Ordered rearrangement of immunoglobulin heavy chain variable region segments

Ontogeny of the Heavy Chain Immunoglobulin Repertoire in Fetal Liver and Bone Marrow

We studied the kinetics of maturation of B cell progenitors in the mouse embryo, from day 15 of development to birth, both in liver and bone marrow. The analysis of Ig heavy chain rearrangements at different time points of late fetal development shows that oligoclonal patterns of VH-D-JH rearrangements are detected by day 15 in fetal liver. The pattern is polyclonal and diverse by day 17; however, 80% of the rearrangements are nonproductive. In bone marrow, the pattern of rearrangements is less diverse at birth, although the percentage of nonproductive rearrangements approaches adult bone marrow levels (35–40%). After day 17 in fetal liver, there is a sudden reversal in the percentage of nonproductive rearrangements that reaches 33% at day 19 (birth). Maturation of B cells, as measured by the fraction of surface Ig+ in total B220+ cells and the presence of N sequence additions in VH-D-JH joints, occurs in the marrow before fetal liver. These results demonstrate that the lymphopoietic environment in fetal liver and bone marrow of animals at the same stage of development is functionally distinct.

Regulation of B lymphocyte development by the truncated immunoglobulin heavy chain protein Dmu

The development of B lymphocytes from progenitor cells is dependent on the expression of a pre-B cell-specific receptor made up by a mu heavy chain associated with the surrogate light chains, immunoglobulin (Ig)alpha, and Igbeta. A variant pre-B cell receptor can be formed in which the mu heavy chain is exchanged for a truncated mu chain denoted Dmu. To investigate the role of this receptor in the development of B cells, we have generated transgenic mice that express the Dmu protein in cells of the B lineage. Analysis of these mice reveal that Dmu expression leads to a partial block in B cell development at the early pre-B cell stage, probably by inhibiting VH to DHJH rearrangement. Furthermore, we provide evidence that Dmu induces VL to JL rearrangements.

Use of D gene segments with irregular spacers in terminal deoxynucleotidyltransferase (TdT)+/+ and TdT-/- mice carrying a human Ig heavy chain transgenic minilocus

D gene segments with irregular spacers (DIR) are D gene segments that are specific to higher primates. Their use is controversial because of their G+C-rich long sequences. In the human, it has always been tempting to assume that a complementarity-determining region 3 sequence has been added by terminal deoxynucleotidyltransferase (TdT) activity and is not derived from DIR recombination. Herein, we examine the use of human DIR gene segments by cross-breeding the human Ig heavy chain minilocus pHC1 transgenic mice and TdT-deficient mice. In the absence of TdT and with a defined set of human D gene segments, it is relatively easy to demonstrate that DIR2 is used to form human Ig heavy chains, contributing to 7% of the human heavy chain rearrangements. VHDJH rearrangements (where H is heavy chain) in the minilocus TdT-/- mice use small portions of DIR2 located throughout the coding sequence. These results constitute the strongest evidence to date that DIR gene segments are used to form human antibodies. Additionally, we show that direct and inverted DIR2JH and VHDIR2 rearrangements occur in the minilocus transgenic mice. During these rearrangements, DM2 3' signal sequence and a new DIR2 5' signal sequence are used. These rearrangements generally follow the 12/23 recombination rule. Our results at the VHDJH, DJH, and VHD levels indicate that DIR2 is used to form human heavy chains in transgenic mice. The rearrangement of this gene segment likely involves, however, other mechanisms in addition to the classical VHDJH recombination.

Precursor B cells showing H chain allelic inclusion display allelic exclusion at the level of pre-B cell receptor surface expression

Within the pools of muH chain-producing precursor and mature B cells from normal and lambda5-defective mice, the frequency of cells in which both H chain alleles were productively VHDJH rearranged was determined. An equally high percentage (2%-4%) of cells carrying two productively VHDJH-rearranged H chain loci was found in precursor and mature B cell pools of both mouse strains. In all of these cells, one allele encodes a muH chain incapable of forming a surface-expressed pre-B cell receptor. Hence, allelic exclusion is maintained at the level of pre-B cell receptor expression. The surprising conservation of H chain allelic exclusion in lambda5-defective B cells suggests that an alternative form of pre-B cell receptor might function to ensure this allelic exclusion.

Biochemical and Functional Analyses of Chromatin Changes at the TCR-β Gene Locus During CD4−CD8− to CD4+CD8+ Thymocyte Differentiation

Allelic exclusion is the process wherein lymphocytes express Ag receptors from only one of two possible alleles, and is effected through a feedback inhibition of further rearrangement of the second allele. The feedback signal is thought to cause chromatin changes that block accessibility of the second allele to the recombinase. To identify the putative chromatin changes associated with allelic exclusion, we assayed for DNase I hypersensitivity, DNA methylation, and transcription in 100 kb of the TCR-β locus. Contrary to current models, we identified chromatin changes indicative of an active and accessible locus associated with the occurrence of allelic exclusion. Of 11 DNase I hypersensitive sites identified, 3 were induced during CD4−CD8− to CD4+CD8+ thymocyte differentiation, and demethylation and increased germline transcription of the locus were evident. We further examined the role of the most prominently induced site near the TCR-β enhancer (Eβ) in allelic exclusion by targeted mutagenesis. Two other sites were also examined in New Zealand White (NZW) mice that have a natural deletion in the TCR-β locus. TCR-β gene recombination and allelic exclusion were normal in both mutant mice, negating dominant roles for the three hypersensitive sites in the control of allelic exclusion. The data suggest that alternative cis-regulatory elements, perhaps contained in the Eβ enhancer and/or in the upstream Vβ region, are involved in the control of TCR-β allelic exclusion.

The specific variable domain of camel heavy-chain antibodies is encoded in the germline

The variable domains of the functional heavy-chain antibodies (VHHs) discovered in camels are related to the human VH subgroup III. They are nevertheless clearly distinguishable from the VHs of conventional four-chain immunoglobulins by the presence of important amino acid substitutions, located in the solvent-exposed surface normally covered by the variable domain of the light chain. The analysis of an unrearranged dromedary DNA library revealed that the specific VHH gene with its characteristic amino acid substitutions is encoded in the germline. Therefore, it is concluded that the VHHs do not arise through an ontogenic process of somatic hypermutation. The presence of putative DNA recombination signals that are more prevalent in the camel VHH, compared to the VH germline gene, might play a role in the formation and efficient expansion of the VHH repertoire.

The B-cell biology of aging

Although both the number and responsiveness of peripheral B cells in aged mice remain relatively intact, there are dramatic changes in B-cell generation. Alterations in B-cell development include both a skewing of V-gene utilization, especially in cells responsive to phosphorylcholine, and a decrease in the generation of various developmental B-cell subsets. The altered representation of these subsets appears to be the consequence of two developmental blocks. The first developmental block occurs during the maturation of pro-B cells and is evidenced by a decrease in the number of pre-B cells. The second developmental block occurs at the earliest stage of sIg(+)-cell maturation (sIgMvery lo). Because of this block in B-cell maturation, in spite of a decrease in incoming pre-B cells, the number of sIgMvery lo cells appears to increase in aged mice. Additionally, the time of residence of cells within this maturational stage increases dramatically, while the proportion of cells in more mature (sIgMhi) stages of bone marrow development are decreased. In addition to the decreased number of maturing bone marrow B cells, the population of splenic B cells that represent recent bone marrow émigrés (HSAvery hi) is markedly decreased. In the face of this decrease in newly emerging cells from the bone marrow, the population of mature splenic B cells is maintained by their increased longevity.

Receptor editing in a transgenic mouse model: site, efficiency, and role in B cell tolerance and antibody diversification

Mice carrying transgenic rearranged V region genes in their IgH and Igkappa loci to encode an autoreactive specificity direct the emerging autoreactive progenitors into a pre-B cell compartment, in which their receptors are edited by secondary Vkappa-Jkappa rearrangements and RS recombination. Editing is an efficient process, because the mutant mice generate normal numbers of B cells. In a similar nonautoreactive transgenic strain, neither a pre-B cell compartment nor receptor editing was seen. Thus, the pre-B cell compartment may have evolved to edit the receptors of autoreactive cells and later been generally exploited for efficient antibody diversification through the invention of the pre-B cell receptor, mimicking an autoreactive antibody to direct the bulk of the progenitors into that compartment.

Early Onset of Immunoglobulin Heavy Chain Gene Rearrangements in Normal Human Bone Marrow CD34+ Cells

To characterize early B-cell precursors in humans, we correlated immunoglobulin heavy chain (IgH) gene rearrangement status with the CD34, CD19, and CD10 cell surface markers. Highly purified adult bone marrow (BM) cell fractions were obtained by two successive rounds of flow cytometric cell sorting, and IgH rearrangements were analyzed by polymerase chain reaction (PCR) amplification. Complete VDJH rearrangements were observed in the CD34+ CD19+ fraction, but not in the more immature CD34+ CD19− fraction. About one quarter of these rearrangements had an open reading frame, thus potentially permitting the synthesis of a μ chain. Partial DJH rearrangements were detected in both CD34+ CD19+ and CD34+ CD19− subsets, although they were less abundant in the latter. When triple labeling was used to better characterize the CD34+ CD19− population, DJH rearrangements were found to be present in the CD34+ CD10+ CD19− fraction, but not in the more primitive CD34+ CD10− CD19−. These results indicate that IgH gene rearrangements occur in CD34+ BM cells and that they initiate in immature progenitors expressing the CD10, but not yet the CD19 surface antigen. Finally, the presence of IgH gene rearrangements in CD34+ BM cells provides a useful marker of clonality to evaluate the possible involvement of these cells in various B-cell lymphoid malignancies.

V(D)J recombination in mature B cells: a mechanism for altering antibody responses

The clonal selection theory states that B lymphocytes producing high-affinity immunoglobulins are selected from a pool of cells undergoing antibody gene mutation. Somatic hypermutation is a well-documented mechanism for achieving diversification of immune responses in mature B cells. Antibody genes were also found to be modified in such cells in germinal centers by recombination of the variable (V), diversity (D), and joining (J) segments. The ability to alter immunoglobulin expression by V(D)J recombination in the selective environment of the germinal center may be an additional mechanism for inactivation or diversification of immune responses.

Immunoglobulin heavy chain junctional diversity in young and aged humans

The causes of observed deficiencies to the humoral immune response in aged humans are unknown. Since a major source of antibody diversity is generated at the VH-D-JH junctional regions of the immunoglobulin heavy chain, we determined whether differences in junctional diversity are manifested with aging. We compared the CDR3 regions of IgM heavy chain transcripts isolated from young adult and aged humans. A PCR assay that measures CDR3 length in the majority of mu-heavy chains showed the same average size and normal range of CDR3 length in aged individuals as observed in young adults. To characterize the features of junctional diversity of aged adults in more detail, we determined the CDR3 sequences of a subset of the mu-heavy chain repertoire that utilizes members of the VH 5 family. In general CDR3 length, D family usage, and JH gene usage were similar in aged compared to young adults. Thus, in contrast to dramatic changes in heavy chain junctional diversity associated with fetal to adult development, no major differences were found between young and aged adults. Since the CDR3 repertoire generated in aged individuals appears to be as diverse as that observed in younger adults, the decline in humoral immunocompetence with aging cannot be attributed to a restriction in heavy chain junctional diversification processes.

Changes in the V(H) gene repertoire of developing precursor B lymphocytes in mouse bone marrow mediated by the pre-B cell receptor

The V(H) repertoire on both H chain alleles of normal and lambda5-deficient B lineage cells were analyzed by single-cell PCR. The mu H chains were tested for their capacity to form a pre-B cell receptor. In bone marrow, D-proximal V(H) genes were found preferentially expressed in lambda5-deficient pre-B cells and in a newly identified early c-kit+ cytoplasmic mu H chain+ pre-B cell population of normal mice. Only half of the mu H chains expressed in these cells have the capacity to form a pre-B cell receptor. Representation of the D-proximal V(H) genes was found suppressed on the productive but not on the nonproductive V(H)DJ(H) rearranged alleles of c-kit preB-II cells and splenic lambda5-deficient B cells. More than 95% of the mu H chains expressed in preB-II cells can form a pre-B cell receptor. These results demonstrate that the pre-B cell receptor in normal mice and the B cell receptor in lambda5-deficient mice mediate a shift in the V(H) repertoire.

T cell receptor (TCR)-beta gene recombination: dissociation from cell cycle regulation and developmental progression during T cell ontogeny

T cell lymphopoiesis involves extensive cell division and differentiation; these must be balanced by export and programmed cell death to maintain thymic homeostasis. Details regarding the nature of these processes, as well as their relationships to each other and to the definitive process of T cell receptor (TCR) gene recombination, are presently emerging. Two widely held concepts are that cell cycle status is inherently and inversely linked to gene recombination and that the outcomes of gene recombination regulate developmental progression. In this study, we analyze TCR-beta recombination and cell cycle status with respect to differentiation during early T cell ontogeny. We find that although differentiation, cell cycle fluctuations, and gene recombination are coincident during normal T cell development, differentiation and cell cycle status are not inherently linked to the recombination process or its products. Rather, recombination appears to occur in parallel with these events as part of a genetically patterned program of development. We propose that the outcome of gene recombination (i.e., TCR expression) may not influence developmental progression per se, but instead serves to perpetuate those developing cells that have been successful in recombination. The potential consequences of this model for the regulation of thymic lymphopoiesis and programmed cell death are discussed.

Lack of allelic exclusion in B cell chronic lymphocytic leukemia

We determined the immunoglobulin (Ig) V(H) subgroup expressed by the leukemia cells of 108 patients with B cell chronic lymphocytic leukemia (CLL). Surprisingly, we found that six samples (5%) each expressed Ig of more than one V(H) subgroup. Southern blot analysis demonstrated that these samples each had rearrangements involving both Ig heavy chain alleles. Nucleic acid sequence analyses of the Ig cDNA revealed each to express two functional Ig V(H) genes: V(H)3-33 and V(H)4-39; V(H)3-7 and V(H)4-39; V(H)3-23 and V(H)4-61; V(H)2-70 and V(H)3-30.3; or V(H)3-30 and V(H)4-b (DP67). One sample expressed three Ig V(H) genes: V(H)2-70, V(H)3-7, and V(H)4-59. Despite having more than one Ig heavy chain transcript, each sample was found to express only one functional Ig light chain. From the primary sequence, we deduced that the Ig of some of these CLL samples should react with Lc1, a monoclonal antibody (mAb) reactive with a supratypic cross-reactive idiotype present on Ig encoded by a subgroup of Ig V(H)4 genes (namely, V(H)4-39, V(H)4-b [DP-67], V(H)4-59, or V(H)4-61), and B6, an mAb that reacts with Ig encoded by certain Ig V(H)3 genes (namely, V(H)3-23, V(H)3-30, or V(H)3-30.3), and/or modified staphylococcal protein A (SpA), a 45-kilodalton bacterial "superantigen" that reacts with most Ig of the V(H)3 subgroup. Flow cytometric analyses revealed that such samples did in fact react with Lc1 and B6 and/or SpA, but not with control mAbs of irrelevant specificity. This study demonstrates that a subset of CLL patients have leukemic B cells that express more than one functional Ig heavy chain.

The roles of preB cell receptor in early B cell development and its signal transduction

The preB cell receptor is expressed for a short period after mu heavy chain is produced, that is, at the large preB cell stage in B cell development. The severe impairment of B cell differentiation observed in mice deficient for the preB cell receptor clearly demonstrated the importance of the preB cell receptor in B cell development. Analyses of bone marrow precursor B cells in normal and B cell-deficient mutant mice indicated the preB cell receptor transduced signals to drive cell cycle and to induce allelic exclusion. The proliferation of the preB cell receptor-expressing cells leads to the selective expansion of cells which have succeeded in the productive rearrangement of mu heavy chain gene. This process builds up a preB cell pool large enough to generate sufficient numbers of mature B cells. The preB cell receptor appears to induce allelic exclusion by shutting off the expression of recombinase activation gene (RAG). In order to analyse the signal transduction pathway downstream of the preB cell receptor, we have developed a new system in which cross-linking of Ig beta expressed on bone marrow proB cells mimics the signalling through the preB cell receptor to induce differentiation from proB to small preB cells.

Control of immunoglobulin gene rearrangements in developing B cells

The antigen receptor on B lymphocytes is the product of a series of gene rearrangements which ends when a functional receptor gene is assembled. Recent work has shown that the receptor-associated molecules Ig alpha and Ig beta provide the signals that lead to inhibition of further recombination. Furthermore, Ig beta has been implicated in initiating the last step of the recombination reaction.

Transcription of germline VH gene elements by normal human fetal liver

Transcription of the gene elements that form the variable region of immunoglobulin heavy chains has been proposed to represent the process that controls access for the recombination enzymes in their sequential steps of catalysis. Evidence for germline transcription of VH gene elements, as part of VH to DJH recombination, has been limited to transcripts of only a few gene elements. We have examined normal fetal liver mRNA by Northern blotting and present evidence for germline transcripts from six human VH gene families. The candidate VH4 transcripts have been confirmed as germline transcripts by hybridization with 3' flanking sequences that would have been removed by recombination from mature VHDJH genes. The candidate transcripts for VH1, VH3, VH4 and VH6 have been confirmed by polymerase chain reaction amplification with primers from the 3' flanking sequences of these gene families and determination of the sequence of these products. Determination of sequence from two clones of VH1, VH3 and VH4 indicates that more than one gene from each of these families is transcribed. PCR amplification of VH4 and VH6 with primers specific for the leader sequence (exon 1) and 3' flanking sequence indicate that these transcripts are spliced, representing RNA processing. Germline transcripts from these families are also present in normal human bone marrow. These results indicate that transcriptional activation of germline VH gene elements is a general phenomenon in tissues undergoing V to DJ recombination.

Immunoglobulin VH gene replacements in a T-cell lymphoblastic lymphoma

We have analysed the rearrangement status of the immunoglobulin heavy (IgH) chain locus during progression of a T-cell lymphoblastic lymphoma displaying multiple IgH rearrangements as demonstrated by variable heavy (VH) gene family specific polymerase chain reaction (PCR) analysis. The tumor was found to undergo diversification at the IgH locus between diagnosis and relapse through a mechanism of VH to VHDJH replacement. In subsets of the tumor at relapse, two separate VH gene segments were found to have replaced the VH gene utilized by a VHDJH rearrangement identified at diagnosis. The observed VH gene replacement events appear to have been mediated by a heptamer sequence homologous to the heptamer of the recombination signal sequence (RSS) located internally in the VH gene segment. These results support the notion that VH replacements contribute to the diversification of immunoglobulin genes.

Changes in locus-specific V(D)J recombinase activity induced by immunoglobulin gene products during B cell development

The process of V(D)J recombination is crucial for regulating the development of B cells and for determining their eventual antigen specificity. Here we assess the developmental regulation of the V(D)J recombinase directly, by monitoring the double-stranded DNA breaks produced in the process of V(D)J recombination. This analysis provides a measure of recombinase activity at immunoglobulin heavy and light chain loci across defined developmental stages spanning the process of B cell development. We find that expression of a complete immunoglobulin heavy chain protein is accompanied by a drastic change in the targeting of V(D)J recombinase activity, from being predominantly active at the heavy chain locus in pro-B cells to being exclusively restricted to the light chain loci in pre-B cells. This switch in locus-specific recombinase activity results in allelic exclusion at the immunoglobulin heavy chain locus. Allelic exclusion is maintained by a different mechanism at the light chain locus. We find that immature, but not mature, B cells that already express a functional light chain protein can undergo continued light chain gene rearrangement, by replacement of the original rearrangement on the same allele. Finally, we find that the developmentally regulated targeting of V(D)J recombination is unaffected by enforced rapid transit through the cell cycle induced by an E mu-myc transgene.

Both E12 and E47 allow commitment to the B cell lineage

The E2A gene products, E12 and E47, are required for proper B cell development. Mice lacking the E2A gene products generate only a very small number of B220+ cells, which lack immunoglobulin DJ(H) rearrangements. We have now generated mice expressing either E12 or E47. B cell development in mice expressing E12 but lacking E47 is perturbed at the pro-B cell stage, and these mice lack IgM+B220+ B cells in both bone marrow and spleen. IgM+B220+ B cells can be detected, albeit at significantly reduced levels, in the bone marrow and spleen of mice lacking E12. Ectopic expression of both E12 and E47 in a null mutant background shows that E12 and E47 act in concert to promote B lineage development. Taken together, the data indicate that both E12 and E47 allow commitment to the B cell lineage and act synergistically to promote B lymphocyte maturation.

Constitutive Bcl-2 expression during immunoglobulin heavy chain-promoted B cell differentiation expands novel precursor B cells

To test for effects on B cell differentiation, we introduced immunoglobulin mu heavy chain (HC) and Bcl-2 transgenes, separately or together, into recombination-activating gene 2 (RAG-2)-deficient mice. Transgenic Bcl-2 expression led to increased numbers of RAG-deficient pro-B cells, but did not promote their further differentiation. Expression of the mu HC transgene promoted the differentiation of RAG-deficient pro-B cells into pre-B cells that also expressed certain differentiation markers characteristic of even more mature B cell stages. However, the extent of the mu HC-dependent differentiation effects was greatly enhanced by coexpression of the transgenic Bcl-2 gene, and a subset of pre-B cells from both HC and HC, Bcl-2-transgenic RAG-2-deficient animals expressed surface mu HCs that were functional as judged by cross-linking experiments. These experiments demonstrate that the pro-B to pre-B transition in vivo cannot be effected by the expression of Bcl-2 alone, and that nontransformed immature B-lineage cells are competent to receive signals through a surface mu complex.

Counterselection against D mu is mediated through immunoglobulin (Ig)alpha-Igbeta

The pre-B cell receptor is a key checkpoint regulator in developing B cells. Early events that are controlled by the pre-B cell receptor include positive selection for cells express membrane immunoglobulin heavy chains and negative selection against cells expressing truncated immunoglobulins that lack a complete variable region (D mu). Positive selection is known to be mediated by membrane immunoglobulin heavy chains through Ig alpha-Ig beta, whereas the mechanism for counterselection against D mu has not been determined. We have examined the role of the Ig alpha-Ig beta signal transducers in counterselection against D mu using mice that lack Ig beta. We found that D mu expression is not selected against in developing B cells in Ig beta mutant mice. Thus, the molecular mechanism for counterselection against D mu in pre-B cells resembles positive selection in that it requires interaction between mD mu and Ig alpha-Ig beta.

Clonal selection and learning in the antibody system

Each antibody-producing B cell makes antibodies of unique specificity, reflecting a series of ordered gene rearrangements which must be successfully performed if the cell is to survive. A second selection process occurs during immune responses in which a new antibody repertoire is generated through somatic hypermutation. Here only mutants binding antigen with high affinity survive to become memory cells. Cells expressing autoreactive receptors are counter-selected at both stages. This stringent positive and negative selection allows the generation and diversification of cells while rigorously controlling their specificity.

Cell type-specific chromatin structure determines the targeting of V(D)J recombinase activity in vitro

A common V(D)J recombinase that recognizes a conserved recombination signal sequence (RSS) mediates the assembly of immunoglobulin (Ig) and T cell receptor (TCR) genes in B and T cell precursors. The rearrangement of particular Ig and TCR gene segments, however, is tightly regulated with respect to cell lineage and developmental stage. Using an in vitro system, we analyzed recombinase cleavage of RSSs flanking Ig and TCR gene segments in nuclei. We found that both the lineage-specificity and temporal ordering of gene rearrangement is reflected in the accessibility of RSSs within chromatin to in vitro cleavage.

Regulation of an early developmental checkpoint in the B cell pathway by Ig beta

Many of the cell fate decisions in precursor B cells and more mature B cells are controlled by membrane immunoglobulin (Ig)M heavy chain (mu) and the Ig alpha-Ig beta signal transducers. The role of Ig beta in regulating early B cell development was examined in mice that lack Ig beta (Ig beta-/-). These mice had a complete block in B cell development at the immature CD43+B220+ stage. Immunoglobulin heavy chain diversity (DH) and joining (JH) segments rearranged, but variable (VH) to DJH recombination and immunoglobulin messenger RNA expression were compromised. These experiments define an unexpected, early requirement for Ig(beta) to produce B cells that can complete VDJH recombination.

The transition of pre-BI to pre-BII cells is dependent on the VH structure of the mu/surrogate L chain receptor

We have demonstrated previously that the majority ( > 90%) of VH12 B cells are absent from the adult peripheral repertoire, and that most that remain have the fourth position at the D-J function (designated 10/G4). We report here that most VH 12-expressing pre-B cells are lost during the transition from the pre-BI to the pre-BII cell stage in normal mice, and that pre-BII cell productive (P) rearrangements ar enriched in 10/G4 CDR3. This coincides with the initial expression of H chain and the generation of the mu/surrogate L chain (SL) receptor. In contrast, there is not enrichment for 10/G4 CDR3 in mu MT mice, and the frequency of P rearrangements is as expected from a random rearrangement mechanism, ruling out a biased rearrangement mechanism unique to VH12. We have also demonstrated that non-10/G4 mu chains can associate with SL and be expressed on the cell surface, suggesting that they are available on the cell surface for selection. Thus, transition of pre-BI to pre-BII cells is dependent on the structure of the VH domain.

Non-stochastic utilization of Ig V region genes in unselected human peripheral B cells

Limited evidence based on a few subjects suggests that human peripheral blood B cells may express a non-stochastic assortment of V region genes. To determine if non-stochastic utilization was a generally applicable rule, the identities of rearranged V region gene segments were determined in unselected peripheral blood B cells from 12 subjects (five male, seven female), ranging in age from 35 to 72 years. The analysis was limited to V region genes belonging to the VH3 gene family. More than 4500 independent VH3-containing rearrangements were analysed. The frequency of occurrence of eight individual VH3 gene segments contained in rearrangements was assessed using gene specific oligonucleotide probes. Usage of elements was not uniform. Three elements, which have been known to encode autoantibodies as well as to be frequently rearranged during fetal development, were represented among rearrangements more frequently than were other members of the VH3 family, and in aggregate, accounted for the majority of rearrangements. These three predominant loci are clustered in an 80 kb region suggesting an influence of chromosomal location on efficiency of rearrangement. The results document a clear, statistically significant, preference for the occurrence of specific V region genes among rearrangements. The modest amount of variation observed between subjects was not associated with either age or gender. Duplications which increased gene dose may have contributed to increased gene usage. These data indicate that, in caucasians, the immunoglobulin rearrangements in adult human B cells are dominated by a few heavy chain V region genes to the exclusion of other putatively equally functional genes. Thus, the conventional notion that the adult repertoire is normalized with respect to family complexity is not confirmed by analysis of individual VH genes.

Surrogate light chain expression is required to establish immunoglobulin heavy chain allelic exclusion during early B cell development

Allelic exclusion at the IgH locus was examined in B lineage cells of wild-type mice and mice unable to express the surrogate light chain molecule lambda 5 using a single-cell PCR approach. By analyzing B precursor cells containing two VHDHJH rearrangements, we found that in wild-type animals, cells are allelically excluded as soon as mu chains are expressed. Furthermore, we provide evidence that in cells expressing D mu proteins VH-->DHJH rearrangement is inhibited. In contrast, in the absence of lambda 5 protein, B precursor cells were allelically "included", indicating that allelic exclusion at the IgH locus requires expression of the pre-B cell receptor either containing a mu chain or a D mu chain. However, although mu chain double-producing B precursor cells are generated in lambda 5-deficient mice, such cells were not detected among surface immunoglobulin positive B cells.

Cellular and molecular analysis of lymphoid development using Rag-deficient mice

The establishment of a functional immune system with diverse antigen receptors is dependent on the V(D)J recombination activating gene products Rag-1 and Rag-2. These two proteins constitute the key lymphoid components required for the activation of antigen receptor rearrangement. Both Rag-1 and Rag-2 are required for the catalysis of the initial stages of V(D)J recombination. Thus, functional disruption of either the Rag-1 or Rag-2 genes by homologous recombination, leads to immunodeficiency due to lymphoid arrest at a stage prior to the recombination of the antigen receptor loci. In Rag-deficient mice, both B- and T-cell differentiation is eliminated due to the absence of antigen receptors. Lymphoid development can be restored by the introduction of rearranged antigen receptor transgenes that give rise to monoclonal populations of fully mature B- or T-cells. The absence of the major conventional populations of B- and T-cells from the Rag-deficient mice provided an excellent background for studying the molecular and cellular mechanisms of lymphoid differentiation. The Rag-deficient background has been used as a system for: the functional analysis of Rag-1 and Rag-2; studying the developmental functions of antigen receptors and other molecules of the immune system; the molecular analysis of the early stages of the B- and T-cell lineages; the co-development of lymphocytes with stroma cells; the identification of minor subpopulations of the developing immune system; the involvement of lymphoid populations in the onset of pathogenesis. In addition, the development of the "blastocyst complementation assay" methodology, based on the phenotype of the Rag-/- mice, allowed the functional analysis of numerous lymphoid specific components.

The 3' enhancer region determines the B/T specificity and pro-B/pre-B specificity of immunoglobulin V kappa-J kappa joining

Using transgenic substrates, we found that the immunoglobulin kappa gene 3' enhancer (E3') acts as a negative regulator in V kappa-J kappa joining. Although the E3' was originally identified as a transcriptional enhancer, it acts in a suppressive manner for recombinational regulation. Base substitution analysis has shown that the PU.1-binding site within the E3' regulates the B/T specificity of V kappa-J kappa joining. In a substrate with a mutated PU.1-binding site (GAGGAA to TCTTCG), V kappa-J kappa joining occurred not only in B cells, but also in T cells. The E3' region is also responsible for determining the pro-B/pre-B specificity of V kappa-J kappa joining. When the E3' region was deleted, kappa gene rearrangement actively occurred at the early pro-B stage of B cell development: nongermline (N) nucleotides were common at recombination junctions.

Down-regulation of RAG1 and RAG2 gene expression in preB cells after functional immunoglobulin heavy chain rearrangement

Two waves of immunoglobulin gene rearrangements, first of the heavy, then of the light chain chain gene loci form functional immunoglobulin genes during B cell development. In mouse bone marrow the differential surface expression of B220 (CD45R), c-kit, CD25, and surrogate light chain as well as the cell cycle status allows FACS separation of the cells in which these two waves of rearrangements occur. The gene products of two recombination activating genes, RAG1 and RAG2 are crucial for this rearrangement process. Here, we show that the expression of the RAG genes is twice up- and down-regulated, at the transcriptional level for RAG1 and RAG2, and at the postranscriptional level for RAG2 protein. Expression levels are high in D-->JH and VH-->DJH rearranging proB and preB-I cells, low in preB cells expressing the preB cell receptor on the cell surface, and high again in VL-->JL rearranging small preB-II cells. In immature B cells expressing on the cell surface RAG1 and RAG2 mRNA is down-regulated, whereas RAG2 protein levels are maintained. Down-regulation of RAG1 and RAG2 gene expression after productive rearrangement at one heavy chain allele might be part of the mechanisms that prevent further rearrangements at the other allele.

Induction of IgA B cell differentiation of bone marrow-derived B cells by Peyer's patch autoreactive helper T cells

The objective of this study was to demonstrate in vitro that bone marrow-derived pro/pre-B cells bearing mu mRNA can switch their Ig heavy-chain isotype to that of alpha mRNA-expressing B cells after contact with Peyer's patches-derived activated autoreactive CD4+ T cells. Bone marrow-derived pro/pre-B cells and activated autoreactive Peyer's patch, mesenteric lymph node, or spleen CD4+ T cells were co-cultured in the presence of recombinant (r) IL-2, rIL-7, and Con A for 3 days. The mixed cultured cells were isolated for preparation of total RNA. Dot/slot hybridization, using murine C mu (pu3741) and C alpha (P alpha J558) Ig heavy-chain cDNA probes, detected C mu and C alpha Ig heavy-chain mRNA transcripts. The magnitude of each mRNA expression was measured demsitometrically. In addition, the secreted class-specific Ig contents from the co-cultured supernatants were measured. The results indicate that activated autoreactive Peyer's patch and mesenteric lymph node CD4+ T cells provide a specific Ig heavy-chain switch from mu to alpha (Peyer's patch CD4+ T cells > mesenteric lymph node CD4+ T cells) in bone marrow-derived pro/pre-B cells and also assist to develop IgA-secreting plasma cells. The alpha heavy-chain switch and IgA production do not occur in the presence of activated autoreactive spleen CD4+ T cells. These results support the view that autoreactive gut Peyer's patch CD4+ T cells, at least, regulate IgA B cell heavy-chain switching and terminal differentiation during gut mucosal B cell development.

Rearrangement of upstream DH and VH genes to a rearranged immunoglobulin variable region gene inserted into the DQ52-JH region of the immunoglobulin heavy chain locus

We investigated gene rearrangements in the mutant IgH locus of a mouse strain generated by insertion of a rearranged heavy chain variable region gene (VT15) into the DQ52-JH region through gene targeting. In more than half of the B cells of heterozygous mutant mice, the mutant IgH locus was silenced by the rearrangement of an endogenous DH or DH and VH gene to the inserted VT15 gene. In these cases, a functional VHDHJH gene was present on the wild-type allele. The silencing rearrangement appeared to be mediated by recombination signal sequence (RSS)-like elements present in the "recipient" VT15 gene. Among the many such elements on the inserted VT15 gene, which apparently met the requirement for an RSS with respect to nucleotide sequence, only two were observed in the actual rearrangements. This indicates that targeting of the recombination machinery involves sequences in addition to the RSS motifs as they have been characterized so far. In homozygous mutant mice, most B cells appeared to carry the intact VT15 gene on both mutant IgH alleles, although single-cell polymerase chain reaction revealed that silencing rearrangements occurred frequently in B cell progenitors in the bone marrow. This observation indicates that once silencing rearrangements are initiated in a cell, they involve both VT15 genes in most cases, reminiscent of normal DH-JH rearrangement. B cells which did not initiate such rearrangements develop to populate the peripheral B cell compartment.

Deletion of the 3' splice site of the leader-variable region intron of immunoglobulin heavy chain genes induces a direct splicing of leader to constant region, resulting in the production of truncated mu-chains

An Abelson virus-transformed immature B cell line, AT8-1-12-5-2, produced truncated mu-chains. Sequencing analysis of the VHDJH complex on the expressed H-chain allele revealed the deletion of 75 nucleotides that involved leader-variable region intron and the 5' end of the variable region, which resulted in the loss of the 3' splice site of leader-variable region intron. Sequence studies of a leader- and CH1-containing cDNA clone showed that leader region was directly spliced to the CH1 exon, resulting in the production of the truncated mu-chains without variable portion. Our results demonstrated for the first time that the only loss of the 3' splice site of leader-variable region intron could induce an aberrant splicing between leader and constant region.

A Xenopus lymphoid tumor cell line with complete Ig genes rearrangements and T-cell characteristics

The first lymphoid cell line derived from an amphibian (Xenopus) thymic tumor shows an extreme form of lineage infidelity. Although it has rearranged in-frame the two alleles of the heavy chain, deleted one light chain locus, and rearranged abortively the two alleles of the second light chain locus, the cell line does not produce immunoglobulin molecules or message. It expresses a variety of T-cell characteristic markers such as Xenopus pan T-cell markers, CD8 equivalent and GATA3 transcription factor. It does not express any major histocompatibility complex class I or class II molecules. It resembles some rare types of mammalian leukemias.

Murine B cell development: commitment and progression from multipotential progenitors to mature B lymphocytes

B lymphocytes, the cellular source of antibody, are critical components of the immune response. They develop from multipotential stem cells, progressively acquiring the traits that allow them to function as mature B lymphocytes. This developmental program is dependent on appropriate interactions with the surrounding environment. These interactions, mediated by cell-cell and cell-matrix interactions, provide the growth and differentiation signals that promote progression along the developmental pathway. This chapter addresses the properties of developing B lineage cells and the nature of the environmental signals that support B lineage progression.

Identical VHD and DJH junctions in monoclonal antibodies derived in response to dextran B512 could be the result of developmental selection

We describe here the CDR3s of a collection of monoclonal antibodies (MoAb) with specificity for the carbohydrate dextran B512 produced in the mouse strain C57BL/6. In spite of the postulated mechanisms for variability in this region, a high proportion of these monoclonals displayed identical VHD (24/30) and DJH (21/30) junctions and 21 of them were identical in the whole CDR3. These 21 independently generated identical CDR3s could be ordered in eight groups indicating that not a particular CDR3, but instead the mechanism for generating identical junctions was preserved. Two of the CDR3s in this study were found to be identical to the CDR3 of the monoclonal B1-8 produced in C57BL/6 in response to proteins bearing the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP). This and other parameters support the notion that the generation of identical junctions could be independent of antigenic selection. We also report here the association between JH usage and amino acid (aa) residues at the VHD and DJH junctions. Since these MoAb were generated in response to dextran B512, immunoglobulin conformation has to be compatible with antigen binding. Nevertheless, no aa residue of CDR3 could be directly related to antigen binding. We postulate therefore, that the observed selection of CDR3s could be directed to the production of variable regions with protein configuration most suitable with immunoglobulin folding and may occur prior to antigenic selection. Selection for junctional residues in relation to JH usage and the generation of identical CDR3s are probably different events. Possible genetic mechanisms operating for CDR3 construction and/or selection by cellular ligands are discussed.

Immune modulating effects of intravenous immunoglobulin (IVIg) in autoimmune diseases

Administration of intravenous immunoglobulins (IVIg) have now been reported to be beneficial in a large number of autoimmune diseases, whether mediated by autoantibodies or by T cells. We have proposed that the immunoregulatory effect of IVIg in autoimmune diseases is dependent on the selection of recipient's immune repertoires by variable (V) regions of infused immunoglobulins. Thus: (a) IVIg contains antibodies reactive with idiotypes of natural and disease-related autoantibodies and surface immunoglobulins of B cells; IVIg also contains antibodies reactive with idiotype, framework and constant regions of the beta chain of the alpha beta T cell receptor; (b) infusion of IVIg results in transient or long-lasting suppression of specific autoantibody clones in vivo and in stimulation of a distinct subset of B cells reactive with F(ab')2 fragments of IVIg; (c) infusion of IVIg alters the general "architecture" of the network as assessed by studying the kinetic patterns of spontaneous fluctuations of natural autoantibodies in serum; (d) infusion of normal mouse Ig in healthy adult mice selects expressed immune repertoire by removing late pre-B and B cells in the bone marrow, mostly those expressing D proximal VH genes, and by activating distinct subsets of B cells and CD4+ T cells in the spleen; and (e) infusion of IVIg results in a modulation of synthesis and release of cytokines. Although dependent on the V-region reactivities (composition) or injected preparations, these effects probably also require that the infused immunoglobulin contains an intact Fc moiety. This review focuses on autoimmune and inflammatory diseases in which IVIg therapy has been beneficial. Recent recommendations from a committee of experts for IVIg therapy have been pointed out.

Structure of the murine interferon alpha/beta receptor-encoding gene: high-frequency rearrangements in the interferon-resistant L1210 cell line

The structure of the murine IFNAR gene, encoding the interferon alpha/beta receptor, is reported. The gene has eleven exons dispersed in about 23 kb of genomic DNA. The nature of the rearrangements affecting this gene in interferon-resistant (IFNR) L1210 mutant cell lines is described.

Entry of B lymphocytes into the persistent cell pool in non-immunized mice is not accompanied by somatic mutation of VH genes

In this study we compare VH-gene repertoires of short-lived and persistent B lymphocytes in normal nonimmunized mice. Enriched populations of persistent peripheral B cells were obtained in vivo either by (i) repeated injections with hydroxyurea or (ii) maintained ganciclovir administration to herpes simplex virus-1 thymidine kinase transgenic mice. Both approaches have previously been shown to deplete newly formed, short-lived B cells. VH genes expressed by persistent or unselected B cell populations were amplified by polymerase chain reaction, cloned using the lambda-ImmunoZAP system (Stratagene) and sequenced. The results presented here concern a total of 116 complete VH sequences from two VH gene families of established germ-line composition: VH7183 and VHX24. No differences were found between the two cell populations as to usage of D or JH segments and to the presence of N sequence additions at D/JH or VH/DJH junctions and CDR3 length. Over 90% of the sequenced VH genes were of germ-line arrangement with no evidence of somatic mutation. These results show that persistent B cells in normal mice are not of embryonic origin and that somatic hypermutation is not necessary for B cell survival. They also suggest that a significant fraction of persistent IgM+ B cells in normal mice are not generated by conventional antigenic stimulation and could represent a novel class of "memory" cells expressing germ-line repertoires.

Analysis of the B-cell progenitor compartment at the level of single cells

BACKGROUND

During B-cell development in the mouse, the VH, DH and JH elements of the immunoglobulin heavy chain (IgH) locus are rearranged, firstly by DH-JH joining, and then by VH-DHJH joining. In-frame ('productive') VHDHJH joints and DHJH joints in reading frame 2 (one of the three possible DH reading frames) allow the expression of mu and truncated mu chains (D mu proteins), respectively. The expression of such molecules from one of the two IgH loci of a cell is thought to interfere with VH-DHJH recombination on the other IgH locus, and to guide the cells through further development.

RESULTS

We have developed a gene amplification assay that permits the examination of rearranged immunoglobulin genes in single cells. Using this assay, we monitored cells bearing DHJH and/or VHDHJH joints at early stages of development: in CD43+ B-cell progenitors, subdivided into fractions A, B, C and C' by flow cytometry, and in CD43- pre-B cells (fraction D). Fraction C was enriched for cells with two non-productive VHDHJH joints. Cells containing both a DHJH joint in DH reading frame 2 and a VHDHJH joint were not seen in any fraction. All fraction D cells harbored an in-frame VHDHJH joint. Cells with two productive VHDHJH joints appear to be selected against throughout development.

CONCLUSIONS

Cells expressing D mu proteins appear to be arrested in development as a result of inhibited VH-DHJH joining. Expression of the mu chain is required for maturation into CD43- pre-B cells; accordingly, cells carrying two non-productive VHDHJH joints accumulate in the CD43+ compartment. Such a developmental arrest may also affect cells that express self-reactive VHDHJH antibody domains. Our results indicate further that allelic exclusion at the IgH locus is already established at the pre-B cell stage.

A lambda 1 transgene under the control of a heavy chain promoter and enhancer does not undergo somatic hypermutation

To identify cis-acting elements responsible for targeting somatic hypermutation to immunoglobulin variable regions, we generated transgenic mice which carry a rearranged lambda 1 gene regulated by the heavy chain intron enhancer, E mu, and the heavy chain promoter PH186.2 from the VH186.2 variable region. C57BL/6 x SJL founders were bred with C57BL/6 mice to establish a line carrying a single copy of the transgene. Somatic hypermutation was studied by generating hybridoma cell lines from mice immunized with the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP) coupled to chicken gamma globulin. The immune response in this transgenic line was dominated by the endogenous VH186.2 heavy chain variable region and the transgenic lambda 1 light chain, and the transgene was actively expressed in all hybridomas analyzed. In this work we show that the transgenic V lambda 1 regions do not undergo hypermutation, despite high levels of expression, while the expressed heavy chain V regions accumulate mutations at a rate typical of the NP response in C57BL/6 mice. Thus, within the same B cell, the PH186.2 promoter in connection with E mu drives efficient expression of both a VH and a V lambda region, but only the VH is a target for somatic hypermutation. Our observations show that cis-acting sequences that activate immunoglobulin gene transcription are not sufficient to target somatic hypermutation.