells (LTC-ICs) so-named because of their ability to produce colony-forming cell (CFC) progeny for > or = 5 weeks when cocultured with stromal fibroblasts. Some expansion of LTC-ICs in vitro has recently been described, but identification of the factors required and whether LTC-IC self-renewal divisions are involved have remained unresolved issues. To address these issues, we examined the maintenance and/or generation of LTC-ICs from single CD34+ CD38- cells cultured for variable periods under different culture conditions. Analysis of the progeny obtained from cultures containing a feeder layer of murine fibroblasts engineered to produce steel factor, interleukin (IL)-3, and granulocyte colony-stimulating factor showed that approximately 20% of the input LTC-ICs (representing approximately 2% of the original CD34+ CD38- cells) executed self-renewal divisions within a 6-week period. Incubation of the same CD34+ CD38- starting populations as single cells in a defined (serum free) liquid medium supplemented with Flt-3 ligand, steel factor, IL-3, IL-6, granulocyte colony-stimulating factor, and nerve growth factor resulted in the proliferation of initial cells to produce clones of from 4 to 1000 cells within 10 days, approximately 40% of which included > or = 1 LTC-IC. In contrast, in similar cultures containing methylcellulose, input LTC-ICs appeared to persist but not divide. Overall the LTC-IC expansion in the liquid cultures was 30-fold in the first 10 days and 50-fold by the end of another 1-3 weeks. Documentation of human LTC-IC self-renewal in vitro and identification of defined conditions that permit their extensive and rapid amplification should facilitate analysis of the molecular mechanisms underlying these processes and their exploitation for a variety of therapeutic applications.