Thyroid-infiltrating B lymphocytes in Graves' disease are related to marginal zone and memory B cell compartments

Thymic B Cells as a New Player in the Type 1 Diabetes Response

Type 1 diabetes (T1d) results from a sustained autoreactive T and B cell response towards insulin-producing β cells in the islets of Langerhans. The autoreactive nature of the condition has led to many investigations addressing the genetic or cellular changes in primary lymphoid tissues that impairs central tolerance- a key process in the deletion of autoreactive T and B cells during their development. For T cells, these studies have largely focused on medullary thymic epithelial cells (mTECs) critical for the effective negative selection of autoreactive T cells in the thymus. Recently, a new cellular player that impacts positively or negatively on the deletion of autoreactive T cells during their development has come to light, thymic B cells. Normally a small population within the thymus of mouse and man, thymic B cells expand in T1d as well as other autoimmune conditions, reside in thymic ectopic germinal centres and secrete autoantibodies that bind selective mTECs precipitating mTEC death. In this review we will discuss the ontogeny, characteristics and functionality of thymic B cells in healthy and autoimmune settings. Furthermore, we explore how in silico approaches may help decipher the complex cellular interplay of thymic B cells with other cells within the thymic microenvironment leading to new avenues for therapeutic intervention.

Marginal zone B cells: From housekeeping function to autoimmunity?

Marginal zone (MZ) B cells comprise a subset of innate-like B cells found predominantly in the spleen, but also in lymph nodes and blood. Their principal functions are participation in quick responses to blood-borne pathogens and secretion of natural antibodies. The latter is important for housekeeping functions such as clearance of apoptotic cell debris. MZ B cells have B cell receptors with low poly-/self-reactivity, but they are not pathogenic at steady state. However, if simultaneously stimulated with self-antigen and pathogen- and/or damage-associated molecular patterns (PAMPs/DAMPs), MZ B cells may participate in the initial steps towards breakage of immunological tolerance. This review summarizes what is known about the role of MZ B cells in autoimmunity, both in mouse models and human disease. We cover factors important for shaping the MZ B cell compartment, how the functional properties of MZ B cells may contribute to breaking tolerance, and how MZ B cells are being regulated.

CD19 regulates ADAM28-mediated Notch2 cleavage to control the differentiation of marginal zone precursors to MZ B cells

As the first line of defence, marginal zone (MZ) B cells play principal roles in clearing blood‐borne pathogens during infection and are over‐primed in autoimmune diseases. However, the basic mechanisms underlying MZ B‐cell development are still unclear. We found here that CD19 deficiency blocked the differentiation of marginal zone precursors (MZP) to MZ B cells, whereas CD19 expression in CD19‐deficient MZP rescues MZ B‐cell generation. Furthermore, CD19 regulates Notch2 cleavage by up‐regulating ADAM28 expression in MZP. Finally, we found that CD19 suppressed Foxo1 expression to promote ADAM28 expression in MZP. These results suggest that CD19 controls the differentiation of MZP to MZ B cells by regulating ADAM28‐mediated Notch2 cleavage. Thus, we demonstrated the basic mechanisms underlying the differentiation of MZP to MZ B cells.

The peripheral blood compartment in patients with Graves' disease: activated T lymphocytes and increased transitional and pre-naive mature B lymphocytes

Graves' disease (GD) is an autoimmune disease that involves aberrant B and T lymphocyte responses. Detailed knowledge about lymphocyte subpopulation composition will therefore enhance our understanding of the pathogenesis of GD and might support the development of new immunomodulatory treatment approaches. The aim of this study was to gain detailed insight into the composition of the peripheral blood lymphocyte compartment in GD before and during anti-thyroid drug therapy. Major B and T lymphocyte subpopulations were investigated by flow cytometry in peripheral blood from newly diagnosed GD patients (n = 5), GD patients treated with anti-thyroid drugs (n = 4), patients with recurrent GD (n = 7) and healthy controls (HC; n = 10). In GD patients, numbers of activated T lymphocytes [human leucocyte antigen D-related (HLA-DR)⁺ and CD25⁺] were increased. The B lymphocyte compartment in GD was characterized by significantly higher numbers of transitional (CD38(high) CD27⁻, P < 0.03) and pre-naive mature (CD38(low) CD27⁻ IgD⁺ CD5⁺, P < 0.04) B lymphocytes, while memory populations were slightly decreased. The increased numbers of CD5⁺, transitional and pre-naive mature B lymphocytes correlated positively with fT4 plasma levels. GD is associated with increased numbers of activated T lymphocytes and transitional and pre-naive mature CD5⁺ B lymphocytes within the peripheral blood. The increase in CD5⁺ B lymphocytes was due mainly to an increase in transitional and pre-naive mature B lymphocytes. Increased fT4 plasma levels might be associated with this increase in transitional and pre-naive mature CD5⁺ B lymphocytes.

Intrinsic molecular factors cause aberrant expansion of the splenic marginal zone B cell population in nonobese diabetic mice

Marginal zone (MZ) B cells are an innate-like population that oscillates between MZ and follicular areas of the splenic white pulp. Differentiation of B cells into the MZ subset is governed by BCR signal strength and specificity, NF-κB activation through the B cell-activating factor belonging to the TNF family (BAFF) receptor, Notch2 signaling, and migration signals mediated by chemokine, integrin, and sphingosine-1-phosphate receptors. An imbalance in splenic B cell development resulting in expansion of the MZ subset has been associated with autoimmune pathogenesis in various murine models. One example is the NOD inbred mouse strain, in which MZ B cell expansion has been linked to development of type 1 diabetes and Sjögren's syndrome. However, the cause of MZ B cell expansion in this strain remains poorly understood. We have determined that increased MZ B cell development in NOD mice is independent of T cell autoimmunity, BCR specificity, BCR signal strength, and increased exposure to BAFF. Rather, mixed bone marrow chimeras showed that the factor(s) responsible for expansion of the NOD MZ subset is B cell intrinsic. Analysis of microarray expression data indicated that NOD MZ and precursor transitional 2-MZ subsets were particularly dysregulated for genes controlling cellular trafficking, including Apoe, Ccbp2, Cxcr7, Lgals1, Pla2g7, Rgs13, S1pr3, Spn, Bid, Cd55, Prf1, and Tlr3. Furthermore, these B cell subsets exhibited an increased steady state dwell time within splenic MZ areas. Our data therefore reveal that precursors of mature B cells in NOD mice exhibit an altered migration set point, allowing increased occupation of the MZ, a niche favoring MZ B cell differentiation.

Small molecule inhibitors of the Pyk2 and FAK kinases modulate chemoattractant-induced migration, adhesion and Akt activation in follicular and marginal zone B cells

B-lymphocytes produce protective antibodies but also contribute to autoimmunity. In particular, marginal zone (MZ) B cells recognize both microbial components and self-antigens. B cell trafficking is critical for B cell activation and is controlled by chemoattactants such as CXCL13 and sphingosine 1-phosphate (S1P). The related tyrosine kinases focal adhesion kinase (FAK) and proline-rich tyrosine kinase (Pyk2) regulate cell migration and adhesion but their roles in B cells are not fully understood. Using a novel Pyk2-selective inhibitor described herein (PF-719), as well as a FAK-selective inhibitor, we show that both Pyk2 and FAK are important for CXCL13- and S1P-induced migration of B-2 cells and MZ B cells. In contrast, LFA-1-mediated adhesion required only Pyk2 whereas activation of the Akt pro-survival kinase required FAK but not Pyk2. Thus Pyk2 and FAK mediate critical processes in B cells and these inhibitors can be used to further elucidate their functions in B cells.

B cell depletion in autoimmune diabetes: insights from murine models

INTRODUCTION

The incidence of type 1 diabetes (T1D) is rising for reasons that largely elude us. New strategies aimed at halting the disease process are needed. One type of immune cell thought to contribute to T1D is the B lymphocyte. The first Phase II trial of B cell depletion in new onset T1D patients indicated that this slowed the destruction of insulin-producing pancreatic beta cells. The mechanistic basis of the beneficial effects remains unclear.

AREAS COVERED

Studies of B cell depletion and deficiency in animal models of T1D. How B cells can influence T cell-dependent autoimmune diabetes in animal models. The heterogeneity of B cell populations and current evidence for the potential contribution of specific B cell subsets to diabetes, with emphasis on marginal zone B cells and B1 B cells.

EXPERT OPINION

B cells can influence the T cell response to islet antigens and B cell depletion or genetic deficiency is associated with decreased insulitis in animal models. New evidence suggests that B1 cells may contribute to diabetes pathogenesis. A better understanding of the roles of individual B cell subsets in disease will permit fine-tuning of therapeutic strategies to modify these populations.

Lymphocytes in peripheral blood and thyroid tissue in children with Graves' disease

BACKGROUND

This study was undertaken to analyze subsets of lymphocytes in peripheral blood in the early phase and in the thyroid tissue in the late phase of Graves' disease (GD) in children.

METHODS

The study included 30 children with GD and 30 healthy children. Monoclonal antibodies were used to define peripheral blood lymphocyte subsets and they were analyzed using the flow cytometer Ortho Diagnostic System. After thyroidectomy, T cells were detected by CD3, CD4, CD8 antibodies, B cells by CD79alpha antibodies, and the antigen presenting dendritic cells (APCs) by CD1alpha antibodies (DakoCytomation) in the thyroid tissue.

RESULTS

Before the treatment, an increased percentage of CD4+ T helper cells and B cells and decreased CD8+ T suppressor/cytotoxic cells were observed in peripheral blood in all the GD children. The number of lymphocytes and dendritic cells in the thyroid tissue increased in the children with GD in comparison to the control group, especially T cells subsets CD4+ and CD8+ and CD79alpha+ B cells. The percentage of T cells in the thyroid tissue was lower and that of B cells was higher than in peripheral blood. In their structure, thyrocytes can have components similar to alpha-chains connected with beta-microglobulins, which were characteristic for APCs.

CONCLUSIONS

The primary defect of immunoregulation in children with GD is probably dependent on a large number and the activity of T helper cells and on a small number and hypofunction of T suppressor cells. The amount of lymphocytes decreased proportionally to the duration of methimazole treatment. The thyrocytes probably can present antigens.

Treatment of Graves' disease and associated ophthalmopathy with the anti-CD20 monoclonal antibody rituximab: an open study

INTRODUCTION

Hyperthyroid Graves' disease (GD) is a B-cell-mediated condition caused by TSH receptor antibodies (TRAb), which decline when GD remits. Anti-CD20 monoclonal antibody rituximab (RTX) induces transient B-cell depletion that may potentially modify the active inflammatory phase of thyroid-associated ophthalmopathy (TAO).

METHODS

Nine patients with GD, (seven with active TAO, two with mild lid signs) were studied. The trial was only approved as an open pilot study; thus we compared the effect of RTX therapy to that of i.v. glucocorticoids (IVGC) in 20 consecutive patients. Patients were treated with RTX (1000 mg i.v. twice at 2-week interval) or with IVGC (500 mg i.v. for 16 weeks). TAO was assessed by the clinical activity score (CAS) and severity was classified using NOSPECS (No signs or symptoms; Only signs (lid); Soft tissue involvement; Proptosis, Extraocular muscle involvement; Corneal involvement; Sight loss). Thyroid function and lymphocyte count were measured by standardized methods.

RESULTS

All patients attained peripheral B-cell depletion with the first RTX infusion. Minor side effects were reported in three patients. Thyroid function was not affected by RTX therapy and hyperthyroid patients required therapy with methimazole. After RTX, the changes in the levels of thyroglobulin antibodies, thyroperoxidase antibodies and TRAb were neither significant nor correlated with CD20+ depletion (P = NS). CAS values before RTX were 4.7 +/- 0.5 and decreased to 1.8 +/- 0.8 at the end of follow-up (P < 0.0001) and more significantly compared with IVGC (P < 0.05). Proptosis decreased significantly after RTX both in patients with active TAO (ANOVA; P < 0.0001) and those with lid signs (ANOVA; P < 0.003). The degree of inflammation (class 2) decreased significantly in response to RTX (ANOVA; P < 0.001). Relapse of active TAO was not observed in patients treated with RTX, but occurred in 10% of those treated with IVGC, who also experienced adverse effects more frequently (45 vs 33% of patients).

CONCLUSIONS

RTX positively affects the clinical course of TAO, independently of either thyroid function or circulating antithyroid antibodies, including TRAb. If our findings are confirmed in large controlled studies, RTX may represent a useful therapeutic tool in patients with active TAO.

Treatment of autoimmune hyperthyroidism in a murine model of Graves' disease with tumor necrosis factor-family ligand inhibitors suggests a key role for B cell activating factor in disease pathology

Hyperthyroid Graves' disease is a common autoimmune disorder mediated by agonistic antibodies to the TSH receptor, termed thyroid stimulating antibodies (TSAbs). Recently members of the TNF superfamily, B cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL), have been identified along with their receptors, B cell maturation antigen and transmembrane activator and calcium-modulator and cyclophilin ligand interactor, and the BAFF-specific receptor. BAFF is a fundamental B cell survival/maturation factor, and both BAFF and APRIL have been implicated in antibody production. We investigated the effect of interfering with BAFF- and APRIL-mediated signals in an induced model of Graves' disease by blockade of these factors using soluble decoy receptors. In a therapeutic setting in mice with established hyperthyroidism, we show that blockade of BAFF or BAFF+APRIL with BAFF-specific receptor-Fc and B cell maturation antigen-Fc, respectively, leads to significant reductions in the induced hyperthyroidism. This was supported by a parallel pattern of declining TSAbs in the responding animals. Histopathological analysis of splenic sections from treated animals revealed marked reductions in the B cell follicle regions, but staining with anti-CD138 revealed the persistence of plasma cells. Thus, the reductions in TSAbs in the treated animals were not related to overall plasma cell numbers in the secondary lymphoid organs. Our results are the first to demonstrate attenuation of established hyperthyroidism by therapeutic intervention aimed at autoreactive B cells and indicate that both BAFF and APRIL appear to play important roles in the development and survival of the autoantibody producing cells in this model.

Development of nephritis but not sialadenitis in autoimmune-prone BAFF transgenic mice lacking marginal zone B cells

B cell-activating factor belonging to the TNF family (BAFF) is a B cell survival factor required for B cell maturation. BAFF transgenic (Tg) mice develop autoimmune disorders characterized by autoantibody production, which leads to nephritis and salivary gland destruction (sialadenitis), features reminiscent of systemic lupus erythematosus and Sjögren's syndrome (SS), respectively. Disease in BAFF Tg mice correlates with the expansion of the marginal zone (MZ) B cell compartment and the abnormal presence of MZ-like B cells in the blood, LN and inflamed salivary glands, suggesting a role for these cells in BAFF-induced autoimmunity. Lymphotoxin-beta (LTbeta)-deficient mice show disrupted splenic architecture, lack MZ B cells and some peripheral LN, and are unable to mount T cell-dependent immune responses. BAFF Tg mice lacking LTbeta (LTbetaDelta-BTg) retained these defects, yet still developed nephritis associated with the presence of B-1 B cells in the kidneys. However, in contrast to old BAFF Tg mice, aging LTbetaDelta-BTg mice no longer developed sialadenitis. Thus, autoimmune disorders in BAFF Tg mice are possibly events coordinated by MZ and B-1 B cells at separate anatomical sites.

A soluble BAFF antagonist, BR3-Fc, decreases peripheral blood B cells and lymphoid tissue marginal zone and follicular B cells in cynomolgus monkeys

BAFF (also known as BLyS), a member of the tumor necrosis factor superfamily, plays a critical role in the maturation and development of B cells. BAFF has three receptors on B cells, the most crucial of which is BR3. In this study, we demonstrate the biological outcome of BAFF blockade in cynomolgus monkeys using a soluble fusion protein consisting of human BR3 and human IgG1 Fc. In vitro, BR3-Fc blocked BAFF-mediated survival and proliferation of cynomolgus monkey B cells. Weekly treatment of cynomolgus monkeys with BR3-Fc for 13 to 18 weeks resulted in significant B-cell reduction in the peripheral blood and in lymphoid organs. CD21(high) B cells in lymphoid tissues, a subset analogous to human marginal zone B cells, expressed nearly twofold higher BR3 levels than did CD21(med) B cells. Lymphoid tissue flow cytometric analysis showed that BR3-Fc reduced this CD21(high) B-cell subset to a greater extent than it reduced CD21(med) B cells. Dual-label immunohistochemistry and morphometric image analysis supported these results by demonstrating that BR3-Fc reduced a significant proportion of the B cells within the splenic inner and outer marginal zones. These findings should prove very useful in guiding the desired therapeutic use of BR3-Fc for autoimmune diseases in the clinic.

MARGINAL ZONE B CELLS

Our views regarding the origins and functions of splenic marginal zone B cells have changed considerably over the past few years. Perspectives regarding the development and function of these cells vary considerably between investigators studying human and rodent immunology. Marginal zone B cells are now recognized to constitute a distinct naive B lymphoid lineage. Considerable progress has been made regarding the mechanisms involved in marginal zone B cell development in the mouse. Many of the molecular events that participate in the retention of this lineage of B cells in the marginal zone have been identified. Here, we discuss the functions of these cells in both innate and adaptive immunity. We also attempt to reconcile differing viewpoints regarding the generation and function of marginal zone B cells in rodents and primates.

B-lymphocytes, innate immunity, and autoimmunity

Having evolved to generate a huge Ag-specific repertoire and to mount T cell-dependent responses and long-term memory, the B lymphocyte is a central player in the adaptive branch of immune defense. However, accumulating evidence indicates that B-1 cells of the peritoneal cavity and marginal zone (MZ) B cells of the spleen also can play innate-like immune functions. Their anatomical locations allow frequent Ag encounter. Secreting essentially germline-encoded, polyreactive Abs, and responding rapidly and vigorously to stimulation, these two B cell subsets have evolved to impart potentially protective responses. With their additional capacities to secrete factors that can directly mediate microbial destruction and to express Toll-like receptors (TLR), B cells provide an important link between the innate and adaptive branches of the immune system. Currently, the relevance of these innate-like B cells to the pathogenesis of autoimmune disease is the focus of investigation. In experimental models of autoimmunity, the sequestration of autoreactive B cells in the MZ has been proposed to be essential for the maintenance of self-tolerance. The low activation threshold of MZ B cells makes them particularly reactive to high loads and/or altered self-Ags, potentially exacerbating autoimmune disease. Their expansion in autoimmune models and their association with autoantibody secretion indicate that they may participate in tissue damage. The demonstration that B cell depletion therapies may represent a highly beneficial therapeutic goal in autoimmune disorders suggests that specific elimination of B-1 and MZ B cells may represent a more efficient immunointervention strategy in systemic autoimmunity.

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.

Effect of the B cell superantigen protein A from S. aureus on the early lupus disease of (NZBxNZW) F1 mice

The (NZBxNZW) F(1) mouse develops a spontaneous autoimmune disease process with striking similarities to human systemic lupus erythematosus (SLE). In female (NZBxNZW) F(1) mice, the production of IgG antinuclear antibodies, including antibodies to double-stranded DNA (dsDNA), is associated with the development of a severe immune complex-mediated glomerulonephritis that results in death from renal failure in virtually all animals by 12 months of age. Since B-1 and marginal zone (MZ) cells represent a potential source of pathogenic antibodies and because B cell superantigens have been demonstrated to reduce B-1 and MZ cells in vivo, we tested the effect of repeated injections of the superantigen protein A (SpA) from S. aureus on the disease of this lupus model. We found that weekly intraperitoneal injections of SpA delay the progression of serum anti-DNA IgG and reduce proteinuria early in young female (NZBxNZW) F(1) mice. This superantigen also induced a specific depression in the numbers of peritoneal B-1 cells, as compared to mice treated with a control protein. These results support the role of B-1 cells in the development of the autoimmune disease in this mouse model and suggest that B cell superantigens may be useful in the management of autoimmune conditions.

Differences in thyroid-infiltrating B lymphocytes in patients with Graves' disease: relationship to autoantibody detection

OBJECTIVE

Thyroid-infiltrating B (Thyr-B) lymphocytes are thought to play an important role in the pathogenic mechanisms underlying Graves' disease.

DESIGN AND METHODS

In this study, a broad phenotypic analysis of these cells has been performed in 15 consecutive patients who underwent thyroidectomy.

RESULTS

Data reveal the occurrence of two distinct types of Thyr-B cell infiltrates. Type 1 was present in most of the cases (10/15) and consisted of a combination of IgM+ IgD(low to-) B lymphocytes showing features of marginal zone B cells, and IgG+ classic memory B cells. In contrast, in 5 of the 15 cases, a second type of Thyr-B cell infiltrate occurred, exhibiting the profile IgM- IgD- CD44(low to-) CD38++ CD71+ CD95+. This phenotype is highly suggestive of germinal center (GC) B cells, a finding not always anticipated from routine histologic examination. The presence of these ectopic GC was closely associated with the elevated serum level of anti-thyroid peroxidase (TPO), but not with anti-thyrotropin receptor (TSHR), autoantibodies. Moreover, local active anti-thyroglobulin (Tg) antibody secretion was only detected in cultures of type 2 Thyr-B cells.

CONCLUSION

These findings indicate that high titers of anti-TPO, but not anti-TSHR antibody, might be associated with intrathyroidal GC development.

TNF deficiency fails to protect BAFF transgenic mice against autoimmunity and reveals a predisposition to B cell lymphoma

TNF is well characterized as a mediator of inflammatory responses. TNF also facilitates organization of secondary lymphoid organs, particularly B cell follicles and germinal centers, a hallmark of T-dependent Ab responses. TNF also mediates defense against tumors. We examined the role of TNF in the development of inflammatory autoimmune disorders resembling systemic lupus erythematosus and Sjögren's syndrome induced by excess B cell-activating factor belonging to the TNF family (BAFF), by generating BAFF-transgenic (Tg) mice lacking TNF. TNF(-/-) BAFF-Tg mice resembled TNF(-/-) mice, in that they lacked B cell follicles, follicular dendritic cells, and germinal centers, and have impaired responses to T-dependent Ags. Nevertheless, TNF(-/-) BAFF-Tg mice developed autoimmune disorders similar to that of BAFF-Tg mice. Disease in TNF(-/-) BAFF-Tg mice correlates with the expansion of transitional type 2 and marginal zone B cell populations and enhanced T-independent immune responses. TNF deficiency in BAFF-Tg mice also led to a surprisingly high incidence of B cell lymphomas (>35%), which most likely resulted from the combined effects of BAFF promotion of neoplastic B cell survival, coupled with lack of protective antitumor defense by TNF. Thus, TNF appears to be dispensable for BAFF-mediated autoimmune disorders and may, in fact, counter any proneoplastic effects of high levels of BAFF in diseases such as Sjögren's syndrome, systemic lupus erythematosus, and rheumatoid arthritis.

BAFF AND APRIL: a tutorial on B cell survival

BAFF, a member of the TNF family, is a fundamental survival factor for transitional and mature B cells. BAFF overexpression leads to an expanded B cell compartment and autoimmunity in mice, and elevated amounts of BAFF can be found in the serum of autoimmune patients. APRIL is a related factor that shares receptors with BAFF yet appears to play a different biological role. The BAFF system provides not only potential insight into the development of autoreactive B cells but a relatively simple paradigm to begin considering the balancing act between survival, growth, and death that affects all cells.

Blood dendritic cells interact with splenic marginal zone B cells to initiate T-independent immune responses

Marginal zone (MZ) and B1 B lymphocytes participate jointly in the early immune response against T-independent (TI) particulate antigens. Here we show that blood-derived neutrophil granulocytes and CD11c(lo) immature dendritic cells (DC) are the primary cells that efficiently capture and transport particulate bacteria to the spleen. In a systemic infection, CD11c(lo) DC, but not neutrophils, provide critical survival signals, which can be inhibited by TACI-Fc, to antigen-specific MZ B cells and promote their differentiation into IgM-secreting plasmablasts. In a local TI response, peritoneal cavity macrophages provide similar support to B1 B-derived Ag-specific blasts. In the absence of soluble TACI ligands, Ag-activated MZ- and B1-derived blasts lack survival signals and undergo apoptosis, resulting in severely impaired antibody responses.

Marginal-zone B cells

Recent advances in genomics and proteomics, combined with the facilitated generation and analysis of transgenic and gene-knockout animals, have revealed new complexities in classical biological systems, including the B-cell compartment. Studies on an 'old', but poorly characterized, B-cell subset--the naive, marginal-zone (MZ) B-cell subset--over the past two years have spawned an avalanche of data that encompass the generation and function of these cells. Now that the initial 'infatuation' is over, it is time to reconsider these data and generate some conclusions that can be incorporated into a working model of the B-cell system.