A temperature-sensitive mutant of Abelson murine leukemia virus confers inducibility of IgM expression to transformed lymphoid cells

The non-Ig parts of the VpreB and λ5 proteins of the surrogate light chain play opposite roles in the surface representation of the precursor B cell receptor

The VpreB and λ5 proteins, together with Igμ-H chains, form precursor BCRs (preBCRs). We established λ5(-/-)/VpreB1(-/-)/VpreB2(-/-) Abelson virus-transformed cell lines and reconstituted these cells with λ5 and VpreB in wild-type form or with a deleted non-Ig part. Whenever preBCRs had the non-Ig part of λ5 deleted, surface deposition was increased, whereas deletion of VpreB non-Ig part decreased it. The levels of phosphorylation of Syk, SLP65, or PLC-γ2, and of Ca(2+) mobilization from intracellular stores, stimulated by μH chain crosslinking Ab were dependent on the levels of surface-bound preBCRs. It appears that VpreB probes the fitness of newly generated VH domains of IgH chains for later pairing with IgL chains, and its non-Ig part fixes the preBCRs on the surface. By contrast, the non-Ig part of λ5 crosslinks preBCRs for downregulation and stimulation.

The nonimmunoglobulin portion of lambda5 mediates cell-autonomous pre-B cell receptor signaling

The pre-B cell receptor (preBCR), composed of mu immunoglobulin (Ig) and surrogate light chains, signals large 'preB-II' cells to proliferate in the apparent absence of ligands or cooperating cells. We deleted the N-terminal, nonimmunoglobulin (nonlg) portion of lambda5, or mutated seven arginine residues in it to serine residues. PreBCRs with such mutant lambda5 proteins showed increased cell surface representation and a diminished rate of aggregation and internalization. Tyrosine phosphorylation of preBCR complexes containing mutant lambda5 proteins was abolished. These results indicate that the nonIg portion of lambda5, and the seven arginine residues in it, are needed for signal transduction, and that signaling could be cell autonomous. We propose two models to explain the apparently constitutive, ligand-independent signal-transducing capacity of the preBCR.

The identification of a nonclassical cadherin expressed during B cell development and its interaction with surrogate light chain

A 130-kDa glycoprotein (p130) has been found to be associated with surrogate light chain on pro- and pre-B I cells. Using peptide sequences obtained from purified p130 we have cloned its gene. The gene encodes a typical cadherin type 1 membrane protein with six extracellular cadherin domains (one pseudo domain) but lacking the catenin-binding site in its cytoplasmic part. Even without this catenin-binding site, p130 mediates Ca(2+)-dependent homotypic adhesion of cells. The interaction of p130 with surrogate light chain is confirmed by co-transfection and co-immunoprecipitation experiments. The expression of p130 is biphasic during the B cell development. Reverse transcriptase-polymerase chain reaction and flow cytometric analyses revealed that it is expressed on B220(+)c-Kit(+) pro-B and pre-B-I cells as well as on B220(+)CD25(-)IgM(+) immature and mature B cells but not on B220(+)CD25(+) pre-B-II cells. It is also expressed in fetal liver, at low levels in myeloid cells, and strongly in intestinal epithelial cells. In the spleen, p130-expressing cells are mainly localized in the marginal zone. We call this B lineage-, intestine-, liver- and leukocyte-expressed gene BILL-cadherin. The possible functions of BILL-cadherin in B cell development are discussed.

Changes in the V(H) gene repertoire of developing precursor B lymphocytes in mouse bone marrow mediated by the pre-B cell receptor

The V(H) repertoire on both H chain alleles of normal and lambda5-deficient B lineage cells were analyzed by single-cell PCR. The mu H chains were tested for their capacity to form a pre-B cell receptor. In bone marrow, D-proximal V(H) genes were found preferentially expressed in lambda5-deficient pre-B cells and in a newly identified early c-kit+ cytoplasmic mu H chain+ pre-B cell population of normal mice. Only half of the mu H chains expressed in these cells have the capacity to form a pre-B cell receptor. Representation of the D-proximal V(H) genes was found suppressed on the productive but not on the nonproductive V(H)DJ(H) rearranged alleles of c-kit preB-II cells and splenic lambda5-deficient B cells. More than 95% of the mu H chains expressed in preB-II cells can form a pre-B cell receptor. These results demonstrate that the pre-B cell receptor in normal mice and the B cell receptor in lambda5-deficient mice mediate a shift in the V(H) repertoire.

Mutation of unique region of Bruton's tyrosine kinase in immunodeficient XID mice

The cytoplasmic tyrosine kinase, Bruton's tyrosine kinase (Btk, formerly bpk or atk), is crucial for B cell development. Loss of kinase activity results in the human immunodeficiency, X-linked agammaglobulinemia, characterized by a failure to produce B cells. In the murine X-linked immunodeficiency (XID), B cells are present but respond abnormally to activating signals. The Btk gene, btk, was mapped to the xid region of the mouse X chromosome by interspecific backcross analysis. A single conserved residue within the amino terminal unique region of Btk was mutated in XID mice. This change in xid probably interferes with normal B cell signaling mediated by Btk protein interactions.

Unlinked regulation of cell growth and differentiation in immature B cell lines

We established many immunoglobulin-null immature B cell lines transformed by tsOS-59, a temperature-sensitive mutant of Abelson murine leukemia virus. In different cell lines cell growth was depressed and cell differentiation (generation of intracytoplasmic mu-positive cells from Ig- cells) was induced by the shift of culture temperature from low (35 degrees C) to high (39 degrees C). Cell lines were categorized into four groups: (i) temperature sensitive (ts) to both cell growth and differentiation, (ii) ts to cell growth but not to cell differentiation, (iii) ts to cell differentiation but not to cell growth, and (iv) ts to neither cell growth nor differentiation. These results indicated that the depression of cell growth did not necessarily induce cell differentiation, and that cell differentiation was induced regardless of whether cell growth was depressed or not. Furthermore, the results showed that the depression of cell growth and the induction of cell differentiation occurred without the reduction of tyrosine kinase activity of P120gag-abl at high, nonpermissive temperature. Our cell growth and differentiation system described here should provide us with the interesting findings of the relation between B cell growth and differentiation.

Two types of mu chain complexes are expressed during differentiation from pre-B to mature B cells

Immunoglobulin mu chains synthesized in murine pre-B cells are known to be associated with surrogate light chains designated as omega (omega), iota (iota) and B34. In addition to these molecules, we identified the complexes of polypeptides (50, 40, 27 and 15.5 kd) associated with surface or intracellular mu chains of pre-B cell lines. Most of these polypeptides were continuously synthesized and associated with mu chains in virgin B cells lines, although some of them scarcely bound to the mu kappa dimer or mu 2 kappa 2 tetramer concomitantly present in the same clone or population. However, in mature B cells they were no longer detectable except B34. Cross-linking of micron chains on the surface of pre-B cells resulted in an increase in intracellular free Ca2+, indicating that the micron chain complex on the surface of pre-B cell lines acted as a signal transduction molecule. However, the receptor cross-linkage of pre-B cell lines did not induce the increased inositol phospholipid metabolism usually observed in virgin and mature B cell lines. These results suggest that, during the differentiation from pre-B to mature B cells, the cells express two types of mu chain complexes which exhibit different structures as a whole and possess different signal transducing capacities.

B cell precursors are present in the thymus during early development

An in vitro system for transforming immature lymphoid cells present in the thymus at early development has been established. By phenotype analysis of the transformants obtained, we observed that B cell precursors, susceptible to Abelson murine leukemia virus (A-MuLV)- or Harvey murine sarcoma virus (H-MuSV)-induced lymphogenesis, were present at high frequency in the fetal thymus of BALB/c mice. These precursors recolonized alymphoid thymus lobes in vitro, as do T cell precursors. It was further observed that B precursors in the fetal liver were also capable of recolonizing alymphoid thymus lobes and were stored in a thymic environment. These results suggest that stroma cells of the fetal thymus may possess the capacity to support the growth of B precursors. On the other hand, B cell precursors sensitive to the viral transformation were undetectable in the fetal thymus of C57BL/6, although immunohistochemical analysis suggested their presence. However, in the fetal liver of the same strain, B precursors recolonizing alymphoid thymus in vitro were sensitive to the viral transformation. Based on these results, we will discuss both the role and fate of thymic B precursors. In addition, we also obtained T cell lymphomas at different stages of differentiation from the fetal thymus of C57BL/6 infected with A-MuLV or H-MuSV. These data indicate the usefulness of our system in establishing cell lines derived from intrathymic lymphogenesis at early development.

The viral and cellular forms of the Abelson (abl) oncogene

The precision of molecular biology has allowed a better definition of the components of the Abelson system. We know the gene structures and gene products for the cellular and viral forms of this family of related tyrosine kinases. However, many basic issues first identified in the early biological observations of Abelson, Rabstein, and others remain unanswered. The precise pathway for transformation in biochemical terms remains unknown for Ab-MLV and all of its relatives. Relatively little can be said to explain the preferential growth stimulation for certain hematopoietic cell types by the viral and other altered forms of the oncogene, and no clear insights into the function of the normal cellular forms of the abl oncogene are available. Future progress will certainly depend on the intensive efforts by many workers in the broader field of cellular growth control mechanisms.

Role of the ABL oncogene tyrosine kinase activity in human leukaemia

A great deal of information has emerged over the past decade regarding the gene structures and corresponding protein products of the cellular and transformation-associated forms of the ABL tyrosine kinase family. Many reports have also detailed the biological effects of these proteins (particularly the viral ABL forms) on a broad range of cell types. However, in spite of all these research efforts, the precise role of the ABL gene in normal and neoplastic growth remains to be determined. To elucidate the mechanism of action of normal and altered ABL proteins, it is imperative to identify their relevant cellular substrates and establish the role of the ABL target proteins in transformation and normal cellular growth. The availability of temperature-sensitive ABL proteins, coupled with the use of sensitive anti-phosphotyrosine antibodies, should be useful in this respect. Purification of enzymatically active, intact forms of the ABL proteins produced in insect cells by employing baculovirus expression vectors should permit direct comparison of the biochemical properties and tertiary structures of the various members of the ABL protein kinase family. Such studies will aid in understanding the nature of the alteration of ABL which results in the activation of its transforming potential. Furthermore, the availability of purified ABL proteins should permit examination of interactions of ABL with other growth-regulatory proteins, such as growth factor receptors. It has been shown that transformation-associated ABL proteins interact with the IL-3, IL-2 and GM-CSF growth-factor pathways. These and other components of the cellular signalling pathways are potential ABL targets. The elucidation of ABL function by a variety of approaches such as those described above will ultimately aid in the development of far-reaching therapeutic treatments for at least two forms of human leukaemia: Ph positive CML and Ph positive ALL.