Immunoglobulin D (IgD)-deficient mice reveal an auxiliary receptor function for IgD in antigen-mediated recruitment of B cells

TGF-beta receptor controls B cell responsiveness and induction of IgA in vivo

To determine the role of the pleiotropic cytokine TGF-beta in B cells, we generated mice lacking the TGF-beta receptor (TbetaR) type II selectively in this cell type through conditional mutagenesis (Cre/loxP). The absence of TbetaRII in B cells leads to a reduced life span of conventional B cells, expansion of peritoneal B-1 cells, B cell hyperplasia in Peyer's patches, elevated serum immunoglobulin, and substantial IgG3 responses to a normally weak immunogen. This B cell hyperresponsiveness is associated with a virtually complete serum IgA deficiency. The data reveal differential roles of TbetaR in homeostasis and antigen responsiveness of B cell subpopulations and establish a critical function of the TGF-beta receptor ligand pair in the induction of IgA responses in vivo.

Stability of the B cell antigen receptor complex

The B cell antigen receptor (BCR) comprises the membrane-bound immunoglobulin (mIg) molecule and the Ig-alpha/Ig-beta heterodimer. By comparing the stability of the IgD-BCR and IgM-BCR in different detergent lysates, we find that the IgD-BCR is more stable than the IgM-BCR. Analysis of chimeric mIgD molecules suggests that the deltam transmembrane region is responsible for the more stable association of mIgD with the Ig-alpha/Ig-beta heterodimer. Further, the differential glycosylation of Ig-alpha molecules, in the two different BCR complexes, is determined solely by the ectodomains of the mIg molecules. The implications of these findings for the intracellular transport and the signalling capacity of the BCRs are discussed.

Apoptosis or plasma cell differentiation of CD38-positive B-chronic lymphocytic leukemia cells induced by cross-linking of surface IgM or IgD

Previously, we demonstrated that B-chronic lymphocytic leukemia (B-CLL) cells could be divided into 2 groups depending on the expression of CD38 by the malignant cells. The 2 groups differed in their signal-transducing capacities initiated by cross-linking of surface IgM; only in CD38-positive cells was an efficient signal delivered, invariably resulting in cell apoptosis. In this study, we investigated the effect of surface IgD cross-linking in 10 patients with CD38-positive B-CLL. Exposure of the malignant cells to goat antihuman δ-chain antibodies (Gaδ-ab) caused [Ca++]i mobilization and tyrosine kinase phosphorylation in a manner not different from that observed after goat antihuman μ-chain antibody (Gaμ-ab) treatment in vitro. However, Gaδ-ab-treated cells failed to undergo apoptosis and instead displayed prolonged survival in culture and differentiated into plasma cells when rIL2 was concomitantly present. Cross-linking of surface IgD failed to induce proliferation of the malignant cells in vitro. Moreover, treatment with Gaδ-ab did not prevent apoptosis of B-CLL cells induced by Gaμ-ab. Collectively, these experiments demonstrated that IgM and IgD expressed by the same cell may deliver opposite signals under particular circumstances and provide some clues for the understanding of the pathophysiology of B-CLL.

Impaired immunity and enhanced resistance to endotoxin in the absence of neutrophil elastase and cathepsin G

While the critical role of reactive oxygen intermediates (ROI) in the microbicidal activity of polymorphonuclear granulocytes is well established, the function of the nonoxidative effector mechanisms in vivo remains unclear. Here we show that mice deficient in the neutrophil granule serine proteases elastase and/or cathepsin G are susceptible to fungal infections, despite normal neutrophil development and recruitment. The protease deficiencies but not the absence of ROI leads to enhanced resistance to the lethal effects of endotoxin LPS, although normal levels of TNFalpha are produced. The data demonstrate a critical role of the nonoxidative effector mechanisms of neutrophils in host immunity and immunopathology and identify elastase and cathepsin G as effectors in the endotoxic shock cascade downstream of TNFalpha.

Differential effects of injections of anti-mu and anti-delta monoclonal antibodies on B-cell populations in adult mice: regulation of xenoreactive natural antibody-producing cells

BACKGROUND

The depletion of differential B cell and xenoreactive natural antibodies (XNA) by anti-delta and anti-mu injections was analyzed in adult mice. Sequential treatment with anti-delta and then anti-mu induces a complete depletion of B cells and XNA and represents a potential approach to induce xenograft tolerance.

METHODS

Adult mice were injected with anti-mu, anti-delta, anti-delta then anti-mu, or control isotype monoclonal antibodies from day 0 to day 14. The different B-cell populations were analyzed by FACS and immunohistology. Ig production was tested by ELISA. XNA were analyzed by FACS.

RESULTS

Anti-mu injections induced a depletion of IgMhigh, immature B cells, marginal zone B cells, and B1 cells and an increase of IgG-XNA production. Anti-delta injections induced mature conventional IgDhigh B-cell depletion and increased IgM-XNA production. Interestingly, sequential injections of anti-delta then anti-mu induced a depletion of immature B cells, mature B cells (MZ, B2, and B1), and XNA.

CONCLUSIONS

These results demonstrate that mature B-cell depletion in adult mice can be obtained by mAb injections and depends on the surface immunoglobulin cross-linking threshold. Indeed, anti-mu mAb depleted IgMhigh B cells (MZ and B1) and anti-delta, IgDhigh B cells (B2). The differential B-cell suppression shows that conventional B cells are responsible in the IgG-XNA production and MZ and B1 cells in the IgM-XNA production. Sequential repeated injections of anti-delta then anti-mu mAb depleted all B-cell populations and suppressed the whole XNA production.

Immunological development and cardiovascular function are normal in annexin VI null mutant mice

Annexins are calcium-binding proteins of unknown function but which are implicated in important cellular processes, including anticoagulation, ion flux regulation, calcium homeostasis, and endocytosis. To gain insight into the function of annexin VI, we performed targeted disruption of its gene in mice. Matings between heterozygous mice produced offspring with a normal Mendelian pattern of inheritance, indicating that the loss of annexin VI did not interfere with viability in utero. Mice lacking annexin VI reached sexual maturity at the same age as their normal littermates, and both males and females were fertile. Because of interest in the role of annexin VI in cardiovascular function, we examined heart rate and blood pressure in knockout and wild-type mice and found these to be identical in the two groups. Similarly, the cardiovascular responses of both sets of mice to septic shock were indistinguishable. We also examined components of the immune system and found no differences in thymic, splenic, or bone marrow lymphocyte levels between knockout and wild-type mice. This is the first study of annexin knockout mice, and the lack of a clear phenotype has broad implications for current views of annexin function.

T(h)2 cytokine dependence of IgD production by normal human B cells

IgD is a minor component of serum Ig and the control of IgD secretion is virtually unknown. We measured concentrations of IgD (and IgE and IgM as controls) in culture supernatants of peripheral blood mononuclear cells (PBMC) from 60 normal donors as well as mononuclear cells from 10 tonsils following culture in the absence or presence of CD40 mAb and cytokines. Low levels of IgD were measured in cultures of PBMC, either unstimulated or stimulated by anti-CD40 antibodies. IL-4 and IL-10 significantly increased IgD production by CD40 mAb-stimulated cells in the majority of normal subjects studied, whereas in a limited number of individuals, spontaneous IgD production was either low or high, but with no increase upon stimulation. Spontaneous IgD production by tonsil-derived mononuclear cells was higher than by PBMC and increased after CD40 stimulation and even more in the presence of IL-10, but not IL-4. IL-2 and IFN-gamma exerted a dose-dependent inhibition on spontaneous as well as CD40- and cytokine-induced IgD production by PBMC, but not by tonsil mononuclear cells. Activation by IL-4 of CD40-stimulated purified B cells from tonsil and PBMC, and by IL-10 of tonsil B cells increased IgD production, whereas IL-2 and IFN-gamma had no detectable inhibitory effect. This suggests that accessory cells indirectly regulate IgD synthesis. IgD production induced in PBMC by IL-4 or IL-10 appeared to result from an active synthesis, and correlated with an increase in the number of IgD-containing plasma cells as demonstrated by immunofluorescence and increased expression of secreted IgD transcripts. These findings suggest that IgD production by normal peripheral blood human B cells is regulated positively by T(h)2 cytokines and negatively by T(h)1 cytokines.

Molecular cloning and phylogenetic analysis of the Atlantic salmon immunoglobulin D gene

A gene homologous to the IgD heavy chain (delta) gene in channel catfish (Ictalurus punctatus) was found 0.9 kb downstream of the IgM heavy chain (mu) gene in Atlantic salmon (Salmo salar). As in catfish, the first constant mu exon is spliced into the delta transcripts. In agreement with the tetraploid ancestry of the salmonid fish family there are two highly similar delta genes in Atlantic salmon. Characterization of these genes showed that they encode seven 'unique' Ig domains, three of which are tandem duplicated, i.e. like delta1-(delta2-delta3-delta4)*-(delta2- delta3-delta4)-delta5-delta6-d elta7. Sequence analysis indicates that delta1-delta7 arose from two duplication events. Accordingly, salmon delta can be reduced to a unit of three Ig domains corresponding to the three C-terminal domains of a prototypic Ig molecule. The ancestral three-domain unit is apparently best conserved in delta1-delta5-delta6. Phylograms indicate a relationship between teleost and mammalian IgD mainly because of the similarity between the teleost delta5 and human delta2. The corresponding domain in mouse IgD has been deleted during evolution. The teleost delta1 and delta6 sequences are most similar to domains of other non-IgM isotypes, including those in cartilaginous fishes.

Immune System Development and Function in Prolactin Receptor-Deficient Mice

Prolactin (PRL) is the primary lactogenic pituitary hormone that plays an essential role in many aspects of reproduction, from fertilization to mammary gland development and maternal behavior. PRL has also been reported to play a role in immunoregulation. Because initial observations indicated that hypophysectomized rats present abnormalities of the immune system, including increased thymic atrophy and lymphopenia, a number of studies have focused on the potential immunomodulatory roles of PRL. This hormone exerts its biological activities following binding to specific cell surface PRL receptors (PRLRs). In this report, we have characterized the development and function of the immune system in PRLR-deficient mice. Compared with wild-type control mice, PRLR−/− mice demonstrate no alterations in thymic or splenic cellularity or in the composition of the lymphocyte subsets present in primary (bone marrow and thymus) or secondary (spleen and lymph nodes) lymphoid organs. Lymphocytes from PRLR−/− mice are functional in vitro, as they can proliferate normally to mitogens, cytokines, and allogeneic cells. PRLR−/− splenocytes display normal NK-mediated cytotoxicity to YAC-1 target cells. In vivo studies have revealed that PRLR−/− mice are able to 1) generate normal steady-state Ig levels, 2) mount a normal specific Ig response following immunization with a T-dependent Ag, 3) eliminate injected allogeneic tumor cells, and 4) effectively control Listeria monocytogenes infection. Taken together, these results show that immune system development and function proceed normally in the absence of PRL-mediated signaling and suggest that PRLR pathways are not essential for immunomodulation in vivo.

Facilitated antigen presentation by B cells expressing IgD when responding T cells express IgD-receptors

In vitro studies have confirmed that cognate interactions between T and B cells are required to demonstrate enhanced helper activity using T cells with upregulated IgD-receptors (IgD-Rs). We studied the mechanism by which IgD-R+ T cells facilitate antibody responses by examining whether T cells also benefit from their expression of IgD-R. Experiments were designed to determine whether upregulation of IgD-R on T cells facilitates antigen presentation by IgD+ B cells. Goat Ig-primed splenic T cells from BALB/c mice were tested for their ability to respond to antigen-presenting B cells treated with goat anti-mouse (GAM) IgM or GAM IgD. T cell responses to GAM IgM and GAM IgD presented by B cells were significantly higher when goat Ig-primed cells were induced to express IgD-R by exposure to oligomeric IgD compared with goat Ig-primed control T cells. This effect was inhibited when monomeric IgD was added to the cultures. No differences in T and IgD-R+ T cell responses were seen using adherent cells as APCs. B cells from IgD-/- mice were also tested. Such B cells present antigen to IgD-R+ T cells without promoting enhanced responses compared with B cells from heterozygous IgD+/- mice. These studies suggest that IgD may play a costimulatory role during antigen presentation. We conclude that when T cells are induced to express IgD-R, these lectin-like receptors can ligate B cell membrane IgD during antigen presentation to facilitate responses of each of the cells engaged in cognate interaction, yielding enhanced antigen-specific T cell and B cell responses.

Postnatally induced inactivation of gp130 in mice results in neurological, cardiac, hematopoietic, immunological, hepatic, and pulmonary defects

The pleiotrophic but overlapping functions of the cytokine family that includes interleukin (IL)-6, IL-11, leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and cardiotrophin 1 are mediated by the cytokine receptor subunit gp130 as the common signal transducer. Although mice lacking individual members of this family display only mild phenotypes, animals lacking gp130 are not viable. To assess the collective role of this cytokine family, we inducibly inactivated gp130 via Cre-loxP-mediated recombination in vivo. Such conditional mutant mice exhibited neurological, cardiac, hematopoietic, immunological, hepatic, and pulmonary defects, demonstrating the widespread importance of gp130-dependent cytokines.

Mice Deficient for the Ecto-Nicotinamide Adenine Dinucleotide Glycohydrolase CD38 Exhibit Altered Humoral Immune Responses

CD38 is a membrane-associated ecto-nicotinamide adenine dinucleotide (NAD+) glycohydrolase that is expressed on multiple hematopoietic cells. The extracellular domain of CD38 can mediate the catalysis of NAD+ to cyclic adenosine diphosphoribose (cADPR), a Ca2+-mobilizing second messenger, adenosine diphosphoribose (ADPR), and nicotinamide. In addition to its enzymatic properties, murine CD38 has been shown to act as a B-cell coreceptor capable of modulating signals through the B-cell antigen receptor. To investigate the in vivo physiological function(s) of this novel class of ectoenzyme we generated mice carrying a null mutation in the CD38 gene. CD38−/− mice showed a complete loss of tissue-associated NAD+ glycohydrolase activity, showing that the classical NAD+ glycohydrolases and CD38 are likely identical. Although murine CD38 is expressed on hematopoietic stem cells as well as on committed progenitors, we show that CD38 is not required for hematopoiesis or lymphopoiesis. However, CD38−/− mice did exhibit marked deficiencies in antibody responses to T-cell–dependent protein antigens and augmented antibody responses to at least one T-cell–independent type 2 polysaccharide antigen. These data suggest that CD38 may play an important role in vivo in regulating humoral immune responses.

© 1998 by The American Society of Hematology.

Differential inhibition of B-cell development and xenoreactive natural antibody production by administration of anti-mu or anti-delta monoclonal antibodies in adult rats

BACKGROUND

Given the role of xenoreactive natural antibodies (XNA) in the pathogenesis of xenograft rejection, we tested whether the administration of anti-mu or anti-delta monoclonal antibodies (mAbs) in adult rats would suppress the generation of XNA.

METHODS

Adult LOU/C (Igkappa-1a) rats were treated with anti-mu or anti-delta mAbs after nonlethal total body irradiation and bone marrow transplantation from congenic LOU/C (Igkappa-1b) rats. The differentiation of donor bone marrow (BM)-driven Igkappa-1b+ B cells and XNA production were analyzed.

RESULTS

Both anti-mu and anti-delta mAbs arrested B-cell differentiation in the BM. In anti-mu-treated rats, there was a total depletion of donor-driven, peripheral Igkappa-1b+ B cells, secreting cells, and circulating XNA of the Igkappa-1b allotype. In anti-delta-treated rats, a significant number of Igkappa-1b+ B cells, which did not express membrane IgD, "escaped" deletion and partially repopulated peripheral lymphoid organs. This B-cell population was active in the production of XNA, as revealed by the high serum levels of XNA in these animals.

CONCLUSIONS

Anti-mu administration resulted in arrest of B-cell differentiation and in down-regulation of IgM and IgG XNA production in adult rats. These data suggest that the use of anti-mu mAbs may be a useful approach to suppress the production of XNA and prevent xenograft rejection. Furthermore, we suggest that the B-cell population responsible for the production of XNA in adult rats belongs to a B-cell lineage expressing low levels of membrane IgD and "escaping" deletion in the BM upon anti-delta treatment.

Mice Deficient for the Ecto-Nicotinamide Adenine Dinucleotide Glycohydrolase CD38 Exhibit Altered Humoral Immune Responses

CD38 is a membrane-associated ecto-nicotinamide adenine dinucleotide (NAD+) glycohydrolase that is expressed on multiple hematopoietic cells. The extracellular domain of CD38 can mediate the catalysis of NAD+ to cyclic adenosine diphosphoribose (cADPR), a Ca2+-mobilizing second messenger, adenosine diphosphoribose (ADPR), and nicotinamide. In addition to its enzymatic properties, murine CD38 has been shown to act as a B-cell coreceptor capable of modulating signals through the B-cell antigen receptor. To investigate the in vivo physiological function(s) of this novel class of ectoenzyme we generated mice carrying a null mutation in the CD38 gene. CD38−/− mice showed a complete loss of tissue-associated NAD+ glycohydrolase activity, showing that the classical NAD+ glycohydrolases and CD38 are likely identical. Although murine CD38 is expressed on hematopoietic stem cells as well as on committed progenitors, we show that CD38 is not required for hematopoiesis or lymphopoiesis. However, CD38−/− mice did exhibit marked deficiencies in antibody responses to T-cell–dependent protein antigens and augmented antibody responses to at least one T-cell–independent type 2 polysaccharide antigen. These data suggest that CD38 may play an important role in vivo in regulating humoral immune responses.

© 1998 by The American Society of Hematology.

IgD can largely substitute for loss of IgM function in B cells

The mu and delta heavy chains of IgM and IgD, the first antibody isotypes expressed during bone-marrow B-cell development, are encoded by a common transcription unit. Expression of the mu chain on the surface of late pre-B cells allows their further development to immature B cells. Coexpression of the delta chain and emigration of the immature B cells to the periphery eventually leads to the development of naive mature IgM/IgD double-positive cells. Although IgM is important in driving B-cell development, the contribution of IgD is not clear. Here we investigate the function of IgD. We generated mice deficient in IgM (IgM-/- mice) by deleting the mu region in embryonic stem cells. IgM-/- mice showed normal B-cell development and maturation, with IgD replacing membrane-bound and secretory IgM. Moreover, specific B-cell responses and isotype class switches occurred during immunization or infection. In contrast to mice deficient in B cells, IgM-/- mice survived infection with vesicular stomatitis virus by developing neutralizing immunoglobulins, but they were more susceptible than wild-type controls with delayed specific immunoglobulin responses. These data lead us to conclude that IgD is largely able to substitute for IgM functions.

Affinity Maturation in Lyn Kinase-Deficient Mice with Defective Germinal Center Formation

Lyn kinase-deficient (lyn−/−) mice show several abnormalities such as reduced numbers of circulating B cells, hyper-IgM, and low proliferative responses induced by CD40 ligand. Lyn−/− mice also develop splenomegaly, produce autoreactive Abs with age, and finally develop glomerulonephritis. Another abnormality observed in lyn−/− mice is that their disability to form germinal centers (GCs). It has been considered that GCs play an important role in affinity maturation and differentiation to B cell memory upon immunization with thymus-dependent Ag. Since Lyn kinase has been thought to be downstream of the signals from the B cell Ag receptor as well as CD40, we studied whether or not lyn−/− mice could exhibit normal Ag-specific class switching and affinity maturation following somatic hypermutation. The mice were immunized with (4-hydroxy-3-nitrophenyl)acetyl-chicken γ-globulin (NP-CG). Production of NP-specific IgG1 Abs was slightly reduced but clearly detectable. The affinity of Abs produced was comparable to that in wild-type mice. Furthermore, somatic hypermutation occurred in the heavy-chain variable region at the same level as that in wild-type mice. Therefore, we conclude that isotype switching and affinity maturation occur normally in lyn−/− mice without the formation of GCs. The results lead to a speculation that Lyn may not play a role in induction of isotype switching or affinity maturation, despite being downstream of the signals from the B cell Ag receptor complex and CD40, and that GC architecture may not be absolutely essential for affinity maturation.

Enhanced B-1 Cell Development, But Impaired IgG Antibody Responses in Mice Deficient in Secreted IgM

The role of endogenous natural IgM in promoting the adaptive Ab response was investigated in newly constructed mutant mice in which B cells do not secrete IgM but still express surface IgM and IgD and undergo class switching to express other Ig isotypes. While the mutant mice had relatively normal numbers of conventional B (B-2) cells in all tissues examined, unexpectedly, B-1 cells in the peritoneum and spleen were approximately threefold more abundant. The elevated levels of B-1 cells were already detectable at 4 wk of age and were stably maintained throughout life. The levels of serum IgG2a, IgG3, and IgA were also elevated in the mutant mice at an early age. IgG2a response to a T cell-independent Ag was augmented, whereas IgG Ab responses to suboptimal doses of a T cell-dependent Ag were impaired. The latter defect was associated with fewer splenic germinal centers, impaired Ab affinity maturation, and less Ag trapping on follicular dendritic cells. Together, these findings demonstrate a physiologic role of natural IgM in the feedback regulation of B-1 cell development, the regulation of IgG2a production, and the promotion of efficient B-2 cell Ab responses.

Cell division number regulates IgG1 and IgE switching of B cells following stimulation by CD40 ligand and IL-4

CD40 ligand (CD40L) and IL-4 are sufficient to induce resting murine B cells to divide and switch isotypes from IgM and IgD to IgG1 and IgE. Tracking of cell division following (5- and 6) carboxyfluorescein diacetate succinimidyl ester (CFSE) labeling revealed that B cells expressed IgG1 after three cell divisions, and IgE after five. The probability of isotype switching at each division was independent of both time after stimulation and of the dose of CD40L. IL-4 concentration regulated the number of divisions that preceded isotype switching. Loss of surface IgM and IgD was also related to cell division and appeared to be differentially regulated. B cell proliferation was typically asynchronous with the proportion of cells in consecutive divisions being markedly affected by the concentration of CD40L and IL-4. Simultaneous (5-bromo)-2'-deoxyuridine labeling and CFSE staining revealed that B cells in each division cycle were dividing at the same rate. Therefore, division cycle asynchrony resulted from dose-dependent variation in the time taken to enter the first division cycle. These results suggest that T-dependent B cell expansion is linked to predictable functional changes that may, in part, explain why IgE is produced in response to prolonged antigenic stimulation.

Residual MHC class II expression on mature dendritic cells and activated B cells in RFX5-deficient mice

Patients with major histocompatibility complex class II (MHC-II) deficiency are known to carry mutations in either the RFX complex or the trans-activator CIITA. While the pivotal role of CIITA for MHC-II gene transcription is supported by the essential absence of MHC-II molecules in CIITA-deficient mice, we demonstrate here that RFX5-/- mice retain expression of MHC-II in thymic medulla, mature dendritic cells, and activated B cells. Nevertheless, RFX5-/- mice develop a severe immunodeficiency due to the lack of MHC-II in thymic cortex, failure of positive selection of CD4+ T cells, and absence of MHC-II on resting B cells and resident or IFNgamma-activated macrophages. This differential requirement for CIITA and RFX5 in subsets of antigen-presenting cells may be specific for the mouse; it may, however, also exist in humans without having been noticed so far.

The effect of aging on IgD receptor expression by T cells and its functional implications

Exposure to oligomeric or aggregated (a), but not to monomeric (m), IgD causes a rapid (within 1 h) upregulation of IgD-R expression on CD4+ T cells from young, but not from aged, mice and on both CD4+ and CD8+ T cells from all young and from approximately 65% of aged humans. In normal young (but not in IgD-/-) mice, this increase in IgD-R expression is associated with a marked increase in primary and secondary antibody responses, transferable to both aged and young mice with T cells from aIgD pretreated donors. In both species, immunization causes a rise in the IgD-R+ expression in vivo in the young. In mice, mIgD abolishes both the induction of IgD-R expression and augmentation of immune responses, suggesting that interaction between IgD-R+ T and IgD+ B cells is needed. In aged humans, the ability of peripheral blood lymphocytes to exhibit IgD-R expression in response to aIgD in vitro or to influenza vaccine in vivo is strongly correlated to the individual's ability to produce antibody. In T cells from aged mice, but not from aged IgD-non-responder humans, IgD-R are able to come to the cell surface if an additional signal has been supplied, such as by (ionomycin/thapsigargin + aIgD). Agents which induce IgD-R and augmentation of antibody production in aged and young mice include phosphatidylcholine and dehydroepiandrosterone sulfate. The immunoaugmenting effect of pretreatment with these agents appears indeed due to IgD-R+ T cells, because it is abolished by mIgD.

Abnormal development of secondary lymphoid tissues in lymphotoxin beta-deficient mice

The tumor necrosis factor (TNF) family cytokines lymphotoxin (LT) alpha and LTbeta form heterotrimers that are expressed on the surface of activated lymphocytes and natural killer cells; LTalpha homotrimers can be secreted as well. Mice with a disrupted LTalpha gene lack lymph nodes (LN), Peyer's patches (PP), and follicular dendritic cell (FDC) networks and reveal profound defects of the splenic architecture. However, it is unclear which of these abnormalities is the result of the absence in LTalpha homotrimers or LTalphabeta heterotrimers. To distinguish between these two possibilities, a mouse strain deficient in LTbeta was created employing Cre/loxP-mediated gene targeting. Mice deficient in LTbeta reveal severe defects in organogenesis of the lymphoid system similar to those of LTalpha-/- mice, except that mesenteric and cervical LN are present in most LTbeta-deficient mice. Both LTbeta- and LTalpha-deficient mice show significant lymphocytosis in the circulation and peritoneal cavity and lymphocytic infiltrations in lungs and liver. After immunization, PNA-positive B cell clusters were detected in the splenic white pulp of LTbeta-deficient mice, but FDC networks were severely underdeveloped. Collectively, these results indicate that LTalpha can signal independently from LTbeta in the formation of PNA-positive foci in the spleen, and especially in the development of mesenteric and cervical LN.

The BCL-6 proto-oncogene controls germinal-centre formation and Th2-type inflammation

Structural alterations of the promoter region of the BCL-6 proto-oncogene represent the most frequent genetic alteration associated with non-Hodgkin lymphoma, a malignancy often deriving from germinal-centre B cells. The BCL-6 gene encodes a zinc-finger transcriptional repressor normally expressed in both B cells and CD4+ T cells within germinal centres, but its precise function is unknown. We show that mice deficient in BCL-6 displayed normal B-cell, T-cell and lymphoid-organ development but have a selective defect in T-cell-dependent antibody responses. This defect included a complete lack of affinity maturation and was due to the inability of follicular B cells to proliferate and form germinal centres. In addition, BCL-6-deficient mice developed an inflammatory response in multiple organs characterized by infiltrations of eosinophils and IgE-bearing B lymphocytes typical of a Th2-mediated hyperimmune response. Thus, BCL-6 functions as a transcriptional switch that controls germinal centre formation and may also modulate specific T-cell-mediated responses. Altered expression of BCL-6 in lymphoma represents a deregulation of the pathway normally leading to B cell proliferation and germinal centre formation.

A novel chimeric Ig heavy chain from a teleost fish shares similarities to IgD

IgD is considered to be a recently evolved Ig, being previously found only in primates and rodents. Here we describe, from a teleost fish (the channel catfish, Ictalurus punctatus), a novel complex chimeric Ig heavy chain, homologous, in part, to the heavy chain (delta) of IgD. In addition to alternative secretory or membrane-associated C termini, this chimeric molecule contains a rearranged variable domain, the first constant domain of mu, and seven constant domains encoded by a delta gene homolog. Identification of the catfish gene as delta is based on the following properties: sequence relatedness to mammalian delta; a location within the IgH locus that is immediately downstream of the mu gene; separate terminal exons for the secretory and membrane forms; coexpression with the complete mu chain in some but not all B cells. These results (i) suggest that IgD is an ancient immunoglobulin that was present in vertebrates ancestral to both the mammals and the ray-finned fishes, and (ii) raise the possibility that this Ig isotype may have served an as yet unidentified important function early in the evolution of the immune system.

The roles of gamma 1 heavy chain membrane expression and cytoplasmic tail in IgG1 responses

In antibody responses, B cells switch from the expression of immunoglobulin (Ig) mu and delta heavy (H) chains to that of other Ig classes (alpha, gamma, or epsilon), each with a distinct effector function. Membrane-bound forms of alpha, gamma, and epsilon, but not mu and delta, have highly conserved cytoplasmic tails. Mutant mice unable to express membrane gamma1 H chains or producing tailless gamma1 H chains failed to generate efficient IgG1 responses and IgG1 memory. H chain membrane expression after class switching is thus required for these functions, and class switching equips the B cell antigen receptor with a regulatory cytoplasmic tail that naïve B cells lack.

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.

Germinal center founder cells display propensity for apoptosis before onset of somatic mutation

B lymphocytes undergo affinity maturation of their antigen receptors within germinal centers. These anatomical structures develop in secondary lymphoid organs from the clonal expansion of a few antigen-specific founder B cells, whose isolation and characterization are reported here. Human germinal center founder cells express the naive B cell markers surface IgM and IgD as well as the germinal center B cell markers CD10 and CD38. They express low levels of Bcl-2, high levels of Fas, and undergo rapid apoptosis in culture. The smaller nonproliferating sIgM+IgD+CD38+ B cells displayed a lower level of somatic mutation in their immunoglobulin variable region genes compared with the large proliferating ones. Unmutated sIgM+IgD-CD38+ tonsillar B cells may thus represent germinal center founder cells in which the program for apoptotic cell death is triggered before the onset of somatic mutation, allowing the selection of the germline antibody repertoire at an early stage.

Characterization of the expression and gene promoter of CD22 in murine B cells

CD22 is a B cell-restricted surface molecule which may play an important role in interactions between B cells and other cells and in regulating signals through the B cell receptor (BCR) complex. Here we have examined whether the mouse is a suitable in vivo model for studying CD22 functions. In primary and secondary lymphoid organs of adult mice CD22 is on all mature B cells, including resting IgM+IgD+ B cells, IgG+ HSA(lo) memory B cells, syndecan+ plasma cells and CD5+ B cells, but it is not on immature IgM+IgD- B cells. Biochemical analysis revealed that murine CD22 is associated with the IgM receptor in some, but not all, CD22+ B leukemic and lymphoma cell lines; as with human CD22, murine CD22 is rapidly phosphorylated on tyrosine after ligation of the BCR. In the CD22- murine pro-B cell line, FEMCL, CD22 expression was inducible by treatment with phorbol 12-myristate 13-acetate. A genomic fragment of the cd22b allele containing 1.3 kb 5' of exon 1 was sequenced in order to identify potential DNA regulatory elements in the CD22 promoter region. Consensus sequences for transcription factor binding sites including PU.1, AP-1, AP-2, C/EBP and SP-1 were present, but no classical TATA elements or initiator motifs were evident at relevant positions. The 1.3-kb promoter fragment 5' of exon 1 was sufficient for directing basal promoter activity in B and T cells. There was no significant sequence similarity between the murine and human cd22 gene promoters, although both contain repetitive elements and Sp-1 and AP1 binding sites. Thus, murine CD22 shares a number of features with human CD22 and the mouse provides a suitable model system for elucidating the function of CD22 in vivo.

Negative signaling in B cells by surface immunoglobulins

Cross-linking of surface immunoglobulins generates negative signals that cause B-cell death unless appropriate rescue signals are provided. Surface IgM is the main transducer of the negative signaling, but surface IgD and IgG may also transduce negative signaling when cross-linked intensively. In the surface IgM+, IgD+ human malignant B lymphoma cell lines B104 and DND-39, cross-linking of surface IgM by anti-IgM antibodies induced cell death. Anti-IgM antibody-induced B104 cell death was inhibited by stimulation with alpha- and beta-interferons but not stimulation with anti-CD40 antibody or IL-4, whereas anti-IgM antibody-induced DND-39 cell death was inhibited by stimulation with anti-CD40 antibody but not stimulation with alpha- and beta-interferons. Anti-IgM antibody-stimulated B104 cells had morphologic features compatible with necrosis, whereas anti-IgM antibody-stimulated DND-39 cells showed morphologic features of apoptosis. CD11a/CD54-dependent cell adhesion induced by stimulation with anti-CD40 antibody was involved in anti-CD40 antibody-mediated inhibition of anti-IgM antibody-induced DND-39 cells. In normal human mature B cells, cross-linking of surface IgM induced different signaling consequences, including DNA synthesis or cell division (positive signaling) or cell cycle arrest or death (negative signaling). In this system, too, CD40-transduced signal inhibited anti-IgM antibody-induced negative signaling, and CD11a/CD54-dependent cell adhesion played a role in the rescue process. It is suggested that quantitatively different intensities of surface IgM cross-linking induce qualitatively different signaling consequences; relatively weak cross-linking may induce DNA synthesis; moderate cross-linking may induce DNA synthesis with cell cycle arrest at the G2/M interphase; and intense cross-linking may induce apoptotic cell death. The reasons for this difference are not yet known. Further elucidation of the molecular mechanisms responsible for surface IgM-mediated negative signaling and its rescue signaling may contribute toward development of therapy for allergic disorders by artificial modulation of specific immunoglobulin production.

Affinity maturation without germinal centres in lymphotoxin-alpha-deficient mice

Affinity maturation by somatic hypermutation is thought to occur within germinal centres. Mice deficient in lymphotoxin-alpha (LT alpha-/- mice) have no lymph nodes or Peyer's patches, and fail to form germinal centres in the spleen. We tested whether germinal centres are essential for maturation of antibody responses to T-cell-dependent antigens. LT alpha-/- mice immunized with low doses of (4-hydroxy-3-nitrophenyl)acetyl-ovalbumin (NP-OVA) showed dramatically impaired production of high-affinity anti-NP IgG1. However, LT alpha-/- mice immunized with high doses of NP-OVA, even though they failed to produce germinal centres, manifested a high-affinity anti-NP IgG1 response similar to wild-type mice. Furthermore, when LT alpha-/- mice were multiply immunized with high doses of NP-OVA, the predominantly expressed anti-NP VH gene segment VH186.2 showed somatic mutations typical of affinity maturation. Thus, B-cell memory and affinity maturation are not absolutely dependent on the presence of germinal centres.

Immunoglobulin D enhances the release of tumor necrosis factor-alpha, and interleukin-1 beta as well as interleukin-1 receptor antagonist from human mononuclear cells

Immunoglobulin D (IgD) is normally present in only low concentrations in serum. In the hyper-IgD and periodic fever syndrome (HIDS), however, serum levels exceed 140 mg/l. This syndrome is further characterized by recurrent inflammatory febrile attacks together with an acute phase response and appearance of cytokines in the circulation. The role of IgD in the pathogenesis of HIDS and its relation to the increased cytokine concentrations is unclear. Therefore, we tested whether IgD, IgG and alpha 1-acid glycoprotein (AGP) isolated from human serum influence the synthesis of interleukin-1 beta (IL-1 beta), tumour necrosis factor-alpha (TNF-alpha), and IL-1ra, as measured by specific radioimmunoassays, in human peripheral blood mononuclear cells (PBMC). Incubation of PBMC with IgD and AGP for 24 hr led to increased release of IL-1 beta, TNF-alpha, and IL-lra. The magnitude of stimulation of IgD exceeded that of AGP; the effect by IgD was dose-dependent and showed a 30-fold (TNF-alpha) to almost 150-fold (IL-1 beta) increase at the highest concentration (50 mg/l), while AGP (750 micrograms/ml) only increased the cytokine secretion fourfold (TNF-alpha) to almost 30-fold (IL-1 beta). The effect of IgD on IL-1ra was less dramatic but a fivefold increase was observed at 50 mg/l compared with a 2.5-fold increase with AGP. IgD potentiated the effect of lipopolysaccharide (LPS) on secretion of both IL-1 beta and TNF-alpha, although the effect was most apparent for TNF-alpha. Apart from inducing IL-1ra synthesis, IgG did not influence cytokine release in human PBMC. These data indicate that IgD is a potent inducer of TNF-alpha, IL-1 beta and IL-1ra and thus may contribute to the pathogenesis of HIDS.

Aberrant B cell development and immune response in mice with a compromised BCR complex

The immunoglobulin alpha (Ig-alpha)-Ig-beta heterodimer is the signaling component of the antigen receptor complex on B cells (BCR) and B cell progenitors (pre-BCR). A mouse mutant that lacks most of the Ig-alpha cytoplasmic tail exhibits only a small impairment in early B cell development but a severe block in the generation of the peripheral B cell pool, revealing a checkpoint in B cell maturation that ensures the expression of a functional BCR on mature B cells. B cells that do develop demonstrate a differential dependence on Ig-alpha signaling in antibody responses such that a signaling-competent Ig-alpha appears to be critical for the response to T-independent, but not T-dependent, antigens.

Normal human IgD+IgM- germinal center B cells can express up to 80 mutations in the variable region of their IgD transcripts

Somatic hypermutation in immunoglobulin variable region genes occurs within germinal centers. Here, we describe a subset of germinal center dark zone centroblasts that express only sIgD and have accumulated up to 80 mutations per heavy chain variable region (IgVH delta gene). Over half of the hypermutated IgVH delta sequences were found to be clonally related. This level of mutation is not observed in either IgVH gamma transcripts from the same sample or IgVH delta transcripts from peripheral blood, suggesting that these cells neither undergo isotype switch nor mature into circulating memory B cells. Optimal growth of these cells in vitro depends on CD40 ligand, T cell cytokines, and a fibroblast stroma, a combination possibly mimicking the dark zone microenvironment. Our hypothesis is that these cells may be sequestered within germinal centers, where their somatic mutation machinery is triggered. The isolation of these hypermutated B cells may represent a critical step for studying both the biology and biochemistry of somatic hypermutation.

Impairment of T-cell-dependent B-cell responses and B-1 cell development in CD19-deficient mice

CD19 is the hallmark differentiation antigen of the B lineage. Its early expression has implicated a role for CD19 during the antigen-independent phases of B-cell development, whereas in mature B cells CD19 can act synergistically with surface immunoglobulin to induce activation. We have generated CD19-deficient mice and found that development of conventional B cells is unperturbed. However, mature CD19-/- B cells show a profound deficiency in responding to protein antigens that require T-cell help. This is accompanied by a lack of germinal centre formation and affinity maturation of serum antibodies. Thus CD19 is crucial for both initial B-cell activation by T-cell-dependent antigens and the maturation and/or selection of the activated cells into the memory compartment. An impairment in ligand-driven selection may also be responsible for the observation of a striking reduction in the B-1 (formerly Ly-1) B-cell subset, thought to develop under the control of self-antigens and bacterial antigens (reviewed in ref. 2).

The rabbit B cell antigen receptor is non-covalently associated with unique heteromeric protein complexes: possible insights into the membrane IgM/IgD coexpression paradox

We describe several proteins that are components of the rabbit B cell receptor complex. Two proteins (37 kDa and 42 kDa) were found in non-covalent association with IgM expressed on B cells from peripheral blood. These proteins were also immunoprecipitated by anti-B29 (Ig-beta) and anti-mb1 (Ig-alpha) monoclonal antibodies. As in the mouse and human, the IgM associated molecules were found as heteromeric structures with non-reduced apparent molecular weights of approximately 70-75 kDa. On rabbit B cells we also found these proteins in a 100-135 kDa complex which may represent trimeric or tetrameric structures. By Western blot, the 37 kDa protein was identified as rabbit Ig-beta (B29), suggesting that the 42 kDa protein is rabbit Ig-alpha. These data suggest that rabbit IgM is associated with both Ig-alpha/beta and Ig-(alpha beta)2 or alpha beta gamma complexes. When similar immunoprecipitation studies were performed on lysates made from B cells isolated from appendix follicles, we found two additional IgM associated protein complexes containing 34 kDa and 36 kDa proteins.

IgD expression on B cells is more efficient than IgM but both receptors are functionally equivalent in up-regulation CD80/CD86 co-stimulatory molecules

The expression and function of IgM and IgD antigen receptors were studied in a series of anti-hen egg lysozymes (HEL) immunoglobulin (Ig)-transgenic mice expressing either IgM alone, IgD alone, or both IgM and IgD. B cell surface expression of IgD was found to be more efficient than that of IgM. Thus antigen receptor density on IgD+, IgM- B cells was twofold higher than on IgM+, IgD- B cells despite the presence of sevenfold lower levels of membrane heavy chain mRNA, and coexpression of IgD with IgM led to almost complete inhibition of surface IgM. In addition, less extensive down-regulation of IgD occurred following exposure to antigen in vitro. When regulation of CD80/CD86 co-stimulatory molecules by surface Ig was examined, up-regulation of the former was initiated at lower antigen concentrations on IgM-, IgD+ compared to IgM+, IgD- B cells. On correcting for antigen receptor density, however, induction of CD80/CD86 by IgM and IgD was comparable. Taken together, these results reinforced the functional similarity of IgM and IgD antigen receptors while at the same time revealing differences in expression which may explain their simultaneous presence on mature B cells.

Mouse anti-mouse IgD monoclonal antibodies generated in IgD-deficient mice

The production of high-affinity monoclonal antibodies (mABs) is generally restricted to antigens recognised as foreign by the immune system. Here we report the generation of mouse mABs specific for mouse IgD. Mice rendered IgD-deficient by gene targeting and consequently immunologically fully responsive to mouse IgD, were used to elicit a humoral response against mouse IgD. Hybridomas producing mABs of high affinity were isolated and clones specific for non-allotypic determinants on the Fc or Fab portion of mouse IgD were obtained. The data show that mice lacking a protein of interest due to targeted gene inactivation can be utilised for the production of high-affinity mABs specific for that mouse protein and should also facilitate the generation of mABs specific for proteins highly conserved between species.

Positive and negative selection events during B lymphopoiesis

Early in B-cell development, large numbers of cells have to be generated, each of which expresses only one type of B-cell receptor (i.e. Ig) on its surface. This is achieved by the surface expression of a pre-B cell receptor containing a mu heavy chain/surrogate light chain which differentially provides signals for two responses of precursor B cells at this stage of development. On the one hand, it signals inhibition of further rearrangements of variable heavy chain to diverse-joining heavy chain loci to achieve allelic exclusion at the heavy-chain locus. On the other hand, it signals proliferative expansion by factors between 20 and 100. Later in B-cell development, tolerance to autoantigens must be established and maintained. Tolerance is achieved by developmental arrest and induction of secondary light-chain gene rearrangements in those IgM+ immature B cells that are reactive to autoantigens presented in the primary B-cell generating organs. Even later in development, when mature surface (s)IgM+/sIgD+ B cells encounter autoantigens presented to them in the periphery, either deletion or anergy of the autoantigen-reactive cells occurs. Anergic cells have a sIg-dependent, sIg-proximal defect in signaling and are short-lived. Anergy can be broken in vitro by polyclonal activation via ligation of CD40 in the presence of IL-4. A small part of the remaining immature B cells not reactive to autoantigens are selected to become mature, antigen-reactive sIgM+/sIgD+ B cells. Molecules which might guide such positive selection of B cells still remain to be identified.

Defective antigen receptor-mediated proliferation of B and T cells in the absence of Vav

Crosslinking of B- or T-cell antigen receptors results in the rapid tyrosine phosphorylation of a number of proteins, including Vav, a protein expressed in cells of the haematopoietic system. Vav contains an array of structural motifs that include Src-homology domains SH2/SH3 and regions of homology to the guanine-nucleotide-exchange protein Dbl, pleckstrin and protein kinase C (refs 5-9). Using the RAG-complementation approach, we have analysed in vivo differentiation and in vitro responses of B- and T-lineage cells generated by injection of embryonic stem cells homozygous for a null mutation in the vav gene into blastocysts of RAG-1- or RAG-2-deficient mice. Here we report that antigen receptor-mediated proliferative responses of B and T cells in vitro are severely reduced in the absence of Vav. We also suggest a direct link between the low proliferative response of Vav-deficient B and T cells and the reduced number of these cells in peripheral lymphoid organs of chimaeric mice.