Cell-type-specific and regulated expression of a human gamma 1 heavy-chain immunoglobulin gene in transgenic mice

Complete cis Exclusion upon Duplication of the Eμ Enhancer at the Immunoglobulin Heavy Chain Locus

Developing lymphocytes somatically diversify their antigen-receptor loci through V(D)J recombination. The process is associated with allelic exclusion, which results in monoallelic expression of an antigen receptor locus. Various cis-regulatory elements control V(D)J recombination in a developmentally regulated manner, but their role in allelic exclusion is still unclear. At the immunoglobulin heavy chain locus (IgH), the Eμ enhancer plays a critical role in V(D)J recombination. We generated a mouse line with a replacement mutation in the constant region of the locus that duplicates the Eμ enhancer and allows premature expression of the γ3 heavy chain. Strikingly, IgM expression was completely and specifically excluded in cis from the mutant allele. This cis exclusion recapitulated the main features of allelic exclusion, including differential exclusion of variable genes. Notably, sense and antisense transcription within the distal variable domain and distal V(H)-DJ(H) recombination were inhibited. cis exclusion was established and stably maintained despite an active endogenous Eμ enhancer. The data reveal the importance of the dynamic, developmental stage-dependent interplay between IgH locus enhancers and signaling in the induction and maintenance of allelic exclusion.

Antigen-specific monoclonal antibodies isolated from B cells expressing constitutively active STAT5

Background

Fully human monoclonal antibodies directed against specific pathogens have a high therapeutic potential, but are difficult to generate.

Methodology/Principal Findings

Memory B cells were immortalized by expressing an inducible active mutant of the transcription factor Signal Transducer and Activator of Transcription 5 (STAT5). Active STAT5 inhibits the differentiation of B cells while increasing their replicative life span. We obtained cloned B cell lines, which produced antibodies in the presence of interleukin 21 after turning off STAT5. We used this method to obtain monoclonal antibodies against the model antigen tetanus toxin.

Conclusions/Significance

Here we describe a novel and relatively simple method of immortalizing antigen-specific human B cells for isolation of human monoclonal antibodies. These results show that STAT5 overexpression can be employed to isolate antigen specific antibodies from human memory B cells.

Human monoclonal antibodies from transgenic mice

Since the 1986 regulatory approval of muromonomab-CD3, a mouse monoclonal antibody (MAb) directed against the T cell CD3epsilon antigen, MAbs have become an increasingly important class of therapeutic compounds in a variety of disease areas ranging from cancer and autoimmune indications to infectious and cardiac diseases. However, the pathway to the present acceptance of therapeutic MAbs within the pharmaceutical industry has not been smooth. A major hurdle for antibody therapeutics has been the inherent immunogenicity of the most readily available MAbs, those derived from rodents. A variety of technologies have been successfully employed to engineer MAbs with reduced immunogenicity. Implementation of these antibody engineering technologies involves in vitro optimization of lead molecules to generate a clinical candidate. An alternative technology, involving the engineering of strains of mice to produce human instead of mouse antibodies, has been emerging and evolving for the past two decades. Now, with the 2006 US regulatory approval of panitumumab, a fully human antibody directed against the epidermal growth factor receptor, transgenic mice expressing human antibody repertoires join chimerization, CDR grafting, and phage display technologies, as a commercially validated antibody drug discovery platform. With dozens of additional transgenic mouse-derived human MAbs now in clinical development, this new drug discovery platform appears to be firmly established within the pharmaceutical industry.

IgG1 B cell receptor signaling is inhibited by CD22 and promotes the development of B cells whose survival is less dependent on Ig alpha/beta

We describe a mouse strain in which B cell development relies either on the expression of membrane-bound immunoglobulin (Ig) gamma1 or mu heavy chains. Progenitor cells expressing gamma1 chains from the beginning generate a peripheral B cell compartment of normal size with all subsets, but a partial block is seen at the pro- to pre-B cell transition. Accordingly, gamma1-driven B cell development is disfavored in competition with developing B cells expressing a wild-type (WT) IgH locus. However, the mutant B cells display a long half-life and accumulate in the mature B cell compartment, and even though partial truncation of the Ig alpha cytoplasmic tail compromises their development, it does not affect their maintenance, as it does in WT cells. IgG1-expressing B cells showed an enhanced Ca(2+) response upon B cell receptor cross-linking, which was not due to a lack of inhibition by CD22. The enhanced Ca(2+) response was also observed in mature B cells that had been switched from IgM to IgG1 expression in vivo. Collectively, these results suggest that the gamma1 chain can exert a unique signaling function that can partially replace that of the Ig alpha/beta heterodimer in B cell maintenance and may contribute to memory B cell physiology.

Gene dose-dependent maturation and receptor editing of B cells expressing immunoglobulin (Ig)G1 or IgM/IgG1 tail antigen receptors

Conserved differences between the transmembrane and cytoplasmic domains of membrane immunoglobulin (Ig)M and IgG may alter the function of antigen receptors on naive versus memory B cells. Here, we compare the ability of these domains to signal B cell allelic exclusion and maturation in transgenic mice. A lysozyme-binding antibody was expressed in parallel sets of mice as IgM, IgG1, or a chimeric receptor with IgM extracellular domains and transmembrane/cytoplasmic domains of IgG1. Like IgM, the IgG1 or chimeric IgM/G receptors triggered heavy chain allelic exclusion and supported development of mature CD21(+) B cells. Many of the IgG or IgM/G B cells became CD21(high) and downregulated their IgG and IgM/G receptors spontaneously, resembling memory B cells and B cells with mutations that exaggerate B cell antigen receptor signaling. Unlike IgM-transgenic mice, "edited" B cells that carry non-hen egg lysozyme binding receptors preferentially accumulated in IgG and IgM/G mice. This was most extreme in lines with the highest transgene copy number and diminished in variant offspring with fewer copies. The sensitivity of B cell maturation to transgene copy number conferred by the IgG transmembrane and cytoplasmic domains may explain the diverse phenotypes found in other IgG-transgenic mouse strains and may reflect exaggerated signaling.

DNA immunization in relB-deficient mice discloses a role for dendritic cells in IgM-->IgG1 switch in vivo

A single intraspleen inoculation of plasmid DNA coding for an immunoglobulin heavy chain gene initiates immunity and establishes immunologic memory against the antigenic determinants of transgenic immunoglobulins, somatic transgene immunization. During priming mice produce IgM but not IgG1 antibodies. Since IgM --> IgG1 class switch occurs spontaneously during the primary immune response to protein antigens we investigated possible mechanisms for failure of spontaneous isotype switch in vivo in this model of immunity. We found that inoculation of plasmid DNA in the form of a chimeric gene coding for granulocyte-macrophage colony-stimulating factor (GM-CSF) was able to drive IgG1 class switch readily after priming. Since GM-CSF activates cells of the dendritic lineage we tested the possibility that dendritic cells (DC) may be involved in regulating IgM --> IgG1 switch. To this end we used bone marrow chimeras constructed from mice carrying the null mutation for the relB member of the NF-kappaB/Rel family as these mice lack bone marrow-derived mature DC. RelB (-/-) mice and (-/-) bone marrow chimeras inoculated with DNA/GM-CSF did not produce IgG1 antibodies during the primary immune response. Since relB (-/-) bone marrow chimeras lack DC of donor origin but possess resident follicular dendritic cells we conclude that Ig class switch in vivo is regulated by the function of interdigitating dendritic cells (IDC). Thus, IDC may contribute to the qualitative aspects of the emerging immune response.

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.

Somatic transgene immunization with DNA encoding an immunoglobulin heavy chain

A plasmid DNA containing a chimeric immunoglobulin heavy-chain gene with tissue-specific promoter and enhancer elements was used as a model system to study the events triggered by a single intraspleen DNA inoculation in adult C57Bl/6 mice. A single intraspleen inoculation was followed in a week by secretion of transgene immunoglobulins and production of immunoglobulin M (IgM) anti-immunoglobulins. Their kinetics of serum appearance were almost superimposable. While anti-immunoglobulin antibodies remained detectable for over 6 months, transgene immunoglobulins disappeared after 3-4 weeks. However, transgene mRNA was detected in the spleen for 4 months. A multiplex polymerase chain reaction (PCR) analysis on splenic genomic DNA 17 days after inoculation demonstrated that the transgene was integrated in the host chromosomal DNA. The nucleotide sequence of the rearranged VDJ region from splenic genomic DNA was identical to that of the parental plasmid DNA, hence ruling out that hypermutation had occurred. A booster injection of immunoglobulin encoded by the transgene on day 200 elicited a typical secondary immune response with IgG1 and IgG2b antibodies. These results demonstrate that a single inoculation of an immunoglobulin heavy-chain DNA targeted to spleen lymphocytes leads to spontaneous integration of the transgene into the host DNA, and that this is sufficient to initiate immunity and establish immunologic memory. Our data also show that minute amounts (<100 ng/ml) of an endogenously produced protein secreted in the microenvironment of a lymphoid tissue generate immunity and establish immunologic memory rather than tolerance.

lambda 5, but not mu, is required for B cell maturation in a unique gamma 2b transgenic mouse line

gamma 2b transgenic mice have a severe B cell defect, apparently caused by strong feedback inhibition of endogenous H-gene rearrangement coupled with an inability of gamma 2b to provide the survival/maturation functions of mu. A unique gamma 2b transgenic line, named the C line, was found to permit B cell development. When the C line is crossed with a mu-membrane knockout line, gamma 2b+ B cells develop in the homozygous knockout. In contrast, a transgenic line representative of all the other gamma 2b lines is completely B cell deficient when mu-mem is deleted. Strikingly, the C phenotype is dominant in C x other gamma 2b transgenic line crosses. There is no evidence for higher gamma 2b transgene expression or other position effects on the transgene in the C mouse. The sequences of the three gamma 2b transgene copies in the C line are identical to that of the original transgene. These results have led to the conclusion that in the C line the transgene integration constitutively induces a gene whose expression can replace mu. To more clearly delineate the stage at which the altered phenotype of the C line is expressed, C mice were crossed onto a lambda 5 knockout background. In the absence of lambda 5, the C line produces no B cells. Since it was also found that gamma 2b can associate with the surrogate light chain (sL; lambda 5/Vpre-B), the crosses between C line gamma 2b mice and lambda 5 knockout mice suggest that gamma 2b/sL is required for B cell maturation in this mouse line. Thus, gamma 2b alone is unable to replace mu for pre-B cell survival/maturation; however, in combination with an unknown factor and the sL, gamma 2b can provide these nurturing functions.

Gamma 2b transgenic mice as a model for the role of immunoglobulins in B cell development

The development of B lymphocytes is tightly linked to the expression of immunoglobulins (Igs). Pro/preB cells which do not correctly rearrange heavy/light chain genes are aborted. Correctly rearranged Ig transgenes are apparently recognized by the developing B cells and can prevent the rearrangement of endogenous Ig genes. Both mu and gamma 2b heavy chain genes cause this feedback inhibition of heavy chain gene rearrangement. Mu transgenes can in addition replace endogenous MU in its preB cell survival/maturation function. However, several different transgenic lines have shown that gamma 2b transgenes do not provide the nurturing functions of mu, except for one unique gamma 2b transgenic line, the C line. In this line mature B cells express gamma 2b only. Presumably, an unknown gene has been activated at the transgene integration site whose product overcomes the need for mu. The function of this gene depends of the presence of the surrogate light chain (sL), and thus must operate in combination with the preB cell receptor or in a downstream signaling/antiapoptosis event requiring the gamma 2b/sL receptor. The analysis of the two types of gamma 2b transgenic mice shows that the signals for preB cell development are highly complex and promises to reveal new insights into the molecular and cellular mechanisms of B cell maturation.

Induction of tolerance to an IgG autoantibody

Nonautoimmune mice transgenic for the heavy chain of an IgG2b anti-double-stranded-DNA antibody express the transgene in lymphoid organs and display partial allelic exclusion of this gamma 2b transgene. The spleens of these mice are characterized by marked B-cell depletion. Although there are B cells in these mice that express the transgene and recognize double-stranded DNA, they are anergic in vivo. Recovery from the state of anergy occurs in vitro after lipopolysaccharide stimulation. Thus this transgenic model demonstrates the induction of self tolerance to an IgG autoantibody.

B-less: a strain of profoundly B cell-deficient mice expressing a human lambda transgene

We have created several transgenic mouse strains that bear the human lambda light chain gene driven by its own promoter and a mouse immunoglobulin heavy chain enhancer. The transgene is expressed in many tissues, with particularly high levels of expression in the bone marrow, thymus, spleen, and lymph nodes. One of these transgenic lines, B-less, displays a dramatic phenotype characterized by an acute susceptibility to bacterial and viral infections. Analysis of this strain shows it to be profoundly deficient in both immature (pre-B) and mature B cells, as well as in circulating immunoglobulin. The pre-B and B cell defects are cell autonomous, as judged by cell culture and bone marrow graft chimeras. Despite this B cell deficiency, the T cell lineage appears grossly normal as assessed by flow cytometric analysis and by its response to mitogen stimulation. Since an independently derived transgenic strain bearing the same human lambda construct displays a partial B-less phenotype, it is likely that the B lineage deficiency is due to a dominant effect of transgene expression rather than to the insertional perturbation of an endogenous mouse gene. It is interesting that the deficiency phenotype is fully expressed in the FVB/N genetic background, but is suppressed in F1 hybrids formed between the FVB/N and C57BL/6 inbred strains. Evidently, there are one or more dominant genetic suppressors of B-less in the C57BL/6 genome.

High-level expression of a human immunoglobulin gamma 1 transgene depends on switch region sequences

We describe that chimeric mouse-human immunoglobulin heavy chain (IgH) genes lacking a switch region and controlled by an IgH promoter and the intronic enhancer are only weakly expressed in transgenic mice. Insertion of part of the human C gamma 1 or murine Cmu switch region into the major intron of the chimeric IgH gene results in a 10(2)-to 10(3)-fold increase in transgene expression. Analysis of B cell hybridoma clones from transgenic mice suggests that switch sequences influence IgH transgene expression at the cellular level. However, the effect of switch region sequences on IgH gene expression observed in vivo is not apparent in transfected B cell lines. These results indicate that switch region sequences which are located proximal to the constant part of the gene and which are normally present in a rearranged IgH gene after class switching represent a novel type of regulatory element that plays a critical role in IgH gene expression in vivo.

Functional analysis of defined mutations in the immunoglobulin heavy-chain enhancer in transgenic mice

We have analyzed the effect of defined mutations in the mouse immunoglobulin heavy-chain enhancer after introduction into the germline of transgenic mice. We have tested a mutation of the enhancer octamer motif, a double mutation of the octamer motif and the microB-site, and a triple mutation in the microE2, microE3 and microE4-sites. All constructs are expressed in the spleen of transgenic mice. Furthermore, expression is exclusively detectable in lymphoid organs and not in several nonlymphoid tissues. Whereas mutations in the microE-sites have a more pronounced effect on transgene activity in thymocytes as compared to bone marrow and spleen cells, the octamer/microB double mutation shows significantly reduced expression levels only in B-cells. Finally, our results demonstrate that the intronic heavy-chain enhancer element does not contribute to the increase steady state levels of heavy-chain mRNA after stimulation of spleen cells with LPS.

Activation of immunoglobulin control elements in transgenic mice

To assess the role interleukins and mitogens play in regulating immunoglobulin (Ig) gene expression via the Ig enhancer and promoter, transgenic mice carrying two different Ig gene regulatory regions were generated. One, EmukCAT, contains the Ig heavy chain enhancer (Emu) and the kappa light chain promoter driving the chloramphenicol acetyltransferase (CAT) gene. In the other, delta EmukCAT, CAT is under the control of the kappa promoter alone. Emu and kappa relative activity were assessed by CAT assay. In EmukCAT mice, low CAT expression was consistently found in spleen, bone marrow, mesenteric lymph node, and thymus but not in brain, lung, or kidney. In delta EmukCAT mice, CAT expression was detectable just above background in lymphoid tissues, suggesting a basic level of tissue specificity in the absence of the enhancer. Whole spleen cell cultures prepared from the mice were treated with lymphokines and mitogens. Lipopolysaccharide (LPS), concanavilin A (Con A), interleukin 6 (IL-6), and interferon-gamma (IFN-gamma) increased CAT expression to varying extents in cells derived from EmukCAT mice but not in spleen cells prepared from delta EmukCAT mice. Thus, the presence of Emu, in addition to the kappa promoter, is essential for the stimulation of CAT expression mediated by these factors. B cells from EmukCAT mice were separated by density into populations of small and large cells. In untreated small B cells, no CAT expression was detected and only addition of LPS resulted in an increase in CAT expression. In large B cells, CAT was expressed at a low level without addition of exogenous factors. Incubation with LPS, IL-6, Con A and IFN-gamma caused CAT expression to increase several-fold. This transgenic system provides a means to identify exogenous factors that activate Ig enhancers and promoters.

Allelic exclusion of membrane but not secreted immunoglobulin in a mature B cell line

Although many B lymphocytes contain two completely rearranged immunoglobulin (Ig) heavy (H) chain genes, only one of the alleles specifies a protein product. This phenomenon is termed allelic exclusion and is controlled in part by the membrane portion of Ig H chains. We have identified a mature B cell line derived from murine bone marrow that expresses two H chain proteins, gamma 3 and gamma 2b. Proteins of appropriate size for the membrane and secreted forms of both H chains are produced and both IgG3 and IgG2b are secreted from the cells. However, only IgG3 is expressed on the membrane. Detailed restriction mapping of the H chain genes indicates that both are rearranged, one containing a VHJ558 gene linked to the gamma 3 constant region and the other a VHS107 gene linked to the gamma 2b constant region. Primer extension sequencing of the RNA reveals that the gamma 2b RNA is transcribed from the allele containing VHS107 sequences while the gamma 3 RNA is transcribed from the allele containing VHJ558 sequences. Thus, this mature B cell line secretes two distinct antibody molecules but is allelically excluded at the level of surface Ig expression. This cell line provides a model system to study factors, in addition to aberrant rearrangement, that influence allelic exclusion.

Hair-specific expression of chloramphenicol acetyltransferase in transgenic mice under the control of an ultra-high-sulfur keratin promoter

We have generated a transgenic mouse line by microinjection of a chimeric DNA fragment (KER-CAT) containing a hair-specific, murine ultra-high-sulfur keratin promoter (KER) fused to the coding region of the bacterial chloramphenicol acetyltransferase (CAT) gene. A 671-base pair (bp) stretch of the 5' promoter region was used to direct the expression of the CAT gene in this construct. Of the tissues tested for CAT activity in these transgenic animals only skin with growing hair, isolated hair follicles, and microdissected vibrissae showed substantial levels of activity. These are the same tissues where the endogenous ultra-high-sulfur keratin gene is expressed as shown by in situ hybridization. Furthermore, analysis of the CAT activity during the developmental stages of the hair growth cycle shows that the chimeric gene is expressed during the anagen phase of the hair growth cycle; this is the expected time during development for its expression. From these results we conclude that 671 bp of the promoter sequence from the ultra-high-sulfur keratin gene is sufficient to direct the correct development-specific and tissue-specific expression of the reporter gene construct in transgenic mice. The appropriate expression of the KER-CAT construct in transgenic mice is an important step in understanding the regulation of this gene during hair organogenesis.

Expression from the transferrin gene promoter in transgenic mice

Transferrin is an iron-binding protein that is expressed as a major product in liver and secreted into the plasma. To study the tissue-specific regulatory regions of this gene, the genomic mouse transferrin (mTf) gene was cloned and characterized by partial sequence analysis and S1 nuclease mapping of the transcriptional start site. Fusion genes containing the transferrin gene promoter and 5'-flanking sequences were ligated to the human growth hormone (hGH) gene and used to produce transgenic mice. A deletion construct containing the -581 to +50 region of the transferrin gene was sufficient to direct a high level of liver-specific expression resembling endogenous transferrin gene expression. Deletion to -139 base pairs of 5'-flanking sequence gave a construct which retained liver specificity, but the magnitude of expression decreased severalfold. These results demonstrate the presence of a liver-specific transcriptional element between -139 and +50 and suggest the presence of a distal element between -581 and -139 that can further increase expression. Surprisingly, fusion constructs containing -3 kilobase pairs (kb) of 5'-flanking sequence gave higher levels of mRNA in nonhepatic tissues than did either the -581 or -139 construct. Further studies indicated that the high levels of circulating hGH in these transgenic mice specifically induced the endogenous transferrin and albumin genes in liver and also stimulated the normally low levels of expression of the endogenous transferrin gene in brain, heart, kidney, and muscle. A mutated hGH gene that does not produce active growth hormone was fused to the -3- to +50-kb transferrin sequences to produce the -3-kb mTf-hGX construct. A liver-specific pattern of expression was observed in transgenic mice harboring the -3-kb mTf-hGX construct, and this mutated transgene was shown to be induced four- to sevenfold by either bovine or human growth hormone. These results demonstrate the presence of a growth hormone-responsive element between -3 and +50 kb in the 5'-flanking region of the mTf gene promoter.

Expression from the transferrin gene promoter in transgenic mice

Transferrin is an iron-binding protein that is expressed as a major product in liver and secreted into the plasma. To study the tissue-specific regulatory regions of this gene, the genomic mouse transferrin (mTf) gene was cloned and characterized by partial sequence analysis and S1 nuclease mapping of the transcriptional start site. Fusion genes containing the transferrin gene promoter and 5'-flanking sequences were ligated to the human growth hormone (hGH) gene and used to produce transgenic mice. A deletion construct containing the -581 to +50 region of the transferrin gene was sufficient to direct a high level of liver-specific expression resembling endogenous transferrin gene expression. Deletion to -139 base pairs of 5'-flanking sequence gave a construct which retained liver specificity, but the magnitude of expression decreased severalfold. These results demonstrate the presence of a liver-specific transcriptional element between -139 and +50 and suggest the presence of a distal element between -581 and -139 that can further increase expression. Surprisingly, fusion constructs containing -3 kilobase pairs (kb) of 5'-flanking sequence gave higher levels of mRNA in nonhepatic tissues than did either the -581 or -139 construct. Further studies indicated that the high levels of circulating hGH in these transgenic mice specifically induced the endogenous transferrin and albumin genes in liver and also stimulated the normally low levels of expression of the endogenous transferrin gene in brain, heart, kidney, and muscle. A mutated hGH gene that does not produce active growth hormone was fused to the -3- to +50-kb transferrin sequences to produce the -3-kb mTf-hGX construct. A liver-specific pattern of expression was observed in transgenic mice harboring the -3-kb mTf-hGX construct, and this mutated transgene was shown to be induced four- to sevenfold by either bovine or human growth hormone. These results demonstrate the presence of a growth hormone-responsive element between -3 and +50 kb in the 5'-flanking region of the mTf gene promoter.

Transgenic animals

The ability to introduce foreign genes into the germ line and the successful expression of the inserted gene in the organism have allowed the genetic manipulation of animals on an unprecedented scale. The information gained from the use of the transgenic technology is relevant to almost any aspect of modern biology including developmental gene regulation, the action of oncogenes, the immune system, and mammalian development. Because specific mutations can be introduced into transgenic mice, it becomes feasible to generate precise animal models for human genetic diseases and to begin a systematic genetic dissection of the mammalian genome.

Rearrangement and expression of rabbit immunoglobulin kappa light chain gene in transgenic mice

To determine whether a foreign unrearranged immunoglobulin gene can be functionally rearranged and expressed in vivo, a rabbit b9 kappa light chain gene construct containing a single germ-line kappa chain variable (V) region gene (V kappa), the five kappa chain joining (J) segments (J kappa), and the kappa chain constant (C) region germ-line gene (C kappa) was introduced into fertilized mouse eggs. Eleven transgenic mice carrying 1-30 copies of the rabbit kappa gene construct were obtained. Rearrangement of the transgene due to V kappa-J kappa recombination was observed in the spleen of all the mice lines analyzed. Only the J kappa 1 and J kappa 2 segments, which have canonical sequences for rearrangement and high-level expression, are utilized in assembly of the rabbit gene. V kappa-J kappa 1 and V kappa-J kappa 2 joining was also observed in the thymus but not in nonlymphoid tissue. Polyadenylylated rabbit kappa chain transcripts of 1.2 kilobases were found in the spleen of the transgenic mice. The level of transcription was low despite a high level of rearrangement. Three transgenic mice lines secreted kappa light chains encoded by the foreign rabbit gene. Serum rabbit kappa chains were associated with mouse mu and gamma 1 heavy chains. However, hybrid antibody molecules containing both rabbit and mouse kappa light chains were also found in the serum of these animals. These results suggest that, although the rabbit kappa chain gene construct contains the necessary sequences for gene assembly, sequences implicated in stage- and tissue-specific regulation of kappa chain gene rearrangement are either missing or not recognized by mouse lymphoid cells.

Allelic exclusion in transgenic mice that express the membrane form of immunoglobulin mu

Antibody-producing cells display a special form of regulation whereby each cell produces immunoglobulin from only one of its two sets of antibody genes. This phenomenon, called allelic exclusion, is thought to be mediated by the product of one heavy chain allele restricting the expression of the other. Heavy chains are synthesized in two molecular forms, secreted and membrane bound. In order to determine whether it is specifically the membrane-bound form of the immunoglobulin M (IgM) heavy chain (mu) that mediates this regulation, transgenic mice were created that carry a human mu chain gene altered so that it can only direct the synthesis of the membrane-bound protein. The membrane-bound form of the human mu chain was made by most of the B cells in these animals as measured by assays of messenger RNA and surface immunoglobulins. Further, the many B cells that express the human gene do not express endogenous mouse IgM, and the few B cells that express endogenous mouse mu do not express the transgene. Thus, the membrane-bound form of the mu chain is sufficient to mediate allelic exclusion. In addition, the molecular structures recognized for this purpose are conserved between human and mouse systems.

Role of replication time in the control of tissue-specific gene expression

Late-replicating chromatin in vertebrates is repressed. Housekeeping (constitutively active) genes always replicate early and are in the early-replicating R-bands. Tissue-specific genes are usually in the late-replicating G-bands and therein almost always replicate late. Within the G-bands, however, a tissue-specific gene does replicate early in those cell types that express that particular gene. While the condition of late replication may simply be coincident with gene repression, we review evidence suggesting that late replication may actively determine repression. As mammals utilize a developmental program to Lyonize (facultatively heterochromatinize) whole X chromosomes to a late-replicating and somatically heritable repressed state, similarly another program seems to Lyonize individual replicons. In frogs, all genes begin embryogenesis by replicating during a very short interval. As the developmental potency of embryonic cells becomes restricted, late-replicating DNA gradually appears. This addition to the repertoire of gene control--i.e., repression via Lyonization of individual replicons--seems to have evolved in vertebrates with G-bands being a manifestation of the mechanism.