In vitro evidence suggests that plasmacytoid dendritic cells (pDCs) are intimately involved in the pathogenesis of lupus. However, it remains to be determined whether these cells are required in vivo for disease development, and whether their contribution is restricted to hyperproduction of type I IFNs. To address these issues, we created lupus-predisposed mice lacking the IFN regulatory factor 8 (IRF8) or carrying a mutation that impairs the peptide/histidine transporter solute carrier family 15, member 4 (SLC15A4). IRF8-deficient NZB mice, lacking pDCs, showed almost complete absence of anti-nuclear, anti-chromatin, and anti-erythrocyte autoantibodies, along with reduced kidney disease. These effects were observed despite normal B-cell responses to Toll-like receptor (TLR) 7 and TLR9 stimuli and intact humoral responses to conventional T-dependent and -independent antigens. Moreover, Slc15a4 mutant C57BL/6-Fas lpr mice, in which pDCs are present but unable to produce type I IFNs in response to endosomal TLR ligands, also showed an absence of autoantibodies, reduced lymphadenopathy and splenomegaly, and extended survival. Taken together, our results demonstrate that pDCs and the production of type I IFNs by these cells are critical contributors to the pathogenesis of lupus-like autoimmunity in these models. Thus, IRF8 and SLC15A4 may provide important targets for therapeutic intervention in human lupus.E xtensive evidence suggests that type I IFNs are major pathogenic effectors in lupus-associated systemic autoimmunity. A well-documented pattern of expression of type I IFN-inducible genes occurs in peripheral blood mononuclear cells of patients with systemic lupus erythematosus (SLE) (1-3), and reduced disease is observed in some lupus-predisposed mice that either lack the common receptor (IFNAR) for these cytokines (4, 5) or have been treated with IFNAR-blocking antibody (6). Consequently, attention has focused on defining the cell subsets and signaling processes involved in type I IFN production, the mechanisms by which these mediators accelerate disease, and approaches to interfere with these pathogenic events.Early in vitro studies showed that type I IFN production can be induced in normal blood leukocytes by SLE autoantibodies complexed with nucleic acid-containing apoptotic/necrotic cell material, and further work demonstrated that this activity is sensitive to RNase and DNase digestion (7,8). These results were integrated in a more comprehensive scheme following the demonstration that type I IFN induction by these complexes is mediated by the engagement of endosomal Toll-like receptors (TLRs) (9-11). Similarly, antigenic cargo containing nucleic acids was found to promote B-cell proliferation in a TLR9-or TLR7-dependent manner, with this effect enhanced by type I IFN signaling (9, 12, 13). The contribution of nucleic acid-sensing TLRs to systemic autoimmunity was further corroborated by studies in lupus-predisposed mice lacking or overexpressing TLR7 and/or TLR9 (14-20), and in Unc93b1 (3d) mutant mice i...

Regulatory T (Treg) cells expressing forkhead box P3 (Foxp3) arise during thymic selection among thymocytes with modestly self-reactive T cell receptors. In vitro studies suggest Foxp3 can also be induced among peripheral CD4+ T cells in a cytokine dependent manner. Treg cells of thymic or peripheral origin may serve different functions in vivo, but both populations are phenotypically indistinguishable in wild-type mice. Here we show that mice with a Carma1 point mutation lack thymic CD4+Foxp3+ Treg cells and demonstrate a cell-intrinsic requirement for CARMA1 in thymic Foxp3 induction. However, peripheral Carma1-deficient Treg cells could be generated and expanded in vitro in response to the cytokines transforming growth factor beta (TGFβ) and interleukin-2 (IL-2). In vivo, a small peripheral Treg pool existed that was enriched at mucosal sites and could expand systemically after infection with mouse cytomegalovirus (MCMV). Our data provide genetic evidence for two distinct mechanisms controlling regulatory T cell lineage commitment. Furthermore, we show that peripheral Treg cells are a dynamic population that may expand to limit immunopathology or promote chronic infection.

The discovery of molecular sensors that enable eukaryotes to recognize microbial pathogens and their products has been a key advance in our understanding of innate immunity. A tripartite sensing apparatus has developed to detect danger signals from infectious agents and damaged tissues, resulting in an immediate but short-lived defense response. This apparatus includes Toll-like receptors, retinoid acid-inducible gene-I-like receptors and other cytosolic nucleic acid sensors, and nucleotide-binding and oligomerization domain-like receptors; adaptors, kinases and other signaling molecules are required to elicit effective responses. Although this sensing is beneficial to the host, excessive activation and/or engagement by self molecules might induce autoimmune and other inflammatory disorders.

The demonstration in humans and mice that nucleic acid-sensing Toll-like receptors (TLRs) and type I interferons (IFNs) are essential disease mediators is a milestone in delineating the mechanisms of lupus pathogenesis. Here, we show that Ifnb gene deletion does not modify disease progression in NZB mice, thereby strongly implicating IFN-α subtypes as the principal pathogenic effectors. We further document that long-term treatment of male BXSB mice with an anti-IFNAR antibody of mouse origin reduced serologic, cellular and histologic disease manifestations and extended survival, suggesting that disease acceleration by the Tlr7 gene duplication in this model is mediated by type I IFN signaling. The efficacy of this treatment in BXSB mice was clearly evident when applied early in the disease process, but only partial reductions in some disease characteristics were observed when treatment was initiated at later stages. A transient therapeutic effect was also noted in the MRL-Faslpr model, although overall mortality was unaffected. The combined findings suggest that IFNAR blockade, particularly when started at early disease stages, may be a useful treatment approach for human SLE and other autoimmune syndromes.

Following exposure to some types of antigen (superantigens), responsive T cells expand and then decline in numbers, a phenomenon that has been called 'peripheral deletion'. This process may play a role in limiting autoimmune reactions and in the maintenance of immune homeostasis. Here we describe experiments on peripheral deletion in mice carrying the lpr/lpr defect, which has been shown to be due to defective production of the CD95/Fas molecule. Young lpr/lpr mice with no apparent immunologic abnormalities display a defect in bacterial superantigen-induced peripheral deletion. Apoptotic death of the expanded T cell population associated with such peripheral deletion. Apoptotic death of the expanded T cell population associated with such peripheral deletion in normal animals is dramatically reduced in the mutant mice. Further, the levels of Fas on responding cells in normal mice increases and decreases together with increases and decreases in cell numbers, suggesting that cells with the highest levels of Fas are preferentially deleted. These observations are consistent with the known ability of CD95 to transduce a signal leading to apoptosis, and they implicate this signal transduction pathway in peripheral deletion. In contrast, bacterial superantigen-induced deletion of thymocytes appears to be fully functional in these mice, and thus Fas/APO-1 does not appear to be required for this process. Further, antibody ligation of the TCR on activated T cells from normal or young lpr/lpr mice can induce apoptosis and therefore under some circumstances this phenomenon is not dependent upon CD95/Fas. Thus, to avoid autoreactivity and ensure immune homeostasis, several different apoptotic mechanisms exist in peripheral T lymphocytes, only some of which involve Fas.(ABSTRACT TRUNCATED AT 250 WORDS)

Among T cell subsets, γδ T cells uniquely display an Ag receptor-based tissue distribution, but what defines their preferential homing and homeostasis is unknown. To address this question, we studied the resources that control γδ T cell homeostasis in secondary lymphoid organs. We found that γδ and αβ T cells are controlled by partially overlapping resources, because acute homeostatic proliferation of γδ T cells was inhibited by an intact αβ T cell compartment, and both populations were dependent on IL-7 and IL-15. Significantly, to undergo acute homeostatic proliferation, γδ T cells also required their own depletion. Thus, γδ T cell homeostasis is maintained by trophic cytokines commonly used by other types of lymphoid cells, as well as by additional, as yet unidentified, γδ-specific factors.

Significance Lassa virus is, after dengue virus, the second most common cause of viral hemorrhagic fever. In susceptible individuals, Lassa virus infection is associated with vascular permeability, leading to tissue edema, organ failure, and death. Hemorrhagic fever viruses efficiently infect vascular endothelial cells, but are generally considered noncytopathic. Thus, the mechanism of virus-induced vascular injury remains unclear. Using the lymphocytic choriomeningitis virus variant clone 13, a prototype of Lassa virus, we show here that lethal vascular leakage in susceptible mice was completely prevented by type I IFN receptor blockade. Therefore, approaches that target type I IFNs or effector molecules induced by these cytokines may be considered for the treatment of Lassa fever and other severe hemorrhagic viral illnesses.

Evidence strongly suggests that excessive or protracted signaling, or both, by cell-surface or intracellular innate immune receptors is central to the pathogenesis of most autoimmune and autoinflammatory rheumatic diseases. The initiation of aberrant innate and adaptive immune responses in autoimmune diseases can be triggered by microbes and, at times, by endogenous molecules—particularly nucleic acids and related immune complexes—under sterile conditions. By contrast, most autoinflammatory syndromes are generally dependent on germline or de novo gene mutations that cause or facilitate inflammasome assembly. The consequent production of proinflammatory cytokines, principally interferon-α/β and tumor necrosis factor in autoimmune diseases, and interleukin-lβ in autoinflammatory diseases, leads to the creation of autoamplification feedback loops and chronicity of these syndromes. These findings have resulted in a critical reappraisal of pathogenetic mechanisms, and provide a basis for the development of novel diagnostic and therapeutic modalities for these diseases.