The pleiotropic functions of type I interferons mirror a complexity of intracellular signaling pathways 6. The type I interferons all utilize a ubiquitously expressed heterodimeric IFNα/β receptor (IFNAR) and generate similar, although not necessarily identical, signals 4.

IFNAR consists of IFNAR1 and IFNAR2 subunits that are associated with cytoplasmic protein tyrosine kinases Tyk2 and Jak1, respectively. IFNAR2 is also constitutively associated with STAT2. Ligation of IFNAR results in activation of Tyk2 and Jak1, and in phosphorylation of tyrosine residues in the IFNAR cytoplasmic domains, thus creating docking sites for signal transducers and activators of transcription (STATs), including STAT2 and proabably STAT1. Both STAT1 and STAT2 are activated at the IFNAR signaling complex by tyrosine phosphorylation, with subsequent formation of STAT1:STAT2 heterodimers and STAT1:STAT1 homodimers 46.

These STAT proteins translocate to the nucleus, where STAT1:STAT1 homodimers bind to gamma-activated sequence (GAS) sites (consensus sequence TTCNNNGAA) in gene promoters and activate transcription. In contrast, STAT1:STAT2 heterodimers associate with IRF9 to form the ISGF3 complex that binds to an interferon-stimulated response element (ISRE) site (consensus sequence TTTCNN TTTC) 446. Type I interferons thus activate expression of genes that have either GAS or ISRE sites, or both, in their promoters 24647. ISGF3 and cognate interferon-stimulated response elements play a key role in activation of antiviral genes and antiviral responses, whereas gamma-activated sequence elements mediate induction of inflammatory genes such as chemokines. Type I interferons can also activate STAT3 and, in a cell-type-specific manner, STAT4, STAT5 and STAT6 47.

In addition to the classical Jak-STAT pathway, other signaling pathways - including the mitogen-activated protein kinase pathways and the PI3K pathways - are activated by type I interferons 6. These nonclassical signaling pathways are activated in a stimulus-specific and cell-type-specific manner downstream of IFNAR. There is evidence that the cytoplasmic protein tyrosine kinases Tyk2 and Jak1 are actively implicated in the induction of the nonclassical signaling pathways. For example, they phosphorylate the insulin receptor sub-strates 1 and 2, providing a docking site for the regulatory domain of PI3K (p85), and they activate upstream regulators of p38, triggering activation of the PI3K and p38 pathways, respectively. The biologic consequences of the activation of these nonclassical signaling pathways have been reviewed recently 6.

Type I interferons are constitutively expressed at low levels; expression is rapidly enhanced upon viral or bacterial infection, aiming for an effective immune response against the pathogens 335. Type I interferons induce an antiviral state in cells by suppressing mechanisms for viral replication, trigger apoptosis in virally infected cells, promote natural killer cell-mediated and CD8⁺ T-cell-mediated cytotoxicity, promote the transition from innate to acquired immunity by augmenting DC maturation, cross-presentation of antigens and migration, and enhance T-helper type 1 responses, generation of T follicular helper cells, and humoral responses 3848495051.

Mounting observations indicate that type I interferons are implicated in the pathogenesis of several diseases, with autoimmune characteristics including SLE, primary Sjögren's syndrome, dermatomyositis, scleroderma and insulin-dependent diabetes mellitus 24. The role of type I interferons in SLE pathogenesis is now a well-established notion that has been supported by several lines of evidence derived from humans and animal models. Serum levels of IFNα in SLE patients can be as high as levels achieved during acute viral infections, and a positive correlation between serum IFNα levels and disease severity has been described 2952. Additionally, gene expression profiling with microarrays and real-time PCR demonstrated an IFNα signature in peripheral blood cells of SLE patients that was correlated with disease activity 5354. Interestingly, IFNα therapy in the setting of cancer or chronic viral infection induces autoantibody formation in 4 to 19% of patients and occasionally induces SLE-like symptoms 2225. Finally, IFNα administration in lupus-prone mice accelerates disease, and deletion of IFNAR substantially decreases morbidity and prolongs survival in these animals 2455. SLE is characterized by autoimmunity and organ inflammation. Type I interferons may contribute to disease pathogenesis by promoting either autoimmunity or inflammation, or both (Figure 2, left-hand side) 222346.

One mechanism by which type I interferons may promote autoimmunity, driving SLE pathogenesis, consists of abrogating peripheral tolerance to autoantigens through activation of immature DCs 22. Normally, autoreactive lymphocytes that have escaped from the central checkpoint of the thymus are controlled in the periphery by immature DCs lacking co-stimulatory molecules. DCs capture, process and present antigens in association with MHC molecules to lymphocytes. Antigen presentation by immature DCs leads to tolerance. Type I interferons promote maturation of DCs through induction of MHC II molecules, as well as CD40, CD80 and CD86 co-stimulatory molecules. Sera from patients with active SLE promote in vitro maturation of DCs, and blood monocytes derived from these patients exhibit DC-like properties such as enhanced antigen presentation and co-stimulatory capacity 29. During the course of SLE, tissue damage and decreased clearance of apoptotic bodies generate an excess of autoantigens. Augmented levels of type I interferons may promote a break of tolerance to these autoantigens, thus driving autoimmunity (by allowing activation of autoreactive lymphocytes by interferon-activated DCs) 22.

A hallmark of SLE is the presence of autoantibodies, including antinuclear antibodies, antibodies directed to cell surface molecules (on erythrocytes, platelets, neurons), antibodies against proteins bound to phospholipids and antibodies against antigens expressed in target organs like the kidney 22. Type I interferons, via complex mechanisms, enhance antibody responses (adjuvant activity) and promote long-term antibody production, class switching and immunological memory 48495051. Recently it has been reported that type I interferons stimulate generation of germinal centers by targeting DCs and stromal cells to promote development of T follicular helper cells 48. The T follicular helper cells represent a distinct T-helper cell population restricted to draining lymph nodes and specialized in supporting differentiation of germinal center B cells, a mandatory process for production of class-switched IgG 48. Additionally, type I interferons stimulate production of B-cell activating factor belonging to the TNF family (BAFF), of a proliferation-inducing ligand (APRIL), and of IL-6 - all of which promote B-cell survival and differentiation to plasmablasts and antibody-producing plasma cells 2250.

Several investigators, including our group, have considered the possibility that type I interferons may contribute to SLE pathogenesis by bolstering inflammatory responses through priming of myeloid cells (DCs and macrophages) and by inducing high-level STAT1 expression 46. The concept of priming is a key aspect of interferon biology and is defined as pre-exposure to low concentrations of interferons leading to enhanced responses to minor subsequent stimuli (including infections, autoantigens, environmental antigens, components of damaged tissues, or cytokines) 35. The classical function of priming is to potentiate innate immune responses during the early phase of infections when cytokine concentrations are low, thereby enhancing host defense. A common theme is that priming results in increased activation of STAT1, with its attendant inflammatory actions. STAT1 further amplifies cell activation by activating STAT1-mediated feedforward mechanisms (described below) and by counteracting feedback pathways such as those mediated by suppressor of cytokine signaling (SOCS) proteins or STAT3 46.

A substantial body of data supports the notion that priming of immunocytes occurs in SLE and other rheumatic diseases and contributes to disease pathogenesis. As previously noted, type I interferons are expressed in SLE at levels that would be expected to elicit priming 29. Furthermore, leukocytes isolated from SLE patients have been found to express increased levels of STAT1, a key marker in interferon priming. Interestingly, with the use of corticosteroids, suppression of STAT1 expression occurs concomitantly with demonstrated clinical improvement in SLE patients, thereby providing a possible mechanism of action for this drug class in systemic lupus 23. Isolated monocytes of SLE patients who had not undergone treatment displayed an enhanced response to TLR activators (such as lipopolysaccharide) and IL-1β, potentially as a result of in vivo interferon-mediated priming 46. By priming the host response, type I interferons can, in turn, enhance and modulate the inflammatory cellular responses to other molecules long implicated in modulating the pathogenesis of autoimmune diseases, such as TNF, IL-1 and IL-6, IFNγ, and TLRs 23354647.

Several mechanisms have been proposed to explain how exposure to interferons sensitizes cells to subsequent stimuli. Stimulation with type I interferons potentiates the physical association between IFNAR1 and IFNGR2 or gp130 35. Formation of such receptor complexes (receptosomes) increases the docking sites for STATs, resulting in more efficient dimerization of these STATs and enhanced responses to IFNγ and IL-6. Another mechanism of priming with relevance to SLE is that type I interferons induce increased expression of TLR3, TLR7 and TLR9, thus sensitizing cells to nucleic acid stimulation. As previously noted, a key feature of interferon priming is the increased expression of STAT1 56. Increased intracellular availability of STAT1 results in increased STAT1 phosphorylation downstream of IFNGR and IL-6 receptors, resulting in enhanced cellular activation in response to IFNγ and IL-6 stimulation 464757. An additional aspect of the biological consequences of the high intracellular levels of STAT1 is the perturbation of IL-10 signaling and function. IL-10, a powerful anti-inflammatory cytokine, is a potent activator of the anti-inflammatory STAT3 and a weak activator of STAT1 in myeloid cells, and does not normally induce STAT1 target genes as does IFNγ. Pre-exposure to type I interferons, however, reprograms STAT activation by IL-10 such that STAT1 is more strongly activated by IL-10 in these cells, resulting in induction of a group of STAT1-dependent genes and thereby a gain of inflammatory function 56.

Additional mechanisms by which type I interferons may promote inflammation and tissue damage in the context of SLE include the induction of pathogenic complement-fixing antibodies and production of chemokines that recruit inflammatory cells at sites of immune complex deposition 23. It is also interesting to consider the possibility that type I interferons may have pathogenic effects by acting on cells in target organs. Recent evidence from our and other laboratories has demonstrated an interferon signature in the kidneys in murine lupus nephritis that is associated with macrophage infiltration and proliferative nephritis 5859. Our study provides in vivo evidence that IFNα increases matrix metalloproteinase activation and growth factor expression to promote glomerular cell proliferation and early fibrosis. This process was mediated by alternatively activated macrophages expressing a dysregulated tissue repair program 58.

Although an interferon signature has been observed in peripheral blood cells derived from a subgroup of patients with MS and active RA 606162, there is evidence suggesting that type I interferons are therapeutic in MS and are potentially beneficial for RA and IBD by reining inflammation and tissue destruction. Firstly, IFNβ has been shown to reduce attack frequency in patients with MS and has become the established treatment of choice for the relapsing remitting form of MS 9. Additionally, in animal models of inflammatory arthritis, MS (EAE) and IBD, administration or triggered production of IFNα/β or IFNβ gene therapy has been shown to reduce disease severity 1011131516206364, while the lack of type I interferon signaling (IFNAR1 knockout or IFNβ knockout) resulted in more severe disease manifestation 151620316566.

Various mechanisms have been proposed to explain such anti-inflammatory and tissue-protective effects of type I interferons (Figure 2, right-hand side) 14262728. Reports suggest that type I interferons regulate the inflammatory milieu by increasing expression of anti-inflammatory factors (including IL-10, IL-27, transforming growth factor beta, IL-1R antagonist, soluble TNF receptors, suppressor of cytokine signaling) and decreasing proinflammatory mediators (including IL-1, IL-6, IL-8, IL-12, IL-18, TNFα, IFNγ, osteopontin, prostaglandin E and cyclooxygenase-2) 814262867. It is important to emphasize that type I interferons modulate the inflammatory microenvironment in a context-dependent manner and not uniformly. Thus, administration of IFN-β in mice with collagen-induced arthritis resulted in significant decrease in synovial expression of TNF-α and IL-6 13, while in rats with adjuvant-induced arthritis expression of IL-1β but not TNF-α and IL-6 was reduced 64.

Type I interferons suppress production of proinflammatory cytokines both at the level of transcription and at the level of mRNA stability. One mechanism may be the capacity of type I interferons to suppress key transcription factors for induction of proinflammatory cytokines (including NF-κB, AP-1 and PU.1) 1323. In a study from our laboratory, IFNα also suppressed TNFα production by inducing expression of the tyrosine kinase Axl that in turn induced expression of Twist, a transcriptional repressor of the TNFα promoter 68. Interferon- induced Axl and related TAM kinases have been implicated in feedback suppression of TLR responses by a SOCS1-mediated mechanism 69. Additionally, type I interferons induce the expression of an RNA-binding protein named tristetraprolin, which destabilizes transcripts of proinflammatory cytokines (including TNFα and IL-6) by mediating mRNA decay 70.

Evidence also indicates that type I interferons not only suppress production but also inhibit the function of proinflammatory mediators. Our group observed, using an in vitro system, that IFNα attenuates IL-1β proinflammatory effects by downregulating the expression of the signaling receptor IL-1RI 71, and others have found that type I interferons induce production of IL-1R antagonist by a PI3K-mediated mechanism 72.

Interestingly, accumulating evidence indicates that type I interferons suppress T-helper type 17 (Th 17) cell development 6667737475. In animal models of arthritis (collagen-induced arthritis) and central nervous system inflammation (EAE), Th 17 cells are principal drivers for disease pathogenesis. Their pathogenic role has also been suspected in human diseases including RA, MS, IBD, psoriasis, juvenile diabetes and autoimmune uveitis. It has been suggested that type I interferons suppress Th 17 cell generation by modulating the cytokine microenvironment that determines the fate of T-helper cells. Type I interferons - by targeting antigen-presenting cells (monocytes, macrophages and DCs) - increase the production of IL-27, which is a potent inhibitor of Th 17, and decrease IL-1β, trans forming growth factor beta 1 and IL-23, which favor Th 17 differentiation 66677475. A recent report indicates that type I interferons induce the production of IL-27 in DCs by downregulating the intracellular isoform of osteopontin (Opn-i) 67.

The anti-angiogenic effect of type I interferons may represent another mechanism contributing to the beneficial function of type I interferons in the context of inflammatory arthritis, where increased vascularization is a prominent component of synovial pathology 14. Type I interferons inhibit angiogenesis either indirectly by targeting nonendothelial cells or directly by targeting endothelial cells. In nonendothelial cells, type I interferons decrease the production of angiogenesis-promoting factors (including basic fibroblast growth factor and vascular endothelial growth factor) and induce angiostatic chemokines 14; in endothelial cells, type I interferons trigger an antiangiogenic program of gene expression 76 and suppress proliferation of endothelial cells and their progenitors 77.

Another important facet of the homeostatic function of type I interferons is their capacity to protect tissue integrity and restrain damage. IFNβ preserves the integrity of the blood-brain barrier and attenuates demyelination in EAE 3132, and prevents destruction of cartilage and bone in animal models of arthritis 14. Proposed explanations for this tissue protection have been that type I interferons regulate inflammatory cell trafficking, modulate the balance between proteases and protease inhibitors and inhibit osteoclastogenesis 142628. Type I interferons restrict tissue invasion of inflammatory cells by regulating the expression of adhesion molecules (downregulate expression on the cell surface and upregulate soluble forms of adhesion molecules) and by reducing MMP-9, which degrades extracellular matrix and facilitates tissue entrance of inflammatory cells 14. Finally, type I interferons are powerful inhibitors of osteoclastogenesis by decreasing expression of c-Fos, an essential transcription factor required for osteoclastogenesis induced downstream of RANKL 34. Additional mechanisms that contribute to the anti-osteoclastogenic effects of type I interferon may include an inhibitory effect on the expression of osteoclastogenic TNFα and IL-1β and decreased c-Cbl and Cbl-b expression 64.