Release of spleen focus-forming virus (SFFV) from differentiation inducible promyelocytic leukemia cell lines transformed in vitro by Friend leukemia virus

Reduced Competitive Repopulation Capacity of Multipotential Hematopoietic Stem Cells in the Bone Marrow of Friend Virus-infected Fv2-resistant Mice

BACKGROUND/AIM

The polycythemia form of Friend leukemia virus (FVP) causes splenomegaly and lethal erythroleukemia in Fv-2^s-susceptible mouse strains. We sought to determine whether the hematopoietic stem cell (HSC) pool was expanded in Fv-2^r-resistant mice.

MATERIALS AND METHODS

The 120-day bone marrow transplantation competitive repopulation assay was used to determine whether FVP-infected Fv-2^r C57BL/6 mice demonstrated expansion of the HSC pool compared to the pool of committed hematopoietic progenitor cells in the same marrow assayed in vitro.

RESULTS

There was a significant expansion of committed hematopoietic progenitors observed in virus-infected Fv-2^s FVB mice, but not Fv-2^r C57BL/6 mice. Furthermore, Fv-2^r mice showed no detectable expansion of either committed hematopoietic progenitor cells or the multipotential stem cell pool by competitive repopulation assay.

CONCLUSION

Friend virus disease in Fv-2^s mice is associated with expansion of committed hematopoietic progenitors. Fv-2^r mice show no expansion of either committed progenitor or pluripotential stem cell numbers.

Hematopoietic cell survival signals are elicited through non-tyrosine-containing sequences in the membrane-proximal region of the erythropoietin receptor (EPOR) by a Stat5-dependent pathway

OBJECTIVE

Erythropoietin is essential for red blood cell development in vivo and is also an important therapeutic agent to treat anemia resulting from kidney failure or bone marrow suppression. The erythropoietin receptor (EPOR) elicits both positive and negative regulatory signaling pathways, primarily through phosphorylated tyrosine residues in the cytoplasmic domain of the activated receptor complex. Surprisingly, however, EPOR tyrosine residues are dispensable for in vivo erythropoiesis under nonstress conditions. One of the key signaling molecules elicited by the EPOR is the Stat5 transcription factor. Stat5 activation has been mapped to tyrosines 343 and 401 in the EPOR cytoplasmic region, although non-tyrosine-containing sequences in the EPOR cytoplasmic region can also stimulate Stat5. To test the functional role of non-tyrosine-containing sequences in the EPOR, we analyzed a series of mutant EPOR isoforms in cell survival and proliferation assays.

METHODS

The IL-3-dependent 32D cell line was stably transfected with cDNAs encoding the wild-type EPOR or mutant EPORs containing or lacking intracellular tyrosines, in the absence or presence of a dominant inhibitory Stat5 isoform. EPO-dependent cell signaling, survival, and proliferation were evaluated.

RESULTS

EPOR isoforms lacking intracellular tyrosine residues elicit an important survival signal in 32D cells. Stat5 function is critical for EPO-dependent cell survival mediated by these non-tyrosine-containing receptor sequences. Interestingly, EPO-dependent survival does not require the presence of fetal calf serum (FCS) in the culture medium, yet FCS is important for 32D cell proliferation in response to EPO.

CONCLUSION

Our results elucidate a previously unrecognized survival pathway elicited by the EPOR. They demonstrate that this pathway requires Stat5 and is serum independent. These findings contribute significantly to our understanding of the complexity by which the EPOR functions in hematopoietic cells.

Selection of highly productive mammalian cells based on an inducible growth advantage using an antibody/receptor chimera

In mammalian cell culture, the selection of high producers is a critical step in efficient recombinant protein production. Drug-resistance selection has been commonly used, but does not always give a pure population of high producers. In this study, we propose a novel selection method in which the growth of high producers is specifically promoted. Two plasmids encoding (i) a hybrid receptor composed of the V(H) portion of anti-hen egg lysozyme antibody HyHEL-10 and an N-terminally truncated erythropoietin receptor (V(H)-EpoR), and (ii) a V(L)-EpoR fusion derived from the same construct as in (i), were employed. The second plasmid contained enhanced green fluorescent protein (EGFP) as a model recombinant protein that was flanked by the internal ribosomal entry sequence. Both plasmids were used simultaneously to transfect an IL-3-dependent murine myeloid cell line, 32D. The transfectants, after antigen selection in the absence of IL-3, showed a clear antigen-induced dose-dependent proliferation. In addition, a high EGFP expression level was observed by flow cytometry in comparison with the cells before antigen selection. The results clearly demonstrate the advantage of our method over conventional drug-resistance selection. We propose the term AMEGA (Antigen MEdiated Genetically-modified cell Amplification) for such an approach.

The inositol 5-phosphatase SHIP is expressed as 145 and 135 kDa proteins in blood and bone marrow cells in vivo, whereas carboxyl-truncated forms of SHIP are generated by proteolytic cleavage in vitro

The inositol polyphosphate 5-phosphatase SHIP plays an important role in negative signalling in B cells and mast cells and in the down-regulation of cytokine receptor-mediated signals in myeloid cells. SHIP is expressed as a 145 kDa full-length protein and an isoform of 135 kDa due to alternative splicing. Additional smaller forms of SHIP which are truncated at the carboxy terminus have been described in bone marrow and peripheral blood mononuclear cells (PBMC). Our data demonstrate that human bone marrow cells and PBMC from healthy donors and patients with acute myeloid leukemia express the 145 kDa form of SHIP and low amounts of a 135 kDa form of SHIP in vivo whereas C-terminal-truncated SHIP proteins are generated by a PMSF-sensitive protease during the preparation of cell lysates in vitro. We have further characterized this protease and identified a proteolytic cleavage site in the human SHIP protein C-terminal to tryptophan residue 941. These data support a physiological role for the 145 and 135 kDa forms of SHIP in bone marrow and peripheral blood cells from normal donors and patients with acute myeloid leukemia.

Cytokine signaling through Stat3 activates integrins, promotes adhesion, and induces growth arrest in the myeloid cell line 32D

Hematopoietic cell development and function is dependent on cytokines and on intercellular interactions with the microenvironment. Although the intracellular signaling pathways stimulated by cytokine receptors are well described, little is known about the mechanisms through which these pathways modulate hematopoietic cell adhesion events in the microenvironment. Here we show that cytokine-activated Stat3 stimulates the expression and function of cell surface adhesion molecules in the myeloid progenitor cell line 32D. We generated an erythropoietin receptor (EpoR) isoform (ER343/401-S3) that activates Stat3 rather than Stat5 by substituting the Stat3 binding/activation sequence motif from gp130 for the sequences surrounding tyrosines 343 and 401 in the receptor cytoplasmic region. Activation of Stat3 leads to homotypic cell aggregation, increased expression of intercellular adhesion molecule 1 (ICAM-1), CD18, and CD11b, and activation of signaling through CD18-containing integrins. Unlike the wild type EpoR, ER343/401-S3 is unable to support long term Epo-dependent proliferation in 32D cells. Instead, Epo-treated ER343/401-S3 cells undergo G(1) arrest and express elevated levels of the cyclin-dependent kinase inhibitor p27(Kip1). Sustained activation of Stat3 in these cells is required for their altered morphology and growth properties since constitutive SOCS3 expression abrogates homotypic cell aggregation, signaling through CD18-containing integrins, G(1) arrest, and accumulation of p27(Kip1). Collectively, our results demonstrate that cytokine-activated Stat3 stimulates the expression and function of cell surface adhesion molecules, indicating that a role for Stat3 is to regulate intercellular contacts in myeloid cells.

Sequential mutations in the interleukin-3 (IL3)/granulocyte-macrophage colony-stimulating factor/IL5 receptor beta-subunit genes are necessary for the complete conversion to growth autonomy mediated by a truncated beta C subunit

An amino-terminally truncated beta C receptor (beta C-R) subunit of the interleukin-3 (IL3)/granulocyte-macrophage colony-stimulating factor/IL5 receptor complex mediates factor-independent and tumorigenic growth in two spontaneous mutants of a promyelocytic cell line. The constitutive activation of the JAK2 protein kinase in these mutants confirms that signaling occurs through the truncated receptor protein. Noteworthily, in addition to a 10-kb deletion in the beta C-R subunit gene encoding the truncated receptor, several secondary and independent mutations that result in the deletion or functional inactivation of the allelic beta C-R subunit and the closely related beta IL3-R subunit genes were observed in both mutants, suggesting that such mutations are necessary for the full oncogenic penetrance of the truncated beta C-R subunit. Reversion of these mutations by the expression of the wild-type beta C-R in the two mutants resulted in a fivefold decrease in cloning efficiency of the mutants in the absence of IL3, confirming a functional interaction between the wild-type and truncated proteins. Furthermore, expression of the truncated beta C-R subunit in factor-dependent myeloid cells did not immediately render the cells autonomous but increased the spontaneous frequency to factor-independent growth by 4 orders of magnitude. Implications for both leukemogenic progression and receptor-subunit interaction and signaling are discussed.

The gene for erythropoietin receptor is expressed in multipotential hematopoietic and embryonal stem cells: evidence for differentiation stage-specific regulation

The principal regulator of erythropoiesis is the glycoprotein erythropoietin, which interacts with a specific cell surface receptor (EpoR). A study aimed at analyzing EpoR gene regulation has shown that both pluripotent embryonal stem cells and early multipotent hematopoietic cells express EpoR transcripts. Commitment to nonerythroid lineages (e.g., macrophage or lymphocytic) results in the shutdown of EpoR gene expression, whereas commitment to the erythroid lineage is concurrent with or followed by dramatic increases in EpoR transcription. To determine whether gene activity could be correlated with chromatin alterations, DNase-hypersensitive sites (HSS) were mapped. Two major HSS located in the promoter region and within the first intron of the EpoR gene are present in all embryonal stem and hematopoietic cells tested, the intensities of which correlate well with EpoR expression levels. In addition, a third major HSS also located within the first intron of the EpoR gene is uniquely present in erythroid cells that express high levels of EpoR. Transfection assays show that sequences surrounding this major HSS impart erythroid cell-specific enhancer activity to a heterologous promoter and that this activity is at least in part mediated by GATA-1. These data, together with concordant expression levels of GATA-1 and EpoR in both early multipotent hematopoietic and committed erythroid cells, support a regulatory role of the erythroid cell-specific transcription factor GATA-1 in EpoR transcription in these cells. However, the lack of significant levels of GATA-1 expression in embryonal stem cells implies an alternative regulatory mechanism of EpoR transcription in cells not committed to the hematopoietic lineage.

The gene for erythropoietin receptor is expressed in multipotential hematopoietic and embryonal stem cells: evidence for differentiation stage-specific regulation

The principal regulator of erythropoiesis is the glycoprotein erythropoietin, which interacts with a specific cell surface receptor (EpoR). A study aimed at analyzing EpoR gene regulation has shown that both pluripotent embryonal stem cells and early multipotent hematopoietic cells express EpoR transcripts. Commitment to nonerythroid lineages (e.g., macrophage or lymphocytic) results in the shutdown of EpoR gene expression, whereas commitment to the erythroid lineage is concurrent with or followed by dramatic increases in EpoR transcription. To determine whether gene activity could be correlated with chromatin alterations, DNase-hypersensitive sites (HSS) were mapped. Two major HSS located in the promoter region and within the first intron of the EpoR gene are present in all embryonal stem and hematopoietic cells tested, the intensities of which correlate well with EpoR expression levels. In addition, a third major HSS also located within the first intron of the EpoR gene is uniquely present in erythroid cells that express high levels of EpoR. Transfection assays show that sequences surrounding this major HSS impart erythroid cell-specific enhancer activity to a heterologous promoter and that this activity is at least in part mediated by GATA-1. These data, together with concordant expression levels of GATA-1 and EpoR in both early multipotent hematopoietic and committed erythroid cells, support a regulatory role of the erythroid cell-specific transcription factor GATA-1 in EpoR transcription in these cells. However, the lack of significant levels of GATA-1 expression in embryonal stem cells implies an alternative regulatory mechanism of EpoR transcription in cells not committed to the hematopoietic lineage.

Granulocyte-macrophage colony-stimulating factor binding sites and oxidative metabolism in human granulocytes

We investigated the interaction between GM-CSF and its receptor on human granulocytes and on several human tumor cell lines. Specific high-affinity binding for GM-CSF was characterized by Scatchard plot analysis. The specific radioactivity of the 125I-labeled derivative of rH. GM-CSF was determined by self-displacement analysis and calculated to be 30 microCi/micrograms. The maximum concentration of binding sites (B max) in granulocytes was 40 fmol/mg protein (2,200 molecules GM-CSF bound/cell) and the dissociation constant (KD) was 0.42 nM. No binding sites for GM-CSF were found in two lung cancer cell lines, SCLC-16HV and NCI-N417 or in the urinary bladder carcinoma cell line 5637, whereas the promyelocytic leukemia cell line HL60 was positive for GM-CSF binding. Time course experiments showed maximum binding of GM-CSF in granulocytes after an incubation period of 60 min and a decrease in binding after an incubation period of 2 h. In parallel, we found a maximum biological signal when granulocytes were preincubated for 90 min with GM-CSF, and a decrease after an incubation time of 120 min. Preincubation of the cells with rH. GM-CSF induced an enhancement of the production of activated oxygen species by the cells in response to PMA.

Selection of lineage-restricted cell lines immortalized at different stages of hematopoietic differentiation from the murine cell line 32D

Erythropoietin (Epo), granulocyte-macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor- (G-CSF) dependent cell lines have been derived from the murine hematopoietic cell line 32D with a selection strategy involving the culture of the cells in FBS-deprived medium supplemented only with pure recombinant Epo, GM-CSF, or G-CSF. The cells retain the diploid karyotype of the original 32D clone, do not grow in the absence of exogenous growth factor, and do not induce tumors when injected into syngeneic recipients. The morphology of the Epo-dependent cell lines (32D Epo1, -2, and -3) was heterogeneous and evolved with passage. The percent of differentiated cells also was a function of the cell line investigated. Benzidine-positive cells ranged from 1-2% (32D Epo3) to 50-60% (32D Epo1). These erythroid cells expressed carbonic anhydrase I and/or globin mRNA but not carbonic anhydrase II. The GM-CSF- and G-CSF-dependent cell lines had predominantly the morphology of undifferentiated myeloblasts or metamyelocytes, respectively. The GM-CSF-dependent cell lines were sensitive to either GM-CSF or interleukin-3 (IL-3) but did not respond to G-CSF. The G-CSF-dependent cell lines grew to a limited extent in IL-3 but did not respond to GM-CSF. These results indicate that the cell line 32D, originally described as predominantly a basophil/mast cell line, has retained the capacity to give rise to cells which proliferate and differentiate in response to Epo, GM-CSF, and/or G-CSF. These cells represent the first nontransformed cell lines which can be maintained in growth factors other than IL-3 and which differentiate in the presence of physiologic signals. As such, they may represent a model to study the molecular mechanisms underlying the process of hematopoietic differentiation, as well as sensitive targets for bioassays of specific growth factors.

Conversion of factor-dependent myeloid cells to factor independence: autocrine stimulation is not coincident with tumorigenicity

It has been postulated that the disruption of the normal hormonal regulation of blood cell formation and proliferation leads to the autonomous growth of hematopoietic progenitors or stem cells and thus to leukeamia. We have utilized established hematopoietic cell lines to establish the different mechanism by which growth autonomy is acquired. The analysis of thirteen spontaneous factor-independent mutants revealed that the majority (12/13) secreted a factor that stimulated growth of the parental cell line. Thus, autocrine stimulation may be a important mechanism by which normal growth control is disrupted. This is supported by the observation of Young and Griffin (1987) that some cells isolated from patients with acute myeloblastic leukemia (AML) autogenously produce growth factor. In the majority of Dind mutants more closely examined, growth factor gene activation was due to the juxtapostion of a retrotransposon. Although the exact nature of the involvement of human retroviruses in inducing leukemia has not been elucidated, one could envisage that altered growth factor regulation due to integration of the virus may play an important role. The existence of a second class of Dind mutants that have obtained factor-independence by a mechanism not involving factor production concurs with the acquisition of factor-independent growth in hematopoietic cells after introduction of some oncogenes. Several models have been proposed to explain how oncogenes may "short circuit" and thus activate the normal signal transduction pathway by mimicking the active receptor, transducer, or effector (Weinberg, 1985). To investigate more closely the role of autocrine stimulation in the induction of growth autonomy and tumorigenicity, retroviral vectors expressing either GM-CSF or IL3 were introduced into factor-dependent hematopoitic cell lines. Non-linear clonability of infected cell lines in the absence of exogenous growth factor and inhibition of proliferation by antiserum supported a model of autocrine stimulation. However, a secondary event, correlated with amount of factor released, often occurred that abrogated the requirement for secreted CSF. Growth of cells in which this alteration had occured was cell-density independent and could not be blocked by antibody. It has been postulated that autogenous factor may react with its receptor intracellularly (Lang et al., 1985). The results presented here cannot exclude that the secondary events may allow the internal interaction of receptor and factor.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of genes involved in growth autonomy of hematopoietic cells by analysis of factor-independent mutants

The factor-dependent myeloid precursor cell line D35 mutates spontaneously at a frequency greater than 2.4 x 10(-7) to growth factor autonomy. This frequency could be increased at least 20-fold by retrovirus insertional mutagenesis. The isolation and characterization of factor-independent mutants allowed the identification of genes involved in growth autonomy. Mutants could be subdivided into two sets: those that secreted a stimulating factor (10/11) and those that did not (1/11). In one case, the factor released was distinct from previously characterized growth factors. In most mutants (6/9), the activation of a growth factor gene was associated with rearrangement that could be attributed to the insertion of a transposable-like element either 5' or 3' of the factor coding region in all cases examined, excluding oncogene involvement. All factor-independent mutants were tumorigenic, consistent with the hypothesis that growth-factor independence initiated by aberrant growth factor gene activation is an important and early step in tumorigenesis.

Macrophage cell lines transformed by the malignant histiocytosis sarcoma virus: increase of CSF receptors suggests a model for transformation

The malignant histiocytosis sarcoma virus (MHSV) contains Ha-v-ras-related oncogenic sequences and rapidly transforms myeloid cells in vivo and in vitro. Myeloid cell lines can be derived which do not require growth factor for continued proliferation. We initiated this work to define the process of transformation leading to autonomy of cell growth in transformed myeloid cells. Five established cell lines were examined. All express macrophage-specific cell-surface antigens and exhibit several other properties typical for mature macrophages. Growth properties, growth factor release, and growth factor receptor presentation were examined: Release of growth factors is not a consistent feature. All cell lines show cell-density-independent colony formation and do not release self-stimulating factors, thus excluding autocrine stimulation as a model leading to transformation. All cell lines express unusually high levels of granulocyte-macrophage (GM)- and multi-CSF receptors and, except for one, M-CSF receptors. The high increase in GM-CSF and other growth factor receptors may be causally related to the transformed state of the cells. MHSV can be used as a tool to easily derive cell lines of the macrophage pathway as a model to study myeloid transformation, differentiation, and macrophage function.

Friend virus-infected long-term bone marrow cultures produce colony stimulating factor dependent and independent granulocyte-macrophage progenitor cells for over four years in vitro

C3H/HeJ mouse long-term bone marrow cultures infected at initiation with a cloned polycythemic strain of Friend spleen focus forming virus in a cloned N-tropic murine leukemia virus helper virus coat, persistently produced: colony-forming unit spleen (CFUs) for 55 weeks that formed macroscopic spleen colonies in syngeneic or allogeneic C57B10.Br/J mice; and L-cell or WEHI-3 cell conditioned medium-dependent granulocyte-macrophage colony forming unit culture (GM-CFUc); and morphologically normal granulocytes for over 245 weeks. Colony stimulating factor (CSF)-independent colony forming progenitor cells were first detectably produced in vitro at 75 weeks, and when subcultured generated karyotypically distinct permanent factor-independent tumorigenic cell lines. Nonadherent cells removed from long-term marrow cultures at 19 but not at 77 weeks reconstituted donor origin hematopoiesis in C57B10.Br/J mice as measured by B-cell lineage surface immunoglobin allotype. Nonadherent cells removed at 77 weeks produced lethal splenomegaly and marrow infiltration with culture origin cells in C57B10.Br/J mice. Despite generation of clonal malignant cell lines, L-cell DSF (CSF-1, M-CSF) responsive GM-CFUc that were simultaneously produced over 4 years in the same long-term marrow cultures, grew to 7 day colonies in semisolid medium and terminally differentiated. Thus, adherent stromal cells in Friend virus-infected long-term bone marrow cultures simultaneously support CSF-responsive and malignant CSF-independent hematopoietic progenitor cells.

Multipotential hemopoietic cell lines isolated from stem cell cultures infected with Friend virus complex (MuLV + F-SFFV) show presence of MuLV but not F-SFFV

The only factor-dependent or factor-independent hemopoietic murine stem cell lines which can be permanently maintained in vitro are cell lines which originate from bone marrow cultures of congenic mice differing in the Fv-2 locus infected with Friend spleen focus forming virus (F-SFFV) in conjunction with either Rauscher or Friend helper virus (R-MuLV or F-MuLV). We determined the viral state of these cells by restriction enzyme analysis, measurement of SFFV and MuLV related RNA, immunoprecipitation analysis of viral related proteins and biological activity to test whether integration and expression of R-MuLV or F-SFFV are obligatory. All stem cell lines (SUT, JUT, 416B) showed expression of MuLV coded proteins or virus but SFFV was not found in these cell lines. A two-fold difference of RNA hybridizing with SFFV-specific cDNAs observed in stem cells of Fv-2rr and of Fv-2ss genotype is in agreement with data published earlier.

Multistep virus-induced leukemogenesis in vitro: description of a model specifying three steps within the myeloblastic malignant process

A helper-independent Friend leukemia virus was used to infect bone marrow cultures. This virus induces myeloblastic leukemia in mice after a long latency period. Infection of the bone marrow cultures resulted in the in vitro production of myeloblastic leukemogenesis after a long latency period. Three steps were observed in the evolution of the infected cultures, and permanent cell lines were derived at each step. This allowed us to individualize three successive events in the course of the myeloblastic transformation: (i) an abnormal responsiveness to the physiological hormone granulo-macrophagic colony-stimulating factor, (ii) the acquisition of growth autonomy, and (iii) the acquisition of in vivo tumorigenicity.

Biologic properties of factor-independent nonadherent hematopoietic and adherent preadipocyte cell lines derived from continuous bone marrow culture

Cell lines dependent for growth upon an inducer T-cell synthesized glycoprotein factor interleukin-3 have been derived from continuous mouse bone marrow cultures. These factor-dependent (FD) lines have been shown to be multipotential (erythroid/basophil/neutrophil) or (eosinophil/basophil/neutrophil); or are unipotent basophil or neutrophil granulocyte cell lines. Both classes of cloned FD lines have maintained self-renewal in vitro for several years with absolute growth dependence on freshly added IL-3. In four instances, factor-independent (FI) variant cell lines were derived, one by subculture in medium containing hydrocortisone and 25% horse serum and three by evolution of variants from cloned FD lines. One class of (FI) lines demonstrated adherent fibroblast-like morphology with differentiation to differentiated adipocytes in medium containing 10(-5) hydrocortisone. A second class of cell lines evolved from cloned FD lines and each grew in suspension culture to a saturation density over 10-fold greater than that for the parent FD line (greater than 10(7)/ml) and each contained no detectable hematopoietic cellular differentiation markers by histochemistry or cell surface receptors. In contrast to IL-3 dependent cell lines, (FI) cell lines failed to differentiate to mature granulocyte morphology in diffusion chambers in vivo. The FI cell lines formed no detectable CFUs in vivo, did not reconstitute hematopoiesis in irradiated mice and did not form tumors in vivo. The failure of the (FI) lines to form tumors and lack of detectable hematopoietic differentiation capacity indicates that these lines may represent an intermediate state between normally regulated hematopoietic cellular self-renewal and malignant transformation.

Multistep virus-induced leukemogenesis in vitro: description of a model specifying three steps within the myeloblastic malignant process

A helper-independent Friend leukemia virus was used to infect bone marrow cultures. This virus induces myeloblastic leukemia in mice after a long latency period. Infection of the bone marrow cultures resulted in the in vitro production of myeloblastic leukemogenesis after a long latency period. Three steps were observed in the evolution of the infected cultures, and permanent cell lines were derived at each step. This allowed us to individualize three successive events in the course of the myeloblastic transformation: (i) an abnormal responsiveness to the physiological hormone granulo-macrophagic colony-stimulating factor, (ii) the acquisition of growth autonomy, and (iii) the acquisition of in vivo tumorigenicity.

Studies on the in vivo production of a lymphokine activity, interleukin 3 (IL-3) elaborated by lymphocytes and a myeloid leukaemic line in vitro and the fate of IL-3 dependent cell lines

Interleukin 3 (IL-3) is produced constitutively by WEHI-3b leukaemic cells and stimulated lymphoid cell populations in vitro. We have investigated the in vivo production of IL-3 in mice rendered leukaemic with WEHI-3b cells and mice stimulated by acute graft versus host disease (GVHD). In leukaemic mice, IL-3 was not found in serum or sonicates of 18-day spleens or bone marrow, although cells from the leukaemic organs were fully competent to elaborate IL-3 in vitro. Further, elaboration of IL-3 by WEHI cells in vitro was not affected by co-culture with normal haemopoietic cells. However, intracellular IL-3 was detected in leukaemic nodules isolated from the liver. Inhibitors specific for IL-3 were not found, although liver-cell conditioned medium and leukaemic nodule sonicates contained potent non-specific inhibitors of cell growth. At 21 days, intracellular IL-3 was also present in spleens and correlated with the presence of non-specific inhibitors. In GVHD, no evidence for IL-3 elaboration in vivo was found, nor did lymphoid populations affected by GVHD spontaneously elaborate it in vitro; however, their competence to produce it was unaffected, as IL-3 was elaborated on subsequent mitogen stimulation in vitro. We also investigated the recovery and circulation of in vitro 111Indium-labelled IL-3 dependent cells after injection in vivo and the half-life of semi-purified IL-3. Dependent cells were not recovered after injection into irradiated recipients, although the cells recirculated for at least 24 hours. Inability to recover dependent cells was explicable on general cytotoxicity which masked potential recovery. The serum half-life of injected partially purified material with IL-3 activity was short (less than 30 min). We conclude that the elaboration of IL-3 by leukaemic WEHI-3b is not an in vitro artifact and these results are discussed in relationship to other growth factors and the leukaemic state, and the origin of IL-3 dependent lines.

Demonstration of permanent factor-dependent multipotential (erythroid/neutrophil/basophil) hematopoietic progenitor cell lines

Multipotential hematopoietic progenitor cell lines have been established from nonadherent cell populations removed from continuous mouse bone marrow cultures. Clonal sublines of lines B6SUtA or B6JUt derived from single cells formed mixed colonies containing erythroid cells, neutrophil-granulocytes, and basophil/mast cells in semisolid medium containing erythropoietin and conditioned medium from pokeweed mitogen-stimulated spleen cells. Each of several subclones of cell line Ro cl formed colonies containing eosinophils, neutrophil-granulocytes, and basophil/mast cells in semisolid medium. Multipotentiality was maintained in vitro for over 2 1/2 years. In contrast, cell line 32D formed basophil/mast cell colonies with no detectable differentiation to other pathways. Multipotential cell lines did not produce detectable spleen colonies (CFUs) in vivo, nor did intravenous inoculation of up to 5 X 10(7) cells protect lethally irradiated mice from bone marrow failure. Newborn and adult mice inoculated with 5 X 10(7) cells showed no detectable leukemia or solid tumors after one year. Both multipotential and committed basophil/mast cell lines demonstrated absolute dependence upon a source of a growth factor(s) found in medium conditioned by WEHI-3 cells. These cell lines should be of value in studies of the regulation of hematopoietic stem cell differentiation in vitro.

Role of stromal and hematopoietic stem cells in Friend spleen focus forming virus effects in continuous bone marrow culture

Replication of the Friend spleen focus forming virus (SFFV) in C3H/HeJ or C57BL/6J mouse continuous bone marrow cultures is associated with an increased cumulative production of pluripotential/hematopoietic stem cells (CFUs), granulocyte-macrophage progenitor cells (GM-CFUc), and total granulocytes, compared to uninfected or helper virus infected control cultures. The site(s) of action of the virus are not known. To determine whether viral effect(s) occurred in adherent stromal and/or non-adherent hematopoietic stem cells, purified cell populations, comprised exclusively of cells from each compartment, were separated from C57BL/6J (Fv-1bb) or C3H/HeJ (Fv-1nn) marrow cultures and were left uninfected or were infected with host range replication restricted B-tropic or N-tropic helper pseudotype viruses of SFFV respectively. Cell populations were then mixed to establish continuous hematopoiesis in allogeneic or syngeneic combinations. Virus host range restriction between compartments was maintained in allogeneic reconstituted cultures which showed active hematopoiesis for 16-17 weeks and no significant differences between: (1) virus infected and uninfected groups or (2) groups comprised of stromal cells from one or the other strain. Thus, these data indicate that the prolongation of hematopoiesis in undisturbed long-term marrow cultures by SFFV occurs through an interaction of adherent hematopoietic stem cells with the marrow stroma.

Extended self-renewal capacity of pluripotent hemopoietic stem cells: association with persistent Friend spleen focus-forming virus

The pluripotential hemopoietic stem cells (CFU-S) of genetically resistant Fv2rr adult C57BL/6 (B6) mice can be persistently infected by Friend virus (FV) when exposed to defective spleen focus-forming virus (SFFV) and helper virus (LLV) at the time of transplantation into heavily irradiated syngeneic recipients (that is, B6 leads to B6 isologous chimeras). Whereas normal and LLV+ marrow were depleted of CFU-S after three serial transplantations at 14-day intervals, SFFV+ donor marrow exhibited prolonged self-renewal capacity. SFFV+ CFU-S from B6. Fv2rr chimeric mice gave rise to normal erythroid and myeloid progenitor cells and were easily detected and quantified over 17 serial marrow transfers equivalent to 260 cumulative days. Marrow cellularity on day 14 of each transfer remained constant during the entire transplantation time. When SFFV fell below detectable levels at passage 15, CFU-S self-renewal exhibited rapid and total decline. Since the frequency of LLV-infected cells in late marrow transfers rose during the period of CFU-S decline, we conclude that SFFV is directly associated with the triggering of CFU-S self-renewal. It is also suggested that this proliferative event may be a prerequisite for the development of FV-induced leukemia.

Immunotherapy of murine leukemia. VII. Prevention of Friend leukemia virus-induced immunosuppression by passive serum therapy

Previous studies have suggested that the passive therapy of Friend leukemia virus (FLV)-induced disease with chimpanzee anti-FLV serum operates by reducing the level of infectious virus in the treated animal below the immunosuppressive threshold, thereby allowing the host to mount anti-viral immune responses which are responsible for long-term protection. The present study was undertaken to examine directly the effect of passive serum therapy on the marked immunosuppression induced by FLV in progressively infected mice, as well as to determine whether virus-specific host cellular immune effector functions are augmented in serum-protected animals. Using a variety of assays of host immunocompetence, including natural killing (NK), antibody-dependent cellular cytotoxicity (ADCC) in vivo and in vitro induction of allogeneic killers, and mitogen blastogenesis, a marked compartmentalization of FLV immunodepression was observed in progressively infected DBA/2 mice, possibly reflecting the distribution of FLV target cells in various host lymphoid populations. Thus, spleen-cell functions were suppressed most rapidly and to the greatest degree, followed by peritoneal cells and peripheral blood lymphocytes, while lymph node cells and thymocytes maintained normal levels of activity. In contrast, serum-protected mice demonstrated no sign of FLV-induced immunosuppression regardless of the host effector-cell population or immune function examined. However, we were not able to identify host anti-viral cellular immune functions which are significantly enhanced in serum-protected animals; thus the specific role of the host immune system in the passive serum therapy of FLV-induced disease remains undefined at the present time.

In vitro quantitation of lethal and physiologic effects of total body irradiation on stromal and hematopoietic stem cells in continuous bone marrow cultures from Rf mice

The role of stromal-supportive cells in hematopoietic stem cell responses to irradiation is poorly understood. The effects of in vivo total body irradiation (TBI) and interval from TBI to explant of marrow on: stromal cell proliferation in vitro; stromal cell support of hematopoiesis in continuous bone marrow culture; and generation of WEHI-3 growth factor (GF)-dependent lines of hematopoietic progenitor cells were evaluated. Continuous marrow cultures from non-irradiated control RfM/UN, C57BL/6J, C3H/HeJ, and N:NIH (Swiss) mice generated pluripotential hematopoietic stem cells (CFUs) and committed granulocyte-macrophage progenitor cells (GM-CFUc) for over 20 weeks. Explant of marrow at 2, 4, 5, or 6 months after single fraction TBI (300-800 rad) was associated with decreased longevity of hemopoiesis (2-12 weeks), and a decrease in the proliferative capacity of fibroblastic adherent-stromal colony forming cells (CFUf) as measured by colony size at 14 days and number of colonies per 10(6) cells plated. In contrast, explant of marrow 8 to 24 months after TBI produced cultures with longevity that was indistinguishable from age-matched control cultures (19-24 weeks). Marrow from irradiated first and second generation recipients of serially transferred marrow demonstrated a similar 7-month in vivo recovery period; however, the plateau maximum duration of hemopoiesis did not return to control levels. Purified stromal cell cultures were prepared by corticosteroid-deprivation of explanted marrow for 28 days and were then engrafted in vitro with marrow from C57BL/6J or RfM/UN mice that had been irradiated 1 month previously. Hemopoiesis in these cultures was restored, and they produced GM-CFUc and granulocytes for 15-24 weeks. Thus, healthy stroma supported growth of recently irradiated hemopoietic cells in vitro. Nonadherent cells removed from the above continuous marrow cultures generated clonal non-leukemogenic WEHI-3 GF-dependent hemopoietic progenitor cell lines with a frequency concordant with radiation effects on culture longevity, and this was increased by the presence of purified healthy stromal cultures. Indirect effects of x-irradiation on hemopoietic stem cells through damage and repair in the stromal cell compartment can be effectively studied with the present bone marrow culture system.

Friend disease in vitro

In long-term marrow cultures, hemopoiesis can be maintained for several months, although erythropoiesis is normally suppressed at the most primitive level of development (the erythroid colony-forming cells). Infection of these cultures with a viral complex combining helper-independent murine leukemia virus (F-MuLV) and a spleen focus-forming virus (SFFVp) results in a productive infection of both the replication defective SFFVp and the F-MuLV. After infection, the cultures show a dramatic elevation in the numbers of late erythroid progenitor cells (CFU-E), many of which will grow in the absence of added erythropoietin, and a transient erythropoietin, independent erythropoiesis, including the production of mature, enucleated erythrocytes. Hemopoiesis eventually declines, with no evidence for the generation of Friend tumor cells. When erythropoiesis is induced in the long-term cultures by addition of anemic mouse serum before infection by polycythemia-inducing Friend virus, the generation of erythropoietin-independent CFU-E and erythrocyte formation is followed by the sustained production (greater than 40 wk) of primitive erythroid cells with low spontaneous levels (less than 5%) of hemoglobinization. Although these cells will produce spleen colonies in irradiated mice and can be cloned in soft-gel media, they do not produce autonomous, permanently growing cell lines in vitro, i.e., they retain a dependency upon the marrow-adherent layer for their continued growth. However, following a further passage on a "virgin" marrow environment, permanent cell lines can be established that are able to grow independently of environmental influences. Thus, this system is the first description of a complete in vitro system for the reproducible production and isolation of Friend virus-induced erythroid cell lines.

Emergence of tumorigenic cells during the course of Friend virus leukemias

Appearance of tumorigenic cells was studied in DBA/2 and ICFW mice infected either with the polycythemia-inducing or the anemia-inducing strain of Friend leukemia virus. Tumorigenicity was defined by transplantability of virus-infected cells into the omentum of an isogeneic preirradiated host. Tumorigenic cells were detected in 50% of the leukemic donors 3 wk after infection by the polycythemia-inducing strain and 7-8 wk after infection by the anemia-inducing strain. These cells appeared first in the spleen and later in peripheral blood, bone marrow, and liver. They consisted of a heterogeneous population at different degrees of malignancy as determined by successive transfers in vivo and in vitro. The observations clearly show that leukemias induced by Friend viruses evolve by multistep processes, in which different stages of malignancy can be detected.

Self-renewal of factor-dependent hemopoietic progenitor cell-lines derived from long-term bone marrow cultures demonstrates significant mouse strain genotypic variation

Long-term bone marrow cultures established from C57Ks/J mice have been shown to spontaneously release endogenous ecotropic RNA type-C virus (retrovirus). C57Ks/J marrow cultures produced granulocyte-macrophage progenitor cells (GM-CFUc) and immature and mature granulocytes for over 45 weeks. In contrast, NIH Swiss mouse marrow cultures failed to release detectable ecotropic virus and generated GM-CFUc and granulocytes for 25-35 weeks and established WEHI-3 conditioned medium (CM) dependent cell lines in vitro and did not establish permanent cell lines. To determine whether viral and/or cellular genes regulated the longevity of C57Ks/J marrow cultures, groups of cultures were established from the marrow of (NIH-Swiss X C47Ks/J) F1 hybrid, F2 hybrid, and (NIH Swiss X C47Ks/J) X NIH Swiss backcross generations. Release of endogenous ecotropic virus was measured weekly in each culture as was the duration of production of immature granulocytic cells and GM-CFUc over a 58-week period. The results demonstrated a complex pattern of inheritance of longevity of long-term in vitro hemopoiesis. Increased longevity did not absolutely correlate with detectable replication of the C57Ks/J N-tropic virus.