Impaired CD40-signalling in CD19-deficient mice selectively affects Th2-dependent isotype switching

Altered Phenotype and Enhanced Antibody-Producing Ability of Peripheral B Cells in Mice with Cd19-Driven Cre Expression

Given the importance of B lymphocytes in inflammation and immune defense against pathogens, mice transgenic for Cre under the control of Cd19 promoter (Cd19^Cre/+ mice) have been widely used to specifically investigate the role of loxP-flanked genes in B cell development/function. However, impacts of expression/insertion of the Cre transgene on the phenotype and function of B cells have not been carefully studied. Here, we show that the number of marginal zone B and B1a cells was selectively reduced in Cd19^Cre/+ mice, while B cell development in the bone marrow and total numbers of peripheral B cells were comparable between Cd19^Cre/+ and wild type C57BL/6 mice. Notably, humoral responses to both T cell-dependent and independent antigens were significantly increased in Cd19^Cre/+ mice. We speculate that these differences are mainly attributable to reduced surface CD19 levels caused by integration of the Cre-expressing cassette that inactivates one Cd19 allele. Moreover, our literature survey showed that expression of Cd19^Cre/+ alone may affect the development/progression of inflammatory and anti-infectious responses. Thus, our results have important implications for the design and interpretation of results on gene functions specifically targeted in B cells in the Cd19^Cre/+ mouse strain, for instance, in the context of (auto) inflammatory/infectious diseases.

CD40-CD154: A perspective from type 2 immunity

The interaction between CD40 and CD154 (CD40 ligand) is central in immunology, participating in CD4⁺ T cell priming by dendritic cells (DC), CD4⁺ T cell help to B cells and classical macrophage activation by CD4⁺ T cells. However, its role in the Th2 side of immunology including helminth infection remains incompletely understood. Contrary to viral and bacterial stimuli, helminth products usually do not cause CD40 up-regulation in DC, and exogenous CD40 ligation drives Th2-biased systems towards Th1. On the other hand, CD40 and CD154 are necessary for induction of most Th2 responses. We attempt to reconcile these observations, mainly by proposing that (i) CD40 up-regulation in DC in Th2 systems is mostly induced by alarmins, (ii) the Th2 to Th1 shift induced by exogenous CD40 ligation is related to the capacity of such ligation to enhance IL-12 production by myeloid cells, and (iii) signals elicited by endogenous CD154 available in Th2 contexts and by exogenous CD40 ligation are probably different. We stress that CD40-CD154 is important beyond cognate cellular interactions. In such a context, we argue that the proliferation response of B-cells to IL-4 plus CD154 reflects a Th2-specific mechanism for polyclonal B-cell amplification and IgE production at infection sites. Finally, we argue that CD154 is a general immune activation signal across immune polarization including Th2, and propose that competition for CD154 at tissue sites may provide negative feedback on response induction at each site.

Mesenteric IL-10-producing CD5+ regulatory B cells suppress cow's milk casein-induced allergic responses in mice

Food allergy is a hypersensitive immune reaction to food proteins. We have previously demonstrated the presence of IL-10-producing CD5(+) B cells and suggested their potential role in regulating cow's milk casein allergy in humans and IgE-mediated anaphylaxis in mice. In this study, we determined whether IL-10-producing CD5(+) regulatory B cells control casein-induced food allergic responses in mice and, if so, the underlying mechanisms. The induction of oral tolerance (OT) by casein suppressed casein-induced allergic responses including the decrease of body temperature, symptom score, diarrhea, recruitment of mast cells and eosinophils into jejunum, and other biological parameters in mice. Notably, the population of IL-10-producing CD5(+) B cells was increased in mesenteric lymph node (MLN), but not in spleen or peritoneal cavity (PeC) in OT mice. The adoptive transfer of CD5(+) B cells from MLN, but not those from spleen and PeC, suppressed the casein-induced allergic responses in an allergen-specific and IL-10-dependent manner. The inhibitory effect of IL-10-producing CD5(+) B cells on casein-induced allergic response was dependent on Foxp3(+) regulatory T cells. Taken together, mesenteric IL-10-producing regulatory B cells control food allergy via Foxp3(+) regulatory T cells and could potentially act as a therapeutic regulator for food allergy.

Fluctuation in the Levels of Immunoglobulin M and Immunoglobulin G Antibodies for Cardiolipin and β2-Glycoprotein among Healthy Pregnant Women

OBJECTIVES

Antiphospholipid antibodies fluctuate during a healthy normal pregnancy. This study aimed to investigate the levels of both immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies for cardiolipin and β2-glycoprotein (β2GP) among healthy pregnant women.

METHODS

This study was conducted between May 2010 and December 2012. A total of 75 healthy Omani pregnant women with no history of autoimmune disease were investigated during their pregnancy and 90 days after delivery at the Armed Forces Hospital in Muscat, Oman. A control group of 75 healthy Omani non-pregnant women were also investigated as a comparison. Levels of IgM and IgG antibodies for both anti-cardiolipin antibodies (ACAs) and β2GP were measured using a standard enzyme-linked immunosorbent assay.

RESULTS

The ACA IgM levels were significantly higher in the control group compared to the pregnant women (P <0.001). No significant differences were observed in the ACA IgM levels between the control group and the pregnant women after delivery. In contrast, ACA IgG levels were significantly higher during pregnancy and after delivery compared with those of the healthy control group (P = 0.007 and 0.002, respectively). The levels of β2GP IgG were significantly higher during pregnancy than after delivery and in the control group (P = 0.001 and <0.001, respectively).

CONCLUSION

In this study, ACA IgG levels increased during healthy pregnancies and after normal deliveries whereas β2GP IgG levels increased transiently during the pregnancies. Both phenomena were found to be significantly associated with a transient decline in the levels of IgM specific for these antigens. Therefore, the levels of these antibodies may be regulated during a healthy pregnancy.

Swiprosin-1/EFhd2 limits germinal center responses and humoral type 2 immunity

Activated B cells are selected for in germinal centers by regulation of their apoptosis. The Ca2+ -binding cytoskeletal adaptor protein Swiprosin-1/EFhd2 (EFhd2) can promote apoptosis in activated B cells. We therefore hypothesized that EFhd2 might limit humoral immunity by repressing both the germinal center reaction and the expected enhancement of immune responses in the absence of EFhd2. Here, we established EFhd2(-/-) mice on a C57BL/6 background, which revealed normal B- and T-cell development, basal Ab levels, and T-cell independent type 1, and T-cell independent type 2 responses. However, T cell-dependent immunization with sheep red blood cells and infection with the helminth Nippostrongylus brasiliensis (N.b) increased production of antibodies of multiple isotypes, as well as germinal center formation in EFhd2(-/-) mice. In addition, serum IgE levels and numbers of IgE+ plasma cells were strongly increased in EFhd2(-/-) mice, both after primary as well as after secondary N.b infection. Finally, mixed bone marrow chimeras unraveled an EFhd2-dependent B cell-intrinsic contribution to increased IgE plasma cell numbers in N.b-infected mice. Hence, we established a role for EFhd2 as a negative regulator of germinal center-dependent humoral type 2 immunity, with implications for the generation of IgE.

B-cell expansion and lymphomagenesis induced by chronic CD40 signaling is strictly dependent on CD19

CD40, a member of the TNF receptor family, is expressed on all mature B cells and on most B-cell lymphomas. Recently, we have shown that constitutive activation of CD40 signaling in B cells induced by a fusion protein consisting of the transmembrane part of the Epstein-Barr viral latent membrane protein 1 (LMP1) and the cytoplasmic part of CD40 (LMP1/CD40) drives B-cell lymphoma development in transgenic mice. Because LMP1/CD40-expressing B cells showed an upregulation of CD19, we investigated CD19's function in CD40-driven B-cell expansion and lymphomagenesis. Here, we demonstrate that ablation of CD19 in LMP1/CD40 transgenic mice resulted in a severe loss and reduced lifespan of mature B cells and completely abrogated development of B-cell lymphoma. CD19 is localized to lipid rafts and constitutively activated by the LMP1/CD40 fusion protein in B cells. We provide evidence that the improved survival and malignant transformation of LMP1/CD40-expressing B cells are dependent on activation of the MAPK Erk that is mediated through CD19 in a PI3K-dependent manner. Our data suggest that constitutively active CD40 is dependent on CD19 to transmit survival and proliferation signals. Moreover, we detected a similarly functioning prosurvival pathway involving phosphorylated CD19 and PI3K-dependent Erk phosphorylation in human diffuse large B-cell lymphoma cell lines. Our data provide evidence that CD19 plays an important role in transmitting survival and proliferation signals downstream of CD40 and therefore might be an interesting therapeutic target for the treatment of lymphoma undergoing chronic CD40 signaling.

Cutting edge: Primary and secondary effects of CD19 deficiency on cells of the marginal zone

Marginal zone (MZ) B cells are absent in CD19(-/-) mice. Possible causes include an intrinsic defect in B cells and/or a secondary defect in the extrinsic MZ microenvironment as a result of changes in B cell differentiation in mice lacking CD19. Cells in the MZ also include MZ macrophages (MZM) and MZ dendritic cells (DC). Although CD19 is only expressed on B cells, SIGN-R1(+) MZM are absent and CD11c(+) MZ DC distribution is abnormal in CD19(-/-) mice. Adoptively transferred B cells from normal mice are able to reconstitute MZ B cells in CD19(-/-) mice. In contrast, CD19(-/-) B cells could not enter the MZ of the normal mice. Furthermore, MZM distribution and MZ DC distribution are restored following MZ B cell reconstitution in CD19(-/-) mice. Thus, MZ B cells are required for MZM differentiation and MZ DC localization, but the deficiency of MZ B cells in CD19(-/-) mice is caused by a defect of intrinsic B cell signaling.

Lateral organization of membrane proteins: tetraspanins spin their web

Despite high expression levels at the plasma membrane or in intracellular vesicles, tetraspanins remain among the most mysterious transmembrane molecules 20 years after their discovery. Several genetic studies in mammals and invertebrates have demonstrated key physiological roles for some of these tetraspanins, in particular in the immune response, sperm-egg fusion, photoreceptor function and the normal function of certain epithelia. Other studies have highlighted their ability to modulate cell migration and metastasis formation. Their role in the propagation of infectious agents has drawn recent attention, with evidence for HIV budding in tetraspanin-enriched plasma membrane domains. Infection of hepatocytic cells by two major pathogens, the hepatitis C virus and the malaria parasite, also requires the tetraspanin CD81. The function of tetraspanins is thought to be linked to their ability to associate with one another and a wealth of other integral proteins, thereby building up an interacting network or 'tetraspanin web'. On the basis of the biochemical dissection of the tetraspanin web and recent analysis of the dynamics of some of its constituents, we propose that tetraspanins tightly regulate transient interactions between a variety of molecules and as such favour the efficient assembly of specialized structures upon proper stimulation.

CD86 regulates IgG1 production via a CD19-dependent mechanism

CD86 signals directly in a B cell to activate PI3K and increase the rate of IgG(1) production, without affecting germline transcription. However, the mechanism by which CD86 activates PI3K in a B cell and the relevance of CD86 stimulation in vivo remains unknown. We show that the addition of CD28/Ig to CD40 ligand/IL-4-activated wild-type, but not CD86- or CD19-deficient, B cells increased the level of phosphorylation for Lyn and CD19, as well as the amount of Lyn, Vav, and PI3K that immunoprecipitated with CD19. Adoptive transfer of CD86-deficient B cells and wild-type CD4(+) T cells into RAG2-deficient mice and immunization with trinitrophenylated keyhole limpet hemocyanin resulted in an IL-4 and germline IgG(1) response equivalent to control mice, but a decrease in serum IgG(1). Thus, our findings suggest that CD86 plays a key role in regulating the level of IgG(1) produced in vitro and in vivo, and that Lyn and CD19 may be the signaling intermediates activated by CD86 proximal to PI3K.

Splenic Marginal Zone Dendritic Cells Mediate the Cholera Toxin Adjuvant Effect: Dependence on the ADP-Ribosyltransferase Activity of the Holotoxin

The in vivo mechanisms of action of most vaccine adjuvants are poorly understood. In this study, we present data in mice that reveal a series of critical interactions between the cholera toxin (CT) adjuvant and the dendritic cells (DC) of the splenic marginal zone (MZ) that lead to effective priming of an immune response. For the first time, we have followed adjuvant targeting of MZ DC in vivo. We used CT-conjugated OVA and found that the Ag selectively accumulated in MZ DC following i.v. injections. The uptake of Ag into DC was GM1 ganglioside receptor dependent and mediated by the B subunit of CT (CTB). The targeted MZ DC were quite unique in their phenotype: CD11c(+), CD8alpha(-), CD11b(-), B220(-), and expressing intermediate or low levels of MHC class II and DEC205. Whereas CTB only delivered the Ag to MZ DC, the ADP-ribosyltransferase activity of CT was required for the maturation and migration of DC to the T cell zone, where these cells distinctly up-regulated CD86, but not CD80. This interaction appeared to instruct Ag-specific CD4(+) T cells to move into the B cell follicle and strongly support germinal center formations. These events may explain why CT-conjugated Ag is substantially more immunogenic than Ag admixed with soluble CT and why CTB-conjugated Ag can tolerize immune responses when given orally or at other mucosal sites.

CD19 Regulates B Cell Maturation, Proliferation, and Positive Selection in the FDC Zone of Murine Splenic Germinal Centers

Mice with mutations in CD19 Y482/Y513 form germinal centers (GC) but fail to produce high-affinity antibodies. In these mice, GC B cell differentiation, proliferation, and class switching occur but are defective. Altered CD19 signaling results in retention of early GC B cells and reduced proliferation in the follicular dendritic cell (FDC) zone of GC, and causes failure to select for high-affinity mutations. In normal mice, the earliest detectable aggregates of GC B cells are in contact with FDC and IgM+ cells are only found in the FDC zone, further evidence that the FDC zone is the site of initial GC B cell proliferation, differentiation, and class switching. Proliferation in the non-FDC zone and somatic mutation are not dependent on CD19, indicating separate signaling requirements for the two GC compartments, but these CD19-independent GC functions are not sufficient to generate high-affinity antibodies and B cell memory.

Strong differential regulation of serum and mucosal IgA responses as revealed in CD28-deficient mice using cholera toxin adjuvant

In this study, we show that costimulation required for mucosal IgA responses is strikingly different from that needed for systemic responses, including serum IgA. Following oral immunization with cholera toxin (CT) adjuvant we found that whereas CTLA4-H1 transgenic mice largely failed to respond, CD28-/- mice developed near normal gut mucosal IgA responses but poor serum Ab responses. The local IgA response was functional in that strong antitoxic protection developed in CT-immunized CD28-/- mice. This was in spite of the fact that no germinal centers (GC) were observed in the Peyer's patches, spleen, or other peripheral lymph nodes. Moreover, significant somatic hypermutation was found in isolated IgA plasma cells from gut lamina propria of CD28-/- mice. Thus, differentiation to functional gut mucosal IgA responses against T cell-dependent Ags does not require signaling through CD28 and can be independent of GC formations and isotype-switching in Peyer's patches. By contrast, serum IgA responses, similar to IgG-responses, are dependent on GC and CD28. However, both local and systemic responses are impaired in CTLA4-Hgamma1 transgenic mice, indicating that mucosal IgA responses are dependent on the B7-family ligands, but require signaling via CTLA4 or more likely a third related receptor. Therefore, T-B cell interactions leading to mucosal as opposed to serum IgA responses are uniquely regulated and appear to represent separate events. Although CT is known to strongly up-regulate B7-molecules, we have demonstrated that it acts as a potent mucosal adjuvant in the absence of CD28, suggesting that alternative costimulatory pathways are involved.

CD19 function in early and late B cell development: I. Maintenance of follicular and marginal zone B cells requires CD19-dependent survival signals

Loss of membrane-bound Ig results in the rapid onset of apoptosis in recirculating B cells. This observation implies that a competent B cell receptor (BCR) is not only required for Ag-dependent differentiation, but also for continued survival in the peripheral immune system. Expression of the B cell coreceptor, CD19, is likewise essential for key B cell differentiative events including the formation of B-1, germinal center, and marginal zone (MZ) B cells. In this study, we report that CD19 also exerts a role before Ag encounter by promoting the survival of naive recirculating B cells. This aspect of CD19 signaling was first suggested by the analysis of mixed bone marrow chimeras, wherein CD19-/- B cells fail to effectively compete with wild-type B cells to reconstitute the peripheral B cell compartment. Consistent with this observation, Bromodeoxyuridine- and CFSE-labeling studies reveal a shorter in vivo life span for CD19-/- B cells vs their wild-type counterparts. Moreover, we find that CD19 is necessary for propagation of BCR-induced survival signals and thus may contribute to homeostatic mechanisms of tonic signaling. To determine whether provision of a constitutive survival signal could compensate for the loss of CD19 in vivo, Bcl-2-transgenic mice were bred onto the CD19-/- background. Here, we observe an increase in follicular B cell numbers and selective recovery of the MZ B cell compartment. Together these findings suggest that maintenance of the follicular and MZ B cell compartments require CD19-dependent survival signals.