Genetic dissection of systemic lupus erythematosus pathogenesis: partial functional complementation between Sle1 and Sle3/5 demonstrates requirement for intracellular coexpression for full phenotypic expression of lupus

Sle1 on chromosome 1 and Sle3/5 on chromosome 7 are two of the most critical lupus susceptibility loci of the New Zealand Black/White-derived NZM2410 mouse strain. In contrast to C57BL/6 mice congenic for either Sle1 (B6.Sle1) or Sle3/5 (B6.Sle3/5), strains that express only a modest lupus-related phenotype, the bicongenic B6.Sle1.Sle3/5 strain has a robust phenotype, suggesting a critical role for epistatic interactions in lupus pathogenesis. Mixed chimera experiments indicated that the two loci are functionally expressed by different cell populations and predicted that phenotypic expression of the phenotypic features of the B6.Sle1.Sle3/5 strain could be fully reproduced with a combination of B6.Sle1 and B6.Sle3/5 bone marrow. Contrary to our expectations, there was only a partial functional complementation in these mixed chimeras. Spleen enlargement, CD4:CD8 ratio elevation, and epitope spreading of autoantibodies were fully developed in B6+B6.Sle1.Sle3/5 but not in B6.Sle1+B6.Sle3/5 mixed chimeras. This study is the first to present evidence that the pathways mediated by two critical lupus susceptibility loci derived from the New Zealand White strain must be integrated intracellularly for epistatic interactions to occur. Our mixed chimera approach continues to provide novel insights into the functional genetic pathways underlying this important murine model of systemic autoimmunity.     

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.     

Murine lupus susceptibility locus Sle1a controls regulatory T cell number and function through multiple mechanisms

The Sle1 locus is a key determinant of lupus susceptibility in the NZM2410 mouse model. Within Sle1, we have previously shown that Sle1a expression enhances activation levels and effector functions of CD4(+) T cells and reduces the size of the CD4(+)CD25(+)Foxp3(+) regulatory T cell subset, leading to the production of autoreactive T cells that provide help to chromatin-specific B cells. In this study, we show that Sle1a CD4(+) T cells express high levels of ICOS, which is consistent with their increased ability to help autoreactive B cells. Furthermore, Sle1a CD4(+)CD25(+) T cells express low levels of Foxp3. Mixed bone marrow chimeras demonstrated that these phenotypes require Sle1a to be expressed in the affected CD4(+) T cells. Expression of other markers generally associated with regulatory T cells (Tregs) was similar regardless of Sle1a expression in Foxp3(+) cells. This result, along with in vitro and in vivo suppression studies, suggests that Sle1a controls the number of Tregs rather than their function on a per cell basis. Both in vitro and in vivo suppression assays also showed that Sle1a expression induced effector T cells to be resistant to Treg suppression, as well as dendritic cells to overproduce IL-6, which inhibits Treg suppression. Overall, these results show that Sle1a controls both Treg number and function by multiple mechanisms, directly on the Tregs themselves and indirectly through the response of effector T cells and the regulatory role of dendritic cells.     

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

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

Murine lupus susceptibility locus Sle1c2 mediates CD4+ T cell activation and maps to estrogen-related receptor γ

Sle1c is a sublocus of the NZM2410-derived Sle1 major lupus susceptibility locus. We have shown previously that Sle1c contributes to lupus pathogenesis by conferring increased CD4(+) T cell activation and increased susceptibility to chronic graft-versus-host disease (cGVHD), which mapped to the centromeric portion of the locus. In this study, we have refined the centromeric sublocus to a 675-kb interval, termed Sle1c2. Mice from recombinant congenic strains expressing Sle1c2 exhibited increased CD4(+) T cell intrinsic activation and cGVHD susceptibility, similar to mice with the parental Sle1c. In addition, B6.Sle1c2 mice displayed a robust expansion of IFN-γ-expressing T cells. NZB complementation studies showed that Sle1c2 expression exacerbated B cell activation, autoantibody production, and renal pathology, verifying that Sle1c2 contributes to lupus pathogenesis. The Sle1c2 interval contains two genes, only one of which, Esrrg, is expressed in T cells. B6.Sle1c2 CD4(+) T cells expressed less Esrrg than B6 CD4(+) T cells, and Esrrg expression was correlated negatively with CD4(+) T cell activation. Esrrg encodes an orphan nuclear receptor that regulates oxidative metabolism and mitochondrial functions. In accordance with reduced Esrrg expression, B6.Sle1c2 CD4(+) T cells present reduced mitochondrial mass and altered mitochondrial functions as well as altered metabolic pathway utilization when compared with B6 CD4(+) T cells. Taken together, we propose Esrrg as a novel lupus susceptibility gene regulating CD4(+) T cell function through their mitochondrial metabolism.     

The major murine systemic lupus erythematosus susceptibility locus Sle1 results in abnormal functions of both B and T cells

Sle1 is a major susceptibility locus in the NZM2410 murine model of systemic lupus erythematosus. When isolated on a C57BL/6 background in the B6.Sle1 congenic strain, Sle1 results in the production of high levels of anti-chromatin IgG Abs, histone-specific T cells, and increased B and T cell activation. We have shown by mixed bone marrow chimeras with allotypic markers that Sle1 is expressed in B cells. Using the same technique, we now show that it is also expressed in T cells. To assess whether Sle1 results in intrinsic defects in B or T cells, we have bred the muMT and Tcralpha(-/-) mutations onto B6.Sle1 resulting in the absence of circulating B cells and alphabeta T cells in B6.Sle1.muMT and B6.Sle1.Tcralpha(-/-), respectively. The immune phenotypes in these two strains were compared with that of B6.Sle1 and B6.muMT or B6.Tcralpha(-/-). Sle1-expressing B cells broke tolerance to chromatin in the absence of T cells, as shown by high levels of anti-ssDNA IgM Abs in B6.Sle1.Tcralpha(-/-) mice, and had an increased expression of activation markers. Conversely, increased expression of activation markers and increased cytokine production were observed in Sle1-expressing T cells in the absence of B cells in B6.Sle1.muMT mice. However, the production of IgG antinuclear Abs required the presence of both T and B cells. These experiments showed that Sle1 expression results in both B and T cells intrinsic defects and demonstrate that the documented involvement of each cell compartment in the production of anti-chromatin Abs corresponds to genetic defects rather than bystander effects.     

Genetic dissection of systemic lupus erythematosus pathogenesis: evidence for functional expression of Sle3/5 by non-T cells

On the non-autoimmune C57BL/6 (B6) background, the chromosome 7-derived lupus susceptibility loci Sle3 and Sle5 have been shown to mediate an elevated CD4:CD8 ratio with an increase in activated CD4(+) T cells, decreased susceptibility to apoptosis, and a break in humoral tolerance. Development of subcongenic strains has subsequently shown that the elevated CD4:CD8 ratio is due to Sle3 but that both loci contribute to the development of autoantibodies. To elucidate the functional expression patterns of these loci, adoptive transfer experiments were conducted. All possible combinations of bone marrow reconstitution, including syngenic, were conducted between the congenic B6 and B6.Sle3/5 strains. It was found that the Sle3/5 locus was functionally expressed by bone marrow-derived cells, but not by host cells, and that the elevated CD4:CD8 phenotype could be reconstituted in radiation chimeras. Using Ly5-marked congenic strains and B6 host mice, additional experiments surprisingly demonstrated that the elevated CD4:CD8 ratio was neither an intrinsic property of the T cells nor of single positive thymocytes. Allotype-marked chimeras indicated that autoantibody production by B cells was also an extrinsic property, as shown by the fact that B cells without the Sle3/5 interval contributed to autoantibody production. These experiments strongly suggest that a gene within the B6.Sle3/5 interval was expressed by a bone marrow-derived, nonlymphocyte population in the thymus and periphery and was affecting T cell selection and/or survival.     

IL-6 produced by dendritic cells from lupus-prone mice inhibits CD4+CD25+ T cell regulatory functions

The B6.Sle1.Sle2.Sle3 triple congenic mouse (B6.TC) is a model of lupus coexpressing the three major NZM2410-derived susceptibility loci on a C57BL/6 background. B6.TC mice produce high titers of antinuclear nephrogenic autoantibodies and a highly penetrant glomerulonephritis. Previous studies have shown the Sle1 locus is associated with a reduced number of regulatory T cells (Treg) and that Sle3 results in intrinsic defects of myeloid cells that hyperactivate T cells. In this report, we show that B6.TC dendritic cells (DCs) accumulate in lymphoid organs and present a defective maturation process, in which bone marrow-derived, plasmacytoid, and myeloid DCs express a significantly lower level of CD80, CD86, and MHC class II. B6.TC DCs also induce a higher level of proliferation in CD4(+) T cells than B6 DCs, and B6.TC DCs block the suppressive activity of Treg. B6.TC DCs overproduce IL-6, which is necessary for the blockade of Treg activity, as shown by the effect of anti-IL-6 neutralizing Ab in the suppression assays. The overproduction of IL-6 by DCs and the blockade of Treg activity maps to Sle1, which therefore not only confers a reduced number of Treg but also blocks their ability to regulate autoreactive T cells. Taken together, these results provide a genetic and mechanistic evidence for systemic autoimmunity resulting from an impaired regulatory T cell compartment in both number and function and for Sle1-expressing DCs playing a major role in the latter defect though their production of IL-6.     

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.     

Sle1ab mediates the aberrant activation of STAT3 and Ras-ERK signaling pathways in B lymphocytes

The Sle1ab genomic interval on murine chromosome 1 mediates the loss of immune tolerance to chromatin resulting in antinuclear Abs (ANA) production in the lupus-prone NZM2410 mouse. Global gene expression analysis was used to identify the molecular pathways that are dysregulated at the initiation of B lymphocyte autoimmunity in B6.Sle1ab mice. This analysis identified that STAT3 and ras-ERK signaling pathways are aberrantly activated in Sle1ab B lymphocytes, consistent with increased production of IL-6 by splenic B lymphocytes and monocytes in B6.Sle1ab mice. In vitro treatment of splenic mononuclear cells isolated from ANA-positive Sle1ab mice with anti-IL-6 Ab or AG490, an inhibitor of STAT3 signaling pathway, suppressed ANA production in short-term culture, indicating that this pathway was essential to the production of autoantibodies. In vivo treatment of ANA-positive B6.Sle1ab mice with the ras pathway inhibitor, perillyl alcohol, suppressed the increase of ANA. These findings identify IL-6 as a early key cytokine in Sle1ab-mediated disease development and indicate that the STAT3 and ras-ERK signaling pathways are potential therapeutic targets for treating systemic lupus erythematosus.     

Mechanisms of peritoneal B-1a cells accumulation induced by murine lupus susceptibility locus Sle2

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.     

A New Zealand Black-derived locus suppresses chronic graft-versus-host disease and autoantibody production through nonlymphoid bone marrow-derived cells

The development of lupus pathogenesis results from the integration of susceptibility and resistance genes. We have used a chronic graft-versus-host disease (cGVHD) model to characterize a suppressive locus at the telomeric end of the NZM2410-derived Sle2 susceptibility locus, which we named Sle2c2. cGVHD is induced normally in Sle2c2-expressing mice, but it is not sustained. The analysis of mixed bone marrow chimeras revealed that cGVHD resistance was eliminated by non-B non-T hematopoietic cells expressing the B6 allele, suggesting that resistance is mediated by this same cell type. Furthermore, Sle2c2 expression was associated with an increased number and activation of the CD11b(+) GR-1(+) subset of granulocytes before and in the early stage of cGVHD induction. We have mapped the Sle2c2 critical interval to a 6-Mb region that contains the Cfs3r gene, which encodes for the G-CSFR, and its NZM2410 allele carries a nonsynonymous mutation. The G-CSFR-G-CSF pathway has been previously implicated in the regulation of GVHD, and our functional data on Sle2c2 suppression suggest a novel regulation of T cell-induced systemic autoimmunity through myeloid-derived suppressor cells. The validation of Csf3r as the causative gene for Sle2c2 and the further characterization of the Sle2c2 MDSCs promise to unveil new mechanisms by which lupus pathogenesis is regulated.     

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.     

Pre-B cell leukemia homeobox 1 is associated with lupus susceptibility in mice and humans

Sle1a.1 is part of the Sle1 susceptibility locus, which has the strongest association with lupus nephritis in the NZM2410 mouse model. In this study, we show that Sle1a.1 results in the production of activated and autoreactive CD4(+) T cells. Additionally, Sle1a.1 expression reduces the peripheral regulatory T cell pool, as well as induces a defective response of CD4(+) T cells to the retinoic acid expansion of TGF-β-induced regulatory T cells. At the molecular level, Sle1a.1 corresponds to an increased expression of a novel splice isoform of Pbx1, Pbx1-d. Pbx1-d overexpression is sufficient to induce an activated/inflammatory phenotype in Jurkat T cells and to decrease their apoptotic response to retinoic acid. PBX1-d is expressed more frequently in the CD4(+) T cells from lupus patients than from healthy controls, and its presence correlates with an increased central memory T cell population. These findings indicate that Pbx1 is a novel lupus susceptibility gene that regulates T cell activation and tolerance.     

Cyclin-dependent kinase inhibitor Cdkn2c regulates B cell homeostasis and function in the NZM2410-derived murine lupus susceptibility locus Sle2c1

Sle2c1 is an NZM2410- and NZB-derived lupus susceptibility locus that induces an expansion of the B1a cell compartment. B1a cells have a repertoire enriched for autoreactivity, and an expansion of this B cell subset occurs in several mouse models of lupus. A combination of genetic mapping and candidate gene analysis presents Cdkn2c, a gene encoding for cyclin-dependent kinase inhibitor p18(INK4c) (p18), as the top candidate gene for inducing the Slec2c1-associated expansion of B1a cells. A novel single nucleotide polymorphism in the NZB allele of the Cdkn2c promoter is associated with a significantly reduced Cdkn2c expression in the splenic B cells and peritoneal cavity B1a cells from Sle2c1-carrying mice, which leads to a defective G1 cell cycle arrest in splenic B cells and increased proliferation of peritoneal cavity B1a cells. As the cell cycle is differentially regulated in B1a and B2 cells, these results suggest that Cdkn2c plays a critical role in B1a cell self-renewal and that its impaired expression leads to an accumulation of these cells with high autoreactive potential.     

Cyclin-dependent kinase inhibitor cdkn2c deficiency promotes b1a cell expansion and autoimmunity in a mouse model of lupus

The lupus-prone NZM2410 mice present an expanded B1a cell population that we have mapped to the Sle2c1 lupus susceptibility locus. The expression of Cdkn2c, a gene encoding for cyclin-dependent kinase inhibitor p18(Ink4c) and located within Sle2c1, is significantly lower in B6.Sle2c1 B cells than in B6 B cells. To test the hypothesis that the B1a cell expansion in B6.Sle2c1 mice was due to a defective p18 expression, we analyzed the B1a cell phenotypes of p18-deficient C57BL/6 mice. We found a dose-dependent negative correlation between the number of B1a cells and p18 expression in B cells, with p18-deficient mice showing an early expansion of the peritoneal B1a cell pool. p18 deficiency enhanced the homeostatic expansion of B1a cells but not of splenic conventional B cells, and the elevated number of B6.Sle2c1 B1a cells was normalized by cyclin D2 deficiency. These data demonstrated that p18 is a key regulator of the size of the B1a cell pool. B6.p18(-/-) mice produced significant amounts of anti-DNA IgM and IgG, indicating that p18 deficiency contributes to humoral autoimmunity. Finally, we have shown that Sle2c1 increases lpr-associated lymphadenopathy and T cell-mediated pathology. B6.p18(-/-).lpr mice showed a greater lymphadenopathy than B6.Sle2c1.lpr mice, but their renal pathology was intermediate between that of B6.lpr and B6.Sle2c1.lpr mice. This indicated that p18-deficiency synergizes, at least partially, with lpr-mediated pathology. These results show that Cdkn2c contributes to lupus susceptibility by regulating the size of the B1a cell compartment and hence their contribution to autoimmunity.     

Myeloid dendritic cells from B6.NZM Sle1/Sle2/Sle3 lupus-prone mice express an IFN signature that precedes disease onset

Patients with systemic lupus erythematosus show an overexpression of type I IFN-responsive genes that is referred to as "IFN signature." We found that B6.NZMSle1/Sle2/Sle3 (Sle1,2,3) lupus-prone mice also express an IFN signature compared with non-autoimmune C57BL/6 mice. In vitro, myeloid dendritic cells (mDCs) (GM-CSF bone marrow-derived dendritic cells; BMDCs) from Sle1,2,3 mice constitutively overexpressed IFN-responsive genes such as IFN-β, Oas-3, Mx-1, ISG-15, and CXCL10 and members of the IFN signaling pathway STAT1, STAT2, and IRF7. The IFN signature was similar in Sle1,2,3 BMDCs from young, pre-autoimmune mice and from mice with high titers of autoantibodies, suggesting that the IFN signature in mDCs precedes disease onset and is independent from the autoantibodies. Sle1,2,3 BMDCs hyperresponded to stimulation with IFN-α and the TLR7 and TLR9 agonists R848 and CpGs. We propose that this hyperresponse is induced by the IFN signature and only partially contributes to the signature, as oligonucleotides inhibitory for TLR7 and TLR9 only partially suppressed the constitutive IFN signature, and pre-exposure to IFN-α induced the same hyperresponse in wild-type BMDCs as in Sle1,2,3 BMDCs. In vivo, mDCs and to a lesser extent T and B cells from young prediseased Sle1,2,3 mice also expressed the IFN signature, although they lacked the strength that BMDCs showed in vitro. Sle1,2,3 plasmacytoid DCs expressed the IFN signature in vitro but not in vivo, suggesting that mDCs may be more relevant before disease onset. We propose that Sle1,2,3 mice are useful tools to study the role of the IFN signature in lupus pathogenesis.     

Abnormal costimulatory phenotype and function of dendritic cells before and after the onset of severe murine lupus

We analyzed the activation and function of dendritic cells (DCs) in the spleens of diseased, lupus-prone NZM2410 and NZB-W/F1 mice and age-matched BALB/c and C57BL/6 control mice. Lupus DCs showed an altered ex vivo costimulatory profile, with a significant increase in the expression of CD40, decreased expression of CD80 and CD54, and normal expression of CD86. DCs from young lupus-prone NZM2410 mice, before the development of the disease, expressed normal levels of CD80 and CD86 but already overexpressed CD40. The increase in CD40-positive cells was specific for DCs and involved the subset of myeloid and CD8alpha+ DCs before disease onset, with a small involvement of plasmacytoid DCs in diseased mice. In vitro data from bone marrow-derived DCs and splenic myeloid DCs suggest that the overexpression of CD40 is not due to a primary alteration of CD40 regulation in DCs but rather to an extrinsic stimulus. Our analyses suggest that the defect of CD80 in NZM2410 and NZB-W/F1 mice, which closely resembles the costimulatory defect found in DCs from humans with systemic lupus erythematosus, is linked to the autoimmune disease. The increase in CD40 may instead participate in disease pathogenesis, being present months before any sign of autoimmunity, and its downregulation should be explored as an alternative to treatment with anti-CD40 ligand in lupus.