Transcription of a productively rearranged Ig VDJC alpha does not require the presence of HS4 in the IgH 3' regulatory region

High-affinity IgG antibodies develop naturally in Ig-knockout rats carrying germline human IgH/Igκ/Igλ loci bearing the rat CH region

Mice transgenic for human Ig loci are an invaluable resource for the production of human Abs. However, such mice often do not yield human mAbs as effectively as conventional mice yield mouse mAbs. Suboptimal efficacy in delivery of human Abs might reflect imperfect interaction between the human membrane IgH chains and the mouse cellular signaling machinery. To obviate this problem, in this study we generated a humanized rat strain (OmniRat) carrying a chimeric human/rat IgH locus (comprising 22 human V(H)s, all human D and J(H) segments in natural configuration linked to the rat C(H) locus) together with fully human IgL loci (12 Vκs linked to Jκ-Cκ and 16 Vλs linked to Jλ-Cλ). The endogenous Ig loci were silenced using designer zinc finger nucleases. Breeding to homozygosity resulted in a novel transgenic rat line exclusively producing chimeric Abs with human idiotypes. B cell recovery was indistinguishable from wild-type animals, and human V(D)J transcripts were highly diverse. Following immunization, the OmniRat strain performed as efficiently as did normal rats in yielding high-affinity serum IgG. mAbs, comprising fully human variable regions with subnanomolar Ag affinity and carrying extensive somatic mutations, are readily obtainable, similarly to conventional mAbs from normal rats.

Dynamic changes in binding of immunoglobulin heavy chain 3' regulatory region to protein factors during class switching

The 3' regulatory region (3' RR) of the Igh locus works at long distances on variable region (V(H)) and switch region (I) region promoters to initiate germ line (non-coding) transcription (GT) and promote class switch recombination (CSR). The 3' RR contains multiple elements, including enhancers (hs3a, hs1.2, hs3b, and hs4) and a proposed insulator region containing CTCF (CCCTC-binding factor) binding sites, i.e. hs5/6/7 and the downstream region ("38"). Notably, deletion of each individual enhancer (hs3a-hs4) has no significant phenotypic consequence, suggesting that the 3' RR has considerable structural flexibility in its function. To better understand how the 3' RR functions, we identified transcription factor binding sites and used chromatin immunoprecipitation (ChIP) assays to monitor their occupancy in splenic B cells that initiate GT and undergo CSR (LPS±IL4), are deficient in GT and CSR (p50(-/-)), or do not undergo CSR despite efficient GT (anti-IgM+IL4). Like 3' RR enhancers, hs5-7 and the 38 region were observed to contain multiple Pax5 binding sites (in addition to multiple CTCF sites). We found that the Pax5 binding profile to the 3' RR dynamically changed during CSR independent of the specific isotype to which switching was induced, and binding focused on hs1.2, hs4, and hs7. CTCF-associated and CTCF-independent cohesin interactions were also identified. Our observations are consistent with a scaffold model in which a platform of active protein complexes capable of facilitating GT and CSR can be formed by varying constellations of 3' RR elements.

Homologous elements hs3a and hs3b in the 3' regulatory region of the murine immunoglobulin heavy chain (Igh) locus are both dispensable for class-switch recombination

Immunoglobulin heavy chain (IgH) genes are formed, tested, and modified to yield diverse, specific, and high affinity antibody responses to antigen. The processes involved must be regulated, however, to avoid unintended damage to chromosomes. The 3' regulatory region of the Igh locus plays a major role in regulating class-switch recombination (CSR), the process by which antibody effector functions are modified during an immune response. Loss of all known enhancer-like elements in this region dramatically impairs CSR, but individual element deletions have no effect on this process. In the present study, we explored the hypothesis that an underlying functional redundancy in the homologous elements hs3a and hs3b was masking the importance of either element to CSR. Several transgenic mouse lines were generated, each carrying a bacterial artificial chromosome transgene that mimicked Igh locus structure but in which hs3a was missing and hs3b was flanked by loxP sites. Matings to Cyclization Recombination Enzyme-expressing mice established "pairs" of lines that differed only in the presence or absence of hs3b. Remarkably, CSR remained robust in the absence of both hs3a and hs3b, suggesting that the remaining two elements of the 3' regulatory region, hs1.2 and hs4, although individually dispensable for CSR, are, together, sufficient to support CSR.

In vivo redundant function of the 3' IgH regulatory element HS3b in the mouse

In the mouse, the regulatory region located at the 3' end of the IgH locus includes four transcriptional enhancers: HS3a, HS1-2, HS3b, and HS4; the first three lie in a quasi-palindromic structure. Although the upstream elements HS3a and HS1-2 proved dispensable for Ig expression and class switch recombination (CSR), the joint deletion of HS3b and HS4 led to a consistent decrease in IgH expression in resting B cells and to a major CSR defect. Within this pair of distal enhancers, it was questionable whether HS3b and HS4 could be considered individually as elements critical for IgH expression and/or CSR. Studies in HS4-deficient mice recently revealed the role of HS4 as restricted to Igmicro-chain expression from the pre-B to the mature B cell stage and left HS3b as the last candidate for CSR regulation. Our present study finally invalidates the hypothesis that CSR could mostly rely on HS3b itself. B cells from HS3b-deficient animals undergo normal proliferation, germline transcription, and CSR upon in vitro stimulation with LPS; in vivo Ag-specific responses are not affected. In conclusion, our study highlights a major effect of the global ambiance of the IgH locus; enhancers demonstrated as being strongly synergistic in transgenes turn out to be redundant in their endogenous context.

Ig synthesis and class switching do not require the presence of the hs4 enhancer in the 3' IgH regulatory region

Several studies have reported that regulatory elements located 3' of the IgH locus (namely hs3a, hs1,2, hs3b, and hs4) might play a role during class switch recombination (CSR) and Ig synthesis. While individual deletion of hs3a or hs1,2 had no effect, pairwise deletion of hs3b (an inverted copy of hs3a) and hs4 markedly affected CSR and Ig expression. Among these two elements, hs4 was tentatively presented with the master role due to its unique status within the 3' regulatory region: distal position outside repeated regions, early activation in pre-B cells, strong activity throughout B cell ontogeny. To clarify its role, we generated mice with a clean deletion of the hs4 after replacement with a floxed neo(R) cassette. Surprisingly, and as for previous deletion of hs3a or hs1,2, deletion of hs4 did not affect either in vivo CSR or the secretion level of any Ig isotype. In vitro CSR and Ig secretion in response to LPS and cytokines was not affected either. The only noticeable effects of the hs4 deletion were a decrease in the number of B splenocytes and a decreased membrane IgM expression. In conclusion, while dispensable for CSR and Ig transcription in plasma cells, hs4 mostly appears to contribute to Ig transcription in resting B lymphocytes.

A role for the IgH intronic enhancer E mu in enforcing allelic exclusion

The intronic enhancer (E mu) of the immunoglobulin heavy chain (IgH) locus is critical for V region gene assembly. To determine E mu's subsequent functions, we created an Igh allele with assembled V(H) gene but with E mu removed. In mice homozygous for this E mu-deficient allele, B cell development was normal and indistinguishable from that of mice with the same V(H) knockin and E mu intact. In mice heterozygous for the E mu-deficient allele, however, allelic exclusion was severely compromised. Surprisingly, this was not a result of reduced suppression of V-DJ assembly on the second allele. Rather, the striking breakdown in allelic exclusion took place at the pre-B to immature B cell transition. These findings reveal both an important role for E mu in influencing the fate of newly arising B cells and a second checkpoint for allelic exclusion.

Analysis of modulation of immunoglobulin gene expression

Immunoglobulins (Ig) are critical in maintaining host immunity to a variety of pathogens. Regulation of Ig expression is a complex process involving transcriptional regulation of different Ig gene loci by many transcription factors and transcriptional regulatory regions. This complexity suggests many possible molecular targets for immunotoxicants. Therefore, thorough evaluation of chemical-induced modulation of Ig expression may necessitate multiple experimental approaches evaluating: (1) number of B cells secreting antibodies by antibody-forming cell response or plaque assay; (2) concentration of total secreted antibodies by enzyme-linked immunosorbent assay (ELISA); (3) cellular proliferation and viability by cell count measurements, [(3)H] thymidine incorporation, and trypan blue exclusion; (4) Ig mRNA expression by quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR); (5) transcriptional activity of specific Ig regulatory regions by reporter gene analysis; and (6) transcription factor binding to specific Ig regulatory regions by electrophoretic mobility shift (EMSA) and chromatin immunoprecipitation (ChIP). These experimental approaches are discussed in the unit, with detailed description of EMSA, EMSA-western analysis, and isolation of nuclear protein.

Evidence for physical interaction between the immunoglobulin heavy chain variable region and the 3' regulatory region

B cell-specific expression of immunoglobulin heavy chain (IgH) genes utilizes two cis regulatory regions, the intronic enhancer (Emicro), located in the J(H)-Cmicro intron, and a complex regulatory region that lies 3' to the IgH gene cluster, 3' RR. We hypothesized that the 3' RR is involved in IgH gene transcription in plasma cells via physical interaction between distal 3' RR enhancers and target V(H) sequences, with loop formation by intervening DNA. In support of this hypothesis we report sequence data at DNA recombination breakpoints as evidence for loop formation preceding DNA inversion in a plasma cell line. In addition, using the chromosome conformation capture technique, physical interactions between V(H) and 3' RR were analyzed directly and detected in MPC11 plasma cells and variants and normal splenic B cells but not detected in splenic T cells or in non-B cells. V(H)-3' RR interactions were present in the absence of Emicro, but when the hs1,2 enhancer was replaced by a Neo(R) gene in a variant cell line lacking Emicro, H chain expression was lost, and interactions between V(H) and 3' RR and among the 3' RR regulators themselves were severely disrupted. In addition, the chromosome conformation capture technique detected interactions between the myc promoter and 3' RR elements in MPC11, which like other plasmacytomas contains a reciprocal translocation between the c-myc and the IgH locus. In sum, our data support a hypothesis that cis V(H)-3' RR and myc-3' RR interactions involve physical interactions between these DNA elements.