Mu Heavy Chain Research Articles

Selective synergy of immunoglobulin enhancer elements in B-cell development: a characteristic of kappa light chain enhancers, but not heavy chain enhancers.

We have examined the interactions of the enhancers of the kappa immunoglobulin light chain gene as well as the interactions of the intron, mu, and 3' alpha enhancers of the heavy chain locus in mouse. We have observed that each of the kappa enhancers is very weak in comparison with the heavy chain intron enhancer. The mouse heavy chain 3' alpha enhancer is relatively weak as well. However, two kappa enhancers together synergistically activate transcription of a luciferase reporter gene to a level that is roughly equivalent to the heavy chain mu enhancer. Additionally, dimerization of either kappa enhancer results in synergistic increases in transcription. This property of synergism appears to be confined to the enhancers of the kappa locus, as addition of the 3' alpha E to mu E containing constructs increases transcription only modestly, and neither heavy chain enhancer synergizes when dimerized. We have gone on to characterize some of the minimal requirements for synergism between the kappa enhancers and find that the KB and E2 sites are required, but not the E3 site. The implications of these results for the coordinate regulation of the heavy and light chain transcription are discussed.

Quality control of ER synthesized proteins: an exposed thiol group as a three-way switch mediating assembly, retention and degradation.

Plasma cells secrete IgM only in the polymeric form: the C-terminal cysteine of the mu heavy chain (Cys575) is responsible for both intracellular retention and assembly of IgM subunits. Polymerization is not quantitative, and part of IgM is degraded intracellularly. Neither chloroquine nor brefeldin A (BFA) inhibits degradation, suggesting that this process occurs in a pre-Golgi compartment. Degradation of IgM assembly intermediates requires Cys575: the monomeric IgMala575 mutant is stable also when endoplasmic reticulum (ER) to Golgi transport is blocked by BFA. Addition of the 20 C-terminal residues of mu to the lysosomal protease cathepsin D is sufficient to induce pre-Golgi retention and degradation of the chimeric protein: the small amounts of molecules which exit from the ER are mostly covalent dimers. By contrast, when retained by the KDEL sequence, cathepsin D is stable in the ER, indicating that retention is not sufficient to cause degradation. Replacing the C-terminal cysteine with serine restores transport through the Golgi. As all chimeric cathepsin D constructs display comparable protease activity in vitro, their different fates are not determined by gross alterations in folding. Thus, also out of its normal context, the mu chain Cys575 plays a crucial role in quality control, mediating assembly, retention and degradation.

Different assembly species of IgM are directed to distinct degradation sites along the secretory pathway.

In 38C B lymphocytes the membrane form of IgM is displayed on the cell surface whereas the secretory form of IgM is degraded. In the EH cell line, a light chain-deficient variant of 38C cells, the mu heavy chains are partially assembled with surrogate light chains characteristic of pre-B cells. In these cells neither the membrane (microns) nor the secretory (microsecond) forms of the mu heavy chain reach their final destination, and both are rapidly degraded. The degradation of mu chains in EH cells, like that of microsecond in 38C cells, is nonlysosomal and occurs prior to the trans-Golgi. However, while microsecond degradation in 38C cells is inhibited by brefeldin A, in EH cells microsecond and micron are retained and degraded by a brefeldin A-insensitive mechanism. These results indicate that degradation in EH cells occurs within the endoplasmic reticulum, whereas degradation in 38C cells requires exit from this compartment. Thus, mu heavy chains can be degraded in multiple sites along the secretory pathway. The location of the degradation process is determined by the developmentally regulated assembly species of the mu chains with either "classical" or surrogate light chains.

Selection for enhanced adhesion to microvessel endothelial cells or resistance to interferon-gamma modulates the metastatic potential of murine RAW117 large-cell lymphoma cells.

Poorly liver metastatic large-cell lymphoma RAW117-P cells were sequentially selected in vitro for increased adhesion to murine hepatic sinusoidal endothelial cells. After three or four sequential selections, the selected sublines showed increased rates of adhesion to target hepatic microvessel endothelial cells and increased formation of experimental metastases in the liver. However, the endothelial cell adhesion-selected RAW117 sublines were generally unstable and gradually lost their enhanced adhesive and metastatic properties during passage in culture. Highly metastatic, liver-selected RAW117-H10 large-cell lymphoma cells were more resistant to the cytostatic effects of interferon-gamma (IFN-gamma) than poorly metastatic unselected parental RAW117-P cells. When tested for their sensitivity to IFN-gamma, the endothelial cell adhesion variants were significantly more resistant than the unselected RAW117-P cells, but after a 72-h treatment with IFN-gamma, the in vitro-selected cells lost their enhanced endothelial cell adhesion characteristics, their potential to colonize the liver, and their ability to grow when injected at subcutaneous or intramuscular sites. In contrast, the metastatic potential of similarly treated RAW117-P cells was unaffected by IFN-gamma during a 72-h treatment. Sequential selection of RAW117-P cells for increased resistance to IFN-gamma in vitro resulted in variant lines that were refractory to the growth-inhibiting effects of IFN-gamma, and these IFN-gamma-selected variants were also less adhesive to liver microvessel endothelial cells. The IFN-gamma-selected variants also lost their experimental metastatic potentials completely and their tumorigenicities at sites of subcutaneous or intramuscular injection. Cytofluorographic analysis indicated reduced cell surface expression of H-2Kd antigen and fibronectin receptor on the selected variant cells but no change in cell surface mu heavy chain immunoglobulin. The unselected and selected RAW117 lines had similar sensitivities to natural killer (NK) cell-mediated cytolysis, indicating that the in vivo differences were probably not due to differences in NK cell-mediated cytolysis. The results suggest that selection for adhesion to organ microvessel endothelial cells or sequential exposure to certain cytokines can affect the adhesive, growth and metastatic properties of RAW117 cells without modifying their responses to NK cells.

Stimulation of kappa light-chain gene rearrangement by the immunoglobulin mu heavy chain in a pre-B-cell line.

B-lymphocyte development exhibits a characteristic order of immunoglobulin gene rearrangements. Previous work has led to the hypothesis that expression of the immunoglobulin mu heavy chain induces rearrangement activity at the kappa light-chain locus. To examine this issue in more detail, we isolated five matched pairs of mu- and endogenously rearranged mu+ cell lines from the Abelson murine leukemia virus-transformed pro-B-cell line K.40. In four of the five mu+ cell lines, substantial expression of mu protein on the cell surface was observed, and this correlated with an enhanced frequency of kappa immunoglobulin gene rearrangement compared with that in the matched mu- cell lines. This increased kappa gene rearrangement frequency was not due to a general increase in the amount of V(D)J recombinase activity in the mu+ cells. Consistently, introduction of a functionally rearranged mu gene into one of the mu- pre-B-cell lines resulted in a fivefold increase in kappa gene rearrangements. In three of the four clonally matched pairs with increased kappa gene rearrangements, the increase in rearrangement frequency was not accompanied by a significant increase in germ line transcripts from the C kappa locus. However, in the fourth pair, K.40D, we observed an increase in germ line transcription of the kappa locus after expression of mu protein encoded by either an endogenously rearranged or a transfected functional heavy-chain allele. In these cells, the amount of the germ line C kappa transcript correlated with the measured frequency of rearranged kappa genes. These results support a regulated model of B-cell development in which mu protein expression in some way targets the V(D)J recombinase to the kappa gene locus.

Stimulation of kappa light-chain gene rearrangement by the immunoglobulin mu heavy chain in a pre-B-cell line.

B-lymphocyte development exhibits a characteristic order of immunoglobulin gene rearrangements. Previous work has led to the hypothesis that expression of the immunoglobulin mu heavy chain induces rearrangement activity at the kappa light-chain locus. To examine this issue in more detail, we isolated five matched pairs of mu- and endogenously rearranged mu+ cell lines from the Abelson murine leukemia virus-transformed pro-B-cell line K.40. In four of the five mu+ cell lines, substantial expression of mu protein on the cell surface was observed, and this correlated with an enhanced frequency of kappa immunoglobulin gene rearrangement compared with that in the matched mu- cell lines. This increased kappa gene rearrangement frequency was not due to a general increase in the amount of V(D)J recombinase activity in the mu+ cells. Consistently, introduction of a functionally rearranged mu gene into one of the mu- pre-B-cell lines resulted in a fivefold increase in kappa gene rearrangements. In three of the four clonally matched pairs with increased kappa gene rearrangements, the increase in rearrangement frequency was not accompanied by a significant increase in germ line transcripts from the C kappa locus. However, in the fourth pair, K.40D, we observed an increase in germ line transcription of the kappa locus after expression of mu protein encoded by either an endogenously rearranged or a transfected functional heavy-chain allele. In these cells, the amount of the germ line C kappa transcript correlated with the measured frequency of rearranged kappa genes. These results support a regulated model of B-cell development in which mu protein expression in some way targets the V(D)J recombinase to the kappa gene locus.

Predictability of the t(l;19)(q23;p13) From Surface Antigen Phenotype: Implications for Screening Cases of Childhood Acute Lymphoblastic Leukemia for Molecular Analysis: A Pediatric Oncology Group Study

The t(1;19)(q23;p13) translocation occurs in approximately 5% of B-precursor acute lymphoblastic leukemias (ALLs) occurring in children. Its presence has been associated with a poor prognosis, which may be overcome with more intensive therapy. Although leukemic cells from cases of t(1;19)-ALL frequently express cytoplasmic mu heavy chains, their complete antigenic profile remains undefined. Among 697 consecutive cases of B-precursor ALL with complete phenotypic studies using a panel of monoclonal antibodies, 22 cases were found to carry the t(1;19). Twenty of 22 cases had an identical, complex phenotype characterized by homogeneous expression of CD19, CD10, and CD9; complete absence of CD34; and at least partial absence of CD20. Overall, this phenotype was seen in only 8.0% (56 of 697) of childhood B-precursor ALL. One of the two remaining t(1;19)-carrying cases conformed to this phenotype, but was lacking data for CD9. The other case differed by virtue of expression of CD34 and was also hyperdiploid with 55 chromosomes. Molecular studies showed E2A-PBX1 abnormalities in all examined cases (12 of 12) with the t(1;19), including the case lacking CD9 data. In contrast, no E2A-PBX1 abnormalities were detected in the sole t(1;19)-ALL with CD34 expression. Seventeen cases with the characteristic phenotype and uninformative cytogenetics were also molecularly analyzed and 5 of 17 (including 4 of 8 with unsatisfactory cytogenetics and 1 of 9 with a normal karyotype) contained E2A gene rearrangements and E2A-PBX1 fusion mRNAs. Our results show that all cases of t(1;19)-ALL with concomitant E2A-PBX1 fusion invariably express a characteristic but uncommon profile of surface antigens. These observations suggest that selective molecular analysis of a small subset of patients (those with uninformative cytogenetics and the characteristic phenotype) can identify a significant number of additional cases of ALL with E2A-PBX1 fusion that might benefit from more intensive therapy.

Predictability of the t(1;19)(q23;p13) from surface antigen phenotype: implications for screening cases of childhood acute lymphoblastic leukemia for molecular analysis: a Pediatric Oncology Group study

The t(1;19)(q23;p13) translocation occurs in approximately 5% of B- precursor acute lymphoblastic leukemias (ALLs) occurring in children. Its presence has been associated with a poor prognosis, which may be overcome with more intensive therapy. Although leukemic cells from cases of t(1;19)-ALL frequently express cytoplasmic mu heavy chains, their complete antigenic profile remains undefined. Among 697 consecutive cases of B-precursor ALL with complete phenotypic studies using a panel of monoclonal antibodies, 22 cases were found to carry the t(1;19). Twenty of 22 cases had an identical, complex phenotype characterized by homogeneous expression of CD19, CD10, and CD9; complete absence of CD34; and at least partial absence of CD20. Overall, this phenotype was seen in only 8.0% (56 of 697) of childhood B-precursor ALL. One of the two remaining t(1;19)-carrying cases conformed to this phenotype, but was lacking data for CD9. The other case differed by virtue of expression of CD34 and was also hyperdiploid with 55 chromosomes. Molecular studies showed E2A-PBX1 abnormalities in all examined cases (12 of 12) with the t(1;19), including the case lacking CD9 data. In contrast, no E2A-PBX1 abnormalities were detected in the sole t(1;19)-ALL with CD34 expression. Seventeen cases with the characteristic phenotype and uninformative cytogenetics were also molecularly analyzed and 5 of 17 (including 4 of 8 with unsatisfactory cytogenetics and 1 of 9 with a normal karyotype) contained E2A gene rearrangements and E2A-PBX1 fusion mRNAs. Our results show that all cases of t(1;19)-ALL with concomitant E2A-PBX1 fusion invariably express a characteristic but uncommon profile of surface antigens. These observations suggest that selective molecular analysis of a small subset of patients (those with uninformative cytogenetics and the characteristic phenotype) can identify a significant number of additional cases of ALL with E2A-PBX1 fusion that might benefit from more intensive therapy.

Development of B Cells secreting Endogenous or Transgene‐Encoded Immunoglobulins in H‐Chain Transgenic Mice

The development of splenic B cells secreting transgene-encoded or endogenous immunoglobulin (Ig) was analysed in the mu heavy (H-)chain transgenic mouse line M54. The results show that cells secreting endogenous Ig are not detectable during the perinatal period, even after lipopolysaccharide stimulation in vitro. At this time, transgene-secreting cells are readily detectable and keep increasing with age of the animals. After a few weeks of age cells secreting endogenous Ig appear in the spleen and keep increasing with age, reaching numbers comparable to non-transgenic littermates by 5 weeks of age. Thereafter, the proportion of transgene-secreting B cells decreases. We conclude that the preferential expression of endogenous Igs by secreting B cells in the adult does not result from peculiar genetic features of those cells, but from age-dependent cellular selection operating on all B cells.

A complex of glycoproteins is associated with VpreB/lambda 5 surrogate light chain on the surface of mu heavy chain-negative early precursor B cell lines.

Monoclonal antibodies (mAbs) have been made specific for the pre-B cell-specific proteins VpreB and lambda 5 which together form the surrogate light (L) chain. mAbs specific for VpreB protein identified the 16-kD molecule associated on precursor B cell lines with lambda 5 protein as the product of the VpreB gene. Surrogate L chain was detectable even in the absence of mu heavy (H) chain on the surface of early precursor cell lines such as pro-B cell lines where all immunoglobulin (Ig) loci are in the germline configuration, as well as early pre-B cell lines where Ig H chain loci are DHJH rearranged in reading frame I or III, which does not allow the expression of a DHJHC mu protein. A complex of glycoproteins (200, 130, 105, and 65-35 kD) was identified as coprecipitated with the Vpreb/lamba 5 surrogate L chain in mu H chain-negative precursor B cell lines. The 130-kD protein was most strongly labeled with iodine and most consistently detected in noncovalent association with surrogate L chain. This protein turned out to be a N-linked glycoprotein with a 100-kD protein core and isoelectric point 5.8, indicating that it is distinct from CD43 and the BP-1/6C3 antigen. The surface deposition of the surrogate L chain in association with the newly identified glycoproteins suggests that the surrogate L chain may function as a receptor even before the association with mu H chain in early precursor B cells.

Modulating the metastatic potential of murine RAW117 large-cell lymphoma cells by selection for resistance to interferon-gamma.

Highly metastatic, in vivo-selected cells of RAW117-H10 large-cell lymphoma have been shown to be more resistant than poorly metastatic parental RAW117-P cells to the cytolytic and cytostatic activities of activated macrophages in co-culture experiments. Activated macrophages are known to produce soluble, cytostatic respiration-inhibiting factors, and such activities can be duplicated by interferon-gamma (IFN-gamma) or by combinations of IFN-gamma and Escherichia coli lipopolysaccharide (LPS). Highly metastatic RAW117-H10 cells are more resistant to the cytostatic effects of IFN-gamma and LPS than poorly metastatic RAW117-P cells, and short-term (up to 72 hr) treatment with IFN-gamma and LPS does not change the metastatic potentials of RAW117 cells. We have studied the effects of sequential selection of RAW117-P cells for increased resistance to IFN-gamma and LPS. After 7 to 13 sequential selections, the resulting variant lines were completely refractory to the growth-inhibitory effects of IFN-gamma and LPS and cross-tolerant to macrophage-conditioned medium. The selected variants also completely lost their experimental metastatic potentials and their tumorigenicities after s.c. or i.m. injection. Cytofluorographic analysis indicated reduced cell-surface expression of H-2Kd antigen and fibronectin receptor on the variant cells but no change in surface mu heavy-chain immunoglobulin. The IFN-gamma-selected lines were less adhesive to liver microvascular endothelial cells than the unselected RAW117 cell lines, but were equally sensitive to NK cytolysis by spleen cells. Our results suggest that exposure to certain cytokines can affect the growth and metastatic potential of RAW117 cells and result in the selection of resistant variants with altered biologic properties.

Regulation of lymphoid-specific immunoglobulin mu heavy chain gene enhancer by ETS-domain proteins.

The enhancer for the immunoglobulin mu heavy chain gene (IgH) activates a heterologous gene at the pre-B cell stage of B lymphocyte differentiation. A lymphoid-specific element, microB, is necessary for enhancer function in pre-B cells. A microB binding protein is encoded by the PU.1/Spi-1 proto-oncogene. Another sequence element, microA, was identified in the mu enhancer that binds the product of the ets-1 proto-oncogene. The microA motif was required for microB-dependent enhancer activity, which suggests that a minimal B cell-specific enhancer is composed of both the PU.1 and Ets-1 binding sites. Co-expression of both PU.1 and Ets-1 in nonlymphoid cells trans-activated reporter plasmids that contained the minimal mu enhancer. These results implicate two members of the Ets family in the activation of IgH gene expression.

Isolation of new nonsense and frameshift mutants in the immunoglobulin mu heavy-chain gene of hybridoma cells.

In order to expand the experimental material available for genetic and biochemical analyses of the natural immunoglobulin genes, we have isolated a variety of mutant mouse hybridoma cell lines. Some of these mutants have partial or complete deletions of the mu gene. Other mutants have nonsense or frameshift mutations in the exons encoding the variable and the second and third constant region domains of the mu heavy chain. When combined with earlier mutant data, this collection of genotypically and phenotypically tight mutants of known sequence spans most of the 10 kb of the mu gene, providing material for a variety of studies of genetic recombination and mRNA metabolism.

RAG-2-deficient blastocyst complementation: an assay of gene function in lymphocyte development.

We describe a system to evaluate the function of lymphocyte-specific and generally expressed genes in the differentiation and/or function of lymphocytes. RAG-2 (recombination-activating gene 2)-deficient mice have no mature B and T lymphocytes due to the inability to initiate VDJ recombination. Blastocysts from RAG-2-deficient mice generate animals with no mature B and T cells following implantation into foster mothers. However, injection of normal ES cells into RAG-2-deficient blastocysts leads to the generation of somatic chimeras with mature B and T cells all of which derive from the injected ES cells (referred to as RAG-2-deficient blastocyst complementation). Complementation of RAG-2-deficient blastocysts with mutant ES cells heterozygous for a targeted mutation that deletes all immunoglobulin heavy-chain joining (JH) gene segments (JH+/-) also leads to generation of chimeras with normal B and T cells. However, complementation with ES cells homozygous for the JH mutation (JH-/-) generates animals with normal T cells but no B cells, due to a block in B-cell development at a very early stage. Transfection of a functionally assembled mu heavy-chain gene into the JH-/- ES cells prior to blastocyst injection rescues the JH-/- mutation and allows the generation of both mature T and mature B cells. The rescued B cells express IgM but not IgD and respond normally to bacterial lipopolysaccharide stimulation by proliferating and by secreting IgM.

The structure of the μ/pseudo light chain complex on human pre‐B cells is consistent with a function in signal transduction

Prior to immunoglobulin (Ig) light (L) chain rearrangement, pre-B cells can express mu heavy (H) chains at the cell surface in association with pseudo (psi) L chains. This complex may be essential for B cell development. We have investigated the composition of the mu/psi L chain complex of a human pre-B cell line, in view of its potential role in transmembrane signal transduction. The mu/lambda receptor of a mature B cell line was analyzed in comparison. The mu/psi L chain complex is associated with disulfide-linked molecules that are homologous or identical to the mb-1 and B29 proteins, known to be integral components of membrane Ig receptors on mature B cells. Both receptors contain tyrosine (Tyr) kinase activity. In the mu/lambda receptor, the lyn and lck Tyr kinases could clearly be identified. The mb-1 and B29 proteins in both mu/lambda and mu/psi L chain receptors are substrates for in vitro phosphorylation on Tyr, but also on serine (Ser) and threonine (Thr) residues. The undefined mu-associated Ser/Thr kinase also phosphorylates the src-related kinases in the mu/lambda receptor and a 43-kDa mu-associated protein that is present in both complexes. The 43-kDa protein may be an integral part of both receptor types, or a transiently associated molecule instrumental in the signaling process. We conclude that the mu/psi L receptor on human pre-B cells fulfills the presently known criteria to function as a signal transduction unit.

Immunoglobulin class switch recombination.

A B lymphocyte that produces the immunoglobulin heavy (H) chain mu may switch to the production of another heavy chain class: gamma, epsilon, or alpha. Since the new heavy chain retains the original variable (V) region, antigenic specificity is maintained. The switch is accompanied by a large deletion of DNA at the heavy chain locus. To explain how this deletion is generated, three models have been proposed: recombination between homologs, unequal sister chromatid exchange, and looping out and deletion. While none of the predicted recombination products of the first two models have been found, both by-products of looping out--inversions and circular DNA--have been isolated. Thus looping out and deletion appears to be the appropriate model to explain the genetic events leading to the immunoglobulin heavy chain class switch. One requirement for switching may be transcription of the constant (C) region to which the cell switches. The switch rearrangement is catalyzed by a switch recombinase, and the isolation of the components of this putative enzyme system is in progress. Although the switch deletion is an accepted fact, the discussion is enlivened by scenarios for switching without DNA rearrangement; such suggestions include processing at the RNA level and trans-splicing.

Somatic hypermutation of an immunoglobulin mu heavy chain transgene.

We have analyzed somatic hypermutation of an immunoglobulin (Ig) heavy chain transgene. Hybridomas expressing the transgene were produced from immunized transgenic mice and transgene copies were sequenced to assay for mutation. In two IgM-producing hybridomas, as well as in several IgG-producing hybridomas, mutations were found in the VDJ region of the transgene. In the IgM-producing hybridomas, both mutated and unmutated transgene copies were present and expressed as mRNA. Several mutated transgene copies were present in a single cell and these showed different patterns of mutation. Two IgG-producing hybridomas isolated from a single animal also showed a hierarchical pattern of mutation indicating that transgene mutations can accumulate during B cell proliferation, similar to the mutational process for endogenous antibody genes. Among hybridomas that expressed both IgG and IgM molecules derived from the transgene, the isotype-switched gamma transgene copy exhibited a higher level of mutation than the mu transgene copies. Our results indicate that the 15-kb ARSmu transgene contains all the sequence information required to target the Ig-specific hypermutational machinery, and raise the possibility that sequences associated with the endogenous CH locus might enhance somatic mutation.

A high frequency of hybridomas from M54 mu heavy chain transgenic mice initially co-express transgenic and rearranged endogenous mu genes.

The M54 transgenic mouse line, which carries the 17.2.25 Ig mu heavy chain gene, rearranges Ig heavy chains and expresses both transgenic and endogenous mu. B cell lineage development is selectively impaired in these mice and cells that simultaneously express transgenic and endogenous mu ('double-producers') are common amongst the B cells and plasma cells that do develop. Weaver, Imanishi-kari, Baltimore and colleagues failed to obtain double-producing hybridomas from M54 mice; however, molecular and serologic studies presented here show that such hybridomas are readily generated. These hybridomas are extremely unstable and rapidly yield variants producing either transgenic or endogenous mu. Therefore the stable cloned lines we obtained, like Weaver et al., were almost all single or non-producers. We also found that the VH gene usage in our hybridomas was skewed towards the JH proximal (VHQ52, VH81X) families, supporting the idea that the expression of the M54 transgene alters the endogenous Ig repertoire.

A topoisomerase II-like protein is part of an inducible DNA-binding protein complex that binds 5' of an immunoglobulin promoter.

We previously identified a B cell-specific protein-DNA complex 5' of an immunoglobulin mu heavy chain promoter. The sequences to which this protein complex bound were required for induction of immunoglobulin mRNA levels with interleukin-5 + antigen. Further studies identified a second sequence 5' of these regulatory sequences that bound to both the nuclear matrix and to a similar interleukin-5 + antigen inducible protein complex. Therefore, we sought to identify the putative regulatory proteins that comprised this DNA-binding complex. In this study, we have used anti-topoisomerase II antibodies to demonstrate that one of the proteins found in the interleukin-5 + antigen inducible complexes is serologically related to topoisomerase II. To our knowledge, this is the first report where a topoisomerase II related protein participates in an inducible mobility shifted protein-DNA complex. These data suggest a model in which the enhanced immunoglobulin gene transcription observed after treatment with interleukin-5 + antigen might be explained by the induction of a protein complex that acts to relieve torsional stress along the gene.

B-lineage cells in mu-transgenic scid mice proliferate in response to IL-7 but fail to show evidence of immunoglobulin light chain gene rearrangement.

The severe combined immunodeficiency (scid) mouse mutation impairs the recombination of Ig and TCR genes. Mice homozygous for this mutation (scid mice) lack pre-B, B, and T lymphocytes. Earlier we introduced a functionally rearranged mu-heavy chain gene into the scid mouse genome and found that this resulted in the development of pre-B cells in the bone marrow of these mice; however, sIgM+ B cells were not detected. We have now investigated the growth properties and rearrangement status of Ig genes in early B-lineage cells arising in mu-transgenic scid mice. We find that the presence of a functional mu-transgene allows pro-B cells from these mice to proliferate in short-term culture with IL-7. Nevertheless, rearrangements of Ig light chain genes are not detected in the bone marrow of such mice. Furthermore, the frequency of rearrangement detected at the endogenous Ig heavy chain locus in scid pro-B and pre-B cells is reduced relative to that in wild-type cells.