Characterization of anti-DNA B cells that escape negative selection

Circulating autoreactive proteinase 3+ B cells and tolerance checkpoints in ANCA-associated vasculitis

BACKGROUND

Little is known about the autoreactive B cells in antineutrophil cytoplasmic antibody–associated (ANCA-associated) vasculitis (AAV). We aimed to investigate tolerance checkpoints of circulating antigen-specific proteinase 3–reactive (PR3⁺) B cells.

METHODS

Multicolor flow cytometry in combination with bioinformatics and functional in vitro studies were performed on baseline samples of PBMCs from 154 well-characterized participants of the RAVE trial (NCT00104299) with severely active PR3-AAV and myeloperoxidase-AAV (MPO-AAV) and 27 healthy controls (HCs). Clinical data and outcomes from the trial were correlated with PR3⁺ B cells (total and subsets).

RESULTS

The frequency of PR3⁺ B cells among circulating B cells was higher in participants with PR3-AAV (4.77% median [IQR, 3.98%–6.01%]) than in participants with MPO-AAV (3.16% median [IQR, 2.51%–5.22%]) and participants with AAV compared with HCs (1.67% median [IQR, 1.27%–2.16%], P < 0.001 for all comparisons), implying a defective central tolerance checkpoint in patients with AAV. Only PBMCs from participants with PR3-AAV contained PR3⁺ B cells capable of secreting PR3-ANCA IgG in vitro, proving they were functionally distinct from those of participants with MPO-AAV and HCs. Unsupervised clustering identified subtle subsets of atypical autoreactive PR3⁺ memory B cells accumulating through the maturation process in patients with PR3-AAV. PR3⁺ B cells were enriched in the memory B cell compartment of participants with PR3-AAV and were associated with higher serum CXCL13 levels, suggesting an increased germinal center activity. PR3⁺ B cells correlated with systemic inflammation (C-reactive protein and erythrocyte sedimentation rate, P < 0.05) and complete remission (P < 0.001).

CONCLUSION

This study suggests the presence of defective central antigen-independent and peripheral antigen-dependent checkpoints in patients with PR3-AAV, elucidating the selection process of autoreactive B cells.

Trial registration

ClinicalTrials.gov NCT00104299.

Funding

The Vasculitis Foundation, the National Institute of Allergy and Infectious Diseases of the NIH, and the Mayo Foundation for Education and Research.

Loss of an IgG plasma cell checkpoint in patients with lupus

BACKGROUND

IgG antinuclear antibodies (ANAs) are a feature of several autoimmune diseases. These antibodies arise through defects in central or peripheral tolerance checkpoints. The specific checkpoints breached in patients with autoimmune disease are not fully understood.

OBJECTIVES

We sought to study whether autoreactive plasma cells in lupus models and patients with systemic lupus erythematosus (SLE) arise as a consequence of defective antigen-specific selection or a global enhancement of IgG plasma cell differentiation.

METHODS

We optimized and validated a novel technique to detect naturally occurring ANA⁺ B cells and plasma cells.

RESULTS

We observed a major checkpoint for generation of ANA⁺ IgG⁺ plasma cells in both nonautoimmune mice and healthy human subjects. Interestingly, we observed increased numbers of ANA⁺ IgG⁺ plasma cells despite normal tolerance checkpoints in immature and naive B cells of lupus-prone MRL/lpr and NZB/W mice, as well as patients with SLE. This increase was due to increased numbers of total IgG⁺ plasma cells rather than lack of selection against ANA⁺ plasma cells.

CONCLUSION

Using a method that permits quick and accurate quantification of autoreactive B cells and plasma cells in vivo within a native B-cell repertoire in mice and human subjects, we demonstrate the importance of a checkpoint that restricts the generation of IgG plasma cells and protects against IgG ANAs. Our observations suggest a fundamentally revised understanding of SLE: that it is a disease of aberrant B-cell differentiation rather than a defect in antigen-specific B-cell tolerance.

Hormonal milieu at time of B cell activation controls duration of autoantibody response

A strong gender bias is seen in many autoimmune diseases including systemic lupus erythematosus (SLE). To investigate the basis for the female preponderance in SLE, we have been studying BALB/c mice in which B cells express the R4A heavy chain of an anti-DNA antibody in association with an endogenous light chain repertoire (R4Atg mice). In unmanipulated mice, approximately 5% of B cells express the R4A transgene. R4Atg mice do not spontaneously develop elevated serum titers of anti-DNA antibodies. Administration of either estradiol (E2) or prolactin (Pr) results in escape from tolerance of autoreactive B cells, expressed as an increase in transgene-expressing B cells and elevated serum titers of anti-DNA antibodies. We previously demonstrated that autoreactive B cells maturing in an estrogenic milieu develop as marginal zone (MZ) B cells; when these same B cells mature in the presence of increased prolactin, they develop as follicular (Fo) B cells. To determine the long term consequence of this differential maturation of DNA-reactive B cells, we treated R4Atg BALB/c mice with E2 or Pr for 6 weeks until serum titers of anti-DNA antibody were high, at which time hormonal exposure was discontinued. In E2-treated mice, the anti-DNA titers remained high even 3 months after discontinuation of hormone exposure. Nascent B cells underwent normal tolerance induction, but existing autoreactive MZ B cells persisted and continued to secrete autoantibody. In contrast, Pr caused only a short-term increase in anti-DNA antibody titers. By 3 months after cessation of hormone treatment, serum anti-DNA antibody titers and B cell subsets were indistinguishable from those in placebo (P) treated mice. These findings suggest that autoantibody responses are sustained for variable lengths of time depending on the B cell subset producing the autoantibodies. This observation may be relevant to understanding the heterogeneous presentation of patients with SLE and to the design of therapies targeting specific B-cell populations in autoimmune disease.

Immune enhancement by novel vaccine adjuvants in autoimmune-prone NZB/W F1 mice: relative efficacy and safety

Background

Vaccines have profoundly impacted global health although concerns persist about their potential role in autoimmune or other adverse reactions. To address these concerns, vaccine components like immunogens and adjuvants require critical evaluation not only in healthy subjects but also in those genetically averse to vaccine constituents. Evaluation in autoimmune-prone animal models of adjuvants is therefore important in vaccine development. The objective here was to assess the effectiveness of experimental adjuvants: two phytol-derived immunostimulants PHIS-01 (phytanol) and PHIS-03 (phytanyl mannose), and a new commercial adjuvant from porcine small intestinal submucosa (SIS-H), relative to a standard adjuvant alum. Phytol derivatives are hydrophobic, oil-in water diterpenoids, while alum is hydrophilic, and SIS is essentially a biodegradable and collagenous protein cocktail derived from extracellular matrices.

Results

We studied phthalate -specific and cross-reactive anti-DNA antibody responses, and parameters associated with the onset of autoimmune disorders. We determined antibody isotype and cytokine/chemokine milieu induced by the above experimental adjuvants relative to alum. Our results indicated that the phytol-derived adjuvant PHIS-01 exceeded alum in enhancing anti-phthalate antibody without much cross reactivity with ds-DNA. Relatively, SIS and PHIS-03 proved less robust, but they were also less inflammatory. Interestingly, these adjuvants facilitated isotype switching of anti-hapten, but not of anti-DNA response. The current study reaffirms our earlier reports on adjuvanticity of phytol compounds and SIS-H in non autoimmune-prone BALB/c and C57BL/6 mice. These adjuvants are as effective as alum also in autoimmune-prone NZB/WF1 mice, and they have little deleterious effects.

Conclusion

Although all adjuvants tested impacted cytokine/chemokine milieu in favor of Th1/Th2 balance, the phytol compounds fared better in reducing the onset of autoimmune syndromes. However, SIS is least inflammatory among the adjuvants evaluated.

Antigen is required for maturation and activation of pathogenic anti-DNA antibodies and systemic inflammation

Systemic lupus erythematosus is an autoimmune disease characterized by autoantibodies and systemic inflammation that results in part from dendritic cell activation by nucleic acid containing immune complexes. There are many mouse models of lupus, some spontaneous and some induced. We have been interested in an induced model in which estrogen is the trigger for development of a lupus-like serology. The R4A transgenic mouse expresses a transgene-encoded H chain of an anti-DNA Ab. This mouse maintains normal B cell tolerance with deletion of high-affinity DNA-reactive B cells and maturation to immunocompetence of B cells making nonglomerulotropic, low-affinity DNA-reactive Abs. When this mouse is given estradiol, normal tolerance mechanisms are altered; high-affinity DNA-reactive B cells mature to a marginal zone phenotype, and the mice are induced to make high titers of anti-DNA Abs. We now show that estradiol administration also leads to systemic inflammation with increased B cell-activating factor and IFN levels and induction of an IFN signature. DNA must be accessible to B cells for both the production of high-affinity anti-DNA Abs and the generation of the proinflammatory milieu. When DNase is delivered to the mice at the same time as estradiol, there is no evidence for an abrogation of tolerance, no increased B cell-activating factor and IFN, and no IFN signature. Thus, the presence of autoantigen is required for positive selection of autoreactive B cells and for the subsequent positive feedback loop that occurs secondary to dendritic cell activation by DNA-containing immune complexes.

Lessons from an anti-DNA autoantibody

R4A is an anti-DNA antibody derived by hybridoma technology from a BALB/c mouse. The study of its properties has helped to elucidate aspects of the physiopathology of Systemic Lupus Erythematosus (SLE). It has been useful to decipher the mechanisms implicated in breaks in B cell self tolerance by B cell extrinsic or B cell intrinsic factors. Furthermore, it illustrates the importance of molecular mimicry in understanding the cellular and molecular effects relevant to some manifestations of neuropsychiatric lupus, as it cross-reacts with the D/E W D/E YS/G peptide sequence present in subunits of the N-methyl d-aspartate receptor (NMDAR).

Anti-dsDNA antibodies bind to mesangial annexin II in lupus nephritis

Production of anti-dsDNA antibodies is a hallmark of lupus nephritis, but how these antibodies deposit in organs and elicit inflammatory damage remains unknown. In this study, we sought to identify antigens on the surface of human mesangial cells (HMC) that mediate the binding of human anti-dsDNA antibodies and the subsequent pathogenic processes. We isolated anti-dsDNA antibodies from patients with lupus nephritis by affinity chromatography. We used multiple methods to identify and characterize antigens from the plasma membrane fraction of mesangial cells that crossreacted with the anti-dsDNA antibodies. We found that annexin II mediated the binding of anti-dsDNA antibodies to HMC. After binding to the mesangial cell surface, anti-dsDNA antibodies were internalized into the cytoplasm and nucleus. This also led to induction of IL-6 secretion and annexin II synthesis, mediated through activation of p38 MAPK, JNK, and AKT. Binding of anti-dsDNA antibodies to annexin II correlated with disease activity in human lupus nephritis. Glomerular expression of annexin II correlated with the severity of nephritis, and annexin II colocalized with IgG and C3 deposits in both human and murine lupus nephritis. Gene silencing of annexin II in HMC reduced binding of anti-dsDNA antibody and partially decreased IL-6 secretion. In summary, our data demonstrate that annexin II mediates the binding of anti-dsDNA antibodies to mesangial cells, contributing to the pathogenesis of lupus nephritis. This interaction provides a potential target for therapeutic intervention.

Selective regulation of autoreactive B cells by FcgammaRIIB

FcgammaRIIB is an inhibitory receptor which plays a role in limiting B cell and DC activation. Since FcgammaRIIB is known to dampen the signaling strength of the BCR, we wished to determine the impact of FcgammaRIIB on the regulation of BCRs which differ in their affinity for DNA. For these studies, FcgammaRIIB deficient BALB/c mice were bred with mice expressing the transgene-encoded H chain of the R4A anti-DNA antibody which gives rise to BCRs which express high, low or no affinity for DNA. The deletion of FcgammaRIIB in R4A BALB/c mice led to an alteration in the B cell repertoire, allowing for the expansion and activation of high affinity DNA-reactive B cells. By 6-8 months of age, R4A x FcgammaRIIB-/- BALB/c mice spontaneously developed anti-DNA antibody titers. These mice also displayed an induction of IFN-inducible genes and an elevation in levels of the B cell survival factor, BAFF. These data demonstrate that FcgammaRIIB preferentially limits activation of high affinity autoreactive B cells and can influence the activation of DC through an immune complex-mediated mechanism.

Hormonal regulation of B-cell function and systemic lupus erythematosus

The prevalence of systemic lupus erythematosus (SLE) is far higher in females than in males and numerous investigations to understand this gender bias have been performed, which propose as casual actors genetic predispositions and sex hormones effects. We will describe in this review how the sex hormones estrogen and prolactin influence B cell maturation and selection, permitting B cells to mature to immunocompetence in a mouse model of lupus. Finally, we will discuss the relevance and implications of these results for human disease.

Auto-reactive B cells in transgenic mice

In order to understand how the natural occurrence of autoreactive B cells is controlled in normal individuals, and how self reactive B cells can escape this control during diverse clinical situations, many different transgenic mice have been generated expressing self reactive antibodies. In this review, we focus our attention on disease-associated self reactive transgenic models which show the variety of the tolerization mechanisms. The same transgenic lines are also used to analyse the effects of the autoimmune genetic background on the self reactive B cell fate, as well as to study the influence of infectious agents on the behaviour of the auto-reactive transgenic B cells.

IgG1 B cell receptor signaling is inhibited by CD22 and promotes the development of B cells whose survival is less dependent on Ig alpha/beta

We describe a mouse strain in which B cell development relies either on the expression of membrane-bound immunoglobulin (Ig) gamma1 or mu heavy chains. Progenitor cells expressing gamma1 chains from the beginning generate a peripheral B cell compartment of normal size with all subsets, but a partial block is seen at the pro- to pre-B cell transition. Accordingly, gamma1-driven B cell development is disfavored in competition with developing B cells expressing a wild-type (WT) IgH locus. However, the mutant B cells display a long half-life and accumulate in the mature B cell compartment, and even though partial truncation of the Ig alpha cytoplasmic tail compromises their development, it does not affect their maintenance, as it does in WT cells. IgG1-expressing B cells showed an enhanced Ca(2+) response upon B cell receptor cross-linking, which was not due to a lack of inhibition by CD22. The enhanced Ca(2+) response was also observed in mature B cells that had been switched from IgM to IgG1 expression in vivo. Collectively, these results suggest that the gamma1 chain can exert a unique signaling function that can partially replace that of the Ig alpha/beta heterodimer in B cell maintenance and may contribute to memory B cell physiology.

Sex and systemic lupus erythematosus: the role of the sex hormones estrogen and prolactin on the regulation of autoreactive B cells

PURPOSE OF REVIEW

For many decades, it has been speculated that sex hormones play a role in systemic lupus erythematosus. Recent data accumulated during the past few years provide striking evidence that hormonal modulation of B cells can have a profound impact on the survival, maturation and repertoire selection of autoreactive B cells and begin to explain the sex bias associated with the condition.

RECENT FINDINGS

While there are still insufficient clinical data to define a role for estrogen or prolactin in human systemic lupus erythematosus, recent studies of anti-DNA antibody transgenic mice clearly demonstrate that an elevation in either estrogen or prolactin breaks tolerance of high affinity DNA-reactive B cells and induces a lupus phenotype. B cells with the same antigenic specificities are rescued by either estrogen or prolactin, but estrogen promotes the survival and activation of the T independent marginal zone B cell subset, while prolactin promotes the survival and activation of the T dependent follicular B cell subset.

SUMMARY

Elevations in the levels of estrogen or prolactin can promote the survival and activation of high affinity autoreactive B cells. These hormones engage different B cell pathways to interfere with B cell tolerance. The identification of systemic lupus erythematosus patients with either an estrogen-responsive or prolactin-responsive disease will further the development of therapeutics that can specifically modulate hormonal responses.

Hormonal regulation of B cell development: 17 beta-estradiol impairs negative selection of high-affinity DNA-reactive B cells at more than one developmental checkpoint

There are increasing data suggesting that sex hormones, such as estrogen, have immunomodulatory effects and play a role in disease progression and pathogenesis in patients with the autoimmune disorder systemic lupus erythematosus. We have shown previously that treatment with 17beta-estradiol (E2) induces a lupus phenotype in BALB/c mice that express a transgene-encoded H chain of an anti-DNA Ab. Because E2 treatment interferes with normal tolerance of naive DNA-reactive B cells, we elected to study the effects of hormonal modulation on the regulation of autoreactive B cells at early developmental checkpoints. Single-cell PCR was performed to study the repertoire of DNA-reactive B cell subsets. High-affinity DNA-reactive B cells were rescued at both the immature and transitional B cell stage in E2-treated mice. Interestingly, although low-affinity DNA-reactive B cells survive negative selection in control mice, the frequency of these cells was significantly reduced in the mature pool of E2-treated mice, suggesting that the high-affinity DNA-reactive cells that mature to immunocompetence out-compete the low-affinity population for survival as mature B cells. These data provide evidence that an elevation in serum levels of E2 facilitates the maturation of a pathogenic naive autoreactive B cell repertoire and hampers the maturation of a potentially protective autoreactive B cell repertoire. Furthermore, these data show that both positive and negative selection occur within the transitional B cell stage.

Cutting Edge: Hormonal milieu, not antigenic specificity, determines the mature phenotype of autoreactive B cells

Although both marginal zone and follicular B cells produce anti-DNA Abs in murine models of systemic lupus erythematosus, it has been unclear whether these distinct B cell subsets make identical or different Abs. Single-cell analysis demonstrates that the same DNA-reactive B cells can mature to either subset, depending on the hormonal environment. Anti-DNA B cells in estradiol-treated mice become marginal zone cells while identical cells from prolactin-treated mice become follicular cells. The B cell receptor signaling pathway is influenced by hormonal milieu. Thus, hormonal milieu and perhaps B cell receptor signaling, but not antigenic specificity, correlates with the differentiation pathway. These observations have implications for the pathogenesis and treatment of autoimmune disease.

Sex hormones and SLE: influencing the fate of autoreactive B cells

The prevalence of systemic lupus erythematosus (SLE) is far higher in females than in males and numerous investigations to understand this gender bias have been conducted. While it is plausible that some sex-linked genes may contribute to the genetic predisposition for the disease, other likely culprits are the sex hormones estrogen and prolactin. In this chapter we review studies that have addressed the influence of sex hormones in SLE activity and discuss the recent data established in a BALB/c mouse transgenic for the heavy chain of an anti-DNA antibody. These mice are prone to develop lupus following exposure to exogenous sex hormones. We describe how estrogen and prolactin influence B cell maturation and selection, permitting B cells to mature to immunocompetence. Finally, we discuss the relevance and implications of these data for human disease.

Spotlight on the role of hormonal factors in the emergence of autoreactive B-lymphocytes

Pathogenic autoimmunity requires a combination of inherited and acquired factors. In as much as hormones influence the sexual dimorphism of the immune system, it is possible that they can initiate or accelerate an autoimmune process, and contribute to gender-biased autoimmune disorders. Not only natural hormones, but also endocrine disruptors, such as environmental estrogens, may act in conjunction with other factors to override immune tolerance to self-antigens. In lupus, murine and human studies demonstrate that female sex hormones are implicated in disease pathogenesis. In the B cell compartment, both prolactin and estrogen are immunomodulators that affect maturation, selection and antibody secretion. Their impact may be based on their capacity to allow autoreactive B cells to escape the normal mechanisms of tolerance and to accumulate in sufficient numbers to cause clinically apparent disease. Both hormones lead to the survival and activation of autoreactive B cells, but they skew B cell maturation towards different directions, with prolactin inducing T cell-dependent autoreactive follicular B cells and estrogen eliciting T cell-independent autoreactive marginal zone B cells. Differential modulation of the cytokine milieu by hormones may also affect the development and activation of specific mature B cell subsets. This novel insight suggests that targeted manipulation of these pathways may represent a promising avenue in the treatment of lupus and other gender-biased autoimmune diseases.

Expression of the Epstein-Barr virus nuclear antigen-1 (EBNA-1) in the mouse can elicit the production of anti-dsDNA and anti-Sm antibodies

Systemic lupus erythematosus (SLE) is a chronic autoimmune disorder characterized by the production of anti-nuclear antibodies. The etiology of SLE is unknown, although several viruses including the Epstein-Barr virus (EBV) have been implicated. An increase in the frequency of EBV infection has been observed in SLE patients relative to normal individuals. Some patients with SLE develop antibodies that recognize a proline rich epitope in the ribonucleoprotein Sm B/B that is similar to an epitope in EBNA-1, a major nuclear antigen of EBV. In the present study we have cloned the EBNA-1 gene under the control of the CMV promoter in the vector pcDNA3. We now report for the first time that expression of the entire EBNA-1 protein in the mouse can elicit the production of IgG antibodies to Sm and to double-stranded DNA (dsDNA). Our data suggest that the anti-Sm response arises as a consequence of antigenic cross-reactivity by anti-EBNA-1 antibodies. These results support a possible association between EBV infection and SLE.

B Cell Selection and Susceptibility to Autoimmunity

Autoreactive B cells arise routinely as part of the naive B cell repertoire. The immune system employs several mechanisms in an attempt to silence these autoreactive cells before they achieve immunocompetence. The BCR plays a central role in B cell development, activation, survival, and apoptosis, and thus is a critical component of the regulation of both protective and autoreactive B cells. The strength of signal mediated by the BCR is determined by numerous factors, both B cell intrinsic and B cell extrinsic. Perturbations in the molecules that regulate the BCR signal strength or that activate pathways that engage in cross talk with the BCR-mediated signaling pathways can lead to the aberrant survival and activation of autoreactive B cells. In this review, we will discuss the some newly identified genetic loci and factors that modulate the BCR signal transduction pathway and, therefore, the regulation of autoreactive B cells. We will also provide evidence for a model of autoreactivity in which a reduction in the strength of the BCR signal allows the survival and the modulation of a naive B cell repertoire replete with autoreactivity.

Prolactin as a modulator of B cell function: implications for SLE

Prolactin is not only a lactigenic hormone but also an immunomodulator involved in lymphocyte survival, activation and proliferation. There is increasing data implicating prolactin in autoimmunity, and specifically in SLE. Increased serum prolactin levels have been reported in lupus patients of both genders, and have been associated with accelerated disease expression and early mortality in lupus-prone mice. Furthermore, suppression of prolactin secretion with bromocriptine provides beneficial effects in murine lupus, and perhaps in some SLE patients as well. Treatment with prolactin that causes mild to moderate hyperprolactinemia, similar to that present in SLE patients, breaks tolerance and induces a lupus-like illness in non-spontaneously autoimmune mice with a susceptible genetic background. These immuno stimulatory effects of prolactin are mediated by a decrease in negative selection and the maturation of autoreactive B cells to the follicular subset. Consistent with the fact that follicular B cells are T cell dependent, CD4+ T cells are necessary for the prolactin-mediated break down of B cell tolerance. In mice, the effects of prolactin on the immune system are genetically determined, suggesting that only a subset of SLE patients are likely to have a prolactin-responsive disease. The manipulation of serum prolactin or, even more specifically, follicular B cells that are susceptible to the immuno stimulatory effects of prolactin, may provide novel therapeutic options for those SLE patients with a prolactin-modulated disease.

A second heavy chain permits survival of high affinity autoreactive B cells

Anti-DNA antibody is the serological hallmark of systemic lupus erythematosus (SLE). While antibodies with this specificity may be generated in many individuals, only patients with SLE fail to regulate them effectively. We have demonstrated previously that in non-autoimmune mice transgenic for the heavy chain of the R4A-gamma2b anti-DNA antibody, the existence of high affinity, IgG2b dsDNA binding B cells is tightly correlated with the co-expression of endogenous IgM heavy chain. These cells are anergic. In contrast, low affinity IgG2b dsDNA binding B cells do not express an endogenous heavy chain and represent a population of immunocompetent autoreactive B cells. In order to determine whether the presence of a second heavy chain permits the high affinity autoreactive B cells to escape deletion, the R4A-gamma2b mouse was mated to a strain with a targeted deletion of the transmembrane portion of the mu heavy chain, muMT mice, to produce R4A-gamma2b/muKO mice. Serum titers of anti-DNA antibodies were negligible in both R4A-gamma2b and R4A-gamma2b/muKO mice. In R4A-gamma2b/muKO mice, however, LPS was able to activate a DNA-reactive population although an LPS inducible DNA-reactive population. Light chain gene usage in transgene expressing B cells from R4A-gamma2b/muKO mice was similar to that of the previously defined low affinity anti-DNA B cells that escape tolerance. These data suggest a requirement for a second heavy chain for the survival of this anergic B cell subset.

Pathogenic profiles and molecular signatures of antinuclear autoantibodies rescued from NZM2410 lupus mice

Two outstanding questions concerning antinuclear antibodies (ANAs) in lupus involve their pathogenic potential and their molecular signatures. To address these questions, a panel of 56 antinuclear and 47 nonnuclear binding monoclonal antibodies was rescued from four seropositive NZM2410 lupus mice. The monoclonals varied in their reactivity to nucleosomes, ssDNA, dsDNA, and glomerular substrate. A large fraction of the antibodies demonstrated apparent polyreactivity (to DNA, histones, and glomerular antigens) due to bound, DNase-1 sensitive nuclear antigenic bridges. Although nephrophilic immunoglobulin (Ig) M and IgG antibodies were the most pathogenic, the dsDNA-binding antibodies were modestly so; in contrast, antinucleosome antibodies were clearly not pathogenic. Compared with the nonnuclear antigen-binding monoclonal antibodies rescued from the same mice, ANAs exhibited increased utilization of VH5/7183 genes and highly cationic heavy chain (HC) CDR3 regions. Most intriguingly, the CDR3 regions of the ANAs exhibited alternating arginine/lysine peaks at H96, H98, and H100, with neutral troughs at H95, H97, and H99. To summarize, glomerular-binding anti-dsDNA antibodies appear to be the most pathogenic variety of lupus autoantibodies. The presence of an alternating charge pattern in their HC CDR3 regions appears to be a prominent hallmark of ANAs.

Autoreactive responses to an environmental factor. 2. Phthalate-induced anti-DNA specificity is downregulated by autoreactive cytotoxic T cells

An environmental factor (phthalate) was shown, in our previous study, to induce serum anti-DNA responses in BALB/c, NZB and lupus-prone NZB/W F1 mice. Out of such anti-phthalate responses, cross-reactive populations were identified that strongly bind phthalate, DNA, or both. A phthalate-specific BALB/c monoclonal antibody, 2C3-Ig (gamma1,kappa), showed considerable affinity for DNA and had extensive sequence homology with the heavy and light chain variable regions of a known anti-DNA immunoglobulin, BV04-01, from lupus-prone NZB/W F1 mice. This study was initiated to address how BALB/c mice, but not NZB/W F1 mice, are protected from these adverse autoreactive B cells. Using 2C3 hybridoma cells as the prototype autoreactive BALB/c B cell, we determined whether its DNA-binding monoclonal antibody would induce any regulatory cell-mediated immune responses. Synthetic idiopeptides corresponding to the heavy and light chain variable regions of 2C3-Ig were found to be effective at inducing specific effector cells in BALB/c mice, but not in lupus-prone F1 mice. The splenocytes from BALB/c mice incubated in vitro with the idiopeptides, particularly the complementarity-determining region 1 (VL1) of the 2C3-Ig light chain, showed significant proliferative and cytolytic responses. A CD8+ cytotoxic T-lymphocyte (CTL) response was elicited that recognized the VL1 peptide presented by the Kd allele, and affected the growth of 2C3 cells. In vivo depletion of CD8+ T cells in BALB/c mice significantly decreased this CTL activity but increased the anti-DNA humoral response. These results suggest that autoreactive CTLs are induced in non-autoimmune prone mice as a mechanism to downregulate self-reactive B cells.

Autoreactive responses to an environmental factor: 1. phthalate induces antibodies exhibiting anti-DNA specificity

Environmental factors have been implicated in the induction of autoimmune disorders. We report here that a common chemical, phthalate, used widely in synthetic polymers and cosmetics induces serum anti-self DNA antibodies in BALB/c, NZB and autoimmune-prone NZB/W F1 mice. The latter group experiences a high mortality, and significantly higher anti-DNA antibody levels along with nephritis and other histopathologic changes in kidney. Comparison of amino acid sequences of an anti-phthalate BALB/c B-cell hybrid, 2C3 with the known database at the National Center for Biotechnology Information reveals a striking homology between the variable regions of 2C3-Ig (gamma1, kappa) and an anti-DNA antibody, BV04-01 (gamma2b,kappa) isolated from the lupus-prone NZB/W F1 mice. The homology is 98% for kappa light chain and 70% for gamma heavy chain. Like 2C3-Ig, BV04-01 also has specificity for d(pT)4. Furthermore, the light chains of both 2C3-Ig and BV04-01 are products of Vkappa1 gene. To understand the nature of anti-phthalate responses in general, hybridomas generated from phthalate-keyhole limpet haemocyanin-primed BALB/c splenocytes were characterized. The study identifies cross-reactive populations that strongly bind phthalate, DNA, or both. Of the 14 hybridomas evaluated, six express the same Vkappa1 gene-derived light chain as 2C3, and bind both phthalate and ds and ss-DNA. They specifically recognize the oligonucleotides, d(pT)4, and d(pT)10. Additionally, when antisera raised against idiopeptides corresponding to 2C3-Ig hypervariable regions are allowed to react with 2C3-Ig, their binding is blocked specifically by both d(pT)4 and phthalate. This study clearly demonstrates that phthalate exposure leads to activation of a significant number of autoreactive B-cells, with the consequence of a significant pathogenic progression in susceptible NZB/W F1 mice but not in non-autoimmune-prone BALB/c.

Transgenic models of tolerance and autoimmunity: with special reference to systemic lupus erythematosus

Transgenic and knockout mouse carrying rearranged antigen-receptor genes have been invaluable for the elucidation of basic mechanisms in autoimmunity and have contributed new models of human autoimmune diseases. Several examples of transgenic models expressing rearranged immunoglobulin genes have been described. These models have provided a window into the events involved in this process, allowing the development and fate of self-reactive lymphocytes to be followed in vivo. In the B cell lineage, as in T cells, self-reactive cells have been found to undergo several distinct fates in vivo: they can be physically eliminated, functionally inactivated, or they can persist unchanged or become activated. Nevertheless the precise understanding of the molecular events leading to lymphocyte deletion, anergy or activation remains a challenge.

Identification of an antigen-specific B cell population

The difficulty in characterizing antigen-specific B cells that arise in the native B cell repertoire has been a formidable obstacle to understanding both protective and pathogenic antibody responses. We have developed a tetramer-based technique for identifying antigen-specific B cells. Biotin-labeled antigen is made tetrameric by interaction with streptavidin. The enhanced avidity of this antigenic compound for the B cell membrane permits the visualization, characterization and isolation of antigen-specific B cells.

Prolactin modulates the naive B cell repertoire

Prolactin is a peptide hormone produced by the anterior pituitary gland that is critical in lactation. Prolactin can also be produced by lymphocytes, and both B and T cells express prolactin receptors. These findings have suggested that prolactin has immunomodulatory functions. Studies in spontaneously autoimmune hosts have demonstrated a role for prolactin in augmenting autoreactivity. We chose to analyze prolactin effects on anti-DNA B cells in nonspontaneously autoimmune female BALB/c mice transgenic for the heavy chain of an anti-DNA antibody. Treatment with prolactin for 4 weeks induced a lupus-like phenotype with an increased number of transgene-expressing B cells, elevated serum anti-DNA antibody titers, and glomerular immunoglobulin deposits. Prolactin caused a decrease in the population of transitional B cells and an increase in mature follicular and marginal zone B cells. The DNA-reactive B cells had a follicular cell phenotype. Anti-DNA hybridomas demonstrated that prolactin alters selection of the naive B cell repertoire. The expansion and activation of anti-DNA B cells in prolactin-treated R4A-gamma2b BALB/c mice was dependent on the presence of CD4(+) T cells. Finally, treatment with prolactin was unable to break tolerance in R4A-gamma2b transgenic C57Bl/6 mice, suggesting that responsiveness of the immune system to prolactin is genetically determined.

Persistence of partially functional double-stranded (ds) DNA binding B cells in mice transgenic for the IgM heavy chain of an anti-dsDNA antibody

One mechanism by which anti-double stranded (ds) DNA B cells are regulated is anergy. Multiple phenotypes have been attributed to anergic B cells in various transgenic models. Differences in the nature of the antigen and in the avidity of antigen-antibody interactions may account for these variations in phenotype. In the present study we describe a population of dsDNA binding B cells that display many of the features of anergic B cells, but have characteristics which suggest they are partially functional as well. These B cells do not spontaneously secrete antibody nor can they be induced to secrete antibody following receptor cross-linking in vitro. Furthermore, they display an immature phenotype and have a shortened lifespan, characteristic of anergic B cells. However, they can be induced to secrete anti-dsDNA antibody following activation with T cell-derived factors as well as with lipopolysaccharide (LPS) and they can be recovered by somatic cell hybridization even in the absence of LPS stimulation prior to fusion. These results suggest that antigen receptor signaling can be uncoupled from signaling induced by T cell-derived factors or LPS and that this may be a mechanism for maintaining tolerance. This may have protective advantages because it may enable B cells to be down-regulated in response to autoantigen yet be available for recruitment in an inflammatory response.

The immune tolerance network and rheumatic disease: immune tolerance comes to the clinic

The development of effective, new, biologically based therapies for RA has created real excitement and justifiable optimism in recent years among rheumatologists and among patients with rheumatic diseases. Recent advances in our understanding of the mechanisms of immune activation and immune tolerance provide further cause for optimism. Against this background, the establishment of the ITN is an important step. However, significant hurdles remain to be cleared. First, despite dramatic scientific progress, restoration of immune tolerance in the face of an established autoimmune response is still an elusive goal, even in the laboratory. Not only does the ITN face this fundamental scientific challenge, but it also faces daunting practical and political challenges. For example, can the ITN influence the research agenda of the pharmaceutical and biotechnology industries? This question and other important questions will only be answered as the ITN matures. Autoimmune disease, although individually uncommon, affects more than 2% of Americans. The rheumatologist is especially aware of the devastating potential of autoimmune diseases. If the ITN succeeds in linking basic research into the mechanisms of autoimmunity with clinical trials of promising new therapies, it can be expected to play a critical role in advancing the practice of clinical rheumatology.

Natural autoreactive B cells in transgenic mice reproduce an apparent paradox to the clonal tolerance theory

Naturally occurring autoreactive B cells are thought to be physically eliminated or rendered functionally silent through different mechanisms of tolerance. However, multireactive low affinity natural autoantibody-producing B cells seem to escape these mechanisms in normal adults and could constitute the B cell pool from which pathological autoantibodies can emerge. To analyze this apparent paradox to the clonal tolerance theory, we have made two transgenic mouse lines (mu(k), mudelta(k)) producing a natural low affinity multireactive human autoantibody. These models enable us to test both the central tolerance mechanisms (reactivity with single-stranded DNA) and the peripheral tolerance mechanisms after Ag administration. Not only are the multireactive B cells not deleted in the bone marrow, they circulate and remain in the periphery even after the prolonged administration of Ag, the presence of membrane IgD increasing the number of mature autoreactive B cells. Self-reactive B cells are shown to be autoantigen ignorant both in vivo and in vitro, but they are not anergic because they can be easily activated through both B cell receptor-dependent and -independent pathways. Thus, these mouse lines reproduce an apparent paradox to the clonal tolerance theory meriting further investigation of the biological significance of this phenomenon.

Virus-induced maturation and activation of autoreactive memory B cells

We have examined B cell populations that participate in distinct phases of the immune response to the influenza virus A/PR/8/34 hemagglutinin (HA) for their susceptibility to negative selection in mice that express the HA as a neo-self-antigen (HA104 mice). We demonstrated previously that specificity for the neo-self-HA causes a population of immunoglobulin G antibody-secreting cells, which dominate the primary response to virus immunization in BALB/c mice, to be negatively selected in HA104 mice. We find here that in contrast to these primary response B cells, HA-specific memory response B cells developed equivalently in HA104 and nontransgenic (BALB/c) mice. Indeed, there was no indication that HA-specific B cells were negatively selected during memory formation in influenza virus-immunized HA104 mice, even though the neo-self-HA can be recognized by memory B cells. Furthermore, HA-specific autoantibodies were induced in the absence of virus immunization by mating HA104 mice with mice transgenic for a CD4(+) HA-specific T cell receptor. These findings indicate that specificity for a self-antigen does not prevent the maturation of autoreactive B cells in the germinal center pathway. Rather, the availability of CD4(+) T cell help may play a crucial role in regulating autoantibody responses to the HA in HA104 mice.

Bromocriptine restores tolerance in estrogen-treated mice

Estrogen can modulate autoimmunity in certain models of systemic lupus erythematosus. Recently, we have shown that it can mediate survival and activation of anti-DNA B cells in a mouse transgenic for the heavy chain of a pathogenic anti-DNA antibody. To identify whether estrogen effects reflect increased prolactin secretion, we characterized B-cell autoreactivity in transgenic mice given both bromocriptine (an inhibitor of prolactin secretion) and estradiol. Treatment of mice with estradiol plus bromocriptine led to reduced titers of anti-DNA antibodies and diminished IgG deposition in kidneys compared with treatment with estradiol alone. However, mice treated with estradiol plus bromocriptine showed an expansion of transgene-expressing B cells and enhanced Bcl-2 expression, similar to those of estradiol-treated mice. We identified anergic high-affinity anti-DNA B cells in mice treated with estradiol plus bromocriptine, and we showed by molecular analysis of anti-DNA hybridomas that their B cells derive from a naive repertoire. Thus, the estradiol-induced breakdown in B-cell tolerance can be abrogated by bromocriptine, which induces anergy in the high-affinity DNA-reactive B cells. These studies demonstrate that some of the effects of estrogen on naive autoreactive B cells require the presence of prolactin and, thus, suggest potential therapeutic interventions in lupus.

Anti-DNA and autoantibodies

Systemic lupus erythematosus is a prototypic autoimmune disease characterized by antinuclear antibodies (ANAs), including pathogenic specificities to DNA. As shown by recent research, ANA production is a genetically determined process in which self antigens drive B and T cells that have escaped the normal mechanisms of tolerance. Although antibodies can bind isolated protein or nucleic acid species, the in vivo driving antigens likely exist as complexes that have been released from apoptotic cells. The clinical measurement of ANAs, although valuable in assessing diagnosis and prognosis, must nevertheless be interpreted with caution because ANAs, despite their disease associations, can occur in healthy individuals.

Estrogen up-regulates Bcl-2 and blocks tolerance induction of naive B cells

Sex hormones are presumed to contribute to sexual dimorphism in the immune system. Estrogen, in particular, has been suggested to predispose women to systemic lupus erythematosus. We report here that estradiol (E(2)) can break B cell tolerance and induce a lupus-like phenotype in nonautoimmune mice transgenic for the heavy chain of a pathogenic anti-DNA antibody. E(2) treatment resulted in a rise in anti-DNA serum titers and in Ig deposition in renal glomeruli. ELISPOT analysis confirmed a significant increase in the number of high-affinity anti-DNA antibody-secreting B cells in the spleens of E(2)-treated mice. Hybridomas generated from E(2)-treated mice express high-affinity, unmutated anti-DNA antibodies, indicating that naive B cells that are normally deleted or anergized are rescued from tolerance induction. Finally, immunohistochemical studies revealed increased Bcl-2 expression in splenic B cells of E(2)-treated mice. These data demonstrate that estrogen interferes with tolerance induction of naive autoreactive B cells and that the presence of these B cells in the periphery is associated with up-regulation of Bcl-2.

Bcl-2 leads to expression of anti-DNA B cells but no nephritis: a model for a clinical subset

Transgenic mice expressing anti-DNA antibodies have been extensively studied as a model for understanding B cell regulation in systemic lupus erythematosus (SLE). BALB/c mice transgenic for the R4A-gamma2b heavy chain of an anti-double-stranded DNA (dsDNA) antibody produce two populations of high-affinity anti-dsDNA B cells, one deleted, and the other anergized. We generated double-transgenic BALB / c mice expressing both the R4A-gamma2b heavy chain and the anti-apoptotic bcl-2 gene in the B cell compartment to study whether bcl-2 overexpression differentially affected anergic and deleted B cells. The double-transgenic mice (R4A/bcl-2) express elevated serum titers of both high- and low-affinity anti-dsDNA antibodies and display rescue of autoreactive B cells that are normally either deleted or anergized. Despite the presence of anti-dsDNA antibodies in their serum, R4A/bcl-2-transgenic mice do not develop nephritis, demonstrating that overexpression of bcl-2 is not by itself sufficient to allow disease progression. This phenotype resembles that of some SLE patients who have high titers of anti-DNA antibodies without nephritis.