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IFN-γ has long been recognized as a signature proinflammatory cytokine that plays a central role in inflammation and autoimmune disease. There is now emerging evidence indicating that IFN-γ possesses unexpected properties as a master regulator of immune responses and inflammation. In this issue of the JCI, Guillonneau et al. show that indefinite allograft survival induced by CD40Ig treatment is mediated by CD8+CD45RClow T cells through the production of IFN-γ (see the related article beginning on page 1096), supporting the emerging view that IFN-γ is critical in the self-regulation of inflammation. These contradictory roles of IFN-γ, perhaps best understood by the principle of yin and yang, represent one of nature’s paradoxes, whereby the same cytokine functions as an inducer as well as a regulator for inflammation. Understanding this complex process of IFN-γ signaling is essential, as it has therapeutic implications.

The classification of T helper cells into type 1 (Th1) and type 2 (Th2) led to the hypothesis that Th1 cells and their cytokines (interleukin [IL]-2, interferon [IFN]-γ) are involved in cell-mediated autoimmune diseases, and that Th2 cells and their cytokines (IL-4, IL-5, IL-10, IL-13) are involved in autoantibody(humoral)-mediated autoimmune diseases. However, this paradigm has been refuted by recent studies in several induced and spontaneous mouse models of systemic lupus erythematosus, which showed that IFN-γ is a major effector molecule in this disease. These and additional findings, reviewed here, suggest that these two cross-talking classes of cytokines can exert autoimmune disease-promoting or disease-inhibiting effects without predictability or strict adherence to the Th1-versus-Th2 dualism.

Summary

The type I interferon (IFN) response initiated by detection of nucleic acids is important for antiviral defense, but is also associated with specific autoimmune diseases. Mutations in the human 3′ repair exonuclease 1 (Trex1) gene cause Aicardi-Goutières syndrome (AGS), an IFN-associated autoimmune disease. However, the source of the type I IFN response and the precise mechanisms of disease in AGS remain unknown. Here, we demonstrate that Trex1 is an essential negative regulator of the STING-dependent antiviral response. We used an in vivo reporter of IFN activity in Trex1-deficient mice to localize the initiation of disease to non-hematopoietic cells. These IFNs drove T cell-mediated inflammation and an autoantibody response that targeted abundant, tissue-restricted autoantigens. However, B cells contributed to mortality independently of T cell-mediated tissue damage. These findings reveal a stepwise progression of autoimmune disease in Trex1-deficient mice, with implications for the treatment of AGS and related disorders.

Interferon (IFN) gamma, a cardinal proinflammatory cytokine, induces expression of the gene products of the class II locus of the major histocompatibility complex (MHC), whereas IFN-alpha or -beta suppresses MHC class II expression. The mechanism of IFN-beta-mediated MHC class II inhibition has been unclear. Recently, a novel factor termed class II transactivator (CIITA) has been identified as essential for IFN- gamma-induced MHC class II transcription. We studied the status of IFN- gamma-induced CIITA messenger RNA (mRNA) accumulation and CIITA-driven transactivation in IFN-beta-treated cells and used cell lines that had defined defects in the type I IFN response pathway to address the roles of IFN signaling components in the inhibition of MHC class II induction. IFN-beta treatment did not suppress IFN-gamma-induced accumulation of CIITA mRNA. After cells were stably transfected with CIITA, endogenous MHC class II genes were constitutively expressed, and MHC class II promoters, delivered by transfection, were actively transcribed in CIITA-expressing cells. Expression of these promoters was significantly impaired by pretreatment with IFN-beta. These results suggest that IFN-beta acts downstream of CIITA mRNA accumulation, and acts in part by reducing the functional competence of CIITA for transactivating MHC class II promoters. IFN stimulated gene factor 3 (ISGF3) gamma was essential for IFN-beta to mediate inhibition of MHC class II induction, regardless of whether MHC class II transcription was stimulated by IFN-gamma or directly by CIITA expression. Results of these experiments suggest that inhibition of MHC class II in IFN-beta- treated cells requires expression of gene(s) directed by the ISGF3-IFN- stimulated response element pathway, and that these gene product(s) may act by blocking CIITA-driven transcription of MHC class II promoters.

Introduction

Interleukin (IL)-17 is a pro-inflammatory cytokine in rheumatoid arthritis (RA) and collagen-induced arthritis (CIA). Since interferon (IFN)-γ inhibits Th17 cell development, IFN-γ receptor knockout (IFN-γR KO) mice develop CIA more readily. We took advantage of this model to analyse the mechanisms of action of IL-17 in arthritis. The role of IFN-γ on the effector mechanisms of IL-17 in an in vitro system was also investigated.

Methods

IFN-γR KO mice induced for CIA were treated with anti-IL-17 or control antibody. The collagen type II (CII)-specific humoral and cellular autoimmune responses, myelopoiesis, osteoclastogenesis, and systemic cytokine production were determined. Mouse embryo fibroblasts (MEF) were stimulated with IL-17, tumor necrosis factor (TNF)-α and the expression of cytokines and chemokines were determined.

Results

A preventive anti-IL-17 antibody treatment inhibited CIA in IFNγR KO mice. In the joints of anti-IL-17-treated mice, neutrophil influx and bone destruction were absent. Treatment reduced the cellular autoimmune response as well as the splenic expansion of CD11b+ cells, and production of myelopoietic cytokines such as granulocyte macrophage colony-stimulating factor (GM-CSF) and IL-6. IL-17 and TNF-α synergistically induced granulocyte chemotactic protein-2 (GCP-2), IL-6 and receptor activator of NFκB ligand (RANKL) in MEF. This induction was profoundly inhibited by IFN-γ in a STAT-1 (signal transducer and activator of transcription-1)-dependent way.

Conclusions

In the absence of IFN-γ, IL-17 mediates its pro-inflammatory effects mainly through stimulatory effects on granulopoiesis, neutrophil infiltration and bone destruction. In vitro IFN-γ profoundly inhibits the effector function of IL-17. Thus, aside from the well-known inhibition of the development of Th17 cells by IFN-γ, this may be an additional mechanism through which IFN-γ attenuates autoimmune diseases.

Interferons (IFNs) were discovered nearly 60 years ago as a family of cytokines induced during and protecting from viral infection. They have been documented to play essential roles in numerous physiological processes beyond innate antiviral defense, including immunomodulation, regulation of the cell cycle, cell survival and differentiation, and the host response to microbial pathogens. Recent data have also uncovered a potentially darker side to the functions of IFN, including roles in autoimmunity and diabetes. Many IFN effects occur in the absence of acute viral infection, highlighting a physiologic role for constitutively produced IFN. Type I IFNs are constitutively produced at vanishingly low quantities and yet exert profound effects, mediated at least in part through modulation of signaling intermediates required for diverse cytokine response pathways. We review evidence for a yin-yang of IFN function through its role in modulating crosstalk between multiple cytokines by both feed-forward and feed-back regulation of common signaling intermediates and postulate that a similar mechanism underlies a homeostatic role for IFN through tonic signaling in the absence of acute infection.

Rheumatoid arthritis (RA) is an autoimmune synovitis characterized by the presence of anticitrullinated protein Abs, although the exact targets and role of anticitrullinated protein autoimmunity in the pathogenesis of RA remain to be defined. Fibrinogen, which can be citrullinated, has recently emerged as a candidate autoantigen. To determine whether autoimmunity against fibrinogen can mediate inflammatory arthritis, we immunized a variety of common mouse strains with fibrinogen and found that DBA/1 and SJL mice developed an inflammatory and erosive arthritis. Mice with fibrinogen-induced arthritis (FIA) possess fibrinogen-reactive T cells that produce the proinflammatory cytokines IL-6, IL-17, TNF-α, and IFN-γ. FIA can be adoptively transferred with either plasma or fibrinogen-specific T cells from diseased mice. Mice with FIA possess rheumatoid factor, circulating immune complexes, and anticyclic citrullinated peptide Abs, all of which are characteristic of human RA. These observations demonstrate that fibrinogen is arthritogenic in mice and that the pathogenesis of FIA is mediated by both autoantibodies and fibrinogen-reactive T cells.

IL-17 is the hallmark cytokine for the newly identified subset of T helper cells, Th17. Th17 cells are important instigators of inflammation in several models of autoimmune disease; in particular, collagen induced arthritis (CIA) and experimental autoimmune encephalomyelitis (EAE), which were previously characterized as Th1-mediated diseases. Although high levels of IFN-γ are secreted in CIA and EAE, disease is exacerbated in IFN-γ or IFN-γ receptor deficient mice due to the ability of IFN-γ to suppress IL-17 secretion. However, in proteoglycan-induced arthritis (PGIA), severe arthritis is dependent on the production of IFN-γ. We were therefore interested in determining the role of IL-17 in PGIA. We assessed the progression of arthritis in IL-17-deficient (IL-17−/−) mice and found the onset and severity of arthritis equivalent in wildtype (WT) and IL-17−/− mice. Despite evidence that IL-17 is involved in neutrophil recruitment, synovial fluid from arthritic joints showed a comparable proportion of Gr1+ neutrophils in WT and IL-17−/− mice. IL-17 is also implicated in bone destruction in autoimmune arthritis, however histological analysis of the arthritic joints from WT and IL-17−/− mice revealed a similar extent of joint cellularity, cartilage destruction and bone erosion despite significantly reduced RANKL expression. There were only subtle differences between WT and IL-17−/− in pro-inflammatory cytokine expression, T cell proliferation and autoanibody production. These data demonstrate that IL-17 is not absolutely required for autoimmune arthritis and production of other proinflammatory mediators are sufficient to compensate for the loss of IL-17 in PGIA.

Summary

IFN-γ is an important mediator of immunity and inflammation that utilizes the Jak-STAT pathway to activate STAT1. Many functions of IFN-γ have been ascribed to direct STAT1-mediated induction of immune effector genes, but recently it has become clear that key IFN-γ functions are mediated by crossregulation of cellular responses to other cytokines and inflammatory factors. Here we review mechanisms by which IFN-γ and STAT1 regulate signaling by TLRs, inflammatory factors, tissue destructive cytokines, anti-inflammatory cytokines, and cytokines that activate opposing STATs. These signaling mechanisms reveal insights about how IFN-γ regulates macrophage activation, inflammation, tissue remodeling, and Th and Treg differentiation, and how Th1 and Th17 responses are integrated in autoimmune diseases.

Summary

Plasmacytoid dendritic cells (pDCss) are bone marrow-derived cells that secrete large amounts of type I interferon (IFN) in response to viruses. Type I IFNs are pleiotropic cytokines with antiviral activity that also enhance innate and adaptive immune responses. Viruses trigger activation of pDCss and type I IFN responses mainly through the Toll-like receptor pathway. However, a variety of activating and inhibitory pDCs receptors fine tune the amplitude of type I IFN responses. Chronic activation and secretion of type I IFN in the absence of infection can promote autoimmune diseases. Furthermore, while activated pDCss promote immunity and autoimmunity, resting or alternatively activated pDCss may be tolerogenic. The various roles of pDCss have been extensively studied in vitro and in vivo with depleting antibodies. However, depleting antibodies cross-react with other cell types that are critical for eliciting protective immunity, potentially yielding ambiguous phenotypes. Here we discuss new approaches to assess pDCs functions in vivo and provide preliminary data on their potential roles during viral infections. Such approaches would also prove useful in the more specific evaluation of how pDCss mediate tolerance and autoimmunity. Finally, we discuss the emergent role of pDCss and one of their receptors, tetherin, in human immunodeficiency virus pathogenesis.

Increased Type I IFNs or IFN-I have been associated with human systemic lupus erythematosus. Interestingly augmenting or negating IFN-I activity in murine lupus not only modulates systemic autoimmunity, but also impacts lupus nephritis, suggesting that IFN-I may be acting at the level of the end-organ. We find resident renal cells to be a dominant source of IFN-I in an experimental model of autoantibody-induced nephritis. In this model, augmenting IFN-I amplified antibody-triggered nephritis, whereas ablating IFN-I activity ameliorated disease. One mechanism through which increased IFN-I drives immune-mediated nephritis might be operative through increased recruitment of inflammatory monocytes and neutrophils, though this hypothesis needs further validation. Collectively, these studies indicate that an important contribution of IFN-I toward the disease pathology seen in systemic autoimmunity may be exercised at the level of the end-organ.

The type I interferon (IFN) response plays a critical role in autoimmunity and is induced by innate receptor ligation and activation of IFN-regulatory factors (IRF). The present study investigated the roles and functional hierarchy of IRF3, IRF5, and IRF7 in expression of cytokines, chemokines, and matrix metalloproteinases in human THP1 monocytic cells. Targeted IRF knockdown was followed by evaluation of gene expression, promoter activation, and mRNA stability to determine the role of IRF as potential targets for modulating IFN responses in patients with autoimmune diseases. IRF played a distinct role in regulation of type I IFN gene expression in human monocytic cells and specifically regulated gene expression through the IFN-stimulated response element, with no contribution to transcription of traditionally AP-1 or NF-κB regulated genes. IRF7 regulated IL-6 gene expression by increasing IL-6 mRNA stability. IRF regulation of inflammation and induction of the IFN signature might contribute to the pathogenesis of autoimmune diseases and therefore represent novel therapeutic targets.

The contribution of the pro-inflammatory cytokines IFN-γ and IL-17 to the pathogenesis of experimental arthritis is controversial. In proteoglycan-induced arthritis (PGIA) severe arthritis is dependent on the production of IFN-γ whereas IL-17 is dispensable. In collagen-induced arthritis (CIA) and antigen-induced arthritis (AIA), although high levels of IFN-γ are secreted, disease is exacerbated in IFN-γ or IFN-γ receptor deficient mice due to the ability of IFN-γ to suppress IL-17 expression. In the present study, we investigated the effect of IFN-γ on the IL-17 response and its consequences in PGIA. In PG-immunized IFN-γ−/− mice, despite reduction in arthritis, the PG-specific CD4+ T cell IL-17 response was significantly increased. Elevated IL-17 contributed to development of arthritis as disease in IFN-γ /IL-17−/− was significantly reduced in comparison to either IFN-γ−/− or IL-17−/− mice. A contribution of IFN-γ and IL-17 to the development of arthritis was also identified in T-bet−/− mice. PG-specific CD4+ T cells from T-bet−/− mice produced reduced IFN-γ and elevated concentrations of IL-17. Both IFN-γ and IL-17 contribute to arthritic as T-bet−/− mice lacking IL-17 (T-bet/IL-17−/−) were resistant whereas WT, T-bet−/−, and IL-17−/− mice were susceptible to PGIA. T cell proliferation and autoantibody production did not correlate with development of disease, however, expression of cytokines and chemokines in joint tissues demonstrate that IFN-γ and IL-17 cooperatively contribute to inflammation. These results demonstrate that both IFN-γ and IL-17 have the potential to induce PGIA but it is the strength of the IFN-γ response that regulates the contribution of each of these T helper effector cytokines to disease.

Dendritic cells (DCs) produce cytokines and are susceptible to cytokine-mediated activation. Thus, interaction of resting immature DCs with TLR ligands, for example nucleic acids, or with microbes leads to a cascade of pro-inflammatory cytokines and skewing of T cell responses. Conversely, several cytokines are able to trigger DC activation (maturation) via autocrine, for example TNF and plasmacytoid DCs, and paracrine, for example type I IFN and myeloid DCs, pathways. By controlling DC activation, cytokines regulate immune homeostasis and the balance between tolerance and immunity. The increased production and/or bioavailability of cytokines and associated alterations in DC homeostasis have been implicated in various human inflammatory and autoimmune diseases. Targeting these cytokines with biological agents as already is the case with TNF and IL-1 represents a success of immunology and the coming years will expand the range of cytokines as therapeutic targets in autoinflammatory and autoimmune pathology.

Natural killer (NK) T cells recognize lipid antigens in the context of the major histocompatibility complex (MHC) class 1–like molecule CD1 and rapidly secrete large amounts of the cytokines interferon (IFN)-γ and interleukin (IL)-4 upon T cell receptor (TCR) engagement. We have asked whether NK T cell activation influences adaptive T cell responses to myelin antigens and their ability to cause experimental autoimmune encephalomyelitis (EAE), a model for multiple sclerosis. While simultaneous activation of NK T cells with the glycolipid α-galactosylceramide (α-GalCer) and myelin-reactive T cells potentiates EAE in B10.PL mice, prior activation of NK T cells protects against disease. Exacerbation of EAE is mediated by an enhanced T helper type 1 (Th1) response to myelin basic protein and is lost in mice deficient in IFN-γ. Protection is mediated by immune deviation of the anti-myelin basic protein (MBP) response and is dependent upon the secretion of IL-4. The modulatory effect of α-GalCer requires the CD1d antigen presentation pathway and is dependent upon the nature of the NK T cell response in B10.PL or C57BL/6 mice. Because CD1 molecules are nonpolymorphic and remarkably conserved among different species, modulation of NK T cell activation represents a target for intervention in T cell–mediated autoimmune diseases.

Summary

Activated macrophages and their inflammatory products play a key role in innate immunity and in pathogenesis of autoimmune/inflammatory diseases. Macrophage activation needs to be tightly regulated to rapidly mount responses to infectious challenges but to avoid toxicity associated with excessive activation. Rapid and potent macrophage activation is driven by cytokine-mediated feedforward loops, while excessive activation is prevented by feedback inhibition. Here we discuss feedforward mechanisms that augment macrophage responses to Toll-like receptor (TLR) ligands and cytokines that are mediated by signal transducer and activator of transcription 1 (STAT1) and induced by interferon-γ (IFN-γ). IFN-γ also drives full macrophage activation by inactivating feedback inhibitory mechanisms, such as those mediated by IL-10 and STAT3. Priming of macrophages with IFN-γ reprograms cellular responses to other cytokines, such as type I IFNs and IL-10, with a shift toward pro-inflammatory STAT1-dominated responses. Similar but partially distinct priming effects are induced by other cytokines that activate STAT1, including type I IFNs and interleukin-27. We propose a model whereby opposing feedforward and feedback inhibition loops crossregulate each other to fine tune macrophage activation. In addition, we discuss how dysregulation of the balance between feedforward and feedback inhibitory mechanisms can contribute to the pathogenesis of autoimmune and inflammatory diseases, such as rheumatoid arthritis and systemic lupus erythematosus.

Immune cytokine interferon-γ (IFN-γ) plays a crucial role in immune-mediated demyelination diseases such as multiple sclerosis and experimental autoimmune encephalomyelitis (EAE). Our previous studies have shown that enforced expression of IFN-γ in the central nervous system (CNS) inhibits developmental myelination or remyelination in EAE demyelinated lesions. While many of the cellular actions of IFN-γ result from its activation of the signal transducer and activator of transcription 1 (STAT1) pathway, recent studies have shown that STAT1-independent pathways regulate some facets of IFN-γ biology. In this study, we dissected the role of STAT1-dependent and STAT1-independent pathways in IFN-γ-induced hypomyelination using a genetic approach. We found that the induction of the STAT1-dependent, IFN-γ responsive genes in response to this cytokine was abolished in the CNS of STAT1 null mice. Moreover, STAT1 deletion diminished oligodendrocyte loss, the reduction of myelinated axons and inflammatory response in the CNS of transgenic mice that ectopically express IFN-γ in the CNS. Nevertheless, IFN-γ-induced reduction of myelin sheath thickness in the CNS of these mice was not altered by STAT1 deletion. Collectively, these data demonstrated that both STAT1-dependent and STAT1-independent pathways are involved in the detrimental effects of IFN-γ on the myelination process.

In autoimmune arthritis, traditionally classified as a T helper (Th) type 1 disease, the activation of T cells results in bone destruction mediated by osteoclasts, but how T cells enhance osteoclastogenesis despite the anti-osteoclastogenic effect of interferon (IFN)-γ remains to be elucidated. Here, we examine the effect of various Th cell subsets on osteoclastogenesis and identify Th17, a specialized inflammatory subset, as an osteoclastogenic Th cell subset that links T cell activation and bone resorption. The interleukin (IL)-23–IL-17 axis, rather than the IL-12–IFN-γ axis, is critical not only for the onset phase, but also for the bone destruction phase of autoimmune arthritis. Thus, Th17 is a powerful therapeutic target for the bone destruction associated with T cell activation.

Cytokines play a major role in regulating both humoral and cell-mediated immune responses. Recent advances in our understanding of cell-mediated immune responses have focused on the antigen presentation machinery and the proteins in the endoplasmic reticulum (ER). These proteins help the formation and stabilization of the major histocompatibility complex (MHC)–peptide interaction. A 96-kDa, ER-resident glycoprotein (gp96) is being evaluated as a therapeutic agent in cancer because of its ability to associate with a vast number of cellular peptides irrespective of size or sequence. Because the antigen presentation complex is assembled in the ER and a number of ER-resident proteins are modulated by cytokines, it is important to examine the regulation of gp96 in response to immune cytokines interferon γ (IFN-γ), and interleukin 2 (IL-2). Defects in signaling pathway in either of the cytokines can result in suboptimal immune response. We examined the effect of the cytokines IFN-γ and IL-2 on the induction of gp96 in different cancer cell lines and examined the induction of DNA-binding proteins that recognize gamma interferon–activating sequence (GAS), present in the promoter region of gp96. The induction of GAS binding protein correlated with the induction of STAT 1 protein, a transcriptional regulator and mediator of IFN-γ–mediated gene expression. The use of cytokines in inducing gp96 levels may have significance in maintaining high levels of gp96 for a sustained immune response.

Different viruses trigger pattern recognition receptor systems, such as Toll-like receptors or cytosolic RIG-I like helicases (RLH), and thus induce early type I interferon (IFN-I) responses. Such responses may confer protection until adaptive immunity is activated to an extent that the pathogen can be eradicated. Interestingly, the same innate immune mechanisms that are relevant for early pathogen defense have a role in ameliorating experimental autoimmune encephalomyelitis (EAE), a rodent model of human multiple sclerosis. We and others found that mice devoid of a component of the IFN-I receptor (Ifnar1−/−) showed significantly enhanced autoimmune disease of the central nervous system (CNS). A detailed analysis revealed that in wild-type mice IFN-I triggering of myeloid cells was instrumental in reducing brain damage. A more recent study indicated that similar to Ifnar1−/− mice, RLH-signaling-deficient mice showed enhanced autoimmune disease of the CNS as well. Moreover, when peripherally treated with synthetic RLH ligands wild-type animals with EAE disease showed reduced clinical scores. Under such conditions, IFN-I receptor triggering of dendritic cells had a crucial role. The therapeutic effect of treatment with RLH ligands was associated with negative regulation of Th1 and Th17 T-cell responses within the CNS. These experiments are consistent with the hypothesis that spatiotemporal conditions of, and cell types involved in, disease-ameliorating IFN-I responses differ significantly, depending on whether they were endogenously induced in the context of EAE pathogenesis within the CNS or upon therapeutic RLH triggering in the periphery. It is attractive to speculate that RLH triggering represents a new strategy to treat multiple sclerosis by stimulating endogenous immunoregulatory IFN-I responses.

Background

Studies of the host response to infection often require quantitative measurement of the antiviral type I interferons (IFN-α/β) in biological samples. The amount of IFN is either determined via its ability to suppress a sensitive indicator virus, by an IFN-responding reporter cell line, or by ELISA. These assays however are either time-consuming and lack convenient readouts, or they are rather insensitive and restricted to IFN from a particular host species.

Results

An IFN-sensitive, Renilla luciferase-expressing Rift Valley fever virus (RVFV-Ren) was generated using reverse genetics. Human, murine and avian cells were tested for their susceptibility to RVFV-Ren after treatment with species-specific IFNs. RVFV-Ren was able to infect cells of all three species, and IFN-mediated inhibition of viral reporter activity occurred in a dose-dependent manner. The sensitivity limit was found to be 1 U/ml IFN, and comparison with a standard curve allowed to determine the activity of an unknown sample.

Conclusions

RVFV-Ren replicates in cells of several species and is highly sensitive to pre-treatment with IFN. These properties allowed the development of a rapid, sensitive, and species-independent antiviral assay with a convenient luciferase-based readout.

Interferon (IFN)-stimulated gene factor 2 (ISGF2) plays a role in transcription of the beta IFN (IFN-beta) gene and IFN-stimulated genes (ISGs) and may function as a central mediator of cytokine responses. Constitutive ISGF2 transgene expression resulted in substantial resistance to three RNA virus families. This phenotype was not a consequence of IFN production and may have arisen directly through ISG expression. ISGF2 acted generally as a positive transcription factor through binding sites from several genes, in the context of transient cotransfection. Constitutive transcription of the endogenous IFN-beta gene, and several genes that are normally induced by either IFN-alpha or IFN-gamma, or only by IFN-alpha, was elevated in cells that constitutively express an ISGF2 transgene. However, constitutive and virus-induced levels of IFN-beta mRNA were unaffected in such cell lines.

Images

Previous studies have shown that EAE can be elicited by the adoptive transfer of either IFN-γ-producing (Th1) or IL-17-producing (Th17) myelin-specific CD4+ T-cell lines. Paradoxically, mice deficient in either IFN-γ or IL-17 remain susceptible to EAE following immunization with myelin antigens in CFA. These observations raise questions about the redundancy of IFN-γ and IL-17 in autoimmune demyelinating disease mediated by a diverse, polyclonal population of autoreactive T cells. In this study, we show that an atypical form of EAE, induced in C57BL/6 mice by the adoptive transfer of IFN-γ-deficient effector T cells, required IL-17 signaling for the development of brainstem infiltrates. In contrast, classical EAE, characterized by predominant spinal cord inflammation, occurred in the combined absence of IFN-γ and IL-17 signaling, but was dependent on GM-CSF and CXCR2. Our findings contribute to a growing body of data, indicating that individual cytokines vary in their importance across different models of CNS autoimmunity.

Mice deficient in interferon (IFN)-γ or IFN-γ receptor develop progressive and fatal experimental autoimmune encephalomyelitis (EAE). We demonstrate that CD4 T cells lacking IFN-γ production were required to passively transfer EAE, indicating that they were disease-mediating cells in IFN-γ knockout (KO) mice. IFN-γ KO mice accumulated 10–16-fold more activated CD4 T cells (CD4+CD44hi) than wild-type mice in the central nervous system during EAE. CD4+CD44hi T cells in the spleen and central nervous system of IFN-γ KO mice during EAE showed markedly increased in vivo proliferation and significantly decreased ex vivo apoptosis compared with those of wild-type mice. IFN-γ KO CD4+CD44hi T cells proliferated extensively to antigen restimulation in vitro and accumulated larger numbers of live CD4+ CD44hi T cells. IFN-γ completely suppressed proliferation and significantly induced apoptosis of CD4+CD44hi T cells responding to antigen and hence inhibited accumulation of live, activated CD4 T cells. We thus present novel in vivo and in vitro evidence that IFN-γ may limit the extent of EAE by suppressing expansion of activated CD4 T cells.

Neutralizing antibodies to type I interferons are of therapeutic significance i.e. are currently evaluated for the treatment of autoimmune diseases with pathogenic IFN-α production such as for systemic lupus erythematosus.

Unexpectedly, we observed that several neutralizing antibodies reportedly known to counteract IFN-α or -β activity triggered an “IFN-like” response in quiescent primary human endothelial cells leading to activation of the transcription factor ISGF3 and the expression of interferon-responsive genes. Furthermore, these antibodies were found to enhance rather than inhibit type I interferon signals, and the effect was also detectable for distinct other cell types such as PBMCs. The stimulatory capacity of anti-IFN-α/β antibodies was mediated by the constitutive autocrine production of sub-threshold IFN levels, involved the type I IFN receptor and was dependent on the Fc antibody domain, as Fab or F(ab’)2 fragments potently inhibited IFN activity.

We thus propose that a combined effect of IFN recognition by the antibody paratope and the concomitant engagement of the Fc domain may trigger an interferon signal via the respective type I IFN receptor which accounts for the observed “IFN-like” response to the neutralizing antibodies. With respect to clinical applications, the finding may be of importance for the design of recombinant antibodies versus Fab or F(ab’)2 fragments to efficiently counteract IFN activity without undesirable activating effects.