Analysis of the Individual Contributions of Igα (CD79a)- and Igβ (CD79b)-Mediated Tonic Signaling for Bone Marrow B Cell Development and Peripheral B Cell Maturation

B-cells absence in patients diagnosed as inborn errors of immunity: a registry-based study

Hypogammaglobulinemia without B-cells is a subgroup of inborn errors of immunity (IEI) which is characterized by a significant decline in all serum immunoglobulin isotypes, coupled with a pronounced reduction or absence of B-cells. Approximately 80 to 90% of individuals exhibit genetic variations in Bruton's agammaglobulinemia tyrosine kinase (BTK), whereas a minority of cases, around 5-10%, are autosomal recessive agammaglobulinemia (ARA). Very few cases are grouped into distinct subcategories. We evaluated phenotypically and genetically 27 patients from 13 distinct families with hypogammaglobinemia and no B-cells. Genetic analysis was performed via whole-exome and Sanger sequencing. The most prevalent genetic cause was mutations in BTK. Three novel mutations in the BTK gene include c.115 T > C (p. Tyr39His), c.685-686insTTAC (p.Asn229llefs5), and c.163delT (p.Ser55GlnfsTer2). Our three ARA patients include a novel homozygous stop-gain mutation in the immunoglobulin heavy constant Mu chain (IGHM) gene, a novel frameshift mutation of the B-cell antigen receptor complex-associated protein (CD79A) gene, a novel bi-allelic stop-gain mutation in the transcription factor 3 (TCF3) gene. Three patients with agammaglobulinemia have an autosomal dominant inheritance pattern, which includes a missense variant in PIK3CD, a novel missense variant in PIK3R1 and a homozygous silent mutation in the phosphoinositide-3-kinase regulatory subunit (RASGRP1) gene. This study broadens the genetic spectrum of hypogammaglobulinemia without B-cells and presented a few novel variants within the Iranian community, which may also have implications in other Middle Eastern populations. Notably, disease control was better in the second affected family member in families with multiple cases.

Naturally occurring autoimmune disease in (NZB X NZW) F1 mice is correlated with suppression of MZ B cell development due to aberrant B Cell Receptor (BCR) signaling, which is exacerbated by exposure to inorganic mercury

Autoimmune diseases are multifactorial and include environmental as well as genetic drivers. Although much progress has been made in understanding the nature of genetic underpinnings of autoimmune disease, by comparison much less is understood regarding how environmental factors interact with genetics in the development of autoimmunity and autoimmune disease. In this report, we utilize the (NZB X NZW) F1 mouse model of Systemic Lupus Erythematosus (SLE). Mercury is a xenobiotic that is environmentally ubiquitous and is epidemiologically linked with the development of autoimmunity. Among other attributes of human SLE, (NZB X NZW) F1 mice spontaneously develop autoimmune-mediated kidney disease. It has been previously shown that if (NZB X NZW) F1 mice are exposed to inorganic mercury (Hg2+), the development of autoimmunity, including autoimmune kidney pathology, is accelerated. We now show that in these mice the development of kidney disease is correlated with a decreased percentage of marginal zone (MZ) B cells in the spleen. In Hg2+-intoxicated mice, kidney disease is significantly augmented, and matched by a greater decrease in MZ B cell splenic percentages than found in control mice. In Hg2+- intoxicated mice, the decrease in MZ B cells appears to be linked to aberrant B Cell Receptor (BCR) signal strength in transitory 2 (T2) B cells, developmental precursors of MZ B cells.

Dysregulated Cyclic Nucleotide Metabolism in Alcohol-Associated Steatohepatitis: Implications for Novel Targeted Therapies

BACKGROUND

Cyclic nucleotides are second messengers, which play significant roles in numerous biological processes. Previous work has shown that cAMP and cGMP signaling regulates various pathways in liver cells, including Kupffer cells, hepatocytes, hepatic stellate cells, and cellular components of hepatic sinusoids. Importantly, it has been shown that cAMP levels and enzymes involved in cAMP homeostasis are affected by alcohol. Although the role of cyclic nucleotide signaling is strongly implicated in several pathological pathways in liver diseases, studies describing the changes in genes regulating cyclic nucleotide metabolism in ALD are lacking.

METHODS

Male C57B/6 mice were used in an intragastric model of alcohol-associated steatohepatitis (ASH). Liver injury, inflammation, and fibrogenesis were evaluated by measuring plasma levels of injury markers, liver tissue cytokines, and gene expression analyses. Liver transcriptome analysis was performed to examine the effects of alcohol on regulators of cyclic AMP and GMP levels and signaling. cAMP and cGMP levels were measured in mouse livers as well as in livers from healthy human donors and patients with alcohol-associated hepatitis (AH).

RESULTS

Our results show significant changes in several phosphodiesterases (PDEs) with specificity to degrade cAMP (Pde4a, Pde4d, and Pde8a) and cGMP (Pde5a, Pde6d, and Pde9a), as well as dual-specificity PDEs (Pde1a and Pde10a) in ASH mouse livers. Adenylyl cyclases (ACs) 7 and 9, which are responsible for cAMP generation, were also affected by alcohol. Importantly, adenosine receptor 1, which has been implicated in the pathogenesis of liver diseases, was significantly increased by alcohol. Adrenoceptors 1 and 3 (Adrb), which couple with stimulatory G protein to regulate cAMP and cGMP signaling, were significantly decreased. Additionally, beta arrestin 2, which interacts with cAMP-specific PDE4D to desensitize G-protein-coupled receptor to generate cAMP, was significantly increased by alcohol. Notably, we observed that cAMP levels are much higher than cGMP levels in the livers of humans and mice; however, alcohol affected them differently. Specifically, cGMP levels were higher in patients with AH and ASH mice livers compared with controls. As expected, these changes in liver cyclic nucleotide signaling were associated with increased inflammation, steatosis, apoptosis, and fibrogenesis.

CONCLUSIONS

These data strongly implicate dysregulated cAMP and cGMP signaling in the pathogenesis of ASH. Future studies to identify changes in these regulators in a cell-specific manner could lead to the development of novel targeted therapies for ASH.

Combination of High-Resolution Structures for the B Cell Receptor and Co-Receptors Provides an Understanding of Their Interactions with Therapeutic Antibodies

B cells are central to the adaptive immune response, providing long lasting immunity after infection. B cell activation is mediated by a cell surface B cell receptor (BCR) following recognition of an antigen. BCR signaling is modulated by several co-receptors including CD22 and a complex that contains CD19 and CD81. Aberrant signaling through the BCR and co-receptors promotes the pathogenesis of several B cell malignancies and autoimmune diseases. Treatment of these diseases has been revolutionized by the development of monoclonal antibodies that bind to B cell surface antigens, including the BCR and its co-receptors. However, malignant B cells can escape targeting by several mechanisms and until recently, rational design of antibodies has been limited by the lack of high-resolution structures of the BCR and its co-receptors. Herein we review recently determined cryo-electron microscopy (cryo-EM) and crystal structures of the BCR, CD22, CD19 and CD81 molecules. These structures provide further understanding of the mechanisms of current antibody therapies and provide scaffolds for development of engineered antibodies for treatment of B cell malignancies and autoimmune diseases.

Oncogene-independent BCR-like signaling adaptation confers drug resistance in Ph-like ALL

Children and adults with Philadelphia chromosome-like B cell acute lymphoblastic leukemia (Ph-like B-ALL) experience high relapse rates despite best-available conventional chemotherapy. Ph-like ALL is driven by genetic alterations that activate constitutive cytokine receptor and kinase signaling, and early-phase trials are investigating the potential of the addition of tyrosine kinase inhibitors (TKIs) to chemotherapy to improve clinical outcomes. However, preclinical studies have shown that JAK or PI3K pathway inhibition is insufficient to eradicate the most common cytokine receptor-like factor 2-rearranged (CRLF2-rearranged) Ph-like ALL subset. We thus sought to define additional essential signaling pathways required in Ph-like leukemogenesis for improved therapeutic targeting. Herein, we describe an adaptive signaling plasticity of CRLF2-rearranged Ph-like ALL following selective TKI pressure, which occurs in the absence of genetic mutations. Interestingly, we observed that Ph-like ALL cells have activated SRC, ERK, and PI3K signaling consistent with activated B cell receptor (BCR) signaling, although they do not express cell surface μ-heavy chain (μHC). Combinatorial targeting of JAK/STAT, PI3K, and "BCR-like" signaling with multiple TKIs and/or dexamethasone prevented this signaling plasticity and induced complete cell death, demonstrating a more optimal and clinically pragmatic therapeutic strategy for CRLF2-rearranged Ph-like ALL.

Optimal Development of Mature B Cells Requires Recognition of Endogenous Antigens

It has long been appreciated that highly autoreactive BCRs are actively removed from the developing B cell repertoire by Ag-dependent receptor editing and deletion. However, there is persistent debate about whether mild autoreactivity is simply tolerated or positively selected into the mature B cell repertoire as well as at what stage, to what extent, under what conditions, and into which compartments this occurs. In this study, we describe two minor, trackable populations of B cells in B1-8i Ig transgenic mice that express the VH186.2 H chain and recognize a common foreign Ag (the hapten 4-hydroxy-3-nitrophenylacetyl) but differ in L chain expression. We use the Nur77-eGFP reporter of BCR signaling to define their reactivity toward endogenous Ags. The less autoreactive of these two populations is strongly counterselected during the development of mature B1a, follicular, and marginal zone B cells. By genetically manipulating the strength of BCR signal transduction via the titration of surface CD45 expression, we demonstrate that this B cell population is not negatively selected but instead displays characteristics of impaired positive selection. We demonstrate that mild self-reactivity improves the developmental fitness of B cell clones in the context of a diverse population of B cells, and positive selection by endogenous Ags shapes the mature B cell repertoire.

Tonic B-cell receptor signaling in diffuse large B-cell lymphoma

We used clustered regularly interspaced short palindromic repeats/Cas9-mediated genomic modification to investigate B-cell receptor (BCR) signaling in cell lines of diffuse large B-cell lymphoma (DLBCL). Three manipulations that altered BCR genes without affecting surface BCR levels showed that BCR signaling differs between the germinal center B-cell (GCB) subtype, which is insensitive to Bruton tyrosine kinase inhibition by ibrutinib, and the activated B-cell (ABC) subtype. Replacing antigen-binding BCR regions had no effect on BCR signaling in GCB-DLBCL lines, reflecting this subtype's exclusive use of tonic BCR signaling. Conversely, Y188F mutation in the immunoreceptor tyrosine-based activation motif of CD79A inhibited tonic BCR signaling in GCB-DLBCL lines but did not affect their calcium flux after BCR cross-linking or the proliferation of otherwise-unmodified ABC-DLBCL lines. CD79A-GFP fusion showed BCR clustering or diffuse distribution, respectively, in lines of ABC and GCB subtypes. Tonic BCR signaling acts principally to activate AKT, and forced activation of AKT rescued GCB-DLBCL lines from knockout (KO) of the BCR or 2 mediators of tonic BCR signaling, SYK and CD19. The magnitude and importance of tonic BCR signaling to proliferation and size of GCB-DLBCL lines, shown by the effect of BCR KO, was highly variable; in contrast, pan-AKT KO was uniformly toxic. This discrepancy was explained by finding that BCR KO-induced changes in AKT activity (measured by gene expression, CXCR4 level, and a fluorescent reporter) correlated with changes in proliferation and with baseline BCR surface density. PTEN protein expression and BCR surface density may influence clinical response to therapeutic inhibition of tonic BCR signaling in DLBCL.

Grouper (Epinephelus coioides) BCR signaling pathway was involved in response against Cryptocaryon irritans infection

B cell antigen receptor (BCR) plays a crucial role in B cell development and antibody production. It comprises membrane immunoglobulin non-covalently associated with CD79a/CD79b heterodimer. After B cell activation, initial extracellular signals are transduced by BCR complex and amplified by two protein tyrosine kinases, LYN and SYK, which then trigger various pathways. In the present study, we cloned grouper genes for BCR accessory molecules, EcCD79a (669 bp) and EcCD79b (639 bp), as well as two protein tyrosine kinases, EcLYN (1482 bp) and EcSYK (1854 bp). Homology analysis showed that all four molecules had a relatively high amino acid identity compared with those in other animals. Among them, they all shared the highest identity with Takifugu rubripes (EcCD79a 49%, EcCD79b 52%, EcLYN 82% and EcSYK 77%). The conserved features and important functional residues were analyzed. Together with IgM and IgT, tissue distribution analysis showed that all six molecules were mainly expressed in immune organs, particularly systematic immune organs. In groupers infected with Cryptocaryon irritans, up-regulation of EcCD79a and b, EcIgM and EcIgT were not seen in the early stage skin and gill until 14-21 days. Up-regulation of EcCD79a was seen in head kidney at most time points, while EcCD79a and b were only significantly up-regulated in day 14 spleen. Significant up-regulation of EcIgT were seen in day 21 head kidney and day 1, day14 spleen. Significant up-regulation of EcIgM were seen in day 1 head kidney and 12 h spleen. In addition, two protein kinase genes, EcLYN and EcSYK, were up-regulated in the skin at most time points, which suggested that B cells may be activated at the skin local infection site.

Survival of Igα-Deficient Mature B Cells Requires BAFF-R Function

Expression of a functional BCR is essential for the development of mature B cells and has been invoked in the control of their maintenance. To test this maintenance function in a new experimental setting, we used the tamoxifen-inducible mb1-CreER(T2) mouse strain to delete or truncate either the mb-1 gene encoding the BCR signaling subunit Igα or the VDJ segment of the IgH (H chain [HC]). In this system, Cre-mediated deletion of the mb-1 gene is accompanied by expression of a GFP reporter. We found that, although the Igα-deficient mature B cells survive for >20 d in vivo, the HC-deficient or Igα tail-truncated B cell population is short-lived, with the HC-deficient cells displaying signs of an unfolded protein response. We also show that Igα-deficient B cells still respond to the prosurvival factor BAFF in culture and require BAFF-R signaling for their in vivo maintenance. These results suggest that, under certain conditions, the loss of the BCR can be tolerated by mature B cells for some time, whereas HC-deficient B cells, potentially generated by aberrant somatic mutations in the germinal center, are rapidly eliminated.

Novel drug targets for personalized precision medicine in relapsed/refractory diffuse large B-cell lymphoma: a comprehensive review

Diffuse large B-cell lymphoma (DLBCL) is a clinically heterogeneous lymphoid malignancy and the most common subtype of non-Hodgkin's lymphoma in adults, with one of the highest mortality rates in most developed areas of the world. More than half of DLBLC patients can be cured with standard R-CHOP regimens, however approximately 30 to 40 % of patients will develop relapsed/refractory disease that remains a major cause of morbidity and mortality due to the limited therapeutic options.Recent advances in gene expression profiling have led to the identification of at least three distinct molecular subtypes of DLBCL: a germinal center B cell-like subtype, an activated B cell-like subtype, and a primary mediastinal B-cell lymphoma subtype. Moreover, recent findings have not only increased our understanding of the molecular basis of chemotherapy resistance but have also helped identify molecular subsets of DLBCL and rational targets for drug interventions that may allow for subtype/subset-specific molecularly targeted precision medicine and personalized combinations to both prevent and treat relapsed/refractory DLBCL. Novel agents such as lenalidomide, ibrutinib, bortezomib, CC-122, epratuzumab or pidilizumab used as single-agent or in combination with (rituximab-based) chemotherapy have already demonstrated promising activity in patients with relapsed/refractory DLBCL. Several novel potential drug targets have been recently identified such as the BET bromodomain protein (BRD)-4, phosphoribosyl-pyrophosphate synthetase (PRPS)-2, macrodomain-containing mono-ADP-ribosyltransferase (ARTD)-9 (also known as PARP9), deltex-3-like E3 ubiquitin ligase (DTX3L) (also known as BBAP), NF-kappaB inducing kinase (NIK) and transforming growth factor beta receptor (TGFβR).This review highlights the new insights into the molecular basis of relapsed/refractory DLBCL and summarizes the most promising drug targets and experimental treatments for relapsed/refractory DLBCL, including the use of novel agents such as lenalidomide, ibrutinib, bortezomib, pidilizumab, epratuzumab, brentuximab-vedotin or CAR T cells, dual inhibitors, as well as mechanism-based combinatorial experimental therapies. We also provide a comprehensive and updated list of current drugs, drug targets and preclinical and clinical experimental studies in DLBCL. A special focus is given on STAT1, ARTD9, DTX3L and ARTD8 (also known as PARP14) as novel potential drug targets in distinct molecular subsets of DLBCL.

Expression of the B-cell receptor component CD79a on immature myeloid cells contributes to their tumor promoting effects

The role of myeloid derived suppressor cells (MDSCs) in promoting tumorigenesis is well-established, and significant effort is being made to further characterize surface markers on MDSCs both for better diagnosis and as potential targets for therapy. Here we show that the B cell receptor adaptor molecule CD79a is unexpectedly expressed on immature bone marrow myeloid cells, and is upregulated on MDSCs generated in multiple different mouse models of metastatic but not non-metastatic cancer. CD79a on MDSCs is upregulated and activated in response to soluble factors secreted by tumor cells. Activation of CD79a on mouse MDSCs, by crosslinking with a specific antibody, maintained their immature phenotype (CD11b+Gr1+), enhanced their migration, increased their suppressive effect on T cell proliferation, and increased secretion of pro-tumorigenic cytokines such as IL-6 and CCL22. Furthermore, crosslinking CD79a on myeloid cells activated signaling through Syk, BLNK, ERK and STAT3 phosphorylation. In vivo, CD79+ myeloid cells showed enhanced ability to promote primary tumor growth and metastasis. Finally we demonstrate that CD79a is upregulated on circulating myeloid cells from lung cancer patients, and that CD79a+ myeloid cells infiltrate human breast tumors. We propose that CD79a plays a functional role in the tumor promoting effects of myeloid cells, and may represent a novel target for cancer therapy.

Association of reactive oxygen species-mediated signal transduction with in vitro apoptosis sensitivity in chronic lymphocytic leukemia B cells

BACKGROUND

Chronic lymphocytic leukemia (CLL) is a B cell malignancy with a variable clinical course and unpredictable response to therapeutic agents. Single cell network profiling (SCNP) utilizing flow cytometry measures alterations in signaling biology in the context of molecular changes occurring in malignancies. In this study SCNP was used to identify proteomic profiles associated with in vitro apoptotic responsiveness of CLL B cells to fludarabine, as a basis for ultimately linking these with clinical outcome.

METHODOLOGY/PRINCIPAL FINDING

SCNP was used to quantify modulated-signaling of B cell receptor (BCR) network proteins and in vitro F-ara-A mediated apoptosis in 23 CLL samples. Of the modulators studied the reactive oxygen species, hydrogen peroxide (H₂O₂), a known intracellular second messenger and a general tyrosine phosphatase inhibitor stratified CLL samples into two sub-groups based on the percentage of B cells in a CLL sample with increased phosphorylation of BCR network proteins. Separately, in the same patient samples, in vitro exposure to F-ara-A also identified two sub-groups with B cells showing competence or refractoriness to apoptotic induction. Statistical analysis showed that in vitro F-ara-A apoptotic proficiency was highly associated with the proficiency of CLL B cells to undergo H₂O₂-augmented signaling.

CONCLUSIONS/SIGNIFICANCE

This linkage in CLL B cells among the mechanisms governing chemotherapy-induced apoptosis increased signaling of BCR network proteins and a likely role of phosphatase activity suggests a means of stratifying patients for their response to F-ara-A based regimens. Future studies will examine the clinical applicability of these findings and also the utility of this approach in relating mechanism to function of therapeutic agents.

Downregulation of the B-cell receptor signaling component CD79b in plasma cell myeloma: a possible post transcriptional regulation

The CD79 molecule, encoded by the CD79a and CD79b genes, is a signaling unit of the B-cell receptor complex, which transmits signals of B-cell activation, growth, and differentiation. They are B-cell-specific and expressed at most stages of B-cell development. Although plasma cells have been believed to lack these gene products, the regulation of CD79 expression in plasma cells is still controversial. In particular, the regulation of CD79b expression remains unclear. We sought to examine CD79b expression in normal and neoplastic plasma cells by immunohistochemical analysis. Out of the 23 clinical samples and 11 cell lines of plasma cell myeloma (PCM), none of the clinical samples and only 1 of 11 cell lines expressed CD79b immunohistologically, whereas non-neoplastic plasma cells in reactive hyperplastic lymph nodes exhibited loss of CD79b protein expression. This finding is quite different from our previous report on CD79a. Not only immunocytochemistry, but also RT-PCR and Western blot analysis of PCM cell lines gave identical results. Interestingly, we detected mRNA transcripts of CD79b in PCM cell lines, although protein translation was lacking. These findings suggest that expression of CD79b is downregulated in both plasma cells and plasma cell myeloma, and this process is possibly under post transcriptional regulation.

IFN regulatory factor 8 restricts the size of the marginal zone and follicular B cell pools

Transcriptional control of marginal zone (MZ) and follicular (FO) B cell development remains incompletely understood. The transcription factor, IFN regulatory factor (IRF)8, is known to play important roles in the differentiation of early B cells. In this article, we demonstrate that IRF8 is also required for normal development of MZ and FO B cells. Mice with a conventional knockout of Irf8 (IRF8(-/-)) or a point mutation in the IRF association domain of IRF8 had increased numbers of MZ B cells. To determine the B cell-intrinsic effects of IRF8 deficiency, we generated mice with a conditional allele of Irf8 crossed with CD19-Cre mice (designated IRF8-conditional knockout [CKO]). These mice had enlarged MZ and increased numbers of MZ and FO B cells compared with controls. The FO B cells of CKO mice exhibited reduced expression of CD23 and moderately increased expression of CD21. Gene-expression profiling showed that increased B cell production in IRF8-CKO mice was associated with changes in expression of genes involved in regulation of transcription, signaling, and inflammation. Functional studies showed that IRF8-CKO mice generated normal Ab responses to T-independent and T-dependent Ags. Thus, IRF8 controls the expansion and maturation of MZ and FO B cells but has little effect on B cell function.

Differential regulation of TLR4 expression in human B cells and monocytes

Toll-like receptor 4 (TLR4) is an innate immune receptor that is constitutively and inducibly activated in monocytes. Although TLR4 is expressed at very low levels on human B cells from healthy individuals, recent reports showed that TLR4 expression and function is elevated in B cells from inflammatory disease patients. New data showed that TLR4 expression on B cells is increased upon stimulation through surface Igμ and CD40 in combination with IL-4. In contrast, monocyte stimulation through CD40 and IL-4 receptors decreased TLR4 surface expression. Analysis of molecular signatures of TLR4 activation in stimulated B cells suggested that TLR4 is regulated by different mechanisms in B cells compared to monocytes. PU.1 and interferon regulatory factor association with the TLR4 promoter are sufficient for TLR4 transcription, but are not sufficient for surface TLR4 expression on B cells. In contrast, the PU.1/IRF combination is sufficient for surface TLR4 expression on monocytes. These data identify mechanisms that can activate B cell TLR4 expression in inflammatory disease patients, and demonstrate that B cells have additional layers of TLR4 regulation absent in monocytes.

Analysis of Syk expression in bovine lymphoma and persistent lymphocytosis induced by bovine leukemia virus

Spleen tyrosine kinase (Syk) is closely related to various cell reactions. In B-cells, Syk is involved in early B-cell receptor signaling, which affects cellular survival, proliferation and differentiation. Although the kinetics of Syk mRNA and its activity are variable in different types of tumor cells, Syk may have a relation to tumor progression in many human tumors, including B-cell lymphoma/leukemia. In this study we examined whether Syk mRNA expression was changed in bovine leukemia virus (BLV)-induced persistent lymphocytosis (PL) and lymphoma. As a result, we demonstrated that the Syk mRNA expression was significantly increased in PL samples, whereas it was decreased in tumor samples. Moreover one cow, which Syk mRNA expression has been lowest among PL cattle, developed lymphoma three months later and the expression significantly decreased. These data suggest that Syk mRNA expression dynamics is closely related to BLV-induced disease.

B cell depletion with anti-CD79 mAbs ameliorates autoimmune disease in MRL/lpr mice

MRL/lpr mice develop a spontaneous systemic lupus erythematosus-like autoimmune syndrome due to a dysfunctional Fas receptor, with contributions from other less well-defined genetic loci. The removal of B cells by genetic manipulation not only prevents autoantibody formation, but it also results in substantially reduced T cell activation and kidney inflammation. To determine whether B cell depletion by administration of Abs is effective in lupus mice with an intact immune system and established disease, we screened several B cell-specific mAbs and found that a combination of anti-CD79alpha and anti-CD79beta Abs was most effective at depleting B cells in vivo. Anti-CD79 therapy started at 4-5 mo of age in MRL/lpr mice significantly decreased B cells (B220(+)CD19(+)) in peripheral blood, bone marrow, and spleens. Treated mice also had a significant increase in the number of both double-negative T cells and naive CD4(+) T cells, and a decreased relative abundance of CD4(+) memory cells. Serum anti-chromatin IgG levels were significantly decreased compared with controls, whereas serum anti-dsDNA IgG, total IgG, or total IgM were unaffected. Overall, survival was improved with lower mean skin scores and significantly fewer focal inflammatory infiltrates in submandibular salivary glands and kidneys. Anti-CD79 mAbs show promise as a potential treatment for systemic lupus erythematosus and as a model for B cell depletion in vivo.

B cell receptor accessory molecules in the channel catfish, Ictalurus punctatus

B cell receptor (BCR) accessory molecules CD79a and CD79b homologs were identified in the channel catfish, Ictalurus punctatus. Both are found as single copy genes that encode proteins containing a signal peptide, an extracellular immunoglobulin domain, a transmembrane region and a cytoplasmic tail containing an immune-receptor tyrosine-dased activation motif (ITAM). IpCD79a and IpCD79b transcripts correlate well with IgM message expression. They are highly expressed in peripheral blood leukocytes (PBL) enriched in membrane (m) IgM+ cells and catfish clonal B cell lines, but not in catfish clonal T cells, indicating that IpCD79a and IpCD79b expression is B cell restricted. Studies using catfish clonal B cells (3B11) transfected with constructs encoding epitope-tagged IpCD79a and IpCD79b revealed that IpCD79a was expressed as a 45 kDa protein and IpCD79b was expressed as a 32 kDa protein. Furthermore, co-immunoprecipitations of epitope-tagged CD79 proteins demonstrate that these molecules are non-covalently associated with mIgM. These data correlate with some of the previous immunoprecipitation data demonstrating that catfish mIgM associates with proteins of 45 and 32 kDa.

Tonic B-cell and viral ITAM signaling: context is everything

The presence of an immunoreceptor tyrosine-based activation motif (ITAM) makes immunoreceptors different from other signaling receptors, like integrins, G-coupled protein receptors, chemokine receptors, and growth factor receptors. This unique motif has the canonical sequence D/Ex(0-2)YxxL/Ix(6-8)YxxL/I, where x represents any amino acid and is present at least once in all immunoreceptor complexes. Immunoreceptors can promote survival, activation, and differentiation by transducing signals through these highly conserved motifs. Traditionally, ITAM signaling is thought to occur in response to ligand-induced aggregation, although evidence indicates that ligand-independent tonic signaling also provides functionally relevant signals. The majority of proteins containing ITAMs are transmembrane proteins that exist as part of immunoreceptor complexes. However, oncogenic viruses also have ITAM-containing proteins. In this review, we discuss what is known about tonic signaling by both cellular and viral ITAM-containing proteins and speculate what we might learn from each context.

Fate decisions regulating bone marrow and peripheral B lymphocyte development

In adult mammals, bone marrow pluripotent hematopoietic stem cells generate B lymphoid-specified progeny that progress through a series of well-characterized stages before generating B-cell receptor expressing B lymphocytes. These functionally immature B lymphocytes then migrate to the spleen wherein they differentiate through transitional stages into follicular or marginal zone B lymphocytes capable of responding to T-dependent and -independent antigens, respectively. During the terminal stages of B lymphocyte development in the bone marrow, as well as immediately following egress into the peripheral compartments, B lymphocytes are counterselected to eliminate B lymphocytes with potentially dangerous self-reactivity. These developmental and selection events in the bone marrow and periphery are dependent on the integration of intrinsic genetic programs with extrinsic microenvironmental signals that drive progenitors toward increasing B lineage commitment and maturation. This chapter provides a comprehensive overview of the various stages of primary and secondary B lymphocyte development with an emphasis on the selection processes that affect decisions at critical checkpoints. Our intent is to stress the concept that at many steps in the developmental process leading to a mature immunocompetent B lymphocyte, B lineage cells are integrating multiple and different signaling inputs that are translated into specific and appropriate cell fate decisions.