Fetal liver pro-B and pre-B lymphocyte clones: expression of lymphoid-specific genes, surface markers, growth requirements, colonization of the bone marrow, and generation of B lymphocytes in vivo and in vitro

Comparative effects of human Ig alpha and Ig beta in inducing autoreactive antibodies against B cells in mice

Human and mouse Ig alpha molecules share only 58% amino acid sequence identity in their extracellular regions. However, mice immunized with a recombinant Fc fusion protein containing the extracellular portion of human Ig alpha produced significant amounts of IgG capable of binding to Ig alpha on mouse B cells. The induced auto/cross-reactive Abs could down-regulate B cell levels and the consequent humoral immune responses against an irrelevant Ag in treated mice. Analogous immunization with an Fc fusion protein containing the extracellular portion of human Ig beta gave a much weaker response to mouse Ig beta, although human and mouse Ig beta, like their Ig alpha counterparts, share 56% sequence identity in their extracellular regions. Protein sequence analyses indicated that a potential immunogenic segment, located at the C-terminal loop of the extracellular domain, has an amino acid sequence that is identical between human and mouse Ig alpha. A mAb A01, which could bind to both human and mouse Ig alpha, was found to be specific to a peptide encompassing this immunogenic segment. These findings suggest that specific auto/cross-reactivity against self Ig alpha can be induced by a molecular mimicry presented by a foreign Ig alpha.

Long-lived polyclonal B-cell lines derived from midgestation mouse embryo lymphohematopoietic progenitors reconstitute adult immunodeficient mice

Lymphohematopoietic progenitors derived from midgestation mouse embryos were established in long-term cultures with stromal cell monolayers and interleukin 7 (IL-7), giving rise to B-lineage cell lines. The initial emergence and in vitro establishment of these early embryo cell lines were highly sensitive to IL-7-mediated signals, in comparison to cell lines similarly obtained using precursors from late fetal liver (> 13 days postcoitum) and adult bone marrow. The early embryo-derived progenitors spontaneously differentiated in vitro to CD19(+)IgM(+) immature B cells in the presence of optimal concentrations of IL-7, in contrast to those progenitors obtained from late gestation and adult mice, whose differentiation only occurred in the absence of IL-7. The newly in vitro-generated B cells of the early embryo cell lines repopulated adult immunodeficient severe combined immunodeficient mice on their adoptive transfer in vivo and generated specific humoral immune responses after immunization.

Spatial and temporal mapping of c-kit and its ligand, stem cell factor expression during human embryonic haemopoiesis

Receptor tyrosine kinases (RTKs) mediate cellular responses to the extracellular signals involved in the regulation of cell differentiation and proliferation. Ligand binding initiates a cascade of events, such as receptor dimerization and tyrosine phosphorylation. The c-kit gene encodes an RTK for stem cell factor (SCF), (c-kit ligand, KL), both of which play a critical role in the differentiation and growth of haemopoietic stem cells (HSCs). We investigated the expression of the c-kit and SCF genes and the presence of the corresponding proteins in haemopoietic tissues during human embryogenesis. We have examined c-kit and SCF transcripts levels in human embryonic yolk sac, the AGM region, and liver at different stages of gestation (days 25 to 63), using RT-PCR amplification combined with PhosphorImager quantitative analysis and RNase Protection Assay (RPA). Weak levels of SCF gene expression were observed in the AGM region (days 25 to 34) and high levels were found in the early-stage liver (day 34). The expression of c-kit transcript was observed in all studied tissues, but at various levels. The restricted presence of SCF protein following mRNA expression was demonstrated in embryonic liver CD38+ haemopoietic cells by immunocytochemistry. These observations suggest that the biological function of the c-kit receptor plays an important role in the early stages of human haemopoiesis, and that c-kit/SCF signalling is particularly involved in early human definitive haemopoiesis.

The role of cytokine receptor signaling in lymphocyte development

Cytokine receptor signaling plays an essential role in the early stages of lymphocyte development. Signals through various cytokine receptors - such as c-kit, flt3/flk2, CXCR4, the IL-7 receptor and the IL-15 receptor - are known to promote the expansion and survival of uncommitted progenitor cells as well as their migration to the appropriate microenvironment and subsequent differentiation into B, T or natural killer cells. The recent generation of mice deficient in one or more of these signaling pathways has revealed which cytokines play unique and/or redundant roles in each of these lymphocyte lineages during this developmental process.

Four-Color Flow Cytometric Investigation of Terminal Deoxynucleotidyl Transferase–Positive Lymphoid Precursors in Pediatric Bone Marrow: CD79a Expression Precedes CD19 in Early B-Cell Ontogeny

Terminal deoxynucleotidyl transferase (TdT)-positive cells in human bone marrow (BM) are a phenotypically inhomogeneous population of precursor cells. In their majority, these TdT+ cells are unambiguously committed to the B lineage, as evidenced by CD19 expression. However, TdT+ precursors that lack CD19 also exist and these may encompass a differentiation potential for the B as well as for other lineages. Because recent data suggested that CD19 expression is not the earliest differentiation event in B-cell ontogeny, we sought to reevaluate TdT+ lymphoid precursors in pediatric BM to define the phenotypic denominator of B-lineage affiliation upstream of CD19. Using four-color flow cytometry, we focused on the assessment of the CD79a antigen, which is highly B-cell specific and which may also be expressed very early in B-cell ontogeny. We found that a majority of TdT+ cells coexpressed CD19 and CD79a in addition to CD10 and CD34, whereas, in all investigated samples, some TdT+ precursors lacked CD19 but expressed CD79a, which suggestively indicates also their B-lineage affiliation. In contrast to the CD19+precursors, which were usually CD10hi and CD79b+, these CD19−CD79a+putative B-cell precursors preferentially expressed CD10 at low levels and were CD79b+ in only 41%. About 17% of these TdT+CD19−CD79a+ precursors also coexpressed CD33 and CD7, but not myeloperoxidase, CD14, or cytoplasmic CD3, which is discussed in the light of cellular activation rather than lineage promiscuity. Our data confirm that the earliest differentiation stages of B cells can be dissected upon expression of the lineage antigens CD79a and CD19 and imply that CD79a is earlier expressed than CD19. This raises the chance to follow the sequential events heralding B-cell commitment in the most immature precursors by correlating phenotypic and genetic differentiation markers.

© 1998 by The American Society of Hematology.

Four-Color Flow Cytometric Investigation of Terminal Deoxynucleotidyl Transferase–Positive Lymphoid Precursors in Pediatric Bone Marrow: CD79a Expression Precedes CD19 in Early B-Cell Ontogeny

Terminal deoxynucleotidyl transferase (TdT)-positive cells in human bone marrow (BM) are a phenotypically inhomogeneous population of precursor cells. In their majority, these TdT+ cells are unambiguously committed to the B lineage, as evidenced by CD19 expression. However, TdT+ precursors that lack CD19 also exist and these may encompass a differentiation potential for the B as well as for other lineages. Because recent data suggested that CD19 expression is not the earliest differentiation event in B-cell ontogeny, we sought to reevaluate TdT+ lymphoid precursors in pediatric BM to define the phenotypic denominator of B-lineage affiliation upstream of CD19. Using four-color flow cytometry, we focused on the assessment of the CD79a antigen, which is highly B-cell specific and which may also be expressed very early in B-cell ontogeny. We found that a majority of TdT+ cells coexpressed CD19 and CD79a in addition to CD10 and CD34, whereas, in all investigated samples, some TdT+ precursors lacked CD19 but expressed CD79a, which suggestively indicates also their B-lineage affiliation. In contrast to the CD19+precursors, which were usually CD10hi and CD79b+, these CD19−CD79a+putative B-cell precursors preferentially expressed CD10 at low levels and were CD79b+ in only 41%. About 17% of these TdT+CD19−CD79a+ precursors also coexpressed CD33 and CD7, but not myeloperoxidase, CD14, or cytoplasmic CD3, which is discussed in the light of cellular activation rather than lineage promiscuity. Our data confirm that the earliest differentiation stages of B cells can be dissected upon expression of the lineage antigens CD79a and CD19 and imply that CD79a is earlier expressed than CD19. This raises the chance to follow the sequential events heralding B-cell commitment in the most immature precursors by correlating phenotypic and genetic differentiation markers.

© 1998 by The American Society of Hematology.

Preferential proliferation of murine colony-forming units in culture in a chemically defined condition with a macrophage colony-stimulating factor-negative stromal cell clone

The establishment of culture conditions that selectively support hematopoietic stem cells is an important goal of hematology. In this study, we investigated the possibility of using for this purpose a defined medium, mSFO2, which was developed for stromal cell-dependent bone marrow cultures. We found that a combination of epidermal growth factor (EGF), the OP9 stromal cell line, which lacks macrophage colony-stimulating factor, recombinant stem cell factor, and the chemically defined medium mSFO2 provides a microenvironment where c-Kit+ Thy-1+/lo Mac-1+/lo B220- TER119- common beta + IL-2R gamma + gp130+ cells are selectively propagated from normal, unfractionated bone marrow cells. This cell population produced an in vitro colony at a very high efficiency (50%), whereas it has only limited proliferative ability in the irradiated recipient. Thus, the cells selected in this culture condition might represent colony-forming units in culture (CFU-c) with short-term reconstituting ability. Transferring this cell population into medium containing differentiation signals resulted in the rapid production of mature myelomonocytic and B cell lineages in vitro and in vivo. The fact that a similar culture condition was created by erb-B2-transduced OP9 in the absence of EGF indicated that EGF exerts its effect by acting on OP9 rather than directly on CFU-c. These results suggested that the balance between self-renewal and differentiation of CFU-c can be regulated by extra-cellular signals.

Targeted disruption of the flk2/flt3 gene leads to deficiencies in primitive hematopoietic progenitors

The flk2 receptor tyrosine kinase has been implicated in hematopoietic development. Mice deficient in flk2 were generated. Mutants developed into healthy adults with normal mature hematopoietic populations. However, they possessed specific deficiencies in primitive B lymphoid progenitors. Bone marrow transplantation experiments revealed a further deficiency in T cell and myeloid reconstitution by mutant stem cells. Mice deficient for both c-kit and flk2 exhibited a more severe phenotype characterized by large overall decreases in hematopoietic cell numbers, further reductions in the relative frequencies of lymphoid progenitors, and a postnatal lethality. Taken together, the data suggest that flk2 plays a role both in multipotent stem cells and in lymphoid differentiation.

B-lymphoid potential in pre-liver mouse embryo

Increasing interest in the initial steps of hemopoiesis in the embryo is prompted by the lack of information about the primordial origin and the nature of hemopoietic stem cells. It appears critical to understand the emergence, diversification and differentiation potential of hemopoietic cells in the preliver embryo. In-vitro studies of B-potential in the mouse embryo prior to liver colonization show that, contrary to interpretations prevalent 10 years ago, there are two sources of B-cell progenitors (and most probably stem cells) at these stages: the yolk sac and the paraaortic splanchnopleura within the embryo proper. The analysis of the phenotype and differentiation potential of precursors from both sources shows them to be very similar, while different from the fetal liver progenitors. Indications for an independent generation of precursors in both sites are reviewed.

Molecular cloning and chromosomal mapping of a bone marrow stromal cell surface gene, BST2, that may be involved in pre-B-cell growth

Bone marrow stromal cells regulate B-cell growth and development through their surface molecules and cytokines. In this study, we generated a mAb, RS38, that recognized a novel human membrane protein, BST-2, expressed on bone marrow stromal cell lines and synovial cell lines. We cloned a cDNA encoding BST-2 from a rheumatoid arthritis-derived synovial cell line. BST-2 is a 30- to 36-kDa type II transmembrane protein, consisting of 180 amino acids. The BST-2 gene (HGMW-approved symbol BST2) is located on chromosome 19p13.2. BST-2 is expressed not only on certain bone marrow stromal cell lines but also on various normal tissues, although its expression pattern is different from that of another bone marrow stromal cell surface molecule, BST-1. BST-2 surface expression on fibroblast cell lines facilitated the stromal cell-dependent growth of a murine bone marrow-derived pre-B-cell line, DW34. The results suggest that BST-2 may be involved in pre-B-cell growth.

Pregnancy: a clue to normal regulation of B lymphopoiesis

The large-scale production of lymphocytes in the bone marrow reflects a delicate balance between positive and negative regulatory signals. For instance, interleukin 7 (IL-7) provides a positive signal, and appears to be both essential and limiting in the mouse. However, much less is understood concerning the negative molecular signals that may limit the output of lymphocytes. Here, Paul Kincade and colleagues discuss how a chance observation with pregnant mice revealed that sex steroids can act as negative regulators of B lymphopoiesis, and may do so under normal steady-state conditions.

Regulation of gene expression at early stages of B-cell differentiation

The phenotype of B lymphocytes at various stages of differentiation is, in part, controlled at the transcriptional level. Recently, a number of B-cell lineage and stage-specific transcription factors have been identified as candidate determinants for the developmental regulation of gene expression in B lymphocytes.

B-lineage differentiation in normal and transformed cells and the microenvironment that supports it

B-cell differentiation is a complex process mediated through interactions with the microenvironment of the bone marrow and fetal liver. These interactions alter patterns of gene expression and allow precursors to develop into Ig+ B cells. Recent work has shown that some of these events can be triggered in B-cell precursors transformed by Abelson virus. Other advances have refined our understanding of the role of cytokines, hormones and stromal cells in the differentiation process.

Developmentally regulated cell surface expression and function of c-kit receptor during lymphocyte ontogeny in the embryo and adult mice

We have used a c-kit-specific monoclonal antibody, immuno-fluorescence staining and flow fluorocytometry or microscopy analysis to assess the cell surface expression of the c-kit receptor on a panel of non-transformed clones representing different stages of T- and B-lymphocyte development, freshly isolated lymphoid cells from thymus, bone marrow and spleen of young adult C57BL/6 mice and cells from yolk sac, thymus and liver of developing C57BL/6 mouse embryos. Pro-T, Pro-B and Pre-B clones derived from thymus or liver of 14-day embryos are c-kit+. Starting at day 8 to 8.5 in yolk sac, day-10 in fetal liver, and day 11 to 12 in fetal thymus, there are many c-kit+ cells. The number of c-kit+ cells in liver and thymus increases up to day 15 and progressively decreases thereafter. Cell sorter purified c-kit+ day 14 fetal liver cells fully reconstitute the T and B cell compartments of immunodeficient Scid mice. Stromal cells or epithelial cells derived from fetal thymus or liver, which can support growth and differentiation of c-kit+ lymphocyte progenitor clones, synthesize mRNA for Steel Factor (SF), the ligand of c-kit. In the adult mouse, however, c-kit expression is restricted to very early stages of T- and B-lymphocyte development (multipotent progenitors, B-cell/myelocytic progenitors, Pro-T and Pro-B lymphocyte progenitors). Most cells at the Pre-T, Pre-B and later stages of development do not bear detectable c-kit. Using Cos-1 cells transfected with mouse SF-cDNA and an antagonistic c-kit receptor-specific antibody, we show that the c-kit/SF system contributes to the survival of lymphocyte progenitors and enhances the proliferative responses of these cells to other growth factors (i.e. IL2, IL3, IL4, IL7). However, the c-kit receptor/SF ligand pair is neither sufficient nor necessary for the differentiation of lymphocyte progenitors into mature T- or B-lymphocytes. Finally, in stromal cell lines from fetal liver and adult bone marrow and thymic epithelial cell lines the level of steady state SF-RNA transcripts is inversely correlated with that of IL-7-mRNA. Moreover, IL7 inhibits the synthesis of SF-mRNA in stromal cells and rIL6 abrogates this inhibitory effect of rIL7. Thus, the expression of SF in stromal cells is subjected to complex regulation by other cytokines produced by the same stromal cells or by neighboring cells in a given microenvironment.(ABSTRACT TRUNCATED AT 400 WORDS)