Efficient peripheral clonal elimination of B lymphocytes in MRL/lpr mice bearing autoantibody transgenes

Meant to B: B cells as a therapeutic target in systemic lupus erythematosus

B cells have a prominent role in the pathogenesis of systemic lupus erythematosus (SLE). They are mediators of inflammation through the production of pathogenic antibodies that augment inflammation and cause direct tissue and cell damage. Multiple therapeutic agents targeting B cells have been successfully used in mouse models of SLE; however, these preclinical studies have led to approval of only one new agent to treat patients with SLE: belimumab, a monoclonal antibody targeting B cell-activating factor (BAFF). Integrating the experience acquired from previous clinical trials with the knowledge generated by new studies about mechanisms of B cell contributions to SLE in specific groups of patients is critical to the development of new treatment strategies that will help to improve outcomes in patients with SLE. In particular, a sharper focus on B cell differentiation to plasma cells is warranted.

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.

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.

Global analysis of B cell selection using an immunoglobulin light chain-mediated model of autoreactivity

The nature of the immunoglobulin light chain affects peripheral B cell tolerance and autoreactivity.

The important subtleties of B cell tolerance are best understood in a diverse immunoglobulin (Ig) repertoire context encoding a full spectrum of autoreactivity. To achieve this, we used mice expressing Igκ transgenes that confer varying degrees of autoreactivity within a diverse heavy chain (HC) repertoire. These transgenes, coupled with a biomarker to identify receptor-edited cells and combined with expression cloning of B cell receptors, allowed us to analyze tolerance throughout B cell development. We found that both the nature of the autoantigen and the Ig HC versus light chain (LC) contribution to autoreactivity dictate the developmental stage and mechanism of tolerance. Furthermore, although selection begins in the bone marrow, over one third of primary tolerance occurs in the periphery at the late transitional developmental stage. Notably, we demonstrate that the LC has profound effects on tolerance and can lead to exacerbated autoantibody production.

B cell tolerance and positive selection in lupus

Systemic lupus erythematosus is considered a prototype of systemic autoimmune diseases; however, despite considerable advances in recent years in the understanding of basic mechanisms in immunology, little progress has been made in elucidating the etiology and pathogenesis of this disease. This even holds for inbred mice, such as the lupus-prone New Zealand Black/New Zealand White F(1) mice, which are all genetically programmed to develop lupus at a predetermined age. This frustrating state of affairs calls for a fundamental change in our scientific thinking and the opening of new directions in lupus research. In this study, we suggest that intrinsic B cell tolerance mechanisms are not grossly impaired in lupus-prone mice, but that an unusually strong positive selection event recruits a small number of autoreactive B cells to the germinal centers. This event could be facilitated by nucleic acid-protein complexes that are created by somatic changes in the susceptible animal.

Liver-expressed Igkappa superantigen induces tolerance of polyclonal B cells by clonal deletion not kappa to lambda receptor editing

Analysis of tolerance in a polyclonal wild-type B cell population demonstrates apoptosis of cells reactive to antigen expressed on liver membrane.

Little is know about the nature of peripheral B cell tolerance or how it may vary in distinct lineages. Although autoantibody transgenic studies indicate that anergy and apoptosis are involved, some studies claim that receptor editing occurs. To model peripheral B cell tolerance in a normal, polyclonal immune system, we generated transgenic mice expressing an Igκ–light chain–reactive superantigen targeted to the plasma membrane of hepatocytes (pAlb mice). In contrast to mice expressing κ superantigen ubiquitously, in which κ cells edit efficiently to λ, in pAlb mice, κ B cells underwent clonal deletion. Their κ cells failed to populate lymph nodes, and the remaining splenic κ cells were anergic, arrested at a semi-mature stage without undergoing receptor editing. In the liver, κ cells recognized superantigen, down-regulated surface Ig, and expressed active caspase 3, suggesting ongoing apoptosis at the site of B cell receptor ligand expression. Some, apparently mature, κ B1 and follicular B cells persisted in the peritoneum. BAFF (B cell–activating factor belonging to the tumor necrosis factor family) overexpression rescued splenic κ B cell maturation and allowed κ cells to populate lymph nodes. Our model facilitates analysis of tissue-specific autoimmunity, tolerance, and apoptosis in a polyclonal B cell population. The results suggest that deletion, not editing, is the major irreversible pathway of tolerance induction among peripheral B cells.

Deletion of IgG-switched autoreactive B cells and defects in Fas(lpr) lupus mice

During a T cell-dependent Ab response, B cells undergo Ab class switching and V region hypermutation, with the latter process potentially rendering previously innocuous B cells autoreactive. Class switching and hypermutation are temporally and anatomically linked with both processes dependent on the enzyme, activation-induced deaminase, and occurring principally, but not exclusively, in germinal centers. To understand tolerance regulation at this stage, we generated a new transgenic mouse model expressing a membrane-tethered gamma2a-reactive superantigen (gamma2a-macroself Ag) and assessed the fate of emerging IgG2a-expressing B cells that have, following class switch, acquired self-reactivity of the Ag receptor to the macroself-Ag. In normal mice, self-reactive IgG2a-switched B cells were deleted, leading to the selective absence of IgG2a memory responses. These findings identify a novel negative selection mechanism for deleting mature B cells that acquire reactivity to self-Ag. This process was only partly dependent on the Bcl-2 pathway, but markedly inefficient in MRL-Fas(lpr) lupus mice, suggesting that defective apoptosis of isotype-switched autoreactive B cells is central to Fas mutation-associated systemic autoimmunity.

Innate pathways to B-cell activation and tolerance

B cells represent an important link between the adaptive and innate immune systems as they express both antigen-specific B-cell receptors (BCRs) as well as various Toll-like receptors (TLRs). Several checkpoints in B-cell development ensure that self-specific cells are eliminated from the mature B-cell repertoire to avoid harmful autoreactive responses. These checkpoints are controlled by BCR-mediated events but are also influenced by TLR-dependent signals from the innate immune system. Additionally, B-cell-intrinsic and extrinsic TLR signaling are critical for inflammatory events required for the clearance of microbial infections. Factors secreted by TLR-activated macrophages or dendritic cells directly influence the fate of protective and autoreactive B cells. Additionally, naive and memory B cells respond differentially to TLR ligands, as do different B-cell subsets. We review here recent literature describing intrinsic and extrinsic effects of TLR stimulation on the fate of B cells, with particular attention to autoimmune diseases.

Defining a transcriptional fingerprint of murine splenic B-cell development

B-cell development occurs in a stepwise fashion that can be followed by the expression of B cell-specific surface markers. In this study, we wished to identify proteins that could contribute to the changes in expression of such markers. By using RNA from freshly isolated B220+ cells, we hoped to reduce the effect of artifacts that occur during the isolation and amplification steps necessary to use flow cytometry analysis-sorted subsets in microarray experiments. Analyses comparing expression patterns from B220+ 2-week bone marrow (pro-B, pre-B, immature B cells), 2-week spleen (predominantly transitional cells) and 8-week spleen (mainly mature B cells) yielded hundreds of genes. We also examined the B cell-activating factor (BAFF)-dependent effects on immature splenic B cells by comparing expression patterns in the spleen between 2-week A/J vs 2-week A/WySnJ mice, which lack functional BAFF receptor signaling. Genes that showed the expression differences between spleen and bone marrow samples were then analyzed through quantitative PCR on B-cell subsets isolated using two different sorting protocols. A comparison of the results from our study with the results from other analyses showed not only some overlap of preferentially expressed genes but also an expansion of other genes potentially involved in B-cell development.

Characterization of a late transitional B cell population highly sensitive to BAFF-mediated homeostatic proliferation

We have characterized a distinct, late transitional B cell subset, CD21^int transitional 2 (T2) B cells. In contrast to early transitional B cells, CD21^int T2 B cells exhibit augmented responses to a range of potential microenvironmental stimuli. Adoptive transfer studies demonstrate that this subset is an immediate precursor of both follicular mature and marginal zone (MZ) B cells. In vivo, a large percentage of CD21^int T2 B cells has entered the cell cycle, and the cycling subpopulation exhibits further augmentation in mitogenic responses and B cell-activating factor of the TNF family (BAFF) receptor expression. Consistent with these features, CD21^int T2 cells exhibit preferential responses to BAFF-facilitated homeostatic signals in vivo. In addition, we demonstrate that M167 B cell receptor (BCR) idiotypic-specific B cells are first selected within the cycling CD21^int T2 population, ultimately leading to preferential enrichment of these cells within the MZ B cell compartment. These data, in association with the coordinate role for BAFF and microenvironmental cues in determining the mature BCR repertoire, imply that this subset functions as a unique selection point in peripheral B cell development.

The regulation of autoreactive B cells during innate immune responses

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

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

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

Class switch recombination: a friend and a foe

The effector functions of the antibody are determined by the heavy chain constant region (CH). During CSR the primarily expressed mu constant region (Cmu) of the heavy chain is replaced with a downstream isotype Cgamma, Calpha or Cepsilon. The murine immunoglobulin heavy chain (IgH) locus contains eight different CH genes. Class switch recombination (CSR) involves a recombination between two different repetitive switch (S) region sequences, located upstream of each CH gene and the deletion of the intervening DNA. However, this protecting mechanism is also involved in aberrant chromosomal translocations and generation of B cell malignancies. It is also involved in susceptibility to autoimmunity. The current review focuses on the basic mechanism of CSR and the adverse outcomes that it may cause.

Selection of anti-double-stranded DNA B cells in autoimmune MRL-lpr/lpr mice

Abs to DNA and nucleoproteins are expressed in systemic autoimmune diseases, whereas B cells producing such Abs are edited, deleted, or inactivated in healthy individuals. Why autoimmune individuals fail to regulate is not well understood. In this study, we investigate the sources of anti-dsDNA B cells in autoimmune transgenic MRL-lpr/lpr mice. These mice are particularly susceptible to lupus because they carry a site-directed transgene, H76R that codes for an anti-DNA H chain. Over 90% of the B cells are eliminated in the bone marrow of these mice, and the few surviving B cells are associated with one of two Vkappa editors, Vkappa38c and Vkappa21D. Thus, it appears that negative selection by deletion and editing are intact in MRL-lpr/lpr mice. However, a population of splenic B cells in the H76R MRL-lpr/lpr mice produces IgG anti-nuclear Abs, and these mice have severe autoimmune organ damage. These IgG Abs are not associated with editors but instead use a unique Vkappa gene, Vkappa23. The H76R/Vkappa23 combination has a relatively high affinity for dsDNA and an anti-nuclear Ab pattern characteristic of lupus. Therefore, this Vkappa gene may confer a selective advantage to anti-DNA Abs in diseased mice.

Spontaneous B cell hyperactivity in autoimmune-prone MRL mice

The MRL-lpr/lpr mouse strain is a commonly used model of the human autoimmune disease systemic lupus erythematosus (SLE). Although much is known about the contribution of the lpr Fas mutation to B cell tolerance breakdown, the role of the genetic background of the MRL strain itself is less well explored. In this study, we use the MD4 anti-hen egg lysozyme Ig (IgHEL) transgenic system to explore B cell function in MRL+/+ and non-autoimmune mice. We demonstrate that MRL IgHEL B cells show spontaneous hyperactivity in the absence of self-antigen, which is associated with low total B cell numbers but an expansion of the marginal zone B cell population. However, B cell anergy is normal in the presence of soluble lysozyme [soluble hen egg lysozyme (sHEL)], and MRL IgHEL B cells undergo normal elimination in the presence of sHEL when competing with a polyclonal C57BL/6 B cell repertoire. We conclude that B cell hyperactivity may contribute to the autoimmune phenotype of MRL+/+ and MRL-lpr/lpr strains when it initiates antibody responses to rare or sequestered antigens that are below the threshold for tolerance induction, but that there is no B cell intrinsic defect in anergy in MRL mice.

Toll-like receptors, endogenous ligands, and systemic autoimmune disease

The critical role of Toll-like receptors (TLRs) as mediators of pathogen recognition by the innate immune system is now firmly established. Such recognition results in the initiation of an inflammatory immune response and subsequent instruction of the adaptive immune system, both of which are designed to rid the host of the invading pathogen. More controversial is the potential role of TLRs in the recognition of endogenous ligands and what effect this might have on the consequent development of autoimmune or other chronic sterile inflammatory disorders. An increasing number of studies implicate TLRs as being involved in the immune response to self-molecules that have in some way been altered from their native state or accumulate in non-physiologic sites or amounts, although questions have been raised about possible contaminants in certain of these studies. In this review, we discuss the evidence for endogenous ligand-TLR interactions with particular emphasis on mammalian chromatin, systemic lupus erythematosus, and atherosclerosis. Overall, the data support the general concept of a role for TLRs in the recognition of endogenous ligands. However, the precise details of the interactions and the extent to which they may contribute to the pathogenesis of human disease remain to be clarified.

Visualizing the Onset and Evolution of an Autoantibody Response in Systemic Autoimmunity

The onset of systemic autoimmunity is variable, making it difficult to identify early events. In this study, we show in rheumatoid factor (RF) Ig-transgenic autoimmune-prone mice that the appearance of RF B cells in blood signifies the onset of RF B cell activation in spleen, providing a novel window into the initiation of an autoantibody response. This allowed us to study the early and late phases of spontaneous induction of the B cell autoimmune response. Using this approach we showed that extensive Ab-forming cell generation in spleen, accompanied by somatic hypermutation, occurred despite the lack of an early germinal center response. The onset of the RF response correlated with the levels of IgG2a-containing immune complexes but not total IgG2a. By identifying the time of onset in individual mice, we were able to track progression of disease. We found remissions of RF Ab-forming cell production in some mice, suggesting that at the clonal level, chronic autoantibody responses are dynamic and episodic, much like acute pathogen responses. Surprisingly, there was little accumulation of long-lived plasma cells in bone marrow of mice with long-standing RF responses in spleen. These studies are among the first to define the early events of a spontaneous B cell autoimmune response.

Regeneration in the ears of immunodeficient mice: identification and lineage analysis of mesenchymal stem cells

Wound healing in the ears of Athymic Nude-nu mice resembles regeneration. Histological analysis of the ear-punched tissues revealed the initial formation of a blastema-like structure followed by dermal, vascular, cartilage, and muscle regrowth exclusively in Athymic Nude-nu mice but not in wild-type controls (C57BL/6J). A subset of stem cells referred to here as ear mesenchymal stem cells (EMSCs) has been isolated from the external ears of regenerative (Athymic Nude-nu) and nonregenerative strains of mice. Morphological, histochemical, and molecular analysis after the induction of EMSC differentiation revealed multiple mesenchymal cell lineages (adipocytes, chondrocytes, and osteocytes) in all murine strains independent of their ability for regeneration. Thus, the absence of regeneration in wounded ears of C57BL/6J wild-type mice is not related to the absence of mesenchymal stem cell differentiation in tissue culture. Because nude mice lack T lymphocytes, it appears that in this model the absence of T lymphocytes in the wounded ears provides a microenvironment conducive to regeneration of mesenchymal tissues. These findings provide a new model to study the influence of the immune system on tissue regeneration.

Class switch recombination in B lymphopoiesis: a potential pathway for B cell autoimmunity

Pathogenic autoantibodies detected in autoimmune diseases are predominantly IgG isotypes, reflecting the generation and activation of an autoimmune memory B cell repertoire. It is not completely understood how such autoreactive cells are generated and escape central and/or peripheral tolerance mechanisms, and several models to explain this have been proposed. It is generally thought that B lymphocytes utilize IgM receptors for development and tolerance establishment, whereas IgG receptors are primarily used to promote memory formation and signal for memory-type responses. In here we review recent findings suggesting that spontaneously occurring class switch recombination in B lymphopoiesis confer B lymphocytes with a novel developmental pathway that is driven by non-IgM receptors. The physiological relevance of this developmental pathway in generating an autoimmune memory repertoire, as well as a Fas-dependent mechanism regulating it, is discussed.

Requirements for follicular exclusion and competitive elimination of autoantigen-binding B cells

Results from several mouse tolerance models indicate that autoreactive B cells in peripheral lymphoid organs develop an anergic phenotype, migrate to the boundary between the T cell zone and the B cell follicle (T/B boundary), and undergo rapid cell death. We have used B cells from mice that are double-transgenic for soluble hen egg lysozyme (HEL) and an Ig that recognizes HEL with a high affinity to characterize the mechanisms underlying the migration and elimination of autoreactive B cells. In contrast to the situation for acutely activated B cells, we find that anergic B cells have reduced levels of CXCR5, the receptor for the follicular chemokine, CXCL13, and this contributes to their exclusion from follicles. CCR7 expression is required for follicular exclusion of anergic cells, although up-regulation of the receptor does not appear to be necessary. By TUNEL analysis, we observe that excluded anergic cells die in situ at the T/B boundary. We also show that this elimination occurs via a Fas-independent mechanism. Using CCR7(-/-)Ig(HEL)-transgenic B cells we find that localization to the T/B boundary is not a necessary event to achieve the competitive elimination of autoantigen-binding B cells. These findings characterize the mechanism for follicular exclusion of autoantigen-binding B cells and they indicate that B cells compete for survival by mechanisms that are separate from competition for the follicular niche.

Contribution of alphabeta and gammadelta T cells to the generation of primary immunoglobulin G-driven autoimmune response in immunoglobulin- mu-deficient/lpr mice

Class switch recombination (CSR) is a T-cell-dependent mechanism regulating isotype switching in activated mature B cells. Recently we showed that T-cell-independent CSRs occur spontaneously during B lymphopoiesis, but such cells are negatively selected by Fas signalling. In immunoglobulin mu-deficient mice, lack of Fas rescues isotype-switched B cells, resulting in generation of an autoimmune primary immunoglobulin G (IgG) repertoire in muMT/lpr mice. In the present study, we studied the role of alphabeta and gammadelta T cells in regulating this primary gammaH-driven repertoire. We found that a lack of alphabeta T cells significantly inhibited IgG production and autoimmunity in muMT/lpr mice, whereas a lack of gammadelta T cells resulted in augmented IgG production and autoimmunity. Also, a lack of T cells in muMT mice rescued isotype-switched B cells and serum IgG, probably owing to the lack of available FasL. We suggest that although CSRs in B-cell lymphopoiesis are T-cell independent, alphabeta T cells are important in the expansion of isotype-switched B-cell precursors and in promoting gammaH-driven autoimmunity, whereas gammadelta T cells regulate these cells.

A fail-safe mechanism for negative selection of isotype-switched B cell precursors is regulated by the Fas/FasL pathway

In B lymphocytes, immunoglobulin (Ig)M receptors drive development and construction of naive repertoire, whereas IgG receptors promote formation of the memory B cell compartment. This isotype switching process requires appropriate B cell activation and T cell help. In the absence of T cell help, activated B cells undergo Fas-mediated apoptosis, a peripheral mechanism contributing to the establishment of self-tolerance. Using Igμ-deficient μMT mouse model, where B cell development is blocked at pro-B stage, here we show an alternative developmental pathway used by isotype-switched B cell precursors. We find that isotype switching occurs normally in B cell precursors and is T independent. Ongoing isotype switching was found in both normal and μMT B cell development as reflected by detection of IgG1 germline and postswitch transcripts as well as activation-induced cytidine deaminase expression, resulting in the generation of IgG-expressing cells. These isotype-switched B cells are negatively selected by Fas pathway, as blocking the Fas/FasL interaction rescues the development of isotype-switched B cells in vivo and in vitro. Similar to memory B cells, isotype-switched B cells have a marginal zone phenotype. We suggest a novel developmental pathway used by isotype-switched B cell precursors that effectively circumvents peripheral tolerance requirements. This developmental pathway, however, is strictly controlled by Fas/FasL interaction to prevent B cell autoimmunity.

Activation and differentiation of autoreactive B-1 cells by interleukin 10 induce autoimmune hemolytic anemia in Fas-deficient antierythrocyte immunoglobulin transgenic mice

The Fas (CD95) gene is among critical genetic factors in some autoimmune diseases, which are characterized by autoantibody (autoAb) productions. In mice, mutations in the Fas gene cause lymphoproliferation (lpr) which predominantly develops glomerulonephritis, whereas the mutations in human cause autoimmune lymphoproliferative syndrome (ALPS) characterized by autoimmune hemolytic anemia (AIHA) and thrombocytopenia. Although the mechanism of antinuclear Ab in Fas-deficient background has been well characterized, that of antierythrocyte Ab production in ALPS has been still unclear. To investigate this mechanism, we developed a mouse line by crossing the antierythrocyte antibody transgenic mice (H+L6 mice) and Fas-deficient mice. Although Fas deficiency did not break tolerance of autoreactive B-2 cells in H+L6 mice, autoreactive B-1 cells in Fas-deficient H+L6 homozygous mice became activated and differentiated into autoAb-producing cells in mesenteric lymph nodes and lamina propria of intestine, resulting in severe anemia. In addition, serum levels of interleukin (IL)-10 significantly increased in Fas-/- x H+L6 homozygous mice and administration of anti-IL-10 Ab prevented exacerbation of autoAb production and AIHA. These results suggest that activation of B-1 cells is responsible for induction of AIHA in Fas-deficient condition and that IL-10 plays a critical role in terminal differentiation of B-1 cells in these mice.

Humoral autoimmunity to basement membrane antigens is regulated in C57BL/6 and MRL/MpJ mice transgenic for anti-laminin Ig receptors

Basement membrane proteins are targeted in organ-limited and systemic autoimmune nephritis, yet little is known about the origin or regulation of immunity to these complex extracellular matrices. We used mice transgenic for a nephrotropic systemic lupus erythematosus (SLE) Ig H chain to test the hypothesis that humoral immunity to basement membrane is actively regulated. The LamH-Cmu Ig H chain transgene combines with diverse L chains to produce nephrotropic Ig reactive with murine laminin alpha1. To determine the fate of transgene-bearing B cells in vivo, transgenic mice were outcrossed onto nonautoimmune B6 and SLE-prone MRL backgrounds and exposed to potent mitogen or Ag in adjuvant. In this work we demonstrate that transgenic autoantibodies are absent in serum from M6 and M29 lineage transgenic mice and transgenic B cells hypoproliferate and fail to increase Ig production upon exposure to endotoxin or when subjected to B cell receptor cross-linking. Administration of LPS or immunization with autologous or heterologous laminin, maneuvers that induce nonoverlapping endogenous anti-laminin IgG responses, fails to induce a transgenic anti-laminin response. The marked reduction in splenic B cell number suggests that selected LamH-Cmu H chain and endogenous L chain combinations generate autospecificities that lead to B cell deletion. It thus appears that SLE-like anti-laminin B cells have access to and engage a tolerizing self-Ag in vivo. Failure to induce autoimmunity by global perturbations in immune regulation introduced by the MRL autoimmune background and exposure to potent environmental challenge suggests that humoral immunity to nephritogenic basement membrane epitopes targeted in systemic autoimmunity is tightly regulated.

Impaired light chain allelic exclusion and lack of positive selection in immature B cells expressing incompetent receptor deficient of CD19

Positive signaling is now thought to be important for B cell maturation, although the nature of such signals has not yet been defined. We are studying the regulatory role of B cell Ag receptor (BCR) signaling in mediating positive selection of immature B cells. To do so, we use Ig transgenic mice (3-83Tg) that are deficient in CD19, thus generating a monoclonal immature B cell population expressing signaling-incompetent BCR. Immature 3-83Tg CD19(-/-) B cells undergo developmental arrest in the bone marrow, allowing maturation only to cells that effectively compensate for the compromised receptor by elevated levels of BCR. We find that developmentally arrested 3-83Tg CD19(-/-) B cells fail to impose L chain allelic exclusion and undergo intensive V(D)J recombination to edit their BCR. Furthermore, immature 3-83Tg CD19(-/-) B cells, which were grown in vitro, failed to undergo positive selection and to survive when adoptively transferred into normal recipients. However, elevation of BCR expression levels, obtained by transgene homozygosity, effectively compensated for the compromised BCR and completely restored BCR-mediated Ca(2+) influx, allelic exclusion, and positive selection. Our results suggest that the BCR signaling threshold mediates positive selection of developing B cells, and that a receptor-editing mechanism has an important role in rescuing cells that fail positive selection because of incompetent receptors.

Low-affinity anti-Smith antigen B cells are regulated by anergy as opposed to developmental arrest or differentiation to B-1

Understanding the regulation of B lymphocytes specific for self-Ags targeted in human and murine systemic lupus erythematosus, such as the ribonucleoprotein Smith Ag (Sm), is crucial to understanding the etiology of this autoimmune disease. To address the role of B cell receptor affinity in the regulation of anti-Sm B cells, we generated low-affinity anti-Sm transgenic mice by combining the anti-Sm 2-12H transgene with a V(kappa)8 transgene. In contrast to 2-12H transgenic mice, in which anti-Sm B cells are predominantly splenic transitional, and peritoneal B-1, low-affinity anti-Sm B cells are long-lived B-2 cells and are found in the spleen, lymph nodes, and peritoneum. However, they are unresponsive to LPS in vitro, indicating that they are anergic, although they do not down-regulate IgM and are not excluded from follicles even in the presence of nonautoreactive B cells. Thus, low-affinity anti-Sm B cells appear to have a partial form of anergy. Interestingly, these cells have elevated levels of MHC class II and CD95, but not CD40, CD80, or CD86, suggesting that they are poised to undergo deletion rather than activation upon T cell encounter. These data identify anergy as a mechanism involved in anti-Sm B cell regulation.

Role of CD5 in growth regulation of B-1 cells

CD5 is a membrane glycoprotein that is expressed on a subset of B lymphocytes called B-1 cells, thymocytes and T cells. The CD5+ B-1 cells are normally unresponsive to surface Ig receptor induced growth signals unless the CD5 gene is deleted or sequestered away. Here we show that CD5 mediated negative regulation is unique to B cell receptor (BCR) signaling. The CD5 molecule in normal B-1 cells is constitutively tyrosine phosphorylated and associates specifically with SHP-1, an SH2 domain containing protein tyrosine phosphatase. CD5 promotes a prolonged interaction between BCR and SHP-1, which may be inhibitory to BCR signaling. CD5 was shown to modulate the function of autoantibody producing B cells in transgenic mice expressing anti-DNA antibodies.

A receptor presentation hypothesis for T cell help that recruits autoreactive B cells

To uncover mechanisms that drive spontaneous expansions of autoreactive B cells in systemic lupus erythematosus, we analyzed somatic mutations in variable region genes expressed by a panel of (NZB x SWR)F(1) hybridomas representing a large, spontaneously arising clone with specificity for chromatin. A single mutation within the Jkappa intron that was shared by all members of the lineage indicated that the clone emanated from a single mutated precursor cell and led to the prediction that a somatic mutation producing a functionally decisive amino acid change in the coding region would also be universally shared. Upon cloning and sequencing the corresponding germline V(H) gene, we found that two replacement somatic mutations in FR1 and CDR2 were indeed shared by all seven clone members. Surprisingly, neither mutation influenced Ab binding to chromatin; however, one of them produced a nonconservative amino acid replacement in a mutationally "cold" region of FR1 and created an immunodominant epitope for class II MHC-restricted T cells. The epitope was restricted by IA(q) (SWR), and the SWR MHC locus is associated with systemic lupus erythematosus in (NZB x SWR)F(1) mice. These, and related findings, provoke the hypothesis that autoreactive B cells may be recruited by a "receptor presentation" mechanism involving cognate interactions between T cells and somatically generated V region peptides that are self-presented by B cells.

Unexpected autoantibody production in membrane Ig-mu-deficient/lpr mice

In the B lymphocyte lineage, Fas-mediated cell death is important in controlling activated mature cells, but little is known about possible functions at earlier developmental stages. In this study we found that in mice lacking the IgM transmembrane tail exons (muMT mice), in which B cell development is blocked at the pro-B stage, the absence of Fas or Fas ligand allows significant B cell development and maturation, resulting in high serum Ig levels. These B cells demonstrate Ig heavy chain isotype switching and autoimmune reactivity, suggesting that lack of functional Fas allows maturation of defective and/or self-reactive B cells in muMT/lpr mice. Possible mechanisms that may allow maturation of these B cells are discussed.

Role of B cell antigen receptor in regulation of V(D)J recombination and cell survival

B lymphocytes learn through the interaction of the B cell receptor with antigens in the context of B cell developmental stage and environmental cues. B cells can respond by proliferation and antibody secretion, programmed cell death, or modification of the antibody genes themselves through secondary immunoglobulin gene rearrangements or somatic point mutation. A critical learning process is that of self/nonself-discrimination. We have shown that one potent mechanism for immune self-tolerance in B cells is ongoing antibody light chain gene rearrangements, which can result in "receptor editing" that changes antigen receptor specificity. This process appears to be developmentally regulated, because it is confined to cells at an immature stage of development. Cells at later stages of development can be tolerized by apoptosis, but probably not by receptor editing.

Somatic mutation and light chain rearrangement generate autoimmunity in anti-single-stranded DNA transgenic MRL/lpr mice

Antibodies to single-stranded (ss)DNA are expressed in patients with systemic lupus erythematosus and in lupus-prone mouse models such as the MRL/Mp-lpr/lpr (MRL/lpr) strain. In nonautoimmune mice, B cells bearing immunoglobulin site-directed transgenes (sd-tgs) that code for anti-ssDNA are functionally silenced. In MRL/lpr autoimmune mice, the same sd-tgs are expressed in peripheral B cells and these autoantibodies gain the ability to bind other autoantigens such as double-stranded DNA and cell nuclei. These new specificities arise by somatic mutation of the anti-ssDNA sd-tgs and by secondary light chain rearrangement. Thus, B cells that in normal mice are anergic can be activated in MRL/lpr mice, which can lead to the generation of pathologic autoantibodies. In this paper, we provide the first direct evidence for peripheral rearrangement in vivo.

The central and multiple roles of B cells in lupus pathogenesis

A standard view of B cells in systemic autoimmunity is that they promote lupus by producing autoantibodies (autoAb). However, this view is incomplete because recent studies have revealed that autoimmune disease can be dissociated from autoAb deposition. Furthermore, the spontaneous T-cell activation and organ infiltration in systemic lupus erythematosus patients and animal models are difficult to explain entirely via a direct autoAb-mediated mechanism. In this review, we describe work addressing the B-cell functions of autoantigen presentation and autoAb production in lupus pathogenesis. In the JHD-MRL-Faslpr strain (JHD/lpr), a B-cell-deficient version of the lupus-prone MRL-Faslpr (MRL/lpr) mouse, spontaneous nephritis and dermatitis is abrogated, demonstrating that B cells have a primary role in disease. B cells play a similar role in Fas-intact, lupus-prone MRL mice. To address the role of autoantigen presentation, we analyzed transgenic mice which have B cells that cannot secrete immunoglobulin (mIgM transgenic mice). The restoration of B cells without antibody caused substantial interstitial nephritis and vasculitis although less marked than the intact MRL/lpr controls. To address the role of autoAb, we infused serum from aged MRL/lpr mice into JHD/lpr mice. At most, mild to no nephritis was observed in the infused mice. These results indicate that B cells are promoting autoimmunity in mechanisms other than autoAb secretion, and we describe a model depicting these B-cell roles in the context of other inflammatory events in lupus.

Production of High Affinity Autoantibodies in Autoimmune New Zealand Black/New Zealand White F1 Mice Targeted with an Anti-DNA Heavy Chain

Lupus-prone, anti-DNA, heavy (H) chain “knock-in” mice were obtained by backcrossing C57BL/6 mice, targeted with a rearranged H chain from a VH11(S107)-encoded anti-DNA hybridoma (D42), onto the autoimmune genetic background of New Zealand Black/New Zealand White (NZB/NZW) F1 mice. The targeted female mice developed typical lupus serologic manifestations, with the appearance of transgenic IgM anti-DNA autoantibodies at a young age (2–3 mo) and high affinity, somatically mutated IgM and IgG anti-DNA Abs at a later age (6–7 mo). However, they did not develop clinical, lupus-associated glomerulonephritis and survived to at least 18 mo of age. L chain analysis of transgenic anti-DNA Abs derived from diseased NZB/NZW mouse hybridomas showed a very restricted repertoire of Vκ utilization, different from that of nonautoimmune (C57BL/6 × BALB/c)F1 transgenic anti-DNA Abs. Strikingly, a single L chain was repetitively selected by most anti-DNA, transgenic NZB/NZW B cells to pair with the targeted H chain. This L chain had the same Vκ-Jκ rearrangement as that expressed by the original anti-DNA D42 hybridoma. These findings indicate that the kinetics of the autoimmune serologic manifestations are similar in wild-type and transgenic lupus-prone NZB/NZW F1 mice and suggest that the breakdown of immunologic tolerance in these mice is associated with the preferential expansion and activation of B cell clones expressing high affinity anti-DNA H/L receptor combinations.

Effector and predisposing genes in murine lupus

This Publication is No. 11789-IMM from the Department of Immunology, The Scripps Research Institute, 10550 North Torrey Pines Road/IMM3, La Jolla, CA 92037. The work of the author reported herein was supported, in part, by NIH grants AR39555, AR31203, and AG15061.