Phospholipase Cgamma2 is essential in the functions of B cell and several Fc receptors

Functions of IgM fc receptor (FcµR) related to autoimmunity

Unlike Fc receptors for switched immunoglobulin (Ig) isotypes, Fc receptor for IgM (FcµR) is selectively expressed by lymphocytes. The ablation of the FcµR gene in mice impairs B cell tolerance as evidenced by concomitant production of autoantibodies of IgM and IgG isotypes. In this essay, we reiterate the autoimmune phenotypes observed in mutant mice, ie IgM homeostasis, dysregulated humoral immune responses including autoantibodies, and Mott cell formation. We also propose the potential phenotypes in individuals with FCMR deficiency and the model for FcµR-mediated regulation of self-reactive B cells.

Glucocorticoids paradoxically promote steroid resistance in B cell acute lymphoblastic leukemia through CXCR4/PLC signaling

Glucocorticoid (GC) resistance in childhood relapsed B-cell acute lymphoblastic leukemia (B-ALL) represents an important challenge. Despite decades of clinical use, the mechanisms underlying resistance remain poorly understood. Here, we report that in B-ALL, GC paradoxically induce their own resistance by activating a phospholipase C (PLC)-mediated cell survival pathway through the chemokine receptor, CXCR4. We identify PLC as aberrantly activated in GC-resistant B-ALL and its inhibition is able to induce cell death by compromising several transcriptional programs. Mechanistically, dexamethasone (Dex) provokes CXCR4 signaling, resulting in the activation of PLC-dependent Ca2+ and protein kinase C signaling pathways, which curtail anticancer activity. Treatment with a CXCR4 antagonist or a PLC inhibitor improves survival of Dex-treated NSG mice in vivo. CXCR4/PLC axis inhibition significantly reverses Dex resistance in B-ALL cell lines (in vitro and in vivo) and cells from Dex resistant ALL patients. Our study identifies how activation of the PLC signalosome in B-ALL by Dex limits the upfront efficacy of this chemotherapeutic agent.

Human PLCG2 haploinsufficiency results in a novel natural killer cell immunodeficiency

BACKGROUND

Although most individuals effectively control herpesvirus infections, some suffer from severe and/or recurrent infections. A subset of these patients possess defects in natural killer (NK) cells, lymphocytes that recognize and lyse herpesvirus-infected cells; however, the genetic etiology is rarely diagnosed. PLCG2 encodes a signaling protein in NK-cell and B-cell signaling. Dominant-negative or gain-of-function variants in PLCG2 cause cold urticaria, antibody deficiency, and autoinflammation. However, loss-of-function variants and haploinsufficiency have not been reported to date.

OBJECTIVES

The investigators aimed to identify the genetic cause of NK-cell immunodeficiency in 2 families and herein describe the functional consequences of 2 novel loss-of-function variants in PLCG2.

METHODS

The investigators employed whole-exome sequencing in conjunction with mass cytometry, microscopy, functional assays, and a mouse model of PLCG2 haploinsufficiency to investigate 2 families with NK-cell immunodeficiency.

RESULTS

The investigators identified novel heterozygous variants in PLCG2 in 2 families with severe and/or recurrent herpesvirus infections. In vitro studies demonstrated that these variants were loss of function due to haploinsufficiency with impaired NK-cell calcium flux and cytotoxicity. In contrast to previous PLCG2 variants, B-cell function remained intact. Plcg2+/- mice also displayed impaired NK-cell function with preserved B-cell function, phenocopying human disease.

CONCLUSIONS

PLCG2 haploinsufficiency represents a distinct syndrome from previous variants characterized by NK-cell immunodeficiency with herpesvirus susceptibility, expanding the spectrum of PLCG2-related disease.

Multi-omics analyses reveal novel effects of PLCγ2 deficiency in the mouse brain

Phospholipase C gamma-2 (PLCγ2) catalyzes the hydrolysis of the membrane phosphatidylinositol-4,5-bisphosphate (PIP2) to form diacylglycerol (DAG) and inositol trisphosphate (IP3), which subsequently feed into numerous downstream signaling pathways. PLCG2 polymorphisms are associated with both reduced and increased risk of Alzheimer's disease (AD) and with longevity. In the brain, PLCG2 is highly expressed in microglia, where it is proposed to regulate phagocytosis, secretion of cytokines/chemokines, cell survival and proliferation. We analyzed the brains of three-month-old PLCγ2 knockout (KO), heterozygous (HET), and wild-type (WT) mice using multiomics approaches, including shotgun lipidomics, proteomics, and gene expression profiling, and immunofluorescence. Lipidomic analyses revealed sex-specific losses of total cerebrum PIP2 and decreasing trends of DAG content in KOs. In addition, PLCγ2 depletion led to significant losses of myelin-specific lipids and decreasing trends of myelin-enriched lipids. Consistent with our lipidomics results, RNA profiling revealed sex-specific changes in the expression levels of several myelin-related genes. Further, consistent with the available literature, gene expression profiling revealed subtle changes on microglia phenotype in mature adult KOs under baseline conditions, suggestive of reduced microglia reactivity. Immunohistochemistry confirmed subtle differences in density of microglia and oligodendrocytes in KOs. Exploratory proteomic pathway analyses revealed changes in KO and HET females compared to WTs, with over-abundant proteins pointing to mTOR signaling, and under-abundant proteins to oligodendrocytes. Overall, our data indicate that loss of PLCγ2 has subtle effects on brain homeostasis that may underlie enhanced vulnerability to AD pathology and aging via novel mechanisms in addition to regulation of microglia function.

Impact of Hemostasis on the Lymphatic System in Development and Disease

Lymphatic vessels form a systemic network that maintains interstitial fluid homeostasis and regulates immune responses and is strictly separated from the circulatory system. During embryonic development, lymphatic endothelial cells originate from blood vascular endothelial cells in the cardinal veins and form lymph sacs. Platelets are critical for separating lymph sacs from the cardinal veins through interactions between CLEC-2 (C-type lectin-like receptor-2) and PDPN (podoplanin) in lymphatic endothelial cells. Therefore, deficiencies of these genes cause blood-filled lymphatic vessels, leading to abnormal lymphatic vessel maturation. The junction between the thoracic duct and the subclavian vein has valves and forms physiological thrombi dependent on CLEC-2/PDPN signaling to prevent blood backflow into the thoracic duct. In addition, platelets regulate lymphangiogenesis and maintain blood/lymphatic separation in pathological conditions, such as wound healing and inflammatory diseases. More recently, it was reported that the entire hemostatic system is involved in lymphangiogenesis. Thus, the hemostatic system plays a crucial role in the establishment, maintenance, and rearrangement of lymphatic networks and contributes to body fluid homeostasis, which suggests that the hemostatic system is a potential target for treating lymphatic disorders. This review comprehensively summarizes the role of the hemostatic system in lymphangiogenesis and lymphatic vessel function and discusses challenges and future perspectives.

Protein restriction impairs the response activation/responsivity of MAPK signaling pathway of hematopoietic stem cells

Protein restriction (PR) leads to bone marrow hypoplasia with changes in stromal cellularity components of the extracellular matrix in hematopoietic stem cells (HSCs). However, the underlying signaling mechanisms are poorly understood. We hypothesize that PR impairs the HSC mitogen-activated protein kinase (MAPK) signaling pathway response activation. Our aim is to evaluate the activation of MAPK and interleukin-3 (IL-3) proteins in HSC to explain PR-induced bone marrow hypoplasia, which causes altered proliferation and differentiation. C57BL/6 male mice were subjected to a low-protein diet (2% protein) or normoproteic (12% protein). PKC, PLCγ2, CaMKII, AKT, STAT3/5, ERK1/2, JNK, and p38d phosphorylation were evaluated by flow cytometry, and GATA1/2, PU.1, C/EBPα, NF-E2, and Ikz-3 genes (mRNAs) assessed by quantitative real-time-polymerase chain reaction. Pathway proteins, such as PLCγ2, JAK2, STAT3/5, PKC, and RAS do not respond to the IL-3 stimulus in PR, leading to lower activation of ERK1/2 and Ca2+ signaling pathways, consequently lowering the production of hematopoietic transcription factors. Colony forming units granulocyte-macrophage and colony forming units macrophage formation are impaired in PR even after being stimulated with IL-3. Long-term hematopoietic stem cells, short-term hematopoietic stem cells, granulocyte myeloid progenitor, and megakaryocyte-erythroid progenitor cells were significantly reduced in PR animals. This study shows for the first time that activation of MAPK pathway key proteins in HSCs is impaired in cases of PR. Several pathway proteins, such as PLCγ2, JAK2, STAT3, PKC, and RAS do not respond to IL-3 stimulation, leading to lower activation of extracellular signal-regulated protein kinase 1/2 and consequently lower production of hematopoietic transcription factors GATA1/2, PU.1, C/EBPa, NF-E2, and Ikz3. These changes result in a reduction in colony-forming units, proliferation, and differentiation, leading to hypocellularity.

Calcineurin-independent NFATc1 signaling is essential for survival of Burkitt lymphoma cells

In Burkitt lymphoma (BL), a tumor of germinal center B cells, the pro-apoptotic properties of MYC are controlled by tonic B cell receptor (BCR) signals. Since BL cells do not exhibit constitutive NF-κB activity, we hypothesized that anti-apoptotic NFATc1 proteins provide a major transcriptional survival signal in BL. Here we show that post-transcriptional mechanisms are responsible for the calcineurin (CN) independent constitutive nuclear over-expression of NFATc1 in BL and Eµ-MYC - induced B cell lymphomas (BCL). Conditional inactivation of the Nfatc1 gene in B cells of Eµ-MYC mice leads to apoptosis of BCL cells in vivo and ex vivo. Inhibition of BCR/SYK/BTK/PI3K signals in BL cells results in cytosolic re-location of NFATc1 and apoptosis. Therefore, NFATc1 activity is an integrated part of tonic BCR signaling and an alternative target for therapeutic intervention in BL.

Activation Mechanisms and Diverse Functions of Mammalian Phospholipase C

Phospholipase C (PLC) plays pivotal roles in regulating various cellular functions by metabolizing phosphatidylinositol 4,5-bisphosphate in the plasma membrane. This process generates two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol, which respectively regulate the intracellular Ca²⁺ levels and protein kinase C activation. In mammals, six classes of typical PLC have been identified and classified based on their structure and activation mechanisms. They all share X and Y domains, which are responsible for enzymatic activity, as well as subtype-specific domains. Furthermore, in addition to typical PLC, atypical PLC with unique structures solely harboring an X domain has been recently discovered. Collectively, seven classes and 16 isozymes of mammalian PLC are known to date. Dysregulation of PLC activity has been implicated in several pathophysiological conditions, including cancer, cardiovascular diseases, and neurological disorders. Therefore, identification of new drug targets that can selectively modulate PLC activity is important. The present review focuses on the structures, activation mechanisms, and physiological functions of mammalian PLC.

CARD19, a Novel Regulator of the TAK1/NF-κB Pathway in Self-Reactive B Cells

The caspase recruitment domain family member (CARD)11-Bcl10-Malt1 signalosome controls TGF-β-activated kinase 1 (TAK1) activation and regulates BCR-induced NF-κB activation. In this study, we discovered that CARD19 interacted with TAK1 and inhibited TAB2-mediated TAK1 ubiquitination and activation. Although CARD19 deficiency in mice did not affect B cell development, it enhanced clonal deletion, receptor editing, and anergy of self-reactive B cells, and it reduced autoantibody production. Mechanistically, CARD19 deficiency increased BCR/TAK1-mediated NF-κB activation, leading to increased expression of transcription factors Egr2/3, as well as the E3 ubiquitin ligases c-Cbl/Cbl-b, which are known inducers of B cell tolerance in self-reactive B cells. RNA sequencing analysis revealed that although CARD19 deficiency did not affect the overall Ag-induced gene expression in naive B cells, it suppressed BCR signaling and increased hyporesponsiveness of self-reactive B cells. As a result, CARD19 deficiency prevented Bm12-induced experimental systemic lupus erythematosus. In summary, CARD19 negatively regulates BCR/TAK1-induced NF-κB activation and its deficiency increases Egr2/3 and c-Cbl/Cbl-b expression in self-reactive B cells, thereby enhancing B cell tolerance.

Carriers of the p.P522R variant in PLCγ2 have a slightly more responsive immune system

BACKGROUND

The rs72824905 single-nucleotide polymorphism in the PLCG2 gene, encoding the p.P522R residue change in Phospholipase C gamma 2 (PLCγ2), associates with protection against several dementia subtypes and with increased likelihood of longevity. Cell lines and animal models indicated that p.P522R is a functional hypermorph. We aimed to confirm this in human circulating peripheral immune cells.

METHODS

We compared effects of p.P522R on immune system function between carriers and non-carriers (aged 59-103y), using in-depth immunophenotyping, functional B-cell and myeloid cell assays, and in vivo SARS-CoV-2 vaccination.

RESULTS

In line with expectations, p.P522R impacts immune cell function only slightly, but it does so across a wide array of immune cell types. Upon B-cell stimulation, we observed increased PLCγ2 phosphorylation and calcium release, suggesting increased B-cell sensitivity upon antigen recognition. Further, p.P522R-carriers had higher numbers of CD20++CD21-CD24+ naive B cells and IgG1+ memory B cells. In myeloid cells, normalized ROS production was higher upon PLCγ2-dependent stimulation. On classical monocytes, CD33 levels were elevated. Furthermore, carriers expressed lower levels of allergy-related FcεRI on several immune cell subsets. Nevertheless, carriers and non-carriers had similar serological responses to SARS-CoV-2 vaccination.

CONCLUSION

The immune system from p.P522R-carriers is slightly more responsive to stimulation than in non-carriers.

Molecular regulation of PLCβ signaling

Phospholipase C (PLC) enzymes convert the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2) into inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 and DAG regulate numerous downstream pathways, eliciting diverse and profound cellular changes and physiological responses. In the six PLC subfamilies in higher eukaryotes, PLCβ is intensively studied due to its prominent role in regulating crucial cellular events underlying many processes including cardiovascular and neuronal signaling, and associated pathological conditions. In addition to GαqGTP, Gβγ generated upon G protein heterotrimer dissociation also regulates PLCβ activity. Here, we not only review how Gβγ directly activates PLCβ, and also extensively modulates Gαq-mediated PLCβ activity, but also provide a structure-function overview of PLC family members. Given that Gαq and PLCβ are oncogenes, and Gβγ shows unique cell-tissue-organ specific expression profiles, Gγ subtype-dependent signaling efficacies, and distinct subcellular activities, this review proposes that Gβγ is a major regulator of Gαq-dependent and independent PLCβ signaling.

Granulomatous inflammation in inborn errors of immunity

Granulomas have been defined as inflammatory infiltrates formed by recruitment of macrophages and T cells. The three-dimensional spherical structure typically consists of a central core of tissue resident macrophages which may merge into multinucleated giant cells surrounded by T cells at the periphery. Granulomas may be triggered by infectious and non-infectious antigens. Cutaneous and visceral granulomas are common in inborn errors of immunity (IEI), particularly among patients with chronic granulomatous disease (CGD), combined immunodeficiency (CID), and common variable immunodeficiency (CVID). The estimated prevalence of granulomas in IEI ranges from 1%-4%. Infectious agents causing granulomas such Mycobacteria and Coccidioides presenting atypically may be 'sentinel' presentations for possible underlying immunodeficiency. Deep sequencing of granulomas in IEI has revealed non-classical antigens such as wild-type and RA27/3 vaccine-strain Rubella virus. Granulomas in IEI are associated with significant morbidity and mortality. The heterogeneity of granuloma presentation in IEI presents challenges for mechanistic approaches to treatment. In this review, we discuss the main infectious triggers for granulomas in IEI and the major forms of IEI presenting with 'idiopathic' non-infectious granulomas. We also discuss models to study granulomatous inflammation and the impact of deep-sequencing technology while searching for infectious triggers of granulomatous inflammation. We summarize the overarching goals of management and highlight the therapeutic options reported for specific granuloma presentations in IEI.

The role of Syk phosphorylation in Fc receptor mediated mIgM+ B lymphocyte phagocytosis in flounder (Paralichthys olivaceus)

Spleen tyrosine kinase (Syk) is a non-receptor protein tyrosine kinase, and it mediates downstream signaling of FcR-mediated immune responses. Our previous work revealed that the expression of Syk was significantly up-regulated in flounder mIgM⁺ B lymphocytes after phagocytosis of antiserum-opsonized Edwardsiella tarda, which suggested Syk might be involved in Ig-opsonized phagocytosis. In this paper, phospho-Syk (pSyk) inhibitor was used to investigate the potential role of phosphorylated Syk in FcR-mediated phagocytosis of IgM⁺ B cells. Indirect immunofluorescence assay (IFA) and Western blotting showed that the level of phosphorylated Syk in the mIgM⁺ B lymphocytes treated with pSyk inhibitor was significantly lower compared to the control group after stimulation with flounder antiserum. Flow cytometry analysis showed that after 3 h incubation with antiserum-opsonized E. tarda, the phagocytosis rates of mIgM⁺ B lymphocytes from peripheral blood, spleen and head kidney pre-treated with pSyk inhibitor were 48.1%, 40.1% and 43.6% respectively, which were significantly lower than that of the control groups with 58.7%, 53.2% and 57.4%, respectively. And likewise, after pSyk inhibitor treatment, the proportions of mIgM⁺ B lymphocytes with higher intracellular reactive oxygen species (ROS) levels in peripheral blood, spleen and head kidney decreased to 15.2%, 12.0% and 12.1% from the control level of 26.5%, 25.9% and 26.3%, respectively. Moreover, the expression of three genes affected by pSyk, including phospholipase Cγ1 (PLCγ1), phospholipase Cγ2 (PLCγ2) and phosphatidylinositol 3 kinase (PI3K) were found to be significantly down-regulated in pSyk inhibitor-treated mIgM⁺ B lymphocytes post phagocytosis. These results suggest that pSyk plays a key role in FcR-mediated phagocytosis and bactericidal activity of mIgM⁺ B lymphocytes, which promotes further understanding of the regulatory role of pSyk in teleost B cells phagocytosis.

The inositol 5-phosphatase INPP5B regulates B cell receptor clustering and signaling

Upon antigen binding, the B cell receptor (BCR) undergoes clustering to form a signalosome that propagates downstream signaling required for normal B cell development and physiology. BCR clustering is dependent on remodeling of the cortical actin network, but the mechanisms that regulate actin remodeling in this context remain poorly defined. In this study, we identify the inositol 5-phosphatase INPP5B as a key regulator of actin remodeling, BCR clustering, and downstream signaling in antigen-stimulated B cells. INPP5B acts via dephosphorylation of the inositol lipid PI(4,5)P2 that in turn is necessary for actin disassembly, BCR mobilization, and cell spreading on immobilized surface antigen. These effects can be explained by increased actin severing by cofilin and loss of actin linking to the plasma membrane by ezrin, both of which are sensitive to INPP5B-dependent PI(4,5)P2 hydrolysis. INPP5B is therefore a new player in BCR signaling and may represent an attractive target for treatment of B cell malignancies caused by aberrant BCR signaling.

Endothelial Phospholipase Cγ2 Improves Outcomes of Diabetic Ischemic Limb Rescue Following VEGF Therapy

Therapeutic vascular endothelial growth factor (VEGF) replenishment has met with limited success for the management of critical limb-threatening ischemia. To improve outcomes of VEGF therapy, we applied single-cell RNA sequencing (scRNA-seq) technology to study the endothelial cells of the human diabetic skin. Single-cell suspensions were generated from the human skin followed by cDNA preparation using the Chromium Next GEM Single-cell 3' Kit v3.1. Using appropriate quality control measures, 36,487 cells were chosen for downstream analysis. scRNA-seq studies identified that although VEGF signaling was not significantly altered in diabetic versus nondiabetic skin, phospholipase Cγ2 (PLCγ2) was downregulated. The significance of PLCγ2 in VEGF-mediated increase in endothelial cell metabolism and function was assessed in cultured human microvascular endothelial cells. In these cells, VEGF enhanced mitochondrial function, as indicated by elevation in oxygen consumption rate and extracellular acidification rate. The VEGF-dependent increase in cell metabolism was blunted in response to PLCγ2 inhibition. Follow-up rescue studies therefore focused on understanding the significance of VEGF therapy in presence or absence of endothelial PLCγ2 in type 1 (streptozotocin-injected) and type 2 (db/db) diabetic ischemic tissue. Nonviral topical tissue nanotransfection technology (TNT) delivery of CDH5 promoter-driven PLCγ2 open reading frame promoted the rescue of hindlimb ischemia in diabetic mice. Improvement of blood flow was also associated with higher abundance of VWF+/CD31+ and VWF+/SMA+ immunohistochemical staining. TNT-based gene delivery was not associated with tissue edema, a commonly noted complication associated with proangiogenic gene therapies. Taken together, our study demonstrates that TNT-mediated delivery of endothelial PLCγ2, as part of combination gene therapy, is effective in diabetic ischemic limb rescue.

A novel likely pathogenic PLCG2 variant in a patient with a recurrent skin blistering disease and B-cell lymphopenia

Pathogenic variants of PLCG2 encoding phospholipase C gamma 2 (PLCγ2) were first reported in 2012 and their clinical manifestations vary widely. PLCG2-associated antibody deficiency and immune dysregulation (PLAID) and autoinflammation and PLCγ2-associated antibody deficiency and immune dysregulation (APLAID) are representative examples of PLCG2 pathogenic variants. In this report, we describe a 17-year-old male with recurrent blistering skin lesions, B-cell lymphopenia, and asthma. Distinct from the patients in previous reports, this patient had the heterozygous de novo c.2119T > C missense variant (NM_002661.4) resulting in a serine to proline amino acid substitution (p.Ser707Pro). The variant located to the PLCγ2 C-terminal Src homology 2 (cSH2) domain, which is a critical site for the restriction of intrinsic enzyme activity. This variant could be classified as "likely pathogenic" according to American College of Medical Genetics and Genomics guidelines. Laboratory results showed a reduction in circulating B cells without a decrease of serum IgG and IgA. Our findings expand the variety of clinical phenotypes for PLCG2 missense variants.

Phospholipase Cγ2 is Essential for Experimental Models of Epidermolysis Bullosa Acquisita

Phospholipase Cγ2 (PLCγ2) mediates tyrosine kinase‒coupled receptor signaling in various hematopoietic lineages. Although PLCγ2 has been implicated in certain human and mouse inflammatory disorders, its contribution to autoimmune and inflammatory skin diseases is poorly understood. In this study, we tested the role of PLCγ2 in a mouse model of epidermolysis bullosa acquisita triggered by antibodies against type VII collagen (C7), a component of the dermo-epidermal junction. PLCγ2-deficient (Plcg2^-/-) mice and bone marrow chimeras with a Plcg2^-/- hematopoietic system were completely protected from signs of anti-C7-induced skin disease, including skin erosions, dermal‒epidermal separation, and inflammation, despite normal circulating levels and skin deposition of anti-C7 antibodies. PLCγ2 was required for the tissue infiltration of neutrophils, eosinophils, and monocytes/macrophages as well as for the accumulation of proinflammatory mediators (including IL-1β, MIP-2, and LTB₄) and reactive oxygen species. Mechanistic experiments revealed a role for PLCγ2 in the release of proinflammatory mediators and reactive oxygen species but not in the intrinsic migratory capacity of leukocytes. The phospholipase C inhibitor U73122 inhibited dermal-epidermal separation of human skin sections incubated with human neutrophils in the presence of anti-C7 antibodies. Taken together, our results suggest a critical role for PLCγ2 in the pathogenesis of the inflammatory form of epidermolysis bullosa acquisita.

PLCγ2 regulates TREM2 signalling and integrin-mediated adhesion and migration of human iPSC-derived macrophages

Human genetic studies have linked rare coding variants in microglial genes, such as TREM2, and more recently PLCG2 to Alzheimer's disease (AD) pathology. The P522R variant in PLCG2 has been shown to confer protection for AD and to result in a subtle increase in enzymatic activity. PLCγ2 is a key component of intracellular signal transduction networks and induces Ca²⁺ signals downstream of many myeloid cell surface receptors, including TREM2. To explore the relationship between PLCγ2 and TREM2 and the role of PLCγ2 in regulating immune cell function, we generated human induced pluripotent stem cell (iPSC)- derived macrophages from isogenic lines with homozygous PLCG2 knockout (Ko). Stimulating TREM2 signalling using a polyclonal antibody revealed a complete lack of calcium flux and IP1 accumulation in PLCγ2 Ko cells, demonstrating a non-redundant role of PLCγ2 in calcium release downstream of TREM2. Loss of PLCγ2 led to broad changes in expression of several macrophage surface markers and phenotype, including reduced phagocytic activity and survival, while LPS-induced secretion of the inflammatory cytokines TNFα and IL-6 was unaffected. We identified additional deficits in PLCγ2- deficient cells that compromised cellular adhesion and migration. Thus, PLCγ2 is key in enabling divergent cellular functions and might be a promising target to increase beneficial microglial functions.

The role of PLCγ2 in immunological disorders, cancer, and neurodegeneration

Phosphatidylinositol-specific phospholipase Cγ2 (PLCγ2) is a critical signaling molecule activated downstream from a variety of cell surface receptors that contain an intracellular immunoreceptor tyrosine-based activation motif. These receptors recruit kinases such as Syk, BTK, and BLNK to phosphorylate and activate PLCγ2, which then generates 1D-myo-inositol 1,4,5-trisphosphate and diacylglycerol. These well-known second messengers are required for diverse membrane functionality including cellular proliferation, endocytosis, and calcium flux. As a result, PLCγ2 dysfunction is associated with a variety of diseases including cancer, neurodegeneration, and immune disorders. The diverse pathologies associated with PLCγ2 are exemplified by distinct genetic variants. Inherited mutations at this locus cause PLCγ2-associated antibody deficiency and immune dysregulation, in some cases with autoinflammation. Acquired mutations at this locus, which often arise as a result of BTK inhibition to treat chronic lymphocytic leukemia, result in constitutive downstream signaling and lymphocyte proliferation. Finally, a third group of PLCγ2 variants actually has a protective effect in a variety of neurodegenerative disorders, presumably by increased uptake and degradation of deleterious neurological aggregates. Therefore, manipulating PLCγ2 activity either up or down could have therapeutic benefit; however, we require a better understanding of the signaling pathways propagated by these variants before such clinical utility can be realized. Here, we review the signaling roles of PLCγ2 in hematopoietic cells to help understand the effect of mutations driving immune disorders and cancer and extrapolate from this to roles which may relate to protection against neurodegeneration.

Comparative analyses of aging-related genes in long-lived mammals provide insights into natural longevity

Extreme longevity has evolved multiple times during the evolution of mammals, yet its underlying molecular mechanisms remain largely underexplored. Here, we compared the evolution of 115 aging-related genes in 11 long-lived species and 25 mammals with non-increased lifespan (control group) in the hopes of better understanding the common molecular mechanisms behind longevity. We identified 16 unique positively selected genes and 23 rapidly evolving genes in long-lived species, which included nine genes involved in regulating lifespan through the insulin/IGF-1 signaling (IIS) pathway and 11 genes highly enriched in immune-response-related pathways, suggesting that the IIS pathway and immune response play a particularly important role in exceptional mammalian longevity. Interestingly, 11 genes related to cancer progression, including four positively selected genes and seven genes with convergent amino acid changes, were shared by two or more long-lived lineages, indicating that long-lived mammals might have evolved convergent or similar mechanisms of cancer resistance that extended their lifespan. This suggestion was further corroborated by our identification of 12 robust candidates for longevity-related genes closely related to cancer.

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Evolution analyses of 115 aging-related genes exploring natural longevity in mammals

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Positively selected genes & rapidly evolved genes enriched in IIS and immune pathways

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Convergent mutations in genes associated with cancer in long-lived species

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Evolution of longevity through cancer resistance in long-lived mammals

Heightened activation of embryonic megakaryocytes causes aneurysms in the developing brain of mice lacking podoplanin

During early embryonic development in mammals, including humans and mice, megakaryocytes (Mks) first originate from primitive hematopoiesis in the yolk sac. These embryonic Mks (eMks) circulate in the vasculature with unclear function. Herein, we report that podoplanin (PDPN), the ligand of C-type lectin-like receptor (CLEC-2) on Mks/platelets, is temporarily expressed in neural tissue during midgestation in mice. Loss of PDPN or CLEC-2 resulted in aneurysms and spontaneous hemorrhage, specifically in the lower diencephalon during midgestation. Surprisingly, more eMks/platelets had enhanced granule release and localized to the lower diencephalon in mutant mouse embryos than in wild-type littermates before hemorrhage. We found that PDPN counteracted the collagen-1-induced secretion of angiopoietin-1 from fetal Mks, which coincided with enhanced TIE-2 activation in aneurysm-like sprouts of PDPN-deficient embryos. Blocking platelet activation prevented the PDPN-deficient embryo from developing vascular defects. Our data reveal a new role for PDPN in regulating eMk function during midgestation.

The Role of BANK1 in B Cell Signaling and Disease

The B cell scaffold protein with ankyrin repeats (BANK1) is expressed primarily in B cells and with multiple but discrete roles in B cell signaling, including B cell receptor signaling, CD40-related signaling, and Toll-like receptor (TLR) signaling. The gene for BANK1, located in chromosome 4, has been found to contain genetic variants that are associated with several autoimmune diseases and also other complex phenotypes, in particular, with systemic lupus erythematosus. Common genetic variants are associated with changes in BANK1 expression in B cells, while rare variants modify their capacity to bind efferent effectors during signaling. A BANK1-deficient model has shown the importance of BANK1 during TLR7 and TLR9 signaling and has confirmed its role in the disease. Still, much needs to be done to fully understand the function of BANK1, but the main conclusion is that it may be the link between different signaling functions within the B cells and they may act to synergize the various pathways within a cell. With this review, we hope to enhance the interest in this molecule.

Mechanosensation and Mechanotransduction by Lymphatic Endothelial Cells Act as Important Regulators of Lymphatic Development and Function

Our understanding of the function and development of the lymphatic system is expanding rapidly due to the identification of specific molecular markers and the availability of novel genetic approaches. In connection, it has been demonstrated that mechanical forces contribute to the endothelial cell fate commitment and play a critical role in influencing lymphatic endothelial cell shape and alignment by promoting sprouting, development, maturation of the lymphatic network, and coordinating lymphatic valve morphogenesis and the stabilization of lymphatic valves. However, the mechanosignaling and mechanotransduction pathways involved in these processes are poorly understood. Here, we provide an overview of the impact of mechanical forces on lymphatics and summarize the current understanding of the molecular mechanisms involved in the mechanosensation and mechanotransduction by lymphatic endothelial cells. We also discuss how these mechanosensitive pathways affect endothelial cell fate and regulate lymphatic development and function. A better understanding of these mechanisms may provide a deeper insight into the pathophysiology of various diseases associated with impaired lymphatic function, such as lymphedema and may eventually lead to the discovery of novel therapeutic targets for these conditions.

TREM2/PLCγ2 signalling in immune cells: function, structural insight, and potential therapeutic modulation

The central role of the resident innate immune cells of the brain (microglia) in neurodegeneration has become clear over the past few years largely through genome-wide association studies (GWAS), and has rapidly become an active area of research. However, a mechanistic understanding (gene to function) has lagged behind. That is now beginning to change, as exemplified by a number of recent exciting and important reports that provide insight into the function of two key gene products – TREM2 (Triggering Receptor Expressed On Myeloid Cells 2) and PLCγ2 (Phospholipase C gamma2) – in microglia, and their role in neurodegenerative disorders. In this review we explore and discuss these recent advances and the opportunities that they may provide for the development of new therapies.

Monogenic autoinflammatory disorders: Conceptual overview, phenotype, and clinical approach

Autoinflammatory diseases are conditions in which pathogenic inflammation arises primarily through antigen-independent hyperactivation of immune pathways. First recognized just over 2 decades ago, the autoinflammatory disease spectrum has expanded rapidly to include more than 40 distinct monogenic conditions. Related mechanisms contribute to common conditions such as gout and cardiovascular disease. Here, we review the basic concepts underlying the "autoinflammatory revolution" in the understanding of immune-mediated disease and introduce major categories of monogenic autoinflammatory disorders recognized to date, including inflammasomopathies and other IL-1-related conditions, interferonopathies, and disorders of nuclear factor kappa B and/or aberrant TNF activity. We highlight phenotypic presentation as a reflection of pathogenesis and outline a practical approach to the evaluation of patients with suspected autoinflammation.

Neutrophil Phospholipase Cγ2 Drives Autoantibody-Induced Arthritis Through the Generation of the Inflammatory Microenvironment

OBJECTIVE

Gain-of-function mutations and genome-wide association studies have linked phospholipase Cγ2 (PLCγ2) to various inflammatory diseases, including arthritis in humans and mice. PLCγ2-deficient (Plcg2^-/- ) mice are also protected against experimental arthritis. This study was undertaken to test how PLCγ2 triggers autoantibody-induced arthritis in mice.

METHODS

PLCγ2 was deleted from various mouse cellular lineages. Deletion efficacy and specificity were tested by immunoblotting and intracellular flow cytometry. Autoantibody-induced arthritis was triggered by K/BxN serum transfer. The role of neutrophil PLCγ2 was further investigated by analysis of the inflammatory exudate, competitive in vivo migration assays, and in vitro functional studies.

RESULTS

PLCγ2 deficiency in the entire hematopoietic compartment completely blocked autoantibody-induced arthritis. Arthritis development was abrogated by deletion of PLCγ2 from myeloid cells or neutrophils but not from mast cells or platelets. Neutrophil infiltration was reduced in neutrophil-specific PLCγ2-deficient (Plcg2^{Δ PMN} ) mice. However, this was not due to an intrinsic migration defect since Plcg2^{Δ PMN} neutrophils accumulated normally when wild-type cells were also present in mixed bone marrow chimeras. Instead, the Plcg2^{Δ PMN} mutation blocked the accumulation of interleukin-1β, macrophage inflammatory protein 2 (MIP-2), and leukotriene B₄ (LTB₄ ) in synovial tissues and reduced the secondary infiltration of macrophages. These findings were supported by in vitro studies showing normal chemotactic migration but defective immune complex-induced respiratory burst and MIP-2 or LTB₄ release in PLCγ2-deficient neutrophils.

CONCLUSION

Neutrophil PLCγ2 is critical for arthritis development, supposedly through the generation of the inflammatory microenvironment. PLCγ2-expressing neutrophils exert complex indirect effects on other inflammatory cells. PLCγ2-targeted therapies may provide particular benefit in inflammatory diseases with a major neutrophil component.

A Versatile Protocol to Quantify BCR-mediated Phosphorylation in Human and Murine B Cell Subpopulations

Signal transduction is the process by which molecular signals are transmitted from the cell surface to its interior, resulting in functional changes inside the cell. B cell receptor (BCR) signaling is of crucial importance for B cells, as it regulates their differentiation, selection, survival, cellular activation and proliferation. Upon BCR engagement by antigen several protein kinases, lipases and linker molecules become phosphorylated. Phosphoflow cytometry (phosphoflow) is a flow cytometry-based method allowing for analysis of protein phosphorylation in single cells. Due to recent advances in methodology and antibody availability - together with the relatively easy quantification of phosphorylation - phosphoflow is increasingly and more commonly used, compared to classical western blot analysis. It can however be challenging to set-up a method that works for all targets of interest. Here, we present a step-by-step phosphoflow protocol allowing the evaluation of the phosphorylation status of signaling molecules in conjunction with extensive staining to identify various human and murine B cell subpopulations, as was previously published in the original paper by Rip et al. (2020). Next to a description of phosphoflow targets from the original paper, we provide directions on additional targets that play a pivotal role in BCR signaling. The step-by-step phosphoflow protocol is user-friendly and provides sensitive detection of phosphorylation of various BCR signaling molecules in human and murine B cell subpopulations.

Use of Toll-Like Receptor (TLR) Ligation to Characterize Human Regulatory B-Cells Subsets

Toll-like receptors (TLRs) are pattern recognition receptors (PRRs), which constitute key components in the recognition of pathogens, thereby initiating innate immune responses and promoting adaptive immune responses. In B cells, TLR ligation is important for their activation and, together with CD40, for their differentiation. TLR ligands are also strong promoters of regulatory B (Breg)-cell development, by enhancing the production of IL-10 and their capacity to induce tolerance. In inflammatory diseases, such as autoimmunity or allergies, Breg-cell function is often impaired, while in chronic infections, such as with helminths, or cancer, Breg-cell function is boosted. Following pathogen exposure, B cells can respond directly by producing cytokines and/or IgM (innate response) and develop into various memory B (Bmem)-cell subsets with class-switched immunoglobulin receptors. Depending on the disease state or chronic infection conditions, various Breg subsets can be recognized as well. Currently, a large array of surface markers is known to distinguish between these large range of B-cell subsets. In recent years, the development of mass cytometers and spectral flow cytometry has allowed for high-dimensional detection of up to 48 markers, including both surface and intracellular/intranuclear markers. Therefore, this novel technology is highly suitable to provide a comprehensive overview of Bmem/Breg-cell subsets in different disease states and/or in clinical intervention trials. Here, we provide detailed instructions of the steps necessary to obtain high-quality data for high-dimensional analysis of multiple human Breg-cell subsets using various TLR ligands.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Kinase inhibition in autoimmunity and inflammation

Despite recent advances in the treatment of autoimmune and inflammatory diseases, unmet medical needs in some areas still exist. One of the main therapeutic approaches to alleviate dysregulated inflammation has been to target the activity of kinases that regulate production of inflammatory mediators. Small-molecule kinase inhibitors have the potential for broad efficacy, convenience and tissue penetrance, and thus often offer important advantages over biologics. However, designing kinase inhibitors with target selectivity and minimal off-target effects can be challenging. Nevertheless, immense progress has been made in advancing kinase inhibitors with desirable drug-like properties into the clinic, including inhibitors of JAKs, IRAK4, RIPKs, BTK, SYK and TPL2. This Review will address the latest discoveries around kinase inhibitors with an emphasis on clinically validated autoimmunity and inflammatory pathways.

Unmet medical needs in the treatment of autoimmune and inflammatory diseases still exist. This Review discusses the activity of kinases that regulate production of inflammatory mediators and the recent advances in developing inhibitors to target such kinases.

Severe Autoinflammatory Manifestations and Antibody Deficiency Due to Novel Hypermorphic PLCG2 Mutations

Autoinflammatory diseases (AIDs) were first described as clinical disorders characterized by recurrent episodes of seemingly unprovoked sterile inflammation. In the past few years, the identification of novel AIDs expanded their phenotypes toward more complex clinical pictures associating vasculopathy, autoimmunity, or immunodeficiency. Herein, we describe two unrelated patients suffering since the neonatal period from a complex disease mainly characterized by severe sterile inflammation, recurrent bacterial infections, and marked humoral immunodeficiency. Whole-exome sequencing detected a novel, de novo heterozygous PLCG2 variant in each patient (p.Ala708Pro and p.Leu845_Leu848del). A clear enhanced PLCγ2 activity for both variants was demonstrated by both ex vivo calcium responses of the patient's B cells to IgM stimulation and in vitro assessment of PLC activity. These data supported the autoinflammation and PLCγ2-associated antibody deficiency and immune dysregulation (APLAID) diagnosis in both patients. Immunological evaluation revealed a severe decrease of immunoglobulins and B cells, especially class-switched memory B cells, with normal T and NK cell counts. Analysis of bone marrow of one patient revealed a reduced immature B cell fraction compared with controls. Additional investigations showed that both PLCG2 variants activate the NLRP3-inflammasome through the alternative pathway instead of the canonical pathway. Collectively, the evidences here shown expand APLAID diversity toward more severe phenotypes than previously reported including dominantly inherited agammaglobulinemia, add novel data about its genetic basis, and implicate the alternative NLRP3-inflammasome activation pathway in the basis of sterile inflammation.

Phosphatidylinositol(4,5)bisphosphate: diverse functions at the plasma membrane

Phosphatidylinositol(4,5) bisphosphate (PI(4,5)P2) has become a major focus in biochemistry, cell biology and physiology owing to its diverse functions at the plasma membrane. As a result, the functions of PI(4,5)P2 can be explored in two separate and distinct roles - as a substrate for phospholipase C (PLC) and phosphoinositide 3-kinase (PI3K) and as a primary messenger, each having unique properties. Thus PI(4,5)P2 makes contributions in both signal transduction and cellular processes including actin cytoskeleton dynamics, membrane dynamics and ion channel regulation. Signalling through plasma membrane G-protein coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and immune receptors all use PI(4,5)P2 as a substrate to make second messengers. Activation of PI3K generates PI(3,4,5)P3 (phosphatidylinositol(3,4,5)trisphosphate), a lipid that recruits a plethora of proteins with pleckstrin homology (PH) domains to the plasma membrane to regulate multiple aspects of cellular function. In contrast, PLC activation results in the hydrolysis of PI(4,5)P2 to generate the second messengers, diacylglycerol (DAG), an activator of protein kinase C and inositol(1,4,5)trisphosphate (IP3/I(1,4,5)P3) which facilitates an increase in intracellular Ca2+. Decreases in PI(4,5)P2 by PLC also impact on functions that are dependent on the intact lipid and therefore endocytosis, actin dynamics and ion channel regulation are subject to control. Spatial organisation of PI(4,5)P2 in nanodomains at the membrane allows for these multiple processes to occur concurrently.

Hemostasis vs. homeostasis: Platelets are essential for preserving vascular barrier function in the absence of injury or inflammation

Platelets are best known for their vasoprotective responses to injury and inflammation. Here, we have asked whether they also support vascular integrity when neither injury nor inflammation is present. Changes in vascular barrier function in dermal and meningeal vessels were measured in real time in mouse models using the differential extravasation of fluorescent tracers as a biomarker. Severe thrombocytopenia produced by two distinct methods caused increased extravasation of 40-kDa dextran from capillaries and postcapillary venules but had no effect on extravasation of 70-kDa dextran or albumin. This reduction in barrier function required more than 4 h to emerge after thrombocytopenia was established, reverting to normal as the platelet count recovered. Barrier dysfunction was also observed in mice that lacked platelet-dense granules, dense granule secretion machinery, glycoprotein (GP) VI, or the GPVI signaling effector phospholipase C (PLC) γ2. It did not occur in mice lacking α-granules, C type lectin receptor-2 (CLEC-2), or protease activated receptor 4 (PAR4). Notably, although both meningeal and dermal vessels were affected, intracerebral vessels, which are known for their tighter junctions between endothelial cells, were not. Collectively, these observations 1) highlight a role for platelets in maintaining vascular homeostasis in the absence of injury or inflammation, 2) provide a sensitive biomarker for detecting changes in platelet-dependent barrier function, 3) identify which platelet processes are required, and 4) suggest that the absence of competent platelets causes changes in the vessel wall itself, accounting for the time required for dysfunction to emerge.

BCALM (AC099524.1) Is a Human B Lymphocyte-Specific Long Noncoding RNA That Modulates B Cell Receptor-Mediated Calcium Signaling

Of the thousands of long noncoding RNAs (lncRNA) identified in lymphocytes, very few have defined functions. In this study, we report the discovery and functional elucidation of a human B cell-specific lncRNA with high levels of expression in three types of B cell cancer and normal B cells. The AC099524.1 gene is upstream of the gene encoding the B cell-specific phospholipase C γ 2 (PLCG2), a B cell-specific enzyme that stimulates intracellular Ca2+ signaling in response to BCR activation. AC099524.1 (B cell-associated lncRNA modulator of BCR-mediated Ca+ signaling [BCALM]) transcripts are localized in the cytoplasm and, as expected, CRISPR/Cas9 knockout of AC099524.1 did not affect PLCG2 mRNA or protein expression. lncRNA interactome, RNA immunoprecipitation, and coimmunoprecipitation studies identified BCALM-interacting proteins in B cells, including phospholipase D 1 (PLD1), and kinase adaptor proteins AKAP9 (AKAP450) and AKAP13 (AKAP-Lbc). These two AKAP proteins form signaling complexes containing protein kinases A and C, which phosphorylate and activate PLD1 to produce phosphatidic acid (PA). BCR stimulation of BCALM-deficient B cells resulted in decreased PLD1 phosphorylation and increased intracellular Ca+ flux relative to wild-type cells. These results suggest that BCALM promotes negative feedback that downmodulates BCR-mediated Ca+ signaling by promoting phosphorylation of PLD1 by AKAP-associated kinases, enhancing production of PA. PA activates SHP-1, which negatively regulates BCR signaling. We propose the name BCALM for B-Cell Associated LncRNA Modulator of BCR-mediated Ca+ signaling. Our findings suggest a new, to our knowledge, paradigm for lncRNA-mediated modulation of lymphocyte activation and signaling, with implications for B cell immune response and BCR-dependent cancers.

Phosphoflow Protocol for Signaling Studies in Human and Murine B Cell Subpopulations

BCR signaling, involving phosphorylation of various downstream molecules, including kinases, lipases, and linkers, is crucial for B cell selection, survival, proliferation, and differentiation. Phosphoflow cytometry (phosphoflow) is a single-cell-based technique to measure phosphorylated intracellular proteins, providing a more quantitative read-out than Western blotting. Recent advances in phosphoflow basically allow simultaneous analysis of protein phosphorylation in B cell (sub)populations, without prior cell sorting. However, fixation and permeabilization procedures required for phosphoflow often affect cell surface epitopes or mAb conjugates, precluding the evaluation of the phosphorylation status of signaling proteins across different B cell subpopulations present in a single sample. In this study, we report a versatile phosphoflow protocol allowing extensive staining of B cell subpopulations in human peripheral blood or various anatomical compartments in the mouse, starting from freshly isolated or frozen cell suspensions. Both human and mouse B cell subpopulations showed different basal and BCR stimulation-induced phosphorylation levels of downstream signaling proteins. For example, peritoneal B-1 cells and splenic marginal zone B cells exhibited significantly increased basal (ex vivo) signaling and increased responsiveness to in vitro BCR stimulation compared with peritoneal B-2 cells and splenic follicular B cells, respectively. In addition, whereas stimulation with anti-IgM or anti-Igκ L chain Abs resulted in strong pCD79a and pPLCγ2 signals, IgD stimulation only induced CD79a but not pPLCγ2 phosphorylation. In summary, the protocol is user friendly and quantifies BCR-mediated phosphorylation with high sensitivity at the single-cell level, in combination with extensive staining to identify individual B cell development and differentiation stages.

RNA-binding protein Ptbp1 is essential for BCR-mediated antibody production

The RNA-binding protein polypyrimidine tract-binding protein-1 (Ptbp1) binds to the pyrimidine-rich sequence of target RNA and controls gene expression via post-transcriptional regulation such as alternative splicing. Although Ptbp1 is highly expressed in B lymphocytes, its role to date is largely unknown. To clarify the role of Ptbp1 in B-cell development and function, we generated B-cell-specific Ptbp1-deficient (P1BKO) mice. B-cell development in the bone marrow, spleen and peritoneal cavity of the P1BKO mice was nearly normal. However, the P1BKO mice had significantly lower levels of natural antibodies in serum compared with those of the control mice. To investigate the effect of Ptbp1 deficiency on the immune response in vivo, we immunized the P1BKO mice with T-cell-independent type-2 (TI-2) antigen NP-Ficoll and T-cell-dependent (TD) antigen NP-CGG. We found that B-cell-specific Ptbp1 deficiency causes an immunodeficiency phenotype due to defective production of antibody against both TI-2 and TD antigen. This immunodeficiency was accompanied by impaired B-cell receptor (BCR)-mediated B-cell activation and plasmablast generation. These findings demonstrate that Ptbp1 is essential for the humoral immune response.

Lymph Flow Induces the Postnatal Formation of Mature and Functional Meningeal Lymphatic Vessels

Recently, the presence of lymphatics has been demonstrated and characterized in the dura mater, which is in contrast to the well-accepted view indicating the lack of a classical lymphatic drainage system of the central nervous system (CNS). Moreover, the role of meningeal lymphatics in the pathogenesis of Alzheimer's disease and multiple sclerosis was suggested. However, the possible regulators of the developmental program and function of meningeal lymphatics remain unclear. Here, we aimed at characterizing the lymph flow dependence of the developmental program and function of the meningeal lymphatics. First, we demonstrated that lymphatics present in the dura mater are involved in the uptake and transport of macromolecules from the CNS. Meningeal lymphatics develop during the postnatal period which process involves the maturation of the vessels. The formation of mature meningeal lymphatics coincides with the increase of the drainage of macromolecules from the CNS to the deep cervical lymph nodes. Importantly, the structural remodeling and maturation of meningeal lymphatics is impaired in Plcγ2^−/− mice with reduced lymph flow. Furthermore, macromolecule uptake and transport by the meningeal lymphatics are also affected in Plcγ2^−/− mice. Collectively, lymph flow-induced mechanical forces are required for the postnatal formation of mature and functional meningeal lymphatic vessels. Defining lymph flow-dependence of the development and function of meningeal lymphatics may lead to better understanding of the pathogenesis of neurological diseases including Alzheimer's disease and multiple sclerosis.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate various functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature on PLC. In addition, references have been provided for more in-depth information regarding areas that are not discussed including tyrosine kinase activation of PLC. Understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the physiological roles of these enzymes in the development of diseases and the maintenance of homeostasis.

A Germline Mutation in the C2 Domain of PLCγ2 Associated with Gain-of-Function Expands the Phenotype for PLCG2-Related Diseases

We report three new cases of a germline heterozygous gain-of-function missense (p.(Met1141Lys)) mutation in the C2 domain of phospholipase C gamma 2 (PLCG2) associated with symptoms consistent with previously described auto-inflammation and phospholipase Cγ2 (PLCγ2)-associated antibody deficiency and immune dysregulation (APLAID) syndrome and pediatric common variable immunodeficiency (CVID). Functional evaluation showed platelet hyper-reactivity, increased B cell receptor-triggered calcium influx and ERK phosphorylation. Expression of the altered p.(Met1141Lys) variant in a PLCγ2-knockout DT40 cell line showed clearly enhanced BCR-triggered influx of external calcium when compared to control-transfected cells. Our results further expand the molecular basis of pediatric CVID and phenotypic spectrum of PLCγ2-related defects.

Differential use of BTK and PLC in FcεRI- and KIT-mediated mast cell activation: A marginal role of BTK upon KIT activation

In mast cells (MCs), the TEC family kinase (TFK) BTK constitutes a central regulator of antigen (Ag)-triggered, FcεRI-mediated PLCγ phosphorylation, Ca²⁺ mobilization, degranulation, and pro-inflammatory cytokine production. Less is known about the function of BTK in the context of stem cell factor (SCF)-induced KIT signaling. In bone marrow-derived MCs (BMMCs), Ag stimulation caused intense phosphorylation of BTK at Y551 in its active center and at Y223 in its SH3-domain, whereas in response to SCF only Y223 was significantly phosphorylated. Further data using the TFK inhibitor Ibrutinib indicated that BTK Y223 is phosphorylated by a non-BTK TFK upon SCF stimulation. In line, SCF-induced PLCγ1 phosphorylation was stronger attenuated by Ibrutinib than by BTK deficiency. Subsequent pharmacological analysis of PLCγ function revealed a total block of SCF-induced Ca²⁺ mobilization by PLC inhibition, whereas only the sustained phase of Ca²⁺ flux was curtailed in Ag-stimulated BMMCs. Despite this severe stimulus-dependent difference in inducing Ca²⁺ mobilization, PLCγ inhibition suppressed Ag- and SCF-induced degranulation and pro-inflammatory cytokine production to comparable extents, suggesting involvement of additional TFK(s) or PLCγ-dependent signaling components. In addition to PLCγ, the MAPKs p38 and JNK were activated by Ag in a BTK-dependent manner; this was not observed upon SCF stimulation. Hence, FcεRI and KIT employ different mechanisms for activating PLCγ, p38, and JNK, which might strengthen their cooperation regarding pro-inflammatory MC effector functions. Importantly, our data clearly demonstrate that analyzing BTK Y223 phosphorylation is not sufficient to prove BTK activation.

Docosahexaenoyl ethanolamide mitigates IgE-mediated allergic reactions by inhibiting mast cell degranulation and regulating allergy-related immune cells

Docosahexaenoic acid (DHA) is a long-chain polyunsaturated fatty acid mainly found in fish oil. Although several studies have suggested that it can alleviate allergy symptoms, its mechanism of action remains to be elucidated. In the present study, we found that docosahexaenoyl ethanolamide (DHEA), a metabolite of DHA produced in the human body, exerts the anti-allergic activity in vitro and in vivo. DHEA suppressed degranulation of rat basophilic leukemia RBL-2H3 cells and bone marrow-derived mast cells in a dose-dependent manner without cytotoxicity. This occurred due to a decrease in Ca²⁺ influx, which is critical for mast cell degranulation. DHEA also suppressed IgE-mediated passive cutaneous anaphylaxis reaction in mice. In addition, DHEA was demonstrated to lessen an allergic symptom in a mouse model of pollinosis and to alter the production of IgE and cytokines secreted by splenocytes collected from the pollinosis mice. Taken together, this study indicates that DHEA is a promising anti-allergic agent as it inhibits mast cell degranulation and modulates other immune cells.

Inositol polyphosphates promote T cell-independent humoral immunity via the regulation of Bruton's tyrosine kinase

T cell-independent (TI) B cell response is critical for the early protection against pathogen invasion. The regulation and activation of Bruton's tyrosine kinase (Btk) is known as a pivotal step of B cell antigen receptor (BCR) signaling in TI humoral immunity, as observed in patients with X-linked agammaglobulinemia (XLA) experiencing a high incidence of encapsulated bacterial infections. However, key questions remain as to whether a well-established canonical BCR signaling pathway is sufficient to regulate the activity of Btk. Here, we find that inositol hexakisphosphate (InsP6) acts as a physiological regulator of Btk in BCR signaling. Absence of higher order inositol phosphates (InsPs), inositol polyphosphates, leads to an inability to mount immune response against TI antigens. Interestingly, the significance of InsP6-mediated Btk regulation is more prominent in IgM+ plasma cells. Hence, the present study identifies higher order InsPs as principal components of B cell activation upon TI antigen stimulation and presents a mechanism for InsP-mediated regulation of the BCR signaling.

Signaling control of antibody isotype switching

Class switch recombination (CSR) generates isotype-switched antibodies with distinct effector functions essential for mediating effective humoral immunity. CSR is catalyzed by activation-induced deaminase (AID) that initiates DNA lesions in the evolutionarily conserved switch (S) regions at the immunoglobulin heavy chain (Igh) locus. AID-initiated DNA lesions are subsequently converted into DNA double stranded breaks (DSBs) in the S regions of Igh locus, repaired by non-homologous end-joining to effect CSR in mammalian B lymphocytes. While molecular mechanisms of CSR are well characterized, it remains less well understood how upstream signaling pathways regulate AID expression and CSR. B lymphocytes express multiple receptors including the B cell antigen receptor (BCR) and co-receptors (e.g., CD40). These receptors may share common signaling pathways or may use distinct signaling elements to regulate CSR. Here, we discuss how signals emanating from different receptors positively or negatively regulate AID expression and CSR.

Alzheimer's disease phospholipase C-gamma-2 (PLCG2) protective variant is a functional hypermorph

BACKGROUND

Recent Genome Wide Association Studies (GWAS) have identified novel rare coding variants in immune genes associated with late onset Alzheimer's disease (LOAD). Amongst these, a polymorphism in phospholipase C-gamma 2 (PLCG2) P522R has been reported to be protective against LOAD. PLC enzymes are key elements in signal transmission networks and are potentially druggable targets. PLCG2 is highly expressed in the hematopoietic system. Hypermorphic mutations in PLCG2 in humans have been reported to cause autoinflammation and immune disorders, suggesting a key role for this enzyme in the regulation of immune cell function.

METHODS

We assessed PLCG2 distribution in human and mouse brain tissue via immunohistochemistry and in situ hybridization. We transfected heterologous cell systems (COS7 and HEK293T cells) to determine the effect of the P522R AD-associated variant on enzymatic function using various orthogonal assays, including a radioactive assay, IP-One ELISA, and calcium assays.

RESULTS

PLCG2 expression is restricted primarily to microglia and granule cells of the dentate gyrus. Plcg2 mRNA is maintained in plaque-associated microglia in the cerebral tissue of an AD mouse model. Functional analysis of the p.P522R variant demonstrated a small hypermorphic effect of the mutation on enzyme function.

CONCLUSIONS

The PLCG2 P522R variant is protective against AD. We show that PLCG2 is expressed in brain microglia, and the p.P522R polymorphism weakly increases enzyme function. These data suggest that activation of PLCγ2 and not inhibition could be therapeutically beneficial in AD. PLCγ2 is therefore a potential target for modulating microglia function in AD, and a small molecule drug that weakly activates PLCγ2 may be one potential therapeutic approach.

Tespa1 Deficiency Dampens Thymus-Dependent B-Cell Activation and Attenuates Collagen-Induced Arthritis in Mice

Thymocyte-expressed, positive selection-associated 1 (Tespa1) plays an important role in both T cell receptor (TCR)-driven thymocyte development and in the FcεRI-mediated activation of mast cells. Herein, we show that lack of Tespa1 does not impair B cell development but dampens the in vitro activation and proliferation of B cells induced by T cell-dependent (TD) antigens, significantly reduces serum antibody concentrations in vivo, and impairs germinal center formation in both aged and TD antigen-immunized mice. We also provide evidence that dysregulated signaling in Tespa1-deficient B cells may be linked to CD40-induced TRAF6 degradation, and subsequent effects on 1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase gamma-2 (PLCγ2) phosphorylation, MAPK activation, and calcium influx. Furthermore, we demonstrate that Tespa1 plays a critical role in pathogenic B cells, since Tespa1-deficient chimeric mice showed a lower incidence and clinical disease severity of collagen-induced arthritis. Overall, our study demonstrates that Tespa1 is essential for TD B cell responses, and suggests an important role for Tespa1 during the development of autoimmune arthritis.

Role of Bruton's tyrosine kinase in B cells and malignancies

Bruton’s tyrosine kinase (BTK) is a non-receptor kinase that plays a crucial role in oncogenic signaling that is critical for proliferation and survival of leukemic cells in many B cell malignancies. BTK was initially shown to be defective in the primary immunodeficiency X-linked agammaglobulinemia (XLA) and is essential both for B cell development and function of mature B cells. Shortly after its discovery, BTK was placed in the signal transduction pathway downstream of the B cell antigen receptor (BCR). More recently, small-molecule inhibitors of this kinase have shown excellent anti-tumor activity, first in animal models and subsequently in clinical studies. In particular, the orally administered irreversible BTK inhibitor ibrutinib is associated with high response rates in patients with relapsed/refractory chronic lymphocytic leukemia (CLL) and mantle-cell lymphoma (MCL), including patients with high-risk genetic lesions. Because ibrutinib is generally well tolerated and shows durable single-agent efficacy, it was rapidly approved for first-line treatment of patients with CLL in 2016. To date, evidence is accumulating for efficacy of ibrutinib in various other B cell malignancies. BTK inhibition has molecular effects beyond its classic role in BCR signaling. These involve B cell-intrinsic signaling pathways central to cellular survival, proliferation or retention in supportive lymphoid niches. Moreover, BTK functions in several myeloid cell populations representing important components of the tumor microenvironment. As a result, there is currently a considerable interest in BTK inhibition as an anti-cancer therapy, not only in B cell malignancies but also in solid tumors. Efficacy of BTK inhibition as a single agent therapy is strong, but resistance may develop, fueling the development of combination therapies that improve clinical responses. In this review, we discuss the role of BTK in B cell differentiation and B cell malignancies and highlight the importance of BTK inhibition in cancer therapy.