Tuning antigen receptor signaling by CD22: integrating cues from antigens and the microenvironment

Non-canonical roles of Siglecs: Beyond sialic acid-binding and immune cell modulation

Siglecs (Sialic acid-binding immunoglobulin-type lectins) are I-type lectins that bind with sialic acid ligands (Sia). Most are expressed on the surface of leukocytes and are involved in immune regulation and possess immune tyrosine-based inhibitory motif (ITIM) in the intracellular domain, thus leading to inhibition of the immune response. This signaling is instrumental in maintaining quiescence under physiological conditions and acts as a brake for inflammatory cascades. By contrast, activating Siglecs carry positively charged residues in the transmembrane domain and interact with immune tyrosine-based activating motif (ITAM)-containing proteins, a DNAX-activating protein of 10-12 kDa (DAP10/12), to activate immune cells. There are various characteristics of Siglecs that do not fit within the classification of Siglec receptors as being either inhibitory or activating in nature. This review focuses on elucidating the non-canonical functions and interactions of Siglec receptors, which include Sia-independent interactions such as protein-protein interactions and interactions with lipids or other sugars. This review also summarizes Siglec expression and function on non-immune cells, and non-classical signaling of the receptor. Thus, this review will be beneficial to researchers interested in the field of Siglecs and sialic acid biology.

Advances in targeted therapy for malignant lymphoma

The incidence of lymphoma has gradually increased over previous decades, and it ranks among the ten most prevalent cancers worldwide. With the development of targeted therapeutic strategies, though a subset of lymphoma patients has become curable, the treatment of refractory and relapsed diseases remains challenging. Many efforts have been made to explore new targets and to develop corresponding therapies. In addition to novel antibodies targeting surface antigens and small molecular inhibitors targeting oncogenic signaling pathways and tumor suppressors, immune checkpoint inhibitors and chimeric antigen receptor T-cells have been rapidly developed to target the tumor microenvironment. Although these targeted agents have shown great success in treating lymphoma patients, adverse events should be noted. The selection of the most suitable candidates, optimal dosage, and effective combinations warrant further investigation. In this review, we systematically outlined the advances in targeted therapy for malignant lymphoma, providing a clinical rationale for mechanism-based lymphoma treatment in the era of precision medicine.

Boronate affinity-based photoactivatable magnetic nanoparticles for the oriented and irreversible conjugation of Fc-fused lectins and antibodies

The utilization of immuno-magnetic nanoparticles (MNPs) for the selective capture, enrichment, and separation of specific glycoproteins from complicated biological samples is appealing for the discovery of disease biomarkers. Herein, MNPs were designed and anchored with abundant boronic acid (BA) and photoreactive alkyl diazirine (Diaz) functional groups to obtain permanently tethered Fc-fused Siglec-2 and antiserum amyloid A (SAA) mAb with the assistance of reversible boronate affinity and UV light activation in an orientation-controlled manner. The Siglec-2-Fc-functionalized MNPs showed excellent stability in fetal bovine serum (FBS) and excellent efficiency in the extraction of cell membrane glycoproteins. The anti-SAA mAb-functionalized MNPs maintained active Ab orientation and preserved antigen recognition capability in biological samples. Thus, the BA-Diaz-based strategy holds promise for the immobilization of glycoproteins, such as antibodies, with the original protein binding activity maintained, which can provide better enrichment for the sensitive detection of target proteins.

CD22: A Regulator of Innate and Adaptive B Cell Responses and Autoimmunity

CD22 (Siglec 2) is a receptor predominantly restricted to B cells. It was initially characterized over 30 years ago and named “CD22” in 1984 at the 2nd International workshop in Boston (1). Several excellent reviews have detailed CD22 functions, CD22-regulated signaling pathways and B cell subsets regulated by CD22 or Siglec G (2–4). This review is an attempt to highlight recent and possibly forgotten findings. We also describe the role of CD22 in autoimmunity and the great potential for CD22-based immunotherapeutics for the treatment of autoimmune diseases such as systemic lupus erythematosus (SLE).

Nanoscale organization and dynamics of the siglec CD22 cooperate with the cytoskeleton in restraining BCR signalling

Receptor organization and dynamics at the cell membrane are important factors of signal transduction regulation. Using super-resolution microscopy and single-particle tracking, we show how the negative coreceptor CD22 works with the cortical cytoskeleton in restraining BCR signalling. In naïve B cells, we found endogenous CD22 to be highly mobile and organized into nanodomains. The landscape of CD22 and its lateral diffusion were perturbed either in the absence of CD45 or when the CD22 lectin domain was mutated. To understand how a relatively low number of CD22 molecules can keep BCR signalling in check, we generated Brownian dynamic simulations and supported them with ex vivo experiments. This combined approach suggests that the inhibitory function of CD22 is influenced by its nanoscale organization and is ensured by its fast diffusion enabling a "global BCR surveillance" at the plasma membrane.

A review of monoclonal antibody therapies in lymphoma

Monoclonal antibodies (moAb) represent a novel way of delivering therapy through specific target antigens expressed on lymphoma cells and minimizes the collateral damage that is common with conventional chemotherapy. The paradigm of this approach is the targeting of CD20 by rituximab. Since its FDA approval in 1997, rituximab has become the standard of care in almost every line of therapy in most B-cell lymphomas. This review will briefly highlight some of the key rituximab trials while looking more closely at the evidence that is bringing other antibodies, including next generation anti-CD20 moAbs, and anti-CD30 moAbs, among others to the forefront of lymphoma therapy.

A reevaluation of CD22 expression in human lung cancer

CD22 is a transmembrane glycoprotein expressed by mature B cells. It inhibits signal transduction by the B-cell receptor and its coreceptor CD19. Recent reports indicate that most human lung cancer cells and cell lines express CD22, making it an important new therapeutic target for lung cancer. The objective of our studies was to independently validate these results with the goal of testing the efficacy of our CD22 immunotoxins on lung cancer cell lines. As determined by quantitative real-time PCR analysis, we found that levels of CD22 mRNA in a panel of human lung cancer cell lines were 200 to 60,000-fold lower than those observed in the human CD22(+) Burkitt lymphoma cells, Daudi. Using flow cytometry with a panel of CD22 monoclonal antibodies and Western blot analyses, we could not detect surface or intracellular expression of CD22 protein in a panel of lung cancer cell lines. In addition, the in vitro proliferation of the lung tumor cell lines was not affected by either CD22 antibodies or our highly potent anti-CD22 immunotoxin. In contrast, CD22(+) Daudi cells expressed high levels of CD22 mRNA and protein, and were sensitive to our CD22 immunotoxin. Importantly, primary non-small cell lung cancers from more than 250 patient specimens did not express detectable levels of CD22 protein as assessed by immunohistochemistry. We conclude that CD22 is not expressed at measurable levels on the surface of lung cancer cells, and that these cells cannot be killed by anti-CD22 immunotoxins.

Associations of genetic polymorphisms of Siglecs with human diseases

Genetic polymorphism studies in humans provide unique opportunities to understand human biology and the mechanisms of diseases. Correlations between polymorphisms in the genes encoding human Siglecs and various diseases have been reported. Leading examples, such as the CD33 polymorphism associated with late-onset Alzheimer's disease, are well supported by genetic replication and mechanistic studies, while some others (such as SIGLEC8 polymorphism associated with bronchial asthma and SIGLEC14 polymorphism associated with exacerbation of chronic obstructive pulmonary disease) may benefit reinforcement by independent genetic replication or mechanistic studies. In a few cases, such as MAG polymorphism associated with psychological disorder and CD22 polymorphism associated with autoimmune disease, the phenotype associated with a genetic polymorphism of a Siglec gene and that of an enzyme gene involved in the biosynthesis of Siglec ligand show some overlap, providing indirect support for the observed genotype-phenotype association. Although studies using engineered mutant mice have provided invaluable insights into the biological functions and mechanisms of diseases, it is not always possible to develop appropriate mouse model to replicate human situations because of significant species-to-species differences, which can be a major obstacle in understanding the biology of some of human CD33/Siglec-3-related Siglecs. Further studies in genetic polymorphisms of human Siglecs, combined with appropriate functional studies, may reveal unexpected biological roles of human Siglecs, and identify possible targets for prevention and/or treatment of certain diseases.

Chemical Approach to a Whole Body Imaging of Sialo-N-Linked Glycans

PET and noninvasive fluorescence imaging of the sialo-N-linked glycan derivatives are described. To establish the efficient labeling protocol for N-glycans and/or glycoconjugates, new labeling probes of fluorescence and ⁶⁸Ga-DOTA, as the positron emission nucleus for PET, through rapid 6π-azaelectrocyclization were designed and synthesized, (E)-ester aldehydes. The high reactivity of these probes enabled the labeling of lysine residues in peptides, proteins, and even amino groups on the cell surfaces at very low concentrations of the target molecules (~10⁻⁸ M) within a short reaction time (~5 min) to result in "selective" and "non-destructive" labeling of the more accessible amines. The first MicroPET of glycoproteins, ⁶⁸Ga-DOTA-orosomucoid and asialoorosomucoid, successfully visualized the differences in the circulatory residence of glycoproteins, in the presence or absence of sialic acids. In vivo dynamics of the new N-glycoclusters, prepared by the "self-activating" Huisgen cycloaddition reaction, could also be affected significantly by their partial structures at the non-reducing end, i.e., the presence or absence of sialic acids, and/or sialoside linkages to galactose. Azaelectrocyclization chemistry is also applicable to the engineering of the proteins and/or the cell surfaces by the oligosaccharides; lymphocytes chemically engineered by sialo-N-glycan successfully target the tumor implanted in BALB/C nude mice, detected by noninvasive fluorescence imaging.

CD72 Stimulation Modulates Anti-IgM Induced Apoptotic Signaling through the Pathway of NF-κB, c-Myc and p27Kip1

Engagement of mIgM induces G1 arrest and apoptosis in immature B cells. The biochemical mechanism(s) regulating the cell death process are poorly understood. Cross-linking of CD72 (a B cell co-receptor) with anti-CD72 antibody was shown to protect B cells from apoptosis. We investigated the molecular mechanism involved in apoptosis preventing signaling mediated by CD72 ligation using a derivative (WEHIdelta) of the WEHI231 cell line which is representative of immature B cells. Apoptotic WEHIdelta cells following cross-linking of mIgM demonstrate a dramatic loss of c-Myc protein after transient up-regulation. In contrast, pre-ligation of CD72 was able to sustain c-Myc expression after transient up-regulation. Cross-linking of mIgM of WEHIdelta cells causes accumulation of the Cdk inhibitor, p27(Kip1). CD72 pre-ligation was shown to inhibit the accumulation of p27(Kip1) protein. Moreover, NF-kappaB activity was not suppressed in WEHIdelta cells after mIgM cross-linking when the cells were pre-treated with anti-CD72 antibody. These results strongly suggest that the apoptosis preventing signal evoked by CD72 ligation is delivered through the pathway of NF-kappaB, c-Myc, p27(Kip1) and cyclin.

B lymphocytes: development, tolerance, and their role in autoimmunity-focus on systemic lupus erythematosus

B lymphocytes are the effectors of humoral immunity, providing defense against pathogens through different functions including antibody production. B cells constitute approximately 15% of peripheral blood leukocytes and arise from hemopoietic stem cells in the bone marrow. It is here that their antigen receptors (surface immunoglobulin) are assembled. In the context of autoimmune diseases defined by B and/or T cell autoreactive that upon activation lead to chronic tissue inflammation and often irreversible structural and functional damage, B lymphocytes play an essential role by not only producing autoantibodies but also functioning as antigen-presenting cells (APC) and as a source of cytokines. In this paper, we describe B lymphocyte functions in autoimmunity and autoimmune diseases with a special focus on their abnormalities in systemic lupus erythematosus.

Rethinking mechanisms of autoimmune pathogenesis

Why exactly some individuals develop autoimmune disorders remains unclear. The broadly accepted paradigm is that genetic susceptibility results in some break in immunological tolerance, may enhance the availability of autoantigens, and may enhance inflammatory responses. Some environmental insults that occur on this background of susceptibility may then contribute to autoimmunity. In this review we discuss some aspects related to inhibitory signaling and rare genetic variants, as well as additional factors that might contribute to autoimmunity including the possible role of clonal somatic mutations, the role of epigenetic events and the contribution of the intestinal microbiome. Genetic susceptibility alleles generally contribute to the loss of immunological tolerance, the increased availability of autoantigens, or an increase in inflammation. Apart from common genetic variants, rare loss-of-function genetic variants may also contribute to the pathogenesis of autoimmunity. Studies of an inhibitory signaling pathway in B cells helped identify a negative regulatory enzyme called sialic acid acetyl esterase. The study of rare genetic variants of this enzyme provides an illustrative example showing the importance of detailed functional analyses of variant alleles and the need to exclude functionally normal common or rare genetic variants from analysis. It has also become clear that pathways that are functionally impacted by either common or rare defective variants can also be more significantly compromised by gene expression changes that may result from epigenetic alterations. Another important and evolving area that has been discussed relates to the role of the intestinal microbiome in influencing helper T cell polarization and the development of autoimmunity.

Intravenous immunoglobulin-mediated immunosuppression and the development of an IVIG substitute

Immunoglobulins are glycoproteins produced by the cells of the immune system. Their primary function is to protect the body from pathogenic infection. Moreover, a concentrated polyclonal mixture of immunoglobulin G (IgG), the so-called intravenous IgG (IVIG), has been used to treat various chronic and systemic disorders of the immune system. Studies on the effects of IVIG in autoimmune disease models have revealed that IgG Fc fragments confer protection against various autoimmune diseases. The identification of this IgG Fc immunomodulatory component is important for the development of IVIG substitutes. The focus of this review is to introduce one of the Fc regulatory entities and to provide a summary of the current knowledge of the putative general mechanisms underlying IVIG activity in vivo on the basis of these Fc fragments. We also address the recent insights into several approaches for the development of IVIG substitutes.

Genomic characterization of remission in juvenile idiopathic arthritis

Introduction

The attainment of remission has become an important end point for clinical trials in juvenile idiopathic arthritis (JIA), although we do not yet have a full understanding of what remission is at the cell and molecular level.

Methods

Two independent cohorts of patients with JIA and healthy child controls were studied. RNA was prepared separately from peripheral blood mononuclear cells (PBMC) and granulocytes to identify differentially expressed genes using whole genome microarrays. Expression profiling results for selected genes were confirmed by quantitative, real-time polymerase chain reaction (RT-PCR).

Results

We found that remission in JIA induced by either methotrexate (MTX) or MTX plus a TNF inhibitor (etanercept, Et) (MTX + Et) is characterized by numerous differences in gene expression in peripheral blood mononuclear cells and in granulocytes compared with healthy control children; that is, remission is not a restoration of immunologic normalcy. Network analysis of the differentially expressed genes demonstrated that the steroid hormone receptor superfamily member hepatocyte nuclear factor 4 alpha (HNF4α) is a hub in several of the gene networks that distinguished children with arthritis from controls. Confocal microscopy revealed that HNF4a is present in both T lymphocytes and granulocytes, suggesting a previously unsuspected role for this transcription factor in regulating leukocyte function and therapeutic response in JIA.

Conclusions

These findings provide a framework from which to understand therapeutic response in JIA and, furthermore, may be used to develop strategies to increase the frequency with which remission is achieved in adult forms of rheumatoid arthritis.

CD22 and autoimmune disease

CD22 is a 140-kDa member of the Siglec family of cell surface proteins that is expressed by most mature B-cell lineages. As a co-receptor of the B-cell receptor (BCR), it is known to contribute to the sensitive control of the B-cell response to antigen. Cross-linking of CD22 and the BCR by antigen triggers the phosphorylation of CD22, which leads to activation of signaling molecules such as phosphatases. Signal transduction pathways involving CD22 have been explored in a number of mouse models, some of which have provided evidence that in the absence of functional CD22, B cells have a "hyperactivated" phenotype, and suggest that loss of CD22 function could contribute to the pathogenesis of autoimmune diseases. Modulating CD22 activity has therefore been suggested as a possible therapeutic approach to such diseases. For example, the novel CD22-targeting monoclonal antibody epratuzumab is currently under investigation as a treatment for the connective tissue disorder systemic lupus erythematosus (SLE).

Increased red cell turnover in a line of CD22-deficient mice is caused by Gpi1c: a model for hereditary haemolytic anaemia

CD22, an inhibitory co-receptor of the BCR, has been identified as a potential candidate gene for the development of autoimmune haemolytic anaemia in mice. In this study, we have examined Cd22(tm1Msn) CD22-deficient mice and identified an increase in RBC turnover and stress erythropoiesis, which might be consistent with haemolysis. We then, however, eliminated CD22 deficiency as the cause of accelerated RBC turnover and established that enhanced RBC turnover occurs independently of B cells and anti-RBC autoanti-bodies. Accelerated RBC turnover in this particular strain of CD22-deficient mice is red cell intrinsic and appears to be the consequence of a defective allele of glucose phosphate isomerase, Gpi1(c). This form of Gpi1 was originally derived from wild mice and results in a substantial reduction in enzyme activity. We have identified the polymorphism that causes impaired catalytic activity in the Gpi1(c) allele, and biochemically confirmed an approximate 75% reduction of GPI1 activity in Cd22(-/-) RBCs. The Cd22(-/-).Gpi1(c) congenic mouse provides a novel animal model of GPI1-deficiency, which is one of the most common causes of chronic non-spherocytic haemolytic anaemia in humans.

Siglecs as sensors of self in innate and adaptive immune responses

Siglecs are expressed on most white blood cells of the immune system and are known to modulate the activity of cell signaling receptors via regulatory motifs in their cytoplasmic domains. This immunoglobulin subfamily of coreceptors recognize sialic acid containing glycans as ligands, which are found on glycoproteins and glycolipids of all mammalian cells. By virtue of their ability to recognize this common structural element, siglecs are increasingly recognized for their ability to help immune cells distinguish between self and nonself, and dampen autoimmune responses.

CD22 serves as a receptor for soluble IgM

CD22 (Siglec-2) is a B-cell membrane-bound lectin that recognizes glycan ligands containing α2,6-linked sialic acid (α2,6Sia) and negatively regulates signaling through the B-cell Ag receptor (BCR). Although CD22 has been investigated extensively, its precise function remains unclear due to acting multiple phases. Here, we demonstrate that CD22 is efficiently activated in trans by complexes of Ag and soluble IgM (sIgM) due to the presence of glycan ligands on sIgM. This result strongly suggests sIgM as a natural trans ligand for CD22. Also, CD22 appears to serve as a receptor for sIgM, which induces a negative feedback loop for B-cell activation similar to the Fc receptor for IgG (FcγRIIB).

Role of PIR-B in autoimmune glomerulonephritis

PIR-B, an inhibitory receptor expressed on murine B cells and myeloid cells, regulates humoral and cellular immune responses via its constitutive binding to the ligand, MHC class I molecules, on the same cells (cis) or on different cells (trans). Although it has been speculated that PIR-B is important for maintaining peripheral tolerance, PIR-B single deficiency does not cause overt autoimmune diseases. Recently, however, the combination of its deficiency with the Fas lpr mutation was found to result in augmented production of autoantibodies such as IgG rheumatoid factor and anti-DNA IgG, leading to glomerulonephritis in mice. Although the precise molecular mechanism for the overall scenario is unclear, PIR-B was found to suppress TLR9-mediated production of naturally autoreactive antibodies by innate B cells or B-1 cells by inhibiting the activation of Bruton's tyrosine kinase. Thus, PIR-B is an important regulator of innate immunity mediated by TLR9 in B-1 cells, which can otherwise provoke autoimmunity when overactivated.

CD22 expression mediates the regulatory functions of peritoneal B-1a cells during the remission phase of contact hypersensitivity reactions

Although contact hypersensitivity (CHS) has been considered a prototype of T cell-mediated immune reactions, recently a significant contribution of regulatory B cell subsets in the suppression of CHS has been demonstrated. CD22, one of the sialic acid-binding immunoglobulin-like lectins, is a B cell-specific molecule that negatively regulates BCR signaling. To clarify the roles of B cells in CHS, CHS in CD22(-/-) mice was investigated. CD22(-/-) mice showed delayed recovery from CHS reactions compared with that of wild-type mice. Transfer of wild-type peritoneal B-1a cells reversed the prolonged CHS reaction seen in CD22(-/-) mice, and this was blocked by the simultaneous injection with IL-10 receptor Ab. Although CD22(-/-) peritoneal B-1a cells were capable of producing IL-10 at wild-type levels, i.p. injection of differentially labeled wild-type/CD22(-/-) B cells demonstrated that a smaller number of CD22(-/-) B cells resided in lymphoid organs 5 d after CHS elicitation, suggesting a defect in survival or retention in activated CD22(-/-) peritoneal B-1 cells. Thus, our study reveals a regulatory role for peritoneal B-1a cells in CHS. Two distinct regulatory B cell subsets cooperatively inhibit CHS responses. Although splenic CD1d(hi)CD5(+) B cells have a crucial role in suppressing the acute exacerbating phase of CHS, peritoneal B-1a cells are likely to suppress the late remission phase as "regulatory B cells." CD22 deficiency results in disturbed CHS remission by impaired retention or survival of peritoneal B-1a cells that migrate into lymphoid organs.

Immunophenotypic profile and role of adhesion molecules in splenic marginal zone lymphoma with bone marrow involvement

Splenic Marginal Zone Lymphoma (SMZL), with or without villous lymphocytes (VL+/-), is a low-grade lymphoproliferative disorder with constant involvement of the bone marrow (BM). Different BM infiltration patterns, mainly intra-sinusoidal, interstitial and nodular, have been described. Adhesion molecules (AMs) constitute a heterogeneous group of antigenic receptors playing a major role in leukocyte recruitment, in lymphocyte homing and in cellular-mediated immune response. Evolution and pattern of the BM infiltrate could be influenced by a variable expression of AM on SMZL lymphocytes. The degree and pattern of BM infiltration and the immunohistochemical expression of AM (H-CAM, BL-CAM, L-selectin, PSGL-1, E-selectin, ICAM-1, VCAM-1 and Beta-1 integrin) among the different infiltration patterns were evaluated in BM biopsies of 38 patients with SMZL and graded according to a semi-quantitative score ranging from 0-4 and based on the percentage of positive cells. An intra-sinusoidal infiltration was constantly observed, alone or in conjunction with other patterns. H-CAM and BL-CAM showed a moderate-to-high degree of positivity in the intra-sinusoidal infiltrate (median expression grade-3) and were expressed in the neoplastic lymphocytes independently from the pattern. PSGL-1 was mostly expressed in the perisinusoidal region and in case of interstitial infiltration (grade-2). ICAM-1 and VCAM-1 were selectively expressed in the nodules as a reticular meshwork located in the core region (grade-2); VCAM-1 was also expressed in the perinodular endothelia. E-selectin, L-selectin and beta-1 integrin proved constantly negative. These data suggest that different expression of AM can influence the modality of BM infiltration in SMZL.

Inositol 1,4,5-trisphosphate 3-kinase B is a negative regulator of BCR signaling that controls B cell selection and tolerance induction

Inositol 1,4,5-trisphosphate 3-kinase B (or Itpkb) converts inositol 1,4,5-trisphosphate to inositol 1,3,4,5-tetrakisphosphate upon Ag receptor activation and controls the fate and function of lymphocytes. To determine the role of Itpkb in B cell tolerance, Itpkb(-/-) mice were crossed to transgenic mice that express a BCR specific for hen egg lysozyme (IgHEL). B cells from Itpkb(-/-) IgHEL mice possess an anergic phenotype, hypoproliferate in response to cognate Ag, and yet they exhibit enhanced Ag-induced calcium signaling. In IgHEL transgenic mice that also express soluble HEL, lack of Itpkb converts anergy induction to deletion. These data establish Itpkb as a negative regulator of BCR signaling that controls the fate of developing B cells and tolerance induction.

Regulation of B-cell entry into the cell cycle

B cells are induced to enter the cell cycle by stimuli including ligation of the B-cell receptor (BCR) complex and Toll-like receptor (TLR) agonists. This review discusses the contribution of several molecules, which act at distinct steps in B-cell activation. The adapter molecule Bam32 (B-lymphocyte adapter of 32 kDa) helps promote BCR-induced cell cycle entry, while the secondary messenger superoxide has the opposite effect. Bam32 and superoxide may fine tune BCR-induced activation by competing for the same limited resources, namely Rac1 and the plasma membrane phospholipid PI(3,4)P(2). The co-receptor CD22 can inhibit BCR-induced proliferation by binding to novel CD22 ligands. Finally, regulators of B-cell survival and death also play roles in B-cell transit through the cell cycle. Caspase 6 negatively regulates CD40- and TLR-dependent G(1) entry, while acting later in the cell cycle to promote S-phase entry. Caspase 6 deficiency predisposes B cells to differentiate rather than proliferate after stimulation. Bim, a pro-apoptotic Bcl-2 family member, exerts a positive regulatory effect on cell cycle entry, which is opposed by Bcl-2. New insights into what regulates B-cell transit through the cell cycle may lead to thoughtful design of highly selective drugs that target pathogenic B cells.

Dendritic cell-dependent inhibition of B cell proliferation requires CD22

Recent studies have shown that dendritic cells (DCs) regulate B cell functions. In this study, we report that bone marrow (BM)-derived immature DCs, but not mature DCs, can inhibit BCR-induced proliferation of B cells in a contact-dependent manner. This inhibition is overcome by treatment with BAFF and is dependent on the BCR coreceptor CD22; however, it is not dependent on expression of the CD22 glycan ligand(s) produced by ST6Gal-I sialyltransferase. We found that a second CD22 ligand (CD22L) is expressed on CD11c(+) splenic and BM-derived DCs, which does not contain ST6Gal-I-generated sialic acids and which, unlike the B cell-associated CD22L, is resistant to neuraminidase treatment and sodium metaperiodate oxidation. Examination of splenic and BM B cell subsets in CD22 and ST6Gal-I knockout mice revealed that ST6Gal-I-generated B cell CD22L plays a role in splenic B cell development, whereas the maintenance of long-lived mature BM B cells depends only on CD22 and not on alpha2,6-sialic acids produced by ST6Gal-I. We propose that the two distinct CD22L have different functions. The alpha2,6-sialic acid-containing glycoprotein is important for splenic B cell subset development, whereas the DC-associated ST6Gal-I-independent CD22L may be required for the maintenance of long-lived mature B cells in the BM.

Glycosylation of IgG B cell receptor (IgG BCR) in multiple myeloma: relationship between sialylation and the signal activity of IgG BCR

Little is known about the glycosylation of the isotype switched B cell receptor (BCR) in multiple myeloma, and the way it might affect receptor function. In this work IgG BCRs isolated from the individual lysates of peripheral blood lymphocytes (PBL) of 32 patients with IgG multiple myeloma and healthy controls were investigated for the expression of sialic acid (SA), galactose (Gal) and N-acetylglucosamine (GlcNAc), the sugars known to specify the glycoforms of human serum IgG. The degree of glycosylation and signaling status of all 32 isolated myeloma IgG BCRs were correlated and compared with the glycosylation of the IgG paraproteins isolated from sera of the same patients. It was shown that BCR IgG in myeloma is more heavily sialylated when compared with normal controls, that the increased sialylation of IgG BCR is associated with higher levels of tyrosine phosphorylation (signaling activity) of the IgG BCR supramolecular complex and that BCR IgG and serum IgG paraprotein from the same patient differed in all cases in the levels of terminal sugar expression. The results suggest that the development of the malignant clone in MM from post-switch B cells expressing IgG BCR at their surfaces to plasma cells secreting IgG paraprotein may be followed by permanent glycosylation changes in the IgG molecules.

Multiple paths to loss of anergy and gain of autoimmunity

B cells and autoimmunity: cells of the immune system have the capacity to recognize/neutralize a myriad array of disease-causing pathogens, while simultaneously minimizing damage to self tissue. Obvious breakdowns in this ability to distinguish between self and non-self are evident in multiple forms of autoimmune disease, where B and T cells mount damaging attacks on cells and organs. B cells may directly damage tissue by producing pathogenic antibodies that bind self antigen, fix complement or form immune complexes. Recent evidence also suggests B cells indirectly induce autoimmunity by concentrating low avidity self antigen through the B cell receptor and presenting self-peptides to autoreactive T cells. B cells may also initiate autoimmunity when provided sufficient help from autoreactive T cells that have escaped deletion in the thymus. Here, we will review the role of anergy in maintenance of tolerance and how alterations in the normal balance of positive and negative signals may contribute to the development of autoimmune disease in mouse models and humans.

CD22: an inhibitory enigma

CD22 is an inhibitory coreceptor of the B-cell receptor (BCR), and plays a critical role in establishing signalling thresholds for B-cell activation. Like other coreceptors, the ability of CD22 to modulate B-cell signalling is critically dependent upon its proximity to the BCR, and this in turn is governed by the binding of its extracellular domain to alpha2,6-linked sialic acid ligands. Manipulation of CD22 ligand binding in various experimental settings has profound effects on B-cell signalling, but as yet there is no complete model for how ligand binding in vivo controls normal CD22 function. Several elegant studies have recently shed light on this issue, although the results appear to suggest two mutually exclusive models for the role of ligand binding; in either promoting or inhibiting, CD22 function. We shall therefore discuss these results in detail, and suggest possible approaches by which these conflicting experimental findings might be reconciled. We shall also consider a second important issue in CD22 biology, which relates to the role that defects in this receptor might play in mediating autoimmune disease. We review the current evidence for this, and discuss the importance of genetic background in modifying CD22 function and predisposition to autoimmunity.

Synthetic glycan ligand excludes CD22 from antigen receptor-containing lipid rafts

CD22/Siglec-2 is a B cell membrane-bound lectin that recognizes glycan ligands containing alpha2,6-linked sialic acid, and negatively regulates signaling through the B cell antigen receptor (BCR). Previous studies demonstrated that synthetic sialosides that bind to CD22 augment BCR signaling by inhibiting CD22-mediated BCR regulation. Here we demonstrate that, after antigen stimulation, CD22 forms a cap together with BCR, and translocates to lipid rafts. Both co-capping of CD22 with BCR and translocation of CD22 to lipid rafts were markedly blocked by a synthetic alpha2,6-linked sialic acid, Neu5Gcalpha2-6GalbetaSE. These results strongly suggest that synthetic glycan ligand excludes CD22 from BCR-containing lipid rafts. Because CD22-mediated signal regulation requires phosphorylation of CD22 by Lyn that localizes in lipid rafts and is activated by BCR, synthetic glycan ligand regulates localization of CD22 crucial for signal regulation.

B cell signaling and autoimmune diseases: CD19/CD22 loop as a B cell signaling device to regulate the balance of autoimmunity

Autoimmune diseases, including connective tissue diseases and bullous diseases, may be life-threatening. Recent clinical and experimental approaches have demonstrated that B cells play critical roles in the manifestation of autoimmune disease not only by well-established autoantibody-mediated mechanisms but also by a variety of other functions. These B cell functions are under the regulation of B cell antigen receptor (BCR)-induced signals and by specialized cell surface coreceptors, or "response regulators", which inform B cells of their microenvironment. These response regulators include CD19 and CD22. CD19 and CD22 do not merely regulate BCR signals independently, but they have their own regulatory network. CD19 regulates CD22 phosphorylation by augmenting Lyn kinase activity, while CD22 inhibits CD19 phosphorylation via SHP-1. Importantly, this "CD19/CD22 loop" is significantly related to an autoimmune phenotype in mice. Thus, the CD19/CD22 loop may be a potential therapeutic target in autoimmune disease for modulating B cell signaling.

Germinal center marker GL7 probes activation-dependent repression of N-glycolylneuraminic acid, a sialic acid species involved in the negative modulation of B-cell activation

Sialic acid (Sia) is a family of acidic nine-carbon sugars that occupies the nonreducing terminus of glycan chains. Diversity of Sia is achieved by variation in the linkage to the underlying sugar and modification of the Sia molecule. Here we identified Sia-dependent epitope specificity for GL7, a rat monoclonal antibody, to probe germinal centers upon T cell-dependent immunity. GL7 recognizes sialylated glycan(s), the alpha2,6-linked N-acetylneuraminic acid (Neu5Ac) on a lactosamine glycan chain(s), in both Sia modification- and Sia linkage-dependent manners. In mouse germinal center B cells, the expression of the GL7 epitope was upregulated due to the in situ repression of CMP-Neu5Ac hydroxylase (Cmah), the enzyme responsible for Sia modification of Neu5Ac to Neu5Gc. Such Cmah repression caused activation-dependent dynamic reduction of CD22 ligand expression without losing alpha2,6-linked sialylation in germinal centers. The in vivo function of Cmah was analyzed using gene-disrupted mice. Phenotypic analyses showed that Neu5Gc glycan functions as a negative regulator for B-cell activation in assays of T-cell-independent immunization response and splenic B-cell proliferation. Thus, Neu5Gc is required for optimal negative regulation, and the reaction is specifically suppressed in activated B cells, i.e., germinal center B cells.

Peripheral B cell receptor editing may promote the production of high-affinity autoantibodies in CD22-deficient mice

CD22-deficient mice are characterized by B cell hyperactivity and autoimmunity. We have constructed knock-in CD22-/- mice, expressing an anti-DNA heavy (H) chain (D42), alone or combined with Vkappa1-Jkappa1 or Vkappa8-Jkappa5 light (L) chains. The Ig-targeted mice produced a lupus-like serology that was age- and sex-dependent. High-affinity IgG autoantibodies were largely dependent on the selection of B cells with a particular H/L combination, in which a non-transgenic, endogenous L chain was assembled by secondary rearrangements through the mechanism of receptor editing. Moreover, we present evidence that these secondary rearrangements are very prominent in splenic peripheral B cells. Since CD22 is primarily expressed on the surface of peripheral B cells, we propose a model for the development of a lupus-like autoimmune disease by a combination of peripheral receptor editing and abnormal B cell activation.

Immobilized alpha2,6-linked sialic acid suppresses caspase-3 activation during anti-IgM antibody-induced apoptosis in Ramos cells

In Ramos cells, a human Burkitt's lymphoma cell line, stimulation of the B cell antigen receptor with anti-IgM antibody (Ab) induces apoptosis as indicated by a decrease in cell viability and an increase in DNA fragmentation and cell surface exposure of phosphatidylserine. Furthermore, these changes are suppressed by incubating the cells in alpha(1)-acid glycoprotein (AGP)-coated tissue culture plates. Here, we found that, during Anti-IgM Ab-induced apoptosis in Ramos cells, caspase-3 is activated downstream of caspase-8 and the mitochondrial pathway is activated, as indicated by a loss of mitochondrial membrane potential, an increase in the release of cytochrome c to the cytoplasm, and enhanced Bax expression. Anti-IgM Ab-induced apoptosis of neuraminidase-treated Ramos cells was suppressed by incubating the cells on plates coated with AGP, which contains a high concentration of alpha2,6-linked sialic acid. The incubation on plates coated with AGP also suppressed anti-IgM Ab-stimulated caspase-3 activity and increased the level of X-linked inhibitor of apoptosis protein (XIAP), but it did not affect caspase-8 activity, the mitochondrial membrane potential, cytochrome c release, or Bax expression. The results indicate that the interaction of Ramos cells with immobilized alpha2,6-linked sialic acid enhances XIAP expression, directly or indirectly suppressing caspase-3 activity and inhibiting anti-IgM Ab-induced apoptosis.

High-affinity ligand probes of CD22 overcome the threshold set by cis ligands to allow for binding, endocytosis, and killing of B cells

CD22 (Siglec-2) is a key regulator of B cell signaling whose function is modulated by interaction with extracellular glycan ligands mediated through its N-terminal Ig domain. Its preferred ligand is the sequence Sia alpha2-6Gal that is abundantly expressed on N-linked glycans of B cell glycoproteins, and by binding to CD22 in cis causes CD22 to appear "masked" from binding to synthetic sialoside probes. Yet, despite the presence of cis ligands, CD22 redistributes to sites of cell contact by binding to trans ligands on neighboring cells. In this study, we demonstrate the dynamic equilibrium that exists between CD22 and its cis and trans ligands, using a high-affinity multivalent sialoside probe that competes with cis ligands and binds to CD22 on native human and murine B cells. Consistent with the constitutive endocytosis reported for CD22, the probes are internalized once bound, demonstrating that CD22 is an endocytic receptor that can carry ligand-decorated "cargo" to intracellular compartments. Conjugation of the sialoside probes to the toxin saporin resulted in toxin uptake and toxin-mediated killing of B lymphoma cell lines, suggesting an alternative approach for targeting CD22 for treatment of B cell lymphomas.

ST6Gal-I restrains CD22-dependent antigen receptor endocytosis and Shp-1 recruitment in normal and pathogenic immune signaling

The ST6Gal-I sialyltransferase produces Siglec ligands for the B-cell-specific CD22 lectin and sustains humoral immune responses. Using multiple experimental approaches to elucidate the mechanisms involved, we report that ST6Gal-I deficiency induces immunoglobulin M (IgM) antigen receptor endocytosis in the absence of immune stimulation. This coincides with increased antigen receptor colocalization with CD22 in both clathrin-deficient and clathrin-enriched membrane microdomains concurrent with diminished tyrosine phosphorylation of Igalpha/beta, Syk, and phospholipase C-gamma2 upon immune activation. Codeficiency with CD22 restores IgM antigen receptor half-life at the cell surface in addition to reversing alterations in membrane trafficking and immune signaling. Diminished immune responses due to ST6Gal-I deficiency further correlate with constitutive recruitment of Shp-1 to CD22 in unstimulated B cells independent of Lyn tyrosine kinase activity and prevent autoimmune disease pathogenesis in the Lyn-deficient model of systemic lupus erythematosus, resulting in a significant extension of life span. Protein glycosylation by ST6Gal-I restricts access of antigen receptors and Shp-1 to CD22 and operates by a CD22-dependent mechanism that decreases the basal rate of IgM antigen receptor endocytosis in altering the threshold of B-cell immune activation.

Analysis of Lyn/CD22 double-deficient B cells in vivo demonstrates Lyn- and CD22-independent pathways affecting BCR regulation and B cell survival

B cell fate is determined by the strength of signals from the antigen receptor and from co-receptors that adjust the activation threshold and tune the B cell to its environment. These co-receptors have been broadly classified into inhibitory and enhancing groups, yet some, such as CD22, may have dual effects. CD22 recruits a variety of signal enhancers at the same time as Lyn-dependent phosphorylation leads to the binding of the inhibitory phosphatase SHP-1. To assess the relative importance of Lyn- and CD22-dependent and -independent pathways, we generated Lyn and CD22 single-deficient mice and Lyn/CD22 double-deficient mice expressing the MD4 immunoglobulin transgene against hen egg lysozyme (IgHEL). This genetic approach has enabled us to compare the contributions of Lyn and CD22 to B cell development in vivo, independent of BCR specificity and in the presence and absence of self-antigen. Our results show that although the effects of Lyn are dominant in negative regulation of B cell hyperactivity, Lyn and CD22 have independent and additive effects on B cell survival. These findings emphasize the subtle nature of regulation at the BCR and the usefulness of genetic complementation to dissect common and parallel pathways.

Signals from a self-antigen induce positive selection in early B cell ontogeny but are tolerogenic in adults

Positive and negative signals from self-Ags shape the B cell repertoire and the development of distinct B cell subsets, but little is known about what distinguishes these signals. To address this question, we have studied the development of anti-hen egg lysozyme MD4 Ig transgene B cells while systematically varying the level, distribution, and timing of exposure to different forms of hen egg lysozyme as a self-Ag. This process has allowed us to explore the effects of Ag independent of BCR specificity. Our findings show how the selection of autoreactive B cells is a competitive process involving immunogenic and tolerogenic forms of self-Ags. Due to a developmental switch during B cell ontogeny, autoreactive anti-hen egg lysozyme MD4 Ig transgene B cells are negatively selected by self-Ags in adult bone marrow but susceptible to positive selection by some of the same self-Ags in fetal and neonatal life. However, the persistence of B1 cells and IgM autoantibodies from early ontogeny enables autoreactive B cells from the adult bone marrow to escape negative selection. Our data suggest that this rescue may be due to the clearance or masking of self-Ag by IgM autoantibody. We discuss the implications of these findings in terms of B cell selection and the maintenance of self-tolerance during early and adult life.

Epratuzumab, a CD22-targeting recombinant humanized antibody with a different mode of action from rituximab

Epratuzumab is a humanized anti-CD22 monoclonal antibody currently in clinical trials for treatment of non-Hodgkin lymphoma (NHL) and certain autoimmune diseases. Here we report the results of investigations of epratuzumab's mode of action in comparison to and in combination with the anti-CD20 mAb, rituximab. In vitro cell growth inhibition, induction of apoptosis, and the ability of the mAbs to mediate complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) were evaluated. We also investigated the potential activity of epratuzumab in the regulation of B-cell antigen receptor (BCR) activation. Epratuzumab and rituximab displayed very distinct modes of action; epratuzumab acts as an immunomodulatory agent, while rituximab is an acutely cytotoxic therapeutic antibody. Epratuzumab has distinct effects on cell growth from rituximab. For example, rituximab+anti-human IgG Fcgamma yielded marked inhibition of proliferation in human NHL cell lines, while epratuzumab had little or no effect in this assay. However, when cells were immobilized and stimulated with anti-IgM, epratuzumab, but not rituximab, caused a significant antiproliferative effect. Unlike rituximab, no CDC could be detected, and ADCC was modest but significant with epratuzumab. Importantly, combining rituximab and epratuzumab did not decrease rituximab's ability to induce apoptosis, CDC, and ADCC. In fact, the combination is more effective than rituximab alone in inhibiting proliferation of Daudi Burkitt lymphoma cells in the presence of second antibody, and at least equally effective to rituximab in the absence of crosslinking. These observations suggest that it may be possible to enhance clinical efficacy by combination therapy comprised of anti-CD20 and anti-CD22 mAbs.

Spontaneous B cell hyperactivity in autoimmune-prone MRL mice

The MRL-lpr/lpr mouse strain is a commonly used model of the human autoimmune disease systemic lupus erythematosus (SLE). Although much is known about the contribution of the lpr Fas mutation to B cell tolerance breakdown, the role of the genetic background of the MRL strain itself is less well explored. In this study, we use the MD4 anti-hen egg lysozyme Ig (IgHEL) transgenic system to explore B cell function in MRL+/+ and non-autoimmune mice. We demonstrate that MRL IgHEL B cells show spontaneous hyperactivity in the absence of self-antigen, which is associated with low total B cell numbers but an expansion of the marginal zone B cell population. However, B cell anergy is normal in the presence of soluble lysozyme [soluble hen egg lysozyme (sHEL)], and MRL IgHEL B cells undergo normal elimination in the presence of sHEL when competing with a polyclonal C57BL/6 B cell repertoire. We conclude that B cell hyperactivity may contribute to the autoimmune phenotype of MRL+/+ and MRL-lpr/lpr strains when it initiates antibody responses to rare or sequestered antigens that are below the threshold for tolerance induction, but that there is no B cell intrinsic defect in anergy in MRL mice.

B cell antigen receptor and CD40 differentially regulate CD22 tyrosine phosphorylation

Cell surface molecules on lymphocytes positively or negatively modulate the Ag receptor signaling, and thus regulate the fate of the cell. CD22 is a B cell-specific cell surface protein that contains multiple ITIMs in the cytoplasmic tail, and critically regulates B cell activation and survival. CD22 regulation on B cell signaling is complex because CD22 can have both positive and negative roles in various contexts. We generated phosphospecific polyclonal Abs reacting four major CD22 tyrosine motifs (Y762, Y807, Y822, and Y842) and analyzed the pattern and intensity of phosphorylation of these tyrosine residues. The tyrosine motifs, Y762, Y822, and Y842, are considered as ITIM, whereas the other, Y807, is suggested to be important for Grb2 recruitment. Approximately 10% of the four tyrosine residues were constitutively phosphorylated. Upon anti-IgM ligation, CD22 Y762 underwent most rapid phosphorylation, whereas all four tyrosine residues were eventually phosphorylated equally at approximately 35% of all CD22 molecules in the cell. By contrast, anti-CD40 stimulation specifically up-regulated anti-IgM-induced phosphorylation of tyrosines within two ITIM motifs, Y762 and Y842, which was consistent with in vivo finding of the negative role of CD22 in CD40 signaling. Thus, CD22 phosphorylation is not only quantitatively but also qualitatively regulated by different stimulations, which may determine the outcome of B cell signaling.

Ablation of CD22 in ligand-deficient mice restores B cell receptor signaling

CD22 is a negative regulator of B cell signaling, an activity modulated by its interaction with glycan ligands containing alpha2-6-linked sialic acids. B cells deficient in the enzyme (ST6Gal I) that forms the CD22 ligand show suppressed BCR signaling. Here we report that mice deficient in both CD22 and its ligand (Cd22-/- St6gal1-/- mice) showed restored B cell receptor (BCR) signaling, suggesting that the suppressed signaling of St6gal1-/- cells is mediated through CD22. Coincident with suppressed BCR signaling, B cells lacking ST6Gal I showed a net redistribution of the BCR to clathrin-rich microdomains containing most of the CD22, resulting in a twofold increase in the localization of CD22 together with the BCR. These studies suggest an important function for the CD22-ligand interaction in regulating BCR signaling and microdomain localization.

Expansion of CD22lo B cells in the spleen of autoimmune-prone flaky skin mice

Similar to murine models with compromised CD22/SHP-1 function, flaky skin (fsn) mutant mice exhibit lymphocyte hyperactivation and an autoimmune phenotype characterized by circulating autoantibodies to dsDNA and glomerulonephritis. Immunophenotyping of fsn/fsn splenic B cells was performed to determine if abnormalities in CD22 expression contributed to the phenotype. We identified an expansion of an IgM(bright) CD22lo population consistent with immature B-lymphocytes. While normal B-lymphocytes require IL-4 to achieve down-modulation of CD22 expression in response to BCR cross-linking, culture with anti-IgM alone led to reduced CD22 expression in fsn/fsn mice. Furthermore, when IL-4 was added to fsn/fsn cultures, no further reduction in CD22 expression was observed. This suggested that fsn/fsn B cells were pre-activated in vivo by chronic IL-4 exposure. A portion of these CD22lo cells expressed the B-1 surface marker CD11b. We contend that decreased activation thresholds among CD22lo B-lymphocytes contributes to the expansion of immature and B-1 B cell populations and to the development of autoimmune pathology in fsn/fsn mice.

Dose-Fractionated Radioimmunotherapy in Non-Hodgkin's Lymphoma Using DOTA-Conjugated, 90Y-Radiolabeled, Humanized Anti-CD22 Monoclonal Antibody, Epratuzumab

PURPOSE

Fractionated radioimmunotherapy may improve therapeutic outcome by decreasing heterogeneity of the dose delivered to the tumor and by decreasing hematologic toxicity, thereby allowing an increased amount of radionuclide to be administered. Because humanized anti-CD22 epratuzumab can be given repeatedly, a single-center study was conducted to establish the feasibility, safety, optimal dosing, and preliminary efficacy of weekly administrations of 90Y-labeled 1,4,7,10-tetra-azacyclodecane-N,N',N'',N'''-tetraacetic acid-conjugated epratuzumab.

EXPERIMENTAL DESIGN

Cohorts of three to six patients with B-cell lymphoma received 185 MBq/m2 [90Y]epratuzumab with unconjugated epratuzumab (total protein dose 1.5 mg/kg) once weekly for two to four infusions, with [(111)In]epratuzumab coadministered at first infusion for scintigraphic imaging and dosimetry.

RESULTS

Sixteen patients received treatment without significant infusional reactions. The overall objective response rate was 62% (95% confidence interval, 39-86%) in both indolent (75%) and aggressive disease (50%). Complete responses (CR/CRu) occurred in 25% of patients and were durable (event-free survival, 14-41 months). Two patients receiving four infusions had hematologic dose-limiting toxicity. Serum epratuzumab levels increased with each weekly dose. Of 13 patients with tumor cell CD22 expression determined by flow cytometry, seven of eight with strongly positive results had objective responses, versus one of five with negative or weakly positive results (P = 0.032).

CONCLUSIONS

Radioimmunotherapy with weekly 185 MBq/m2 [90Y]epratuzumab achieved a high objective response rate (62%) across lymphoma subtypes, including durable CRs. The findings that three weekly infusions (555 MBq/m2, total dose) can be administered safely with only minor toxicity, that antibody levels increased during treatment weeks, and that therapeutic response predominantly occurs in patients with unequivocal CD22 tumor expression provide guidance for future studies.

Homomultimeric complexes of CD22 in B cells revealed by protein-glycan cross-linking

CD22 is a negative regulator of B-cell receptor signaling, an activity mediated by recruitment of SH2 domain-containing phosphatase 1 through a phosphorylated immunoreceptor tyrosine inhibitory motif in its cytoplasmic domain. As in other members of the sialic acid-binding immunoglobulin-like lectin, or siglec, family, the extracellular N-terminal immunoglobulin domain of CD22 binds to glycan ligands containing sialic acid, which are highly expressed on B-cell glycoproteins. B-cell glycoproteins bind to CD22 in cis and 'mask' the ligand-binding domain, modulating its activity as a regulator of B-cell signaling. To assess cell-surface cis ligand interactions, we developed a new method for in situ photoaffinity cross-linking of glycan ligands to CD22. Notably, CD45, surfaceIgM (sIgM) and other glycoproteins that bind to CD22 in vitro do not appear to be important cis ligands of CD22 in situ. Instead, CD22 seems to recognize glycans of neighboring CD22 molecules as cis ligands, forming homomultimeric complexes.

SLP-76 is recruited to CD22 and dephosphorylated by SHP-1, thereby regulating B cell receptor-induced c-Jun N-terminal kinase activation

Despite the important role in the development and activation of T cells, NK cells, mast cells, and macrophages, the expression and function of SLP-76 in B cells have been largely unknown. Here we demonstrate that SLP-76 is expressed in all mouse B cell lines tested and in normal splenic B cells, and serves as an SHP-1 substrate. Dephosphorylation of SLP-76 by SHP-1 inhibits its association with Nck, down-regulating c-Jun N-terminal kinase (JNK) activation and exerting a positive effect on apoptosis. Knockdown of SLP-76 in WEHI-231 cells by small interfering RNA attenuated JNK activation, but showed little effects on extracellular signal-regulated kinase (ERK) or p38 activation. Although WEHI-231 does not express linker for activation of T cells (LAT), SLP-76 localizes in membrane fraction, which increases following B cell receptor (BCR) cross-linking. Further analyses revealed that SLP-76 complexed with Gads is associated with tyrosine-phosphorylated CD22 through the SH2 domains of SLP-76 and Gads. Given that SHP-1 binds to CD22 upon BCR ligation, our findings suggest that dephosphorylation of SLP-76 recruited to CD22 by SHP-1 inhibits BCR-induced JNK activation, dictating apoptosis.

CD22: A Multifunctional Receptor That Regulates B Lymphocyte Survival and Signal Transduction

Recent advances in the study of CD22 indicate a complex role for this transmembrane glycoprotein member of the immunoglobulin superfamily in the regulation of B lymphocyte survival and proliferation. CD22 has been previously recognized as a potential lectin-like adhesion molecule that binds alpha2,6-linked sialic acid-bearing ligands and as an important regulator of B-cell antigen receptor (BCR) signaling. However, genetic studies in mice reveal that some CD22 functions are regulated by ligand binding, whereas other functions are ligand-independent and may only require expression of an intact CD22 cytoplasmic domain at the B-cell surface. Until recently, most of the functional activity of CD22 has been widely attributed to CD22's ability to recruit potent intracellular phosphatases and limit the intensity of BCR-generated signals. However, a more complex role for CD22 has recently emerged, including a central role in a novel regulatory loop controlling the CD19/CD21-Src-family protein tyrosine kinase (PTK) amplification pathway that regulates basal signaling thresholds and intensifies Src-family kinase activation after BCR ligation. CD22 is also central to the regulation of peripheral B-cell homeostasis and survival, the promotion of BCR-induced cell cycle progression, and is a potent regulator of CD40 signaling. Herein we discuss our current understanding of how CD22 governs these complex and overlapping processes, how alterations in these tightly controlled regulatory activities may influence autoimmune disease, and the current and future applications of CD22-directed therapies in oncology and autoimmunity.