Characterization of a 70Z/3 pre-B cell derived macrophage clone. Differential expression of Hox family genes

Epigenetic histone modifications do not control Igkappa locus contraction and intranuclear localization in cells with dual B cell-macrophage potential

Somatic rearrangement of the Ig genes during B cell development is believed to be controlled, at least in part, by accessibility of the loci to the recombinational machinery. Accessibility is poorly understood, but appears to be controlled by a combination of histone posttranslational modifications, large scale Ig locus contractions, and changes in intranuclear localization of the loci. These changes are regulated by developmental stage-specific as well as tissue-specific mechanisms. We previously isolated a murine B cell lymphoma line, Myc5, that can oscillate between the B cell and macrophage lineages depending upon growth conditions. This line provides an opportunity to study tissue-specific regulation of epigenetic mechanisms operating on the Ig loci. We found that when Myc5 cells are induced to differentiate from B cells into macrophages, expression of macrophage-specific transcripts was induced (M-CSFR, F4/80, and CD14), whereas B cell-specific transcripts decreased dramatically (mb-1, E47, IRF4, Pax5, and Igkappa). Loss of Igkappa transcription was associated with reduced Igkappa locus contraction, as well as increased association with heterochromatin protein-1 and association of the Igkappa locus with the nuclear periphery. Surprisingly, however, we found that histone modifications at the Igkappa locus remained largely unchanged whether the cells were grown in vivo as B cells, or in vitro as macrophages. These results mechanistically uncouple histone modifications at the Igkappa locus from changes in locus contraction and intranuclear localization.

Oscillation between B-lymphoid and myeloid lineages in Myc-induced hematopoietic tumors following spontaneous silencing/reactivation of the EBF/Pax5 pathway

B lymphomagenesis is an uncontrolled expansion of immature precursors that fail to complete their differentiation program. This failure could be at least partly explained by inappropriate expression of several oncogenic transcription factors, such as Pax5 and Myc. Both Pax5 and c-Myc are implicated in the pathogenesis of non-Hodgkin lymphomas. To address their role in lymphomagenesis, we analyzed B-cell lymphomas derived from p53-null bone marrow progenitors infected in vivo by a Myc-encoding retrovirus. All Myc-induced lymphomas invariably maintained expression of Pax5, which is thought to be incompatible with terminal differentiation. However, upon culturing in vitro, several cell lines spontaneously down-regulated Pax5 and its target genes CD19, N-Myc, and MB1. Unexpectedly, other B-cell markers (eg, CD45R) were also down-regulated, and markers of myeloid lineage (CD11b and F4/80 antigen) were acquired instead. Moreover, cells assumed the morphology reminiscent of myeloid cells. A pool of F4/80-positive cells as well as several single-cell clones were obtained and reinjected into syngeneic mice. Remarkably, pooled cells rapidly re-expressed Pax5 and formed tumors of relatively mature lymphoid phenotype, with surface immunoglobulins being abundantly expressed. Approximately half of tumorigenic single-cell clones also abandoned myeloid differentiation and gave rise to B lymphomas. However, when secondary lymphoma cells were returned to in vitro conditions, they once again switched to myeloid differentiation. This process could be curbed via enforced expression of retrovirally encoded Pax5. Our data demonstrate that some Myc target cells are bipotent B-lymphoid/myeloid progenitors with the astonishing capacity to undergo successive rounds of lineage switching.

Absence of CD5 dramatically reduces progression of pulmonary inflammatory lesions in SHP-1 protein-tyrosine phosphatase-deficient 'viable motheaten' mice

Mice homozygous for the viable motheaten (Hcph(me-v)) mutation are deficient in SHP-1 protein-tyrosine phosphatase, resulting in severe systemic autoimmunity and immune dysfunction. A high percentage of B-cells in viable motheaten mice express the cell surface glycoprotein CD5, in contrast to wild type mice that express CD5 on only a small percentage of B-cells. CD5(+) B-cells have been associated with autoantibody production. To determine the role of CD5 in the development of the inflammatory disease in me(v)/ me(v) mice, we created a stock of CD5(null)me(v)/ me(v) mice. The longevity of CD5(null)me(v)/ me(v) mice was increased 69% in comparison to me(v)/ me(v) mice on a similar (B6;129) background. The increased lifespan was associated with a marked reduction in pulmonary inflammation. Flow cytometry analysis of spleen cells from CD5(null)me(v)/ me(v) mice at 9-12 weeks of age revealed significant decreases in percentages of IgM/B220 double positive B-cells, Mac-1/Gr-1 double positive cells and CD4(+) T-cells compared with me(v)/ me(v) mice. CD5(null)me(v)/ me(v) mice also had significantly lower serum IgM levels in comparison to me(v)/ me(v) mice. Study of CD5(null)me(v)/ me(v) mice may provide further insight into the role of CD5 in cell signaling and may help explain the observed association of CD5(+) B-cells with autoimmune disease.

Generation of bone-resorbing osteoclasts from B220+ cells: its role in accelerated osteoclastogenesis due to estrogen deficiency

Estrogen deficiency stimulates both osteoclastic bone resorption and pre-B lymphopoiesis, the interrelationships between which remain unknown. To investigate the involvement of an increase in the number of B220+ cells in accelerated osteoclastogenesis after estrogen deficiency, we first examined whether ovariectomy (OVX) increased the frequency of clonogenic osteoclast precursors in bone marrow. The results were that after OVX, the frequency of clonogenic osteoclast precursors is increased in bone marrow, suggesting that accumulated osteoclast precursors contribute to accelerated osteoclastogenesis. Further, we found that cocultures of B220+ cells purified from bone marrow cells and stromal ST2 cells in the presence of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] gave rise to osteoclasts that can resorb bone and express calcitonin receptors. When the frequencies of clonogenic osteoclast precursors in the purified B220+ and B220- cell fractions were compared, it was found that the fractions gave rise to osteoclasts at similar frequencies, which rules out the possibility of cross-contamination and suggests that the two fractions contain comparable numbers of osteoclast precursors. Furthermore, we identified cells that are positive for both tartrate-resistant acid phosphatase (TRAP) and B220, not only in cocultures of B220+ and ST2 cells, but also in freshly isolated unfractionated bone cells. Therefore, it is concluded that at least a subfraction of B220+ cells are capable of generating osteoclasts and that the increase in the number of B220+ cells caused by estrogen deficiency may contribute to accelerated bone resorption by this novel osteoclastogenesis pathway.

Lymphocyte development from hematopoietic stem cells

The recent application of new techniques, such as multi-color cell sorting and the production of transgenic and gene-knockout mice, has contributed to a better understanding of lymphocyte development from hematopoietic stem cells. Now that we can purify progenitors at different maturational stages during lymphocyte development, the challenge is to understand the processes that govern each developmental stage transition.

The relationship of CD5+ B lymphocytes to macrophages: insights from normal biphenotypic B/macrophage cells

For decades, numerous investigators have reported derivation of macrophage-like cells from CD5(+) pre-B cell lymphomas. Recently, it has become clear that biphenotypic CD5(+) B/macrophage cells are not a spurious result of malignancy. Indeed, the existence of normal biphenotypic cells with CD5(+) B lymphocyte and macrophage characteristics has been demonstrated in the mouse. This review considers normal B/macrophage cell function in an evolutionary context where a primitive, flexible cell type could perform dual roles in adaptive and innate immunity.

Bipotential B-macrophage progenitors are present in adult bone marrow

According to the current model of adult hematopoiesis, differentiation of pluripotential hematopoietic stem cells into common myeloid- and lymphoid-committed progenitors establishes an early separation between the myeloid and lymphoid lineages. This report describes a rare and previously unidentified CD45R-CD19+ B cell progenitor population in postnatal bone marrow that can also generate macrophages. In addition to the definition of this B-lineage intermediate, the data indicate that a developmental relationship between the B and macrophage lineages is retained during postnatal hematopoiesis.

BSAP/Pax5A Expression Blocks Survival and Expansion of Early Myeloid Cells Implicating Its Involvement in Maintaining Commitment to the B-Lymphocyte Lineage

Early B lymphopoiesis is marked by plasticity between the myeloid and B lineages. An attractive model for B-lineage development is that commitment to this lineage is partly determined by the ordered expression of genes that prohibit switching to the myeloid lineage. In this regard, whereas the role of the B-cell–specific transcription factor BSAP/Pax5A in regulating B-lymphoid–restricted gene expression has been well-established, its role in maintaining B-lineage commitment is unclear. Thus, BSAP/Pax5A was constitutively expressed in the multipotent EML cell line, which can be directed toward the myeloid lineage by culture with interleukin-3 (IL-3) and retinoic acid. EML cells expressing BSAP/Pax5A successfully acquired the myeloid lineage markers CD11b and F4/80 in response to IL-3 and retinoic acid, indicating differentiation to the myeloid lineage. However, these early myeloid cells failed to expand in culture with granulocyte-macrophage colony-stimulating factor and were directed instead toward an apoptotic pathway. In parallel, primary bone marrow stem cells transduced with retrovirus constitutively expressing BSAP/Pax5A began myeloid cell differentiation, but like the transformed EML model failed to expand in response to myeloid growth factors. These studies identify a role for BSAP/Pax5A in suppressing the response to myeloid growth factors, which may be a component of the regulatory processes that limit plasticity of early B-lymphoid progenitors.

BSAP/Pax5A Expression Blocks Survival and Expansion of Early Myeloid Cells Implicating Its Involvement in Maintaining Commitment to the B-Lymphocyte Lineage

Early B lymphopoiesis is marked by plasticity between the myeloid and B lineages. An attractive model for B-lineage development is that commitment to this lineage is partly determined by the ordered expression of genes that prohibit switching to the myeloid lineage. In this regard, whereas the role of the B-cell–specific transcription factor BSAP/Pax5A in regulating B-lymphoid–restricted gene expression has been well-established, its role in maintaining B-lineage commitment is unclear. Thus, BSAP/Pax5A was constitutively expressed in the multipotent EML cell line, which can be directed toward the myeloid lineage by culture with interleukin-3 (IL-3) and retinoic acid. EML cells expressing BSAP/Pax5A successfully acquired the myeloid lineage markers CD11b and F4/80 in response to IL-3 and retinoic acid, indicating differentiation to the myeloid lineage. However, these early myeloid cells failed to expand in culture with granulocyte-macrophage colony-stimulating factor and were directed instead toward an apoptotic pathway. In parallel, primary bone marrow stem cells transduced with retrovirus constitutively expressing BSAP/Pax5A began myeloid cell differentiation, but like the transformed EML model failed to expand in response to myeloid growth factors. These studies identify a role for BSAP/Pax5A in suppressing the response to myeloid growth factors, which may be a component of the regulatory processes that limit plasticity of early B-lymphoid progenitors.

Expression of an immunoglobulin heavy chain transgene in macrophage as well as lymphocyte lineages in vivo

A rearranged immunoglobulin heavy chain (IgH) transgene-encoded protein is expressed in macrophage lineage cells, in addition to B and T lineages, in transgenic mouse bone marrow. Peripheral macrophages also express transgenic IgH protein. Mature T cells express lower levels than immature thymocytes. Almost all B220+ cells in the bone marrow express transgenic IgH protein, and this early expression in the B lineage is accompanied by a reduction of cell frequency even in the early B220+ CD43+ BP-1- stages, although it is more prominent in BP-1+ pre-B cells. Thus, an IgH transgene can be expressed not only in lymphoid but also in myeloid cells, although its developmental effects are restricted to the B cell lineage.

Both Stat3-Activation and Stat3-Independent BCL2 Downregulation Are Important for Interleukin-6–Induced Apoptosis of 1A9-M Cells

A unique subclone of a bone marrow-derived stromal cell line, BMS2.4, produces soluble factors that inhibit proliferation of several types of hematopoietic cell lines. An understanding of these molecules may be informative about negative regulatory circuits that can potentially limit blood cell formation. We used expression cloning to identify interleukin-6 (IL-6) as one factor that suppressed growth of a pre-B–cell variant line, 1A9-M. Moreover, IL-6 induced macrophage-differentiation and apoptosis of 1A9-M cells. During this process, IL-6 downregulated expression of BCL2 in 1A9-M cells and stimulated BCL-XL expression, but had no effect on p53, Bax, or Bak gene expression. Mechanisms for transduction of IL-6–induced signals were then evaluated in IL-6–stimulated 1A9-M cells. Whereas the signal transducer and activator of transcription 3 (Stat3) was phosphorylated and activated, there was no effect on either Stat1 or Stat5. The importance of BCL2 and Stat3 on IL-6–induced macrophage-differentiation and apoptosis was studied with 1A9-M cells expressing human BCL2 or a dominant-negative form of Stat3, respectively. IL-6–induced apoptosis, but not macrophage-differentiation, was blocked by continuously expressed BCL2. A dominant-negative form of Stat3 inhibited both macrophage-differentiation and apoptosis induced by IL-6. However, diminished Stat3 activity did not prevent IL-6–induced downregulation of the BCL2 gene. Therefore, activation of Stat3 is essential for IL-6–induced macrophage-differentiation and programmed cell death in this model. Whereas overexpression of BCL2 abrogates the apoptotic response, Stat3-independent signals appear to downregulate expression of the BCL2 gene.

Both Stat3-Activation and Stat3-Independent BCL2 Downregulation Are Important for Interleukin-6–Induced Apoptosis of 1A9-M Cells

A unique subclone of a bone marrow-derived stromal cell line, BMS2.4, produces soluble factors that inhibit proliferation of several types of hematopoietic cell lines. An understanding of these molecules may be informative about negative regulatory circuits that can potentially limit blood cell formation. We used expression cloning to identify interleukin-6 (IL-6) as one factor that suppressed growth of a pre-B–cell variant line, 1A9-M. Moreover, IL-6 induced macrophage-differentiation and apoptosis of 1A9-M cells. During this process, IL-6 downregulated expression of BCL2 in 1A9-M cells and stimulated BCL-XL expression, but had no effect on p53, Bax, or Bak gene expression. Mechanisms for transduction of IL-6–induced signals were then evaluated in IL-6–stimulated 1A9-M cells. Whereas the signal transducer and activator of transcription 3 (Stat3) was phosphorylated and activated, there was no effect on either Stat1 or Stat5. The importance of BCL2 and Stat3 on IL-6–induced macrophage-differentiation and apoptosis was studied with 1A9-M cells expressing human BCL2 or a dominant-negative form of Stat3, respectively. IL-6–induced apoptosis, but not macrophage-differentiation, was blocked by continuously expressed BCL2. A dominant-negative form of Stat3 inhibited both macrophage-differentiation and apoptosis induced by IL-6. However, diminished Stat3 activity did not prevent IL-6–induced downregulation of the BCL2 gene. Therefore, activation of Stat3 is essential for IL-6–induced macrophage-differentiation and programmed cell death in this model. Whereas overexpression of BCL2 abrogates the apoptotic response, Stat3-independent signals appear to downregulate expression of the BCL2 gene.

Induction of early B cell factor (EBF) and multiple B lineage genes by the basic helix-loop-helix transcription factor E12

The transcription factors encoded by the E2A and early B cell factor (EBF) genes are required for the proper development of B lymphocytes. However, the absence of B lineage cells in E2A- and EBF-deficient mice has made it difficult to determine the function or relationship between these proteins. We report the identification of a novel model system in which the role of E2A and EBF in the regulation of multiple B lineage traits can be studied. We found that the conversion of 70Z/3 pre-B lymphocytes to cells with a macrophage-like phenotype is associated with the loss of E2A and EBF. Moreover, we show that ectopic expression of the E2A protein E12 in this macrophage line results in the induction of many B lineage genes, including EBF, IL7Ralpha, lambda5, and Rag-1, and the ability to induce kappa light chain in response to mitogen. Activation of EBF may be one of the critical functions of E12 in regulating the B lineage phenotype since expression of EBF alone leads to the activation of a subset of E12-inducible traits. Our data demonstrate that, in the context of this macrophage line, E12 induces expression of EBF and together these transcription factors coordinately regulate numerous B lineage-associated genes.

The interferon-inducible protein kinase PKR modulates the transcriptional activation of immunoglobulin kappa gene

PKR is an interferon (IFN)-induced serine/threonine protein kinase that regulates protein synthesis through phosphorylation of eukaryotic translation initiation factor-2 (eIF-2). In addition to its demonstrated role in translational control, recent findings suggest that PKR plays an important role in regulation of gene transcription, as PKR phosphorylates I kappa B alpha upon double-stranded RNA treatment resulting in activation of NF-kappa B DNA binding in vitro (Kumar, A., Haque, J., Lacoste, J., Hiscott, J., and Williams, B.R.G. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 6288-6292). To further investigate the role of PKR in transcriptional signaling, we expressed the wild type human PKR and a catalytically inactive dominant negative PKR mutant in the murine pre-B lymphoma 70Z/3 cells. Here, we report that expression of wild type PKR had no effect on kappa-chain transcriptional activation induced by lipopolysaccharide or IFN-gamma. However, expression of the dominant negative PKR mutant inhibited kappa gene transcription independently of NF-kappa B activation. Phosphorylation of eIF-2 alpha was not increased by lipopolysaccharide or IFN-gamma, suggesting that PKR mediates kappa gene transcriptional activation without affecting protein synthesis. Our findings further support a transcriptional role for PKR and demonstrate that there are at least two distinct PKR-mediated signal transduction pathways to the transcriptional machinery depending on cell type and stimuli, NF-kappa B-dependent and NF-kappa B-independent.

Characterization of the 3' untranslated region of the mouse homeobox gene HoxB5

The mouse pre-B cell line, 70Z/3, expresses multiple transcripts of the homeobox gene, HoxB5. We show here that this heterogeneity is due, at least in part, to the usage of alternative poly-A addition sites in the 3' untranslated region (UT) of the primary HoxB5 transcript. Furthermore, upon analysis of the subcellular distribution of the different HoxB5 RNA species, we found that the transcripts are present mainly in the nucleus, with two-to-five-fold less RNA present in the cytoplasm. These studies suggest that multiple post-transcriptional regulatory mechanisms are involved in the expression of HoxB5 RNA.

Murine B cell development: commitment and progression from multipotential progenitors to mature B lymphocytes

B lymphocytes, the cellular source of antibody, are critical components of the immune response. They develop from multipotential stem cells, progressively acquiring the traits that allow them to function as mature B lymphocytes. This developmental program is dependent on appropriate interactions with the surrounding environment. These interactions, mediated by cell-cell and cell-matrix interactions, provide the growth and differentiation signals that promote progression along the developmental pathway. This chapter addresses the properties of developing B lineage cells and the nature of the environmental signals that support B lineage progression.

Characterization of the B cell-macrophage lineage transition in 70Z/3 cells

The 70Z/3 cell line is able to undergo lineage conversion from a pre-B to macrophage phenotype. Data presented here show that the transition from pre-B to macrophage follows a reproducible pathway via a stable intermediate stage. The cells in the intermediate stage are adherent and have lost the ability to respond to lipopolysaccharide or interferon-gamma by induction of immunoglobulin kappa light chains. However, these cells do not yet display the full range of macrophage-specific properties such as receptors for macrophage-colony stimulating factor or the beta 2 integrin CD11b/CD18. Subcloning experiments with the intermediate cells revealed that they retain the options of either persisting along the macrophage line of differentiation, acquiring additional macrophage traits, or reverting to the pre-B phenotype. Further differentiation to the macrophage stage is accompanied by the apparent loss of the ability to revert. Thus, these studies define relationships among lineage-specific traits, and begin to reveal critical stages in lineage commitment.

Hypersomy of chromosome 15 with retrovirally rearranged c-myc, loss of germline c-myc and IgK/c-myc juxtaposition in a macrophage-monocytic tumour line

From a lymphoid tumour induced by 7,12-dimethylbenz-[a]-anthracene (DMBA) + methyl-N-nitrose-N-urea (MNU) in an [AKR Rb(6.15) x CBAT6T6]F1 mouse, a macrophage- monocyte line (KT-10) was isolated. Following ethyl methanesulfonate (EMS) treatment, a bromodeoxyuridine (BUdR) resistant subline was selected. Serial propagation of this line in vitro in the presence of BUdR (28 months) with periodic cytogenetic and molecular examinations, has led to the definition of four successive stages. During stage I, the cells were trisomic for chromosome 15. They contained Rb(6.15) and Rb(del6.15) of AKR and T(14;15) of CBA origin. Southern blotting showed the presence of both germline (G) and rearranged (R) c-myc. At stage II, Rb(del6.15) has duplicated. Both Rb(6.15) and T(14;15) persisted together with G-myc and R-myc. In stage III, the CBA-derived T(14;15) was lost, in parallel with G-myc. At this stage, a Dic.In(6.15) was detected. One of its arms was cytogenetically identical with the long arm of In(6.15) in the variant IgK/myc translocations. This chromosome carried R-myc and IgK in juxtaposition, as indicated by comigration on pulsed field electrophoresis (PFGE). At stage IV, the R-myc carrying AKR-derived chromosome 15s were present in six copies. Possible relationships between the increasing R/G myc ratio and changed growth characteristics in vivo and in vitro are discussed.

Characteristic patterns of HOX gene expression in different types of human leukemia

Homeobox-containing genes are a network of genes encoding DNA-binding proteins highly conserved throughout evolution. They are involved in the control of normal development as well as in the regulation of gene expression in adult differentiating systems, including hematopoiesis. Aberrant expression of homeobox-containing genes has recently been related to leukemic phenotype. Human homeobox-containing genes of the HOX family are organized into 4 large clusters. We have analyzed the expression of HOX genes in different types of human leukemia to investigate whether the physical organization of HOX loci reflects a regulatory hierarchy involved in the differentiation of hematopoietic cells or whether HOX gene expression might contribute to the leukemic phenotype. Our results show that HOX genes are coordinately regulated in blocks in myeloid cells whereas they appear to function as isolated genes in lymphoid cells. Six contiguous genes of the HOX2 locus, highly expressed in acute non-lymphocytic leukemia, are switched off in chronic myelogenous leukemia, suggesting that down-regulation of HOX2 genes might be required for cell maturation of the myeloid lineages. In contrast, a few scattered genes are active in lymphoid populations. These observations suggest that hematopoietic cells express a repertoire of HOX genes characteristic of a particular cell lineage at a specific stage of differentiation. The characteristic patterns of HOX gene expression may reflect the potentially important role that these genes play in cell lineage determination during both normal and leukemic hematopoiesis.