CD22 ligation inhibits downstream B cell receptor signaling and Ca(2+) flux upon activation

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

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

SM03, an Anti-CD22 Antibody, Converts Cis-to-Trans Ligand Binding of CD22 against α2,6-Linked Sialic Acid Glycans and Immunomodulates Systemic Autoimmune Diseases

SM03, an anti-CD22 recombinant IgG1 mAb, is currently in a phase III clinical trial for the treatment of rheumatoid arthritis (NCT04312815). SM03 showed good safety and efficacy in phase I systemic lupus erythematosus and phase II moderate to severe rheumatoid arthritis clinical trials. We propose the success of SM03 as a therapeutic to systemic autoimmune diseases is through the utilization of a novel mechanism of action unique to SM03. CD22, an inhibitory coreceptor of the BCR, is a potential immunotherapeutic target against autoimmune diseases. SM03 could disturb the CD22 homomultimeric configuration through disrupting cis binding to α2,6-linked sialic acids, induce rapid internalization of CD22 from the cell surface of human B cells, and facilitate trans binding between CD22 to human autologous cells. This in turn increased the activity of the downstream immunomodulatory molecule Src homology region 2 domain-containing phosphatase 1 (SHP-1) and decreased BCR-induced NF-κB activation in human B cells and B cell proliferation. This mechanism of action gives rationale to support the significant amelioration of disease and good safety profile in clinical trials, as by enabling the "self" recognition mechanism of CD22 via trans binding to α2,6 sialic acid ligands on autologous cells, SM03 specifically restores immune tolerance of B cells to host tissues without affecting the normal B cell immune response to pathogens.

Role of inhibitory signaling in peripheral B cell tolerance*

At least 20% of B cells in the periphery expresses an antigen receptor with a degree of self-reactivity. If activated, these autoreactive B cells pose a risk as they can contribute to the development of autoimmune diseases. To prevent their activation, both B cell-intrinsic and extrinsic tolerance mechanisms are in place in healthy individuals. In this review article, I will focus on B cell-intrinsic mechanisms that prevent the activation of autoreactive B cells in the periphery. I will discuss how inhibitory signaling circuits are established in autoreactive B cells, focusing on the Lyn-SHIP-1-SHP-1 axis, how they contribute to peripheral immune tolerance, and how disruptions of these circuits can contribute to the development of autoimmunity.

Effects of alternative splicing events and transcriptome changes on kidney stone formation

During the development of urinary stone disease, the formation of tiny crystals that adhere to the renal tubular epithelium induces epithelial cell damage. This damage and repair of the epithelium is associated with the establishment of more crystal adhesion sites, which in turn stimulates further crystal adhesion and, eventually, stone formation. Deposited crystals typically cause changes in epithelial cell gene expression, such as transcriptome changes and alternative splicing events. Although considered important for regulating gene expression, alternative splicing has not been reported in studies related to kidney stones. To date, whether alternative splicing events are involved in the regulation of stone formation and whether crystallographic cell interactions are regulated by alternative splicing at the transcriptional level have remained unknown. Therefore, we conducted RNA sequencing and alternative splicing-related bioassays by modeling the in vitro stone environment. Many alternative splicing events were associated with crystallographic cell interactions. Moreover, these events regulated transcription and significantly affected the capacity of crystals to adhere to renal tubular epithelial cells and regulate apoptosis.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

Kinobead Profiling Reveals Reprogramming of BCR Signaling in Response to Therapy within Primary CLL Cells

PURPOSE

B-cell receptor (BCR) signaling is critical for the pathogenesis of chronic lymphocytic leukemia (CLL), promoting both malignant cell survival and disease progression. Although vital, understanding of the wider signaling network associated with malignant BCR stimulation is poor. This is relevant with respect to potential changes in response to therapy, particularly involving kinase inhibitors. In the current study, we describe a novel high-resolution approach to investigate BCR signaling in primary CLL cells and track the influence of therapy on signaling response.

EXPERIMENTAL DESIGN

A kinobead/mass spectrometry-based protocol was used to study BCR signaling in primary CLL cells. Longitudinal analysis of samples donated by clinical trial patients was used to investigate the impact of chemoimmunotherapy and ibrutinib on signaling following surface IgM engagement. Complementary Nanostring and immunoblotting analysis was used to verify our findings.

RESULTS

Our protocol isolated a unique, patient-specific signature of over 30 kinases from BCR-stimulated CLL cells. This signature was associated with 13 distinct Kyoto Encyclopedia of Genes and Genomes pathways and showed significant change in cells from treatment-naïve patients compared with those from patients who had previously undergone therapy. This change was validated by longitudinal analysis of clinical trials samples where BCR-induced kinome responses in CLL cells altered between baseline and disease progression in patients failing chemoimmunotherapy and between baseline and treatment in patients taking ibrutinib.

CONCLUSIONS

These data comprise the first comprehensive proteomic investigation of the BCR signaling response within CLL cells and reveal unique evidence that these cells undergo adaptive reprogramming of this signaling in response to therapy.

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.

B Cells in Atherosclerosis: Mechanisms and Potential Clinical Applications

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B cells regulate atherosclerotic plaque formation through production of antibodies and cytokines, and effects are subset specific (B1 and B2).

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Putative human atheroprotective B1 cells function similarly to murine B1 in their spontaneous IgM antibody production. However, marker strategies in identifying human and murine B1 are different.

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IgM antibody to oxidation specific epitopes produced by B1 cells associate with human coronary artery disease.

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Neoantigen immunization may be a promising strategy for atherosclerosis vaccine development, but further study to determine relevant antigens still need to be done.

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B-cell–targeted therapies, used in treating autoimmune diseases as well as lymphoid cancers, might have potential applications in treating cardiovascular diseases. Short- and long-term cardiovascular effects of these agents need to be assessed.

Because atherosclerotic cardiovascular disease is a leading cause of death worldwide, understanding inflammatory processes underpinning its pathology is critical. B cells have been implicated as a key immune cell type in regulating atherosclerosis. B-cell effects, mediated by antibodies and cytokines, are subset specific. In this review, we focus on elaborating mechanisms underlying subtype-specific roles of B cells in atherosclerosis and discuss available human data implicating B cells in atherosclerosis. We further discuss potential B cell–linked therapeutic approaches, including immunization and B cell–targeted biologics. Given recent evidence strongly supporting a role for B cells in human atherosclerosis and the expansion of immunomodulatory agents that affect B-cell biology in clinical use and clinical trials for other disorders, it is important that the cardiovascular field be cognizant of potential beneficial or untoward effects of modulating B-cell activity on atherosclerosis.

Therapeutic implications of the anergic/postactivated status of B cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterised by numerous abnormalities in B lineage cells, including increased CD27++ plasmablasts/plasma cells, atypical CD27-IgD- B cells with increased CD95, spleen tyrosine kinase (Syk)++, CXCR5- and CXCR5+ subsets and anergic CD11c+Tbet+ age-associated B cells. Most findings, together with preclinical lupus models, support the concept of B cell hyperactivity in SLE. However, it remains largely unknown whether these specific B cell subsets have pathogenic consequences and whether they provide relevant therapeutic targets. Recent findings indicate a global distortion of B cell functional capability, in which the entire repertoire of naïve and memory B cells in SLE exhibits an anergic or postactivated (APA) functional phenotype. The APA status of SLE B cells has some similarities to the functional derangement of lupus T cells. APA B cells are characterised by reduced global cytokine production, diminished B cell receptor (BCR) signalling with decreased Syk and Bruton's tyrosine kinase phosphorylation related to repeated in vivo BCR stimulation as well as hyporesponsiveness to toll-like receptor 9 engagement, but intact CD40 signalling. This APA status was related to constitutive co-localisation of CD22 linked to phosphatase SHP-1 and increased overall protein phosphatase activities. Notably, CD40 co-stimulation could revert this APA status and restore BCR signalling, downregulate protein tyrosine phosphatase transcription and promote B cell proliferation and differentiation. The APA status and their potential rescue by bystander help conveyed through CD40 stimulation not only provides insights into possible mechanisms of escape of autoreactive clones from negative selection but also into novel ways to target B cells therapeutically.

Sialic Acid-Siglec Axis in Human Immune Regulation, Involvement in Autoimmunity and Cancer and Potential Therapeutic Treatments

Siglecs are sialic acid-binding immunoglobulin-like lectins. Most Siglecs function as transmembrane receptors mainly expressed on blood cells in a cell type-specific manner. They recognize and bind sialic acids in specific linkages on glycoproteins and glycolipids. Since Sia is a self-molecule, Siglecs play a role in innate immune responses by distinguishing molecules as self or non-self. Increasing evidence supports the involvement of Siglecs in immune signaling representing immune checkpoints able to regulate immune responses in inflammatory diseases as well as cancer. Although further studies are necessary to fully understand the involvement of Siglecs in pathological conditions as well as their interactions with other immune regulators, the development of therapeutic approaches that exploit these molecules represents a tremendous opportunity for future treatments of several human diseases, as demonstrated by their application in several clinical trials. In the present review, we discuss the involvement of Siglecs in the regulation of immune responses, with particular focus on autoimmunity and cancer and the chance to target the sialic acid-Siglec axis as novel treatment strategy.

Identification of novel long non-coding RNAs involved in bisphenol A induced immunotoxicity in fish primary macrophages

Bisphenol A (BPA), a well-known environmental endocrine-disrupting chemical (EDC), could pose a great toxicity risk to aquatic organisms. The present study aimed to evaluate the underlying role of long non-coding RNAs (lncRNAs) in BPA-induced immunotoxicity in head kidney (HK) macrophages of the red common carp (Cyprinus carpio), using lncRNA-RNA sequencing (RNA-Seq). In BPA-exposed HK macrophages group, 2,095 and 1,138 differentially expressed mRNAs (DEGs) and lncRNAs (DE-lncRNAs) were obtained, respectively, compared with controls. The qRT-PCR validation results of DEGs and DE-lncRNAs were similar to the RNA-Seq results. The KEGG analysis of DEGs and target genes of DE-lncRNAs have shown that some immune-related signaling pathways, including NF-kappa B, Toll-like receptor, B-cell receptor, Jak-STAT, and Hippo signaling pathways, were severely disrupted by BPA exposure. Moreover, we observed the synergic regulation of some mRNAs involved in immune response such as two hub genes traf6 and mapk1/3 and their upstream lncRNAs in HK macrophages upon the BPA exposure or its analogue bisphenol S (BPS) exposure. This suggested the dysregulation of lncRNAs by BPA or BPS may lead to a change in the expression of hub genes, which affects the cross-talk of various signaling pathways by interaction with other network genes. In conclusion, the present study demonstrates the potential role of lncRNAs in immunotoxicity of bisphenol compounds in red common carp HK macrophages, and our results provide evidence for further exploring lncRNA's role in EDC-induced toxicity in aquatic organisms.

Autoantibodies and B Cells: The ABC of rheumatoid arthritis pathophysiology

Rheumatoid arthritis (RA) is an autoimmune disease characterized by joint inflammation. In the last few decades, new insights into RA‐specific autoantibodies and B cells have greatly expanded our understanding of the disease. The best‐known autoantibodies in RA—rheumatoid factor (RF) and anti‐citrullinated protein antibodies (ACPA)—are present long before disease onset, and both responses show signs of maturation around the time of the first manifestation of arthritis. A very intriguing characteristic of ACPA is their remarkably high abundance of variable domain glycans. Since these glycans may convey an important selection advantage of citrulline‐reactive B cells, they may be the key to understanding the evolution of the autoimmune response. Recently discovered autoantibodies targeting other posttranslational modifications, such as anti‐carbamylated and anti‐acetylated protein antibodies, appear to be closely related to ACPA, which makes it possible to unite them under the term of anti‐modified protein antibodies (AMPA). Despite the many insights gained about these autoantibodies, it is unclear whether they are pathogenic or play a causal role in disease development. Autoreactive B cells from which the autoantibodies originate have also received attention as perhaps more likely disease culprits. The development of autoreactive B cells in RA largely depends on the interaction with T cells in which HLA “shared epitope” and HLA DERAA may play an important role. Recent technological advances made it possible to identify and characterize citrulline‐reactive B cells and acquire ACPA monoclonal antibodies, which are providing valuable insights and help to understand the nature of the autoimmune response underlying RA. In this review, we summarize what is currently known about the role of autoantibodies and autoreactive B cells in RA and we discuss the most prominent hypotheses aiming to explain the origins and the evolution of autoimmunity in RA.

Identification and Characterization of Post-activated B Cells in Systemic Autoimmune Diseases

Autoimmune diseases (AID) such as systemic lupus erythematosus (SLE), primary Sjögren's syndrome (pSS), and rheumatoid arthritis (RA) are chronic inflammatory diseases in which abnormalities of B cell function play a central role. Although it is widely accepted that autoimmune B cells are hyperactive in vivo, a full understanding of their functional status in AID has not been delineated. Here, we present a detailed analysis of the functional capabilities of AID B cells and dissect the mechanisms underlying altered B cell function. Upon BCR activation, decreased spleen tyrosine kinase (Syk) and Bruton's tyrosine kinase (Btk) phosphorylation was noted in AID memory B cells combined with constitutive co-localization of CD22 and protein tyrosine phosphatase (PTP) non-receptor type 6 (SHP-1) along with hyporesponsiveness to TLR9 signaling, a Syk-dependent response. Similar BCR hyporesponsiveness was also noted specifically in SLE CD27⁻ B cells together with increased PTP activities and increased transcripts for PTPN2, PTPN11, PTPN22, PTPRC, and PTPRO in SLE B cells. Additional studies revealed that repetitive BCR stimulation of normal B cells can induce BCR hyporesponsiveness and that tissue-resident memory B cells from AID patients also exhibited decreased responsiveness immediately ex vivo, suggesting that the hyporesponsive status can be acquired by repeated exposure to autoantigen(s) in vivo. Functional studies to overcome B cell hyporesponsiveness revealed that CD40 co-stimulation increased BCR signaling, induced proliferation, and downregulated PTP expression (PTPN2, PTPN22, and receptor-type PTPs). The data support the conclusion that hyporesponsiveness of AID and especially SLE B cells results from chronic in vivo stimulation through the BCR without T cell help mediated by CD40–CD154 interaction and is manifested by decreased phosphorylation of BCR-related proximal signaling molecules and increased PTPs. The hyporesponsiveness of AID B cells is similar to a form of functional anergy.

Moxetumomab pasudotox-tdfk for relapsed/refractory hairy cell leukemia: a review of clinical considerations

PURPOSE

Hairy cell leukemia (HCL) is a rare mature B cell leukemia. Purine analogs are the mainstay of treatment of HCL, but relapse after purine analog therapy is common. Outcomes of treatment of relapsed/refractory HCL typically diminish with each successive line of therapy. Moxetumomab pasudotox-tdfk is a novel recombinant immunotoxin approved for the treatment of patients with relapsed/refractory HCL who have received at least two prior therapies, including a purine analog. This article reviews HCL treatment, focusing on moxetumomab pasudotox-tdfk, its place in therapy, considerations for preparation and administration, and strategies for prevention and management of toxicities.

METHODS

A literature search was conducted in the PubMed database from inception to January 2019, using the following terms: moxetumomab, hairy cell leukemia, relapsed/refractory hairy cell leukemia, immunotoxin, and CD22. The package insert and available posters and abstracts were also reviewed.

RESULTS

FDA approval of moxetumomab pasudotox-tdfk was based on a phase III single-arm, open-label trial in 80 patients. Treatment with moxetumomab pasudotox-tdfk yielded a durable complete response rate of 30% with a median duration of response that had not yet been reached at a median follow-up of 16.7 months. The objective response rate was 75% based on blinded independent central review. The most common adverse reactions were infusion-related reactions, edema, nausea, fatigue, headache, pyrexia and anemia. Serious adverse events include capillary leak syndrome and hemolytic uremic syndrome.

CONCLUSIONS

Clinicians providing care for patients receiving moxetumomab pasudotox-tdfk should be aware of the strategies required for safe administration, including the management of serious adverse events.

B Cell Siglecs-News on Signaling and Its Interplay With Ligand Binding

CD22 and Siglec-G are members of the Siglec family. Both are inhibitory co-receptors on the surface of B cells and inhibit B-cell receptor induced signaling, characterized by inhibition of the calcium mobilization and cellular activation. CD22 functions predominantly as an inhibitor on conventional B cells, while Siglec-G is an important inhibitor on the B1a-cell subset. These two B-cell Siglecs do not only inhibit initial signaling, but also have an important function in preventing autoimmunity, as double deficient mice develop a lupus-like phenotype with age. Siglecs are characterized by their conserved ability to bind terminal sialic acid of glycans on the cell surface, which is important to regulate the inhibitory role of Siglecs. While CD22 binds α2,6-linked sialic acids, Siglec-G can bind both α2,6-linked and α2,3-linked sialic acids. Interestingly, ligand binding is differentially regulating the ability of CD22 and Siglec-G to control B-cell activation. Within the last years, quite a few studies focused on the different functions of B-cell Siglecs and the interplay of ligand binding and signal inhibition. This review summarizes the role of CD22 and Siglec-G in regulating B-cell receptor signaling, membrane distribution with the importance of ligand binding, preventing autoimmunity and the role of CD22 beyond the naïve B-cell stage. Additionally, this review article features the long time discussed interaction between CD45 and CD22 with highlighting recent data, as well as the interplay between CD22 and Galectin-9 and its influence on B-cell receptor signaling. Moreover, therapeutical approaches targeting human CD22 will be elucidated.

Decreased CD22 expression and intracellular signaling aberrations in B cells of patients with systemic sclerosis

The objective of the study was to explore the phenotype and intracellular signaling events of B cells in patients with systemic sclerosis (SSc). Peripheral blood B cell surface markers CD19 and CD22 were evaluated by flow cytometry in 23 patients with SSc and seven healthy individuals. Levels of intracellular kinases Lyn, Syk and P-Y 348 Syk along with phosphatase SHP-1 were examined with Western immunoblotting in selected patients. P-Y 822 CD22 was subsequently evaluated flow cytometrically in antigen receptor stimulated B cells. A statistically significant decrease in CD22 B cell surface expression was found in the diffuse subset of patients (median CD22 MFI ± SD was 5.90 ± 2.35 vs 10.20 ± 1.88 for patients vs healthy controls respectively; p = 0.021), while no statistically significant change was found regarding CD19. CD22 underexpression was more pronounced when interstitial lung disease (ILD) was present (median CD22 MFI ± SD was 5.90 ± 2.25 vs 10.20 ± 1.88 for patients with ILD vs healthy controls respectively; p = 0.011). CD22 phosphorylation following B cell receptor (BCR) stimulation was also found to be impaired in patients with diffuse SSc (median change in MFI ± SD was 0.28 ± 0.09 vs 0.38 ± 0.08 for patients vs healthy controls respectively; p = 0.034). Low CD22 expression was arithmetically correlated with kinase Lyn underexpression (Pearson coefficient 0.926; p = ns) in B cells from a small sample of patients. These results suggest that CD22 underexpression and impaired phosphorylation along with implications for Lyn kinase aberrations could contribute to the activated B cell phenotype in SSc.

Advances in the diagnosis and treatment of Sjogren's syndrome

Sjogren's syndrome (SS) is a systemic autoimmune disease that primarily affects the exocrine glands, resulting in dryness of the eyes and mouth due to lymphocytic infiltration of the salivary and lacrimal glands along with arthritis, kidney, liver, and lung involvement, chronic fatigue, musculoskeletal pain, vasculitis, and so on. Considerable advance has been made for the classification and treatment of primary SS in the past few years. This article reviews the recent classification criteria for primary SS and briefly discusses the conventional and novel therapies of the disease.

B Cell Modulation Strategies in Autoimmune Diseases: New Concepts

B cells are major effector cells in autoimmunity through antibody production, T cell help and pro-inflammatory cytokine production. Major advances have been made in human B cell biology knowledge using rituximab and type II new anti-CD20 antibodies, anti-CD19 antibodies, anti-CD22 antibodies, autoantigen specific B cell depleting therapy (chimeric antigen receptor T cells), and B cell receptor signaling inhibition (Bruton’s tyrosine kinase inhibitors). However, in certain circumstances B cell depleting therapy may lead to the worsening of the autoimmune disease which is in accordance with the existence of a regulatory B cell population. Current concepts and future directions for B cell modulating therapies in autoimmune diseases with a special focus on pemphigus are discussed.

Epratuzumab for the treatment of systemic lupus erythematosus

INTRODUCTION

Systemic lupus erythematosus (SLE) is a multi-system autoimmune disease. There are three drugs licensed for the treatment of lupus: corticosteroids, hydroxychloroquine and belimumab. Immunosuppressants such as azathioprine, methotrexate and mycophenolate are also used. Despite these treatments there is still considerable morbidity. New treatments are needed for the management of active lupus. Epratuzumab a humanized IgG1 monoclonal antibody that targets CD22 resulting in selective B cell modulation that has been considered a potential treatment for SLE. Areas covered: Summary of the relevant pathogenesis and disease activity measurements used in SLE patients, current treatments and unmet needs in SLE, pharmacokinetics and pharmacodynamics of epratuzumab therapy, and a summary of the 7 clinical trials that have investigated the efficacy and safety of epratuzumab in SLE. Expert commentary: It is not clear why trials have failed to demonstrate efficacy but high placebo response rates from optimisation of standard of care and a sub-optimal dosing regimen may have played a role. Post-hoc analysis suggested that there may be subgroups that did respond, such as anti-SSA positive patients with features of Sjogren's syndrome. Further research is needed to explore this and other potential sub-groups that might respond.

Besides an ITIM/SHP-1-dependent pathway, CD22 collaborates with Grb2 and plasma membrane calcium-ATPase in an ITIM/SHP-1-independent pathway of attenuation of Ca2+i signal in B cells

CD22 is a surface immunoglobulin implicated in negative regulation of B cell receptor (BCR) signaling; particularly inhibiting intracellular Ca2+ (Ca2+i)signals. Its cytoplasmic tail contains six tyrosine residues (Y773/Y783/Y817/Y828/Y843/Y863, designated Y1~Y6 respectively), including three (Y2/5/6) lying within immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that serve to recruit the protein tyrosine phosphatase SHP-1 after BCR activation-induced phosphorylation. The mechanism of inhibiting Ca2+i by CD22 has been poorly understood. Previous study demonstrated that CD22 associated with plasma membrane calcium-ATPase (PMCA) and enhanced its activity (Chen, J. et al. Nat Immunol 2004;5:651-7). The association is dependent on BCR activation-induced cytoplasmic tyrosine phosphorylation, because CD22 with either all six tyrosines mutated to phenylalanines or cytoplasmic tail truncated loses its ability to associate with PMCA. However, which individual or a group of tyrosine residues determine the association and how CD22 and PMCA interacts, are still unclear. In this study, by using a series of CD22 tyrosine mutants, we found that ITIM Y2/5/6 accounts for 34.3~37.1% Ca2+i inhibition but is irrelevant for CD22/PMCA association. Non-ITIM Y4 and its YEND motif contribute to the remaining 69.4~71.7% Ca2+i inhibition and is the binding site for PMCA-associated Grb2. Grb2, independently of BCR cross-linking, is constitutively associated with and directly binds to PMCA in both chicken and human B cells. Knockout of Grb2 by CRISPR/Cas9 completely disrupted the CD22/PMCA association. Thus, our results demonstrate for the first time that in addition to previously-identified ITIM/SHP-1-dependent pathway, CD22 holds a major pathway of negative regulation of Ca2+i signal, which is ITIM/SHP-1-independent, but Y4/Grb2/PMCA-dependent.

Targeting CD22 with the monoclonal antibody epratuzumab modulates human B-cell maturation and cytokine production in response to Toll-like receptor 7 (TLR7) and B-cell receptor (BCR) signaling

Background

Abnormal B-cell activation is implicated in the pathogenesis of autoimmune diseases, including systemic lupus erythematosus (SLE). The B-cell surface molecule CD22, which regulates activation through the B-cell receptor (BCR), is a potential target for inhibiting pathogenic B cells; however, the regulatory functions of CD22 remain poorly understood. In this study, we determined how targeting of CD22 with epratuzumab (Emab), a humanized anti-CD22 IgG1 monoclonal antibody, affects the activation of human B-cell subsets in response to Toll-like receptor 7 (TLR7) and BCR engagement.

Methods

B-cell subsets were isolated from human tonsils and stimulated with F(ab′)₂ anti-human IgM and/or the TLR7 agonist R848 in the presence of Emab or a human IgG1 isotype control. Changes in mRNA levels of genes associated with B-cell activation and differentiation were analyzed by quantitative PCR. Cytokine production was measured by ELISA. Cell proliferation, survival, and differentiation were assessed by flow cytometry.

Results

Pretreatment of phenotypically naïve CD19⁺CD10^–CD27^– cells with Emab led to a significant increase in IL-10 expression, and in some but not all patient samples to a reduction of IL-6 production in response to TLR7 stimulation alone or in combination with anti-IgM. Emab selectively inhibited the expression of PRDM1, the gene encoding B-lymphocyte-induced maturation protein 1 (Blimp-1) in activated CD10^–CD27^– B cells. CD10^–CD27^–IgD^– cells were highly responsive to stimulation through TLR7 as evidenced by the appearance of blasting CD27^hiCD38^hi cells. Emab significantly inhibited the activation and differentiation of CD10^–CD27^–IgD^– B cells into plasma cells.

Conclusions

Emab can both regulate cytokine expression and block Blimp1-dependent B-cell differentiation, although the effects of Emab may depend on the stage of B-cell development or activation. In addition, Emab inhibits the activation of CD27^–IgD^– tonsillar cells, which correspond to so-called double-negative memory B cells, known to be increased in SLE patients with more active disease. These data may be relevant to the therapeutic effect of Emab in vivo via modulation of the production of pro-inflammatory and anti-inflammatory cytokines by B cells. Because Blimp-1 is required by B cells to mature into antibody-producing cells, inhibition of Blimp1 may reduce autoantibody production.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1284-2) contains supplementary material, which is available to authorized users.

Clinical Pharmacokinetics and Pharmacodynamics of Biologic Therapeutics for Treatment of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a multifactorial autoimmune disease with potentially severe clinical manifestation that mainly affects women of child-bearing age. Patients who do not respond to standard-of-care therapies, such as corticosteroids and immunosuppressants, require biologic therapeutics that specifically target a single or multiple SLE pathogenesis pathways. This review summarizes the clinical pharmacokinetic and pharmacodynamic characteristics of biologic agents that are approved, used off-label, or in the active pipeline of drug development for SLE patients. Depending on the type of target, the interacting biologics may exhibit linear (non-specific) or non-linear (target-mediated) disposition profiles, with terminal half-lives varying from approximately 1 week to 1 month. Biologics given by subcutaneous administration, which offers dosing flexibility over intravenous administration, demonstrated a relatively slow absorption with a time to maximum concentration of approximately 1 day to 2 weeks and a variable bioavailability of 30-82 %. The population pharmacokinetics of monoclonal antibodies were best described by a two-compartment model with central clearance and steady-state volume of distribution ranging from 0.176 to 0.215 L/day and 3.60-5.29 L, respectively. The between-subject variability in pharmacokinetic parameters were moderate (20-79 %) and could be partially explained by body size. The development of linked pharmacokinetic-pharmacodynamic models incorporating SLE disease biomarkers are an attractive strategy for use in dosing regimen simulation and optimization. The relationship between efficacy/adverse events and biologic concentration should be evaluated to improve clinical trial outcomes, especially for biologics in the advanced phase of drug development. New strategies, such as model-based precision dosing dashboards, could be utilized to incorporate information collected from therapeutic drug monitoring into pharmacokinetic/pharmacodynamic models to enable individualized dosing in real time.

Beyond pan-B-cell-directed therapy - new avenues and insights into the pathogenesis of SLE

New insights into the mechanisms of autoimmune diseases have been obtained not only from preclinical studies, but also from clinical trials of pan-B-cell-directed therapy. Overall, the results of these clinical trials suggest that more-specific approaches focusing on pathogenic B-cell functions, and perhaps sparing or even enhancing regulatory B-cell activity, might be attractive alternatives. Importantly, pathogenic B-cell subpopulations function within a network of cellular interactions, many of which might require additional interventions to restore immunologic balance and suppress autoimmune disease. Thus, approaches that simultaneously target innate immune cells as well as multiple nodes of T-cell and B-cell interactions might hold the promise of improved therapeutic efficacy. Interfering with B-cell intracellular signalling pathways, altering their intracellular metabolic pathways and perturbing transcription factors are additional options. This Review critically analyses these approaches, examines the role of cytokines and other functions of B-lineage cells separate from antibody secretion, and provides insights into the potential next generation of therapies targeting B-lineage cells.

Targeting B cells in treatment of autoimmunity

B cells have emerged as effective targets for therapeutic intervention in autoimmunities in which the ultimate effectors are antibodies, as well as those in which T cells are primary drivers of inflammation. Proof of this principle has come primarily from studies of the efficacy of Rituximab, an anti-CD20 mAb that depletes B cells, in various autoimmune settings. These successes have inspired efforts to develop more effective anti-CD20s tailored for specific needs, as well as biologicals and small molecules that suppress B cell function without the risks inherent in B cell depletion. Here we review the current status of B cell-targeted therapies for autoimmunity.

A Critical Review of the Effects of Hydroxychloroquine and Chloroquine on the Eye

Hydroxychloroquine (HCQ) and chloroquine have been used for more than 50 years to treat systemic lupus erythematosus (SLE) and other rheumatic diseases. In general, these drugs are well tolerated and rarely need to be discontinued because of an adverse systemic reaction. However, both medications can be irreversibly toxic to the retina. A new study indicates that toxicity is not as rare as once believed, but depends critically on daily dosage and duration of use, as well as other risk factors. With attention to dosage and other factors, and with proper screening for early signs of toxicity, HCQ can be prescribed with relative safety even over long periods of time.

Long-Term Safety and Efficacy of Epratuzumab in the Treatment of Moderate-to- Severe Systemic Lupus Erythematosus: Results From an Open-Label Extension Study

OBJECTIVE

The primary objective was to assess the long-term safety of repeated courses of epratuzumab therapy in patients with moderate-to-severe systemic lupus erythematosus. Secondary objectives were to assess long-term efficacy and health-related quality of life (HRQOL).

METHODS

Eligible patients from the 12-week, phase IIb, randomized, placebo-controlled EMBLEM study enrolled into the open-label extension (OLE) study, SL0008. In the SL0008 study, patients received 1,200 mg epratuzumab infusions at weeks 0 and 2 of repeating 12-week cycles, plus standard of care. Safety measures included treatment-emergent adverse events (TEAEs) and serious TEAEs. Efficacy measures included combined treatment response, the British Isles Lupus Assessment Group score, the Systemic Lupus Erythematosus Disease Activity Index score, and the physician's and patient's global assessment of disease activity. Total daily corticosteroid dose and HRQOL (by the Short Form 36 health survey) were also assessed.

RESULTS

A total of 113 of the 203 patients (55.7%) who entered the SL0008 study continued epratuzumab therapy until study closure (total cumulative exposure: 381.3 patient-years, median exposure: 845 days, and maximum exposure: 1,185 days/approximately 3.2 years). TEAEs were reported in 192 patients (94.6%); most common were infections and infestations (68.0%, 138 patients). Serious TEAEs were reported in 51 patients (25.1%), and 14 patients (6.9%) had serious infections. In patients treated for 108 weeks (n = 116), the median corticosteroid dose was reduced from 10.0 mg/day at OLE screening to 5.0 mg/day at week 108. Improvements in efficacy and HRQOL measures in EMBLEM were maintained in the OLE, while placebo patients exhibited similar improvements in disease activity upon a switch to epratuzumab.

CONCLUSION

Open-label epratuzumab treatment was well tolerated for up to 3.2 years, and associated with sustained improvements in disease activity and HRQOL, while steroids were reduced.

Epratuzumab modulates B-cell signaling without affecting B-cell numbers or B-cell functions in a mouse model with humanized CD22

Treatment of systemic lupus erythematosus patients with epratuzumab (Emab), a humanized monoclonal antibody targeting CD22, leads to moderately reduced B-cell numbers but does not completely deplete B cells. Emab appears to induce immunomodulation of B cells, but the exact mode of action has not been defined. In the present study, we aimed to understand the effects of Emab on B cells using a humanized mouse model (Huki CD22), in which the B cells express human instead of murine CD22. Emab administration to Huki CD22 mice results in rapid and long-lasting CD22 internalization. There was no influence on B-cell turnover, but B-cell apoptosis ex vivo was increased. Emab administration to Huki CD22 mice had no effect on B-cell numbers in several lymphatic organs, nor in blood. In vitro exposure of B cells from Huki CD22 mice to Emab resulted in decreased B-cell receptor (BCR) induced Ca(2+) mobilization, whereas B-cell proliferation after Toll-like receptor (TLR) stimulation was not affected. In addition, IL-10 production was slightly increased after TLR and anti-CD40 stimulation, whereas IL-6 production was unchanged. In conclusion, Emab appears to inhibit BCR signaling in a CD22-dependent fashion without strong influence on B-cell development and B-cell populations.

Use of Biologics in Sjögren's Syndrome

The management of patients suffering from primary Sjögren syndrome (pSS) has long been mainly symptomatic and demonstration of effectiveness of systemic drugs was lacking. However, progress made in the understanding of pSS pathogenesis has allowed moving into a more targeted approach to therapeutic intervention. Given the key role of chronic B-cell activation, B-cell target therapies were the first candidates. New pathways are currently being investigated, including costimulation and ectopic germinal center. In this review, we summarize the current evidence regarding B-cell targeted and anti-TNF therapies and provide an overview of promising drugs in the pipeline.

Engagement of CD22 on B cells with the monoclonal antibody epratuzumab stimulates the phosphorylation of upstream inhibitory signals of the B cell receptor

The binding of antigen to the B cell receptor (BCR) results in a cascade of signalling events that ultimately drive B cell activation. Uncontrolled B cell activation is regulated by negative feedback loops that involve inhibitory co-receptors such as CD22 and CD32B that exert their functions following phosphorylation of immunoreceptor tyrosine-based inhibition motifs (ITIMs). The CD22-targeted antibody epratuzumab has previously been shown to inhibit BCR-driven signalling events, but its effects on ITIM phosphorylation of CD22 and CD32B have not been properly evaluated. The present study therefore employed both immunoprecipitation and flow cytometry approaches to elucidate the effects of epratuzumab on direct phosphorylation of key tyrosine (Tyr) residues on both these proteins, using both transformed B cell lines and primary human B cells. Epratuzumab induced the phosphorylation of Tyr(822) on CD22 and enhanced its co-localisation with SHP-1. Additionally, in spite of high basal phosphorylation of other key ITIMs on CD22, in primary human B cells epratuzumab also enhanced phosphorylation of Tyr(807), a residue involved in the recruitment of Grb2. Such initiation events could explain the effects of epratuzumab on downstream signalling in B cells. Finally, we were able to demonstrate that epratuzumab stimulated the phosphorylation of Tyr(292) on the low affinity inhibitory Fc receptor CD32B which would further attenuate BCR-induced signalling. Together, these data demonstrate that engagement of CD22 with epratuzumab leads to the direct phosphorylation of key upstream inhibitory receptors of BCR signalling and may help to explain how this antibody modulates B cell function.

The Influence of Immunosuppressive Agents on the Risk of De Novo Donor-Specific HLA Antibody Production in Solid Organ Transplant Recipients

Production of de novo donor-specific antibodies (dnDSA) is a major risk factor for acute and chronic antibody-mediated rejection and graft loss after all solid organ transplantation. In this article, we review the data available on the risk of individual immunosuppressive agents and their ability to prevent dnDSA production. Induction therapy with rabbit antithymocyte globulin may achieve a short-term decrease in dnDSA production in moderately sensitized patients. Rituximab induction may be beneficial in sensitized patients, and in abrogating rebound antibody response in patients undergoing desensitization or treatment for antibody-mediated rejection. Use of bortezomib for induction therapy in at-risk patients is of interest, but the benefits are unproven. In maintenance regimens, nonadherent and previously sensitized patients are not suitable for aggressive weaning protocols, particularly early calcineurin inhibitor withdrawal without lymphocyte-depleting induction. Early conversion to mammalian target of rapamycin inhibitor monotherapy has been reported to increase the risk of dnDSA formation, but a combination of mammalian target of rapamycin inhibitor and reduced-exposure calcineurin inhibitor does not appear to alter the risk. Early steroid therapy withdrawal in standard-risk patients after induction has no known dnDSA penalty. The available data do not demonstrate a consistent effect of mycophenolic acid on dnDSA production. Risk minimization for dnDSA requires monitoring of adherence, appropriate risk stratification, risk-based immunosuppression intensity, and prospective DSA surveillance.

New biological therapies in Sjögren's syndrome

Primary Sjögren's syndrome (pSS) is a systemic autoimmune disease characterized by dryness and systemic involvement in more than a third of the patients. Patient management has suffered from the lack of effective treatments. However, progresses made in the understanding of pSS pathogenesis have allowed a move to a more targeted approach to therapeutic intervention. Given the key role of chronic B cell activation, B cell-targeted therapies were the first candidate. New pathways are currently investigated including costimulation and ectopic germinal centre formation. In this review, we have summarized the new tools available in clinical research in the field of pSS, the current evidence regarding B cell-targeted therapies and an overview of the promising drugs in the pipeline.

Safety, pharmacokinetics, and pharmacodynamics of epratuzumab in Japanese patients with moderate-to-severe systemic lupus erythematosus: Results from a phase 1/2 randomized study

OBJECTIVES

This 12-week, randomized, double-blind, placebo-controlled, multicenter phase 1/2 study (NCT01449071) assessed the safety, pharmacokinetics, and pharmacodynamics of epratuzumab in Japanese patients with moderate-to-severe systemic lupus erythematosus despite standard of care.

METHODS

Twenty patients were randomized 1:1:1:1:1 to placebo or one of four epratuzumab dose regimens (100 mg every other week [Q2W], 400 mg Q2W, 600 mg every week [QW], or 1200 mg Q2W) administered during an initial 4-week dosing period. Adverse events (AEs), pharmacokinetics and pharmacodynamics were assessed.

RESULTS

Nineteen of 20 patients completed the study. All placebo patients and 13 of 16 epratuzumab patients reported ≥1 AE, 2 of 16 epratuzumab patients reported a serious AE. C(max) and AUC(τ) increased proportionally with dose after first and last infusion, t(1/2) was similar across groups (∼13 days). Epratuzumab treatment was associated with decreased CD22 mean fluorescence intensity in total B cells (CD19(+)CD22(+)) and unswitched memory B cells (CD19(+)IgD(+)CD27(+)). Small-to-moderate decreases were observed in total B cell (CD20(+)) count.

CONCLUSIONS

Epratuzumab was well-tolerated, with no new safety signals identified. The pharmacokinetics appeared linear after first and last infusions. Treatment with epratuzumab was associated with CD22 downregulation and with small-to-moderate decreases in total B cell count.

Therapeutic Targeting of Siglecs using Antibody- and Glycan-Based Approaches

The sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of immunomodulatory receptors whose functions are regulated by their glycan ligands. Siglecs are attractive therapeutic targets because of their cell type-specific expression pattern, endocytic properties, high expression on certain lymphomas/leukemias, and ability to modulate receptor signaling. Siglec-targeting approaches with therapeutic potential encompass antibody- and glycan-based strategies. Several antibody-based therapies are in clinical trials and continue to be developed for the treatment of lymphoma/leukemia and autoimmune disease, while the therapeutic potential of glycan-based strategies for cargo delivery and immunomodulation is a promising new approach. Here we review these strategies with special emphasis on emerging approaches and disease areas that may benefit from targeting the Siglec family.

Extensive crosslinking of CD22 by epratuzumab triggers BCR signaling and caspase-dependent apoptosis in human lymphoma cells

Epratuzumab has demonstrated therapeutic activity in patients with non-Hodgkin lymphoma, acute lymphoblastic leukemia, systemic lupus erythematosus, and Sjögren's syndrome, but its mechanism of affecting normal and malignant B cells remains incompletely understood. We reported previously that epratuzumab displayed in vitro cytotoxicity to CD22-expressing Burkitt lymphoma cell lines (Daudi and Ramos) only when immobilized on plates or combined with a crosslinking antibody plus a suboptimal amount of anti-IgM (1 μg/mL). Herein, we show that, in the absence of additional anti-IgM ligation, extensive crosslinking of CD22 by plate-immobilized epratuzumab induced intracellular changes in Daudi cells similar to ligating B-cell antigen receptor with a sufficiently high amount of anti-IgM (10 μg/mL). Specifically, either treatment led to phosphorylation of CD22, CD79a and CD79b, along with their translocation to lipid rafts, both of which were essential for effecting caspase-dependent apoptosis. Moreover, such immobilization induced stabilization of F-actin, phosphorylation of Lyn, ERKs and JNKs, generation of reactive oxygen species (ROS), decrease in mitochondria membrane potential (Δψ_m), upregulation of pro-apoptotic Bax, and downregulation of anti-apoptotic Bcl-xl and Mcl-1. The physiological relevance of immobilized epratuzumab was implicated by noting that several of its in vitro effects, including apoptosis, drop in Δψ_m, and generation of ROS, could be observed with soluble epratuzumab in Daudi cells co-cultivated with human umbilical vein endothelial cells. These results suggest that the in vivo mechanism of non-ligand-blocking epratuzumab may, in part, involve the unmasking of CD22 to facilitate the trans-interaction of B cells with vascular endothelium.

The mechanistic impact of CD22 engagement with epratuzumab on B cell function: Implications for the treatment of systemic lupus erythematosus

Epratuzumab is a B-cell-directed non-depleting monoclonal antibody that targets CD22. It is currently being evaluated in two phase 3 clinical trials in patients with systemic lupus erythematosus (SLE), a disease associated with abnormalities in B-cell function and activation. The mechanism of action of epratuzumab involves perturbation of the B-cell receptor (BCR) signalling complex and intensification of the normal inhibitory role of CD22 on the BCR, leading to reduced signalling and diminished activation of B cells. Such effects may result from down-modulation of CD22 upon binding by epratuzumab, as well as decreased expression of other proteins involved in amplifying BCR signalling capability, notably CD19. The net result is blunting the capacity of antigen engagement to induce B-cell activation. The functional consequences of epratuzumab binding to CD22 include diminished B-cell proliferation, effects on adhesion molecule expression, and B-cell migration, as well as reduced production of pro-inflammatory cytokines, such as IL-6 and TNF. Studies in patients treated with epratuzumab have revealed a number of pharmacodynamic effects that are linked to the mechanism of action (i.e., a loss of the target molecule CD22 from the B-cell surface followed by a modest reduction in peripheral B-cell numbers after prolonged therapy). Together, these data indicate that epratuzumab therapy affords a unique means to modulate BCR complex expression and signalling.

Epratuzumab inhibits the production of the proinflammatory cytokines IL-6 and TNF-α, but not the regulatory cytokine IL-10, by B cells from healthy donors and SLE patients

Introduction

Cytokines produced by B cells are believed to play important roles in autoimmune diseases. CD22 targeting by epratuzumab has been demonstrated to inhibit phosphorylation of B cell receptor (BCR) downstream signaling in B cells. It has been shown that other sialoadhesin molecules related to CD22 have immunoregulatory functions; therefore, in the present study, we addressed the role of epratuzumab on the production of key cytokines by B cells of patients with systemic lupus erythematosus (SLE) and of healthy donors (HD).

Methods

Peripheral blood B cells were purified and activated by BCR with or without Toll-like receptor 9 (TLR9) stimulation in the presence or absence of epratuzumab. Cytokine production by B cells (interleukin [IL]-6, tumor necrosis factor [TNF]-α and IL-10) in the supernatant and the induction of IL-10⁺ B cells from patients with SLE and HD were analyzed.

Results

The secretion of the proinflammatory cytokines TNF-α and IL-6 by anti-BCR and BCR- and/or TLR9-activated B cells from HD and patients with SLE was inhibited by epratuzumab. In contrast, the production of IL-10 by B cells was not affected by epratuzumab under either stimulation condition. Consistently, the induction of IL-10–producing B cells in culture was not affected by epratuzumab.

Conclusions

Epratuzumab, by targeting CD22, was able to inhibit the production of the proinflammatory cytokines IL-6 and TNF-α by B cells, in contrast to IL-10, in vitro. These data suggest that targeting CD22 alters the balance between proinflammatory cytokines (TNF-α, IL-6) and the regulatory cytokine IL-10 as another B cell effector mechanism.

Systems pharmacology-based drug discovery for marine resources: an example using sea cucumber (Holothurians)

ETHNOPHARMACOLOGICAL RELEVANCE

Sea cucumber, a kind of marine animal, have long been utilized as tonic and traditional remedies in the Middle East and Asia because of its effectiveness against hypertension, asthma, rheumatism, cuts and burns, impotence, and constipation. In this study, an overall study performed on sea cucumber was used as an example to show drug discovery from marine resource by using systems pharmacology model. The value of marine natural resources has been extensively considered because these resources can be potentially used to treat and prevent human diseases. However, the discovery of drugs from oceans is difficult, because of complex environments in terms of composition and active mechanisms. Thus, a comprehensive systems approach which could discover active constituents and their targets from marine resource, understand the biological basis for their pharmacological properties is necessary.

MATERIALS AND METHODS

In this study, a feasible pharmacological model based on systems pharmacology was established to investigate marine medicine by incorporating active compound screening, target identification, and network and pathway analysis.

RESULTS

As a result, 106 candidate components of sea cucumber and 26 potential targets were identified. Furthermore, the functions of sea cucumber in health improvement and disease treatment were elucidated in a holistic way based on the established compound-target and target-disease networks, and incorporated pathways.

CONCLUSIONS

This study established a novel strategy that could be used to explore specific active mechanisms and discover new drugs from marine sources.

Evaluation of epratuzumab as a biologic therapy in systemic lupus erythematosus

B cells play a key role in the pathogenesis of systemic lupus erythematosus. Some of the current biologic therapies target B cells or B-cell activating factors. Epratuzumab is a humanized monoclonal antibody, which targets CD22 on B cells. This review focuses on the safety and efficacy of epratuzumab in systemic lupus erythematosus based on the information from various published clinical trials and presentations at international meetings. Epratuzumab acts as a B-cell modulator through inhibition of B-cell receptor signaling. It has been shown to be efficacious in open-label and Phase I and Phase II randomized controlled trials. The drug has steroid-sparing properties and treatment is associated with significant improvements in Health Related Quality of Life and its safety profile is comparable to placebo.

B cells: depletion or functional modulation in rheumatic diseases

PURPOSE OF REVIEW

The availability of drugs directly and indirectly targeting the B cells has refocussed attention on the role of B lymphocytes in rheumatic autoimmune/inflammatory diseases (RAIDs), but their distinct therapeutic potential for certain diseases remains to be further assessed.

RECENT FINDINGS

Although additional drugs are currently in clinical development targeting surface molecules (CD19, CD20, CD22, etc.) and cytokines (IL-6, IL-21, BAFF and APRIL) with key effects on B cell/plasma cell survival and differentiation, respectively, recent studies have also provided further insights into the effects of currently available drugs on protective immunity and mechanisms of the initiation and progression of RAIDs (i.e. rituximab, belimumab, mycophenolate and azathioprine). A key aspect of B-cell-directed drugs is their impact on continuous immune activation and chronic maintenance which may differ between individual RAIDs.

SUMMARY

The translational advances in the area of B-cell-depleting therapies and more sophisticated approaches to modulate key B-cell functions, such as blocking B-cell receptor downstream effects, interfering with the differentiation and survival of antigen-experienced memory B and plasma cells are of central interest. Differences in the efficacy and safety profiles of B-cell depletion compared with B-cell-modulating therapies (including antigen-specific tolerance induction) need to be further delineated.

CD22 and Siglec-G regulate inhibition of B-cell signaling by sialic acid ligand binding and control B-cell tolerance

CD22 and Siglec-G are two B-cell expressed members of the Siglec (sialic acid-binding immunoglobulin (Ig)-like lectin) family and are potent inhibitors of B-cell signaling. Genetic approaches have provided evidence that this inhibition of B-cell antigen receptor (BCR) signaling by Siglecs is dependent on ligand binding to sialic acids in specific linkages. The cis-ligand-binding activity of CD22 leads to homo-oligomer formation, which are to a large extent found in membrane domains that are distinct from those containing the BCR. In contrast, Siglec-G is recruited via sialic acid binding to the BCR. This interaction of Siglec-G with mIgM leads to an inhibitory function that seems to be specific for B-1 cells. Both CD22 and Siglec-G control B-cell tolerance and loss of these proteins, its ligands or its inhibitory pathways can increase the susceptibility for autoimmune diseases. CD22 is a target protein both in B-cell leukemias and lymphomas, as well as in B-cell mediated autoimmune diseases. Both antibodies and synthetic chemically modified sialic acids are currently tested to target Siglecs on B cells.

The role of CD22 and Siglec-G in B-cell tolerance and autoimmune disease

A high proportion of peripheral human B cells produce polyreactive or autoreactive antibodies, which indicates that they have escaped the elimination of self-reactive B cells in the bone marrow. CD22 and Siglec-G are two inhibitory receptors of the sialic-acid-binding immunoglobulin-like lectin (Siglec) family that inhibit the B-cell antigen receptor (BCR) signal. The ability of these two receptors to bind sialic acids is crucial for regulating inhibition and inducing tolerance to self-antigens. Sialylated glycans are usually absent on microbes (although several pathogenic microorganisms have evolved strategies to mimic self by decorating their surfaces with sialic acids) but abundant in higher vertebrates and might, therefore, provide an important tolerogenic signal. Combined Siglec-G deficiency and CD22 deficiency leads to spontaneous autoimmunity in mice, and mutations in an enzyme that modifies Siglec ligands are directly linked to several autoimmune diseases in humans. New data show that high-affinity ligands for CD22 and Siglec-G can be used to induce antigen-specific B-cell tolerance, which might be one strategy for the treatment of autoimmune diseases in the future.