Positive signaling through CD72 induces mitogen-activated protein kinase activation and synergizes with B cell receptor signals to induce X-linked immunodeficiency B cell proliferation

Cellular assays to evaluate B-cell function

Inborn errors of immunity (IEI) that present with recurrent infections are largely due to antibody (Ab) deficiencies. Therefore, assessment of the B-cell and Ab compartment is a major part of immunologic evaluation. Here we provide an overview about cellular assays used to study B-cell function and focus on lymphocyte proliferation assay (LPA), opsonophagocytic assay (OPA), and the Enzyme-linked Immunosorbent Spot Assay (ELISPOT) including clinical applications and limitations of these techniques. LPAs assess ex-vivo cell proliferation in response to various stimuli. Clinically available LPAs utilize peripheral blood mononuclear cells and mostly assess T-cell proliferation with pokeweed mitogen considered the most B-cell specific stimulus. In the research setting, isolating B cells or using more B-cell specific stimuli such as CD40L with IL-4/IL-21 or the TLR9 ligand CpG can more specifically capture the proliferative ability of B cells. OPAs are functional in-vitro killing assays used to evaluate the ability of IgG Ab to induce phagocytosis applied when assessing the potency of vaccine candidates or along with avidity assays to evaluate the quality of secreted IgG. The B-cell ELISPOT assesses Ab production at a cellular level and can characterize the Ab response of particular B-cell subtypes. It can be used in patients on IgG therapy by capturing specific Abs produced by individual B cells, which is not affected by exogenous IgG from plasma donors, and when assessing the vaccine response in patients on immunomodulatory drugs that can affect memory B-cell function. Emerging approaches that are only available in research settings are also briefly introduced.

JNK signaling as a target for anticancer therapy

The JNKs are members of mitogen-activated protein kinases (MAPK) which regulate many physiological processes including inflammatory responses, macrophages, cell proliferation, differentiation, survival, and death. It is increasingly clear that the continuous activation of JNKs has a role in cancer development and progression. Therefore, JNKs represent attractive oncogenic targets for cancer therapy using small molecule kinase inhibitors. Studies showed that the two major JNK proteins JNK1 and JNK2 have opposite functions in different types of cancers, which need more specification in the design of JNK inhibitors. Some of ATP- competitive and ATP non-competitive inhibitors have been developed and widely used in vitro, but this type of inhibitors lack selectivity and inhibits phosphorylation of all JNK substrates and may lead to cellular toxicity. In this review, we summarized and discussed the strategies of JNK binding inhibitors and the role of JNK signaling in the pathogenesis of different solid and hematological malignancies.

Biology and function of neuroimmune semaphorins 4A and 4D

Semaphorins belong to a family of membrane-bound and secreted molecules that regulate the functional activity of axons in the nervous system. Sema4A and Sema4D were the first semaphorins also found to be expressed in immune cells and were, therefore, termed "immune semaphorins". It is known that Sema4A has three functional receptors, namely Plexin D1, Plexin B1, and Tim-2, whereas Sema4D binds to Plexin B1 and CD72. Recent studies suggest that immune semaphorins play critical roles in many physiological and pathological processes and such. In this review, we summarize the current knowledge on the biology of neuroimmune semaphorins and their corresponding receptors, their distribution in organs and tissues, function in the immune response, and critical regulatory roles in various diseases.

[Evaluation of percentage of lymphocytes B with expression of co-receptors CD 40, CD22 and CD72 in hypertrophied adenoid at children with otitis media with effusion]

INTRODUCTION

In hypertrophied adenoid lymphocytes B make up about 60% all lymphocytes. When the lymphocytes B come in interaction with antigens this membranes signal be passed through their receptor (BCR) to interior of cell. This signal affect modulation on gene expression, activation from which depends activation, anergy or apoptosis of lymphocyte B. Accompany BCR co-receptors regulate his functions influence stimulate or inhibitive. To the most important co-receptors stepping out on lymphocyte B belong: CD40, CD22, CD72.

AIM OF STUDY

The aim of study was evaluation of lymphocytes B (CD19) with co-expression with CD72 and CD40 receptors in hypertrophied adenoid with at children with otitis media with effusion.

MATERIAL

An investigation was executed in hypertrophied adenoids with or without otitis media with effusion.

METHODS

By flow cytometry percentage of lymphocytes B with co-receptors CD 40, CD22 and CD72 in was analyzed.

RESULTS

The percentages of CD19+CD72+ lymphocytes in the group of children with adenoid hypertrophy and exudative otitis media were lower as compared to the reference group. However, the percentages of CD19+CD22+, CD19+CD40+ in the study group was approximate to the reference group.

CONCLUSIONS

The lower percentage of lymphocytes B CD72 + near approximate percentages of lymphocytes B CD40+ and BCD22+ at children with otitis media with effusion can be the cause of incorrect humoral response in hypertrophied adenoid at children. Maybe it is cause reduced spontaneous production IgA and IgG through lymphocyte at children with otitis media with effusion.

CD72 negatively regulates KIT-mediated responses in human mast cells

KIT activation, through binding of its ligand, stem cell factor, is crucial for normal mast cell growth, differentiation, and survival. Furthermore, KIT may also contribute to mast cell homing and cytokine generation. Activating mutations in KIT lead to the dysregulated mast cell growth associated with the myeloproliferative disorder, mastocytosis. We investigated the potential of downregulating such responses through mast cell inhibitory receptor activation. In this study, we report that the B cell-associated ITIM-containing inhibitory receptor, CD72, is expressed in human mast cells. Ligation of CD72 with the agonistic Ab, BU40, or with recombinant human CD100 (rCD100), its natural ligand, induced the phosphorylation of CD72 with a resulting increase in its association with the tyrosine phosphatase SH2 domain-containing phosphatase-1. This, in turn, resulted in an inhibition of KIT-induced phosphorylation of Src family kinases and extracellular-regulated kinases (ERK1/2). As a consequence of these effects, KIT-mediated mast cell proliferation, chemotaxis, and chemokine production were significantly reduced by BU40 and rCD100. Furthermore, BU40 and rCD100 also downregulated the growth of the HMC1.2 human mast cell line. Thus, targeting CD72 may provide a novel approach to the suppression of mast cell disease such as mastocytosis.

Anomalous constitutive Src kinase activity promotes B lymphoma survival and growth

Background

Previously we have shown that B cell receptor (BCR) expression and B cell receptor signaling pathways are important for the basal growth of B lymphoma cells. In particular we have shown that the activation of Syk, a non-src family protein tyrosine kinase and the mitogen activated protein kinases (MAPK), ERK and JNK that mediate BCR signals are required for the constitutive growth of B lymphoma cells. Since src family protein tyrosine kinases (SFKs) like Lyn are known to be needed for the phosphorylation of BCR co-receptors, Ig-α and Ig-β, we hypothesized that one or more SFKs will be constitutively activated in B lymphoma cells and may be necessary for B lymphoma growth.

Results

Src kinase activity was found to be constitutively high in many murine and human B lymphoma cell lines and primary lymphoma samples. The specific pharmacological inhibitors of SFKs, PP1 and PP2 inhibited the proliferation of a number of both murine and human B lymphomas in a dose-dependent manner. Importantly, dasatinib (BMS-354825), an oral dual BCR-ABL and SFK specific inhibitor inhibited the growth of B lymphomas in the nanomolar range in vitro and strongly inhibited a mouse lymphoma growth in vivo. Among the SFKs, Lyn is predominantly phosphorylated and Lyn-specific small interfering RNA inhibited the growth of B lymphomas, supporting an important role for Lyn in B lymphoma growth. Suppression of SFK activity blocks BCR mediated signaling pathways. PMA or CpG can partially reverse the growth inhibition induced by SFK inhibition. Although blocking SFK activity inhibited the growth of a number of B lymphomas, some lymphomas such as SudHL-4, SudHL-6, OCI-Ly3 and OCI-Ly10 are more resistant due to an increased expression of the anti-apoptotic proteins Bcl-2 and Bcl-x_L.

Conclusions

These studies further support our concept that BCR signaling pathways are important for the continued growth of established B lymphoma cells. Some of the intermediates in this BCR pathway are potential immunotherapeutic targets. In particular, inhibition of SFK activity alone or in synergy with inhibition of the prosurvival Bcl-2 proteins holds promise in developing more effective treatments for B lymphoma patients.

Structure and Function of CD72 in the Non-obese Diabetic (NOD) Mouse

Type I or insulin dependent diabetes mellitus develops in the non-obese diabetic (NOD) mouse as a consequence of T cell mediated autoimmune attack on pancreatic beta cells. B lymphocytes are required for disease progression in NOD and loss of tolerance in the B cell compartment is one of the earliest manifestation of the autoimmune process. To understand how the fate and function of B lymphocytes may be regulated in the context of an organ specific autoimmune disease, the B cell co-receptor CD72 (Lyb-2) was examined in NOD mice. Mab that recognize a,b, and d alleles of CD72 reacted poorly with NOD B cells while western blots of B cell extracts show that CD72 is abundant in NOD B cells. Nucleotide sequencing of CD72 cDNA confirms that an uncommon allele, CD72c, is expressed in NOD. Functional studies using monoclonal antibodies indicate that the CD72c allele of NOD can serve as a positive regulator of B cell responses both as a single signal and in synergy with BCR or IL-4 stimulation. Since CD72c differs principally in the extra cellular or ligand binding portion of the molecule, interactions with its natural ligand in vivo may contribute to functional differences in mouse strains that express this allele. NOD and lupus prone strains share the CD72c allele and its functions may contribute to overlapping features of organ specific and systemic autoimmune disorders.

CD72, a coreceptor with both positive and negative effects on B lymphocyte development and function

INTRODUCTION

B lymphocytes remain in a resting state until activated by antigenic stimuli through interaction with the B cell receptor (BCR). Coreceptors on B cells can modulate the thresholds for signaling through the BCR for growth and differentiation. CD72 is a B cell coreceptor that has been shown to interact with CD100, a semaphorin, and to enhance BCR signaling.

DISCUSSION

CD72 ligation induces a variety of early signaling events such as activation of the Src kinases Blk and Lyn and the non-src kinase Btk leading to activation of the mitogen-activated protein (MAP) kinases, events usually associated with positive signaling. CD72 signals can enable Btk-deficient B cells to overcome their unresponsiveness to BCR signaling. On the other hand, BCR-mediated signals are enhanced in CD72-deficient cells but are reduced in CD100 null cells. The dual effects of CD72 on B cells can be explained by its association with positive and negative signaling molecules. Thus, CD72 interacts with SHP-1, an SH2-domain containing protein tyrosine phosphatase, a negative regulator of signaling, and Grb2, an adaptor protein associated with the Ras/MAPK pathway. Ligation of CD72 also triggered its association with CD19, a positive modulator of B cell receptor signaling. We propose a dual signaling hypothesis to explain the growth and differentiation promoting properties of CD72. Deficiency in either CD72 or CD100 leads to autoimmunity in mouse models. CD72 expression and polymorphisms exhibit some association with autoimmune diseases such as lupus, Sjogren's syndrome, and type 1 diabetes.

Modulation of peripheral B cell tolerance by CD72 in a murine model

OBJECTIVE

B cells play a dominant role in the pathogenesis of several autoimmune diseases, including systemic lupus erythematosus. It is not well understood how B cell signaling contributes to autoantibody production. The goal of this study was to elucidate the role of CD72 in modulating B cell receptor (BCR)-mediated tolerogenic signaling and peripheral B cell tolerance.

METHODS

A mouse model utilizing hen egg lysozyme (HEL) "anergic" B cells was studied. CD72-deficient mice carrying the BCR-specific IgHEL and/or soluble HEL (sHEL) transgenes were generated by breeding IgHEL-transgenic MD4 mice and/or sHEL-transgenic ML5 mice with congenic, CD72-deficient C57BL/6J mice. Normal and anergic B cells were isolated for analyses of B cell signaling. Aged wild-type and CD72-deficient mice were also examined for autoimmune phenomena.

RESULTS

In the absence of CD72, anergic B cells inappropriately proliferated and survived in response to stimulation with self antigen. Biochemical analyses indicated that in anergic B cells, CD72 dominantly down-regulated BCR signaling to limit the antigen-induced elevation in [Ca2+]i and the activation of NFATc1, NF-kappaB, MAPK, and Akt. Mechanistically, CD72 was associated with, and regulated, the molecular adaptor Cbl-b in anergic B cells, suggesting that Cbl-b may play a role in mediating the negative effects of CD72 on BCR signaling. Moreover, in aged CD72-deficient mice, spontaneous production of antinuclear and anti-double-stranded DNA autoantibodies and features of lupus-like autoimmune disease were observed.

CONCLUSION

CD72 is required to maintain B cell anergy and functions as a regulator of peripheral B cell tolerance. Thus, altered CD72 expression may play a role during the development of systemic lupus erythematosus.

Various domains of the B-cell regulatory molecule CD72 has diverged at different rates in mammals: cloning, transcription and mapping of porcine CD72

We report the cloning of the porcine B-cell co-receptor CD72, as well as genomic mapping and examination of transcription. The B-cell receptor (BCR) complex mediates signalling upon antigen recognition by the membrane bound BCR. Several co-receptors modulate this signal positively or negatively. CD72 has been shown to be a negatively regulating BCR co-receptor. We isolated and sequenced three porcine CD72 transcript variants. Using a pig radiation hybrid panel we found the porcine CD72 gene to be located on chromosome 1q21-28 in a region syntenic to human chromosome 9. The porcine CD72 gene is highly transcribed in lymph node, thymus and lung tissues as well as in pulmonary alveolar macrophages. The predicted porcine CD72 polypeptide shows conservation of immunoreceptor tyrosine-based inhibitory motifs and an extracellular C-type lectin domain. Compared to CD72 sequences from other mammals as well as from chicken, the polypeptide is highly conserved in the intracellular part and much less conserved in the extracellular part. We suggest that this difference might be due to the different nature of ligands and the constrains on these to co-evolve.

Increased mutations of CD72 transcript in B-lymphocytes from adolescent patients with systemic lupus erythematosus

Recent studies have shown that B cells play a central role in the pathogenesis of systemic lupus erythematosus (SLE). Abnormal expression of molecules engaging in B-cell receptor (BCR) signaling and resultant hyperactivity of B cells has been reported in both mouse models of lupus and patients with SLE. CD72 on B cells is unique in that it regulates BCR signaling both positively and negatively. We analyzed the expression of CD72 protein and mRNA in peripheral blood B cells from adolescent patients with SLE. The expression level of CD72 on B cells of the patients was decreased compared with that on B cells of controls. Sequence analysis of CD72 mRNA showed significantly increased nucleotide mutations, including both nucleotide substitutions and deletions. Almost all (95.6%) of the CD72 transcripts from the patients had different nucleotide sequences from those of the wild type. About half (41.3%) of the mutations were point mutations located close to the sequence of the immunoreceptor tyrosine-based inhibitory motif (ITIM), which negatively regulates BCR signaling. These results indicate that increased nucleotide mutation of CD72 mRNA accounts for the decreased expression level of CD72 in B cells, and it might be related to hyperactivity of B cells in patients with SLE.

The role of MAPKs in B cell receptor-induced down-regulation of Egr-1 in immature B lymphoma cells

Cross-linking of the B cell receptor (BCR) on the immature B lymphoma cell line BKS-2 induces growth inhibition and apoptosis accompanied by rapid down-regulation of the immediate-early gene egr-1. In these lymphoma cells, egr-1 is expressed constitutively and has a prosurvival role, as Egr-1-specific antisense oligonucleotides or expression of a dominant-negative inhibitor of Egr-1 also prevented the growth of BKS-2 cells. Moreover, enhancement of Egr-1 protein with phorbol 12-myristate 13-acetate or an egr-1 expression vector rescued BKS-2 cells from BCR signal-induced growth inhibition. Nuclear run-on and mRNA stability assays indicated that BCR-derived signals act at the transcriptional level to reduce egr-1 expression. Inhibitors of ERK and JNK (but not of p38 MAPK) reduced egr-1 expression at the protein level. Transcriptional regulation appears to have a role because egr-1 promoter-driven luciferase expression was reduced by ERK and JNK inhibitors. Promoter truncation experiments suggested that several serum response elements are required for MAPK-mediated egr-1 expression. Our study suggests that BCR signals reduce egr-1 expression by inhibiting activation of ERK and JNK. Unlike ERK and JNK, p38 MAPK reduces constitutive expression of egr-1. Unlike the immature B lymphoma cells, normal immature B cells did not exhibit constitutive MAPK activation. BCR-induced MAPK activation was modest and transient with a small increase in egr-1 expression in normal immature B cells consistent with their inability to proliferate in response to BCR cross-linking.

A unique CD72 epitope suggests a potential interaction with Fc gamma RII/CD32 on B lineage lymphocytes

It has long been known that ligation of the transmembrane CD72 glycoprotein delivers signals to B lymphocytes, with the outcome depending on context. Of particular interest is its ability to function as a counter-receptor/ ligand for the CD100 semaphorin protein. We have now obtained evidence that CD72 physically interacts on the lymphocyte membrane with Fcgamma receptor II (CD32). The association was first revealed with a new monoclonal antibody that recognizes polymorphic determinants on murine CD72. Although the specificity for CD72 was clear from immunoblotting, transfection and other experiments, staining with this reagent was inhibited when cells were pretreated with an Fc receptor-blocking antibody (CD16/CD32 specific). Furthermore, confocal microscopy revealed that the two molecules co-distributed on viable B cells. We also used the antibody to determine when CD72 becomes available to maturing lymphocytes. The marker is first acquired as large pre-B cells and enter the IL-7 independent phase of maturation within bone marrow. Subsequent interactions between CD72 and CD32 may cooperatively deliver negative signals that modulate humoral immune responses.

CD72 down-modulates BCR-induced signal transduction and diminishes survival in primary mature B lymphocytes

CD72, a 45-kDa type II transmembrane glycoprotein carrying an ITIM motif, is believed to be an inhibitory coreceptor of the BCR. Mature B cells lacking CD72 show enhanced Ca(2+) mobilization and are hyperproliferative in response to BCR ligation. However, the signal transduction pathways downstream of BCR signaling that transmit the inhibitory effect of CD72 in mature B cells remain unknown. To address this question, we used hen egg lysozyme-specific BCR transgenic mice to elucidate the differential cell signaling between wild-type and CD72-deficient B cells in response to hen egg lysozyme Ag stimulation. Our results demonstrate that CD72 predominantly down-regulates the major signal transduction pathways downstream of the BCR, including NF-AT, NF-kappaB, ERK, JNK, p38-MAPK, and PI3K/Akt in mature B cells. CD72 ligation with anti-CD72 Ab (K10.6), which mimics the binding of CD100 (a natural ligand for CD72) to release the inhibitory function of CD72, augments cell proliferation, Ca(2+) flux, IkappaBalpha activation, and ERK MAPK activity upon Ag stimulation in wild-type B cells. In addition, we show direct evidence that CD72 promotes cell cycle arrest and apoptosis after Ag stimulation in mature B cells. Taken together, our findings conclude that CD72 plays a dominant role as a negative regulator of BCR signaling in primary mature B lymphocytes.

Components of the antigen processing and presentation pathway revealed by gene expression microarray analysis following B cell antigen receptor (BCR) stimulation

Background

Activation of naïve B lymphocytes by extracellular ligands, e.g. antigen, lipopolysaccharide (LPS) and CD40 ligand, induces a combination of common and ligand-specific phenotypic changes through complex signal transduction pathways. For example, although all three of these ligands induce proliferation, only stimulation through the B cell antigen receptor (BCR) induces apoptosis in resting splenic B cells. In order to define the common and unique biological responses to ligand stimulation, we compared the gene expression changes induced in normal primary B cells by a panel of ligands using cDNA microarrays and a statistical approach, CLASSIFI (Cluster Assignment for Biological Inference), which identifies significant co-clustering of genes with similar Gene Ontology™ annotation.

Results

CLASSIFI analysis revealed an overrepresentation of genes involved in ion and vesicle transport, including multiple components of the proton pump, in the BCR-specific gene cluster, suggesting that activation of antigen processing and presentation pathways is a major biological response to antigen receptor stimulation. Proton pump components that were not included in the initial microarray data set were also upregulated in response to BCR stimulation in follow up experiments. MHC Class II expression was found to be maintained specifically in response to BCR stimulation. Furthermore, ligand-specific internalization of the BCR, a first step in B cell antigen processing and presentation, was demonstrated.

Conclusion

These observations provide experimental validation of the computational approach implemented in CLASSIFI, demonstrating that CLASSIFI-based gene expression cluster analysis is an effective data mining tool to identify biological processes that correlate with the experimental conditional variables. Furthermore, this analysis has identified at least thirty-eight candidate components of the B cell antigen processing and presentation pathway and sets the stage for future studies focused on a better understanding of the components involved in and unique to B cell antigen processing and presentation.

Mouse splenic B lymphocyte activation using different activation stimuli induces in vitro splicing of tumor necrosis factor-α nuclear pre-mRNA

The pleiotropic functions of tumor necrosis factor-alpha (TNFalpha) have brought considerable attention in the past decade to its physiological and pathological roles in inflammatory and autoimmune diseases. However, little is known about how the production of TNFalpha is regulated at the transcriptional and translational levels in immune cells such as T and B lymphocytes. Our previous study demonstrated that unspliced "pre-mRNA" of TNFalpha is present in resting T cells. Initiation of splicing of TNFalpha pre-mRNA to mature mRNA requires T cell activation, which is unique and necessary for TNFalpha production when compared to its production in mononuclear phagocytes, including different lineages of macrophages (Mvarphi) and dendritic cells (DC). In this study, we further demonstrate that resting mouse B cells also contain pre-existing TNFalpha mRNA. The physiological process of B cell activation induced by (1) either the cross-linking of the B cell receptor (BCR) or CD40, (2) treatment with LPS, or PMA plus ionomycin, induces TNFalpha mRNA splicing in vitro. The kinetic response of TNFalpha splicing in B cells is much slower when compared to that in activated T cells. Studies using well-known kinase inhibitors demonstrated that MAP kinase kinase (MEK) and protein kinase C (PKC) are required for TNFalpha splicing upon stimulation through the BCR. These studies demonstrate that the production of TNFalpha in activated B cells is regulated differently than in activated T cells, and these differences may allow for the selective inhibition of TNFalpha in various autoimmune diseases depending on the mechanism of action of the selected anti-TNFalpha therapy.

BCR-crosslinking induces a transcription of protein phosphatase component G5PR that is required for mature B-cell survival

BCR-crosslinking triggers activation-induced cell death (AICD) selectively in the restricted stage of B-cell differentiation. We examined the transcription of a protein phosphatase subunit G5PR in immature and mature B-cells, because absence of this factor augmented cell sensitivity to AICD, associated with increased activation of JNK and Bim. BCR-crosslinking-induced G5pr transcription in AICD-resistant mature splenic IgM(lo)IgD(hi) B-cells but not in AICD susceptible immature IgM(hi)IgD(lo) B-cells. Thus, G5pr induction correlated with the prevention of AICD; High in mature splenic CD23(hi) B-cells but low in immature B-cells of neonatal mice, sub-lethally irradiated mice, or xid mice. Lack of G5pr upregulation was associated with the prolonged activation of JNK. The G5pr cDNA transfection protected an immature B-cell line WEHI-231 from BCR-mediated AICD. The differential expression of G5PR might be responsible for the antigen-dependent selection of B-cells.

CD72-mediated suppression of human naive B cell differentiation by down-regulating X-box binding protein 1

B cells can differentiate into antibody-secreting plasma cells, however the signals that control the entry into this pathway are not clearly understood. We have investigated the role of human CD72 in mature B cell differentiation. Human CD72 is preferentially expressed in naive B cells, but marginal levels of expression can be found in switched memory B cells. CD72 cross-linking promoted an increase in B cell activation and proliferation. Interestingly, expression of CD27, whose signal induces the differentiation of B cells into plasma cells, was down-modulated by CD72 stimulation. This CD72 signaling also induced tyrosine phosphorylation of various proteins such as Blk. Plasma cell differentiation and Ig syntheses were diminished by CD72 ligation in the presence of Staphylococcus aureus Cowan strain (SAC) plus IL-2 but not in the presence of CD40 signaling or CpG oligodeoxynucleotide. Our results show that CD72 signaling reduces the expression of X-box binding protein 1 in B cells stimulated with SAC plus IL-2, but the expression of PRDI-BF1 was unaffected. Taken together, these data demonstrate that CD72 is a key molecule in regulating mature B cell differentiation, particularly in preventing the differentiation of naive B cells into plasma cells, thus blocking the production of low-affinity antibodies.

The role of CD22 and other inhibitory co-receptors in B-cell activation

Inhibitory co-receptors downmodulate B-cell receptor (BCR) signalling by setting a signalling threshold that prevents overstimulation of B cells. Activation of these inhibitory co-receptors occurs by phosphorylation on their cytoplasmic inhibitory immunoreceptor tyrosine-based inhibition motifs (ITIMs), followed by recruitment of the tyrosine phosphatase SHP-1 or the lipid phosphatase SHIP, and depends on their association with the BCR. Recent evidence shows that B-cell signal inhibition is regulated by ligand binding of inhibitory receptors.

c-Jun N-terminal kinase (JNK) is required for survival and proliferation of B-lymphoma cells

Several primary murine and human B lymphomas and cell lines were found to constitutively express high levels of the activated form of c-jun N-terminal kinase (JNK), a member of the mitogen-activated protein (MAP) kinase family. Proliferation of murine B lymphomas CH31, CH12.Lx, BKS-2, and WEHI-231 and the human B lymphomas BJAB, RAMOS, RAJI, OCI-Ly7, and OCI-Ly10 was strongly inhibited by SP600125, an anthrapyrazolone inhibitor of JNK, in a dose-dependent manner. The lymphoma cells underwent apoptosis and arrested at the G2/M phase of cell cycle. Furthermore, JNK-specific small interfering RNA (siRNA) inhibited the growth of both murine and human B lymphomas. Thus in the B-lymphoma model, JNK appears to have a unique prosurvival role. Survival signals provided by CD40 and interleukin-10 (IL-10) together reversed the growth inhibition induced by the JNK inhibitor. c-Myc protein levels were reduced in the presence of both SP600125 and JNK-specific siRNA, and CD40 ligation restored c-Myc levels. Moreover, Bcl-xL rescued WEHI-231 cells from apoptosis induced by the JNK inhibitor. The JNK inhibitor also reduced levels of early growth response gene-1 (Egr-1) protein, and overexpressing Egr-1 partially rescued lymphoma cells from apoptosis. Thus, JNK may act via c-Myc and Egr-1, which were shown to be important for B-lymphoma survival and growth.

Dual regulation of BCR-mediated growth inhibition signaling by CD72

CD72 has been reported to regulate BCR-mediated signals both positively and negatively. SHP-1 and Grb2 bind, respectively, to ITIM1 and ITIM2 of CD72. We generated transformed B cell lines with an immature phenotype following J2 virus infection of splenocytes from CD72(-/-) and wild-type (Wt) mice. The transformed lines were infected with retroviral vectors carrying Tyr (Y) to Phe (F) substitutions in the ITIM sequences (ITIM1 mutated: Y7/F; ITIM2 mutated: Y39/F; and both ITIM mutated: Y7,39/F). Cross-linking of the BCR induced growth inhibition in transfectants expressing Wt CD72, but this response was less sensitive in transfectants with Y7,39/F. The Y7/F transfectants demonstrated the least sensitive response. We were not able to obtain transfectants with Y39/F, suggesting that CD72 associated with SHP-1, but not with Grb2, delivers a strong negative signal. Pre-ligation of CD72, which induces dephosphorylation of the molecule, partially rescued the Wt transfectants from growth inhibition, leading to a growth response profile similar to that of Y7,39/F transfectants. These results suggest that ITIM1/SHP-1 delivers a very strong negative signal that is down-modulated by signals through ITIM2/Grb2, leading to delivery of an attenuated negative signal. Thus, pre-ligation of CD72 results in the manifestation of an ostensible positive signal.

CD72 polymorphisms associated with alternative splicing modify susceptibility to human systemic lupus erythematosus through epistatic interaction with FCGR2B

We previously reported association of FCGR2B-Ile232Thr with systemic lupus erythematosus (SLE) in three Asian populations. Because polymorphism of CD72, another inhibitory receptor of B cells, was associated with murine SLE, we identified human CD72 polymorphisms, tested their association with SLE and examined genetic interaction with FCGR2B in the Japanese (160 SLE, 277 controls), Thais (87 SLE, 187 controls) and Caucasians (94 families containing SLE members). Four polymorphisms and six rare variations were detected. The former constituted two major haplotypes that contained one or two repeats of 13 nucleotides in intron 8 (designated as *1 and *2, respectively). Although association with susceptibility to SLE was not detected, the *1 allele was significantly associated with nephritis among the Japanese patients (P=0.024). RT-PCR identified a novel alternatively spliced (AS) transcript that was expressed at the protein level in COS-7 transfectants. The ratio of AS/common isoforms was strikingly increased in individuals with *2/*2 genotype when compared with *1/*1 (P=0.000038) or *1/*2 (P=0.0085) genotypes. Using the two Asian cohorts, significant association of FCGR2B-232Thr/Thr with SLE was observed only in the presence of CD72-*1/*1 genotype (OR 4.63, 95% CI 1.47-14.6, P=0.009 versus FCGR2B-232Ile/Ile plus CD72-*2/*2). Minigene assays demonstrated that the 13-nucleotide repeat and 4 bp deletion within the same haplotype of intron 8 could regulate alternative splicing. The AS isoform lacks exon 8, and is deduced to contain 49 amino acid changes in the membrane-distal portion of the extracellular domain, where considerable amino acid changes are known in CD72(c) allele associated with murine SLE. These results indicated that the presence of CD72-*2 allele decreases risk for human SLE conferred by FCGR2B-232Thr, possibly by increasing the AS isoform of CD72.

Increased expression of the B-cell-regulatory molecule CD72 in primary Sjögren's syndrome

To determine whether there is an intrinsic abnormality of B-cell signaling in primary Sjögren's syndrome (pSS), the expression of B-cell coreceptors was determined in patients with primary Sjögren's syndrome and healthy and disease controls. Peripheral blood mononuclear cells were labeled with monoclonal antibodies to CD21, CD22, or CD72, and then the pan B-cell marker CD19. The expression of these coreceptors on the total CD19(+) population was determined. There was a significant increased expression of CD72 on the B cells of pSS patients (MFI, 215 +/- 6) compared to normal controls (MFI, 141 +/- 6). The increased CD72 expression was disease specific for pSS, as it was not observed in systemic lupus erythematosus or rheumatoid arthritis. The effect of B-cell stimulation on coreceptor expression was determined by culturing cells with B-lymphocyte-activating factor (BAFF) and/or pokeweed mitogen (PWM) or without either. Following culture, CD72 expression was decreased in both pSS and normal controls, regardless of the presence of BAFF or PWM. The upregulation of CD72 in pSS might be a compensatory response to increased B-cell receptor stimulation or a primary abnormality leading to uncontrolled B-cell activation.

Inhibitory coreceptors activated by antigens but not by anti-Ig heavy chain antibodies install requirement of costimulation through CD40 for survival and proliferation of B cells

Ag-induced B cell proliferation in vivo requires a costimulatory signal through CD40, whereas B cell Ag receptor (BCR) ligation by anti-Ig H chain Abs, such as anti-Ig micro H chain Ab and anti-Ig delta H chain Ab, alone induces proliferation of B cells in vitro, even in the absence of CD40 ligation. In this study, we demonstrate that CD40 signaling is required for survival and proliferation of B cells stimulated by protein Ags in vitro as well as in vivo. This indicates that the in vitro system represents B cell activation in vivo, and that protein Ags generate BCR signaling distinct from that by anti-Ig H chain Abs. Indeed, BCR ligation by Ags, but not by anti-Ig H chain Abs, efficiently phosphorylates the inhibitory coreceptors CD22 and CD72. When these coreceptors are activated, anti-Ig H chain Ab-stimulated B cells can survive and proliferate only in the presence of CD40 signaling. Conversely, treatment of Ag-stimulated B cells with anti-CD72 mAb blocks CD72 phosphorylation and induces proliferation, even in the absence of CD40 signaling. These results strongly suggest that activation of B cells by anti-Ig H chain Abs involves their ability to silence the inhibitory coreceptors, and that the inhibitory coreceptors install requirement of CD40 signaling for survival and proliferation of Ag-stimulated B cells.

Transitional type 1 and 2 B lymphocyte subsets are differentially responsive to antigen receptor signaling

Mature B-lymphocytes develop sequentially from transitional type 1 (T1) and type 2 (T2) precursors in the spleen. To elucidate the mechanisms that regulate the developmental fate of these distinct B cell subsets, we investigated their biochemical and biological responses following stimulation through the B-cell antigen receptor (BCR). As compared with the T1 subset, T2 cells are more responsive to BCR engagement, as evidenced by their robust induction of activation markers, expression of the prosurvival protein Bcl-x(L), and enhanced proliferation. BCR stimulation of T2 cells leads to the appearance of B cells with mature phenotypic characteristics, whereas T1 cells die. All of these T2 responses are dependent on the BCR signal transducer Bruton's tyrosine kinase, which is dispensable for the T1 to T2 transition. Furthermore, the serine/threonine kinases ERK, p38 MAPK, and Akt are predominantly activated in T2 compared with T1 B cells following BCR cross-linking. We conclude that T1 and T2 B cells respond differentially to BCR engagement via the induction of stage-specific signaling pathways. In turn, these signaling pathways probably govern the development and selection processes that are critical for the formation of the mature B cell compartment.

Requirement for a hsp90 chaperone-dependent MEK1/2-ERK pathway for B cell antigen receptor-induced cyclin D2 expression in mature B lymphocytes

A requirement for cyclin D2 in G(1)-to-S phase progression has been definitively established in mature B cells stimulated via the B cell antigen receptor (BCR). However, the identity of constituents of the BCR signaling cascade that leads to cyclin D2 accumulation remains incomplete. We report that inhibition of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK)-1/2 blocked BCR-induced activation of extracellular signal-regulated kinase (ERK). Inhibition of the MEK1/2-ERK pathway was sufficient to abrogate BCR-induced cyclin D2 expression at the mRNA and protein levels. Disruption of endogenous heat shock protein 90 (hsp90) function with geldanamycin abrogated BCR-induced cyclin D2 expression and proliferation. Geldanamycin effects were attributed to a selective depletion of cellular Raf-1 that interrupted BCR-coupled activation of MEK1/2 and ERK. By contrast, signaling through the phosphatidylinositol 3-kinase and protein kinase C pathways was not affected, suggesting that disruption of hsp90 function did not cause a general impairment of BCR signaling. These results suggest that the MEK1/2-ERK pathway is essential for BCR signaling to cyclin D2 accumulation in ex vivo splenic B lymphocytes. Furthermore, these findings imply that hsp90 function is required for BCR signaling through the Raf-1-MEK1/2-ERK pathway but not through the phosphatidylinositol 3-kinase- or protein kinase C-dependent pathways.