Truncation of the mu heavy chain alters BCR signalling and allows recruitment of CD5+ B cells

Ig are multifunctional molecules with distinct properties assigned to individual domains. To assess the importance of IgM domain assembly in B cell development we generated two transgenic mouse lines with truncated muH chains by homologous integration of the neomycin resistance gene (neo(r)) into exons C(mu)1 and C(mu)2. Upon DNA rearrangement shortened muH chain transcripts, V(H)-D-J(H)-C(mu)3-C(mu)4, are produced independent of the transcriptional orientation and termination signals provided by neo(r). The truncated muH chain of approximately 52 kDa associates non-covalently with the L chain to form a monovalent HL heterodimer. Surface IgM is assembled into a defective BCR complex which has lost important signalling capacity. In immunizations with T-dependent and T-independent antigens, specific IgM antibodies cannot be detected, whilst IgG responses remain normal. B cell development in the bone marrow is characterized by an increase in early B cells, but a decrease of B220(+) cells from the stage when muH chain rearrangement is completed. The peritoneal lymphocyte population has elevated levels of CD5(+) B cells and their expansion may be the result of a negative feedback mechanism. The results show that antigenic stimulation is compromised by truncated monovalent IgM and that this deficit in stimulation leads to reduced levels of conventional B-2 lymphocytes, but dramatically increased levels of B-1 cells.

Novel control motif cluster in the IgH delta-gamma 3 interval exhibits B cell-specific enhancer function in early development

The majority of the human Ig heavy chain (IgH) constant (C) region locus has been cloned and mapped. An exception is the region between C delta and C gamma 3, which is unstable and may be a recombination hot spot. We isolated a pBAC clone (pHuIgH3'delta-gamma 3) that established a 52-kb distance between C delta and C gamma 3. Sequence analysis identified a high number of repeat elements, explaining the instability of the region, and an unusually large accumulation of transcription factor-binding motifs, for both lymphocyte-specific and ubiquitous transcription activators (IKAROS, E47, Oct-1, USF, Myc/Max), and for factors that may repress transcription (Delta EF1, Gfi-1, E4BP4, C/EBP beta). Functional analysis in reporter gene assays revealed the importance of the C delta-C gamma 3 interval in lymphocyte differentiation and identified independent regions capable of either enhancement or silencing of reporter gene expression and interaction with the IgH intron enhancer E mu. In transgenic mice, carrying a construct that links the beta-globin reporter to the novel delta-gamma 3 intron enhancer (E delta-gamma 3), transgene transcription is exclusively found in bone marrow B cells from the early stage when IgH rearrangement is initiated up to the successful completion of H and L locus recombination, resulting in Ab expression. These findings suggest that the C delta-C gamma 3 interval exerts regulatory control on Ig gene activation and expression during early lymphoid development.

Rearrangement of the human heavy chain variable region gene V3-23 in transgenic mice generates antibodies reactive with a range of antigens on the basis of VHCDR3 and residues intrinsic to the heavy chain variable region

To formulate a 'logic' for how a single immunoglobulin variable region gene generates antibodies with different antigen specificity and polyreactivity, we analysed chimeric antibodies produced in transgenic mice carrying the germ-line human V3-23 gene, multiple diversity (D) and joining (J) gene segments. Hybridomas producing antibodies encoded by the V3-23 gene in combination with different mouse Vkappa genes were obtained by fusion of splenocytes from transgenic mice. All antibodies had human mu-chains and mouse light chains, were multimeric in structure and expressed the human V3-23 gene. Nucleotide sequence analyses of genes encoding the heavy and light chains of 12 antibodies in relation to antigen specificity highlighted the importance of heavy chain variable region CDR3 in determining reactivity with different antigens. However, the results also suggest that non-CDR3 sequences intrinsic to the V3-23 gene itself may be involved in, or determine, the binding of the chimeric antibodies to some of the antigens tested in the current study.

Improved assessment of minimal residual disease in B cell malignancies using fluorogenic consensus probes for real-time quantitative PCR

PCR of clonally rearranged immunoglobulin heavy chain (IgH) gene sequences is increasingly used for detection of minimal residual disease (MRD) in lymphoid malignancies. Inherent quantitating problems are the main drawbacks of traditional PCR technologies. These limitations have been overcome by the recently developed real-time quantitative PCR (RQ PCR) technology. However, clinical application of the few published RQ PCR assays targeting immune gene rearrangements is hampered by the expensive and time-consuming need for individual hybridization probes for each patient. We have developed a new RQ PCR strategy targeting clonally rearranged IgH sequences that solves this problem. The method uses only two different JH hybridization probes and four downstream JH primers homologous to consensus germline JH gene segments. In combination with an allele-specific upstream (ASO) primer the consensus JH probes and primers allow quantitation of about 90% of possible IgH rearrangements. In a series of 22 B-lineage ALL the new assay allowed the detection of one to 10 blasts in a background of 10(5) normal cells. To prove the clinical utility we quantified MRD in 23 follow-up samples of six ALL patients with the new assay in comparison with a published RQ PCR technique that used individually designed primer/probe sets. We showed that the sensitivity of the new RQ PCR assay was slightly higher for four of the six cases and about 100-fold higher for one case, enabling detection of an increasing MRD level as an indicator of subsequent relapse 44 weeks earlier compared to the ASO probe assay in this particular patient. The results suggest, that the novel RQ PCR assay is a rapid, technically simple, reliable, and sensitive alternative to traditional quantification assays and simplifies current approaches of monitoring MRD in clinical trials.

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.

Selection of a human anti-progesterone antibody fragment from a transgenic mouse library by ARM ribosome display

In antibody-ribosome-mRNA complex (ARM) ribosome display, stable complexes of nascent protein, mRNA and ribosomes are produced in a eukaryotic in vitro expression system, through coupled transcription and translation of DNA lacking a 3' stop codon. Selection of the protein simultaneously captures the relevant mRNA, which is recovered as DNA by coupled reverse transcription-polymerase chain reaction (RT-PCR) performed on the intact complexes. Here, we describe the use of ARM display to select a specific human antibody fragment from a transgenic mouse library. The mice carry unrearranged gene segments of the human heavy (H) and kappa light (L) chain loci, while the endogenous murine H and kappa loci are functionally silenced; they respond to immunisation by production of fully human IgM antibodies. A library encoding human single-chain (sc) antibody (V(H)/K) fragments, in which V(H) domains and kappa light chains were combined at random by PCR, was prepared from spleen cells of transgenic mice immunised with progesterone-bovine serum albumin (BSA). Library diversity was demonstrated by sequencing. Progesterone-binding fragments were selected over five cycles of ARM display and the selected DNA cloned and expressed in Escherichia coli. Soluble V(H)/K fragments obtained in periplasmic extracts had the same specificity as ribosome-bound V(H)/K, supporting the view that folding and specificity of the displayed and soluble proteins are equivalent. The affinity of the expressed V(H)/K was approximately 10(-8) M. Sequencing showed that ARM display selected a single V(H)/V(L) combination (V(H)1-2, Vkappa4-1) and rearrangement, with a few mutational differences between clones. Monoclonal antibodies against progesterone-BSA obtained from hybridomas were encoded by the same V(H) and V(L) segments and had similar properties to the fragments obtained in vitro. The combination of ribosome display and transgenic mouse technologies is a rapid means of generating fully human antibody fragments in vitro for expression and further manipulation.

Dominance of intrinsic genetic factors in shaping the human immunoglobulin Vlambda repertoire

The expressed human immunoglobulin Vlambda repertoire demonstrates a strong bias in the use of individual Vlambda segments. Mechanisms that underlie such biases can be divided into two categories: intrinsic genetic processes that lead to the preferential rearrangement and/or expression of certain segments; and selection following light chain expression. Here, we have used two approaches to investigate the factors that shape the human Vlambda repertoire. Firstly, we characterised 136 Vlambda rearrangements (59 productive and 77 non-productive) amplified from the human genomic DNA of peripheral blood cells. Secondly, we analysed Vlambda segment use in a library of 2000 cDNA clones from a transgenic mouse containing a 380 kb region (including 15 functional Vlambda segments) from the human immunoglobulin lambda locus. By hybridisation and sequencing we found that the patterns of use of human Vlambda segments in the transgenic mouse were similar to those found in the expressed human peripheral blood repertoire and in productive and non-productive genomic DNA rearrangements. These data indicate the importance of intrinsic genetic factors in shaping the human Vlambda repertoire and highlight the remarkable conservation of the molecular mechanisms involved in the production of the antibody repertoire in mouse and man. Therefore, transgenic mice represent a good model for analysis of the human antibody repertoire and for the production of human antibodies.

B-cell tumorigenesis in mice carrying a yeast artificial chromosome-based immunoglobulin heavy/c-myc translocus is independent of the heavy chain intron enhancer (Emu)

We have used YAC (yeast artificial chromosome) technology to create large translocation regions where the c-myc proto-oncogene is coupled to the core region of the human immunoglobulin heavy chain (IgH) locus (from VH2-5 through to Cdelta). Chimeric mice were obtained from embryonic stem cells carrying a single copy of the 240-kb IgH/c-myc translocation region. B-cell tumorigenesis occurs in the translocus mice, even when the entire Emu intron enhancer region between the joining segments and switch mu is deleted. This demonstrates that as yet unidentified regulatory elements in the IgH locus, independent from the known enhancers, are sufficient to cause B-cell specific activation of c-myc after translocation. The phenotype of tumors from IgH/c-myc YAC transgenic mice with or without Emu (B220+, IgM+/IgD+) is reminiscent of Burkitt's lymphoma. A rapidly expanding abnormal B-cell population is present at birth and accumulates in bone marrow, periphery, and spleen, well before discrete tumor establishment. Molecular analysis identified a clonal origin, with rearrangement of one mouse heavy chain allele retained in tumor cells from different sites, whereas subsequent rearrangements of heavy or light chain loci can be diverse. These mice routinely develop mature B-cell tumors early in life and may provide an invaluable resource of a B-cell lymphoma model.

A human immunoglobulin lambda locus is similarly well expressed in mice and humans

Transgenic mice carrying a 380-kb region of the human immunoglobulin (Ig) lambda light (L) chain locus in germline configuration were created. The introduced translocus on a yeast artificial chromosome (YAC) accommodates the most proximal Iglambda variable region (V) gene cluster, including 15 Vlambda genes that contribute to >60% of lambda L chains in humans, all Jlambda-Clambda segments, and the 3' enhancer. HuIglambdaYAC mice were bred with animals in which mouse Igkappa production was silenced by gene targeting. In the kappa-/- background, human Iglambda was expressed by approximately 84% of splenic B cells. A striking result was that human Iglambda was also produced at high levels in mice with normal kappa locus. Analysis of bone marrow cells showed that human Iglambda and mouse Igkappa were expressed at similar levels throughout B cell development, suggesting that the Iglambda translocus and the endogenous kappa locus rearrange independently and with equal efficiency at the same developmental stage. This is further supported by the finding that in hybridomas expressing human Iglambda the endogenous L chain loci were in germline configuration. The presence of somatic hypermutation in the human Vlambda genes indicated that the Iglambda-expressing cells function normally. The finding that human lambda genes can be utilized with similar efficiency in mice and humans implies that L chain expression is critically dependent on the configuration of the locus.

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.

Production of human antibody repertoires in transgenic mice

Transgenic mice have been created that carry human immunoglobulin heavy and light chain genes in germline configuration and that have the corresponding endogenous genes silenced. The transgenes are either minigene constructs or large, almost authentic, transloci on yeast artificial chromosomes and undergo B-cell-specific DNA rearrangement and hypermutation in the mouse lymphoid tissue. Monoclonal antibodies with good affinities for human antigens have been obtained after immunisation. These mice may be a future source of human antibodies for therapy.

Yeast colony size reflects YAC copy number

A novel strategy for separation of co-cloned YACs was developed. For this, yeast cells were grown under non-selective conditions to allow the mitotic loss of multiple YACs. Yeast colonies of different size appear on 'drop-out' selection plates with small clones consistently containing a single-copy YAC. Different auxotrophic marker genes can be used to separate co-cloned YACs or reduce their copy number, which is essential for most YAC-modification procedures.

Rapid induction of B-cell lymphomas in mice carrying a human IgH/c-mycYAC

Activation of the c-myc proto-oncogene by one of the immunoglobulin (Ig) loci after chromosomal translocation is a consistent feature of Burkitt's lymphoma. Different subtypes of this tumor vary in the molecular architecture of the translocation region. In most cases there are no known regulatory elements of the Ig locus neighboring the oncogene and this considerably obscures the mechanism of its deregulation. In order to assess possible oncogene activation signals, we produced an experimental translocation region by insertion of a c-myc gene about 50 kb from the IgH intron enhancer in a yeast artificial chromosome (YAC) containing a 220 kb region of the human Ig heavy chain (IgH) locus. Single copy integration of this YAC into the genome of mouse embryonic stem (ES) cells was achieved by spheroplast fusion. Chimeric mice derived from these ES cells developed monoclonal B-cell lymphomas expressing surface IgM by 8-16 weeks of age. The IgH/c-myc translocus showed different V(h)DJ(H) rearrangement in almost all tumors without any alterations of the distance between c-myc and the IgH intron enhancer. This mouse model can be used for the in vivo analysis of c-myc deregulation and the tumor formation capacity of the IgH locus in aberrant rearrangements.

Assembly and extension of yeast artificial chromosomes to build up a large locus

For the assembly of a large human locus, overlapping regions on yeast artificial chromosomes (YACs) and cosmids were linked up using their regions of homology. By site-specific recombination a YAC of 410 kb was created accommodating the major part of the human lambda light chain locus in authentic configuration with 28 variable (V) genes, all joining (J) segments, all constant (C) genes and the downstream enhancer. A contiguous region was first constructed from three overlapping cosmids. Each of these was linearized at unique sites in the vectors and YAC arms were ligated to the 5' and 3' ends. After cells of Saccharomyces cerevisiae were transformed with the three cosmids, YACs of 120 kb were obtained which contained the reassembled 3' J-C region in authentic configuration. The assembled YAC was further extended by mitotic recombination with a YAC containing a 280-kb region of the C-proximal part of the V gene cluster with a 15-kb 3' overlap. For this, a simple three-way selection procedure was developed involving the integration of different selectable marker genes at specific sites in the left and right YAC arms. Rare recombination events between two overlapping YACs could be identified in yeast clones able to grow in lysine- and adenine-deficient medium in the presence of 5-fluoro-orotic acid which is toxic for yeast cells containing a YAC with a functional URA3 gene. This approach made it possible to assemble and extend large YACs from an unlimited number of smaller overlapping YACs by positive-negative selection.

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.

Dominant expression of a 1.3 Mb human Ig kappa locus replacing mouse light chain production

Expression studies of multigene families, such as the immunoglobulin (Ig) loci, are difficult because of their large size and the necessity to introduce germline configured regions into an animal. Antibody diversity from Ig gene miniloci is limited by the number of variable (V) region genes and the need for distal regulatory elements to control expression. Here, we show germline transfer into mice of a 1300 kb human Ig kappa light chain locus on a yeast artificial chromosome that resulted in early DNA rearrangement and highly efficient human light chain expression. The human locus was assembled from a 300 kb authentic region using contig extension by addition of cosmid multimers to supplement the variable gene cluster. This resulted in the addition of about 100 V region genes in germline configuration from different families. In transgenic animals with Ig kappa disruption, this large human kappa locus replaced the endogenous locus, and subsequent down-regulation of Ig lambda light chain contribution led to a dominant expression of the rearranged human genes. Contrary to expectation, rather than providing a solely selective advantage for ensuring repertoire formation controlled by the sheer number of introduced genes, the lambda/kappa ratio in serum appears to be the result of competition for early surface Ig expression maintained in the developing B cell.-Zou, X., Xian, J., Davies, N. P., Popov, A. V., Brüggemann, M. Dominant expression of a 1.3 Mb human Ig kappa locus replacing mouse light chain production.

Chimaeric monoclonal antibodies encoded by the human VH26 gene from naïve transgenic mice display a wide range of antigen-binding specificities

To elucidate the molecular basis for the ability of antibodies encoded by the human VH26 heavy-chain variable region gene to react with diverse antigens, we have generated 34 hybridomas secreting chimaeric monoclonal antibodies (human mu heavy chain/mouse light chains) from transgenic mice. The transgenic mice carry an immunoglobulin minilocus containing the human VH26 gene, human DH and JH gene segments, and genes encoding the human C mu region. The minilocus in these animals undergoes functional rearrangement resulting in the production of chimaeric antibodies in which human mu heavy chains utilizing the VH26 gene are paired with mouse kappa or lambda light chains. The hybridomas described in this study were generated from naïve animals and were selected solely on the basis of human mu-chain expression. The antibodies described have covalently attached mouse light chains and are multimeric in structure. The binding properties of the antibodies were examined using a panel of both self- and foreign antigens using enzyme-linked immunosorbent assays, agglutination or radio-immunoprecipitation assays and immunofluorescence. Chimaeric immunoglobulins from 21 of the 34 hybridoma clones (61.7%) reacted with one or more antigens, of which 13 (38.2%) reacted with more than two antigens. These studies demonstrate that the VH26 gene, in combination with human DH and JH gene segments, and mouse light-chain genes, is able to encode antibodies with a wide range of ligand-binding specificities. These findings have important implications in the context of the possible origins of autoantibodies encoded by VH26 which may play a role in the pathogenesis of a number of autoimmune conditions.

Subtle differences in antibody responses and hypermutation of lambda light chains in mice with a disrupted chi constant region

Analysis of lambda light chain use in normal mice is made difficult by the dominant chi light chain repertoire. We produced mice rendered deficient in chi light chain expression by gene targeting and focused on questions concerned with the generation of lambda light chain diversity. Whilst these mice compensate the chi deficiency with increased lambda liters, and their Ig level is therefore not significantly reduced, they show major differences in immunization titers, germinal center (GC) development and somatic hypermutation. After immunization, using antigens that elicit a restricted IgL response in normal mice, we obtained in the chi-/- mice elevated primary antibody titers but a subsequent lack in titer increase after repeated antigen challenge. Analysis of the Peyer's patches (PP) revealed a dramatically reduced cell content with rather small but highly active GC. Flow cytometric analysis showed different cell populations in the PP with enriched peanut agglutinin (PNA)hi/CD45R(B220)+ B cells, implying that the apparent compensation for the lack of lambda light chain expression involves the GC microenvironment in cell selection, the initiation of hypermutation and high affinity expansion. The three V lambda genes, V1, V2 and Vx, are mutated in the GC B cells, but show no junctional diversity. In contrast, a reduced rate of V lambda hypermutation is found in the hybridoma antibodies, which appears to reflect a selection bias rather than structural constraints. However, mechanisms of somatic mutation and specificity selection can operate with equal efficiency on the few V lambda genes.

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.

Antibodies generated from human immunoglobulin miniloci in transgenic mice

One approach to the production of human monoclonal antibodies focusses on the creation of transgenic mice bearing human immunoglobulin gene miniloci. Whilst such loci undergo lymphoid-specific gene rearrangement, only a small proportion of mouse B cells express the human immunoglobulin chains; the miniloci thus contribute poorly to serum immunoglobulin. Attributing this poor performance to competition between the transgenic and endogenous immunoglobulin loci, we crossed mice bearing a human immunoglobulin heavy-chain (HulgH) minilocus with animals that had been rendered B cell-deficient by disruption of their endogenous heavy-chain locus. The results were dramatic: the human minilocus rescued B cell differentiation such that effectively all B cells now expressed human mu chains. The concentration of antibody in the mouse serum recognised by anti-human mu increased to a concentration about one sixth that in human serum. The HulgH antibodies are heterogenous with diversity being generated by both combinatorial and junctional processes. Following antigen challenge, specific antibody is elicited but at low titre.

Creation of mice expressing human antibody light chains by introduction of a yeast artificial chromosome containing the core region of the human immunoglobulin kappa locus

We have previously described a strategy for integrating selectable marker genes into yeast artificial chromosomes (YACs) to facilitate their transfer into embryonic stem (ES) cells. Here we apply this technology to create mice carrying the core region of the human immunoglobulin (Ig) kappa light chain locus. A YAC was isolated which contains a 300 kb insert spanning three V kappa segments, the J kappa cluster, the C kappa region and extending downstream of the Kde element. After modification of this YAC to integrate the selectable neo marker gene, the YAC was introduced into ES cells by protoplast fusion. Several ES cell clones were obtained which appeared to harbor one complete copy of the YAC while retaining little or no other yeast DNA. The ES cells were injected into blastocysts and the chimaeric mice were shown to rearrange the introduced human light chain genes with the resultant production of antibodies containing human kappa light chains in the serum.

Extension of yeast artificial chromosomes by cosmid multimers

Images