Chronic graft-versus-host in Ig knockin transgenic mice abrogates B cell tolerance in anti-double-stranded DNA B cells

Overexpressed CD44 is associated with B-cell activation via the HA-CD44-AIM2 pathway in lupus B cells

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by aberrant development of B cells and excess production of autoantibodies. Our team previously reported that absent in melanoma 2 (AIM2) regulates B-cell differentiation via the Bcl-6-Blimp-1 axis. Notably, in keyhole limpet hemocyanin (KLH)-immunized CD19creAim2f/f mice, the frequency of CD19+CD44+ B cells was decreased, accompanied by a weakened KLH response, indicating that AIM2 deficiency suppressed the antigen-induced B-cell immune response by downregulating the expression of CD44. CD44, a surface marker of T-cell activation and memory, was overexpressed in T cells of SLE patients, but its roles and mechanism in B cells have not been elucidated. In the current work, we revealed that CD44 expression was upregulated in the B cells of SLE patients and MRL/lpr mice, accompanied by elevated AIM2 expression in CD19+CD44+ B-cell subsets, and that its ligand hyaluronan (HA) was also abnormally increased in the serum of SLE patients. Notably, the extrafollicular (EF) region serves as an important site of B-cell activation and differentiation separate from the germinal center, while CD44 expression is concentrated in EF B cells. In addition, in vitro experiments demonstrated that the HA-CD44 interaction stimulated B-cell activation and upregulated the expression of AIM2 and the transcription factor STAT3. Either blocking CD44, knocking down AIM2 expression or suppressing the activity of STAT3 in B cells suppressed B-cell activation and proliferation. Moreover, blocking CD44 downregulated the expression of STAT3 and AIM2, while suppressing the activity of STAT3 decreased the expression of CD44 and AIM2. In summary, overexpressed CD44 in B cells might participate in B-cell activation and proliferation in the EF region via the HA-CD44-AIM2 pathway, providing potential targets for SLE therapy.

Role of inhibitory B cell co-receptors in B cell self-tolerance to non-protein antigens

Antibodies to non-protein antigens such as nucleic acids, polysaccharides, and glycolipids play important roles in both host defense against microbes and development of autoimmune diseases. Although non-protein antigens are not recognized by T cells, antibody production to non-protein antigens involve T cell-independent mechanisms such as signaling through TLR7 and TLR9 in antibody production to nucleic acids. Although self-reactive B cells are tolerized by various mechanisms including deletion, anergy, and receptor editing, T cell tolerance is also crucial in self-tolerance of B cells to protein self-antigen because self-reactive T cells induce autoantibody production to these self-antigens. However, presence of T cell-independent mechanism suggests that T cell tolerance is not able to maintain B cell tolerance to non-protein self-antigens. Lines of evidence suggest that B cell response to non-protein self-antigens such as nucleic acids and gangliosides, sialic acid-containing glycolipids, are suppressed by inhibitory B cell co-receptors CD72 and Siglec-G, respectively. These inhibitory co-receptors recognize non-protein self-antigens and suppress BCR signaling induced by these antigens, thereby inhibiting B cell response to these self-antigens. Inhibitory B cell co-receptors appear to be involved in B cell self-tolerance to non-protein self-antigens that can activate B cells by T cell-independent mechanisms.

The Complex Association of FcγRIIb With Autoimmune Susceptibility

FcγRIIb is the only inhibitory Fc receptor and controls many aspects of immune and inflammatory responses. The observation 19 years ago that Fc γ RIIb ^-/- mice generated by gene targeting in 129 derived ES cells developed severe lupus like disease when backcrossed more than 7 generations into C57BL/6 background initiated extensive research on the functional understanding of this strong autoimmune phenotype. The genomic region in the distal part of Chr1 both in human and mice in which the Fc γ R gene cluster is located shows a high level of complexity in relation to the susceptibility to SLE. Specific haplotypes of closely linked genes including the Fc γ RIIb and Slamf genes are associated with increased susceptibility to SLE both in mice and human. Using forward and reverse genetic approaches including in human GWAS and in mice congenic strains, KO mice (germline and cell type specific, on different genetic background), knockin mice, overexpressing transgenic mice combined with immunological models such as adoptive transfer of B cells from Ig transgenic mice the involved genes and the causal mutations and their associated functional alterations were analyzed. In this review the results of this 19 years extensive research are discussed with a focus on (genetically modified) mouse models.

Nur77 Links Chronic Antigen Stimulation to B Cell Tolerance by Restricting the Survival of Self-Reactive B Cells in the Periphery

Nur77 (Nr4a1) belongs to a small family of orphan nuclear receptors that are rapidly induced by BCR stimulation, yet little is known about its function in B cells. We have previously characterized a reporter of Nr4a1 transcription, Nur77-eGFP, in which GFP expression faithfully detects Ag encounter by B cells in vitro and in vivo. In this study, we report that Nur77 expression correlates with the degree of self-reactivity, counterselection, and anergy among individual B cell clones from two distinct BCR transgenic mouse models but is dispensable for all of these tolerance mechanisms. However, we identify a role for Nur77 in restraining survival of self-reactive B cells in the periphery under conditions of competition for a limited supply of the survival factor BAFF. We find that Nur77 deficiency results in the progressive accumulation of self-reactive B cells in the mature repertoire with age and is sufficient to break B cell tolerance in V_H3H9 H chain transgenic mice. We thus propose that Nur77 is upregulated in self-reactive B cells in response to chronic Ag stimulation and selectively restricts the survival of these cells, gradually pruning self-reactivity from the mature repertoire to impose a novel layer of peripheral B cell tolerance.

Human Ig knockin mice to study the development and regulation of HIV-1 broadly neutralizing antibodies

A major challenge for HIV-1 vaccine research is developing a successful immunization approach for inducing broadly neutralizing antibodies (bnAbs). A key shortcoming in meeting this challenge has been the lack of animal models capable of identifying impediments limiting bnAb induction and ranking vaccine strategies for their ability to promote bnAb development. Since 2010, immunoglobulin knockin (KI) technology, involving inserting functional rearranged human variable exons into the mouse IgH and IgL loci has been used to express bnAbs in mice. This approach has allowed immune tolerance mechanisms limiting bnAb production to be elucidated and strategies to overcome such limitations to be evaluated. From these studies, along with the wealth of knowledge afforded by analyses of recombinant Ig-based bnAb structures, it became apparent that key functional features of bnAbs often are problematic for their elicitation in mice by classic vaccine paradigms, necessitating more iterative testing of new vaccine concepts. In this regard, bnAb KI models expressing deduced precursor V(D)J rearrangements of mature bnAbs or unrearranged germline V, D, J segments (that can be assembled into variable region exons that encode bnAb precursors), have been engineered to evaluate novel immunogens/regimens for effectiveness in driving bnAb responses. One promising approach emerging from such studies is the ability of sequentially administered, modified immunogens (designed to bind progressively more mature bnAb precursors) to initiate affinity maturation. Here, we review insights gained from bnAb KI studies regarding the regulation and induction of bnAbs, and discuss new Ig KI methodologies to manipulate the production and/or expression of bnAbs in vivo, to further facilitate vaccine-guided bnAb induction studies.

Humanized Immunoglobulin Mice: Models for HIV Vaccine Testing and Studying the Broadly Neutralizing Antibody Problem

A vaccine that can effectively prevent HIV-1 transmission remains paramount to ending the HIV pandemic, but to do so, will likely need to induce broadly neutralizing antibody (bnAb) responses. A major technical hurdle toward achieving this goal has been a shortage of animal models with the ability to systematically pinpoint roadblocks to bnAb induction and to rank vaccine strategies based on their ability to stimulate bnAb development. Over the past 6 years, immunoglobulin (Ig) knock-in (KI) technology has been leveraged to express bnAbs in mice, an approach that has enabled elucidation of various B-cell tolerance mechanisms limiting bnAb production and evaluation of strategies to circumvent such processes. From these studies, in conjunction with the wealth of information recently obtained regarding the evolutionary pathways and paratopes/epitopes of multiple bnAbs, it has become clear that the very features of bnAbs desired for their function will be problematic to elicit by traditional vaccine paradigms, necessitating more iterative testing of new vaccine concepts. To meet this need, novel bnAb KI models have now been engineered to express either inferred prerearranged V(D)J exons (or unrearranged germline V, D, or J segments that can be assembled into functional rearranged V(D)J exons) encoding predecessors of mature bnAbs. One encouraging approach that has materialized from studies using such newer models is sequential administration of immunogens designed to bind progressively more mature bnAb predecessors. In this review, insights into the regulation and induction of bnAbs based on the use of KI models will be discussed, as will new Ig KI approaches for higher-throughput production and/or altering expression of bnAbs in vivo, so as to further enable vaccine-guided bnAb induction studies.

Predominant role for activation-induced cytidine deaminase in generating IgG anti-nucleosomal antibodies of murine SLE

Serum IgG anti-nuclear antibodies (ANA) directed to complexes of DNA and histones are a hallmark of systemic lupus erythematosus (SLE) and reflect a failure in lymphocyte self-tolerance. A prior study utilizing spontaneously autoimmune B6.Nba2 mice deficient in terminal deoxynucleotidyl transferase (TdT) and with heterozygous deficiencies in Jh and Igk loci underscored the importance of somatic hypermutation (SHM) as a major generator of SLE-associated ANA. This interpretation had to be qualified because of severely limited opportunities for receptor editing and restricted VHCDR3 diversity. Therefore, we performed the converse study using mice that carried functional Tdt genes and wild type Jh and Igk loci but that could not undergo SHM. Analyses of ANA and ANA-producing hybridomas from B6.Nba2 Aicda(-/-) mice revealed that few animals produced high titers of the prototypical ANA directed to complexes of histones and DNA, that this response was delayed and that those cells that did produce such antibody exhibited limited clonal expansion, unusual Jk use and only infrequent dual receptor expression. This, together with the additional finding of an intrinsic propensity for SHM to generate Arg codons selectively in CDRs, reinforce the view that most IgG autoimmune clones producing prototypical anti-nucleosome antibodies in wild type mice are created by SHM.

Induced murine models of systemic lupus erythematosus

Induced mouse models of systemic lupus erythematosus (SLE) have been developed to complement the spontaneous models. This chapter describes the methods used in the pristane-induced model and the chronic graft-versus-host disease (cGVHD) model, both of which have been extensively used. We will also outline the specific mechanisms of systemic autoimmunity that can be best characterized using each of these models.

Somatic mutagenesis in autoimmunity

Our laboratory investigates systemic autoimmune disease in the context of mouse models of systemic lupus erythematosus (SLE). SLE is associated with high titers of serum autoantibodies of the IgG class that are predominantly directed against nuclear antigens, with pathological manifestations that are considered by many to be characteristic of an immune-complex mediated disease. In this review, we focus on the known and potential roles of somatic mutagenesis in SLE. We will argue that anti-nuclear antibodies (ANA) arise predominantly from nonautoreactive B cells that are transformed into autoreactive cells by the process of somatic hypermutation (SHM), which is normally associated with affinity maturation during the germinal center reaction. We will also discuss the role of SHM in creating antigenic peptides in the V region of the B cell receptor (BCR) and its potential to open an avenue of unregulated T cell help to autoreactive B cells. Finally, we will end this review with new experimental evidence suggesting that spontaneous somatic mutagenesis of genes that regulate B cell survival and activation is a rate-limiting causative factor in the development of ANA.

T cells, murine chronic graft-versus-host disease and autoimmunity

The chronic graft-versus-host disease (cGVHD) in mice is characterized by the production of autoantibodies and immunopathology characteristic of systemic lupus erythematosus (lupus). The basic pathogenesis involves the cognate recognition of foreign MHC class II of host B cells by alloreactive CD4 T cells from the donor. CD4 T cells of the host are also necessary for the full maturation of host B cells before the transfer of donor T cells. CD8 T cells play critical roles as well. Donor CD8 T cells that are highly cytotoxic can ablate or prevent the lupus syndrome, in part by killing recipient B cells. Host CD8 T cells can reciprocally downregulate donor CD8 T cells, and thus prevent them from suppressing the autoimmune process. Thus, when the donor inoculum contains both CD4 T cells and CD8 T cells, the resultant syndrome depends on the balance of activities of these various cell populations. For example, in one cGVHD model (DBA/2(C57BL/6xDBA/2)F1, the disease is more severe in females, as it is in several of the spontaneous mouse models of lupus, as well as in human disease. The mechanism of this female skewing of disease appears to depend on the relative inability of CD8 cells of the female host to downregulate the donor CD4 T cells that drive the autoantibody response. In general, then, the abnormal CD4 T cell help and the modulating roles of CD8 T cells seen in cGVHD parallel the participation of T cells in genetic lupus in mice and human lupus, although these spontaneous syndromes are presumably not driven by overt alloreactivity.

The pathophysiology of chronic graft-versus-host disease: the unveiling of an enigma

Chronic graft-versus-host disease (CGVHD) is one of the most significant complications of long-term survivors after allogeneic hematopoietic stem cell transplantation (allo-HSCT). CGVHD may have protean manifestations and can pose unique diagnostic and therapeutic challenges. New recommendations that emphasize the importance of qualitative differences, as opposed to time of onset after HSCT, are now being used to standardize the diagnosis and clinical assessment of CGVHD, but they require validation. During the past 3 decades, experimental studies and clinical observations have elucidated the mechanisms of acute GVHD, but its biology is much less well-understood. Experimental studies have generated at least 4 theories to explain the pathophysiology of CGVHD: (1) thymic damage and the defective negative selection of T cells, (2) regulatory T cell deficiencies, (3) auto-antibody production by aberrant B cells, and (4) the formation of profibrotic lesions. Mouse models have provided important insights into the pathophysiology of CGVHD, and these have helped improve clinical outcomes following allo-HSCT, but no animal model fully replicates all of the features of CGVHD in humans. In this article, recent clinical changes, the pathogenesis of CGHVD, the cellular and cytokine networks implicated in its pathogenesis, and the animal models used to devise strategies to prevent and treat CGVHD are reviewed.

B cell receptor editing in tolerance and autoimmunity

Receptor editing is the process of ongoing antibody gene rearrangement in a lymphocyte that already has a functional antigen receptor. The expression of a functional antigen receptor will normally terminate further rearrangement (allelic exclusion). However, lymphocytes with autoreactive receptors have a chance at escaping negative regulation by "editing" the specificities of their receptors with additional antibody gene rearrangements. As such, editing complicates the Clonal Selection Hypothesis because edited cells are not simply endowed for life with a single, invariant antigen receptor. Furthermore, if the initial immunoglobulin gene is not inactivated during the editing process, allelic exclusion is violated and the B cell can exhibit two specificities. Here, we describe the discovery of editing, the pathways of receptor editing at the heavy (H) and light (L) chain loci, and current evidence regarding how and where editing happens and what effects it has on the antibody repertoire.

Disrupted Mer receptor tyrosine kinase expression leads to enhanced MZ B-cell responses

Control of lymphocyte homeostasis is essential to ensure efficient immune responses and to prevent autoimmunity. Splenic marginal zone B cells are important producers of autoantibodies, and are subject to stringent tolerance mechanisms to prevent autoimmunity. In this paper, we explore the role of the Mer tyrosine kinase (Mertk) in regulating autoreactive B cells. This receptor tyrosine kinase serves to bind apoptotic cells, to mediate their phagocytosis, and to regulate subsequent cytokine production. Mice lacking Mertk suffer from impaired apoptotic cell clearance and develop a lupus-like autoimmune syndrome. Here we show that such Mertk-KO mice have expanded numbers of splenic marginal zone B cells. Mertk-KO mice bearing a DNA-specific immunoglobulin heavy-chain transgene (3H9) produced anti-DNA antibodies that appeared to be secreted largely by marginal zone B cells. Finally, Mertk-KO mice developed greater antibody responses after NP-Ficoll immunization than their B6 counterparts. Taken together, our data show that Mertk has a major effect on the development of the marginal zone B-cell compartment. Mertk is also important in establishing DNA-specific B-cell tolerance in 3H9 anti-DNA transgenic mice.

A new site-directed transgenic rheumatoid factor mouse model demonstrates extrafollicular class switch and plasmablast formation

The AM14 rheumatoid factor (RF) transgenic (Tg) mouse has been valuable for studying how self-reactive B cells are regulated beyond central tolerance, because they remain ignorant in normal mice. AM14 B-cell activation can be studied on autoimmune-prone strains or by inducing activation with IgG2a anti-chromatin antibodies (Abs). Despite the utility of conventional Ig-Tg mice, site-directed Ig-Tg (sd-Tg) mice provide a more physiological model for B-cell responses, allowing class switch and somatic hypermutation. We report here the creation of an AM14 sd-Tg mouse and describe its phenotype on both normal and autoimmune-prone backgrounds. AM14 sd-Tg B cells develop normally but remain unactivated in the BALB/c background, even after significant aging. In contrast, in the autoimmune-prone strain MRL/lpr, AM14 sd-Tg B cells become activated and secrete large amounts of IgG RF Ab into the serum. Class-switched Ab-forming cells were found in the spleen and bone marrow. IgG RF plasmablasts were also observed in extrafollicular clusters in the spleens of aged AM14 sd-Tg MRL/lpr mice. Class switch and Ab secretion were observed additionally in AM14 sd-Tg BALB/c B cells activated in vivo using IgG2a anti-chromatin Abs. Development of IgG auto-Abs is a hallmark of severe autoimmunity and is related to pathogenesis. Using the AM14 sd-Tg, we now show that switched auto-Ab-forming cells develop robustly outside germinal centers, further confirming the extrafollicular expression of activation induced cytidine deaminase (AID). This model will allow more physiological studies of B-cell biology in the future, including memory responses marked by class switch.

ICOS expression by effector T cells influences the ability of regulatory T cells to inhibit anti-chromatin B cell responses in recipient mice

T regulatory cells are critical for the prevention of autoimmunity. Specifically, Treg cells can control anti-chromatin antibody production in vivo, and this correlates with decreased ICOS expression on CD4(+) T helper cells. Here we test the significance of high ICOS expression by T effector cells, firstly in terms of the anti-chromatin B cell response, and secondly on the ability of Treg cells to suppress T cell help. We bred CD4(+) T cell receptor transgenic mice with mice that carry the Roquin(san/san) mutation. The Roquin gene functions to limit ICOS mRNA such that CD4 T cells from mutant mice express elevated ICOS. Using an in vivo model, TS1.Roquin(san/san) Th cells were compared with wild-type TS1 Th cells with regard to their ability to help anti-chromatin B cells in the presence or absence of Treg cells. Both TS1 and TS1.Roquin(san/san) Th cells induced anti-chromatin IgM(a) antibodies, but the TS1.Roquin(san/san) Th cells resulted in the recovery of more class-switched and germinal center B cells. Neither source of Th cells were capable of inducing long-lived autoantibodies. Treg cells completely suppressed anti-chromatin IgM(a) antibody production and reduced anti-chromatin B cell recovery induced by TS1 Th cells. Importantly, this suppression was less effective when TS1.Roquin(san/san) Th cells were used. Thus, high ICOS levels on effector T cells results in autoimmunity by augmenting the autoreactive B cell response and by dampening the effect of Treg cell suppression.

Spontaneous autoimmunity in mice that carry an IghV partial transgene: a required arginine in VHCDR3

We describe a unique spontaneous mouse model of autoimmunity, which occurs on a non-autoimmune-prone SWR genetic background. In this model, SWR mice carry an IghV partial transgene (pTg) encoding only the heavy chain variable domain of an antibody directed against chromatin. Autoimmune disease in pTg mice was manifested by some of the features of systemic lupus erythematosus (SLE), including the presence of serum anti-nuclear antibodies, splenomegaly, skin lesions and a moderate degree of kidney pathology, in various combinations among individuals. Autoimmunity was observed in three independent transgenic lines, but not in three control lines carrying a nearly identical pTg, in which a VHCDR3 codon for Arg was replaced by one for Ser to ablate chromatin reactivity. Various features of disease were often but not always accompanied by anti-chromatin antibodies. Unexpectedly, the anti-chromatin antibodies detected in seropositive animals were not encoded by the pTg. These observations strongly implicate a role for the transgene product in disease initiation but not necessarily for end-state pathology, and they raise the possibility that autoreactive B cells may play a previously unappreciated role in initiating the development of systemic autoimmunity.

T cell-independent spontaneous loss of tolerance by anti-double-stranded DNA B cells in C57BL/6 mice

Systemic lupus erythematosus is characterized by loss of tolerance to DNA and other nuclear Ags. To understand the role of T cells in the breaking of tolerance, an anti-DNA site-specific transgenic model of spontaneous lupus, B6x56R, was studied. T cells were eliminated by crossing B6x56R with CD4(-/)(-) or TCRbeta(-/-)delta(-/-) mice, and the effects on anti-dsDNA serum levels, numbers of anti-dsDNA Ab-secreting cells, and isotypes of anti-dsDNA were analyzed. In addition, the development and activation of B cells in these mice were examined. Surprisingly, the presence of T cells made little difference in the development and character of the serum anti-dsDNA Ab in B6x56R mice. At 1 mo of age, anti-dsDNA Abs were somewhat lower in mice deficient in alphabeta and gammadelta T cells. Levels of Abs later were not affected by T cells, nor was autoantibody class switching. B cell activation was somewhat diminished in T cell-deficient mice. Thus, in the B6 background, the presence of an anti-dsDNA transgene led the production of autoantibodies with a specificity and isotype characteristic of murine systemic lupus erythematosus with little influence from T cells. TLR9 also did not appear to play a role. Although we do not yet understand the mechanism of this failure of immunoregulation, these results suggest that similar processes may influence autoimmunity associated with clinical disease.

Therapeutic potential of CD8+ cytotoxic T lymphocytes in SLE

Recent evidence supports the idea that following a break in tolerance, CD8 cytotoxic T lymphocytes (CTL) may be an important but unrecognized mechanism for limiting expansion of autoreactive B cells. Failure of this mechanism could allow persistence of CD4 T cell driven polyclonal B cell activation resulting in clinical lupus. Although CD8 CTL failure may occur early in disease, work in mice supports the concept that therapeutic CTL enhancement may be both practical and beneficial in lupus. Devising such therapy for humans will first require an understanding of the in vivo mechanisms critical in CTL expansion and down regulation, particularly in the lupus setting which may differ from CTL generation in other clinical settings (e.g. tumors, infections).

Regulation of B cell tolerance by 129-derived chromosome 1 loci in C57BL/6 mice

OBJECTIVE

Systemic lupus erythematosus is a multifactorial disease with a strong genetic component. Previous studies have shown that a 129-derived chromosome 1 interval (Sle16) on the C57BL/6 (B6) background is sufficient to induce humoral autoimmunity. The aim of the present study was to elucidate the mechanisms by which this locus contributes to the loss of peripheral tolerance.

METHODS

Anti-single-stranded DNA (anti-ssDNA)-knockin transgenic mice (V(H)3H9R/Vkappa8R and V(H)3H9R) were crossed with a B6 congenic line named B6.129chr1b that carries the Sle16 locus. A parallel study of a gene-targeted animal, whose mutated gene is located within the 129chr1b interval on chromosome 1, was also performed.

RESULTS

The combination of V(H)3H9R/Vkappa8R with the 129chr1b interval resulted in impaired B cell anergy, and transgenic IgM and IgG anti-ssDNA antibodies were found in the circulation. The presence of IgG2a(a) anti-ssDNA and IgM(a) anti-Sm antibodies in sera indicated that the autoreactive transgenic B cells underwent class switching and epitope spreading. The 129chr1b locus appeared to have a dominant effect, since transgenic antibodies were also detected in mice carrying a single allele. The gene-targeted animals showed a similar phenotype.

CONCLUSION

The presence of a single 129chr1b locus on the B6 background impaired B cell anergy, prevented deletion of anti-DNA transgenic B cells, and induced receptor revision. The findings of this study also emphasize that the autoimmune phenotype observed in mice with targeted genes located on chromosome 1 may simply arise from epistatic interactions between the 129 and B6 parental strains.

Sites and stages of autoreactive B cell activation and regulation

B cells are essential for the development and pathogenesis of both systemic and organ-specific autoimmune diseases. Autoreactive B cells are typically thought of as sources of autoantibody, but their most important pathogenetic roles may be to present autoantigens to T cells and to secrete proinflammatory cytokines. A rate-limiting step in the genesis of autoimmunity then is the activation of autoreactive B cells. Here, mechanisms are discussed that normally prevent such activation and how they break down during disease. Integrating classic work with recent insights, emphasis is placed on efforts to pinpoint the precursor cells for autoantibody-secreting cells and the unique stimuli and pathways by which they are activated.

Accelerated atherosclerosis in ApoE deficient lupus mouse models

The accelerated development of atherosclerosis with increased risk of cardiovascular disease in systemic lupus erythematosus (SLE) patients is not well understood. An appropriate mouse model would greatly help to understand the mechanisms of this association. We have therefore combined the ApoE(-/-) model of atherosclerosis with three different murine models of SLE. We found that induction of cGVH in B6.ApoE(-/-) mice, breeding a Fas null gene onto the B6.ApoE(-/-) mice, and breeding the ApoE(-/-) defect onto MRL/lpr mice all caused a modest increase of atherosclerosis at 24 weeks of age compared to B6.ApoE(-/-) controls. B cells in B6.ApoE(-/-) mice had certain phenotypic differences compared to congenic C57BL/6 mice, as indicated by high expression of MHC II, Fas, CD86, and by increased number of cells bearing marginal zone phenotype. Furthermore, B6ApoE(-/-) mice had significant titers of anti-oxLDL and anti-cardiolipin autoantibodies compared to their B6 counterparts. Our studies also indicate that, following induction of cGVH, marginal zone B cells in B6.ApoE(-/-) are depleted, and there is considerable increase in anti-oxLDL and anti-cardiolipin abs along with secretion of lupus-specific autoantibodies, such as anti-dsDNA and anti-chromatin abs. Histological sections showed that cGVH and/or Fas deficiency could exacerbate atherosclerosis. The production of anti-oxLDL and anti-cardiolipin in ApoE(-/-) mice was also increased. These observations define a connection between induction of lupus-like symptoms and development of severe atherosclerosis in ApoE deficient lupus mouse models.

TWEAK/Fn14 interactions are instrumental in the pathogenesis of nephritis in the chronic graft-versus-host model of systemic lupus erythematosus

TNF-like weak inducer of apoptosis (TWEAK), a member of the TNF superfamily, is a prominent inducer of proinflammatory cytokines in vitro and in vivo. We previously found that kidney cells display the TWEAK receptor Fn14, and that TWEAK stimulation of mesangial cells and podocytes induces a potent proinflammatory response. Several of the cytokines up-regulated in the kidney in response to TWEAK are instrumental in Lupus nephritis; we therefore hypothesized that TWEAK/Fn14 interactions may be important in the cascade(s) leading to renal damage in systemic Lupus erythematosus. In this study, we analyzed the effects of Fn14 deficiency in the chronic graft-vs-host model of SLE, and the benefits of treatment with an anti-TWEAK mAb in this mouse model. We found that anti-nuclear Ab titers were no different between C57BL/6 Fn14 wild-type and deficient mice injected with alloreactive bm12 splenocytes. However, kidney disease was significantly less severe in Fn14 knockout mice. Furthermore, kidney IgG deposition, IL-6, MCP-1, RANTES, and IP-10, as well as macrophage infiltration, were significantly decreased in Fn14-deficient mice with induced lupus. Similarly, mice with induced Lupus treated with an anti-TWEAK neutralizing mAb had significantly diminished kidney expression of IL-6, MCP-1, IL-10, as well as proteinuria, but similar autoantibody titers, as compared with control-treated mice. We conclude that TWEAK is an important mediator of kidney damage that acts by promoting local inflammatory events, but without impacting adaptive immunity in this experimental LN model. Thus, TWEAK blockade may be a novel therapeutic approach to reduce renal damage in SLE.

Mature B Cells Preferentially Lose Tolerance in the Chronic Graft-versus-Host Disease Model of Systemic Lupus Erythematosus

Chronic graft-vs-host (cGVH) disease is a well-characterized systemic lupus erythematosus (SLE) model. Induction of cGVH in anti-DNA H chain knockin (3H9KI) transgenic mice results in specific activation of anti-dsDNA B cells. In this study, we show that B cells from 3H9KI mice were activated by cGVH even when adoptively transferred into irradiated JHT-/- recipients that lack endogenous B cells. This process of activation was reflected by high autoantibody titers and changes in phenotypic markers. We have used this system to characterize the particular B cell subsets that were responsible for secreting autoantibodies during cGVH response. We isolated splenic B cell subsets based on their expression of specific cell surface markers and used them in our adoptive transfer studies. We found that mature B cells were the most vulnerable to the allostimulus and were the major source of autoantibodies compared with immature B cells. The greater susceptibility of mature B cells to become activated and thereby lose tolerance was unanticipated and has implications for maintenance of peripheral tolerance and for the development of autoimmunity. Furthermore, of the mature B cells, marginal zone B cells were particularly responsible for mounting the initial response to the cGVH stimulus. This observation underscores the critical role of marginal zone B cells in activation and production of autoantibodies.

Lupus susceptibility genes may breach tolerance to DNA by impairing receptor editing of nuclear antigen-reactive B cells

An NZM2410-derived lupus susceptibility locus on murine chromosome 4, Sle2(z), has previously been noted to engender generalized B cell hyperactivity. To study how Sle2(z) impacts B cell tolerance, two Ig H chain site-directed transgenes, 3H9 and 56R, with specificity for DNA were backcrossed onto the C57BL/6 background with or without Sle2(z). Interestingly, the presence of the NZM2410 "z" allele of Sle2 on the C57BL/6 background profoundly breached B cell tolerance to DNA, apparently by thwarting receptor editing. Whereas mAbs isolated from the spleens of B6.56R control mice demonstrated significant usage of the endogenous (i.e., nontargeted) H chain locus and evidence of vigorous L chain editing; Abs isolated from B6.Sle2(z).56R spleens were largely composed of the transgenic H chain paired with a spectrum of L chains, predominantly recombined to J(k)1 or J(k)2. In addition, Sle2(z)-bearing B cells adopted divergent phenotypes depending on their Ag specificity. Whereas Sle2(z)-bearing anti-DNA transgenic B cells were skewed toward marginal zone B cells and preplasmablasts, B cells from the same mice that did not express the transgene were skewed toward the B1a phenotype. This work illustrates that genetic loci that confer lupus susceptibility may influence B cell differentiation depending on their Ag specificity and potentially contribute to antinuclear autoantibody formation by infringing upon B cell receptor editing. Taken together with a recent report on Sle1(z), these studies suggest that dysregulated receptor-editing of nuclear Ag-reactive B cells may be a major mechanism through which antinuclear Abs arise in lupus.

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

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

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

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

Modulation of autoimmunity by TLR9 in the chronic graft-vs-host model of systemic lupus erythematosus

Chronic graft-vs-host (cGVH) disease is induced in nonautoimmune mice by the transfer of alloreactive T cells that recognize foreign MHC class II. It closely resembles systemic lupus erythematosus, with antinuclear Abs and immune-mediated nephritis. Recent work has implicated TLRs, particularly TLR9, in the recognition of certain autoantigens in vitro and in vivo. To explore further the role of TLR9 in systemic autoimmunity, we induced cGVH disease in C57BL/6 (B6) mice lacking TLR9, including B6 mice expressing the anti-DNA-encoding IgH transgenes 3H9 or 56R (B6.3H9.TLR9(-/-), B6.56R.TLR9(-/-)). We found that cGVH disease caused breakdown of B cell tolerance to chromatin and DNA in TLR9(-/-) recipients of alloreactive cells, yet that nephritis was less severe and that some autoantibody titers were lower compared with B6-cGVH controls. Spleen lymphocyte analysis showed that cGVH disease strikingly depleted marginal zone B cells in B6 mice, but did not influence T cell subsets in either B6 or B6-TLR9(-/-) hosts. B6.56R.TLR9(-/-) mice had less spontaneous production of autoantibodies than B6.56R mice, but there were no significant differences between B6.56R and B6.56R.TLR9(-/-) postinduction of cGVH disease. Taken together, these results suggested that TLR9 may worsen some aspects of systemic autoimmunity while alleviating others.

Peripheral B cell receptor editing may promote the production of high-affinity autoantibodies in CD22-deficient mice

CD22-deficient mice are characterized by B cell hyperactivity and autoimmunity. We have constructed knock-in CD22-/- mice, expressing an anti-DNA heavy (H) chain (D42), alone or combined with Vkappa1-Jkappa1 or Vkappa8-Jkappa5 light (L) chains. The Ig-targeted mice produced a lupus-like serology that was age- and sex-dependent. High-affinity IgG autoantibodies were largely dependent on the selection of B cells with a particular H/L combination, in which a non-transgenic, endogenous L chain was assembled by secondary rearrangements through the mechanism of receptor editing. Moreover, we present evidence that these secondary rearrangements are very prominent in splenic peripheral B cells. Since CD22 is primarily expressed on the surface of peripheral B cells, we propose a model for the development of a lupus-like autoimmune disease by a combination of peripheral receptor editing and abnormal B cell activation.

Consequences of receptor editing at the lambda locus: multireactivity and light chain secretion

To investigate the manner in which B cells with lambda light (L) chains undergo receptor editing, we have studied hybridoma panels from 56R/kappa-deleted (kdel) mice. 56R/kdel mice only produce four L chains (lambda1, lambda2, lambda3, and lambdaX). They also have a simplified heavy (H) chain repertoire: All B cells start out with a 56R anti-DNA H chain. A few frankly autoreactive 56R lambda1 cells appear to escape into the periphery, but the majority of the peripheral B cell repertoire in 56R/kdel is made up of B cells expressing the 56R H chain with the lambdaX L chain. Surprisingly, 56R lambdaX B cells are multireactive, binding to a variety of self and nonself antigens, including dsDNA (albeit at reduced affinity compared with the other lambda L chains). Another significant population in the 56R/kdel mouse consists of allelically included B cells that express lambdaX along with another L chain. The multireactivity of both 56R lambdaX and 56R lambdaX/lambda1 receptors could contribute to autoimmunity if these B cells were to become activated. Also found among 56R/kdel hybridomas are clones that have inactivated the H chain and secrete only L chains. These clones may represent products of exhaustive rearrangement. Multireactivity, allelic inclusion, and L chain secretion are three consequences of editing at the lambda locus that may predispose toward the development of autoimmunity.

Light chain editing generates polyreactive antibodies in chronic graft-versus-host reaction

The chronic graft-versus-host (cGvH) reaction is a model of induced lupus caused by alloreactive CD4(+) T cells from a Bm-12 mouse in a C57BL/6 recipient. We used this cGvH reaction in C57BL/6 anti-DNA H chain transgenic mice, 56R/B6, to understand the structure, specificity, and origin of the induced autoantibodies (auto-Abs). We found anti-DNA Abs that reacted to several different antigens, such as phosphatidylserine, myelin basic protein, thyroglobulin, histone, insulin, cytochrome C, and beta-galactosidase. This polyreactivity was found for Abs from B cells that expressed the 56R H chain transgene with "editor" L chains that did not completely veto autoreactivity. We suggest that such incomplete editing results in polyreactivity and that incompletely edited polyreactive B cells influence the subsequent expression of pathogenic auto-Abs in disease. We also found B cells that coexpress kappa and lambda L chain. These B cells contributed to the autoimmune response and are possibly in the marginal zone of the spleen.

Development of Functional B Cells in a Line of SCID Mice with Transgenes Coding for Anti-Double-Stranded DNA Antibody

Deletion or inactivation of anti-self (DNA) B cells has been reported in non-autoimmune mice bearing Ig transgenes that code for Abs with specificity for dsDNA or ssDNA. However, we report a case in which anti-dsDNA B cells appear to escape both deletion and inactivation. We show that B cells (B220+IgM+) can develop in non-autoimmune SCID mice bearing two site-directed transgenes, 3H9(56R) and Vkappa8, that together code for an anti-dsDNA Ab. The B cells appear inactive, because the mice (56RVkappa8 SCID mice) generally lack serum Ig. However, 56RVkappa8 SCID mice are able to produce IgG Ab with specificity for dsDNA when they become "leaky" for T cells or are reconstituted with exogenous T cells from B cell-deficient JH-/- donors. Thus, anti-dsDNA B cells that escape deletion in 56RVkappa8 SCID mice appear fully functional and can differentiate, class switch, and give rise to IgG-producing cells in the presence of T cells and self-Ag.

Donor CD4+ T and B cells in transplants induce chronic graft-versus-host disease with autoimmune manifestations

Chronic graft-vs-host disease (GVHD) is a major cause of morbidity and mortality of long-term survivors of allogeneic hemato-poietic cell transplantation (HCT). Chronic GVHD can have features of an autoimmune collagen vascular disease with clinical manifestations similar to autoimmune scleroderma and systemic lupus erythematosus (SLE). However, the pathogenesis of chronic GVHD is poorly understood. It is unclear how autoreactive T and B cells are generated in chronic GVHD recipients. We have recently developed a new chronic GVHD model by transplantation of donor DBA/2 (H-2d) spleen cells into major histocompatibility complex (MHC)-matched but minor antigen-mismatched sublethally irradiated BALB/c (H-2d) recipients as well as athymic BALB/c(nu/nu) and adult-thymectomized BALB/c recipients. Both euthymic and athymic BALB/c recipients developed high levels of serum IgG autoantibodies, sclerodermatous skin damage, and glomerulonephritis. Disease induction required both donor CD25-CD4+ T and B cells in transplants. In contrast, donor CD25+CD4+ T regulatory (Treg) cells prevented the disease induction. These results indicate that host thymus is not required for induction of chronic GVHD and that quiescent autoreactive T and B cells in transplants from nonautoimmune donors may be activated and expanded to cause chronic GVHD with autoimmune manifestations in allogeneic recipients, and donor Treg cells can suppress this process.

Potential Contribution of VH Gene Replacement in Immunity and Disease

VH replacement occurs through RAG-mediated recombination between a cryptic recombination signal sequence (cRSS) presented in a rearranged VH gene and a 23-bp RSS from an upstream VH gene. VH replacement renews the entire VH coding region and extends the immunoglobulin heavy-chain (IgH) CDR3 regions preferentially with charged amino acids. VH replacement occurs in bone marrow-immature B cells and contributes significantly to the primary B-cell repertoire in humans. However, the biological significance of VH replacement is not clear. Our recent studies revealed elevated frequencies of VH replacement products in different autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, and Sjögren's syndrome. Moreover, elevated frequencies of VH replacement products were also found in patients with human immunodeficiency virus or hepatitis C virus infections. These results provide the first clue that VH replacement contributes to autoimmune disease and antiviral immunity, and they also suggest a potential link between viral infection and autoimmune disease.

The regulation and activation of lupus-associated B cells

Anti-double-stranded DNA (anti-dsDNA) B cells are regulated in non-autoimmune mice. While some are deleted or undergo receptor editing, a population of anti-dsDNA (VH3H9/V lambda 1) B cells that emigrate into the periphery has also been identified. These cells have an altered phenotype relative to normal B cells in that they have a reduced lifespan, appear developmentally arrested, and localize primarily to the T/B-cell interface in the spleen. This phenotype may be the consequence of immature B cells encountering antigen in the absence of T-cell help. When provided with T-cell help, the anti-dsDNA B cells differentiate into antibody-forming cells. In the context of the autoimmune-prone lpr/lpr or gld/gld mutations, the VH3H9/V lambda 1 anti-dsDNA B cells populate the B-cell follicle and by 12 weeks of age produce serum autoantibodies. The early event of anti-dsDNA B-cell follicular entry, in the absence of autoantibody production, is dependent upon CD4(+) T cells. We hypothesize that control of autoantibody production in young autoimmune-prone mice may be regulated by the counterbalancing effect of T-regulatory (T(reg)) cells. Consistent with this model, we have demonstrated that T(reg) cells are able to prevent autoantibody production induced by T-cell help. Additional studies are aimed at investigating the mechanisms of this suppression as well as probing the impact of distinct forms of T-cell-dependent and -independent activation on anti-dsDNA B cells.

Several Genes Contribute to the Production of Autoreactive B and T Cells in the Murine Lupus Susceptibility Locus Sle1c

The systemic lupus erythematosus 1 (Sle1) locus mediates the loss of tolerance to nuclear Ags in the NZM2410 mouse model of lupus through intrinsic defects in both B and T cells. Congenic analysis has shown that Sle1 corresponds to at least three genetic loci, Sle1a, Sle1b, and Sle1c. Telomeric Sle1c is associated with abnormal B cell responses to subthreshold stimulation with anti-IgM and C3d and with decreased T-dependent humoral immune responses. We have proposed that these phenotypes resulted from polymorphisms in the C3 complement receptor Cr2 gene. We have also found that Sle1c was associated with the production of histone-specific autoreactive CD4(+) T cells, which correlated with higher activation and proliferative responses, and a reduction in the CD4(+)CD25(+)CD62L(+)forkhead/winged helix transcription factor gene (Foxp3(+)) compartment. In this study we showed, using congenic recombinants, that the decreased humoral immune response and impaired GC formation map to the NZM2410 Cr2 allele. A chronic graft-vs-host disease model also showed that Sle1c produces significantly more autoreactive B cells than B6 controls, and that this phenotype maps to two regions excluding the Cr2 gene. Mixed bone marrow chimera demonstrated that the increased activation, proliferative response, and reduced regulatory T cell compartment were intrinsic to Sle1c-expressing CD4(+) T cells. These phenotypes mapped to the same two loci identified with the chronic graft-vs-host disease model, excluding the Cr2 region. Overall, these results show that Sle1c results in the production of autoreactive B and T cells through the expression of three different genes, one of which is consistent with Cr2, based on the phenotypes of the Cr2-deficient mice, and the other two corresponding to as yet unidentified genes.

The Role of Host CD4 T Cells in the Pathogenesis of the Chronic Graft-versus-Host Model of Systemic Lupus Erythematosus

Systemic lupus erythematosus is characterized by production of autoantibodies and glomerulonephritis. The murine chronic graft-vs-host (cGVH) model of systemic lupus erythematosus is induced by allorecognition of foreign MHC class II determinants. Previous studies have shown that cGVH could not be induced in CD4 knockout (CD4KO) mice. We have further explored the role of host CD4 T cells in this model. Our studies now show that B cells in CD4KO mice have intrinsic defects that prevent them from responding to allohelp. In addition, B cells in CD4KO mice showed phenotypic differences compared with congeneic C57BL/6 B cells, indicating some degree of in vivo activation and increased numbers of cells bearing a marginal zone B cell phenotype. The transfer of syngeneic CD4 T cells at the time of initiation of cGVH did not correct these B cell abnormalities; however, if CD4 T cells were transferred during the development and maturation of B cells, then the B cells from CD4KO mice acquire the ability to respond in cGVH. These studies clearly indicate that B cells need to coexist with CD4 T cells early in their development to develop full susceptibility to alloactivation signals.

The inhibitory Fcgamma receptor modulates autoimmunity by limiting the accumulation of immunoglobulin G+ anti-DNA plasma cells

Deletion of the gene encoding the Fc immunoglobulin G receptor IIB (FcgammaRIIB) results in a fulminant, lupus-like disease in C57BL/6 but not BALB/c mice. Here we have investigated this strain-specific, epistatic loss of tolerance using gene-targeted immunoglobulin variable heavy-chain (V(H)) alleles 3H9 or 56R, which encode DNA-specific heavy chains, expressed on the C57BL/6 or BALB/c background. The combination of C57BL/6 and V(H) 56R (B6.56R) resulted in a loss of tolerance; hybridoma and single-cell analysis indicated an FcgammaRIIB-independent difference in immunoglobulin light-chain usage, consistent with an alteration in receptor editing. FcgammaRIIB deficiency resulted in an increase in immunoglobulin G (IgG) antibodies to DNA in the serum, an increased frequency of anti-DNA-reactive IgG(+) B cells with a plasma cell phenotype and immune complex deposition in the glomeruli and renal disease in B6.56R mice. Thus, FcgammaRIIB provides a distal peripheral checkpoint to limit the accumulation of autoreactive plasma cells, thereby maintaining tolerance.

The molecular basis and biological significance of VH replacement

First observed in mouse pre-B-cell lines and then in knock-in mice carrying self-reactive IgH transgenes, VH replacement has now been shown to contribute to the primary B-cell repertoire in humans. Through recombination-activating gene (RAG)-mediated recombination between a cryptic recombination signal sequence (RSS) present in almost all VH genes and the flanking 23 base pair RSS of an upstream VH gene, VH replacement renews the entire VH-coding region, while leaving behind a short stretch of nucleotides as a VH replacement footprint. In addition to extending the CDR3 region, the VH replacement footprints preferentially contribute charged amino acids. VH replacement rearrangement in immature B cells may either eliminate a self-reactive B-cell receptor or contribute to the generation of self-reactive antibodies. VH replacement may also rescue non-productive or dysfunctional VHDJH rearrangement in pro-B and pre-B cells. Conversely, VH replacement of a productive immunoglobulin H gene may generate non-productive VH replacement to disrupt or temporarily reverse the B-cell differentiation process. VH replacement can thus play a complex role in the generation of the primary B-cell repertoire.

Activation and tolerance in CD4(+) T cells reactive to an immunoglobulin variable region

Antibody diversity creates an immunoregulatory challenge for T cells that must cooperate with B cells, yet discriminate between self and nonself. To examine the consequences of T cell reactions to the B cell receptor (BCR), we generated a transgenic (Tg) line of mice expressing a T cell receptor (TCR) specific for a κ variable region peptide in monoclonal antibody (mAb) 36-71. The κ epitope was originally generated by a pair of somatic mutations that arose naturally during an immune response. By crossing this TCR Tg mouse with mice expressing the κ chain of mAb 36-71, we found that κ-specific T cells were centrally deleted in thymi of progeny that inherited the κTg. Maternally derived κTg antibody also induced central deletion. In marked contrast, adoptive transfer of TCR Tg T cells into κTg recipients resulted in T and B cell activation, lymphadenopathy, splenomegaly, and the production of IgG antichromatin antibodies by day 14. In most recipients, autoantibody levels increased with time, Tg T cells persisted for months, and a state of lupus nephritis developed. Despite this, Tg T cells appeared to be tolerant as assessed by severely diminished proliferative responses to the Vκ peptide. These results reveal the importance of attaining central and peripheral T cell tolerance to BCR V regions. They suggest that nondeletional forms of T tolerance in BCR-reactive T cells may be insufficient to preclude helper activity for chromatin-reactive B cells.

Activation of diverse repertoires of autoreactive T cells enhances the loss of anti-dsDNA B cell tolerance

CD4+ helper T cells play a critical role in the production of the antinuclear autoantibodies that characterize systemic lupus erythematosus in mice and humans. A key issue is whether this help is derived from a diverse repertoire of autoreactive CD4+ T cells or from a select number of T cells of limited specificity. We used the chronic graft-versus-host disease model to define the diversity of the CD4+ T cell repertoire required to induce the autoantibody response. By transferring clonally restricted versus clonally diverse populations of MHC class II-reactive CD4+ T cells, we show that the loss of B cell tolerance to nuclear antigens has two distinct components with different CD4+ cell requirements. Activation of limited repertoires of CD4+ T cells was sufficient for the expansion of anergized anti-double-stranded DNA B cells and production of IgM autoantibodies. Unexpectedly, we found that CD4+ T cell diversity was necessary for CD4+ T cell trafficking into the follicle and for the generation of isotype-switched IgG autoantibodies. Importantly, combining two limited repertoires of T cells provides sufficient CD4+ T cell diversity to drive antinuclear Ab production. These data demonstrate that a diverse CD4+ T cell repertoire is required to generate a sustained effector B cell response capable of mediating systemic autoimmunity.

Secondary heavy chain rearrangement: a mechanism for generating anti-double-stranded DNA B cells

The chronic graft-versus-host (cGVH) reaction results in a syndrome that closely resembles systemic lupus erythematosus (SLE). It is induced in nonautoimmune mice by the transfer of alloreactive T cells. The availability of anti-DNA transgenes allows us to study the genetic origins of autoantibodies in this model. We induced cGVH in two anti-DNA H chain site-directed transgenic mouse strains. This resulted in clonal expansion and selection of specific mutations in the anti-double-stranded (ds) DNA B cell population. These data, together with a high frequency of anti-dsDNA B cell clones recovered as hybridomas, suggested that anti-dsDNAs are the product of an antigen-driven immune response. Genetic analysis associated this response with the generation of anti-dsDNA B cells through secondary rearrangements that replaced the site-directed transgene (sd-tg) with endogenous VH genes.

Making and breaking tolerance

The lymphocyte's decision between tolerance and immunity/autoimmunity is regulated at many levels. Two important parameters in this decision are the maturation state of the antigen presenting cells (APCs) and the amount of self antigen that is detected by the immune system. Maturation of APCs occurs as a consequence of signals received by the innate immune system and may lead to the breakdown of tolerance. Particularly relevant to this process are the Toll-like receptors and mechanisms of cross presentation of self antigens. In addition, genetic alterations in a variety of cell surface receptors, signalling components and regulators of apoptosis/survival can break tolerance and lead to autoimmunity in vivo.