Regulation of genome rearrangement events during lymphocyte differentiation

Alzheimer's disease and the immune system: A comprehensive overview with a focus on B cells, humoral immunity, and immunotherapy

Alzheimer's disease (AD) is a complex neurodegenerative disorder and the major cause of dementia. Amyloid-β (Aβ) and tau aggregation, mitochondrial dysfunction, and microglial dysregulation are key contributors to AD pathogenesis. Impairments in the blood-brain barrier have unveiled the contribution of the immune system, particularly B cells, in AD pathology. B cells, a crucial component of adaptive immunity, exhibit diverse functions, including antigen presentation and antibody production. While their role in neuroinflammatory disorders has been well-documented, their specific function in AD lacks adequate data. This review examines the dual role of the B cells and humoral immunity in modulating brain inflammation in AD and explores recent advancements in passive and active immunotherapeutic strategies targeting AD pathobiology. We summarize preclinical and clinical studies investigating B cell frequency, altered antibody levels, and their implications in neuroinflammation and immunotherapy. Notably, B cells demonstrate protective and pathological roles in AD, influencing neurodegeneration through antibody-mediated clearance of toxic aggregates and inflammatory activation inflammation. Passive immunotherapies targeting Aβ have shown potential in reducing amyloid plaques, while active immunotherapies are emerging as promising strategies, requiring further validation. Understanding the interplay between B cells, humoral immunity, microglia, and mitochondrial dysfunction is critical to unraveling AD pathogenesis. Their dual nature in disease progression underscores the need for precise therapeutic interventions to optimize immunotherapy outcomes and mitigate neuroinflammation effectively.

Immunoglobulin Heavy Chain Gene Rearrangement in B-Cell Non-Hodgkin's Lymphomas Detected by the Polymerase Chain Reaction

Aims and background

Immunoglobulin heavy chain gene rearrangement serves as a marker of clonality and cell lineage in B-cell lymphoproliferative disorders. In this study we used the polymerase chain reaction (PCR) to detect clonal rearrangements of the immunoglobulin heavy chain gene in a group of patients with B-cell lymphomas.

Methods

DNA was extracted from frozen tissue of 40 B-cell non-Hodgkin's lymphomas and subjected to PCR amplification using primers that recognize conserved sequences of the variable and joining regions of the immunoglobulin heavy chain gene.

Results

Monoclonal rearrangements were detected in 23 of 40 malignant B-cell lymphomas. No clonal rearrangements were detected in the 10 control cases.

Conclusions

We conclude that this PCR-based technique may provide a simplified and rapid approach for the detection of clonal immunoglobulin heavy chain gene rearrangements in B-cell lymphomas without recourse to Southern blotting, which can be reserved for cases in which PCR is negative.

Exploration of the Germline Genome of the Ciliate Chilodonella uncinata through Single-Cell Omics (Transcriptomics and Genomics)

Separate germline and somatic genomes are found in numerous lineages across the eukaryotic tree of life, often separated into distinct tissues (e.g., in plants, animals, and fungi) or distinct nuclei sharing a common cytoplasm (e.g., in ciliates and some foraminifera). In ciliates, germline-limited (i.e., micronuclear-specific) DNA is eliminated during the development of a new somatic (i.e., macronuclear) genome in a process that is tightly linked to large-scale genome rearrangements, such as deletions and reordering of protein-coding sequences. Most studies of germline genome architecture in ciliates have focused on the model ciliates Oxytricha trifallax, Paramecium tetraurelia, and Tetrahymena thermophila, for which the complete germline genome sequences are known. Outside of these model taxa, only a few dozen germline loci have been characterized from a limited number of cultivable species, which is likely due to difficulties in obtaining sufficient quantities of “purified” germline DNA in these taxa. Combining single-cell transcriptomics and genomics, we have overcome these limitations and provide the first insights into the structure of the germline genome of the ciliate Chilodonella uncinata, a member of the understudied class Phyllopharyngea. Our analyses reveal the following: (i) large gene families contain a disproportionate number of genes from scrambled germline loci; (ii) germline-soma boundaries in the germline genome are demarcated by substantial shifts in GC content; (iii) single-cell omics techniques provide large-scale quality germline genome data with limited effort, at least for ciliates with extensively fragmented somatic genomes. Our approach provides an efficient means to understand better the evolution of genome rearrangements between germline and soma in ciliates.

Our understanding of the distinctions between germline and somatic genomes in ciliates has largely relied on studies of a few model genera (e.g., Oxytricha, Paramecium, Tetrahymena). We have used single-cell omics to explore germline-soma distinctions in the ciliate Chilodonella uncinata, which likely diverged from the better-studied ciliates ~700 million years ago. The analyses presented here indicate that developmentally regulated genome rearrangements between germline and soma are demarcated by rapid transitions in local GC composition and lead to diversification of protein families. The approaches used here provide the basis for future work aimed at discerning the evolutionary impacts of germline-soma distinctions among diverse ciliates.

Generation and CRISPR/Cas9 editing of transformed progenitor B cells as a pseudo-physiological system to study DNA repair gene function in V(D)J recombination

Antigen receptor gene assembly is accomplished in developing lymphocytes by the V(D)J recombination reaction, which can be separated into two steps: DNA cleavage by the recombination-activating gene (RAG) nuclease and joining of DNA double strand breaks (DSBs) by components of the nonhomologous end joining (NHEJ) pathway. Deficiencies for NHEJ factors can result in immunodeficiency and a propensity to accumulate genomic instability, thus highlighting the importance of identifying all players in this process and deciphering their functions. Bcl2 transgenic v-Abl kinase-transformed pro-B cells provide a pseudo-physiological cellular system to study V(D)J recombination. Treatment of v-Abl/Bcl2 pro-B cells with the Abl kinase inhibitor Imatinib leads to G1 cell cycle arrest, the rapid induction of Rag1/2 gene expression and V(D)J recombination. In this system, the Bcl2 transgene alleviates Imatinib-induced apoptosis enabling the analysis of induced V(D)J recombination. Although powerful, the use of mouse models carrying the Bcl2 transgene for the generation of v-Abl pro-B cell lines is time and money consuming. Here, we describe a method for generating v-Abl/Bcl2 pro-B cell lines from wild type mice and for performing gene knock-out using episomal CRISPR/Cas9 targeting vectors. Using this approach, we generated distinct NHEJ-deficient pro-B cell lines and quantified V(D)J recombination levels in these cells. Furthermore, this methodology can be adapted to generate pro-B cell lines deficient for any gene suspected to play a role in V(D)J recombination, and more generally DSB repair.

Molecular Analysis of Genetic Markers for Non-Hodgkin Lymphomas

Molecular analysis complements the clinical and histopathologic tools used to diagnose and subclassify hematologic malignancies. The presence of clonal antigen-receptor gene rearrangements can help to confirm the diagnosis of a B or T cell lymphoma and can serve as a fingerprint of that neoplasm to be used in identifying concurrent disease at disparate sites or recurrence at future time points. Certain lymphoid malignancies harbor a characteristic chromosomal translocation, a finding that may have significant implications for an individual's prognosis or response to therapy. The polymerase chain reaction (PCR) is typically used to detect antigen-receptor gene rearrangements as well as specific translocations that can be supplemented by fluorescence in situ hybridization (FISH) and karyotype analysis. © 2017 by John Wiley & Sons, Inc.

Lineage tracing of human B cells reveals the in vivo landscape of human antibody class switching

Antibody class switching is a feature of the adaptive immune system which enables diversification of the effector properties of antibodies. Even though class switching is essential for mounting a protective response to pathogens, the in vivo patterns and lineage characteristics of antibody class switching have remained uncharacterized in living humans. Here we comprehensively measured the landscape of antibody class switching in human adult twins using antibody repertoire sequencing. The map identifies how antibodies of every class are created and delineates a two-tiered hierarchy of class switch pathways. Using somatic hypermutations as a molecular clock, we discovered that closely related B cells often switch to the same class, but lose coherence as somatic mutations accumulate. Such correlations between closely related cells exist when purified B cells class switch in vitro, suggesting that class switch recombination is directed toward specific isotypes by a cell-autonomous imprinted state.

DOI:http://dx.doi.org/10.7554/eLife.16578.001

The human immune system comprises cells and processes that protect the body against infection and disease. B cells are immune cells that once activated produce antibodies, or proteins that help identify and neutralize infectious microbes and diseased host cells.

Antibodies fall into one of ten different classes, and each class has a different, specialized role. Certain antibody classes are responsible for eradicating viruses, while others recruit and help activate additional cells of the immune system.

B cells multiply quickly once they are activated. During this proliferation process, dividing B cells can switch from making one class of antibody to another. As such, a single activated B cell can yield a group of related B cells that produce distinct classes of antibodies. Although much has been learned about antibody class switching and its role in generating a diverse set of antibodies, the process of creating different antibody classes in humans remains unknown.

Horns, Vollmers et al. now reveal how antibodies of every class are created in living humans. By developing a way to reconstruct the B cell proliferation process and thereby trace the lineage of individual B cells, the occurrence of class switching events could be measured and mapped. This approach revealed that most antibodies are produced via a single dominant pathway that involves first switching through one of two antibody classes.

Horns, Vollmers et al. also determined that closely related B cells, which were recently born through division of a common ancestor, often switched to the same class. The shared fate is likely explained by the existence of similar conditions inside each cell, which are inherited during cell division and direct switching toward a particular class. All together, these new findings lay a foundation for developing techniques to direct antibody class switching in ways that support the immune system. Future work will aim to understand the conditions inside a cell that direct switching toward a particular class of antibody.

DOI:http://dx.doi.org/10.7554/eLife.16578.002

Re-evaluation of the immunological Big Bang

Classically the immunological 'Big Bang' of adaptive immunity was believed to have resulted from the insertion of a transposon into an immunoglobulin superfamily gene member, initiating antigen receptor gene rearrangement via the RAG recombinase in an ancestor of jawed vertebrates. However, the discovery of a second, convergent adaptive immune system in jawless fish, focused on the so-called variable lymphocyte receptors (VLRs), was arguably the most exciting finding of the past decade in immunology and has drastically changed the view of immune origins. The recent report of a new lymphocyte lineage in lampreys, defined by the antigen receptor VLRC, suggests that there were three lymphocyte lineages in the common ancestor of jawless and jawed vertebrates that co-opted different antigen receptor supertypes. The transcriptional control of these lineages during development is predicted to be remarkably similar in both the jawless (agnathan) and jawed (gnathostome) vertebrates, suggesting that an early 'division of labor' among lymphocytes was a driving force in the emergence of adaptive immunity. The recent cartilaginous fish genome project suggests that most effector cytokines and chemokines were also present in these fish, and further studies of the lamprey and hagfish genomes will determine just how explosive the Big Bang actually was.

Epigenetic aspects of lymphocyte antigen receptor gene rearrangement or 'when stochasticity completes randomness'

To perform their specific functional role, B and T lymphocytes, cells of the adaptive immune system of jawed vertebrates, need to express one (and, preferably, only one) form of antigen receptor, i.e. the immunoglobulin or T-cell receptor (TCR), respectively. This end goal depends initially on a series of DNA cis-rearrangement events between randomly chosen units from separate clusters of V, D (at some immunoglobulin and TCR loci) and J gene segments, a biomolecular process collectively referred to as V(D)J recombination. V(D)J recombination takes place in immature T and B cells and relies on the so-called RAG nuclease, a site-specific DNA cleavage apparatus that corresponds to the lymphoid-specific moiety of the VDJ recombinase. At the genome level, this recombinase's mission presents substantial biochemical challenges. These relate to the huge distance between (some of) the gene segments that it eventually rearranges and the need to achieve cell-lineage-restricted and developmentally ordered routines with at times, mono-allelic versus bi-allelic discrimination. The entire process must be completed without any recombination errors, instigators of chromosome instability, translocation and, potentially, tumorigenesis. As expected, such a precisely choreographed and yet potentially risky process demands sophisticated controls; epigenetics demonstrates what is possible when calling upon its many facets. In this vignette, we will recall the evidence that almost from the start appeared to link the two topics, V(D)J recombination and epigenetics, before reviewing the latest advances in our knowledge of this joint venture.

Somatic mosaicism in healthy human tissues

From the fertilization of an egg until the death of an individual, somatic cells can accumulate genetic changes, such that cells from different tissues or even within the same tissue differ genetically. The presence of multiple cell clones with distinct genotypes in the same individual is referred to as 'somatic mosaicism'. Many endogenous factors such as mobile elements, DNA polymerase slippage, DNA double-strand break, inefficient DNA repair, unbalanced chromosomal segregation and some exogenous factors such as nicotine and UV exposure can contribute to the generation of somatic mutations, thereby leading to somatic mosaicism. Such changes can potentially affect the epigenetic patterns and levels of gene expression, and ultimately the phenotypes of cells. Although recent studies suggest that somatic mosaicism is widespread during normal development and aging, its implications for heightened disease risks are incompletely understood. Here, I discuss the origins, prevalence and implications of somatic mosaicism in healthy human tissues.

Id1 has a physiological role in regulating early B lymphopoiesis

Basic helix-loop-helix E proteins play critical roles in B-cell development by stimulating B cell-specific gene expression and immunoglobulin gene rearrangement. The function of E proteins can be effectively suppressed by their naturally occurring inhibitors, Id1 to 4. Ectopic expression of Id1 has been shown to block B-cell development at the early pro-B cell stage. However, whether Id1 plays a physiological role in controlling B lymphopoiesis was not known. Although Id1-deficient mice do not exhibit significant abnormalities in steady-state B lymphopoiesis, we detected more robust B-cell engraftment in transplant recipients of Id1-deficient bone marrow compared to those of wild-type donor cells. In culture, Id1 ablation dramatically enhances B-lineage cell production without any marked effects on myeloid differentiation. Consistently, Id1 expression was found in pro-B but not pre-B cells as measured by enhanced green fluorescent protein (EGFP) fluorescence and by quantitative reverse transcription-PCR. Although loss of Id1 did not alter the number of B-cell colonies generated from whole bone marrow or the proliferation rate of developing B cells, B-cell colonies were detectable at a much earlier time point and the size of the colonies were larger. Therefore, we infer that Id1-deficient progenitors possess higher potential to differentiate to the pre-B cell stage when a proliferative burst occurs. Taken together, we present evidence to suggest that Id1 plays a physiological role in restraining the developmental progression, which may be important for proper B-cell differentiation in the bone marrow.

The peritoneal cavity B-2 antibody repertoire appears to reflect many of the same selective pressures that shape the B-1a and B-1b repertoires

To assess the extent and nature of somatic categorical selection of CDR-3 of the Ig H chain (CDR-H3) content in peritoneal cavity (PerC) B cells, we analyzed the composition of V(H)7183DJCμ transcripts derived from sorted PerC B-1a, B-1b, and B-2 cells. We divided these sequences into those that contained N nucleotides (N(+)) and those that did not (N(-)) and then compared them with sequences cloned from sorted IgM(+)IgD(+) B cells from neonatal liver and both wild-type and TdT-deficient adult bone marrow. We found that the PerC B-1a N(-) repertoire is enriched for the signatures of CDR-H3 sequences present in neonatal liver and shares many features with the B-1b N(-) repertoire, whereas the PerC B-1a N(+), B-1b N(+), and B-2 N(+) repertoires are enriched for adult bone marrow sequence signatures. However, we also found several sequence signatures that were not shared with other mature perinatal or adult B cell subsets but were either unique or variably shared between the two or even among all three of the PerC subsets that we examined. These signatures included more sequences lacking N nucleotides in the B-2 population and an increased use of D(H) reading frame 2, which created CDR-H3s of greater average hydrophobicity. These findings provide support for both ontogenetic origin and shared Ag receptor-influenced selection as the mechanisms that shape the unique composition of the B-1a, B-1b, and B-2 repertoires. The PerC may thus serve as a general reservoir for B cells with Ag binding specificities that are uncommon in other mature compartments.

The center of accessibility: Dβ control of V(D)J recombination

Developmental patterning of antigen receptor gene assembly in lymphocyte precursors correlates with decondensation of the chromatin surrounding individual gene segments. Ongoing V(D)J recombination is associated with hyperacetylation of histones H3 and H4 and the expression of sterile germline transcripts across the region of recombinational accessibility. Likewise, histone acetyltransferase and SWI/SNF chromatin remodeling complexes each appear to be required for recombination, and the PHD-finger of RAG-2 preferentially associates with recombination signal sequence (RSS) chromatin that contains H3 trimethylated on lysine 4. However, the regulatory mechanisms that direct chromatin alteration and rearrangement have proven elusive, due in large part to the interdependency of individual stages in gene activation, our limited understanding of functional significance of changes to the histone code, and the difficulty of modeling recombinational accessibility in existing experimental systems. Examining Tcrb assembly in developing thymocytes, we review the central roles of RSS elements and germline promoters as foci for epigenetic reorganization of recombinationally accessible gene segments in light of recent findings and persistent questions.

Molecular analysis of genetic markers for non-Hodgkin lymphomas

Molecular analysis complements the clinical and histopathologic tools used to diagnose and subclassify hematologic malignancies. The presence of clonal antigen-receptor gene rearrangements can help to confirm the diagnosis of a B or T cell lymphoma and can serve as a fingerprint of that neoplasm to be used in identifying concurrent disease at disparate sites or recurrence at future time points. Certain lymphoid malignancies harbor a characteristic chromosomal translocation, a finding that may have significant implications for an individual's prognosis or response to therapy. The polymerase chain reaction (PCR) is typically used to detect antigen-receptor gene rearrangements as well as specific translocations that can be supplemented by fluorescence in situ hybridization (FISH) and karyotype analysis.

Transcription-associated recombination in eukaryotes: link between transcription, replication and recombination

Homologous recombination (HR) is an important DNA repair pathway and is essential for cellular survival. It plays a major role in repairing replication-associated lesions and is functionally connected to replication. Transcription is another cellular process, which has emerged to have a connection with HR. Transcription enhances HR, which is a ubiquitous phenomenon referred to as transcription-associated recombination (TAR). Recent evidence suggests that TAR plays a role in inducing genetic instability, for example in the THO mutants (Tho2, Hpr1, Mft1 and Thp2) in yeast or during the development of the immune system leading to genetic diversity in mammals. On the other hand, evidence also suggests that TAR may play a role in preventing genetic instability in many different ways, one of which is by rescuing replication during transcription. Hence, TAR is a double-edged sword and plays a role in both preventing and inducing genetic instability. In spite of the interesting nature of TAR, the mechanism behind TAR has remained elusive. Recent advances in the area, however, suggest a link between TAR and replication and show specific genetic requirements for TAR that differ from regular HR. In this review, we aim to present the available evidence for TAR in both lower and higher eukaryotes and discuss its possible mechanisms, with emphasis on its connection with replication.

B lymphocytes: how they develop and function

The discovery that lymphocyte subpopulations participate in distinct components of the immune response focused attention onto the origins and function of lymphocytes more than 40 years ago. Studies in the 1960s and 1970s demonstrated that B and T lymphocytes were responsible primarily for the basic functions of antibody production and cell-mediated immune responses, respectively. The decades that followed have witnessed a continuum of unfolding complexities in B-cell development, subsets, and function that could not have been predicted. Some of the landmark discoveries that led to our current understanding of B lymphocytes as the source of protective innate and adaptive antibodies are highlighted in this essay. The phenotypic and functional diversity of B lymphocytes, their regulatory roles independent of antibody production, and the molecular events that make this lineage unique are also considered. Finally, perturbations in B-cell development that give rise to certain types of congenital immunodeficiency, leukemia/lymphoma, and autoimmune disease are discussed in the context of normal B-cell development and selection. Despite the significant advances that have been made at the cellular and molecular levels, there is much more to learn, and cross-disciplinary studies in hematology and immunology will continue to pave the way for new discoveries.

Duck immunoglobulins: structure, functions and molecular genetics

Four types of immunoglobulin (Ig) have been identified in ducks: IgM, a secretory Ig resembling IgM, a 7.8S IgG, and a 5.7S IgG. Structurally and antigenically the 5.7S IgG resembles an F(ab')2 fragment of the 7.8S IgG. When ducks mount serum antibody responses, the sequence of Ig involvement is IgM --> 7.8S IgG --> 5.7S IgG. Although serum Ig levels increase, and antigen-binding Igs can be demonstrated, sera from repeatedly immunized ducks commonly lack secondary antibody activities such as agglutination, precipitation, complement fixation and tissue sensitization. These deficiencies are most likely attributable to the absence of functionally important components of the predominant Ig (5.7S IgG) and/or a possibly unusual steric structure of duck Igs. A related issue concerns production of the two antigenically related IgGs: what are the cellular and molecular events involved, and how are they controlled? Evidence from current molecular genetic studies has confirmed the similarity of the VH, CH1 and CH2 domains of the 7.8S and 5.7S IgGs and shown, by virtue of the existence of separate mature messages for the heavy (H) chains of these molecules, that they are biosynthesized independently. Models for the possibilities that the two H chains are products of one gene or of two genes are presented. Cloning and sequencing the duck H chain gene locus, which is in progress, is providing data supporting the one gene hypothesis. The results obtained from cDNA sequencing also confirm that the duck IgGs are unusual in terms of the anatomy of the hinge region and of the number and location of intra- and inter-chain disulphide bonds, observations which will be of importance for understanding structure/function relationships of these unusual and interesting molecules.

Molecular analysis of DNA rearrangements in leukemias and non-Hodgkin's lymphomas

Genetic markers for leukemias and lymphomas include chromosomal translocations and antigen-receptor gene rearrangements. Clonal rearrangements of immunoglobulin or T cell receptor (TCR) genes reflect clonal proliferations of lymphocytes, a characteristic feature of lymphoid neoplasia. These rearrangements can be detected as described in this unit by Southern blot hybridization or, in many instances, the polymerase chain reaction (PCR). Specific chromosomal translocations can also serve as markers for clonality, for malignant transformation, and for various defined subtypes of hematopoietic cancers. PCR protocols are described for detection of the two most commonly assayed translocations, t(9;22) of chronic myelogenous leukemia or acute lymphoblastic leukemia, and t(14;18) of follicular lymphomas.

From gene amplification to V(D)J recombination and back: a personal account of my early years in B cell biology

I have been invited to write a short historical feature in the context of being a co-recipient with Klaus Rajewsky and Fritz Melchers of the 2007 Novartis Prize in Basic Immunology that was given in the general area of the molecular biology of B cells. In this feature, I cover the main points of the short talk that I presented at the Award Ceremony at the International Immunology Congress in Rio de Janeiro, Brazil. This talk focused primarily on the work and people involved early on in generating the models and ideas that have formed the basis for my ongoing efforts in the areas of V(D)J recombination and B cell development.

Immunoglobulin light chain repertoire in hairy cell leukemia

Of 166 hairy cell leukemia (HCL) patients, 81 had kappa and 80 had lambda expression. IGKV-J and IGLV-J rearrangement structure was analyzed in 21 HCL patients (11 kappa, 10 lambda). For kappa, IGKV1-5 was most frequent, and the KJ2 gene was over-utilized. For lambda HCL, LJ3 was over-utilized compared to normal. This study significantly adds to previous studies of light chain usage in HCL and is the first to report light chain gene usage. In HCL, we confirm the lack of kappa predominance observed in normal lymphocytes and in chronic lymphocytic leukemia, and note over-representation of several light chain genes.

Germline transcription from T-cell receptor Vbeta gene is uncoupled from allelic exclusion

Allelic exclusion operates in B and T lymphocytes to ensure clonal expression of antigen receptors after V(D)J recombination. Germline transcription, which proceeds V(D)J recombination, has been postulated to provide an instructive signal for allelic exclusion. Here, we use a genetic marker to track germline transcription from a Vbeta gene within the TCRbeta locus. We find that developing thymocytes exhibit uniformed, bi-allelic activation of the Vbeta gene before V-DJ recombination, a process subject to allelic exclusion. We further show that V-DJ rearrangement promotes activation rather than silencing of germline transcription from the remaining Vbeta genes on either the functionally or non-functionally rearranged chromosome. Results presented here suggest that germline transcription, although necessary for V(D)J recombination, is not sufficient to instruct allelic exclusion.

Visualization of the earliest steps of γδ T cell development in the adult thymus

The checkpoint in gammadelta cell development that controls successful T cell receptor (TCR) gene rearrangements remains poorly characterized. Using mice expressing a reporter gene 'knocked into' the Tcrd constant region gene, we have characterized many of the events that mark the life of early gammadelta cells in the adult thymus. We identify the developmental stage during which the Tcrd locus 'opens' in early T cell progenitors and show that a single checkpoint controls gammadelta cell development during the penultimate CD4- CD8- stage. Passage through this checkpoint required the assembly of gammadelta TCR heterodimers on the cell surface and signaling via the Lat adaptor protein. In addition, we show that gammadelta selection triggered a phase of sustained proliferation similar to that induced by the pre-TCR.

Mechanism and control of V(D)J recombination at the immunoglobulin heavy chain locus

V(D)J recombination assembles antigen receptor variable region genes from component germline variable (V), diversity (D), and joining (J) gene segments. For B cells, such rearrangements lead to the production of immunoglobulin (Ig) proteins composed of heavy and light chains. V(D)J is tightly controlled at the Ig heavy chain locus (IgH) at several different levels, including cell-type specificity, intra- and interlocus ordering, and allelic exclusion. Such controls are mediated at the level of gene segment accessibility to V(D)J recombinase activity. Although much has been learned, many long-standing questions regarding the regulation of IgH locus rearrangements remain to be elucidated. In this review, we summarize advances that have been made in understanding how V(D)J recombination at the IgH locus is controlled and discuss important areas for future investigation.

An antibody VH gene that promotes marginal zone B cell development and heavy chain allelic inclusion

The Ig heavy (H) chain plays a pivotal role in the regulation of primary B cell development through its association with a variety of other proteins including Igalpha and Igbeta, the surrogate light chain components and bona fide L chains, to form transmembrane signaling complexes. Little is known about how alterations in the structure of the H chain variable region influence association with these proteins, or the signaling capacity of the complexes that form. Here we describe a line of VH 'knockin' mice in which the transgene-encoded VH region differs by eight amino acid residues from the VH region in a VH knockin line we previously constructed and characterized. The transgenic H chain locus in the line of mice we characterized earlier efficiently promotes H chain allelic exclusion and all phases of primary B cell development, resulting in the generation of mature B1, marginal zone (MZ) and follicular (FO) B cell compartments. In contrast, the transgenic H chain locus in the new line fails to enforce allelic exclusion, as evidenced by the majority of peripheral B cells expressing two H chains on their surfaces. Moreover, this locus inefficiently drives bone marrow B lymphopoiesis and FO B cell development. However, this H chain locus does promote MZ B cell development, from precursors that appear to be generated during fetal and neonatal life. We discuss these data in the context of previous findings on the influence of Ig H chain structure on primary B cell development.

Qualitatively and quantitatively similar effects of active and passive maternal tobacco smoke exposure on in utero mutagenesis at the HPRT locus

BACKGROUND

Induced mutagenesis in utero is likely to have life-long repercussions for the exposed fetus, affecting survival, birth weight and susceptibility to both childhood and adult-onset diseases, such as cancer. In the general population, such exposures are likely to be a consequence of the lifestyle choices of the parents, with exposure to tobacco smoke one of the most pervasive and easily documented. Previous studies attempting to establish a direct link between active smoking and levels of somatic mutation have largely discounted the effects of passive or secondary exposure, and have produced contradictory results.

METHODS

Data from three studies of possible smoking effects on in utero mutagenesis at the HPRT locus were compiled and reanalyzed, alone and in combination. Where possible, passive exposure to environmental tobacco smoke was considered as a separate category of exposure, rather than being included in the non-smoking controls. Molecular spectra from these studies were reanalyzed after adjustment for reported mutation frequencies from the individual studies and the entire data set.

RESULTS

A series of related studies on mutation at the X-linked HPRT locus in human newborn cord blood samples has led to the novel conclusion that only passive maternal exposure to tobacco mutagens has a significant effect on the developing baby. We performed a pooled analysis of the complete data from these studies, at the levels of both induced mutation frequency and the resulting mutational spectrum.

CONCLUSION

Our analysis reveals a more commonsensical, yet no less cautionary result: both active maternal smoking and secondary maternal exposure produce quantitatively and qualitatively indistinguishable increases in fetal HPRT mutation. Further, it appears that this effect is not perceptibly ameliorated if the mother adjusts her behavior (i.e. stops smoking) when pregnancy is confirmed, although this conclusion may also be affected by continued passive exposure.

Proviral integration of an Abelson-murine leukemia virus deregulates BKLF-expression in the hypermutating pre-B cell line 18-81

The transcription factor BKLF (basic Krüppel-like factor, KLF3) is a member of the Krüppel-like factors (KLF) family. KLF members harbor a characteristic C-terminal zinc-finger DNA-binding domain and bind preferentially to CACCC-motifs. BKLF is highly expressed in haematopoietic and erythoid cells and works either as repressor or activator of transcription in various genes. BKLF-deficient mice display myeloproliferative disorders and abnormalities in haematopoiesis. Other members of the KLF-family such as GKLF and BCL11A have been implicated in tumorigenesis, however, for BKLF such association has not yet been demonstrated. We report here that a single Abelson-murine leukemia virus (A-MuLV) provirus is present in the genome of the hypermutating murine pre-B cell line 18-81. The provirus has integrated into the locus of the transcription factor BKLF. In contrast to other A-MuLV transformed pre-B cell lines, BKLF is highly transcribed in cell line 18-81. BKLF transcripts originate from the retroviral long terminal repeats (LTRs) and BKLF protein can be detected by gel shift retardation assay. We hypothesize on a potential role of BKLF deregulation in tumorigenesis and/or in the induction of somatic hypermutation in cell line 18-81.

An integrated view of suppressor T cell subsets in immunoregulation

The immune system evolved to protect organisms from a virtually infinite variety of disease-causing agents but to avoid harmful responses to self. Because immune protective mechanisms include the elaboration of potent inflammatory molecules, antibodies, and killer cell activation--which together can not only destroy invading microorganisms, pathogenic autoreactive cells, and tumors, but also mortally injure normal cells--the immune system is inherently a "double-edged sword" and must be tightly regulated. Immune response regulation includes homeostatic mechanisms intrinsic to the activation and differentiation of antigen-triggered immunocompetent cells and extrinsic mechanisms mediated by suppressor cells. This review series will focus on recent advances indicating that distinct subsets of regulatory CD4+ and CD8+ T cells as well as NK T cells control the outgrowth of potentially pathogenic antigen-reactive T cells and will highlight the evidence that these suppressor T cells may play potentially important clinical roles in preventing and treating immune-mediated disease. Here we provide a historical overview of suppressor cells and the experimental basis for the existence of functionally and phenotypically distinct suppressor subsets. Finally, we will speculate on how the distinct suppressor cell subsets may function in concert to regulate immune responses.

A novel L3-acute lymphoblastic leukemia cell line (BALM-27) carrying cytoplasmic immunoglobulin 8 chain but lacking expression of cell surface immunoglobulin chains

A human acute lymphoblastic leukemia (ALL) cell line, BALM-27, was established from the peripheral blood specimen of a patient with B-cell ALL L3 type (ALL-L3) at diagnosis. As with the original leukemia cells, the established line was negative for all cell surface immunoglobulin (Ig) chains, but carrying only cytoplasmic Ig delta heavy chain. Southern blot analysis of the various Ig chain genes demonstrated homozygous deletion of the Jkappa gene, germ line configuration of the Jlambda and rearrangement of IgJH genes. Cytogenetic analysis of both primary leukemic bone marrow and BALM-27 cells showed the der(8;15)(q10;q10) chromosomal alteration, in addition to the t(8;22)(q24;q11) abnormality which is highly associated with ALL-L3 and Burkitt's lymphoma. The established cell line BALM-27 represents a rich resource of abundant, accessible, and manipulable cell material for analyzing the unique expression of Ig chain and for investigating the pathogenesis of B-cell malignancy. The scientific significance of BALM-27 lies in (1) the rarity of this type of leukemia cell lines, and (2) its unique chromosomal aberrations.

Differential surrogate light chain expression governs B-cell differentiation

Surrogate light chain expression during B lineage differentiation was examined by using indicator fluorochrome-filled liposomes in an enhanced immunofluorescence assay. Pro-B cells bearing surrogate light chain components were found in mice, but not in humans. A limited subpopulation of relatively large pre-B cells in both species expressed pre-B cell receptors. These cells had reduced expression of the recombinase activating genes, RAG-1 and RAG-2. Their receptor-negative pre-B cell progeny were relatively small, expressed RAG-1 and RAG-2, and exhibited selective down-regulation of VpreB and λ5expression. Comparative analysis of the 2 pre-B cell subpopulations indicated that loss of the pre-B cell receptors from surrogate light chain gene silencing was linked with exit from the cell cycle and light chain gene rearrangement to achieve B-cell differentiation.

Truncated immunoglobulin Dmu causes incomplete developmental progression of RAG-deficient pro-B cells

Early stages of B cell development are dependent on the expression of a pre-B cell receptor (BCR), composed of a mu heavy chain (HC) in association with surrogate light chain (SLC) proteins and the signaling molecules, Igalpha and Igbeta. During the formation of the variable region of the mu chain by somatic gene rearrangement, a truncated form of the mu protein (called Dmu) is sometimes produced by the rearrangement of a D(H) segment to a J(H) segment using one of three reading frames (designated rf2). When a Dmu protein is formed, subsequent B cell development is blocked by down-regulation of further HC rearrangements, so that a full-length muHC cannot be formed. In this study, we demonstrate that in recombinase activating gene (RAG)-2-deficient B220(+) CD43(+) pro-B cells in which B lymphopoiesis has been arrested at fraction C, transgenic expression of Dmu promoted partial developmental progression to fraction C', but was unable to mediate the pro-B to pre-B cell transition to fraction D effected by full-length muHC protein. These data suggest that the intracellular signaling pathways engaged by the Dmu pre-BCR are insufficient to facilitate the expansion and/or survival of pre-B cells, and are distinct from those engaged by the pre-BCR-containing full-length muHC.

Establishment of novel B-cell precursor leukemia sister cell lines NALM-36 and NALM-37: shift of immunoglobulin phenotype to double light chain positive B-cell

Two novel B-cell precursor (BCP) acute lymphoblastic leukemia (ALL) sister cell lines, designated NALM-36 and NALM-37, were established from the peripheral blood (at diagnosis) and bone marrow (at relapse) of a 37-year-old woman with ALL. Immunophenotyping showed BCP type III pre-B cell characteristics including TdT, CD10, CD19, CD22, CD79a and HLA class II. T cell and myeloid-associated antigens tested were negative except CD5 which was 100% positive for both cell lines. The surrogate light chains lambda5 and VpreB were positive for both cell lines. Cytogenetic analysis of NALM-36 revealed an abnormal karyotype with 46, XX, add(1)(q?42), -14, +mar. Southern blot analysis of the immunoglobulin (Ig) genes status of NALM-36 at 10 months after establishment showed germ line configuration of the kappa light chain gene, and rearrangement of the lambda light and mu heavy chain genes. At 16 months we detected a phenotypic shift of Ig chain protein expression from a BCP-III pre-B cell phenotype to a BCP-IV mature B cell phenotype, with kappa and lambda double Ig light chain and mu heavy chain expression, both on the cell surface and in the cytoplasm. We designated this subline as NALM-36KL. Authenticity of the NALM-36KL, NALM-36 and NALM-37 cell lines was demonstrated by DNA fingerprinting. The extensive characterization of the sister cell lines suggests that these three novel cell lines, derived from a single patient, may represent unique and relevant in vitro model systems for BCP-type leukemia cells. They may provide useful models and unprecedented opportunities for analyzing the multitude of biological aspects of normal and neoplastic B-lymphocytes and their precursors.

B-cell precursors differentiated from cord blood CD34+ cells are more immature than those derived from granulocyte colony-stimulating factor-mobilized peripheral blood CD34+ cells

Umbilical cord blood (CB) has been widely used instead of bone marrow (BM) and peripheral blood (PB) for stem cell transplantation (SCT). However, problems of sustained immunodeficiency after CB transplantation remain to be resolved. To elucidate the mechanism of immunodeficiency, we compared the characteristics of B cells differentiated in vitro from CD34+ cells of CB with those of PB. Purified CD34+ cells from CB and PB were cultured on murine stroma cell-line MS-5 with stem cell factor and granulocyte colony-stimulating factor for 6 weeks. The B-cell precursors (pre-B cells) that differentiated in this culture system, were analysed as to their immunoglobulin heavy chain (IgH) variable region gene repertoire and the expression of B-cell differentiation-related genes. CD10+ CD19+ pre-B cells were differentiated from both PB and CB. Although the usages of IgH gene segments in pre-B cells differentiated from CB and PB were similar, the N region was significantly shorter in CB-derived than PB-derived cells. Productive rearrangements were significantly fewer in cells of CB than PB in the third week. Among a number of B-cell differentiation-related genes, the terminal deoxynucleotidyl transferase (TdT) gene was not expressed in CB-derived cells during the culture. These results indicated that immature features of pre-B cells from CB, such as lack of TdT expression, and a short N region and few productive rearrangements in the IgH gene, might cause the delay in mature B-cell production.

Early death and severe lymphopenia caused by ubiquitous expression of the Rag1 and Rag2 genes in mice

The recombination activating proteins (RAG1 and RAG2) are essential for V(D)J recombination of immunoglobulin chains. Expression of both genes is lymphocyte-specific and RAG levels are tightly regulated throughout lymphopoiesis and cell cycle. To assess the significance of this pattern of expression, we generated transgenic mice expressing the Rag genes both continuously throughout lymphocyte development and constitutively in most non-lymphoid tissues. The transgenes partially complement an endogenous Rag2 null mutation and lead to a partial block in early B and T lymphopoiesis when introduced on a Rag2 sufficient background. The defect in thymocyte number is restricted to the alpha beta lineage leaving the gamma delta T cell pool intact, while neither IgH phenotypic allelic exclusion nor the kappa/lambda light chain ratio are altered. Finally, the ectopic expression of the Rag genes associates with growth retardation and early death of the animals, a phenotype reminiscent of those reported for mice deficient in double-strand break repair molecules.

Novel T-cell receptor delta gene rearrangement involving a recombining element located 2.6 kb 3' from the Vdelta2 gene segment

In this study, we describe a novel T-cell receptor delta (TCRdelta) gene rearrangement observed in acute myeloid leukemia with coexpression of T-lymphoid antigens (Ly+AML) and in peripheral blood leukocytes (PBL) from one out of ten healthy donors. The rearrangement was identified by Southern blot analysis using a joining region (Jdelta1) specific probe and amplified by polymerase chain reaction (PCR) with a variable region (Vdelta2) and Jdelta1 specific primers. The nucleotide sequence analysis of an atypical 3000 bp PCR product allowed localization of the breakpoint within the TCRdelta gene locus, 2.6 kb 3' from the Vdelta2 gene segment. A regular Ddelta2-Ddelta3-Jdelta1 joining was found at the 3' end of the breakpoint, indicating that the rearrangement was mediated by the VDJ recombinase, but no TCRdelta gene segment was detected at the 5' end. Analysis of the germline sequence 3' from the breakpoint revealed an isolated recombination signal sequence (RSS) capable of initiating a rearrangement. The RSS motif described by us is the second TCRdelta recombining element (deltaRec2). The deltaRec2(Ddelta)Jdelta1 recombination is a rather rare event and can be found in acute leukemia and in PBL from healthy individuals. Most likely, the nonfunctional deltaRec2(Ddelta)Jdelta1 rearrangement is a transient step during the VDJ recombination. It may potentially lead to deletion of the deltaRec2(Ddelta)Jdelta1 complex and either to direct joining of a Vdelta region to one of the downstream Jdelta regions or to a rearrangement of the TCRalpha gene.

Immunoglobulin variable region structure and B-cell malignancies

The enormous diversity of immunoglobulin (Ig) variable (V) gene sequences encoding the antibody repertoire are formed by the somatic recombination of relatively few genetic elements. In B-lineage malignancies, Ig gene rearrangements have been widely used for determining clonality and cell origin. The recent development of rapid cloning and sequencing techniques has resulted in a substantial accumulation of IgV region sequences at various stages of B-cell development and has revealed stage-specific trends in the use of V, diversity, joining genes, the degree of noncoding nucleotide addition, and the rate of somatic mutations. Furthermore, sequences from B-lineage malignant cells nearly reflect the characteristics of the normal counterpart at each respective stage of development. Alternatively, from the IgV region structure of the malignant cells, it is possible to speculate at which stage of B-cell development the cells were transformed. As the complete nucleotide sequences of the human Ig heavy and Ig light V region loci have now been determined, the study of Ig genetics has entered into the super-information era.

Multipotent hematopoietic progenitor cells immortalized by Lhx2 self-renew by a cell nonautonomous mechanism

OBJECTIVE

Direct molecular and cellular studies of hematopoietic stem cells (HSCs) are hampered by the low levels of HSCs in hematopoietic tissues. To address these issues, we generated immortalized multipotent hematopoietic precursor cell (HPC) lines by expressing the LIM-homeobox gene Lhx2 (previously LH2) in hematopoietic progenitors derived from embryonic stem cells differentiated in vitro.

MATERIALS AND METHODS

To validate further the relevance of the HPC lines as a model for normal HSCs, we analyzed in detail the growth requirements of HPC lines in vitro.

RESULTS

Lhx2 immortalized the HPC lines by a putatively novel and cell nonautonomous mechanism. Self-renewal of the HPC lines is dependent on functional Lhx2 expression. Most early-acting hematopoiesis-related growth factors show synergistic effects on the HPC lines, whereas late-acting factors do not induce differentiation by themselves. Transforming growth factor-beta(1) is a potent inhibitor of proliferation of the HPC lines. HPC lines form cobblestone areas with high efficiency when seeded onto stromal cell lines, and the cobblestone area-forming cell can be maintained in these cultures for several months.

CONCLUSIONS

Our data show that, in many respects, HPC lines are similar to normal hematopoietic progenitor/stem cells on the cellular level, in contrast to most previously described multipotent hematopoietic cell lines. The cell nonautonomous mechanism for immortalization of the HPC lines suggests that Lhx2 regulates, directly or indirectly, soluble mediators involved in self-renewal of the HPC lines.

Novel B-cell precursor leukemia sister cell lines, NALM-33 and NALM-34, established from a patient with acute lymphoblastic leukemia

Two novel B-cell precursor (BCP) acute lymphoblastic leukemia (ALL) cell lines, designated NALM-33 and NALM-34, were established from a 72 year-old male patient with ALL at relapse. Subcultures of each initial flask were first made after eight weeks of continuous incubation; thus, the two cell lines are simultaneous sister cell lines. The cells proliferate consistently singly and free-floating in suspension. They are negative for Epstein-Barr virus (EBV) infection by polymerase chain reaction (PCR) and are negative for mycoplasma infection. They have the morphological appearance of lymphoblasts with a scanty rim of cytoplasm, fine nuclear chromatin and distinct nucleoli. The primary leukemic blasts showed a common ALL phenotype with CD19+, CD10+, CD13-, HLA-DR+ and Igs-; the cell lines NALM-33/-34 display an identical immunophenotype. They fulfill "European Group for the Immunological Characterization of Leukemias (EGIL)" criteria as BCP leukemia B-II type. While the immunoglobulin heavy chain genes were found uniquely to be in their germline configuration, rearrangement of both kappa and lambda light chain genes was noted by Southern blot analysis. CDR-II detection by reverse transcriptase-PCR was also not detected. NALM-33/-34 did not respond significantly to the proliferative stimuli of various hematopoietic cytokines. In the cytogenetic analysis, they revealed the t(8;14)(q24.1;q32) with additional numerical and structural chromosomal abnormalities. The extensive immunological, cytogenetic and functional characterization of NALM-33/-34 suggests that these two novel cell lines may represent unique and relevant in vitro model systems for BCP-type leukemia cells.

An extrachromosomal switch recombination substrate reveals kinetics and substrate requirements of switch recombination in primary murine B cells

Ig class switch recombination occurs in B lymphocytes upon activation, and is targeted to distinct switch (S) regions by cytokine-mediated induction of switch transcripts spanning the entire S region and the adjacent constant region gene segments. Using a novel type of switch recombination substrate, constructed according to the intron-exon structure of the IgH locus, but with heterologous elements, we here have tested the structural requirements for targeting and the kinetics of switch recombination in activated primary murine B cells. When transfected at various times after activation, up to 10% of the transfected B cells perform recombination of the substrate within 12 h. Switch recombination in primary B cells is restricted to the first 72 h after onset of activation, then rapidly decreases to background levels, as obtained in plasmacytoma cells or with substrates carrying no S region sequences. In terms of structural requirements, switch recombination is targeted to any transcription unit that contains an intronic S region and depends on processing of the primary transcript by splicing.

Processing of switch transcripts is required for targeting of antibody class switch recombination

Antibody class switching is mediated by somatic recombination between switch regions of the immunoglobulin heavy chain gene locus. Targeting of recombination to particular switch regions is strictly regulated by cytokines through the induction of switch transcripts starting 5' of the repetitive switch regions. However, switch transcription as such is not sufficient to target switch recombination. This has been shown in mutant mice, in which the I-exon and its promoter upstream of the switch region were replaced with heterologous promoters. Here we show that, in the murine germline targeted replacement of the endogenous gamma1 promoter, I-exon, and I-exon splice donor site by heterologous promoter and splice donor sites directs switch recombination in activated B lymphocytes constitutively to the gamma1 switch region. In contrast, switch recombination to IgG1 is inhibited in mutant mice, in which the replacement does not include the heterologous splice donor site. Our data unequivocally demonstrate that targeting of switch recombination to IgG1 in vivo requires processing of the Igamma1 switch transcripts. Either the processing machinery or the processed transcripts are involved in class switch recombination.

Immunoglobulin variable region sequences of human monoclonal anti-DNA antibodies

OBJECTIVE

Anti-DNA antibodies are believed to be important in the pathogenesis of systemic lupus erythematosus (SLE). Antibodies that bind specifically and with high affinity to dsDNA are most closely involved in tissue damage. Analysis of the sequences of the variable regions of human monoclonal anti-DNA antibodies is useful in defining the structural features that give rise to these binding properties. This article systematically reviews the evidence derived from such sequences.

METHOD

Previous reviews of this subject have been hampered by incomplete knowledge of the human immunoglobulin variable region repertoire. In this article, the original sequence data from reports of over 50 human monoclonal antibodies (mAb) are reinterpreted by alignment to the most similar alleles of the most similar germline genes. This allows accurate estimation of the site and nature of somatic mutations.

RESULTS

Human IgG monoclonal anti-DNA antibodies generally carry more mutations than IgM. In many cases these have been selected by an antigen-driven process. In many of the more specific, higher affinity dsDNA binders, there is an accumulation of basic residues in the complementarity determining regions. However, many exceptions to this rule exist, particularly among IgM mAb.

CONCLUSIONS

Unlike murine anti-DNA antibodies, these human mAb show little evidence for preferential use of particular V(H), V(K) and V(lambda) genes or families to encode antibodies of this specificity.

A Regulatory Role for Fcγ Receptors CD16 and CD32 in the Development of Murine B Cells

Early in development, murine B-lineage progenitor cells express two classes of IgG Fc receptors (FcγR) designated as FcγRII (CD32) and FcγRIII (CD16), but mature B lymphocytes only express FcγRII (CD32), which functions as an inhibitor of B-cell activation when it is induced to associate with mIgM. The functions of CD16 and CD32 on B-lineage precursor cells have not previously been investigated. To search for FcγR functions on developing B-lineage cells, normal murine bone marrow cells were cultured in the presence of 2.4G2, a rat monoclonal antibody that binds to CD16 and CD32, or in the presence of control normal rat IgG, and then the B-lineage compartment was analyzed for effects. Cultures that contained 2.4G2 showed enhanced growth and differentiation of B-lineage cells compared with control cultures. The enhancing effect of 2.4G2 also occurred when fluorescence-activated cell-sorted B-cell precursors (B220+, sIgM−, HSAhigh, FcγR+) from normal bone marrow were cocultured with BMS2, a bone marrow stromal cell line, but not when they were cultured in BMS2-conditioned media. The enhancement of B-lineage development induced by 2.4G2 was CD16-dependent and CD32-dependent, because 2.4G2 did not effect B-lineage growth or differentiation in cultures of bone marrow from mice in which either the gene encoding CD16 or CD32 had been disrupted. Analysis of fresh bone marrow from the CD16 gene-disrupted mice showed normal numbers and distribution of cells within the B-cell compartment, but in CD32 gene-disrupted mice, the B-cell compartment was significantly enlarged. These experiments provide several lines of evidence that the FcγR expressed on murine B-cell precursors can influence their growth and differentiation.

© 1998 by The American Society of Hematology.

A Regulatory Role for Fcγ Receptors CD16 and CD32 in the Development of Murine B Cells

Early in development, murine B-lineage progenitor cells express two classes of IgG Fc receptors (FcγR) designated as FcγRII (CD32) and FcγRIII (CD16), but mature B lymphocytes only express FcγRII (CD32), which functions as an inhibitor of B-cell activation when it is induced to associate with mIgM. The functions of CD16 and CD32 on B-lineage precursor cells have not previously been investigated. To search for FcγR functions on developing B-lineage cells, normal murine bone marrow cells were cultured in the presence of 2.4G2, a rat monoclonal antibody that binds to CD16 and CD32, or in the presence of control normal rat IgG, and then the B-lineage compartment was analyzed for effects. Cultures that contained 2.4G2 showed enhanced growth and differentiation of B-lineage cells compared with control cultures. The enhancing effect of 2.4G2 also occurred when fluorescence-activated cell-sorted B-cell precursors (B220+, sIgM−, HSAhigh, FcγR+) from normal bone marrow were cocultured with BMS2, a bone marrow stromal cell line, but not when they were cultured in BMS2-conditioned media. The enhancement of B-lineage development induced by 2.4G2 was CD16-dependent and CD32-dependent, because 2.4G2 did not effect B-lineage growth or differentiation in cultures of bone marrow from mice in which either the gene encoding CD16 or CD32 had been disrupted. Analysis of fresh bone marrow from the CD16 gene-disrupted mice showed normal numbers and distribution of cells within the B-cell compartment, but in CD32 gene-disrupted mice, the B-cell compartment was significantly enlarged. These experiments provide several lines of evidence that the FcγR expressed on murine B-cell precursors can influence their growth and differentiation.

© 1998 by The American Society of Hematology.

The role of in vitro expression systems in the investigation of antibodies to DNA

OBJECTIVES

Antibodies to DNA are believed to be important in the development of tissue inflammation and clinical activity in systemic lupus erythematosus (SLE). Sequence analysis of monoclonal murine and human anti-DNA antibodies suggests that somatic mutations and basic residues are important features at the DNA-binding site. To test this hypothesis, it is possible to alter these residues by site-directed mutagenesis of cloned variable region cDNA. The mutagenized cDNA sequence is then expressed in the form of a protein molecule whose properties can be tested in assays of binding or pathogenicity. The purpose of this article is to provide a systematic review of the evidence derived by such methods in the study of anti-DNA antibodies.

METHODS

Various different expression systems are available. Experiments using bacterial and eukaryotic expression systems are considered in turn. The advantages and disadvantages of the systems are described and the results obtained are compared.

RESULTS AND CONCLUSIONS

High yields of antibody fragments such as scFv and Fab can be achieved by expression in bacteria. Such studies tend to confirm that reversion of somatic mutations or removal of basic residues at the antigen binding site reduce affinity for DNA. Tests of pathogenicity can only be performed by expressing whole antibodies in eukaryotic cells. The limited data available from expression of mutagenized cDNA in such systems argue against a simple relationship between changes in DNA binding affinity and changes in pathogenic potential. Further studies are therefore required to analyze the sequence requirements for pathogenicity.

Induction of early B cell factor (EBF) and multiple B lineage genes by the basic helix-loop-helix transcription factor E12

The transcription factors encoded by the E2A and early B cell factor (EBF) genes are required for the proper development of B lymphocytes. However, the absence of B lineage cells in E2A- and EBF-deficient mice has made it difficult to determine the function or relationship between these proteins. We report the identification of a novel model system in which the role of E2A and EBF in the regulation of multiple B lineage traits can be studied. We found that the conversion of 70Z/3 pre-B lymphocytes to cells with a macrophage-like phenotype is associated with the loss of E2A and EBF. Moreover, we show that ectopic expression of the E2A protein E12 in this macrophage line results in the induction of many B lineage genes, including EBF, IL7Ralpha, lambda5, and Rag-1, and the ability to induce kappa light chain in response to mitogen. Activation of EBF may be one of the critical functions of E12 in regulating the B lineage phenotype since expression of EBF alone leads to the activation of a subset of E12-inducible traits. Our data demonstrate that, in the context of this macrophage line, E12 induces expression of EBF and together these transcription factors coordinately regulate numerous B lineage-associated genes.

Somatic mutations in the Ig variable region genes and expression of novel Cmu-germline transcripts in a B-lymphoma cell line ("Farage") not producing Ig polypeptide chains

Non-Hodgkin's B-lymphomas (B-NHL) are a very heterogeneous group of B-cell neoplasias originating from the germinal centers of lymphatic follicles. Thus, they represent a suitable experimental model to study the molecular basis of certain key events which take place in the lymphatic follicles, including somatic hypermutation and heavy chain isotypic switch. An unusual B-NHL cell line ("Farage") not producing Ig polypeptide chains was previously shown to rearrange its IgH and Igkappa genes and transcribe seemingly normal size mu and kappa mRNAs. In an attempt to characterize the phenotype of Farage cells better and to elucidate the molecular basis of the failure of Farage cells to synthesize Ig chains, we sequenced its VH and Vkappa rearranged gene segments by PCR and RT-PCR. It was found that both V genes are somatically, heavily mutated compared to their germline counterparts. In addition, this rearranged VDJ gene of the heavy chain is not transcribed. Instead, the Farage cells express a low level of a new family of germline transcripts starting with a VH like sequence, continuing with a small segment of the 3'VH germline flanking region, and ending within the Cmu region. These transcripts lack D and J segments and do not contain the open reading frame of the full-length Cmu protein. Thus, Farage cells fail to produce mu heavy chains due to silencing of the expression of the conventional VDJCmu transcript and expression of unusual Cmu-germline transcripts. In contrast to the IgH genes, the rearranged VJ gene of Farage is transcribed and gives rise to a full-size kappa-mRNA. This transcript, however, is not translated to a full-length kappa-chain, as it contains a stop codon in its coding region. All the above show that Farage cells are unable to produce Ig polypeptide chains, due to somatic mutations altering the kappa-chain gene, and mutations and/or regulatory events that shutoff the transcription of the IgH gene. The heavily mutated Vkappa and Vkappa genes found, support the conclusion that the Farage cell line originated either from germinal center cells or from the mantle zone of the lymphoid follicle.