Development and function of the early B cell repertoire

Drivers and regulators of humoral innate immune responses to infection and cancer

The complement cascade consists of cell bound and serum proteins acting together to protect the host from pathogens, remove cancerous cells and effectively links innate and adaptive immune responses. Despite its usefulness in microbial neutralization and clearance of cancerous cells, excessive complement activation causes an immune imbalance and tissue damage in the host. Hence, a series of complement regulatory proteins present at a higher concentration in blood plasma and on cell surfaces tightly regulate the cascade. The complement cascade can be initiated by B-1 B cell production of natural antibodies. Natural antibodies arise spontaneously without any known exogenous antigenic or microbial stimulus and protect against invading pathogens, clear apoptotic cells, provide tissue homeostasis, and modulate adaptive immune functions. Natural IgM antibodies recognize microbial and cancer antigens and serve as an activator of complement mediated lysis. This review will discuss advances in complement activation and regulation in bacterial and viral infections, and cancer. We will also explore the crosstalk of natural antibodies with bacterial populations and cancer.

Mapping an epitope in EBNA-1 that is recognized by monoclonal antibodies to EBNA-1 that cross-react with dsDNA

Introduction

The Epstein Barr Virus (EBV) has been associated with the autoimmune disease, Systemic Lupus Erythematosus (SLE). EBV nuclear antigen‐I (EBNA‐1) is the major nuclear protein of EBV. We previously generated an IgG monoclonal antibody (MAb) to EBNA‐1, 3D4, and demonstrated that it cross‐reacts with double stranded DNA (dsDNA) and binds the 148 amino acid viral binding site (VBS) in the carboxyl region of EBNA‐1. The aim of the present study was to characterize another antibody to EBNA‐1 that cross‐reacts with dsDNA, compare its immunoglobulin genes to 3D4, and finely map the epitope in EBNA‐1 that is recognized by these cross‐reactive antibodies.

Methods

We generated an IgM MAb to EBNA‐1, 16D2, from EBNA‐1 injected mice and demonstrated by ELISA that it cross‐reacts with dsDNA and binds the 148 amino acid VBS. We sequenced the variable heavy and light chain genes of 3D4 and 16D2 and compared V gene usage. To more finely map the epitope in EBNA‐1 recognized by these MAbs, we examined their binding by ELISA to 15 overlapping peptides spanning the 148 amino acid domain.

Results

Sequence analysis revealed that 3D4 and 16D2 utilize different V_H and V_L genes but identical J_H and J_k regions with minimal junctional diversity. This accounts for similarities in their CDR3 regions and may explain their similar dual binding specificity. Epitope mapping revealed 3D4 and 16D2 bind the same peptide in the VBS. Based on the crystal structure of EBNA‐1, we observed that this peptide resides at the base of an exposed proline rich loop in EBNA‐1.

Conclusion

We have demonstrated that two MAbs that bind EBNA‐1 and cross‐react with dsDNA, recognize the same peptide in the VBS. This peptide may serve as a mimetope for dsDNA and may be of diagnostic and therapeutic value in SLE.

Characterization of omental immune aggregates during establishment of a latent gammaherpesvirus infection

Herpesviruses are characterized by their ability to establish lifelong latent infection. The gammaherpesvirus subfamily is distinguished by lymphotropism, establishing and maintaining latent infection predominantly in B lymphocytes. Consequently, gammaherpesvirus pathogenesis is closely linked to normal B cell physiology. Murine gammaherpesvirus 68 (MHV68) pathogenesis in laboratory mice has been extensively studied as a model system to gain insights into the nature of gammaherpesvirus infection in B cells and their associated lymphoid compartments. In addition to B cells, MHV68 infection of macrophages contributes significantly to the frequency of viral genome-positive cells in the peritoneal cavity throughout latency. The omentum, a sheet of richly-vascularized adipose tissue, resides in the peritoneal cavity and contains clusters of immune cell aggregates termed milky spots. Although the value of the omentum in surgical wound-healing has long been appreciated, the unique properties of this tissue and its contribution to both innate and adaptive immunity have only recently been recognized. To determine whether the omentum plays a role in gammaherpesvirus pathogenesis we examined this site during early MHV68 infection and long-term latency. Following intraperitoneal infection, immune aggregates within the omentum expanded in size and number and contained virus-infected cells. Notably, a germinal-center B cell population appeared in the omentum of infected animals with earlier kinetics and greater magnitude than that observed in the spleen. Furthermore, the omentum harbored a stable frequency of viral genome-positive cells through early and into long-term latency, while removal of the omentum prior to infection resulted in a slight decrease in the establishment of splenic latency following intraperitoneal infection. These data provide the first evidence that the omentum is a site of chronic MHV68 infection that may contribute to the maintenance of chronic infection.

Activation-induced deaminase-deficient MRL/lpr mice secrete high levels of protective antibodies against lupus nephritis

OBJECTIVE

We previously generated MRL/lpr mice deficient in activation-induced deaminase (AID) that lack isotype switching and immunoglobulin hypermutation. These mice have high levels of unmutated (germline) autoreactive IgM, yet they experienced an increase in survival and an improvement in lupus nephritis that exceeded that of MRL/lpr mice lacking IgG. The purpose of the present study was to test the hypothesis that high levels of germline autoreactive IgM in these mice confer protection against lupus nephritis.

METHODS

Autoreactive IgM antibodies of various specificities, including antibodies against double-stranded DNA (dsDNA), from AID-deficient MRL/lpr mice were given to asymptomatic MRL/lpr mice, and the levels of cytokines, proteinuria, immune complex deposition in the kidneys, and glomerulonephritis were examined. Novel AID-deficient MRL/lpr mice that lack any antibodies were generated for comparison to AID-deficient MRL/lpr mice that secrete only IgM.

RESULTS

Treatment with IgM anti-dsDNA resulted in a dramatic improvement in lupus nephritis. Other autoreactive IgM antibodies, such as antiphospholipid and anti-Sm, did not alter the pathologic changes. Secretion of proinflammatory cytokines by macrophages and the levels of inflammatory cells and apoptotic debris in the kidneys were lower in mice receiving IgM anti-dsDNA. Protective IgM derived from AID-deficient MRL/lpr mice displayed a distinct B cell repertoire, with a bias toward members of the V(H) 7183 family.

CONCLUSION

IgM anti-dsDNA protected MRL/lpr mice from lupus nephritis, likely by stopping the inflammatory cascade leading to kidney damage. A distinct repertoire of V(H) usage in IgM anti-dsDNA hybridomas from AID-deficient mice suggests that there is enrichment of a dedicated B cell population that secretes unmutated protective IgM in these mice.

Omental immune aggregates and tumor metastasis within the peritoneal cavity

The omentum, an important peritoneal tissue, is studded with a high number of immune aggregates, or "milky spots," the number, function, and phenotype of which is largely unknown. We have analyzed the immune composition on the normal omentum and also have shown that both free immune cells and tumor cells in the peritoneal fluid bind preferentially to these immune aggregates. This binding may be mediated by the network of collagen I fibers, which overlay these areas. In addition, we have shown that not only do omental vessels express vascular endothelial growth factor receptor 3 (VEGFR3), a receptor that is only found on angiogenic blood vessels, but that tumor cells co-localize with these vessels, possibly increasing the ability of tumor to induce neovascularization and therefore thrive.

Repertoire-based selection into the marginal zone compartment during B cell development

Marginal zone (MZ) B cells resemble fetally derived B1 B cells in their innate-like rapid responses to bacterial pathogens, but the basis for this is unknown. We report that the MZ is enriched in “fetal-type” B cell receptors lacking N regions (N⁻). Mixed bone marrow (BM) chimeras, made with adult terminal deoxynucleotidyl transferase (TdT)^+/+ and TdT^−/− donor cells, demonstrate preferential repertoire-based selection of N⁻ B cells into the MZ. Reconstitution of irradiated mice with adult TdT^+/+ BM reveals that the MZ can replenish N⁻ B cells in adult life via repertoire-based selection and suggest the possibility of a TdT-deficient precursor population in the adult BM. The mixed chimera data also suggest repertoire-based bifurcations into distinct BM and splenic maturation pathways, with mature “recirculating” BM B cells showing a very strong preference for N⁺ complementarity-determining region (CDR) 3 compared with follicular B cells. Because the T1 and MZ compartments are both the most enriched for N⁻ H-CDR3, we propose a novel direct T1→MZ pathway and identify a potential T1–MZ precursor intermediate. We demonstrate progressive but discontinuous repertoire-based selection throughout B cell development supporting multiple branchpoints and pathways in B cell development. Multiple differentiation routes leading to MZ development may contribute to the reported functional heterogeneity of the MZ compartment.

Antibody repertoire development in fetal and neonatal piglets: XIX. Undiversified B cells with hydrophobic HCDR3s preferentially proliferate in the porcine reproductive and respiratory syndrome

Porcine respiratory and reproductive syndrome virus (PRRSV) causes an extraordinary increase in the proportion of B cells resulting in lymphoid hyperplasia, hypergammaglobulinemia, and autoimmunity in neonatal piglets. Spectratypic analysis of B cells from neonatal isolator piglets show a non-Gaussian pattern with preferential expansion of clones bearing certain H chain third complementary region (HCDR3) lengths. However, only in PRRSV-infected isolator piglets was nearly the identical spectratype observed for all lymphoid tissues. This result suggests dissemination of the same dominant B cell clones throughout the body. B cell expansion in PRRS was not associated with preferential VH gene usage or repertoire diversification and these cells appeared to bear a naive phenotype. The B cell population observed during infection comprised those with hydrophobic HCDR3s, especially sequences encoded by reading frame 3 of DHA that generates the AMVLV motif. Thus, the hydropathicity profile of B cells after infection was skewed to favor those with hydrophobic binding sites, whereas the normally dominant region of the hydropathicity profile containing neutral HCDR3s was absent. We believe that the hypergammaglobulinemia results from the products of these cells. We speculate that PRRSV infection generates a product that engages the BCR of naive B cells, displaying the AMVLV and similar motifs in HCDR3 and resulting in their T-independent proliferation without repertoire diversification.

IgM-enriched human intravenous immunoglobulin suppresses T lymphocyte functions in vitro and delays the activation of T lymphocytes in hu-SCID mice

Previous studies of an experimental human immunoglobulin preparation for intravenous use, containing normal pooled IgM (IVIgM), have shown its beneficial therapeutic effect in experimental autoimmune diseases. The mechanisms of its immunomodulatory activity remain however, poorly understood. In the experiments reported here, IVIgM inhibited the proliferation of various autonomously growing human lymphoid cell lines in vitro, as well as of MLR- and of PHA-stimulated human T-lymphocytes. These effects of IVIgM were observed at non-apoptotic concentrations and were stronger on a molar basis than those of normal pooled IgG for intravenous use (IVIg). Both preparations, when administered to SCID mice, repopulated with human peripheral blood mononuclear cells, delayed the expression of the early activation marker CD69 on both human CD4+ and CD8+ T-lymphocytes, activated by the mouse antigenic environment. The data obtained show that normal pooled human IgM exerts a powerful antiproliferative effect on T-cells that is qualitatively similar but quantitatively superior to that of therapeutic IVIg. Our results suggest that infusions with IVIgM might have a significant beneficial immunomodulating activity in patients with selected autoimmune diseases.

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.

Reconstitution of self-reactive antibody repertoires of autologous plasma IgM in patients with non-Hodgkin's lymphoma following myeloablative therapy

In healthy individuals, natural self-reactive antibody repertoires are restricted to a limited subset of autoantigens that is selected early in development and that remains invariant between individuals through aging. In the present study, we addressed the question of whether self-reactive antibody repertoires of plasma IgM change during high-dose chemotherapy (HDCT) with autologous blood stem cell support and whether antibody repertoires generated during immune reconstitution are similar to those present under physiological conditions. We followed the development of antibody repertoires in patients undergoing HDCT for the treatment of B-cell non-Hodgkin's lymphoma (NHL). Antibody repertoires were investigated by quantitative immunoblotting on whole tissue extracts as sources of self-antigens and by multiparametric statistical analysis of the data. We demonstrate that self-reactive antibody repertoires of plasma IgM of NHL patients prior to HDCT differ from those of healthy individuals, that they change during recovery of immune functions, and that antibody repertoires similar to those of healthy individuals are generated during immune reconstitution. We conclude that the mechanisms responsible for the selection of self-reactive repertoires of autologous plasma IgM during immune reconstitution after HDCT may follow those present under physiological conditions and that immune reconstitution may include a shift from altered toward normal patterns of self-reactivity.

Antibodies binding granulocyte-macrophage colony stimulating factor produced by cord blood-derived B cell lines immortalized by Epstein-Barr virus in vitro

We detected natural antibodies (auto-Abs) binding human granulocyte-macrophage colony stimulating factor (GM-CSF) in umbilical cord blood (CB) (23 of 94 samples screened) and peripheral blood of women at the end of pregnancy (6 of 42 samples tested). To demonstrate that Abs detected in CB were produced by the fetus, CB mononuclear cells were infected with Epstein-Barr virus in vitro. Ten cell lines producing constitutively anti-recombinant human GM-CSF (rhGM-CSF) Abs were isolated and characterized. These cells displayed a male karyotype, an early activated B cell phenotype, coexpressed surface IgM and IgD, and secreted only IgM with prevailing lambda clonal restriction. Specific cell surface binding of biotinylated rhGM-CSF and high-level anti-rhGM-CSF IgM Ab production were typical features of early cell cultures. In late cell passages the frequency of more undifferentiated B cells increased. Serum Abs of either maternal or fetal origin or Abs produced in culture did not affect the granulocyte and macrophage colony stimulating activity of rhGM-CSF from bone marrow progenitors in soft agar, suggesting that the Abs produced were nonneutralizing.

Generation of B lymphoma cell lines from knockout mice by transformation in vivo with an Emu-myc transgene

While a great deal has been learned about the genetic control of B lymphocyte behavior through the observation of primary B cells from mice bearing targeted gene mutations, such studies can be restricted by the limited number and longevity of the cells ex vivo, by their heterogeneity, and by the inability to apply further genetic manipulations. Here we describe a protocol for the efficient derivation of mutant B lymphoma cell lines, by crossing mice bearing targeted mutations in genes affecting mature B cell function with transgenic mice bearing the lymphomagenic Emu-myc transgene. Pre-B and B lymphomas were obtained with high frequency, and the cells were readily adaptable to culture. The B lymphoma lines bore surface markers consistent with an immature phenotype and were amenable to cloning and to stable transfection. They are currently being used as a well-defined and unlimited cell source to study antigen receptor signalling and B cell-specific gene regulation, and the dependence of these processes on the products of the vav, CD45, lyn, oct-2, btk and OBF-1 genes.

The role of components of recombination signal sequences in immunoglobulin gene segment usage: a V81x model

It has long been appreciated that some immunoglobulin (and T-cell receptor) gene segments are used much more frequently than others. The VHsegment V81x is a particularly striking case of overusage. Its usage varies with the stage of B-cell development and with the strain of mice, but it is always high in B cell progenitors. We have found that the coding sequence and the recombination signal sequences (RSS) are identical in five mouse strains, including CAST/Ei, a strain derived from the species Mus castaneus. Thus, the strain differences cannot be attributed to sequences within V81x itself. V81x RSS mediated recombination at rates significantly higher than another VHRSS. Although the V81x nonamer differs at one base pair from the consensus sequence, an RSS with this nonamer and a consensus heptamer recombines as well as the consensus RSS. When the V81x spacer is replaced by that of VA1, the frequency of recombination decreases by approximately 5-fold; thus, the contribution of variation in natural spacers to variability in VHusage in vivo is likely to be more than has been previously appreciated. Furthermore, the contribution of the heptamer and nonamer to differential VHusage in our assay is correlated inversely with their conservation throughout the VHlocus.

Antibody Repertoire Development in Fetal and Neonatal Piglets. I. Four VH Genes Account for 80 Percent of VH Usage During 84 Days of Fetal Life

VDJ rearrangement and VH gene usage during fetal development in 35 outbred piglets was examined by PCR amplification of VDJs; VDJs were subsequently characterized by hybridization with VH-specific gene probes and by sequencing. VDJ rearrangement was first seen in the fetal liver on day 30 of a 114-day gestation. Four VH genes (VHA, VHB, VHC, and VHE) accounted for ~80% of all VH gene usage regardless of gestational age, choice of piglet, or lymphoid tissue tested; DHA and DHB were used in >90% of the fetal VDJs examined. Evidence of somatic hypermutation during fetal development was not found. The proportion of the four prominent fetal VH genes did not differ significantly between cDNA and DNA, suggesting the absence of selective B cell differentiation. A comparison of recombination signal sequences, flanking sequences, and framework sequences of these fetal genes with other germline VH genes of swine offered no clue as to their selective usage. N-region additions were prominent on day 40 but not on day 30, suggesting that the onset of terminal deoxynucleotidyltransferase activity occurs after 30 days of fetal development. These collective findings indicate that the preimmune, “natural Ab” repertoire of the fetal piglet is largely restricted to the use of four nonpolymorphic and nonmutated VH genes and two nonmutated DH segments. This suggests that the preimmune repertoire of swine is either highly restricted or almost entirely determined by junctional diversity in complementarity-determining region-3.

The role of interleukin-5 (IL-5) in vivo: studies with IL-5 deficient mice

Eosinophil recruitment is a characteristic feature of a number of pathological conditions and was the topic of the recent International Symposium on allergic inflammation, asthma, parasitic and infectious diseases (Rio de Janeiro, June 3-5, 1996). Since interleukin-5 (IL-5) is believed to regulate the growth, differentiation and activation of eosinophils (Coffman et al. 1989, Sanderson 1992), the role of eosinophils and IL-5 are closely linked. Although IL-5 specifically regulates eosinophilia in vivo and this is its most well established activity, it is becoming clear that IL-5 also has other biological effects. The recent derivation of an IL-5 deficient mouse (Kopf et al. 1996), provides a model for exploring not only the role of IL-5 and eosinophils but also other novel activities of IL-5. Of note is that although the IL-5 deficient mice cannot elicit a pronounced eosinophilia in response to inflammatory stimulation following aeroallergen challenge or parasite infection they still produce basal levels of eosinophils that appear to be morphologically and functionally normal. However, the basal levels of eosinophils appear insufficient for normal host defense as IL-5 deficiency has now been shown to compromise defense against several helminth infections. In addition, IL-5 deficient mice appear to have functional deficiencies in B-1 B lymphocytes and in IgA production.

Natural and therapeutically-induced antibodies to cytokines

Serum samples obtained from non-immunocompromised patients treated therapeutically with recombinant cytokines (e.g. Il-1alpha; Il-1beta; Il-2 to Il-18; IFNalpha; GM-CSF; G-CSF; etc.) often reveal the presence of high affinity anti-cytokine antibodies. Antibody Fab binding in a saturable manner by ELISA and RIA or western immunoblotting prove their specificity. Antibody level often increases in these patients with repeated cytokine administration, suggesting boosts of antigen stimulation. However, the appearance in circulation of auto-antibodies to exogenous cytokine is not always associated with a decreased clinical response to therapy. The demonstration that non-neutralizing auto-antibodies to several natural cytokines can be found even in sera of normal healthy individuals never treated before with cytokines and particularly during the last trimester of pregnancy and in cord-blood, suggests that these naturally- occurring and therapeutically-induced auto-antibodies may exert different functions, not only as inhibitors or antagonists but also as beneficial physiological cytokine carriers or regulators of their activity.

Generation of the Germline Peripheral B Cell Repertoire: VH81X-λ B Cells Are Unable to Complete All Developmental Programs

The generation of VH81X heavy chain λ-light chain-expressing B cells (VH81X-λ+ B cells) was studied in VH81X heavy chain transgenic mice as well as in VH81X JH −/− and VH81X JH −/− Ck −/− mice, in which competition resulting from expression of heavy and light chains from the endogenous heavy and κ light chain loci was prevented. We show that although λ light chain gene rearrangements occur normally and give rise to light chains that associate with the transgenic heavy chain to form surface and soluble IgM molecules, further B cell development is almost totally blocked. The few VH81X-λ+ B cells that are generated progress into a mature compartment (expressing surface CD21, CD22, CD23, and low CD24 and having a relatively long life span) but they also have reduced levels of surface Ig receptor and express higher amounts of Fas Ag than VH81X-κ+ B cells. These VH81X-λ+ B cells reach the peripheral lymphoid organs and accumulate in the periarteriolar lymphoid sheath but are unable to generate primary B cell follicles. In other heavy chain transgenic mice (MD2, M167, and M54), λ+ B cells are generated. However, they seem to be preferentially selected in the peripheral repertoire of some transgenic heavy chain mice (M54) but not in others (MD2, M167). These studies show that a crucial selection step is necessary for B cell survival and maintenance in which B cells, similar to T cells, receive signals depending on their clonal receptors.

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.

Natural and therapy-induced anti-GM-CSF and anti-G-CSF antibodies in human serum

Serum samples were obtained from patients with lymphoid and plasma cell malignancies who received after chemotherapy human recombinant GM-CSF or G-CSF. Sera from some patients revealed the presence of anti-cytokine antibodies, particularly after repetitive cytokine injections. Antibody Fab binding in a saturable manner by ELISA and Western immuno-blotting confirmed antibody specificity. Anti-cytokine antibodies were detected before the exogenous cytokine injections in some patients, but increasing antibody levels were found after one or subsequent treatments. Low levels of anti-GM-CSF and anti-G-CSF antibodies were also detected in a relatively large proportion (about 10-30%) of normal sera from different adult healthy volunteers who had never been treated before with exologous cytokines as well as from cord blood. EBV-immortalized cord blood derived B-cell cultures were also found to produce anti GM-CSF and/or anti-G-CSF antibodies with high frequency.

Ig DH Gene Segment Transcription and Rearrangement Before Surface Expression of the Pan-B–Cell Marker CD19 in Normal Human Bone Marrow

The onset of IgH transcription and rearrangement is a defining characteristic of the progenitor population in which B-lineage commitment occurs. These features were used to better define the earliest stage of B-cell commitment in humans and to determine if these stages differ as a function of human ontogeny. Fetal and adult bone marrow mononuclear cells were sorted into B-lineage subpopulations on the basis of surface expression of the stem cell marker CD34, the pan-B–cell marker CD19, and IgM and analyzed for transcription and rearrangement of the IgH locus. The locus was found to be transcriptionally active before surface expression of CD19, as indicated by the presence of germline Iμ, Cμ, and DHQ52 transcripts in the CD34+ CD19− subpopulation. Transcripts from IgH alleles that had undergone DJCμ rearrangements were also detected in the CD34+ CD19− subpopulation. Within this subpopulation, low levels of DXP-containing DJCμ transcripts were detected in both fetal and adult cells. Although DHQ52 DJCμ transcripts were abundant in fetal CD34+ CD19− cells, they were not detected in cells of the same phenotype derived from adult bone marrow. In both fetus and adult, VH3-and VH6-containing VDJCμ transcripts were detected only in the CD19+ subpopulations. These data indicate that transcription of DHQ52-JH and DXP-JH rearrangements differs during fetal and adult B lymphopoiesis. Moreover, in both fetus and adult, transcription of unrearranged components of the IgH locus and DJ rearrangements can proceed before the surface expression of CD19.

Ig DH Gene Segment Transcription and Rearrangement Before Surface Expression of the Pan-B–Cell Marker CD19 in Normal Human Bone Marrow

The onset of IgH transcription and rearrangement is a defining characteristic of the progenitor population in which B-lineage commitment occurs. These features were used to better define the earliest stage of B-cell commitment in humans and to determine if these stages differ as a function of human ontogeny. Fetal and adult bone marrow mononuclear cells were sorted into B-lineage subpopulations on the basis of surface expression of the stem cell marker CD34, the pan-B–cell marker CD19, and IgM and analyzed for transcription and rearrangement of the IgH locus. The locus was found to be transcriptionally active before surface expression of CD19, as indicated by the presence of germline Iμ, Cμ, and DHQ52 transcripts in the CD34+ CD19− subpopulation. Transcripts from IgH alleles that had undergone DJCμ rearrangements were also detected in the CD34+ CD19− subpopulation. Within this subpopulation, low levels of DXP-containing DJCμ transcripts were detected in both fetal and adult cells. Although DHQ52 DJCμ transcripts were abundant in fetal CD34+ CD19− cells, they were not detected in cells of the same phenotype derived from adult bone marrow. In both fetus and adult, VH3-and VH6-containing VDJCμ transcripts were detected only in the CD19+ subpopulations. These data indicate that transcription of DHQ52-JH and DXP-JH rearrangements differs during fetal and adult B lymphopoiesis. Moreover, in both fetus and adult, transcription of unrearranged components of the IgH locus and DJ rearrangements can proceed before the surface expression of CD19.

Signaling thresholds and interclonal competition in preimmune B-cell selection

The need to eliminate autoreactive B cells must be checked against the need for a diverse B-cell repertoire. Protein tyrosine phosphatases SHP1 and CD45 act antagonistically within B cells to set the threshold level of antigen-receptor engagement required for B-cell elimination. The fate of B cells binding weak autoantigens is also regulated by interclonal competition. In the presence of a normal diverse repertoire of competitor B cells, the autoantigen-binding cells are excluded from follicles in spleen and lymph nodes and undergo rapid cell death. In the absence of interclonal competition, the autoreactive cells enter the follicular microenvironment and survive. A model in which B cells compete for access to limiting follicular niches in order to survive is discussed.

Targeted disruption of the V(H) 81X gene: influence on the B cell repertoire

We have generated a mutant mouse in which the most D-proximal V(H) gene (V(H)81X) has been disrupted by introducing a neomycin-resistance gene into the V(H)81X exon by means of gene targeting in embryonic stem cells. The mutant mice generated are unable to express the V(H)81X gene but appear to display a normal pattern of B cell differentiation as well as normal numbers of bone marrow and peripheral B cells from fetal life all through ontogeny. They mount normal immune responses to several different antigens tested. In contrast, the distribution of V(H) gene rearrangements in the V(H)7183 family is altered in homozygous mutant mice. Thus, the antibody repertoire of the targeted mice is modified, at least as far as the expression of V(H)7183 genes is concerned.

B cell development in mice

The development and establishment of the B Cell Repertoire is the net result of both genetic and environmental forces. The primary event at the genetic level is Ig gene rearrangement resulting in numerous possible combination of genes which can be further modified by somatic events such as N segment addition and somatic mutation. Environmental forces in the form of self and exogenous Ags also shape the repertoire by positively or negatively selecting B cells according to the specificity of their Ig receptors. These are dynamic processes beginning with the earliest expression of immunoglobulins in fetal life and continuing throughout life. In this review we discuss the genetic and selective mechanisms responsible for differences in the early immune system compared to that of the adult.

Balancing immunity and tolerance: deleting and tuning lymphocyte repertoires

Immunological self-tolerance is ensured by eliminating or inhibiting self-reactive lymphocyte clones, creating physical or functional holes in the B- and T-lymphocyte antigen receptor repertoires. The nature and size of these gaps in our immune defenses must be balanced against the necessity of mounting rapid immune responses to an everchanging array of foreign pathogens. To achieve this balance, only a fraction of particularly hazardous self-reactive clones appears to be physically eliminated from the repertoire in a manner that fully prevents their recruitment into an antimicrobial immune response. Many self-reactive cells are retained with a variety of conditional and potentially flexible restraints: (i) their ability to be triggered by antigen is diminished by mechanisms that tune down signaling by their antigen receptors, (ii) their ability to carry out inflammatory effector functions can be inhibited, and (iii) their capacity to migrate and persist is constrained. This balance between tolerance and immunity can be shifted, altering susceptibility to autoimmune disease and to infection by genetic or environmental differences either in the way antigens are presented, in the tuning molecules that adjust triggering set points for lymphocyte responses to antigen, or in the effector molecules that eliminate, retain, or expand particular clones.

IL-5-deficient mice have a developmental defect in CD5+ B-1 cells and lack eosinophilia but have normal antibody and cytotoxic T cell responses

Mice deficient in interleukin-5 (IL-5-/- mice) were generated by gene targeting in embryonal stem cells. Contrary to previous studies, no obligatory role for IL-5 was demonstrated in the regulation of conventional B (B-2) cells, in normal T cell-dependent antibody responses or in cytotoxic T cell development. However, CD5+ B cells (B-1 cells) in the peritoneal cavity were reduced by 50%-80% in 2-week-old IL-5-/- mice, returning to normal by 6-8 weeks of age. The IL-5-/- mice did not develop blood and tissue eosinophilia when infected with the helminth Mesocestoides corti, but basal levels of eosinophils with normal morphology were produced in the absence of IL-5. IL-5 deficiency did not affect the worm burden of infected mice, indicating that increased eosinophils do not play a significant role in the host defence in this parasite model.

The regulation of self-reactive B cells

Self-reactive B cells are eliminated in a series of checkpoints that are triggered by antigen binding. Recent reports have shown that in addition to the processes of elimination at the immature B-cell stage, B-cell anergy and regulation of T-cell help, self-reactive cells are also controlled by follicular competition, Fas-mediated elimination by T cells and censoring in the germinal centres. Each checkpoint operates at a threshold that reflects the need to maintain immune diversity at the same time as suppressing autoimmune disease. Analysis of the motheaten mutation has given a direct demonstration of how such thresholds can be modulated by genetic effects.

Development of the mouse B-cell repertoire

The mouse B-cell repertoire early in ontogeny contains B cells with receptor immunoglobulins exhibiting high connectivity, multi/self-reactivity, and generally low affinity. This is due structurally to extensive restriction in the germline components used to generate the B-cell receptor. The selective pressure acting on the nascent repetoire has both negative and positive components as seen in our in vivo models. VH81X-bearing B cells from the VH81X transgenic mice (and probably from normal mice) are subject to self-selective pressure with two components: a positive one favoring a certain (self-reactive) specificity in the CD23-IgM+ population and a negative one preventing the entry of B cells with this specificity into the CD23+IgM+ compartment.

Abnormal B lymphocyte development, activation, and differentiation in mice that lack or overexpress the CD19 signal transduction molecule

CD19-deficient mice were generated to examine the role of CD19 in B cell growth regulation in vivo. Deletion of CD19 had no deleterious effects on the generation of B cells in the bone marrow, but there was a significant reduction in the number of B cells in peripheral lymphoid tissues. B cells from CD19-deficient mice exhibited markedly decreased proliferative responses to mitogens, and serum immunoglobulin levels were also significantly decreased. In contrast, mice that overexpressed CD19 had significant defects in early B cell development in the bone marrow, augmented mitogenic responses, and increased serum immunoglobulin levels. These experiments indicate that CD19 functions to define signaling thresholds for cell surface receptors that regulate B lymphocyte selection, activation, and differentiation.

Molecular characterization of monoclonal IgM derived from human B cell lines expressing the 4C9 rheumatoid factor associated idiotype

Ten human monoclonal B cell lines that express the RF associated Id 4C9 were analyzed using an immunogenetic approach. Five of eight tested lines were also strongly positive for the 6B6.6 Id. We found that all the 4C9/6B6.6 positive lines expressed VkIIIa light chain genes. In contrast, 4C9 reactivity was also found on a cell line expressing a VkIIIb light chain gene and on a line expressing a V light chain gene. The two anti-Ids recognized a linear light chain determinant on Humkv328 encoded light chains but also a conformational determinant on Vg encoded light chains that appeared to be dependent on the presence of a heavy chain. Idiotypic reactivity occurred on both RF positive and RF negative antibodies. Within this idiotypic system, the basis for idiotypic reactivity and RF reactivity is complex, subject to both heavy and light chain gene usage and sensitive to small numbers of somatic mutations.

Protein tyrosine phosphatase 1C negatively regulates antigen receptor signaling in B lymphocytes and determines thresholds for negative selection

Motheaten viable (mev) mice are deficient in the cytosolic protein tyrosine phosphatase, PTP1C, and exhibit severe B cell immunodeficiency and autoantibody production. The role of PTP1C in B cell selection and function was analyzed by breeding immunoglobulin transgenes specific for a defined antigen, hen egg lysozyme, into mev mice. Antigen triggered a greater and more rapid elevation of intracellular calcium in PTP1C-deficient B cells, indicating that this phosphatase negatively regulates immunoglobulin signaling. Elimination of self-reactive B cells carrying this signal-enhancing mutation was triggered during their development by binding a lower valency form of self-antigen than is normally required. These findings establish that activation of distinct repertoire-censoring mechanisms depends on quantitative differences in antigen receptor signaling, whose thresholds are determined by negative regulation through PTP1C.

Competition for follicular niches excludes self-reactive cells from the recirculating B-cell repertoire

Two different approaches to follow clones of B lymphocytes in a diverse preimmune repertoire reveal a new process for eliminating self-reactive cells in the periphery which depends on competition between cells with different specificities. A key feature of this censoring mechanism is the selective exclusion of self-antigen-binding B cells from the normal migration route into lymphoid follicles, resulting in their premature death. This is a striking example of homeostasis by cellular competition for limiting niches and may explain the paradoxical association between immunodeficiency and autoimmunity.

Selection in the expression of functionally distinct B-cell subsets

Although the isolation and characterization of functionally distinct B-cell subsets has been a major preoccupation of immunologists for the past two decades, only recently has it been recognized that the various B-cell subsets may be disparately selected during their development and maturation on the basis of their expressed Ig V regions. Thus, pre-B cells are selected for clonal expansion and maturation by virtue of the amino-acid sequence of their nascent H chains and newly emerging B cells may be selected for longevity and subset distribution on the basis of both clonotype-specific and relatively non-specific interactions of their surface Ig receptors.

Maternal immunization to prevent infectious diseases in the neonate or infant

The approach of providing passive protection to young infants by immunizing pregnant women can bypass the problems of immunological immaturity in the neonate, avoid or delay active immunization of the infant in the first year of life, and prevent transmission of an infection from the mother to the neonate. Optimal vaccines for this approach should induce high immunoglobulin G antibody titers that quickly reach their maximum level after immunization and persist at protective levels for several years, thus providing passive protection in subsequent pregnancies. Specific applications of this approach include the worldwide practice of maternal immunization with tetanus toxoid vaccine and ongoing studies of maternal immunization to prevent Haemophilus influenzae type b, group B streptococcal, pneumococcal, meningococcal, and human immunodeficiency virus infection in the infant. Addressing the cultural, sociological, and legal aspects of maternal immunization will be required to ensure the success of this approach.

Normal B lymphocyte differentiation

Normal differentiation of B lineage cells has been the subject of intensive investigation over the past three decades. Current models of this process in humans are melded from the results of studies in a variety of organisms, including humans, mice and birds. Several recent developments have significantly reshaped and refined these models. The technique of homologous recombination in embryonic stem cells has allowed the production of mice with selectively disrupted genes that are important for B cell development in mice. At the same time, functional studies of human B cell differentiation, together with analysis of naturally occurring mutations that disrupt this process, have progressed rapidly. This has provided insight into the pathogenesis of lymphoproliferative and immunodeficiency diseases as well as a clearer view of normal developmental events. In this chapter we have reviewed human B cell differentiation with particular emphasis on newly emerging concepts. We also discussed CD5, a pan-T cell antigen that is expressed in low levels on a subpopulation of B cells implicated in the pathogenesis of chronic lymphocytic leukaemia (CLL). Finally, we discussed the issue of restricted variable region gene usage during B cell ontogeny and in CLL.

Isolation of germinal centerlike events from human spleen RNA. Somatic hypermutation of a clonally related VH6DJH rearrangement expressed with IgM, IgG, and IgA

12 rearranged human VH6 immunoglobulin heavy chain genes arising from the same rearrangement were isolated without preselection from the RNA of a fragment of human spleen. The 12 clones were isolated from a pool of 31 unique VH6 clones arising from 18 unique rearrangements. 2 of the 12 related clones were expressed with IgM, 2 with IgG, and 8 with IgA1. All the clones, including those expressing IgM, showed extensive somatic mutation of germline bases (5.6%), which was consistent with antigen-driven activation of these VH6-expressing clones with recruitment into the immune repertoire. On the basis of significant sharing of somatic mutations between the IgM clones and clones expressing the other isotypes (six mutations shared with IgG clones and eight mutations shared with IgA clones), it was apparent that the IgM-expressing precursor in this diversified family had undergone extensive antigen-driven somatic mutation prior to isotype switching. This family of related clones suggests that a germinal centerlike event had been sampled. The highly mutated IgM clones suggest that there may exist memory B cells capable of further somatic mutation and differential isotype-switching depending on the specific antigenic stimulus.