Patients with systemic lupus erythematosus (SLE) show an over-expression of Type I Interferon (IFN) responsive genes called “Interferon Signature”. We found that the B6.NZMSle1/Sle2/Sle3 (Sle1,2,3) lupus-prone mice also express an Interferon Signature compared to non autoimmune C57BL/6 mice. In vitro, myeloid dendritic cells (mDCs)(GM-CSF bone marrow-derived BMDCs) from Sle1,2,3 mice constitutively over-expressed IFN responsive genes such as IFNb, Oas-3, Mx-1, ISG-15 and CXCL10, and the members of IFN signaling pathway STAT1, STAT2, and IRF7. The Interferon Signature was similar in Sle1,2,3 BMDCs from young, pre-autoimmune mice and from mice with high titers of autoantibodies, suggesting that the Interferon Signature in mDCs precedes disease onset and it is independent from the autoantibodies. Sle1,2,3 BMDCs hyper-responded to stimulation with IFNa and the TLR7 and TLR9 agonists R848 and CpGs. We propose that this hyper-response is induced by the Interferon Signature and only partially contributes to the Signature, since oligonucleotides inhibitory for TLR7 and TLR9 only partially suppressed the constitutive Interferon Signature and pre-exposure to IFNa induced the same hyper-response in wild type BMDCs than in Sle1,2,3 BMDCs. In vivo, mDCs and with lesser extent T and B cells from young pre-diseased Sle1,2,3 mice also expressed the Interferon Signature, although they lacked the strength that BMDCs showed in vitro. Sle1,2,3 plasmacytoid DCs expressed the Interferon 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 Interferon Signature in lupus pathogenesis.

A number of recent studies show that activation of CR3 on dendritic cells (DCs) suppresses TLR-induced TNF-α and IL-12 production and inhibits effective Ag presentation. Although the proposed physiologic role for these phenomena is immune suppression due to recognition of iC3b opsonized apoptotic cells by CR3, all of the aforementioned investigations used artificial means of activating CR3. We investigated whether iC3b opsonized apoptotic cells could induce the same changes reported with artificial ligands such as mAbs or iC3b-opsonized RBC. We explored the kinetics of iC3b opsonization in two models of murine cell apoptosis, γ-irradiated thymocytes and cytokine deprivation of the IL-3 dependent cell line BaF3. Using a relatively homogenous population of early apoptotic cells (IL-3 deprived BaF3 cells), we show that iC3b opsonized apoptotic cells engage CR3, but this interaction is dispensable in mediating the anti-inflammatory effects of apoptotic cells. TLR-induced TNF-α and IL-12 production by bone marrow-derived DCs occurs heterogeneously, with apoptotic cells inhibiting only certain populations depending on the TLR agonist. In contrast, although apoptotic cells induced homogeneous IL-10 production by DCs, IL-10 was not necessary for the inhibition of TNF-α and IL-12. Furthermore, because the ability of iC3b opsonization to enhance phagocytosis of apoptotic cells has been controversial, we report that iC3b opsonization does not significantly affect apoptotic cell ingestion by DCs. We conclude that the apoptotic cell receptor system on DCs is sufficiently redundant such that the absence of CR3 engagement does not significantly affect the normal anti-inflammatory processing of apoptotic cells.

Type I Interferons (IFNs) are pathogenic in Systemic Lupus Erythematosus. PBMCs from lupus patients over-express Type I IFN responsive genes. The cause and cellular source of this "Interferon Signature" are still unclear. Since we found that dendritic cells (DCs) from lupus-prone mice express the IFN Signature, we asked whether this IFN Signature was intrinsic to the DCs. We found that bone marrow-derived dendritic cells (BMDCs) from young pre-diseased Sle1,2,3 lupus-prone mice constitutively over-express Type I IFN responsive genes, analyzed by real-time RT-PCR (IFN-a, IFN-b, STAT1, STAT2, Mx1, IL-6, IL-15, IRF-7), as compared to autoimmune C57BL/6 BMDCs, as well as ELISA and flow cytometry. We also found that STAT1 is constitutively phosphorylated, suggesting an ongoing response to Type I IFN in the Sle1,2,3 DCs. Treatment with ligands for TLR7 and TLR9 showed that lupus BMDCs are hyper-sensitive to nucleic acids. To determine the mechanism of the constitutive hyper-activation of the IFN response in lupus DCs, we treated Sle1,2,3 BMDCs with inhibitors of TLR7 and TLR9 and found a partial role for chronic exposure to nucleic acids. We are now testing other mechanisms to explain the IFN Signature in lupus DCs. We have reported earlier that IL-4 suppresses Type I IFN responses in DCs of normal mice. We have now found that IL-4 inhibits the IFN Signature in Sle1,2,3 BMDCs, suggesting IL-4 as a potential therapeutic strategy for lupus by suppressing the Type I IFN effects.