Establishment of a novel factor-dependent myeloid cell line from primary cultures of mouse bone marrow

Reducing the genotoxic potential of retroviral vectors

The recent development of leukemia in gene therapy patients with X-linked severe combined immunodeficiency disease because of retroviral vector insertional mutagenesis has prompted reassessment of the genotoxic potential of integrating vector systems. In this chapter, various strategies are described to reduce the associated risks of retroviral genomic integration. These include deletion of strong transcriptional enhancer-promoter elements in the retroviral long terminal repeats, flanking the retroviral transcriptional unit with enhancer blocking sequences and designing vectors with improved RNA 3' end processing. Protocols are provided to evaluate the relative biosafety of the modified vectors based on their ability to immortalize hematopoietic progenitor cells and propensity to trigger clonal hematopoiesis or leukemogenesis following hematopoietic stem cell transplantation.

Induction of Synaptosomal-Associated Protein-23 kD (SNAP-23) by Various Cytokines

Cytokines manifest their function through regulation of gene expression. We searched for immediate-early cytokine responsive genes by the mRNA differential display technique using interleukin-3 (IL-3)–dependent OTT-1 cells, and have isolated a novel cDNA which encodes 210 amino acids and shows 87% amino acid identity to human SNAP-23 (synaptosomal-associated protein of 23 kD). The message for this protein (mouse SNAP-23) was induced in OTT-1 cells by IL-3, granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-5. The experiment using C-terminal deletion mutants of the common β subunit (βc) of IL-3/GM-CSF/IL-5 receptors showed that expression of SNAP-23 was associated with the Ras-Raf-MAPK pathway, but not with the JAK-STAT pathway. Moreover, SNAP-23 was induced in response to a wide variety of cytokines, including IL-2, IL-3, IL-5, IL-10, stem cell factor, G-CSF, GM-CSF, leukemia inhibitory factor, and erythropoietin. Constitutive expression of SNAP-23 was seen in various tissues, including heart, lung, kidney, liver, spleen, and small intestine. Possible involvement of SNAP-23 in cytokine signal transduction is discussed.

Identification and characterization of a constitutively active STAT5 mutant that promotes cell proliferation

STAT (signal transducers and activators of transcription) proteins are transcription factors which are activated by phosphorylation on tyrosine residues upon stimulation by cytokines. Seven members of the STAT family are known, including the closely related STAT5A and STAT5B, which are activated by various cytokines. Except for prolactin-dependent beta-casein production in mammary gland cells, the biological consequences of STAT5 activation in various systems are not clear. We applied PCR-driven random mutagenesis and a retrovirus-mediated expression screening system to identify constitutively active forms of STAT5. By this strategy, we have identified a constitutively active STAT5 mutant which has two amino acid substitutions; one is located upstream of the putative DNA binding domain (H299R), and the other is located in the transactivation domain (S711F). The mutant STAT5 was constitutively phosphorylated on tyrosine residues, localized in the nucleus, and was transcriptionally active. Expression of the mutant STAT5 partially dispenses with interleukin 3 (IL-3) as a growth stimulant of IL-3-dependent cell lines. Further analyses of the mutant STAT5 have demonstrated that both of the mutations are required for nuclear localization, efficient transcriptional activation, and induction of IL-3-independent growth of an IL-3-dependent cell line, Ba/F3, and have indicated that a molecular basis for the constitutive activation is the stability of the phosphorylated form of the mutant STAT5.

Multi-colony stimulating activity of interleukin 5 (IL-5) on hematopoietic progenitors from transgenic mice that express IL-5 receptor alpha subunit constitutively

The interleukin 3 (IL-3), IL-5, and granulocyte/macrophage colony-stimulating factor receptors consist of a cytokine-specific alpha subunit and the common beta subunit. Whereas IL-3 stimulates various lineages of hematopoietic cells, including multipotential progenitors, IL-5 acts mainly as an eosinophil lineage-specific factor. To investigate whether the lineage specificity of IL-5 is due to restricted expression of the IL-5 receptor alpha subunit (IL-5R alpha), we generated transgenic mice that express the mouse IL-5R alpha constitutively by phosphoglycerate kinase promoter. The transgenic mouse expressed IL-5R alpha ubiquitously, and the bone marrow cells formed various types of colonies, including multi-lineage colonies, in response to IL-5. IL-5 also supported formation of both multi-lineage and blast cell colonies from dormant progenitors of the 5-fluorouracil-treated transgenic mice. The cells composing the blast cell colony gave rise to many colonies including multi-lineage colonies when they were replated in secondary culture containing either Il-5 or IL-3. There was no significant difference in replating efficiency or in types of secondary colonies between IL-5- and IL-3-stimulated cultures. Conversely, the cells from the IL-3-induced blast cell colonies of the transgenic mice proliferated in response to either IL-3 or IL-5. Thus, the development of the progenitors can be equally supported by either IL-5 or IL-3, suggesting that intracellular signals from the IL-3R can be replaced by those from IL-5. These results strongly suggest that the lineage specificity of IL-5 is mainly due to the restricted expression of IL-5R alpha.

Progenitor cell hyperplasia with rare development of myeloid leukemia in interleukin 11 bone marrow chimeras

Post 5-fluorouracil-treated murine bone marrow cells infected with a recombinant retrovirus (murine stem cell virus-interleukin 11 [MSCV-IL-11]) bearing a human IL-11 gene were transplanted into lethally irradiated syngeneic mice. Analysis of proviral integration sites in DNA prepared from hematopoietic tissues and purified cell populations of long-term reconstituted primary and secondary recipients demonstrated polyclonal engraftment by multipotential stem cells. High levels (100-1,500 U/ml) of IL-11 were detected in the plasma of the MSCV-IL-11 mice. Systemic effects of chronic IL-11 exposure included loss of body fat, thymus atrophy, some alterations in plasma protein levels, frequent inflammation of the eyelids, and often a hyperactive state. A sustained rise in peripheral platelet levels (approximately 1.5-fold) was seen throughout the observation period (4-17 wk). No changes were observed in the total number of circulating leukocytes in the majority of the transplanted animals (including 10 primary and 18 secondary recipients) despite a > 20-fold elevation in myeloid progenitor cell content in the spleen. The exceptions were members of one transplant pedigree which presented with myeloid leukemia during the secondary transplant phase. A clonal origin of the disease was determined, with significant expansion of the MSCV-IL-11-marked clone having occurred in the spleen of the primary host. Culturing of leukemic spleen cells from a quaternary recipient led to the establishment of a permanent cell line (denoted PGMD1). IL-11-producing PGMD1 myeloid leukemic cells are dependent on IL-3 for continuous growth in vitro and they differentiate into granulocytes and macrophages in response to granulocyte/macrophage colony-stimulating factor. The inability of autogenously produced IL-11 to support autonomous growth of PGMD1 cells argues against a mechanism of transformation involving a classical autocrine loop.

Association between ICAM-1 expression and metastatic capacity of murine B-cell hybridomas

We previously reported that a derivative of the interleukin-6 (IL-6)-dependent B9 B-cell hybridoma (B9/LPNU1L) constitutively expressing an interleukin-1 alpha (IL-1 alpha) gene introduced by retrovirus-mediated gene transfer preferentially metastasized to bone marrow following intravenous injection into unirradiated syngeneic BALB/c mice. B9/LPNU1L cells recovered from the femoral marrow of a recipient with hind limb paralysis (denoted B9/BM1) retained their IL-6-dependency yet displayed enhanced metastatic capacity during serial transplantation in vivo. In contrast, autonomously-growing B9 variants spontaneously arising in vitro or IL-6-independent B9 derivatives created by infection with recombinant IL-6 retroviruses rarely gave rise to experimental metastases in syngeneic BALB/c or nude mice. Examination of cell adhesion molecule profiles by immunofluorescence flow cytometry has revealed high levels of CD44, moderate levels of VLA-4 and low levels of LFA-1 on all B9-series cells. By comparison, ICAM-1 expression was significantly elevated on B9/BM1 cells, with independent isolates stably expressing about 4-fold higher levels which were paralleled by corresponding increases in the steady-state levels of ICAM-1 mRNA. L-Selectin was not expressed by any of the cell lines. Despite higher ICAM-1 levels, cell aggregation assays revealed that LFA-1-ICAM-1 adhesive interactions were not involved in the homotypic adhesion of B9/BM1 cells but rather that binding of CD44 to endogenously-synthesized hyaluronan was responsible. Furthermore, B9/BM1 cells expressing high levels of ICAM-1 were found to be less susceptible to cytolysis by natural killer (NK) cells than their weakly metastatic or nonmetastatic counterparts.

Transplantable myeloproliferative disease induced in mice by an interleukin 6 retrovirus

Lethally irradiated mice transplanted with bone marrow cells infected with a novel recombinant retrovirus (murine stem cell virus-interleukin 6 [MSCV-IL-6]) bearing a mouse IL-6 gene developed a fatal myeloproliferative disease within 4 wk of engraftment. The hematologic manifestations of the syndrome included elevated peripheral leukocyte counts (up to 430 x 10(3) cells/mm3) with a predominance of neutrophilic granulocytes, microcytic anemia, and thrombocytosis or thrombocytopenia. The mice showed extensive neutrophil infiltration of the lungs, liver, and occasionally lymph nodes, plus splenomegaly resulting from enhanced splenic myelopoiesis (30-60-fold increase in progenitor numbers). Despite the chronic stimulation of neutrophil excess by IL-6, bone marrow from affected mice was capable of repopulating the hematopoietic tissues (bone marrow and spleen) of lethally irradiated hosts during repeated serial transplantation. In the longest documented case, the progeny of a single MSCV-IL-6-marked cell transferred the myeloproliferative disease to two secondary, four tertiary, and two quaternary recipients (the clone endured for a total of 72 wk). These results, demonstrating considerable proliferative longevity of the IL-6-producing cells, support an in vivo role of IL-6 in the maintenance of hematopoietic precursors. Dysregulated IL-6 production also had significant systemic effects. The mice displayed increased mesangial cell proliferation in the kidney, frequent liver abnormalities, and marked alterations in plasma protein levels. Unlike previous studies where constitutive expression of exogenous IL-6 genes resulted in lymphoproliferative disorders characterized by massive plasmacytosis, minimal plasma cell expansion occurred in the MSCV-IL-6 mice during the observation period. Potential explanations for the differences in disease phenotypes observed in the present and previous studies are different cell types expressing the exogenous IL-6 genes, higher sustained circulating levels of IL-6 achieved using the MSCV-IL-6 retroviral delivery system, and/or the premature death (3-15 wk after transplantation) of the MSCV-IL-6 mice before the onset of plasmacytosis. This animal model should prove useful for further investigation of the function of IL-6 in normal and abnormal hematopoiesis and in inflammatory responses.

Increased frequency of both total and specific monoclonal antibody producing hybridomas using a fusion partner that constitutively expresses recombinant IL-6

The addition of auxiliary feeder cells or conditioned medium has been shown to augment the yield of mouse hybridomas obtained following the cell-cell fusion of myeloma and B lymphocytes. The addition of one of these factors, interleukin-6 (IL-6) has been found to increase the proportion of hybridomas secreting monoclonal antibodies of desired specificity. As an alternative genetic approach, we have examined the efficacy of a retroviral infectant of Sp2/0 cells that constitutively expresses recombinant murine IL-6 (Sp2/mIL-6) as fusion partner. The results demonstrated that the yields of both viable Ig-secreting hybridomas, and antigen-specific monoclonal antibodies were increased 3-15-fold and 5-9-fold, respectively, with the Sp2/mIL-6 relative to Sp2/0 or Sp2/neo cells as fusion partner. Sp2/mIL-6 cells generated hybridomas with comparable growth rates, stability, and Ig production. The results of staining nascent hybridoma colonies immunohistochemically for Ig production suggest that Sp2/mIL-6 cells as a fusion partner increased the viability and/or stability of nascent hybrid cells that are producing Ig. Thus the Sp2/mIL-6 cells are an improved myeloma parent for the generation of large numbers of antibody-producing hybridomas against specific antigens.

Expression of retrovirally transduced IL-1 alpha in IL-6-dependent B cells: a murine model of aggressive multiple myeloma

Retroviral-mediated gene transfer was employed to introduce an IL-1 alpha cDNA into an IL-6-dependent murine B-cell line. Bone marrow metastases and bone lesions were frequently observed following intravenous injection of these B cells into syngeneic mice. Because the retroviral vector also contained the neomycin phosphotransferase gene, metastatic cells could be easily recovered from bone marrow by addition of G418 to the culture medium. Interestingly, the metastatic B cells were found to retain their IL-6 dependency through several transplant generations. By comparison, intravenous injection of autonomously-growing B-cell lines generated in vitro by retroviral introduction of an IL-6 cDNA rarely resulted in bone marrow metastases. These results demonstrate that abrogation of growth factor dependency is neither necessary nor sufficient for the in vivo growth and dissemination of tumor cells in this experimental system. It is proposed that the increased metastasis of the IL-1 alpha-producing B-cells to bone marrow is due to alterations in cell adhesion molecules. The B-cell bone marrow metastasis model described here may be useful for studies of bone marrow homing and for evaluation of therapeutic regimens for multiple myeloma.

Leukocytosis in mice following long-term reconstitution with genetically-modified bone marrow cells constitutively expressing interleukin 1 alpha or interleukin 6

Leukemic cells of patients with acute myeloid leukemia have recently been shown to spontaneously produce autostimulatory IL-1 and IL-6. In order to investigate the effects of systemic production of these cytokines on normal hematopoietic cells, mice were engrafted with bone marrow cells infected with high-titer retroviral vectors carrying the murine IL-1 alpha or IL-6 genes and the neomycin phosphotransferase gene. Sustained expression of the introduced IL-1 alpha and IL-6 genes was documented by Northern-blot analysis of RNA from G418-resistant mast cells and T cells, derived from bone marrow and spleen, respectively, of successfully reconstituted mice 6-10 months after transplantation. A single mouse engrafted with IL-1 alpha-infected cells which presented with a dramatic neutrophilic granulocytosis (54-fold elevation in circulating neutrophils) was sacrificed for health concerns 2 months post-transplant. Modest changes in peripheral leukocyte counts (at most a 2-fold rise) were observed in all of the other mice, and they remained healthy throughout the observation period. The majority displayed increased hematopoietic activity in bone marrow and spleen, predominantly granulopoiesis, with moderate lymphoid hyperplasia seen in the spleens of mice constitutively expressing IL-1 alpha. These mouse models provide the opportunity to evaluate the potential of persistent IL-1 alpha and IL-6 expression to contribute to leukemogenic transformation.