Somatic diversification and affinity maturation of IgM and IgG anti-DNA antibodies in murine lupus

B-1 cells in immunotoxicology: Mechanisms underlying their response to chemicals and particles

Since their discovery nearly 40 years ago, B-1 cells have continued to challenge the boundaries between innate and adaptive immunity, as well as myeloid and lymphoid functions. This B-cell subset ensures early immunity in neonates before the development of conventional B (B-2) cells and respond to immune injuries throughout life. B-1 cells are multifaceted and serve as natural- and induced-antibody-producing cells, phagocytic cells, antigen-presenting cells, and anti-/pro-inflammatory cytokine-releasing cells. This review retraces the origin of B-1 cells and their different roles in homeostatic and infectious conditions before focusing on pollutants comprising contact-sensitivity-inducing chemicals, endocrine disruptors, aryl hydrocarbon receptor (AHR) ligands, and reactive particles.

Natural antibodies bridge innate and adaptive immunity

Natural Abs, belonging to isotypes IgM, IgG3, and IgA, were discovered nearly half a century ago. Despite knowledge about the role of the polyreactive natural IgM in pathogen elimination, B cell survival and homeostasis, inflammatory diseases, and autoimmunity, there is a lack of clarity about the physiological role of natural IgG and natural IgA because they appear incapable of recognizing Ags on their own and are perceived as nonreactive. However, recent research revealed exciting functions of natural IgG in innate immunity. Natural IgG:lectin collaboration swiftly and effectively kills invading pathogens. These advances prompt further examination of natural Abs in immune defense and homeostasis, with the potential for developing novel therapeutics. This review provides new insights into the interaction between natural Abs and lectins, with implications on how interactions between molecules of the innate and adaptive immune systems bridge these two arms of immunity.

Mechanisms involved in the p62-73 idiopeptide-modulated delay of lupus nephritis in SNF(1) mice

The F(1) progeny of the (SWR × NZB) cross develop a lupus-like disease with high serum titers of autoantibodies, and increased frequency and severity of immune complex-mediated glomerulonephritis in females. In previous work, we found that an idiotypic peptide corresponding to aa62-73 (p62-73) of the heavy chain variable region of autoantibody 540 (Id(LN)F(1)) induced the proliferation of p62-73 idiotype-reactive T cell clones. Further, monthly immunization of pre-nephritic SNF(1) female mice with p62-73 resulted in decreased nephritis and prolonged life spans. Here we show that this treatment modulated proliferative responses to Id(LN)F(1) antigen, including a reduction in the population of idiopeptide-presenting antigen-presenting cells (APCs), as early as two weeks after immunization (10 weeks of age). Th1-type cytokine production was increased at 12 weeks of age. The incidence and severity of nephritis was reduced by 14 weeks compared to controls. Clinical indicators of nephritis, specifically histological evidence of glomerulonephritis and urine protein levels, were reduced by 20 weeks. Together these data suggest that events involved in the mechanism(s) whereby p62-73 immunization delayed nephritis occurred early after immunization, and involved modulation of APCs, B and T cell populations.

AID dysregulation in lupus-prone MRL/Fas(lpr/lpr) mice increases class switch DNA recombination and promotes interchromosomal c-Myc/IgH loci translocations: modulation by HoxC4

Immunoglobulin gene somatic hypermutation (SHM) and class switch DNA recombination (CSR) play important roles in the generation of autoantibodies in systemic lupus erythematosus. Systemic lupus is characterized by the production of an array of pathogenic high-affinity mutated and class-switched, mainly IgG, antibodies to a variety of self-antigens, including nuclear components, such as dsDNA, histones, and chromatin. We previously found that MRL/Fas(lpr/lpr) mice, which develop a systemic autoimmune syndrome sharing many features with human lupus, display greatly upregulated CSR, particularly to IgG2a, in B cells of the spleen, lymph nodes, and Peyer's patches. In MRL/Fas(lpr/lpr) mice, the significant upregulation of CSR is associated with increased expression of activation-induced cytidine deaminase (AID), which is critical for CSR and SHM. We also found that HoxC4 directly activates the promoter of the AID gene to induce AID expression, CSR and SHM. Here, we show that in both lupus patients and lupus-prone MRL/Fas(lpr/lpr) mice, the expression of HoxC4 and AID is significantly upregulated. To further analyze the role of HoxC4 in lupus, we generated HoxC4(-/-) MRL/Fas(lpr/lpr) mice. In these mice, HoxC4-deficiency resulted in reduced AID expression, impaired CSR, and decreased serum anti-dsDNA IgG, particularly IgG2a, autoantibodies, which were associated with a reduction in IgG deposition in kidney glomeruli. In addition, consistent with our previous findings in MRL/Fas(lpr/lpr) mice that upregulated AID expression is associated with extensive DNA lesions, comprising deletions and insertions in the IgH locus, we found that c-Myc to IgH (c-Myc/IgH) translocations occur frequently in B cells of MRL/Fas(lpr/lpr) mice. The frequency of such translocations was significantly reduced in HoxC4(-/-) MRL/Fas(lpr/lpr) mice. These findings suggest that in lupus B cells, upregulation of HoxC4 plays a major role in dysregulation of AID expression, thereby increasing CSR and autoantibody production and promoting c-Myc/IgH translocations.

Lupus-prone MRL/faslpr/lpr mice display increased AID expression and extensive DNA lesions, comprising deletions and insertions, in the immunoglobulin locus: concurrent upregulation of somatic hypermutation and class switch DNA recombination

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the production of an array of pathogenic autoantibodies, including high-affinity anti-dsDNA IgG antibodies. These autoantibodies are mutated and class-switched, mainly to IgG, indicating that immunoglobulin (Ig) gene somatic hypermutation (SHM) and class switch DNA recombination (CSR) are important in their generation. Lupus-prone MRL/fas(lpr/lpr) mice develop a systemic autoimmune syndrome that shares many features with human SLE. We found that Ig genes were heavily mutated in MRL/fas(lpr/lpr) mice and contained long stretches of DNA deletions and insertions. The spectrum of mutations in MRL/fas(lpr/lpr) B cells was significantly altered, including increased dG/dC transitions, increased targeting of the RGYW/WRCY mutational hotspot and the WGCW AID-targeting hotspot. We also showed that MRL/fas(lpr/lpr) greatly upregulated CSR, particularly to IgG2a and IgA in B cells of the spleen, lymph nodes and Peyer's patches. In MRL/fas(lpr/lpr) mice, the significant upregulation of SHM and CSR was associated with increased expression of activation-induced cytidine deaminase (AID), which mediates DNA lesion, the first step in SHM and CSR, and translesion DNA synthesis (TLS) polymerase (pol) theta, pol eta and pol zeta, which are involved in DNA synthesis/repair process associated with SHM and, possibly, CSR. Thus, in lupus-prone MRL/fas(lpr/lpr) mice, SHM and CSR are upregulated, as a result of enhanced AID expression and, therefore, DNA lesions, and dysregulated DNA repair factors, including TLS polymerases, which are involved in the repair process of AID-mediated DNA lesions.

Genetic control of the spontaneous activation of CD4+ Th cells in systemic lupus erythematosus-prone (NZB x NZW) F1 mice

The F(1) hybrid of autoimmune hemolytic anemia-prone NZB and nonautoimmune NZW strains of mice has been studied as a murine model of systemic lupus erythematosus. Both NZB and F(1) hybrid mice show age-dependent spontaneous activation of peripheral CD4(+) T cells as reflected by the elevated frequencies of CD4(+) T cells positive for CD69 early activation marker. Both strains also show age-dependent abnormal decrease of the frequencies of CD62L(+) naive CD4(+) T cells and/or NTA260(+) memory CD4(+) T cells in the spleen. We studied the multigenic control of these abnormal features of peripheral CD4(+) T cells in (NZB x NZW) F(1) x NZW backcross mice by quantitative trait loci mapping and by association rule analysis. The abnormally elevated frequencies of CD69(+)CD4(+) T cells and decreased frequencies of CD62L(+) naive and/or NTA260(+) memory CD4(+) T cells were under the common genetic control, in which the interaction between MHC and a hitherto unknown locus, designated Sta-1 (spontaneous T-cell activation) on chromosome 12, plays a major role. The allelic effects of these loci likely predispose CD4(+) T cells to the loss of self-tolerance, and are responsible for the accelerated autoimmune phenotypes of (NZB x NZW) F(1) hybrid mice.

Significance of MHC class II haplotypes and IgG Fc receptors in SLE

Systemic lupus erythematosus (SLE) is a systemic antibody-mediated autoimmune disease that develops under the control of multiple susceptibility genes. Genetic studies in murine and human SLE have identified several chromosomal intervals that contain candidate susceptibility genes. However, the ultimate identification of the genes and their roles in disease process need much further investigation. Spontaneous murine SLE models provide useful tools in this respect. In this chapter, we show this line of investigation, particularly focusing on the roles of major histocompatibility complex (MHC) class II and immunoglobulin G Fc receptors (FcgammaRs). The existence of high-affinity autoantibodies is evidence that autoimmunity in SLE is antigen-driven. Thereby, MHC class II haplotypes have been implicated in SLE susceptibility; however, because of the linkage disequilibrium that exists among the class I, II and III genes within the MHC complex, it has been difficult to discriminate the relative contributions of individual loci. On the other hand, the extent of antibody synthesis upon antigen stimulation and associated inflammatory cascades are controlled in several ways by the balance of stimulatory and inhibitory signaling molecules on immune cells. Stimulatory/inhibitory FcgammaRs mediate one such mechanism, and there are reports indicating the association between polymorphic FcgammaRs and SLE. However, as stimulatory and inhibitory FcgammaRs cluster on the telomeric chromosome 1, the absolute contribution of individual genes has been difficult to dissect. In studies of genetic dissection using interval-congenic and intragenic recombinant mouse strains of SLE models, we show evidence and discuss how and to what extent MHC class II molecules and stimulatory/inhibitory FcgammaRs are involved in SLE susceptibility.

Latest technologies for the enhancement of antibody affinity

High affinity antibodies are crucial both for the discovery and validation of biomarkers for human health and disease and as clinical diagnostic and therapeutic reagents. This review describes some of the latest technologies for the design, mutation and selection of high affinity antibodies that provide a paradigm for molecular evolution of a far wider range of proteins including enzymes. Strategies include both in vivo and in vitro methods and embrace the latest concepts for antibody display and selection. Specifically, affinity enhancement can be tailored to the target-binding surface, typically the complementary determining region (CDR) loops in antibodies, whereas enhanced stability, expression or catalytic properties can be affected by selected changes to the core protein scaffold. Together, these technologies provide a rapid and powerful strategy to drive the next generation of protein-based reagents for numerous clinical, environmental and agribusiness applications.

B1 lymphocytes and myeloid dendritic cells in lymphoid organs are preferential extratumoral sites of parvovirus minute virus of mice prototype strain expression

Due to their oncolytic properties and apathogenicity, autonomous parvoviruses have attracted significant interest as possible anticancer agents. Recent preclinical studies provided evidence of the therapeutic potential of minute virus of mice prototype strain (MVMp) and its recombinant derivatives. In a murine model of hemangiosarcoma, positive therapeutic outcome correlated with high intratumoral expression of MVMp-encoded genes in tumors and lymphoid organs, especially in tumor-draining lymph nodes. The source and relevance of this extratumoral expression, which came as a surprise because of the known fibrotropism of MVMp, remained unclear. In the present study, we investigated (i) whether the observed expression pattern occurs in different tumor models, (ii) which cell population is targeted by the virus, and (iii) the immunological consequences of this infection. Significant MVMp gene expression was detected in lymphoid tissues from infected tumor-free as well as melanoma-, lymphoma-, and hemangiosarcoma-bearing mice. This expression was especially marked in lymph nodes draining virus-injected tumors. Fluorescent in situ hybridization analysis, multicolor fluorescence-activated cell sorting, and quantitative reverse transcription-PCR revealed that MVMp was expressed in rare subpopulations of CD11b (Mac1)-positive cells displaying CD11c+ (myeloid dendritic cells [MDC]) or CD45B (B220+ [B1 lymphocytes]) markers. Apart from the late deletion of cytotoxic memory cells (CD8+ CD44+ CD62L-), this infection did not lead to significant alteration of the immunological profile of cells populating lymphoid organs. However, subtle changes were detected in the production of specific proinflammatory cytokines in lymph nodes from virus-treated animals. Considering the role of B1 lymphocytes and MDC in cancer and immunological surveillance, the specific ability of these cell types to sustain parvovirus-driven gene expression may be exploited in gene therapy protocols.

Retinoic acid reduces autoimmune renal injury and increases survival in NZB/W F1 mice

Retinoic acids, a group of natural and synthetic vitamin A derivatives, have potent antiproliferative and anti-inflammatory properties. Recently, retinoic acids were reported to inhibit Th1 cytokine production. We investigated the effects of retinoic acid on lupus nephritis in a model of NZB/NZW F(1) (NZB/W F(1)) mice. Three-month-old NZB/W F(1) mice were separated into two groups: one treated with all-trans-retinoic acid (ATRA; 0.5 mg i.p., three times weekly for 7 mo) and one with saline as a control. Compared with controls, ATRA-treated mice survived longer and exhibited a significant reduction of proteinuria, renal pathological findings including glomerular IgG deposits, and serum anti-DNA Abs. Splenomegaly was less marked in the treated mice than in controls. Transcripts encoding IFN-gamma, IL-2, and IL-10 in splenic CD4(+) T cells were significantly reduced in treated mice compared with controls. We conclude that treatment with ATRA in SLE-prone NZB/W F(1) mice significantly alleviates autoimmune renal disorder and prolongs survival; this may thus represent a novel approach to the treatment of patients with lupus nephritis.

Genome screening for susceptibility loci in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a complex, multigenic autoimmune disease with a wide spectrum of clinical manifestations. Much of the pathology is attributed to deposition to various tissues of immune complexes continuously formed with autoantibodies; thus, the pathogenesis is related to dysregulation of self-reactive B cells. Recent family linkage studies and allele-sharing linkage analyses of affected sibling pairs have advanced genome screening for susceptibility loci in SLE, and a considerable number of chromosomal intervals with significant or suggestive linkage to SLE have been identified. However, there are still several inherent difficulties in precisely identifying loci and genes, as the complexity of polygenic inheritance of SLE phenotypes is considerable. One must note that each specific aspect of diverse SLE phenotypes (clinical manifestations and immunological abnormalities) is mostly controlled separately by a different set of susceptibility loci. Involvement of positive and negative epistatic gene interactions often puzzles genetic analyses. Studies on SLE using murine lupus models are ongoing to solve some of these difficulties. Comparative studies have identified several syntenic chromosomal intervals with susceptibility loci in both mouse models and humans. Thus, combining knowledge derived from both human and murine studies is vital. The ultimate identification of susceptibility genes and their functions will probably depend largely on studies using genetically manipulated mutant mice, including those with homologous recombination of potent polymorphic target genes. The up-coming completion of genomic sequences in mice and humans is predicted to limit the numbers of potent candidate genes in particular genomic intervals and accelerates this line of studies. Such knowledge will lead to elucidation of genetic and cellular mechanisms involved in the dysregulation of self-reactive lymphocytes in the pathogenesis of SLE. Prophylactic and therapeutic clinical approaches can then be better designed.

Origins and functions of B-1 cells with notes on the role of CD5

Whether B-1a (CD5+) cells are a distinct lineage derived from committed fetal/neonatal precursors or arise from follicular B-2 cells in response to BCR ligation and other, unknown signals remains controversial. Recent evidence indicates that B-1a cells can derive from adult precursors expressing an appropriate specificity when the (self-) antigen is present. Antibody specificity determines whether a B cell expressing immunoglobulin transgenes has a B-2, B-1a or marginal zone (MZ) phenotype. MZ cells share many phenotypic characteristics of B-1 cells and, like them, appear to develop in response to T independent type 2 antigens. Because fetal-derived B cell progenitors fail to express terminal deoxynucleotidyl transferase (TdT) and for other reasons, they are likely to express a repertoire that allows selection into the B-1a population. As it is selected by self-antigen, the B-1 repertoire tends to be autoreactive. This potentially dangerous repertoire is also useful, as B-1 cells are essential for resistance to several pathogens and they play an important role in mucosal immunity. The CD5 molecule can function as a negative regulator of BCR signaling that may help prevent inappropriate activation of autoreactive B-1a cells.

Lupus nephritis: lessons from experimental animal models

Lupus nephritis is a frequent and severe complication of SLE. In the last decades, animal models for SLE have been studied widely to investigate the immunopathology of this autoimmune disease because abnormalities can be studied and manipulated before clinical signs of the disease become apparent. In this review an overview is given of our current knowledge on the development of lupus nephritis, as derived from animal models, and a hypothetical pathway for the development of lupus nephritis is postulated. The relevance of the studies in experimental models in relationship with our knowledge of human SLE is discussed.

Genetic aspects of inherent B-cell abnormalities associated with SLE and B-cell malignancy: lessons from New Zealand mouse models

Genes that predispose to SLE are closely related to key events in pathogenesis of this disease. As much of the pathology can be attributed to high affinity autoantibodies and/or their immune complexes, some of the genes may exert effects in the process of emergence, escape from tolerance mechanisms, activation, clonal expansion, differentiation, class switching and affinity maturation of self-reactive B cells. A number of growth and differentiation factors and signaling molecules, including positive and negative regulators, are involved in this process. Genetic variations associated with functional deficits in some of such molecules can be involved in the susceptibility for SLE. As is the case with SLE, hereditary factors play significant roles in the pathogenesis of B cell chronic lymphocytic leukemia (B-CLL). Patients with B-CLL or their family members frequently have immunological abnormalities, including those associated with SLE. It is suggested that certain genetically determined regulatory abnormalities of B cells may be a crossroad between B-CLL and SLE. A thorough understanding of the genetic pathways in B cell abnormalities leading to either SLE or B-CLL is expected to shed light on their association. New Zealand mouse strains are pertinent laboratory models for these studies. Chromosomal locations of several major genetic loci for abnormal proliferation, differentiation and maturation of B cells and relevant candidate genes, located in close proximity to these intervals and potentially related to the SLE pathogenesis, have been identified in these mice. Further studies make for a wider knowledge and understanding of the pathogenesis of SLE and related B-cell malignancy.

IL-18 prevents the development of chronic graft-versus-host disease in mice

The development of chronic graft-versus-host disease (GVHD), which is induced by the transfer of DBA/2 spleen cells into (C57BL/6 x DBA/2)F1 (BDF1) mice, is closely related to diminished donor anti-host CTL activity and host B cell hyperactivation. Therefore, an approach which activates donor CD8+ T cells or suppresses donor CD4+ T cell-host B cell interaction may have clinical utility in the treatment of chronic GVHD. We have previously demonstrated that IL-18 induces the development of naive CD8+ T cells into type I effector cells in DBA/2 anti-BDF1 MLC. In this paper we examined the effect of IL-18 administration on the development of chronic GVHD in mice. The treatment was started before or after the onset of clinical evidence of the disease. Regardless of the treatment schedule, IL-18 significantly decreased immunological parameters indicative of chronic GVHD, such as elevated serum IgG antinuclear Abs, IgG1, and IgE levels, and host B cell numbers and their activation. Importantly, IL-18-treated mice did not show the same acute GVHD-like symptoms reported for IL-12 treatment, because there was no weight loss, death, or severe immunodeficiency as indicated by a decrease in IL-2 and IFN-gamma production by Con A-stimulated spleen cells. In contrast, IL-18 treatment partially but significantly restored the production of these cytokines. Data further suggested that these IL-18-mediated therapeutic effects may be due to the induction of donor CD8+ CTL, the decrease in donor CD4+ T cell numbers, and a down-regulation of host B cell MHC class II expression. Thus, our results suggest that IL-18 has beneficial effects in the prevention and treatment of chronic GVHD.

Molecular analysis of the autoantibody response in peptide-induced autoimmunity

Immunization of nonautoimmune BALB/c mice with multimeric DWEYSVWLSN, a peptide mimotope of DNA, induces anti-DNA and other lupus-associated Abs. To further investigate the pathogenesis of the autoantibody response induced by peptide immunization, we generated hybridomas from peptide-immunized mice that bound peptide, dsDNA, cardiolipin, Sm/ribonucleoprotein (RNP), or some combination of these Ags. Analysis of 24 IgM Abs led to the identification of three groups of Abs: 1) Abs reactive with peptide alone, 2) anti-peptide Abs cross-reactive with one or more autoantigens, and 3) autoantibodies that do not bind to peptide. The gene families and particular VH-VL combinations used in those hybridomas binding DNA were similar to those used in the anti-DNA response in spontaneous murine lupus. Another similarity to the spontaneous anti-DNA response was the generation of arginines in the complementarity-determining region-3 of DNA-binding hybridomas. Interestingly, one Ab had the VH-VL combination present in the original R4A anti-DNA Ab used to select the DWEYSVWLSN peptide from a phage display library. Many of the heavy and light chains displayed evidence of somatic mutation, suggesting that they were made by Ag-activated B cells. Analysis of the Ab repertoire in peptide-induced autoimmunity may provide insights into the generation of anti-DNA Abs following exposure to foreign Ag. Furthermore, the recovery of an Ab with the heavy and light chain combination of the Ab originally used to isolate the immunizing peptide confirms the utility of phage display peptide libraries in generating true molecular mimics.

High pathogenic potential of low-affinity autoantibodies in experimental autoimmune hemolytic anemia

To assess the potency of low-affinity anti-red blood cell (RBC) autoantibodies in the induction of anemia, we generated an immunoglobulin (Ig)G2a class-switch variant of a 4C8 IgM anti-mouse RBC autoantibody, and compared its pathogenic potential with that of its IgM isotype and a high-affinity 34-3C IgG2a autoantibody. The RBC-binding activity of the 4C8 IgG2a variant was barely detectable, at least 1,000 times lower than that of its IgM isotype, having a high-binding avidity, and that of the 34-3C IgG2a monoclonal antibody (mAb). This low-affinity feature of the 4C8 mAb was consistent with the lack of detection of opsonized RBCs in the circulating blood from the 4C8 IgG2a-injected mice. However, the 4C8 IgG2a variant was highly pathogenic, as potent as its IgM isotype and the 34-3C IgG2a mAb, due to its capacity to interact with Fc receptors involved in erythrophagocytosis. In addition, our results indicated that the pentameric form of the low-affinity IgM isotype, by promoting the binding and agglutination of RBCs, is critical for its pathogenic activity. Demonstration of the remarkably high pathogenic potency of low-affinity autoantibodies, if combined with appropriate heavy chain effector functions, highlights the critical role of the Ig heavy chain constant regions, but the relatively minor role of autoantigen-binding affinities, in autoimmune hemolytic anemia.

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

Systemic lupus erythematosus (SLE) is a multigenic disease associated with IgG hypergammaglobulinemia, IgG anti-nuclear antibodies and immune complex (IC)-type glomerulonephritis. In both human and murine SLE, one susceptibility allele has been mapped to the interval linked to the IgG Fc receptor II (FcgammaRII) gene on chromosome 1. In spontaneous SLE models of NZB and (NZB x NZW) F(1) mice, expression of FcgammaRIIB1, which acts as a negative regulator for B cells, was abnormally down-regulated in follicular germinal center B cells from aged mice, compared to findings in non-SLE NZW, while levels in non-germinal center B cells were practically identical. Such strain differences were also evident in young mice upon in vivo stimulation with foreign antigens. In the FcgammaRIIB promoter region, the NZB allele has two deletion sites, including transcription factor-binding sites. Analyses using (NZB x NZW) F(1) x NZW backcross mice showed that this NZB allele was significantly linked to hyper-IgG, irrespective of the MHC haplotype, while high levels of IgG antibodies specific for DNA were regulated by a combinatorial effect of the F(1)-unique MHC haplotype and the NZB FcgammaRIIB allele. Therefore, the FcgammaRIIB promoter polymorphism may possibly predispose to SLE through germinal center B cells abnormally down-regulating FcgammaRIIB1 expression upon autoantigen stimulations and thus escaping negative signals for IgG production.

Enhanced mutational activity of Vkappa gene rearrangements in systemic lupus erythematosus

To determine the differential impact of somatic hypermutation and selective influences on the light chain repertoire in systemic lupus erythematosus (SLE), the frequency and pattern of somatic hypermutations were compared between the productive and nonproductive Vkappa gene repertoire manifested by individual CD19(+) B cells in a patient with SLE. The mutational frequency of nonproductive rearrangements in the SLE patient was significantly (P < 0.001) increased (3.7 x 10(-2)) compared to normals (4.8 x 10(-3)). Similarly, the mutational frequency of the productive Vkappa rearrangements was also significantly increased in the SLE patient (2.8 x 10(-2) vs 1.1 x 10(-2)) (P < 0.001). There were no differences in the R/S ratios of mutations in productive and nonproductive Vkappa rearrangements. Moreover, a variety of mutational "hot spots" were noted, but, unexpectedly, in the FRs. As in normals, mutations were found most frequently in RGYW/WRCY sequences accounting for 39.3% (nonproductive) and 40.1% (productive) of all mutations. Of note, nonproductive Vkappa rearrangements harbored significantly more mutations than productive rearrangements (P < 0.05) indicating that there was overall selection against mutations in the expressed repertoire. This was most apparent in the CDR3. These data are most consistent with the conclusion that, in this SLE patient, the mutational machinery was markedly enhanced compared to normals, but with no subsequent positive selection of mutations. The enhanced mutational activity may play a role in the emergence of autoreactivity in this SLE patient.

Susceptibility Alleles for Aberrant B-1 Cell Proliferation Involved in Spontaneously Occurring B-Cell Chronic Lymphocytic Leukemia in a Model of New Zealand White Mice

B-cell chronic lymphocytic leukemia (B-CLL) and autoimmune disease are a related event, and genetic factors are linked to both diseases. As B-CLL is mainly of B-1 cell type that participates in autoantibody production, genetically-determined regulatory abnormalities in proliferation and/or differentiation of B-1 cells may determine their fate. We earlier found that, in H-2–congenic (NZB × NZW) F1 mice, while H-2d/z heterozygosity predisposes to autoimmune disease, H-2z/z homozygosity predisposes to B-CLL. Studies also suggested the involvement of non–H-2-linked NZW allele(s) in leukemogenesis. Using H-2–congenic NZW and B10 mouse strains, their F1 and backcross progeny, we have now identified three major NZW susceptibility loci for abnormal proliferation of B-1 cells, which form the basis of leukemogenesis; one H-2–linked locus on chromosome 17 and the other two non–H-2-linked loci, each on chromosome 13 and chromosome 17. Each susceptibility allele functioned independently, in an incomplete dominant fashion, the sum of effects determining the extent of aberrant B-1 cell frequencies. The development of leukemia was associated with age-related increase in B-1 cell frequencies in the blood. Thus, these alleles probably predispose B-1 cells to accumulate genetic alterations, giving rise to B-CLL. Potentially important candidate genes and correlation of the findings with autoimmune disease are discussed.

Susceptibility Alleles for Aberrant B-1 Cell Proliferation Involved in Spontaneously Occurring B-Cell Chronic Lymphocytic Leukemia in a Model of New Zealand White Mice

B-cell chronic lymphocytic leukemia (B-CLL) and autoimmune disease are a related event, and genetic factors are linked to both diseases. As B-CLL is mainly of B-1 cell type that participates in autoantibody production, genetically-determined regulatory abnormalities in proliferation and/or differentiation of B-1 cells may determine their fate. We earlier found that, in H-2–congenic (NZB × NZW) F1 mice, while H-2d/z heterozygosity predisposes to autoimmune disease, H-2z/z homozygosity predisposes to B-CLL. Studies also suggested the involvement of non–H-2-linked NZW allele(s) in leukemogenesis. Using H-2–congenic NZW and B10 mouse strains, their F1 and backcross progeny, we have now identified three major NZW susceptibility loci for abnormal proliferation of B-1 cells, which form the basis of leukemogenesis; one H-2–linked locus on chromosome 17 and the other two non–H-2-linked loci, each on chromosome 13 and chromosome 17. Each susceptibility allele functioned independently, in an incomplete dominant fashion, the sum of effects determining the extent of aberrant B-1 cell frequencies. The development of leukemia was associated with age-related increase in B-1 cell frequencies in the blood. Thus, these alleles probably predispose B-1 cells to accumulate genetic alterations, giving rise to B-CLL. Potentially important candidate genes and correlation of the findings with autoimmune disease are discussed.

Co-existence of somatic hypermutation and gene conversion in hypervariable regions of single Igkappa clones

In the rabbit, recent investigations have provided evidence that gene conversion leads to the generation of diversity of heavy chain rearranged VH-DH-JH genes. No data have been published on a similar mechanism for rabbit light chains. In our laboratory, we initially infected rabbits with Trypanosoma brucei, which stimulates B-cell hyperplasia and hypergammaglobulinaemia. The heterozygous rabbits exhibited the Ckappa1 b4 and b9 kappa light chain allotypes. After reverse transcription of mRNA, and cloning and sequencing of cDNA, the Vkappa-Jkappa-Ckappa genes provided evidence for both somatic hypermutation and gene conversion. We saw that in each of the b4 and b9 kappa light chain cDNA, CDR1 and CDR3 carried both point mutation and provisional gene conversion traits. In the CDR2 region, point mutation and gene conversion inserts were observed in the b4 genes, with only gene conversion in two b9 genes. In the CDR regions, although some genes exhibited only somatic hypermutation or gene conversion, others showed linkage of both somatic hypermutation and gene conversion in the same sequence. This also marks the first time that somatic hypermutation and gene conversion in the same cloned CDR region has been observed in Vkappa1 genes; however, it has been seen earlier in rabbit heavy chain VH sequences. Furthermore, the addition of several codons to the CDR3 segment by gene conversion may have provided a mechanism for length variation. In addition, we demonstrated that Jkappa and framework region segments contained examples of somatic hypermutation. Confirmation of gene conversion necessitates that donor sequences be identified as providing the templated inserts. Thus after cloning two pseudogenes we found putative CDR3 donor segments for two CDR3 rearranged genes. The results offer additional mechanisms for the generation of diversity among rearranged rabbit kappa light chain genes. Whether there is a relationship or influence of gene conversion upon somatic hypermutation or vice versa is not discernable at present.

Multigenic control of lupus-associated antiphospholipid syndrome in a model of (NZW x BXSB) F1 mice

In a subset of systemic lupus erythematosus (SLE) patients, antiphospholipid syndrome, characterized by occurrence of anti-cardiolipin (CL) antibodies, thrombocytopenia, thrombosis and recurrent intrauterine fetal death occurs. Male (NZW x BXSB)F1 mice, carrying the BXSB Yaa gene, serve as a model for SLE-associated antiphospholipid syndrome. Using microsatellite markers in the NZW x (NZW x BXSB)F1 backcross male progeny, we mapped BXSB alleles contributing to the generation of anti-CL antibodies, platelet-binding antibodies, thrombocytopenia and myocardial infarction. Generation of each disease character was controlled by two major independently segregating dominant alleles, i.e. those on chromosomes (Chr.) 4 and 17 for anti-CL antibodies, Chr. 8 and 17 for both anti-platelet antibodies and thrombocytopenia and, to our surprise, Chr. 7 and 14 for myocardial infarction, and that a combination of the two alleles appeared to produce full expression of each character, as a complementary gene action. The alleles on Chr. 17 linked to the above three characters were all mapped in close proximity to the H-2 complex. Therefore, no single factor such as anti-CL antibodies can explain the pathogenesis of SLE-associated antiphospholipid syndrome. Rather, a combination of susceptibility alleles such as described here, along with additional modifying loci, i.e. BXSB Yaa and some from NZW, characterizes unique SLE features in male (NZW x BXSB) F1 mice. There are potentially important candidate genes which may be linked to the syndrome.

Immunoglobulin double isotype-producing hybridomas isolated from an autoimmune (NZB x NZW)F1 mouse

In the sera of (NZB x NZW)F1 (B/W) mice that develop a lupus-like syndrome, increased levels of IgG antibodies (Ab) reacting with TNP have been detected before the appearance of IgG anti-DNA Ab and clinical symptoms. A single injection of trinitrophenyl-bovine serum albumin (TNP/BSA) in physiological saline into a young B/W mouse (3 months old), followed by fusion of its splenocytes 3 days later, gave rise to hybridomas simultaneously secreting IgM and IgG Ab with anti-TNP reactivity. Both mu and gamma chains were detected in culture supernatants by ELISA, and double isotype-producing cells were labeled by immunofluorescence. Molecular analysis of two of these double isotype-producing hybridomas showed the presence of mRNA coding for both mu and gamma chains of Ig, and this gamma mRNA could be translated in vitro into a gamma heavy (H) chain. Comparison of the H chain variable-region sequences of IgM and IgG revealed 100% homology between mu and gamma V(H) genes in one clone, while mu and gamma V(H) genes showed only 80% homology in the other clone. Both clones produced a single kappa light (L) chain. These two hybridomas, isolated from a B/W mouse, thus represent two different mechanisms of double isotype expression: the first one corresponds to an IgM to IgG switch, while the second one reflects a lack of allelic exclusion.

Monoclonal Anti-Cardiolipin Antibodies from New Zealand Black × New Zealand White F1 Mice React to Thrombomodulin

The reactivity with and affinity for thrombomodulin (TM) of monoclonal anti-cardiolipin Abs (MoaCL), derived from a New Zealand Black × New Zealand White F1 (NZB/W F1) mouse, were studied to investigate the pathogenicity of anti-cardiolipin Abs (aCL). Four of eighteen MoaCL were found to react with rabbit TM when examined using ELISA. These four MoaCL also reacted with synthetic peptide that included the epidermal growth factor-like domain of human TM, a binding site for thrombin. The reaction with TM of these four MoaCL was inhibited by bovine thrombin. When the affinity for TM of the MoaCL was determined, the dissociation constants (Kd) ranged from 4.8 × 10−9 to 4.7 × 10−8 M. By contrast, examination of the affinity for cardiolipin (CL) gave values from 8.3 × 10−6 to 7.4 × 10−5 M. Thus, these MoaCL reacted to TM with a higher affinity than to CL. Moreover, these MoaCL also bound to TM on HUVEC and down-regulated the expression level of TM on the surface of HUVEC due to internalization of TM. The binding of thrombin to TM is known to initiate rapid protein C activation, and complexes of activated protein C and protein S show anticoagulatory activity. Thus, the present studies suggest that certain pathogenic aCL cross-react with TM and induce down-regulation of TM on endothelial cells, followed by induction of thrombosis.

A novel approach for detecting an immunodominant antigen of Porphyromonas gingivalis in diagnosis of adult periodontitis

In the course of long-term infection with Porphyromonas gingivalis in adult periodontitis, a specific antibody response to this organism is generated. We describe a potential novel approach for identifying an immunodominant antigen in human periodontitis patients. First, various monoclonal antibodies (MAbs) were established from mice immunized with crude antigen preparations of P. gingivalis FDC 381. The antigen specificities of these MAbs were compared with those of serum antibodies of 10 periodontitis patients in a competitive enzyme-linked immunosorbent assay. The binding of one MAb (termed PF18) was readily inhibited by sera from all patients but not by sera from healthy volunteers. The antigen recognized by PF18 existed on the cell surface, presumably in the capsule layer, shown by immunoelectron microscopic analysis. Purification of the antigenic substance, termed PF18-Ag, was performed by immunoaffinity chromatography with the MAb. Characterization of PF18-Ag suggested that the epitope was composed of carbohydrates but not peptides and that the substance was different from lipopolysaccharide. Measurement of levels of serum antibody to PF18-Ag better discriminated periodontitis patients from healthy individuals than measurement of antibodies to crude antigen preparations of P. gingivalis. Immunoglobulin G2 was the predominant isotype among the antibodies to PF18-Ag in the patients' sera. These results suggest that PF18-Ag, which is possibly a novel substance, is an important antigenic substance and is potentially useful for the clinical diagnosis of adult periodontitis. The approach that was used would also be relevant to detecting immunodominant antigens of other infectious microorganisms.

Precursor B cells for autoantibody production in genomically Fas-intact autoimmune disease are not subject to Fas-mediated immune elimination

The Fas/Fas ligand (FasL) system participates in regulation of the immune system through the apoptotic process. However, the extent to which abnormalities in this system are involved in the loss of self-tolerance and development of autoimmune disease not associated with Fas/FasL mutations remains unknown. The present study addresses this issue in Fas/FasL-intact, systemic lupus erythematosus (SLE)-prone (NZB x NZW) (NZB/W) F1 mice. While splenic B cells from 2-month-old mice before overt SLE expressed Fas poorly, in vitro stimulation with an agonistic anti-CD40 mAb up-regulated their Fas expression, thus revealing the existence of two populations: one was Fashigh and highly susceptible to anti-Fas mAb-induced apoptosis, and the other was Faslow and apoptosis-resistant. The Faslow cells were included in the CD5(+) B cell subpopulation and contained most of the cells that produced IgM anti-DNA antibodies. The isotype of anti-DNA antibodies switches from IgM to IgG in NZB/W F1 mice at ages beginning at about 6 months. These IgG anti-DNA antibodies were produced almost exclusively by a subpopulation of splenic B cells that spontaneously expressed low levels of Fas in vivo and were apoptosis-resistant. The findings indicate that precursor B cells for autoantibody production and presumably autoantibody-secreting cells in these mice are relatively resistant to Fas-mediated apoptosis, a finding supporting the concept that abnormalities of Fas-mediated apoptotic process are involved in the development of autoreactive B cells in Fas/FasL-intact autoimmune disease.

CD40-mediated stimulation of B1 and B2 cells: implication in autoantibody production in murine lupus

B1 cells usually show preferential responses to T cell-independent antigens. To ask whether B1 cells could respond to CD40-mediated stimulation for proliferation and differentiation, and whether CD40-mediated signals are involved in the production of autoantibodies by B1 cells, we compared responses to our newly established agonistic anti-mouse CD40 monoclonal antibody (mAb) between B1 and B2 cells from autoimmune-prone (NZB x NZW) F1 mice. Stimulation with this mAb induced a similar level of proliferative responses of both B1 and B2 cells, as well as an increase in expression of cell surface molecules I-A, CD54, CD23, CD80, and CD86. While co-stimulation with interleukin (IL)-4 markedly augmented proliferative as well as IgG1 and IgE antibody responses of both B and B2 cells, co-stimulation with IL-5 augmented proliferative and IgM antibody responses of only B1 cells. Splenic B1, but not B2 cells from young (NZB x NZW) F1 mice spontaneously produced substantial amounts of IgM including IgM anti-DNA antibodies, and the levels increased in case of stimulation with anti-CD40 mAb alone, or to a greater extent with the mAb plus IL-4 and IL-5. Collectively, these results indicate that splenic B1 cells from autoimmune (NZB x NZW) F1 mice have a comparable responsiveness to the CD40-mediated stimulation to that of B2 cells, which would be a potent regulatory mechanism involved in the spontaneous production of autoantibodies by B1 cells.

Induction of cross-reactive anti-dsDNA antibodies in preautoimmune NZB/NZW mice by immunization with bacterial DNA

To investigate the role of antigen drive in anti-double-stranded (ds) DNA production, the antibody response induced in lupus-prone NZB/NZW mice by E. coli (EC) dsDNA was evaluated. Preautoimmune NZB/NZW female mice were immunized with complexes of EC dsDNA with methylated bovine serum albumin (mBSA) in complete Freund's adjuvant; control mice received either mBSA complexes with calf thymus (CT) dsDNA or mBSA alone in adjuvant. IgG antibody responses were assessed by ELISA. Similar to normal mice, immunized NZB/NZW mice produced significant levels of anti-dsDNA when measured with EC dsDNA as antigen. Whereas normal mice produce antibodies which are specific for the immunizing bacterial DNA, NZB/NZW mice produced antibodies that bound crossreactively to CT dsDNA by ELISA. Furthermore, the induced antibodies resembled lupus anti-DNA in their fine specificity for polynucleotide antigens and reactivity with Crithidia luciliae DNA. Despite their response to EC dsDNA, NZB/NZW mice immunized with CT dsDNA failed to generate significant anti-dsDNA responses. These results provide further evidence for the enhanced immunogenicity of bacterial DNA and suggest that immune cell abnormalities in NZB/NZW mice promote the generation of crossreactive autoantibody responses when confronted with a foreign DNA.

How unique are pathogenic anti-DNA autoantibody V regions?

Unique characteristics of the molecular structure of V regions encoding pathogenic anti-DNA autoantibodies have become apparent upon comparison of a large number of nucleotide sequences encoding this autospecificity. Moreover, the generation of transgenic animals expressing V regions encoding anti-DNA autoantibodies has shed light on the tolerizing mechanisms that regulate B cells producing antibodies against DNA.