Activating mutations of RTK/ras signal transduction pathway in pediatric acute myeloid leukemia

Activating mutations of receptor tyrosine kinases (RTKs) and their downstream affectors are common in acute myeloid leukemia (AML). We performed mutational analysis of FLT3, c-kit, c-fms, vascular endothelial growth factor (VEGF) receptors (Flt-1, KDR [kinase domain receptor]), and ras genes in a group of 91 pediatric patients with AML treated on Children's Cancer Group clinical trial CCG-2891. Forty-six percent of patients had activating mutations of FLT3 (24.5%), c-kit (3%), or ras (21%) genes. Mutation-positive patients had a higher median diagnostic white blood cell (WBC) count (71.5 vs 19.6 x 10(9)/L; P =.005) and lower complete remission rate (55% versus 76%; P =.046) than mutation-negative patients. The Kaplan-Meier estimate of overall survival (OS) for patients with and without an activating mutation was 34% versus 57%, respectively (P =.035). However, within this group, patients with FLT3/ALM (activation loop mutation) had good outcomes (OS, 86%). Exclusion of the FLT3/ALM from analysis decreased the OS for the remaining mutation-positive patients to 26% (P =.003). Ten of the 23 mutation-positive and 11 of the 34 mutation-negative patients received an allogeneic bone marrow transplant (BMT) in first complete remission (CR). In the mutation-positive group, the disease-free survival (DFS) for the allogeneic BMT recipients was 72% versus 23% for the 13 patients who received chemotherapy or autologous BMT (P =.01). DFS for the mutation-free patients with and without allogeneic BM transplantation was 55% and 40%, respectively (P =.38). Activating mutations in the RTK/ras signaling pathway are common in pediatric AML, and their presence may identify a population at higher risk of poor outcome who may benefit from allogeneic BM transplantation.

Multidrug resistance protein attenuates gemtuzumab ozogamicin-induced cytotoxicity in acute myeloid leukemia cells

Gemtuzumab ozogamicin (GO) is a novel immunoconjugate therapy for acute myeloid leukemia (AML). P-glycoprotein (Pgp) confers resistance to GO and is associated with a worse clinical response. To address whether multidrug resistance protein (MRP) affects GO susceptibility, we characterized Pgp, MRP1, and MRP2 expression in CD33+ cell lines and CD33+ AML samples and analyzed the effect of the Pgp inhibitor cyclosporine (CSA) and the MRP inhibitor MK-571 on GO-induced cytotoxicity. MRP1, but not MRP2, expression correlated with MRP activity. MK-571 enhanced GO-induced cytotoxicity in Pgp-negative/MRP-positive NB4 and HL-60 cells. CSA, but not MK-571 alone, restored GO susceptibility in Pgp-positive/MRP-positive TF1 cells; however, MK-571 enhanced cytotoxicity in the presence of CSA. All patient samples exhibited MRP activity, and 17 of 23 exhibited Pgp activity. CSA increased GO-induced cytotoxicity in 12 Pgp-positive samples, whereas MK-571 alone was effective in only one sample with minimal Pgp activity. In 3 Pgp-positive/MRP-positive samples, MK-571 enhanced GO-induced cytotoxicity in the presence of CSA. Thus, MRP1 may attenuate susceptibility to GO. This effect was comparatively less than that for Pgp and required the inhibition of Pgp for detection in cells that coexpressed both transporters. Because MK-571 and CSA failed to affect cytotoxicity in a portion of Pgp-positive/MRP-positive AML samples, additional resistance mechanisms are likely important.

Immunophenotypic evidence of leukemia after induction therapy predicts relapse: results from a prospective Children's Cancer Group study of 252 patients with acute myeloid leukemia

Approximately 40% of children with acute myeloid leukemia (AML) who respond to initial therapy subsequently relapse. Multidimensional flow cytometry employing a standardized panel of monoclonal antibodies enables the detection of small numbers of occult leukemic cells that persist during therapy using technology adaptable by most clinical laboratories. We performed a prospective, blinded evaluation of bone marrow specimens obtained from 252 pediatric patients with de novo AML to determine whether detection of occult leukemia defined as more than or equal to 0.5% blasts with aberrant surface antigen expression as determined by flow cytometry was predictive of subsequent relapse. Occult leukemia was detected in 41 (16%) of the 252 patients who responded to initial induction therapy. In time-dependent multivariate analyses that controlled for allogeneic marrow transplantation, variable intervals between sample submission, age, sex, white blood cell count at diagnosis, presence of splenomegaly or hepatomegaly, and presence of more than 15% blasts in the marrow after the first course of induction, patients harboring occult leukemia were 4.8 times more likely to relapse (95% confidence interval [CI] = 2.8 to 8.4, P <.0001) and 3.1 times more likely to die (95% CI; 1.9 to 5.1, P <.0001) than those lacking leukemia detectable by flow cytometry. In this analysis, flow cytometric evidence of leukemia after the initiation of therapy emerged as the most powerful independent prognostic factor associated with poor outcome. Among patients in whom a marrow sample was available for analysis at the end of consolidation therapy, overall survival at 3 years was 41% versus 69% for patients with and without occult leukemia, respectively (P =.0058).

Notch signalling in hematopoiesis

The Notch pathway is a widely utilized, evolutionarily conserved regulatory system that plays a central role in the fate decisions of multipotent precursor cells. Notch often acts by inhibiting differentiation along a particular pathway while permitting or promoting self-renewal or differentiation along alternative pathways. Haematopoietic cells and stromal cells express Notch receptors and their ligands, and Notch signalling affects the survival, proliferation, and fate choices of precursors at various stages of haematopoietic development, including whether haematopoietic stem cells self-renew or differentiate, common lymphoid precursors undergo T or B cell differentiation, or monocytes differentiate into macrophage or dendritic cells. These findings suggest that the Notch pathway plays a fundamental role in regulating haematopoietic development.

Combined effects of Notch signaling and cytokines induce a multiple log increase in precursors with lymphoid and myeloid reconstituting ability

We investigated whether combined signaling induced by engineered Notch ligands and hematopoietic growth factors influences hematopoietic stem-cell differentiation. We show that incubation of murine marrow precursors with Delta1(ext-IgG), a Notch ligand consisting of the Delta1 extracellular domain fused to the Fc portion of human immunoglobulin G1 (IgG1), and growth factors stem cell factor (SCF), interleukin 6 (IL-6), IL-11, and Flt3-l inhibited myeloid differentiation and promoted a several-log increase in the number of precursors capable of short-term lymphoid and myeloid repopulation. Addition of IL7 promoted early T-cell development, whereas addition of granulocyte-macrophage colony-stimulating factor (GM-CSF) led to terminal myeloid differentiation. These results support a role for combinatorial effects by Notch and cytokine-induced signaling pathways in regulating hematopoietic cell fate and suggest the usefulness of Notch ligand in increasing hematopoietic precursor numbers for clinical stem-cell transplantation.

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.

Antibody-targeted chemotherapy of older patients with acute myeloid leukemia in first relapse using Mylotarg (gemtuzumab ozogamicin)

We analyzed the safety and efficacy of Mylotarg (gemtuzumab ozogamicin, an antibody-targeted chemotherapy consisting of a humanized anti-CD33 antibody linked to calicheamicin, a potent antitumor antibiotic) in the treatment of 101 patients > or =60 years of age with acute myeloid leukemia (AML) in untreated first relapse in three open-label trials. Mylotarg is administered as a 2-h intravenous infusion at 9 mg/m(2) for two doses with 14 days between doses. The overall remission rate was 28%, with complete remission (CR) in 13% of patients and complete remission with incomplete platelet recovery (CRp) in 15%. Median survival was 5.4 months for all patients and 14.5 months and 11.8 months for patients achieving CR and CRp, respectively. CD33 antigen is present on normal hematopoietic progenitor cells; thus, an expected high incidence of grade 3 or 4 neutropenia (99%) and thrombocytopenia (99%) was observed. The incidences of grade 3 or 4 elevations of bilirubin and hepatic transaminases were 24% and 15%, respectively. There was a low incidence of grade 3 or 4 mucositis (4%) and infections (27%) and no treatment-related cardiotoxicity, cerebellar toxicity, or alopecia. Mylotarg is an effective treatment for older patients with CD33-positive AML in first relapse and has acceptable toxicity.

The notch pathway: modulation of cell fate decisions in hematopoiesis

The hematopoietic system is maintained by a rare population of hematopoietic stem cells (HSC) that are thought to undergo self-renewal as well as continuously produce progeny that differentiate into the various hematopoietic lineages. However, the mechanisms regulating cell fate choices by HSC and their progeny have not been understood. Results of most studies support a stochastic model of cell fate determination in which growth factors support only the survival or proliferation of the progeny specified along a particular lineage. In other developmental systems, however, Notch signaling has been shown to play a central role in regulating fate decisions of numerous types of precursors, often inhibiting a particular (default) pathway while permitting self-renewal or differentiation along an alternative pathway. There is also accumulating evidence that the Notch pathway affects survival, proliferation, and cell fate choices at various stages of hematopoietic cell development, including the decisions of HSC to self-renew or differentiate and of common lymphoid precursors to undergo T- or B-cell differentiation. These data suggest that the Notch pathway plays a fundamental role in the development and maintenance of the hematopoietic system.

The use of radioimmunoconjugates in stem cell transplantation

Radiolabeled monoclonal antibodies have been used with encouraging results in conjunction with stem cell transplantation for patients with hematologic malignancies targeting a variety of surface antigens including CD33, CD45 and CD66 for leukemias, CD20 and CD22 for non-Hodgkin's lymphomas, and ferritin for Hodgkin's disease. The results obtained targeting epithelial antigens on solid tumors, however, have generally been less encouraging, primarily due to the relative insensitivity of these malignancies to ionizing radiation. In this report we review clinical studies that have incorporated myeloablative doses of targeted radiation using radiolabeled antibodies in conjunction with stem cell transplant regimens.

High-dose chemo-radioimmunotherapy with autologous stem cell support for relapsed mantle cell lymphoma

Relapsed mantle cell lymphoma is a radiation-sensitive malignancy that is unlikely to be cured by treatment with conventional high-dose therapy and autologous stem cell transplantation. We tested the safety and efficacy of using a CD20-specific monoclonal antibody conjugated with (131)I to deliver high-dose radiation selectively to all lymphoma sites. Patients with relapsed or refractory mantle cell lymphoma received infusions of (131)I-labeled CD20-specific monoclonal antibody (Tositumomab). The antibody dose was 1.7 mg/kg body weight, and the amount of (131)I was calibrated to deliver 20 to 25 Gy to vital normal organs. This treatment was followed 10 days later by administration of high-dose etoposide (30-60 mg/kg), cyclophosphamide (60-100 mg/kg), and infusion of cryopreserved autologous stem cells. The 16 patients in this study had received a median of 3 prior treatments, and 7 had chemotherapy-resistant disease. The median dose of (131)I was 510 mCi (18.87 GBq). There were no therapy-related deaths. Among the 11 patients with conventionally measurable disease at the time of treatment, the respective complete and overall response rates were 91% and 100%. Fifteen patients remain alive, and 12 have had no progression of lymphoma at 6 to 57 months from transplantation and 16 to 97 months from diagnosis. Overall survival at 3 years from transplantation is estimated at 93%, and progression-free survival is estimated at 61%. High-dose treatment with (131)I-Tositumomab, etoposide, and cyclophosphamide results in a high remission rate and may provide long-term disease-free survival for patients with relapsed or refractory mantle cell lymphoma.

CD33 as a target for selective ablation of acute myeloid leukemia

The CD33 antigen is a 67-kd glycosylated transmembrane protein of the sialic acid-binding immunoglobulinlike lectin (siglec) family with immunoreceptor tyrosine-based inhibitory motifs. It is expressed on the surface of normal mature and immature myeloid cells, including colony-forming progenitor cells, and on leukemic blasts from the majority of patients with acute myeloid leukemia (AML). CD33 is not expressed by the normal stem cells, suggesting that in vivo ablation of CD33-bearing normal and leukemic myeloid cells might lead to the establishment of normal hematopoiesis by the remaining normal stem cells. However, whether there are significant numbers of CD33- leukemic stem cells is controversial. Therapeutic trials using unmodified anti-CD33 antibodies have, thus far, met with limited success. Studies with a radiolabeled anti-CD33 antibody have demonstrated rapid saturation of, and internalization by, leukemic blast cells after intravenous administration, suggesting the possibility of using an anti-CD33 antibody to deliver a cytotoxic drug. Using gemtuzumab ozogamicin (Mylotarg, a humanized anti-CD33 antibody conjugated with calicheamicin, the effectiveness of in vivo ablation of CD33+ cells to treat patients with AML was borne out by the portion of patients who achieved remission. To what extent CD33- leukemic precursors are responsible for failure to respond or for relapse following gemtuzumab ozogamicin therapy remains to be determined.

Antibody-directed therapies for hematological malignancies

Malignant hematopoietic cells express lineage-restricted antigens that serve as suitable targets for antibody-directed therapy. Although several highly specific, potent and relatively nontoxic, engineered antibodies, immunoglobulin fragments and antibody conjugates have been developed, only three have gained approval for clinical use. Of these, a chimeric mouse-human anti-CD20 antibody has yielded the most impressive clinical results. Encouraging data with the other approved antibodies, and with agents in clinical trials, suggest that rational antibody design will generate effective products for several different hematological malignancies. Despite these advances, significant challenges remain in the identification of optimal cellular targets, antibody forms and treatment schedules for therapeutic applications.

All-trans retinoic acid facilitates oncoretrovirus-mediated transduction of hematopoietic repopulating stem cells

A major limiting factor in achieving high levels of gene transfer into hematopoietic stem cells is the ability to retain significant repopulating activity of the stem cells during the ex vivo exposure to oncoretroviral vectors. Recently, we reported that pharmacological levels (1 microM) of all-trans retinoic acid (ATRA) enhanced the maintenance of in vivo repopulating hematopoietic stem cells during liquid suspension culture. Therefore, we investigated the use of ATRA to improve transduction of hematopoietic repopulating cells. Hematopoietic precursors cultured and transduced with a GFP-containing oncoretroviral vector with or without ATRA were transplanted immediately post-transduction (day 3 post-culture initiation) or following extended culture without further transduction (day 7 post-culture initiation). Mice transplanted with 3-day ATRA-treated cells had four-fold more donor cells than the untreated cells. In contrast, there were more GFP-expressing donor cells in recipients of cells cultured without ATRA (31.31 +/- 8.47% no ATRA vs. 16.52 +/- 9.35% ATRA). After 7 days of culture, however, the repopulating ability of the hematopoietic precursors was the same for both treatment groups, but the ATRA-treated cells had significantly more green fluorescence protein (GFP)-expressing donor cells (5.57 +/- O.53% no ATRA vs. 13.67 +/- 2.14% ATRA). Secondary recipients of marrow from recipients of the 3 day cultured cells had similar donor cell levels, but the percentage of GFP-expressing cells within the donor cell population was higher in the recipients of ATRA-treated cells (3.25 +/- 0.70% no ATRA vs. 7.97 +/- 2.71% ATRA). Our data show that the addition of ATRA to cultures of hematopoietic precursors resulted in increased gene transfer into murine hematopoietic repopulating cells. These data suggest that ATRA may be useful in clinical gene therapy protocols using oncoretroviral vectors.

The Notch ligand, Delta-1, inhibits the differentiation of monocytes into macrophages but permits their differentiation into dendritic cells

Notch-mediated cellular interactions are known to regulate cell fate decisions in various developmental systems. A previous report indicated that monocytes express relatively high amounts of Notch-1 and Notch-2 and that the immobilized extracellular domain of the Notch ligand, Delta-1 (Delta(ext-myc)), induces apoptosis in peripheral blood monocytes cultured with macrophage colony-stimulating factor (M-CSF), but not granulocyte-macrophage CSF (GM-CSF). The present study determined the effect of Notch signaling on monocyte differentiation into macrophages and dendritic cells. Results showed that immobilized Delta(ext-myc) inhibited differentiation of monocytes into mature macrophages (CD1a+/-CD14+/- CD64+) with GM-CSF. However, Delta(ext-myc) permitted differentiation into immature dendritic cells (CD1a+CD14-CD64-) with GM-CSF and interleukin 4 (IL-4), and further differentiation into mature dendritic cells (CD1a+CD83+) with GM-CSF, IL-4, and tumor necrosis factor-alpha (TNF-alpha). Notch signaling affected the differentiation of CD1a-CD14+ macrophage/dendritic cell precursors derived in vitro from CD34+ cells. With GM-CSF and TNF-alpha, exposure to Delta(ext-myc) increased the proportion of precursors that differentiated into CD1a+CD14- dendritic cells (51% in the presence of Delta(ext-myc) versus 10% in control cultures), whereas a decreased proportion differentiated into CD1a-CD14+ macrophages (6% versus 65%). These data indicate a role for Notch signaling in regulating cell fate decisions by bipotent macrophage/dendritic precursors.

Multidrug-resistance phenotype and clinical responses to gemtuzumab ozogamicin

Expression of multidrug resistance (MDR) features by acute myeloid leukemia (AML) cells predicts a poor response to many treatments. The MDR phenotype often correlates with expression of P-glycoprotein (Pgp), and Pgp antagonists such as cyclosporine (CSA) have been used as chemosensitizing agents in AML. Gemtuzumab ozogamicin, an immunoconjugate of an anti-CD33 antibody linked to calicheamicin, is effective monotherapy for CD33(+) relapsed AML. However, the contribution of Pgp to gemtuzumab ozogamicin resistance is poorly defined. In this study, blast cell samples from relapsed AML patients eligible for gemtuzumab ozogamicin clinical trials were assayed for Pgp surface expression and Pgp function using a dye efflux assay. In most cases, surface expression of Pgp correlated with Pgp function, as indicated by elevated dye efflux that was inhibited by CSA. Among samples from patients who either failed to clear marrow blasts or failed to achieve remission, 72% or 52%, respectively, exhibited CSA-sensitive dye efflux compared with 29% (P =.003) or 24% (P <.001) among samples from responders. In vitro gemtuzumab ozogamicin--induced apoptosis was also evaluated using an annexin V--based assay. Low levels of drug-induced apoptosis were associated with CSA-sensitive dye efflux, whereas higher levels correlated strongly with achievement of remission and marrow blast clearance. In vitro drug-induced apoptosis could be increased by CSA in 14 (29%) of 49 samples exhibiting low apoptosis in the absence of CSA. Together, these findings indicate that Pgp plays a role in clinical resistance to gemtuzumab ozogamicin and suggest that treatment trials combining gemtuzumab ozogamicin with MDR reversal agents are warranted. (Blood. 2001;98:988-994)

Loss of heterozygosity in childhood de novo acute myelogenous leukemia

A genome-wide screening for loss of heterozygosity (LOH), a marker for possible involvement of tumor suppressor genes, was conducted in 53 children with de novo acute myelogenous leukemia (AML). A total of 177 highly polymorphic microsatellite repeat markers were used in locus-specific polymerase chain reactions. This comprehensive allelotyping employed flow-sorted cells from diagnostic samples and whole-genome amplification of DNA from small, highly purified samples. Nineteen regions of allelic loss in 17 patients (32%) were detected on chromosome arms 1q, 3q, 5q, 7q (n = 2), 9q (n = 4), 11p (n = 2), 12p (n = 3), 13q (n = 2), 16q, 19q, and Y. The study revealed a degree of allelic loss underestimated by routine cytogenetic analysis, which failed to detect 9 of these LOH events. There was no evidence of LOH by intragenic markers for p53, Nf1, or CBFA2/AML1. Most lymphocytes lacked the deletions, which were detected only in the leukemic myeloid blast population. Analysis of patients' clinical and biologic characteristics indicated that the presence of LOH was associated with a white blood cell count of 20 x 10(9)/L or higher but was not correlated with a shorter overall survival. The relatively low rate of LOH observed in this study compared with findings in solid tumors and in pediatric acute lymphoblastic leukemia and adult AML suggests that tumor suppressor genes are either infrequently involved in the development of pediatric de novo AML or are inactivated by such means as methylation and point mutations. Additional study is needed to determine whether these regions of LOH harbor tumor suppressor genes and whether specific regions of LOH correlate with clinical characteristics. (Blood. 2001;98:1188-1194)

Efficacy and safety of gemtuzumab ozogamicin in patients with CD33-positive acute myeloid leukemia in first relapse

PURPOSE

Three open-label, multicenter trials were conducted to evaluate the efficacy and safety of single-agent Mylotarg (gemtuzumab ozogamicin; CMA-676; Wyeth Laboratories, Philadelphia, PA), an antibody-targeted chemotherapy agent, in patients with CD33-positive acute myeloid leukemia (AML) in untreated first relapse.

PATIENTS AND METHODS

The study population comprised 142 patients with AML in first relapse with no history of an antecedent hematologic disorder and a median age of 61 years. All patients received Mylotarg as a 2-hour intravenous infusion, at a dose of 9 mg/m(2), at 2-week intervals for two doses. Patients were evaluated for remission, survival, and treatment-emergent adverse events.

RESULTS

Thirty percent of patients treated with Mylotarg obtained remission as characterized by 5% or less blasts in the marrow, recovery of neutrophils to at least 1,500/microL, and RBC and platelet transfusion independence. Although patients treated with Mylotarg had relatively high incidences of myelosuppression, grade 3 or 4 hyperbilirubinemia (23%), and elevated hepatic transaminase levels (17%), the incidences of grade 3 or 4 mucositis (4%) and infections (28%) were relatively low. There was a low incidence of severe nausea and vomiting (11%) and no treatment-related cardiotoxicity, cerebellar toxicity, or alopecia. Many patients received Mylotarg on an outpatient basis (38% and 41% of patients for the first and second doses, respectively). Among the 142 patients, the median total duration of hospitalization was 24 days; 16% of patients required 7 days of hospitalization or less.

CONCLUSION

Administration of the antibody-targeted chemotherapy agent Mylotarg to patients with CD33-positive AML in first relapse induces complete remissions with what appears to be a favorable safety profile.

Immunosuppressive effects of (131)I-anti-CD45 antibody in unsensitized and donor antigen-presensitized H2-matched, minor antigen-mismatched murine transplant models

Iodine-131-labeled anti-CD45 antibody has been added to conventional hematopoietic stem cell transplant preparative regimens to deliver targeted radiation to hematopoietic tissues, with the goal of decreasing relapse rates without increasing toxicity. However, higher radiation doses could be delivered to leukemia cells by antibody if the systemic therapy were decreased or eliminated. To examine the ability of (131)I-anti-CD45 antibody to provide sufficient immunosuppression for transplantation across allogeneic barriers, T-cell-depleted BALB.B marrow was transplanted into H2-compatible B6-Ly5(a) mice after (131)I-30F11 (rat antimurine CD45) antibody with or without varying dose levels of total body irradiation (TBI). Groups of five or six recipient mice per (131)I or TBI dose level per experiment were given tail vein injections of 100 microg of (131)I-labeled 30F11 antibody 4 days before marrow infusion, with or without TBI on day 0. Engraftment, defined as > or =50% donor B cells at 3 months posttransplant, was determined by two-color flow cytometric analysis of peripheral blood granulocytes, T cells, and B cells using antibodies specific for donor and host CD45 allotypes and for CD3. Donor engraftment of > or =80% recipient mice was achieved with either 8 Gy of TBI or 0.75 mCi of (131)I-30F11 antibody, which delivers an estimated 26 Gy to bone marrow. Subsequent experiments determined the dose of TBI alone or TBI plus 0.75 mCi of (131)I-30F11 antibody necessary for engraftment in recipient mice that had been presensitized to donor antigens before transplant, a setting requiring more stringent immunosuppression. Engraftment was seen in > or =80% of presensitized recipients surviving after 14-16 Gy of TBI or 12-14 Gy of TBI and 0.75 mCi of (131)I-30F11 antibody. However, only 28 of 69 (41%) presensitized mice receiving 10-16 Gy of TBI alone survived, presumably because of rejection of donor marrow and ablation of host hematopoiesis. In contrast, 29 of 35 (83%) presensitized mice receiving (131)I-30F11 antibody and 10-14 Gy of TBI survived, presumably because the additional immunosuppression provided by estimated radiation doses of 53 Gy to lymph nodes and 81 Gy to spleen from 0.75 mCi of (131)I-30F11 antibody permitted engraftment of donor marrow. These results suggest that targeted radiation delivered by (131)I-anti-CD45 antibody provides sufficient immunosuppression to replace an appreciable portion of the TBI dose used in matched sibling hematopoietic stem cell transplant.

Targeting of the CD33-calicheamicin immunoconjugate Mylotarg (CMA-676) in acute myeloid leukemia: in vivo and in vitro saturation and internalization by leukemic and normal myeloid cells

Antibody-targeted chemotherapy is a promising therapy in patients with acute myeloid leukemia (AML). In a phase II study of Mylotarg (CMA-676, gemtuzumab ozogamicin), which consists of a CD33 antibody linked to calicheamicin, saturation and internalization by leukemic and normal myeloid cells were analyzed in 122 patients with relapsed AML. Peripheral blood samples were obtained just before and 3 and 6 hours after the start of the first and second Mylotarg treatment cycles. Within 3 to 6 hours after infusion, near complete saturation of CD33 antigenic sites by Mylotarg was reached for AML blasts, monocytes, and granulocytes, whereas Mylotarg did not bind to lymphocytes. Saturation levels prior to the start of the second Mylotarg treatment cycle were significantly increased compared with background levels before the start of the first cycle. This apparently was caused by remaining circulating Mylotarg from the first treatment cycle (approximately 2 weeks earlier). On binding of Mylotarg to the CD33 antigen, Mylotarg was rapidly internalized, as determined by the decrease in maximal surface membrane Mylotarg binding. Internalization of Mylotarg was also demonstrated in myeloid cells in vitro and was confirmed by confocal laser microscopy. In vitro studies using pulse labeling with Mylotarg showed a continuous renewed membrane expression of CD33 antigens, which can significantly increase the internalization process and thereby the intracellular accumulation of the drug. Finally, Mylotarg induced dose-dependent apoptosis in myeloid cells in vitro. These data indicate that Mylotarg is rapidly and specifically targeted to CD33(+) cells, followed by internalization and subsequent induction of cell death.

Prevalence and prognostic significance of Flt3 internal tandem duplication in pediatric acute myeloid leukemia

The Flt3 gene encodes a tyrosine kinase receptor that regulates proliferation and differentiation of hematopoietic stem cells. An internal tandem duplication of the Flt3 gene (Flt3/ITD) has been reported in acute myelogenous leukemia (AML) and may be associated with poor prognosis. We analyzed diagnostic bone marrow specimens from 91 pediatric patients with AML treated on Children's Cancer Group (CCG)-2891 for the presence of the Flt3/ITD and correlated its presence with clinical outcome. Fifteen of 91 samples (16.5%) were positive for the Flt3/ITD. Flt3/ITD-positive patients had a median diagnostic white count of 73 800 compared with 28 400 for the Flt3/ITD-negative patients (P =.05). The size of the duplication ranged from 21 to 174 base pairs (bp). Nucleotide sequencing of the abnormal polymerase chain reaction products demonstrated that all duplications involved exon 11 of the Flt3 gene and also preserved the reading frame. Lineage restriction analysis revealed that Flt3/ITD was not present in the lymphocytes, suggesting a lack of stem cell involvement for this mutation. None of the Flt3/ITD-positive patients had unfavorable cytogenetic markers, and there was no predominance of a particular FAB class. The remission induction rate was 40% in Flt3/ITD-positive patients compared with 74% in Flt3/ITD-negative ones (P =.005). The Kaplan-Meier estimates of event-free survival at 8 years for patients with and without Flt3/ITD were 7% and 44%, respectively (P =.002). Multivariate analysis demonstrated that presence of the Flt3/ITD was the single most significant, independent prognostic factor for poor outcome (P =.009) in pediatric AML.

Immobilization of Notch ligand, Delta-1, is required for induction of notch signaling

Cell-cell interactions mediated by Notch and its ligands are known to effect many cell fate decisions in both invertebrates and vertebrates. However, the mechanisms involved in ligand induced Notch activation are unknown. Recently it was shown that, in at least some cases, endocytosis of the extracellular domain of Notch and ligand by the signaling cell is required for signal induction in the receptive cell. These results imply that soluble ligands (ligand extracellular domains) although capable of binding Notch would be unlikely to activate it. To test the potential activity of soluble Notch ligands, we generated monomeric and dimeric forms of the Notch ligand Delta-1 by fusing the extracellular domain to either a series of myc epitopes (Delta-1(ext-myc)) or to the Fc portion of human IgG-1 (Delta-1(ext-IgG)), respectively. Notch activation, assayed by inhibition of differentiation in C2 myoblasts and by HES1 transactivation in U20S cells, occurred when either Delta-1(ext-myc) or Delta-1(ext-IgG) were first immobilized on the plastic surface. However, Notch was not activated by either monomeric or dimeric ligand in solution (non-immobilized). Furthermore, both non-immobilized Delta-1(ext-myc) and Delta-1(ext-IgG) blocked the effect of immobilized Delta. These results indicate that Delta-1 extracellular domain must be immobilized to induce Notch activation in C2 or U20S cells and that non-immobilized Delta-1 extracellular domain is inhibitory to Notch function. These results imply that ligand stabilization may be essential for Notch activation.

A phase I/II trial of iodine-131-tositumomab (anti-CD20), etoposide, cyclophosphamide, and autologous stem cell transplantation for relapsed B-cell lymphomas

Relapsed B-cell lymphomas are incurable with conventional chemotherapy and radiation therapy, although a fraction of patients can be cured with high-dose chemoradiotherapy and autologous stem-cell transplantation (ASCT). We conducted a phase I/II trial to estimate the maximum tolerated dose (MTD) of iodine 131 ((131)I)-tositumomab (anti-CD20 antibody) that could be combined with etoposide and cyclophosphamide followed by ASCT in patients with relapsed B-cell lymphomas. Fifty-two patients received a trace-labeled infusion of 1.7 mg/kg (131)I-tositumomab (185-370 MBq) followed by serial quantitative gamma-camera imaging and estimation of absorbed doses of radiation to tumor sites and normal organs. Ten days later, patients received a therapeutic infusion of 1.7 mg/kg tositumomab labeled with an amount of (131)I calculated to deliver the target dose of radiation (20-27 Gy) to critical normal organs (liver, kidneys, and lungs). Patients were maintained in radiation isolation until their total-body radioactivity was less than 0.07 mSv/h at 1 m. They were then given etoposide and cyclophosphamide followed by ASCT. The MTD of (131)I-tositumomab that could be safely combined with 60 mg/kg etoposide and 100 mg/kg cyclophosphamide delivered 25 Gy to critical normal organs. The estimated overall survival (OS) and progression-free survival (PFS) of all treated patients at 2 years was 83% and 68%, respectively. These findings compare favorably with those in a nonrandomized control group of patients who underwent transplantation, external-beam total-body irradiation, and etoposide and cyclophosphamide therapy during the same period (OS of 53% and PFS of 36% at 2 years), even after adjustment for confounding variables in a multivariable analysis.

Pluripotent, cytokine-dependent, hematopoietic stem cells are immortalized by constitutive Notch1 signaling

Hematopoietic stem cells give rise to progeny that either self-renew in an undifferentiated state or lose self-renewal capabilities and commit to lymphoid or myeloid lineages. Here we evaluated whether hematopoietic stem cell self-renewal is affected by the Notch pathway. Notch signaling controls cell fate choices in both invertebrates and vertebrates by inhibiting certain differentiation pathways, thereby permitting cells to either differentiate along an alternative pathway or to self-renew. Notch receptors are present in hematopoietic precursors and Notch signaling enhances the in vitro generation of human and mouse hematopoietic precursors, determines T- or B-cell lineage specification from a common lymphoid precursor and promotes expansion of CD8(+) cells. Here, we demonstrate that constitutive Notch1 signaling in hematopoietic cells established immortalized, cytokine-dependent cell lines that generated progeny with either lymphoid or myeloid characteristics both in vitro and in vivo. These data support a role for Notch signaling in regulating hematopoietic stem cell self-renewal. Furthermore, the establishment of clonal, pluripotent cell lines provides the opportunity to assess mechanisms regulating stem cell commitment and demonstrates a general method for immortalizing stem cell populations for further analysis.

Monocytes express high amounts of Notch and undergo cytokine specific apoptosis following interaction with the Notch ligand, Delta-1

Notch signaling has been shown to play a key role in cell fate decisions in numerous developmental systems. Using a reverse transcriptase-polymerase chain reaction (RT-PCR) assay, we reported the expression of human Notch-1 in CD34+ progenitors. In this study, we evaluated the expression of human Notch-1 and Notch-2 protein by hematopoietic cells. In immunofluoresence study, we detected low amounts of Notch-1 and Notch-2 protein in both CD34+ and CD34+Lin- cells, high amounts in CD14+ monocytes as well as B and T cells, but no expression in CD15+ granulocytes. We further found that an immobilized truncated form of the Notch ligand, Delta-1, induced apoptosis in monocytes in the presence of macrophage colony-stimulating factor (M-CSF), but not granulocyte-macrophage colony-stimulating factor (GM-CSF). The widespread expressions of Notch proteins suggest multiple functions for this receptor during hematopoiesis. These studies further indicate a novel role for Notch in regulating monocyte survival. (Blood. 2000;95:2847-2854)

Monoclonal antibodies to the myeloid stem cells: therapeutic implications of CMA-676, a humanized anti-CD33 antibody calicheamicin conjugate

There are several competing models of stem cell involvement in acute myeloid leukemia (AML). At issue is whether the disease origin is in the pluripotent stem cell or whether it arises later in a more mature progenitor cell. The observation that the CD33 antigen is present on AML cells, and on normal and leukemic progenitors, suggested that one might be able to target these cells while sparing the normal stem cells. Response rates of acute myelogenous leukemia patients treated with the newly developed anti-CD33 antibody-calicheamicin conjugate suggest that at least for a proportion of patients early precursors responsible for re-establishing hematopoiesis are likely to be predominantly normal in origin.

All-trans retinoic acid enhances the long-term repopulating activity of cultured hematopoietic stem cells

The retinoic acid receptor (RAR) agonist, all-trans retinoic acid (ATRA), is a potent inducer of terminal differentiation of malignant promyelocytes, but its effects on more primitive hematopoietic progenitors and stem cells are less clear. We previously reported that pharmacologic levels (1 micromol) of ATRA enhanced the generation of colony-forming cell (CFC) and colony-forming unit-spleen (CFU-S) in liquid suspension cultures of lin- c-kit+ Sca-1+ murine hematopoietic precursors. In this study, we further investigated the effects of ATRA as well as an RAR antagonist, AGN 193109, on the generation of transplantable cells, including pre-CFU-S, short-term repopulating stem cells (STRCs), and long-term repopulating stem cells (LTRCs). ATRA enhanced the ex vivo maintenance and production of competitive repopulating STRCs and LTRCs from lin- c-kit+ Sca-1+ cells cultured in liquid suspension for 14 days. In addition, ATRA prevented the differentiation of these primitive stem cells into more mature pre-CFU-S during the 14 days of culture. In marked contrast, lin- c-kit+ Sca-1+ cells cultured with AGN 193109 for 7 days had virtually no short- or long-term repopulating ability, but displayed an approximately 6-fold increase in the pre-CFU-S population. The data suggest that the RAR agonist ATRA enhances the maintenance and self-renewal of short- and long-term repopulating stem cells. In contrast, the RAR antagonist AGN 193109 abrogates reconstituting ability, most likely by promoting the differentiation of the primitive stem cells. These results imply an important and unexpected role of retinoids in regulating hematopoietic stem cell differentiation. (Blood. 2000;95:470-477)

Phase I study of (131)I-anti-CD45 antibody plus cyclophosphamide and total body irradiation for advanced acute leukemia and myelodysplastic syndrome

Delivery of targeted hematopoietic irradiation using radiolabeled monoclonal antibody may improve the outcome of marrow transplantation for advanced acute leukemia by decreasing relapse without increasing toxicity. We conducted a phase I study that examined the biodistribution of (131)I-labeled anti-CD45 antibody and determined the toxicity of escalating doses of targeted radiation combined with 120 mg/kg cyclophosphamide (CY) and 12 Gy total body irradiation (TBI) followed by HLA-matched related allogeneic or autologous transplant. Forty-four patients with advanced acute leukemia or myelodysplasia received a biodistribution dose of 0.5 mg/kg (131)I-BC8 (murine anti-CD45) antibody. The mean +/- SEM estimated radiation absorbed dose (centigray per millicurie of (131)I) delivered to bone marrow and spleen was 6.5 +/- 0.5 and 13.5 +/- 1.3, respectively, with liver, lung, kidney, and total body receiving lower amounts of 2.8 +/- 0.2, 1.8 +/- 0.1, 0.6 +/- 0.04, and 0.4 +/- 0.02, respectively. Thirty-seven patients (84%) had favorable biodistribution of antibody, with a higher estimated radiation absorbed dose to marrow and spleen than to normal organs. Thirty-four patients received a therapeutic dose of (131)I-antibody labeled with 76 to 612 mCi (131)I to deliver estimated radiation absorbed doses to liver (normal organ receiving the highest dose) of 3.5 Gy (level 1) to 12.25 Gy (level 6) in addition to CY and TBI. The maximum tolerated dose was level 5 (delivering 10.5 Gy to liver), with grade III/IV mucositis in 2 of 2 patients treated at level 6. Of 25 treated patients with acute myeloid leukemia (AML)/myelodysplastic syndrome (MDS), 7 survive disease-free 15 to 89 months (median, 65 months) posttransplant. Of 9 treated patients with acute lymphoblastic leukemia (ALL), 3 survive disease-free 19, 54, and 66 months posttransplant. We conclude that (131)I-anti-CD45 antibody can safely deliver substantial supplemental doses of radiation to bone marrow (approximately 24 Gy) and spleen (approximately 50 Gy) when combined with conventional CY/TBI.

All-trans retinoic acid delays the differentiation of primitive hematopoietic precursors (lin-c-kit+Sca-1(+)) while enhancing the terminal maturation of committed granulocyte/monocyte progenitors

All-trans retinoic acid (ATRA) is a potent inducer of terminal differentiation of malignant promyelocytes, but its effects on more primitive hematopoietic progenitors and stem cells are less clear. In this study, we investigated the effect of ATRA on highly enriched murine hematopoietic precursor cells (lin-c-kit+Sca-1(+)) grown in liquid suspension culture for 28 days. ATRA initially slowed the growth of these hematopoietic precursors but prolonged and markedly enhanced their colony-forming cell production compared with the hematopoietic precursors cultured in its absence. At 7 and 14 days of culture, a substantially greater percentage of cells cultured with ATRA did not express lineage-associated antigens (55.4% at day 7 and 68.6% at day 14) and retained expression of Sca-1 (44.7% at day 7 and 79.9% at day 14) compared with cells grown in its absence (lin- cells: 31.5% at day 7 and 4% at day 14; Sca-1(+): 10.4% at day 7 and 0.7% at day 14). Moreover, a marked inhibition of granulocyte production was observed in cultures continuously incubated with ATRA. Significantly, ATRA markedly prolonged and enhanced the production of transplantable colony-forming unit-spleen (CFU-S) during 14 days of liquid suspension culture. In contrast with its effects on primitive lin-c-kit+Sca-1(+) hematopoietic precursors, ATRA did not exert the same effects on the more committed lin-c-kit+Sca-1(-) progenitor cells. Moreover, the late addition of ATRA (7 days post-culture initiation) to cultures of primitive hematopoietic precursors resulted in a marked decrease in colony-forming cell production in these cultures, which was associated with enhanced granulocyte differentiation. These observations indicate that ATRA has different effects on hematopoietic cells depending on their maturational state, preventing and/or delaying the differentiation of primitive hematopoietic precursors while enhancing the terminal differentiation of committed granulocyte/monocyte progenitors.

Selective ablation of acute myeloid leukemia using antibody-targeted chemotherapy: a phase I study of an anti-CD33 calicheamicin immunoconjugate

Leukemic blast cells express the CD33 antigen in most patients with acute myeloid leukemia (AML), but this antigen is not expressed by hematopoietic stem cells. We conducted a study to determine whether normal hematopoiesis could be restored in patients with AML by selective ablation of cells expressing the CD33 antigen. In a dose escalation study, 40 patients with relapsed or refractory CD33(+) AML were treated with an immunoconjugate (CMA-676) consisting of humanized anti-CD33 antibody linked to the potent antitumor antibiotic calicheamicin. The capacity of leukemic cells to efflux 3, 3'-diethyloxacarbocyanine iodide (DiOC2) was used to estimate pretreatment functional drug resistance. Leukemia was eliminated from the blood and marrow of 8 (20%) of the 40 patients; blood counts returned to normal in three (8%) patients. A high rate of clinical response was observed in leukemias characterized by low dye efflux in vitro. Infusions of CMA-676 were generally well tolerated, and a postinfusion syndrome of fever and chills was the most common toxic effect. Two patients who were treated at the highest dose level (9 mg/m2) were neutropenic >5 weeks after the last dose of CMA-676. These results show that an immunoconjugate targeted to CD33 can selectively ablate malignant hematopoiesis in some patients with AML.

Marrow ablative and immunosuppressive effects of 131I-anti-CD45 antibody in congenic and H2-mismatched murine transplant models

Targeted hematopoietic irradiation delivered by 131I-anti-CD45 antibody has been combined with conventional marrow transplant preparative regimens in an effort to decrease relapse. Before increasing the proportion of therapy delivered by radiolabeled antibody, the myeloablative and immunosuppressive effects of such low dose rate irradiation must be quantitated. We have examined the ability of 131I-anti-CD45 antibody to facilitate engraftment in Ly5-congenic and H2-mismatched murine marrow transplant models. Recipient B6-Ly5(a) mice were treated with 30F11 antibody labeled with 0.1 to 1.5 mCi 131I and/or total body irradiation (TBI), followed by T-cell-depleted marrow from Ly5(b)-congenic (C57BL/6) or H2-mismatched (BALB/c) donors. Engraftment was achieved readily in the Ly5-congenic setting, with greater than 80% donor granulocytes and T cells after 0.5 mCi 131I (estimated 17 Gy to marrow) or 8 Gy TBI. A higher TBI dose (14 Gy) was required to achieve engraftment of H2-mismatched marrow, and engraftment occurred in only 3 of 11 mice receiving 1.5 mCi 131I delivered by anti-CD45 antibody. Engraftment of H2-mismatched marrow was achieved in 22 of 23 animals receiving 0.75 mCi 131I delivered by anti-CD45 antibody combined with 8 Gy TBI. Thus, targeted radiation delivered via 131I-anti-CD45 antibody can enable engraftment of congenic marrow and can partially replace TBI when transplanting T-cell-depleted H2-mismatched marrow.

Follow-up of relapsed B-cell lymphoma patients treated with iodine-131-labeled anti-CD20 antibody and autologous stem-cell rescue

PURPOSE

Radioimmunotherapy (RIT) is a promising treatment approach for B-cell lymphomas. This is our first opportunity to report long-term follow-up data and late toxicities in 29 patients treated with myeloablative doses of iodine-131-anti-CD20 antibody (anti-B1) and autologous stem-cell rescue.

PATIENTS AND METHODS

Trace-labeled biodistribution studies first determined the ability to deliver higher absorbed radiation doses to tumor sites than to lung, liver, or kidney at varying amounts of anti-B1 protein (0.35, 1.7, or 7 mg/kg). Twenty-nine patients received therapeutic infusions of single-agent (131)I-anti-B1, given at the protein dose found optimal in the biodistribution study, labeled with amounts of (131)I (280 to 785 mCi [10.4 to 29.0 GBq]) calculated to deliver specific absorbed radiation doses to the normal organs, followed by autologous stem-cell support.

RESULTS

Major responses occurred in 25 patients (86%), with 23 complete responses (CRs; 79%). The nonhematopoietic dose-limiting toxicity was reversible cardiopulmonary insufficiency, which occurred in two patients at RIT doses that delivered > or = 27 Gy to the lungs. With a median follow-up time of 42 months, the estimated overall and progression-free survival rates are 68% and 42%, respectively. Currently, 14 of 29 patients remain in unmaintained remissions that range from 27+ to 87+ months after RIT. Late toxicities have been uncommon except for elevated thyroid-stimulating hormone (TSH) levels found in approximately 60% of the subjects. Two patients developed second malignancies, but none have developed myelodysplasia (MDS).

CONCLUSION

Myeloablative (131)I-anti-B1 RIT is relatively well tolerated when given with autologous stem-cell support and often results in prolonged remission durations with few late toxicities.

The Notch ligand, Jagged-1, influences the development of primitive hematopoietic precursor cells

We examined the expression of two members of the Notch family, Notch-1 and Notch-2, and one Notch ligand, Jagged-1, in hematopoietic cells. Both Notch-1 and Notch-2 were detected in murine marrow precursors (Lin-Sca-1+c-kit+). The Notch ligand, Jagged-1, was not detected in whole marrow or in precursors. However, Jagged-1 was seen in cultured primary murine fetal liver stroma, cultured primary murine bone marrow stroma, and in stromal cell lines. These results indicate a potential role for Notch-Notch ligand interactions in hematopoiesis. To further test this possibility, the effect of Jagged-1 on murine marrow precursor cells was assessed by coculturing sorted precursor cells (Lin-Sca-1+c-kit+) with a 3T3 cell layer that expressed human Jagged-1 or by incubating sorted precursors with beads coated with the purified extracellular domain of human Jagged-1 (Jagged-1(ext)). We found that Jagged-1, presented both on the cell surface and on beads, promoted a twofold to threefold increase in the formation of primitive precursor cell populations. These results suggest a potential use for Notch ligands in expanding precursor cell populations in vitro.

Importance of pre-treatment radiation absorbed dose estimation for radioimmunotherapy of non-Hodgkin's lymphoma

Non-Hodgkin's lymphoma I-131 radioimmunotherapy data were analyzed to determine whether a predictive relationship exists between radiation absorbed doses calculated from biodistribution studies and doses derived from patient size. Radioactivity treatment administrations scaled to patient size (MBq/kg or MBq/m2) or fixed MBq doses do not produce consistent radiation absorbed dose to critical organs. Treatment trials that do not provide dose estimates for critical normal organs are less likely to succeed in identifying a clinical role for radioimmunotherapy.

Normal immunologic response to a neoantigen, bacteriophage phiX-174, in baboons with long-term lymphohematopoietic reconstitution from highly purified CD34+ Lin- allogeneic marrow cells

The CD34 antigen is thought to be expressed by hematopoietic stem cells in adult humans and nonhuman primates. We present data that baboons transplanted with highly purified allogeneic CD34+ marrow cells devoid of detectable mature and immature T and B lymphocytes and myeloid cells, isolated from sex-mismatched mixed lymphocyte culture (MLC) nonreactive siblings, have maintained stable lymphohematopoietic engraftment with donor cells for greater than 4.9, greater than 6.0, and 5.0 years. Cytogenetic analysis of unfractionated marrow and peripheral blood cells at multiple time points after transplantation show virtually all donor cells in two animals and stable mixed chimerism in the third. We used polymerase chain reaction to show that colony-forming unit-granulocyte-macrophage, burst-forming unit-erythroid, and high proliferative potential colony-forming cells (HPP-CFC) were virtually all of donor origin in two animals and present at lower levels in the stable mixed chimera. CD20+ B-lymphoblastoid cell lines derived by Herpesvirus Papio transformation of peripheral blood cells were virtually all donor in two animals and 50% donor in the mixed chimera. CD4+ and CD8+ T cells and neutrophils purified from the peripheral blood of the two female animals also were all donor-derived. To assess immunologic function after transplantation, we immunized the three long-term chimeric animals and two normal control animals with bacteriophage phiX-174, a neoantigen that requires the interaction of antigen-presenting cells, T lymphocytes, and B lymphocytes to mount a normal antibody response. Experimental and control animals, when immunized with bacteriophage, had similar serum Ig levels. The experimental and control animals generated similar titers of antibacteriophage antibodies after primary and secondary immunizations with evidence of amplification and class switching. These findings further support the hypothesis that the CD34+ antigen is expressed on hematopoietic stem cells that can mediate stable long-term lymphohematopoiesis in vivo and, importantly, that normal immunologic function can be reconstituted in vivo after transplantation of the highly purified CD34+ Lin- cells alone.

MLL gene rearrangement, cytogenetic 11q23 abnormalities, and expression of the NG2 molecule in infant acute myeloid leukemia

To study prognostic factors in infant acute myeloid leukemia (AML), we analyzed 44 children treated on Childrens Cancer Group protocols for MLL gene rearrangement by Southern blot, cytogenetic 11q23 abnormalities, and reactivity with monoclonal antibody 7.1. This antibody detects the human homologue of the rat NG2 chondroitin sulfate proteoglycan molecule, which has previously been reported to be expressed on human melanoma. NG2 has been found to be expressed on human leukemic blasts but not on other hematopoietic cells. In childhood AML, NG2 cell surface expression correlated with poor outcome and with some but not all 11q23 rearrangements. In childhood acute lymphoblastic leukemia, NG2 expression correlated with poor outcome and with balanced 11q23 translocations. In this study, 29 of 44 (66%) of infants with AML showed MLL rearrangement and, as expected, this group had a high incidence of French-American-British M4/M5 morphology (22/29). Of the cases tested, 35.1% (13/37) were NG2 positive. All (13/13) NG2-positive cases were rearranged at MLL, whereas only 46% (11/24) of NG2-negative cases had MLL rearrangement. NG2 expression did not correlate with poor outcome (P = .31); there was a trend towards a worse outcome with MLL rearrangement (P = .13). Thus monoclonal antibody 7.1 does not detect all cases of MLL rearrangement in infant AML.

Inhibition of granulocytic differentiation by mNotch1

Effective hematopoiesis requires the commitment of pluripotent and multipotent stem cells to distinct differentiation pathways, proliferation and maturation of cells in the various lineages, and preservation of pluripotent progenitors to provide continuous renewal of mature blood cells. While the importance of positive and negative cytokines in regulating proliferation and maturation of hematopoietic cells has been well documented, the factors and molecular processes involved in lineage commitment and self-renewal of multipotent progenitors have not yet been defined. In other developmental systems, cellular interactions mediated by members of the Notch gene family have been shown to influence cell fate determination by multipotent progenitors. We previously described the expression of the human Notch1 homolog, TAN-1, in immature hematopoietic precursors. We now demonstrate that constitutive expression of the activated intracellular domain of mouse Notch1 in 32D myeloid progenitors inhibits granulocytic differentiation and permits expansion of undifferentiated cells, findings consistent with the known function of Notch in other systems.

The role of radioimmunotherapy in bone marrow transplantation

Radioimmunotherapy offers an exciting new therapeutic modality for patients with recurrent hematologic malignancies and solid tumors resistant to conventional chemotherapy. In this review, a brief overview of tumor radiobiology as well as various obstacles to treatment is presented. Early radiolabeled antibody trials documented myelosuppression as the dose-limiting toxicity. Ongoing trials in solid tumors and hematologic malignancies are testing the hypothesis that myeloablative doses of radiation in conjunction with hematopoietic stem cell rescue will improve long-term survival. For solid tumors, there are many barriers to achieving this goal. The most encouraging trials in metastatic breast cancer have documented significant symptomatic relief and a 50% partial response in patients. In contrast, trials involving hematologic malignancies have produced more impressive results. With a median follow-up of 33 months, 67% of patients with recurrent acute myelogenous leukemia or myelodysplasia treated with radiolabeled antibodies, total-body irradiation, and high-dose chemotherapy remain disease free. Alone, myeloablative doses of radioimmunotherapy have documented a 41% complete response in patients with Hodgkin's disease. Seattle trials with recurrent non-Hodgkin's lymphoma have demonstrated objective responses in 90% of patients, complete responses in 85% of patients, a progression-free survival of 62%, and an overall survival of 93% with a median follow-up of 2 years.

Prediction of relapse of pediatric acute myeloid leukemia by use of multidimensional flow cytometry

BACKGROUND

Most patients receiving therapy for acute myeloid leukemia (AML) enter an interval in which leukemic blast cells cannot be detected by light microscopy (i.e., morphologic remission). However, many of these patients experience a subsequent relapse. Multidimensional flow cytometry, which allows the discrimination of antigens expressed on normal and malignant cells, can detect small numbers of cancer cells in bone marrow or peripheral blood specimens. This technique enables the detection of one leukemic blast cell among 10(3) to 10(2) normal regenerating hematopoietic cells.

PURPOSE

We determined whether the presence of residual leukemic blast cells, identified in the bone marrow of pediatric patients with AML by use of multidimensional flow cytometry, would be predictive of subsequent leukemic relapse.

METHODS

Multidimensional flow cytometry was performed on 205 marrow specimens collected throughout the course of treatment from 39 patients who had achieved morphologic remission. The analyses employed monoclonal antibodies directed against CD45 in combination with mixed pairs of monoclonal antibodies directed against 10 other antigens. A time-varying Cox regression analysis that controlled for sample time intervals, age, sex, morphologic classification of disease, and white blood cell count at diagnosis was used to relate the multidimensional flow cytometric results to the risk of relapse after achieving remission. Reported P values are two-sided.

RESULTS

Thirty-five of the 39 patients had bone marrow specimens available from the time that first morphologic remission was achieved. Leukemic blast cells were detected in the specimens from 19 (54%) of these 35 patients. Twenty-five of the 35 patients did not receive an allogeneic (i.e., from a different genetic background) bone marrow transplant during first morphologic remission, and 13 of 14 with residual leukemic cells experienced a relapse at a median time of 153 days after diagnosis (range, 48-863 days). Nine of the 11 patients who did not receive an allogeneic bone marrow transplant and lacked evidence of leukemic blast cells at first morphologic remission relapsed at a median time of 413 days after diagnosis (range, 321-794 days). Among the 10 individuals who received an allogeneic bone marrow transplant during first morphologic remission, five were positive for leukemic blast cells and five were negative; one of these patients (positive for leukemic blast cells) experienced a relapse 265 days after diagnosis, and three others died of transplant-related complications. The estimated risk of relapse during intervals of multidimensional flow cytometric positivity (i.e., intervals of remission for which the immediately preceding cytometry measurement was positive) was 2.8 times greater than that during negative intervals (95% confidence interval = 1.1-7.0; P = .02).

CONCLUSIONS AND IMPLICATIONS

Multidimensional flow cytometry identifies residual leukemia in more than half of the patients with AML who are in morphologic remission. The detection of leukemic blast cells in these patients by multidimensional flow cytometry is predictive of a more rapid relapse.

Regulation of colony forming cell generation by flt-3 ligand

The recently cloned ligand for the flt-3/flk-2 receptor was examined for its effect on colony formation by subpopulations of CD34+ cells including the least mature CD34+lin-CD38- small-medium lymphocyte-sized cell population. Flt-3 ligand (flt-3l) had little or no effect when added alone to cells. Isolated CD34+lin+ cells formed increased numbers of colony-forming cells (CFC) when flt-3l was added together with IL-3, IL-6, G-CSF, GM-CSF or c-kit ligand (KL), or with the combination of IL-3 and KL. Significant increases in CFC formation from CD34+lin- cells were consistently seen when flt-3l was added to the IL-3 and KL combination, with variable effects observed when it was added to individual growth factors. Studies of the generation of CFC from CD34+lin- cells in liquid cultures showed that cultures containing IL-3 and KL continued to produce CFC after 3 weeks of culture, whereas cultures with IL-3, KL and flt-3l produced few CFC past 2 weeks of culture. Flt-3l alone or the combination of IL-3 and KL did not stimulate significant growth of CD34+lin-CD38- small-medium lymphocyte-sized cells, although these cells reproducibly generated CFC when grown in the combination of IL-1 beta, IL-3, IL-6, G-CSF, GM-CSF and KL. Addition of flt-3l to either IL-3 and KL or to a combination of growth factors induced increased CFC in three of four experiments. These data therefore demonstrate a role for flt-3l in the induction of myelopoiesis by haemopoietic precursors, including the least mature subpopulation population of CD34+ cells.

Comparative metabolism and retention of iodine-125, yttrium-90, and indium-111 radioimmunoconjugates by cancer cells

Radiolabeled antibodies have produced encouraging remissions in patients with chemotherapy-resistant hematological malignancies; however, the selection of therapeutic radionuclides for clinical trials remains controversial. In this study, we compared the internalization, lysosomal targeting, metabolism, and cellular retention of radiolabeled murine and humanized monoclonal antibodies targeting the CD33 antigen (monoclonal antibodies mP67 and hP67, respectively) on myeloid leukemia cell lines (HEL and HL-60) and of anti-carcinoma antibodies (monoclonal antibodies hCTM01 and hA33) targeting breast cancer and colorectal carcinoma cell lines (MCF7 and Colo 205, respectively). Each antibody was labeled with 125I (by the IodoGen method) and with 111In and 90Y using macrocyclic chelation technology. Targeted tumor cells were analyzed for retention and metabolism of radioimmunoconjugates using cellular-radioimmunoassays, Percoll gradient fractionation of cell organelles, SDS-PAGE, and TLC of cell lysates and culture supernatants. Our results suggest that antibodies are routed to lysosomes after endocytosis, where they are proteolytically degraded. [125I]monoiodotyrosine is rapidly excreted from cells after lysosomal catabolism of antibodies radioiodinated by conventional methods, whereas small molecular weight 111In and 90Y catabolites remain trapped in lysosomes. As a consequence of the differential disposition of small molecular weight catabolites, 111In and 90Y conjugates displayed superior retention of radioactivity compared with 125I conjugates when tumor cells were targeted using rapidly internalizing antibody-antigen systems (e.g., hP67 with HEL cells and hCTM01 with MCF7 cells). When tumor cells were targeted using antibody-antigen systems exhibiting slow rates of endocytosis (e.g., hP67 on HL-60 cells and hA33 on Colo 205 cells), little differences in cellular retention of radioactivity was observed, regardless of whether 125I, 111In, or 90Y was used.

The human homologue of rat NG2, a chondroitin sulfate proteoglycan, is not expressed on the cell surface of normal hematopoietic cells but is expressed by acute myeloid leukemia blasts from poor-prognosis patients with abnormalities of chromosome band 11q23

In our efforts to produce monoclonal antibodies that recognize cell-surface antigens expressed by hematopoietic precursor and stromal cells, we generated a monoclonal antibody, 7.1, which recognizes a 220- to 240-kD cell-surface protein whose N-terminal amino acid sequence is identical to the rat NG2 chondroitin sulfate proteoglycan molecule. This chondroitin sulfate proteoglycan, previously reported to be expressed by human melanoma cells, was not found to be expressed by normal hematopoietic cells, nor was it expressed on the cell surface of cell lines of hematopoietic origin including cell lines with 11q23 abnormalities. It was found on the cell surface of acute myeloid leukemia (AML) blasts and cell lines derived from nonhematopoietic tissues. Samples of leukemic marrow from 166 children with AML enrolled on Childrens Cancer Group protocol 213 were evaluated for cell-surface expression of this proteoglycan molecule. In 18 of 166 (11%) patient samples, greater than 25% of leukemic blasts expressed the NG2 molecule. These 18 patients had a poorer outcome with respect to survival (P = .002) and event-free survival (P = .035) with an actuarial survival at 4 years of 16.7%. Blast cell expression of the NG2 molecule was strongly associated with French-American-British M5 morphology (P < .0001) and abnormalities in chromosome band 11q23, site of the MLL gene. These results show that the NG2 molecule is expressed by malignant hematopoietic cells that have abnormalities in chromosome band 11q23, suggesting that antibody 7.1 may be useful in the rapid identification of this group of poor-prognosis patients.

Human homologue of the rat chondroitin sulfate proteoglycan, NG2, detected by monoclonal antibody 7.1, identifies childhood acute lymphoblastic leukemias with t(4;11)(q21;q23) or t(11;19)(q23;p13) and MLL gene rearrangements

Monoclonal antibody 7.1, which recognizes the chondroitin sulfate proteoglycan molecule NG2, was used to screen prospectively blast cells from 104 consecutive children at initial presentation with acute lymphoblastic leukemia (ALL). Reactivity with this antibody was found in 9 cases (8.6%), of whom 5 had a t(4;11)(q21;q23) and 4 had a t(11;19)(p13;q23). None of the NG2- cases had either translocation. Southern blot analysis disclosed MLL gene rearrangement in only the 9 cases with 7.1 reactivity plus the t(4;11)(q21;q23) or t(11;19)(q23;p13) translocation. MLL gene rearrangements were not detected in 89 patient leukemic samples that did not express NG2, including 7 patients with del(11)(q23) or inv(11)(p13q23). As expected from the association with t(4;11) and t(11;19), NG2+ cases were significantly more likely to be infants, to have hyperleukocytosis and central nervous system involvement, to be CD10-, and to express myeloid-associated antigens CD15 and CD65. Despite short follow-up duration, 3 of the NG2+ cases have relapsed while the other 101 patients remain in remission. Thus, blast cell surface expression of NG2 is useful for identifying patients with ALL having t(4;11) or t(11;19) translocations that are associated with poor prognosis, especially in the infant age group.

Effects of flt3 ligand on acute myeloid and lymphocytic leukemic blast cells from children

A ligand for the flt3 tyrosine kinase receptor (flt3R) has recently been cloned. Forty-three cases of childhood acute myeloid leukemia (AML) and 27 cases of childhood acute lymphocytic leukemia (ALL) were examined by flow cytometric analysis for cell-surface flt3R and proliferative response in vitro to flt3 ligand (flt3L). Flt3R was commonly expressed on the cell surface of leukemic cells from all AML subclasses and B-ALL, but we did not detect cell-surface flt3R on T-ALL. Flt3L alone induced the proliferation of the monocytic AML-M5 cells and some erythroleukemic AML-M6 cells. Some isolated instances of weak proliferative responses were also noted in other AML subclasses. Interleukin-4 (IL-4) alone inhibited the proliferation of a group of AML-M5 cells and, when combined with flt3L, suppressed the proliferative effect of flt3L. In general, B-ALL and T-ALL cells failed to respond to flt3L alone or in the presence of combinations of IL-2, IL-3, or IL-7.

Cell surface expression of the multidrug resistance P-glycoprotein (P-170) as detected by monoclonal antibody MRK-16 in pediatric acute myeloid leukemia fails to define a poor prognostic group: a report from the Childrens Cancer Group

Expression of the multidrug resistance (MDR-1) gene product, P-glycoprotein (P-170), and the stem cell antigen, CD34, at diagnosis were determined using monoclonal antibodies (MoAbs) MRK-16 and 12.8 respectively, in 130 pediatric acute myeloid leukemia (AML) patients entered onto Childrens Cancer Group (CCG) study CCG-2891. Fluorescein isothiocyanate (FITC) as a second step reagent was employed for the measurement of P-170 expression since it is commonly used in clinical laboratories. Nine of 30 (30%) infant ( < 1 year of age) de novo specimens expressed P-170 at levels > or = 20% of control cells. In contrast, eight of 100 (8%) AML samples from older children ( > or = 1 year of age) expressed the multidrug resistance surface protein at diagnosis. With the exception of one infant, all de novo samples that expressed P-170 also expressed CD34. Pediatric patients of any age with positive P-170 expression using MoAb MRK-16 with a FITC-conjugated second step reagent fared no worse than remaining patients treated on the same treatment with regard to induction failure, incidence of relapse, event-free survival, or overall survival. Further investigation is necessary to determine whether P-170 assay systems with greater sensitivity will distinguish pediatric AML patients with poor prognosis.

Primitive human hematopoietic precursors express Bcl-x but not Bcl-2

Bcl-2 and its homologue, bcl-xL, encode membrane-associated proteins that suppress programmed cell death of hematopoietic cell lines after growth factor withdrawal, and are expressed in hematopoietic precursor cells. To better understand the maintenance of long-term survival in the hematopoietic stem cell population, we evaluated the expression patterns of Bcl-2 and Bcl-x in primitive hematopoietic precursor populations. Hematopoietic precursor cells expressing CD34 (CD34+) and lacking maturation-linked surface antigens (lin-) were isolated from adult human bone marrow using two-color immunofluorescence cell sorting and fractionated on the basis of forward light scatter characteristics into blast-sized and small to medium lymphocyte-sized cell populations. Bcl-2 expression was shown in 78% to 90% of CD34+ lin- blast-sized cells versus less than 10% of small to medium lymphocyte-sized CD34+ lin- cells by immunohistochemical analysis. Small to medium lymphocyte-sized CD34+ lin- cells were further enriched for primitive precursors by selecting cells that lacked expression of CD38 (CD34+ lin- CD38-). In parallel experiments, only 1% to 4% of CD34+ lin- CD38- cells expressed Bcl-2, whereas 45% to 56% of these cells generated colony-forming cells. In contrast, > or = 94% of cells in all bone marrow subpopulations studied expressed Bcl-x protein. Both alternatively spliced bcl-x transcripts, bcl-xL and bcl-xs, were present. Our data show that the most primitive hematopoietic precursors express Bcl-x but not Bcl-2. Thus, the functional bcl-2 homologue, bcl-xL, may be essential for the long-term survival of the hematopoietic stem cell population.

Generation of hematopoietic colony-forming cells from embryonic stem cells: synergy between a soluble factor from NIH-3T3 cells and hematopoietic growth factors

Murine embryonic stem cells are able to differentiate into embryoid bodies (EBs) in vitro in the absence of leukemia-inhibitory factor with the formation of different types of hematopoietic precursors within these EBs. With the aim of determining the in vitro requirements for the continued development of hematopoietic colony-forming cells (CFCs) and their progeny from embryonic stem-derived cells, cells from EBs disrupted after 9 days of formation in the absence of leukemia-inhibitor factor were cultured under different conditions. Low numbers of day-9 EB cells (5 x 10(5) or less) cultured in the presence of several growth factors (interleukin-3 [IL-3], IL-1, c-kit ligand, basic fibroblast growth factor, insulin growth factor-1, IL-6, granulocyte colony-stimulating factor, fetal liver kinase-2 ligand) develop few or no CFCs after 1 week of culture. When these cells are plated on irradiated NIH-3T3 with IL-3 or c-kit ligand or combinations containing these and other growth factors, they are able to generate CFCs for at least 3 weeks. These cultures were found to include granulocytic, monocytic, erythrocytic, and megakaryocytic cells. Transwell cultures in which NIH-3T3 cells were separated from the EB cells and cultures in which cells were replaced by NIH-3T3 conditioned medium showed that the interaction between EB-derived cells and NIH-3T3 is via a soluble factor(s). These studies show that maximal generation of hematopoietic CFCs from precursors present in day-9 EBs is stimulated by a combination of known hematopoietic growth factors and a soluble factor(s) produced by NIH-3T3 cells.