Genetically determined aberrant down-regulation of FcgammaRIIB1 in germinal center B cells associated with hyper-IgG and IgG autoantibodies in murine systemic lupus erythematosus

Serum immunoglobulin and the threshold of Fc receptor-mediated immune activation

Antibodies can mediate immune recruitment or clearance of immune complexes through the interaction of their Fc domain with cellular Fc receptors. Clustering of antibodies is a key step in generating sufficient avidity for efficacious receptor recognition. However, Fc receptors may be saturated with prevailing, endogenous serum immunoglobulin and this raises the threshold by which cellular receptors can be productively engaged. Here, we review the factors controlling serum IgG levels in both healthy and disease states, and discuss how the presence of endogenous IgG is encoded into the functional activation thresholds for low- and high-affinity Fc receptors. We discuss the circumstances where antibody engineering can help overcome these physiological limitations of therapeutic antibodies. Finally, we discuss how the pharmacological control of Fc receptor saturation by endogenous IgG is emerging as a feasible mechanism for the enhancement of antibody therapeutics.

FcɣRIIB regulates autoantibody responses by limiting marginal zone B cell activation

FcɣRIIB is an inhibitory receptor expressed throughout B cell development. Diminished expression or function is associated with lupus in mice and humans, in particular through an effect on autoantibody production and plasma cell differentiation. Here, we analysed the effect of B cell-intrinsic FcɣRIIB expression on B cell activation and plasma cell differentiation. Loss of FcɣRIIB on B cells (Fcgr2b cKO mice) led to a spontaneous increase in autoantibody titers. This increase was most striking for IgG3, suggestive of increased extrafollicular responses. Marginal zone (MZ) B cells had the highest expression of FcɣRIIB in both mouse and human. This high expression of FcɣRIIB was linked to increased MZ B cell activation, Erk phosphorylation, and calcium fluxin the absence of FcɣRIIB triggering. Marked increases in IgG3+ plasma cells and B cells were observed during extrafollicular plasma cell responses in Fcgr2b cKO mice. The increased IgG3 response following immunization of Fcgr2b cKO mice was lost in MZ-deficient Notch2/Fcgr2b cKO mice. Importantly, SLE patients exhibited decreased expression of FcɣRIIB, most strongly in MZ B cells. Thus, we present a model where high FcɣRIIB expression in MZ B cells prevents their hyperactivation and ensuing autoimmunity.

Genetic dissection of a major haplotype associated with arthritis reveal FcγR2b and FcγR3 to act additively

A haplotype with tightly linked Fc gamma receptor (FcγR) genes is known as a major locus controlling immune responses and autoimmune diseases, including arthritis. Here, we split a congenic fragment derived from the NOD mouse (Cia9) to study its effect on immune response and arthritis in mice. We found that arthritis susceptibility was indeed controlled by the FcγR gene cluster and a recombination between the FcγR2b and FcγR3 loci gave us the opportunity to separately study their impact. We identified the NOD-derived FcγR2b and FcγR3 alleles as disease-promoting for arthritis development without impact on antibody secretion. We further found that macrophage-mediated phagocytosis was directly correlated to FcγR3 expression in the congenic mice. In conclusion, we positioned FcγR2b and FcγR3 alleles as disease regulatory and showed that their genetic polymorphisms independently and additively control innate immune cell activation and arthritis.

Monocyte subsets involved in the development of systemic lupus erythematosus and rheumatoid arthritis

AbstractMonocytes are evolutionally conserved innate immune cells that play essential roles for the protection of the host against pathogens and also produce several inflammatory cytokines. Thus, the aberrant functioning of monocytes may affect not only host defense but also the development of inflammatory diseases. Monocytes are a heterogeneous population with phenotypical and functional differences. Most recent studies have shown that monocytes are divided into three subsets, namely classical, intermediate and non-classical subsets, both in humans and mice. Accumulating evidence showed that monocyte activation is associated with the disease progression in autoimmune diseases, such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). However, it remains to be determined how monocytes contribute to the disease process and which subset is involved. In this review, we discuss the pathogenic role of monocyte subsets in SLE and RA on the basis of current studies by ourselves and others to shed light on the suitability of monocyte-targeted therapies in these diseases.

Immunosuppression overcomes insulin- and vector-specific immune responses that limit efficacy of AAV2/8-mediated insulin gene therapy in NOD mice

We report the restoration of euglycaemia in chemically induced diabetic C57BL/6 mice and spontaneously diabetic Non Obese Diabetic (NOD) mice by intravenous systemic administration of a single-stranded adeno-associated virus (ssAAV2/8) codon optimised (co) vector encoding furin cleavable human proinsulin under a liver-specific promoter. There were no immunological barriers to efficacy of insulin gene therapy in chemically induced C57BL/6 mice, which enjoyed long-lasting correction of hyperglycaemia after therapy, up to 250 days. Euglycaemia was also restored in spontaneously diabetic NOD mice, although these mice required a 7-10-fold higher dose of vector to achieve similar efficacy as the C57BL/6 mice and the immunodeficient NODscid mice. We detected CD8+ T cell reactivity to insulin and mild inflammatory infiltration in the livers of gene therapy recipient NOD mice, neither of which were observed in the treated C57BL/6 mice. Efficacy of the gene therapy in NOD mice was partially improved by targeting the immune system with anti-CD4 antibody treatment, while transfer of NOD mouse AAV2/8-reactive serum to recipients prevented successful restoration of euglycaemia in AAV2/8-HLP-hINSco-treated NODscid mice. Our data indicate that both immune cells and antibodies form a barrier to successful restoration of euglycaemia in autoimmune diabetic recipient mice with insulin gene therapy, but that this barrier can be overcome by increasing the dose of vector and by suppressing immune responses.

Fc-gamma receptor expression profile in a North-Indian cohort of pediatric-onset systemic lupus erythematosus: An observational study

BACKGROUND

Polymorphisms in the Fcγ-receptor (FcγR) have been associated with increased susceptibility to systemic lupus erythematosus (SLE). There is a paucity of data on FcγR expression pattern in pediatric subjects with SLE. The aim of the study was to assess the expression of various FcγRs by flow cytometry in children with pediatric-onset SLE (pSLE).

METHODS

Thirty-one children aged 0-15 years fulfilling 2012 Systemic Lupus International Collaborating Clinics Classification Criteria for SLE were enrolled. Disease-active (n = 14) and the inactive group were delineated using the SLE Disease Activity Index (SLEDAI). Thirteen age- and sex-matched controls were also enrolled. Blood samples of cases and controls were assessed for CD64, CD32B and CD16 expression on B lymphocytes, neutrophils and monocytes by flow cytometry using standard techniques. Median fluorescence intensity (MFI) and percentage expression were calculated using the FACS DIVA software and Kaluza software.

RESULTS

Median fluorescence intensity and percentage expression of CD64 on monocytes (MFI: 1.71 vs 1.51, P = 0.86) and neutrophils (MFI: 0.42 vs 0.64, P = 0.3) were comparable between patients and controls. MFIs of CD16 expression on neutrophils (3.47 vs 11.4, P = 0.05) and monocytes (1.28 vs 3.45, P = 0.07) were lower in patients compared to controls. CD32B expression on lymphocytes (MFI: 0.56 vs 1.37; % expression:18.3% vs 12.32%) was also comparable between cases and controls. Expression of CD64, CD16, and CD32B were also comparable between patients with active and inactive disease.

CONCLUSION

No significant differences were observed in FcγR expression between patients and controls. However, the overall trends of FcγR expression and decreased CD16 on monocytes and neutrophils are in consonance with data from larger cohorts of adult SLE patients.

FcγRIIb on B Cells and Myeloid Cells Modulates B Cell Activation and Autoantibody Responses via Different but Synergistic Pathways in Lupus-Prone Yaa Mice

C57BL/6 (B6).FcγRIIb^-/- Yaa mice spontaneously develop lethal lupus nephritis. To define the cell type-specific role of FcγRIIb in Yaa-associated lupus, we established B cell- (CD19^Cre Yaa), myeloid cell- (C/EBPα^Cre Yaa), and dendritic cell- (DC) (CD11c^Cre Yaa) specific FcγRIIb-deficient B6.Yaa mouse strains. CD19^Cre Yaa mice developed milder lupus than B6.FcγRIIb^-/- Yaa mice, indicating that FcγRIIb deficiency on B cells is not sufficient for the development of severe disease. Surprisingly, C/EBPα^Cre Yaa mice also showed autoantibody production and mild lupus similar to that in CD19^Cre Yaa mice, whereas CD11c^Cre Yaa mice stayed disease free. These observations indicate that FcγRIIb deficiency in B cells and myeloid cells, but not DCs, contributes to the severe disease in B6.FcγRIIb^-/- Yaa mice. Flow cytometric analysis showed that the frequency of peripheral Gr-1^- but not Gr-1⁺ monocyte was increased in B6.FcγRIIb^-/- Yaa and C/EBPα^Cre Yaa but not CD19^Cre Yaa mice, suggesting a link between FcγRIIb deficiency on myeloid cells and the high frequency of Gr-1^- monocytes. RNA sequencing revealed that compared with Gr-1⁺ monocytes, Gr-1^- monocytes expressed higher levels of the B cell-stimulating cytokines BSF-3, IL-10, and IL-1β, the DC markers CD11c, CD83, and Adamdec1, and the antiapoptotic factors Bcl2 and Bcl6. In conclusion, in Yaa-associated lupus nephritis, FcγRIIb on B cells and myeloid cells modulates B cell activation via different but synergistic pathways. Gr-1^- monocytes are the most likely candidate myeloid cells involved.

Serologic features of cohorts with variable genetic risk for systemic lupus erythematosus

BACKGROUND

Systemic lupus erythematosus (SLE) is an autoimmune disease with genetic, hormonal, and environmental influences. In Western Europe and North America, individuals of West African descent have a 3-4 fold greater incidence of SLE than Caucasians. Paradoxically, West Africans in sub-Saharan Africa appear to have a low incidence of SLE, and some studies suggest a milder disease with less nephritis. In this study, we analyzed sera from African American female SLE patients and four other cohorts, one with SLE and others with varying degrees of risk for SLE in order to identify serologic factors that might correlate with risk of or protection against SLE.

METHODS

Our cohorts included West African women with previous malaria infection assumed to be protected from development of SLE, clinically unaffected sisters of SLE patients with high risk of developing SLE, healthy African American women with intermediate risk, healthy Caucasian women with low risk of developing SLE, and women with a diagnosis of SLE. We developed a lupus risk index (LRI) based on titers of IgM and IgG anti-double stranded DNA antibodies and levels of C1q.

RESULTS

The risk index was highest in SLE patients; second highest in unaffected sisters of SLE patients; third highest in healthy African-American women and lowest in healthy Caucasian women and malaria-exposed West African women.

CONCLUSION

This risk index may be useful in early interventions to prevent SLE. In addition, it suggests new therapeutic approaches for the treatment of SLE.

Intracellular B Lymphocyte Signalling and the Regulation of Humoral Immunity and Autoimmunity

B lymphocytes are critical for effective immunity; they produce antibodies and cytokines, present antigens to T lymphocytes and regulate immune responses. However, because of the inherent randomness in the process of generating their vast repertoire of antigen-specific receptors, B cells can also cause diseases through recognizing and reacting to self. Therefore, B lymphocyte selection and responses require tight regulation at multiple levels and at all stages of their development and activation to avoid diseases. Indeed, newly generated B lymphocytes undergo rigorous tolerance mechanisms in the bone marrow and, subsequently, in the periphery after their migration. Furthermore, activation of mature B cells is regulated through controlled expression of co-stimulatory receptors and intracellular signalling thresholds. All these regulatory events determine whether and how B lymphocytes respond to antigens, by undergoing apoptosis or proliferation. However, defects that alter regulated co-stimulatory receptor expression or intracellular signalling thresholds can lead to diseases. For example, autoimmune diseases can result from altered regulation of B cell responses leading to the emergence of high-affinity autoreactive B cells, autoantibody production and tissue damage. The exact cause(s) of defective B cell responses in autoimmune diseases remains unknown. However, there is evidence that defects or mutations in genes that encode individual intracellular signalling proteins lead to autoimmune diseases, thus confirming that defects in intracellular pathways mediate autoimmune diseases. This review provides a synopsis of current knowledge of signalling proteins and pathways that regulate B lymphocyte responses and how defects in these could promote autoimmune diseases. Most of the evidence comes from studies of mouse models of disease and from genetically engineered mice. Some, however, also come from studying B lymphocytes from patients and from genome-wide association studies. Defining proteins and signalling pathways that underpin atypical B cell response in diseases will help in understanding disease mechanisms and provide new therapeutic avenues for precision therapy.

FcγRIIB and autoimmunity

Autoimmune diseases are characterized by adaptive immune responses against self-antigens, including humoral responses resulting in the production of autoantibodies. Autoantibodies generate inflammation by activating complement and engaging Fcγ receptors (FcγRs). The inhibitory receptor FcγRIIB plays a central role in regulating the generation of autoantibodies and their effector functions, which include activation of innate immune cells and the cellular arm of the adaptive immune system, via effects on antigen presentation to CD4 T cells. Polymorphisms in FcγRIIB have been associated with susceptibility to autoimmunity but protection against infections in humans and mice. In the last few years, new mechanisms by which FcγRIIB controls the adaptive immune response have been described. Notably, FcγRIIB has been shown to regulate germinal center B cells and dendritic cell migration, with potential impact on the development of autoimmune diseases. Recent work has also highlighted the implication of FcγRIIB on the regulation of the innate immune system, via inhibition of Toll-like receptor- and complement receptor-mediated activation. This review will provide an update on the role of FcγRIIB in adaptive immune responses in autoimmunity, and then focus on their emerging function in innate immunity.

Apoptotic Debris Accumulates on Hematopoietic Cells and Promotes Disease in Murine and Human Systemic Lupus Erythematosus

Apoptotic debris, autoantibody, and IgG-immune complexes (ICs) have long been implicated in the inflammation associated with systemic lupus erythematosus (SLE); however, it remains unclear whether they initiate immune-mediated events that promote disease. In this study, we show that PBMCs from SLE patients experiencing active disease, and hematopoietic cells from lupus-prone MRL/lpr and NZM2410 mice accumulate markedly elevated levels of surface-bound nuclear self-antigens. On dendritic cells (DCs) and macrophages (MFs), the self-antigens are part of IgG-ICs that promote FcγRI-mediated signal transduction. Accumulation of IgG-ICs is evident on ex vivo myeloid cells from MRL/lpr mice by 10 wk of age and steadily increases prior to lupus nephritis. IgG and FcγRI play a critical role in disease pathology. Passive transfer of pathogenic IgG into IgG-deficient MRL/lpr mice promotes the accumulation of IgG-ICs prior to significant B cell expansion, BAFF secretion, and lupus nephritis. In contrast, diminishing the burden IgG-ICs in MRL/lpr mice through deficiency in FcγRI markedly improves these lupus pathologies. Taken together, our findings reveal a previously unappreciated role for the cell surface accumulation of IgG-ICs in human and murine lupus.

Testing the role of the FcγRIIB immunoreceptor tyrosine-based inhibitory motif in regulation of the B cell immune response

In vitro studies have demonstrated that the immunoreceptor tyrosine-based inhibitory motif (ITIM) of the inhibitory Fc receptor FcγRIIB is critical for mediating attenuation of signaling via immunoreceptor tyrosine-based activation motif (ITAM) containing receptors, such as the B cell antigen receptor (BCR), when FcγRIIB is co-cross-linked to these activation receptors. To test the role of the FcγRIIB ITIM motif in regulation of the B cell immune response in vivo, we constructed lines of transgenic mice expressing a form of FcγRIIB with an inactivating tyrosine (Y) to phenylalanine (F) mutation in the ITIM motif. Detailed studies of one of these lines, in which the mutant FcγRIIB was expressed on B cells and other cell types that normally express this receptor, were performed. No quantitative differences in germinal center (GC) B cell responses were observed between the mutant FcγRIIB transgenic line and control mice. However, serum antibody and antibody forming cell responses were often observed to be elevated in the ITIM mutant FcγRIIB transgenic mice as compared to controls, though not to the same extent as mice deficient in expression of FcγRIIB. Moreover, primary B cells from the ITIM mutant FcγRIIB line did not display the same level of augmented BCR signaling as primary FcγRIIB deficient B cells under conditions inducing co-cross-linking of FcγRIIB and the BCR. In total, these data suggest that a functional ITIM motif is not required for all in vivo inhibitory activity of this receptor. However, we also found that the transgenic ITIM mutant FcγRIIB receptor was expressed at abnormal levels in several hematopoietic lineages. Thus, confirmation of our findings will require the generation and analysis of mice in which an ITIM mutant form of FcγRIIB is expressed in vivo as is the endogenous receptor.

BXSB-type genome causes murine autoimmune glomerulonephritis: pathological correlation between telomeric region of chromosome 1 and Yaa

The autoimmune-prone BXSB/MpJ-Yaa mouse is a model of membranous proliferative glomerulonephritis (MPGN). Severe MPGN has been reported only in male BXSB/MpJ-Yaa mice because of the Y-linked autoimmune accelerator (Yaa) locus. However, we show that female BXSB/MpJ mice develop age-related MPGN without Yaa. Female BXSB/MpJ mice clearly developed MPGN characterized by increased mesangial cells, thickening of the glomerular basement membrane (GBM), double contouring and spike formation of GBM with T-cell infiltrations and podocyte injuries corresponding with increased autoantibody production and albuminuria. Analysis of the renal levels of the Fc gamma receptor (Fcgr) and interferon-activated gene 200 (Ifi200) family genes, which are MPGN candidate genes localized to the telomeric region of chromosome 1 (Chr.1), showed that Fcgr2b levels decreased, whereas Fcgr3 and Ifi202b levels increased in female BXSB/MpJ mice compared with healthy C57BL/6 mice. Furthermore, in isolated glomeruli, microarray analysis revealed that Fcgr3, Fcgr4 and Ifi202b expression was higher in male BXSB/MpJ-Yaa mice than in male BXSB/MpJ mice. These findings indicate that the BXSB/MpJ-type genome causes age-related MPGN with significant contribution from the telomeric region of Chr.1, and Yaa enhances the expression of genes localizing to this locus, thereby leading to severe MPGN in male mice.

Deletion of microRNA-155 reduces autoantibody responses and alleviates lupus-like disease in the Fas(lpr) mouse

MicroRNA-155 (miR-155) regulates antibody responses and subsequent B-cell effector functions to exogenous antigens. However, the role of miR-155 in systemic autoimmunity is not known. Using the death receptor deficient (Fas(lpr)) lupus-prone mouse, we show here that ablation of miR-155 reduced autoantibody responses accompanied by a decrease in serum IgG but not IgM anti-dsDNA antibodies and a reduction of kidney inflammation. MiR-155 deletion in Fas(lpr) B cells restored the reduced SH2 domain-containing inositol 5'-phosphatase 1 to normal levels. In addition, coaggregation of the Fc γ receptor IIB with the B-cell receptor in miR-155(-/-)-Fas(lpr) B cells resulted in decreased ERK activation, proliferation, and production of switched antibodies compared with miR-155 sufficient Fas(lpr) B cells. Thus, by controlling the levels of SH2 domain-containing inositol 5'-phosphatase 1, miR-155 in part maintains an activation threshold that allows B cells to respond to antigens.

Aberrant B-lymphocyte responses in lupus: inherent or induced and potential therapeutic targets

BACKGROUND

Lupus is a prototype autoimmune disease of unknown aetiology. The disease is complex; manifest diverse clinical symptoms and disease mechanisms. This complexity has provided many leads to explore: from disease mechanisms to approaches for therapy. B-lymphocytes play a central role in the pathogenesis of the disease. However, the cause of aberrant B-lymphocyte responses in patients and, indeed, its causal relationship with the disease remain unclear.

DESIGN

This article provides a synopsis of current knowledge of immunological abnormalities in lupus with an emphasis on abnormalities in the B-lymphocyte compartment.

RESULTS

There is evidence for abnormalities in most compartments of the immune system in animal models and patients with lupus including an ever expanding list of abnormalities within the B-lymphocyte compartment. In addition, recent genome-wide linkage analyses in large cohorts of patients have identified new sets of genetic association factors some with potential links with defective B-lymphocyte responses although their full pathophysiological effects remain to be determined. The accumulating knowledge may help in the identification and application of new targeted therapies for treating lupus disease.

CONCLUSIONS

Cellular, molecular and genetic studies have provided significant insights into potential causes of immunological defects associated with lupus. Most of this insight relate to defects in B- and T-lymphocyte tolerance, signalling and responses. For B-lymphocytes, there is evidence for altered regulation of inter and intracellular signalling pathways at multiple levels. Some of these abnormalities will be discussed within the context of potential implications for disease pathogenesis and targeted therapies.

B-lymphocyte signalling abnormalities and lupus immunopathology

Lupus is a complex autoimmune rheumatic disease of unknown aetiology. The disease is associated with diverse features of immunological abnormality in which B-lymphocytes play a central role. However, the cause of atypical B-lymphocyte responses remains unclear. In this article, we provide a synopsis of current knowledge on intracellular signalling abnormalities in B-lymphocytes in lupus and their potential effects on the response of these cells in mouse models and in patients. There are numerous reported defects in the regulation of intracellular signalling proteins and pathways in B-lymphocytes in lupus that, potentially, affect critical biological responses. Most of the evidence for these defects comes from studies of disease models and genetically engineered mice. However, there is also increasing evidence from studying B-lymphocytes from patients and from genome-wide linkage analyses for parallel defects to those observed in mice. These studies provide molecular and genetic explanations for the key immunological abnormalities associated with lupus. Most of the new information appears to relate to defects in intracellular signalling that impact B-lymphocyte tolerance, cytokine production and responses to infections. Some of these abnormalities will be discussed within the context of disease pathogenesis.

Interferon-inducible Ifi200-family genes as modifiers of lupus susceptibility

Both genetic and environmental factors contribute to the development and progression of systemic lupus erythematosus (SLE), a complex autoimmune disease. The disease exhibits a strong gender bias and develops predominantly in females. Additionally, most SLE patients exhibit increased serum levels of interferon-α (IFN-α) and the "IFN signature". Studies using the mouse models of lupus have identified several lupus susceptibility loci, including the New Zealand Black (NZB)-derived autoimmunity 2 (Nba2) interval on the chromosome 1. The interval, which is syntenic to the human chromosome 1q region, harbors the FcγR family, SLAM/CD2-family, and the IFN-inducible Ifi200-family genes (encoding for the p200-family proteins). Studies involving the B6.Nba2 congenic mice revealed that the development of antinuclear autoantibodies (ANAs) depends on the age, gender, and activation of type I IFN-signaling. Interestingly, recent studies involving the generation of Nba2 subcongenic mouse lines and generation of mice deficient for the Fcgr2b or Aim2 gene within the interval have provided evidence that epistatic interactions among the Nba2 genes contribute to increased lupus susceptibility. Given that the expression of some of the p200-family proteins is differentially regulated by sex hormones and these proteins differentially regulate cytosolic DNA-induced production of type I IFN and proinflammatory cytokines (IL-1β and IL-18), the major known contributors of SLE-associated inflammation, we discuss the recent advancements in our understanding of the role of p200-family proteins in lupus susceptibility modification. An improved understanding of the role of p200-family proteins in the development of autoimmunity is likely to identify new approaches to treat SLE patients.

Susceptibility loci for the defective foreign protein-induced tolerance in New Zealand Black mice: implication of epistatic effects of Fcgr2b and Slam family genes

In contrast to normal mice, autoimmune-prone New Zealand Black (NZB) mice are defective in susceptibility to tolerance induced by deaggregated bovine γ globulin (DBGG). To examine whether this defect is related to the loss of self-tolerance in autoimmunity, susceptibility loci for this defect were examined by genome-wide analysis using the F(2) intercross of nonautoimmune C57BL/6 (B6) and NZB mice. One NZB locus on the telomeric chromosome 1, designated Dit (Defective immune tolerance)-1, showed a highly significant linkage. This locus overlapped with a locus containing susceptibility genes for autoimmune disease, namely Fcgr2b and Slam family genes. To investigate the involvement of these genes in the defective tolerance to DBGG, we took advantage of two lines of Fcgr2b-deficient B6 congenic mice: one carries autoimmune-type, and the other carries B6-type, Slam family genes. Defective tolerance was observed only in Fcgr2b-deficient mice with autoimmune-type Slam family genes, indicating that epistatic effects of both genes are involved. Thus, common genetic mechanisms may underlie the defect in foreign protein antigen-induced tolerance and the loss of self-tolerance in NZB mouse-related autoimmune diseases.

Aim2 deficiency in mice suppresses the expression of the inhibitory Fcgamma receptor (FcgammaRIIB) through the induction of the IFN-inducible p202, a lupus susceptibility protein

Murine Aim2 and Ifi202 genes (encoding for the Aim2 and p202 proteins) are members of the IFN-inducible Ifi200 gene family. The Aim2 deficiency in mice activates IFN signaling and stimulates the expression of the lupus susceptibility gene, the Ifi202, located within the NZB autoimmunity 2 (Nba2) interval. Given that the deficiency in the expression of the Fcgr2b gene (encoding for the inhibitory FcγRIIB receptor) is associated with increased lupus susceptibility in mice, we investigated whether the Aim2 protein could regulate the expression of Fcgr2b gene. In this article, we report that Aim2 deficiency in mice suppresses the expression of the FcγRIIB receptor. Interestingly, the Fcgr2b-deficient cells expressed increased levels of the IFN-β, activated IFN signaling, and expressed reduced levels of the Aim2 protein. Treatment of splenic cells with IFN-α or -γ reduced levels of the FcγRIIB mRNA and protein and also decreased the activity of the FcγRIIB p(-729/+585) Luc reporter. Moreover, levels of the FcγRIIB receptor were significantly higher in the Stat1-deficient splenic cells than in the wild-type cells. Accordingly, increased expression of IFN-β in lupus-prone B6.Nba2-ABC mice, as compared with non-lupus-prone C57BL/6 (B6) or B6.Nba2-C mice, was associated with reduced expression of the FcγRIIB receptor. Notably, overexpression of the p202 protein in cells decreased the expression of the Aim2 gene, activated the IFN response, and suppressed the expression of the Fcgr2b gene. These observations demonstrate that the expression of Aim2 protein is required to maintain the expression of the Fcgr2b gene and also predict epistatic interactions between the Ifi200 genes and the Fcgr2b gene within the Nba2 interval.

Defining the immunological phenotype of Fc receptor-like B (FCRLB) deficient mice: Confounding role of the inhibitory FcγRIIb

Fc receptor-like A (FCRLA) and FCRLB have homology to the transmembrane FCRL family members (FCRL 1-6) and to the conventional receptors for the Fc portion of immunoglobulin, but uniquely are cytosolic proteins expressed in B cells. Here we describe the phenotype of Fcrlb-gene targeted mice. B cell development and in vitro responses are normal; however, antibody responses to a T-dependent antigen are elevated. The gene encoding the inhibitory FcγRIIb is located nearby Fcrlb. Although Fcrlb-gene targeting had no effect on the function or basal expression of FcγRIIb, its expression was reduced following activation. This abnormal regulation was due to co-inheritance of Fcgr2b and the mutant Fcrlb allele from the 129 ES cells. A promoter polymorphism in the 129/Sv Fcgr2b allele results in diminished upregulation of FcγRIIb following B cell activation. Thus, we speculate that the enhanced antibody response seen in the FCRLB-deficient mice may be due to the Fcgr2b promoter.

Development of self-reactive germinal center B cells and plasma cells in autoimmune Fc gammaRIIB-deficient mice

The leukemogenic effects of Myc drive recurrent trisomy in a mouse model of acute myeloid leukemia.

Abnormalities in expression levels of the IgG inhibitory Fc gamma receptor IIB (FcγRIIB) are associated with the development of immunoglobulin (Ig) G serum autoantibodies and systemic autoimmunity in mice and humans. We used Ig gene cloning from single isolated B cells to examine the checkpoints that regulate development of autoreactive germinal center (GC) B cells and plasma cells in FcγRIIB-deficient mice. We found that loss of FcγRIIB was associated with an increase in poly- and autoreactive IgG⁺ GC B cells, including hallmark anti-nuclear antibody–expressing cells that possess characteristic Ig gene features and cells producing kidney-reactive autoantibodies. In the absence of FcγRIIB, autoreactive B cells actively participated in GC reactions and somatic mutations contributed to the generation of highly autoreactive IgG antibodies. In contrast, the frequency of autoreactive IgG⁺ B cells was much lower in spleen and bone marrow plasma cells, suggesting the existence of an FcγRIIB-independent checkpoint for autoreactivity between the GC and the plasma cell compartment.

B cell activating factor (BAFF) and T cells cooperate to breach B cell tolerance in lupus-prone New Zealand Black (NZB) mice

The presence of autoantibodies in New Zealand Black (NZB) mice suggests a B cell tolerance defect however the nature of this defect is unknown. To determine whether defects in B cell anergy contribute to the autoimmune phenotype in NZB mice, soluble hen egg lysozyme (sHEL) and anti-HEL Ig transgenes were bred onto the NZB background to generate double transgenic (dTg) mice. NZB dTg mice had elevated levels of anti-HEL antibodies, despite apparently normal B cell functional anergy in-vitro. NZB dTg B cells also demonstrated increased survival and abnormal entry into the follicular compartment following transfer into sHEL mice. Since this process is dependent on BAFF, BAFF serum and mRNA levels were assessed and were found to be significantly elevated in NZB dTg mice. Treatment of NZB sHEL recipient mice with TACI-Ig reduced NZB dTg B cell survival following adoptive transfer, confirming the role of BAFF in this process. Although NZB mice had modestly elevated BAFF, the enhanced NZB B cell survival response appeared to result from an altered response to BAFF. In contrast, T cell blockade had a minimal effect on B cell survival, but inhibited anti-HEL antibody production. The findings suggest that the modest BAFF elevations in NZB mice are sufficient to perturb B cell tolerance, particularly when acting in concert with B cell functional abnormalities and T cell help.

Mouse Models of Systemic Lupus Erythematosus Reveal a Complex Pathogenesis

The mammalian immune system is remarkable in that it can respond to an essentially infinite number of foreign antigens. The ability to mount a long-lasting (adaptive) immune response against foreign antigen requires the participation of cells selected from an enormously diverse population of B and T cells. Because the B and T cell receptors expressed by these cells are generated at random, a significant percentage of B and T cells are invariably directed against self-antigen. Under normal circumstances, autoreactive B and T cells are eliminated, reprogrammed, or inactivated in the primary and secondary lymphoid organs. Despite these checks and balances, a small but significant number of people and animals still develop autoimmune disease. One such autoimmune disease—systemic lupus erythematosus—is characterized by the loss of B- and T-cell tolerance to self-antigens (principally nuclear), culminating in multisystemic inflammation. Multiple genetic defects, drug exposure, infectious agents, and environmental factors can contribute to the pathogenesis of the disease. Loss of B- and T-cell tolerance precipitates activation of plasmacytoid and myeloid dendritic cells; collectively, these cells cooperate to form a complex positive feedback loop, continually stimulated by the persistence of self-antigen. Novel treatment strategies now focus on specific inhibition of various aspects of the feedback loop. These specific inhibitors have the potential to be more effective and lack the side effects associated with generalized immunosuppression.

FcgammaRIIB in autoimmunity and infection: evolutionary and therapeutic implications

FcgammaRIIB is the only inhibitory Fc receptor. It controls many aspects of immune and inflammatory responses, and variation in the gene encoding this protein has long been associated with susceptibility to autoimmune disease, particularly systemic lupus erythematosus (SLE). FcgammaRIIB is also involved in the complex regulation of defence against infection. A loss-of-function polymorphism in FcgammaRIIB protects against severe malaria, the investigation of which is beginning to clarify the evolutionary pressures that drive ethnic variation in autoimmunity. Our increased understanding of the function of FcgammaRIIB also has potentially far-reaching therapeutic implications, being involved in the mechanism of action of intravenous immunoglobulin, controlling the efficacy of monoclonal antibody therapy and providing a direct therapeutic target.

Dysregulation of germinal centres in autoimmune disease

In germinal centres, somatic hypermutation and B cell selection increase antibody affinity and specificity for the immunizing antigen, but the generation of autoreactive B cells is an inevitable by-product of this process. Here, we review the evidence that aberrant selection of these autoreactive B cells can arise from abnormalities in each of the germinal centre cellular constituents--B cells, T follicular helper cells, follicular dendritic cells and tingible body macrophages--or in the supply of antigen. As the progeny of germinal centre B cells includes long-lived plasma cells, selection of autoreactive B cells can propagate long-lived autoantibody responses and cause autoimmune diseases. Elucidation of crucial molecular signals in germinal centres has led to the identification of novel therapeutic targets.

Development of murine lupus involves the combined genetic contribution of the SLAM and FcgammaR intervals within the Nba2 autoimmune susceptibility locus

Autoantibodies are of central importance in the pathogenesis of Ab-mediated autoimmune disorders. The murine lupus susceptibility locus Nba2 on chromosome 1 and the syntenic human locus are associated with a loss of immune tolerance that leads to antinuclear Ab production. To identify gene intervals within Nba2 that control the development of autoantibody-producing B cells and to determine the cellular components through which Nba2 genes accomplish this, we generated congenic mice expressing various Nba2 intervals where genes for the FcgammaR, SLAM, and IFN-inducible families are encoded. Analysis of congenic strains demonstrated that the FcgammaR and SLAM intervals independently controlled the severity of autoantibody production and renal disease, yet are both required for lupus susceptibility. Deregulated homeostasis of terminally differentiated B cells was found to be controlled by the FcgammaR interval where FcgammaRIIb-mediated apoptosis of germinal center B cells and plasma cells was impaired. Increased numbers of activated plasmacytoid dendritic cells that were distinctly CD19+ and promoted plasma cell differentiation via the proinflammatory cytokines IL-10 and IFNalpha were linked to the SLAM interval. These findings suggest that SLAM and FcgammaR intervals act cooperatively to influence the clinical course of disease through supporting the differentiation and survival of autoantibody-producing cells.

The lupus susceptibility locus Sle1 breaches peripheral B cell tolerance at the antibody-forming cell and germinal center checkpoints

We have described a line of V(H) knock-in mice termed HKIR in which the transgenic Igh locus partially encodes "dual-reactive" antichromatin and anti-p-azophenylarsonate (Ars) BCRs. HKIR B cells termed canonical, expressing a particular Vkappa L chain, evade central tolerance by down-regulating BCR levels. Canonical HKIR B cells can be recruited into the primary germinal center (GC) and Ab-forming cell (AFC) compartments via Ars immunization. However, their participation in the GC response rapidly wanes and they do not efficiently contribute to the memory compartment, indicating that they are regulated by a GC tolerance checkpoint. We analyzed the influence of the Sle1 genetic interval, shown to break tolerance of chromatin-reactive B cells, on the behavior of HKIR B cells during the anti-Ars response. Canonical B cells from congenic HKIR.Sle1 mice gave rise to elevated short and long-lived AFC responses, and the attenuated GC and memory responses characteristic of these B cells were relieved in adoptive, wild-type recipients. HKIR GC B cells containing Sle1 expressed increased levels of Bcl-2 and c-FLIP and decreased levels of Fas RNA compared with HKIR controls, suggesting direct alteration of the regulation of the GC response by Sle1. High titers of canonical and anti-dsDNA Abs spontaneously developed in many aged HKIR.Sle1 mice. Together, these data indicate that Sle1 perturbs the action of peripheral tolerance checkpoints operative on antinuclear Ag B cells in both the AFC and GC pathways in a cell autonomous fashion.

Modulation of single-cell IgG secretion frequency and rates in human memory B cells by CpG DNA, CD40L, IL-21, and cell division

During the recall response by CD27(+) IgG class-switched human memory B cells, total IgG secreted is a function of the following: 1) the number of IgG-secreting cells (IgG-SC), and 2) the secretion rate of each cell. In this study, we report the quantitative ELISPOT method for simultaneous estimation of single-cell IgG secretion rates and secreting cell frequencies in human B cell populations. We found that CD27(+) IgM(-) memory B cells activated with CpG and cytokines had considerable heterogeneity in the IgG secretion rates, with two major secretion rate subpopulations. BCR cross-linking reduced the frequency of cells with high per-cell IgG secretion rates, with a parallel decrease in CD27(high) B cell blasts. Increased cell death may account for the BCR-stimulated reduction in high-rate IgG-SC CD27(high) B cell blasts. In contrast, the addition of IL-21 to CD40L plus IL-4-activated human memory B cells induced a high-rate IgG-SC population in B cells with otherwise low per-cell IgG secretion rates. The profiles of human B cell IgG secretion rates followed the same biphasic distribution and range irrespective of division class. This, along with the presence of non-IgG-producing, dividing B cells in CpG plus cytokine-activated B memory B cell populations, is suggestive of an on/off switch regulating IgG secretion. Finally, these data support a mixture model of IgG secretion in which IgG secreted over time is modulated by the frequency of IgG-SC and the distribution of their IgG secretion rates.

FcgammaRIIB deficiency leads to autoimmunity and a defective response to apoptosis in Mrl-MpJ mice

Data suggests that modulation of FcgammaRIIB expression represents a significant risk factor for the development of autoimmunity. In this study, we investigated this notion in mice that possess genetics permissible for the development of autoimmunity. To this end, Mrl-MpJ Fcgr2b-/- mice were monitored for the development of autoreactivity. We found that FcgammaRIIB deficiency led to chronic B cell activation associated with increased germinal center and plasma cell accumulation in the spleen. Likewise, Mrl-MpJ Fcgr2b-/- mice exhibited significant serum IgG reactivity against DNA. We further analyzed the IgG isotype contribution to the anti-dsDNA response and found increases in all subtypes with the exception of IgG3. In particular, we found large increases in IgG1 and IgG2b autoreactivity correlating with significant increases in immune complex deposition and kidney pathology. Finally, we found dendritic cells derived from Mrl-MpJ Fcgr2b-/- mice greatly increased IL-12 expression upon coincubation with apoptotic thymocytes compared with wild-type controls. The results indicate that FcgammaRIIB is an important regulator of peripheral tolerance and attenuation of the inhibitory signal it provides enhances autoimmune disease on susceptible backgrounds. Additionally, the data indicates FcgammaRIIB function has a significant impact on APC activity, suggesting a prominent role in dendritic cell activity in response to interaction with particulate autoantigens.

Fc receptor genes and the systemic lupus erythematosus diathesis

Fc receptors represent a distinct group of hematopoeitic cell surface glycoproteins that have a characterized role in affecting the efficiency of the mononuclear phagocyte system to clear IgG immune complexes. Functional genetic variations in this family of receptors have been identified as heritable susceptibility factors for SLE and lupus nephritis across diverse populations. In this review, we describe the roles of the classical Fc receptors for IgG (Fc gamma) and non-classical Fc-like receptors (FCR1-FCRL6L), Fc receptors for IgE (Fc epsilon RI) and IgA and IgM (Fc alpha/mu R) in SLE diathesis. The combined effects of these genes on SLE pathogenesis, either via linkage disequilibrium or epistasis with additional genetic or environmental factors, provide a challenge for future investigations. The pursuit of a polygenic SLE-profile that includes longitudinal evaluations of SLE and markers involved in the protean clinical manifestations associated with SLE will facilitate our understanding of the cascade of inflammatory events associated with the diathesis.

Distinct cell-specific control of autoimmunity and infection by FcgammaRIIb

FcγRIIb is an inhibitory Fc receptor expressed on B cells and myeloid cells. It is important in controlling responses to infection, and reduced expression or function predisposes to autoimmunity. To determine if increased expression of FcγRIIb can modulate these processes, we created transgenic mice overexpressing FcγRIIb on B cells or macrophages. Overexpression of FcγRIIb on B cells reduced the immunoglobulin G component of T-dependent immune responses, led to early resolution of collagen-induced arthritis (CIA), and reduced spontaneous systemic lupus erythematosus (SLE). In contrast, overexpression on macrophages had no effect on immune responses, CIA, or SLE but increased mortality after Streptococcus pneumoniae infection. These results help define the role of FcγRIIb in immune responses, demonstrate the contrasting roles played by FcγRIIb on B cells and macrophages in the control of infection and autoimmunity, and emphasize the therapeutic potential for modulation of FcγRIIb expression on B cells in inflammatory and autoimmune disease.

Fcgamma receptors as regulators of immune responses

In addition to their role in binding antigen, antibodies can regulate immune responses through interacting with Fc receptors (FcRs). In recent years, significant progress has been made in understanding the mechanisms that regulate the activity of IgG antibodies in vivo. In this Review, we discuss recent studies addressing the multifaceted roles of FcRs for IgG (FcgammaRs) in the immune system and how this knowledge could be translated into novel therapeutic strategies to treat human autoimmune, infectious or malignant diseases.

Dependence of surface monoclonal antibody binding on dynamic changes in FcgammaRIIb expression

Receptors for the Fc region of immunoglobulin G (FcgammaRs) are expressed on a broad range of haematopoietic cell types and are responsible for regulating antibody production and linking the humoral and effector responses. In response to a number of stimuli, such as cytokine signals or inflammation, FcgammaR expression at the cell surface is dynamically regulated. On B cells, we observed what appeared to be a correlation between CD22 expression and FcgammaRIIb expression when the latter was varied in a number of models. Further investigation revealed that this was specific to a particular anti-CD22 monoclonal antibody, which appeared to require stabilization by interaction with FcgammaRIIb for optimal binding to CD22. Since alterations in the regulation of FcgammaR expression are important in controlling immune responses and have been associated with a number of immune-mediated disease states, we suggest that it might be prudent to confirm the expression of cell surface markers by two independent methods. Furthermore, because the efficacy of therapeutic antibodies may depend upon their interaction with FcgammaRs, our results are relevant to their design and assessment.

Fc-receptors as regulators of immunity

Receptors for immunoglobulins [Fc-receptors (FcRs)] are widely expressed throughout the immune system. By binding to the antibody Fc-portion, they provide a link between the specificity of the adaptive immune system and the powerful effector functions triggered by innate immune effector cells. By virtue of coexpression of activating and inhibitory FcRs on the same cell, they set a threshold for immune cell activation by immune complexes (ICs). Besides their involvement in the efferent phase of an immune response, they are also important for modulating adaptive immune responses by regulating B cell and dendritic cell (DC) activation. Deletion of the inhibitory FcR leads to the loss of tolerance in the humoral immune system and the development of autoimmune disease. Uptake of ICs by FcRs on DCs and the concommitant triggering of activating and inhibitory signaling pathways will determine the strength of the initiated T-cell response. Loss of this balanced signaling results in uncontrolled responses that can lead to the damage of healthy tissues and ultimately to the initiation of autoimmune processes. In this chapter, we will discuss how coexpression of different activating and inhibitory receptors on different immune cells of the innate and adaptive immune system modulates cell activity. Moreover, we will focus on exogenous factors that can influence the balanced triggering of activating and inhibitory FcRs, such as the cytokine milieu and the role of differential antibody glycosylation.

The lupus-related Lmb3 locus contains a disease-suppressing Coronin-1A gene mutation

Here, we show that a lupus-suppressing locus is caused by a nonsense mutation of the filamentous actin-inhibiting Coronin-1A gene. This mutation was associated with developmental and functional alterations in T cells including reduced migration, survival, activation, and Ca2+ flux. T-dependent humoral responses were impaired, but no intrinsic B cell defects were detected. By transfer of T cells, it was shown that suppression of autoimmunity could be accounted for by the presence of the Coro1a(Lmb3) mutation in T cells. Our results demonstrate that Coronin-1A is required for the development of systemic lupus and identify actin-cytoskeleton regulatory proteins as potential targets for modulating autoimmune diseases.

FcgammaRIIB as a modulator of autoimmune disease susceptibility

Antibodies are secreted to recognize and in some cases directly neutralize pathogens. Another important means by which they are essential components of the immune system is through binding to Fc receptors. Effector responses triggered by antibody binding of Fc receptors affect a host of important cellular responses such as phagocytosis, inflammatory cytokine release, antigen presentation, and regulation of humoral responses. A crucial check on this antibody-mediated signal is through the inhibitory receptor, FcgammaRIIB. In this review we discuss how dysregulation of FcgammaRIIB can result in a lowered threshold for autoimmunity in mice and humans. We close with a discussion of the potential for applying these findings to immunotherapy.

Monocytosis in BXSB mice is due to epistasis between Yaa and the telomeric region of chromosome 1 but does not drive the disease process

The BXSB murine model of systemic lupus erythematosus is differentiated from other murine models of lupus by a severe monocytosis. The recently identified Y-linked autoimmune accelerator locus, Yaa, which is fundamental to accelerated disease in male BXSB mice, is required for the monocytic phenotype in BXSB. It has also recently been shown to induce monocytosis in combination with the Nba2 locus from NZB. To dissect the genetic basis and associated pathogenicity of BXSB-related monocytosis, a panel of existing congenic mice were studied and a novel sub-congenic mouse B10.Y(BXSB).BXSB-Bxs3 was generated. Monocytosis was found to be caused by an epistatic interaction between Yaa and the telomeric region of chromosome 1, an area of approximately 30 cM. Bxs3 and Yaa together were sufficient to generate monocytosis equivalent to that of BXSB. In contrast to the NZB model, however, where monocytosis tightly correlated with autoantibody production and lethal lupus nephritis, this was not the case in BXSB. While Yaa(+) mice bearing the Bxs3 locus drive monocytosis, glomerulonephritis and autoantibody production, both autoantibody production and nephritis are discreet events that occur in the absence of the Bxs3 locus. Yaa is a pre-requisite for monocytosis, demonstrating a novel synergistic interaction between Yaa and Bxs3.

Expression of the autoimmune Fcgr2b NZW allele fails to be upregulated in germinal center B cells and is associated with increased IgG production

The inhibitory receptor FcgammaRIIb regulates B-cell functions. Genetic studies have associated Fcgr2b polymorphisms and lupus susceptibility in both humans and murine models, in which B cells express reduced FcgammaRIIb levels. Furthermore, FcgammaRIIb absence results in lupus on the appropriate genetic background, and lentiviral-mediated FcgammaRIIb overexpression prevents disease in the NZM2410 lupus mouse. The NZM2410/NZW allele Fcgr2b is, however, located in-between Sle1a and Sle1b, two potent susceptibility loci, making it difficult to evaluate Fcr2b(NZW) independent contribution. By using two congenic strains that each carries only Sle1a (B6.Sle1a(15-353)), or Fcr2b(NZW) in the absence of Sle1a or Sle1b (B6.Sle1(111-148)), we show that the Fcr2b(NZW) allele does not upregulate its expression on germinal center B cells and plasma cells, as does the C57BL/6 allele on B6.Sle1a(15-353) B cells. Furthermore, in the absence of the flanking Sle1a and Sle1b, Fcr2b(NZW) does not produce an autoimmune phenotype, but is associated with an increased number of class-switched plasma cells. These results show that while a lower level of FcgammaRIIb does not by itself induce the development of autoreactive B cells, it has the potential to amplify the contribution of autoreactive B cells induced by other lupus-susceptibility loci by enhancing the production of class-switched plasma cells.

Altered Expression of Fc and Complement Receptors on B Cells in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by B cell hyper-reactivity, autoantibody production, immune complex (IC) deposition, and multiple organ damage. The contribution of IC and B cell-mediated changes in the pathogenesis of SLE is well established, however, the exact role of IC-binding receptors expressed on B cells, Fcgamma receptors, and complement receptors CR1 and CR2 in these pathological processes is unclear. Development of lupus-like symptoms in mice defective for the inhibitory Fc-gammaRIIb and genetic association of certain FcgammaR genes with SLE demonstrate a significant role for these receptors but reports indicating alterations of Fcgamma or complement receptor-mediated B cell functions in human SLE are relatively few. The present review highlights a selected set of data including our own discussing the significance of animal models, genetics, and functional alterations of these IC-binding receptors in the etiopathogenesis of SLE.

FcgammaRIIB regulates autoreactive primary antibody-forming cell, but not germinal center B cell, activity

The low-affinity FcR for IgG FcgammaRIIB suppresses the development of IgG autoantibodies and autoimmune disease in normal individuals, but how this effect is mediated is incompletely understood. To investigate this issue, we created FcgammaRIIB-deficient versions of two previously described targeted BCR-transgenic lines of mice that contain follicular B cells with specificity for the hapten arsonate, but with different levels of antinuclear autoantigen reactivity. The primary development and tolerance of both types of B cells were unaltered by the absence of FcgammaRIIB. Moreover, the reduced p-azophenylarsonate-driven germinal center and memory responses characteristic of the highly autoreactive clonotype were not reversed by an intrinsic FcgammaRIIB deficiency. In contrast, the p-azophenylarsonate-driven primary Ab-forming cell responses of both clonotypes were equivalently increased by such a deficiency. In total, our data do not support the idea that FcgammaRIIB directly participates in the action of primary or germinal center tolerance checkpoints. In contrast, this receptor apparently contributes to the prevention of autoimmunity by suppressing the production of autoreactive IgGs from B cells that have breached tolerance checkpoints and entered the Ab-forming cell pathway due to spontaneous, or cross-reactive, Ag-mediated activation.

FcγRIIb controls bone marrow plasma cell persistence and apoptosis

The survival of long-lived plasma cells, which produce most serum immunoglobulin, is central to humoral immunity. We found here that the inhibitory Fc receptor FcgammaRIIb was expressed on plasma cells and controlled their persistence in the bone marrow. Crosslinking FcgammaRIIb induced apoptosis of plasma cells, which we propose contributes to the control of their homeostasis and suggests a method for therapeutic deletion. Plasma cells from mice prone to systemic lupus erythematosus did not express FcgammaRIIb and were protected from apoptosis. Human plasmablasts expressed FcgammaRIIb and were killed by crosslinking, as were FcgammaRIIb-expressing myeloma cells. Our results suggest that FcgammaRIIb controls bone marrow plasma cell persistence and that defects in it may contribute to autoantibody production.