Deficiency in CD22, a B cell-specific inhibitory receptor, is sufficient to predispose to development of high affinity autoantibodies

Theory of proteolytic antibody occurrence

Antibodies (Abs) with proteolytic and other catalytic activities have been characterized in the blood and mucosal secretions of humans and experimental animals. The catalytic activity can be traced to nucleophilic sites of innate origin located in Ab germline variable regions. Discoveries of the natural chemical reactivity of Abs were initially met with bewilderment, as the notion had taken hold that catalytic activities can be introduced into Abs by artificial means, but somatically operative selection pressures are designed only to adapt non-covalent Ab binding to antigen ground states. Unsurprisingly, initial efforts to engineer Abs with catalytic activity were oriented towards improving the non-covalent binding at the atoms immediately within the transition state reaction center. Slowly, however, dogmatic approaches to Ab catalysis have given way to the realization that efficient and specific catalytic Abs can be prepared by improving the natural nucleophilic reactivity combined with non-covalent recognition of epitope regions remote from the reaction center. The field remains beset, however, with controversy. This article attempts to provide a rational basis for natural Ab catalysis, in the hope that understanding this phenomenon will stimulate medical and basic science advances in the field.

B-Cell-Targeted Therapy for Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a complex disease characterised by numerous autoantibodies and clinical involvement in multiple organ systems. The immunological events triggering the onset of clinical manifestations have not yet been fully defined, but a central role for B cells in the pathogenesis of this disease has more recently gained prominence as a result of research in both mice and humans. Both antibody-dependent and -independent mechanisms of B cells are important in SLE. Autoantibodies contribute to autoimmunity by multiple mechanisms, including immune complex-mediated type III hypersensitivity reactions, type II antibody-dependent cytotoxicity, and by instructing innate immune cells to produce pathogenic cytokines such as interferon-alpha, tumour necrosis factor and interleukin-1. Suggested autoantibody-independent B-cell functions include antigen presentation, T-cell activation and polarisation, and dendritic-cell modulation. Several of these functions are mediated by the ability of B cells to produce immunoregulatory cytokines, chemokines and lymphangiogenic growth factors, and by their critical contribution to lymphoid tissue development and organisation, including the development of ectopic tertiary lymphoid tissue. Given the large body of evidence implicating abnormalities in the B-cell compartment in SLE, a recent therapeutic focus has been to develop interventions that target the B-cell compartment by multiple mechanisms.Rituximab, a mouse-human chimeric monoclonal antibody against CD20 that specifically depletes B cells, has been studied the most extensively. Although promising open-label data await confirmation in ongoing multicentre placebo-controlled trials, a number of preliminary conclusions can be drawn. The adequacy of peripheral B-cell depletion depends on achieving high and sustained serum rituximab concentrations, pharmacokinetics that can be varied with treatment dose and factors that may affect drug clearance, such as human anti-chimeric antibodies. In SLE patients with effective B-cell depletion, the clinical response can be significant, with favourable responses observed in a diverse array of disease manifestations. Moreover, rituximab appears to have the potential to induce clinical remission in severe, refractory disease. B-cell depletion has the potential to induce disease amelioration by inhibiting autoantibody production and/or by interfering with other B-cell pathogenic functions. The fact that clinical improvement correlates with B-cell depletion and precedes by several months any decline in serum levels of relevant autoantibodies suggests a predominant effect of autoantibody-independent functions of B cells, although the subset of patients with disease remission ultimately also experience autoantibody normalisation. Significant questions remain about rituximab therapy in SLE, including the immunological determinants of treatment response and remission, the role of combination therapy, and the safety of repeated courses of rituximab. In addition, the efficacy and role of other B-cell-depleting approaches, such as humanised anti-CD20 antibodies and anti-CD22, remain to be defined. Another B-cell-targeted therapeutic approach is to block costimulatory interactions between T and B cells. Blockade of the CD40-CD40 ligand pathway has met with variable clinical benefit and unfortunate thromboembolic complications, although inhibition of the B7 pathway with cytotoxic T-lymphocyte antigen-4Ig is currently under early investigation in SLE clinical trials. Preliminary data on the treatment of SLE with belimumab, a fully human monoclonal antibody that specifically binds to and neutralises the B-lymphocyte stimulator (BLyS or B-cell-activating factor [BAFF]), are now available. In a phase II double-blind, placebo-controlled trial of the safety and efficacy of three different doses administered in addition to standard therapy, belimumab was well tolerated but reportedly did not meet primary efficacy endpoints. Blockade of BAFF is still viewed as a promising therapeutic approach and additional agents that interfere with the BAFF pathway are under study.Overall, therapies targeting B cells appear to be promising in the treatment of SLE, provide additional evidence for the importance of B cells to disease pathogenesis, and will continue to elucidate the diverse roles of B cells in this disease.

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.

Expression of aberrant forms of CD22 on B lymphocytes in Cd22a lupus-prone mice affects ligand binding

CD22 functions primarily as a negative regulator of B-cell receptor signaling. The Cd22a allele has been proposed as a candidate allele for murine systemic lupus erythematosus. In this study, we explored the possible expression of aberrant forms of CD22, which differ in the N-terminal sequences constituting the ligand-binding site due to synthesis of abnormally processed Cd22 mRNA, in several Cd22a mouse strains, including C57BL/6 Cd22 congenic mice. The staining pattern of splenic B cells obtained with CY34 anti-CD22 mAb, which was expected to bind poorly to the aberrant CD22, was more heterogeneous in Cd22(a) mice than in Cd22b mice. Moreover, CD22 detected on B cells of Cd22a mice was expressed more weakly and as a smaller-sized protein, compared with Cd22b mice. Significantly, analysis with a synthetic CD22 ligand demonstrated that Cd22a mice carried a larger proportion of CD22 that was not bound by cis ligands on the B-cell surface than Cd22b mice. Finally, the study of C57BL/6 Cd22 congenic mice revealed that Cd22a B cells displayed a phenotype reminiscent of constitutively activated B cells (reduced surface IgM expression and augmented MHC class II expression), as reported for B cells expressing a mutant CD22 lacking the ligand-binding domain. Our demonstration that Cd22a B cells express aberrant forms of CD22, which can potentially deregulate B-cell signaling because of their decreased ligand-binding capacity, provides further support for Cd22a as a potential candidate allele for murine systemic lupus erythematosus.

B lymphocytes and systemic sclerosis

PURPOSE OF REVIEW

Systemic sclerosis is characterized by fibrosis and autoimmunity. Systemic sclerosis displays a variety of abnormal immune activations, including the production of disease-specific autoantibodies, although the pathogenic relation between systemic autoimmunity and the clinical manifestations of systemic sclerosis remains unknown. Recent studies have rediscovered that B cells play critical roles in systemic autoimmunity and disease expression through various functions more than autoantibody production, such as antigen presentation and cytokine production. This review focuses on recent advances in understanding the B cell's role in systemic sclerosis.

RECENT FINDINGS

Patients with systemic sclerosis have altered B-cell homeostasis characterized by expanded naive B cells and diminished memory B cells. Although memory B cells are decreased in number, they are chronically activated, possibly because of CD19 over-expression in B cells from patients with systemic sclerosis. CD19 over-expression can be genetically explained in part by a polymorphism of CD19 promoter region. Similarly, B cells from a tight-skin mouse, a genetic model of systemic sclerosis, show augmented CD19 signaling and chronic hyper-reactivity. CD19 hyper-phosphorylation in tight-skin B cells is caused by impaired function of CD22, a negative response regulator expressed on B cells. Classic roles of autoantibody secretion may also be important in systemic sclerosis because autoantibodies to matrix metalloproteinases can be pathogenic in vivo.

SUMMARY

B cells may have more pathogenic roles in systemic sclerosis than had been appreciated. Further studies are required to clarify the precise molecular basis that links B cells and fibrosis. Collectively, B cells and B-cell-specific response regulators such as CD19/CD22 appear to be potential therapeutic targets of the disease.

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.

Immune receptor signaling, aging, and autoimmunity

With advancing age, the immune system undergoes changes that predispose to autoimmune reactivity. Aging reduces the efficiency of physical barriers, decreasing protection against invasive pathogens, and exposing previously hidden antigens in the body's own tissues. Self-antigens acquire alterations that increase their immunogenicity. In addition, the ability of innate immunity to eliminate infectious agents deteriorates, resulting in inappropriate persistence of immune stimulation and antigen levels exceeding the threshold for the activation of B or T cells. B cell turnover is reduced and numbers of naïve T cells decline to the advantage of increasing numbers of memory T cells. In parallel, the loss of co-stimulatory T cell molecules may increase reactivity of T cells, and render them less susceptible to downregulation. Since optimal immune reactivity requires a tight balance of transduction pathways in both T and B lymphocytes, and because these pathways are altered in systemic autoimmune diseases, we would like to propose that, with age, alterations of the immune receptor signaling machinery underlie the higher incidence of autoimmune phenomena in the elderly. Consistently, aging is associated with alterations in several components of the signaling complex in B cells, memory and naïve T cells, and a reduced activation of several lipid rafts-associated proteins. Because the coincidence of autoimmune disease with other ailments increases the burden of disease and limits therapeutic options in the aged, further investigation of these pathways in the elderly represents a challenge that will need to be addressed in order to devise effective preventive and therapeutic interventions.

Siglecs--the major subfamily of I-type lectins

Animal glycan-recognizing proteins can be broadly classified into two groups-lectins (which typically contain an evolutionarily conserved carbohydrate-recognition domain [CRD]) and sulfated glycosaminoglycan (SGAG)-binding proteins (which appear to have evolved by convergent evolution). Proteins other than antibodies and T-cell receptors that mediate glycan recognition via immunoglobulin (Ig)-like domains are called "I-type lectins." The major homologous subfamily of I-type lectins with sialic acid (Sia)-binding properties and characteristic amino-terminal structural features are called the "Siglecs" (Sia-recognizing Ig-superfamily lectins). The Siglecs can be divided into two groups: an evolutionarily conserved subgroup (Siglecs-1, -2, and -4) and a CD33/Siglec-3-related subgroup (Siglecs-3 and -5-13 in primates), which appear to be rapidly evolving. This article provides an overview of historical and current information about the Siglecs.

Autoimmune alterations induced by the New Zealand Black Lbw2 locus in BWF1 mice

The New Zealand Black (NZB) Lbw2 locus (lupus NZB x New Zealand White (NZW) 2 locus) was previously linked to mortality and glomerulonephritis, but not to IgG autoantibodies, suggesting that it played a role in a later disease stage. To define its contribution, (NZB x NZW)F1 hybrids (BWF1) containing two, one, or no copies of this locus were generated. Lack of the NZB Lbw2 indeed reduced mortality and glomerulonephritis, but not serum levels of total and anti-DNA IgG Abs. There were, however, significant reductions in the B cell response to LPS, total and anti-DNA IgM and IgG Ab-forming cells, IgM Ab levels, and glomerular Ig deposits. Furthermore, although serum IgG autoantibody levels correlated poorly with kidney IgG deposits, the number of spontaneous IgG Ab-forming cells had a significant correlation. Genome-wide mapping of IgM anti-chromatin levels identified only Lbw2, and analysis of subinterval congenics tentatively reduced Lbw2 to approximately 5 Mb. Because no known genes associated with B cell activation and lupus are in this interval, Lbw2 probably represents a novel B cell activation gene. These findings establish the importance of Lbw2 in the BWF1 hybrid and indicate that Lbw2, by enhancing B cell hyperactivity, promotes the early polyclonal activation of B cells and subsequent production of autoantibodies.

Lyn tyrosine kinase: accentuating the positive and the negative

Lyn, one of several Src-family tyrosine kinases in immune cells, is noted for its ability to negatively regulate signaling pathways through phosphorylation of inhibitory receptors, enzymes, and adaptors. Somewhat paradoxically, it is also a key mediator in several pathways of B cell activation, such as CD19 and CD180. Whether Lyn functions to promote or inhibit immune cell activation depends on the stimulus and the developmental state, meaning that the consequences of Lyn activity are context dependent. The importance of regulating Lyn activity is exemplified by the pathological conditions that develop in both lyn-/- and lyn gain-of-function mice (lynup/up), including lethal antibody-mediated autoimmune diseases and myeloid neoplasia. Here, we review the outcomes of altered Lyn activity within the framework of B cell development and differentiation and the circumstances that appear to dictate the outcome.

Differential activation of anti-erythrocyte and anti-DNA autoreactive B lymphocytes by the Yaa mutation

An as-yet-unidentified mutation, Y-linked autoimmune acceleration (Yaa), is responsible for the accelerated development of lupus-like autoimmune syndrome in mice. In view of a possible role for Yaa as a positive regulator of BCR signaling, we have explored whether the expression of the Yaa mutation affects the development and activation of transgenic autoreactive B cells expressing either 4C8 IgM anti-RBC or Sp6 IgM anti-DNA. In this study, we show that the expression of the Yaa mutation induced a lethal form of autoimmune hemolytic anemia in 4C8 transgenic C57BL/6 mice, likely as a result of activation of 4C8 anti-RBC autoreactive B cells early in life. This was further supported, although indirectly, by increased T cell-independent IgM production in spleens of nontransgenic C57BL/6 mice bearing the Yaa mutation. In contrast, Yaa failed to induce activation of Sp6 anti-DNA autoreactive B cells, consistent with a lack of increased IgM anti-DNA production in nontransgenic C57BL/6 Yaa mice. Our results suggest that Yaa can activate autoreactive B cells in a BCR-dependent manner, related to differences in the form and nature of autoantigens.

B Cell Selection and Susceptibility to Autoimmunity

Autoreactive B cells arise routinely as part of the naive B cell repertoire. The immune system employs several mechanisms in an attempt to silence these autoreactive cells before they achieve immunocompetence. The BCR plays a central role in B cell development, activation, survival, and apoptosis, and thus is a critical component of the regulation of both protective and autoreactive B cells. The strength of signal mediated by the BCR is determined by numerous factors, both B cell intrinsic and B cell extrinsic. Perturbations in the molecules that regulate the BCR signal strength or that activate pathways that engage in cross talk with the BCR-mediated signaling pathways can lead to the aberrant survival and activation of autoreactive B cells. In this review, we will discuss the some newly identified genetic loci and factors that modulate the BCR signal transduction pathway and, therefore, the regulation of autoreactive B cells. We will also provide evidence for a model of autoreactivity in which a reduction in the strength of the BCR signal allows the survival and the modulation of a naive B cell repertoire replete with autoreactivity.

Antibodies as defensive enzymes

Antibodies (Abs) and enzymes are structural and functional relatives. Abs with promiscuous peptidase activity are ubiquitous in healthy humans, evidently derived from germline variable domain immunoglobulin genes encoding the serine protease-like nucleophilic function. Exogenous and endogenous electrophilic antigens can bind the nucleophilic sites covalently, and recent evidence suggests that immunization with such antigens can induce proteolytic antibodies. Previously, Ab catalytic activities have been linked to pathogenic autoimmune reactions, but recent studies indicate that proteolytic Abs may also serve beneficial functions. An example is the rapid and selective cleavage of the HIV-1 coat protein gp120 by IgMs found in uninfected humans. The selectivity of this reaction appears to derive from recognition of gp120 as a superantigen. A second example is the cleavage of amyloid beta-peptide by IgM and IgG from aged humans, a phenomenon that may represent a specific proteolytic response to a neurotoxic endogenous peptide implicated in the pathogenesis of Alzheimer's disease.

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.

Decreased Lyn expression and translocation to lipid raft signaling domains in B lymphocytes from patients with systemic lupus erythematosus

OBJECTIVE

B lymphocytes from patients with systemic lupus erythematosus (SLE) are hyperactive and produce anti-double-stranded DNA (anti-dsDNA) autoantibodies. The cause or causes of B cell defects in SLE are unknown. In this study, we determined the level and subcellular distribution of Lyn protein, a key negative regulator of B cell receptor signaling, and assessed whether altered Lyn expression is characteristic of B cells in the setting of SLE.

METHODS

Negative selection was used to isolate B lymphocytes from blood. Lipid raft signaling domains were purified from B cells obtained from 62 patients with SLE, 15 patients with rheumatoid arthritis, and 31 healthy controls, by gradient ultracentrifugation. The total Lyn protein level was determined by Western blotting, confocal microscopy, and fluorescein-activated cell sorting (FACS). The distribution of Lyn into lipid raft and nonlipid raft domains was determined by Western blotting and confocal microscopy. Lyn content in B cell subpopulations was determined by FACS. In order to assess B lymphocyte activity, we used (3)H-thymidine incorporation and enzyme-linked immunosorbent assay to measure spontaneous proliferation and IgG and cytokine production by B cells.

RESULTS

This study revealed that B lymphocytes from a majority of patients with SLE have a reduced level of Lyn and manifest altered translocation to lipid rafts. An investigation into the mechanisms of Lyn reduction suggested that increased ubiquitination is involved. This was evident from increased ubiquitination of Lyn and translocation of c-Cbl into lipid rafts. Studies of B cell responses showed that altered Lyn expression was associated with heightened spontaneous proliferation, anti-dsDNA autoantibodies, and increased interleukin-10 production.

CONCLUSION

This study provides evidence for altered Lyn expression in B cells from a majority of patients with SLE. Altered Lyn expression in SLE may influence the B cell receptor signaling and B cell hyperactivity that are characteristic of the disease.

Restoration of C1q levels by bone marrow transplantation attenuates autoimmune disease associated with C1q deficiency in mice

C1q deficiency in both humans and mice is strongly associated with autoimmunity. We have previously shown that bone marrow transplantation (BMT) restored C1q levels in C1q-deficient (C1qa(-/-)) mice. Here, we studied the effect of BMT on autoimmunity in C1qa(-/-) mice. Following irradiation, young C1qa(-/-) or wild-type MRL/Mp mice received bone marrow cells (BMC) from strain-matched wild-type or C1qa(-/-) animals. C1q levels increased rapidly when C1qa(-/-) mice received BMC from wild-type mice. Conversely, they decreased slowly in wild-type mice transplanted with C1qa(-/-) BMC. C1qa(-/-) animals transplanted with C1qa(-/-) BMC demonstrated accelerated disease when compared with wild-type mice given wild-type BMC. In contrast, a significant delay in the development of autoantibodies and glomerulonephritis was observed in C1qa(-/-) mice reconstituted with wild-type BMC, and the impaired clearance of apoptotic cells, previously described in C1qa(-/-) mice, was rectified. Moreover, the autoimmune disease was accelerated in wild-type mice given C1qa(-/-) BMC compared to animals transplanted with wild-type cells. These results provide supporting evidence that BMT may be a therapeutic option in the treatment of autoimmunity associated with human C1q deficiency.

B cells in human and murine systemic lupus erythematosus

PURPOSE OF REVIEW

The purpose of this review is to discuss recent publications dealing with the control of autoreactive B cells, how this control is subverted in human systemic lupus erythematosus and in murine models of systemic lupus erythematosus, and how dysregulated autoreactive B cells may then contribute to disease expression through both regulatory and effector mechanisms.

RECENT FINDINGS

Autoreactive B cells are abundant in the mature peripheral B-cell repertoire and need to be censored to avoid autoimmunity. This censoring is accomplished in diverse ways and may be broken down by multiple mechanisms both intrinsic and extrinsic to the B cells.

SUMMARY

The work reviewed here paints a suggestive picture while confirming the pathogenic potential of autoreactive B cells and pointing to specific defects that warrant further exploration and could represent future therapeutic targets for this autoimmune disease.

Dissection of BXSB lupus phenotype using mice congenic for chromosome 1 demonstrates that separate intervals direct different aspects of disease

To dissect the individual effects of the four non-MHC, autosomal loci (Bxs1 to Bxs4) that contribute to SLE susceptibility in BXSB mice, we generated congenic lines from chromosome 1 on a C57BL/10.Y(BXSB) (B10.Yaa) background for the intervals (values in megabases (Mb)) Bxs1 (46.3-89.2 Mb), Bxs1/4 (20.0-65.9 Mb), Bxs1/2 (64.4-159.0 Mb), and Bxs2/3 (105.4-189.0 Mb). Glomerulonephritis, qualitatively similar to that seen in the parental BXSB strain, developed in three of these congenic strains. Early onset, severe disease was observed in B10.Yaa.BXSB-Bxs2/3 congenic mice and caused 50% mortality by 12 mo. In B10.Yaa.BXSB-Bxs1/4 mice disease progressed more slowly, resulting in 13% mortality at 12 mo. The progression of renal disease in both of these strains was correlated with the level of anti-dsDNA Abs. B10.Yaa.BXSB-Bxs1 mice, despite their genetic similarity to B10.Yaa.BXSB-Bxs1/4 mice, developed a low-grade glomerulonephritis in the absence of anti-dsDNA Abs. Thus, Bxs4 directed an increase in titer and spectrum of autoantibodies, whereas Bxs1 promoted the development of nephritis. The Bxs2 interval was linked to the production of anti-dsDNA Abs without concomitant glomerulonephritis. In contrast, the Bxs3 interval was sufficient to generate classic lupus nephritis in a nonautoimmune-prone strain. Immune phenotype differed between controls and congenics with a significant increase in B220(+) cells in BXSB and B10.Yaa.BXSB-Bxs2/3, and an increase in CD4 to CD8 ratio in both BXSB and B10.Yaa.BXSB-Bxs1/4. Disease in the Bxs3 mice was delayed in comparison to the BXSB parental strain, emphasizing the necessity for multiple interactions in the production of the full BXSB phenotype.

Genetic basis of murine lupus

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by the formation of a variety of autoantibodies and subsequent development of severe glomerulonephritis. Etiology of SLE remains unknown even if it is now well established that SLE is under polygenic control as well as the contribution of hormonal and environmental factors. The availability of several murine strains that spontaneously develop an autoimmune syndrome resembling human SLE, such as New Zealand, MRL and BXSB mice has provided useful tools for the genetic dissection of susceptibility to SLE. Moreover, development of various transgenic and mutant mice has made it possible to identify a number of susceptibility genes such as those involved in the regulation of apoptosis or B cell receptor signaling that can trigger lupus-like phenotypes. Obviously, further identification of the genetic defects present in lupus-prone mice is of paramount importance for understanding the immunopathogenesis of SLE.

Peripheral B lymphocyte tolerance

This lecture discusses two interrelated topics, B cell tolerance in the peripheral immune system and BAFF. Using the 3-83 antibody transgenic mouse bred to mice carrying cognate antigen in the liver, we previously found that clonal elimination drastically reduced the precursor frequency of autoreactive cells. The consensus model to explain this tolerance is the 2-signal hypothesis, which proposes that in the absence of T cell help BCR stimulation is a negative signal for B cells. However, this model fails to explain how these same B cells can respond to T-independent type II (TI-2) antigens, raising the question of how they distinguish TI-2 antigens from multimeric self determinants. We propose that B cells use NK-like missing self recognition to provide the needed specificity, as foreign antigens are unlikely to carry self markers. The model has implications for the evolution of the immune system, B lymphocyte signaling, tissue specificity of autoimmunity, and microbial subversion of the immune system. Overexpression of the critical B cell survival cytokine BAFF/BLyS has been associated with autoimmunity. We have discovered a novel splice isoform that regulates BAFF activity and may play a role in limiting B cell activity. The novel form, called DBAFF, is able to heteromultimerize with normal BAFF and can suppress receptor binding and proteolytic release from the cell surface. Preliminary studies from transgenic mice overexpressing wild type or DBAFF are consistent with a possible regulatory role for DBAFF, raising the possibility that the relative expression levels of BAFF and DBAFF regulates tolerance.

A Critical Role for FcγRIIB in the Induction of Rheumatoid Factors

Rheumatoid factors (RF) are autoantibodies with specificity for the Fc portion of IgG, and IgG-containing immune complexes are likely to be the major source of RF autoantigens. Therefore, the activation of RF-producing B cells could be controlled specifically through recognition of IgG immune complexes by the low-affinity IgG FcR, FcgammaRIIB, a potent negative regulator of the BCR. To test this possibility, we determined the development of RF in C57BL/6 (B6) mice lacking FcgammaRIIB, in relation to the H2 haplotype, complement C3, and the Y-linked autoimmune acceleration (Yaa) mutation. FcgammaRIIB-null B6 mice displayed substantial anti-IgG2a RF activities in their sera, in addition to anti-DNA autoantibodies. Their RF and anti-DNA responses were linked to the H2(b) haplotype, but were suppressed almost completely by the H2(d) haplotype. Strikingly, the absence of C3 failed to modulate RF production, but strongly inhibited anti-DNA production. Furthermore, we observed that partial FcgammaRIIB deficiency (i.e., heterozygous level of FcgammaRIIB expression) was sufficient to induce the production of RF and anti-DNA autoantibodies in the presence of the Yaa mutation. In contrast to FcgammaRIIB, the deficiency in another BCR negative regulator, CD22, was unable to promote RF and anti-DNA autoimmune responses in B6 mice. Our results indicate that RF autoimmune responses are critically controlled by FcgammaRIIB, together with the H2(b) and Yaa gene, while C3 regulates positively and specifically anti-DNA, but not RF autoimmune responses.

Spontaneous autoimmunity in 129 and C57BL/6 mice-implications for autoimmunity described in gene-targeted mice

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disorder in which complex genetic factors play an important role. Several strains of gene-targeted mice have been reported to develop SLE, implicating the null genes in the causation of disease. However, hybrid strains between 129 and C57BL/6 mice, widely used in the generation of gene-targeted mice, develop spontaneous autoimmunity. Furthermore, the genetic background markedly influences the autoimmune phenotype of SLE in gene-targeted mice. This suggests an important role in the expression of autoimmunity of as-yet-uncharacterised background genes originating from these parental mouse strains. Using genome-wide linkage analysis, we identified several susceptibility loci, derived from 129 and C57BL/6 mice, mapped in the lupus-prone hybrid (129 × C57BL/6) model. By creating a C57BL/6 congenic strain carrying a 129-derived Chromosome 1 segment, we found that this 129 interval was sufficient to mediate the loss of tolerance to nuclear antigens, which had previously been attributed to a disrupted gene. These results demonstrate important epistatic modifiers of autoimmunity in 129 and C57BL/6 mouse strains, widely used in gene targeting. These background gene influences may account for some, or even all, of the autoimmune traits described in some gene-targeted models of SLE.

Several strains of gene-targeted mice develop systemic lupus erythematosus (SLE). Analysis of these strains demonstrates that the genetic background profoundly influences the development of autoimmunity

Severely impaired B lymphocyte proliferation, survival, and induction of the c-Myc:Cullin 1 ubiquitin ligase pathway resulting from CD22 deficiency on the C57BL/6 genetic background

Understanding the molecular mechanisms through which CD22 regulates B lymphocyte homeostasis, signal transduction, and tolerance is critical to defining normal B cell function and understanding the role of CD22 in autoimmunity. Therefore, CD22 function was examined in vivo and in vitro using B cells from CD22-deficient (CD22(-/-)) mice. Backcrossing of founder CD22(-/-) mice onto the C57BL/6 (B6) genetic background from a B6/129 mixed background resulted in a dramatically reduced B cell proliferative response following IgM ligation, characterized by a paucity of lymphoblasts and augmented apoptosis. Also, the phenotype of splenic B6 CD22(-/-) B cells was uniquely HSA(high) and IgD(low)/CD21(low) with intermediate levels of CD5 expression, although the percentages of mature and transitional B cells were normal. That B6 CD22(-/-) B cells predominantly underwent apoptosis following IgM ligation correlated with this unique tolerant phenotype, as well as defective induction of the c-Myc:Cullin 1 (CUL1) ubiquitin ligase pathway that is necessary for progression to the S phase of cell cycle. CD40 ligation compensated for CD22 deficiency by restoring lymphoblast development, proliferation, c-Myc and CUL1 expression, and protein ubiquitination/degradation in IgM-stimulated B6 CD22(-/-) B cell cultures. Thereby, this study expands our current understanding of the complex role of CD22 during B cell homeostasis and Ag responsiveness, and reveals that the impact of CD22 deficiency is dictated by the genetic background on which it is rendered. Moreover, this study defines CD22 and CD40 as the first examples of lymphocyte coreceptors that influence induction of the c-Myc:CUL1 ubiquitin ligase pathway.

The role of B cell CD22 expression in Staphylococcus aureus arthritis and sepsis

Severe Staphylococcus aureus infections give rise to a pronounced antigen-specific and polyclonal B cell response with elevated serum immunoglobulin levels. However, it has been difficult to correlate the antibody levels with the clinical outcome of sepsis and/or arthritis concerning both protection and pathogenic aspects. Earlier studies have shown that macrophages and neutrophils are of great importance for bacterial clearance. However, deletion of the complete B cell compartment affected neither S. aureus-induced arthritis nor survival. MZ B cells are believed to be of importance for clearance of blood-borne antigens and have been implicated in protection against S. aureus infections. CD22 is a B-cell-specific inhibitory receptor binding to alpha2,6-linked sialic acids, and deficiency in CD22 leads to a 75% reduction of the MZ B cell compartment. CD22-/- mice and congeneic controls were inoculated intravenously with an arthritogenic dose of live S. aureus. No differences between the groups were detected regarding frequency and severity of arthritis, survival, bacterial clearance, or induction of inflammatory response. This study shows explicitly that a reduced MZ B cell compartment in the absence of CD22 expression does not interfere with the inflammatory response during S. aureus infection.

CD22 attenuates calcium signaling by potentiating plasma membrane calcium-ATPase activity

Binding of antigen to the B cell receptor induces a calcium response, which is required for proliferation and antibody production. CD22, a B cell surface protein, inhibits this signal through mechanisms that have been obscure. We report here that CD22 augments calcium efflux after B cell receptor crosslinking. Inhibition of plasma membrane calcium-ATPase (PMCA) attenuated these effects, as did disruption by homologous recombination of the gene encoding PMCA4a and PMCA4b. PMCA coimmunoprecipitated with CD22 in an activation-dependent way. CD22 cytoplasmic tyrosine residues were required for association with PMCA and enhancement of calcium efflux. Moreover, CD22 regulation of efflux and the calcium response required the tyrosine phosphatase SHP-1. Thus, SHP-1 and PMCA provide a mechanism by which CD22, a tissue-specific negative regulator, can affect calcium responses.

Masking of CD22 by cis ligands does not prevent redistribution of CD22 to sites of cell contact

CD22, a negative regulator of B cell signaling, is a member of the siglec family that binds to alpha2-6-linked sialic acids on glycoproteins. Previous reports demonstrated that binding of multivalent sialoside probes to CD22 is blocked, or "masked," by endogenous (cis) ligands, unless they are first destroyed by sialidase treatment. These results suggest that cis ligands on B cells make CD22 functionally unavailable for binding to ligands in trans. Through immunofluorescence microscopy, however, we observed that CD22 on resting B cells redistributes to the site of contact with other B or T lymphocytes. Redistribution is mediated by interaction with trans ligands on the opposing cell because it does not occur with ligand-deficient lymphocytes from ST6GalI-null mice. Surprisingly, CD45, proposed as both a cis and trans ligand of CD22, was not required for redistribution to sites of cell contact, given that redistribution of CD22 was independent of CD45 and was observed with lymphocytes from CD45-deficient mice. Furthermore, CD45 is not required for CD22 masking as similar levels of masking were observed in the WT and null mice. Comparison of the widely used sialoside-polyacrylamide probe with a sialoside-streptavidin probe revealed that the latter bound a subset of B cells without sialidase treatment, suggesting that cis ligands differentially impacted the binding of these two probes in trans. The combined results suggest that equilibrium binding to cis ligands does not preclude binding of CD22 to ligands in trans, and allows for its redistribution to sites of contact between lymphocytes.

B-cell homeostasis, competition, resources, and positive selection by self-antigens

In adult mice, the number of B lymphocytes remains constant under homeostatic control, in spite of the fact that B cells are produced continuously in numbers that largely exceed the number required to replenish the peripheral pools. It follows that each newly formed lymphocyte can only persist if another lymphocyte dies. In an immune system where the total number of cells is limited, cell survival is no longer a passive phenomenon but rather a continuous active process where each lymphocyte must compete with other lymphocytes to survive. Consequently, the number and the life expectancy of a B-cell clone vary according to the presence or absence of competitor populations. This process of lymphocyte competition is likely controlled by a common need for resources that are in limited supply. The number of peripheral B-cells varies according to the availability of B-cell receptor (BCR) ligands. Indeed, it is possible to modify steady-state B-cell numbers by antigen manipulation. Moreover, conventional self-reactive B cells can undergo positive selection. We showed that the fate of a self-reactive B cell is determined by the quantity of self-antigens, the number of antigen-specific receptors engaged, and its overall antigen-binding avidity rather than the affinity of individual BCRs.

B lymphocyte selection and survival in systemic lupus

B lymphocytes are an essential element in the body's immune system. Engagement of the B cell receptor is responsible for initiating the signaling events that can activate, inactivate or physically eliminate B cells, depending on the magnitude and duration of the signal. Control of B cell signaling occurs through both positive and negative regulation, as well as through the actions of molecular scaffolds that contribute to the formation of signaling complexes. Inactivation of genes encoding signaling molecules was shown to result in clinical manifestations reminiscent of systemic autoimmunity in experimental animals. Aberrant expression of some signaling molecules was also observed in patients with systemic autoimmune diseases. Understanding the mechanisms that subvert B cell receptor transduction pathways is likely to aid in the development of therapeutic agents to treat autoimmune diseases.

Chromatin breakdown during necrosis by serum Dnase1 and the plasminogen system

OBJECTIVE

Dnase1-deficient mice with the 129 x C57BL/6 genetic background develop symptoms of systemic lupus erythematosus, such as high titers of antinuclear autoantibodies directed against nucleosomes. In this study we analyzed a potential molecular pathomechanism leading to this autoimmunity, by exploring the influence of extracellular Dnase1 present in serum on the breakdown of chromatin in necrotic cells in vitro.

METHODS

Human breast adenocarcinoma cells (MCF-7) and other cell lines were subjected to necrosis induced by hydrogen peroxide, streptolysin O, or freeze-thawing. Subsequently, the influence of sera from Dnase1-deficient and wild-type mice as well as the influence of purified enzymes present in the culture medium on the process of necrotic chromatin breakdown was investigated.

RESULTS

Necrotic chromatin breakdown resembled apoptotic DNA laddering and was catalyzed by serum Dnase1 in conjunction with plasmin. During necrosis, Dnase1 and plasminogen penetrated the cell and accumulated in the cytoplasm and nucleus. Plasminogen bound to the cytoskeleton and nuclear structures, was activated to plasmin by either tissue-type or urokinase-type plasminogen activator, and degraded histone H1, thereby facilitating internucleosomal DNA cleavage by Dnase1.

CONCLUSION

Our results suggest that serum Dnase1 in cooperation with the plasminogen system guarantees a fast and effective breakdown of chromatin during necrosis by the combined cleavage of DNA as well as of DNA binding proteins. The failure of such a clearance mechanism might lead to antinuclear autoimmunity similar to that observed in the Dnase1-deficient mouse.

Neuronal expression of CD22: Novel mechanism for inhibiting microglial proinflammatory cytokine production

Although considered an immunologically privileged site, the central nervous system (CNS) can display significant inflammatory responses, which may play a pathogenic role in a number of neurological diseases. Microglia appear to be particularly important for initiating and sustaining CNS inflammation. These cells exist in a quiescent form in the normal CNS, but acquire macrophage-like properties (including active phagocytosis, upregulation of proteins necessary for antigen presentation, and production of proinflammatory cytokines) after stimulation with inflammatory substances such as lipopolysaccharide (LPS). Recent studies have focused on elucidating the role of neurons in the regulation of microglial inflammatory responses. In the present study, we demonstrate, using neuron-microglial cocultures, that neurons are capable of inhibiting LPS-induced tumor necrosis factor-alpha (TNF-alpha) production by microglia. This inhibition appears to be dependent on secretion of substances at axon terminals, as treatment with the presynaptic calcium channel blocker omega-conotoxin abolishes this inhibitory effect. Moreover, we show that conditioned medium from neuronal cultures similarly inhibits microglial TNF-alpha production, which provides additional evidence that neurons secrete inhibitory substances. We previously demonstrated that the transmembrane protein-tyrosine phosphatase CD45 plays an important role in negatively regulating microglial activation. The recent characterization of CD22 as an endogenous ligand of this receptor led us to investigate whether neurons express this protein. Indeed, we were able to demonstrate CD22 mRNA and protein expression in cultured neurons and mouse brain, using reverse transcriptase-polymerase chain reaction and antibody-based techniques. Furthermore, we show that neurons secrete CD22, which functions as an inhibitor of microglial proinflammatory cytokine production.

Altering immune tolerance therapeutically: the power of negative thinking

The etiology of most human autoimmune diseases remains largely unknown. However, investigators have identified several negative regulatory mechanisms acting at the level of innate and/or adaptive immunity. Mutations resulting in a deficiency of some key regulatory molecules are associated with systemic or organ-specific inflammatory disorders, which often have a prominent autoimmune component. Genetic studies have implicated the negative regulator cytotoxic T-lymphocyte antigen 4 (CTLA-4) and other regulatory molecules in human autoimmune diseases. In addition to CTLA-4, key inhibitory molecules include programmed death 1 and B and T lymphocyte attenuator. Transforming growth factor beta1 and interleukin-10 also play major anti-inflammatory and regulatory roles. Tumor cells and infectious agents use negative regulatory pathways to escape immunity. The therapeutic blockage of negative signaling (particularly of CTLA-4) increases immunity against tumor antigens but also induces or aggravates autoimmune diseases. It appears that under normal conditions, the immune system is under strong "negative influences" that prevent autoimmunity and that release of this suppression results in disease. Regulation involves communication between the immune system and nonlymphoid tissues, and the latter can deliver inhibitory or stimulatory signals. Recent studies reveal that the generation of negative signals by selective engagement of inhibitory molecules is feasible and is likely to be of therapeutic benefit in autoimmune diseases and allograft rejection.

Identification of the CD33-related Siglec receptor, Siglec-5 (CD170), as a useful marker in both normal myelopoiesis and acute myeloid leukaemias

Sialic acid-binding immunoglobulin-like lectin (Siglec)-5 or CD170 is a CD33-related receptor, containing cytoplasmic immune receptor-based tyrosine signalling motifs, that has previously been reported to be myeloid-specific like CD33 and thus may be useful in the characterization of both normal and malignant haemopoiesis. This study showed that Siglec-5 had a distinct expression pattern to CD33 both on normal myeloid cells and in acute myeloid leukaemia (AML). In normal bone marrow and cord blood, myeloid cells predominantly expressed Siglec-5 at the later stages of granulocytic differentiation. Siglec-5 was not expressed at significant levels by CD34+ progenitors either from bone marrow or mobilized peripheral blood. During in vitro myeloid differentiation of cord blood purified CD34+ cells, Siglec-5 was upregulated later than CD33. Siglec-5 expression remained absent or very low on cultured CD34+ cells, unlike CD33, which was present on almost all CD34+ cells by day 4. However, analysis of blasts from 23 patients with AML revealed aberrant expression of Siglec-5 with CD34 in 50% (seven of 14) of patients with CD34+ AML; 61% (14 of 23) of AML cases were positive for Siglec-5 with an increased frequency in the French-American-British subtypes M3-5 (80%) compared with M0-2 (25%). All 13 acute lymphoblastic leukaemic (ALL) samples tested, including a CD33+ ALL, were Siglec-5 negative. These results support the further evaluation of Siglec-5 antibodies in the diagnosis and monitoring of AML.

Reduced dosage of Bruton's tyrosine kinase uncouples B cell hyperresponsiveness from autoimmunity in lyn-/- mice

The development of autoimmunity is correlated with heightened sensitivity of B cells to B cell Ag receptor (BCR) cross-linking. BCR signals are down-regulated by Lyn, which phosphorylates inhibitory receptors. lyn(-/-) mice have reduced BCR signaling thresholds and develop autoantibodies, glomerulonephritis, splenomegaly due to myeloid hyperplasia, and increased B-1 cell numbers. Bruton's tyrosine kinase (Btk), a critical component of BCR signaling pathways, is required for autoantibody production in lyn(-/-) mice. It is unclear whether Btk mediates autoimmunity at the level of BCR signal transduction or B cell development, given that lyn(-/-)Btk(-/-) mice have a severe reduction in conventional B and B-1 cell numbers. To address this issue, we crossed a transgene expressing a low dosage of Btk (Btk(low)) in B cells to lyn(-/-)Btk(-/-) mice. Conventional B cell populations were restored to levels similar to those in lyn(-/-) mice. These cells were as hypersensitive to BCR cross-linking as lyn(-/-) B cells as measured by proliferation, Ca(2+) flux, and activation of extracellular signal-regulated kinase and Akt. However, lyn(-/-)Btk(low) mice did not produce anti-ssDNA, anti-dsDNA, anti-histone, or anti-histone/DNA IgM or IgG. They also lacked B-1 cells and did not exhibit splenomegaly. Thus, B cell hyperresponsiveness is insufficient for autoimmunity in lyn(-/-) mice. These studies implicate B-1 and/or myeloid cells as key contributors to the lyn(-/-) autoimmune phenotype.

Differential control of CD22 ligand expression on B and T lymphocytes, and enhanced expression in murine systemic lupus

OBJECTIVE

CD22, a B cell-restricted transmembrane glycoprotein, regulates B cell antigen receptor signaling upon interaction with alpha2,6-linked sialic acid-bearing glycans, which act as ligands and are expressed on B and T cells. In this study, we investigated how the expression of CD22 ligand (CD22L) is modulated following lymphocyte activation or during the course of systemic lupus erythematosus (SLE).

METHODS

The expression levels of CD22L on B and T cells in nonautoimmune mice were assessed by flow cytometric analysis using a soluble recombinant form of CD22, following stimulation with antigen or mitogen in vitro. In addition, the expression levels of CD22L on circulating lymphocytes were correlated with the progression of SLE in lupus-prone mice.

RESULTS

We observed a constitutive expression of CD22L on mature B cells, but not T cells, in nonautoimmune mice. However, CD22L levels were up-regulated selectively on T cells (but not B cells) stimulated with antigens in vitro, while their expression levels on B cells was up-modulated following polyclonal activation with lipopolysaccharide. Furthermore, expression of CD22L was increased on circulating B cells (and to a lesser extent on T cells) in parallel with progression of SLE in several different lupus-prone mice and in a cohort of (C57BL/6 x [NZB x C57BL/6.Yaa]F(1)) backcross mice.

CONCLUSION

The expression of CD22L is differentially regulated in B and T cells, and high expression of CD22L on circulating B cells is a marker for development of severe SLE, suggesting a role for CD22-CD22L interactions in SLE as well as in the regulation of humoral immunity.

The scent of age

In many species, older males are often preferred mates because they carry 'good' genes that account for their viability. How females discern a male's age is a matter of question. However, for animals that rely heavily on chemical communication there is some indication that an animal's age can be determined by its scent. To investigate whether there are changes in body odours with age, and if so their composition, mice were trained in a Y-maze to discriminate urine odours of donor mice of different ages: Adult (3-10 months old) and Aged (more than 17 months old). Trained mice could discriminate between these two age groups by odour alone. To determine the chemical basis for these discriminations, studies were performed using gas chromatography and mass spectrometry. These analyses demonstrated differences in the ratio of urinary volatiles with age. The most prominent differences involved significantly greater amounts of 2-phenylacetamide and significantly lower amounts of methylbutyric acids in Aged animals relative to Adult animals. Fractionating and manipulating the levels of these compounds in the urine demonstrated that the mice can distinguish age based on variation in amounts of these specific compounds in the combined urine.

The B cell coreceptor CD22 associates with AP50, a clathrin-coated pit adapter protein, via tyrosine-dependent interaction

The B cell coreceptor CD22 plays an important role in regulating signal transduction via the B cell Ag receptor. Studies have shown that surface expression of CD22 can be modulated in response to binding of ligand (i.e., mAb). Thus, it is possible that alterations in the level of CD22 expression following binding of natural ligand(s) may affect its ability to modulate the Ag receptor signaling threshold at specific points during B cell development and differentiation. Therefore, it is important to delineate the physiologic mechanism by which CD22 expression is controlled. In the current study, yeast two-hybrid analysis was used to demonstrate that CD22 interacts with AP50, the medium chain subunit of the AP-2 complex, via tyrosine-based internalization motifs in its cytoplasmic domain. This interaction was further characterized using yeast two-hybrid analysis revealing that Tyr(843) and surrounding amino acids in the cytoplasmic tail of CD22 comprise the primary binding site for AP50. Subsequent studies using transfectant Jurkat cell lines expressing wild-type or mutant forms of CD22 demonstrated that either Tyr(843) or Tyr(863) is sufficient for mAb-mediated internalization of CD22 and that these motifs are involved in its interaction with the AP-2 complex, as determined by coprecipitation of alpha-adaptin. Finally, experiments were performed demonstrating that treatment of B cells with either intact anti-Ig Ab or F(ab')(2) blocks ligand-mediated internalization of CD22. In conclusion, these studies demonstrate that internalization of CD22 is dependent on its association with the AP-2 complex via tyrosine-based internalization motifs.

The Yaa mutation promoting murine lupus causes defective development of marginal zone B cells

The accelerated development of systemic lupus erythematosus (SLE) in BXSB male mice is associated with the presence of an as yet unidentified mutant gene, Yaa (Y-linked autoimmune acceleration). In view of a possible role of marginal zone (MZ) B cells in murine SLE, we have explored whether the expression of the Yaa mutation affects the differentiation of MZ and follicular B cells, thereby implicating the acceleration of the disease. In this study, we show that both BXSB and C57BL/6 Yaa mice, including two different substrains of BXSB Yaa males that are protected from SLE, displayed an impaired development of MZ B cells early in life. Studies in bone marrow chimeras revealed that the loss of MZ B cells resulted from a defect intrinsic to B cells expressing the Yaa mutation. The lack of selective expansion of MZ B cells in diseased BXSB Yaa males strongly argues against a major role of MZ B cells in the generation of pathogenic autoantibodies in the BXSB model of SLE. Furthermore, a comparative analysis with mice deficient in CD22 or expressing an IgM anti-trinitrophenyl/DNA transgene suggests that the hyperreactive phenotype of Yaa B cells, as judged by a markedly increased spontaneous IgM secretion, is likely to contribute to the enhanced maturation toward follicular B cells and the block in the MZ B cell generation.

B lymphocytes contribute to autoimmune disease pathogenesis: current trends and clinical implications

Abnormal B lymphocytes influence the pathogenesis of many autoimmune diseases, in addition to serving as the origin of pathogenic autoantibodies. Although aberrant B cell function and autoimmunity have complex polygenic origins, recent studies in mouse models of autoimmune diseases have revealed overlapping defects in signal transduction pathways that alter B cell survival or activation, and lead to an autoimmune phenotype. Discovery of these important signaling pathways in mice has lead to an intense search for B cell abnormalities that correlate with autoimmune diseases in humans. This search has identified potential targets for therapeutic intervention that are the focus of planned and ongoing human clinical trials. This promises an arsenal of highly targeted, less toxic therapies focused on restoring normal B cell function that will eliminate pathogenic autoantibodies and replace the current use of immunosuppressive drugs.

How do inhibitory phosphatases work?

We present a hypothesis regarding the mode of induction of the inhibitory phosphatases SHP-1 and SHIP in hematopoietic cells. One mode is a general one in which the phosphatase regulates but does not abort signal transduction and biology. Regulator phosphatases are induced by directly or indirectly engaging the amino acid motifs present in the activating receptor, and act to control the biochemical and biological output. The other mode of induction is a specific one, which critically involves paired co-clustering of activating and inhibitory receptors. Phosphatases working in this way act only under conditions of paired co-clustering of activating and inhibitory receptors, and directly bind amino acid motifs present in the inhibitory receptor. However, this mode of induction is apparently more efficient, as cellular activation is completely aborted. This review presents several examples of each mode of inhibition and speculates on their mechanisms.

Making and breaking tolerance

The lymphocyte's decision between tolerance and immunity/autoimmunity is regulated at many levels. Two important parameters in this decision are the maturation state of the antigen presenting cells (APCs) and the amount of self antigen that is detected by the immune system. Maturation of APCs occurs as a consequence of signals received by the innate immune system and may lead to the breakdown of tolerance. Particularly relevant to this process are the Toll-like receptors and mechanisms of cross presentation of self antigens. In addition, genetic alterations in a variety of cell surface receptors, signalling components and regulators of apoptosis/survival can break tolerance and lead to autoimmunity in vivo.

Constitutively unmasked CD22 on B cells of ST6Gal I knockout mice: novel sialoside probe for murine CD22

The interaction of CD22 with glycoprotein ligands bearing the Siaalpha2,6Gal-R sequence is believed to modulate its function as a regulator of B cell signaling. Although a commercial sialoside-polyacrylamide (PAA) probe, NeuAc- alpha2,6Gal-PAA, has facilitated studies on ligand binding by human CD22, murine CD22 binds instead with high affinity to NeuGcalpha2,6Gal-R. A multivalent probe with this sequence was constructed to facilitate investigations of ligand binding in CD22 function using genetically defined murine models. The probe is based on the sialoside-PAA platform, which is then biotinylated for easy detection. A series of sialoside probes were constructed with two different length linker arms between the sialoside and the backbone and three different sialoside to PAA molar ratios. The NeuGcalpha2,6Gal-PAA probe is specific for CD22: it binds to sialidase-treated B cells of wild-type mice but not B cells of CD22-null mice. Additionally, because the probe only binds to sialidase-treated wild-type cells, it confirms that CD22 is constitutively "masked" on most B cells from wild-type mice by binding to ligands in cis. In contrast, the probe bound equally well to native or sialidase-treated B cells from the immunocompromised ligand-deficient ST6Gal I knockout mice, demonstrating that CD22 is constitutively "unmasked" in these cells.

CD19, CD21, and CD22: multifaceted response regulators of B lymphocyte signal transduction

B lymphocyte development and function depend upon the activity of intrinsic and B cell antigen receptor (BCR)-induced signals. These signals are interpreted, amplified, fine-tuned, or suppressed through the precise actions of specialized cell surface coreceptors, or "response regulators," that inform B cells of their extracellular environment. Important cell surface response regulators include the CD19/CD21 complex, CD22, and CD72. CD19 establishes a novel Src-family protein tyrosine kinase (PTK) amplification loop that regulates basal signaling thresholds and intensifies Src-family PTK activation following BCR ligation. In turn, CD22 limits the intensity of CD19-dependent, BCR-generated signals through the recruitment of potent phosphotyrosine and phosphoinositide phosphatases. Herein we discuss our current understanding of how CD19/CD21 and CD22 govern the emergence and intensity of BCR-mediated signals, and how alterations in these tightly controlled regulatory activities contribute to autoimmunity in mice and humans.

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) acts as a regulator of B-cell development, B-cell antigen receptor (BCR)-mediated activation, and autoimmune disease

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) is an immunoglobulin-immunoreceptor tyrosine-based inhibitory motif (Ig-ITIM) superfamily member that recruits and activates protein-tyrosine phosphatases, SHP-1 and SHP-2, through its intrinsic ITIMs. PECAM-1-deficient (PECAM-1(-/-) ) mice exhibit a hyperresponsive B-cell phenotype, increased numbers of B-1 cells, reduced B-2 cells, and develop autoantibodies. In the periphery, there are reduced mature recirculating B-2 cells and increased B-1a cells within the peritoneal cavity. In addition, PECAM-1(-/-) B cells display hyperproliferative responses to lipopolysaccharide and anti-IgM stimulation and showed enhanced kinetics in their intracellular Ca(++) response following IgM cross-linking. PECAM-1(-/-) mice showed increased serum levels of IgM with elevated IgG isotypes and IgA antidinitrophenol antibody in response to the T-independent antigen, dinitrophenol-Ficoll. Finally, PECAM-1(-/-) mice developed antinuclear antibodies and lupuslike autoimmune disease with age.

Estrogen alters thresholds for B cell apoptosis and activation

Estrogen is thought to contribute to the increased frequency of autoimmune disorders occurring in females, but a molecular basis for its effects on autoimmunity remains to be elucidated. We have shown previously that estrogen leads to the survival and activation of autoreactive cells in the naive repertoire. To identify the molecular pathways involved in B cell tolerance, we sought to identify genes that are differentially regulated by estrogen in mouse B cells. Several genes involved in B cell activation and survival, including cd22, shp-1, bcl-2, and vcam-1, were upregulated by estrogen in B cells. We found that overexpression of CD22 and SHP-1 in B cells decreased B cell receptor signaling. Estrogen receptors alpha and beta are expressed on B cells and are functional, since they can directly upregulate expression of CD22, SHP-1, and Bcl-2. Estrogen treatment protected isolated primary B cells from B cell receptor-mediated apoptosis. These results suggest that estrogen induces a genetic program that alters survival and activation of B cells in a B cell-autonomous fashion and thus skews the naive immune system toward autoreactivity.