Introduction of a mu immunoglobulin gene into the mouse germ line: specific expression in lymphoid cells and synthesis of functional antibody

TFG is required for autophagy flux and to prevent endoplasmic reticulum stress in CH12 B lymphoma cells

Plasma cells depend on quality control of newly synthesized antibodies in the endoplasmic reticulum (ER) via macroautophagy/autophagy and proteasomal degradation. The cytosolic adaptor protein TFG (Trk-fused gene) regulates ER-Golgi transport, the secretory pathway and proteasome activity in non-immune cells. We show here that TFG is upregulated during lipopolysaccharide- and CpG-induced differentiation of B1 and B2 B cells into plasmablasts, with the highest expression of TFG in mature plasma cells. CRISPR-CAS9-mediated gene disruption of tfg in the B lymphoma cell line CH12 revealed increased apoptosis, which was reverted by BCL2 but even more by ectopic TFG expression. Loss of TFG disrupted ER structure, leading to an expanded ER and increased expression of ER stress genes. When compared to wild-type CH12 cells, tfg KO CH12 cells were more sensitive toward ER stress induced by tunicamycin, monensin and proteasome inhibition or by expression of an ER-bound immunoglobulin (Ig) μ heavy (µH) chain. CH12 tfg KO B cells displayed more total LC3, lower LC3-II turnover and increased numbers and size of autophagosomes. Tandem-fluorescent-LC3 revealed less accumulation of GFP-LC3 in starved and chloroquine-treated CH12 tfg KO B cells. The GFP:RFP ratio of tandem-fluorescent-LC3 was higher in tunicamycin-treated CH12 tfg KO B cells, suggesting less autophagy flux during induced ER stress. Based on these data, we suggest that TFG controls autophagy flux in CH12 B cells and propose that TFG is a survival factor that alleviates ER stress through the support of autophagy flux in activated B cells and mature plasma cells.Abbreviations: Ab, antibody; Ag, antigen; ASC, antibody-secreting cells; ATG, autophagy-related; BCR, B cell receptor; COPII, coat protein complex II; CpG, non-methylated CpG oligonucleotide; ER, endoplasmic reticulum; ERAD, ER-associated degradation; FO, follicular; GFP, green fluorescent protein; HC, heavy chain; Ig, immunoglobulin; IRES, internal ribosomal entry site; LC, light chain; MZ, marginal zone; NFKB, nuclear factor of kappa light polypeptide gene enhancer in B cells; TLR, toll-like receptor; UPR, unfolded protein response.

Antigen-Specific Human Monoclonal Antibodies from Transgenic Mice

Due to the difficulties found when generating fully human monoclonal antibodies (mAbs) by the traditional method, several efforts have attempted to overcome these problems, with varying levels of success. One approach has been the development of transgenic mice carrying immunoglobulin (Ig) genes in germline configuration. The engineered mouse genome can undergo productive rearrangement in the B-cell population, with the generation of mouse B lymphocytes expressing human Ig (hIg) chains. To avoid the expression of mouse heavy or light chains, the endogenous mouse Ig (mIg) loci must be silenced by gene-targeting techniques. Subsequently, to obtain antigen-specific mAbs, conventional immunization protocols can be followed and the mAb technique used (fusion of activated B cells with mouse myeloma cells, screening, cloning, freezing, and testing) with these animThis chapter summarizes the most common chromatographic mAb andals expressing human Ig genes. This chapter describes the type of transgenic-knockout mice generated for various research groups, provides examples of human mAbs developed by research groups and companies, and includes protocols of immunization, generation, production, and purification of human mAbs from such mice. In addition, it also addresses the problems detected, and includes some of the methods that can be used to analyze functional activities with human mAbs.

Competitive Promoter-Associated Matrix Attachment Region Binding of the Arid3a and Cux1 Transcription Factors

Arid3a/Bright/Dril1 is a B cell-specific transactivator that regulates immunoglobulin heavy chain (IgH) gene transcription by binding promoter and enhancer-associated matrix attachment regions (MARs) within the IgH gene locus. Promoter MAR-mediated Arid3a transactivation is antagonized by direct competition of MAR binding by Cux1/CDP-a ubiquitously expressed repressor originally termed NF-μNR. We report that the NF-μNR complex includes Arid3a in B cells but not in non-B cells through mobility shift assays. The binding activity of NF-μNR and Arid3a in B cells is reciprocally altered during the cell division cycle and by the B cell mitogen lipopolysaccharide LPS. LPS treatment had no effect on Arid3a localization but increased its total abundance within the nucleus and cytoplasm. We show that this increased level of Arid3a is capable of displacing Cux from the MARs to facilitate IgH gene transcription. Finally, we showed that the MARs (termed Bf150 and Tx125) associated with the V_H1 rearranged variable region expressed in the S107 murine plasmacytoma, can repress reporter gene transcription in non-B cells and that they can relieve the repression mediated by Eμ enhancer in B cells. These results have significant implications for early human development and demonstrate that MARs in IgH locus, NF-µNR and Arid3a regulate IgH gene expression in a concerted fashion. This paves the way for future studies examining the misregulation of this pathway in pediatric disease.

Antigen-specific human monoclonal antibodies from transgenic mice

Due to the difficulties found when generating fully human monoclonal antibodies (mAbs) by the traditional method, several efforts have attempted to overcome these problems, with varying levels of success. One approach has been the development of transgenic mice carrying immunoglobulin (Ig) genes in germ line configuration. The engineered mouse genome can undergo productive rearrangement in the B cell population, with the generation of mouse B lymphocytes expressing human Ig (hIg) chains. To avoid the expression of mouse heavy or light chains, the endogenous mouse Ig (mIg) loci must be silenced by gene-targeting techniques. Subsequently, to obtain antigen-specific mAbs, conventional immunization protocols can be followed and the mAb technique used (fusion of activated B cells with mouse myeloma cells, screening, cloning, freezing, and testing) with these animals expressing human Ig genes. This chapter describes the type of transgenic knockout mice generated for various research groups, provides examples of human mAbs developed by research groups and companies, and includes protocols of immunization, generation, production, and purification of human mAbs from such mice. In addition, it also addresses the problems detected, and includes some of the methods that can be used to analyze functional activities with human mAbs.

Transgenic mouse models--a seminal breakthrough in oncogene research

Transgenic mouse models are an integral part of modern cancer research, providing a versatile and powerful means of studying tumor initiation and progression, metastasis, and therapy. The present repertoire of these models is very diverse, with a wide range of strategies used to induce tumorigenesis by expressing dominant-acting oncogenes or disrupting the function of tumor-suppressor genes, often in a highly tissue-specific manner. Much of the current technology used in the creation and characterization of transgenic mouse models of cancer will be discussed in depth elsewhere. However, to gain a complete appreciation and understanding of these complex models, it is important to review the history of the field. Transgenic mouse models of cancer evolved as a new and, compared with the early cell-culture-based techniques, more physiologically relevant approach for studying the properties and transforming capacities of oncogenes. Here, we will describe early transgenic mouse models of cancer based on tissue-specific expression of oncogenes and discuss their impact on the development of this still rapidly growing field.

Murine IL-4 is able to signal via chimeric human IL-4Ralpha/mouse gamma-chain receptor

Human IL-4Ralpha binds to mouse gammac resulting in a chimeric receptor specific for human IL-4 but not mouse IL-4, providing in principle an inducible hIL-4 system. We investigated the in vitro and in vivo characteristics of human IL-4Ralpha transgenic mice on a mouse IL-4Ralpha-deficient background (hIL-4Ralpha Tg/mIL-4Ralpha(-/-)). The integrity of lymphocyte-specific hIL-4Ralpha expression in hIL-4Ralpha Tg/mIL-4Ralpha(-/-) mice was demonstrated by FACS analysis. This was confirmed in functional studies as lymphocytes responded to recombinant hIL-4 but not mIL-4 or mIL-13 in proliferation and T helper differentiation assays, demonstrating species-specificity and inducibility of the chimeric receptor in vitro. We then infected transgenic mice with Nippostrongylus brasiliensis, known to induce a strong Type 2 response in wild-type mice. As expected hIL-4Ralpha Tg/mIL-4Ralpha(-/-) mice were unable to expel N. brasiliensis worms which confirms unresponsiveness in non-lymphocytes. However they developed a Th2 cytokine and IgE response in the absence of induction with hIL-4. These results suggested that lymphocyte-specific IL-4Ralpha responsiveness was still present in vivo. Neutralization of endogenous mIL-4 resulted in inhibition of N. brasiliensis-induced Th2 cytokine and total IgE production in hIL-4Ralpha Tg/mIL-4Ralpha(-/-) mice suggesting that mIL-4 was involved. Intercrossing hIL-4Ralpha Tg/mIL-4Ralpha(-/-) mice with mIL-4(-/-)/mIL-13(-/-) mice completely abrogated Type 2 responses in N. brasiliensis infections. Together, these data demonstrate that mIL-4 triggered the hIL-4Ralpha/mgammac chimeric receptor in vivo.

Oncogenic role of Pax5 in the T-lymphoid lineage upon ectopic expression from the immunoglobulin heavy-chain locus

Four of 9 PAX transcription factor genes have been associated with chromosomal translocations in human tumors, although their oncogenic potential has not yet been demonstrated in transgenic mouse models. The B-lymphoidPAX5 gene participates in the generation of the t(9;14)(p13;q32) translocation in germinal center B cells, which leads to deregulated PAX5 expression under the control of the immunoglobulin heavy-chain (IgH) locus in a subset of B-cell non-Hodgkin lymphomas. Here we reconstructed a human t(9;14) translocation in a knock-in mouse by inserting a PAX5 minigene into the IgH locus. The IgHP5ki allele, which corresponds to a germline rather than somatic mutation, is activated in multipotent hematopoietic progenitors and is subsequently expressed in dendritic cells (DCs) and in natural killer (NK), T, and B cells. Ectopic Pax5 expression interferes with normal T-cell development and causes immature T-lymphoblastic lymphomas in IgHP5ki/+ and IgHP5ki/P5ki mice. Aggressive T-cell lymphomas develop even faster in IkPax5/+ mice expressing Pax5 from the Ikaros locus. Pax5 expression in thymocytes activates B-cell–specific genes and represses T-lymphoid genes, suggesting that Pax5 contributes to lymphomagenesis by deregulating the T-cell gene-expression program. These data identify Pax5 as a potent oncogene and demonstrate that the T-lymphoid lineage is particularly sensitive to the oncogenic action of Pax5.

Foundation Review: Transgenic animals and their impact on the drug discovery industry

The ability to direct genetic changes at the molecular level has resulted in a revolution in biology. Nowhere has this been more apparent than in the production of transgenic animals. Transgenic technology lies at the junction of several enabling techniques in such diverse fields as embryology, cell biology and molecular genetics. A host of techniques have been used to effect change in gene expression and develop new pharmaceutical and nutraceutical compounds cost-effectively. Scientific advances gained by transgenic capabilities enable further understanding of basic biological pathways and yield insights into how changes in fundamental processes can perturb programmed development or culminate in disease pathogenesis.

IgM monomers accelerate disease manifestations in autoimmune-prone Fas-deficient mice

The monomeric form of IgM, also known as low molecular weight IgM, is found in increased concentrations in patients chronically infected with a variety of viral and bacterial pathogens or suffering from various autoimmune diseases. Whether monomeric IgM contributes to the disease process, however, is not known. To address this question, transgenic mice were created that secreted elevated levels of IgM monomers. In normal mice (C57BL/6), the presence of IgM monomers did not alter the composition of the immune system significantly: lymphocyte subsets and serum antibody levels were normal, with the exception of increased levels of IgM due to the presence of the monomers. Immune responses also appeared to be normal. Transgenic mice did develop antinuclear antibodies (ANA) earlier than non-transgenic littermates, but did not develop further indications of autoimmune disease. When the transgene was expressed in the autoimmune-prone strain of mice, B6.MRL-Tnfrsf6(lpr) (B6/lpr), these mice developed autoimmune manifestations more rapidly than non-transgenic littermates, including hypergammaglobulinemia, splenomegaly, and ANA production. Transgenic mice also displayed earlier evidence of immune complex deposition in the kidneys. From these results, we conclude that monomeric IgM does not induce autoimmune disease, but its presence can accelerate the onset of autoimmune manifestations in otherwise autoimmune prone animals.

Retroviral vectors to monitor somatic hypermutation

The recent expansion of studies on hypermutation may benefit from a fast and uncomplicated way to measure mutation rates. In this paper we compare different retroviral vector designs for monitoring hypermutation in vivo. Retroviral vectors combine a high transduction rate with integration at random sites within the host cell genome, thus equalizing positional effects on the reporter gene. The vectors contain a reporter gene with a premature TAG termination codon; upon reversion, a full-length fluorescent protein is expressed. Any single point mutation at the amber codon activates the reporter--except the transition from G to A, which only creates the stop codon TAA. In the construct, the reporter gene is followed by an internal ribosome entry site and a second marker that allows selection of stably transduced cells. As a reporter gene, we tested the green and yellow fluorescence proteins (GFP and YFP); and various proteins with red fluorescence (dsRed). The second marker was either a drug resistance gene, or a second fluorescent protein. We also introduced various cis-acting enhancer elements into the reporter construct, to study the simultaneous activity of enhancers on transcription and hypermutation. We found that GFP as a reporter, combined with a drug selection marker, gave the most consistent and convenient mutation rate measurements. DsRed is a good alternative to GFP, but variants with greater fluorescence intensity are needed when combined with green fluorescence measurements. We also confirm that no immunoglobulin specific sequence is needed to target hypermutation. Depending on their position in these ectopically expressed constructs, enhancers can have positive or negative effects on hypermutation.

Locus 'decontraction' and centromeric recruitment contribute to allelic exclusion of the immunoglobulin heavy-chain gene

Allelic exclusion of immunoglobulin genes ensures the expression of a single antibody molecule in B cells through mostly unknown mechanisms. Large-scale contraction of the immunoglobulin heavy-chain (Igh) locus facilitates rearrangements between Igh variable (V(H)) and diversity gene segments in pro-B cells. Here we show that these long-range interactions are mediated by 'looping' of individual Igh subdomains. The Igk locus also underwent contraction by looping in small pre-B and immature B cells, demonstrating that immunoglobulin loci are in a contracted state in rearranging cells. Successful Igh recombination induced the rapid reversal of locus contraction in response to pre-B cell receptor signaling, which physically separated the distal V(H) genes from the proximal Igh domain, thus preventing further rearrangements. In the absence of locus contraction, only the four most proximal V(H) genes escaped allelic exclusion in immature mu-transgenic B lymphocytes. Pre-B cell receptor signaling also led to rapid repositioning of one Igh allele to repressive centromeric domains in response to downregulation of interleukin 7 signaling. These data link both locus 'decontraction' and centromeric recruitment to the establishment of allelic exclusion at the Igh locus.

The role of p53 in suppression of KSHV cyclin-induced lymphomagenesis

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a cyclin D homolog, K cyclin, that is thought to promote viral oncogenesis. However, expression of K cyclin in cultured cells not only triggers cell cycle progression but also engages the p53 tumor suppressor pathway, which probably restricts the oncogenic potential of K cyclin. Therefore, to assess the tumorigenic properties of K cyclin in vivo, we transgenically targeted expression of K cyclin to the B and T lymphocyte compartments via the E micro promoter/enhancer. Around 17% of E micro -K cyclin animals develop lymphoma by 9 months of age, and all such lymphomas exhibit loss of p53. A critical role of p53 in suppressing K cyclin-induced lymphomagenesis was confirmed by the greatly accelerated onset of B and T lymphomagenesis in all E micro -K cyclin/p53(-/-) mice. However, absence of p53 did not appear to accelerate K cyclin-induced lymphomagenesis by averting apoptosis: E micro -K cyclin/p53(-/-) end-stage lymphomas contained abundant apoptotic cells, and transgenic E micro -K cyclin/p53(-/-) lymphocytes in vitro were not measurably protected from DNA damage-induced apoptosis compared with E micro -K cyclin/p53(wt) cells. Notably, whereas aneuploidy was frequently evident in pre-lymphomatous tissues, end-stage E micro -K cyclin/p53(-/-) tumors showed a near-diploid DNA content with no aberrant centrosome numbers. Nonetheless, such tumor cells did harbor more restricted genomic alterations, such as single-copy chromosome losses or gains or high-level amplifications. Together, our data support a model in which K cyclin-induced genome instability arises early in the pre-tumorigenic lymphocyte population and that loss of p53 licenses subsequent expansion of tumorigenic clones.

Control of pre-BCR signaling by Pax5-dependent activation of the BLNK gene

The developmental progression from pro-B to pre-B cells is controlled by pre-B cell receptor (pre-BCR) signaling which depends on BLNK (SLP-65) for coupling the Syk kinase to its downstream effector pathways. Here we identified BLNK as a direct target of the transcription factor Pax5 (BSAP). Restoration of BLNK expression in Ig(mu) transgenic Pax5(-/-) pro-B cells resulted in constitutive pre-BCR signaling and increased cell proliferation without inducing progression to the pre-B cell stage. Ig(mu)(+) Pax5(-/-) pro-B cells expressing a BLNK-estrogen receptor fusion protein initiated signaling immediately upon hormone addition, which facilitated analysis of pre-BCR-induced gene expression changes. The pre-BCR was shown to execute its checkpoint function by regulating genes involved in cell proliferation, intracellular signaling, growth factor responsiveness, and V(D)J recombination.

Immunoglobulin heavy chain transgenic mice expressing Galalpha(1,3)Gal-reactive antibodies

BACKGROUND

Natural antibodies that bind the carbohydrate antigen Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) mediate rigorous rejection of porcine xenografts and represent a major immunological hurdle to successful discordant xenotransplantation. However, little is known about how production of antibodies specific for alphaGal is regulated.

METHODS

Transgenic mice expressing an IgM heavy chain isolated from a B-cell hybridoma that produces antibodies specific for alphaGal were constructed. These mice were bred to mutant mice that lack the alphaGal epitope (GT0 mice) or wild-type (GT+) mice to generate animals in which the transgene is expressed in the presence or absence of alphaGal as a "self"-antigen. Development of transgene-expressing B cells and production of alphaGal-specific serum antibodies were then analyzed in transgenic mice on GT0 and GT+ backgrounds.

RESULTS

B cells expressing the transgenic heavy chain and transgene-encoded serum antibodies specific for alphaGal were readily detected in mice on the GT0 background. Most alphaGal-reactive antibodies in GT0 mice used the transgene rather than endogenous Ig heavy chains. In contrast, transgene-encoded serum antibodies specific for alphaGal were not detected in GT+ mice. In transgenic mice on the GT+ background, B cells expressing the transgene underwent deletion as a result of encountering alphaGal during their development, indicating that expression of alphaGal as part of self-mediated efficient negative selection of B cells expressing transgene-encoded alphaGal-specific antibodies.

CONCLUSIONS

The development of transgenic mice expressing a B cell receptor specific for alphaGal provides a novel system to study developmental regulation of B cells making carbohydrate-specific antibodies. In addition, these mice may be useful for examining methods to prevent production of alphaGal-reactive antibodies.

The expression of ETV6/CBFA2 (TEL/AML1) is not sufficient for the transformation of hematopoietic cell lines in vitro or the induction of hematologic disease in vivo

ETV6/CBFA2 (TEL/AML1) is the most frequent genetic abnormality associated with acute lymphoblastic leukemias in children, and is associated with a favorable prognosis. To investigate the influence of ETV6/CBFA2 on cellular transformation, the fusion gene was cloned into a murine ecotropic retroviral vector and transduced into IL-3-dependent Ba/F3 and 32Dcl.3 and IL-7-dependent IxN/2b murine hematopoietic cell lines. Different variants of ETV6/CBFA2, corresponding to CBFA2 alternatively spliced variants, and the reciprocal product CBFA2/ETV6, were stably expressed in each of these cell lines. However, although Western blot analysis demonstrated expression of each variant, none of the stable cell lines expressing CBFA2/ETV6 or the variants conferred factor-independent growth. We further investigated the effect of ETV6/CBFA2 expression in vivo by generating transgenic mice in which expression of the fusion was directed to lymphoid cells using the immunoglobulin heavy chain enhancer/promoter. Four founder mice were identified showing transmission and expression of the chimeric product. The mice were bred for five generations and followed for more than 24 months. The mice did not develop a malignant hematologic disorder, nor did they display histopathologic, morphologic, or immunophenotypic abnormalities, although ETV6/CBFA2 expression was confirmed in each line. We conclude that the expression of ETV6/CBFA2 alone is not sufficient for induction of growth factor independence in hematopoietic cell lines or hematologic disease in transgenic mice.

HS1,2 enhancer regulation of germline epsilon and gamma2b promoters in murine B lymphocytes: evidence for specific promoter-enhancer interactions

During an immune response, activated B cells develop into high rate Ig-secreting plasma cells. They also switch from production of IgM to IgG, IgA, or IgE. This process requires a DNA recombination event, which is regulated at the transcriptional level by the production of isotype-specific, sterile germline (GL) transcripts. Induction of these transcripts is controlled by GL promoters and, possibly, by IgH 3' enhancers. We investigated the interaction of the GL epsilon and gamma2b promoters with the HS1,2 enhancer using transiently transfected mouse primary B cells and cell lines. The constructs used for the transfections contained a GL promoter upstream and HS1,2 downstream of a luciferase reporter gene. Both GL epsilon and gamma2b promoters synergized strongly with the HS1,2 enhancer in activated primary B cells, a mature B cell line, and a plasma cell line. We show that the major activity of HS1,2 in activated primary B cells occurs within a 310-bp fragment that includes NF-kappaB, OCT, and NF of activated B cells (Ets/AP-1) sites. By mutating the consensus sequences for various transcription factors, we have determined which sites in HS1,2 are important for synergy with the GL epsilon and gamma2b promoters. Our findings indicate that different sites in HS1,2 might selectively interact with the GL epsilon and gamma2b promoters. We also provide evidence that B cell-specific activator protein is not an absolute suppressor of HS1,2 activity.

Cutting Edge: IFN-γ Regulated Germline Transcripts Are Expressed from γ2a Transgenes Independently of the Heavy Chain 3′ Enhancers

Several results indicate that transcriptional enhancers lying 3′ of the Cα gene regulate RNA expression and switch recombination of heavy chain genes. To investigate this regulation we prepared transgenic mice with a 10.5-kb transgene that included the germline form of the murine γ2α gene, including promoter, I, S, and C regions. RNA was expressed from these γ2a transgenes with correct IFN-γ regulation, in spite of the fact that they lacked the 3′ enhancers. This RNA expression was independent of insertion site and dependent on copy number, indicating that the γ2a gene includes locus control region-like elements. Addition of either a cassette containing 3′ enhancer DNase I hypersensitive sites 1, 2, 3B, and 4 or the intronic μ enhancer increased transcription from the γ2a transgene by ∼75-fold in B cells. However, this increased transcription was not responsive to IFN-γ treatment of the transgenic B cells.

Differential effect of an Ig mu transgene on development of pre-B cells in fetal and adult SCID mice

Progression of pro-B lymphocytes to the pre-B stage depends on the expression of a pre-B cell receptor (pre-BCR), consisting of an Ig mu H chain, Ig surrogate light chain, and associated signal transducing chains. Mice that are unable to express a pre-BCR show an arrest of B cell development at the pro-B stage. Such is the case for severe combined immune deficient (SCID) mice in which mu chains are not made because of a defect in V(D)J recombination. When mu chains are made, as in SCID mice bearing a functional mu transgene, then B cell differentiation can proceed to the pre-B stage. However, as reported here, a mu transgene (M54) that promotes development of SCID pre-B cells in adult bone marrow fails to do so in fetal liver. We suggest that a pre-BCR containing the M54 mu chain cannot signal progression of pro-B cells to the pre-B stage in the fetal liver microenvironment.

The Ig heavy chain intronic enhancer core region is necessary and sufficient to promote efficient class switch recombination

The intronic IgH enhancer E(mu), which consists of the core enhancer (cE(mu) flanked by 5' and 3' matrix attachment regions (MAR), has been implicated in the control of IgH locus recombination and transcription. Both cE(mu) and the MAR are required to enhance transcription of an IgH transgene. To elucidate the regulatory functions of cE(mu) versus its associated MAR in IgH class switch recombination (CSR), we have assayed ES cell lines which have targeted deletions of these elements, both individually and in combination, by the Rag-2-deficient blastocyst complementation method. Mutant B cells from chimeric mice were activated in culture and the influence of the mutations on CSR was assessed by analysis of B cell hybridomas. We find that the cE(mu) is necessary and sufficient for providing the functions of E(mu) required for efficient CSR at the IgH locus. However, the 5' and 3' MAR sequences, as well as the known I(mu) transcription start sites and the bulk of I(mu) coding sequences, were dispensable for the process.

Extended duration of DH-JH rearrangement in immunoglobulin heavy chain transgenic mice: implications for regulation of allelic exclusion

Here we show that suppression of VH-DJH rearrangement in mice bearing a mu heavy (H) chain transgene (mu-tg mice) is associated with an extended period of DH-JH rearrangement, the first step of Immunoglobulin H chain gene rearrangement. Whereas DH-JH rearrangement is normally initiated and completed at the pro-B cell stage, in mu-tg mice it continues beyond this stage and occurs most frequently at the small (late) pre-B stage. Despite ongoing DH-JH rearrangement in late pre-B cells of mu-tg mice, VH-DJH rearrangement is not detectable in these cells. We infer that the lack of VH-DJH rearrangement primarily reflects tg-induced acceleration of B cell differentiation past the stage at which rearrangement of VH elements is permissible. In support of this inference, we find that the normal representation of early B lineage subsets is markedly altered in mu-tg mice. We suggest that the effect of a productive VH-DJH rearrangement at an endogenous H chain allele may be similar to that of a mu-tg; i.e., cells that make a productive VH-DJH rearrangement on the first attempt rapidly progress to a developmental stage that precludes VH-DJH rearrangement at the other allele (allelic exclusion).

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.

Recombination and transcription of the endogenous Ig heavy chain locus is effected by the Ig heavy chain intronic enhancer core region in the absence of the matrix attachment regions

The intronic Ig heavy chain (IgH) enhancer, which consists of the core enhancer flanked by 5' and 3' matrix attachment regions, has been implicated in control of IgH locus recombination and transcription. To elucidate the regulatory functions of the core enhancer and its associated matrix attachment regions in the endogenous IgH locus, we have introduced targeted deletions of these elements, both individually and in combination, into an IgHa/b-heterozygous embryonic stem cell line. These embryonic stem cells were used to generate chimeric mice by recombination activating gene-2 (Rag-2)-deficient blastocyst complementation, and the effects of the introduced mutations were assayed in mutant B cells. We find that the core enhancer is necessary and sufficient to promote normal variable (V), diversity (D), and joining (J) segment recombination in developing B lineage cells and IgH locus transcription in mature B cells. Surprisingly, the 5' and 3' matrix attachment regions were dispensable for these processes.

Early function of Pax5 (BSAP) before the pre-B cell receptor stage of B lymphopoiesis

The formation of the pre-B cell receptor (BCR) corresponds to an important checkpoint in B cell development that selects pro-B (pre-BI) cells expressing a functionally rearranged immunoglobulin mu (Igmu) heavy chain protein to undergo the transition to the pre-B (pre-BII) cell stage. The pre-BCR contains, in addition to Igmu, the surrogate light chains lambda5 and VpreB and the signal transducing proteins Igalpha and Igbeta. The absence of one of these pre-BCR components is known to arrest B cell development at the pre-BI cell stage. Disruption of the Pax5 gene, which codes for the B cell-specific activator protein (BSAP), also blocks adult B lymphopoiesis at the pre-BI cell stage. Moreover, expression of the mb-1 (Igalpha) gene and VH-to-DHJH recombination at the IgH locus are reduced in Pax5-deficient B lymphocytes approximately 10- and approximately 50-fold, respectively. Here we demonstrate that complementation of these deficiencies in pre-BCR components by expression of functionally rearranged Ig mu and chimeric Igmu-Igbeta transgenes fails to advance B cell development to the pre-BII cell stage in Pax5 (-/-) mice in contrast to RAG2 (-/-) mice. Furthermore, the pre-BCR is stably expressed on cultured pre-BI cells from Igmu transgenic, Pax5-deficient bone marrow, but is unable to elicit its normal signaling responses. In addition, the early developmental block is unlikely to be caused by the absence of a survival signal, as it could not be rescued by expression of a bcl2 transgene in Pax5-deficient pre-BI cells. Together, these data demonstrate that the absence of Pax5 arrests adult B lymphopoiesis at an early developmental stage that is unresponsive to pre-BCR signaling.

Repressible antisense inhibition in B lymphocytes

The tetracycline-responsive promoter (TRP) system has been adopted in an attempt to obtain repressible antisense inhibition in a B lymphocyte model in vitro. Levels of secreted IgM protein and mRNA were assessed following the stable transfection of B cell line, HO-2.2, with a series of plasmid constructs containing antisense or sense target sequence DNA (the 3'-untranslated region adjacent to the secreted exon of IgM gene) under the control of the TRP. Significant reduction (approximately 90%) in IgM secretion was observed for clones transfected with antisense plasmids driven by the TRP and containing the IgH enhancer element and the polyadenylation signal sequence from membrane IgM, when compared with untransfected and sense controls. Tetracycline (1 microgram/ml) addition to the culture medium restored the level of IgM secretion in these clones to control values, demonstrating repressibility of antisense inhibition. Transfection of HO-2.2 cells with antisense (or sense) constructs had no detectable effect on membrane IgM protein levels. Hybridisation studies demonstrated that decreased protein production observed in the antisense-transfected clones was most likely attributable to reduced RNA levels. These data show that the TRP can be used for repressible and specific antisense inhibition of gene product expression in B lymphocytes.

Essential functions of Pax-5 (BSAP) in pro-B cell development

Pax-5 codes for the transcription factor BSAP which is expressed in all B-lymphoid tissues in addition to the developing central nervous system and testis. Within the B-lymphoid lineage, Pax-5 expression is already detected in the earliest B cell progenitors and persists up to the mature B cell stage. Targeted inactivation of the Pax-5 gene in the mouse germline revealed a differential dependency of fetal and adult B-lymphopoiesis on this transcription factor. Pax-5 is required for the differentiation of the earliest B-lineage-committed precursor cells in the fetal liver. In contrast, B cell development in the adult bone marrow progresses up to an early pro-B cell stage in the absence of Pax-5 function. The expression of CD19, Ig alpha (mb-1) and N-myc is severely reduced in Pax-5-deficient pro-B cells. These BSAP target genes are, however, unlikely to explain the early developmental block based on their known function in B cell development. Moreover, VH-to-DHJH rearrangements at the immunoglobulin heavy-chain locus are approximately 50-fold reduced in Pax-5-deficient pro B-cells, while the DH-to-JH rearrangements occur at a normal frequency. However, the expression of rearranged mu heavy-chain transgenes does not allow Pax-5-deficient pro-B cells to develop further to the pre-B cell stage. Together these data demonstrate therefore that B cell development in the Pax-5 deficient bone marrow is arrested at an early pro-B cell stage which is not yet responsive to pre-B cell receptor signaling.

An example of idiotypic mimicry

We have evaluated the impact of transgenic immunoglobulin (TGIg) expression on endogenous antibody repertoires. The transgenic system was chosen as to allow for normal recombination of endogenous Ig genes, secretion of TGIg from early development on, and distinguishing the TGIg from endogenous Ig by several serological markers on the C and V regions of the molecules. The transgenic construct encodes a complete anti-(4-hydroxy-3-iodo-5-nitrophenyl)acetyl (NP) antibody molecule carrying a well-defined idiotype, bearing a lambda 1 light chain and a chimeric heavy chain encoded by a human alpha 2 C region devoid of its membrane exon, and the murine B1.8 VDJ-region. Endogenous antibody repertoires were analyzed in mitogen-driven limiting dilution cultures, in single-cell assays for naturally activated Ig-secreting cells, and in hybridomas derived by direct fusion of spleen cells from unmanipulated animals. The results show that a very high frequency of splenic resting B cells and plasma cells in transgenic animals produce IgM with B1.8-cross-reactive idiotypes. This was confirmed by hybridoma analysis which also established that the levels of transgene expression and of idiotype-positive IgM production by the same cell are not correlated. The affinities of idiotype-positive endogenous Ig varied, but were generally several orders of magnitude lower than the transgene-encoded idiotype. V regions from idiotype-cross-reactive IgM heavy chains showed marked diversity in sequences that were all different from the transgenic B1.8. These results are compatible with idiotypic mimicry resulting from intercellular selection based on degenerate, whole V region reactivities.

In vivo role of B lymphocytes in somatic transgene immunization

Immunity generated by in vivo inoculation of plasmid DNA is a straightforward and potentially valuable new approach to immunization. Little is known about the type of cells involved, the various immunological aspects, and the destiny of the transgene. In this report, we describe a system in which immunity is the result of in vivo targeting of B lymphocytes. This was accomplished using plasmid DNA encoding an immunoglobulin heavy-chain gene under the control of immunoglobulin promoter and enhancer elements. We show persistence of the transgene in splenic B lymphocytes for at least 3 months, i.e., the average life span of long-lived B lymphocytes in the mouse. The transgene could not be detected in any other lymphoid or nonlymphoid organs over a period of 6 months. We also established that the transgene is integrated in the host DNA. These studies bring new understanding to the events underlying the in vivo use of plasmid DNA. Moreover, the characteristics of this new approach make somatic transgene immunization a model system to study the immunogenicity of endogenous antigens in adult animals.

Does positive selection determine the B cell repertoire?

To know whether each newly formed B cell has an equal chance of survival in the organism, we analyzed the composition of the B cell repertoire of extremely limited diversity by generating mu-transgenic kappa-knockout mice. Surprisingly, in both types of mice studied, the B cell repertoire is mainly composed of cells expressing the mu-transgene-encoded chain associated with only one out four available lambda types depending on the mu transgene. Moreover, B cell differentiation cultures in vitro show that newly formed B cells can express the various lambda types regardless of the presence or absence of the mu transgenes. These results show a drastic impact of the heavy chain on the lambda light chain repertoire expressed in the periphery. The overexpression of a unique heavy/light chain pairing therefore results from selective processes. The immature B cells may be positively selected to provide the immunocompetent B cells in the periphery.

B cell development in mice

The development and establishment of the B Cell Repertoire is the net result of both genetic and environmental forces. The primary event at the genetic level is Ig gene rearrangement resulting in numerous possible combination of genes which can be further modified by somatic events such as N segment addition and somatic mutation. Environmental forces in the form of self and exogenous Ags also shape the repertoire by positively or negatively selecting B cells according to the specificity of their Ig receptors. These are dynamic processes beginning with the earliest expression of immunoglobulins in fetal life and continuing throughout life. In this review we discuss the genetic and selective mechanisms responsible for differences in the early immune system compared to that of the adult.

Transgenesis in rats: technical aspects and models

The production of transgenic rats by DNA-microinjection into fertilized ova has now become an established procedure, although fewer than 20 lines have been described during the last 5 years. Overall, transgenic rats remain more difficult to produce than transgenic mice, but satisfactory yields have been obtained by several laboratories. A review of the methods used to generate transgenic rats shows considerable variation between different laboratories, particularly in choice of strain, superovulation protocols and the use of embryo culture before reimplantation. In some instances, the production of transgenic rats has provided data that are new and relevant, compared to data obtained in mice bearing the same transgene. Models have been developed for human diseases such as hypertension and autoimmunity, and applications have been found in the study of carcinogenesis and in pharmacological research. Transgenic rat technology also opens up interesting perspectives for transplantation research, in which microsurgery is an essential procedure. Intensive research is in progress in several laboratories to produce rat embryonic stem (ES) cell lines, but existing lines have not participated in germ line formation a prerequisite for their use in gene knock out experiments.

A quasi-monoclonal mouse

As a model for studying the generation of antibody diversity, a gene-targeted mouse was produced that is hemizygous for a rearranged V(D)J segment at the immunoglobulin (Ig) heavy chain locus, the other allele being nonfunctional. The mouse also has no functional kappa light chain allele. The heavy chain, when paired with any lambda light chain, is specific for the hapten (4-hydroxy-3-nitrophenyl) acetyl (NP). The primary repertoire of this quasi-monoclonal mouse is monospecific, but somatic hypermutation and secondary rearrangements change the specificity of 20 percent of the antigen receptors on B cells. The serum concentrations of the Ig isotypes are similar to those in nontransgenic littermates, but less than half of the serum IgM binds to NP, and none of the other isotypes do. Thus, neither network interactions nor random activation of a small fraction of the B cell population can account for serum Ig concentrations.

Assembly of the truncated immunoglobulin heavy chain D mu into antigen receptor-like complexes in pre-B cells but not in B cells

Rearrangements of the IgH locus with JH joined to reading frame 2 of DH are greatly underrepresented in B cells. These rearrangements encode the truncated heavy chain D mu. In pre-B cells, we found D mu protein expressed on the cell surface and assembled into a complex with surrogate light chains, Ig alpha, and Ig beta. Cross-linking of either mu m- or D mu m- containing pre-B cell receptors triggered signal transduction reactions. In contrast, when expressed in mature B cell lines, D mu was not detected on the cell surface and did not efficiently bind kappa immunoglobulin light chains, but did associate with Ig alpha and Ig beta. These results characterize the interactions of D mu chain with other components of the B cell antigen receptor complex and suggest possible mechanisms by which D mu expression may interfere with B cell development.

An immunoglobulin mutator that targets G.C base pairs

Hypermutation can be defined as an enhancement of the spontaneous mutation rate which the organism uses in certain types of differentiated cells where a high mutation rate is advantageous. At the immunoglobulin loci this process increases the mutation rate > 10(5)-fold over the normal, spontaneous rate. Its proximate cause is called the immunoglobulin mutator system. The most important function of this system is to improve antibody affinity in an ongoing response; it is turned on and off during the differentiation of B lymphocytes. We have established an in vitro system to study hypermutation by transfecting a rearranged mu gene into a cell line in which an immunoglobulin mutator has been demonstrated. A construct containing the mu gene and the 3' kappa enhancer has all the cis-acting elements necessary for hypermutation of the endogenous gene segments encoding the variable region. The activity of the mutator does not seem to depend strongly on the position of the transfected gene in the genome. The mutator is not active in transformed cells of a later differentiation stage. It is also not active on a transfected lacZ gene. These results are consistent with the specificity of the mutator system being maintained and make it possible to delineate cis and trans mutator elements in vitro. Surprisingly, the mutator preferentially targets G-C base pairs. Two hypotheses are discussed: (i) the immunoglobulin mutator system in mammals consists of several mutators, of which the mutator described here is only one; or (ii) the primary specificity of the system is biased toward mutation of G-C base pairs, but this specificity is obscured by antigenic selection.

The cytoplasmic domains of immunoglobulin (Ig) alpha and Ig beta can independently induce the precursor B cell transition and allelic exclusion

In mature B cells, signals transduced through membrane immunoglobulin (Ig) produce cellular activation, yet the same receptor can also mediate deletion and silencing of autoreactive B cells. In addition, Ig expression during the antigen-independent phase of B cell development regulates the precursor B (pre-B) cell transition and allelic exclusion. To account for the diverse regulatory functions induced by membrane Ig, it has been proposed that individual receptor components have independent physiologic activities. Here we establish a role for Ig alpha in the pre-B cell transition and allelic exclusion. We find that the cytoplasmic domain of Ig alpha contains sufficient information to trigger both of these antigen-independent events. Direct comparisons of the cytoplasmic domains of Ig alpha and Ig beta show that the two are indistinguishable in the induction of the pre-B cell transition and allelic exclusion. Our experiments suggest that, despite the reported differences in certain biochemical assays, Ig alpha and Ig beta have redundant functions in the developing B cell.

Identification and analysis of the start site of ribosomal RNA transcription of Entamoeba histolytica

In this article we report the identification of the start site of ribosomal RNA transcription unit of the enteric parasite E. histolytica. We cloned the upstream region of the ribosomal RNA and we defined the 5' boundary of the transcription unit with nuclear run-on assays. We report that ribosomal transcription starts 2447 bp upstream the SSU ribosomal gene, at an adenosine residue. This data was supported both by S1 mapping and by primer extension analysis; that the mapped site was indeed the transcription start point was demonstrated by RNAse protection of the in vitro capped RNA. Our sequence data around the transcription start point shows two different tandem repeat clusters immediately downstream from the transcription start point.

Development of B cells in scid mice with immunoglobulin transgenes: implications for the control of V(D)J recombination

The inability of scid pro-B cells to progress to the pre-B and B cell stages is believed to be caused by a defective recombinase activity that fails to resolve chromosomal breaks resulting from attempted V(D)J recombination. In support of this model, we report that certain immunoglobulin transgenes, specifically those which strongly inhibit endogenous VH-to-DJH and V kappa-to-J kappa rearrangement in wild-type mice, allow scid pro-B cells to progress to the pre-B and B cell stages. This rescue of scid B cell differentiation is associated with a dramatic reduction in expression of the recombination activation genes, RAG1 and RAG2, and with reduced transcription of the kappa locus.

T cell receptor (beta chain) transgenic mice have selective deficits in gamma delta T cell subpopulations

TCR-beta (T cell receptor-beta chain) transgenic mice have altered lymphocyte development. TCR-beta transgenic mice are hyporesponsive to alloantigens in vivo and are deficient in gamma delta T cells. In order to begin a study of the relationship between a deficiency of alloreactive gamma delta cells and the defective function of in vivo alloantigen recognition, we analysed the gamma delta T cell development in TCR-beta mice. The presence of the TCR-V beta 8.2 chain transgene is associated with inhibition of gamma chain gene rearrangement. In order to determine how the presence of the TCR-beta transgene affects gamma delta T cell development, gamma delta T cells were studied in the skin, intestine and spleen. TCR-beta mice have dramatically reduced numbers of gamma delta T cells in the spleen and moderately reduced numbers of gamma delta T cells among intestinal intraepithelial lymphocytes. In contrast, these mice have normal numbers of gamma delta dendritic epidermal cells (DEC). These selective deficits could be due to the developmental regulation of transgene transcription during fetal life. We examined transcription of the TCR-beta transgene in the fetal thymus and found that the TCR-beta transgene is first transcribed at high levels on day 16 of fetal life, after DEC have already migrated from the thymus to the epidermis. Furthermore, mRNA from the transgene was detected in DEC, ruling out the formal possibility that DEC bear a gamma delta receptor only because they are incapable of expressing the transgene.(ABSTRACT TRUNCATED AT 250 WORDS)

Dependence of enhancer-mediated transcription of the immunoglobulin mu gene on nuclear matrix attachment regions

Transcription of the immunoglobulin mu heavy chain locus is regulated by an intronic enhancer that is flanked on both sides by nuclear matrix attachment regions (MARs). These MARs have now been shown to be essential for transcription of a rearranged mu gene in transgenic B lymphocytes, but they were not required in stably transfected tissue culture cells. Normal rates of transcriptional initiation at a variable region promoter and the formation of an extended deoxyribonuclease I (DNase I)--sensitive chromatin domain were dependent on MARs, although DNase I hypersensitivity at the enhancer was detected in the absence of MARs. Thus, transcriptional activation of the mu gene during normal lymphoid development requires a synergistic collaboration between the enhancer and flanking MARs.

Lipopolysaccharide-dependent transactivation of the temporally regulated immunoglobulin heavy chain 3' enhancer

To execute different biological functions, the expression pattern of immunoglobulin heavy chain genes (IgH) is altered during B lymphocyte differentiation. Early in B cell differentiation, it is assumed that the heavy chain promoter and the intragenic enhancer (E mu) ensure VDJ recombination. This leads to the expression of the immunoglobulin receptor on the cell surface. An additional strong enhancer in the far 3' end of the IgH locus has, however, prompted a re-evaluation of the regulation of immunoglobulin gene expression. To define the temporal and spatial regulation of the IgH 3' enhancer, transgenic mice harboring an enhancer-dependent reporter gene construct were generated. Here we demonstrate that IgH 3' enhancer activity is largely restricted to activated immunocompetent B cells. Furthermore, the enhancer can be transactivated following mitogen stimulation with lipopolysaccharide and 12-O-tetradecanoylphorbol 13-acetate. We propose a model whereby 3' enhancer activation is linked to the activation of resting immunocompetent B cells. The implications of the enhancer being active in late B lymphocyte differentiation, when heavy chain class switching occurs, are discussed.

Tissue-specific expression of the human CD19 gene in transgenic mice inhibits antigen-independent B-lymphocyte development

CD19 is a B-cell-specific member of the immunoglobulin superfamily expressed from early pre-B-cell development until plasma cell differentiation. In vitro studies demonstrate that the CD19 signal transduction molecule can serve as a costimulatory molecule for activation through other B-lymphocyte cell surface molecules. However, much remains to be known regarding how CD19 functions in vivo and whether CD19 has different roles at particular stages of B-cell differentiation. Therefore, transgenic mice overexpressing the human CD19 (hCD19) gene were generated to determine whether this transgene would be expressed in a B-lineage-specific fashion and to dissect the in vivo role of CD19 in B-cell development and activation. Expression of the human transgene product was specifically restricted to all B-lineage cells and appeared early in development as occurs with hCD19. In addition, expression of hCD19 severely impaired the development of immature B cells in the bone marrow, with dramatically fewer B cells found in the spleen, peripheral circulation, and peritoneal cavity. The level of hCD19 expressed on the cell surface correlated directly with the severity of the defect in different transgenic lines. These results demonstrate that the hCD19 gene is expressed in a lineage-specific fashion in mice, indicating that the hCD19 gene may be useful for mediating B-lineage-specific expression of other transgene products. In addition, these results indicate an important role for the lineage-specific CD19 molecule during early B-cell development before antigen-dependent activation.

Transgenic mice: a decade of progress in technology and research

In little more than a decade, the techniques developed for altering the genetic makeup of laboratory and livestock animals and plants have changed the landscape of biological research. It is now possible to introduce virtually any cloned gene into the germ line and study the expression pattern and effects of the introduced gene, or transgene. This has allowed the extension of in vitro and in vivo cell-culture studies into whole animal systems in which the introduced gene is subject to all normal regulatory processes from the onset of development. Although there have been reports of foreign gene expression resulting from direct injection of DNA in animals (e.g., Wolff et al., 1990; Zhu et al., 1993), transgenic animals are the primary model system for examining molecular genetic phenomena in vivo.

Tissue-specific expression of the human CD19 gene in transgenic mice inhibits antigen-independent B-lymphocyte development

CD19 is a B-cell-specific member of the immunoglobulin superfamily expressed from early pre-B-cell development until plasma cell differentiation. In vitro studies demonstrate that the CD19 signal transduction molecule can serve as a costimulatory molecule for activation through other B-lymphocyte cell surface molecules. However, much remains to be known regarding how CD19 functions in vivo and whether CD19 has different roles at particular stages of B-cell differentiation. Therefore, transgenic mice overexpressing the human CD19 (hCD19) gene were generated to determine whether this transgene would be expressed in a B-lineage-specific fashion and to dissect the in vivo role of CD19 in B-cell development and activation. Expression of the human transgene product was specifically restricted to all B-lineage cells and appeared early in development as occurs with hCD19. In addition, expression of hCD19 severely impaired the development of immature B cells in the bone marrow, with dramatically fewer B cells found in the spleen, peripheral circulation, and peritoneal cavity. The level of hCD19 expressed on the cell surface correlated directly with the severity of the defect in different transgenic lines. These results demonstrate that the hCD19 gene is expressed in a lineage-specific fashion in mice, indicating that the hCD19 gene may be useful for mediating B-lineage-specific expression of other transgene products. In addition, these results indicate an important role for the lineage-specific CD19 molecule during early B-cell development before antigen-dependent activation.