The motheaten mutation rescues B cell signaling and development in CD45-deficient mice

Age-related changes of the human splenic marginal zone B cell compartment

In this review, we will summarize the growing body of knowledge on the age-related changes of human splenic B cell composition and molecular evidence of immune maturation and discuss the contribution of these changes on splenic protective function. From birth on, the splenic marginal zone (sMZ) contains a specialized B cell subpopulation, which recruits and archives memory B cells from immune responses throughout the organism. The quality of sMZ B cell responses is augmented by germinal center (GC)-dependent maturation of memory B cells during childhood, however, in old age, these mechanisms likely contribute to waning of splenic protective function.

The inhibitory coreceptor CD22 restores B cell signaling by developmentally regulating Cd45-/- immunodeficient B cells

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

Galectin-9 binds IgM-BCR to regulate B cell signaling

The galectin family of secreted lectins have emerged as important regulators of immune cell function; however, their role in B-cell responses is poorly understood. Here we identify IgM-BCR as a ligand for galectin-9. Furthermore, we show enhanced BCR microcluster formation and signaling in galectin-9-deficient B cells. Notably, treatment with exogenous recombinant galectin-9 nearly completely abolishes BCR signaling. We investigated the molecular mechanism for galectin-9-mediated inhibition of BCR signaling using super-resolution imaging and single-particle tracking. We show that galectin-9 merges pre-existing nanoclusters of IgM-BCR, immobilizes IgM-BCR, and relocalizes IgM-BCR together with the inhibitory molecules CD45 and CD22. In resting naive cells, we use dual-color super-resolution imaging to demonstrate that galectin-9 mediates the close association of IgM and CD22, and propose that the loss of this association provides a mechanism for enhanced activation of galectin-9-deficient B cells.

The galectin family of secreted lectins are important regulators of immune cell function; however, their role in B cell responses is poorly understood. Here, the authors identify IgM-BCR as a ligand for galectin-9. In resting naive cells, they show that galectin-9 mediates a close association between IgM and CD22.

Differential expression and functional diversification of diverse immunoglobulin domain-containing protein (DICP) family in three gynogenetic clones of gibel carp

Diverse immunoglobulin (Ig) domain-containing protein (DICP) family is a novel bony fish-specific multi-gene family encoding diversified immune receptors. However, their function and the implication of binding partners remain unknown. In this study, we first identified 28 DICPs from three gibel carp gynogenetic clones and revealed their high variability and clone-specific feature. After crucian carp herpesvirus (CaHV) infection, these DICPs were significantly upregulated in head kidney, kidney and spleen. The up-regulation folds in clone A⁺, F and H were related to the susceptibility to CaHV, progressively increasing from resistant clone to susceptible clone. Overexpression of gibel carp DICPs inhibited interferon (IFN) and viperin promoter-driven luciferase activity. The additions of E. coli extracts and lipid A significantly enhanced the inhibition effect. In addition, gibel carp DICPs can interact with SHP-1 and SHP-2. These findings suggest that gible carp DICPs, as inhibitory receptors, might specifically recognize lipid A, and then interact with SHP-1 and SHP-2 to inhibit the induction of IFN and ISGs.

Shp1 function in myeloid cells

The motheaten mouse was first described in 1975 as a model of systemic inflammation and autoimmunity, as a result of immune system dysregulation. The phenotype was later ascribed to mutations in the cytoplasmic tyrosine phosphatase Shp1. This phosphatase is expressed widely throughout the hematopoietic system and has been shown to impact a multitude of cell signaling pathways. The determination of which cell types contribute to the different aspects of the phenotype caused by global Shp1 loss or mutation and which pathways within these cell types are regulated by Shp1 is important to further our understanding of immune system regulation. In this review, we focus on the role of Shp1 in myeloid cells and how its dysregulation affects immune function, which can impact human disease.

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.

An extracatalytic function of CD45 in B cells is mediated by CD22

The receptor-like tyrosine phosphatase CD45 regulates antigen receptor signaling by dephosphorylating the C-terminal inhibitory tyrosine of the src family kinases. However, despite its abundance, the function of the large, alternatively spliced extracellular domain of CD45 has remained elusive. We used normally spliced CD45 transgenes either incorporating a phosphatase-inactivating point mutation or lacking the cytoplasmic domain to uncouple the enzymatic and noncatalytic functions of CD45 in lymphocytes. Although these transgenes did not alter T-cell signaling or development irrespective of endogenous CD45 expression, both partially rescued the phenotype of CD45-deficient B cells. We identify a noncatalytic role for CD45 in regulating tonic, but not antigen-mediated, B-cell antigen receptor (BCR) signaling through modulation of the function of the inhibitory coreceptor CD22. This finding has important implications for understanding how naïve B cells maintain tonic BCR signaling while restraining inappropriate antigen-dependent activation to preserve clonal "ignorance."

Syk inhibition with fostamatinib leads to transitional B lymphocyte depletion

Cell signaling initiated by the B cell receptor is critical to normal development of B lymphocytes, most notably at the transitional B cell stage. Inhibition of this signaling pathway with the syk inhibitor, fostamatinib, has produced significant efficacy in lymphoid malignancies and autoimmune conditions. Here, we demonstrate that short-term use of fostamatinib impairs B lymphocyte development at the transitional stage without affecting mature B cell populations. Additionally, IL-10 producing B cells remained relatively constant throughout the treatment period. These findings provide insight into the mechanism of action of B cell receptor inhibition in autoimmune disease. As the development of agents targeting B cell receptor signaling proceeds, monitoring for long-term consequences as well as functional evaluation of B cell subsets may further improve our understanding of this rapidly growing class of novel agents.

Disturbed homeostasis and multiple signaling defects in the peripheral blood B-cell compartment of patients with severe chronic sarcoidosis

The presence of hypergammaglobulinemia, autoantibodies, and circulating immune complexes suggests that humoral immunity may contribute to the pathogenesis of sarcoidosis. However, little is known about the role played by B cells in the development of this disease. Here we investigated the subpopulation distribution, response to stimulation, and levels of the nuclear transcription factor NF-κB/p65 in peripheral blood B cells from patients with severe chronic sarcoidosis. Patients with severe chronic sarcoidosis had absolute B-cell lymphopenia and exhibited significantly decreased frequencies and total numbers of memory (CD19(+) CD27(+)) B cells. The reduced numbers of memory B cells in these patients reflected a decrease in the total numbers of class-switched (CD19(+) CD27(+) IgD(-)) and unswitched (CD19(+) CD27(+) IgD(+)) memory B cells and coincided with an increased frequency of circulating (CD19(+/-) CD20(-) CD27(++)) plasmablasts. Polyclonal stimulation of sarcoid B cells resulted in reduced expression of activation markers (i.e., CD25, CD69, and CD86), decreased proliferation, and impaired plasma cell differentiation. Baseline expression of p65 in B cells was reduced in 65% of the patients. These results suggest disturbed homeostasis, intrinsic signaling defects, and anergy within the peripheral B-cell compartments of patients with severe chronic sarcoidosis.

Modulation of immune cell signalling by the leukocyte common tyrosine phosphatase, CD45

CD45 is a leukocyte specific transmembrane glycoprotein and a receptor-like protein tyrosine phosphatase (PTP). CD45 can be expressed as several alternatively spliced isoforms that differ in the extracellular domain. The isoforms are regulated in a cell type and activation state-dependent manner, yet their function has remained elusive. The Src family kinase members Lck and Lyn are key substrates for CD45 in T and B lymphocytes, respectively. CD45 lowers the threshold of antigen receptor signalling, which impacts T and B cell activation and development. CD45 also regulates antigen triggered Fc receptor signalling in mast cells and Toll-like receptor (TLR) signalling in dendritic cells, thus broadening the role of CD45 to other recognition receptors involved in adaptive and innate immunity. In addition, CD45 can affect immune cell adhesion and migration and can modulate cytokine production and signalling. Here we review what is known about the substrate specificity and regulation of CD45 and summarise its effect on immune cell signalling pathways, from its established role in T and B antigen receptor signalling to its emerging role regulating innate immune cell recognition and cytokine production.

The follicular versus marginal zone B lymphocyte cell fate decision

Bone marrow-derived B cells make an important cell fate choice to develop into either follicular B cells or marginal zone B cells in the spleen, which depends on signalling through the B cell receptor, Notch2, the receptor for B cell-activating factor and the canonical nuclear factor-kappaB pathway, as well as signals involved in the migration and anatomical retention of marginal zone B cells. Recent information discussed in this Review reconciles some of the controversies regarding the role of the B cell receptor in this cell fate decision and a clearer picture has also emerged regarding the anatomical location of ligands for Notch2 in the spleen. This cell fate decision could provide mechanistic insights that are relevant to other commitment events in lymphocytes.

Survival of monocytes and macrophages and their role in health and disease

Macrophages are versatile cells involved in health and disease. These cells act as scavengers to rid the body of apoptotic and senescent cells and debris through their phagocytic function. Although this is a primary function of these cells, macrophages play vital roles in inflammation and repair of damaged tissue. Macrophages secrete a large number of cytokines, chemokines and growth factors that recruit and activate a variety of cell types to inflamed tissue compartments. These cells are also critical in cell-mediated immunity and in the resolution of inflammation. Since macrophages, and their precursors, blood monocytes, are important in regulating and resolving inflammation, prolonged cellular survival in tissue compartments could be detrimental. Thus, factors that regulate the fate of monocyte and macrophage survival are important in cellular homeostasis. In this article, we will explore stimuli and the intracellular pathways important in regulating macrophage survival and implication in human disease.

Fate decisions regulating bone marrow and peripheral B lymphocyte development

In adult mammals, bone marrow pluripotent hematopoietic stem cells generate B lymphoid-specified progeny that progress through a series of well-characterized stages before generating B-cell receptor expressing B lymphocytes. These functionally immature B lymphocytes then migrate to the spleen wherein they differentiate through transitional stages into follicular or marginal zone B lymphocytes capable of responding to T-dependent and -independent antigens, respectively. During the terminal stages of B lymphocyte development in the bone marrow, as well as immediately following egress into the peripheral compartments, B lymphocytes are counterselected to eliminate B lymphocytes with potentially dangerous self-reactivity. These developmental and selection events in the bone marrow and periphery are dependent on the integration of intrinsic genetic programs with extrinsic microenvironmental signals that drive progenitors toward increasing B lineage commitment and maturation. This chapter provides a comprehensive overview of the various stages of primary and secondary B lymphocyte development with an emphasis on the selection processes that affect decisions at critical checkpoints. Our intent is to stress the concept that at many steps in the developmental process leading to a mature immunocompetent B lymphocyte, B lineage cells are integrating multiple and different signaling inputs that are translated into specific and appropriate cell fate decisions.

B-1 B cell development

CD5+ B cells have attracted considerable interest because of their association with self-reactivity, autoimmunity, and leukemia. In mice, CD5+ B cells are readily generated from fetal/neonatal precursors, but inefficiently from precursors in adult. One model proposed to explain this difference is that their production occurs through a distinctive developmental process, termed B-1, that enriches pre-B cells with novel germline VDJs and that requires positive selection of newly formed B cells by self-Ag. In contrast, follicular B cells are generated throughout adult life in a developmental process termed B-2, selecting VDJs that pair well with surrogate L chain, and whose maturation appears relatively independent of antigenic selection. In the present study, I focus on processes that shape the repertoire of mouse CD5+ B cells, describing the differences between B-1 and B-2 development, and propose a model encompassing both in the generation of functional B cell subpopulations.

Tyrosine phosphatases epsilon and alpha perform specific and overlapping functions in regulation of voltage-gated potassium channels in Schwann cells

Tyrosine phosphatases (PTPs) epsilon and alpha are closely related and share several molecular functions, such as regulation of Src family kinases and voltage-gated potassium (Kv) channels. Functional interrelationships between PTPepsilon and PTPalpha and the mechanisms by which they regulate K+ channels and Src were analyzed in vivo in mice lacking either or both PTPs. Lack of either PTP increases Kv channel activity and phosphorylation in Schwann cells, indicating these PTPs inhibit Kv current amplitude in vivo. Open probability and unitary conductance of Kv channels are unchanged, suggesting an effect on channel number or organization. PTPalpha inhibits Kv channels more strongly than PTPepsilon; this correlates with constitutive association of PTPalpha with Kv2.1, driven by membranal localization of PTPalpha. PTPalpha, but not PTPepsilon, activates Src in sciatic nerve extracts, suggesting Src deregulation is not responsible exclusively for the observed phenotypes and highlighting an unexpected difference between both PTPs. Developmentally, sciatic nerve myelination is reduced transiently in mice lacking either PTP and more so in mice lacking both PTPs, suggesting both PTPs support myelination but are not fully redundant. We conclude that PTPepsilon and PTPalpha differ significantly in their regulation of Kv channels and Src in the system examined and that similarity between PTPs does not necessarily result in full functional redundancy in vivo.

CD45 links the B cell receptor with cell survival and is required for the persistence of germinal centers

To segregate the many contributions that B cell receptor (BCR)-mediated signals make to immune responses, we have analyzed here B cells deficient in the 'pan-leukocyte' marker CD45. BCR ligation of Cd45-/- B cells failed to activate phosphatidylinositol-3-OH kinase, NF-kappaB, Erk1 or Erk2 kinases or to upregulate cell survival proteins and instead induced apoptosis. Immunization of Cd45-/- B cell chimeras induced germinal centers and antigen-specific immunoglobulin G1 antibody-forming cells early, but both cellular compartments decreased by day 14. Proliferation of Cd45-/- B cells induced by CD40 ligand in vitro was impaired as a result of abrogation by BCR ligation of the upregulation of prosurvival proteins. In contrast, enforced expression of the antiapoptotic factor Bcl-xL prevented the collapse of Cd45-/- B cell germinal centers. These results show mechanistic differences in B cell survival during germinal center initiation and propagation; CD40 signaling is sufficient for the former, whereas the latter requires signaling from the BCR.

Tamoxifen Blocks Estrogen-Induced B Cell Maturation but Not Survival

Estrogen treatment has been shown not only to exacerbate disease activity and accelerate death in spontaneous murine models of lupus but also to induce a lupus-like phenotype in non-spontaneously autoimmune mice. In mice transgenic for the H chain of an anti-DNA Ab, estrogen rescues naive autoreactive B cells that normally are deleted and causes them to mature to a marginal zone phenotype. Estrogen further leads to the activation of this population causing an elevation of serum anti-DNA Ab titers and renal disease. This study was designed to evaluate the therapeutic potential of tamoxifen, a selective estrogen receptor modulator, on estrogen-induced lupus. Mice treated with both estradiol and tamoxifen showed no elevation in anti-DNA Ab titers and consequently no glomerular IgG. The DNA-reactive B cell population that is rescued by estrogen was present in an anergic state in mice treated with both estradiol and tamoxifen. Estradiol enhances transitional B cell resistance to apoptosis and expands the population of marginal zone B cells; tamoxifen did not impede the enhanced resistance to apoptosis, but prevented the development of autoreactive cells as marginal zone B cells. Thus, estrogen-induced autoimmunity proceeds through two distinct molecular pathways, one affecting survival and the other maturation. Activation, but not survival, of autoreactive B cells can be abrogated by tamoxifen. Drugs that modulate even some of the effects of estrogen may be beneficial in patients with lupus. Eventually, understanding the pathways involved in survival and activation of autoreactive B cells will permit the development of therapeutics that target all relevant pathways.

CD45: direct and indirect government of immune regulation

The protein tyrosine phosphatase (PTP) CD45 is abundantly expressed on all nucleated hematopoietic cells and is critical for classical antigen receptor signalling indicated by the arrested development of B and T cells in mice deficient for CD45. Despite its clear role in positive regulation of signalling through the activation of the Src family of tyrosine kinases, many reports have shown CD45 to also negatively regulate this process. Given such a dichotomy in CD45 function and a poor understanding of the mechanism underlying the phenotype of CD45(-/-) lymphocytes, we considered it timely to review the existing data and attempt to determine whether aspects of the CD45(-/-) phenotype result from excessive positive or negative kinase activity and the target molecules that may mediate such effects.

Altered B lymphocyte function induces systemic autoimmunity in systemic sclerosis

Systemic sclerosis (SSc) is a connective tissue disease characterized by excessive extracellular matrix deposition in the skin and visceral organs. SSc is associated with immune activation characterized by autoantibody production, lymphocyte activation, and release of various cytokines. The presence of autoantibodies is a central feature of immune activation in SSc. Although autoantibodies are thought to be closely linked to the pathogenesis of SSc, the pathogenic relationship between systemic autoimmunity and the clinical manifestations of SSc, including skin fibrosis, remains unknown. Recent studies have revealed that B cells play a critical role in systemic autoimmunity and disease expression through various functions, including cytokine production in addition to autoantibody production. The B cell signaling thresholds are regulated by response regulators that augment or diminish B cell signals during responses to self and foreign antigens. Abnormal regulation of the response regulator function and expression may result in autoantibody production. Among these response regulators, CD19, which is a critical cell-surface signal transduction molecule of B cells, is the most potent positive regulator. Transgenic mice that overexpress CD19 by approximately 3-fold lose tolerance and generate autoantibodies spontaneously. B cells from SSc patients exhibit a 20%-increase in CD19 expression that induces SSc-specific autoantibody production in transgenic mice. Furthermore, SSc patients have intrinsic B cell abnormalities characterized by expanded naive B cells, activated but diminished memory B cells, and chronic hyper-reactivity of memory B cells, possibly due to CD19 overexpression. Similarly, B cells from a tight-skin mouse, a model of SSc, show augmented CD19 signaling and chronic hyper-reactivity. Remarkably, CD19 loss results in inhibition of chronic B cell hyper-reactivity and elimination of autoantibody production, which is associated with improvement in skin fibrosis and a parallel decrease in IL-6 production by B cells. Thus, chronic B cell activation resulting from augmented CD19 signaling leads to skin fibrosis possibly through IL-6 overproduction, as well as autoantibody production, in tight-skin mice and SSc patients.

B Lymphocyte signaling established by the CD19/CD22 loop regulates autoimmunity in the tight-skin mouse

Systemic sclerosis (SSc) is characterized by fibrosis and autoimmmunity. Peripheral blood B cells from SSc patients specifically overexpress CD19, a critical cell-surface signal transduction molecule in B cells. CD19 deficiency in B cells also attenuates skin fibrosis in the tight-skin (TSK/+) mouse, a genetic model for SSc. Herein we analyzed two transgenic mouse lines that overexpress CD19. Remarkably, 20% increase of CD19 expression in mice spontaneously induced SSc-specific anti-DNA topoisomerase I (topo I) antibody (Ab) production, which was further augmented by 200% overexpression. In TSK/+ mice overexpressing CD19, skin thickness did not increase, although anti-topo I Ab levels were significantly augmented, indicating that abnormal CD19 signaling influences autoimmunity in TSK/+ mice and also that anti-topo I Ab does not have a pathogenic role. The molecular mechanisms for abnormal CD19 signaling were further assessed. B-cell antigen receptor crosslinking induced exaggerated calcium responses and augmented activation of extracellular signal-regulated kinase in TSK/+ B cells. CD22 function was specifically impaired in TSK/+ B cells. Consistently, CD19, a major target of CD22-negative regulation, was hyperphosphorylated in TSK/+ B cells. These findings indicate that reduced inhibitory signal provided by CD22 results in abnormal activation of signaling pathways including CD19 in TSK/+ mice and also suggest that this disrupted B cell signaling contribute to specific autoantibody production.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

Transitional B cells: step by step towards immune competence

Transitional B cells mark the crucial link between bone-marrow (BM) immature and peripheral mature B cells. Examination reveals unexpected heterogeneity, consisting of contiguous subsets with phenotypic and functional differences. Data point to the late transitional B-cell stage as a crucial juncture at which developing B cells gain access to splenic follicles, become responsive to T-cell help and lose sensitivity to negative selection, characterizing the immature B-cell response to B-cell antigen receptor (BCR) signaling in vitro and in vivo. The biological and molecular processes directing maturation through this stage are becoming clearer through biochemical studies and murine models deficient in various components of the BCR signaling pathway.

Mechanisms of anemia in SHP-1 protein tyrosine phosphatase-deficient "viable motheaten" mice

OBJECTIVE

Viable motheaten mice (abbreviated gene symbol me(v)) are deficient in SHP-1, a critical negative regulator of signal transduction in hematopoietic cells. These mice exhibit severe immune dysfunction accompanied by hyperproliferation of myeloid cells, widespread inflammatory lesions, and regenerative anemia. The aim of this study was to investigate the mechanisms underlying anemia in me(v)/me(v) mice.

MATERIALS AND METHODS

Multiple hematologic parameters, osmotic fragility, and erythropoietin levels were measured to characterize the anemia in me(v)/me(v) mice. B-cell-deficient me(v)/me(v) Igh-6(null) mice were generated to assess the role of anti-erythrocyte antibodies. Coombs assays and flow cytometry were carried out for detection of anti-erythrocyte antibodies. Oxidant production by macrophages, glutathione levels, and lipid peroxidation products in erythrocytes were measured, as was the impact of oxidant on the ultrastructure of me(v)/me(v) erythrocytes. Erythroid maturation and erythrocyte plasma membrane integrity were assessed with flow cytometry by evaluating CD71 expression and annexin V labeling.

RESULTS

The regenerative anemia of me(v)/me(v) mice was associated with erythrocyte changes that were independent of the presence of anti-erythrocyte antibodies. Erythrocytes from me(v)/me(v) mice had increased fragility and heightened susceptibility to oxidant damage. Macrophages from me(v)/me(v) mice demonstrated a higher basal level of oxidant production and enhanced production after stimulation. Oxidant damage in me(v)/me(v) erythrocytes was evidenced by a significant elevation of lipid peroxidation and diminished levels of glutathione.

CONCLUSION

Our results support the hypothesis that as a consequence of severe inflammatory disease, me(v)/me(v) erythrocytes are subject to exceptionally high oxidative stress resulting in oxidation of phospholipids in the erythrocyte membrane with subsequent hemolysis.

Positive and negative roles of CD72 in B cell function

Regulation of B cell activation depends on integration of signals transmitted by the B cell receptor (BCR) and a variety of co-receptors. CD72 is a B cell co-receptor that is expressed in all stages of B cell development except plasma cells. Ligation of CD72 enhances B cell growth and differentiation. Recently, the class IV semaphoring, CD100, has been identified as the natural ligand for CD72. Cytoplasmic domain of CD72 has been shown to be associated with SHP-1 leading to the proposal that the positive effects of CD72 on B cell response may result from sequestration of negative signals from BCR. However, association of CD72 with Grb2 and/or CD19 suggests that CD72 could transmit positive signals. Based on these data, we propose a dual signaling model of CD72.

Absence of CD5 dramatically reduces progression of pulmonary inflammatory lesions in SHP-1 protein-tyrosine phosphatase-deficient 'viable motheaten' mice

Mice homozygous for the viable motheaten (Hcph(me-v)) mutation are deficient in SHP-1 protein-tyrosine phosphatase, resulting in severe systemic autoimmunity and immune dysfunction. A high percentage of B-cells in viable motheaten mice express the cell surface glycoprotein CD5, in contrast to wild type mice that express CD5 on only a small percentage of B-cells. CD5(+) B-cells have been associated with autoantibody production. To determine the role of CD5 in the development of the inflammatory disease in me(v)/ me(v) mice, we created a stock of CD5(null)me(v)/ me(v) mice. The longevity of CD5(null)me(v)/ me(v) mice was increased 69% in comparison to me(v)/ me(v) mice on a similar (B6;129) background. The increased lifespan was associated with a marked reduction in pulmonary inflammation. Flow cytometry analysis of spleen cells from CD5(null)me(v)/ me(v) mice at 9-12 weeks of age revealed significant decreases in percentages of IgM/B220 double positive B-cells, Mac-1/Gr-1 double positive cells and CD4(+) T-cells compared with me(v)/ me(v) mice. CD5(null)me(v)/ me(v) mice also had significantly lower serum IgM levels in comparison to me(v)/ me(v) mice. Study of CD5(null)me(v)/ me(v) mice may provide further insight into the role of CD5 in cell signaling and may help explain the observed association of CD5(+) B-cells with autoimmune disease.

B cell receptor signaling and autoimmunity

The immune receptors of lymphocytes are able to sense the nature of bound ligands. Through coupled signaling pathways the generated signals are appropriately delivered to the intracellular machinery, allowing specific functional responses. A central issue in contemporary immunology is how the fate of B lymphocytes is determined at the successive developmental stages and how the B cell receptor distinguishes between signals that induce immune response or tolerance. Experiments with mice expressing transgenes or lacking signal transduction molecules that lead to abnormal lymphocyte development and/or response are providing important clues to the mechanisms that regulate signaling thresholds at different developmental stages. The studies are also revealing novel potential mechanisms of induction of autoimmunity, which may have a bearing on the understanding of human diseases.

Expression of B cell receptor-associated signaling molecules in human lupus

B cell receptor (BcR) signaling requires a tight regulation of several protein tyrosine kinases and phosphatases, and associated co-receptors. Mounting evidence indicates that abnormal BcR signaling, such as occurs in SHP-1 and Lyn-deficient mice, results in production of pathogenic autoantibodies and lupus-like glomerulonephritis, suggesting that altered signaling thresholds could underlie the development of systemic autoimmunity. To test this hypothesis, we investigated expression of BcR-associated signaling molecules in lymphocytes from patients with systemic lupus erythematosus (SLE) during inactive phases of the disease. We found that the transmembrane regulatory protein tyrosine phosphatase CD45 is expressed at abnormal levels. Strikingly, this reduction persisted during four months of follow-up. By contrast, despite its potent role as a regulator of thymus-independent immune responses and of B cell life span, the CD22 co-receptor is expressed at normal levels in B lymphocytes isolated ex vivo from SLE patients. We also noted unusual levels of the cytosolic protein tyrosine kinase Lyn and the protein tyrosine phosphatase SHP-1 in the lymphocytes of the patients. Since in normal B cells Lyn and SHP-1 act in concert within a common negative pathway in which CD45 counteracts SHP-I regulatory role, we propose that this feedback regulatory pathway is crippled to different degrees in human SLE B cells. Break of the balance between positive and negative signaling molecules likely modifies the BcR signaling thresholds. Such alterations, together with other factors, may contribute to the disruption of self-tolerance in this disease.

The SH2 domain containing tyrosine phosphatase-1 down-regulates activation of Lyn and Lyn-induced tyrosine phosphorylation of the CD19 receptor in B cells

SHP-1 is a cytosolic tyrosine phosphatase implicated in down-regulation of B cell antigen receptor signaling. SHP-1 effects on the antigen receptor reflect its capacity to dephosphorylate this receptor as well as several inhibitory comodulators. In view of our observation that antigen receptor-induced CD19 tyrosine phosphorylation is constitutively increased in B cells from SHP-l-deficient motheaten mice, we investigated the possibility that CD19, a positive modulator of antigen receptor signaling, represents another substrate for SHP-1. However, analysis of CD19 coimmunoprecipitable tyrosine phosphatase activity in CD19 immunoprecipitates from SHP-1-deficient and wild-type B cells revealed that SHP-1 accounts for only a minor portion of CD19-associated tyrosine phosphatase activity. As CD19 tyrosine phosphorylation is modulated by the Lyn protein-tyrosine kinase, Lyn activity was evaluated in wild-type and motheaten B cells. The results revealed both Lyn as well as CD19-associated Lyn kinase activity to be constitutively and inducibly increased in SHP-1-deficient compared with wild-type B cells. The data also demonstrated SHP-1 to be associated with Lyn in stimulated but not in resting B cells and indicated this interaction to be mediated via Lyn binding to the SHP-1 N-terminal SH2 domain. These findings, together with cyanogen bromide cleavage data revealing that SHP-1 dephosphorylates the Lyn autophosphorylation site, identify Lyn deactivation/dephosphorylation as a likely mechanism whereby SHP-1 exerts its influence on CD19 tyrosine phosphorylation and, by extension, its inhibitory effect on B cell antigen receptor signaling.

Roles of the SHP-1 tyrosine phosphatase in the negative regulation of cell signalling

The critical role for the SH2 domain-containing SHP-1 tyrosine phosphatase in regulating haemopoietic cell behaviour was initially revealed by data linking SHP-1 deficiency to the systemic autoimmunity and severe inflammation exhibited by motheaten mice. This discovery laid the groundwork for the identification of SHP-1 as an inhibitor of activation-promoting signalling cascades and for the coincident demonstration that protein tyrosine phosphatases (PTPs) such as SHP-1 show considerable specificity with respect to the mechanisms whereby they modulate the biochemical and biological sequelae of extracellular simulation. As outlined in this review, SHP-1 has now been implicated in the regulation of a myriad of signalling cascades and cell functions. As a result, the cumulative data generated from studies of this PTP have elucidated not only the functional relevance of SHP-1, but also a number of novel paradigms as to the molecular mechanisms whereby signalling cascades are regulated so as to either augment or abrogate specific cell behaviours.

The CD45 tyrosine phosphatase: a positive and negative regulator of immune cell function

The CD45 phosphotyrosine phosphatase is one of the most abundant glycoproteins expressed on immune cells. Previously, the serpentine twists and turns of the CD45 research field have tended to draw attention to CD45 either as a positive or negative regulator of immune cell function. This review draws heavily on CD45 knockout mouse data to emphasize that CD45 has both positive and negative actions in regulating receptor thresholds, and these roles vary according to cell lineage and developmental stage. Previously conflicting results are reconciled in a model suggesting how CD45 regulates the p56(lck)tyrosine kinase in T cell signalling and development.

Cytoplasmic protein tyrosine phosphatases SHP-1 and SHP-2: regulators of B cell signal transduction

One of the areas of greatest recent progress in immunology has been the elucidation of inhibitory receptors and their mode of signal transduction. A common feature of members of this growing family is expression of a conserved cytoplasmic sequence motif, the immunoreceptor tyrosine-based inhibitory motif, which functions to recruit and activate phosphatases that mediate the receptors' function. Family members include the protein tyrosine phosphatases SHP-1 (Src-homology-2-domain-containing protein tyrosine phosphatase 1) and SHP-2, which function to dephosphorylate key intermediaries in antigen receptor signaling pathways. Surprisingly, whereas most data to date support a role for SHP-1 in inhibitory signaling, SHP-2 exhibits distinct functions that appear to positively regulate receptor function.

The tyrosine phosphatase SHP-1 influences thymocyte selection by setting TCR signaling thresholds

Modulation of the strength of signals from the TCR determines the outcome of positive and negative selection in thymocyte development. Previous studies have demonstrated that SHP-1 plays a role in determining signal strength from the TCR. Here, we have taken a genetic approach to test whether SHP-1 plays a role in T cell selection in the thymus. Experiments in which a dominant negative mutant of SHP-1 was expressed in the BYDP hybridoma cell line confirmed that SHP-1 regulated TCR signaling in a cell-autonomous manner and suggested that Lck is one of its targets. To examine the role of SHP-1 in T cell development, we crossed the ovalbumin-specific DO11.10 TCR transgene onto the motheaten background, which lacks SHP-1 expression. Analysis of the progeny of these crosses provided evidence that SHP-1 regulates thymocyte selection: (i) flow cytometric analyses revealed alterations in the percentages of thymocyte subpopulations in the me/me background; (ii) ex vivo deletion experiments demonstrated that me/me:Tg thymocytes undergo negative selection at lower concentrations of OVA peptide compared to +/+:Tg thymocytes; and (iii) ex vivo proliferation analyses indicated that me/me:Tg thymocytes were hyper-sensitive to stimulation by the specific OVA peptide. Our observation that the absence of SHP-1 leads to altered selection of TCR transgenic thymocytes demonstrates that SHP-1 regulates the strength of TCR-mediated signals in vivo and, in turn, helps to set the threshold for thymocyte selection.

CD19 is a central response regulator of B lymphocyte signaling thresholds governing autoimmunity

The CD19/CD21 complex is categorized among the 'response regulator' class of receptors which determine the magnitude and outcomes of B cell receptor signals. Small changes in CD19 expression have dramatic effects on signaling thresholds within B cells, which in turn has considerable impact on the balance between humoral immune responses and tolerance induction. B cell signaling thresholds lowered by increased CD19 expression may significantly augment host susceptibility to the development of autoimmunity. Signals generated by C3d-antigen complex binding to CD21 may also be involved in the development of autoimmunity by regulating CD19 function. Since CD19 serves as a central regulator of signaling thresholds in B cells, the CD19/CD21 complex may be an appropriate target for suppressing the development of autoimmunity.

The tyrosine phosphatase SHP-1 is a negative regulator of osteoclastogenesis and osteoclast resorbing activity: increased resorption and osteopenia in me(v)/me(v) mutant mice

Naturally occuring inactivating mutations of the Src homology 2 (SH2) domain-containing tyrosine phosphatase 1 (SHP-1) in mice give rise to the motheaten (me) phenotype. me/me mice have multiple hematopoietic abnormalities, suggesting that this phosphatase plays an important role in hematopoiesis. SHP-1 binds to and is activated by several hematopoietic surface receptors, including the colony-stimulating factor type 1 receptor. We have examined the role of SHP-1 in osteoclastogenesis and osteoclast function using mice with the viable motheaten (me(v)/me(v)) mutation, which has markedly decreased SHP-1 activity. Histomorphometric analysis of 6-week-old me(v)/me(v) mice and control littermates showed a marked osteopenia with an increase in bone resorption indices. The number of formed osteoclast-like cells (OCLs) in cocultures of me(v)/me(v) hematopoietic cells with normal osteoblasts was significantly increased. In contrast, the number of OCLs formed in the coculture of normal bone marrow cells with the me(v)/me(v) osteoblasts was not significantly different from controls. The bone-resorbing activity of me(v)me(v) OCLs and authentic osteoclasts was also found to be increased. Finally, Western blotting of proteins from me(v)/me(v) and control OCLs revealed an overall increase in tyrosine phosphorylation in the me(v)/me(v) lysates. These in vivo and in vitro results suggest that SHP-1 is a negative regulator of bone resorption, affecting both the formation and the function of osteoclasts.

Modulation of B lymphocyte antigen receptor signal transduction by a CD19/CD22 regulatory loop

CD19 and CD22 are B lymphocyte cell-surface molecules that positively and negatively regulate antigen receptor signal transduction, respectively. Biochemical studies with B cells from CD19-deficient and CD22-deficient mice indicated that these two regulatory molecules influenced each other's functions: CD22 expression negatively regulated CD19 tyrosine phosphorylation, while optimal CD22 function was dependent on CD19 expression. Functional CD19 and CD22 interactions were also assessed in vivo by generating CD19/CD22 double-deficient mice. Remarkably, the CD19 mutation was dominant to the CD22 mutation in most instances. B lymphocytes from CD19/CD22-deficient and CD19-deficient mice were functionally equivalent despite the negative influence normally provided by CD22 expression. These data collectively suggest that CD19 activates the CD22/SHP1 inhibitory pathway that then acts primarily on CD19.

B cell development in the spleen takes place in discrete steps and is determined by the quality of B cell receptor-derived signals

Only mature B lymphocytes can enter the lymphoid follicles of spleen and lymph nodes and thus efficiently participate in the immune response. Mature, long-lived B lymphocytes derive from short-lived precursors generated in the bone marrow. We show that selection into the mature pool is an active process and takes place in the spleen. Two populations of splenic B cells were identified as precursors for mature B cells. Transitional B cells of type 1 (T1) are recent immigrants from the bone marrow. They develop into the transitional B cells of type 2 (T2), which are cycling and found exclusively in the primary follicles of the spleen. Mature B cells can be generated from T1 or T2 B cells. Mice with genetic deletions of elements participating in the B cell receptor signaling cascade display developmental arrest at the T1 or T2 stage. The analysis of these defects showed that the development of T2 and mature B cells from T1 precursors requires defined qualitative and quantitative signals derived from the B cell receptor and that the induction of longevity and maturation requires different signals.

Genetic analysis of B cell antigen receptor signaling

In B lymphocytes, a signaling complex that contributes to cell fate decisions is the B cell antigen receptor (BCR). Data from knockout experiments in cell lines and mice have revealed distinct functions for the intracellular protein tyrosine kinases (Lyn, Syk, Btk) in BCR signaling and B cell development. Combinations of intracellular signaling pathways downstream of these PTKs determine the quality and quantity of BCR signaling. For example, concerted actions of the PLC-gamma 2 and PI3-K pathways are required for proper calcium responses. Similarly, the regulation of ERK and JNK responses involves both PLC-gamma 2 and GTPases pathways. Since the immune response in vivo is regulated by alteration of these signaling outcomes, achieving a precise understanding of intracellular molecular events leading to B lymphocyte proliferation, deletion, anergy, receptor editing, and survival still remains a challenge for the future.

The Src-Homology Domain 2-Bearing Protein Tyrosine Phosphatase-1 Inhibits Antigen Receptor-Induced Apoptosis of Activated Peripheral T Cells

Restimulation of Ag receptors on peripheral T lymphocytes induces tyrosine phosphorylation-based signaling cascades that evoke Fas ligand expression and induction of Fas-mediated programmed cell death. In view of the role for the Src homology domain 2-bearing protein tyrosine phosphatase-1 (SHP-1) in modulating TCR signaling, we investigated the influence of SHP-1 on TCR-mediated apoptosis by assaying the sensitivity of peripheral T cells from SHP-1-deficient viable motheaten (mev) mice to cell death following TCR restimulation. The results of these studies revealed mev peripheral T cells to be markedly more sensitive than wild-type cells to induction of cell death following TCR stimulation. By contrast, PMA/ionophore and anti-Fas Ab-induced apoptotic responses were no different in mev compared with wild-type activated cells. Enhanced apoptosis of TCR-restimulated mev lymphocytes was associated with marked increases in Fas ligand expression as compared with wild-type cells, but was almost abrogated in both mev and wild-type cells by Fas-Fc treatment. Thus, the increased sensitivity of mev T cells to apoptosis following TCR restimulation appears to reflect a TCR-driven phenomenon mediated through up-regulation of Fas-Fas ligand interaction and induction of the Fas signaling cascade. These findings, together with the hyperproliferative responses of mev peripheral T cells to initial TCR stimulation, indicate that SHP-1 modulation of TCR signaling translates to the inhibition of both T cell proliferation and activation and, as such, is likely to play a pivotal role in regulating the expansion of Ag-stimulated T cells during an immune response.

CD45 Regulates Tyrosine Phosphorylation of CD22 and Its Association with the Protein Tyrosine Phosphatase SHP-1

Cross-linking of CD45 induced capping and physical sequestration from CD22 leading to an increase in tyrosine phosphorylation of CD22 and SHP-1 recruitment. Additionally, CD22 isolated from a CD45-deficient B cell line exhibited increased basal/inducible tyrosine phosphorylation and enhanced recruitment of SHP-1 compared with CD22 isolated from CD45-positive parental cells. Subsequent experiments were performed to determine whether enhanced SHP-1 recruitment to CD22 is responsible for attenuation of receptor-mediated Ca2+ responses in CD45-deficient cells. Catalytically inactive SHP-1 expressed in CD45-deficient cells interacted with CD22 and decreased phosphatase activity in CD22 immunoprecipitates to levels that were comparable to those in CD45-positive cells. Expression of catalytically inactive SHP-1 restored intracellular mobilization of Ca2+ in response to MHC class II cross-linking, but did not affect B cell Ag receptor- or class II-mediated Ca2+ influx from the extracellular space. These results indicate that CD45 regulates tyrosine phosphorylation of CD22 and binding of SHP-1. The data further indicate that enhanced recruitment and activation of SHP-1 in CD45-deficient cells affect intracellular mobilization of Ca2+, but are not responsible for abrogation of receptor-mediated Ca2+ influx from the extracellular space.

The protein tyrosine phosphatase SHP-1 regulates integrin-mediated adhesion of macrophages

The Src homology 2 domain phosphatase-1 (SHP-1) is a tyrosine phosphatase containing two amino-terminal SH2 domains and is expressed primarily by hematopoietic-derived cells [1]. The viable motheaten (Hcphme-v) mutant mice (mev) suffer from progressive inflammation due to a deficiency of SHP-1 enzyme activity [2,3] and die at 3-4 months of age from macrophage and neutrophil accumulation in the lung [4]. The mechanism by which SHP-1 deficiency leads to inflammation is unknown. We found that macrophages from mev mice adhered and spread to a greater extent than normal macrophages through alpha m beta 2 integrin-mediated contacts. Whereas macrophages deficient in the transmembrane tyrosine phosphatase CD45 (CD45-/-) spontaneously detached from alpha m beta 2 integrin contacts [5], cells deficient in both CD45 and SHP-1 did not. In SHP-1 deficient macrophages there was a 10-15-fold increase in D-3 phospholipid products of phosphatidylinositol (PI) 3-kinase. Concomitantly, there was a 2-5-fold increase in membrane-associated PI 3-kinase activity in mev macrophages relative to normal macrophages. Treatment of macrophages with the PI 3-kinase inhibitors wortmannin or LY294002 resulted in a dramatic detachment of cells, indicating that PI 3-kinase activity is required for adhesion. These data demonstrate that SHP-1 is necessary for detachment from alpha m beta 2 integrin-mediated contacts in primary macrophages and suggest that a defect in this pathway may contribute to inflammatory disease.

Independent and opposing roles for Btk and lyn in B and myeloid signaling pathways

Transphosphorylation by Src family kinases is required for the activation of Bruton's tyrosine kinase (Btk). Differences in the phenotypes of Btk-/- and lyn-/- mice suggest that these kinases may also have independent or opposing functions. B cell development and function were examined in Btk-/-lyn-/- mice to better understand the functional interaction of Btk and Lyn in vivo. The antigen-independent phase of B lymphopoiesis was normal in Btk-/-lyn-/- mice. However, Btk-/-lyn-/- animals had a more severe immunodeficiency than Btk-/- mice. B cell numbers and response to T cell-dependent antigens were reduced. Btk and Lyn therefore play independent or partially redundant roles in the maintenance and function of peripheral B cells. Autoimmunity, hypersensitivity to B cell receptor (BCR) cross-linking, and splenomegaly caused by myeloerythroid hyperplasia were alleviated by Btk deficiency in lyn-/- mice. A transgene expressing Btk at approximately 25% of endogenous levels (Btklo) was crossed onto Btk-/- and Btk-/-lyn-/- backgrounds to demonstrate that Btk is limiting for BCR signaling in the presence but not in the absence of Lyn. These observations indicate that the net outcome of Lyn function in vivo is to inhibit Btk-dependent pathways in B and myeloid cells, and that Btklo mice are a useful sensitized system to identify regulatory components of Btk signaling pathways.

Receptor editing and commitment in B lymphocytes

B cells that fail to pass a developmental checkpoint, either as immature or mature B cells, can be rescued by creating a new B cell antigen receptor through nested secondary immunoglobulin gene rearrangements, a process termed receptor editing. Tolerance-mediated receptor editing occurs in self-reactive immature bone marrow B cells, while peripheral receptor editing probably occurs in low-affinity B cells competing for antigen and for survival signals within the germinal center response.

Analysis of antigen receptor signalling using mouse gene targeting

Gene targeting in mice has enabled the study of antigen receptor signalling in primary lymphocytes. Furthermore, it has provided the tools to directly assess the function of individual signalling proteins by mutation of the genes that code for them. Some of the results that gene targeting has produced have confirmed previous views of the function of particular proteins. Others have given surprising results and overturned accepted viewpoints.