Pathway-selective suppression of chemokine receptor signaling in B cells by LPS through downregulation of PLC-β2

Calcium-dependent signalling in B-cell lymphomas

Induced waves of calcium fluxes initiate multiple signalling pathways that play an important role in the differentiation and maturation of B-cells. Finely tuned transient Ca⁺² fluxes from the endoplasmic reticulum in response to B-cell receptor (BCR) or chemokine receptor activation are followed by more sustained calcium influxes from the extracellular environment and contribute to the mechanisms responsible for the proliferation of B-cells, their migration within lymphoid organs and their differentiation. Dysregulation of these well-balanced mechanisms in B-cell lymphomas results in uncontrolled cell proliferation and resistance to apoptosis. Consequently, several cytotoxic drugs (and anti-proliferative compounds) used in standard chemotherapy regimens for the treatment of people with lymphoma target calcium-dependent pathways. Furthermore, ~10% of lymphoma associated mutations are found in genes with functions in calcium-dependent signalling, including those affecting B-cell receptor signalling pathways. In this review, we provide an overview of the Ca²⁺-dependent signalling network and outline the contribution of its key components to B cell lymphomagenesis. We also consider how the oncogenic Epstein-Barr virus, which is causally linked to the pathogenesis of a number of B-cell lymphomas, can modify Ca²⁺-dependent signalling.

Investigation into the Mechanism(s) That Leads to Platelet Decreases in Cynomolgus Monkeys During Administration of ISIS 104838, a 2'-MOE-Modified Antisense Oligonucleotide

ISIS 104838, a 2'-O-methoxyethyl (2'-MOE)-modified antisense oligonucleotide (ASO), causes a moderate, reproducible, dose-dependent, but selflimiting decrease in platelet (PLT) counts in monkeys and humans. To determine the etiology of PLT decrease in cynomolgus monkeys, a 12-week repeat dose toxicology study in 5 cynomolgus monkeys given subcutaneous injections of ISIS 104838 (30-60 mg/kg/week). Monkeys were also injected intravenously with 111Indium(In)-oxine-labeled PLTs to investigate PLT sequestration. In response to continued dosing, PLT counts were decreased by 50%-90% by day 30 in all monkeys. PLT decreases were accompanied by 2- to 4.5-fold increases in immunoglobulin M(IgM), which were typified by a 2- to 5-fold increase in antiplatelet factor 4 (antiPF4) IgM and antiPLT IgM, respectively. Monocyte chemotactic protein 1 increased upon dosing of ISIS 104838, concomitant with a 2- to 6-fold increase in monocyte-derived extracellular vesicles (EVs), indicating monocyte activation but not PLT activation. Despite a 2- to 3-fold increase in von Willebrand factor antigen in all monkeys following ASO administration, only 2 monkeys showed a 2- to 4-fold increase in endothelial EVs. Additionally, a ∼60 - 80%% increase in PLT sequestration in liver and spleen was also observed. Collectively, these results suggest the overall increase in total IgM, antiPLT IgM and/or antiPF4 IgM, in concert with monocyte activation contributed to increased PLT sequestration in spleen and liver, leading to decreased PLTs in peripheral blood.

PLCβ2 negatively regulates the inflammatory response to virus infection by inhibiting phosphoinositide-mediated activation of TAK1

Excessive or uncontrolled release of proinflammatory cytokines caused by severe viral infections often results in host tissue injury or even death. Phospholipase C (PLC)s degrade phosphatidylinositol-4, 5-bisphosphate (PI(4,5)P2) lipids and regulate multiple cellular events. Here, we report that PLCβ2 inhibits the virus-induced expression of pro-inflammatory cytokines by interacting with and inhibiting transforming growth factor-β-activated kinase 1 (TAK1) activation. Mechanistically, PI(4,5)P2 lipids directly interact with TAK1 at W241 and N245, and promote its activation. Impairing of PI(4,5)P2's binding affinity or mutation of PIP2-binding sites on TAK1 abolish its activation and the subsequent production of pro-inflammatory cytokines. Moreover, PLCβ2-deficient mice exhibit increased expression of proinflammatory cytokines and a higher frequency of death in response to virus infection, while the PLCβ2 activator, m-3M3FBS, protects mice from severe Coxsackie virus A 16 (CVA16) infection. Thus, our findings suggest that PLCβ2 negatively regulates virus-induced pro-inflammatory responses by inhibiting phosphoinositide-mediated activation of TAK1.

NALT M cells are important for immune induction for the common mucosal immune system

Nasopharynx-associated lymphoid tissue (NALT) is one of the major constituents of the mucosa-associated lymphoid tissue (MALT), and has the ability to induce antigen-specific immune responses. However, the molecular mechanisms responsible for antigen uptake from the nasal cavity into the NALT remain largely unknown. Immunohistochemical analysis showed that CCL9 and CCL20 were co-localized with glycoprotein 2 (GP2) in the epithelium covering NALT, suggesting the existence of M cells in NALT. In analogy with the reduced number of Peyer's patch M cells in CCR6-deficient mice, the number of NALT M cells was drastically decreased in CCR6-deficient mice compared with the wild-type mice. Translocation of nasally administered Salmonella enterica serovar Typhimurium into NALT via NALT M cells was impaired in CCR6-deficient mice, whereas S. Typhimurium demonstrated consistent co-localization with NALT M cells in wild-type mice. When wild-type mice were nasally administered with an attenuated vaccine strain of S. Typhimurium, the mice were protected from a subsequent challenge with wild-type S. Typhimurium. Antigen-specific fecal and nasal IgA was detected after nasal immunization with the attenuated vaccine strain of S. Typhimurium only in wild-type mice but not in CCR6-deficient mice. Taken together, these observations demonstrate that NALT M cells are important as a first line of defense against infection by enabling activation of the common mucosal immune system (CMIS).

The Phosphorylation of CCR6 on Distinct Ser/Thr Residues in the Carboxyl Terminus Differentially Regulates Biological Function

CCR6 is a G protein-coupled receptor (GPCR) that recognizes a single chemokine ligand, CCL20 and is primarily expressed by leukocytes. Upon ligand binding, CCR6 activates Gαi heterotrimeric G proteins to induce various potential cellular outcomes through context-specific cell signaling. It is well known that differential phosphorylation of Ser and Thr residues in the C-terminal domains or intracellular loops of GPCRs can generate barcodes that regulate GPCR function by regulating the recruitment of β-arrestins. In this study, we demonstrate that ligand binding to CCR6 induces receptor phosphorylation at Ser/Thr residues in the C-terminal tail, rather than intracellular loops. Using mutagenesis experiments, we determined that distinct clusters of Ser/Thr residues in the C-terminal domain differentially regulate CCL20-induced signaling and cellular response. Substituting the Thr360/Ser361/Thr363 cluster or the Ser370/Ser371 cluster with Ala residues modulated cellular response upon CCL20 stimulation. Notably, receptor internalization, chemotaxis, F-actin distribution, transient ERK1/2 activation, and β-arrestin 2 recruitment were oppositely affected by mutating the two clusters, suggesting that phosphorylation of CCR6 C-terminal Ser/Thr residues directs the cell signaling response upon receptor activation. Moreover, activated CCR6 weakly recruited β-arrestin 1 in comparison with β-arrestin 2, and the two arrestin proteins seemed to play overlapping but distinct roles in mediating CCL20/CCR6-induced cellular responses. Taken together, the effects of site-specific Ser/Thr phosphorylation on CCR6 demonstrate the existence of barcodes on the protein that dictate the activation of different cell signaling profiles and lead to distinct biological outcomes.

CCR6 Deficiency Impairs IgA Production and Dysregulates Antimicrobial Peptide Production, Altering the Intestinal Flora

Intestinal immunity exists as a complex relationship among immune cells, epithelial cells, and microbiota. CCR6 and its ligand-CCL20 are highly expressed in intestinal mucosal tissues, such as Peyer's patches (PPs) and isolated lymphoid follicles (ILFs). In this study, we investigated the role of the CCR6-CCL20 axis in intestinal immunity under homeostatic conditions. CCR6 deficiency intrinsically affects germinal center reactions in PPs, leading to impairments in IgA class switching, IgA affinity, and IgA memory B cell production and positioning in PPs, suggesting an important role for CCR6 in T-cell-dependent IgA generation. CCR6 deficiency impairs the maturation of ILFs. In these follicles, group 3 innate lymphoid cells are important components and a major source of IL-22, which stimulates intestinal epithelial cells (IECs) to produce antimicrobial peptides (AMPs). We found that CCR6 deficiency reduces IL-22 production, likely due to diminished numbers of group 3 innate lymphoid cells within small-sized ILFs. The reduced IL-22 levels subsequently decrease the production of AMPs, suggesting a critical role for CCR6 in innate intestinal immunity. Finally, we found that CCR6 deficiency impairs the production of IgA and AMPs, leading to increased levels of Alcaligenes in PPs, and segmented filamentous bacteria in IECs. Thus, the CCR6-CCL20 axis plays a crucial role in maintaining intestinal symbiosis by limiting the overgrowth of mucosa-associated commensal bacteria.

Activation of the Ca2+-sensing receptors increases currents through inward rectifier K+ channels via activation of phosphatidylinositol 4-kinase

Inward rectifier K⁺ channels are important for maintaining normal electrical function in many cell types. The proper function of these channels requires the presence of membrane phosphoinositide 4,5-bisphosphate (PIP₂). Stimulation of the Ca²⁺-sensing receptor CaR, a pleiotropic G protein-coupled receptor, activates both G_q/11, which decreases PIP₂, and phosphatidylinositol 4-kinase (PI-4-K), which, conversely, increases PIP₂. How membrane PIP₂ levels are regulated by CaR activation and whether these changes modulate inward rectifier K⁺ are unknown. In this study, we found that activation of CaR by the allosteric agonist, NPSR568, increased inward rectifier K⁺ current (I _K1) in guinea pig ventricular myocytes and currents mediated by Kir2.1 channels exogenously expressed in HEK293T cells with a similar sensitivity. Moreover, using the fluorescent PIP₂ reporter tubby-R332H-cYFP to monitor PIP₂ levels, we found that CaR activation in HEK293T cells increased membrane PIP₂ concentrations. Pharmacological studies showed that both phospholipase C (PLC) and PI-4-K are activated by CaR stimulation with the latter played a dominant role in regulating membrane PIP₂ and, thus, Kir currents. These results provide the first direct evidence that CaR activation upregulates currents through inward rectifier K⁺ channels by accelerating PIP₂ synthesis. The regulation of I _K1 plays a critical role in the stability of the electrical properties of many excitable cells, including cardiac myocytes and neurons. Further, synthetic allosteric modulators that increase CaR activity have been used to treat hyperparathyroidism, and negative CaR modulators are of potential importance in the treatment of osteoporosis. Thus, our results provide further insight into the roles played by CaR in the cardiovascular system and are potentially valuable for heart disease treatment and drug safety.

The Phospholipase Cγ2 Mutants R665W and L845F Identified in Ibrutinib-resistant Chronic Lymphocytic Leukemia Patients Are Hypersensitive to the Rho GTPase Rac2 Protein

Mutations in the gene encoding phospholipase C-γ2 (PLCγ2) have been shown to be associated with resistance to targeted therapy of chronic lymphocytic leukemia (CLL) with the Bruton's tyrosine kinase inhibitor ibrutinib. The fact that two of these mutations, R665W and L845F, imparted upon PLCγ2 an ∼2-3-fold ibrutinib-insensitive increase in the concentration of cytosolic Ca2+ following ligation of the B cell antigen receptor (BCR) led to the assumption that the two mutants exhibit constitutively enhanced intrinsic activity. Here, we show that the two PLCγ2 mutants are strikingly hypersensitive to activation by Rac2 such that even wild-type Rac2 suffices to activate the mutant enzymes upon its introduction into intact cells. Enhanced "basal" activity of PLCγ2 in intact cells is shown using the pharmacologic Rac inhibitor EHT 1864 and the PLCγ2F897Q mutation mediating Rac resistance to be caused by Rac-stimulated rather than by constitutively enhanced PLCγ2 activity. We suggest that R665W and L845F be referred to as allomorphic rather than hypermorphic mutations of PLCG2 Rerouting of the transmembrane signals emanating from BCR and converging on PLCγ2 through Rac in ibrutinib-resistant CLL cells may provide novel drug treatment strategies to overcome ibrutinib resistance mediated by PLCG2 mutations or to prevent its development in ibrutinib-treated CLL patients.

Phosphoproteomic analyses reveal that galectin-1 augments the dynamics of B-cell receptor signaling

B-cell activation is important for mounting humoral immune responses and antibody production. Galectin-1 has multiple regulatory functions in immune cells. However, the effects of galectin-1 modulation and the mechanisms underlying the coordination of B-cell activation are unclear. To address this issue, we applied label-free quantitative phosphoproteomic analysis to investigate the dynamics of galectin-1-induced signaling in comparison with that following anti-IgM treatment. A total of 3247 phosphorylation sites on 1245 proteins were quantified, and 70-80% of the 856 responsive phosphoproteins were commonly activated during various biological functions. The similarity between galectin-1- and anti-IgM-elicited B-cell receptor (BCR) signaling pathways was also revealed. Additionally, the mapping of the 149 BCR-responsive phosphorylation sites provided complementary knowledge of BCR signaling. Compared to anti-IgM induction, the phosphoproteomic profiling of BCR signaling, along with validation by western blot analysis and pharmacological inhibitors, revealed that the activation of Syk, Btk, and PI3K may be dominant in galectin-1-mediated activation. We further demonstrated that the proliferation of antigen-primed B cells was diminished in the absence of galectin-1 in an animal model. Together, these findings provided evidence for a new role and insight into the mechanism of how galectin-1 augments the strength of the immunological synapse by modulating BCR signaling.

BIOLOGICAL SIGNIFICANCE

The current study revealed the first systematic phosphorylation-mediated signaling network and its dynamics in B cell activation. The comparative phosphoproteomic analysis on the dynamics of galectin-1 induced activation profiles not only showed that exogenously added galectin-1 augmented B-cell activation but also revealed its relatively enhanced activation in PI3K pathway. Together with proliferation assay, we further delineated that galectin-1 is important for B-cell proliferation in response to antigen challenge. Our phosphoproteomic study reveals a new role for galectin-1 in augmenting the strength of immunological synapse by modulating BCR signaling.

Phospholipase C-β in immune cells

Great progress has recently been made in structural and functional research of phospholipase C (PLC)-β. We now understand how PLC-β isoforms (β1-β4) are activated by GTP-bound Gαq downstream of G protein-coupled receptors. Numerous studies indicate that PLC-βs participate in the differentiation and activation of immune cells that control both the innate and adaptive immune systems. The PLC-β3 isoform also interplays with tyrosine kinase-based signaling pathways, to inhibit Stat5 activation by recruiting the protein-tyrosine phosphatase SHP-1, with which PLC-β3 and Stat5 form a multi-molecular signaling platform, named SPS complex. The SPS complex has important regulatory roles in tumorigenesis and immune cell activation.

The effects of EBV transformation on gene expression levels and methylation profiles

Epstein-Barr virus (EBV) transformed lymphoblastoid cell lines (LCLs) provide a conveniently accessible and renewable resource for functional genomic studies in humans. The ability to accumulate multidimensional data pertaining to the same individual cell lines, from complete genomic sequences to detailed gene regulatory profiles, further enhances the utility of LCLs as a model system. A lingering concern, however, is that the changes associated with EBV transformation of B cells reduce the usefulness of LCLs as a surrogate model for primary tissues. To evaluate the validity of this concern, we compared global gene expression and methylation profiles between CD20+ primary B cells sampled from six individuals and six independent replicates of transformed LCLs derived from each sample. These data allowed us to obtain a detailed catalog of the genes and pathways whose regulation is affected by EBV transformation. We found that the expression levels and promoter methylation profiles of more than half of the studied genes were affected by the EBV transformation, including enrichments of genes involved in transcription regulation, cell cycle and immune response. However, we show that most of the differences in gene expression levels between LCLs and B cells are of small magnitude, and that LCLs can often recapitulate the naturally occurring gene expression variation in primary B cells. Thus, our observations suggest that inference of the genetic architecture that underlies regulatory variation in LCLs can typically be generalized to primary B cells. In contrast, inference based on functional studies in LCLs may be more limited to the cell lines.