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R Pettengell

Publications and source records attributed to R Pettengell.

At least 19 recordsLinked to original sources
PubMed2006

Lack of clinical efficacy of rituximab in the treatment of autoimmune neutropenia and pure red cell aplasia: implications for their pathophysiology.

We describe 11 patients with severe refractory autoimmune cytopenias treated with the anti-CD20 monoclonal antibody rituximab. Six patients had autoimmune neutropenia (AIN), two had pure red cell aplasia (PRCA), one had AIN and autoimmune haemolytic anaemia, one had AIN and immune thrombocytopaenia purpura (ITP) and one had PRCA and ITP. Rituximab was administered at a dose of 375 mg/m(2) as an intravenous infusion weekly for 4 weeks. Six of eight patients with AIN and all three patients with PRCA did not respond. Two patients died: one with resistant AIN and autoimmune haemolytic anaemia died of pneumocytis pneumonia infection, and one with PRCA and ITP died of an acute exacerbation of bronchiectasis. Rituximab in AIN and PRCA appears to be less effective than Campath-1H when compared to historical data from our group. This supports the hypothesis that T cells may be important in the pathophysiology of AIN and PRCA.

Adult↗
PubMed2006

EORTC guidelines for the use of granulocyte-colony stimulating factor to reduce the incidence of chemotherapy-induced febrile neutropenia in adult patients with lymphomas and solid tumours.

Chemotherapy-induced neutropenia is not only a major risk factor for infection-related morbidity and mortality, but is also a significant dose-limiting toxicity in cancer treatment. Patients developing severe (grade 3/4) or febrile neutropenia (FN) during chemotherapy frequently receive dose reductions and/or delays to their chemotherapy. This may impact on the success of treatment, particularly when treatment intent is either curative or to prolong survival. The incidence of severe or FN can be reduced by prophylactic treatment with granulocyte-colony stimulating factors (G-CSFs), such as filgrastim, lenograstim or pegfilgrastim. However, the use of G-CSF prophylactic treatment varies widely in clinical practice, both in the timing of therapy and in the patients to whom it is offered. While several academic groups have produced evidence-based clinical practice guidelines in an effort to standardise and optimise the management of FN, there remains a need for generally applicable, European-focused guidelines. To this end, we undertook a systematic literature review and formulated recommendations for the use of G-CSF in adult cancer patients at risk of chemotherapy-induced FN. We recommend that patient-related adverse risk factors such as elderly age (>or=65 years), be evaluated in the overall assessment of FN risk prior to administering each cycle of chemotherapy. In addition, when using a chemotherapy regimen associated with FN in >20% patients, prophylactic G-CSF is recommended. When using a chemotherapy regimen associated with FN in 10-20% patients, particular attention should be given to patient-related risk factors that may increase the overall risk of FN. In situations where dose-dense or dose-intense chemotherapy strategies have survival benefits, prophylactic G-CSF support is recommended. Similarly, if reductions in chemotherapy dose intensity or density are known to be associated with a poor prognosis, primary G-CSF prophylaxis may be used to maintain chemotherapy. Finally, studies have shown that filgrastim, lenograstim and pegfilgrastim have clinical efficacy and we recommend the use of any of these agents to prevent FN and FN-related complications, where indicated.

Antineoplastic Agents↗
PubMed2004

Long-term follow-up of patients treated with radiotherapy alone for early-stage histologically aggressive non-Hodgkin's lymphoma.

Historically localised aggressive non-Hodgkin's lymphoma (NHL) has been treated with involved field radiotherapy (RT), chemotherapy, or a combination of both modalities. The current weight of evidence supports a preference for combined modality treatment (CMT). Increased patient age at diagnosis is well recognised as a poor prognostic indicator in NHL, but despite this some perceive CMT as too toxic for use in the elderly. As a result, some older patients continue to be offered RT alone. Here, we present long-term follow-up of 377 adults of all ages treated with RT alone for early-stage diffuse large-cell lymphoma on British National Lymphoma Investigation trials between 1974 and 1997. 10-year cause-specific survival in patients older than 60 years was poor and significantly inferior to that in younger patients (47 and 75% respectively; P<0.001). There is growing evidence that short-course chemotherapy, with or without RT, is superior to RT alone in early-stage aggressive NHL, in elderly as well as in younger patients. Increased age alone should not exclude patients from systemic treatment for early-stage aggressive NHL.

Adolescent↗
PubMed2004

Thrombopoietin, flt3-ligand and c-kit-ligand modulate HOX gene expression in expanding cord blood CD133 cells.

Haemopoietic stem/progenitor cell (HSPC) development is regulated by extrinsic and intrinsic stimuli. Extrinsic modulators include growth factors and cell adhesion molecules, whereas intrinsic regulation is achieved with many transcription factor families, of which the HOX gene products are known to be important in haemopoiesis. Umbilical cord blood CD133+ HSPC proliferation potential was tested in liquid culture with 'TPOFLK' (thrombopoietin, flt-3 ligand and c-kit ligand, promoting HSPC survival and self-renewal), in comparison to 'K36EG' (c-kit-ligand, interleukins-3 and -6, erythropoietin and granulocyte colony-stimulating factor, inducing haemopoietic differentiation). TPOFLK induced a higher CD133+ HSPC proliferation (up to 60-fold more, at week 8) and maintained a higher frequency of the primitive colony-forming cells than K36EG. Quantitative polymerase chain reaction analysis revealed opposite expression patterns for specific HOX genes in expanding cord blood CD133+ HSPC. After 8 weeks in liquid culture, TPOFLK increased the expression of HOX B3, B4 and A9 (associated with uncommitted HSPC) and reduced the expression of HOX B8 and A10 (expressed in committed myeloid cells) when compared to K36EG. These results suggest that TPOFLK induces CD133+ HSPC proliferation, self-renewal and maintenance, up-regulation of HOX B3, B4 and A9 and down-regulation of HOX B8 and A10 gene expression.

AC133 Antigen↗
PubMed2002

Autologous stem cell transplantation in follicular non-Hodgkin's lymphoma.

The failure of conventional chemotherapy to improve overall survival rates in follicular non-Hodgkin's lymphoma (NHL) has led to the development of alternative treatment regimens. One such regimen is high-dose chemotherapy (HDT) with autologous stem cell transplantation (ASCT). In ASCT stem cells, harvested predominantly from peripheral blood, are used to repopulate the haemopoietic system after high-dose chemotherapy. Comparison of failure-free survival rates following ASCT, with those previously obtained with conventional chemotherapy, suggests a benefit for ASCT in patients who have previously responded to chemotherapy. Other factors that adversely influence the outcome of ASCT include large tumour burden and bcl-2 overexpression. Although ASCT in follicular NHL can prolong the period of remission, relapse is still common and can be caused either by contamination of the stem cell harvest with tumour cells, or regrowth of residual malignant cells not eradicated by the high-dose chemotherapy. Several strategies have been developed to reduce the rate of relapse, including in vitro purging of the stem cell product to remove tumour cells and using allogenic stem cells from HLA-matched donors with no history of malignant disease. While both these methods may have some benefit, they also have limitations. In vitro purging is labour intensive, costly and, as yet, the effect on relapse is unclear. Allogenic stem cell transplants have been associated with a reduced risk of relapse, but this is offset by increased transplant-related mortality. The most promising strategy to reduce the rate of relapse following ASCT is in vivo purging using rituximab, a monoclonal antibody to CD20. Rituximab mobilises mechanisms to kill lymphoma cells, and causes a rapid depletion of B cells from peripheral blood. Rituximab has demonstrated good efficacy as monotherapy in patients with both aggressive and indolent lymphoma and has shown very high response rates (>95%) when used in combination with HDT.

Antibodies, Monoclonal↗
PubMed2002

Growth factors in haematological cancers.

Since their discovery just under a century ago, growth factors (GFs) have been used almost ubiquitously in haematology. Many haematological cancers are associated with bone marrow failure, either as a direct consequence of the disease or its treatment. Colony stimulating factors (CSFs) have been used to address the problems associated with the resulting cytopenias, however, concerns about the potential leukaemogenic effects of some of these CSFs led to a degree of initial hesitancy in usage, particularly in the management of acute myeloid leukaemia (AML). This has now been largely overcome. Other limitations have included cost and side effect profiles (the latter particularly with the multilineage factors). There has been wide variation locally, nationally and internationally in the usage of GFs. The American Society of Clinical Oncologists (ASCO) attempted to rationalise the usage of GFs by producing a consensus document enumerating the evidence-based indications for use of GFs. There is little information on cost effectiveness, this remains an important issue for the future. Peripheral blood stem cell transplantation (PBSCT) has revolutionised the management of many malignant conditions and has contributed to the increased use of growth factors. Many other indications are emerging for GFs used singly or in combination. Current clinical applications of GFs include: i) amelioration of cytopenias following chemotherapy and stem cell transplantation, ii) chemotherapy dose maintenance and escalation, iii) chemosensitisation and modification of disease states, iv) optimisation of methods for mobilisation of progenitor stem cells, v) immunotherapy, and vi) as therapeutic targets for treatment of haematolgical malignancies.

Journal Article↗
PubMed2001

Use of hematopoietic progenitors in whole blood to support dose-dense chemotherapy: a randomized phase II trial in small-cell lung cancer patients.

PURPOSE: Small-cell lung cancer (SCLC) is exquisitely chemosensitive, but few patients are cured by conventional chemoradiotherapy. Recent studies suggest that increased cytotoxic dose-intensity might improve survival. In this randomized phase II study, we tested the feasibility of dose intensification using sequential reinfusion of hematopoietic progenitors in whole blood. PATIENTS AND METHODS: SCLC patients with a favorable prognosis were treated with six cycles of ifosfamide, carboplatin, and etoposide (ICE), at 4-week (standard treatment) or 2-week (intensified treatment) intervals. Intensified treatment was supported by daily subcutaneous filgrastim injections and reinfusion of 750 mL of autologous blood collected immediately before each cycle. RESULTS: Fifty consecutive patients were randomized to standard (n = 25) or intensified (n = 25) ICE. A total of 94% completed at least three treatment cycles, and 70% completed six cycles; 96% of treatments were given at full dose. The planned dose-intensity was 1.0 for standard and 2.0 for intensified ICE. The median received dose-intensity for cycles 1 through 3 was 0.99 (range, 0.33 to 1.02) for the standard treatment arm and 1.80 (range, 0.99 to 1.97) for the intensified treatment arm (P <.001). Over all six cycles, the median received dose-intensity was 0.95 (range, 0.17 to 1.03) for the standard treatment arm and 1.60 (range, 0.60 to 2.01) for the intensified treatment arm (P <.001). Febrile neutropenia was more common on the standard treatment arm (84% v 56%), resulting in more days of intravenous antibiotics (median, 10 v 3 days; P =.035). Transfusion requirements were similar in the two groups. CONCLUSION: Sequential reinfusion of hematopoietic progenitors in whole blood can safely support substantial increases in dose-intensity of ICE chemotherapy for SCLC.

Adult↗
PubMed2001

In vivo CAMPATH-1H prevents GvHD following nonmyeloablative stem-cell transplantation.

BACKGROUND: We have investigated a novel nonmyeloablative conditioning regimen in 44 patients with hematological malignancies. The median patient age was 41 years. Many of the patients had high-risk features, including 19 patients with a previous failed transplant. METHODS: Recipient conditioning consisted of CAMPATH-1H 20 mg/day on Days -8 to -4, fludarabine 30 mg/m(2) on Days -7 to -3 and melphalan 140 mg/m(2) on Day -2. Thirty-six recipients received unmanipulated G-CSF mobilized PBSC from HLA identical siblings and eight received unmanipulated BM from MUD. GvHD prophylaxis was with CYA alone for 38 patients and CYA plus MTX for six sibling recipients. RESULTS: Forty-two of the 43 evaluable patients had sustained engraftment. Results of chimerism analysis using microsatellite PCR indicate that 18 of 31 patients studied were full donor chimeras, while the other patients were mixed chimeras in one or more lineages. At a median follow-up of 9 months (range, 3-29 months) 33 patients remain alive in CR, or with no evidence of disease progression. Seven patients relapsed or progressed post-transplant and four of them subsequently died. Four patients died from regimen-related complications. There were no cases of Grades III-IV acute GvHD. Only two patients developed Grade II acute GvHD and only one had chronic GvHD. The estimated probability of non-relapse mortality at 1 year was 11%.Results: Although longer follow-up is needed to establish the long-term remission rates, this study demonstrates that this nonmyeloablative preparative regimen is associated with durable engraftment, minimal toxicity and low incidence of GvHD.

Adolescent↗
PubMed2000

In vivo CAMPATH-1H prevents graft-versus-host disease following nonmyeloablative stem cell transplantation.

A novel nonmyeloablative conditioning regimen was investigated in 44 patients with hematologic malignancies. The median patient age was 41 years. Many of the patients had high-risk features, including 19 patients with a previous failed transplant. Recipient conditioning consisted of CAMPATH-1H, 20 mg/day on days -8 to -4; fludarabine, 30 mg/m(2) on days -7 to -3; and melphalan, 140 mg/m(2) on day -2. Thirty-six recipients received unmanipulated granculocyte colony-stimulating factor-mobilized peripheral blood stem cells from HLA-identical siblings, and 8 received unmanipulated marrow from matched unrelated donors. GVHD prophylaxis was with cyclosporine A alone for 38 patients and cyclosporine A plus methotrexate for 6 sibling recipients. Forty-two of the 43 evaluable patients had sustained engraftment. Results of chimerism analysis using microsatellite polymerase chain reaction indicate that 18 of 31 patients studied were full-donor chimeras while the other patients were mixed chimeras in one or more lineages. At a median follow-up of 9 months (range 3 to 29 months), 33 patients remain alive in complete remission or with no evidence of disease progression. Seven patients relapsed or progressed post-transplantation, and 4 of them subsequently died. Four patients died of regimen-related complications. There were no cases of grades III-IV acute GVHD. Only 2 patients developed grade II acute GVHD, and only 1 had chronic GVHD. The estimated probability of nonrelapse mortality was 11%. Although longer follow-up is needed to establish the long-term remission rates, this study demonstrates that this nonmyeloablative preparative regimen is associated with durable engraftment, minimal toxicity, and low incidence of GVHD.

Adolescent↗
PubMed2000

Human tumour and dendritic cell hybrids generated by electrofusion: potential for cancer vaccines.

Hybrid cells created by fusion of antigen presenting and tumour cells have been shown to induce potent protective and curative anti-tumour immunity in rodent cancer models. The application of hybrid cell vaccines for human tumour therapy and the timely intervention in disease control are limited by the requirement to derive sufficient autologous cells to preserve homologous tumour antigen presentation. In this study, the efficiency of various methods of electrofusion in generating hybrid human cells have been investigated with a variety of human haemopoietic, breast and prostate cell lines. Cell fusion using an electrical pulse is enhanced by a variety of stimuli to align cells electrically or bring cells into contact. Centrifugation of cells after an exponential pulse from a Gene Pulser electroporation apparatus provided the highest yield of mixed cell hybrids by FACS analysis. An extensive fusogenic condition generated in human cells after an electrical pulse contradicts the presumption that prior cell contact is necessary for cell fusion. Alignment of cells in a concurrent direct current charge and osmotic expansion of cells in polyethylene glycol also generated high levels of cell fusion. Waxing of one electrode of the electroporation cuvette served to polarize the fusion chamber and increase cell fusion 5-fold. Optimisation of a direct current charge in combination with a fusogenic pulse in which fusion of a range of human cells approached or exceeded 30% of the total pulsed cells. The yield of hybrid prostate and breast cancer cells with dendritic cells was similar to the homologous cell fusion efficiencies indicating that dendritic cells were highly amenable to fusion with human tumour cells under similar electrical parameters. Elimination of unfused cells by density gradient and culture is possible to further increase the quantity of hybrid cells. The generation and purification of quantities of hybrid cells sufficient for human vaccination raises the possibility of rapid, autologous tumour antigen presenting vaccines for trial with common human tumours.

Cancer Vaccines↗
PubMed2000

A UK multicentre phase II study of rituximab (chimaeric anti-CD20 monoclonal antibody) in patients with follicular lymphoma, with PCR monitoring of molecular response.

Follicular lymphoma (FL) cells express CD20 and are associated in most cases with the t(14;18) chromosomal translocation. A multicentre study was undertaken between January 1997 and January 1998 to assess the complete response rate (CR) and overall response rate (RR) to rituximab, a chimaeric anti-CD20 monoclonal antibody. Seventy patients with previously treated FL received rituximab (375 mg/m2/week x4, by intravenous infusion). Restaging studies were performed 1 and 2 months after therapy. Molecular monitoring for the presence of cells harbouring the Bcl-2/JH gene rearrangement in the peripheral blood (PB) and bone marrow (BM) was performed before and after treatment using a two-step semi-nested polymerase chain reaction (PCR) assay. The overall RR was 32/70 (46%), being highest in patients who had received only one previous treatment (12/15, 80%). However, only two patients achieved a CR. The median duration of response was 11 months. Thirteen of 21 evaluable 'PCR-positive' patients (62%) became 'PCR-negative' in PB and/or BM samples 1 month after rituximab, although this did not correlate with clinical response. Treatment was generally well tolerated, although one patient developed Stevens-Johnson syndrome. Rituximab was shown to be active in FL, and in some cases PB and/or BM became PCR negative. Studies in combination with cytotoxic chemotherapy to increase the CR rate are warranted.

Adult↗
PubMed2000

Effective treatment of a patient with a high-grade endometrial stromal sarcoma with an accelerated regimen of carboplatin and paclitaxel.

The rarity of endometrial stromal sarcoma (ESS) and its poor response to treatment provides fertile ground for investigational therapies. An accelerated regimen of carboplatin and paclitaxel is investigated. A patient with a recent history of treated tuberculosis of the lung represented with infertility and acute abdominal pain from suspected fibroids, and underwent a laparotomy with a diagnosis of a high-grade ESS. A novel therapeutic approach using a regimen of carboplatin and paclitaxel with the reinfusion of filgrastim-mobilized peripheral blood progenitor cells is described. A partial response was observed following six cycles of chemotherapy. Grade IV thrombocytopenia occurred after the last cycle, with recovery prior to pelvic radiotherapy. The patient remained well 1 year post-diagnosis. High-grade ESS is responsive to combination chemotherapy with paclitaxel and carboplatin, and requires further evaluation. The use of an accelerated regimen may also have contributed to the response and this question awaits randomized trials.

Adult↗
PubMed1998

Recombinant human granulocyte colony-stimulating factor (filgrastim) following high-dose chemotherapy and peripheral blood progenitor cell rescue in high-grade non-Hodgkin's lymphoma: clinical benefits at no extra cost.

In order to evaluate the potential clinical and economic benefits of granulocyte colony-stimulating factor (G-CSF, filgrastim) following peripheral blood progenitor cells (PBPC) rescue after high-dose chemotherapy (HDCT), 23 consecutive patients aged less than 60 years with poor-prognosis, high-grade non-Hodgkin's lymphoma (NHL) were entered into a prospective randomized trial between May 1993 and September 1995. Patients were randomized to receive either PBPC alone (n = 12) or PBPC+G-CSF (n = 11) after HDCT with busulphan and cyclophosphamide. G-CSF (300 microg day[-1]) was given from day +5 until recovery of granulocyte count to greater than 1.0 x 10(9) l(-1) for 2 consecutive days. The mean time to achieve a granulocyte count > 0.5 x 10(9) l(-1) was significantly shorter in the G-CSF arm (9.7 vs 13.2 days; P<0.0001) as was the median duration of hospital stay (12 vs 15 days; P = 0.001). In addition the recovery periods (range 9-12 vs 11-17 days to achieve a count of 1.0 x 10(9) l[-1]) and hospital stays (range 11-14 vs 13-22 days) were significantly less variable in patients receiving G-CSF in whom the values clustered around the median. There were no statistically significant differences between the study arms in terms of days of fever, documented episodes of bacteraemia, antimicrobial drug usage and platelet/red cell transfusion requirements. Taking into account the costs of total occupied-bed days, drugs, growth factor usage and haematological support, the mean expenditure per inpatient stay was pound sterling 6500 (range pound sterling 5465-pound sterling 8101) in the G-CSF group compared with pound sterling 8316 (range pound sterling 5953-pound sterling 15,801) in the group not receiving G-CSF, with an observed mean saving of 1816 per patient (or 22% of the total cost) in the G-CSF group. This study suggests that after HDCT and PBPC rescue, the use of G-CSF leads to more rapid haematological recovery periods and is associated with a more predictable and shorter hospital stay. Furthermore, and despite the additional costs for G-CSF, these clinical benefits are not translated into increased health care expenditure.

Adult↗
PubMed1998

Randomized comparison of progenitor-cell mobilization using chemotherapy, stem-cell factor, and filgrastim or chemotherapy plus filgrastim alone in patients with ovarian cancer.

PURPOSE: This was the first randomized study to investigate the efficacy of peripheral-blood progenitor cell (PBPC) mobilization using stem-cell factor (SCF) in combination with filgrastim (G-CSF) following chemotherapy compared with filgrastim alone following chemotherapy. PATIENTS AND METHODS: Forty-eight patients with ovarian cancer were treated with cyclophosphamide and randomized to receive filgrastim 5 microg/kg alone or filgrastim 5 microg/kg plus SCF. The dose of SCF was cohort-dependent (5, 10, 15, and 20 microg/kg), with 12 patients in each cohort, nine of whom received SCF plus filgrastim and the remaining three patients who received filgrastim alone. On recovery from the WBC nadir, patients underwent a single apheresis. RESULTS: SCF in combination with filgrastim following chemotherapy enhanced the mobilization of progenitor cells compared with that produced by filgrastim alone following chemotherapy. This enhancement was dose-dependent for colony-forming unit-granulocyte-macrophage (CFU-GM), burst-forming unit-erythrocyte (BFU-E), and CD34+ cells in both the peripheral blood and apheresis product. In the apheresis product, threefold to fivefold increases in median CD34+ and progenitor cell yields were obtained in patients treated with SCF 20 microg/kg plus filgrastim compared with yields obtained in patients treated with filgrastim alone. Peripheral blood values of CFU-GM, BFU-E, and CD34+ cells per milliliter remained above defined threshold levels longer with higher doses of SCF. The higher doses of SCF offer a greater window of opportunity in which to perform the apheresis to achieve high yields. CONCLUSION: SCF (15 or 20 microg/kg) in combination with filgrastim following chemotherapy is an effective way of increasing progenitor cell yields compared with filgrastim alone following chemotherapy.

Adult↗
PubMed1998

Myeloid growth factors in oncology.

Within the last decade haemopoietic growth factors have become established in the pharmacopoeia of oncology. In the form of granulocyte colony-stimulating factor (G-CSF), and to a lesser extent granulocyte-macrophage colony-stimulating factor (GM-CSF), these proteins are routinely used to accelerate restoration of neutrophil count after chemotherapy or bone marrow transplant. Their main advance has been the development of mobilisation protocols. Peripheral blood progenitor cells are induced to egress from the bone marrow and re-transfusion after myelosuppressive chemotherapy allows for a simple and more rapid form of autologous transplantation than bone marrow transplantation. This review will give a brief overview of the biology of haemopoiesis in relation to growth factors and the potential lines of further research. Although the established clinical uses of G-CSF will be discussed the main focus will be on the developmental applications, such as ex vivo haemopoiesis, dose intensification schedules and the application of growth factors in the therapy of haematological malignancies. The relevance of novel or more recently introduced recombinant haemopoietic growth factors will also be discussed in relation to these indications.

Journal Article↗
PubMed1997

Telomerase regulation, cell cycle, and telomere stability in primitive hematopoietic cells.

Low levels of telomerase activity have recently been detected in human primitive hematopoietic cells, however, blood cells exhibit telomere shortening on cell proliferation. This challenging observation led us to study telomerase regulation and telomere length in human hematopoietic progenitor cells from fetal liver (FL), cord blood (CB), peripheral blood (PB), and bone marrow (BM). We found telomerase activity in CD34+/CD38+ cells exceeding levels in CD34+/CD38-, CD34- , and mononuclear cells (P < .05). Baseline telomerase activity was highest in BM (n = 5) CD34+ cells, followed by PB (n = 20), CB (n = 11), and FL (n = 1). Within 48 hours to 72 hours of in vitro culture of CD34+ cells in the presence of cytokines (KL, interleukin-3 [IL-3], IL-6, erythropoietin, granulocyte colony-stimulating factor), telomerase activity was upregulated, peaked after 1 week of culture, and decreased to baseline levels or below detection after 3 to 4 weeks. Stimulation of CD34+ cells with single cytokines resulted in no or minor telomerase upregulation, whereas cytokine combinations resulted in a significant telomerase increase (P < .001). There was a correlation between telomerase activity, cell cycle status by BrdU incorporation, and induction of phosphorylated retinoblastoma protein, CDC2, CDK2, cyclin D1, and cyclin A, but not cyclin E and B1 after 72 hours with multiple (but not single) cytokines. In nonexpanding CD34+ cells, telomerase was low or undetectable. Secondary CD34+ cells showed a reduced ability to upregulate telomerase activity. Antiproliferative cytokines such as transforming growth factor-beta1 and high concentrations of all-trans-retinoic acid in cytokine-supported CD34+ cultures downmodulated telomerase activity. Average telomere lengths were 10.4 kbp, 7.4 kbp, and 7.6 kbp in CB, PB, and BM CD34+ cells, respectively. In ex vivo expansion cultures, an average telomeric DNA loss of 1 to 2 kbp over 4 weeks was observed. However, the rate of base pair loss per population doubling was significantly lower during the first 2 weeks, when telomerase was upregulated, than during weeks 3 and 4 of culture. In summary, telomerase is upregulated in response to cytokine-induced proliferation and cell cycle activation in primitive hematopoietic cells. Telomerase is downregulated between weeks 3 and 4 of ex vivo expansion culture linked with decreased proliferation and greater expansion of more mature cell subsets. Our data suggest that telomerase activity in hematopoietic cells reduces, but does not prevent, telomere shortening on proliferation.

Antigens, CD34↗