CD86 (B7-2) on human B cells. A functional role in proliferation and selective differentiation into IgE- and IgG4-producing cells

The costimulatory molecules CD80, CD86 and OX40L are up-regulated in Aspergillus fumigatus sensitized mice

Aspergillus fumigatus (Af) is a fungus associated with allergic bronchopulmonary aspergillosis (ABPA) and other allergic diseases. Immune responses in these diseases are due to T and B cell responses. T cell activation requires both Af-specific engagement of the T-cell-receptor as well as interaction of antigen independent costimulatory molecules including CD28-CD80/CD86 and OX40-OX40L interactions. Since these molecules and their interactions have been suggested to have a potential involvement in the pathogenesis of ABPA, we have investigated their role in a model of experimental allergic aspergillosis. BALB/c mice were primed and sensitized with Af allergens, with or without exogenous IL-4. Results showed up-regulation of both CD86 and CD80 molecules on lung B cells from Af-sensitized mice (79% CD86+ and 24% CD80+) and Af/rIL-4-treated mice (90% CD86+ and 24% CD80+) compared to normal controls (36% and 17%, respectively). Lung macrophages in Af-sensitized mice treated or not with IL-4 showed enhanced expression of these molecules. OX40L expression was also up-regulated on lung B cells and macrophages from both Af-sensitized and Af/rIL-4 exposed mice as compared to normal controls. All Af-sensitized animals showed peripheral blood eosinophilia, enhanced total serum IgE and allergen-specific IgG1 antibodies and characteristic lung inflammation. The up-regulation of CD80, CD86 and OX40L molecules on lung B cells and macrophages from Af-allergen exposed mice suggests a major role for these molecules in the amplification and persistence of immunological and inflammatory responses in ABPA.

Different cell surface oligomeric states of B7-1 and B7-2: implications for signaling

The costimulatory ligands B7-1 and B7-2 are expressed on the surface of antigen-presenting cells and interact with the costimulatory receptors CD28 and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) expressed on T cells. Although B7-1 and B7-2 are homologous ligands having common receptors, they exhibit distinct biochemical features and roles in immune regulation. Several biochemical and structural studies have indicated differences in the oligomeric state of B7-1 and B7-2. However, the organization of B7 ligands on the cell surface has not been examined. By using photobleaching-based FRET (pbFRET), we demonstrate that B7-1 and B7-2 adopt different oligomeric states on the cell surface. Our study shows that B7-2 exists as a monomer on the cell surface whereas B7-1 exists predominantly as dimers on the cell surface. A series of mutations in B7-1 result in the expression of a predominantly monomeric species on the cell surface and validate the dimer interface proposed by prior crystallographic analysis. The difference in the oligomeric states of B7-1 and B7-2 provides insight into the geometric organization of the costimulatory receptor-ligand complexes in the immunological synapse and suggests constraints on signal transduction mechanisms involved in T cell activation.

CD86 and beta2-adrenergic receptor stimulation regulate B-cell activity cooperatively

IgG1 functions to neutralize and clear foreign antigens, such as extracellular bacteria. Therefore, it is important to understand the multiple mechanisms by which the level of IgG1) is regulated to maintain immune system and host homeostasis. Recent data show that the level of IgG1 produced by a B cell is increased following CD86 and beta2-adrenergic receptor (beta2AR) stimulation, through increased expression of the transcription factor Oct-2 and its coactivator OCA-B (Oct coactivator from B cells), respectively. This finding suggests that signaling pathways that are activated by an immunoreceptor (CD86) and a neuroreceptor (beta2AR) converge to regulate the IgG1 response.

Immunopathogenesis of allergic bronchopulmonary aspergillosis in cystic fibrosis

Allergic bronchopulmonary aspergillosis (ABPA) is a hypersensitivity lung disease mediated by an allergic late-phase inflammatory response to Aspergillus fumigatus antigens. ABPA is characterized by markedly elevated Aspergillus-specific and total IgE levels and eosinophilia, and manifested by wheezing, pulmonary infiltrates, and bronchiectasis and fibrosis, which afflict asthmatic and cystic fibrosis (CF) patients. We propose that ABPA develops in genetically susceptible CF patients due to HLA-DR2 and DR5 restriction, increased sensitivity to IL-4 stimulation, and increased A. fumigatus allergen-specific Th2 CD4+ T-cell-mediated responses. In addition, A. fumigatus proteases play a role in facilitation of antigen transport across the epithelial cell layer by damaging the epithelial integrity and by a direct interaction with epithelial cell surface receptors, resulting in pro-inflammatory cytokine production and corresponding inflammatory responses.

Ligation of B7-1/B7-2 by human CD4+ T cells triggers indoleamine 2,3-dioxygenase activity in dendritic cells

Human monocyte-derived dendritic cells (DCs) are capable of expressing the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO), which allows them to suppress Ag-driven proliferation of T cells in vitro. In DCs that express IDO, the activity of the enzyme is tightly regulated, with the protein being constitutively expressed, but functional activity requiring an additional set of triggering signals supplied during Ag presentation. We now show that triggering of functional IDO obligately requires ligation of B7-1/B7-2 molecules on the DCs by CTLA4/CD28 expressed on T cells. When this interaction was disrupted, IDO remained in the inactive state, and the DCs were unable to inhibit T cell proliferation. Inhibition could be fully restored by direct Ab-mediated cross-linking of B7-1/B7-2. Although both CD4(+) and CD8(+) T cells were susceptible to inhibition once IDO was induced, the ability to trigger functionally active IDO was strictly confined to the CD4(+) subset. Thus, the ability of CD4(+) T cells to induce IDO activity in DCs allowed the CD4(+) population to dominantly inhibit proliferation of the CD8(+) population via the bridge of a conditioned DC. We hypothesize that IDO activation via engagement of B7-1/B7-2 molecules on DCs, specifically, engagement by CTLA4 expressed on regulatory CD4(+) T cells, may function as a physiologic regulator of T cell responses in vivo.

CD86 and beta2-adrenergic receptor signaling pathways, respectively, increase Oct-2 and OCA-B Expression and binding to the 3'-IgH enhancer in B cells

Stimulation of CD86 (formerly known as B7-2) and/or the beta2-adrenergic receptor on a CD40 ligand/interleukin-4-activated B cell increased the rate of mature IgG1 transcription. To identify the mechanism responsible for this effect, we determined whether CD86 and/or beta2-adrenergic receptor stimulation regulated transcription factor expression and binding to the 3'-IgH enhancer in vitro and in vivo. We showed that CD86 stimulation increased the nuclear localization of NF-kappaB1 (p50) and phosphorylated RelA (p65) and increased Oct-2 expression and binding to the 3'-IgH enhancer, in a protein kinase C-dependent manner. These effects were lost when CD86-deficient or NF-kappaB1-deficient B cells were used. CD86 stimulation also increased the level of IkappaB-alpha phosphorylation but in a protein kinase C-independent manner. Beta2-adrenergic receptor stimulation increased CREB phosphorylation, OCA-B expression, and OCA-B binding to the 3'-IgH enhancer in a protein kinase A-dependent manner, an effect lost when beta2-adrenergic receptor-deficient B cells were used. Also, the beta2-adrenergic receptor-induced increase in the level of mature IgG1 transcript was lost when OCA-B-deficient B cells were used. These data are the first to show that CD86 stimulation up-regulates the expression of the transcription factor Oct-2 in a protein kinase C- and NF-kappaB1-dependent manner, and that beta2-adrenergic receptor stimulation up-regulates the expression of the coactivator OCA-B in a protein kinase A-dependent manner to cooperate with Oct-2 binding to the 3'-IgH enhancer.

Antigen-specific versus total immunoglobulin synthesis: total IgE and IgG1, but not IgG2a levels predict murine antigen-specific responses

BACKGROUND

Induction of an effective antibody (Ab) response requires delivery of multiple signals to B cells. Cross-linking of the B cell antigen receptor (BCR), signaling through CD40 and CD80/86 and cytokine signals combine to induce class switching and expression of specific isotypes. These signals are principally derived from activated, antigen (Ag)-specific T cells. In contrast, IFNgamma, the only cytokine known to induce class switch to IgG2a, can be produced systemically by activated NK or NKT cells, suggesting that Ag-nonspecific signals may also regulate IgG2a production.

METHODS

Given the potential differences in regulation between IgE/IgG1 versus IgG2a, we immunized mice on day 0 with ovalbumin (OVA) in the presence of strong type-1- or type-2-immunity-inducing adjuvants and boosted mice 4 weeks later. Mice were bled during the primary immune response and after boost to assess primary and recall Ab responses.

RESULTS

Regardless of strain of mice used, phenotype (type 1 versus type 2 dominated) or nature of the immune response induced (primary versus recall), strong correlations between OVA-specific and total IgE and IgG1 were demonstrated. In contrast, a consistent lack of correlation between OVA-specific and total IgG2a levels was observed in all but BALB/c mice.

CONCLUSION

These data indicate that the increase in total levels of IgE/IgG1 isotypes is primarily a result of increased levels of OVA-specific Ab. In contrast, the lack of correlation between total and OVA-specific IgG2a suggests broader activation of IgG2a-producing B cells routinely occurs following exogenous Ag immunization.

New B7 family members with positive and negative costimulatory function

The B7 family of T-cell costimulatory molecules has expanded considerably in recent years. Among the new costimulatory molecules discovered are inhibitory and activating pathways. Both ligands and receptors often have multiple binding partners, adding to the complexity of T-cell regulation. Some B7 molecules also exhibit reverse signaling, affecting activation of both antigen-presenting cells and T cells. An increased understanding of these pathways of T-cell regulation results in promising new therapeutics because T-cell interference can be better targeted to specific states of activation or location. This will decrease side-effects such as systemic immunosuppression and increase efficiency. Targeting B7 molecular pathways for either inhibiting or increasing cell-mediated immunity has so far shown promising results in models of autoimmunity, transplant rejection and tumor immunotherapy.

Increased sensitivity to IL-4 in cystic fibrosis patients with allergic bronchopulmonary aspergillosis

BACKGROUND

Allergic bronchopulmonary aspergillosis (ABPA) is characterized by a heightened Th2 CD4+ T-cell response to Aspergillus fumigatus allergens and a hyper-immunoglobulin (Ig)E state compared with cystic fibrosis patients without ABPA. We hypothesize that one reason for this response is increased sensitivity to interleukin (IL)-4 in ABPA resulting in increased expression of CD23 and CD86 and leading to a positive amplification mechanism that increases Th2 CD4+ T cell responses.

METHODS

Peripheral blood mononuclear cells (PBMC) were isolated from seven ABPA CF and 19 non-ABPA CF patients and 16 nonatopic controls and stimulated with rIL-4 (range 0.1-10 ng/ml) and rIL-13 (range 1-10 ng/ml) for 48 h. The number of CD23 molecules and percentages of CD23+ B cells were quantified by flow cytometry. Both phorbol 12-myristate 13-acetate (PMA)/ionomycin (IO) and antigen stimulated, toxoid and Asp f2/f3/f4, PBMC were examined for cytoplasmic cytokine synthesis enumerated by cytokine staining using flow cytometry to measure Th2 and Th1 CD3+ T cells.

RESULTS

The numbers of CD23 molecules on B-cells were significantly elevated at time 0 in ABPA CF patients compared with both non-ABPA CF patients and nonatopic controls. Following IL-4 stimulation in vitro, the numbers and percentages of CD23 expression on B cells were significantly up-regulated in ABPA CF patients compared with non-ABPA CF patients and controls. The IL-13 stimulation up-regulated CD23 expression; however, there was no significant difference in ABPA CF patients compared with non-ABPA CF patients and controls. The percentages of interferon (IFN)-gamma+ CD3+T cells following PMA/IO stimulation were significantly decreased in both ABPA and non-ABPA CF patients compared with controls. There were no significant differences of IL-4+ and IL-13+ CD3+ T cells between ABPA and non-ABPA CF patients. When tetanus toxoid stimulated T cells were examined, both ABPA and non-ABPA CF patients had significantly decreased IFN-gamma+ CD3+ T cells compared with controls. In Asp f2/f3/f4 stimulated T cells, ABPA CF patients had significantly increased IL-4+ CD3+ T cells compared with non-ABPA CF patients and controls.

CONCLUSIONS

ABPA CF patients have increased sensitivity to IL-4 but not to IL-13 up-regulation of CD23 molecules compared with non-ABPA CF patients. There were decreased percentages of IFN-gamma+ and IL-2+ Th1 T cells in CF patients compared with nonatopic controls but similar percentages of IL-4+ Th2 T cells in all three groups. However, ABPA CF patients had increased frequency of Aspergillus-stimulated Th2 T cells. This indicated that there is skewing of Th2 T cells in ABPA CF patients. Thus, in CF ABPA patients there is increased Th2 T cells and increased sensitivity to IL-4.

Changes in the number of CD80(+), CD86(+), and CD28(+) peripheral blood lymphocytes have prognostic value in melanoma patients

Our aim was to evaluate whether the number of circulating lymphocytes expressing costimulatory molecules can be associated with melanoma prognosis. We determined the concentration of peripheral blood lymphocytes, which expressed the CD80/CD86 or CD28/CTLA-4 molecules, in 38 patients with cutaneous melanoma and 27 controls. The number of each cell subset was compared between patients and controls, as well as between groups of patients stratified according to Breslow thickness of the primary tumor (< or = 2 mm vs > 2 mm) or to survival after 3 years. The concentration of circulating lymphocytes expressing the CD80/CD86 molecules was not significantly different between patients and controls. There was a lower number of CD3(+)CD8(+)CD28(+) cells, as well as a higher CD3(+)CD8(+)/CD3(+)CD8(+)CD28(+) cell ratio, in melanoma patients than in controls. Melanoma patients with thinner tumors and those surviving revealed an increase of CD19(+)CD80(+) and CD19(+)CD80(+)CD86(+) cells, as well as a higher concentration of CD3(+)CD4(+)CD28(+) cells. CD80(+) B cells and a low CD8 suppressor/cytolytic cell ratio correlate with a good prognosis in melanoma. Our findings support our conclusion that CD80(+) B cells may be important antigen presenting cells that can contribute to an antimelanoma immune response and are candidates to be monitored in melanoma patients.

Functional characterization of IL-13 receptor alpha2 gene promoter: a critical role of the transcription factor STAT6 for regulated expression

Interleukin (IL)-4 and IL-13 are two structurally and functionally related cytokines that have overlapping but also distinct biological activities. One of the components of the IL-13 receptor, the alpha2 chain (IL-13Ralpha2), has been reported to downregulate the cell responsiveness to IL-13, without affecting IL-4 signaling. Here, we report that TNFalpha synergizes with either IL-4 or IL-13 in inducing the IL-13Ralpha2 chain at both the mRNA and protein levels in the HaCaT human keratinocyte cell line. Further studies by 5'RACE identified as yet undescribed exonic sequences of the IL-13Ralpha2 5'UTR, provided evidence for the expression of alternatively spliced IL-13Ralpha2 transcripts and defined the transcription start of the IL-13Ralpha2 gene. A 1.5 kb region upstream of the first exon of the IL-13Ralpha2 gene displayed basal promoter activity when inserted in a reporter plasmid and transiently transfected in HaCaT cells. This promoter activity was further increased in response to IL-4 and IL-13. Furthermore, by electrophoretic mobility shift assay and site-directed mutagenesis, we showed that the IL-4/IL-13-induced promoter activity depended upon a positively acting STAT6 response element. Finally, TNFalpha was shown to potentiate IL-4/IL-13-induced IL-13Ralpha2 promoter activity when the same reporter construct was studied in stably but not in transiently transfected cells. These results suggest that the synergistic effect of TNFalpha on IL-4/IL-13-induced IL-13Ralpha2 expression is dependent upon chromatin re-modeling events.

Selective regulation of mature IgG1 transcription by CD86 and beta 2-adrenergic receptor stimulation

Stimulation of CD86 and the beta(2)-adrenergic receptor (beta(2)AR) on a B cell, either alone or together, is known to increase the level of IgG1 protein produced by a CD40 ligand/IL-4-activated B cell. It is also known that the mechanism by which CD40 and IL-4R stimulation on a B cell increases the level of IgG1 protein is by increasing germline gamma 1 transcription, IgG1 class switching, and mature IgG1 transcription, while the molecular mechanism responsible for mediating the CD86- and beta(2)AR-induced effect remains unknown. In the present study using real-time PCR we show that the level of mature IgG1 transcription increases in CD40 ligand/IL-4-activated B cells following stimulation of either CD86 and/or beta(2)AR, and that this increase reflects the increase in IgG1 protein. Furthermore, we show that the CD86- and/or beta(2)AR-induced increase in mature IgG1 transcript is due to an increase in the rate of mature IgG1 transcription, as determined by nuclear run-on analysis. This effect is additive when both receptors are stimulated and is lost when B cells from CD86- and beta(2)AR-deficient mice are used. In contrast, the level of germline gamma 1 transcription, the stability of mature IgG1 transcript, the number of IgG1-positive B cells, and the number of IgG1-secreting B cells did not change. These results provide the first evidence that CD86 and/or beta(2)AR stimulation on a CD40 ligand/IL-4-activated B cell increases the level of IgG1 protein produced per cell by increasing the rate of mature IgG1 transcription.

B7-H1 pathway and its role in the evasion of tumor immunity

B7-H1 is a recently identified member of the B7 family molecules. Upon ligation to its receptors on T cells it regulates activation and differentiation of T cells. B7-H1 preferentially costimulates IL-10 production in resting T cells and further induces the apoptosis of activated T cells. PD-1 is a receptor of B7-H1 and is shown to mediate the inhibition of activated T cell response, presumably by inhibiting cell cycle progression. The expression of B7-H1 protein is limited to macrophage lineage of cells in normal tissues, although its mRNA transcription is found in a broad range of tissues. In contrast, B7-H1 is abundant in various human cancers. The tumor-associated B7-H1 increases apoptosis of antigen specific T cells, leading to growth of immunogenic tumor growth in vivo. Current data suggest that B7-H1 regulates the organ-specific tolerance in normal tissue and may contribute to immune evasion by cancers. Selective manipulation of B7-H1 pathway thus aids in the design of new regimens in the treatment of human autoimmune disease and the control of malignant cancers.

Crystal structure of the receptor-binding domain of human B7-2: insights into organization and signaling

B7-1 and B7-2 are homologous costimulatory ligands expressed on the surfaces of antigen-presenting cells. Their interactions with CD28/CTLA-4 receptors expressed on T cell surfaces are crucial for the proper regulation of T cell activity. B7-1 and B7-2 display distinct roles in immune regulation, although they are usually considered to have redundant functions. Here, we report the crystal structure of the receptor-binding (Ig V-type) domain of human B7-2 at 2.7-A resolution. Structures of unliganded and liganded B7-1 and B7-2 suggest a physical-chemical basis for the observed functional similarities and differences between these two costimulatory ligands. Of particular note, whereas the majority of the residues mediating B7-1 dimerization are hydrophobic, the B7-2 dimer observed in the B7-2/CTLA-4 complex displays a very hydrophilic dimer interface. These differences provide a mechanism for preventing the formation of B7-1/B7-2 heterodimers. The divergence at the putative dimer interface is also consistent with the lower tendency of B7-2 to dimerize, as shown by the monomeric state of unliganded B7-2 both in solution and crystalline form, and may result in detailed differences in signaling mechanisms associated with B7-1 and B7-2.

Costimulating aberrant T cell responses by B7-H1 autoantibodies in rheumatoid arthritis

A pathogenic hallmark of rheumatoid arthritis (RA) is persistent activation of self-reactive CD4(+) T cells. The cause of this aberrant activity remains elusive. We report here detection of autoantibodies against B7-H1, a recently described member of the B7 family, in 29% of patients with RA versus 4% of healthy donors. High-level expression of cell surface B7-H1 are found on activated human CD4(+), CD8(+), and CD45RO(+) T cells. Immobilized autoantibodies to B7-H1 are capable of costimulating the proliferation of CD4(+) T cells in vitro, and the presence of these autoantibodies correlates with active disease status. Using immobilized B7-H1 mAb's and programmed death 1Ig, we demonstrate that engagement of B7-H1 on CD4(+) T cells costimulates proliferation and secretion of IL-10, and subsequently leads to programmed cell death, accompanied with upregulated expression of TNF-related apoptosis-inducing ligand and activation of caspase-3. Taken together with our previous findings, these data indicate a bidirectional signaling role of B7-H1 in T cell costimulation and apoptosis and implicate B7-H1 autoantibodies as contributing to the progression of RA by inducing aberrant T cell responses.

Differential requirements for expression of CD80/86 and CD40 on B cells for T-dependent antibody responses in vivo

The CD80/86-CD28 and CD40-CD40 ligand costimulatory pathways are essential for Th cell-dependent B cell responses that generate high-affinity, class-switched Ab in vivo. Disruption of either costimulatory pathway results in defective in vivo humoral immune responses, but it remains unclear to what extent this is due to deficient activation of Th cells and/or of B cells. To address this issue, we generated mixed chimeras in which CD80/86- or CD40-deficient bone marrow-derived cells coexist with wild-type (WT) cells, thereby providing the functional T cell help and accessory cell functions required for fully competent B cell responses. We were then able to assess the requirement for CD80/86 or CD40 expression on B cells producing class-switched Ig in response to a T-dependent Ag. In CD80/86 WT plus CD80/86 double-knockout mixed chimeras, both WT- and CD80/86-deficient B cells produced IgG1 and IgE responses, indicating that direct signaling by CD80/86 is not essential for efficient B cell activation. In marked contrast, only WT IgG1 and IgE responses were detected in the chimeras containing CD40-deficient cells, demonstrating that CD40 expression on B cells is essential for class switching by those B cells. Thus, while disrupting either the CD80/86-CD28 or the CD40-CD40 ligand costimulatory pathway abrogates T-dependent B cell immune responses, the two pathways are nonredundant and mediated by distinct mechanisms.

Cross-linking the B7 family molecule B7-DC directly activates immune functions of dendritic cells

B7-DC molecules are known to function as ligands on antigen-presenting cells (APCs), enhancing T cell activation. In this study, cross-linking B7-DC with the monoclonal antibody sHIgM12 directly potentiates dendritic cell (DC) function by enhancing DC presentation of major histocompatibility complex-peptide complexes, promoting DC survival; and increasing secretion of interleukin (IL)-12p70, a key T helper cell type 1 promoting cytokine. Furthermore, ex vivo treatment of DCs or systemic treatment of mice with sHIgM12 increases the number of transplanted DCs that reach draining lymph nodes and increases the ability of lymph node APCs to activate naive T cells. Systemic administration of the antibody has an equivalent effect on DCs transferred at a distant site. These findings implicate B7-DC expressed on DCs in bidirectional communication. In addition to the established costimulatory and inhibitory functions associated with B7-DC, this molecule can also function as a conduit for extracellular signals to DCs modifying DC functions.

IL-10 enhances B-cell IgE synthesis by promoting differentiation into plasma cells, a process that is inhibited by CD27/CD70 interaction

Interleukin-10 (IL-10) is a major regulatory cytokine of inflammatory responses that is considered to play an important role in specific immunotherapy. However, whether IL-10 enhances or inhibits B-cell IgE production has remained a matter of contention. To clarify the effect of IL-10 on IgE synthesis in the presence of IL-4 and CD40 signalling, we examined B-cell proliferation, germline epsilon transcripts and plasma cell differentiation. In addition, the effect of CD27 signalling on IgE synthesis in the presence of IL-10, IL-4 and CD40 signalling was investigated. IL-10 facilitated the production of IgE in mononuclear cells and highly purified B-cells, enhanced B-cell proliferation and, most importantly, promoted the generation of plasma cells. However, IL-10 did not enhance expression of germline epsilon transcripts. The addition of CD27 signalling through the use of CD32-CD27 ligand (CD70) double transfectants significantly diminished the B-cell proliferation, IgE synthesis and plasma cell differentiation enhanced by IL-10. IL-10 enhances B-cell IgE production by promoting differentiation into plasma cells. CD27/CD70 interactions under IL-10 and sufficient CD40 cosignalling exert the opposite effect on IgE synthesis. The results of this study indicate that precautions are critical when planning immunotherapy using IL-10 in IgE-related allergic diseases.

B cell receptor- and beta 2-adrenergic receptor-induced regulation of B7-2 (CD86) expression in B cells

The costimulatory molecule B7-2 (CD86) is expressed on the surface of APCs, including B cells. Considering the importance of B7-2 in regulating both T and B cell function, it may be important to understand the regulatory mechanisms governing its expression. We report in this study that stimulation of the B cell receptor (BCR) and/or a neurotransmitter receptor, the beta(2)-adrenergic receptor (beta(2)AR), may cooperate to regulate B cell-associated B7-2 expression in vitro and in vivo. beta(2)AR stimulation further enhanced the level of BCR-induced B7-2 expression in B cells potentially via protein tyrosine kinase-, protein kinase A-, protein kinase C-, and mitogen-activated protein kinase-dependent mechanisms. Importantly, BCR and/or beta(2)AR stimulation, but not histone hyperacetylation and DNA hypomethylation alone, increased B cell-associated B7-2 expression by increasing B7-2 mRNA stability, NF-kappa B nuclear binding, and NF-kappa B-dependent gene transcription. Thus, this study provides additional insight into the signaling intermediates and molecular mechanisms by which stimulation of the BCR and beta(2)AR may regulate B cell-associated B7-2 expression.

Structural mechanisms of costimulation

Recent studies have highlighted the structural requirements for T cell costimulation and have revealed unusual modes of dimerization for the cytolytic T lymphocyte-associated antigen 4 (CTLA-4) costimulatory receptor and its B7 ligands. These distinctive quaternary structures potentially endow both receptor and ligand with bivalent binding properties, which suggests a number of mechanistic features relevant to signaling. These include the potential to form a highly ordered, alternating network of CTLA-4 and B7 homodimers that may represent the organization of these molecules and their associated signaling partners within the immunological synapse. Primary sequence and structural considerations suggest that some aspects of the organizational and mechanistic features associated with the CTLA-4-B7 complexes may extend to other members of the costimulatory receptor-ligand family. An examination of the signaling mechanisms within the costimulatory receptor-ligand family provides an excellent framework to consider the general principles that are relevant to cell surface receptor-mediated signaling events.

Distinct role of CD80 and CD86 in the regulation of the activation of B cell and B cell lymphoma

To date, not much has been known regarding the role of CD80 and CD86 molecules in signaling of B cells. The CD28/CTLA4 ligands, CD80 (B7-1) and CD86 (B7-2), are expressed on the surface of freshly isolated splenic B cells, and their expression is up-regulated by lipopolysaccharides. In the present study, we have investigated whether signaling via CD80/CD86 could alter the proliferation and immunoglobulin synthesis of B cells. Splenic B cells were stimulated with lipopolysaccharides in the presence of anti-B7-1 (16-10A1) and anti-B7-2 (GL1) monoclonal antibodies (mAbs). Exciting features observed during the study were that cross-linking of CD86 with GL1 enhanced the proliferation and production of IgG1 and IgG2a isotypes. In contrast, anti-B7-1 (16-10A1) mAb could efficiently block the proliferation and production of IgG1 and IgG2a. Furthermore, GL1 mAb could also induce the secretion of IgG isotypes from B cell lymphomas. Importantly, 16-10A1 could retard the growth of lymphomas and favored the up-regulation of pro-apoptotic molecules caspase-3, caspase-8, Fas, FasL, Bak, and Bax and down-regulation of anti-apoptotic molecule Bcl-x(L). In contrast, GL1 augmented the level of anti-apoptotic molecules Bcl-w and Bcl-x(L) and decreased the levels of pro-apoptotic molecule caspase-8, thereby providing a novel insight into the mechanism whereby triggering through CD80 and CD86 could deliver regulatory signals. Thus, this study is the first demonstration of a distinct signaling event induced by CD80 and CD86 molecules in B cell lymphoma. Finally, the significance of the finding is that CD80 provided negative signal for the proliferation and IgG secretion of normal B cells and B cell lymphomas. In contrast, CD86 encouraged the activity of B cells.

Antigen receptor triggered upregulation of CD86 and CD80 in human B cells: augmenting role of the CD21/CD19 co-stimulatory complex and IL-4

The impact of BCR:CD21 co-engagement on B cell expression of molecules critical for T cell activation was investigated with receptor-specific mAbs conjugated to high MW dextran as stimulatory ligands. In the absence of IL-4, BCR:CD21 co-ligation augmented BCR-triggered CD86 only under conditions of very low BCR ligand dose or affinity, and CD80 was minimally induced by BCR and/or CD21 crosslinking. In the presence of IL-4, BCR:CD21 co-ligation augmented CD86 and CD80 expression under conditions of greater BCR engagement. However, with very high level BCR engagement, no bonus effect of BCR:CD21 crosslinking was observed. Co-ligation-promoted CD86 and CD80 expression was associated with heightened B cell activation of resting allogeneic T cells. The data suggest that co-clustering of BCR and the CD21/CD19 co-stimulatory complex following B cell engagement with C3d-bound microbial or self-antigens will enhance B cell recruitment of T cell help only when IL-4 is present and/or BCR engagement is very limiting.

Wasp venom rush immunotherapy induces transient downregulation of B cell surface molecule expression

BACKGROUND

Little is known about the involvement of B cells in venom immunotherapy (VIT). To elucidate changes in the B cell phenotype during this process, we examined the expression of several surface molecules on peripheral B cells before and during VIT.

METHODS

15 venom-allergic patients with a history of systemic reactions after a wasp sting and venom-specific skin test reactivity as well as serum IgE were investigated before VIT (day 1), 1 day after reaching a maintenance dose of 100 microg (day 6) during inpatient rush VIT, and again on day 26 during continued outpatient maintenance therapy. Changes in the serum levels of total IgE, allergen-specific IgE (sIgE) and sIgG4 were measured by ELISA. Expression of several surface molecules on double-labelled B cells was studied by flow cytometry of peripheral blood mononuclear cells.

RESULTS

Levels of total IgE, sIgE and sIgG4 showed a significant increase after 26 days of VIT. On day 6, cell surface expression of HLA- II-DR, CD5, CD32 and CD54 was decreased in intensity and numbers of positive cells compared to day 1, while on day 26, expression of these molecules approached again baseline levels. Furthermore, a trend to decreased CD23 was noted on day 6. No changes were observed for CD40, CD86, CD95 and HLA-I-ABC.

CONCLUSION

These data show that during initiation of rush VIT, B cell expression of surface molecules involved in T-B cell cooperation and antigen presentation is downmodulated. B cells may thus be additional direct or indirect targets of high-dose antigen therapy and contribute to the persistence of TH1 responses during maintenance VIT treatment.

Increased immunoglobulin secretion by B lymphocytes from Trypanosoma cruzi infected mice after B lymphocytes-natural killer cell interaction

In the present study, we investigated whether natural killer (NK) cells modulate immunoglobulin (Ig) secretion by B cells from Trypanosoma cruzi-infected mice. B cells from infected mice increased IgM and IgG2a secretion in the presence of a NK cell line, and this response was cell contact-dependent. Stimulation of splenic B cells with polyinosinic-polycytidylic acid, a NK cell activator, also increased Ig secretion by B cells from infected mice. B cells from infected mice expressed higher levels of the B7.2 molecule. Our results suggest that NK cells may be involved in the control of the abnormal B cell activation observed during T. cruzi infection.

Multiple transcription factors regulate the inducible expression of the human complement receptor 2 promoter

Complement receptor 2 (CR2) is regulated at the transcriptional level, but the promoter elements and the transcription factors that bind to them and contribute to its regulation are unknown. After documenting that PMA and cAMP induced the activity of the CR2 promoter by 10-fold, we conducted promoter truncation and mutagenesis experiments, in conjunction with shift assays, to determine the functionally important regions of the promoter and the proteins that bind to them. We identified two regions, separated by approximately 900 nucleotides, which together were responsible for inducible promoter activity. Mutagenesis of single promoter elements demonstrated a functional upstream stimulatory factor/E box in the TATA box-proximal region and three equally important, closely spaced, CREB/AP-1 half-sites in the upstream promoter region. The cAMP response element-binding protein (CREB)/AP-1 half-sites bound in vitro Jun and CREB that are induced by protein kinases A and/or C. The 900-nucleotide segment stretching between the above two regions had no functional impact on the induced transcription, and its deletion increased the promoter activity. Finally, a region upstream of the distal site had a repressor activity on CR2 transcription. Moreover, IL-4 induced binding of CREB and AP-1 to the upstream promoter elements and resulted in increased CR2 surface protein expression. These studies have characterized regions of the CR2 promoter and the transcription factors that bind to them and are crucial to induced CR2 expression. Our studies may provide insights to novel approaches to modulate B cell function by regulating CR2 gene transcription.

Increased sensitivity to IL-4 in patients with allergic bronchopulmonary aspergillosis

BACKGROUND

Allergic bronchopulmonary aspergillosis (ABPA) is characterized by a heightened Th2 CD4+ cell response to Aspergillus fumigatus allergens and a hyper-IgE state compared to atopic asthmatic and cystic fibrosis patients without ABPA. We hypothesized that one reason for this response is increased sensitivity to IL-4 in ABPA, resulting in increased expression of CD23 and CD86, leading to a positive amplification mechanism which increases Th2 CD4+ T cell responses.

METHODS

Peripheral blood mononuclear cells isolated from 10 ABPA, 9 atopic, and 8 nonatopic subjects and stimulated for 48 h with varying concentrations of rIL-4 ranging from 0.1 to 50 ng/ml. The percentages of CD23+ and CD86+ B cells and the number of CD23+ molecules on CD20+ and CD86+CD20+ B cells were quantified by flow cytometry.

RESULTS

Total serum IgE levels were elevated in ABPA patients compared to atopic and nonatopic controls. At day 0 prior to culture, CD23 molecules per CD20+ B cell were significantly elevated in ABPA patients compared to atopic and to nonatopic patients. CD23 molecules per CD20+ B cell in ABPA and atopic patients decreased after 48 h in culture without IL-4 added and were similar. With IL-4 stimulation, ABPA patients had significantly increased rates of CD23 expression per B cell compared to atopic and nonatopic subjects (p < 0.001). Furthermore, ABPA had significantly increased numbers of CD23+ molecules per B cell and CD86+ B cell following IL-4 stimulation compared to atopic and nonatopic patients. Both ABPA and atopic patients at day 0 prior to culture had increased expression of CD86+ and CD23+CD86+ B cells compared to nonatopic patients. After 48 h in culture without IL-4, the percentages of CD86+ and CD23+CD86+ B cells decreased in ABPA and atopic patients. After stimulation with IL-4, ABPA patients had significant upregulation of CD23+CD86+ B cells compared to atopic and nonatopic patients. Similarly, the number of CD23 molecules per CD86+CD20+ B cell was significantly upregulated following IL-4 stimulation in ABPA patients compared to atopic and to nonatopic subjects.

CONCLUSIONS

This is the first study to demonstrate that ABPA patients have increased sensitivity to IL-4 stimulation compared to other atopic individuals, such that ABPA > atopic >> nonatopic patients. The B cells from ABPA patients were significantly more sensitive to IL-4 stimulation compared to atopic and nonatopic patients with upregulation of CD23 and CD86 expression. ABPA subjects had increased CD86+ and CD23+CD86+ B cell expression on day 0 prior to culture and with upregulation of CD23+ molecules on CD86+CD20+ B cells. IL-4 also stimulated upregulated CD86+ expression on B cells in atopic patients with little effect on nonatopic patients. This study supports the premise that IL-4, IL-4R alpha and CD86 are central targets in the treatment of ABPA and atopic disease.

Stimulation of the B cell receptor, CD86 (B7-2), and the beta 2-adrenergic receptor intrinsically modulates the level of IgG1 and IgE produced per B cell

Our findings using B cells from either wild-type, CD86-deficient, or beta 2-adrenergic receptor (beta2AR)-deficient mice suggest three mechanisms by which the level of IgG1 and IgE production can be increased on a per cell basis. Trinitrophenyl-specific B cells enriched from unimmunized mouse spleens were pre-exposed to Ag and/or the beta 2AR ligand terbutaline for 24 h before being activated by either a beta 2AR-negative Th2 cell clone or CD40 ligand/Sf9 cells and IL-4 in the presence or absence of an anti-CD86 Ab. Data suggest that the first mechanism involves a B cell receptor (BCR)-dependent up-regulation of CD86 expression that, when CD86 is stimulated, increases the amount of IgG1 and IgE produced in comparison to unstimulated cells. The second mechanism involves a BCR- and beta 2AR-dependent up-regulation of CD86 to a level higher than that induced by stimulation of either receptor alone that, when CD86 is stimulated, further increases the amount of IgG1 and IgE produced. The third mechanism is BCR-independent and involves a beta 2AR-dependent increase in the ability of a B cell to respond to IL-4. Flow cytometric and limiting dilution analyses suggest that the increase in IgG1 and IgE occurs independently from the isotype switching event. These findings suggest that the BCR, the beta 2AR, and CD86 are involved in regulating IL-4-dependent IgG1 and IgE production.

alpha-galactosylceramide induces early B-cell activation through IL-4 production by NKT cells

alpha-Galactosylceramide (alpha-GalCer), a glycolipid antigen, specifically activates natural killer T (NKT) cells by a CD1d-restricted mechanism. In this work, we found that in vivo administration of alpha-GalCer resulted in the activation of B cells in addition to NKT cells, namely, alpha-GalCer administration caused upregulation of the early activation marker, CD69, on both NKT and B cells. In addition, expression of B7.2 and I-A(b) on B cells was greatly upregulated by alpha-GalCer. However, serum levels of IgE, IgG1, and IgG2a were not significantly changed within 48 h. In the present experiments, it was also demonstrated that the upregulation of CD69 expression by alpha-GalCer was strongly blocked by anti-IL-4 monoclonal antibody. Moreover, B-cell activation by alpha-GalCer was not observed in NKT-deficient mice. These results suggested that antigen-stimulated NKT cells might play a critical role not only in early defense mechanisms but also in early B-cell activation through IL-4 production.

Expression of the costimulator molecules, CD80, CD86, CD28, and CD152 on lymphocytes from neonates and young children

The expression of CD80, CD86, CD28, and CD152 were examined on peripheral blood lymphocytes from adults, neonates (cord blood lymphocytes) and young children (2-20 months of age). There was no difference in the expression of CD80 or CD86 between adult and neonatal B cells, either resting or activated. A higher percentage of resting T cells expressed CD28 in neonates and young children compared to adults. CD28 expression was similar on adult and neonatal T cells activated with PMA and ionomycin. However, CD28 was expressed at greater intensity on a higher percentage of neonatal T cells than adult T cells stimulated with CD3. CD152 expression was lower on neonatal T cells than adult T cells stimulated with PMA and ionomycin and undetectable on neonatal T cells stimulated with CD3. In contrast, intracellular CD152 was equivalent in adult and neonatal T cells stimulated with PMA and ionomycin, suggesting trafficking of CD152 to the cell surface may be differentially regulated in neonatal T cells. Since the T cell response is determined by the balance of signals received from CD28 and CD152, high levels of CD28 expression and lower surface expression of CD152 on neonatal T cells may represent specialisation to promote activation of neonatal T cells.

The interplay between T helper subset cytokines and IL-12 in directing human B lymphocyte differentiation

This study asks how T helper (TH) subset cytokines impact upon IL-12-directed change in B cells engaged in signaling via the B cell receptor and CD40, essential components in the initiation of T-dependent B cell responses. For B cells stimulated in this way, IL-12 promoted a distinct phenotype highlighted by the hyper-expression of CD38: the Th1 cytokine IFN-gamma reproduced the IL-12 effects while neutralizing antibody to IFN-gamma reversed IL-12-dependent change. The divergent pathway of differentiation promoted by the Th2 cytokine IL-4 (characterized by hyper-induction of CD23) was left unchecked by IL-12. IL-10 was found to dampen IL-12 actions by suppressing IL-12-dependent IFN-gamma production but failed to perturb the effects of exogenous IFN-gamma. Thus, IL-12--by invoking autocrine IFN-gamma production--promotes phenotypic deviation in B cells engaging T-dependent signals. The reversal of such Th1 driving of B cells by IL-10 only when the source of IFN-gamma is endogenous and the inability of IL-12 to impact upon IL-4-directed differentiation suggest a progressive and hierarchical commitment of B cells to polarization during a developing T-dependent response dominated at the level of the Th cell rather than that of the dendritic cell.

Topical CTLA4-Ig suppresses ongoing mucosal immune response in presensitized murine model of allergic rhinitis

Allergic rhinitis is thought to be mediated by CD4+ T cells producing Th2-associated cytokines. Optimal Ag-specific T-cell activation requires the engagement of T-cell receptor with antigen (Ag) in the context of MHC, and the engagement of appropriate costimulatory molecules. One of the most well-characterized costimulatory pathways is the interaction of B7/CD28-CTLA4 molecules. Recent studies have suggested that the costimulatory pathway may influence the development of Th2 immune responses. The objective of this study was the examination of the role of B7/CD28-CTLA4 costimulatory pathway in the pathogenesis of ovalbumin (OVA)-induced immune response in presensitized murine model of allergic rhinitis. Systemically presensitized BALB/c mice significantly developed Ag-induced early phase nasal symptoms, nasal hyperresponsiveness to histamine, nasal eosinophilia, serum levels of OVA- specific IgE and Th2-associated cytokines following repeated topical Ag challenges. Topical administration of CTLA4-Ig during nasal challenges inhibited Ag-induced nasal symptoms and histamine hyperresponsiveness. We also found a significant reduction in nasal lavage eosinophilia and serum levels of OVA-specific IgE. Furthermore, CTLA4-Ig treatment significantly decreased interleukin (IL)-4 content in nasal tissue, while there was no significant change in IL-5 or IFN-gamma levels. These results suggest that B7/CD28-CTLA4 costimulatory pathway mediates the development of ongoing Th2 immune responses and plays a major role in regulating allergic disease, such as allergic rhinitis.

Modulation of costimulatory molecules on follicular lymphoma cells by TNF and CD40

TNF has recently been implicated in the formation of germinal center cells in lymphoid organs. Follicular lymphoma (FL) is thought to represent the pathological counterpart of germinal center B-cell. High levels of TNF and its soluble receptors were found in the plasma of FL patients whereas the transcripts of these molecules were previously found to be present in FL patients lymph nodes. We therefore studied here the effects of TNF on the expression of costimulatory molecules implicated in the cytotoxic T cell response on purified FL cells. In contrast to results described with B-type chronic lymphocytic leukemia, also characterized by high levels of circulating TNF, none of the tested samples showed a regulation of CD80, CD86, CD27 and CD70 in response to TNF. To confirm that the lack of regulation of these molecules was not due to the FL cells inability to modulate their expression, we therefore analyzed costimulatory molecules expression after CD40 pathway stimulation. After culture with human CD40L-transfected L-cells, an up-regulation of CD80, CD86 and CD70 expression was observed, while TNF addition in this model did not influence these changes. In this context, the CD27 molecule was down-regulated except in a single case, where its expression was increased. Taken together, this data demonstrates that in vitro expression of costimulatory molecules such as CD80, CD86, CD27 and CD70, which are implicated in the anti-tumoral response, can be regulated by CD40 ligand but not by TNF.

Suppressive effects of CTLA4-Ig on nasal allergic reactions in presensitized murine model

Ag-specific T cell activation requires the engagement of T cell receptor (TCR) with antigen in the context of MHC, and the engagement of appropriate costimulatory molecules. It is well established that B7/CD28-CTLA4 costimulatory pathway plays an important role in the induction of T helper (Th) cells in T-cell dependent immune reactions. In this study, we evaluated the effects of blocking the costimulatory pathway by systemic administration of CTLA4-Ig during repeated nasal antigen challenges in systemically presensitized mouse. The antigen-induced early phase nasal symptoms, nasal hyperresponsiveness to histamine and nasal eosinophilia were significantly suppressed by CTLA4-Ig treatment. Elevation of serum level of antigen-specific IgE, but not IgG1 or IgG2a was inhibited by the treatment. In relation to cytokine levels in the tissue extracts of the nasal mucosa, an up-regulation of IL-4 was significantly inhibited, however, the levels of IL-5 and IFN-gamma were not affected by the treatment. These results suggest that B7/CD28-CTLA4 costimulatory pathway plays an important role in on-going Th2-related allergic reactions in the nose.

Staphylococcal toxins augment specific IgE responses by atopic patients exposed to allergen

Microbial agents are known to play a significant role in aggravating allergic diseases. Recently described viral and bacterial superantigens represent one important strategy by which infectious agents can stimulate the immune response. In previous work, we reported that the staphylococcal toxin toxic shock toxin-1 (TSST-1), a prototypic superantigen, induces in vitro total IgE synthesis after cross-linking T and B cells. This study was carried out to establish a potential link between superantigens and the enhanced IgE response to specific allergens in allergic patients. Peripheral blood mononuclear cells from atopic patients were isolated during and outside the pollen allergen season and stimulated with TSST-1, a prototypic superantigen. Total IgE and interferon-gamma production were measured in supernatants of these cultures. Outside the pollen season, TSST-1 significantly increased total IgE production only in the presence of exogenous interleukin-4, whereas during the pollen season IgE production was significantly enhanced without the need of exogenous interleukin-4. This increase in the absence of exogenous interleukin-4 was associated with significantly lower interferon-gamma production by peripheral blood mononuclear cells stimulated by TSST-1 during the pollen season. Moreover, TSST-1 stimulation of peripheral blood mononuclear cells from inhalant allergic patients was followed by an increased production of allergen-specific IgE that was restricted to the allergen to which the patient was allergic and recently exposed. In addition, TSST-1 induced on B cells the expression of B7.2, a molecule that has recently been demonstrated to enhance T helper 2 responses and to be involved in IgE regulation. This study, by demonstrating that superantigens can augment allergen-specific IgE synthesis and B7.2 expression, provides a mechanism by which microbial superantigens may modulate allergic responses.

The gp200-MR6 molecule which is functionally associated with the IL-4 receptor modulates B cell phenotype and is a novel member of the human macrophage mannose receptor family

The human gp200-MR6 molecule has previously been shown to have either an antagonistic or agonistic effect on IL-4 function, demonstrated by inhibition of IL-4-induced proliferation of T cells or mimicking of IL-4-induced maturation of epithelium, respectively. We now show that gp200-MR6 ligation can also mimic IL-4 and have an anti-proliferative pro-maturational influence within the immune system, causing up-regulation of co-stimulatory molecules on B lymphocytes. Biochemical analysis and cDNA cloning reveal that gp200-MR6 belongs to the human macrophage mannose receptor family of multidomain molecules. It comprises 1722 amino acids in toto (mature protein, 1695 amino acids; signal sequence, 27 amino acids) organized into 12 external domains (an N-terminal cysteine-rich domain, a fibronectin type II domain and 10 C-type carbohydrate recognition domains), a transmembrane region and a small cytoplasmic C terminus (31 amino acids) containing a single tyrosine residue (Y1679), but no obvious kinase domain. Strong amino acid sequence identity (77%) suggests that gp200-MR6 is the human homologue of the murine DEC-205, indicating that this molecule has much wider functional activity than its classical endocytic role. We also show that the gp200-MR6 molecule is closely associated with tyrosine kinase activity; the link between gp200-MR6 and the IL-4 receptor may therefore be via intracellular signaling pathways, with multifunctionality residing in its extracellular multidomain structure.

Differential expression of costimulatory molecules in chronic inflammatory periodontal disease tissue

Although B cell activation and subsequent immunoglobulin production are the immunopathological features of chronic inflammatory periodontal disease, in situ expression of costimulatory molecules in humoral immunity has not been investigated. In the present study we examined the expression of CD40, CD40 ligand (CD40L), CD80, CD86, CD28 and cytolytic T lymphocyte-associated antigen-4 (CTLA-4) on lymphocytes immunohistochemically. Cryostat sections were prepared from the gingival tissue samples of 14 patients with moderate to advanced adult periodontitis. In vitro kinetics of the expression of CD40L and CTLA-4 by peripheral blood T cells and that of CD80 and CD86 by peripheral blood B cells were also investigated by flow cytometry. Positive percentage expression of CD40L, CD28 and CTLA-4, and CD40, CD80 and CD86 was calculated for the number of CD3+ and CD19+ cells, respectively. Flow cytometric analysis demonstrated that the expression of CD40L and CTLA-4 on T cells, and CD80 and CD86 on B cells of peripheral blood was up-regulated upon activation. While most T cells and B cells expressed CD28, and CD80 and CD86, respectively, in gingival tissues, the expression of CD40L and CTLA-4 was lower but highly variable between specimens. Furthermore, these two molecules seemed to be expressed reciprocally in the lesion. As both CD40L and CTLA-4 expression are induced transiently by stimulation, variability in the expression of the molecules may reflect immunological activities and participation in the regulation of B cell activation of the lesion.

The gp200-MR6 molecule which is functionally associated with the IL-4 receptor modulates B cell phenotype and is a novel member of the human macrophage mannose receptor family

The human gp200-MR6 molecule has previously been shown to have either an antagonistic or agonistic effect on IL-4 function, demonstrated by inhibition of IL-4-induced proliferation of T cells or mimicking of IL-4-induced maturation of epithelium, respectively. We now show that gp200-MR6 ligation can also mimic IL-4 and have an anti-proliferative pro-maturational influence within the immune system, causing up-regulation of co-stimulatory molecules on B lymphocytes. Biochemical analysis and cDNA cloning reveal that gp200-MR6 belongs to the human macrophage mannose receptor family of multidomain molecules. It comprises 1722 amino acids in toto (mature protein, 1695 amino acids; signal sequence, 27 amino acids) organized into 12 external domains (an N-terminal cysteine-rich domain, a fibronectin type II domain and 10 C-type carbohydrate recognition domains), a transmembrane region and a small cytoplasmic C terminus (31 amino acids) containing a single tyrosine residue (Y1679), but no obvious kinase domain. Strong amino acid sequence identity (77%) suggests that gp200-MR6 is the human homologue of the murine DEC-205, indicating that this molecule has much wider functional activity than its classical endocytic role. We also show that the gp200-MR6 molecule is closely associated with tyrosine kinase activity; the link between gp200-MR6 and the IL-4 receptor may therefore be via intracellular signaling pathways, with multifunctionality residing in its extracellular multidomain structure.

Direct cellular interaction with activated CD4(+) T cells overcomes hyporesponsiveness of B-cell chronic lymphocytic leukemia in vitro

The proliferative response of clonal B cells from patients with chronic lymphocytic leukemia (B-CLL) is drastically reduced compared to normal B lymphocytes stimulated via the B cell antigen receptor complex or by CD40 ligation. In the present study we demonstrate that hyporesponsiveness of CLL-B cells can be overcome by stimulatory pathways mediated by activated CD4(+) T cells. In contrast to CD40 ligation, costimulation with activated T cells promotes a proliferative response in CLL-B cells identical to that in normal B cells. Furthermore, coculture with activated T cells improved survival of CLL-B cells in vitro. Differentiation of CLL-B cells into IgM producing cells was promoted, as well. However, the capacity for IgM secretion remained impaired compared to that of normal B cells. For T-cell-mediated B cell activation direct cellular contact with activated T helper cells is absolutely required. Prevention of CD40/CD40L interaction by CD40 antibody caused only partial inhibition of B cell activation, suggesting that additional signals are involved in T-B cell interaction. Whereas interruption of the ligand pairs CD11a/CD54, CD5/CD72, CD27/CD70 had no influence, the addition of CD58 antibody completely inhibited B cell activation by activated T cells. In costimulation with cellular signals the presence of B-cell-tropic cytokines, such as IL-2 and IL-4, was required to optimize B-CLL proliferation, as demonstrated by the use of neutralizing antibodies. We conclude from these results that proliferative hyporesponsiveness by CLL-B cells can be circumvented by antigen-nonspecific signals in addition to CD40 which are mediated by direct contact with activated T helper cells.

Repression of B7.2 on self-reactive B cells is essential to prevent proliferation and allow Fas-mediated deletion by CD4(+) T cells

Peripheral tolerance mechanisms normally prevent delivery of T cell help to anergic self-reactive B cells that accumulate in the T zones of spleen and lymph nodes. Chronic exposure to self-antigens desensitizes B cell antigen receptor (BCR) signaling on anergic B cells so that they are not stimulated into clonal expansion by CD4(+) T cells but instead are eliminated by Fas (CD95)-induced apoptosis. Because a range of BCR-induced signals and responses are repressed in anergic B cells, it is not known which of these are critical to regulate for Fas-mediated peripheral tolerance. Display of the costimulatory molecule, B7.2 (CD86), represents a potentially important early response to acute BCR engagement that is poorly induced by antigen on anergic B cells. We show here that restoring B7.2 expression on tolerant B cells using a constitutively expressed B7.2 transgene is sufficient to prevent Fas-mediated deletion and to trigger extensive T cell-dependent clonal expansion and autoantibody secretion in the presence of specific T cells. Dysregulated expression of B7.2 on tolerant B cells caused a more extreme reversal of peripheral tolerance than that caused by defects in Fas or Fas ligand, and resulted in T cell-dependent clonal expansion and antibody secretion comparable in magnitude to that made by foreign antigen-specific B cells. These findings demonstrate that repression of B7.2 is critical to eliminate autoreactive B cells by Fas in B cell-T cell interactions. The possible role of B7.2 dysregulation in systemic autoimmune diseases is discussed.

Alpha-1 antitrypsin up-regulates human B cell differentiation selectively into IgE- and IgG4- secreting cells

Numerous allergens have proteolytic activities. It has been speculated that this property may contribute to their allergenicity. Therefore, we have evaluated the effect of different physiological protease inhibitors (PI) on the regulation of human IgE synthesis. Unexpectedly, the serine PI, alpha-1 antitrypsin, also called alpha-1 protease inhibitor (alpha1PI), induced a potent and selective dose-dependent increase of IgE and IgG4 production by human tonsillar B cells stimulated with the IgE and IgG4 switch factors, IL-4 and anti-CD40 mAb. The other serine PI tested were inefficient. Furthermore, this effect of alpha1PI was accompanied by an increase in (1) germ-line and mature sigma mRNA transcription, (2) proliferation and (3) membrane CD23 and CD21 expression, while the expression of other molecules involved in the regulation of IgE synthesis was unchanged. Since CD23-CD21 pairing plays a crucial role in the up-regulation of IgE synthesis, we have tested whether blocking this interaction affected alpha1PI-increased IgE production. The neutralizing anti-CD23 mAb, Mab 25, partly reversed the IgE increase caused by alpha1PI. Moreover, alpha1PI potentiation of IgE synthesis was prevented by elastase, a natural substrate of alpha1PI, thereby suggesting that alpha1PI may inhibit endogenous B cell enzyme(s) involved in the down-regulation of IgE synthesis. Alpha1PI also potentiated IgE and IgG4 production by IL-4-stimulated peripheral blood mononuclear cells but was not a switch factor for IgE and IgG4 as it was unable to replace IL-4 or anti-CD40 mAb in inducing IgE and IgG4 production. In conclusion, this study shows that alpha1PI acts as a potent co-stimulus for IgE and IgG4 synthesis and suggests that the equilibrium between protease/ protease inhibitor participates in the control of human IgE and IgG4 synthesis.

IgE Versus IgG4 Production Can Be Differentially Regulated by IL-10

Allergen-specific IgE plays a key role in the physiopathology of allergic disorders. This IgE response is usually accompanied by a production of IgG4. Indirect evidence suggests that IgG4 may not be a sensitizing Ab but, in contrast, could be protective. As such, it may be of potential therapeutic interest to selectively modulate IgE vs IgG4 production. To date, IgE and IgG4 switching seems to be controlled by common mechanisms. We report here that IL-10 has a differential effect on IgE vs IgG4 production by PBMC. IL-10 decreases ε transcript expression and IgE production induced by IL-4 when added during the first 3 days of in vitro culture, suggesting that IL-10 decreases IL-4-induced IgE switching. In contrast, if added later on B cells that are already IgE switched, IL-10 potentiates IgE production. Interestingly, whatever the time of addition, IL-10 augments IL-4-induced γ4 transcript expression and IgG4 production, with a maximal effect when added during the first 3 days. As IL-10 is not a switch factor for IgG4, it is likely that IL-10 enhances IgG4 production by potentiating IL-4-induced IgG4 switching. However, IL-10 may also act by enhancing the growth and/or differentiation of cells that are already IgG4 committed. Finally, CD40 ligation reverses the early down-regulating effect of IL-10 on IgE production. These results are the first evidence of a molecule that differentially regulates IgE vs IgG4 production, thereby suggesting the existence of a pathway(s) selectively controlling their production.

Cross-Linking CD21/CD35 or CD19 Increases Both B7-1 and B7-2 Expression on Murine Splenic B Cells

Activation of the complement cascade and ligation of complement C3 receptors on B cells represent an important bridge between innate and Ag-specific acquired immunity. We show here that cross-linking of mouse CD21 (complement receptor type 2, CR2, C3d receptor) and CD35 (complement receptor type 1, CR1, C3b/C4b receptor) or co-cross-linking of CD21/CD35 and surface IgM rapidly up-regulates both B7-1 and B7-2 expression on murine resting splenic B cells. CD21/CD35-mediated up-regulation of both B7-1 and B7-2 expression is observed within 14 h, while other stimuli up-regulate only B7-2 but not B7-1 at this early time point. Consistent with the increase in B7 levels, BALB/c B cells on which surface IgM and CD21/CD35 have been co-cross-linked stimulate C57BL/6 T cells more effectively than controls. This CD21/CD35-enhanced allogeneic MLR is blocked nearly completely by anti-B7-2 mAbs and partially by anti-B7-1 mAbs. In addition, cross-linking of CD19, which is physically associated with CD21/CD35, leads to increased B7-1 and B7-2 expression. These data suggest that CD21/CD35 ligation results in enhanced B cell Ag presentation using costimulatory mechanisms shared with other activators and thus works cooperatively in this process. Rapid up-regulation of B7-1 expression, a unique response to CD21/CD35 and CD19 cross-linking, may be a particularly important effect of C3-containing ligands. We propose that CD21/CD35- and CD19-mediated B7-1 and B7-2 up-regulation is an important mechanism by which complement activation links innate and acquired immunity.

Interleukin-7 (IL-7) Enhances Class Switching to IgE and IgG4 in the Presence of T Cells Via IL-9 and sCD23

Interleukin-7 (IL-7) is a B-cell growth factor produced by both bone marrow stroma cells and follicular dendritic cells (FDCs) located in primary lymphoid follicles and germinal centers. In this study, we have evaluated the role of IL-7 on human Ig class switching. IL-7 was added to peripheral blood mononuclear cells (PBMCs) or tonsillar B cells in the absence or presence of IL-4 and/or anti-CD40 monoclonal antibody (MoAb). Alone, IL-7 did not affect Ig production by PBMCs or by anti-CD40 MoAb-stimulated B cells. Rather, IL-7 potentiated IL-4–induced IgE and IgG4 production by PBMCs. In parallel, IgG3 production was also enhanced but to a lesser extent, whereas the production of the other isotypes was unaltered. The activity of IL-2, IL-9, or IL-15, which share usage of the common γ chain for signaling, was also assessed. IL-9, like IL-7, potentiated mainly IgE and IgG4 production by IL-4–stimulated PBMCs. IL-15, in contrast, was ineffective, whereas IL-2 enhanced the production of all isotypes. More precisely, IL-7 potentiation of IgE and IgG4 production required the presence of T cells and was accompanied by an increase of the expression of two soluble molecules favoring preferentially IgE and IgG4 synthesis: CD23 (sCD23) and IL-9. Moreover, neutralizing anti-CD23 and anti–IL-9 antibodies partly inhibited the increase of IgE synthesis induced by IL-7. Thus, IL-7 produced locally in the germinal centers by FDCs may interact with T cells and potentiate human IgE and IgG4 switching by favoring IL-9 and sCD23 production.