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CD22 (also known as Siglec-2) is an inhibitory receptor expressed in B cells. CD22 specifically recognizes α2,6 sialic acid and interacts with α2,6 sialylated membrane proteins expressed on the same cell (cis-ligands) and those derived from outside of the cell (trans-ligands). Previously, CD22 cis-ligands were shown to regulate the activity of CD22, thereby regulating both BCR ligation-induced signaling and low-level "tonic" signaling in the absence of BCR ligation that regulates the survival and differentiation of B cells. Mouse CD22 prefers Neu5Gc to Neu5Ac thereby binding to α2,6-linked Neu5Gc with high affinity. Although human CD22 binds to a distinct α2,6 sialylated glycan with high affinity, expression of high-affinity ligands is regulated in a conserved and stringent manner. However, how high- versus low-affinity CD22 ligands regulate B cells is poorly understood. Here we demonstrate that the interaction of CD22 with the endogenous ligands enhances BCR ligation-induced signaling but reduces tonic signaling in Cmah -/- mouse B cells deficient in Neu5Gc as well as wild-type B cells. Moreover, Cmah -/- B cells do not show alterations in the phenotypes correlated to tonic signaling. These results indicate that low-affinity interaction of the CD22 cis-ligands with CD22 is sufficient for the regulation of B cell signaling, and suggest that expression of high-affinity CD22 ligands might be involved in the regulation of B cells by competing for the binding of CD22 with exogenous trans-ligands of CD22., Competing Interests: Conflict of interest The authors declare that they have no conflicts of interest with the contents of this article., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

B cell responses to nucleic acid-containing self-antigens that involve intracellular nucleic acid sensors play a crucial role in autoantibody production in SLE. CD72 is an inhibitory B cell co-receptor that down-regulates BCR signaling, and prevents the development of SLE. We previously showed that CD72 recognizes the RNA-containing self-antigen Sm/RNP, a target of SLE-specific autoantibodies, and induces B cell tolerance to Sm/RNP by specifically inhibiting B cell response to this self-antigen. Here, we address whether CD72 inhibits B cell response to ribosomes because the ribosome is an RNA-containing self-antigen and is a target of SLE-specific autoantibodies as well as Sm/RNP. We demonstrate that CD72 recognizes ribosomes as a ligand, and specifically inhibits BCR signaling induced by ribosomes. Although conventional protein antigens by themselves do not induce proliferation of specific B cells, ribosomes induce proliferation of B cells reactive to ribosomes in a manner dependent on RNA. This proliferative response is down-regulated by CD72. These results suggest that ribosomes activate B cells by inducing dual signaling through BCR and intracellular RNA sensors and that CD72 inhibits B cell response to ribosomes. Moreover, CD72 -/- but not CD72 +/+ mice spontaneously produce anti-ribosome autoantibodies. Taken together, CD72 induces B cell self-tolerance to ribosomes by recognizing ribosomes and inhibiting RNA-dependent B cell response to this self-antigen. CD72 appears to prevent development of SLE by inhibiting autoimmune B cell responses to multiple RNA-containing self-antigens. Because these self-antigens but not protein self-antigens induce RNA-dependent B cell activation, self-tolerance to RNA-containing self-antigens may require a distinct tolerance mechanism mediated by CD72., Competing Interests: Declaration of competing interest The authors declare no conflicts of interests., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Introduction: Giant cell arteritis (GCA) is characterized by headaches, but few studies have examined the detailed characteristics of pathologically confirmed cases. We investigated the characteristics of GCA patients, particularly headache, and their correlation with pathological findings., Methods: We retrospectively analyzed 26 patients (median age: 77.5 years, male: 38.4%) with GCA who underwent superficial temporal artery (STA) biopsy at the Japanese Red Cross Shizuoka Hospital between May 2001 and February 2022. All patients fulfilled the American College of Rheumatology and European League Against Rheumatism classification criteria for GCA. We focused on the relationship between clinical features, especially headache, and pathological findings., Results: Twenty-four patients had unilateral, nonpulsatile, intermittent headaches. Transmural inflammation (TMI), a characteristic pathology of GCA, was present in 14 patients. Bivariate analysis revealed significant associations between the TMI and STA-related tenderness (odds ratio [OR]=11, 95% confidence interval [CI]=1.14 to 106.43, p=0.046) and the TMI and STA-related chordal thickening (OR=0.19, 95% CI=0.068 to 0.52, p=0.021)., Conclusions: Headache in GCA patients was often unilateral, nonpulsatile, and intermittent. This study highlights the significant association of TMI with STA tenderness and ligamentous thickening, which has not been reported previously. Abnormal STA findings were significantly associated with pathological changes in GCA patients, emphasizing the importance of these lesions in predicting GCA., Competing Interests: The authors have declared that no competing interests exist., (Copyright © 2024, Shimohama et al.)

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of membrane molecules that recognize sialic acid. Most of them are inhibitory receptors that inhibit immune-cell activation by recognizing sialic acid as a self-motif. Human B cells express CD22 (also known as Siglec-2), Siglec-5, Siglec-6 and Siglec-10 whereas mouse B cells express CD22 and Siglec-G (ortholog of human Siglec-10). Siglecs recognize both sialylated molecules expressed on the same cell (cis-ligands) and those expressed by other cells (trans-ligands). In Guillain-Barré syndrome (GBS), antibody production to gangliosides (which are sialic acid-containing glycolipids) expressed by neurons plays a pathogenic role. A Siglec-10 variant deficient in recognition of gangliosides is genetically associated with GBS, suggesting that Siglec-10 induces self-tolerance to gangliosides by recognizing gangliosides as trans-ligands. Recognition of the BCR as a cis-ligand by Siglec-G and CD22 suppresses BCR signaling in B-1 cells and conventional B cells, respectively. This signal suppression prevents excess expansion of B-1 cells and is involved in the quality control of signaling-competent B cells by setting a threshold for tonic signaling during B cell development. CD22 recognizes other cis-ligands including CD22 and β7 integrin. Interaction of CD22 with other CD22 molecules induces CD22 clustering that suppresses CD22-mediated signal inhibition upon BCR ligation, and interaction with β7 integrin maintains its function in the gut-homing of B cells. Taken together, interactions of B cell Siglecs with multiple trans- and cis-ligands play important roles in B cell homeostasis and immune responses., (© The Author(s) 2023. Published by Oxford University Press on behalf of The Japanese Society for Immunology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Sialic acid-binding immunoglobulin-like lectins are a family of membrane molecules primarily expressed in immune cells. Most of them are inhibitory receptors containing immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic tail. On the cell surface, sialic acid-binding immunoglobulin-like lectins are mostly bound by sialylated glycans on membrane molecules expressed in the same cell (cis-ligands). Although ligands of sialic acid-binding immunoglobulin-like lectins are not efficiently identified by conventional methods such as immunoprecipitation, in situ labeling including proximity labeling is useful in identifying both cis-ligands and the sialylated ligands expressed by other cells (trans-ligands) of sialic acid-binding immunoglobulin-like lectins. Interaction of the inhibitory sialic acid-binding immunoglobulin-like lectins with cis-ligands including both those with and without signaling function modulates the inhibitory activity of sialic acid-binding immunoglobulin-like lectins by multiple different ways. This interaction also modulates signaling function of the cis-ligands. So far, little is known about the role of the interaction between sialic acid-binding immunoglobulin-like lectins and the cis-ligands. Nonetheless, recent studies showed that the inhibitory activity of CD22 (also known as Siglec-2) is regulated by endogenous ligands, most likely cis-ligands, differentially in resting B cells and those in which B-cell antigen receptor is ligated. This differential regulation plays a role in quality control of signaling-competent B cells and also partial restoration of B-cell antigen receptor signaling in immunodeficient B cells., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Low-intensity pulsed ultrasound treatment is known to shorten the healing period of bone fractures by 30%-40%, but the initial mechanism of the healing process remains unknown. One possible mechanism is related to the piezoelectricity of bone. However, the complex geometry of bones results in inherent challenges to evaluating electric fields induced therein. Therefore, in this study, we investigate the piezoelectric responses of bones by using simulations to study the wave propagation and induced potentials in bone, according to the piezoelectric finite-difference time-domain (PE-FDTD) method. First, we verify the suitability of the PE-FDTD method by comparing the simulated electric field results with the experimental data obtained by an ultrasound receiver using bone as the piezoelectric element. Next, ultrasound irradiation into a real bone model (the radius of a 66-year-old woman) is simulated at different incident angles. At normal incidence and off-axis incidence (45°), the maximum electric field strength was 4.3 and 5.6 mV/cm, respectively. We also present evidence of significant shear wave contribution to the induced potential. The results of this study confirm the existence of ultrasonically induced potentials in heterogenous bones with complex shapes, equal in magnitude to potentials generated in electrically stimulated bone healing., (© 2023 Acoustical Society of America.)

Competing Interests: Conflict of interest The authors declare that they have no conflict of interest.

CD22, one of the sialic acid-binding immunoglobulin-like lectins (Siglecs), regulates B lymphocyte signaling via its interaction with glycan ligands bearing the sequence Neu5Ac/Gcα(2→6)Gal. We have developed the synthetic sialoside GSC-718 as a ligand mimic for CD22 and identified it as a potent CD22 inhibitor. Although the synthesis of CD22-binding sialosides including GSC-718 has been reported by our group, the synthetic route was unfortunately not suitable for large-scale synthesis. In this study, we developed an improved scalable synthetic procedure for sialosides which utilized 1,5-lactam formation as a key step. The improved procedure yielded sialosides incorporating a series of aglycones at the C2 position. Several derivatives with substituted benzyl residues as aglycones were found to bind to mouse CD22 with affinity comparable to that of GSC-718. The new procedure developed in this study affords sialosides in sufficient quantities for cell-based assays, and will facilitate the search for promising CD22 inhibitors that have therapeutic potential., (© 2023. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)

Elimination of autoreactive developing B cells is an important mechanism to prevent autoantibody production. However, how B cell receptor (BCR) signaling triggers apoptosis of immature B cells remains poorly understood. We show that BCR stimulation up-regulates the expression of the lysosomal-associated transmembrane protein 5 (LAPTM5), which in turn triggers apoptosis of immature B cells through two pathways. LAPTM5 causes BCR internalization, resulting in decreased phosphorylation of SYK and ERK. In addition, LAPTM5 targets the E3 ubiquitin ligase WWP2 for lysosomal degradation, resulting in the accumulation of its substrate PTEN. Elevated PTEN levels suppress AKT phosphorylation, leading to increased FOXO1 expression and up-regulation of the cell cycle inhibitor p27Kip1 and the proapoptotic molecule BIM. In vivo, LAPTM5 is involved in the elimination of autoreactive B cells and its deficiency exacerbates autoantibody production. Our results reveal a previously unidentified mechanism that contributes to immature B cell apoptosis and B cell tolerance.

NAD(P)H quinone oxidoreductase 1 (NQO1) is a flavoprotein that catalyzes two-electron reduction of quinone to hydroquinone by using nicotinamide adenine dinucleotide (NADPH), and functions as a scavenger for reactive oxygen species (ROS). The function of NQO1 in the immune response is not well known. In the present study, we demonstrated that Nqo1-deficient T cells exhibited reduced induction of T helper 17 cells (Th17) in vitro during Th17(23)- and Th17(β)- skewing conditions. Nqo1-deficient mice showed ameliorated symptoms in a Th17-dependent autoimmune Experimental autoimmune encephalomyelitis (EAE) model. Impaired Th17-differentiation was caused by overproduction of the immunosuppressive cytokine, IL-10. Increased IL-10 production in Nqo1-deficient Th17 cells was associated with elevated intracellular Reactive oxygen species (ROS) levels. Furthermore, overproduction of IL-10 in Th17 (β) cells was responsible for the ROS-dependent increase of c-avian musculoaponeurotic fibrosarcoma (c-maf) expression, despite the lack of dependency of c-maf in Th17(23) cells. Taken together, the results reveal a novel role of NQO1 in promoting Th17 development through the suppression of ROS mediated IL-10 production., Competing Interests: The authors have declared that no competing interests exist.

Antibodies to non-protein antigens such as nucleic acids, polysaccharides, and glycolipids play important roles in both host defense against microbes and development of autoimmune diseases. Although non-protein antigens are not recognized by T cells, antibody production to non-protein antigens involve T cell-independent mechanisms such as signaling through TLR7 and TLR9 in antibody production to nucleic acids. Although self-reactive B cells are tolerized by various mechanisms including deletion, anergy, and receptor editing, T cell tolerance is also crucial in self-tolerance of B cells to protein self-antigen because self-reactive T cells induce autoantibody production to these self-antigens. However, presence of T cell-independent mechanism suggests that T cell tolerance is not able to maintain B cell tolerance to non-protein self-antigens. Lines of evidence suggest that B cell response to non-protein self-antigens such as nucleic acids and gangliosides, sialic acid-containing glycolipids, are suppressed by inhibitory B cell co-receptors CD72 and Siglec-G, respectively. These inhibitory co-receptors recognize non-protein self-antigens and suppress BCR signaling induced by these antigens, thereby inhibiting B cell response to these self-antigens. Inhibitory B cell co-receptors appear to be involved in B cell self-tolerance to non-protein self-antigens that can activate B cells by T cell-independent mechanisms., (© 2022 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

The protein tyrosine phosphatase CD45 plays a crucial role in B cell antigen receptor (BCR) signaling by activating Src family kinases. Cd45 -/- mice show altered B cell development and a phenotype likely due to reduced steady-state signaling; however, Cd45 -/- B cells show relatively normal BCR ligation-induced signaling. In our investigation of how BCR signaling was restored in Cd45 -/- cells, we found that the coreceptor CD22 switched from an inhibitory to a stimulatory function in these cells. We disrupted the ability of CD22 to interact with its ligands in Cd45 -/- B cells by generating Cd45 -/- St6galI -/- mice, which cannot synthesize the glycan ligand of CD22, or by treating Cd45 -/- B cells in vitro with the sialoside GSC718, which inhibits ligand binding to CD22. BCR ligation-induced signaling was reduced by ST6GalI deficiency, but not by GSC718 treatment, suggesting that CD22 restored BCR ligation-induced signaling in Cd45 -/- mature B cells by altering cellular phenotypes during development. CD22 was required for the increase in the surface amount of IgM-BCR on Cd45 -/- B cells, which augmented signaling. Because B cell survival depends on steady-state BCR signaling, IgM-BCR abundance was likely increased by the selective survival of IgM-BCR hi Cd45 -/- B cells because of CD22-mediated signaling under conditions of substantially reduced steady-state signaling. Because the amount of surface IgM-BCR is increased on B cells from patients with other BCR signaling deficiencies, including X-linked agammaglobulinemia, our findings suggest that CD22 may contribute to the partial restoration of B cell function in these patients.

Marginal zone B cells (MZBs) represent a unique B-cell sub-population that rapidly differentiate into IgM-secreting plasma cells in response to T-independent (T-I) antigen. Sphingosine 1-phosphate (S1P) promotes MZB localization to the marginal zone. However, intracellular molecules involved in MZB localization and migration remain largely unknown. Here, we show that MZBs lacking the glia maturation factor-γ (GMFG) are impaired in chemotaxis toward S1P under both in vitro and in vivo conditions, suggesting that GMFG is an effector downstream of S1P receptors. GMFG undergoes serine phosphorylation upon S1P stimulation and is required for S1P-induced desensitization of S1P receptor 1 (S1PR1). Compared with wild-type mice, Gmfg-/- mice produce elevated levels of 4-hydroxy-3-nitrophenyl-acetyl (NP)-specific IgM against a T-I type II antigen, NP-Ficoll, accompanied by dysregulated MZB localization. These results identify GMFG as a regulator of S1P-induced MZB chemotaxis and reveal a role for MZB localization in the marginal zone for optimal IgM production against a T-I antigen., (© The Japanese Society for Immunology. 2021. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Carriers that augment delivery, immunogenicity or both are crucial in the development of vaccines especially component vaccines as components of pathogens are often poorly immunogenic. Cholesteryl pullulan (CHP) that forms nano-sized hydrogel (nanogel) and encapsulates proteins was shown to be useful in the delivery of vaccines. Here we demonstrate that subcutaneous immunization of mice with bovine serum albumin (BSA) chemically conjugated to NH 2 -CHP nanogel induces strong antibody production. This augmented antibody production requires covalent conjugation between BSA and CHP, but does not require nanogel formation. Conjugation of NH 2 -CHP nanogel induces persistence of BSA in dendritic cells (DCs) in vivo. As resistance to lysosomal degradation was previously shown to augment antigen presentation by DCs, conjugation of antigens with CHP nanogel may enhance antibody production to antigens by delaying lysosomal degradation. Therefore, delayed degradation of antigens by covalent conjugation with nanoparticles may be a good strategy for the development of effective vaccines., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

CD22 is an inhibitory B cell coreceptor that regulates B cell development and activation by downregulating BCR signaling through activation of SH2-containing protein tyrosine phosphatase-1 (SHP-1). CD22 recognizes α2,6 sialic acid as a specific ligand and interacts with α2,6 sialic acid-containing membrane molecules, such as CD45, IgM, and CD22, expressed on the same cell. Functional regulation of CD22 by these endogenous ligands enhances BCR ligation-induced signaling and is essential for normal B cell responses to Ags. In this study, we demonstrate that CD45 plays a crucial role in CD22-mediated inhibition of BCR ligation-induced signaling. However, disruption of ligand binding of CD22 enhances CD22 phosphorylation, a process required for CD22-mediated signal inhibition, upon BCR ligation in CD45 -/- as well as wild-type mouse B cells but not in mouse B cells expressing a loss-of-function mutant of SHP-1. This result indicates that SHP-1 but not CD45 is required for ligand-mediated regulation of CD22. We further demonstrate that CD22 is a substrate of SHP-1, suggesting that SHP-1 recruited to CD22 dephosphorylates nearby CD22 as well as other substrates. CD22 dephosphorylation by SHP-1 appears to be augmented by homotypic CD22 clustering mediated by recognition of CD22 as a ligand of CD22 because CD22 clustering increases the number of nearby CD22. Our results suggest that CD22 but not CD45 is an endogenous ligand of CD22 that enhances BCR ligation-induced signaling through SHP-1-mediated dephosphorylation of CD22 in CD22 clusters., (Copyright © 2021 by The American Association of Immunologists, Inc.)

The integrin α 4 β 7 selectively regulates lymphocyte trafficking and adhesion in the gut and gut-associated lymphoid tissue (GALT). Here, we describe unexpected involvement of the tyrosine phosphatase Shp1 and the B cell lectin CD22 (Siglec-2) in the regulation of α 4 β 7 surface expression and gut immunity. Shp1 selectively inhibited β 7 endocytosis, enhancing surface α 4 β 7 display and lymphocyte homing to GALT. In B cells, CD22 associated in a sialic acid-dependent manner with integrin β 7 on the cell surface to target intracellular Shp1 to β 7 . Shp1 restrained plasma membrane β 7 phosphorylation and inhibited β 7 endocytosis without affecting β 1 integrin. B cells with reduced Shp1 activity, lacking CD22 or expressing CD22 with mutated Shp1-binding or carbohydrate-binding domains displayed parallel reductions in surface α 4 β 7 and in homing to GALT. Consistent with the specialized role of α 4 β 7 in intestinal immunity, CD22 deficiency selectively inhibited intestinal antibody and pathogen responses.

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) expressed in T cells may regulate immune responses in the gut. In addition to T cells, B cells are also an important population in the gut-associated lymphoid tissues that orchestrate mucosal homeostasis. However, the role of CEACAM1 in B cells has not been elucidated. We herein analyzed mature B cells to determine the functions of CEACAM1. Flow cytometry revealed high expression of CEACAM1 on B cells in secondary lymphoid tissues. Cytokine production induced by activation of B cell receptor (BCR) signaling was suppressed by CEACAM1 signaling in contrast to that associated with either Toll-like receptor 4 or CD40 signaling. Confocal microscopy revealed co-localization of CEACAM1 and BCR when activated with anti-Igμ F(ab') 2 fragment. Overexpression of CEACAM1 in a murine B cell line, A20, resulted in reduced expressions of activation surface markers with decreased Ca 2+ influx after BCR signal activation. Overexpression of CEACAM1 suppressed BCR signal cascade in A20 cells in association with decreased spontaneous proliferation. Our results suggest that CEACAM1 can regulate BCR-mediated mature B cell activation in lymphoid tissues. Therefore, further studies of this molecule may lead to greater insights into the mechanisms of immune responses within peripheral tissues and the potential treatment of inflammatory diseases., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Guillain-Barré syndrome (GBS), including its variant Miller Fisher syndrome (MFS), is an acute peripheral neuropathy that involves autoimmune mechanisms leading to the production of autoantibodies to gangliosides; sialic acid-containing glycosphingolipids. Although association with various genetic polymorphisms in the major histocompatibility complex (MHC) is shown in other autoimmune diseases, GBS is an exception, showing no such link. No significant association was found by genome wide association studies, suggesting that GBS is not associated with common variants. To address the involvement of rare variants in GBS, we analyzed Siglec-10, a sialic acid-recognizing inhibitory receptor expressed on B cells. Here we demonstrate that two rare variants encoding R47Q and A108V substitutions in the ligand-binding domain are significantly accumulated in patients with GBS. Because of strong linkage disequilibrium, there was no patient carrying only one of them. Recombinant Siglec-10 protein containing R47Q but not A108V shows impaired binding to gangliosides. Homology modeling revealed that the R47Q substitution causes marked alteration in the ligand-binding site. Thus, GBS is associated with a rare variant of the SIGLEC10 gene that impairs ligand binding of Siglec-10. Because Siglec-10 regulates antibody production to sialylated antigens, our finding suggests that Siglec-10 regulates development of GBS by suppressing antibody production to gangliosides, with defects in its function predisposing to disease., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

B-cell novel protein 1 (BCNP1) has recently been identified as a new B-cell receptor (BCR) signaling molecule but its physiological function remains unknown. Here, we demonstrate that mice deficient in BCNP1 exhibit impaired B-cell maturation and a reduction of B-1a cells. BCNP1-deficient spleen B cells show enhanced survival, proliferation and Ca2+ influx in response to BCR cross-linking as compared with wild-type spleen B cells. Consistently, mutant B cells show elevated phosphorylation of SYK, B-cell linker protein (BLNK) and PLCγ2 upon BCR cross-linking. In vivo, BCNP1-deficient mice exhibit enhanced humoral immune responses to T-independent and T-dependent antigens. Moreover, aged mutant mice contain elevated levels of serum IgM and IgG3 antibodies and exhibit polyclonal and monoclonal B-cell expansion in lymphoid organs. These results reveal distinct roles for BCNP1 in B-cell development, activation and homeostasis., (© The Japanese Society for Immunology. 2019. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Reactive oxygen species (ROS) are not only toxic substances inducing oxidative stress but also play a role in receptor signaling as a second messenger, which augments signaling through various receptors by oxidizing ROS-sensitive signaling molecules. Among ROS, H 2 O 2 is suggested to be an important second messenger because of its relative stability. H 2 O 2 is generated by superoxide dismutase (SOD)-mediated conversion of superoxide produced by membrane-localized NADPH oxidases (NOXes). Superoxide and H 2 O 2 are also produced as a by-product of mitochondrial respiratory chain and various other metabolic reactions. BCR ligation induces ROS production in two phases. ROS production starts immediately after BCR ligation and ceases in 1 h, then re-starts 2 h after BCR ligation and lasts 4-6 h. ROS production in the early phase is mediated by NOX2, a NOX isoform, but does not regulate BCR signaling. In contrast, ROS production at the late phase augments BCR signaling. Although the involvement of mitochondrial respiration was previously suggested in prolonged BCR ligation-induced ROS production, we recently demonstrated that NOX3, another NOX isoform, plays a central role in ROS production at the late phase. NOXes are shown to be a component of ROS-generating signaling endosome called redoxosome together with endocytosed receptors and receptor-associated signaling molecules. In redoxosome, ROS generated by NOXes augment signaling through the endocytosed receptor. The role of NOXes and redoxosome in BCR signaling needs to be further elucidated.

Siglecs are known to be bound and regulated by membrane molecules that display specific sialic acid-containing ligands and are present on the same cell (cis-ligands). Because of the low-affinity binding of Siglecs to the glycan ligands, conventional methods such as immunoprecipitation are not suitable for identification of Siglec cis-ligands. Here we describe efficient and specific labeling of cis-ligands of CD22 (also known as Siglec-2) on B lymphocytes by proximity labeling using tyramide. This method may also be applicable to labeling of cis-ligands of other Siglecs.

B cells express various inhibitory co-receptors including CD22 (also known as Siglec-2), Siglec-10 (Siglec-G in mice), CD72, LILRB (PIR-B in mice) and FcγRIIB that contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) in the cytoplasmic region and negatively regulate BCR signaling by recruiting phosphatases to the ITIMs. Some of the inhibitory B cell co-receptors suppress development of SLE. Among these, CD72 most strongly regulates SLE. CD72 recognizes Sm/RNP, a lupus self-antigen and an endogenous TLR7 ligand, as a specific ligand, and suppresses B cell response to this TLR7 ligand. This suppression may inhibit development of SLE because TLR7 is indispensable in multiple mouse SLE models. In contrast, inhibitory B cell co-receptors such as CD22 and CD72 inhibit expansion of regulatory B cells that are known to regulate development of autoimmune diseases including type 1 diabetes (T1D) and multiple sclerosis. CD72 strongly exacerbate development of T1D in NOD mice probably by limiting expansion of regulatory B cells. Thus, inhibitory B cell co-receptors especially CD72 regulates distinct autoimmune diseases either positively or negatively. As B cell depletion therapy clearly reveals crucial roles of B cells in the regulation of various autoimmune diseases, CD72 may be a novel therapeutic target for treatment of autoimmune diseases.

IgA is the most abundantly produced antibody in the body and plays a crucial role in gut homeostasis and mucosal immunity. IgA forms a dimer that covalently associates with the joining (J) chain, which is essential for IgA transport into the mucosa. Here, we demonstrate that the marginal zone B and B-1 cell-specific protein (MZB1) interacts with IgA through the α-heavy-chain tailpiece dependent on the penultimate cysteine residue and prevents the intracellular degradation of α-light-chain complexes. Moreover, MZB1 promotes J-chain binding to IgA and the secretion of dimeric IgA. MZB1-deficient mice are impaired in secreting large amounts of IgA into the gut in response to acute inflammation and develop severe colitis. Oral administration of a monoclonal IgA significantly ameliorated the colitis, accompanied by normalization of the gut microbiota composition. The present study identifies a molecular chaperone that promotes J-chain binding to IgA and reveals an important mechanism that controls the quantity, quality, and function of IgA., Competing Interests: The authors declare no conflict of interest.

The BCR plays a central role in B cell development, survival, activation, and differentiation. We have identified the B cell novel protein 1 (BCNP1) as a new regulator of BCR signaling. BCNP1 contains a pleckstrin homology domain, three proline-rich motifs, and a potential SH2 binding site, and is predominantly expressed by B cells. We found that BCNP1 overexpression in WEHI231 immature B cells potentiated α-IgM-induced apoptosis. Conversely, BCNP1-deficient WEHI231 cells, generated by CRISPR-Cas9-mediated genome editing, exhibited reduced apoptosis after BCR crosslinking. Biochemical analyses revealed that BCNP1 physically interacted with the B cell linker protein (BLNK), Grb2, and PLCγ2. Moreover, absence of BCNP1 resulted in accelerated dephosphorylation of BLNK, reduced phosphorylation of SYK and PLCγ2, and decreased Ca 2+ influx after BCR crosslinking. These results demonstrate that BCNP1 promotes BCR signaling by modulating the phosphorylation of BLNK, SYK, and PLCγ2., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Reactive oxygen species (ROS) are not only toxic substances inducing oxidative stress but also play a role as a second messenger in signal transduction through various receptors. Previously, B cell activation was shown to involve prolonged ROS production induced by ligation of BCR. However, the mechanisms for ROS production and ROS-mediated activation in B cells are still poorly understood. In this study, we demonstrate that BCR ligation induces biphasic ROS production in both mouse spleen B cells and the mouse B cell line BAL17; transient and modest ROS production is followed by sustained and robust ROS production at 2-6 h after BCR ligation. ROS production in the late phase but not in the early phase augments activation of signaling pathways, such as the NF-κB and PI3K pathways, and is essential for B cell proliferation. ROS production in the late phase appears to be mediated by NADPH oxidases (NOXes) because prolonged ROS production is inhibited by various NOX inhibitors, including the specific inhibitor VAS2870. BCR ligation-induced ROS production is also inhibited by CRISPR/Cas9-mediated deletion of either the Cyba gene encoding p22 phox , the regulator of NOX1-4 required for their activation, or NOX3 , whereas ROS production is not affected by double deficiency of the DUOXA1 and DUOXA2 genes essential for the activation of the NOX isoforms DUOX1 and DUOX2. These results indicate that NOXes play a crucial role in sustained but not early BCR signaling and suggest an essential role of NOX-dependent sustained BCR signaling in B cell activation., (Copyright © 2019 by The American Association of Immunologists, Inc.)

Systemic lupus erythematosus (SLE) is the prototypic systemic autoimmune disease characterized by production of autoantibodies to various nuclear antigens and overexpression of genes regulated by IFN-I called IFN signature. Genetic studies on SLE patients and mutational analyses of mouse models demonstrate crucial roles of nucleic acid (NA) sensors in development of SLE. Although NA sensors are involved in induction of anti-microbial immune responses by recognizing microbial NAs, recognition of self NAs by NA sensors induces production of autoantibodies to NAs in B cells and production of IFN-I in plasmacytoid dendritic cells. Among various NA sensors, the endosomal RNA sensor TLR7 plays an essential role in development of SLE at least in mouse models. CD72 is an inhibitory B cell co-receptor containing an immunoreceptor tyrosine-based inhibition motif (ITIM) in the cytoplasmic region and a C-type lectin like-domain (CTLD) in the extracellular region. CD72 is known to regulate development of SLE because CD72 polymorphisms associate with SLE in both human and mice and CD72 -/- mice develop relatively severe lupus-like disease. CD72 specifically recognizes the RNA-containing endogenous TLR7 ligand Sm/RNP by its extracellular CTLD, and inhibits B cell responses to Sm/RNP by ITIM-mediated signal inhibition. These findings indicate that CD72 inhibits development of SLE by suppressing TLR7-dependent B cell response to self NAs. CD72 is thus involved in discrimination of self-NAs from microbial NAs by specifically suppressing autoimmune responses to self-NAs., Competing Interests: Conflict of Interest: The author declares no potential conflicts of interest.