Elevated expression levels of an Ig transgene in mice links the IgH 3' enhancer to the regulation of IgH expression

Transgenes of the mouse immunoglobulin heavy chain locus, lacking distal elements in the 3' regulatory region, are impaired for class switch recombination

The immunoglobulin heavy (H) chain class switch is mediated by a deletional recombination event between µ and γ, α, or ε constant region genes. This recombination event is upregulated during immune responses by a regulatory region that lies 3′ of the constant region genes. We study switch recombination using a transgene of the entire murine H chain constant region locus. We isolated two lines of mice in which the H chain transgenes were truncated at their 3′ ends. The truncation in both transgenic lines results in deletion of the 3′-most enhancer (HS4) and a region with insulator-like structure and activities. Even though both truncated transgenes express the µ H chain gene well, they undergo very low or undetectable switch recombination to transgenic γ and α constant region genes. For both transgenic lines, germline transcription of some H chain constant regions genes is severely impaired. However, the germline transcription of the γ1 and γ2a genes is at wild type levels for the transgenic line with the larger truncation, but at reduced levels for the transgenic line with the smaller truncation. The dramatic reduction in class switch recombination for all H chain genes and the varied reduction in germline transcription for some H chain genes could be caused by (i) insertion site effects or (ii) deletion of enhancer elements for class switch recombination and transcription, or (iii) a combination of both effects.

The 3' end of the heavy chain constant region locus enhances germline transcription and switch recombination of the four gamma genes

The switch in immunoglobulin (Ig) heavy chain class is preceded by germline transcription and then mediated by a DNA recombination event. To study germline transcription and class switch recombination we used transgenic mice with a 230-kilobase bacterial artificial chromosome that included a rearranged VDJ gene and the entire heavy chain constant region locus. In addition to several lines with intact transgenes, we identified two lines in which the heavy chain locus transgene lacked Cα and everything 3′ of it, including the regulatory elements HS3a, HS1-2, HS3b, and HS4. B cells from both lines with the truncated transgenes make abundant transgenic (Tg) VDJCμ transcripts and IgM protein. Deletion of the 3′ end of the locus results in dramatically reduced expression of both germline transcripts and switched VDJCH transcripts of the γ3, γ2b, γ2a, and ɛ genes. In addition, the transgenes lacking the 3′ end of the locus express reduced amounts of γ1 germline transcripts and 2–3% of the amount of Tg IgG1 in tissue culture compared with intact transgenes. Finally, switch recombination to γ1 is undetectable in the transgenes lacking the 3′ elements, as measured by digestion circularization–polymerase chain reaction or by the expression of VDJCγ1 transcripts.

A pivotal role for DNase I-sensitive regions 3b and/or 4 in the induction of somatic hypermutation of IgH genes

Chimeric mice were prepared from embryonic stem cells transfected with IgH genes as transgenes and RAG-2-deficient blastocysts for the purpose of identifying the cis-acting elements responsible for the induction of somatic hypermutation. Among the three transgene constructs used, the V(H) promoter, the rearranged V(H)-D-J(H), an intron enhancer/matrix attachment region, and human Cmu were common to all, but the 3'-untranslated region in each construct was different. After immunization of mice with a T cell-dependent Ag, the distribution and frequency of hypermutation in transgenes were analyzed. The transgene lacking the 3' untranslated region showed a marginal degree of hypermutation. Addition of the 3' enhancer resulted in a slight increase in the number of mutations. However, the transgene containing DNase I-sensitive regions 3b and 4 in addition to the 3' enhancer showed more than a 10-fold increase in hypermutation, reaching levels comparable to those observed in endogenous V(H)186.2 genes of C57BL/6 mice.

The lymphoid-specific cofactor OBF-1 is essential for the expression of a V(H) promoter/HS1,2 enhancer-linked transgene in late B cell development

Mice deficient for the lymphoid-specific cofactor OBF-1 display reduced levels of IgG, IgA and IgE. To examine whether the lowered immunoglobulin expression is linked to reduced activity of IgH cis-regulatory elements, OBF-1(-/-) mice were crossed with mice expressing transgenes driven by a V(H) or beta-globin promoter linked to the HS1,2 enhancer. Here we show that OBF-1 is essential for the induced expression of a V(H) promoter-linked transgene, in contrast to a beta-globin promoter-dependent transgene, in LPS/IL-4 or CD40-stimulated splenic B cells. Furthermore, impaired transgene expression is observed in OBF-1(-/-) peritoneal B cells. This deficiency may be linked to OBF-1, as peritoneal cells from normal mice express OBF-1 protein constitutively. Our data link OBF-1 to IgH gene expression in late B lymphoid development.

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.

Class switching in B cells lacking 3' immunoglobulin heavy chain enhancers

The 40-kb region downstream of the most 3' immunoglobulin (Ig) heavy chain constant region gene (Calpha) contains a series of transcriptional enhancers speculated to play a role in Ig heavy chain class switch recombination (CSR). To elucidate the function of this putative CSR regulatory region, we generated mice with germline mutations in which one or the other of the two most 5' enhancers in this cluster (respectively referred to as HS3a and HS1,2) were replaced either with a pgk-neor cassette (referred to as HS3aN and HS1,2N mutations) or with a loxP sequence (referred to as HS3aDelta and HS1,2Delta, respectively). B cells homozygous for the HS3aN or HS1,2N mutations had severe defects in CSR to several isotypes. The phenotypic similarity of the two insertion mutations, both of which were cis-acting, suggested that inhibition might result from pgk-neor cassette gene insertion rather than enhancer deletion. Accordingly, CSR returned to normal in B cells homozygous for the HS3aDelta or HS1,2Delta mutations. In addition, induced expression of the specifically targeted pgk-neor genes was regulated similarly to that of germline CH genes. Our findings implicate a 3' CSR regulatory locus that appears remarkably similar in organization and function to the beta-globin gene 5' LCR and which we propose may regulate differential CSR via a promoter competition mechanism.

Temporal control of IgH gene expression in developing B cells by the 3' locus control region

The suggested roles of the downstream 3' regions acting as a Locus Control Region (LCR), have allowed comparisons to be made between the regulation of the IgH locus with other model systems whose gene expression is governed by LCR activity. Here we summarize the importance of the IgH 3'LCR and its putative functional role in IgH gene expression and compare it with the 5'LCR regulatory region of the human beta-globin locus.

The enhancer shift: a model to explain the developmental control of IgH gene expression in B-lineage cells

The roles attributed to the E mu enhancer and the downstream 3' regions acting as locus control regions (LCRs) have allowed comparisons to be drawn between regulation of the IgH locus in early and late B-cell development and other systems governed by LCR activity. Here, the importance of the IgH 3'LCR and its putative functional role in the control of IgH gene expression is evaluated and compared with the 5'LCR regulatory region of the human beta-globin locus.

Evaluation of novel control elements by construction of eukaryotic expression vectors

A novel mammalian eukaryotic expression vector for the production of immunoglobulin heavy chain (IgH) genes has been designed. This expression vector contains the variable heavy chain (VH) promoter, the IgH intron enhancer (muE) and the IgH 3' enhancer (3'E). This construct, designated pTIF-1, was stably transfected into the myeloma cell line J558L. A fivefold increase in the expression level of a rearranged IgH gene was observed when using the pTIF-1 vector containing the 3'E compared to an expression vector lacking this enhancer. Interestingly, this positive effect on the expression level of the 3' enhancer appears to be position independent. The introduction of two recently identified Ig control elements, HS3 and HS4, to the vector cassette did not further elevate the expression level in the cell line tested. The pTIF-1 vector can be used for expression of any antibody specificity, using PCR amplification of the VDJ region of interest. Furthermore, the constant region can easily be exchanged, which further facilitates studies to dissect different effector functions of IgH constant genes.

NFE, a new transcriptional activator that facilitates p50 and c-Rel-dependent IgH 3' enhancer activity

The induction of immunoglobulin heavy chain (IgH) 3' enhancer activity has been coupled to ligand/receptor-dependent activation of resting B cells. To search for transcriptional target sites that account for this induction, extracts from lipopolysaccharide (LPS)-stimulated B cells and cell lines were used. Here we describe, by gel-retardation analysis, the identification of an NF-kappaB site and an adjacent nuclear factor ets-like (NFE) site in the 3' enhancer. The NFE motif binds four protein complexes in resting B cell extracts, of which two are down-regulated upon LPS stimulation. Gel shift-shift experiments of the NF-kappaB complexes with specific antibodies identified p50 and c-Rel proteins to be the predominant factors in primary LPS-stimulated cell extracts. Site-directed mutagenesis of these motifs demonstrates that they contribute to part of the enhancer activity in plasma cells. One copy of the NFkappaB/NFE motifs, linked to a heterologous reporter construct, displays lymphoid-restricted reporter gene activity in transient transfection assays. Mutation of either site abrogates all promoter activity. Complementation experiments demonstrate that although p50 and c-Rel expression vectors reconstitute transcription of an intact NF-kappaB/NFE reporter construct in a dose-dependent manner, mutation of the NFE site or the NF-kappaB site abrogates essentially all transcriptional activity in both plasma cells and in COS cells. Taken together, we provide evidence for the existence of an activator, NFE, which in combination with the p50 and c-Rel proteins, are part of the transcription factor machinery that regulates 3' enhancer activity, and thus the control of the IgH locus in late B lymphocyte development.