Development of the primary antibody repertoire

B cell development in mice with a defective lambda 5 gene

The surrogate light chain encoded by the two pre-B cell-specific genes VpreB and lambda 5 plays a critical role in B cell development of the mouse. It has been shown that targeted disruption of the lambda 5 gene results in a depletion of B220+ CD43- IgM-pre-B cells in bone marrow, and in a delayed appearance both of CD5+ as well as CD5- surface immunoglobulin (sIg)+ B cells in the periphery. In this report we show that DHJH-rearranged B220- and B220+, CD43+, c-kit+, sIgM- pro- and pre-B-I cells with long-term capacity to proliferate in vitro on stromal cells in the presence of interleukin-7 are present in normal numbers in the bone marrow of lambda 5 T/lambda 5 T mice at various ages. They express normal levels of VpreB mRNA but, in contrast to normal pre-B-I cells, do not express surrogate light chain on their surface. Pre-B-I cells from fetal liver and bone marrow of lambda 5 T/lambda 5 T mice differentiate with normal kinetics and in normal numbers to sIg+, mitogen-reactive B cells. These results suggest that the delayed generation of sIg+ B cells in the peripheral, mature compartments of CD5+ and CD5- cells could be accounted for by the daily production of approximately 5 x 10(5) sIg+ B cells from the pre-B-I cell pool in the absence of a normal pool of pre-B-II cells.

Both LyF-1 and an Ets protein interact with a critical promoter element in the murine terminal transferase gene

Terminal deoxynucleotidyltransferase (TdT) is a template-independent DNA polymerase that is expressed transiently during the earliest stages of B- and T-cell ontogeny. Previously, we characterized the promoter for the murine TdT gene and identified a novel DNA-binding protein, called LyF-1, that interacts with a DNA sequence element found to be critical for transcriptional activity in lymphoid cell lines. Here, we present a more detailed analysis of this 30-bp control element, called the TdT D' element, which is centered approximately 60 bp upstream of the transcription start site. We found that both the murine and human D' elements are recognized by multiple proteins, including LyF-1 and at least two Ets family proteins, Ets-1 and Fli-1. Additional protein-DNA interactions were identified through studies using unfractionated nuclear extracts, in which the D' element was apparently incorporated into a multiprotein complex, possibly containing an Ets protein as a core component. By analyzing a series of substitution mutations, two adjacent binding sites for LyF-1 were identified in the murine D' element, with the Ets protein binding site closely coinciding with the proximal, lower-affinity LyF-1 site. Transient transfection analysis with these mutations revealed that only a 10-bp region, containing precisely the Ets and proximal LyF-1 binding sites, was needed for D' activity. These results suggest an important role for an Ets family protein in the expression of the TdT gene. The role of LyF-1 is less clear; it might act in conjunction with the Ets protein bound at the D' element or it might be unnecessary for D' activity.

RAG-2-deficient blastocyst complementation: an assay of gene function in lymphocyte development

We describe a system to evaluate the function of lymphocyte-specific and generally expressed genes in the differentiation and/or function of lymphocytes. RAG-2 (recombination-activating gene 2)-deficient mice have no mature B and T lymphocytes due to the inability to initiate VDJ recombination. Blastocysts from RAG-2-deficient mice generate animals with no mature B and T cells following implantation into foster mothers. However, injection of normal ES cells into RAG-2-deficient blastocysts leads to the generation of somatic chimeras with mature B and T cells all of which derive from the injected ES cells (referred to as RAG-2-deficient blastocyst complementation). Complementation of RAG-2-deficient blastocysts with mutant ES cells heterozygous for a targeted mutation that deletes all immunoglobulin heavy-chain joining (JH) gene segments (JH+/-) also leads to generation of chimeras with normal B and T cells. However, complementation with ES cells homozygous for the JH mutation (JH-/-) generates animals with normal T cells but no B cells, due to a block in B-cell development at a very early stage. Transfection of a functionally assembled mu heavy-chain gene into the JH-/- ES cells prior to blastocyst injection rescues the JH-/- mutation and allows the generation of both mature T and mature B cells. The rescued B cells express IgM but not IgD and respond normally to bacterial lipopolysaccharide stimulation by proliferating and by secreting IgM.

A zonal organization of odorant receptor gene expression in the olfactory epithelium

The mechanisms by which mammals discriminate a vast array of diverse odors are poorly understood. To gain insight into the organizational strategies underlying this discriminatory capacity, we have examined the spatial distribution of odorant receptor RNAs in the mouse olfactory epithelium. We have observed topographically distinct patterns of receptor RNAs suggesting that the nasal cavity is divided into a series of expression zones. The zones exhibit bilateral symmetry in the two nasal cavities and are organized along the dorsal-ventral and medial-lateral axes. Within each zone, a neuron may select a gene for expression from a zonal gene set via a stochastic mechanism. The observed zonal patterning may serve as an initial organizing step in olfactory sensory information coding.

The structure of the mu/pseudo light chain complex on human pre-B cells is consistent with a function in signal transduction

Prior to immunoglobulin (Ig) light (L) chain rearrangement, pre-B cells can express mu heavy (H) chains at the cell surface in association with pseudo (psi) L chains. This complex may be essential for B cell development. We have investigated the composition of the mu/psi L chain complex of a human pre-B cell line, in view of its potential role in transmembrane signal transduction. The mu/lambda receptor of a mature B cell line was analyzed in comparison. The mu/psi L chain complex is associated with disulfide-linked molecules that are homologous or identical to the mb-1 and B29 proteins, known to be integral components of membrane Ig receptors on mature B cells. Both receptors contain tyrosine (Tyr) kinase activity. In the mu/lambda receptor, the lyn and lck Tyr kinases could clearly be identified. The mb-1 and B29 proteins in both mu/lambda and mu/psi L chain receptors are substrates for in vitro phosphorylation on Tyr, but also on serine (Ser) and threonine (Thr) residues. The undefined mu-associated Ser/Thr kinase also phosphorylates the src-related kinases in the mu/lambda receptor and a 43-kDa mu-associated protein that is present in both complexes. The 43-kDa protein may be an integral part of both receptor types, or a transiently associated molecule instrumental in the signaling process. We conclude that the mu/psi L receptor on human pre-B cells fulfills the presently known criteria to function as a signal transduction unit.

Both LyF-1 and an Ets protein interact with a critical promoter element in the murine terminal transferase gene

Terminal deoxynucleotidyltransferase (TdT) is a template-independent DNA polymerase that is expressed transiently during the earliest stages of B- and T-cell ontogeny. Previously, we characterized the promoter for the murine TdT gene and identified a novel DNA-binding protein, called LyF-1, that interacts with a DNA sequence element found to be critical for transcriptional activity in lymphoid cell lines. Here, we present a more detailed analysis of this 30-bp control element, called the TdT D' element, which is centered approximately 60 bp upstream of the transcription start site. We found that both the murine and human D' elements are recognized by multiple proteins, including LyF-1 and at least two Ets family proteins, Ets-1 and Fli-1. Additional protein-DNA interactions were identified through studies using unfractionated nuclear extracts, in which the D' element was apparently incorporated into a multiprotein complex, possibly containing an Ets protein as a core component. By analyzing a series of substitution mutations, two adjacent binding sites for LyF-1 were identified in the murine D' element, with the Ets protein binding site closely coinciding with the proximal, lower-affinity LyF-1 site. Transient transfection analysis with these mutations revealed that only a 10-bp region, containing precisely the Ets and proximal LyF-1 binding sites, was needed for D' activity. These results suggest an important role for an Ets family protein in the expression of the TdT gene. The role of LyF-1 is less clear; it might act in conjunction with the Ets protein bound at the D' element or it might be unnecessary for D' activity.

Gene rearrangement and B-cell development

The differentiation of B lymphocytes from their progenitors progresses through a series of successive stages that are defined by sequential rearrangement of Ig loci and surface expression of various stage-specific markers, including Ig heavy and light chain proteins. Considerable evidence suggests that the appearance of cells with an orderly progression of Ig gene rearrangements is linked to the expression of the rearranged Ig gene products. Recent experiments have clarified our understanding of mechanisms by which rearrangement of Ig gene segments is controlled and how Ig gene products participate in the regulation of the B-cell differentiation program.

The immunoglobulin heavy chain locus contains another B-cell-specific 3' enhancer close to the alpha constant region

The transcription of immunoglobulin genes is controlled by variable region promoters and by enhancers, both of which are lymphoid specific. Because immunoglobulin genes are subject to an extremely complex regulation, we anticipated that there might be additional control elements for these genes. We therefore sought additional enhancers and demonstrate here that there is indeed another weak transcriptional enhancer just 3' to the mouse alpha constant region. This novel immunoglobulin enhancer is lymphoid specific and at two positions can bind members of the Oct family of transcription factors.

Oct-2, although not required for early B-cell development, is critical for later B-cell maturation and for postnatal survival

Oct-2, a POU homeo domain transcription factor, is believed to stimulate B-cell-restricted expression of immunoglobulin genes through binding sites in immunoglobulin gene promoters and enhancers. To determine whether Oct-2 is required for B-cell development or function, or has other developmental roles, the gene was disrupted by homologous recombination. Oct-2-l- mice develop normally but die within hours of birth for undetermined reasons. Mutants contain normal numbers of B-cell precursors but are somewhat deficient in IgM+ B cells. These B cells have a marked defect in their capacity to secrete immunoglobulin upon mitogenic stimulation in vitro. Thus, Oct-2 is not required for the generation of immunoglobulin-bearing B cells but is crucial for their maturation to immunoglobulin-secreting cells and for another undetermined organismal function.

Distinctive growth requirements and gene expression patterns distinguish progenitor B cells from pre-B cells

Long-term bone marrow cultures have been useful in determining gene expression patterns in pre-B cells and in the identification of cytokines such as interleukin 7 (IL-7). We have developed a culture system to selectively grow populations of B lineage restricted progenitors (pro-B cells) from murine bone marrow. Pro-B cells do not grow in response to IL-7, Steel locus factor (SLF), or a combination of the two. c-kit, the SLF receptor, and the IL-7 receptor are both expressed by pro-B cells, indicating that the lack of response is not simply due to the absence of receptors. Furthermore, SLF is not necessary for the growth of pro-B cells since they could be expanded on a stromal line derived from Steel mice that produces no SLF. IL-7 responsiveness in pre-B cells is associated with an increase in n-myc expression and is correlated with immunoglobulin (Ig) gene rearrangements. Although members of the ets family of transcription factors and the Pim-1 kinase are expressed by pro-B cells, n-myc is not expressed. Pro-B cells maintain Ig genes in the germline configuration, which is correlated with a low level of recombination activating genes 1 and 2 (Rag-1 and 2) mRNA expression, but high expression of sterile mu and terminal deoxynucleotidyl transferase. Pro-B cells are unable to grow separated from the stromal layer by a porous membrane, indicating that stromal contact is required for growth. These results suggest that pro-B cells are dependent on alternative growth signals derived from bone marrow stroma and can be distinguished from pre-B cells by specific patterns of gene expression.

VH family utilization by IgG anti-DNA-secreting lymphocytes derived from autoimmune MRL-lpr/lpr mice

OBJECTIVE

To evaluate the heterogeneity of the IgG anti-DNA autoantibody response of MRL/lpr mice.

METHODS

B cell clones were grown on nitrocellulose membranes. Those producing IgG anti-DNA antibodies were identified by a modified enzyme-linked immunospot assay, and their utilization of IgVH genes was determined by hybridization.

RESULTS

Four hundred sixty-eight IgG anti-DNA-secreting colonies were derived from 9 autoimmune MRL/lpr mice. Individual VH families contributed to anti-DNA production at a frequency roughly proportional to their representation in the expressed repertoire (except for the more frequent use of VH 7183 and less frequent use of VH 36-60). In individual mice, anti-DNA antibodies were encoded by 2-5 different IgVH families, with no single family constituting more than 27-65% of any animal's anti-DNA response.

CONCLUSION

The IgG anti-DNA response of individual MRL/lpr mice is oligoclonal.

Molecular analysis of rheumatoid factors derived from rheumatoid synovium suggests an antigen-driven response in inflamed joints

OBJECTIVE

Understanding the molecular genetic basis for rheumatoid factor (RF) production is necessary to a better understanding of the etiology and pathogenesis of rheumatoid arthritis (RA). We sought to define the genetic basis of RF in RA.

METHODS

The heavy and light chain variable region genes encoding 4 human monoclonal RF were cloned and sequenced using the polymerase chain reaction and the dideoxynucleotide chain-termination method.

RESULTS

The heavy and light chains of the C6 RF and the light chain of the G9 RF were encoded by 3 new RF-related Ig V-region genes. The heavy and light chains of D5 and G4 RFs were identical; most of their mutations caused amino acid substitutions.

CONCLUSIONS

The RF-related Ig V-region gene repertoire is large and is still expanding. The data from D5 and G4 strongly suggest that these 2 RFs arise in an antigen-driven response in rheumatoid synovium. The presumed germline V genes for C6 may represent disease-specific RF-related V genes.

The immunoglobulin heavy chain locus contains another B-cell-specific 3' enhancer close to the alpha constant region

The transcription of immunoglobulin genes is controlled by variable region promoters and by enhancers, both of which are lymphoid specific. Because immunoglobulin genes are subject to an extremely complex regulation, we anticipated that there might be additional control elements for these genes. We therefore sought additional enhancers and demonstrate here that there is indeed another weak transcriptional enhancer just 3' to the mouse alpha constant region. This novel immunoglobulin enhancer is lymphoid specific and at two positions can bind members of the Oct family of transcription factors.

The surrogate light chain in B-cell development

The proteins encoded by the VpreB and lambda 5 genes associate with each other to form a light (L) chain-like structure, the surrogate L chain. It can form Ig-like complexes with three partners-the classical heavy (H) chain, the DHJHC mu-protein, or the newly discovered p55 chain; these are expressed on the surface of pre-B cells at different stages of development. Here, Fritz Melchers and colleagues review the structures of the VpreB and lambda 5 genes in mouse and their relatives in humans, describe their pattern of expression, and speculate on their possible evolution and functions.

A role for immunoglobulin D: interference with tolerance induction

We have studied the induction and maintenance of tolerance in the B lymphocyte compartment. Neonatal and adult transgenic mice which expressed either surface IgM (sIgM) or sIgM and sIgD anti-2,4,6-trinitrophenyl (TNP) were treated with soluble mono- and multivalent forms of TNP-modified carriers. We compared the B cell compartment of mice treated with antigen and of littermates injected with phosphate-buffered saline. Antigen-mediated cross-linking of membrane-bound IgM (sIgM) caused deletion of B cells both in neonatal and adult mice with mu and kappa transgenes. Deletion was the result of apoptosis. In mice that carried an additional delta transgene sIgD interfered with tolerance induction. The stage in which the cells were sensitive to deletion was characterized as a transitional stage between immature (sIgMdull, heat-stable antigenbright, B220dull, sIgD-) and more mature (IgMbright, heat-stable antigendull, B220bright, sIgD-) B cells. Surviving cells were functional as measured by receptor-mediated changes in the intracellular free Ca2+ concentration. We propose that when the immature B cells have reached the final stages of maturation IgM always transmits negative signals in the absence of T cell help. When B cells need to be screened against self reactivity IgM is the only antigen receptor expressed. The presence of sIgD protects resting B cells from deletion and allows them to initiate an effective immune response.

Chimeric T cell receptor-immunoglobulin molecules: function and applications

The antigen-specific receptors on T and B cells are related by sequence similarities, mechanisms for the generation of diversity, and a common protein domain structure. In contrast, the form of antigen recognition for T- and B-cell antigen receptors is entirely different. Whereas the B cell antigen receptor, i.e., membrane-bound immunoglobulin (Ig), has the potential to recognize a vast diversity of chemical determinants, the T cell antigen receptor (TCR) invariably recognizes oligomeric peptides bound to major histocompatibility complex molecules. A question is whether the variable domains of the TCR and Ig are similar in structure, and if so, can they be substituted one for the other. Recent experiments show that, in some combinations, the variable region of Ig can substitute for the variable region of a TCR, and convey, to a reactive T lymphocyte, the antigen specificity of an Ig molecule. This type of receptor engineering may have interesting applications in disease therapy.

International Commission for Protection against Environmental Mutagens and Carcinogens. Recombination and gene conversion

Recombination is an important aspect of DNA metabolism. It leads to rearrangements of DNA sequences within genomes. Such genome rearrangements seem to be ubiquitous, since they play a role in evolution, human health and biotechnology. In medicine one important aspect of recombination is its role as one possible step in the multistep process of carcinogenesis. Since recombination may occur as a cellular response to DNA damage, the protection of cells from recombination-inducing agents, so-called recombinagen, should eliminate possible deleterious effects resulting from damage-induced DNA recombination. During the last few years, the awareness of the importance of recombination phenomena has substantially increased and the development of assay systems detecting recombinagens has progressed. The need for considering recombinagenic effects as a safety aspect of chemicals has gained ground in the field of genetic toxicology. This paper summarizes present knowledge concerning the occurence, inducibility, detection and toxicological interpretation of DNA recombination.

Immunoglobulin kappa light chain germ-line transcripts in human precursor B lymphocytes

B lymphoblastoid cell lines (BLCL), established from bone marrow and peripheral blood mononuclear cells from two severe combined immunodeficiency (SCID) patients, manifested a complete absence of genomic rearrangements of the immunoglobulin (Ig) heavy (H) and light (L) chain loci. The BLCL contained germ-line transcripts of the Ig kappa region locus of approximately 1.2 kilobase (kb). By cDNA cloning and sequence analysis the transcripts were shown to consist of a C kappa segment, a J kappa 1 gene segment, 160 base pairs (bp) of J kappa 1 5' intervening sequence, containing the heptamer/nonamer recombination recognition sequences and at the 5' end a 523-bp segment designated human kappa zero, The first 206 bp of this 5' segment were homologous to the reported murine kappa zero region. Genomic restriction mapping and DNA sequence analysis demonstrated that the human kappa zero segment is located approximately 4 kb upstream of J kappa 1. The kappa zero segment contains a putative promoter region with an OCT2 binding site, and has a splice donor site to accomplish splicing to an acceptor site 160 bp upstream of J kappa 1. Expression of the kappa zero gene segment was found in BLCL derived from normal fetal bone marrow, in which both Ig kappa loci were in the germ-line configuration. These findings indicate that the described transcripts are not only present in SCID, but also in normal developing pre-B lymphocytes. The expression of germ-line Ig kappa L chain transcripts may be associated with the locus becoming accessible to gene rearrangement.

Chromosomal organization of the heavy chain variable region gene segments comprising the human fetal antibody repertoire

The adult repertoire of antibody specificities is acquired in a developmentally programmed fashion that, in mouse and man, parallels the ordered rearrangement of a limited number of germ-line heavy chain variable region (VH) gene segments during development. It has been hypothesized that this developmental bias is a consequence of gene organization. In the mouse, rearrangement of VH gene segments proximal to the heavy chain joining region (JH) locus precedes rearrangement of genes located more distal to the JH locus. Similarly, in man, two VH elements located proximal to JH are expressed during fetal development. To test further this hypothesis in man, we have determined in a single individual the positions of an additional eight distinct VH elements known to comprise a significant fraction of the human developmental repertoire. These developmentally expressed VH elements were found to be dispersed over a region of 890 kilobases of the VH locus and were interspersed with other VH elements that are not known to be developmentally expressed. Thus, the ordered developmental expression of VH gene segments in man must involve mechanisms beyond physical proximity to the JH locus. Further, these results support the notion that fetal expression of VH gene segments is a regulated process and suggest that this regulation is important in the acquisition of immunocompetence.

Ectopic recombination within homologous immunoglobulin mu gene constant regions in a mouse hybridoma cell line

We have transferred a pSV2neo vector containing the wild-type constant region of the immunoglobulin mu gene (C mu) into the mutant hybridoma igm482, which bears a 2-bp deletion in the third constant-region exon of its haploid chromosomal mu gene (C mu 3). Independent igm482 transformants contain the wild-type immunoglobulin C mu region stably integrated in ectopic chromosomal positions. We report here that the wild-type immunoglobulin C mu region can function as the donor sequence in a gene conversion event which corrects the 2-bp deletion in the mutant igm482 chromosomal C mu 3 exon. The homologous recombination event restores normal immunoglobulin M production in the mutant cell.

Ectopic recombination within homologous immunoglobulin mu gene constant regions in a mouse hybridoma cell line

We have transferred a pSV2neo vector containing the wild-type constant region of the immunoglobulin mu gene (C mu) into the mutant hybridoma igm482, which bears a 2-bp deletion in the third constant-region exon of its haploid chromosomal mu gene (C mu 3). Independent igm482 transformants contain the wild-type immunoglobulin C mu region stably integrated in ectopic chromosomal positions. We report here that the wild-type immunoglobulin C mu region can function as the donor sequence in a gene conversion event which corrects the 2-bp deletion in the mutant igm482 chromosomal C mu 3 exon. The homologous recombination event restores normal immunoglobulin M production in the mutant cell.

Preferential usage of JH2 in D-J joinings with DQ52 is determined by the primary DNA sequence and is largely dependent on recombination signal sequences

Rearrangement of DQ52 with JH in IgH genes takes place in a large part of lymphocytes at a very early stage of differentiation although usage of DQ52 seems to be limited in the functional IgH genes of mature B cells. A possible and likely explanation for this, is secondary D-JH joinings occurring after the initial DQ52-JH joinings, which becomes possible only when DQ52 segments join 5'-located JH segments. Previously we reported the non-random use of JH in joinings with DQ52, in which DQ52-JH2 joinings are dominant. We questioned how the preferential usage of JH2 in DQ52-JH joinings is determined by utilizing extrachromosomal DNA substrates. Extrachromosomal DNA substrates with a combination of DQ52 and multiple JH segments were prepared. These plasmids with a polyoma-derived replication system were transfected into pre-B cell lines and recombination profiles of recovered plasmids were analyzed semi-quantitatively. Experiments with plasmids containing all four JH segments in normal configuration showed an apparent high frequency of JH2 usage, similar to the rearrangement previously observed in thymocyte genomes. In plasmids containing two JH segments, when one was JH2, it was rearranged more frequently than the other, independent of its proximity to DQ52. The JH3 with recombination signal sequences (RSS) substituted for JH2 was rearranged more frequently than the naive JH2 itself indicating the importance of RSS. A JH3 hybrid with the 200 bp upstream sequence of JH2 but excluding RSS, however, failed to increase JH3 use. The addition of one nucleotide to the 22-bp spacer of JH1 RSS increased the frequency of JH1 usage. These observations suggest that the preferential usage of JH2 is determined by the primary DNA sequence and is largely dependent on RSS.

Molecular approaches to the study of human B-cell and (auto)antibody repertoire generation and selection

We have shown that the restricted repertoire of VH genes expressed in second trimester human fetal liver is not solely determined by JH proximity. Furthermore, by following the fate of two VH gene segments in different B-cell repertoires, we have provided evidence that multiple factors contribute to the frequency with which individual VH genes are utilized. We found that the repertoire of adult blood IgM-bearing B cells contains a high proportion of B lymphocytes that express extensively mutated VH genes. Finally, we show that somatically-mutated variants of particular VH and VL genes that, in germline configuration, are frequently found in the early B-cell repertoire and in natural autoantibodies, encode pathogenic IgG autoantibodies characteristic of human SLE. These VH and VL genes harbor all the characteristics of an antigen-driven B-cell activation and selection process.

Autocrine regulation of murine B lymphocyte growth by an IgM antibody

Culture supernatants of LPS-stimulated murine B lymphocytes are able to inhibit the growth of freshly isolated splenic B cells via an IgM antibody. The binding specificity of this IgM is not yet defined, but appears to be a B lymphocyte surface structure distinct from membrane immunoglobulin, MHC class II antigen, transferrin and Fc gamma receptors, and B220. The regulatory autoantibody allows the normal progression of early, but not late steps in the cycle of polyclonally-stimulated B lymphocytes and does not affect the increased antigen-presenting capacity of activated B cells. Therefore, this autoregulatory cycle is apparently ubiquitous and may be a major component of B lymphocyte homeostasis under physiological, as well as pathological conditions. Moreover, these findings bring into focus a possible regulating role of B lymphocytes in the humoral immune response.

VDJ recombination

The ability of lymphocyte receptor V, D and J gene segments to rearrange generates much of the receptor diversity that is the hallmark of the immune system. Naturally, the mechanisms of immunoglobulin and T-cell receptor gene recombination are of enormous interest. Here, Fred Alt and colleagues review current understanding of the process and speculate on future findings.

Ongoing V kappa-J kappa recombination after formation of a productive V kappa-J kappa coding joint

V kappa genes of man can recombine with the J kappa gene segments either by an inversion or by a deletion mechanism. Back-to-back fusion products of the respective recombination signal sequences (signal joints) are retained on the chromosome after the formation of a V kappa-J kappa coding joint by an inversion. Our knowledge of the structure of the human kappa locus and the application of the polymerase chain reaction allowed us now to establish a direct relationship between different kappa recombination products in the lymphoid cell line JI. Two consecutive inversions fully explain the existence of two coding joints and two signal joints on the same chromosome of this cell line. Although the initially formed coding joint is productively rearranged and expressed, a second V kappa-J kappa rearrangement took place which leads to an aberrant joint. In this process a J kappa gene segment of the signal joint that had been created in the first V kappa-J kappa joining was used as the recombination target. The sequence of the two rearrangements is unequivocal since a product of the first (productive) reaction is a partner in the second (aberrant) one.

Interallelic V(D)J trans-rearrangement within the beta T cell receptor gene is infrequent and occurs preferentially during attempted D beta to J beta joining

Previous work has demonstrated that intergenic V(D)J rearrangement, a process referred to as trans-rearrangement, occurs at an unexpectedly high frequency. These rearrangements generate novel V(D)J combinations which could conceivably have some role in the normal immune system, and since they probably arise through chromosomal rearrangements akin to those associated with lymphoid neoplasia, they may also serve as a model for investigating recombinational events which underlie oncogenesis. In view of the existence of a mechanism that permits relatively frequent intergenic trans-rearrangements, it seems reasonable that interallelic trans-rearrangements involving segments belonging to each of the two alleles of a single antigen receptor gene might also occur. To determine the frequency of such rearrangements, we examined thymocytes of F1 progeny of a cross between SWR mice, which have a deletion spanning 10 of the known V beta segments, and NZW mice, which have a deletion involving all J beta 2 segments. Rearranged TCR-beta genes containing V beta segments from the NZW chromosome and J beta segments from the SWR chromosome were amplified from the DNA of F1 thymocytes with the polymerase chain reaction. Using this approach, we found that such rearrangements are relatively uncommon, being present in about 1 in 10(5) thymocytes, a frequency lower than that of V gamma/J beta intergenic trans-rearrangements. The ratio of conventional cis-rearrangement to interallelic trans-rearrangement for any particular V beta segment appears to be about 10(4):1. The structure of the junctions in all trans-rearrangements analyzed closely resembles conventional cis-rearrangements, indicating involvement of V(D)J recombinase in the ultimate joining event. However, in contrast to cis-rearrangements, a strong bias for inclusion of D beta 1 segments over D beta 2 segments was noted, suggesting that interallelic trans-rearrangement may occur preferentially during attempted D-J joining. J beta 2 segment usage in trans-rearrangements also appeared to differ from that expected from previously studied cis-rearrangements. The results have implications with respect to the events and timing of conventional cis-rearrangement during thymocyte differentiation, and the prevalence of various types of trans-rearrangements.

Differential expression of VH gene families in peripheral B cell repertoires of newborn or adult immunoglobulin H chain congenic mice

The pattern of VH gene family expression in the primary B cell repertoire of the mouse is strain dependent. In C57Bl/6 mice, the VH J558 family is expressed by more than 45% of the cells, while the expression of VH 7183, VH Q52, and VH 36-60 families together does not exceed 20%. In BALB/c mice, relative expression of VH J558 is lower than 35%, while the sum of the other three families reaches 25%. To assess which genetic loci control strain-specific VH gene family expression, we studied VH gene family usage in splenic B cell repertoires of different congenic strains of mice. Changes in major histocompatibility complex or immunoglobulin (Ig) K light chain genes did not modify VH gene family expression in adult mice. Differences at the IgH locus, however, modified VH gene family usage. In 1-d-old mice, the strain-specific VH gene family expression pattern is determined by the IgH haplotype. In adult mice, the VH gene family expression pattern of resting B cells is independent of the IgH locus and follows the genetic background of the congenic strain, while it is determined by the IgH haplotype among Ig-secreting spleen cells. In F1(B6 x BALB/c) mice, each of the two spleen B cell populations, sorted on the basis of mu heavy chain allotype expression, shows an independent VH gene family expression pattern, determined by the IgH locus. The implications of these results in the control of VH gene family expression, and in the selection of peripheral B cell repertoires are discussed.

Transcriptional regulatory elements stimulate recombination in extrachromosomal substrates carrying immunoglobulin switch-region sequences

We have developed a sensitive genetic assay to analyze DNA sequences and regulatory elements required for immunoglobulin heavy chain isotype switch recombination. Recombination substrates containing mu and gamma 3 chain switch (S)-region sequences, S mu and S gamma 3, are transiently introduced into primary murine B cells cultured with lipopolysaccharide to induce isotype switching. Recombination involving S-region sequences deletes a conditionally lethal marker, the leftward promoter of phage lambda (lambda PL), enabling recovered plasmids to transform Escherichia coli. In substrates carrying S mu-lambda PL-S gamma 3, about 2% of replicated molecules undergo deletion of lambda PL during transfection; insertion of either the immunoglobulin heavy chain promoter and enhancer sequences or cytomegalovirus IE1 promoter region upstream of S mu increases recombination 10-fold or more to 25% of replicated molecules. Guanosine-rich S-region sequences are essential for efficient recombination of these substrates.

Use of antibody fragments in immunoaffinity chromatography. Comparison of FV fragments, VH fragments and paralog peptides

Some new antibody fragments have recently been described: FV fragments (Mr 25,000), VH fragments or "dAbs" (12,500) and paralog peptides (1000-2000). FV fragments, VH fragments and a paralog peptide that had been derived from a parent antibody with a specificity for hen lysozyme were produced. All three reagents were immobilized on Sepharose and evaluated for their ability to recover hen lysozyme from "spiked" serum and to separate hen lysozyme from turkey lysozyme. The FV column had excellent specificity for hen lysozyme, the VH column had significantly reduced specificity and the paralog peptide column did not bind lysozyme at all.

Transcription of germ line V alpha segments correlates with ongoing T-cell receptor alpha-chain rearrangement

M14T is a virally transformed immature T-cell line which continues to rearrange its T-cell antigen receptor (TCR) alpha-chain genes in vitro and thus represents a dynamic system for studying TCR assembly. In an effort to investigate whether the TCR alpha locus is accessible for V(D)J rearrangement events, we examined M14T cells for the presence of germ line TCR alpha transcripts. Several unrearranged V alpha segments were found to be transcriptionally active in M14T cells. By comparison, germ line V alpha transcripts are absent in nonlymphoid and pro-T-cell lines and barely detectable in mature T-cell lines, suggesting that this phenomenon is likely stage and tissue specific. We demonstrate a perfect correlation between transcriptionally active V alpha segments and their involvement in ongoing V alpha-to-J alpha rearrangements. In addition, data suggesting that the unrearranged J alpha locus is also transcriptionally active in the M14T line are presented. Furthermore, the recombination-activating genes RAG-1 and RAG-2 are differentially expressed, with RAG-2 detectable only by polymerase chain reaction, implying that very low levels of one of these gene products are sufficient to complement the other to facilitate VJ rearrangements. These findings provide the first direct evidence for an accessibility model of antigen receptor rearrangement in T cells.

Early and late B-cell development in the mouse

A common principle in B-cell development is the stringent selection of cells expressing appropriate antibody V regions as surface receptors. Cells failing to do so appear destined to rapid death. These life-death decisions are mediated by signals whose nature is not yet understood but whose generation involves immunoglobulin receptor complexes on B cells and B-cell progenitors.

Transcription of germ line V alpha segments correlates with ongoing T-cell receptor alpha-chain rearrangement

M14T is a virally transformed immature T-cell line which continues to rearrange its T-cell antigen receptor (TCR) alpha-chain genes in vitro and thus represents a dynamic system for studying TCR assembly. In an effort to investigate whether the TCR alpha locus is accessible for V(D)J rearrangement events, we examined M14T cells for the presence of germ line TCR alpha transcripts. Several unrearranged V alpha segments were found to be transcriptionally active in M14T cells. By comparison, germ line V alpha transcripts are absent in nonlymphoid and pro-T-cell lines and barely detectable in mature T-cell lines, suggesting that this phenomenon is likely stage and tissue specific. We demonstrate a perfect correlation between transcriptionally active V alpha segments and their involvement in ongoing V alpha-to-J alpha rearrangements. In addition, data suggesting that the unrearranged J alpha locus is also transcriptionally active in the M14T line are presented. Furthermore, the recombination-activating genes RAG-1 and RAG-2 are differentially expressed, with RAG-2 detectable only by polymerase chain reaction, implying that very low levels of one of these gene products are sufficient to complement the other to facilitate VJ rearrangements. These findings provide the first direct evidence for an accessibility model of antigen receptor rearrangement in T cells.

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.

Autoimmune VH gene family: PCR-generated murine germline VH10 genes

A relatively large number of variable region genes (V) contribute, via gene rearrangements with smaller numbers of additional gene elements (D and J), to generate diversity in the immune response. While some VH gene families are thought to contain 100- 1000 members, the VH10 family has only two known functioning members with 99% sequence homology. Both members (monoclonal antibodies) are capable of binding DNA, and since they were derived from inbred mice afflicted with the lupus syndrome they are considered autoimmune antibodies. Relative uniqueness of the VH10 primary nucleotide sequence presents a model system with which to examine unrearranged VH genes and attempt to identify germline genes eventually expressed as autoantibodies. PCR amplified germline sequences of the VH10 family are highly conserved, with few base substitutions evenly distributed between both framework and CDR regions. It was determined that the PCR amplified germline sequences are highly similar to the DNA sequences of the two monoclonal VH10 antibodies, and a non-functional psuedo-germline gene was found that is identical to a non-functional cDNA derived from a hybridoma cell line. These findings indicate that the use of unique CDR DNA sequences for the identification and amplification of specific germline V genes via PCR can yield vital information that may answer fundamental questions about the origins of autoimmune anti-DNA antibodies in afflicted individuals. The nature of the germline gene populations and the possible microheterogeniety of these genes may prove to be important in understanding the role of autoimmune antibodies in normal and diseased individuals.

Immunoglobulin light chain variable region gene sequences for human antibodies to Haemophilus influenzae type b capsular polysaccharide are dominated by a limited number of V kappa and V lambda segments and VJ combinations

The immune repertoire to Haemophilus influenzae type b capsular polysaccharide (Hib PS) appears to be dominated by certain light chain variable region genes (IgVL). In order to examine the molecular basis underlying light chain bias, IgVL genes have been cloned from a panel of heterohybridomas secreting human anti-Hib PS (antibody) (anti-Hib PS Ab). One hybridoma, representative of the predominant serum clonotype of anti-Hib PS Ab in older children and adults following immunization or Hib infection, uses a V kappa II segment identical to the germline gene A2, and a JK3 segment. A second kappa hybridoma uses a member of the V kappa I family and a JK4 segment. Four lambda antibodies, all cross-reactive with the structurally related antigen Escherichia coli K100 PS, use V lambda VII segments which are 96-98% homologous to one another, and may originate from a single germline gene. Two additional lambda antibodies, not K100-cross-reactive, are encoded by members of the V lambda II family. All lambda antibodies use highly homologous J lambda 2 or J lambda 3 segments. The VJ joints of all lambda antibodies and the V kappa II-encoded antibody are notable for the presence of an arginine codon, suggesting an important role in antigen binding. Although more complex than heavy chain variable region gene usage, a significant portion of serum anti-Hib PS Ab is likely to be encoded by a limited number of V kappa and V lambda segments and VJ combinations, which may be selectively expressed during development, or following antigen exposure.

An exceptional mouse plasmacytoma with a new kappa/N-myc [T(6; 12) (C1; B)] translocation expresses N-myc but not c-myc

Mouse plasmacytomas (MPC) carry one of three reciprocal translocations that juxtapose c-myc to one of the three immunoglobulin (Ig) loci. Here we describe an exceptional MPC, induced by pristane oil and Abelson (A-MuLV) virus. It does not carry any of the three c-myc/Ig translocations, but contains a previously unknown reciprocal T(6;12) translocation affecting the bands known to carry the IgK (6C/1) and N-myc (12B) loci, respectively. Northern blot analysis showed high N-myc but no c-myc expression. This is consistent with the constitutive activation of N-myc by a juxtaposition of the IgK and N-myc loci. Reciprocal translocation in B-cell derived tumors are believed to involve the Ig loci by the action of some enzyme that participates in the physiological rearrangement of the Ig loci. Only transcriptionally active chromatin regions are accessible to such recombinases (Alt et al. 1987). N-myc is not expressed in B-cells, but it is transcriptionally active during the early pro- and pre-B cell stage, whereafter it and the surrounding chromatin region becomes inactive (Smith et al. 1992). It is therefore most likely that the N-myc/Kappa translocation has arisen at an early stage of B-cell differentiation. This would imply that the myc/Ig translocations do not block B-cell differentiation. They also reaffirm the functional equivalence of N- and c-myc in relation to B-cell carcinogenesis, as shown by our previous work on tumor induction in N-myc transgenic mice (Wang et al. 1992).

Novel excision products of T cell receptor gamma gene rearrangements and developmental stage specificity implied by the frequency of nucleotide insertions at signal joints

We have cloned circular DNA excised by T cell receptor (TcR) gamma 1, gamma 2 and gamma 3 gene rearrangements in fetal and adult mouse thymocytes. Circular DNA contained a signal joint reciprocal to the genomic V-J coding joint. Although signal joints without nucleotide insertions are common in immunoglobulin (Ig) and TcR gene rearrangements, the signal joint of gamma found in adult thymocytes contained non-germ-line element (N) insertions at high frequency, while no insertions were found in fetal thymocytes. Thus developmental stage specificity of TcR gamma gene rearrangements is faithfully reflected on the signal joint of excision products. In addition, examination of gamma gene excision products revealed circular DNA products of TcR gamma-alpha transrearrangements, but no evidence of V gamma gene replacement in a rearranged segment.

Comparative expression of adult and fetal V gene repertoires

A hallmark of the immune system is the extraordinary diversity associated with antibodies. This is made possible by a series of genetic rearrangements involving variable region gene segments. Considerable detail is known about these genetic mechanisms except for the enzymatic machinery involved. An important question in studies of the generation of diversity is whether V genes are selected for rearrangement mainly in a random manner or selected by particular developmental rules. Past studies have indicated that the acquisition of fetal and neonatal specificity repertoires is a nonrandom process. In this report, we review our studies that directly compare the adult and fetal/neonatal V gene repertoires. The evidence suggests that the adult repertoire is more diverse with indications of a random use of VH gene families. However, whether V genes are indeed randomly used in the adult remains to be clarified at the VH gene member level. The fetal repertoire, on the other hand, appears nonrandom in V gene usage. In addition, the fetal repertoire is mostly germline encoded with little evidence of junctional diversity. Taken together, the results indicate different rules for generation of the adult and fetal repertoires, findings most likely explain by distinct B cell subsets and B cell progenitors at early stages in ontogeny.

Natural genetic engineering in evolution

The results of molecular genetics have frequently been difficult to explain by conventional evolutionary theory. New findings about the genetic conservation of protein structure and function across very broad taxonomic boundaries, the mosaic structure of genomes and genetic loci, and the molecular mechanisms of genetic change all point to a view of evolution as involving the rearrangement of basic genetic motifs. A more detailed examination of how living cells restructure their genomes reveals a wide variety of sophisticated biochemical systems responsive to elaborate regulatory networks. In some cases, we know that cells are able to accomplish extensive genome reorganization within one or a few cell generations. The emergence of bacterial antibiotic resistance is a contemporary example of evolutionary change; molecular analysis of this phenomenon has shown that it occurs by the addition rearrangement of resistance determinants and genetic mobility systems rather than by gradual modification of pre-existing cellular genomes. In addition, bacteria and other organisms have intricate repair systems to prevent genetic change by sporadic physicochemical damage or errors of the replication machinery. In their ensemble, these results show that living cells have (and use) the biochemical apparatus to evolve by a genetic engineering process. Future research will reveal how well the regulatory systems integrate genomic change into basic life processes during evolution.