Genetic interactions between susceptibility loci reveal epistatic pathogenic networks in murine lupus

Glycolysis inhibition functionally reprograms T follicular helper cells and reverses lupus

Systemic lupus erythematosus (SLE) is an autoimmune disease in which the production of pathogenic autoantibodies depends on T follicular helper (T FH ) cells. This study was designed to investigate the mechanisms by which inhibition of glycolysis with 2-deoxy-d-glucose (2DG) reduces the expansion of T FH cells and the associated autoantibody production in lupus-prone mice. Integrated cellular, transcriptomic, epigenetic and metabolic analyses showed that 2DG reversed the enhanced cell expansion and effector functions, as well as mitochondrial and lysosomal defects in lupus T FH cells, which include an increased chaperone-mediated autophagy induced by TLR7 activation. Importantly, adoptive transfer of 2DG-reprogrammed T FH cells protected lupus-prone mice from disease progression. Orthologs of genes responsive to 2DG in murine lupus T FH cells were overexpressed in the T FH cells of SLE patients, suggesting a therapeutic potential of targeting glycolysis to eliminate aberrant T FH cells and curb the production of autoantibodies inducing tissue damage.

TLR7 Activation Accelerates Cardiovascular Pathology in a Mouse Model of Lupus

We report a novel model of lupus-associated cardiovascular pathology accelerated by the TLR7 agonist R848 in lupus-prone B6.Sle1.Sle2.Sle3 (TC) mice. R848-treated TC mice but not non-autoimmune C57BL/6 (B6) controls developed microvascular inflammation and myocytolysis with intracellular vacuolization. This histopathology was similar to antibody-mediated rejection after heart transplant, although it did not involve complement. The TC or B6 recipients of serum or splenocytes from R848-treated TC mice developed a reactive cardiomyocyte hypertrophy, which also presents spontaneously in old TC mice as well as in TC.Rag-/- mice that lack B and T cells. Each of these cardiovascular lesions correspond to abnormalities that have been reported in lupus patients. Lymphoid and non-lymphoid immune cells as well as soluble factors contribute to lupus-associated cardiovascular lesions in TC mice, which can now be dissected using this model with and without R848 treatment.

Kidney tissue hypoxia dictates T cell-mediated injury in murine lupus nephritis

The kidney is a frequent target of autoimmune injury, including in systemic lupus erythematosus; however, how immune cells adapt to kidney's unique environment and contribute to tissue damage is unknown. We found that renal tissue, which normally has low oxygen tension, becomes more hypoxic in lupus nephritis. In the injured mouse tissue, renal-infiltrating CD4⁺ and CD8⁺ T cells express hypoxia-inducible factor-1 (HIF-1), which alters their cellular metabolism and prevents their apoptosis in hypoxia. HIF-1-dependent gene-regulated pathways were also up-regulated in renal-infiltrating T cells in human lupus nephritis. Perturbation of these environmental adaptations by selective HIF-1 blockade inhibited infiltrating T cells and reversed tissue hypoxia and injury in murine models of lupus. The results suggest that targeting HIF-1 might be effective for treating renal injury in autoimmune diseases.

Estrogen Receptor Alpha Signaling Is Responsible for the Female Sex Bias in the Loss of Tolerance and Immune Cell Activation Induced by the Lupus Susceptibility Locus Sle1b

The dramatic female sex bias observed in human lupus is thought to be due, at least in part, to estrogens. Using mouse models, we have shown that estrogens, acting through estrogen receptor alpha (ERα) promote lupus development and contribute significantly to the female sex bias observed in this disease. C57Bl/6 (B6) mice carrying the lupus susceptibility locus Sle1 locus exhibit immune cell hyperactivation and loss of tolerance, and the action of Sle1 displays a strong female sex bias. Previously, we showed that disruption of ERα completely eliminates the female sex bias in the effects of Sle1. Here we report that ERα signaling selectively modulates the action of Sle1b, one of the three subloci that together constitute Sle1. We observed that disruption of ERα signaling attenuated T cell hyperactivation, formation of spontaneous germinal centers, loss of tolerance, and the development of anti-chromatin autoantibodies in B6.Sle1b female mice, but had no impact on these phenotypes in B6.Sle1b male mice. In fact, disruption of ERα completely abolished the female sex bias that is seen in each of these phenotypes in B6.Sle1b mice. Strikingly, Sle1b-induced B cell hyperactivation, a female sex-specific manifestation of Sle1b, was completely abrogated by disruption of ERα in B6.Sle1b females. Altogether, these results demonstrate that ERα signaling is responsible for the female sex bias in the actions of Sle1b, and is absolutely required for the female-specific B cell hyperactivation phenotype associated with this lupus susceptibility locus. By contrast, we found that ERα signaling had no impact on Sle1a, the other Sle1 sublocus that exerts effects that show a female sex bias.

Toll-like receptors in lupus nephritis

The pathogenesis of systemic autoimmune diseases such as systemic lupus erythematosus (SLE) is based on the loss of self-tolerance against ubiquitous autoantigens involving all mechanisms of adaptive immunity. However, data accumulating over the last decade imply an important role also for numerous elements of innate immunity, namely the Toll-like receptors in the pathogenesis of SLE. Here we discuss their role in the most common organ complication of SLE, i.e. lupus nephritis. We summarize experimental and clinical data on the expression and functional contribution of the Toll-like receptors in immune complex glomerulonephritis, and intrarenal inflammation. Based on these discoveries Toll-like receptors are evolving as therapeutic targets for the treatment of SLE and lupus nephritis.

Resveratrol counters systemic lupus erythematosus-associated atherogenicity by normalizing cholesterol efflux

Resveratrol is a bioactive molecule used in dietary supplements and herbal medicines and consumed worldwide. Numerous investigations by our group and others have indicated cardioprotective and anti-inflammatory properties of resveratrol. The present study explored potential atheroprotective actions of resveratrol on cholesterol efflux in cultured human macrophages exposed to plasma from systemic lupus erythematosus (SLE) patients. These results were confirmed in ApoE(-/-)Fas(-/-) double knockout mice, displaying a lupus profile with accelerated atherosclerosis. Resveratrol treatment attenuated atherosclerosis in these mice. THP-1 human macrophages were exposed to 10% pooled or individual plasma from patients who met diagnostic criteria for SLE. Expression of multiple proteins involved in reverse cholesterol transport (ABCA1, ABCG1, SR-B1, and cytochrome P450 27-hydroxylase) was assessed using QRT-PCR and Western blotting techniques. Ten-week-old ApoE(-/-)Fas(-/-) double knockout mice (n = 30) were randomly divided into two equal groups of 15, one of which received 0.01% resveratrol for 10 consecutive weeks. Atherosclerosis progression was evaluated in murine aortas. Bone marrow-derived macrophages (BMDM) were cultured and expression of cholesterol efflux proteins was analyzed in each group of mice. Our data indicate that inhibition of cholesterol efflux by lupus plasma in THP-1 human macrophages is rescued by resveratrol. Similarly, administration of resveratrol in a lupus-like murine model reduces plaque formation in vivo and augments cholesterol efflux in BMDM. This study presents evidence for a beneficial role of resveratrol in atherosclerosis in the specific setting of SLE. Therefore, resveratrol may merit investigation as an additional resource available to reduce lipid deposition and atherosclerosis in humans, especially in such vulnerable populations as lupus patients.

B cell-intrinsic TLR7 signaling is essential for the development of spontaneous germinal centers

Spontaneous germinal center (Spt-GC) B cells and follicular helper T cells generate high-affinity autoantibodies that are involved in the development of systemic lupus erythematosus. TLRs play a pivotal role in systemic lupus erythematosus pathogenesis. Although previous studies focused on the B cell-intrinsic role of TLR-MyD88 signaling on immune activation, autoantibody repertoire, and systemic inflammation, the mechanisms by which TLRs control the formation of Spt-GCs remain unclear. Using nonautoimmune C57BL/6 (B6) mice deficient in MyD88, TLR2, TLR3, TLR4, TLR7, or TLR9, we identified B cell-intrinsic TLR7 signaling as a prerequisite to Spt-GC formation without the confounding effects of autoimmune susceptibility genes and the overexpression of TLRs. TLR7 deficiency also rendered autoimmune B6.Sle1b mice unable to form Spt-GCs, leading to markedly decreased autoantibodies. Conversely, B6.yaa and B6.Sle1b.yaa mice expressing an extra copy of TLR7 and B6.Sle1b mice treated with a TLR7 agonist had increased Spt-GCs and follicular helper T cells. Further, TLR7/MyD88 deficiency led to compromised B cell proliferation and survival after B cell stimulation both in vitro and in vivo. In contrast, TLR9 inhibited Spt-GC development. Our findings demonstrate an absolute requirement for TLR7 and a negative regulatory function for TLR9 in Spt-GC formation under nonautoimmune and autoimmune conditions. Our data suggest that, under nonautoimmune conditions, Spt-GCs initiated by TLR7 produce protective Abs. However, in the presence of autoimmune susceptibility genes, TLR7-dependent Spt-GCs produce pathogenic autoantibodies. Thus, a single copy of TLR7 in B cells is the minimal requirement for breaking the GC-tolerance checkpoint.

The NZM2410-derived lupus susceptibility locus Sle2c1 increases Th17 polarization and induces nephritis in fas-deficient mice

OBJECTIVE

Sle2 is a lupus susceptibility locus that has been linked to glomerulonephritis in the NZM2410 mouse. By itself, Sle2 does not induce any autoimmune pathology but results in the accumulation of B-1a cells. This study was designed to assess the contribution of Sle2 to the pathogenesis of autoimmunity.

METHODS

Sle2 or its subcongenic intervals (Sle2a, Sle2b, and Sle2c1) were bred to Fas-deficient B6.lpr mice. Lymphoid phenotypes, which were focused on T cells, were assessed by flow cytometry, and histopathologic changes were compared between cohorts of B6.Sle2.lpr congenic mice and B6.lpr mice of ages up to 6 months.

RESULTS

Sle2 synergized with lpr, resulting in a greatly accelerated lymphadenopathy that largely targeted T cells and mapped to the Sle2c1 locus. This locus has been identified as the main contributor to B-1a cell expansion. Further analyses showed that Sle2c1 expression skewed the differentiation and polarization of Fas-deficient T cells, with a reduction of the CD4+CD25+FoxP3+ regulatory T cell subset and an expansion of the Th17 cells. This was associated with a high number of T cell infiltrates that promoted severe nephritis and dermatitis in the B6.Sle2c1.lpr mice.

CONCLUSION

These results show that Sle2c1 contributes to lupus pathogenesis by affecting T cell differentiation in combination with other susceptibility loci, such as lpr. The significance of the cosegregation of this phenotype and B-1a cell expansion in Sle2c1-expressing mice in relation to the pathogenesis of lupus is discussed.

TLR7 and TLR9 in SLE: when sensing self goes wrong

Autoreactive B and T cells are present in healthy, autoimmunity-free individuals, but they are kept in check by various regulatory mechanisms. In systemic lupus erythematosus (SLE) patients, however, autoreactive cells are expanded, activated, and produce large quantities of autoantibodies, directed especially against nuclear antigens. These antibodies form immune complexes with self-nucleic acids present in SLE serum. Since self-DNA and self-RNA in the form of protein complexes can act as TLR9 and TLR7 ligands, respectively, TLR stimulation is suggested as an additional signal contributing to activation and/or modulation of the aberrant adaptive immune response. Data from mouse models suggest a pathogenic role for TLR7 and a protective role for TLR9 in the pathogenesis of SLE. Future investigations are needed to elucidate the underlying modulatory mechanisms and the role of TLR7 and TLR9 in the complex pathogenesis of human SLE.

The lupus-prone NZM2410/NZW strain-derived Sle1b sublocus alters the germinal center checkpoint in female mice in a B cell-intrinsic manner

C57BL/6 (B6) mice carrying the Sle1b sublocus (named B6.Sle1b), which harbors the lupus-associated NZM2410/NZW SLAM family genes, produce antinuclear Abs (ANAs). However, the role and mechanism(s) involved in the alteration of the germinal center (GC) tolerance checkpoint in the development of ANAs in these mice is not defined. In this study, we show significantly higher spontaneously formed GCs (Spt-GCs) in B6.Sle1b female mice compared with B6 controls. We also found a significant increase in CD4(+)CXCR5(hi)PD-1(hi) spontaneously activated follicular Th cells in B6.Sle1b female mice. Compared with B6 controls, B6.Sle1b female mice had increased numbers of proliferating B cells predominantly located in Spt-GCs. The elevated Spt-GCs in B6.Sle1b female mice were strongly associated with increased ANA-specific Ab-forming cells and ANA titers. The increased numbers of Spt-GCs and spontaneously activated follicular Th cells in B6.Sle1b mice were not the result of a generalized defect in B cells expressing Sle1b. Consistent with the elevated spontaneous response in B6.Sle1b mice, the attenuated GC response characteristic of DNA and p-azophenylarsonate reactive B cells from Ig V(H) knock-in mice (termed HKIR) were relieved in adoptively transferred recipients in the presence of Sle1b. Finally, by generating mixed bone marrow chimeras, we showed that the effect of Sle1b on Spt-GC, follicular Th cell, and autoantibody responses in B6.Sle1b mice was B cell autonomous. These data indicate that the NZM2410/NZW-derived Sle1b sublocus in conjunction with the female sex primarily affects B cells, leading to the alteration of the GC tolerance checkpoint and the generation of ANA-specific Ab-forming cells.

Cutting edge: an NK cell-independent role for Slamf4 in controlling humoral autoimmunity

Several genes within a syntenic region of human and mouse chromosome 1 are associated with predisposition to systemic lupus erythematosus. Analyses of lupus-prone congenic mice have pointed to an important role for the signaling lymphocyte activation molecule family (slamf)6 surface receptor in lupus pathogenesis. In this article, we demonstrate that a second member of the Slamf gene family, Slamf4 (Cd244), contributes to lupus-related autoimmunity. B6.Slamf4(-/-) mice spontaneously develop activated CD4 T cells and B cells and increased numbers of T follicular helper cells and a proportion develop autoantibodies to nuclear Ags. B6.Slamf4(-/-) mice also exhibit markedly increased autoantibody production in the B6.C-H-2bm12/KhEg → B6 transfer model of lupus. Although slamf4 function is best characterized in NK cells, the enhanced humoral autoimmunity of B6.Slamf4(-/-) mice is NK cell independent, as judged by depletion studies. Taken together, our findings reveal that slamf4 has an NK cell-independent negative regulatory role in the pathogenesis of lupus a normally non-autoimmune prone genetic background.

Murine lupus susceptibility locus Sle1a requires the expression of two sub-loci to induce inflammatory T cells

The NZM2410-derived Sle1a lupus susceptibility locus induces activated autoreactive CD4⁺ T cells and reduces the number and function of Foxp3⁺ regulatory T cells. In this study, we first showed that Sle1a contributes to autoimmunity by increasing anti-nuclear antibody production when expressed on either NZB or NZW heterozygous genomes, and by enhancing the chronic graft vs. host disease response indicating an expansion of the autoreactive B cell pool. Screening two non-overlapping recombinants, the Sle1a.1 and Sle1a.2 intervals that cover the entire Sle1a locus, revealed that both Sle1a.1 and Sle1a.2 were necessary for the full Sle1a phenotype. Sle1a.1, and to a lesser extent Sle1a.2, significantly affected CD4⁺ T cell activation as well as Treg differentiation and function. Sle1a.2 also increased the production of autoreactive B cells. Since the Sle1a.1 and Sle1a.2 intervals contain only one and 15 known genes, respectively, this study considerably reduces the number of candidate genes responsible for the production of autoreactive T cells. These results also demonstrate that the Sle1 locus is an excellent model for the genetic architecture of lupus, in which a major obligate phenotype results from the co-expression of multiple genetic variants with individual weak effects.

SLAM family receptors and the SLAM-associated protein (SAP) modulate T cell functions

One or more of the signaling lymphocytic activation molecule (SLAM) family (SLAMF) of cell surface receptors, which consists of nine transmembrane proteins, i.e., SLAMF1-9, are expressed on most hematopoietic cells. While most SLAMF receptors serve as self-ligands, SLAMF2 and SLAMF4 use each other as counter structures. Six of the receptors carry one or more copies of a unique intracellular tyrosine-based switch motif, which has high affinity for the single SH2-domain signaling molecules SLAM-associated protein and EAT-2. Whereas SLAMF receptors are costimulatory molecules on the surface of CD4+, CD8+, and natural killer (NK) T cells, they also involved in early phases of lineage commitment during hematopoiesis. SLAMF receptors regulate T lymphocyte development and function and modulate lytic activity, cytokine production, and major histocompatibility complex-independent cell inhibition of NK cells. Furthermore, they modulate B cell activation and memory generation, neutrophil, dendritic cell, macrophage and eosinophil function, and platelet aggregation. In this review, we will discuss the role of SLAM receptors and their adapters in T cell function, and we will examine the role of these receptors and their adapters in X-linked lymphoproliferative disease and their contribution to disease susceptibility in systemic lupus erythematosus.

Interleukin-6 aborts lymphopoiesis and elevates production of myeloid cells in systemic lupus erythematosus-prone B6.Sle1.Yaa animals

We previously reported the inhibitory action of interleukin-6 (IL-6) on B lymphopoiesis with SHIP(-/-) mice and showed that IL-6 biases lineage commitment toward myeloid cell fates in vitro and in vivo. Because elevated IL-6 is a feature of chronic inflammatory diseases, we applied an animal model of systemic lupus erythematosus (SLE) to determine whether IL-6 has similar effects on hematopoiesis. We found that IL-6 levels were elevated in the B6.Sle1.Yaa mice, and the increase was accompanied by losses of CD19(+) B cells and more primitive B-lymphoid progenitors in bone marrow. Both the CD19(+) B-cell population and their progenitors recovered in an IL-6(-/-) background. The uncommitted progenitors, containing precursors for both lymphoid and myeloid fates, expressed IL-6 receptor-alpha chain and responded to IL-6 by phosphorylation of STAT3. IL-6 stimulation caused uncommitted progenitors to express the Id1 transcription factor, which is known to inhibit lymphopoiesis and elevate myelopoiesis, and its expression was MAPK dependent. We conclude that chronic inflammatory conditions accompanied by increased IL-6 production bias uncommitted progenitors to a myeloid fate by inducing Id1 expression.

Defective B-cell response to T-dependent immunization in lupus-prone mice

Lupus anti-nuclear Ab show the characteristics of Ag-driven T-cell-dependent (TD) humoral responses. If autoAg elicit the same response as exogenous Ag, lupus should enhance humoral responses to immunization. Blunted responses to various immunizations have, however, been reported in a significant portion of lupus patients. In this study, we show that lupus-prone C57BL/6.Sle1.Sle2.Sle3 (B6.TC) mice produce significantly less Ab in response to TD immunization than congenic controls, while producing significantly more total Ig. This blunted Ab response to TD Ag could be reconstituted with B6.TC B and CD4+ T cells. Multiple defects were found in the B6.TC response to 4-hydroxy-3-nitrophenylacetyl-keyhole limpet hemocyanin (NP-KLH) compared with total Ig, including a smaller percentage of B cells participating in the NP-response, a reduced entry into germinal centers, and highly defective production of NP-specific long-lived plasma cells (PC) in the bone marrow. B6.TC PC expressed reduced levels of FcgammaRIIb, which suggests that reduced apoptosis in resident PC prevents the establishment of newly formed NP-specific PC in bone marrow niches. Overall, these results show that lupus-prone mice responded differently to auto- and exogenous Ag and suggest that low FcgammaRIIb, hypergammaglobulinemia, and high autoAb production would be predictive of a poor response to immunization in lupus patients.

The SLAM and SAP gene families control innate and adaptive immune responses

The nine SLAM-family genes, SLAMF1-9, a subfamily of the immunoglobulin superfamily, encode differentially expressed cell-surface receptors of hematopoietic cells. Engagement with their ligands, which are predominantly homotypic, leads to distinct signal transduction events, for instance those that occur in the T or NK cell immune synapse. Upon phosphorylation of one or more copies of a unique tyrosine-based signaling motif in their cytoplasmic tails, six of the SLAM receptors recruit the highly specific single SH2-domain adapters SLAM-associated protein (SAP), EAT-2A, and/or EAT-2B. These adapters in turn bind to the tyrosine kinase Fyn and/or other protein tyrosine kinases connecting the receptors to signal transduction networks. Individuals deficient in the SAP gene, SH2D1A, develop an immunodeficiency syndrome: X-linked lympho-proliferative disease. In addition to operating in the immune synapse, SLAM receptors initiate or partake in multiple effector functions of hematopoietic cells, for example, neutrophil and macrophage killing and platelet aggregation. Here we discuss the current understanding of the structure and function of these recently discovered receptors and adapter molecules in the regulation of adaptive and innate immune responses.

Monocytosis in BXSB mice is due to epistasis between Yaa and the telomeric region of chromosome 1 but does not drive the disease process

The BXSB murine model of systemic lupus erythematosus is differentiated from other murine models of lupus by a severe monocytosis. The recently identified Y-linked autoimmune accelerator locus, Yaa, which is fundamental to accelerated disease in male BXSB mice, is required for the monocytic phenotype in BXSB. It has also recently been shown to induce monocytosis in combination with the Nba2 locus from NZB. To dissect the genetic basis and associated pathogenicity of BXSB-related monocytosis, a panel of existing congenic mice were studied and a novel sub-congenic mouse B10.Y(BXSB).BXSB-Bxs3 was generated. Monocytosis was found to be caused by an epistatic interaction between Yaa and the telomeric region of chromosome 1, an area of approximately 30 cM. Bxs3 and Yaa together were sufficient to generate monocytosis equivalent to that of BXSB. In contrast to the NZB model, however, where monocytosis tightly correlated with autoantibody production and lethal lupus nephritis, this was not the case in BXSB. While Yaa(+) mice bearing the Bxs3 locus drive monocytosis, glomerulonephritis and autoantibody production, both autoantibody production and nephritis are discreet events that occur in the absence of the Bxs3 locus. Yaa is a pre-requisite for monocytosis, demonstrating a novel synergistic interaction between Yaa and Bxs3.

Expression of the autoimmune Fcgr2b NZW allele fails to be upregulated in germinal center B cells and is associated with increased IgG production

The inhibitory receptor FcgammaRIIb regulates B-cell functions. Genetic studies have associated Fcgr2b polymorphisms and lupus susceptibility in both humans and murine models, in which B cells express reduced FcgammaRIIb levels. Furthermore, FcgammaRIIb absence results in lupus on the appropriate genetic background, and lentiviral-mediated FcgammaRIIb overexpression prevents disease in the NZM2410 lupus mouse. The NZM2410/NZW allele Fcgr2b is, however, located in-between Sle1a and Sle1b, two potent susceptibility loci, making it difficult to evaluate Fcr2b(NZW) independent contribution. By using two congenic strains that each carries only Sle1a (B6.Sle1a(15-353)), or Fcr2b(NZW) in the absence of Sle1a or Sle1b (B6.Sle1(111-148)), we show that the Fcr2b(NZW) allele does not upregulate its expression on germinal center B cells and plasma cells, as does the C57BL/6 allele on B6.Sle1a(15-353) B cells. Furthermore, in the absence of the flanking Sle1a and Sle1b, Fcr2b(NZW) does not produce an autoimmune phenotype, but is associated with an increased number of class-switched plasma cells. These results show that while a lower level of FcgammaRIIb does not by itself induce the development of autoreactive B cells, it has the potential to amplify the contribution of autoreactive B cells induced by other lupus-susceptibility loci by enhancing the production of class-switched plasma cells.

Augmentation of NZB autoimmune phenotypes by the Sle1c murine lupus susceptibility interval

The Sle1c lupus susceptibility interval spans a 7-Mb region on distal murine chromosome 1. Cr2 is the strongest candidate gene for lupus susceptibility in this interval, as its protein products are structurally and functionally altered. B6.Sle1c congenic mice develop Abs to chromatin by 9 mo of age with a 30% penetrance and do not develop GN. To determine whether the New Zealand White (NZW)-derived Sle1c interval would interact with New Zealand Black (NZB) genes to result in enhanced autoimmune phenotypes, NZB mice were bred with B6 or B6.Sle1c congenic mice and approximately 20 female offspring were selected from each breeding for longitudinal study. These mice differ only at the Sle1c locus at which they have either a NZB/B6 or NZB/NZW genotype. NZB x B6.Sle1c mice had an accelerated onset of anti-chromatin Abs (100 vs 68% at 6 mo, p = 0.006) and anti-dsDNA Abs (45 vs 5% at 9 mo, p = 0.0048). Furthermore, median titers of anti-chromatin and anti-dsDNA Abs were significantly higher in the NZB x B6.Sle1c group compared with the NZB x B6 group. This corresponded with a higher prevalence of proliferative GN at 12 mo (55 vs 16%, p = 0.0214) as well as increased glomerular deposition of C3 (p = 0.0272) and IgG (p = 0.032), although blood urea nitrogen remained normal and significant proteinuria was not identified in either group. These data show that the Sle1c interval accelerates and augments the loss of tolerance to chromatin and dsDNA induced by NZB genes and induces significantly greater end-organ damage.

Systemic lupus erythematosus: multiple immunological phenotypes in a complex genetic disease

Systemic lupus erythematosus (SLE) is a complex polygenic autoimmune disease characterized by the presence of anti-nuclear autoantibodies (ANAs) that are often detectable years prior to the onset of clinical disease. The disease is associated with a chronic activation of the immune system, with the most severe forms progressing to inflammatory damage that can impact multiple organ systems in afflicted individuals. Current therapeutic strategies poorly control disease manifestations and are generally immunosuppressive. Recent studies in human patient populations and animal models have associated elements of the innate immune system and abnormalities in the immature B lymphocyte receptor repertoires with disease initiation. A variety of cytokines, most notably type I interferons, play important roles in disease pathogenesis and effector mechanisms. The genetic basis for disease susceptibility is complex, and analyses in humans and mice have identified multiple susceptibility loci, several of which are located in genomic regions that are syntenic between humans and mice. The complexities of the genetic interactions that mediate lupus have been investigated in murine model systems by characterizing the progressive development of disease in strains expressing various combinations of susceptibility alleles. These analyses indicate that genetic epistasis dramatically impact disease development and support the feasibility of identifying molecular pathways that can suppress disease progression without completely impairing normal immune function.

Genetics of SLE in mice

Genetic studies in spontaneous, induced, and gene-manipulated mouse models of SLE have provided significant insights into the potential number and diversity of genes that can promote, resist, and modify lupus susceptibility. Novel genes and mechanisms of disease pathogenesis have also been identified. Importantly, mouse models have provided an initial view of the genomic landscape of lupus-affecting genes, and have documented the complexities of verifying and determining the role of specific candidate loci and genes. Mouse models of lupus should continue to serve as a vital approach to defining the genetics of SLE.

A Tlr7 translocation accelerates systemic autoimmunity in murine lupus

The y-linked autoimmune accelerating (yaa) locus is a potent autoimmune disease allele. Transcription profiling of yaa-bearing B cells revealed the overexpression of a cluster of X-linked genes that included Tlr7. FISH analysis demonstrated the translocation of this segment onto the yaa chromosome. The resulting overexpression of Tlr7 increased in vitro responses to Toll-like receptor (TLR) 7 signaling in all yaa-bearing males. B6.yaa mice are not overtly autoimmune, but the addition of Sle1, which contains the autoimmune-predisposing Slam/Cd2 haplotype, causes the development of fatal lupus with numerous immunological aberrations. B6.Sle1yaa CD4 T cells develop the molecular signature for T(FH) cells and also show expression changes in numerous cytokines and chemokines. Disease development and all component autoimmune phenotypes were inhibited by Sles1, a potent suppressor locus. Sles1 had no effect on yaa-enhanced TLR7 signaling in vitro, and these data place Sles1 downstream from the lesion in innate immune responses mediated by TLR7, suggesting that Sles1 modulates the activation of adaptive immunity in response to innate immune signaling.

Genetic dissection of the murine lupus susceptibility locus Sle2: contributions to increased peritoneal B-1a cells and lupus nephritis map to different loci

Lupus pathogenesis in the NZM2410 mouse model results from the expression of multiple interacting susceptibility loci. Sle2 on chromosome 4 was significantly linked to glomerulonephritis in a linkage analysis of a NZM2410 x B6 cross. Yet, Sle2 expression alone on a C57BL/6 background did not result in any clinical manifestation, but in an abnormal B cell development, including the accumulation of B-1a cells in the peritoneal cavity and spleen. Analysis of B6.Sle2 congenic recombinants showed that at least three independent loci, New Zealand White-derived Sle2a and Sle2b, and New Zealand Black-derived Sle2c, contribute to an elevated number of B-1a cells, with Sle2c contribution being the strongest of the three. To determine the contribution of these three Sle2 loci to lupus pathogenesis, we used a mapping by genetic interaction strategy, in which we bred them to B6.Sle1.Sle3 mice. We then compared the phenotypes of these triple congenic mice with that of previously characterized B6.Sle1.Sle2.Sle3, which express the entire Sle2 interval in combination with Sle1 and Sle3. Sle2a and Sle2b, but not Sle2c, contributed significantly to lupus pathogenesis in terms of survival rate, lymphocytic expansion, and kidney pathology. These results show that the Sle2 locus contains several loci affecting B cell development, with only the two NZW-derived loci having the least effect of B-1a cell accumulation significantly contributing to lupus pathogenesis.

Epistatic suppression of systemic lupus erythematosus: fine mapping of Sles1 to less than 1 mb

Sle is a susceptibility locus for systemic autoimmunity derived from the lupus-prone NZM2410 mouse. The New Zealand White-derived suppressive modifier Sles1 was identified as a specific modifier of Sle1 and prevents the development of IgG anti-chromatin autoantibodies mediated by Sle1 on the C57BL/6 (B6) background. Fine mapping of Sles1 with truncated congenic intervals localizes it to a approximately 956-kb segment of mouse chromosome 17. Sles1 completely abrogates the development of activated T and B cell populations in B6.Sle1. Despite this suppression of the Sle1-mediated cell surface activation phenotypes, B6.Sle1 Sles1 splenic B cells still exhibit intrinsic ERK phosphorylation. Classic genetic complementation tests using the nonautoimmmune 129/SvJ mouse suggests that this strain possesses a Sles1 allele complementary to that of New Zealand White, as evidenced by the lack of glomerulonephritis, splenomegaly, and antinuclear autoantibody production seen in (129 x B6.Sle1 Sles1)F(1)s. These findings localize and characterize the suppressive properties of Sles1 and implicate 129 as a useful strain for aiding in the identification of this elusive epistatic modifier gene.

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

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

Genetic Determination of T Cell Help in Loss of Tolerance to Nuclear Antigens

Sle1 is a major lupus susceptibility locus in NZM2410 lupus model that is associated with a loss of tolerance to nuclear Ags. At least three genes, Sle1a, Sle1b, and Sle1c contribute to Sle1, and their relative role in lupus pathogenesis is unknown. We show here that Sle1-expressing CD4(+) T cells present an activated phenotype associated with increased proliferation and cytokine production. In addition, Sle1 CD4(+) T cells provide help to anti-chromatin B cells to produce anti-nuclear antibodies, whether or not these B cells express Sle1. The Sle1a locus alone accounts for all these Sle1 phenotypes, implying that a specific genetic defect in Sle1a is necessary and sufficient to produce autoreactive T cells. However, Sle1c induces intermediate T cell activation and only provides help to Sle1-expressing anti-chromatin-producing B cells, demonstrating the synergic interactions between Sle1c T and Sle1 B cells. Moreover, Sle1a and Sle1c were associated with a significantly reduced level of CD4(+)CD25(+) regulatory T cells that precedes autoantibody production, suggesting a causal relationship with the generation of autoreactive T cells. Our study identifies for the first time that a specific genetic defect is responsible for lupus pathogenesis by inducing autoreactive T cells to break self-tolerance and that this genetic defect is also associated with a decreased number of regulatory T cells.

Association of extensive polymorphisms in the SLAM/CD2 gene cluster with murine lupus

Susceptibility to autoimmunity in B6.Sle1b mice is associated with extensive polymorphisms between two divergent haplotypes of the SLAM/CD2 family of genes. The B6.Sle1b-derived SLAM/CD2 family haplotype is found in many other laboratory mouse strains but only causes autoimmunity in the context of the C57Bl/6 (B6) genome. Phenotypic analyses have revealed variations in the structure and expression of several members of the SLAM/CD2 family in T and B lymphocytes from B6.Sle1b mice. T lymphocytes from B6.Sle1b mice have modified signaling responses to stimulation at 4-6 weeks of age. While autoimmunity may be mediated by a combination of genes in the SLAM/CD2 family cluster, the strongest candidate is Ly108, a specific isoform of which is constitutively upregulated in B6.Sle1b lymphocytes.

Mechanisms of Peritoneal B-1a Cells Accumulation Induced by Murine Lupus Susceptibility LocusSle2

The abundance of B-1a cells found in the peritoneal cavity of mice is under genetic control. The lupus-prone mouse New Zealand Black and New Zealand White (NZB x NZW)F(1) and its derivative NZM2410 are among the strains with the highest numbers of peritoneal B1-a cells. We have previously identified an NZM2410 genetic locus, Sle2, which is associated with the production of large numbers of B-1a cells. In this paper, we examined the mechanisms responsible for this phenotype by comparing congenic C57BL/6 mice with or without Sle2. Fetal livers generated more B-1a cells in B6.Sle2 mice, providing them with a greater starting number of B-1a cells early in life. Sle2-expressing B1-a cells proliferated significantly more in vivo than their B6 counterparts, and reciprocal adoptive transfers showed that this phenotype is intrinsic to Sle2 peritoneal B cells. The rate of apoptosis detected was significantly lower in B6.Sle2 peritoneal cavity B-1a cells than in B6, with or without exogenous B cell receptor cross-linking. Increased proliferation and decreased apoptosis did not affect Sle2 peritoneal B-2 cells. In addition, a significant number of peritoneal cavity B-1a cells were recovered in lethally irradiated B6.Sle2 mice reconstituted with B6.Igh(a) bone marrow, showing radiation resistance in Sle2 B-1a cells or its precursors. Finally, B6.Sle2 adult bone marrow and spleen were a significant source of peritoneal B-1a cells when transferred into B6.Rag2(-/-) mice. This suggests that peritoneal B-1a cells are replenished throughout the animal life span in B6.Sle2 mice. These results show that Sle2 regulates the size of the B-1a cell compartment at multiple developmental checkpoints.

The centromeric region of chromosome 7 from MRL mice (Lmb3) is an epistatic modifier of Fas for autoimmune disease expression

Lupus is a prototypic systemic autoimmune disease that has a significant genetic component in its etiology. Several genome-wide screens have identified multiple loci that contribute to disease susceptibility in lupus-prone mice, including the Fas-deficient MRL/Fas(lpr) strain, with each locus contributing in a threshold liability manner. The centromeric region of chromosome 7 was identified as a lupus susceptibility locus in MRL/Fas(lpr) mice as Lmb3. This locus was backcrossed onto the resistant C57BL/6 (B6) background, in the presence or absence of Fas, resulting in the generation of B6.MRLc7 congenic animals. Detailed analysis of these animals showed that Lmb3 enhances and accelerates several characteristics of lupus, including autoantibody production, kidney disease, and T cell activation, as well as accumulation of CD4(-)CD8(-) double-negative T cells, the latter a feature of Fas-deficient mice. These effects appeared to be dependent on the interaction between Lmb3 and Fas deficiency, as Lmb3 on the B6/+(Fas-lpr) background did not augment any of the lupus traits measured. These findings confirm the role of Lmb3 in lupus susceptibility, as a modifier of Fas(lpr) phenotype, and illustrate the importance of epistatic interaction between genetic loci in the etiology of lupus. Furthermore, they suggest that the genetic lesion(s) in MRLc7 is probably different from those in NZMc7 (Sle3/5), despite a significant overlap of these two intervals.