Control of separate pathogenic autoantibody responses marks MHC gene contributions to murine lupus

Susceptibility quantitative trait loci for pathogenic leucocytosis in SCG/Kj mice, a spontaneously occurring crescentic glomerulonephritis and vasculitis model

The spontaneous crescentic glomerulonephritis-forming/Kinjoh (SCG/Kj) mouse, a model of human crescentic glomerulonephritis (CrGN) and systemic vasculitis, is characterized by the production of myeloperoxidase-specific anti-neutrophil cytoplasmic autoantibody (MPO-ANCA) and marked leucocytosis. This study was performed to identify the specific populations of leucocytes associated with CrGN and susceptibility loci for pathogenic leucocytosis. Four hundred and twenty female (C57BL/6 × SCG/Kj) F2 intercross mice were subjected to serial flow cytometry examination of the peripheral blood (PB). Kidney granulocytes and monocytes were examined histopathologically. Linkage analyses were performed with 109 polymorphic microsatellite markers. Correlation studies revealed that increase of the granulocytes, F4/80(+) cells, CD3(+) CD4(-) CD8(-) T cells and dendritic cells (DCs) in peripheral blood (PB) were associated significantly with glomerulonephritis, crescent formation and vasculitis. In kidney sections, F4/80(low) cells were observed in crescent, while F4/80(high) cells were around the Bowman's capsules and in the interstitium. Numbers of F4/80(+) cells in crescents correlated significantly with F4/80(+) cell numbers in PB, but not with numbers of F4/80(+) cells in the interstitium. Genome-wide quantitative trait locus (QTL) mapping revealed three SCG/Kj-derived non-Fas QTLs for leucocytosis, two on chromosome 1 and one on chromosome 17. QTLs on chromosome 1 affected DCs, granulocytes and F4/80(+) cells, but QTL on chromosome 17 affected DCs and granulocytes. We found CrGN-associated leucocytes and susceptibility QTLs with their positional candidate genes. F4/80(+) cells in crescents are considered as recruited inflammatory macrophages. The results provide information for leucocytes to be targeted and genetic elements in CrGN and vasculitis.

Regulatory B cells in autoimmune diseases

B cells are generally considered to be positive regulators of the immune response because of their capability to produce antibodies, including autoantibodies. The production of antibodies facilitates optimal CD4(+) T-cell activation because B cells serve as antigen-presenting cells and exert other modulatory functions in immune responses. However, certain B cells can also negatively regulate the immune response by producing regulatory cytokines and directly interacting with pathogenic T cells via cell-to-cell contact. These types of B cells are defined as regulatory B (Breg) cells. The regulatory function of Breg cells has been demonstrated in mouse models of inflammation, cancer, transplantation, and particularly in autoimmunity. In this review, we focus on the recent advances that lead to the understanding of the development and function of Breg cells and the implications of B cells in human autoimmune diseases.

Protective and pathogenic roles for B cells during systemic autoimmunity in NZB/W F1 mice

Delineating the relative contributions of B lymphocytes during the course of autoimmune disease has been difficult. Therefore, the effects of depleting all mature B cells using a potent CD20 mAb, or of depleting circulating and marginal zone B cells using a ligand-blocking CD22 mAb, were compared in NZB/W F(1) mice, a model for human systemic lupus erythematosus. Single low-dose mAb treatments depleted B cells efficiently in both NZB/W F(1) and C57BL/6 mice. Prophylactic B cell depletion by repeated CD20 mAb treatments prolonged survival during pristane-accelerated lupus in NZB/W F(1) mice, whereas CD22 mAb had little effect. Despite effective B cell depletion, neither mAb treatment prevented autoantibody generation. In addition, CD20, CD22, and control mAb-treated NZB/W F(1) mice developed anti-mouse IgG autoantibodies in contrast to parental NZB and NZW strains, which may have reduced the effectiveness of B cell depletion. Despite this, low-dose CD20 mAb treatment initiated in 12-28-wk-old mice, and administered every 4 wk thereafter, significantly delayed spontaneous disease in NZB/W F(1) mice. By contrast, B cell depletion initiated in 4-wk-old mice hastened disease onset, which paralleled depletion of the IL-10-producing regulatory B cell subset called B10 cells. B10 cells were phenotypically similar in NZB/W F(1) and C57BL/6 mice, but were expanded significantly in young NZB/W F(1) mice. Thus, B cell depletion had significant effects on NZB/W F(1) mouse survival that were dependent on the timing of treatment initiation. Therefore, distinct B cell populations can have opposing protective and pathogenic roles during lupus progression.

Homeostatically proliferating CD4 T cells are involved in the pathogenesis of an Omenn syndrome murine model

Patients with Omenn syndrome (OS) have hypomorphic RAG mutations and develop varying manifestations of severe combined immunodeficiency. It is not known which symptoms are caused directly by the RAG mutations and which depend on other polymorphic genes. Our current understanding of OS is limited by the lack of an animal model. In the present study, we identified a C57BL/10 mouse with a spontaneous mutation in, and reduced activity of, RAG1. Mice bred from this animal contained high numbers of memory-phenotype T cells and experienced hepatosplenomegaly and eosinophilia, had oligoclonal T cells, and demonstrated elevated levels of IgE, major symptoms of OS. Depletion of CD4+ T cells in the mice caused a reduction in their IgE levels. Hence these "memory mutant" mice are a model for human OS; many symptoms of their disease were direct results of the Rag hypomorphism and some were caused by malfunctions of their CD4+ T cells.

Effects of MHC and gender on lupus-like autoimmunity in Nba2 congenic mice

The lupus-like disease that develops in hybrids of NZB and NZW mice is genetically complex, involving both MHC- and non-MHC-encoded genes. Studies in this model have indicated that the H2d/z MHC type, compared with H2d/d or H2z/z, is critical for disease development. C57BL/6 (B6) mice (H2b/b) congenic for NZB autoimmunity 2 (Nba2), a NZB-derived susceptibility locus on distal chromosome 1, produce autoantibodies to nuclear Ags, but do not develop kidney disease. Crossing B6.Nba2 to NZW results in H2b/z F1 offspring that develop severe lupus nephritis. Despite the importance of H2z in past studies, we found no enhancement of autoantibody production or nephritis in H2b/z vs H2b/b B6.Nba2 mice, and inheritance of H2z/z markedly suppressed autoantibody production. (B6.Nba2 x NZW)F1 mice, compared with MHC-matched B6.Nba2 mice, produced higher levels of IgG autoantibodies to chromatin, but not to dsDNA. Although progressive renal damage with proteinuria only occurred in F1 mice, kidneys of some B6.Nba2 mice showed similar extensive IgG and C3 deposition. We also studied male and female B6.Nba2 and F1 mice with different MHC combinations to determine whether increased susceptibility to lupus among females was also expressed within the context of the Nba2 locus. Regardless of MHC or the presence of NZW genes, females produced higher levels of antinuclear autoantibodies, and female F1 mice developed severe proteinuria with higher frequencies. Together, these studies help to clarify particular genetic and sex-specific influences on the pathogenesis of lupus nephritis.

MHC class I expression regulates susceptibility to spontaneous autoimmune disease in (NZBxNZW)F1 mice

(NZBxNZW)F1 mice spontaneously develop with age an autoimmune disease that resembles the human disease, systemic lupus erythematosus (SLE). Previous studies have demonstrated that susceptibility to experimentally induced SLE depended on the expression of MHC class I molecules: mice deficient in beta2-microglobulin did not express cell surface class I and were resistant to the induction of experimental SLE. Furthermore, the spontaneous SLE-like disease of (NZBxNZW)F1 mice was ameliorated by treatment with an agent that reduces MHC class I expression, methimazole (MMI). In the present study, the role of MHC class I has been examined in (NZBxNZW)F1 mice deficient in beta2-microglobulin expression. Homozygous (NZBxNZW)F1 beta2m-/- mice do not express class I or develop CD8+ T cells. Surprisingly, they show an increased susceptibility to disease. In sharp contrast, heterozygous (NZBxNZW)F1 beta2m+/- express class I, albeit at reduced levels, develop normal levels of CD8+ T cells and are less susceptible to autoimmune disease, relative to their wild-type litter mates. Taken together, these findings suggest that class I expression regulates the development of disease, both positively and negatively. We speculate that MHC class I expression itself confers susceptibility to disease through presentation of self-peptides, while also selecting for a CD8+ suppressor T cell population that mitigates disease.

New insights into disease pathogenesis from mouse lupus genetics

Full manifestation of mouse lupus, similar to the human disease in its severe form, is characterized by elevated antinuclear autoantibody levels and the development of kidney disease. Considerable evidence supports a genetic basis for lupus. The functional dissection of susceptibility loci in multigenic mouse models of lupus has provided insight into the immune abnormalities associated with autoantibody production and other processes critical for inflammation and damage in the kidney. The elucidation of models with single-gene manipulations has also identified immune mechanisms in the pathway to lupus. Recent advances have challenged previously accepted truths and new layers of complexity have become apparent.

Differential role of three major New Zealand Black-derived loci linked with Yaa-induced murine lupus nephritis

By assessing the development of Y-linked autoimmune acceleration (Yaa) gene-induced systemic lupus erythematosus in C57BL/6 (B6) x (New Zealand Black (NZB) x B6.Yaa)F(1) backcross male mice, we mapped three major susceptibility loci derived from the NZB strain. These three quantitative trait loci (QTL) on NZB chromosomes 1, 7, and 13 differentially regulated three different autoimmune traits: anti-nuclear autoantibody production, gp70-anti-gp70 immune complex (gp70 IC) formation, and glomerulonephritis. Contributions to the disease traits were further confirmed by generating and analyzing three different B6.Yaa congenic mice, each carrying one individual NZB QTL. The chromosome 1 locus that overlapped with the previously identified Nba2 (NZB autoimmunity 2) locus regulated all three traits. A newly identified chromosome 7 locus, designated Nba5, selectively promoted anti-gp70 autoantibody production, hence the formation of gp70 IC and glomerulonephritis. B6.Yaa mice bearing the NZB chromosome 13 locus displayed increased serum gp70 production, but not gp70 IC formation and glomerulonephritis. This locus, called Sgp3 (serum gp70 production 3), selectively regulated the production of serum gp70, thereby contributing to the formation of nephritogenic gp70 IC and glomerulonephritis, in combination with Nba2 and Nba5 in NZB mice. Among these three loci, a major role of Nba2 was demonstrated, because B6.Yaa Nba2 congenic male mice developed the most severe disease. Finally, our analysis revealed the presence in B6 mice of an H2-linked QTL, which regulated autoantibody production. This locus had no apparent individual effect, but most likely modulated disease severity through interaction with NZB-derived susceptibility loci.

The effect of the phytoestrogen coumestrol on the NZB/W F1 murine model of systemic lupus

Coumestrol is a naturally occurring plant estrogen. As estrogen influences cellular and humoral immunity, and has known effects on murine models of lupus, we investigated the effect of coumestrol on disease expression in the NZB/W F1 mouse. Female NZB/W F1 mice were fed a "standard" rodent diet including soy proteins, a non-soy diet, or a non-soy diet with 0.01% coumestrol. Outcome measures included survival, autoantibody expression, immunoglobulin levels, proteinuria, renal histology and B cell immunohistochemistry, and renal mRNA expression. At 24 weeks, the treatment group had decreased prevalence of autoantibodies detected by immunofluorescence and less splenomegaly. At 39 weeks, the prevalence of autoantibodies was similar but the treatment group had less proteinuria. Overall, there was little effect of treatment on renal mRNA levels as assessed by gene array analysis, but functional ontology mapping revealed that genes encoding proteins involved in the immune response were most often affected. These results suggest that treatment with coumestrol may ameliorate some aspects of disease progression in this model of systemic autoimmunity.

Association between nuclear antigens and endogenous retrovirus in the generation of autoantibody responses in murine lupus

OBJECTIVE

(NZB x NZW)F(1) (NZB/NZW) mice and other strains of mice with experimental lupus frequently produce autoantibodies to both chromatin constituents and murine leukemia virus envelope gp70. These autoantibody responses are involved in the glomerulonephritis that develops in these mice. This study was undertaken to explore possible connections between these 2 antigen systems.

METHODS

We used monoclonal antibodies (mAb) derived from unmanipulated NZB/NZW mice to investigate the specificity of anti-gp70 and antichromatin autoantibodies for chromatin constituents, recombinant gp70, NZB retroviruses, and retrovirally infected cells. NZB mice were also immunized with retroviral particles and followed up for study of autoantibody responses.

RESULTS

Spontaneous autoantibody production in NZB/NZW mice reflects high-level autoimmune responses to nuclear antigens and gp70 that do not cross-react with the other antigen. However, both types of autoantibodies have the capability to bind to the endogenous xenotropic virions NZB-X1 or NZB-X2. The mAbs to recombinant gp70 cross-reacted only with the NZB-X2 virus, whereas the antichromatin mAb frequently bound to both retroviruses. The binding of antichromatin autoantibodies was mediated by nuclear material complexed to the retrovirus, and studies showed that this material can be acquired through the budding process. Immunization with NZB-X1 or NZB-X2 virions induced strong responses to gp70 and was much more effective than chromatin at inducing autoantibody responses to chromatin and double-stranded DNA in NZB mice.

CONCLUSION

These studies suggest that retroviral virions may harbor nuclear antigens and may link together the autoimmune responses to the disparate antigens, chromatin and gp70.

Anti-DNA antibody subpopulations and lupus nephritis

As a consequence of increased insight into the cellular and molecular mechanisms responsible for induction of B cell and T cell autoimmunity to DNA and nucleosomes, there is an obvious need to reconsider the dogma stating that anti-dsDNA antibodies serve as marker antibodies for SLE and also that anti-dsDNA antibodies per se are responsible for the initiation of lupus nephritis. Given that the potential to produce anti-dsDNA antibodies is an inherent property of the normal immune system and that few anti-DNA antibodies have nephritogenic potential, we must try to solve the problem whether it is avidity for DNA, specificity for unique DNA structures or cross-reactivity with non-DNA molecules, that make such antibodies pathogenic and thus potential markers for SLE and lupus nephritis. In this review, we will summarize contemporary problems related to these questions; (1) try to focus on phenotypic differences with respect to the ability to produce anti-dsDNA antibodies between individuals suffering from SLE and those not belonging to this diagnostic group, and (2) to describe differences between pathogenic and non-pathogenic anti-dsDNA antibodies.

Dichotomous effects of complete versus partial class II major histocompatibility complex deficiency on circulating autoantibody levels in autoimmune-prone mice

OBJECTIVE

To assess the effects of altered class II major histocompatibility complex (MHCII) expression on circulating autoantibody levels in C57BL/6 (B6) mice congenic for the Sle1 (B6.Sle1 mice) or Nba2 (B6.Nba2 mice) regions.

METHODS

H-2Ab(+/+) (MHCII-intact), H-2Ab(+/-) (MHCII-intermediate), and H-2Ab(-/-) (MHCII-deficient) littermate B6.Sle1 and B6.Nba2 mice were evaluated for spleen cell phenotype, numbers of splenic Ig-secreting cells, and serum levels of total IgM, total IgG, IgG antichromatin, IgG antihistone, and IgG anti-double-stranded DNA (anti-dsDNA).

RESULTS

Compared with their MHCII-intact littermates, MHCII-deficient B6.Sle1 and B6.Nba2 mice developed markedly decreased circulating levels of IgG autoantibodies, along with decreased circulating levels of total IgG. In sharp contrast, MHCII-intermediate mice developed increased circulating levels of IgG autoantibodies. This was associated with increased numbers of splenic Ig-secreting cells and serum levels of total IgG in B6.Sle1 mice, but it occurred without concomitant increases in the numbers of splenic Ig-secreting cells or serum total IgG levels in B6.Nba2 mice.

CONCLUSION

In 2 clinically healthy strains of mice with a genetic proclivity for developing autoantibodies, the effects of class II MHC expression on levels of circulating IgG autoantibodies were found to be complex. In the absence of MHCII expression, circulating IgG autoantibody levels were minimal. With full MHCII expression, circulating IgG autoantibody levels were considerable. With intermediate MHCII expression, circulating IgG autoantibody levels were even greater. These last findings may help explain why heterozygosity at the H-2 locus is associated with increased autoantibody titers and aggravated disease in certain lupus-prone mice.

Inhibition of B cell death causes the development of an IgA nephropathy in (New Zealand white x C57BL/6)F(1)-bcl-2 transgenic mice

Little is known about the pathogenic mechanisms of IgA nephropathy, despite being the most prevalent form of glomerulonephritis in humans. We report in this study that in (New Zealand White (NZW) x C57BL/6)F(1) mice predisposed to autoimmune diseases, the expression of a human bcl-2 (hbcl-2) transgene in B cells promotes a CD4-dependent lupus-like syndrome characterized by IgG and IgA hypergammaglobulinemia, autoantibody production, and the development of a fatal glomerulonephritis. Histopathological analysis of glomerular lesions reveals that the glomerulonephritis observed in these animals resembles that of human IgA nephropathy. The overexpression of Bcl-2 in B cells selectively enhances systemic IgA immune responses to T-dependent Ags. Significantly, serum IgA purified from (NZW x C57BL/6)F(1)-hbcl-2 transgenic mice, but not from nontransgenic littermates, shows reduced levels of galactosylation and sialylation and an increased ability to deposit in the glomeruli, as observed in human patients with IgA nephropathy. Our results indicate that defects in the regulation of B lymphocyte survival associated with aberrant IgA glycosylation may be critically involved in the pathogenesis of IgA nephropathy, and that (NZW x C57BL/6)F(1)-hbcl-2 Tg mice provide a new experimental model for this form of glomerulonephritis.

The Sgp3 locus on mouse chromosome 13 regulates nephritogenic gp70 autoantigen expression and predisposes to autoimmunity

By interval mapping of a backcross progeny between New Zealand White (NZW) and C57BL/6 (B6) mice bearing the Y chromosome-linked autoimmune acceleration gene Yaa, we previously identified a genetic locus on mid-chromosome 13, here designated as Sgp3, showing a major effect on the expression of a nephritogenic autoantigen, gp70. In this study, the NZW-derived Sgp3 region was transferred by backcross procedure and marker-assisted selection on the B6 background to produce three independent congenic strains B6.NZW-Sgp3/1, -Sgp3/2, and -Sgp3/3. We show that NZW homozygosity at a single 3 centiMorgans ( approximately 12 megabases (Mb)) interval between markers D13Mit142 and D13Mit254 mediates increased basal serum levels of gp70 in B6.NZW-Sgp3/1 and B6.NZW-Sgp3/2 mice and with a higher degree in males ( approximately 15 micro g/ml) than in females ( approximately 9 micro g/ml) as compared with B6 ( approximately 2 micro g/ml), revealing a gender effect. However, their gp70 levels are still lower than that of NZW mice ( approximately 60 micro g/ml). In addition, B6.NZW-Sgp3/1 and B6.NZW-Sgp3/2 mice showed a moderate 2- to 3-fold increase in serum gp70 in response to LPS, which contrasted with over a 10-fold increase in NZW mice. Although both B6.NZW-Sgp3/1 and B6.NZW-Sgp3/2 mice failed to produce significant amounts of gp70 anti-gp70 immune complexes, unexpectedly, aged B6.NZW-Sgp3/2 congenic males bearing the Yaa gene developed increased titers of IgG autoantibodies to DNA and chromatin. Our data indicate that Sgp3 is involved in a complex process of gp70 production under polygenic control and may provide a significant contribution to lupus susceptibility not only through up-regulation of gp70 autoantigen production but also predisposition to autoimmunity.

Defects in the regulation of B cell apoptosis are required for the production of citrullinated peptide autoantibodies in mice

OBJECTIVE

Protein deimination, a process to modify arginine residues to citrulline by the addition of a neutral oxygen group, is associated with apoptosis. The presence of autoantibodies recognizing citrullinated peptides is highly specific to rheumatoid arthritis (RA) and is therefore a useful marker for the early diagnosis of RA. In this study, we explored whether anti-cyclic citrullinated peptide (anti-CCP) autoantibodies are produced in several experimental models of autoimmune diseases in mice.

METHODS

The levels of anti-CCP autoantibodies were analyzed by enzyme-linked immunosorbent assay in several lupus-prone strains of mice, in animals with type II collagen (CII)-induced arthritis, and after induction of neonatal tolerance to alloantigens.

RESULTS

We observed the production of these autoantibodies in 2 different lupus-prone mice, MRL-lpr/lpr and (NZW x B6)F(1)-hbcl-2 transgenic mice, characterized by the presence of abnormalities in the regulation of B cell apoptosis. Other genetic defects, determining autoimmune susceptibility, present in MRL and NZW mice were additionally required for anti-CCP autoantibody production. The induction of autoantibodies in normal BALB/c mice injected at birth with semiallogeneic spleen cells from (BALB/c x B6)F(1)-hbcl-2 transgenic mice suggested that these additional autoimmune defects may be related, at least in part, to the establishment of abnormal interactions between T cells and B cells. In addition, anti-CCP autoantibodies were not produced in the course of CII-induced arthritis, an experimental model of RA in mice.

CONCLUSION

Our study provides evidence for the association between defects in the regulatory cell death machinery of B lymphocytes and the production of certain autoantibody specificities.

Role of the chemokine stromal cell-derived factor 1 in autoantibody production and nephritis in murine lupus

In normal mice, stromal cell-derived factor 1 (SDF-1/CXCL12) promotes the migration, proliferation, and survival of peritoneal B1a (PerB1a) lymphocytes. Because these cells express a self-reactive repertoire and are expanded in New Zealand Black/New Zealand White (NZB/W) mice, we tested their response to SDF-1 in such mice. PerB1a lymphocytes from NZB/W mice were exceedingly sensitive to SDF-1. This greater sensitivity was due to the NZB genetic background, it was not observed for other B lymphocyte subpopulations, and it was modulated by IL-10. SDF-1 was produced constitutively in the peritoneal cavity and in the spleen. It was also produced by podocytes in the glomeruli of NZB/W mice with nephritis. The administration of antagonists of either SDF-1 or IL-10 early in life prevented the development of autoantibodies, nephritis, and death in NZB/W mice. Initiation of anti-SDF-1 mAb treatment later in life, in mice with established nephritis, inhibited autoantibody production, abolished proteinuria and Ig deposition, and reversed morphological changes in the kidneys. This treatment also counteracted B1a lymphocyte expansion and T lymphocyte activation. Therefore, PerB1a lymphocytes are abnormally sensitive to the combined action of SDF-1 and IL-10 in NZB/W mice, and SDF-1 is key in the development of autoimmunity in this murine model of lupus.

A genetic analysis of lupus

Systemic lupus erythematosus (SLE) is a complex multigenic inherited disease with susceptibility determined by a combination of genetic, environmental and stochastic factors. Although not yet defined, recent technical advances have provided the means to dissect the component genetic contributions of polygenic traits. We have applied such approaches to mouse models of spontaneous SLE and, in this report, summarize our genome wide mapping studies that identified loci predisposing to several major lupus-related traits. Through the generation and study of interval congenic lines, precise mapping, and screening of candidate genes, identification of the specific genes and mechanisms associated with some of the major loci is currently being pursued.

Expansion of marginal zone B cells is not sufficient for the development of renal disease in NZBxNZW F1 mice

The mechanisms which govern the production of autoantibodies and of tissue damage during systemic lupus (SLE) are still unclear. In the NZBxNZW F1 (BW) model of SLE glomerulonephritis, the activation and commitment of B cells are thought to play a major role in disease progression. Previous analysis has suggested that these mice have a substantial increase of marginal zone (MZ) B cells before the occurrence of the disease. Owing to the probable role of this B cell subset in autoantibody production, it was important to define the possible link between this abnormality and the occurrence of kidney damage. Using cytofluorometry analysis, we followed the splenic MZ B cell phenotype in different series of mice with shared autoimmune genetic background and histologically defined renal status. By comparing BW females and BW males, NZB and NZW mice, we confirm that BW mice have an increase in MZ B cells but this MZ B cells expansion is not directly linked to tissue lesions. Genetically modified BW female mice with a restricted repertoire of B and T cell antigen receptors, and which do not develop nephritis, exhibit the same increase of MZ B cells, suggesting that this increase does not depend on a specific set of antigens. Moreover, our analysis brings to light a pre-disease state in BW males, with autoantibody production and mesangial deposits.

A novel susceptibility locus on chromosome 2 in the (New Zealand Black x New Zealand White)F1 hybrid mouse model of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is inherited as a complex polygenic trait. (New Zealand Black (NZB) x New Zealand White (NZW)) F(1) hybrid mice develop symptoms that remarkably resemble human SLE, but (NZB x PL/J)F(1) hybrids do not develop lupus. Our study was conducted using (NZW x PL/J)F(1) x NZB (BWP) mice to determine the effects of the PL/J and the NZW genome on disease. Forty-five percent of BWP female mice had significant proteinuria and 25% died before 12 mo of age compared with (NZB x NZW)F(1) mice in which >90% developed severe renal disease and died before 12 mo. The analysis of BWP mice revealed a novel locus (chi(2) = 25.0; p < 1 x 10(-6); log of likelihood = 6.6 for mortality) designated Wbw1 on chromosome 2, which apparently plays an important role in the development of the disease. We also observed that both H-2 class II (the u haplotype) and TNF-alpha (TNF(z) allele) appear to contribute to the disease. A suggestive linkage to proteinuria and death was found for an NZW allele (designated Wbw2) telomeric to the H-2 locus. The NZW allele that overlaps with the previously described locus Sle1c at the telomeric part of chromosome 1 was associated with antinuclear autoantibody production in the present study. Furthermore, the previously identified Sle and Lbw susceptibility loci were associated with an increased incidence of disease. Thus, multiple NZW alleles including the Wbw1 allele discovered in this study contribute to disease induction, in conjunction with the NZB genome, and the PL/J genome appears to be protective.

Evidence for an interferon-inducible gene, Ifi202, in the susceptibility to systemic lupus

The Nba2 locus is a major genetic contribution to disease susceptibility in the (NZB x NZW)F(1) mouse model of systemic lupus. We generated C57BL/6 mice congenic for this NZB locus, and these mice produced antinuclear autoantibodies characteristic of lupus. F(1) offspring of congenic and NZW mice developed high autoantibody levels and severe lupus nephritis similar to (NZB x NZW)F(1) mice. Expression profiling with oligonucleotide microarrays revealed only two differentially expressed genes, interferon-inducible genes Ifi202 and Ifi203, in congenic versus control mice, and both were within the Nba2 interval. Quantitative PCR localized increased Ifi202 expression to splenic B cells and non-T/non-B cells. These results, together with analyses of promoter region polymorphisms, strain distribution of expression, and effects on cell proliferation and apoptosis, implicate Ifi202 as a candidate gene for lupus.

Autoantigen glycoprotein 70 expression is regulated by a single locus, which acts as a checkpoint for pathogenic anti-glycoprotein 70 autoantibody production and hence for the corresponding development of severe nephritis, in lupus-prone PXSB mice

Retroviral envelope glycoprotein gp70 is present in the sera of immunologically normal and autoimmune-prone strains of mice. However, only lupus-prone mice spontaneously develop gp70-anti-gp70 immune complexes (gp70IC), and these have been implicated in the development of nephritis. We investigated the genetic factors that affect the production of both free serum gp70 and gp70IC in the lupus-prone BXSB mouse strain by analyzing (BXSB x (C57BL/10 x BXSB)F(1))- and (C57BL/10 x (C57BL/10 x BXSB)F(1))-backcrossed male mice. Production of gp70 mapped to a single major locus located on chromosome 13 (Bxs6) with a maximum log likelihood of the odds of 36.7 (p = 1.6 x 10(-38)). The level of gp70IC was highly dependent on Bxs6-related gp70 production, and high titer autoantibody production only occurred when serum gp70 levels were greater than a threshold value of approximately 4.0 microg/ml. The subdivision of the (BXSB x (C57BL/10 x BXSB)F(1))-backcrossed mice into those homozygous or heterozygous for Bxs6 enabled a remarkable association to be observed between high levels of gp70IC and severe nephritis in the Bxs6 homozygote population. A further mapping study in these two subgroups identified a previously unrecognized interval associated with the production of autoantibodies.

Lessons from the NZM2410 model and related strains

SLE susceptibility requires the interplay of an unknown number of genes and equally unidentified triggering events. The past few years have seen significant advances in our understanding of SLE susceptibility through the genetic analysis of murine models. The NZM2410 strain, which is derived from the NZB/WF1 model has played a significant role in these advances. The main advantages presented by this strain over other models are the genetic homozygozity at all loci and an highly penetrant early onset lupus nephritis in both males and females, indicating that the strongest BWF1 susceptibility loci were retained in NZM2410. After identification of NZM2410 susceptibility loci via linkage analyses, congenic strains have been derived in order to convert a polygenic system into a series of monogenic traits. These congenic strains have been analyzed in an integrated process which has provided simultaneously 1) novel functional characterization of the Sle susceptibility loci, 2) high resolution genetic maps that will lead to the identification of the corresponding susceptibility genes by either candidate locus or positional cloning, and 3) insights into the mechanisms by which these loci interact to produce systemic autoimmunity with fatal end-organ damage.

Genetics of systemic autoimmunity in mouse models of lupus

Systemic lupus erythematosus (SLE) is inherited as a complex polygenic trait, involving genetic, environmental and stochastic factors. Although definition of these etiologic processes has been elusive, solid progress has been made toward elucidating the genetic basis for susceptibility. Herein, we summarize our genome wide mapping effort that has defined loci for component phenotypes for lupus-prone NZB, NZW, MRL-Fas(lpr) and BXSB strains. With this framework in place, identification of the specific genetic alterations and mechanisms is now proceeding through the generation of interval congenic lines, precise mapping and screening of candidate genes. In addition to this approach, transgenic and gene knockout studies have begun to identify genes that can induce or modify autoimmunity in nonautoimmune and lupus-prone background mice, including studies by us and others on Th1 and Th2 cytokine genes in lupus. It is apparent that a diversity of genes and mechanisms can independently or in combination promote systemic autoimmunity in mice. This complexity, which is also observed in human lupus, emphasizes the importance of using experimental and less complex mouse models to define these processes, a tactic that has already yielded new insights. With current technologies and the anticipated definition of mammalian genomes, identification of genes predisposing to lupus and elucidation of processes critical for disease pathogenesis appear within grasp.

Lessons from BXSB and related mouse models

The BXSB murine strain spontaneously develops an autoimmune syndrome with features of systemic lupus erythematosus (SLE) that affects males much earlier than females, due to the presence of an as yet unidentified mutant gene located on its Y chromosome, designated Yaa (Y-linked autoimmune acceleration). The Yaa gene by itself is unable to induce significant autoimmune responses in mice without an apparent SLE background, while it can induce and accelerate the development of an SLE in combination with autosomal susceptibility alleles present in lupus-prone mice. Although the genes encoded within or closely linked to the MHC locus play an important role in the development or protection of SLE, the MHC effect can be completely masked by the presence of the Yaa gene in mice highly predisposed to SLE. The role of the Yaa gene for the acceleration of SLE is apparently two-fold; it enhances overall autoimmune responses against autoantigens to which mice respond relatively weakly, and promotes Th 1 responses against autoantigens to which mice respond relatively well, leading to the production of more pathogenic autoantibodies, i.e., FcgammaR-fixing IgG2a and cryoglobulin IgG3 autoantibodies. Yaa+ - Yaa- double bone marrow chimera experiments revealed that the Yaa defect is expressed in B cells, but not in T cells, and that T cells from non-autoimmune mice are capable of providing help for autoimmune responses by collaborating Yaa+ B cells. We speculate that the Yaa defect may decrease the threshold for antigen receptor-dependent stimulation, leading to the triggering and excessive stimulation of autoreactive T and B cells.

Suggestive evidence for association of human chromosome 18q12-q21 and its orthologue on rat and mouse chromosome 18 with several autoimmune diseases

Some immune system disorders, such as type 1 diabetes, multiple sclerosis (MS), and rheumatoid arthritis (RA), share common features: the presence of autoantibodies and self-reactive T-cells, and a genetic association with the major histocompatibility complex. We have previously published evidence, from 1,708 families, for linkage and association of a haplotype of three markers in the D18S487 region of chromosome 18q21 with type 1 diabetes. Here, the three markers were typed in an independent set of 627 families and, although there was evidence for linkage (maximum logarithm of odds score [MLS] = 1.2; P = 0.02), no association was detected. Further linkage analysis revealed suggestive evidence for linkage of chromosome 18q21 to type 1 diabetes in 882 multiplex families (MLS = 2.2; lambdas = 1.2; P = 0.001), and by meta-analysis the orthologous region (also on chromosome 18) is linked to diabetes in rodents (P = 9 x 10(-4)). By meta-analysis, both human chromosome 18q12-q21 and the rodent orthologous region show positive evidence for linkage to an autoimmune phenotype (P = 0.004 and 2 x 10(-8), respectively, empirical P = 0.01 and 2 x 10(-4), respectively). In the diabetes-linked region of chromosome 18q12-q21, a candidate gene, deleted in colorectal carcinoma (DCC), was tested for association with human autoimmunity in 3,380 families with type 1 diabetes, MS, and RA. A haplotype ("2-10") of two newly characterized microsatellite markers within DCC showed evidence for association with autoimmunity (P = 5 x 10(-6)). Collectively, these data suggest that a locus (or loci) exists on human chromosome 18q12-q21 that influences multiple autoimmune diseases and that this association might be conserved between species.

Characterization of CD4(+) T cell autoreactivity to self-MHC in New Zealand hybrid mice

The New Zealand white (NZW) H2(z) locus is strongly associated with the development of autoimmune disease in (NZB x NZW)F(1) mice, a model of systemic lupus erythematosus. To better understand the role of H2(z) in autoimmunity, we generated CD4(+) T cell hybridomas from the spleen and lymph nodes of unimmunized (NZB x NZW)F(1) mice and characterized their specificity. We found that over 50% of the hybridomas responded to syngeneic (H2(d/z)) spleen cells in the absence of exogenous antigen. Many of these autoreactive hybridomas responded to spleen cells expressing H2(z) and used H2(z) class II (I-A(z) or I-E(z)) molecules for presentation. Interestingly, nearly one third of the H2(z)-reactive hybridomas could not respond to spleen cells expressing only H2(z) class II molecules. These studies characterize a frequent population of autoreactive CD4(+) T cells in lupus mice and indicate that major histocompatibility complex molecules in addition to class II may be important for this self-recognition.

Induction of anti-DNA antibody with DNA-peptide complexes

Spontaneous anti-DNA antibodies in autoimmune mice have the characteristics of antibodies produced by antigen-specific, clonally selective B cell stimulation. The nature of the somatically derived antibody variable region structures recurrent among spontaneous anti-DNA antibodies suggests that DNA or DNA-protein complexes may provide the antigenic stimulus for autoimmune anti-DNA antibody. Previously we have demonstrated that native mammalian DNA in complexes with an immunogenic DNA-binding peptide Fus1 from Trypanosoma cruzi can induce anti-DNA antibody in mice not genetically prone to autoimmune disease. The induced anti-DNA has similar specificity, structure and immunopathological function as autoimmune anti-DNA. The present experiments were designed to further characterize the immune response to DNA-peptide complexes. There was considerable variation in the antibody responses of mice from different strains to DNA-Fus1 immunizations. The range was from virtually no response in C57BL/6 mice to most robust responses in NZW mice. The full-length 52 amino acid carboxy-extension protein of ubiquitin (CEP) in T. cruzi (TCEP) protein from which Fus1 was derived functions equally well as an immunogenic carrier for DNA. Anti-DNA responses were generally weak even though anti-Fus1 and anti-TCEP responses were very strong. The results are discussed with respect to the contrasting roles of T cell help and peripheral B cell tolerance in controlling immune and autoimmune antibody responses to DNA.

Anti-DNA and autoantibodies

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

Mixed connective tissue disease

A defining feature of mixed connective tissue disease (MCTD) is the presence of antibodies against the U1-ribonucleoprotein (RNP) complex, but other autoantibodies in MCTD have recently been described. Research has also further elucidated the immune responses directed against U1-RNP in humans and in murine models of disease. Hypotheses implicating modified self-antigens and/or infectious agents in the pathogenesis of MCTD have been advanced. Links between the immunologic and clinical phenomena in MCTD are emerging. Longitudinal study of patients with MCTD highlights the impact of pulmonary hypertension on disease outcome.

Genetic control of glycoprotein 70 autoantigen production and its influence on immune complex levels and nephritis in murine lupus

The F1 hybrids of New Zealand Black (NZB) and New Zealand White (NZW) mice spontaneously develop an autoimmune disease that serves as a model for human systemic lupus erythematosus. Autoimmunity in (NZB x NZW)F1 mice includes the production of autoantibodies to the endogenous retroviral envelope glycoprotein, gp70, and gp70-anti-gp70 immune complexes (gp70 IC) have been implicated in the development of lupus nephritis in these animals. We used backcross and intercross combinations of C57BL/6 (B6; low gp70 levels) and NZB mice (high gp70 levels) to examine the contribution of serum gp70 Ag levels to the development of gp70 IC and nephritis. Analysis of (B6.H2z x NZB)F1 x NZB backcross mice and (NZB x B6)F2 mice showed a much stronger association of gp70 IC with kidney disease compared with IgG anti-chromatin autoantibodies in both populations of mice. Serum levels of gp70 correlated with production of gp70 IC in mice producing autoantibodies, although the overall effect on nephritis appeared to be small. Genetic mapping revealed three NZB-derived regions on chromosomes 2, 4, and 13 that were strongly linked with increased gp70 levels, and together, accounted for over 80% of the variance for this trait. However, additional linkage analyses of these crosses showed that loci controlling autoantibody production rather than gp70 levels were most important in the development of nephritogenic immune complexes. Together, these studies characterize a set of lupus-susceptibility loci distinct from those that control autoantibody production and provide new insight into the components involved in the strong association of gp70 IC with murine lupus nephritis.

The genetics of systemic lupus erythematosus

BACKGROUND

There is a genetic predisposition to human systemic lupus erythematosus (SLE). The genes that contribute to susceptibility are, for the most part, unknown. The introduction of new gene mapping techniques has opened the way to explore lupus genetics on a genome-wide basis.

METHODS

Microsatellites are simple sequence repeats widely distributed throughout eukaryotic genomes. They exhibit length variation. This polymorphism can be exploited to provide a panoply of genome-wide markers. Thereby, loci linked with lupus have been mapped in lupus-prone mouse strains and in recently published studies in multi-case human families.

RESULTS

More than 20 non-MHC (major histocompatibility complex) loci have now been linked with murine lupus. Nine non-MHC loci have been corroborated in human SLE. Some of the mouse intervals are syntenic with human loci raising the tantalizing possibility of common susceptibility genes. Although we await the results of formal gene identification, functional studies in back-cross and congenic analyses indicate that, in the mouse at least, disease genes act at multiple levels in disease development.

CONCLUSIONS

A large number of genes are involved in the pathogenesis of SLE. The data also suggest that even the MHC contribution is multiple. Having mapped disease loci, geneticists now face the task of closing down on the actual aetiological alleles and demonstrating how they might operate. This undertaking will add significantly to our understanding of disease development.

Enhanced susceptibility to lupus contributed from the nonautoimmune C57BL/10, but not C57BL/6, genome

Genes from New Zealand Black and New Zealand White mice have been implicated in the development of a disease similar to human systemic lupus erythematosus. In an attempt to define the MHC class II genes involved in disease, we previously studied similarly designed backcrosses of New Zealand Black mice with C57BL/6 (B6) mice transgenic for Ez genes or with C57BL/10 (B10) mice transgenic for Az genes. Although the transgenes showed no effect on the development of autoantibody production or lupus nephritis in either backcross, surprisingly, there was greatly increased expression of these disease traits in the backcrosses involving B10 compared with B6 mice. These studies therefore implicated genetic contributions in B10 vs B6 backgrounds, despite their 98% identity. A genome-wide linkage analysis uncovered a B10 locus on mid-chromosome 13, which enhanced nephritis and was strongly linked with the production of pathogenic retroviral gp70-anti-gp70 immune complexes when contributed by B10, but not B6, mice. The subsequent identification of a single marker polymorphic between B10 and B6, along with the extreme genetic similarity between the two strains in this region, is likely to permit expedited identification of the lupus-susceptibility gene from this nonautoimmune strain.

Mapping autoimmunity genes

Rat and mouse models for the major human autoimmune/inflammatory diseases are under intense genetic scrutiny. Genome-wide linkage studies reveal that each model is regulated by multiple genetic loci. Many of these loci colocalize to homologous genomic regions associated with several different autoimmune diseases of mice, rats and humans. Candidate genes are being identified. Polymorphic alleles associated with these chromosomal segments may represent predisposing genetic elements common to a number of human diseases with very different clinical presentations.