Production of antigen-specific human monoclonal antibodies: comparison of mice carrying IgH/kappa or IgH/kappa/lambda transloci

Here we compare human monoclonal antibody (MAb) production from mouse strains that carry disruptions of their endogenous mouse IgH/IgK loci and harbor human IgM + Igkappa(BABkappa) or human IgM + Igkappa + IgA transloci (BABkappa,lambda). We found that whereas both strains proved effective for the isolation of antigen-specific IgM antibodies, many of the IgM MAbs elicited from BABkappa comprise human mu chains that are associated with mouse lambda chains. In contrast, BABkappa,lambda mice gave rise to fully functional, polymeric human IgM antibodies comprising both human IgH and human IgL chains. Therefore, the inclusion of a human Iglambda translocus (in addition to the human IgH + Igkappa transloci) not only diminishes problems of endogenous mouse Iglambda expression but also provides a strain of mice that yields fully human MAbs to a wide range of antigens, as witnessed by the isolation of MAbs to human blood cells, tumor cell lines, and an immunoglobulin idiotype.

Novartis Medal Lecture. Antibodies: a paradigm for the evolution of molecular recognition

Novel proteins have been elaborated over evolutionary time by an iterative alternation of mutation and selection. In a similar way, the humoral immune system also uses an iterative alternation of mutation and selection to generate novel antibodies that display a high affinity for their cognate antigen -- but this is achieved in a matter of a days. Gene rearrangement is used to produce a primary repertoire of antibodies and, on entering the body, antigen triggers the clonal expansion of those B lymphocytes that express a cognate antibody, albeit one of low affinity. Rapid and specific affinity maturation is then achieved by subjecting the immunoglobulin genes in the rapidly expanding B cells to a period of intense mutation. The intensity of this mutational assault is tolerated because it is targeted specifically to the immunoglobulin genes, causing relatively little damage to other loci. Antigen-mediated selection then allows the preferential expansion of those mutants expressing antibodies displaying improved binding characteristics. Here, studies are described that have been performed to glean insight into the mechanisms of the hypermutation and selection processes. Experiments are also described in which an attempt has been made to recapitulate aspects of physiological antibody generation in vitro, allowing the development of novel approaches to the generation of proteins with high-affinity binding sites.

Switch junction sequences in PMS2-deficient mice reveal a microhomology-mediated mechanism of Ig class switch recombination

Isotype switching involves a region-specific, nonhomologous recombinational deletion that has been suggested to occur by nonhomologous joining of broken DNA ends. Here, we find increased donor/acceptor homology at switch junctions from PMS2-deficient mice and propose that class switching can occur by microhomology-mediated end-joining. Interestingly, although isotype switching and somatic hypermutation show many parallels, we confirm that PMS2 deficiency has no major effect on the pattern of nucleotide substitutions generated during somatic hypermutation. This finding is in contrast to MSH2 deficiency. With MSH2, the altered pattern of switch recombination and hypermutation suggests parallels in the mechanics of the two processes, whereas the fact that PMS2 deficiency affects only switch recombination may reflect differences in the pathways of break resolution.

Epstein-Barr virus and the somatic hypermutation of immunoglobulin genes in Burkitt's lymphoma cells

It has been suggested that Epstein-Barr virus (EBV) might suppress antibody maturation either by facilitating bypass of the germinal center reaction or by inhibiting hypermutation directly. However, by infecting the Burkitt's lymphoma (BL) cell line Ramos, which hypermutates constitutively and can be considered a transformed analogue of a germinal center B cell, with EBV as well as by transfecting it with selected EBV latency genes, we demonstrate that expression of EBV gene products does not lead to an inhibition of hypermutation. Moreover, we have identified two natural EBV-positive BL cell lines (ELI-BL and BL16) that hypermutate constitutively. Thus, contrary to expectations, EBV gene products do not appear to affect somatic hypermutation.

Ablation of XRCC2/3 transforms immunoglobulin V gene conversion into somatic hypermutation

After gene rearrangement, immunoglobulin V genes are further diversified by either somatic hypermutation or gene conversion. Hypermutation (in man and mouse) occurs by the fixation of individual, non-templated nucleotide substitutions. Gene conversion (in chicken) is templated by a set of upstream V pseudogenes. Here we show that if the RAD51 paralogues XRCC2, XRCC3 or RAD51B are ablated the pattern of diversification of the immunoglobulin V gene in the chicken DT40 B-cell lymphoma line exhibits a marked shift from one of gene conversion to one of somatic hypermutation. Non-templated, single-nucleotide substitutions are incorporated at high frequency specifically into the V domain, largely at G/C and with a marked hotspot preference. These mutant DT40 cell lines provide a tractable model for the genetic dissection of immunoglobulin hypermutation and the results support the idea that gene conversion and somatic hypermutation constitute distinct pathways for processing a common lesion in the immunoglobulin V gene. The marked induction of somatic hypermutation that is achieved by ablating the RAD51 paralogues is probably a consequence of modifying the recombination-mediated repair of such initiating lesions.

B cells acquire antigen from target cells after synapse formation

Soluble antigen binds to the B-cell antigen receptor and is internalized for subsequent processing and the presentation of antigen-derived peptides to T cells. Many antigens are not soluble, however, but are integral components of membrane; furthermore, soluble antigens will usually be encountered in vivo in a membrane-anchored form, tethered by Fc or complement receptors. Here we show that B-cell interaction with antigens that are immobilized on the surface of a target cell leads to the formation of a synapse and the acquisition, even, of membrane-integral antigens from the target. B-cell antigen receptor accumulates at the synapse, segregated from the CD45 co-receptor which is excluded from the synapse, and there is a corresponding polarization of cytoplasmic effectors in the B cell. B-cell antigen receptor mediates the gathering of antigen into the synapse and its subsequent acquisition, thereby potentiating antigen processing and presentation to T cells with high efficacy. Synapse formation and antigen acquisition will probably enhance the activation of B cells at low antigen concentration, allow context-dependent antigen recognition and enhance the linking of B- and T-cell epitopes.

Somatic hypermutation of immunoglobulin kappa transgenes: association of mutability with demethylation

Following antigen encounter, immunoglobulin genes are diversified by somatic hypermutation. The mechanism by which this mutational process preferentially targets immunoglobulin genes is not known, but is likely linked to transcription. However, transcription is not sufficient to ensure mutability. Here, by polymerase chain reaction amplification of bisulfite-modified DNA, the pattern of demethylation within the Igkappa mutation domain is analysed and transgenes are used to identify an association between demethylation and mutability. In mice carrying an Igkappa transgene that is well transcribed but only poorly targeted for hypermutation, the mutated transgene copies have been demethylated within the mutation domain, whereas the methylated copies remain unmutated. Thus, the hypermutation mechanism only acts on immunoglobulin gene targets that are demethylated as well as transcribed, although transcription and demethylation do not themselves guarantee mutability.

In vivo and in vitro studies of immunoglobulin gene somatic hypermutation

Following antigen encounter, two distinct processes modify immunoglobulin genes. The variable region is diversified by somatic hypermutation while the constant region may be changed by class-switch recombination. Although both genetic events can occur concurrently within germinal centre B cells, there are examples of each occurring independently of the other. Here we compare the contributions of class-switch recombination and somatic hypermutation to the diversification of the serum immunoglobulin repertoire and review evidence that suggests that, despite clear differences, the two processes may share some aspects of their mechanism in common.

Somatic hypermutation in the absence of DNA-dependent protein kinase catalytic subunit (DNA-PK(cs)) or recombination-activating gene (RAG)1 activity

Somatic hypermutation and isotype switch recombination occur in germinal center B cells, are linked to transcription, and are similarly affected by deficiency in MutS homologue (MSH)2. Class-switch recombination is abrogated by disruption of genes encoding components of the catalytic subunit of DNA-dependent protein kinase (DNA-PK(cs))/Ku complex and likely involves nonhomologous end joining (NHEJ). That somatic hypermutation might also be associated with end joining is suggested by its association with the creation of deletions, duplications, and sites accessible to terminal transferase. However, a requirement for NHEJ in the mutation process has not been demonstrated. Here we show that somatic mutation in mice deficient in NHEJ can be tested by introduction of rearranged immunoglobulin and T cell receptor transgenes: the transgene combination not only permits reconstitution of peripheral lymphoid compartments but also allows formation of germinal centers, despite the wholly monoclonal nature of the lymphocyte antigen receptors in these animals. Using this strategy, we confirm that somatic hypermutation like class-switching can occur in the absence of recombination-activating gene (RAG)1 but show that the two processes differ in that hypermutation can proceed essentially unaffected by deficiency in DNA-PK(cs) activity.

Disruption of mouse polymerase zeta (Rev3) leads to embryonic lethality and impairs blastocyst development in vitro

Multiple DNA polymerases exist in eukaryotes. Polymerases alpha, delta and epsilon are mainly responsible for chromosomal DNA replication in the nucleus and are required for proliferation. In contrast, the repair polymerases beta and eta are not essential for cellular proliferation in yeast or mice, but a lack of either polymerase can lead, respectively, to defects in base excision repair or the ability to replicate past lesions induced by ultraviolet (UV) radiation [1-3]. Here, we have focused on polymerase zeta. This was first described as a non-essential product of the yeast REV3/REV7 genes involved in UV-induced mutagenesis, and was later implicated in trans-lesion synthesis [4,5]. Unlike in yeast, the mouse homologue (mRev3) was found to be essential for life. Homozygous mutant mice died in utero. Mutant embryos were considerably reduced in size at day 10.5 of development and usually aborted around day 12.5. It is likely that this block reflects a need for mRev3 in proliferative clonal expansion (rather than in the production of a particular cell type) as mutant blastocysts showed greatly diminished expansion of the inner cell mass in culture. Thus, mRev3 could be required to repair a form of externally induced DNA damage that otherwise accumulates during clonal expansion or, consistent with the high homology shared between its Rev7 partner and the mitotic checkpoint gene product Mad2 [6], mRev3 might play a role in cell proliferation and genomic stability even in the absence of environmentally induced damage.

Dysregulated expression of the Cd22 gene as a result of a short interspersed nucleotide element insertion in Cd22a lupus-prone mice

The Cd22 gene encodes a B cell-specific adhesion molecule that modulates B cell Ag receptor-mediated signal transduction, and is allelic to a lupus-susceptibility locus in New Zealand White (NZW) mice. In this study, we show that, in addition to the wild-type transcripts, NZW (Cd22a) mice synthesize aberrant CD22 mRNAs that contain approximately 20-120 nucleotide insertions upstream of the coding region between exons 2 and 3, and/or approximately 100-190 nucleotide deletions of exon 4. Sequence analysis revealed that these aberrant mRNA species arose by alternative splicing due to the presence in the NZW strain of a 794-bp sequence insertion in the second intron, containing a cluster of short interspersed nucleotide elements. Both the presence of sequence insertion and aberrantly spliced mRNAs were specific to mice bearing the Cd22a and Cd22c alleles. Up-regulation of CD22 expression after LPS activation appeared impaired in Cd22a spleen cells (twice lower than in Cd22b B cells). Furthermore, we show that partial CD22 deficiency, i.e., heterozygous level of CD22 expression, markedly promotes the production of IgG anti-DNA autoantibodies in C57BL/6 (Cd22b) mice bearing the Y chromosome-linked autoimmune acceleration gene, Yaa. Taken together, these results suggest that a lower up-regulation of CD22 on activated B cells (resulting from Cd22 gene anomaly in Cd22a mice or from CD22 heterozygosity in mutants obtained by gene targeting) is implicated in autoantibody production, providing support for Cd22a as a possible candidate allele contributing to lupus susceptibility.

Immunology. RNA editing AIDs antibody diversification?

How do B cells generate the enormous diversity of antibodies that are able to recognize and bind to whichever antigen a B cell might happen to encounter in the body? Several genetic mechanisms that manipulate different combinations of immunoglobulin genes are known. In their Perspective, Neuberger and Scott, highlight another genetic mechanism called RNA editing now shown to be involved in the production of antibody diversity.

The contribution of somatic hypermutation to the diversity of serum immunoglobulin: dramatic increase with age

Although somatic mutation contributes to the diversity of only a minor fraction of B cells in mouse spleen or blood, its contribution to the diversity of serum immunoglobulin is unknown. We have devised an immunoassay to monitor mutated antibodies in serum using a monoclonal antibody that recognizes a VK only when mutated at its major intrinsic hot spot. Mutation makes essentially no contribution to the diversity of endogenous serum IgM, IgG, or IgA in young mice. However, in response to environmental antigens, the titer of mutated immunoglobulin in T cell-proficient mice rises strikingly with age, such that the major proportion of serum immunoglobulin in adults is somatically mutated, with the mutation load in IgG being some 10-fold greater than in IgM.

Expression and regulation of immunoglobulin heavy chain gene transfected into lymphoid cells

A plasmid including a mouse immunoglobulin mu gene was transfected into the IgG-secreting human lymphoid line HMy2 and mouse B- and pre-B-cell lines WEHI 231 and 18-81; stably transfected cells were selected. Transfected HMy2 cells synthesized mouse immunoglobulin mu chains as a major secreted protein but the WEHI 231 and 18-81 transfectants transcribed the introduced mu gene at lower levels. In HMy2 transfectants, most of the transcription of the introduced heavy chain gene initiated 40 and 62 bp upstream of the beginning of the VH exon translation start, although a small proportion of transcripts initiating further upstream was detected. WEHI 231 and 18-81 transfectants gave a much higher proportion of upstream initiation. Transient expression of the VH exon was monitored following transfection of mouse myeloma with the VH gene DNA in various plasmid constructs. VH transcription was only observed if the plasmids contained a segment derived from the large VH-CH intron of the immunoglobulin heavy chain locus. This segment, located between JH and switch regions, functioned both downstream of the VH exon and upstream in either orientation. The existence of a transcription enhancer element in this region is therefore proposed.

The c-MYC allele that is translocated into the IgH locus undergoes constitutive hypermutation in a Burkitt's lymphoma line

Burkitt's lymphomas harbour chromosomal translocations bringing c-MYC into the vicinity of one of the immunoglobulin gene loci. Point mutations have been described within c-MYC in several Burkitt's lymphomas and it has been proposed that translocation into the Ig loci might have transformed c-MYC into a substrate for the antibody hypermutation mechanism. Here we test this hypothesis by exploiting a Burkitt's lymphoma line (Ramos) that we have previously shown to hypermutate its immunoglobulin genes constitutively. We find that, during in vitro culture, Ramos mutates the c-MYC allele that is translocated into the IgH locus whilst leaving the untranslocated c-MYC and other control genes essentially unaffected. The mutations are introduced downstream of the c-MYC transcription start with the pattern of substitutions being characteristic of the antibody hypermutation mechanism; the mutation frequency is 2-3-fold lower than for the endogenous functional IgH allele. Thus chromosomal translocations involving the Ig loci may not only contribute to transformation by deregulating oncogene expression but could also act by potentiating subsequent oncogene hypermutation.

Memory in the B-cell compartment: antibody affinity maturation

In the humoral arm of the immune system, the memory response is not only more quickly elicited and of greater magnitude than the primary response, but it is also different in quality. In the recall response to antigen, the antibodies produced are of higher affinity and of different isotype (typically immunoglobulin G rather than immunoglobulin M). This maturation rests on the antigen dependence of B-cell maturation and is effected by programmed genetic modifications of the immunoglobulin gene loci. Here we consider how the B-cell response to antigen depends on the affinity of the antigen receptor interaction. We also compare and draw parallels between the two processes, which underpin the generation of secondary-response antibodies: V gene somatic hypermutation and immunoglobulin heavy-chain class switching.

B cells extract and present immobilized antigen: implications for affinity discrimination

Binding of antigen to B-cell antigen receptor (BCR) leads to antigen internalization and presentation to T cells, a critical process in the initiation of the humoral immune response. However, antigen internalization has been demonstrated for soluble antigen, in vivo antigen is often encountered in insoluble form or tethered to a cell surface. Here, we show that not only can B cells internalize and present large particulate antigen (requiring a signalling-competent BCR to drive antigen uptake), but they can also extract antigen that is tethered tightly to a non-internalizable surface. The form in which the antigen is displayed affects the B cell's ability to discriminate antigen-BCR affinity. Thus, arraying an antigen on a particle or surface allows efficient presentation of low affinity antigens. However, the presentation efficiency of antigen arrayed on an internalizable particle plateaus at low affinity values. In contrast, extraction and presentation of antigen from a non-internalizable surface depends on antigen-BCR affinity over a wide affinity range. The results have implications for understanding both the initiation and affinity maturation of the immune response.

Diversification and selection mechanisms for the production of protein repertoires: lessons from the immune system

The physiological mechanism for producing antigen-specific antibodies is based on a two-phase neo-Darwinian process: the first phase consists of diversity generation (formation of the repertoire), and the second phase is antigen-mediated selection. In this article, we consider how the natural immunoglobulin gene-diversification processes can be exploited both in vivo and in vitro in order to allow the generation of novel antibody (and heterologous protein) repertoires.

Antibody repertoires of four- and five-feature translocus mice carrying human immunoglobulin heavy chain and kappa and lambda light chain yeast artificial chromosomes

We have produced mice that carry the human Ig heavy (IgH) and both kappa and lambda light chain transloci in a background in which the endogenous IgH and kappa loci have been inactivated. The B lymphocyte population in these translocus mice is restored to about one-third of normal levels, with preferential (3:1) expression of human lambda over human kappa. Human IgM is found in the serum at levels between 50 and 400 microg/ml and is elevated following immunization. This primary human Ab repertoire is sufficient to yield diverse Ag-specific responses as judged by analysis of mAbs. The use of DH and J segments is similar to that seen in human B cells, with an analogous pattern of N nucleotide insertion. Maturation of the response is accompanied by somatic hypermutation, which is particularly effective in the light chain transloci. These mice therefore allow the production of Ag-specific repertoires of both IgM,kappa and IgM,lambda Abs and should prove useful for the production of human mAbs for clinical use.

Deficiency in Msh2 affects the efficiency and local sequence specificity of immunoglobulin class-switch recombination: parallels with somatic hypermutation

During maturation of the immune response, IgM+ B cells switch to expression of one of the downstream isotypes (IgG, A or E). This class switching occurs by region-specific recombination within the IgH locus through an unknown mechanism. A lack of switch recombination in mice deficient in components of the DNA-dependent protein kinase (DNA-PK)-Ku complex has pointed to a role for non-homologous end joining. Here we characterize a switching defect in mice lacking a protein involved in DNA mismatch recognition. Mice deficient in Msh2 give diminished IgG (but not IgM) responses following challenge with both T cell-dependent and T cell-independent antigens. This appears to reflect a B cell-intrinsic defect since B cells from Msh2-deficient mice also exhibit impaired switching (but not blasting or proliferation) on in vitro culture with lipopolysaccharide. Furthermore, those switches that do occur in Msh2-deficient B cells reveal a shift in the distribution of recombination sites used: the breakpoints are more likely to occur in consensus motifs. These results, which intriguingly parallel the effects of Msh2 deficiency on hypermutation, suggest a role for Msh2 in the mechanics of class-switch recombination.

Deficiency in CD22, a B cell-specific inhibitory receptor, is sufficient to predispose to development of high affinity autoantibodies

CD22 is a B cell-specific transmembrane glycoprotein that acts to dampen signals generated through the B cell antigen receptor (BCR): B cells from CD22-deficient mice give increased Ca2+ fluxes on BCR ligation. Here we show that this B cell hyperresponsiveness correlates with the development of autoantibodies. After the age of eight months, CD22-deficient mice developed high titers of serum IgG directed against double-stranded DNA; these antibodies were of multiclonal origin, somatically mutated, and high affinity. Increased titers of antibodies to cardiolipin and myeloperoxidase were also noted. The results demonstrate that a single gene defect exclusive to B lymphocytes is, without additional contrivance, sufficient to trigger autoantibody development in a large proportion of aging animals. Thus, CD22 might have evolved specifically to regulate B cell triggering thresholds for the avoidance of autoimmunity.

TdT-accessible breaks are scattered over the immunoglobulin V domain in a constitutively hypermutating B cell line

Searching for an in vitro model for somatic hypermutation, we have identified an IgM-expressing Burkitt lymphoma line that constitutively diversifies its immunoglobulin V domain at high rate during culture. As in in vivo, the mutations are largely nucleotide substitutions with the pattern of substitutions revealing a component of the human hypermutation program that is preferentially targeted to G/C residues. The substitutions frequently create stop codons with IgM-loss variants also being generated by V domain-specific deletions and duplications. However, in transfectants expressing terminal deoxynucleotidyl transferase, many IgM-loss variants additionally arise through short nontemplated nucleotide insertions into the V (but not C) domain. Thus, antibody hypermutation is likely accompanied by DNA strand breaks scattered within the mutation domain.

Targeted gene disruption reveals a role for natural secretory IgM in the maturation of the primary immune response

Accelerated development of the secondary immune response may be attributable in part to the rapid delivery of antigen to lymphoid follicles by circulating antibody elicited on primary immunization. Here we provide evidence indicating that the nonspecific IgM present in naive mice (natural antibody) plays a role in the acceleration of the primary response. Targeted deletion of the Ig microseconds polyadenylation site by use of Cre recombinase allowed the creation of mice that, although harboring a normal number of B cells expressing surface IgM, completely lacked serum IgM while retaining the other Ig isotypes. These mice retained a broadly normal B lymphocyte distribution (although containing a somewhat expanded peritoneal B1a subset) but exhibited substantial delays in mounting affinity-matured IgG responses to T cell-dependent antigens. The T cell-independent response, however, was augmented. The data indicate that the IgM present before antigen challenge (as well, possibly, as that elicited immediately after immunization) accelerates maturation of the primary response, presumably by complexing with the antigen and facilitating lymphocyte activation and/or antigen trapping.

Both DNA strands of antibody genes are hypermutation targets

During the maturation of the immune response, antibody genes are subjected to localized hypermutation. Mutations are not evenly distributed along the V gene; intrinsic hot spots exist that are correlated with primary sequence motifs. Although the mechanism of hypermutation remains unknown, it has been proposed to exhibit DNA strand polarity because purine residues on the coding strand are more frequently targeted for mutation than pyrimidines. However, this polarity may not be an intrinsic property of the hypermutation mechanism but a consequence of evolutionary-selected peculiarities of V gene sequences. Furthermore, the possibility that both strands are hypermutation targets has received little attention. To discriminate between these possibilities, we have analyzed the average frequency of mutations of each of the three bases of all nucleotide triplets by using large databases taken from both V and non-V mutation targets. We also have reassessed the sequence motifs associated with hot spots. We find that even in non-Ig sequences, A mutates more than T, consistent with a strand-dependent component to targeting. However, the mutation biases of triplets and of their inverted complements are correlated, demonstrating that there is a sequence-specific but strand-independent component to mutational targeting. Thus, there are two aspects of the hypermutation process that are sensitive to local DNA sequences, one that is DNA strand-dependent and the other that is not.

Hot spot focusing of somatic hypermutation in MSH2-deficient mice suggests two stages of mutational targeting

Likely creation of mismatches during somatic hypermutation has stimulated interest in the effect of mismatch repair deficiency on the process. Analysis of unselected mutations in the 3' flank of VH rearrangements in germinal center B cells revealed that MSH2 deficiency caused a 5-fold reduced mutation accumulation. This might reflect ectopic effects of the Msh2 disruption; indeed, the mice exhibit other perturbations within the B cell compartment. However, that MSH2 (or factors dependent upon it) plays a role in the mechanism of mutation fixation is indicated by a strikingly increased focusing of the mutations on intrinsic hot spots. We propose two phases to hypermutation targeting. The first is hot spot focused and MSH2 independent; the second, MSH2-dependent phase yields a more even spread of mutation fixation.

Mice carrying a CD20 gene disruption

CD20 is a hallmark antigen of B lymphocytes. Its expression is restricted to precursor and mature B cells but it is not expressed on plasma cells. The protein is a membrane-embedded phosphoprotein that appears likely to transverse the membrane four times. Its function is unknown although CD20 has been variously proposed to play a role in B-cell activation, proliferation, and calcium transport. A unique homologue of human CD20 has been described in mouse, which also shows a B-cell-specific pattern of expression. Here we describe the generating of mice carrying a CD20 gene disruption. So far, we have failed to detect any major effect of the gene disruption on the differentiation and function of B lymphocytes as judged by the expression of surface markers, antigen receptor signaling, proliferative responses, or calcium uptake. We did note, however, that the mice homozygous for the gene disruption [generated by intercrossing (129 x C57BL/6)F1 CD20+/- heterozygotes] showed a substantial depletion of the sub-population of peritoneal B cells that lack expression of the B220 (RA3-6B2) isoform of CD45. The loss of the IgM+ 6B2- peritoneal B cells is not, however, attributable to the CD20 gene disruption itself. Rather, it segregates with a polymorphic difference between the 129 and C57BL/6 strains that is linked to the CD20 locus which, intriguingly, is itself close to the CD5 gene. This demonstrates that caution must be exercised when comparing the phenotypes of F2 litter-mates generated from crosses between 129 embryonic stem-cell-derived chimeras and mice of other strains.

Affinity dependence of the B cell response to antigen: a threshold, a ceiling, and the importance of off-rate

Initiation and affinity maturation of the humoral immune response is driven by antigen interaction with BCR. To study how signaling and antigen presentation through BCR depend on antigen/BCR affinity, lysozyme-specific B cell transfectants were challenged with mutated lysozymes differing in their binding kinetics. For detectable triggering, the antigen/BCR complex needed a Ka > 10(6) M(-1) (dissociation half-life > approximately 1 s). Mutated lysozymes whose binding was below this threshold could nevertheless be presented if complexed with soluble antibody. Above the threshold, the concentration of antigen required to trigger a response decreased as the affinity (particularly dissociation half-life) increased. However, a plateau was reached at Kas > approximately 10(10) M(-1) (dissociation half-life > 0.5 hr), supporting the idea of a ceiling to affinity maturation.

Monitoring and interpreting the intrinsic features of somatic hypermutation

We have used both normal and transgenic mice to analyse the recruitment and targeting of somatic hypermutation to the immunoglobulin loci. We compare methods for analysing hypermutation and discuss how large databases of mutations can be assembled by PCR amplification of the rearranged V-gene flanks from the germinal centre B cells of normal mice as well as by transgene-specific amplification from transgenic B cells. Such studies confirm that hypermutation is preferentially targeted to the immunoglobulin V gene with the bcl6 gene, for example, escaping this intense mutational targeting in germinal centre B cells. We review our data concerning the nature of the hypermutation domain and the targeting of hotspots within that domain. We consider how enhancer-mediated recruitment of hypermutation to the immunoglobulin loci operates in a clonally maintained fashion and illustrate how both the degree of expression and demethylation of the transgene broadly correlate with its mutability.

Multiple sequences from downstream of the J kappa cluster can combine to recruit somatic hypermutation to a heterologous, upstream mutation domain

Recruitment of somatic hypermutation to the Ig kappa locus has previously been shown to depend on the enhancer elements, Ei/MAR and E3'. Here we show that these elements are not sufficient to confer mutability. However, hypermutation is effectively targeted to a chimeric beta-globin/Ig kappa transgene whose 5' end is composed of the human beta-globin gene (promoter and first two exons) and whose 3' end consists of selected sequences derived from downstream of the J kappa cluster (Ei/MAR, C kappa + flank and E3'). Thus, multiple downstream Ig kappa sequences (all derived from 3' of the J kappa cluster) can combine to recruit mutation to a heterologous mutation domain. The location of this hypermutation domain is defined by the position of the transcription start site and this applies even if the Ig kappa Ei/MAR is positioned upstream of the promoter. Hotspots within the mutation domain are, however, defined by local DNA sequence as evidenced by a new hotspot being created within the beta-globin domain by a mutation within the transgene. We propose that multiple, moveable Ig kappa sequences (that are normally located downstream of the transcription start site) cooperate to bring a hypermutation priming factor to the transcription initiation complex; a mutation domain is thereby created downstream of the promoter but the local sequence defines the detailed pattern of mutation within that domain.

Cells strongly expressing Ig(kappa) transgenes show clonal recruitment of hypermutation: a role for both MAR and the enhancers

The V regions of immunoglobulin kappa transgenes are targets for hypermutation in germinal centre B cells. We show by use of modified transgenes that the recruitment of hypermutation is substantially impaired by deletion of the nuclear matrix attachment region (MAR) which flanks the intron-enhancer (Ei). Decreased mutation is also obtained if Ei, the core region of the kappa3'-enhancer (E3') or the E3'-flank are removed individually. A broad correlation between expression and mutation is indicated not only by the fact that the deletions affecting mutation also give reduced transgene expression, but especially by the finding that, within a single mouse, transgene mutation was considerably reduced in germinal centre B cells that poorly expressed the transgene as compared with strongly expressing cells. We also observed that the diminished mutation in transgenes carrying regulatory element deletions was manifested by an increased proportion of B cells in which the transgene had not been targeted at all for mutation rather than in the extent of mutation accumulation once targeted. Since mutations appear to be incorporated stepwise, the results point to a connection between transcription initiation and the clonal recruitment of hypermutation, with hypermutation being more fastidious than transcription in requiring the presence of a full complement of regulatory elements.

Acceleration of intracellular targeting of antigen by the B-cell antigen receptor: importance depends on the nature of the antigen-antibody interaction

The B-cell antigen receptor (BCR) internalizes bound antigen such that antigen-derived peptides become associated with emigrating major histocompatibility complex (MHC) class II molecules for presentation to T cells. Experiments with B-cell transfectants reveal that BCR confers a specificity of intracellular targeting since chimeric antigen receptors which internalize antigen by virtue of a heterologous cytoplasmic domain do not necessarily give rise to presentation. In contrast, however, previous studies have shown that antigen binding to irrelevant cell surface molecules (e.g. transferrin receptor, MHC class I) can ultimately lead to presentation. The solution to this paradox appears to be that the intracellular targeting by BCR actually reflects an acceleration of antigen delivery. Depending on the nature of the BCR-antigen interaction, this accelerated targeting can be essential in determining whether or not internalization leads to significant presentation. Physiologically, the accelerated delivery of antigen by BCR could prove of particular importance early in the immune response when antigen-BCR interaction is likely to be poor.

The immunoglobulin (Ig)alpha and Igbeta cytoplasmic domains are independently sufficient to signal B cell maturation and activation in transgenic mice

The B cell antigen receptor, composed of membrane immunoglobulin (Ig) sheathed by the Igalpha/Igbeta heterodimer plays a critical role in mediating B cell development and responses to antigen. The cytoplasmic tails of Igalpha and Igbeta differ substantially but have been well conserved in evolution. Transfection experiments have revealed that, while these tails share an esssential tyrosine-based activation motif (ITAM), they perform differently in some but not all assays and have been proposed to recruit distinct downstream effectors. We have created transgenic mouse lines expressing chimeric receptors comprising an IgM fused to the cytoplasmic domain of each of the sheath polypeptides. IgM/alpha and IgM/beta chimeras (but not an IgM/beta with mutant ITAM) are each independently sufficient to mediate allelic exclusion, rescue B cell development in gene-targeted Igmu- mice that lack endogenous antigen receptors, as well as signal for B7 upregulation. While the (IgM/alpha) x (IgM/beta) double-transgenic mouse revealed somewhat more efficient allelic exclusion, our data indicate that each of the sheath polypeptides is sufficient to mediate many of the essential functions of the B cell antigen receptor, even if the combination gives optimal activity.

Rapid methods for the analysis of immunoglobulin gene hypermutation: application to transgenic and gene targeted mice

Hypermutation of immunoglobulin genes is a key process in antibody diversification. Little is known about the mechanism, but the availability of rapid facile assays for monitoring immunoglobulin hypermutation would greatly aid the development of culture systems for hypermutating B cells as well as the screening for individuals deficient in the process. Here we describe two such assays. The first exploits the non-randomness of hypermutation. The existence of a mutational hotspot in the Ser31 codon of a transgenic immunoglobulin V gene allowed us to use PCR to detect transgene hypermutation and identify cell populations in which this mutation had occurred. For animals that do not carry immunoglobulin transgenes, we exploited the fact that hypermutation extends into the region flanking the 3'-side of the rearranged J segments. We show that PCR amplification of the 3'-flank of VDJH rearrangements that involve members of the abundantly-used VHJ558 family provides a large database of mutations where the germline counterpart is unequivocally known. This assay was particularly useful for analysing endogenous immunoglobulin gene hypermutation in several mouse strains. As a rapid assay for monitoring mutation in the JH flanking region, we show that one can exploit the fact that, following denaturation/renaturation, the PCR amplified JH flanking region DNA from germinal centre B cells yields mismatched heteroduplexes which can be quantified in a filter binding assay using the bacterial mismatch repair protein MutS -Wagner et al. (1995) Nucleic Acids Res. 23, 3944-3948-. Such assays enabled us, by example, to show that antibody hypermutation proceeds in the absence of the p53 tumour suppressor gene product.

Hyperresponsive B cells in CD22-deficient mice

CD22 is a surface glycoprotein of B lymphocytes that is rapidly phosphorylated on cytoplasmic tyrosines after antigen receptor cross-linking. Splenic B cells from mice with a disrupted CD22 gene were found to be hyperresponsive to receptor signaling: Heightened calcium fluxes and cell proliferation were obtained at lower ligand concentrations. The mice gave an augmented immune response, had an expanded peritoneal B-1 cell population, and contained increased serum titers of autoantibody. Thus, CD22 is a negative regulator of antigen receptor signaling whose onset of expression at the mature B cell stage may serve to raise the antigen concentration threshold required for B cell triggering.

The Ig kappa 3'-enhancer triggers gene expression in early B lymphocytes but its activity is enhanced on B cell activation

Ig kappa gene expression is controlled by two enhancers, one located within the major intron (Ei) and the other located downstream of C kappa (E3'). Whereas loss of E3' has previously been shown to diminish kappa expression, we show here that a rearranged kappa transgene lacking Ei is well expressed, even at the pre-B cell stage. This suggests that E3' alone might be sufficient to give properly regulated transcription throughout B cell development. Indeed, we show that a transgene composed of a beta-globin reporter linked to E3' is expressed in a B cell-specific manner, becoming activated at the late pro-B to pre-B cell stage but with dramatically enhanced activity on B cell activation. Thus, E3' becomes active as a transcription enhancer at the stage when V kappa-J kappa rearrangement is being initiated and is sufficient to yield an expression pattern in a linked reporter gene similar to that of fully rearranged kappa genes.

Strategies for expressing human antibody repertoires in transgenic mice

Repertoires of human antibodies can be created in transgenic mice carrying human immunoglobulin-gene loci in germline configuration. These 'transloci', introduced either as miniloci or as almost locus-sized regions, undergo rearrangement and hypermutation in mouse lymphoid tissue. Here, Marianne Brüggemann and Michael Neuberger review the use of such mice for raising antigen-specific human monoclonal antibodies, as well as their exploitation for studying regulatory aspects of antibody repertoire formation.

Antibody expression from the core region of the human IgH locus reconstructed in transgenic mice using bacteriophage P1 clones

Mice carrying transgenic human immunoglobulin gene miniloci can be used for the production of human monoclonal antibodies. The human variable region (V) gene segments in these miniloci undergo productive rearrangement in mouse lymphoid tissue to yield a population of B lymphocytes expressing a repertoire of antibodies. Many of the miniloci studied to date have included only a small number of germline gene segments in an artificially compact configuration. Here we describe the use of the bacteriophage P1 cloning system to create mice carrying the core region of the human immunoglobulin heavy chain (IgH) locus. Three P1 clones carrying overlapping regions of the human IgH locus (spanning the five JH-proximal VH segments, the entire DH and JH clusters, and the C mu and C delta constant regions) were injected into mouse eggs and appear to have reconstituted the core region of the locus (> 180 kb) following homologous recombination with each other. While this translocus yielded a titer of serum immunoglobulin similar to that obtained with a smaller plasmid-based minilocus, the P1-based locus gave rise to substantially greater diversification by somatic hypermutation. Such diversification is important for obtaining high-affinity antibodies. The results show the usefulness of the P1 system in facilitating the manipulation and recreation of large transgenes.

The targeting of somatic hypermutation

Somatic hypermutation does not occur randomly within immunoglobulin V genes but, rather, is preferentially targeted to certain nucleotide positions (hot spots) and away from others (cold spots). Cold spots often coincide with residues essential for V gene folding. Hotspots, which appear to be strategically located to favour affinity maturation, are most frequently located in the CDRs (particularly CDR1) though conserved hotspots are also found at the base of FR3. Hotspots are in part created by local DNA sequence and the strong biases of codon usage in V genes indicate that the genes have evolved such that somatic hypermutation is targeted to those parts of the V where it is likely to prove most useful. These features of mutational hotspots and biased codon usage are also evident in V genes of lower animals suggesting that diversification by strategic targeting of non-templated mutation may have evolved early in antigen receptor evolution.

Somatic hypermutation of Ig genes in patients with xeroderma pigmentosum (XP-D)

Antibody diversification by somatic hypermutation occurs by the introduction of nucleotide substitutions in and around the rearranged Ig V gene segments. Several characteristics of the process suggest that the introduction of mutations is linked to Ig gene transcription. Since there is a connection between mutation and repair with indications that both processes might show linkage to transcription, we asked whether defects in a component of the transcription factor TFIIH which lead to an inability to carry out nucleotide excision repair also affect somatic hypermutation. A PCR strategy was devised that required small samples of peripheral blood and enabled us to monitor hypermutation of a single, abundantly used VH gene. However, the results showed that in xeroderma pigmentosum patients (complementation group D), somatic hypermutaton appears to take place unaffected as regard both extent and distribution.

Somatic hypermutation of immunoglobulin genes

The relationship between somatic hypermutation and affinity maturation in the mouse is delineated. Recent work on the anatomical and cellular site of this process is surveyed. The molecular characteristics of somatic hypermutation are described in terms of the region mutated and the distinctive patterns of nucleotide changes that are observed. The results of experiments utilizing transgenic mice to find out the minimum cis-acting sequences required to recruit hypermutation are summarized. The hypothesis that V gene sequences have evolved in order to target mutation to certain sites but not others is discussed. The use that different species make of somatic hypermutation to generate either the primary or secondary B cell repertoire is considered. Possible molecular mechanisms for the hypermutation process and future goals of research are outlined.

Targeting of non-Ig sequences in place of the V segment by somatic hypermutation

Affinity maturation of antibodies is characterized by localized hypermutation of the DNA around the V segment. Here we show, using mice containing single or multiple transgene constructs, that an immunoglobulin V kappa segment can be replaced by human beta-globin or prokaryotic neo or gpt genes without affecting the rate of hypermutation; the V gene itself is not necessary for recruiting hypermutation. The ability to target hypermutation to heterologous genes in vivo could find more general applications in biology.

Somatic hypermutation

For the generation of secondary response antibodies, immunoglobulin genes are subjected to hypermutation. Cells expressing antibodies with higher affinity are then selected by antigen. Recent clues to the mechanism of hypermutation come from experiments using transgenic mice enabling analysis of the controlling cis-acting elements and the intrinsic features of the hypermutation, dissociated from the effects of antigenic selection.

Regulated activity of the IgH intron enhancer (E mu) in the T lymphocyte lineage

The activity of the IgH (E mu) enhancer in the T lymphocyte lineage has been investigated using both transgenic mice and transfection studies. Thymocyte fractionation experiments indicate that a transgene consisting of the bacterial chloramphenicol acetyl transferase (CAT) gene, linked to E mu and the SV40 early promoter (E mu-CAT), is expressed only in thymocytes with a mature medullary phenotype and not in immature cells. Transfection of this same construct into two thymoma cell lines representing different stages of thymocyte development mimics the pattern of activity observed in vivo. Further transfection experiments suggest that this pattern of expression might be attributed to the differential activity of the E2E3 and octanucleotide motifs of E mu during development. In contrast, an Ig lambda transgene (linked to E mu and an Ig V lambda promoter) is expressed in the majority of thymocytes. We envisage that the different patterns of expression of the two transgenes reflect interactions between their respective promoters and the factors which are bound to E mu at different stages of thymocyte development. Although differing in their pattern of expression within the thymus, the two transgenes share the property of extinction in peripheral T lymphocytes. These results indicate that the expression of E mu-linked transgenes in the thymus cannot simply be explained by activation of the enhancer in a lymphoid progenitor cell prior to B/T lineage divergence. Rather, the enhancer (or components of it) must be independently activated (and inactivated) during T lymphocyte development. Furthermore, this activity is consistent with the developmental timing of Ig DH-JH rearrangements in these cells.

Somatic mutation of immunoglobulin lambda chains: a segment of the major intron hypermutates as much as the complementarity-determining regions

The rate and nature of hypermutation of immunoglobulin genes are of prime importance in the affinity maturation of antibodies. Although a considerable body of information has been gathered for kappa light chains, there is much less data for lambda chains. We have derived a large data base of somatic mutants of mouse lambda 1 light chains from Peyer's patches germinal center B cells. The endogenous lambda 1 genes mutate at a rate comparable to that previously found for a kappa transgene (V kappa ox1). There are intrinsic hot spots of mutation common to both in-frame and out-of-frame rearrangements; these hot spots cluster in hypermutating domains. In contrast to the pattern seen for V kappa Ox1, the hot spot clusters are found not only in complementarity-determining region (CDR)1 but also in CDR2 and CDR3; mutations also cluster in the joining/constant region intron. The differences between the pattern of mutations in V kappa Ox1 and lambda 1 light chains are discussed.

The diversity of antigen-specific monoclonal antibodies from transgenic mice bearing human immunoglobulin gene miniloci

An approach to the preparation of antigen-specific human monoclonal antibodies focuses on mice transgenic for human immunoglobulin gene miniloci; the V gene segments in these miniloci undergo productive rearrangement to yield mouse B cells expressing human immunoglobulin (Ig) chains. The general usefulness of this strategy hinges on whether it is feasible to obtain specific, high-affinity antibodies following immunization of such animals with a variety of antigens. To test this, we have investigated the antigen-specific responses in mice which carry human IgH miniloci (constaining just one or two VH segments) instead of a functional mouse IgH locus. Although serum responses were relatively weak, monoclonal antibodies were readily obtained to all immunogens tested (a hapten, foreign proteins and human lymphoma cells). The affinities of two of the hapten-specific (anti-2-phenyl-oxazol-5-one) antibodies were 60 and 160 nM, values intermediate between what is typically obtained in the primary and secondary response of normal mice. Sequence analysis of the rearranged V genes revealed that junctional events made a major contribution to diversity with a considerable amount of apparently non-templated sequence at the V-D and D-J borders. Somatic hypermutation was also evident within the expressed V gene segments of many of the antigen-specific hybridomas. These findings augur well for the general usefulness of the transgenic approach for the isolation of high-affinity human antibodies to a wide range of antigens and suggests that the miniloci need not be particularly large.