Implantation and maintenance of functional human bone marrow in SCID-hu mice.

Human fetal bone fragments implanted in the immunodeficient C.B-17 scid/scid (SCID) mouse were shown to sustain active human hematopoiesis in vivo. Human progenitor cell activity was maintained for as long as 20 weeks after implantation and was associated with multilineage differentiation in the engrafted bone. Thus, the bone implants provided stem cells as well as the microenvironment requisite for their long-term maintenance and multilineage differentiation. Administration of human erythropoietin (Epo) stimulated human erythropoiesis in human bone implants. This animal model may facilitate direct analysis of a wide variety of physiologic and pathologic conditions of human bone marrow (BM) in vivo.     

Long-term human hematopoiesis in SCID-hu mice bearing transplanted fragments of adult bone and bone marrow cells

An attempt was made to establish an SCID-hu murine model of long-term human hematopoiesis by coimplantation of a bone fragment and bone marrow (BM) cells from an adult human. The SCID-hu mice were treated with a cytokine mixture (recombinant human stem cell factor, interleukin-3, granulocyte/macrophage colony-stimulating factor, and granulocyte colony-stimulating factor) for 4 months and were then maintained for further 8 months under cytokine-free conditions. In the peripheral blood, spleen, and implanted bone fragments in the SCID-hu mice that had received both a bone fragment and BM cells, human CD59+ cells were detected 1 year after transplantation; however, they were not detectable in SCID-hu mice that had received either a bone fragment or BM cells only. Thus, implantation of both a bone fragment and BM cells appears to provide a model of long-term adult human hematopoiesis in SCID mice.     

Growth of primary T-cell non-Hodgkin's lymphomata in SCID-hu mice: requirement for a human lymphoid microenvironment.

We reasoned that the SCID-hu mouse could provide an appropriate lymphoid or stromal microenvironment to support the growth of primary human lymphoma. Heterotransplantation of nine cases of primary T-cell non-Hodgkin's lymphoma (NHL) into untreated SCID mice and SCID mice reconstituted with human fetal thymus, spleen, and liver (SCID-hu) resulted in the development of lymphoid tumors in five (56%) cases. Two clonal T-cell NHL grew after a mean of 90 days after injection of primary lymphoma cell suspensions into the thymus xenografts in SCID-hu mice and failed to grow in a variety of sites in SCID mice, except for small tumors that developed after a long (157-day) latency period after intracranial injection of tumor cell suspensions into weanling SCID mice. Successful serial transplantation of NHL in SCID and SCID-hu mice required the presence of a human lymphoid or tumor microenvironment, and was enhanced by pretreating the SCID mice with 175 rad radiation and antiasialo antisera. Analysis of the primary and transplanted T-cell tumors showed identical patterns of T-cell surface markers by flow cytometry and immunophenotyping of fixed tissue sections, and, in one case, reactivity with a specific monoclonal antibody to V beta 5.1. Genotyping of the transplanted tumors showed T-cell receptor gene rearrangements identical to those present in the primary tumors. In one case, the presence of Epstein-Barr virus-positive B cells in association with the primary tumor resulted in the growth of a lymphoblastoid B-cell neoplasm in addition to the malignant T-cell lymphoma after transplantation of tumor fragments to SCID mice. The data support the hypothesis that a human lymphoid microenvironment enhances the growth of T-cell NHL in SCID mice. The SCID-hu thymus graft provides an apparently unique microenvironment that supports the growth of primary T-cell NHL, and can be used to study the interaction between lymphoma cells, nontransformed lymphoid cells, and the surrounding stromal microenvironment in vivo.     

Propagation of human blastic myeloid leukemias in the SCID mouse.

Existing in vitro culture technology does not permit the routine propagation of most human myeloid leukemias. Previous work has shown the usefulness of mice with severe combined immunodeficiency (SCID) for the growth of human lymphoblastic leukemia. We show here that human myeloid cell lines and bone marrow samples from patients with acute myeloid leukemia (AML) and blast crisis of chronic myeloid leukemia (CML) also grow in SCID mice. Human AML or CML cell lines (three of three lines tested) grew in the bone marrow and peripheral blood of the mice after intravenous (IV) inoculation in a pattern closely resembling human AML. To define the best conditions for the growth of primary human myeloid leukemia cells, samples were transplanted into mice at several alternative sites. Using flow cytometry and Southern analysis, mice were analyzed at defined intervals up to 36 weeks after transplantation for the presence of human cells in various tissues. For four of four patients with AML and two of two patients with blast crisis of CML, myeloblasts grew locally at the site of implantation and were detected in the murine hematopoietic tissues. In contrast, marrow implants from patients in the chronic phase of CML (six patients) showed infrequent and limited myeloid growth in the mice. These findings demonstrate that the SCID mouse is a reproducible system for the propagation of blastic human myeloid leukemias. The differential growth of early- versus late-phase CML suggests that the SCID mouse may be a useful assay for identifying biologically aggressive leukemias early in their clinical presentation.     

Retrovirus-mediated gene transfer of MLL-ELL transforms primary myeloid progenitors and causes acute myeloid leukemias in mice

The MLL-ELL fusion gene results from the translocation t(11;19)(q23;p13.1) that is associated with de novo and therapy-related acute myeloid leukemia. To study its transforming properties, we retrovirally transduced primary murine hematopoietic progenitors and assessed their growth properties both in vitro and in vivo. MLL-ELL increased the proliferation of myeloid colony-forming cells in methylcellulose cultures upon serial replating, whereas overexpression of ELL alone had no effect. We reconstituted lethally irradiated congenic mice with bone marrow progenitors transduced with MLL-ELL or the control MIE vector encoding the enhanced green fluorescent protein. When the peripheral blood of the mice was analyzed 11-13 weeks postreconstitution, we found that the engraftment of the MLL-ELL-transduced cells was superior to that of the MIE controls. At this time point, the contribution of the donor cells was normally distributed among the myeloid and nonmyeloid compartments. Although all of the MIE animals (n = 10) remained healthy for more than a year, all of the MLL-ELL mice (n = 20) succumbed to monoclonal or pauciclonal acute myeloid leukemias within 100-200 days. The leukemic cells were readily transplantable to secondary recipients and could be established as immortalized cell lines in liquid cultures. These studies demonstrate the enhancing effect of MLL-ELL on the proliferative potential of myeloid progenitors as well as its causal role in the genesis of acute myeloid leukemias.     

Targeted therapies in myeloid leukemias

There is still a compelling need to improve therapeutic outcome in AML. However, during the past several years our understanding of the genetic basis of AML, and the nature of the mutations that contribute to the phenotype, have been elucidated in cell culture and murine models of leukemia. The validation of various mutant leukemogenic gene products has in turn led to the development of an expanding group of molecular targeted therapies that have potential to improve the therapeutic window for treatment of AML.     

Evidence for a rat granulocyte chalone effect on the proliferation of normal human bone marrow and of myeloid leukemias.

Evidence is presented that rat ascites cell extracts, acting as granulocyte chalone, temporarily inhibit the cell doubling of granulocytopoietic precursors in suspension cultures of human bone marrow. The extracts do not affect the proliferation of erythroblasts and do not show any cytotoxicity. In addition, a relative increase of granulocytic precursors capable of proliferation was found, suggesting an increase in the resting population of the granulocytopoietic proliferation pool due to extract treatment. However, the extract capable of depressing the proliferation of the normal granulocytopoiesis did not affect the granulocytopoiesis of 7 chronic myeloid and 2 chronic myelomonocytic leukemias at the same dose level. In contrast to these chronic leukemias, blast proliferation of 4 acute myeloid and monocytic leukemias was greatly depressed not only for 6 h but for the whole culture period of 48 h.     

Engraftment of human hematopoietic precursor cells with secondary transfer potential in SCID-hu mice

Human fetal bone fragments implanted subcutaneously in immunodeficient (SCID) mice maintain active human hematopoiesis. In this study, we show that this human hematopoietic microenvironment supports the engraftment and differentiation of HLA-mismatched, CD34+ primitive hematopoietic progenitor cells isolated from fetal and adult human bone marrow (BM). The BM CD34+ cells were depleted of CD2, CD14, CD15, CD16, glycophorin A, and CD19 lineage-committed cells (CD34+Lin-). Donor cell engraftment was manifested by the presence of B (CD19+) and myeloid (CD33+) cells of donor HLA phenotype. Successful engraftment was observed as early as 4 weeks after fetal BM donor cell injection and sustained for at least 12 weeks, with engraftment success rates of 100% (11/11 grafts) and 92% (11/12 grafts) at 8 and 12 weeks, respectively. Mixed BM chimerism of donor and endogenous cells was consistently observed in SCID-hu bones successfully engrafted with HLA-mismatched CD34+Lin- donor cells. Preconditioning of the SCID-hu bone with a single dose of sublethal (350 rad) whole body irradiation (WBI) immediately before cell injection enhanced the repopulation of the bone grafts with donor cells and, in some instances, resulted in complete repopulation. After WBI, as few as 500 fetal bone marrow CD34+Lin- cells injected in the human bone grafts resulted in donor-derived hematopoiesis. Donor progenitor cells recovered from the SCID-hu bone grafts 8 weeks postinjection had the capacity to repopulate secondary groups of HLA-disparate fetal human bones in SCID-hu mice with B and myeloid cells as well as CD34+ cells in some recipients. In addition, these cells repopulated fetal human thymus fragments in SCID mice with donor thymocytes including immature CD4+CD8+ and mature CD4+CD8- as well as CD4-CD8+ subsets. These results indicate that the fetal human bone implants of SCID-hu mice can support the maintenance of a cell population that has both multilineage potential and repopulating potential for periods of time as long as 16 weeks. The SCID-hu bone model consistently supported the engraftment of both fetal and adult CD34+Lin- cells without the administration of exogenous human cytokines to these animals. This model is currently being used to permit the isolation and characterization of candidate human hematopoietic stem cells (HSCs) and provide important information critical for human HSC therapy in humans.     

Growth pattern and clinical correlation of subcutaneously inoculated human primary acute leukemias in severe combined immunodeficiency mice

We examined the ability of patient-derived human leukemic blasts to generate leukemic growth and dissemination in severe combined immunodeficiency (SCID) mice by subcutaneous inoculation without conditioning treatment or administration of growth-promoting cytokines. Additionally, we correlated the growth pattern with the clinical outcome of patients from whom the leukemic cells were derived. The leukemias displayed three distinct growth patterns, ie, either aggressive, indolent, or no tumor growth. Leukemic cells from 6 of 13 patients with acute myeloid leukemia (AML), 4 of 7 T-cell acute lymphoblastic leukemia (T-ALL), and 11 of 16 patients with B-lineage ALL grew as subcutaneous tumors, with a significant number subsequently disseminating into distant organs in SCID mice. Patients whose leukemic blasts displayed an aggressive growth and dissemination pattern in SCID mice had a relatively poor clinical outcome, whereas patients with AML and T- or B-lineage ALL whose leukemic blasts grew indolently or whose cells failed to induce growth had a more favorable clinical course. Our study has shown that the subcutaneous inoculation of patient-derived human leukemic cells in SCID mice can engraft and grow as subcutaneous tumors with subsequent dissemination to distant organs in a manner analogous to their pattern of growth in humans. Additionally, these data suggest a clinical correlation to the growth and dissemination of some leukemic subtypes that may represent not only an additional prognosticator for patient outcome, but also a vehicle for the study of the biologic behavior of human leukemias and the development of novel therapeutic strategies.     

c-fms expression is a molecular marker of human acute myeloid leukemias

The c-fms protooncogene product was identified as the CSF-1 or M-CSF receptor, a polypeptide growth factor that plays a major role in myelomonocytic differentiation. This led us to look for expression of c-fms in fresh acute myeloid leukemia (AML) cells, using Northern blot analysis. c-fms expression was found in the leukemic cells of 28 AML patients, regardless of their stage of differentiation, which was assessed in the French-American-British (FAB) classification. However, the level of c-fms expression was especially high in AML of the M5 stage. High levels of expression were not accompanied by either amplification or rearrangements of the c-fms gene in AML cell DNAs. In contrast, c-fms expression was not found in acute lymphoid leukemias, whether of T or B origin. Thus, c-fms expression appears as a specific molecular marker of leukemogenesis in the myeloid lineage.     

Modulation of nitrosourea resistance in myeloid leukemias

Drug resistance in myeloid leukemias may be mediated by an increased capacity to repair chemotherapy-induced DNA damage. Some tumor cell lines that are resistant to nitrosoureas contain the DNA repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase). This protects cells by removing cytotoxic, nitrosourea-induced O6-alkylguanine adducts. We measured the level of alkyltransferase activity in myeloid leukemic cells freshly obtained from patients to determine whether the alkyltransferase was an important factor in nitrosourea resistance in these cells and whether inactivation of this protein could sensitize leukemic cells to nitrosoureas. Myeloid leukemic cells from patients with acute nonlymphocytic leukemia and chronic myelogenous leukemia had higher levels of alkyltransferase than did myeloid precursors from normal donors (P less than .01). This difference did not appear to be due to the state of differentiation of the leukemic or normal cells. To show that this repair protein mediated nitrosourea resistance in leukemic cells, cells were treated with the modified base O6- methylguanine to selectively and irreversibly inactivate the alkyltransferase and then exposed to 1,3-bis (2-chloroethyl)-1- nitrosourea (BCNU). An 18-hour incubation in 0.5 mmol/L O6- methylguanine caused an 87% +/- 3.6% decrease in alkyltransferase activity in leukemic cells and a 73% +/- 8.6% decrease in normal myeloid precursors. After treatment with O6-methylguanine, clonogenic leukemic cells from ten different donors became much more sensitive to BCNU, with a decrease in the dose needed to reduce colony survival by 50% (LD50) of 6.3 +/- 1.4-fold. A lesser effect was seen on CFU-GM, BFU- E, and CFU-GEM where the LD50 decreased two- to threefold. These studies show that nitrosourea resistance in myeloid leukemic cells can be abrogated by inactivation of the DNA repair protein O6-alkylguanine- DNA alkyltransferase. This method of biochemical modulation of DNA repair will sensitize leukemic cells to nitrosoureas in vitro and has the potential of increasing the therapeutic index of nitrosoureas in this disease.     

High-dose hydroxyurea in the treatment of poor-risk myeloid leukemias

The aim of the study was to evaluate the antileukemic effectiveness and toxicity of high-dose hydroxyurea (HHY) and to assess its acute toxicity. Between August 1997 and October 1998, 12 consecutive adult patients (>18 years) with high-risk acute myeloid leukemia (AML) (four patients in first early relapse, seven patients with secondary AML, and one patient with de novo AML concomitant to a lymphoproliferative disorder) were enrolled to receive a single course of HY (100 mg/kg per day) until bone marrow aplasia or for a maximum of 30 days. Of the 12 patients, 5 (41.6%) achieved complete remission (CR), 1 achieved partial remission (PR), 4 were resistant to treatment, and 2 died during induction from infection. No patient with relapsed AML achieved CR, while it was achieved by five of eight patients with secondary AML at diagnosis; five of six MDR1+ patients achieved CR. As concerns follow-up of the CR patients, one did not receive any further treatment and died in CR from pulmonary aspergillosis, and one with a concomitant chronic lymphocytic leukemia (CLL) received two courses of FLAG (fludarabine, cytarabine, granulocyte colony-stimulating factor) regimen with disappearance of the clonal Ig rearrangement, but relapsed after 11 months and died from pneumonia. The remaining three patients were consolidated with two courses of high-dose cytosine arabinoside (AraC), followed by peripheral blood stem cell transplantation (PBSCT) in one patient. One of them relapsed after 3 months, while the other two are still in continuous complete remission (CCR) after 16 and 28 months, respectively. This study has demonstrated the safety and efficacy of HHY in inducing CR in AML patients with unfavorable prognosis. Despite the small number of patients, these encouraging results warrant further studies.     

Cytoreduction with iodine-131-anti-CD33 antibodies before bone marrow transplantation for advanced myeloid leukemias

The monoclonal antibodies M195 and HuM195 target CD33, a glycoprotein found on myeloid leukemia cells. When labeled with iodine-131 ((131)I), these antibodies can eliminate large disease burdens and produce prolonged myelosuppression. We studied whether (131)I-labeled M195 and HuM195 could be combined safely with busulfan and cyclophosphamide (BuCy) as conditioning for allogeneic BMT. A total of 31 patients with relapsed/refractory acute myeloloid leukemia (AML) (n=16), accelerated/myeloblastic chronic myeloid leukemia (CML) (n=14), or advanced myelodysplastic syndrome (n=1) received (131)I-M195 or (131)I-HuM195 (122-437 mCi) plus busulfan (16 mg/kg) and cyclophosphamide (90-120 mg/kg) followed by infusion of related-donor bone marrow (27 first BMT; four second BMT). Hyperbilirubinemia was the most common extramedullary toxicity, occurring in 69% of patients during the first 28 days after BMT. Gamma camera imaging showed targeting of the radioisotope to the bone marrow, liver, and spleen, with absorbed radiation doses to the marrow of 272-1470 cGy. The median survival was 4.9 months (range 0.3-90+ months). Three patients with relapsed AML remain in complete remission 59+, 87+, and 90+ months following bone marrow transplantation (BMT). These studies show the feasibility of adding CD33-targeted radioimmunotherapy to a standard BMT preparative regimen; however, randomized trials will be needed to prove a benefit to intensified conditioning with radioimmunotherapy.     

FLT3 internal tandem duplication mutations associated with human acute myeloid leukemias induce myeloproliferative disease in a murine bone marrow transplant model.

FLT3 receptor tyrosine kinase is expressed on lymphoid and myeloid progenitors in the hematopoietic system. Activating mutations in FLT3 have been identified in approximately 30% of patients with acute myelogenous leukemia, making it one of the most common mutations observed in this disease. Frequently, the mutation is an in-frame internal tandem duplication (ITD) in the juxtamembrane region that results in constitutive activation of FLT3, and confers interleukin-3 (IL-3)-independent growth to Ba/F3 and 32D cells. FLT3-ITD mutants were cloned from primary human leukemia samples and assayed for transformation of primary hematopoietic cells using a murine bone marrow transplantation assay. FLT3-ITDs induced an oligoclonal myeloproliferative disorder in mice, characterized by splenomegaly and leukocytosis. The myeloproliferative phenotype, which was associated with extramedullary hematopoiesis in the spleen and liver, was confirmed by histopathologic and flow cytometric analysis. The disease latency of 40 to 60 days with FLT3-ITDs contrasted with wild-type FLT3 and enhanced green fluorescent protein (EGFP) controls, which did not develop hematologic disease (> 200 days). These results demonstrate that FLT3-ITD mutant proteins are sufficient to induce a myeloproliferative disorder, but are insufficient to recapitulate the AML phenotype observed in humans. Additional mutations that impair hematopoietic differentiation may be required for the development of FLT3-ITD-associated acute myeloid leukemias. This model system should be useful to assess the contribution of additional cooperating mutations and to evaluate specific FLT3 inhibitors in vivo.     

Acute myeloid leukemias, multiple myelomas, and chronic leukemias in the setting of HIV infection

B-cell lineage-derived high-grade malignant lymphomas are a well-recognized complication of HIV infection. However, isolated cases of unusual hematologic malignancies such as acute myeloid leukemias (AML), multiple myeloma (MM) or plasmacytomas, and chronic leukemias have been reported. This review focuses on these uncommon malignancies supervening in the setting of HIV infection. Eighteen cases of AML have been reported. Extramedullary localizations are frequently noticed. Nontreated patients have a survival of 2.7 weeks, compared with 9.8 months for patients treated with chemotherapy; being HIV-positive is not a contraindication to the treatment of AML. Based on the observed 72% incidence of AML M4 and M5 in an HIV-infected population versus 19% to 36% expected in a non-HIV-infected population, we postulate that the association of AML and HIV is not coincidental. The monocytotropism of HIV, the chronic cytokine-mediated activation of monocytes/macrophages, and the immunodeficiency may explain this association. Twenty-two cases of MM or plasmacytomas have been described, most of them in young patients. Again, extramedullary plasma cell tumors are recorded in many patients. Physiopathologic studies suggest that MM may develop because of an antigen-driven response to the circulating viral antigens. A role for Epstein-Barr virus (EBV) in the pathogenesis, as previously described in high-grade non-Hodgkin's lymphomas, is suggested by the presence of EBV genomes in plasma cell tumors. Finally, a broad spectrum of chronic leukemias derived from B- or T-cell lymphocyte lineage has been reported. These associations seem coincidental.     

Molecular evaluation of acute myeloid leukemias.

Traditionally, acute myeloid leukemia (AML) has been diagnosed and classified based on the morphologic and cytochemical criteria of the French-American-British (FAB) classification system. However, more recent studies have demonstrated that the cytogenetic and molecular genetic abnormalities consistently associated with distinct forms of AML confer the most important prognostic information. Each broad category of de novo AML that arises in infants and children, young adults, and the elderly, or, AML arising secondary to antecedent myelodysplasia (MDS) or prior exposure to chemotherapy, radiotherapy, or environmental agents is characterized by distinct cytogenetic and molecular genetic abnormalities. Over the past 15 years, a number of these recurrent cytogenetic abnormalities have been cloned and characterized, yielding valuable insights into the mechanisms of leukemogenesis and providing powerful molecular tools for precise diagnosis and monitoring of minimal residual disease. As cytogenetic and molecular genetic characterization of AML cells is now considered crucial for both diagnostic and therapeutic decision-making, it is essential to have a working knowledge of the molecular basis of AML. This chapter reviews the molecular genetic features of different forms of AML and the new automated molecular technologies for residual disease assessment; the emerging molecular paradigms in this disease that are being exploited for therapeutic decision-making are presented.