Structure of primary anti-(4-hydroxy-3-nitrophenyl)acetyl (NP) antibodies in normal and idiotypically suppressed C57BL/6 mice

Lsc regulates marginal-zone B cell migration and adhesion and is required for the IgM T-dependent antibody response

The humoral immune response to protein antigens is composed of a rapid low-affinity IgM antibody response followed by an IgG response exhibiting higher affinity. Here, we demonstrate that Lsc, a protein that regulates G protein-coupled-receptor signaling and RhoA activation, is required by B lymphocytes for the antigen-specific IgM antibody response to a protein antigen. We further show that in lsc(-/-) mice, MZB cells are selectively affected such that naive and in vivo-activated MZB cells migrate toward sphingosine-1-phosphate at increased proportions but release inefficiently from integrin ligands. Consequently, lsc(-/-) MZB cells do not traffick appropriately in an immune response and do not contribute to the TD antibody response. These data demonstrate that Lsc regulates the migration and adhesion of MZB cells, and this regulation appears to be required for these cells to contribute to the antibody response to TD antigens.

The development of functional B lymphocytes in conditional PU.1 knock-out mice

An abundance of research has entrenched the view that the Ets domain containing transcription factor PU.1 is fundamental to the development and function of B lymphocytes. In this study, we have made use of a conditional PU.1 allele to test this notion. Complete deletion of PU.1 resulted in the loss of B cells and all other lineage-positive cells in the fetal liver and death between E18.5 and birth; however, specific deletion of PU.1 in the B lineage had no effect on B-cell development. Furthermore, deletion of PU.1 in B cells did not compromise their ability to establish and maintain an immune response. An increased level of apoptosis was observed in vitro upon B-cell receptor (BCR) cross-linking; however, this was partially rescued by interleukin-4 (IL-4). These findings suggest that PU.1 is not essential for the development of functional B lymphocytes beyond the pre-B stage. (Blood. 2005;106:2083-2090)

Short-Lived Plasmablasts Dominate the Early Spontaneous Rheumatoid Factor Response: Differentiation Pathways, Hypermutating Cell Types, and Affinity Maturation Outside the Germinal Center

We used a newly validated approach to identify the initiation of an autoantibody response to identify the sites and cell differentiation pathways at early and late stages of the rheumatoid factor response. The autoimmune response is mainly comprised of rapidly turning over plasmablasts that, according to BrdU labeling, TUNEL, and hypermutation data, derive from an activated B cell precursor. Surprisingly, few long-lived plasma cells were generated. The response most likely initiates at the splenic T-B zone border and continues in the marginal sinus bridging channels. Both activated B cells and plasmablasts harbor V gene mutations; large numbers of mutations in mice with long-standing response indicate that despite the rapid turnover of responding cells, clones can persist for many weeks. These studies provide insights into the unique nature of an ongoing autoimmune response and may be a model for understanding the response to therapies such as B cell depletion.

A landscape for the dynamics of an immune response

Dynamic shaping of the antibody repertoire is essential for effective immunity. We describe here a novel approach for clarifying how the antibody repertoire is shaped over time for development of a specific immune response. We obtained over 500 immunoglobulin G1 clones harboring VH186.2 from the spleen, bone marrow, and microdissected individual germinal centers of (4-hydroxy-3-nitrophenyl) acetyl-immunized C57BL/6 mice at various time points postimmunization. Statistical analyses provided an index for defining clonal diversity and cluster analyses gave us a three-dimensional landscape with which clone distance was visualized with the expression level of antibodies. This landscape approach facilitated our understanding of the dynamics shaping the actual antibody repertoire, in which pre-existing naturally occurring population persisted and provided a significant impact upon the repertoire. To the established model for describing production of the antibody-forming cells, we were able to append an indispensable issue in considering the maturation of humoral immune response.

The primary structure and specificity determining residues displayed by recombinant salmon antibody domains

Previously, single chain fragments of salmon (Salmo salar L.) immunoglobulin variable regions (scFv) were isolated by reactivity towards trinitrophenyl (TNP) or fluorescein (FITC) using phage display technology. The fine specificity of six scFv clones were analysed by ELISA, while the primary structure was determined by DNA sequencing. In addition, preliminary models of one anti-TNP and one anti-FITC clone were built. Here, a follow-up analysis of the primary and tertiary structure of all six clones is focused on the structural basis for hapten specificity. Tertiary structure was analysed by molecular modelling of the antigen combining site. The analysis shows that reactivity to each hapten is maintained by a number of different combinations of VH, D, JH and VL sequences. Accordingly, various sizes of CDR3 on both the heavy and light chain and CDR2 of IgH may support TNP binding. Due to variability of the antigen combining site each clone probably has a distinct binding affinity. However, a feature common among the four scFv antibodies that recognise TNP is a positively charged Arg in CDR2 of either the heavy or light chain. In the majority of the anti-TNP clones localisation of this side-chain is stabilised by a negatively charged Asp in LCDR1. In addition, a Trp in LCDR3 is conserved in all the anti-TNP clones. Also, the anti-FITC clones display a Trp in the LCDR3, suggesting its participation in binding of FITC as well. In combination with a large aromatic amino acid near the N-terminus of HCDR2 and a positively charged Arg in CDR1, these residues probably determine both specificity and affinity towards the FITC moiety.

Germinal center-associated nuclear protein contributes to affinity maturation of B cell antigen receptor in T cell-dependent responses

Acquired immunity depends on proliferation and differentiation of antigen (Ag)-specific B cells in germinal centers (GCs) of lymphoid follicles in response to T cell-dependent Ags. Here, we studied the function of GC-associated nuclear protein that is selectively up-regulated in GC-B cells. B cell-specific ganp-deficient mice were compromised in affinity maturation of hapten-specific antibodies against T cell-dependent Ags with retarded development of GCs. B cell numbers and development, serum Ig levels, mitogen-induced B cell proliferation in vitro, and responses to T cell-independent Ag were nearly normal; however, the mutant B cells showed a decrease in anti-CD40-induced proliferation and an increased susceptibility to B cell apoptosis in vitro and in vivo. B cell-specific ganp-deficient mice showed a decreased frequency of variable-region somatic mutations, especially of the high-affinity type (W(33) --> L) in the V(H)186.2 region against nitrophenyl-chicken gamma globulin, whereas the class switching was normal. We conclude that GC-associated nuclear protein is necessary for generation or maintenance of B cells with high-affinity B cell Ag receptors during the maturation in GCs.

Segmental flexibility and avidity of IgM in the interaction of polyvalent antigens

We prepared IgG and IgM with identical combining sites to a hapten, (4-hydroxy-3-nitrophenyl)acetic acid (NP), and used surface plasmon resonance to evaluate the association constants (Ka) in interactions of these antibodies (Abs) with antigens (Ags) which differed in the size of carriers and NP valence as well as in the stoichiometry of Ag to Ab in the immune complexes. It was found that IgM was unable to form an Ag1Ab1 complex with the highly haptenated Ag, NP(18.6)-bovine serum albumin (BSA), such that one NP(18.6)-BSA molecule was held by multiple contacts with Fab arms from five subunits, although IgM was capable of forming an Ag4Ab1 complex in which each subunit was bound to one NP(18.6)-BSA molecule. IgM was superior to IgG in interactions with large Ags of low hapten density. The Ka values of IgM to these Ags were estimated to be approximately 1x10(9) M(-1), about 20-fold higher than those of IgG. Reduction of inter-subunit and inter-chain disulfide bonds resulted in a decrease in Ka values to large Ags but no change in those to small Ags.

Estimation of the relative affinity of B cell receptor by flow cytometry

We have developed a simple method using flow cytometry to estimate the relative affinity of B cell receptor (BCR) possessing the hapten-binding activity. Bovine serum albumin (BSA) was conjugated with a hapten, (4-hydroxy-3-nitrophenyl)acetyl (NP) and biotin (NP-BSA-bio). The interaction between NP-BSA-bio and anti-NP monoclonal antibodies (mAb) was studied as a model of the BCR reaction by surface plasmon resonance (SPR) using a biosensor chip immobilized with mAbs through anti-Fc antibody (Ab). The relative affinity of these mAbs was estimated on the basis of resonance units for the binding of NP(0.5)-BSA-bio(21) relative to that of NP(7.4)-BSA-bio(21) expressed as a ratio (NP(0.5)-BSA-bio(21)/NP(7.4)-BSA-bio(21)). In combination with streptavidin (SA)-R-phycoerythrin (PE), we measured the binding of NP-BSA-bio to BCR by flow cytometry and found that a high number of biotin molecules was necessary to improve the sensitivity of detection of the bound NP-BSA-bio without steric hindrance in the NP-BCR interaction. We demonstrated that the ratio of the mean fluorescence intensity (MFI) of NP(0.5)-BSA-bio(21)/NP(7.4)-BSA-bio(21) at a concentration of 10(-8) M could be used as a practical measure of the affinity. This method is expected to be useful for the study of affinity maturation on the cellular level.

Generalization of single immunological experiences by idiotypically mediated clonal connections

Clonal interactions of B cells by idiotope-specific mutual recognition of their antigen receptors with the participation of T cells were assumed to form a web of unknown density, referred to as the idiotypic network. Although these clonal connections were proposed to fulfill important internal regulatory functions, their biological significance, especially in relation to antigen-induced immune responses, remained a mystery. In view of this, we postulate that the basic function of the idiotypic internal connection between B and T cell antigen receptors is to transform antigen-induced cellular activations, by idiotypic crossreactivity, into the regulation of cell clones with different antigen specificities. This process leads not only to the suppression of major clones but also to the activation of minor ones. The latter activating property may allow the generalization of single antigenic experiences, so that the immune system in its entirety benefits in its battle against environmental microbes. Such idiotypic clonal interactions are particularly effective in early ontogeny. During a short neonatal imprinting period, maternal immunological knowledge in the form of somatically mutated, high-affinity IgG antibodies, acquired through a continuous encounter with external antigens, guides the initial ontogenetic development of the immune system and so exerts long-lasting transgenerational advantageous effects in the offspring.

Memory B cells without somatic hypermutation are generated from Bcl6-deficient B cells

After immunization with T cell-dependent antigens, the high-affinity B cells selected in germinal centers differentiate into memory B cells or long-lived antibody-forming cells. However, a role for germinal centers in development of these B lineage cells is still controversial. We show here that Bcl6-deficient B cells, which cannot develop germinal centers, differentiated into IgM and IgG1 memory B cells in the spleen but barely differentiated into long-lived IgG1 antibody-forming cells in the bone marrow. Mutation in the V-heavy gene was null in these memory B cells. Therefore, Bcl6 and germinal center formation are essential for somatic hypermutation, and generation of memory B cells can occur independently of germinal center formation, somatic hypermutation, and Ig class switching.

CDK inhibitor p18(INK4c) is required for the generation of functional plasma cells

B cell terminal differentiation is associated with the onset of high-level antibody secretion and cell cycle arrest. Here the cyclin-dependent kinase (CDK) inhibitor p18(INK4c) is shown to be required within B cells for both terminating cell proliferation and differentiation of functional plasma cells. In its absence, B cells hyperproliferate in germinal centers and extrafollicular foci in response to T-dependent antigens but serum antibody titers are severely reduced, despite unimpaired germinal center formation, class switch recombination, variable region-directed hypermutation, and differentiation to antibody-containing plasmacytoid cells. The novel link between cell cycle control and plasma cell differentiation may, at least in part, relate to p18(INK4c) inhibition of CDK6. Cell cycle arrest mediated by p18(INK4C) is therefore requisite for the generation of functional plasma cells.

Dissection of B cell differentiation during primary immune responses in mice with altered CD40 signals

CD40 is essential for efficient humoral immune responses. CD40 has two cytoplasmic domains required for binding of tumor necrosis factor receptor-associated factors (TRAF). The TRAF6-binding site is within the membrane proximal cytoplasmic (Cmp) region, while a PXQXT motif in the membrane distal cytoplasmic (Cmd) region needs to engage TRAF2/3/5. To dissect CD40 signals necessary for B cell differentiation, we generated transgenic mice expressing wild-type and mutant human CD40 (hCD40) molecules in a mouse CD40-deficient (mCD40(-/-)) background. The B cell-specific expression of hCD40 in mCD40(-/-) mice resulted in T-dependent antibody responses including germinal center (GC) formation. Mutant hCD40 molecules that carry either a point mutation of the TRAF2/3/5-binding site or a deletion of the Cmd region rescued extrafollicular B cell differentiation but not GC formation. A mutant hCD40 that comprises of only the TRAF2/3/5-binding site in the cytoplasmic region also rescued low but significant titers of antigen-specific IgG1 without GC formation. These results demonstrated that two distinct signals either from the Cmp or from the Cmd region induced the extrafollicular B cell differentiation and Ig class switching; however, GC formation required both. We conclude that combinations of these two signals determine which of the extrafollicular or the follicular (GC) differentiation pathway B cells enter.

BCR signal through alpha 4 is involved in S6 kinase activation and required for B cell maturation including isotype switching and V region somatic hypermutation

alpha 4 potentially mediates BCR signals through a rapamycin-sensitive TOR pathway. To investigate a potential role for alpha 4 in B cell activation, the alpha 4 gene was disrupted conditionally in B cells by mating male CD19-Cre mice with female alpha 4-floxed mice. CD19-Cre+/alpha 4flox mice showed loss of alpha 4 protein in B lineage cells and a decreased number of phenotypically normal mature B cells. Compared to normal B cells, alpha 4(-) B cells showed a decreased proliferation in response to the B cell stimulants (anti-IgM antibody plus IL-4, anti-CD40 mAb and lipopolysaccharide), and a reduced S6 kinase activation and rapamycin sensitivity. While CD19-Cre+/alpha 4flox mice showed impaired antibody responses to both T cell-independent and T cell-dependent (TD) antigens, the TD antigen response was markedly impaired as demonstrated by reduced isotype switching, reduced germinal center formation and reduced V region somatic hypermutation. These results show that alpha 4 plays a pivotal role in antigen-specific signal transduction during B cell activation and differentiation in vivo.

Isotype-specific selection of high affinity memory B cells in nasal-associated lymphoid tissue

Mucosal immunoglobulin (Ig)A dominance has been proposed to be associated with preferential class switch recombination (CSR) to the IgA heavy chain constant region, Calpha. Here, we report that B cell activation in nasal-associated lymphoid tissue (NALT) upon stimulation with the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP) coupled to chicken gamma globulin caused an anti-NP memory response dominated by high affinity IgA antibodies. In the response, however, NP-specific IgG(+) B cells expanded and sustained their number as a major population in germinal centers (GCs), supporting the view that CSR to IgG heavy chain constant region, Cgamma, operated efficiently in NALT. Both IgG(+) and IgA(+) GC B cells accumulated somatic mutations, indicative of affinity maturation to a similar extent, suggesting that both types of cell were equally selected by antigen. Despite the selection in GCs, high affinity NP-specific B cells were barely detected in the IgG memory compartment, whereas such cells dominated the IgA memory compartment. Taken together with the analysis of the V(H) gene clonotype in GC and memory B cells, we propose that NALT is equipped with a unique machinery providing IgA-specific enrichment of high affinity cells into the memory compartment, facilitating immunity with high affinity and noninflammatory secretory antibodies.

Interference with immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation modulates or blocks B cell development, depending on the availability of an Igbeta cytoplasmic tail

To determine the function of immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation, we generated mice in which Igalpha ITAM tyrosines were replaced by phenylalanines (Igalpha(FF/FF)). Igalpha(FF/FF) mice had a specific reduction of B1 and marginal zone B cells, whereas B2 cell development appeared to be normal, except that lambda1 light chain usage was increased. The mutants responded less efficiently to T cell-dependent antigens, whereas T cell-independent responses were unaffected. Upon B cell receptor ligation, the cells exhibited heightened calcium flux, weaker Lyn and Syk tyrosine phosphorylation, and phosphorylation of Igalpha non-ITAM tyrosines. Strikingly, when the Igalpha ITAM mutation was combined with a truncation of Igbeta, B cell development was completely blocked at the pro-B cell stage, indicating a crucial role of ITAM phosphorylation in B cell development.

A Role of the Third Complementarity-determining Region in the Affinity Maturation of an Antibody

We recently found that there are two distinct antibody maturation pathways for the immune response of C57BL/6 mice to (4-hydroxy-3-nitrophenyl) acetyl and that a junctional amino acid introduced at a point far in advance of somatic hypermutation determined which pathway of affinity maturation was used. We describe here the structural basis for this aspect of maturation using recently developed H3 rules, which allow for reliable identification of the conformation of the third complementarity-determining region of the heavy chain (CDR-H3) from the primary amino acid sequences only. By the application of these rules, the anti-(4-hydroxy-3-nitrophenyl) acetyl antibodies examined here were classified into two major groups on the basis of their CDR-H3 structure, and these groups were found to be consistent with the maturation pathways. In addition, circular dichroism measurements revealed that the versatile nature of the antigen binding of the antibodies was significantly influenced by the pathway employed. We postulated in this study that flexibility in the CDR-H3 structure in the antigen-combining site could facilitate efficient antibody maturation supported by a plurality of possible antigen binding modes.

Fas is required for clonal selection in germinal centers and the subsequent establishment of the memory B cell repertoire

In T cell-dependent immune responses, high-affinity B cells are selected and differentiate into memory cells; however, the mechanism behind this process remains largely unknown. Here, we report that the selection of high-affinity B cells within germinal centers (GCs) is impaired in Fas-deficient lpr mice in the primary response, probably owing to inefficient negative selection. The memory compartment in control mice is mostly established by precursors generated from the early GCs, whereas the lpr defect expands the memory compartment by the increased recruitment of newly generated precursors from the late GCs, resulting in the accumulation of heavily mutated memory B cells at high frequency. These results suggest that Fas is required for clonal selection within GCs and the establishment of the memory B cell repertoire.

Commitment of B lymphocytes to a plasma cell fate is associated with Blimp-1 expression in vivo

B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor that is sufficient to trigger terminal differentiation in the B cell lymphoma BCL-1. In this study, we have determined the expression pattern of Blimp-1 in vivo in primary and secondary lymphoid organs of humans and immunized mice. Blimp-1 is expressed in plasma cells derived from either a T-independent or T-dependent response in plasma cells that have undergone isotype switching and those resulting from secondary immunization. Blimp-1 is also present in long-lived plasma cells residing in the bone marrow. However, Blimp-1 was not detected in memory B cells. This expression pattern provides further evidence of a critical role for Blimp-1 in plasma cell development, supporting earlier studies in cultured lines. Significantly, Blimp-1 was also found in a fraction (4-15%) of germinal center B cells in murine spleen and human tonsils. Blimp-1 expression in the germinal center is associated with an interesting subset of cells with a phenotype intermediate between germinal center B cells and plasma cells. In the mouse, Blimp-1(+) germinal center B cells peak at day 12 postimmunization and disappear soon thereafter. They are not apoptotic, some are proliferating, they express germinal center markers peanut agglutinin or CD10 but not Bcl-6, and most express CD138 (syndecan-1), IRF4, and cytoplasmic Ig. Together, these data support a model in which B cell fate decisions occur within the germinal center and Blimp-1 expression is critical for commitment to a plasma cell, rather than a memory cell, fate.

Open sandwich enzyme-linked immunosorbent assay for the quantitation of small haptens

The quantitation of low-molecular-weight haptens has been difficult with conventional sandwich immunoassays due to their small size. Many researchers have attempted to develop sandwich assays for haptens due to the significant advantages of the sandwich format over competitive assays including greater dynamic range, ease of automation, and sensitivity. Here we apply the open-sandwich ELISA (OS-ELISA), an immunoassay based on antigen-dependent stabilization of antibody variable regions (V(H) and V(L) domains), to hapten quantitation. Two fusion proteins, the high-affinity mutant V(H) domain from anti-4-hydroxy-3-nitrophenacetyl (NP) antibody B1-8 tethered with Escherichia coli alkaline phosphatase (V(H)(W33L)-PhoA) and the V(L) domain from the same antibody tethered with Streptococcus sp. protein G, were made. These fusion proteins when added together achieved Fv reassociation consequent to the addition of NP. Signal was generated in a direct relationship to the NP concentration with better sensitivity compared with competitive immunoassay, demonstrating this assay to be a quick noncompetitive alternative to the conventional assays for small compounds, such as environmental pollutants, drugs of abuse, and therapeutic drugs. With our previous demonstration that the OS-ELISA works well with large proteins, the OS-ELISA becomes the first practical immunoassay approach capable of quantifying any molecule regardless of their size.

Memory B-cell persistence is independent of persisting immunizing antigen

Immunological memory in the antibody system is generated in T-cell-dependent responses and carried by long-lived memory B cells that recognize antigen by high-affinity antibodies. But it remains controversial whether these B cells represent true 'memory' cells (that is, their maintenance is independent of the immunizing antigen), or whether they are a product of a chronic immune response driven by the immunizing antigen, which can be retained in the organism for extended time periods on the surface of specialized antigen-presenting cells (follicular dendritic cells). Cell transfer experiments provided evidence in favour of a role of the immunizing antigen; however, analysis of memory cells in intact animals, which showed that these cells are mostly resting and can persist in the absence of detectable T-cell help or follicular dendritic cells, argued against it. Here we show, by using a genetic switch mediated by Cre recombinase, that memory B cells switching their antibody specificity away from the immunizing antigen are indeed maintained in the animal over long periods of time, similar to cells retaining their original antigen-binding specificity.

Mimicry of CD40 signals by Epstein-Barr virus LMP1 in B lymphocyte responses

The effect of the Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) on the activation and differentiation of normal B cells was investigated. B cells of transgenic mice expressing LMP1 under the control of immunoglobulin promoter/enhancer displayed enhanced expression of activation antigens and spontaneously proliferated and produced antibody. Humoral immune responses of LMP1 transgenic mice in CD40-deficient or normal backgrounds revealed that LMP1 mimics CD40 signals to induce extrafollicular B cell differentiation but, unlike CD40, blocks germinal center formation. Thus, these specific properties of LMP1 may determine the site of primary B cell infection and the state of infection in the natural course of EBV infection, whereas subsequent loss of LMP1 expression may affect the site of persistent latent infection.

Junctional amino acids determine the maturation pathway of an antibody

We found that two distinct antibody maturation pathways exist in the immune response of C57BL/6 mice to (4-hydroxy-3-nitrophenyl)acetyl and that the junctional amino acid introduced by a process far preceding somatic hypermutation determined the pathway of affinity maturation. Antibodies belonging to each pathway clearly separated into two separate branches of a phylogenic tree. We also constructed a three-dimensional fitness landscape for antibody evolution by introducing the association constants of the antibodies into the phylogenic tree as the third axis, allowing us to comprehend the significance of junctional diversity in the "evolvability" of antibodies. Thermodynamic analyses of the antigen-antibody interactions suggested that a high conformational versatility in the antigen-combining site allows for the enhanced evolvability of antibodies.

Somatic hypermutation and the three R's: repair, replication and recombination

Somatic hypermutation introduces single base changes into the rearranged variable (V) regions of antigen activated B cells at a rate of approximately 1 mutation per kilobase per generation. This is nearly a million-fold higher than the typical mutation rate in a mammalian somatic cell. Rampant mutation at this level could have a devastating effect, but somatic hypermutation is accurately targeted and tightly regulated. Here, we provide an overview of immunoglobulin gene somatic hypermutation; discuss mechanisms of mutation in model organisms that may be relevant to the hypermutation mechanism; and review recent advances toward understanding the possible role(s) of DNA repair, replication, and recombination in this fascinating process.

Lymph node germinal centers form in the absence of follicular dendritic cell networks

Follicular dendritic cell networks are said to be pivotal to both the formation of germinal centers (GCs) and their functions in generating antigen-specific antibody affinity maturation and B cell memory. We report that lymphotoxin beta-deficient mice form GC cell clusters in the gross anatomical location expected of GCs, despite the complete absence of follicular dendritic cell networks. Furthermore, antigen-specific GC generation was at first relatively normal, but these GCs then rapidly regressed and GC-phase antibody affinity maturation was reduced. Lymphotoxin beta-deficient mice also showed substantial B cell memory in their mesenteric lymph nodes. This memory antibody response was of relatively low affinity for antigen at week 4 after challenge, but by week 10 after challenge was comparable to wild-type, indicating that affinity maturation had failed in the GC phase but developed later.

Mature follicular dendritic cell networks depend on expression of lymphotoxin beta receptor by radioresistant stromal cells and of lymphotoxin beta and tumor necrosis factor by B cells

The formation of germinal centers (GCs) represents a crucial step in the humoral immune response. Recent studies using gene-targeted mice have revealed that the cytokines tumor necrosis factor (TNF), lymphotoxin (LT) alpha, and LTbeta, as well as their receptors TNF receptor p55 (TNFRp55) and LTbetaR play essential roles in the development of GCs. To establish in which cell types expression of LTbetaR, LTbeta, and TNF is required for GC formation, LTbetaR-/-, LTbeta-/-, TNF-/-, B cell-deficient (BCR-/-), and wild-type mice were used to generate reciprocal or mixed bone marrow (BM) chimeric mice. GCs, herein defined as peanut agglutinin-binding (PNA+) clusters of centroblasts/centrocytes in association with follicular dendritic cell (FDC) networks, were not detectable in LTbetaR-/- hosts after transfer of wild-type BM. In contrast, the GC reaction was restored in LTbeta-/- hosts reconstituted with either wild-type or LTbetaR-/- BM. In BCR-/- recipients reconstituted with compound LTbeta-/-/BCR-/- or TNF-/-/BCR-/- BM grafts, PNA+ cell clusters formed in splenic follicles, but associated FDC networks were strongly reduced or absent. Thus, development of splenic FDC networks depends on expression of LTbeta and TNF by B lymphocytes and LTbetaR by radioresistant stromal cells.

Antigen Drives Very Low Affinity B Cells to Become Plasmacytes and Enter Germinal Centers

In the first week of the primary immune response to the (4-hydroxy-3-nitrophenyl)acetyl (NP) hapten, plasmacytic foci and germinal centers (GCs) in C57BL/6 mice are comprised of polyclonal populations of B lymphocytes bearing the λ1 L-chain (λ1+). The Ig H-chains of these early populations of B cells are encoded by a variety of VH and D exons undiversified by hypermutation while later, oligoclonal populations are dominated by mutated rearrangements of the VH186.2 and DFL16.1 gene segments. To assess directly Ab affinities within these defined splenic microenvironments, representative VDJ rearrangements were recovered from B cells participating in the early immune response to NP, inserted into Ig H-chain expression cassettes, and transfected into J558L (H−; λ1+) myeloma cells. These transfectoma Abs expressed a remarkably wide range of measured affinities (Ka = 5 × 104-1.3 × 106 M−1) for NP. VDJs recovered from both foci and early GCs generated comparable affinities, suggesting that initial differentiation into these compartments occurs stochastically. We conclude that Ag normally activates B cells bearing an unexpectedly wide spectrum of Ab affinities and that this initial, promiscuous clonal activation is followed by affinity-driven competition to determine survival and clonal expansion within GCs and entry into the memory and bone marrow plasmacyte compartments.

Role of κII-A2 Light Chain CDR-3 Junctional Residues in Human Antibody Binding to the Haemophilus influenzae Type b Polysaccharide

Abs using the κII-A2 V gene segment predominate the human Ab repertoire to the Haemophilus influenzae b (Hib) polysaccharide (PS). All A2 anti-Hib PS Abs sequenced to date possess a 10-amino acid L chain complementarity-determining region-3 (CDR-3) having an insertional arginine (Arg) at position 95a, the V-J junction. These findings suggest an essential requirement for this conserved Arg residue in determining Hib PS-binding affinity. We examined this requirement by performing chain recombination experiments in which a series of A2 L chains, differing at position 95a, were combined individually with an Fd region known to generate a Hib PS-combining site when paired with an A2-Arg(95a)-Jκ1 V region. Hib PS binding of the recombinant Fabs was evaluated quantitatively using a radioantigen-binding assay. Fabs having A2 L chains with either Arg or lysine in position 95a in combination with Jκ1 gave equivalent and strongest binding to Hib PS. Fabs having A2-Jκ1 L chains with either tyrosine, glycine, alanine, leucine, serine, or threonine in position 95a, or having an A2-Arg(95a)-Jκ3 L chain, gave intermediate binding. Fabs having A2-Jκ1 L chains with glutamate or aspartate at 95a or with no junctional residue showed little or no Hib PS binding. These results demonstrate the importance of L chain junctional residue, as well as Jκ usage and CDR-3 length, in determining Hib PS-binding affinity. Contrary to expectation, an Arg junctional residue is not essential for generating either high or intermediate affinity-binding sites.

In situ studies of the primary immune response to (4-hydroxy-3-nitrophenyl)acetyl. V. Affinity maturation develops in two stages of clonal selection

To examine the role of germinal centers (GCs) in the generation and selection of high affinity antibody-forming cells (AFCs), we have analyzed the average affinity of (4-hydroxy-3-nitrophenyl)acetyl (NP)-specific AFCs and serum antibodies both during and after the GC phase of the immune response. In addition, the genetics of NP-binding AFCs were followed to monitor the generation and selection of high affinity AFCs at the clonal level. NP-binding AFCs gradually accumulate in bone marrow (BM) after immunization and BM becomes the predominant locale of specific AFCs in the late primary response. Although the average affinity of NP-specific BM AFCs rapidly increased while GCs were present (GC phase), the affinity of both BM AFCs and serum antibodies continued to increase even after GCs waned (post-GC phase). Affinity maturation in the post-GC phase was also reflected in a shift in the distribution of somatic mutations as well as in the CDR3 sequences of BM AFC antibody heavy chain genes. Disruption of GCs by injection of antibody specific for CD154 (CD40 ligand) decreased the average affinity of subsequent BM AFCs, suggesting that GCs generate the precursors of high affinity BM AFCs; inhibition of CD154-dependent cellular interactions after the GC reaction was complete had no effect on high affinity BM AFCs. Interestingly, limited affinity maturation in the BM AFC compartment still occurs during the late primary response even after treatment with anti-CD154 antibody. Thus, GCs are necessary for the generation of high affinity AFC precursors but are not the only sites for the affinity-driven clonal selection responsible for the maturation of humoral immune responses.

The extent of affinity maturation differs between the memory and antibody-forming cell compartments in the primary immune response

Immunization with protein-containing antigens results in two types of antigen-specific B cell: antibody forming cells (AFCs) producing antibody of progressively higher affinity and memory lymphocytes capable of producing high affinity antibody upon re-exposure to antigen. The issue of the inter-relationship between affinity maturation of memory B cells and AFCs was addressed through analysis of single, antigen-specific B cells from the memory and AFC compartments during the primary response to a model antigen. Only 65% of splenic memory B cells were found capable of producing high affinity antibody, meaning that low affinity cells persist into this compartment. In contrast, by 28 days after immunization all AFCs produced high affinity antibody. We identified a unique, persistent sub-population of bone marrow AFCs containing few somatic mutations, suggesting they arose early in the response, yet highly enriched for an identical affinity-enhancing amino acid exchange, suggesting strong selection. Our results imply that affinity maturation of a primary immune response occurs by the early selective differentiation of high affinity variants into AFCs which subsequently persist in the bone marrow. In contrast, the memory B-cell population contains few, if any, cells from the early response and is less stringently selected.

Germinal center development

Using a set of surface markers including IgD and CD38, human tonsillar B cells were classified into discrete subpopulations. Molecular and functional analysis allowed us to identify: i) two sets of naive B cells (Bm1 and Bm2); ii) germinal center founder cells (Bm2'); iii) an obscure population of germinal center B cells, displaying a high load of somatic mutations in IgV genes, C mu to C delta switch and preferential Ig lambda light chain usage: these cells may represent the precursors of normal and malignant IgD-secreting plasma cells; iv) the centroblasts (Bm3) in which somatic mutation machinery is activated; v) the centrocytes (Bm4) in which isotype switch occurs; vi) the memory B cells. The characterization of these subpopulations showed that: i) programmed cell death is set before somatic mutations, possibly providing an efficient way for affinity maturation; ii) only high affinity centrocytes are allowed to switch isotype; iii) CD40-ligation inhibits plasmacytic differentiation of mature B lymphocytes; iv) memory B cells preferentially differentiate into plasma cells; v) IgD isotype switch occurs in normal B cells; vi) receptor editing may be induced by somatic mutations in germinal centers. We also characterized two types of antigen-presenting cells in germinal centers: follicular dendritic cells that select high affinity B cells, and a new subset of germinal center dendritic cells that activate germinal center T cells.

Correlation between immune maturation and idiotypic network recognition

The maturation of T-dependent humoral immune responses is mediated by somatic mutations. Antigen selection is one mechanism for the activation of B cell clones which express antibodies with progressively increased affinity and which are derived as somatic variants from germ-line-encoded genes. However, the emergence of B cell clones secreting rather low-affinity antibodies and the shift to alternative germ-line V region gene combinations during secondary and tertiary responses cannot be explained by antigen selection. It has been considered that idiotypic suppression may favor this clonal shift. Such an involvement would require that idiotypic recognition in the syngeneic host must be highly restricted to private idiotopes of each clone sequentially activated during immune maturation. To test this possibility, we produced 19 syngeneic anti-idiotypic antibodies to the germ-line-encoded major Ox1 idiotype (IgM-IdOx1 H11.5) of the anti-2-phenyl-oxazolone (phOx) immune response in BALB/c mice. The fine specificity of these anti-IdOx1 was tested with a set of anti-phOx monoclonal antibodies, representing the first steps of maturation. About half of the anti-IdOx1 showed almost no reactivity with the IdOx1 after the switch to IgG and none of the anti-IdOx1 reacted with anti-phOx antibodies which carried a glycine or histidine instead of arginine as the middle amino acid of the D region. These observations suggest a strong correlation between immune maturation and the idiotypic network. A model is presented in which idiotypic suppression may function as a driving force for diversification and maturation of the antigen-induced immune response.

The direct cloning of the immunoglobulin VH genes from primary cultured B cells specific for a short peptide

A new and simple method was devised to obtain immunoglobulin VH genes directly from primary cultured B cells specific for a short peptide. Peptide-specific B cells were separated from splenocytes of peptide-immunized BALB/c mice with antigen-coated magnetic beads, and were cloned by a limitedly diluted culture in the presence of lipopolysaccharide, recombinant interleukin (rIL) -2, rIL-4 and rIL-5, and 3T3 fibroblasts as filler cells for 7 days. Seventeen clones were obtained from 3 x 10(3) fractionated cells by screening the positive wells containing anti-peptide antibody-secreting cells by an enzyme-linked immunosorbent assay (ELISA). The VH cDNAs of these clones were amplified by a set of primers; a primer complementary to the mu-chain constant region gene and the other with high complementarity to most of the VH genes. This is the first report of success in obtaining unknown VH genes directly from primary B cell clones, after their antigen-specificity has been confirmed by ELISA. This new method will provide a powerful tool for designing specific recombinant antibodies.

Within germinal centers, isotype switching of immunoglobulin genes occurs after the onset of somatic mutation

Human tonsillar B cells were separated into naive IgD+CD38-CD23- (Bm1) and IgD+CD38-CD23 (Bm2), germinal center IgD-CD38+CD23- centroblasts (Bm3) and IgD-CD38+CD77- centrocytes (Bm4) and memory IgD-CD38- (Bm5) subsets. Previous IgVH sequence analysis concluded that the triggering of somatic mutations occurs during the transition from Bm2 subset into the Bm3 subset. To determine the initiation of isotype switching, sterile transcript expression was analyzed by amplification, cloning, and sequencing. A selective sterile I gamma, I alpha, and I epsilon expression was observed at centrocyte (Bm4) stage, suggesting that isotype switch is triggered within germinal centers, after somatic mutation is initiated with centroblasts (Bm3). Finally, the high level of 5'S gamma-S mu 3' DNA switching circles observed in germinal center B cells indicates that within human tonsils, germinal center is a major location for isotype switching.

In situ studies of the primary immune response to (4-hydroxy-3-nitrophenyl)acetyl. IV. Affinity-dependent, antigen-driven B cell apoptosis in germinal centers as a mechanism for maintaining self-tolerance

Germinal centers (GCs) are the sites of antigen-driven V(D)J gene hypermutation and selection necessary for the generation of high affinity memory B lymphocytes. Despite the antigen dependence of this reaction, injection of soluble antigen during an established primary immune response induces massive apoptotic death in GC B cells, but not in clonally related populations of nonfollicular B lymphoblasts and plasmacytes. Cell death in GCs occurs predominantly among light zone centrocytes, is antigen specific, and peaks within 4-8 h after injection. Antigen-induced programmed death does not involve cellular interactions mediated by CD40 ligand (CD40L) or Fas; disruption of GCs by antibody specific for CD40L was not driven by apoptosis and C57BL/6.lpr mice, though unable to express the Fas death trigger, remained fully susceptible to soluble antigen. Single injections of antigen did not significantly decrease GC numbers or average size, but repeated injections during an 18-h period resulted in fewer and substantially smaller GCs. As cell loss appeared most extensive in the light zone, decreased GC cellularity after prolonged exposure to soluble antigen implies that the Ig- centroblasts of the dark zone may require replenishment from light zone cells that have survived antigenic selection. GC cell death is avidity-dependent; oligovalent antigen induced relatively little apoptosis and GC B cells that survived long exposures to multivalent antigen expressed atypical VDJ rearrangements unlikely to encode high affinity antibody. Antigen-induced apoptotic death in GCs may represent a mechanism for the peripheral deletion of autoreactive B cell mutants much as the combinatorial repertoire of immature B lymphocytes is censored in the bone marrow.

The affinity maturation of anti-4-hydroxy-3-nitrophenylacetyl mouse monoclonal antibody. A calorimetric study of the antigen-antibody interaction

To understand the mechanism of affinity maturation, we examined the antigen-antibody interactions between 4-hydroxy-3-nitrophenylacetyl (NP) caproic acid and the Fab fragments of three anti-NP antibodies, N1G9, 3B44, and 3B62, by isothermal titration calorimetry. The analyses have revealed that all of these interactions are mainly driven by negative changes in enthalpy. The enthalpy changes decreased linearly with temperature in the range of 25-45 degrees C, producing negative changes in heat capacity. On the basis of the dependence of binding constants on the sodium chloride concentration, we have shown that, during the affinity maturation of the anti-NP antibody, the electrostatic effect does not significantly contribute to the increase in the binding affinity. We have found that, as the logarithm of the binding constants increases during the affinity maturation of the anti-NP antibody, the magnitudes of the corresponding enthalpy, heat capacity, and unitary entropy changes increase almost linearly. On the basis of this correlation, we have concluded that, during the affinity maturation of the anti-NP antibody, a better surface complementarity is attained in the specific complex in order to obtain a higher binding affinity.

Role of conserved amino acid residues in the complementarity determining regions on hapten-antibody interaction of anti-(4-hydroxy-3-nitrophenyl) acetyl antibodies

Monoclonal antibodies (mAbs) specific to (4-hydroxy-3-nitrophenyl)acetyl (NP) were prepared at various times after immunization and the amino acid sequences of VH and V lambda 1 in these mAbs were deduced from cDNA nucleotide sequences. Replacements due to somatic mutation were not found in day 7 mAbs but were found in those of days 14, 84 and 294. The affinity of day 7 mAbs to NP-glycine(NP-Gly) was in the order of 10(4) M-1 and it increased about 8000-fold with time after immunization. The extrinsic circular dichroism (CD) spectrum of the NP-epsilon-aminocaproic acid (NP-Cap)/Ab complex was unique for each mAb, although the spectra were grouped into two types, which tended to shift from one type to another with time, suggesting a variation in the micro-environments around NP-Cap in the combining sites. All these data indicate that the structure of the combining site was altered by somatic mutation; however, the fine-specificity measured by cross-reactivity with hapten analogues did not change significantly with time. We examined the amino acid residues in CDRs responsible for recognition of NP-haptens by comparing the amino acid sequences of anti-NP mAbs. Analyses revealed the presence of several conserved amino acid residues in CDRs of VH and V lambda 1, such as Tyr-32H, and Tyr-60H, in addition to a core segment involving Arg-50H.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Coordination of immunoglobulin chain folding and immunoglobulin chain assembly is essential for the formation of functional IgG

The first constant domain (CH1) of immunoglobulin heavy (H) chains is essential for BiP-mediated retention of unassembled H chains in the endoplasmic reticulum (ER). Here, we demonstrated that both wild-type and a mutant gamma chain lacking the CH1 domain bind BiP when they are reduced in vivo. However, only oxidized mutant H chain dimers are released from BiP interaction, whereas oxidized wild-type gamma chain dimers still bind BiP. In light (L) chain-producing cells, some of the mutant H chains accumulate with L chains in ER-derived vesicles and some are secreted as IgG. Furthermore, only half of the secreted antibodies bind antigen. We found the same with a mutant gamma chain, in which the CH1 domain was replaced by a CH3 domain. Therefore, we propose that BiP interaction with incompletely folded CH1 domains is required to mediate correct assembly of H and L chains.

Measurement of affinity in serum samples of antigen-free, germ-free and conventional mice after hyperimmunization with 2,4-dinitrophenyl keyhole limpet hemocyanin, using surface plasmon resonance

We previously investigated the primary and secondary responses and hyperimmunization to the T cell-dependent antigen 2,4-dinitrophenyl keyhole limpet hemocyanin (DNP-KLH) in antigen-free (AF), germ-free (GF) and conventional (CV) mice. Both the absolute and relative numbers of DNP-specific IgG-secreting cells in the spleen of AF mice were considerably higher compared to GF and CV mice, especially after hyperimmunization. In the present study we measured the total and DNP-specific IgG concentration in the sera of these hyperimmunized mice using a sensitive sandwich enzyme-linked immunosorbent assay. With respect to the total IgG concentration before and after hyperimmunization, the AF mice showed an almost 13-fold increase after boosting with the antigen; the GF mice showed an approximately 8-fold increase. A slight but non-significant increase was observed in the CV mice. The total as well as the DNP-specific IgG levels in the AF-immunized mice were 2-fold and 5-fold higher compared to GF and CV mice, respectively. With the use of Surface Plasmon Resonance instrumentation (BIAcore, Pharmacia, Uppsala, Sweden) we obtained mean binding affinities (KA) of the polyclonal samples of the three groups of hyperimmunized mice. IgA and IgM samples displayed low affinity for DNP-lysine. The AF mice displayed the highest KA value among IgG antibodies, followed by GF mice, while CV mice showed a 3-fold lower KA compared to AF mice. These differences were mainly determined by the dissociation rate constant (kdiss), since no significant changes were observed in the association rate constant (kass). Furthermore, the sera of the CV mice have a lower percentage of high-affinity antibodies compared to GF and AF mice. These results suggest that besides a higher overall binding affinity seen in AF mice, and to a lesser extent in GF mice, the relative contribution of high-affinity IgG is greater in AF mice compared to CV mice.

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

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

Kinetics of somatic mutation in lymph node germinal centres

Somatic mutation activity in immunoglobulin V kappa genes during the response to the hapten 2-phenyl-5-oxazolone was measured in lymph node B-cell populations at various timepoints after footpad immunization. When the V kappa Ox1 genes rearranged to the J kappa 5 segment were amplified from genomic DNA using the polymerase chain reaction and sequenced, somatic mutations could be detected as early as day 4 after immunization. Somatic mutations were also detected after sequencing RNA from oxazolone-specific hybridomas derived from lymph node cells at day 4 after immunization. These early mutations were found mostly in cells with a germinal centre phenotype. No indication of selection at the population level by apoptosis was detected until day 7 after immunization. These results suggest somatic mutations can be induced very early during the immune response in lymph node cells, prior to the peak of clonal expansion and selection with regard to antigen binding.

Lymphocyte populations and immune responses in CD5-deficient mice

The CD5 antigen is expressed at a high level on T cells from early on in thymocyte ontogeny and continues to be expressed on the surface of all mature T cells. In addition, it marks a population of B lymphocytes (B-1a) with distinct physiological properties. To study the in vivo function of CD5, the murine gene was inactivated using the technique of homologous recombination in embryonic stem cells. In homozygous mutant mice the CD5 antigen is not expressed on the surface of either T or B lineage cells, indicating that in both cell populations this antigen is encoded by the same gene. CD5-deficient (CD5T) mice are healthy and populations of T and B lymphocytes in these mice look unchanged when compared to control mice. The mutant mice are able to mount effective immune responses to T cell-dependent and -independent antigens.

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

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

Frequency of immunoglobulin E class switching is autonomously determined and independent of prior switching to other classes

Both, in humans and in mice, a major fraction of immunoglobulin E (IgE)-expressing B lymphocytes develops by sequential Ig class switching from IgM via IgG to IgE. This sequential class switch might have functional implications for the frequency and repertoire of IgE+ cells. Here we show that in mutant mice, in which sequential switching to IgE via IgG1 is blocked, the frequency of cells switching to IgE is not affected. Thus, sequential class switching to IgE merely reflects the simultaneous accessibility of two acceptor switch regions for switch recombination, induced by one cytokine, but with markedly distinct efficiency. Analysis of switch recombination on both IgH alleles of switched cells shows that the low frequency of switching to IgE is an inherent feature of the S epsilon switch region and its control elements.

Induction of interleukin 4 (IL-4) expression in T helper (Th) cells is not dependent on IL-4 from non-Th cells

Interleukin 4 (IL-4) is essential for the induction of immunoglobulin E (IgE) responses in mice. Recent in vitro studies have suggested that IL-4 derived from non T helper (Th) cells, in particular from mast cells and basophils, may be essential for triggering of IL-4 expression in Th cells and may directly contribute to IgE isotype switch induction. Here, we have generated mice carrying a functional IL-4 gene only in Th cells or non-Th cells, respectively, by reconstitution of IL-4-deficient mice (IL-4T mice) with CD4+ or CD4- spleen cells from congenic wild-type animals. In mice in which only CD4+ cells are able to express IL-4, antigen-specific IgE is produced in a T cell-dependent immune response. Thus, induction of IL-4 expression in Th cells can occur in the absence of IL-4 from non-Th cells, which suggests that at least some Th cells can express IL-4 in response to another signal which has yet to be identified. No IgE is detectable, however, in mice in which only CD4- cells can express IL-4, suggesting that Th cells are the primary, if not the only source of IL-4 for initial induction of IgE synthesis.

B cell function in mice transgenic for mCTLA4-H gamma 1: lack of germinal centers correlated with poor affinity maturation and class switching despite normal priming of CD4+ T cells

This report outlines the B cell phenotype of transgenic mice that overexpresses the mouse CTLA-4-human gamma 1 (mCTLA4-H gamma 1) protein. Despite the fact that these mice prime CD4+ T cells (Ronchese, F., B. Housemann, S. Hubele, and P. Lane. 1994. J. Exp. Med. 179:809), antibody responses to T-dependent antigens are severely impaired. In contrast, T-independent responses are normal which suggests mCTLA4-H gamma 1 does not act directly on B cells, but acts indirectly by impairing T cell help. The impaired antibody defect is associated with impaired class switching, with low total immunoglobulin (Ig)G and antigen-specific IgG responses, and an absence of germinal center formation in spleen and lymph nodes but not gut-associated tissues. The defective germinal center formation is associated with a reduction in the degree of somatic mutation in hybridomas made from transgenic mice in comparison with those made from normal mice. It seems likely that mCTLA4-H gamma 1 exerts its effect by blocking an interaction between T and B cells that induce T cell help for B cells.

Comparison of somatic mutation frequency among immunoglobulin genes

We analyzed the frequency of somatic mutation in immunoglobulin genes from hybridomas that secrete anti-(4-hydroxy-3-nitrophenyl)acetyl (NP) monoclonal antibodies. A high frequency of mutation (3.3-4.4%) was observed in both the rearranged VH186.2 and V lambda 1 genes, indicating that somatic mutation occurs with similar frequency in these genes in spite of the absence of an intron enhancer in lambda 1 chain genes. In contrast to the high frequency in J-C introns, only two nucleotide substitutions occurred at positions -462 and -555 in the 5' noncoding region in one of the lambda 1-chain genes and in none of the other three so far studied. Since a similar low frequency of somatic mutation was observed in the 5' noncoding region of inactive lambda 2-chain genes rendered inactive because of incorrect rearrangement, this region may not be a target or alternatively, may be protected from the mutator system. We observed a low frequency of nucleotide substitution in unrearranged V lambda 1 genes (approximately 1/15 that of rearranged genes). Together with previous results (Azuma T., N. Motoyama, L. Fields, and D. Loh, 1993. Int. Immunol. 5:121), these findings suggest that the 5' noncoding region, which contains the promoter element, provides a signal for the somatic mutator system and that rearrangement, which brings the promoter into close proximity to the enhancer element, should increase mutation efficiency.