Dendritic cells from lupus-prone mice are defective in repressing immunoglobulin secretion

Dendritic Cells Regulate Extrafollicular Autoreactive B Cells via T Cells Expressing Fas and Fas Ligand

The extrafollicular (EF) plasmablast response to self-antigens that contain Toll-like receptor (TLR) ligands is prominent in murine lupus models and some bacterial infections, but the inhibitors and activators involved have not been fully delineated. Here, we used two conventional dendritic cell (cDC) depletion systems to investigate the role of cDCs on a classical TLR-dependent autoreactive EF response elicited in rheumatoid-factor B cells by DNA-containing immune complexes. Contrary to our hypothesis, cDC depletion amplified rather than dampened the EF response in Fas-intact but not Fas-deficient mice. Further, we demonstrated that cDC-dependent regulation requires Fas and Fas ligand (FasL) expression by T cells, but not Fas expression by B cells. Thus, cDCs activate FasL-expressing T cells that regulate Fas-expressing extrafollicular helper T (Tefh) cells. These studies reveal a regulatory role for cDCs in B cell plasmablast responses and provide a mechanistic explanation for the excess autoantibody production observed in Fas deficiency.

Breakdown of Immune Tolerance in Systemic Lupus Erythematosus by Dendritic Cells

Dendritic cells (DC) play an important role in the pathogenesis of systemic lupus erythematosus (SLE), an autoimmune disease with multiple tissue manifestations. In this review, we summarize recent studies on the roles of conventional DC and plasmacytoid DC in the development of both murine lupus and human SLE. In the past decade, studies using selective DC depletions have demonstrated critical roles of DC in lupus progression. Comprehensive in vitro and in vivo studies suggest activation of DC by self-antigens in lupus pathogenesis, followed by breakdown of immune tolerance to self. Potential treatment strategies targeting DC have been developed. However, many questions remain regarding the mechanisms by which DC modulate lupus pathogenesis that require further investigations.

Enhanced activation of dendritic cells by autologous apoptotic microvesicles in MRL/lpr mice

Introduction

Systemic lupus erythematosus is associated with a persistent circulation of modified autoantigen-containing apoptotic debris that might be capable of breaking tolerance. We aimed to evaluate apoptotic microvesicles obtained from lupus or control mice for the presence of apoptosis-associated chromatin modifications and for their capacity to stimulate dendritic cells (DC) from lupus and control mice.

Method

Apoptotic microvesicles were in vitro generated from splenocytes, and ex vivo isolated from plasma of both MRL/lpr lupus mice and normal BALB/c mice. Microvesicles were analyzed using flow cytometry. Bone marrow-derived (BM)-DC cultured from MRL/lpr or BALB/c mice were incubated with microvesicles and CD40 expression and cytokine production were determined as measure of activation.

Results

Microvesicles derived from apoptotic splenocytes or plasma of MRL/lpr mice contained more modified chromatin compared to microvesicles of BALB/c mice, and showed enhanced activation of DC, either from MRL/lpr or BALB/c mice, and consecutively an enhanced DC-mediated activation of splenocytes. The content of apoptosis-modified chromatin in microvesicles of apoptotic splenocytes correlated with their potency to induce interleukin-6 (IL-6) production by DC. Microvesicle-activated MRL/lpr DC showed a significant higher production of IL-6 and tumor growth factor-β (TGF-β) compared to BALB/c DC, and were more potent in the activation of splenocytes.

Conclusion

Apoptotic microvesicles from MRL/lpr mice are more potent activators of DC, and DC from MRL/lpr mice appear relatively more sensitive to activation by apoptotic microvesicles. Our findings indicate that aberrations at the level of apoptotic microvesicles and possibly DC contribute to the autoimmune response against chromatin in MRL/lpr mice.

Expansion of follicular helper T cells in the absence of Treg cells: implications for loss of B-cell anergy

The maintenance of B-cell anergy is essential to prevent the production of autoantibodies and autoimmunity. However, B-cell extrinsic mechanisms that regulate B-cell anergy remain poorly understood. We previously demonstrated that regulatory T (Treg) cells are necessary for the maintenance of B-cell anergy. We now show that in Treg-cell-deficient mice, helper T cells are necessary and sufficient for loss of B-cell tolerance/anergy. In addition, we show that the absence of Treg cells is associated with an increase in the proportion of CD4(+) cells that express GL7 and correlated with an increase in germinal center follicular helper T (GC-T(FH) ) cells. These GC-T(FH) cells, but not those from Treg-cell-sufficient hosts, were sufficient to drive antibody production by anergic B cells. We propose that a function of Treg cells is to prevent the expansion of T(FH) cells, especially GC-T(FH) cells, which support autoantibody production.

Autoreactive preplasma cells break tolerance in the absence of regulation by dendritic cells and macrophages

The ability to induce Ab responses to pathogens while maintaining the quiescence of autoreactive cells is an important aspect of immune tolerance. During activation of TLR4, dendritic cells (DCs) and macrophages (MFs) repress autoantibody production through their secretion of IL-6 and soluble CD40L (sCD40L). These soluble mediators selectively repress B cells chronically exposed to Ag, but not naive cells, suggesting a means to maintain tolerance during TLR4 stimulation, yet allow immunity. In this study, we identify TNF-α as a third repressive factor, which together with IL-6 and CD40L account for nearly all the repression conferred by DCs and MFs. Similar to IL-6 and sCD40L, TNF-α did not alter B cell proliferation or survival. Instead, it reduced the number of Ab-secreting cells. To address whether the soluble mediators secreted by DCs and MFs functioned in vivo, we generated mice lacking IL-6, CD40L, and TNF-α. Compared to wild-type mice, these mice showed prolonged anti-nuclear Ab responses following TLR4 stimulation. Furthermore, adoptive transfer of autoreactive B cells into chimeric IL-6(-/-) × CD40L(-/-) × TNF-α(-/-) mice showed that preplasma cells secreted autoantibodies independent of germinal center formation or extrafollicular foci. These data indicate that in the absence of genetic predisposition to autoimmunity, loss of endogenous IL-6, CD40L, and TNF-α promotes autoantibody secretion during TLR4 stimulation.

Potentially autoreactive naturally occurring transitional T3 B lymphocytes exhibit a unique signaling profile

B lymphocytes exhibit phenotypic differences that correlate with their developmental or functional stages and affect humoral immune responses. One recently described subset of naturally occurring immature transitional type 3 (T3) B lymphocytes is believed to consist of potentially autoimmune cells whose signaling properties have not been studied in detail. This study characterizes intracellular signaling in T3 B cells in wildtype C57BL/6 mice. Protein phosphorylation and Ca(2+) responses upon B-cell antigen receptor (BCR) engagement were measured by multicolor flow cytometry. We observed high baseline signaling activity and reduced BCR-mediated responses in T3 cells, which confirmed their anergy - a functional state in which lymphocytes recognize chronically present self-antigens but cannot produce immune response due to intrinsic signaling inhibition. Our results also revealed a previously unknown T3-specific phosphorylation pattern of 24 key signaling molecules involved in BCR signal transduction. These characteristics reflect the balance between stimulatory and inhibitory BCR signaling pathways in anergy. Results obtained in the collagen-induced arthritis model demonstrate the loss of anergy in T3 B cells during the onset of the disease. Our findings provide rationale for further investigating alterations in B-cell signaling patterns as earliest functional biomarkers of changes in the immune tolerance of autoreactive B cells.

Activation-induced deaminase-deficient MRL/lpr mice secrete high levels of protective antibodies against lupus nephritis

OBJECTIVE

We previously generated MRL/lpr mice deficient in activation-induced deaminase (AID) that lack isotype switching and immunoglobulin hypermutation. These mice have high levels of unmutated (germline) autoreactive IgM, yet they experienced an increase in survival and an improvement in lupus nephritis that exceeded that of MRL/lpr mice lacking IgG. The purpose of the present study was to test the hypothesis that high levels of germline autoreactive IgM in these mice confer protection against lupus nephritis.

METHODS

Autoreactive IgM antibodies of various specificities, including antibodies against double-stranded DNA (dsDNA), from AID-deficient MRL/lpr mice were given to asymptomatic MRL/lpr mice, and the levels of cytokines, proteinuria, immune complex deposition in the kidneys, and glomerulonephritis were examined. Novel AID-deficient MRL/lpr mice that lack any antibodies were generated for comparison to AID-deficient MRL/lpr mice that secrete only IgM.

RESULTS

Treatment with IgM anti-dsDNA resulted in a dramatic improvement in lupus nephritis. Other autoreactive IgM antibodies, such as antiphospholipid and anti-Sm, did not alter the pathologic changes. Secretion of proinflammatory cytokines by macrophages and the levels of inflammatory cells and apoptotic debris in the kidneys were lower in mice receiving IgM anti-dsDNA. Protective IgM derived from AID-deficient MRL/lpr mice displayed a distinct B cell repertoire, with a bias toward members of the V(H) 7183 family.

CONCLUSION

IgM anti-dsDNA protected MRL/lpr mice from lupus nephritis, likely by stopping the inflammatory cascade leading to kidney damage. A distinct repertoire of V(H) usage in IgM anti-dsDNA hybridomas from AID-deficient mice suggests that there is enrichment of a dedicated B cell population that secretes unmutated protective IgM in these mice.

Dendritic cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) persists as a chronic inflammatory autoimmune disease and is characterized by the production of autoantibodies and immune complexes that affect multiple organs. The underlying mechanism that triggers and sustains disease are complex and involve certain susceptibility genes and environmental factors. There have been several immune mediators linked to SLE including cytokines and chemokines that have been reviewed elsewhere [ 1-3 ]. A number of articles have reviewed the role of B cells and T cells in SLE [ 4-10 ]. Here, we focus on the role of dendritic cells (DC) and innate immune factors that may regulate autoreactive B cells.

Expression of high mobility group box chromosomal protein 1 and its modulating effects on downstream cytokines in systemic lupus erythematosus

OBJECTIVE

To compare the expression of high mobility group box chromosomal protein 1 (HMGB1) and the modulating effects on its downstream cytokines in patients with systemic lupus erythematosus (SLE) and healthy controls.

METHODS

HMGB1 concentrations in serum from SLE patients and controls were measured by immunoblot analysis. HMGB1 messenger RNA (mRNA) expression in peripheral blood mononuclear cells (PBMC) was detected by real-time reverse transcription-polymerase chain reaction. Immunofluorescence assay was employed to examine the translocation of HMGB1 in monocytes after endotoxin stimulation. Release of tumor necrosis factor-alpha (TNF-alpha) and interleukin 6 (IL-6) by PBMC after rHMGB1 stimulation was also measured.

RESULTS

Serum HMGB1 levels and HMGB1 mRNA expressions in PBMC were elevated in SLE patients compared with controls. A positive correlation was demonstrated between HMGB1 concentrations and SLE Disease Activity Index. There was an inverse correlation between HMGB1 levels and C4 and C3 concentrations in SLE patients. HMGB1 concentrations were higher in patients with vasculitis and myositis. Lipopolysaccharide stimulated a temporarily elevated release of HMGB1 in SLE patients compared with controls. The pattern and localization of HMGB1 staining in monocytes were similar in both groups. After stimulation with rHMGB1, TNF-alpha level decreased but IL-6 level increased in SLE patients compared with controls.

CONCLUSION

Our findings suggest that increased serum levels of HMGB1 in SLE may be associated with lupus disease activity. The altered production of TNF-alpha and IL-6 in response to rHMGB1 stimulation may participate in the disruption of cytokine homeostasis in SLE.

Function and dysfunction of dendritic cells in autoimmune rheumatic diseases

Dendritic cells (DC) have been implicated both in initiation of immunity and in immune tolerance. The mechanisms whereby tolerogenic DC may induce and maintain peripheral tolerance include the generation or expansion of regulatory T cells (Treg) and the promotion of T-cell anergy or deletion. A wide spectrum of hematopoietic growth factors and cytokines are endowed with the ability to differentiate tolerogenic DC both in vitro and in vivo. Based on this knowledge, therapeutic vaccination with cytokine-modulated tolerogenic DC has been applied to animal models of autoimmune disorders. This article will review the current experimental evidence underpinning DC dysfunction in rheumatic autoimmune diseases and will discuss how the manipulation of DC and Treg number and function may control undesired T-cell responses.

B cell receptor affinity and B cell subset identity integrate to define the effectiveness, affinity threshold, and mechanism of anergy

In this study we show that BCR affinity and subset identity make unique contributions to anergy. Analysis of anti-Smith (Sm) B cells of different affinities indicates that increasing affinity improves anergy's effectiveness while paradoxically increasing the likelihood of marginal zone (MZ) and B-1 B cell differentiation rather than just follicular (FO) B cell differentiation. Subset identity in turn determines the affinity threshold and mechanism of anergy. Subset-specific affinity thresholds for anergy induction allow discordant regulation of low-affinity anti-Sm FO and MZ B cells and could account for the higher frequency of autoreactive MZ B cells than that of FO B cells in normal mice. The mechanism of anergy changes during differentiation and differs between subsets. This is strikingly illustrated by the observation that blockade of BCR-mediated activation of FO and MZ B cells occurs at different levels in the signaling cascade. Thus, attributes unique to B cells of each subset integrate with signals from the BCR to determine the effectiveness, affinity threshold, and mechanism of anergy.

The regulation of autoreactive B cells during innate immune responses

Systemic lupus erythematosus (SLE) highlights the dangers of dysregulated B cells and the importance of initiating and maintaining tolerance. In addition to central deletion, receptor editing, peripheral deletion, receptor revision, anergy, and indifference, we have described a new mechanism of B cell tolerance wherein dendritic cells (DCs) and macrophages (MPhis) regulate autoreactive B cells during innate immune responses. In part, DCs and MPhis repress autoreactive B cells by releasing IL-6 and soluble CD40L (sCD40L). This mechanism is selective in that IL-6 and sCD40L do not affect Ig secretion by naïve cells during innate immune responses, allowing immunity in the absence of autoimmunity. In lupus-prone mice, DCs and MPhis are defective in secretion of IL-6 and sCD40L and cannot effectively repress autoantibody secretion suggesting that defects in DC/MPhi-mediated tolerance may contribute to the autoimmune phenotype. Further, these studies suggest that reconstituting DCs and MPhis in SLE patients might restore regulation of autoreactive B cells and provide an alternative to immunosuppressive therapies.

Macrophages prevent the differentiation of autoreactive B cells by secreting CD40 ligand and interleukin-6

Activation of the innate immune system promotes polyclonal antibody secretion to eliminate invading pathogens. Inherent in this process is the potential to activate autoreactive B cells and induce autoimmunity. We showed previously that TLR-stimulated dendritic cells and macrophages regulate B cell tolerance to Smith antigen, in part through the secretion of interleukin-6 (IL-6). In this manuscript, we show that neutralization of IL-6 fails to abrogate macrophage-mediated repression and identify soluble CD40 ligand (CD40L) as a second repressive factor secreted by macrophages. CD40L selectively repressed Ig secretion by chronically antigen-experienced (anergic) immunoglobulin transgenic and nontransgenic B cells but not by transiently stimulated B cells. The importance of macrophages in maintaining B cell tolerance was apparent in lupus-prone MRL/lpr mice. Compared with C57BL/6 mice, macrophages from MRL/lpr mice were significantly less efficient at repressing immunoglobulin secretion coincident with diminished IL-6 and CD40 ligand production. These data indicate that macrophages regulate autoreactive B cells by secreting repressive factors that prohibit terminal differentiation of B cells. The regulation of autoreactive B cells by macrophages is diminished in lupus-prone mice suggesting a role in autoimmunity.

Macrophages prevent the differentiation of autoreactive B cells by secreting CD40 ligand and interleukin-6

Activation of the innate immune system promotes polyclonal antibody secretion to eliminate invading pathogens. Inherent in this process is the potential to activate autoreactive B cells and induce autoimmunity. We showed previously that TLR-stimulated dendritic cells and macrophages regulate B cell tolerance to Smith antigen, in part through the secretion of interleukin-6 (IL-6). In this manuscript, we show that neutralization of IL-6 fails to abrogate macrophage-mediated repression and identify soluble CD40 ligand (CD40L) as a second repressive factor secreted by macrophages. CD40L selectively repressed Ig secretion by chronically antigen-experienced (anergic) immunoglobulin transgenic and nontransgenic B cells but not by transiently stimulated B cells. The importance of macrophages in maintaining B cell tolerance was apparent in lupus-prone MRL/lpr mice. Compared with C57BL/6 mice, macrophages from MRL/lpr mice were significantly less efficient at repressing immunoglobulin secretion coincident with diminished IL-6 and CD40 ligand production. These data indicate that macrophages regulate autoreactive B cells by secreting repressive factors that prohibit terminal differentiation of B cells. The regulation of autoreactive B cells by macrophages is diminished in lupus-prone mice suggesting a role in autoimmunity.

B-cell anergy: from transgenic models to naturally occurring anergic B cells?

Anergy, a condition in which cells persist in the periphery but are unresponsive to antigen, is responsible for silencing many self-reactive B cells. Loss of anergy is known to contribute to the development of autoimmune diseases, including systemic lupus erythematosus and type 1 diabetes. Multiple transgenic mouse models have enabled the dissection of mechanisms that underlie anergy, and recently, anergic B cells have been identified in the periphery of wild-type mice. Heterogeneity of mechanistic concepts developed using model systems has complicated our understanding of anergy and its biological features. In this Review, we compare and contrast the salient features of anergic B cells with a view to developing unifying mechanistic hypotheses that explain their lifestyles.