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

Related Mechanisms of Antibody Somatic Hypermutation and Class Switch Recombination

The primary antibody repertoire is generated by mechanisms involving the assembly of the exons that encode the antigen-binding variable regions of immunoglobulin heavy (IgH) and light (IgL) chains during the early development of B lymphocytes. After antigen-dependent activation, mature B lymphocytes can further alter their IgH and IgL variable region exons by the process of somatic hypermutation (SHM), which allows the selection of B cells in which SHMs resulted in the production of antibodies with increased antigen affinity. In addition, during antigen-dependent activation, B cells can also change the constant region of their IgH chain through a DNA double-strand-break (DSB) dependent process referred to as IgH class switch recombination (CSR), which generates B cell progeny that produce antibodies with different IgH constant region effector functions that are best suited for a elimination of a particular pathogen or in a particular setting. Both the mutations that underlie SHM and the DSBs that underlie CSR are initiated in target genes by activation-induced cytidine deaminase (AID). This review describes in depth the processes of SHM and CSR with a focus on mechanisms that direct AID cytidine deamination in activated B cells and mechanisms that promote the differential outcomes of such cytidine deamination.

Low-affinity IgM antibodies lacking somatic hypermutations are produced in the secondary response of C57BL/6 mice to (4-hydroxy-3-nitrophenyl)acetyl hapten

Class-switched memory B cells, which are generated through the processes of somatic hypermutation (SHM) and affinity-based selection in germinal centers, contribute to the production of affinity-matured IgG antibodies in the secondary immune response. However, changes in the affinity of IgM antibodies during the immune response have not yet been studied, although IgM(+) memory B cells have been shown to be generated. In order to understand the relationship between IgM affinity and the recall immune response, we prepared hybridomas producing anti-(4-hydroxy-3-nitrophenyl)acetyl (NP) IgM antibodies from C57BL/6 mice and from activation-induced cytidine deaminase (AID)-deficient mice. Binding analysis by ELISA showed that mAbs obtained from the secondary immune response contained IgM mAbs with affinity lower than the affinity of mAbs obtained from the primary response. By analyzing sequences of the IgM genes of hybridomas and plasma cells, we found many unmutated VH genes. VH genes that had neither tyrosine nor glycine at position 95 were frequent. The repertoire change may correlate with the lower affinity of IgM antibodies in the secondary response. The sequence and affinity changes in IgM antibodies were shown to be independent of SHM by analyzing hybridomas from AID-deficient mice. A functional assay revealed a reciprocal relationship between affinity and complement-dependent hemolytic activity toward NP-conjugated sheep RBCs; IgM antibodies with lower affinities had higher hemolytic activity. These findings indicate that lower affinity IgM antibodies with enhanced complement activation function are produced in the secondary immune response.

The IgH 3' regulatory region controls somatic hypermutation in germinal center B cells

Somatic hypermutation in variable heavy chain rearranged regions is abrogated in the absence of the 3′ regulatory region enhancer, whereas transcription rate in the Ig heavy chain is only partially reduced.

Interactions with cognate antigens recruit activated B cells into germinal centers where they undergo somatic hypermutation (SHM) in V(D)J exons for the generation of high-affinity antibodies. The contribution of IgH transcriptional enhancers in SHM is unclear. The E_μ enhancer upstream of C_μ has a marginal role, whereas the influence of the IgH 3′ regulatory region (3′RR) enhancers (hs3a, hs1,2, hs3b, and hs4) is controversial. To clarify the latter issue, we analyzed mice lacking the whole 30-kb extent of the IgH 3′RR. We show that SHM in V_H rearranged regions is almost totally abrogated in 3′RR-deficient mice, whereas the simultaneous Ig heavy chain transcription rate is only partially reduced. In contrast, SHM in κ light chain genes remains unaltered, acquitting for any global SHM defect in our model. Beyond class switch recombination, the IgH 3′RR is a central element that controls heavy chain accessibility to activation-induced deaminase modifications including SHM.

AID targeting in antibody diversity

Antibody maturation requires class switch recombination (CSR) and somatic hypermutation (SHM), both of which are initiated by activation-induced cytidine deaminase (AID). AID deaminates cytosine residues resulting in mismatches that are differentially processed to produce double-strand breaks in Ig switch (S) regions that lead to CSR, or to point mutations in variable (V) exons resulting in SHM. Although AID was first thought to be Ig-specific, recent work indicates that it also targets a diverse group of non-Ig loci, including genes such as Bcl6 and c-myc, whose modification by AID results in lymphoma-associated mutations and translocations. Here, we review the recent literature on AID targeting and the role for transcriptional stalling in recruitment of this enzyme to Ig and non-Ig loci. We propose a model for AID recruitment based on transcriptional stalling, which reconciles several of the key features of SHM, CSR, and lymphoma-associated translocation.

In-vitro derived germinal centre B cells differentially generate memory B or plasma cells in vivo

In response to T cell-dependent antigens, B cells proliferate extensively to form germinal centres (GC), and then differentiate into memory B (B(mem)) cells or long-lived plasma cells (LLPCs) by largely unknown mechanisms. Here we show a new culture system in which mouse naïve B cells undergo massive expansion and isotype switching, and generate GC-phenotype B (iGB) cells. The iGB cells expressing IgG1 or IgM/D, but not IgE, differentiate into B(mem) cells in vivo after adoptive transfer and can elicit rapid immune responses with the help of cognate T cells. Secondary culture with IL-21 maintains the proliferation of the iGB cells, while shifting their in vivo developmental fate from B(mem) cells to LLPCs, an outcome that can be reversed by withdrawal of IL-21 in tertiary cultures. Thus, this system enables in vitro manipulation of B-cell fate, into either B(mem) cells or LLPCs, and will facilitate dissection of GC-B cell differentiation programs.

The IgH locus 3' regulatory region: pulling the strings from behind

Antigen receptor gene loci are among the most complex in mammals. The IgH locus, encoding the immunoglobulin heavy chain (IgH) in B-lineage cells, undergoes major transcription-dependent DNA remodeling events, namely V(D)J recombination, Ig class-switch recombination (CSR), and somatic hypermutation (SHM). Various cis-regulatory elements (encompassing promoters, enhancers, and chromatin insulators) recruit multiple nuclear factors in order to ensure IgH locus regulation by tightly orchestrated physical and/or functional interactions. Among major IgH cis-acting regions, the large 3' regulatory region (3'RR) located at the 3' boundary of the locus includes several enhancers and harbors an intriguing quasi-palindromic structure. In this review, we report progress insights made over the past decade in order to describe in more details the structure and functions of IgH 3'RRs in mouse and human. Generation of multiple cellular, transgenic and knock-out models helped out to decipher the function of the IgH 3' regulatory elements in the context of normal and pathologic B cells. Beside its interest in physiology, the challenge of elucidating the locus-wide cross talk between distant cis-regulatory elements might provide useful insights into the mechanisms that mediate oncogene deregulation after chromosomal translocations onto the IgH locus.

Comparison of identical and functional Igh alleles reveals a nonessential role for Eμ in somatic hypermutation and class-switch recombination

Somatic hypermutation (SHM), coupled with Ag selection, provides a mechanism for generating Abs with high affinity for invading pathogens. Class-switch recombination (CSR) ensures that these Abs attain pathogen-appropriate effector functions. Although the enzyme critical to both processes, activation-induced cytidine deaminase, has been identified, it remains unclear which cis-elements within the Ig loci are responsible for recruiting activation-induced cytidine deaminase and promoting its activity. Studies showed that Ig gene-transcription levels are positively correlated with the frequency of SHM and CSR, making the intronic, transcriptional enhancer Eμ a likely contributor to both processes. Tests of this hypothesis yielded mixed results arising, in part, from the difficulty in studying B cell function in mice devoid of Eμ. In Eμ's absence, V(H) gene assembly is dramatically impaired, arresting B cell development. The current study circumvented this problem by modifying the murine Igh locus through simultaneous insertion of a fully assembled V(H) gene and deletion of Eμ. The behavior of this allele was compared with that of a matched allele carrying the same V(H) gene but with Eμ intact. Although IgH transcription was as great or greater on the Eμ-deficient allele, CSR and SHM were consistently, but modestly, reduced relative to the allele in which Eμ remained intact. We conclude that Eμ contributes to, but is not essential for, these complex processes and that its contribution is not as a transcriptional enhancer but, rather, is at the level of recruitment and/or activation of the SHM/CSR machinery.

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.

Switch recombination and somatic hypermutation are controlled by the heavy chain 3' enhancer region

Both class switch recombination (CSR) and somatic hypermutation (SHM) require transcription and the trans-acting factor activation-induced cytidine deaminase (AID), and must be up-regulated during antigen-dependent differentiation of B lymphocytes. To test the role of the heavy chain 3′ enhancers in both CSR and SHM, we used a BAC transgene of the entire heavy chain constant region locus. Using Cre-loxP recombination to delete a 28-kb region that contains the four known 3′ heavy chain enhancers, we isolated lines of BAC transgenic mice with an intact heavy chain locus and paired lines in the same chromosomal insertion site lacking the 3′ enhancers. Intact heavy chain transgenes undergo CSR to all heavy chain genes and mutate their transgenic VDJ exon. In paired transgenes lacking the 3′ enhancer region, CSR to most heavy chain genes is reduced to ∼1% of the levels for intact heavy chain loci; SHM is also reduced. Finally, we find that in B cells with a transgene lacking the 3′ enhancers, interchromosomal recombination between the transgenic VDJ exon and the endogenous heavy chain C genes is more easily detected than CSR within the transgene.

NF-kappaB family of transcription factor facilitates gene conversion in chicken B cells

Activation-induced cytidine deaminase (AID) is critical for immunoglobulin (Ig) diversification in B cells. The majority of evidence supports the model that AID modifies Ig genes at the DNA level by deaminating cytosines into uracils. The mutagenic activity is largely restricted to Ig genes to avoid genomic damage in general, but the underlying mechanism is not understood. We addressed this question in chicken B cell line DT40. We characterized a regulatory region within the Iglambda locus. This regulatory region is important for AID-mediated gene conversion at the Iglambda locus, and is capable of targeting AID activity to ectopic loci. This regulatory region contains binding sites for transcription factors NF-kappaB, Mef2 and octamer binding proteins. Mutation of these binding sites or ablation of NF-kappaB family member, p50 or c-Rel, impairs the AID targeting function of this regulatory region. These results suggest that NF-kappaB family of transcription factors contribute to AID-mediated gene conversion.

In a model of immunoglobulin heavy-chain (IGH)/MYC translocation, the Igh 3' regulatory region induces MYC expression at the immature stage of B cell development

Reciprocal translocations involving the immunoglobulin loci and the cellular oncogene MYC are hallmark mutations of the human postgerminal center B cell neoplasm, Burkitt's lymphoma. They are occasionally found in other B cell lymphomas, as well. Translocations involving the heavy chain locus (IGH) place the MYC gene either in cis with both the intronic enhancer Emu and the IGH 3' regulatory region (3'RR) or in cis with only the 3'RR. The result is deregulated MYC expression. Recent studies have led to some controversy as to when, during B lymphocyte development, IGH/MYC chromosome translocations take place. A related issue, relevant not only to lymphoma development but also to normal controls on IGH gene expression, is the stage, during B lymphocyte development, at which the 3'RR is capable of activating MYC expression. We have developed mice transgenic for a human MYC (hMYC) gene under control of the four core enhancers from the mouse Igh 3'RR. Unlike other transgenic mouse models where premature and inappropriate MYC expression disrupts normal B cell development, the hMYC transgene in these studies carries a mutation that prohibits MYC protein synthesis. As a result, hMYC expression can be analyzed in all of the normal B cell compartments. Our data show that hMYC is expressed almost exclusively in B-lineage cells and is induced to high levels as soon as bone marrow cells reach the immature B cell stage.

Analysis of 6912 unselected somatic hypermutations in human VDJ rearrangements reveals lack of strand specificity and correlation between phase II substitution rates and distance to the nearest 3' activation-induced cytidine deaminase target

The initial event of somatic hypermutation (SHM) is the deamination of cytidine residues by activation-induced cytidine deaminase (AID). Deamination is followed by the replication over uracil and/or different error-prone repair events. We sequenced 659 nonproductive human IgH rearrangements (IGHV3-23*01) from blood B lymphocytes enriched for CD27-positive memory cells. Analyses of 6,912 unique, unselected substitutions showed that in vivo hot and cold spots for the SHM of C and G residues corresponded closely to the target preferences reported for AID in vitro. A detailed analysis of all possible four-nucleotide motifs present on both strands of the V(H) gene showed significant correlations between the substitution frequencies in reverse complementary motifs, suggesting that the SHM machinery targets both strands equally well. An analysis of individual J(H) and D gene segments showed that the substitution frequencies in the individual motifs were comparable to the frequencies found in the V(H) gene. Interestingly, J(H)6-carrying sequences were less likely to undergo SHM (average 15.2 substitutions per V(H) region) than sequences using J(H)4 (18.1 substitutions, p = 0.03). We also found that the substitution rates in G and T residues correlated inversely with the distance to the nearest 3' WRC AID hot spot motif on both the nontranscribed and transcribed strands. This suggests that phase II SHM takes place 5' of the initial AID deamination target and primarily targets T and G residues or, alternatively, the corresponding A and C residues on the opposite strand.

Regulation of epsilon germline transcription and switch region mutations by IgH locus 3' enhancers in transgenic mice

Germline (GL) transcription is regulated by specific promoters and immunoglobulin heavy chain (IgH) 3' locus enhancers and is necessary for Ig class-switch recombination (CSR). We have generated different transgenic lines containing the GL epsilon promoter, switch (S) epsilon region, and constant (C) epsilon region with or without the DNase I-sensitive regions (HS) 3A-HS1,2 or HS3B-HS4 3' IgH enhancer pairs. The enhancerless construct was expressed in B cells activated by interleukin (IL)-4 and CD40, thus resembling regulation of the endogenous gene. Both enhancer-containing transgenes efficiently increased expression in B cells and were strongly up-regulated by stimuli. In addition, Sepsilon regions of the transgene containing HS3B-HS4 were mutated in activated, sorted B cells. Such mutations are known to precede CSR and are dependent on activation-induced cytidine deaminase (AID). Our findings show that all elements necessary for recruitment of the recombination machinery are present in the transgene containing HS3 and HS4. These enhancers probably provide something more specific than mere increased accessibility of switch regions. We propose that transcription factors binding the enhancers help to target the recombination machinery to the switch regions.

Targeting of somatic hypermutation

Somatic hypermutation (SHM) introduces mutations in the variable region of immunoglobulin genes at a rate of approximately 10(-3) mutations per base pair per cell division, which is 10(6)-fold higher than the spontaneous mutation rate in somatic cells. To ensure genomic integrity, SHM needs to be targeted specifically to immunoglobulin genes. The rare mistargeting of SHM can result in mutations and translocations in oncogenes, and is thought to contribute to the development of B-cell malignancies. Despite years of intensive investigation, the mechanism of SHM targeting is still unclear. We review and attempt to reconcile the numerous and sometimes conflicting studies on the targeting of SHM to immunoglobulin loci, and highlight areas that hold promise for further investigation.

Biased dA/dT somatic hypermutation as regulated by the heavy chain intronic iEmu enhancer and 3'Ealpha enhancers in human lymphoblastoid B cells

Somatic hypermutation (SHM) in immunoglobulin gene (Ig) variable (V) regions is critical for the maturation of the antibody response. It is dependent on the expression of activation-induced cytidine deaminase (AID) and translesion DNA polymerases in germinal center B cells as well as Ig V transcription, as regulated by the Ig heavy chain (H) intronic enhancer (iEmu) and the 3' enhancer (3'Ealpha) region. We analyzed the role of these cis elements in SHM by stably transfecting Ramos human lymphoblastoid B cells with a rearranged human IgH chain VD (diversity) J (joining) DNA construct containing a V(H) promoter at the 5' end and C(H)1 and C(H)2 exons of Cgamma1 at the 3' end. In this construct, mutations preferentially targeted dA/dT basepairs in the RGYW/WRCY hotspot. Most of the dA/dT mutations and accompanying dC/dG mutations were transitions. Deletion of iEmu resulted in decreased SHM which could be partially restored by insertion of the IgH hs1,2 enhancer. Other two 3'Ealpha enhancers, hs3-hs4, did not significantly increase the mutation frequency, but further strengthened the dA/dT bias. The frequency and spectrum of the mutations were independent of the genomic integration of the transgene or V gene transcription level. Thus, we have established a novel in vitro system to analyze SHM and identify the role of multiple cis-regulatory elements in regulating dA/dT biased SHM. This model system will be useful to further address the role of other cis-regulating elements and recruited trans-acting factors in expressing the modalities of SHM.

DNA Polymerases for Translesion DNA Synthesis: Enzyme Purification and Mouse Models for Studying Their Function

This chapter discusses experimental methods and protocols for the purification and preliminary characterization of DNA polymerases that are specialized for the replicative bypass (translesion DNA synthesis) of base or other types of DNA damage that typically arrest high-fidelity DNA synthesis, with particular emphasis on DNA polymerase kappa (Polkappa from mouse cells). It also describes some of the methods employed in the evaluation of mouse strains defective in genes that encode these enzymes.

Generation of monoclonal antibodies against chromosomal antigens that have a high sequence similarity between human and mouse

We raised monoclonal antibodies by immunizing mice with total chromosome proteins extracted from isolated human metaphase chromosomes. The indirect immunofluorescence screening of hybridoma cell lines provided 15 monoclonal antibodies against the chromosomal antigens. The antigen proteins of the mAbs were identified by immunoblotting as core histones or by immunoprecipitation followed by a peptide mass fingerprinting method as nuclear mitotic apparatus protein, heterogeneous nuclear ribonucleoprotein A2/B1, ribosomal protein S4, linker histone and beta-actin. During mitosis, localizations of these proteins on chromosomes were clearly observed using the obtained antibodies. These results indicate that the current strategy is effective for obtaining monoclonal antibodies useful for immunoblotting and/or immunofluorescent staining of human proteins, using the antigens with high homology to mouse proteins.

Elucidation of IgH intronic enhancer functions via germ-line deletion

Studies of chimeric mice demonstrated that the core Ig heavy chain (IgH) intronic enhancer (iEmu) functions in V(D)J and class switch recombination at the IgH locus. To more fully evaluate the role of this element in these and other processes, we generated mice homozygous for germ-line mutations in which the core sequences of iEmu (cEmu) were either deleted (cEmu(Delta/Delta) mice) or replaced with a pgk-Neo(R) cassette (cEmu(N/N) mice). The cEmu(Delta/Delta) mice had reduced B cell numbers, in association with impaired D to J(H) and V(H) to DJ(H) rearrangement, whereas cEmu(N/N) mice had a complete block in IgH V(D)J(H) recombination, confirming that additional cis elements cooperate with iEmu to enforce D to J(H) recombination. In addition, developing cEmu(Delta/Delta) and cEmu(N/N) B lineage cells had correspondingly decreased levels of germ-line transcripts from the J(H) region of the IgH locus (mu0 and Imu transcripts); although both had normal levels of germ-line V(H) transcripts, suggesting that cEmu may influence IgH locus V(D)J recombination by influencing accessibility of J(H) proximal regions of the locus. Consistent with chimera studies, peripheral cEmu(Delta/Delta) B cells had normal surface Ig and relatively normal class switch recombination. However, cEmu(Delta/Delta) B cells also had relatively normal somatic hypermutation of their IgH variable region genes, showing unexpectedly that the cEmu is not required for this process. The availability of mice with the iEmu mutation in their germ line will facilitate future studies to elucidate the roles of iEmu in V(H)(D)J(H) recombination in the context of IgH chromatin structure and germ-line transcription.

Hypothesis: biological role for J-C intronic matrix attachment regions in the molecular mechanism of antigen-driven somatic hypermutation

A major function of J-C intronic matrix attachment regions (MAR) during immune diversification via somatic hypermutation (SHM) at immunoglobulin loci may be to manipulate the topology of DNA within the upstream target domain. The suggestion that SHM induction requires MAR-induced torsional strain, in conjunction with DNA remodelling at the J-C intron, completes the definition of a cogent paradigm within which all extant molecular data on the issue may be interpreted. Moreover, the suggestion that a mutagenic mechanism relieves MAR-generated superhelicity could provide an indication as to the evolutionary basis of SHM.

A negative regulatory element in the rabbit 3′IgH chromosomal region

Mouse and human IgH loci contain several 3'IgH enhancers. In rabbit, a single hs1,2 enhancer is located 3' of the distal germ line Calpha gene, Calpha13. We searched for additional regulatory elements in this region by using a luciferase reporter assay and nucleotide sequence analysis. Within 8 kb 3' of Calpha13, we identified a 1-kb fragment that negatively regulated the hs1,2 enhancement of the Ialpha promoter. This negative regulatory element, Calpha-NRE, contains a conserved 300-bp region that is associated with 8 of the 13 germ line Calpha genes. This conserved region contains an E box that, by electrophoretic mobility shift assay, binds an E47-like protein. At the 5' end, Calpha-NRE also includes a 270-bp region with 20-bp repeats nearly identical to those 3' of mouse and human Calpha genes, and these repeats bind unidentified nuclear protein(s). Calpha-NRE appears to be a novel regulatory element that may contribute to the regulation of IgH gene expression.

The HoxC4 homeodomain protein mediates activation of the immunoglobulin heavy chain 3' hs1,2 enhancer in human B cells. Relevance to class switch DNA recombination

The immunoglobulin heavy chain (IgH) 3' regulatory region modulates IgH locus transcription, upon induction by specific trans-acting factors, and plays a significant role in class switch DNA recombination (CSR) and, perhaps, somatic hypermutation (SHM). CSR and SHM are central to the maturation of the antibody response. In contrast to the single 5'-hs3a-hs1,2-hs3b-hs4-3 ' mouse IgH 3 ' regulatory region, the human IgH 3 ' regulatory region exists as a 5'-hs3-hs1,2-hs4-3' cluster duplicated 3 ' of Calpha1 and Calpha2. We show here that the human hs1,2 element is the strongest enhancer of transcription, as directed by a V(H)1 or the ECS-Igamma3 promoter, thereby suggesting a dominant role for hs1,2 over hs3 and hs4 in the overall activity of the 3 ' regulatory region. Within hs1,2, we identified three regions (1, 2, and 3) that are all necessary, but individually not sufficient, for enhancement of transcription. In region 2, a HoxC4 site and a HoxC4/embedded octamer (HoxC4/Oct) site are conserved across human, mouse, rat, and rabbit. These two sites recruit HoxC4 and Oct-1/Oct-2, which act synergistically with the Oca-B coactivator to effect the full hs1,2-enhancing activity. HoxC4, Oct-1/Oct-2, and Oca-B recruitment is negligible in pro-B cells, moderate in pre-B cells, and maximal in germinal center B cells and plasma cells, where HoxC4, Oct-2, and Oca-B expression correlates with hs1,2 activation and ongoing CSR. The hs1,2mediated enhancement of V(H) and C(H) promoter-driven transcription as induced by HoxC4 and Oct-1/Oct-2 suggests an important role of these homeodomain proteins in the overall regulation of the IgH locus expression.

NF-κB and Oct-2 Synergize to Activate the Human 3′ Igh hs4 Enhancer in B Cells

In B cells, the Igh gene locus contains several DNase I-hypersensitive (hs) sites with enhancer activity. These include the 3' Igh enhancers, which are located downstream of the Calpha gene(s) in both mouse and human. In vivo experiments have implicated murine 3' enhancers, hs3B and/or hs4, in class switching and somatic hypermutation. We previously reported that murine hs4 was regulated by NF-kappaB, octamer binding proteins, and Pax5 (B cell-specific activator protein). In this study we report that human hs4 is regulated differently. EMSAs and Western analysis of normal B cells before and after stimulation with anti-IgM plus anti-CD40 showed the same complex binding pattern formed by NF-kappaB, Oct-1, and Oct-2 (but not by Pax5). A similar EMSA pattern was detected in mature human B cell lines (BL-2, Ramos, and HS-Sultan) and in diffuse large B cell lymphoma cell lines, although yin yang 1 protein (YY1) binding was also observed. We have confirmed the in vivo association of these transcription factors with hs4 in B cells by chromatin immunoprecipitation assays. The diffuse large B cell lymphoma cell lines had a distinctive slow-migrating complex containing YY1 associated with Rel-B. We have confirmed by endogenous coimmunoprecipitation an association of YY1 with Rel-B, but not with other NF-kappaB family members. Transient transfection assays showed robust hs4 enhancer activity in the mature B cell lines, which was dependent on synergistic interactions between NF-kappaB and octamer binding proteins. In addition, human hs4 enhancer activity required Oct-2 and correlated with expression of Oct coactivator from B cells (OCA-B).

Segmental flexibility and avidity of IgM in the interaction of polyvalent antigens

We prepared IgG and IgM with identical combining sites to a hapten, (4-hydroxy-3-nitrophenyl)acetic acid (NP), and used surface plasmon resonance to evaluate the association constants (Ka) in interactions of these antibodies (Abs) with antigens (Ags) which differed in the size of carriers and NP valence as well as in the stoichiometry of Ag to Ab in the immune complexes. It was found that IgM was unable to form an Ag1Ab1 complex with the highly haptenated Ag, NP(18.6)-bovine serum albumin (BSA), such that one NP(18.6)-BSA molecule was held by multiple contacts with Fab arms from five subunits, although IgM was capable of forming an Ag4Ab1 complex in which each subunit was bound to one NP(18.6)-BSA molecule. IgM was superior to IgG in interactions with large Ags of low hapten density. The Ka values of IgM to these Ags were estimated to be approximately 1x10(9) M(-1), about 20-fold higher than those of IgG. Reduction of inter-subunit and inter-chain disulfide bonds resulted in a decrease in Ka values to large Ags but no change in those to small Ags.

The immunoglobulin heavy-chain locus hs3b and hs4 3' enhancers are dispensable for VDJ assembly and somatic hypermutation

The more distal enhancers of the immunoglobulin heavy-chain 3' regulatory region, hs3b and hs4, were recently demonstrated as master control elements of germline transcription and class switch recombination to most immunoglobulin constant genes. In addition, they were shown to enhance the accumulation of somatic mutations on linked transgenes. Since somatic hypermutation and class switch recombination are tightly linked processes, their common dependency on the endogenous locus 3' enhancers could be an attractive hypothesis. VDJ structure and somatic hypermutation were analyzed in B cells from mice carrying either a heterozygous or a homozygous deletion of these enhancers. We find that hs3b and hs4 are dispensable both for VDJ assembly and for the occurrence of mutations at a physiologic frequency in the endogenous locus. In addition, we show that cells functionally expressing the immunoglobulin M (IgM) class B-cell receptor encoded by an hs3b/hs4-deficient locus were fully able to enter germinal centers, undergo affinity maturation, and yield specific antibody responses in homozygous mutant mice, where IgG1 antibodies compensated for the defect in other IgG isotypes. By contrast, analysis of Peyer patches from heterozygous animals showed that peanut agglutinin (PNAhigh) B cells functionally expressing the hs3b/hs4-deficient allele were dramatically outclassed by B cells expressing the wild-type locus and normally switching to IgA. This study thus also highlights the role of germinal centers in the competition between B cells for affinity maturation and suggests that membrane IgA may promote recruitment in an activated B-cell compartment, or proliferation of activated B cells, more efficiently than IgM in Peyer patches.

The absence of DNA polymerase kappa does not affect somatic hypermutation of the mouse immunoglobulin heavy chain gene

During the immune response to T cell-dependent antigen, somatic hypermutation (SHM) is introduced into immunoglobulin (Ig) genes. The variable region is the target for SHM and it is here that DNA lesions are introduced and mutations are generated. It has been suggested that error-prone DNA polymerase(s) may play an important role in this mutagenesis phase. Recently, DNA polymerase kappa (Polkappa), which belongs to the Y-family of DNA polymerases, was identified. Since a hot spot of SHMs (RGYW motif) is also a hot spot of mutations by human Polkappa, this enzyme was suggested to be an SHM instigator. In order to address the question whether Polkappa is involved in SHM, we immunized Polkappa-deficient mice and analyzed the SHM of the Ig heavy chain gene. We found that the SHM frequency and spectrum were indistinguishable between the Polkappa knockout mice and control mice. These results suggested that Polkappa is not essential for this process.

The origin of a developmentally regulated Igh replicon is located near the border of regulatory domains for Igh replication and expression

The 3' Ig heavy chain locus (Igh) regulatory region is the most downstream known element of the murine Igh gene cluster. We report here that the nearest non-Igh genes-Crip, Crp2, and Mta1-are located approximately 70 kb further downstream and are beyond the end of the domain of Igh transcriptional regulation. We have localized an origin of replication in MEL cells to a 3-kb segment located between the 3' Igh regulatory region and Crip. Sequences downstream of this origin are replicated by forks that move in both directions. Sequences upstream of this origin (Igh-C, -D, and -J) are replicated in a single direction through a 500-kb segment in which no active bidirectional origins can be detected. We propose that this origin may lie at or near the end of the Igh regulation domain.

Mechanism and control of class-switch recombination

Class-switch recombination (CSR) occurs by an unusual and intriguing mechanism that has not been clearly elucidated as yet. Currently, we know that this mechanism involves recombination between large and highly repetitive switch (S) regions, is targeted by S-region transcription and requires the activity of the newly discovered activation-induced deaminase (AID). In this review, we discuss the potential role of these factors in CSR, discuss potential relationships between CSR and somatic hypermutation, and speculate how CSR and related mechanisms might contribute to genomic instability.