Heavy-chain class switch does not terminate somatic mutation

Molecular footprints of a germinal center derivation of human IgM+(IgD+)CD27+ B cells and the dynamics of memory B cell generation

The origin of IgM(+)CD27(+) B lymphocytes with mutated IgV genes, which account for approximately 20% of human peripheral blood (PB) B cells, is controversially discussed. A generation in a primary diversification pathway, in T cell-independent immune responses, or in T cell-dependent germinal center (GC) reactions has been proposed. We show here that IgM(+)IgD(+)CD27(+) and IgM(+)IgD(-/low)CD27(+) B cell subsets carry, like class-switched memory B cells, mutations in the Bcl6 gene as a genetic trait of a GC experience. Moreover, the identification of PB IgM(+)IgD(+)CD27(+) B cells clonally related to GC-derived IgG(+) memory B cells with shared and distinct IgV gene mutations demonstrates the GC origin also of the former subset. These findings provide genetic evidence for a GC derivation of somatically mutated IgM(+) B cells and indicate that adult humans harbor a large population of IgM(+)IgD(+) post-GC memory B cells. Furthermore, the analysis revealed that a highly diverse and often very large population of memory B cells is generated from a given GC B cell clone, and that (preferentially IgM) memory B cells are generated already early in the GC reaction. This provides novel insights into the dynamics of GC reactions and the generation of a memory B cell repertoire.

Germinal centres: role in B-cell physiology and malignancy

Over the past several years, studies on normal and malignant B cells have provided new insights into the unique physiology of the germinal centre (GC). In particular, advances in technology have allowed a more precise dissection of the phenotypes of GC B cells and the specific transcriptional programmes that are responsible for this phenotype. Furthermore, substantial progress has been made in the understanding of the mechanism controlling the exit of B cells from the GC and the decision to become a memory B cell or plasma cell. This Review focuses on these recent advances and discusses their implications for the pathogenesis of B-cell lymphomas.

Re-entry of tumour B cells into the cycle of somatic mutation and isotype switching in follicular lymphoma

Sequencing of the immunoglobulin (Ig) gene transcripts of the tumour B cells in lymph node (LN) and bone marrow (BM) from a follicular lymphoma (FL) patient associated with multiple myeloma identified two dominant clones. One of the clones, present in both LN and BM, had somatic mutations and extensive clonal diversity. Among the diversified clones, two dominant populations of identical sequences (group I and II) were present. Group II was a descendant population of group I and had nine more somatic mutations. Group I contained micro-, delta-, gamma- and alpha-expressing clones. Group II clones contained mainly micro- and delta-expressing clones. These findings showed that somatic mutation and isotype switching occurred repeatedly in this patient.

Natural antibodies with the T15 idiotype may act in atherosclerosis, apoptotic clearance, and protective immunity

The immune response to oxidized LDL (OxLDL) may play an important role in atherogenesis. Working with apoE-deficient mice, we isolated a panel of OxLDL-specific B-cell lines that secrete IgM Abs that specifically bind to oxidized phospholipids such as 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC). These Abs block uptake of OxLDL by macrophages, recognize similar oxidation-specific epitopes on apoptotic cells, and are deposited in atherosclerotic lesions. The Abs were found to be structurally and functionally identical to classic "natural" T15 anti-PC Abs that are of B-1 cell origin and are reported to provide optimal protection from virulent pneumococcal infection. These findings suggest that there has been natural selection for B-1 cells secreting oxidation-specific/T15 antibodies, both for their role in natural immune defense and for housekeeping roles against oxidation-dependent neodeterminants in health and disease.

Natural antibodies with the T15 idiotype may act in atherosclerosis, apoptotic clearance, and protective immunity

The immune response to oxidized LDL (OxLDL) may play an important role in atherogenesis. Working with apoE-deficient mice, we isolated a panel of OxLDL-specific B-cell lines that secrete IgM Abs that specifically bind to oxidized phospholipids such as 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC). These Abs block uptake of OxLDL by macrophages, recognize similar oxidation-specific epitopes on apoptotic cells, and are deposited in atherosclerotic lesions. The Abs were found to be structurally and functionally identical to classic "natural" T15 anti-PC Abs that are of B-1 cell origin and are reported to provide optimal protection from virulent pneumococcal infection. These findings suggest that there has been natural selection for B-1 cells secreting oxidation-specific/T15 antibodies, both for their role in natural immune defense and for housekeeping roles against oxidation-dependent neodeterminants in health and disease.

Disparity in the kinetics of onset of hypermutation in immunoglobulin heavy and light chains

The present paper describes a comparative analysis of light chains associated with primary and secondary IgM, as well as with secondary IgG antibodies to fluorescein, undertaken in order to explore the relationship between light chain somatic hypermutation and the isotype switch. The data reveal a disparity in the frequency of somatic hypermutation of secondary IgM heavy versus light chains. Among 20 secondary IgM light chains, a mutation frequency of 1/777 nucleotides was defined. In contrast, our previous analysis of the heavy chains of these molecules had identified a mutation frequency of 1/129. Among 17 IgG-derived light chains, obtained from animals killed at the same time point as those from which the secondary IgM antibodies were obtained, we measured a mutation frequency of 1/77. Finally, analysis of 20 light chains derived from primary IgM antibodies revealed a mutation frequency of only 1/1192 nucleotides. These data demonstrate that, prior to the class switch, light chain mutation occurs at a frequency considerably lower than that measured for the associated heavy chain gene. Six additional apparent mutations in the secondary IgM antibody 95B3 were all shared with a set of IgG antifluorescein antibodies belonging to the Vkappa 34 family. It is suggested that these light chains represent the products of a previously uncharacterized germ line gene.

Regulation of B lymphocyte differentiation

LEARNING OBJECTIVES

Type I hypersensitivity reactions uniquely involve the IgE class of immunoglobulins (Ig). IgE differs from other classes of Ig in that the majority of the antibodies are bound to high affinity IgE Fc(epsilon)Rs that are expressed on a variety of cell types. Some of these cell types, most notably, mast cells and basophils, are triggered to undergo rapid activation, degranulation, and release of bioactive mediators following binding of antigen to Fc(epsilon)RI-bound IgE. Because of the central role that IgE antibodies and these mediators play in the tissue injury typical of type I hypersensitivity, this article will review the various stages of B lymphocyte development, activation, and differentiation and comment, where appropriate on potential sites of deregulation in allergic disease.

DATA SOURCES

A literature search of the stages of B lymphocyte differentiation with emphasis on events that concern IgE expression was performed.

RESULTS

B lymphocyte differentiation into IgE expressing cells is dependent upon three types of signals. The first signal is delivered through the B cell antigen receptor and is pivotal in determining the antigenic specificity of the response. The second signal is provided primarily by cytokines derived from T helper 2 (TH2) cells, ie, interleukin (IL)-4 and IL-13. These cytokines are under tight regulation and their role appears to be the stimulation of transcription through the Ig constant region genes. Finally, the third signal is provided via the interaction between the constitutively expressed CD40 molecule on B lymphocytes and CD154 (CD40 ligand), a molecule expressed on T lymphocytes following activation. Elevated levels of IgE in atopic individuals may result from the preferential activation of TH2 cells.

CONCLUSIONS

A greater understanding of the regulation of IgE expression may be central to the development of more effective immunotherapy strategies designed to attenuate IgE synthesis.

Induction of Ig Somatic Hypermutation and Class Switching in a Human Monoclonal IgM+ IgD+ B Cell Line In Vitro: Definition of the Requirements and Modalities of Hypermutation

Partly because of the lack of a suitable in vitro model, the trigger(s) and the mechanism(s) of somatic hypermutation in Ig genes are largely unknown. We have analyzed the hypermutation potential of human CL-01 lymphocytes, our monoclonal model of germinal center B cell differentiation. These cells are surface IgM+ IgD+ and, in the absence of T cells, switch to IgG, IgA, and IgE in response to CD40:CD40 ligand engagement and exposure to appropriate cytokines. We show here that CL-01 cells can be induced to effectively mutate the expressed VHDJH-Cμ, VHDJH-Cδ, VHDJH-Cγ, VHDJH-Cα, VHDJH-Cε, and VλJλ-Cλ transcripts before and after Ig class switching in a stepwise fashion. In these cells, induction of somatic mutations required cross-linking of the surface receptor for Ag and T cell contact through CD40:CD40 ligand and CD80:CD28 coengagement. The induced mutations showed intrinsic features of Ig V(D)J hypermutation in that they comprised 110 base substitutions (97 in the heavy chain and 13 in the λ-chain) and only 2 deletions and targeted V(D)J, virtually sparing CH and Cλ. These mutations were more abundant in secondary VHDJH-Cγ than primary VHDJH-Cμ transcripts and in V(D)J-C than VλJλ-Cλ transcripts. These mutations were also associated with coding DNA strand polarity and showed an overall rate of 2.42 × 10−4 base changes/cell division in VHDJH-CH transcripts. Transitions were favored over transversions, and G nucleotides were preferentially targeted, mainly in the context of AG dinucleotides. Thus, in CL-01 cells, Ig somatic hypermutation is readily inducible by stimuli different from those required for class switching and displays discrete base substitution modalities.

Induction of Ig somatic hypermutation and class switching in a human monoclonal IgM+ IgD+ B cell line in vitro: definition of the requirements and modalities of hypermutation

Partly because of the lack of a suitable in vitro model, the trigger(s) and the mechanism(s) of somatic hypermutation in Ig genes are largely unknown. We have analyzed the hypermutation potential of human CL-01 lymphocytes, our monoclonal model of germinal center B cell differentiation. These cells are surface IgM+ IgD+ and, in the absence of T cells, switch to IgG, IgA, and IgE in response to CD40:CD40 ligand engagement and exposure to appropriate cytokines. We show here that CL-01 cells can be induced to effectively mutate the expressed VHDJH-C mu, VHDJH-C delta, VHDJH-C gamma, VHDJH-C alpha, VHDJH-C epsilon, and V lambda J lambda-C lambda transcripts before and after Ig class switching in a stepwise fashion. In these cells, induction of somatic mutations required cross-linking of the surface receptor for Ag and T cell contact through CD40:CD40 ligand and CD80: CD28 coengagement. The induced mutations showed intrinsic features of Ig V(D)J hypermutation in that they comprised 110 base substitutions (97 in the heavy chain and 13 in the lambda-chain) and only 2 deletions and targeted V(D)J, virtually sparing CH and C lambda. These mutations were more abundant in secondary VHDJH-C gamma than primary VHDJH-C mu transcripts and in V(D)J-C than V lambda J lambda-C lambda transcripts. These mutations were also associated with coding DNA strand polarity and showed an overall rate of 2.42 x 10(-4) base changes/cell division in VHDJH-CH transcripts. Transitions were favored over transversions, and G nucleotides were preferentially targeted, mainly in the context of AG dinucleotides. Thus, in CL-01 cells, Ig somatic hypermutation is readily inducible by stimuli different from those required for class switching and displays discrete base substitution modalities.

VH Gene Analysis of Primary Cutaneous B-Cell Lymphomas: Evidence for Ongoing Somatic Hypermutation and Isotype Switching

Primary cutaneous B-cell lymphomas are B-cell non-Hodgkin’s lymphomas that arise in the skin. The major subtypes discerned are follicle center cell lymphomas, immunocytomas (marginal zone B-cell lymphomas), and large B-cell lymphomas of the leg. In this study, we analyzed the variable heavy chain (VH) genes of 7 of these lymphomas, ie, 4 follicle center cell lymphomas (diffuse large-cell lymphomas) and 3 immunocytomas. We show that all these lymphomas carry heavily mutated VH genes, with no obvious bias in VH gene usage. The low ratios of replacement versus silent mutations observed in the framework regions of 5 of the 7 lymphomas suggest that the structure of the B-cell antigen receptor was preserved, as in normal B cells that are selected for antibody expression. Moreover, evidence for ongoing mutation was obtained in 3 immunocytomas and in one lymphoma of large-cell type. In addition, in 1 immunocytoma, both IgG- and IgA-expressing clones were found, indicative of isotype switching. Our data provide insight into the biology of primary cutaneous B-cell lymphomas and may be of significance for their classification.

VH Gene Analysis of Primary Cutaneous B-Cell Lymphomas: Evidence for Ongoing Somatic Hypermutation and Isotype Switching

Primary cutaneous B-cell lymphomas are B-cell non-Hodgkin’s lymphomas that arise in the skin. The major subtypes discerned are follicle center cell lymphomas, immunocytomas (marginal zone B-cell lymphomas), and large B-cell lymphomas of the leg. In this study, we analyzed the variable heavy chain (VH) genes of 7 of these lymphomas, ie, 4 follicle center cell lymphomas (diffuse large-cell lymphomas) and 3 immunocytomas. We show that all these lymphomas carry heavily mutated VH genes, with no obvious bias in VH gene usage. The low ratios of replacement versus silent mutations observed in the framework regions of 5 of the 7 lymphomas suggest that the structure of the B-cell antigen receptor was preserved, as in normal B cells that are selected for antibody expression. Moreover, evidence for ongoing mutation was obtained in 3 immunocytomas and in one lymphoma of large-cell type. In addition, in 1 immunocytoma, both IgG- and IgA-expressing clones were found, indicative of isotype switching. Our data provide insight into the biology of primary cutaneous B-cell lymphomas and may be of significance for their classification.

Somatic hypermutation of antibody genes: a hot spot warms up

In the course of an immune response, antibodies undergo affinity maturation in order to increase their efficiency in neutralizing foreign invaders. Affinity maturation occurs by the introduction of multiple point mutations in the variable region gene that encodes the antigen binding site. This somatic hypermutation is restricted to immunoglobulin genes and occurs at very high rates. The precise molecular basis of this process remains obscure. However, recent studies using a variety of in vivo and in vitro systems have revealed important regulatory regions, base motifs that are preferred targets of mutation and evidence that transcription may play an active role in hypermutation.

The roles of gamma 1 heavy chain membrane expression and cytoplasmic tail in IgG1 responses

In antibody responses, B cells switch from the expression of immunoglobulin (Ig) mu and delta heavy (H) chains to that of other Ig classes (alpha, gamma, or epsilon), each with a distinct effector function. Membrane-bound forms of alpha, gamma, and epsilon, but not mu and delta, have highly conserved cytoplasmic tails. Mutant mice unable to express membrane gamma1 H chains or producing tailless gamma1 H chains failed to generate efficient IgG1 responses and IgG1 memory. H chain membrane expression after class switching is thus required for these functions, and class switching equips the B cell antigen receptor with a regulatory cytoplasmic tail that naïve B cells lack.

Germinal center development

Using a set of surface markers including IgD and CD38, human tonsillar B cells were classified into discrete subpopulations. Molecular and functional analysis allowed us to identify: i) two sets of naive B cells (Bm1 and Bm2); ii) germinal center founder cells (Bm2'); iii) an obscure population of germinal center B cells, displaying a high load of somatic mutations in IgV genes, C mu to C delta switch and preferential Ig lambda light chain usage: these cells may represent the precursors of normal and malignant IgD-secreting plasma cells; iv) the centroblasts (Bm3) in which somatic mutation machinery is activated; v) the centrocytes (Bm4) in which isotype switch occurs; vi) the memory B cells. The characterization of these subpopulations showed that: i) programmed cell death is set before somatic mutations, possibly providing an efficient way for affinity maturation; ii) only high affinity centrocytes are allowed to switch isotype; iii) CD40-ligation inhibits plasmacytic differentiation of mature B lymphocytes; iv) memory B cells preferentially differentiate into plasma cells; v) IgD isotype switch occurs in normal B cells; vi) receptor editing may be induced by somatic mutations in germinal centers. We also characterized two types of antigen-presenting cells in germinal centers: follicular dendritic cells that select high affinity B cells, and a new subset of germinal center dendritic cells that activate germinal center T cells.

Evidence for a Large Compartment of IgM-Expressing Memory B Cells in Humans

The recent finding of somatically mutated μ heavy chain transcripts in human peripheral blood (PB) B lymphocytes suggests that T-dependent B-cell memory might not be restricted to class-switched cells. We provide here evidence that IgM-only PB B cells are likely to be the IgM-expressing counterpart of classical (IgM−IgD−) memory B cells in humans. As shown by molecular single cell analysis, most IgM-only cells carry mutated V region genes, like class-switched cells. Although both subsets represent populations of nonactivated, resting cells, they express higher levels of Ig mRNA than naive (IgM+IgD+) B cells. IgM-only and class-switched cells are CD38−CD77−, and mostly CD23−, thus neither resembling germinal center nor naive B cells. Because many IgM-expressing B cells located in secondary lymphoid tissues resemble IgM-only PB B cells in terms of cell phenotype, we propose that the human lymphoid system contains a large compartment of IgM-expressing memory cells. Moreover, these cells seem to represent the nonmalignant counterparts of IgM-expressing tumor cells in sporadic Burkitt's lymphoma, MALT lymphoma, monocytoid B-cell lymphoma, and diffuse large-cell lymphoma that were found to harbor somatically mutated V genes.

The transcriptional promoter regulates hypermutation of the antibody heavy chain locus

A somatic process introduces mutations into antibody variable (V) region genes at a high rate in many vertebrates, and is a major source of antibody diversity. The mechanism of this hypermutation process remains enigmatic, although retrospective studies and transgenic experiments have recently suggested a role for transcriptional regulatory elements. Here, we demonstrate that mouse heavy (H) chain loci in which the natural VH promoter has been replaced by a heterologous promoter undergo hypermutation. However, while the distribution of mutation in such loci appears normal, the frequency of mutation does not. Conversely, moving the VH promoter 750 bp upstream of its normal location results in a commensurate change in the site specificity of hypermutation in H chain loci, and the foreign DNA inserted into the VH leader intron to produce this promoter displacement is hypermutated in a manner indistinguishable from natural Ig DNA. These data establish a direct mechanistic link between the IgH transcription and hypermutation processes.

Differential V region mutation of two transfected Ig genes and their interaction in cultured B cell lines

We have established B cell culture systems in which transfected and stably integrated Ig constructs spontaneously undergo high rates of variable (V) region mutation. Mutation rates were determined using reversion analysis of an Ig V region nonsense codon (Vn). A construct (Vn/gamma2a) in which a Vn was associated with the gamma2a constant region and its intervening and immediate flanking sequences mutated at a high rate of 2.2 x 10(-4)/bp/generation in the NSO myeloma cell line. This same Vn, when associated with the mu constant region (Vn/mu), mutated at a 1000-fold lower rate in NSO. The Vn/gamma2a construct also mutated at high rates in the 18.81 pre-B and the S107 myeloma cell lines and at a low rate in the J558 myeloma cell line. In NSO, the presence of the gamma2a construct raised the mutation rate of the mu construct and the mu decreased the mutation rate of gamma2a. These results suggest that there is both positive and negative regulation of V region mutation and that different cell lines express different combinations and/or amounts of trans-acting factors that are involved in the mutational process.

Gene family use and somatic mutation in primary and secondary fluorescein-specific IgM antibody responses

A comparative analysis of the DNA sequences of primary and secondary IgM, fluorescein-specific antibodies was performed. These antibodies were secreted by hybridomas generated following fusion of immunized BALB/c mouse lymphocytes and SP2/0 myeloma cells. Our results show that primary and secondary fluorescein-specific IgM antibodies use a variety of segments from the variable region of the immunoglobulin heavy chain locus (VH), with members of the J558 and 7183 VH gene families predominating in both populations. D regions from the DF116 and DSP2 families were used exclusively in our primary antibody sample and predominated in the secondary response. In the primary antibodies, 15 out of 18 definable D regions were transcribed in reading frame one, but in the secondary antibodies the three reading frames were used stochastically. Secondary IgM antibodies showed a higher frequency of somatic mutation than their primary counterparts, but we could detect no evidence of selection for mutations in the complementarity determining regions as compared with the framework regions. It appears that fusion of secondary cells, 3-6 days after immunization, is able to 'capture' the IgM-producing population of B cells at a stage in their development following mutation but prior to antigenic selection.

Within germinal centers, isotype switching of immunoglobulin genes occurs after the onset of somatic mutation

Human tonsillar B cells were separated into naive IgD+CD38-CD23- (Bm1) and IgD+CD38-CD23 (Bm2), germinal center IgD-CD38+CD23- centroblasts (Bm3) and IgD-CD38+CD77- centrocytes (Bm4) and memory IgD-CD38- (Bm5) subsets. Previous IgVH sequence analysis concluded that the triggering of somatic mutations occurs during the transition from Bm2 subset into the Bm3 subset. To determine the initiation of isotype switching, sterile transcript expression was analyzed by amplification, cloning, and sequencing. A selective sterile I gamma, I alpha, and I epsilon expression was observed at centrocyte (Bm4) stage, suggesting that isotype switch is triggered within germinal centers, after somatic mutation is initiated with centroblasts (Bm3). Finally, the high level of 5'S gamma-S mu 3' DNA switching circles observed in germinal center B cells indicates that within human tonsils, germinal center is a major location for isotype switching.

Facultative role of germinal centers and T cells in the somatic diversification of IgVH genes

The development of memory B cells takes place in germinal centers (GC) of lymphoid follicles where antigen-driven lymphocytes undergo somatic hypermutation and affinity selection, presumably under the influence of helper T cells. However, the mechanisms that drive this complex response are not well understood. We explored the relationship between GC formation and the onset of hypermutation in response to the hapten phosphorylcholine (PC) coupled to antigenic proteins in mice bearing different frequencies of CD4+ T cells. PC-reactive GC were identified by staining frozen splenic sections with peanut agglutinin (PNA) and with monoclonal Abs against AB1-2, a dominant idiotope of T15+ anti-PC antibody. The nucleotide sequences of rearranged T15 VH1 genes were determined from polymerase chain reaction amplifications of genomic DNA from microdissected GC B cells. T15+ GC became fully developed by day 6-7 after primary immunization of euthymic mice with either PC-keyhole limpet hemocyanin (KLH) or PC-chicken gamma globulin (CGG). Yet the VH1 gene segments recovered from the primary GC as late as day 10-14 had low numbers of mutations, in contrast to responses to the haptens nitrophenyl or oxazolone that sustain high levels of hypermutation after GC formation. PC-reactive B cells proliferate in histologically typical GC for considerable periods with no or little somatic hypermutation; the signals for GC formation are independent of those for the activation of hypermutation. We then examined GC 7 d after secondary immunization with PC-KLH in euthymic mice, in nu/nu mice reconstituted with limited numbers of normal CD4+ cells before priming (CD4(+)-nu/nu) and in nu/nu mice. All of these animals develop T15+ GC after antigen priming, however, the patterns of V gene mutations in the secondary GC reflected the levels of CD4+ cells present during the primary response. VDJ sequences from secondary GC of euthymic mice were heavily mutated, but most of these mutations were shared among all related (identical VDJ joints) sequences suggesting the proliferation of mutated, memory B cells, with little de novo somatic hypermutation. In contrast, the patterns of V gene diversity in secondary GC from CD4(+)-nu/nu mice suggested that there was ongoing mutation and clonal diversification during the first week after rechallenge. The secondary GC from T cell-deficient, nu/nu mice showed little evidence for mutational and/or recombinational diversity of T15+ B cells. We conclude that the participation of CD4+ helper cells is required for full activation of the mutator in GC and takes place in a dose-dependent fashion.

Human splenic IgM immunoglobulin transcripts are mutated at high frequency

Human spleen immunoglobulin gene rearrangements that used the VH6 gene and were expressed with IgM were characterized for their frequency of somatic hypermutation from PCR amplified spleen cDNA. A high frequency of rearrangements that were somatically mutated was demonstrated by restriction endonuclease analysis and sequencing of cloned rearrangements. The 24 rearrangements cloned from three different spleens had an overall mutation frequency of 3.1% mutations/bp sequenced and ranged from 0.4 to 6.0%. These mutations appeared to have been antigenically selected based on both the high frequency and high amino acid replacement to silent (R/S) ratios in the complementarity determining regions. Five clones that arose from two different rearrangements showed evidence of intraclonal diversification with both shared and unique mutations. The mutated clones of one spleen donor were lower in frequency and were not concentrated in the CDR, which suggested these mutations had not been antigenically selected. These findings support the dissociation of somatic mutation and isotype switching and the possibility that IgM-expressing B cells may serve as human memory B cells.

Isolation of germinal centerlike events from human spleen RNA. Somatic hypermutation of a clonally related VH6DJH rearrangement expressed with IgM, IgG, and IgA

12 rearranged human VH6 immunoglobulin heavy chain genes arising from the same rearrangement were isolated without preselection from the RNA of a fragment of human spleen. The 12 clones were isolated from a pool of 31 unique VH6 clones arising from 18 unique rearrangements. 2 of the 12 related clones were expressed with IgM, 2 with IgG, and 8 with IgA1. All the clones, including those expressing IgM, showed extensive somatic mutation of germline bases (5.6%), which was consistent with antigen-driven activation of these VH6-expressing clones with recruitment into the immune repertoire. On the basis of significant sharing of somatic mutations between the IgM clones and clones expressing the other isotypes (six mutations shared with IgG clones and eight mutations shared with IgA clones), it was apparent that the IgM-expressing precursor in this diversified family had undergone extensive antigen-driven somatic mutation prior to isotype switching. This family of related clones suggests that a germinal centerlike event had been sampled. The highly mutated IgM clones suggest that there may exist memory B cells capable of further somatic mutation and differential isotype-switching depending on the specific antigenic stimulus.

Hypermutation is observed only in antibody H chain V region transgenes that have recombined with endogenous immunoglobulin H DNA: implications for the location of cis-acting elements required for somatic mutation

Mice with transgenes containing an antibody H chain V region (VHDJH) gene were used in an analysis of the cis-acting elements required for hypermutation of immunoglobulin (Ig) V genes. These transgenes can somatically recombine with endogenous IgH DNA, leading to the formation of functional heavy (H) chains partially encoded by the transgenic VHDJH. The transgenomes in the five different lines of mice analyzed contain as little as 150 bp, and as much as 2.8 kb of natural DNA flanking the 5' side of the VH and either 1.5 or 2.3 kb (including the intronic enhancer and 5' matrix attachment region [MAR]) flanking the 3' side of VH. Hybridomas were constructed from immunized transgenic mice, and transgenes present in these hybridomas that had or had not recombined to form functional H chain loci were sequenced. The data obtained show that: (a) the recombined transgenes contain hypermutated VH genes; and (b) among such transgenes, even those containing only 150 bp of natural VH 5' flanking sequence and several kilobases of 5' plasmid vector sequence display a frequency, distribution, and type of mutation characteristic of conventional IgH loci. The data also indicate that transgenic VHDJH genes that have not recombined with endogenous IgH DNA are not substrates for hypermutation, even if they are flanked by 2.8 kb of natural 5' DNA, and 2.3 kb of natural 3' DNA, including the JH2-JH4 region, a MAR, and the intronic enhancer. Collectively, the data suggest that sequences 5' of the VH promoter are dispensable, a VH promoter and the intronic IgH enhancer region are not sufficient, and a region(s) within or 3' of the IgH constant region locus is requisite, for hypermutation of Ig VH transgenes.

DNA sequences at immunoglobulin switch region recombination sites

The immunoglobulin heavy chain switch from synthesis of IgM to IgG, IgA or IgE is mediated by a DNA recombination event. Recombination occurs within switch regions, 2-10 kb segments of DNA that lie upstream of heavy chain constant region genes. A compilation of DNA sequences at more than 150 recombination sites within heavy chain switch regions is presented. Switch recombination does not appear to occur by homologous recombination. An extensive search for a recognition motif failed to find such a sequence, implying that switch recombination is not a site-specific event. A model for switch recombination that involves illegitimate priming of one switch region on another, followed by error-prone DNA synthesis, is proposed.

High frequency of somatically mutated IgM molecules in the human adult blood B cell repertoire

Nucleotide sequence analysis of cDNA encoded by the single member of the human immunoglobulin VH6 gene family show that blood B cells in adults, but not in neonates, frequently express somatically mutated IgM molecules. The number of mutations in VH6-encoded cDNA from adult blood ranged from 2 to 19 mutations/VH gene (average 10.1/VH gene). The distribution of silent and replacement mutations suggests that at least some of the VH6 genes were derived from B cells that were activated and selected by antigen. We conclude that the blood B cell repertoire in adult humans, in contrast to its much-studied murine splenic counterpart, is a rich source of highly mutated IgM molecules.

Natural occurrence and origin of somatically mutated memory B cells in mice

While most murine peripheral B cells express germline-encoded antibodies of classes M and D (mu+ delta+ cells), small numbers of memory B cells expressing somatically mutated immunoglobulin G antibodies are generated upon T cell-dependent immunization. Analyzing the antibody repertoire of the mu-delta- B cell pool in unimmunized mice, we show that these cells express somatically mutated VH genes and that most of these genes derive from a set of germline VH genes dominantly expressed by mu+delta+ B cells. Thus, class-switched memory B cells are generated in the absence of intentional immunization, presumably in response to environmental antigens. These cells are either recruited from mu+delta+ B cells or selected from newly arising B cells in parallel to the latter, by the same antigens.

Immunoglobulin heavy chain gene expression in peripheral blood B lymphocytes

cDNA libraries for IgM heavy chain variable regions were prepared from unmanipulated peripheral blood lymphocytes of two healthy people. Partial sequencing of 103 clones revealed VH gene family use and complete CDR3 and JH sequences. The libraries differed in the two subjects. In one person's cDNA the VH5 family was overexpressed and the VH3 family underexpressed relative to genomic complexity. In the second person's cDNA, VH3 was most frequently expressed. In both libraries, JH4 was most frequent. VH segments of several clones were closely related to those in fetal repertoires. However, there was also evidence of mutation in many cDNAs. Three clones differed from the single nonpolymorphic VH6 germline gene by 7-13 bases. Clones with several differences from VH5 germline gene VH251 were identified. CDR3 segments were highly diverse. JH portions of several CDR3's differed from germline JH sequences. 44% of the clones had DH genes related to the DLR and DXP families, most with differences from germline sequences. In 11 DLR2-related sequences, several base substitutions could not be accounted for by polymorphism. Thus, circulating IgM-producing B cell populations include selected clones, some of which are encoded by variable region gene segments that have mutated from the germline form.

B-cell proliferation initiated by Ia cross-linking and sustained by interleukins leads to class switching but not somatic mutation in vitro

Somatic mutations that are acquired by antibody V genes of antigen-stimulated B cells ultimately provide the clonal diversity from which memory B cells are selected during immune responses to T-cell-dependent antigens. Somatic mutations apparently are not acquired when B cells are stimulated by mitogens nor when they participate in immune responses to T-cell-independent antigens. Since the basis of T-cell-dependent humoral immunity is T-cell recognition of processed antigen in the context of class II major histocompatibility glycoproteins (Ia) on the B-cell surface, we sought to determine whether the ligation of Ia on B cells induces somatic mutation. B cells were stimulated in vitro by a procedure in which their proliferation was dependent upon ligation of surface Ia with antibody. Sequences of hybridoma V genes derived from these B cells revealed no somatic mutations despite prolonged stimulation in vitro and the induction of immunoglobulin secretion and switching to isotypes characteristic of T cell-dependent humoral immunity. We infer that Ia-mediated signalling and isotype switching are not causally related to somatic mutation. The avenue of differentiation that leads to somatic mutation in memory B cells is apparently separable from that leading to proliferation, immunoglobulin secretion and switching.

Characterization of somatically mutated S107 VH11-encoded anti-DNA autoantibodies derived from autoimmune (NZB x NZW)F1 mice

We have studied 19 S107 heavy chain variable region gene (VH11)-encoded monoclonal antibodies from NZBWF1 mice. These studies show that a single VH gene can encode both antibodies to foreign antigens (anti-phosphorylcholine) and to self antigens (anti-double-stranded DNA) in the same animal. All of the anti-DNA antibodies contain many somatic mutations compared with the relevant germline genes. Since the anti-DNA antibodies were extensively somatically mutated and had undergone isotype switching, the response seems to be T cell dependent. While some of the antibodies appear to be the products of an antigen-driven and antigen-selected response, a number of characteristics of the antibodies suggest that forces other than antigen are contributing to the stimulation and selection of this response.