Spontaneous B cell hyperactivity in autoimmune-prone MRL mice

Defective LAT signalosome pathology in mice mimics human IgG4-related disease at single-cell level

Mice with a loss-of-function mutation in the LAT adaptor (LatY136F) develop an autoimmune and type 2 inflammatory disorder called defective LAT signalosome pathology (DLSP). We analyzed via single-cell omics the trajectory leading to LatY136F DLSP and the underlying CD4+ T cell diversification. T follicular helper cells, CD4+ cytotoxic T cells, activated B cells, and plasma cells were found in LatY136F spleen and lung. Such cell constellation entailed all the cell types causative of human IgG4-related disease (IgG4-RD), an autoimmune and inflammatory condition with LatY136F DLSP-like histopathological manifestations. Most previously described T cell-mediated autoimmune manifestations require persistent TCR input. In contrast, following their first engagement by self-antigens, the autoreactive TCR expressed by LatY136F CD4+ T cells hand over their central role in T cell activation to CD28 costimulatory molecules. As a result, all subsequent LatY136F DLSP manifestations, including the production of autoantibodies, solely rely on CD28 engagement. Our findings elucidate the etiology of the LatY136F DLSP and qualify it as a model of IgG4-RD.

Cosmc deficiency causes spontaneous autoimmunity by breaking B cell tolerance

Factors regulating the induction and development of B cell–mediated autoimmunity are not well understood. Here, we report that targeted deletion in murine B cells of X-linked Cosmc, encoding the chaperone required for expression of core 1 O-glycans, causes the spontaneous development of autoimmune pathologies due to a breakdown of B cell tolerance. BC-CosmcKO mice display multiple phenotypic abnormalities, including severe weight loss, ocular manifestations, lymphadenopathy, and increased female-associated mortality. Disruption of B cell tolerance in BC-CosmcKO mice is manifested as elevated self-reactive IgM and IgG autoantibodies. Cosmc-deficient B cells exhibit enhanced basal activation and responsiveness to stimuli. There is also an elevated frequency of spontaneous germinal center B cells in BC-CosmcKO mice. Mechanistically, loss of Cosmc confers enhanced B cell receptor (BCR) signaling through diminished BCR internalization. The results demonstrate that Cosmc, through control of core 1 O-glycans, is a previously unidentified immune checkpoint gene in maintaining B cell tolerance.

Suppressor of cytokine signaling-1 mimetic peptides attenuate lymphocyte activation in the MRL/lpr mouse autoimmune model

Autoimmune diseases are driven largely by a pathogenic cytokine milieu produced by aberrantly activated lymphocytes. Many cytokines, including interferon gamma (IFN-γ), utilize the JAK/STAT pathway for signal propagation. Suppressor of Cytokine Signaling-1 (SOCS1) is an inducible, intracellular protein that regulates IFN-γ signaling by dampening JAK/STAT signaling. Using Fas deficient, MRL/MpJ-Faslpr/J (MRL/lpr) mice, which develop lupus-like disease spontaneously, we tested the hypothesis that a peptide mimic of the SOCS1 kinase inhibitory region (SOCS1-KIR) would inhibit lymphocyte activation and modulate lupus-associated pathologies. Consistent with in vitro studies, SOCS1-KIR intraperitoneal administration reduced the frequency, activation, and cytokine production of memory CD8+ and CD4+ T lymphocytes within the peripheral blood, spleen, and lymph nodes. In addition, SOCS1-KIR administration reduced lymphadenopathy, severity of skin lesions, autoantibody production, and modestly reduced kidney pathology. On a cellular level, peritoneal SOCS1-KIR administration enhanced Foxp3 expression in total splenic and follicular regulatory T cells, reduced the effector memory/naïve T lymphocyte ratio for both CD4+ and CD8+ cells, and reduced the frequency of GL7+ germinal center enriched B cells. Together, these data show that SOCS1-KIR treatment reduced auto-reactive lymphocyte effector functions and suggest that therapeutic targeting of the SOCS1 pathway through peptide administration may have efficacy in mitigating autoimmune pathologies.

Altered B lymphocyte homeostasis and functions in systemic sclerosis

Beyond the production of autoantibodies, B-cells are thought to play a role in systemic sclerosis (SSc) by secreting proinflammatory/profibrotic cytokines. B-cells are a heterogeneous population with different subsets distinguished by their phenotypes and cytokine production. Data about B-cell subsets, cytokine production and intracellular pathways leading to this production are scarce in SSc. The aim of our study was to describe B-cell homeostasis, activation, proliferation, cytokine production in B-cells and serum and B-cell intracellular signaling pathways in SSc. We hypothezided that B-cell homeostasis and cytokine production were altered in SSc and could be explained by serum cytokine as well as by intracellular signaling pathway abnormalities. Forty SSc patients and 20 healthy controls (HC) were prospectively included. B-cell subsets were determined by flow cytometry using CD19, CD21, CD24, CD38, CD27, IgM and IgD. CD25, CD80, CD95, HLA-DR were used to assess B-cell activation. Intracellular production of IL-10 and IL-6 were assessed by flow cytometry after TLR9 and CD40 stimulation. IL-6, IL-10, Ki67, Bcl2 mRNA were quantified in B-cells. Cytokine production was also assessed in sera and supernatants of B-cell culture, using a multiplex approach. Signaling pathways were studied through phosphorylation of mTOR, ERK, STAT3, STAT5 using a flow cytometry approach. We found that SSc patients exhibited an altered peripheral blood B-cell subset distribution, with decreased memory B-cells but increased proportion of naive and CD21^LoCD38^Lo B-cell subsets. We observed an increased expression of activation markers (CD80, CD95, HLA-DR) on some B-cell subsets, mainly the memory B-cells. Secretion of IL-6, BAFF and CXCL13 were increased in SSc sera. There was no correlation between the peripheral blood B-cell subsets and the serum concentrations of these cytokines. After stimulation, we observed a lower proportion of IL-10 and IL-6 producing B-cells in SSc. Finally, we observed a significant decrease of mTOR phosphorylation in SSc patient B-cells. In conclusion, we observed an altered B-cell homeostasis in SSc patients compared to HC. Memory B-cells were both decreased and activated in patients. IL-10 producing B-cells were decreased in SSc. This decrease was associated with an alteration of mTOR phosphorylation in B-cells. Conversely, there was no correlation between serum cytokine profile and B-cell homeostasis alterations.

MRL Strains Have a BAFFR Mutation without Functional Consequence

It has been shown that B cell activating factor receptor (BAFFR) is critical for B cell development and survival. In this study, we sought to evaluate the expression and function of BAFFR across multiple stains of mice that vary in their potential to develop systemic autoimmune disease. The inability of a commercial antibody to bind to BAFFR in the autoimmune prone mouse strains, MRL and MRL/Lpr led to the discovery of a mutation in TNFRSF13C gene (encoding BAFFR) that resulted in a Pro44Ser substitution in the N-terminus near the BAFF binding site in these strains. To define the biological consequences of mutant BAFFR, we compared the expression and activity of BAFFR in MRL and MRL/Lpr mice to BALB/c, which express the consensus version of TNFRSF13C. B cells from MRL and MRL/Lpr mice expressed mutant BAFFR on surface and were capable of responding to BAFF as exhibited by BAFF-mediated reduction in apoptosis and NF-κB2 activation. Signaling through MAPK ERK1/2 was not significantly induced by BAFF in MRL/Lpr mice; however, MAPK ERK1/2 signaling was intact in MRL mice. The inability of MRL/Lpr B cells to significantly activate ERK1/2 in response to BAFF was due to the high basal activity of the signaling pathway in these cells. In fact, basal activity of ERK1/2 in B cells correlated with the degree of autoimmune susceptibility exhibited by each strain. In addition, aged MRL/Lpr mice with severe autoimmune disease had high BAFF levels, low surface BAFFR, and high basal NF-κB2 activation, a pattern which is attributed to the high frequency of antibody secreting cells. We conclude that P44S BAFFR mutation does not hinder BAFFR function or enhance B cell activity in MRL/Lpr and MRL mice and that other susceptibility loci on the MRL background contributed to the hyperactivity of these cells.

Prevention of immune nephritis by the small molecular weight immunomodulator iguratimod in MRL/lpr mice

OBJECTIVE

This study was performed to investigate the therapeutic effects of iguratimod in a lupus mouse model.

METHODS

Female MRL/lpr mice were treated with iguratimod, vehicle solution or cyclophosphamide. Proteinuria was monitored and kidney injury was blindly scored by a renal pathologist. Serum anti-double-stranded DNA antibodies were monitored by radioimmunoassay. Kidney IgG and CD20 were stained by immunohistochemistry. Splenic lymphocyte phenotypes were analyzed by flow cytometry. BAFF, IL-17A, IL-6, and IL-21 levels in serum and splenic lymphocytes were detected by ELISA or quantitative PCR.

RESULTS

Compared with the vehicle-treated controls, MRL/lpr mice treated with iguratimod showed less protenuria, less acute pathological lesions and no chronic changes in the kidneys. There were significant differences in glomerular injury and vasculitis scores, as well as in the semi-quantitative analysis of immune complex deposition between the two groups. Disease activity markers in sera (anti-dsDNA antibodies and immunoglobulin levels) were reduced and hypocomplementemia was attenuated. Lymphocyte expression of BAFF, IL-6, IL-17A and IL-21 was decreased. The abnormal splenic B220+ T cell and plasma cell populations in MRL/lpr mice were reduced by iguratimod treatment, with recovery of the total B cell population and inhibition of B cell infiltration of the kidney tissue. The dosage of iguratimod used in this study showed no significant cytotoxic effects in vivo and no overt side-effects were observed.

CONCLUSION

Iguratimod ameliorates immune nephritis in MRL/lpr mice via a non-antiproliferative mechanism. Our data suggest a potential therapeutic role of iguratimod in lupus.

A bacterial artificial chromosome transgene with polymorphic Cd72 inhibits the development of glomerulonephritis and vasculitis in MRL-Faslpr lupus mice

Systemic lupus erythematosus is considered to be under the control of polygenic inheritance, developing according to the cumulative effects of susceptibility genes with polymorphic alleles; however, the mechanisms underlying the roles of polygenes based on functional and pathological genomics remain uncharacterized. In this study, we substantiate that a CD72 polymorphism in the membrane-distal extracellular domain impacts on both the development of glomerulonephritis and vasculitis in a lupus model strain of mice, MRL/MpJ-Fas(lpr), and the reactivity of BCR signal stimulation. We generated mice carrying a bacterial artificial chromosome transgene originating from C57BL/6 (B6) mice that contains the Cd72(b) locus (Cd72(B6) transgenic [tg]) or the modified Cd72(b) locus with an MRL-derived Cd72(c) allele at the polymorphic region corresponding to the membrane-distal extracellular domain (Cd72(B6/MRL) tg). Cd72(B6) tg mice, but not Cd72(B6/MRL) tg mice, showed a significant reduction in mortality following a marked improvement of disease associated with decreased serum levels of IgG3 and anti-dsDNA Abs. The number of splenic CD4(-)CD8(-) T cells in Cd72(B6) tg mice was decreased significantly in association with a reduced response to B cell receptor signaling. These results indicate that the Cd72 polymorphism affects susceptibility to lupus phenotypes and that novel functional rescue by a bacterial artificial chromosome transgenesis is an efficient approach with wide applications for conducting a genomic analysis of polygene diseases.

TLR9 drives the development of transitional B cells towards the marginal zone pathway and promotes autoimmunity

Maturation of B cells depends on environmental stimuli. Peripheral immature B cells develop into follicular pathway when antigenic stimulation is combined with T cell signals. Here, we wished to identify stimuli contributing to the development into marginal zone B cells known to be involved in autoimmune response. We found that TLR9 stimulation of transitional B cells induces proliferation and specific maturation into CD24(-) CD38(+) CD21(high) CD23(low) IgM(high) IgD(low) and Notch2(high) B cells characteristics of marginal zone B cells. Terminal differentiation into antibody-secreting cell associated with isotype switch commitment is also triggered which leads to a striking production of autoantibodies. Interestingly, mature B cells do not differentiate into marginal zone pathway following TLR9 stimulation, nor do transitional B cells under antigenic and T cell combined signals. These results suggest that transitional B cells are specifically sensitive to TLR9 stimulation to produce autoreactive marginal zone B cells.

Heterologous protein incites abnormal plasma cell accumulation and autoimmunity in MRL-MpJ mice

Although it is evident that there is complex interplay among genetic and environmental factors contributing to systemic autoimmunity, the events inciting autoreactivity are incompletely understood. Previously we demonstrated that MRL-MpJ mice posses a genetic background susceptible to autoimmunity development under conditions of altered inhibitory signaling. To gain better understanding of the influence of exogenous factors on autoreactivity in susceptible individuals, young MRL-MpJ mice were challenged with a single injection of heterologous protein and evaluated for evidence of autoimmunity. We found that MRL-MpJ mice developed high titer serum reactivity to DNA within 1 week of protein administration reaching maximal levels within 1 month. Importantly, the level of autoimmunity was sustained for an extended period of time (6 months). This was accompanied by a substantial increase in germinal center B cell and plasma cell numbers. In contrast, control mice showed no change in autoreactivity or lymphocyte homeostasis. Autoimmunity was dependent on marginal zone B cells as their depletion reduced serum auto-reactivity after challenge, thus suggesting immune stimulation with heterologous proteins can precipitate loss of B cell tolerance and autoimmunity in genetically prone individuals. This model may provide an important tool to further investigate the mechanisms whereby environmental stimuli trigger autoimmune reactivity in susceptible hosts.

Neutrophil extracellular traps that are not degraded in systemic lupus erythematosus activate complement exacerbating the disease

Ongoing inflammation including activation of the complement system is a hallmark of systemic lupus erythematosus (SLE). Antimicrobial neutrophil extracellular traps (NETs) are composed of secreted chromatin that may act as a source of autoantigens typical for SLE. In this study, we investigated how complement interacts with NETs and how NET degradation is affected by complement in SLE patients. We found that sera from a subset of patients with active SLE had a reduced ability to degrade in vitro-generated NETs, which was mostly restored when these patients were in remission. Patients that failed to degrade NETs had a more active disease and they also displayed lower levels of complement proteins C4 and C3 in blood. We discovered that NETs activated complement in vitro and that deposited C1q inhibited NET degradation including a direct inhibition of DNase-I by C1q. Complement deposition on NETs may facilitate autoantibody production, and indeed, Abs against NETs and NET epitopes were more pronounced in patients with impaired ability to degrade NETs. NET-bound autoantibodies inhibited degradation but also further increased C1q deposition, potentially exacerbating the disease. Thus, NETs are a potent complement activator, and this interaction may play an important role in SLE. Targeting complement with inhibitors or by removing complement activators such as NETs could be beneficial for patients with SLE.

The transcription factor Bright plays a role in marginal zone B lymphocyte development and autoantibody production

Previous data suggested that constitutive expression of the transcription factor Bright (B cell regulator of immunoglobulin heavy chain transcription), normally tightly regulated during B cell differentiation, was associated with autoantibody production. Here we show that constitutive Bright expression results in skewing of mature B lineage subpopulations toward marginal zone cells at the expense of the follicular subpopulation. C57Bl/6 transgenic mice constitutively expressing Bright in B lineage cells generated autoantibodies that were not the result of global increases in immunoglobulin or of breaches in key tolerance checkpoints typically defective in other autoimmune mouse models. Rather, autoimmunity correlated with increased numbers of marginal zone B cells and alterations in the phenotype and gene expression profiles of lymphocytes within the follicular B cell compartment. These data suggest a novel role for Bright in the normal development of mature B cell subsets and in autoantibody production.

Lymphoproliferative disorders involving T helper effector cells with defective LAT signalosomes

Linker for activation of T cells (LAT) is a membrane adaptor protein that is expressed in T cells and coordinates the assembly of a multiprotein complex-the LAT signalosome-that links the T cell-specific and the ubiquitous components of the T cell antigen receptor (TCR) signaling pathway. The present review focuses on recent LAT knock-in mice that were found to develop lymphoproliferative disorders involving polyclonal CD4(+) T cells that produced excessive amounts of T helper-type 2 cytokines. These mouse models revealed that LAT constitutes more than just a positive regulator of TCR signaling and plays a negative regulatory role that contributes to terminate antigen-driven T cell responses by exerting a repressive function on components of the TCR signaling cassette that lie upstream of LAT or function independently of LAT. In the absence of such a LAT-operated negative regulatory loop that is intrinsic to conventional CD4(+) T cells and of no lesser importance than the extrinsic regulatory mechanisms mediated by regulatory T cells, physiologic, antigen-specific CD4(+) T cell responses evolve into chronic pro-inflammatory responses that perpetuate themselves in a manner that does not depend on engagement of the TCR and that induce the production of massive amounts of antibodies and autoantibodies in a major histocompatibility complex-II-independent, "quasi-mitogenic" mode. As discussed, these data underscore that a novel immunopathology proper to defective LAT signalosomes is likely taking shape, and we propose to call it "LAT signaling pathology."

Critical role of TLR2 and TLR4 in autoantibody production and glomerulonephritis in lpr mutation-induced mouse lupus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by pathogenic autoantibodies directed against nuclear Ags and immune complex deposits in damaged organs. Environmental factors have been thought to play a role in the onset of the disease. The recognition of these factors is mediated by TLRs, in particular TLR2 and TLR4 which bind pathogen-associated molecular patterns of Gram(+) and Gram(-) bacteria, respectively. We attempted to determine the role of these TLRs in SLE by creating TLR2- or TLR4-deficient C57BL/6(lpr/lpr) mice. These mice developed a less severe disease and fewer immunological alterations. Indeed, in C57BL/6(lpr/lpr)-TLR2 or -TLR4-deficient mice, glomerular IgG deposits and mesangial cell proliferation were dramatically decreased and antinuclear, anti-dsDNA, and anti-cardiolipin autoantibody titers were significantly reduced. However, the response against nucleosome remained unaffected, indicating a role of TLR2 and TLR4 in the production of Abs directed against only certain categories of SLE-related autoantigens. Analysis of B cell phenotype showed a significant reduction of marginal zone B cells, particularly in C57BL/6(lpr/lpr)-TLR4-deficient mice, suggesting an important role of TLR4 in the sustained activation of these cells likely involved in autoantibody production. Interestingly, the lack of TLR4 also affected the production of cytokines involved in the development of lupus disease.

B cells flying solo

Systemic autoimmunity such as systemic lupus erythematosus (SLE) is associated with the loss of B-cell tolerance, B-cell dysregulation and autoantibody production. While some autoantibodies may contribute to the pathology seen with SLE, numerous studies have shown that dysregulation of T-cell function is another critical aspect driving disease. The positive results obtained in clinical trials using T-cell- or B-cell-specific treatments have suggested that cooperation between T and B cells probably underlies disease progression in many patients. A similar cooperative mechanism seemed to explain SLE developing in mice overexpressing the B-cell-activating factor from the tumor necrosis factor family (BAFF). However, surprisingly, T-cell-deficient BAFF transgenic (Tg) mice develop SLE similar to T-cell-sufficient BAFF Tg mice, and the disease was linked to innate activation of B cells and production of proinflammatory autoantibody isotypes. In conclusion, dysregulated innate activation of B cells alone can drive disease independently of T cells, and as such this aspect represents a new pathogenic mechanism in autoimmunity.

Th2 lymphoproliferative disorder of LatY136F mutant mice unfolds independently of TCR-MHC engagement and is insensitive to the action of Foxp3+ regulatory T cells

Mutant mice where tyrosine 136 of linker for activation of T cells (LAT) was replaced with a phenylalanine (Lat(Y136F) mice) develop a fast-onset lymphoproliferative disorder involving polyclonal CD4 T cells that produce massive amounts of Th2 cytokines and trigger severe inflammation and autoantibodies. We analyzed whether the Lat(Y136F) pathology constitutes a bona fide autoimmune disorder dependent on TCR specificity. Using adoptive transfer experiments, we demonstrated that the expansion and uncontrolled Th2-effector function of Lat(Y136F) CD4 cells are not triggered by an MHC class II-driven, autoreactive process. Using Foxp3EGFP reporter mice, we further showed that nonfunctional Foxp3(+) regulatory T cells are present in Lat(Y136F) mice and that pathogenic Lat(Y136F) CD4 T cells were capable of escaping the control of infused wild-type Foxp3(+) regulatory T cells. These results argue against a scenario where the Lat(Y136F) pathology is primarily due to a lack of functional Foxp3(+) regulatory T cells and suggest that a defect intrinsic to Lat(Y136F) CD4 T cells leads to a state of TCR-independent hyperactivity. This abnormal status confers Lat(Y136F) CD4 T cells with the ability to trigger the production of Abs and of autoantibodies in a TCR-independent, quasi-mitogenic fashion. Therefore, despite the presence of autoantibodies causative of severe systemic disease, the pathological conditions observed in Lat(Y136F) mice unfold in an Ag-independent manner and thus do not qualify as a genuine autoimmune disorder.

Hypomethylation of interleukin-4 and -6 promoters in T cells from systemic lupus erythematosus patients

AIM

DNA methylation regulates gene expression, and hypomethylation is associated with abnormal T-cell function in systemic lupus erythematosus (SLE). However, little is known about the methylation levels of the interleukin (IL)-4 and -6 promoters in SLE patients.

METHODS

T cells were isolated from 20 SLE patients and 10 healthy controls, activated in vitro in the presence or absence of 5- azacytidine (5-azaC), and their IL-4 and -6 transcripts were characterized using semiquantitative RT-PCR. Following bisulfate modification of their genomic DNA, the levels of DNA methylation in the IL-4 or -6 promoter were determined by nested PCR and direct sequencing.

RESULTS

The levels of IL-4 and -6 mRNA transcripts were significantly higher in SLE T cells, as compared with that in the controls. Furthermore, the treatment of healthy T cells with 5-azaC demethylated the CpG islands in the IL-4 or -6 promoter and increased IL-4 and -6 mRNA transcriptions. Importantly, the hypomethylation of the CpG islands in the IL-4 and -6 promoters displayed in SLE patients was similar to that of healthy T cells treated with 5-azaC. Finally, the hypomethylation levels of the CpG islands in the IL-4 and -6 promoters in lupus patients were significantly correlated to the IL-4 and -6 expressions.

CONCLUSION

The hypomethylation of the CpG islands of the IL-4 and -6 promoters accrued in T cells from SLE patients and was associated with the severity of SLE at the clinic.

A genetic locus controlling aging-sensitive regression of B lymphopoiesis in an autoimmune-prone MRL/lpr strain of mice

Aging readily affects immune system under the influence of environmental and/or intrinsic factors while accelerating the development of various immune disorders including autoimmune diseases. Little is known about molecular and cellular mechanisms connecting between immune senescence and development of autoimmune diseases. Here, we first show strain-specific and aging-sensitive onset of B-cell abnormality in a lupus-prone MRL/Mp.Fas(lpr) (MRL/lpr) strain of mice. This abnormality was characterized by the regression of B lymphopoiesis in the bone marrow of this strain. We next examined the association between the B-cell regression and onset of autoimmune diseases in aged (MRL/lpr x C3H/He.Fas(lpr)) F2 mice, in which pathologic phenotypes, such as glomerulonephritis, vasculitis, sialoadenitis and arthritis, variously developed. We also searched whole genome to identify genetic loci linked to the B-cell regression by using the same F2 mice. The B-cell regression manifested in the spleen of F2 mice was retrospectively evaluated by reverse transcriptase-based PCR quantification. The results demonstrated that the onset of autoimmune diseases in the F2 mice was not associated with the aging-sensitive B-cell regression. The genetic study identified a significant locus responsible for the B-cell regression in the vicinity of D5Mit233 (29 cM). This is first evidence for the presence of a genetic locus that affects B lymphopoiesis in an aging-sensitive manner.

Shared signaling networks active in B cells isolated from genetically distinct mouse models of lupus

Though B cells play key roles in lupus pathogenesis, the molecular circuitry and its dysregulation in these cells as disease evolves remain poorly understood. To address this, a comprehensive scan of multiple signaling axes using multiplexed Western blotting was undertaken in several different murine lupus strains. PI3K/AKT/mTOR (mTOR, mammalian target of rapamycin), MEK1/Erk1/2, p38, NF-kappaB, multiple Bcl-2 family members, and cell-cycle molecules were observed to be hyperexpressed in lupus B cells in an age-dependent and lupus susceptibility gene-dose-dependent manner. Therapeutic targeting of the AKT/mTOR axis using a rapamycin (sirolimus) derivative ameliorated the serological, cellular, and pathological phenotypes associated with lupus. Surprisingly, the targeting of this axis was associated with the crippling of several other signaling axes. These studies reveal that lupus pathogenesis is contingent upon the activation of an elaborate network of signaling cascades that is shared among genetically distinct mouse models and raise hope that targeting pivotal nodes in these networks may offer therapeutic benefit.

C1q deficiency promotes the production of transgenic-derived IgM and IgG3 autoantibodies in anti-DNA knock-in transgenic mice

C1q-deficient mice have been shown to develop a lupus-like disease and to display an impaired clearance of apoptotic cells that are enriched in lupus autoantigens. However, the role of C1q in the regulation of autoreactive B cells remains debatable. To explore this we crossed MRL/Mp C1q-deficient mice with knock-in transgenic (Tg) mice expressing an anti-ssDNA antibody (V_H3H9R and V_H3H9R/V_Lκ8R). Analysis of the V_H3H9R mice showed that in the absence of C1q higher titres of Tg-derived IgM and IgG₃ anti-ssDNA antibodies were detectable. In contrast, in the V_H3H9R/V_Lκ8R C1q-deficient animals no increase in Tg antibody levels was observed. In both models the lack of C1q induced a marked reduction of marginal zone B cells and this was paralleled by a significant increase in the percentage of plasmocytes. Thus, one could postulate that in the absence of C1q the failure to clear efficiently dying cells provides an additional stimulus to the autoreactive Tg B cells resulting in their emigration from the marginal zone B cell compartment with subsequent increase in plasmocytes. However, the lack of C1q led to an increased production of Tg IgM and IgG₃ antibodies only in V_H3H9R mice indicating that additional genetic susceptibility factors are required to break self-tolerance.

BAFF and MyD88 signals promote a lupuslike disease independent of T cells

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease characterized by the production of autoantibodies. However, the underlying cause of disease appears to relate to defects in T cell tolerance or T cell help to B cells. Transgenic (Tg) mice overexpressing the cytokine B cell–activating factor of the tumor necrosis factor family (BAFF) develop an autoimmune disorder similar to SLE and show impaired B cell tolerance and altered T cell differentiation. We generated BAFF Tg mice that were completely deficient in T cells, and, surprisingly, these mice developed an SLE-like disease indistinguishable from that of BAFF Tg mice. Autoimmunity in BAFF Tg mice did, however, require B cell–intrinsic signals through the Toll-like receptor (TLR)–associated signaling adaptor MyD88, which controlled the production of proinflammatory autoantibody isotypes. TLR7/9 activation strongly up-regulated expression of transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), which is a receptor for BAFF involved in B cell responses to T cell–independent antigens. Moreover, BAFF enhanced TLR7/9 expression on B cells and TLR-mediated production of autoantibodies. Therefore, autoimmunity in BAFF Tg mice results from altered B cell tolerance, but requires TLR signaling and is independent of T cell help. It is possible that SLE patients with elevated levels of BAFF show a similar basis for disease.