RA (rheumatoid arthritis) is a chronic rheumatic condition hallmarked by joint inflammation and destruction by self-reactive immune responses. Clinical management of RA patients is often hampered by its heterogeneous nature in both clinical presentation and outcome, thereby highlighting the need for new predictive biomarkers. In this sense, several studies have recently revealed a role for type I IFNs (interferons), mainly IFNα, in the pathogenesis of a subset of RA patients. Genetic variants associated with the type I IFN pathway have been linked with RA development, as well as with clinical features. Moreover, a role for IFNα as a trigger for RA development has also been described. Additionally, a type I IFN signature has been associated with the early diagnosis of RA and clinical outcome prediction in patients undergoing biological drug treatment, two challenging issues for decision-making in the clinical setting. Moreover, these cytokines have been related to endothelial damage and vascular repair failure in different autoimmune disorders. Therefore, together with chronic inflammation and disease features, they could probably account for the increased cardiovascular disease morbidity and mortality of these patients. The main aim of the present review is to provide recent evidence supporting a role for type I IFNs in the immunopathology of RA, as well as to analyse their possible role as biomarkers for disease management.

INTRODUCTION

Type I IFNs (interferons) are a family of cytokines discovered more than 50 years ago by their ability to inhibit (interfere with) viral replication. Since their discovery, they have been the subject of intense research not only from a basic science perspective, but also for their roles in autoimmunity, as well as their therapeutic use in a variety of conditions.

The human type I IFN family comprises 13 functional IFNα genes and single genes for IFNβ, IFNκ, IFNε and IFNω, all derived from one common ancestral gene. All type I IFN cytokines bind to a single heterodimeric receptor composed of two subunits [IFNAR1 and IFNAR2 (IFN-α receptor 1 and 2 respectively)] [1], resulting in the activation of two receptor-associated protein tyrosine kinases JAK1 (Janus kinase 1) and TYK2 (tyrosine kinase 2), which phosphorylate STAT (signal transducer and activator of transcription) 1 and STAT2, leading to their activation and dimerization (Figure 1). Then, the activated STAT1/STAT2 complex assembles with IRF9 (IFN-regulatory factor 9) and this trimeric complex binds to specific DNA target sequences, known as ISREs (IFN-stimulated response elements), thereby activating the transcription of several related genes, coined as IRGs (IFN-response genes) or IFI (IFN-inducible) genes (such as IRF5, IRF7, etc.) (Figure 1) [1]. These IRGs are responsible for the antiviral and immunomodulatory effects triggered by type I IFNs. Actually, type I IFNs have been reported to modulate up to 200 genes, highlighting their pleiotropic character.

Type I IFN pathway in pDCs

Figure 1
Type I IFN pathway in pDCs

Type I IFNs can activate the transcription of IRGs through the JAK/STAT pathway. The type I IFNs can regulate almost 200 genes, whose simultaneous increased expression is referred to as the so-called IFN signature. These genes are responsible for the antiviral effects of type I IFNs and are also involved in the triggering of type I IFNs, as well as in a variety of immunomodulatory effects of type I IFNs. On the other hand, interferogenic immune complexes in autoimmune disorders can stimulate type I IFN gene transcription by engaging FcγRIIa receptors, thereby leading to higher type I IFN expression. Molecules reported to be associated with genetic susceptibility to RA are highlighted in red.

Figure 1
Type I IFN pathway in pDCs

Type I IFNs can activate the transcription of IRGs through the JAK/STAT pathway. The type I IFNs can regulate almost 200 genes, whose simultaneous increased expression is referred to as the so-called IFN signature. These genes are responsible for the antiviral effects of type I IFNs and are also involved in the triggering of type I IFNs, as well as in a variety of immunomodulatory effects of type I IFNs. On the other hand, interferogenic immune complexes in autoimmune disorders can stimulate type I IFN gene transcription by engaging FcγRIIa receptors, thereby leading to higher type I IFN expression. Molecules reported to be associated with genetic susceptibility to RA are highlighted in red.

During viral infections, although most human cell types are able to secrete type I IFNs, DCs (dendritic cells), and mainly pDCs (plasmacitoid DCs), are the most efficient IFNα producers [2]. Virus and viral RNA/DNA stimulate type I IFN production through cell-associated pattern recognition receptors, such as TLRs (Toll-like receptors), this synthesis being tightly regulated and limited in time. However, interferogenic immune complexes, containing self-nucleic acids, have been reported in some autoimmune disorders [3,4] to induce type I IFNs, predominantly IFNα, after internalization in endosomes via FcγRIIa (Figure 1). At this point, TLR7/TLR9 present in pDC endosomes activate the MyD88 (myeloid differentiation factor 88) adaptor protein, which associates with TRAF3/TRAF6 [TNF (tumour necrosis factor) receptor-associated factor 3 and 6] and IRAK1/IRAK4 [IL (interleukin) receptor-associated kinase 1 and 4]. This complex promotes the phosphorylation and translocation to the nucleus of IRF3, IRF5 and IFR7, which ultimately activate the transcription of type I IFN genes [5,6].

Regarding their functions, type I IFNs efficiently inhibit viral replication, thus being the major innate immune response against viral infections. However, type I IFNs can also carry out a wide range of immunomodulatory functions, including the up-regulation of MHC-I and MHC-II, maturation of DCs, activation natural killer cells, and induction of chemokines, chemokine receptors and co-stimulatory molecules (CD80 and CD86), as well as stimulating B-cell differentiation, antibody production and isotype class switching [57]. In addition, these cytokines have been associated with a Th1 shift [5,7]. Therefore type I IFNs can influence both innate and adaptive responses, leading to the activation of cellular and humoral immunity.

TYPE I IFNS AND AUTOIMMUNITY

Owing to their immune-stimulatory effects, type I IFNs and their signalling pathways represent a key factor in the breakdown of tolerance and the subsequent development and/or perpetuation of autoimmune phenomena (Figure 2). Therefore their role in autoimmune diseases has been intensely studied. The most relevant results come from SLE (systemic lupus erythematosus), where IFNα seems to have a prominent role. However, many other autoimmune conditions have been related to the IFN pathway, such as Sjögren syndrome, systemic sclerosis and, more recently, RA (rheumatoid arthritis). Actually, some of these autoimmune conditions (or, specifically, some subsets of patients) may be classified as ‘Type I interferonopathies’, a recently coined term to cover the broad spectrum of conditions hallmarked by aberrant type I IFN signalling [8,9].

Proposed roles of type I IFNs on immune activation and CV disease in RA

Figure 2
Proposed roles of type I IFNs on immune activation and CV disease in RA

Type I IFNs are pleiotropic cytokines that carry out a wide variety of cellular and molecular processes. Immunomodulatory effects of type I IFNs can lead to an enhanced immune activation by affecting both innate and acquired immunity, thus precipitating a breakdown in the tolerance and, hence, onset and perpetuation of the autoimmune phenomena characteristic of RA. Additionally, effects other than immune stimulation may have a role in premature atherosclerosis and impaired vascular repair, therefore promoting CVD (CV disease) progression in RA patients.

Figure 2
Proposed roles of type I IFNs on immune activation and CV disease in RA

Type I IFNs are pleiotropic cytokines that carry out a wide variety of cellular and molecular processes. Immunomodulatory effects of type I IFNs can lead to an enhanced immune activation by affecting both innate and acquired immunity, thus precipitating a breakdown in the tolerance and, hence, onset and perpetuation of the autoimmune phenomena characteristic of RA. Additionally, effects other than immune stimulation may have a role in premature atherosclerosis and impaired vascular repair, therefore promoting CVD (CV disease) progression in RA patients.

Consistent data indicate a role for type I IFNs (and especially IFNα) in SLE pathogenesis. High plasma levels of IFNα were found in SLE patients [10], and were reported to be associated with serological features, as well as with disease activity [11]. Similar findings were observed when expression arrays were performed in peripheral blood cells and tissue from SLE patients [12]. Additionally, IFNα therapy is associated with the occurrence and increase in the titre of antinuclear antibodies (the prototypical autoantibodies in SLE) in healthy subjects and also with worsening of pre-existing SLE conditions [13,14].

Further evidence for a pathogenic function of IFNα in SLE has come from animal models [15], where a role for the genetic background in IFNα signalling [16,17] has been suggested. These findings are in line with the heterogeneity of SLE and thus probably account for the different expression patterns of the type I IFN pathway in different patients [12]. Later studies have delineated the cellular network where IFNα was involved in SLE pathogenesis (reviewed in [18]), with a prominent role for pDCs and immunocomplexes in a self-perpetuating circle of chronic IFN stimulation and B-cell activation. These lines of evidence and the fact that anti-IFNα vaccination resulted in a notable amelioration of clinical features in murine models [19] led to the consideration of IFNα as a therapeutic target in SLE. In this sense, two monoclonal antibodies have been designed and the first clinical trials have been conducted [20,21]. Information concerning clinical benefit it is not yet sufficient to evaluate the efficacy of this therapy. However, despite being a promising therapeutic tool, it should be taken into account that only patients with increased IFNα serum levels or type I IFN signature expression might respond favourably, thus highlighting a clear need for predictive biomarkers in this field.

In spite of the consistent body of evidence that supports a role for IFNα in SLE, it has received little attention in RA (Table 1). RA is a complex disease, characterized by high heterogeneity in genetic predisposition, clinical presentation, disease progression, co-morbidities and response to therapies. Some lines of evidence support a pathogenic role for type I IFNs in RA (Table 2), although some controversy also exists.

Table 1
Comparison between the state of the art knowledge concerning type I IFN between SLE, the prototypical IFNα-mediated autoimmune condition, and RA

The number of ticks represents the strength of the evidence for each issue.

 SLE RA 
Type I IFN-related genes association with genetic susceptibility ✓✓✓ [45,165168✓✓ [36,45,169,170
Disease and/or disease-specific autoantibodies occurrence after treatment with type I IFNs ✓✓✓ [13,14,56] (reviewed in [55]) ✓ [5760,63
Disease triggered in mouse models by IFNα ✓ [15✓ [71
Increased type I IFN signature in peripheral blood from patients (% of patients) ✓✓✓ (up to 90%) [11,12,82] (reviewed in [7]) ✓ (25–65%) [68,82,87
Increased type I IFN plasma levels ✓✓ [10,114✓ [75,142
Association with clinical and serological features ✓✓ [11,12,171] (reviewed in [18]) ✓ [68,75,142
Targeted therapy against type I IFN approved and/or clinical trials performed ✓ [20,21,122,125
Association with CV risk and CV disease occurrence in patients ✓✓ [134,136138,147,158,172✓ [142,143,160,161
 SLE RA 
Type I IFN-related genes association with genetic susceptibility ✓✓✓ [45,165168✓✓ [36,45,169,170
Disease and/or disease-specific autoantibodies occurrence after treatment with type I IFNs ✓✓✓ [13,14,56] (reviewed in [55]) ✓ [5760,63
Disease triggered in mouse models by IFNα ✓ [15✓ [71
Increased type I IFN signature in peripheral blood from patients (% of patients) ✓✓✓ (up to 90%) [11,12,82] (reviewed in [7]) ✓ (25–65%) [68,82,87
Increased type I IFN plasma levels ✓✓ [10,114✓ [75,142
Association with clinical and serological features ✓✓ [11,12,171] (reviewed in [18]) ✓ [68,75,142
Targeted therapy against type I IFN approved and/or clinical trials performed ✓ [20,21,122,125
Association with CV risk and CV disease occurrence in patients ✓✓ [134,136138,147,158,172✓ [142,143,160,161
Table 2
Outline of the current evidence for and against a pathogenic role for type I IFNs in RA
Study For Against 
Pre-clinical studies Pathogenic role in RA animal models [70,71Anti-inflammatory effects (IFNβ) [22,23,26
 Type I IFN blockade is clinically effective in mouse models [19Inhibition of cartilage and bone destruction (IFNβ) [25,27
 IFNAR-deficient mice do not develop RA [69,71Inhibition of vascularization and cell trafficking [173175
 IRF-deficient mouse are resistant to RA [43Treatment with IFNβ ameliorate clinical symptoms in animal models [27
 IFNα injection in joints induces RA development in mice [71 
 IFNα-producing cells detected in synovial tissue [77,79 
 Increased ex vivo IFNα-production by pDCs from RA patients [77 
   
Clinical studies Genetic variants linked to susceptibility and clinical features [35,37,40,42,44,45,4750,169,176,177]Viral infections promote disease exacerbations [66,67RA remission after IFNβ treatment [178]Type I IFN signature is only found in a subset of patients [68,82,87
 IFNα treatment associated with disease occurrence or aggravation [13,5664 
 Induction of RA-specific autoantibodies during IFNα therapy [56,62,65 
 Increased expression in peripheral blood and target tissue (synovia) [68,82,84,87 
 Associations with clinical features [68,142 
 Association with poor clinical response [9496,98101,106 
 Association with erosive lesions [75 
 Promotion of vascular repair failure [142,143,160,161 
Study For Against 
Pre-clinical studies Pathogenic role in RA animal models [70,71Anti-inflammatory effects (IFNβ) [22,23,26
 Type I IFN blockade is clinically effective in mouse models [19Inhibition of cartilage and bone destruction (IFNβ) [25,27
 IFNAR-deficient mice do not develop RA [69,71Inhibition of vascularization and cell trafficking [173175
 IRF-deficient mouse are resistant to RA [43Treatment with IFNβ ameliorate clinical symptoms in animal models [27
 IFNα injection in joints induces RA development in mice [71 
 IFNα-producing cells detected in synovial tissue [77,79 
 Increased ex vivo IFNα-production by pDCs from RA patients [77 
   
Clinical studies Genetic variants linked to susceptibility and clinical features [35,37,40,42,44,45,4750,169,176,177]Viral infections promote disease exacerbations [66,67RA remission after IFNβ treatment [178]Type I IFN signature is only found in a subset of patients [68,82,87
 IFNα treatment associated with disease occurrence or aggravation [13,5664 
 Induction of RA-specific autoantibodies during IFNα therapy [56,62,65 
 Increased expression in peripheral blood and target tissue (synovia) [68,82,84,87 
 Associations with clinical features [68,142 
 Association with poor clinical response [9496,98101,106 
 Association with erosive lesions [75 
 Promotion of vascular repair failure [142,143,160,161 

In contrast with SLE, up-regulation of type I IFNs have only been found in a subset of RA patients and unexpected differences between the two subtypes of type I IFNs (IFNα and IFNβ) have been reported, thereby highlighting the relevance of these cytokines in the pathogenesis of this disease. Interestingly, IFNβ has predominantly anti-inflammatory properties, inhibits metalloproteinases and plays a role in bone homoeostasis. Proposed explanations for these tissue protection capabilities have been that IFNβ: (i) regulates inflammation and cell migration by decreasing the expression of adhesion molecules, (ii) modulates the balance between (metallo)-proteases and protease inhibitors, thus inhibiting tissue remodelling, and (iii) inhibits cartilage destruction and osteoclastogenesis by inhibiting RANKL (receptor activator of nuclear factor κB ligand)-mediated c-Fos activation [2227]. Although these regulatory mechanisms of IFNβ may contrast with the immunomodulatory effects proposed for IFNα, IFNβ therapy did not show clinical benefit in RA patients [28], in contrast with the pre-clinical studies. Probably mode of administration and doses may account for the lack of efficacy in this clinical trial. Additionally, the beneficial effect of IFNβ is supposed to be IL-1RA (IL-1 receptor antagonist)-mediated [29]. However, IL-1RA treatment exhibits a modest clinical effect in RA [30,31]. Moreover, the same schedule for IFNβ administration was followed as in multiple sclerosis patients, but these diseases differ in terms of immunopathology and type I IFN activation [32]. As previously stated, differences in type I IFN subtypes (IFNα and IFNβ) arise in RA and may also explain these contradictory findings. It is important to note that, whereas IFNα is mainly produced by pDCs, IFNβ is produced widely, with several cell types involved. More recently, a potential role for type III IFNs (IFNλ) in RA has been proposed [33], as common downstream mediators of type I and type III IFNs have been identified and type III IFNs can induce the expression of a number of type I IFN-induced genes [34]. However, the role for IFNλ is yet to be clarified. All of these features make the study of the type I IFN system in RA challenging.

The search for new biomarkers that could assist in the clinical management of RA patients is not only a necessity, but also a constant challenge and, recently, the possibility of considering type I IFNs as biomarkers in RA has emerged.

IRGs AND RA SUSCEPTIBILITY

Extensive research has shed light on the genetic predisposition of RA, thus underlining the relevance of more than 60 risk loci that account for 50% of the total heritability in RA. Most of these genes are related to antigen presentation, the threshold of immune responses and cytokine regulation. Given the immune effects of type I IFNs and the reported role of their genetic variants in their dysregulation in autoimmunity [5,7], polymorphisms in IRGs may be a plausible explanation of the genetic susceptibility in RA. Actually, among the identified risk genes for RA, a considerable number are involved in the regulation or production of the type I IFN pathway, (Figure 1).

Several studies revealed a role for IRF5 in RA susceptibility, with three polymorphisms being documented [35]. Additional studies confirmed this association, especially in the case of patients harbouring the shared epitope [36,37], thereby (i) restricting the genetic predisposition to a subset of patients with special characteristics, and (ii) associating IFN-related genetic susceptibility to patients with enhanced immune responses to viral antigens [38,39]. Interestingly, the susceptibility to RA associated with IRF5 variants seems to be similar to that of SLE [40]. Furthermore, these variants conferred increased IFN activity in SLE patients, therefore suggesting a functional role of these polymorphisms at the protein level. Whether this effect is also present in RA is still unknown; however, some controversy exists regarding IRF5 and RA susceptibility, probably due to the ethnicity of the subjects and the approach used [41,42]. Likewise, a lack of activation of the IFN pathway in mice lacking IRF1 has been described to result in a decreased frequency and severity of RA [43], and copy number variations in this locus are implicated in RA development in human studies [44]. Additionally, different polymorphisms in STAT4 [45], STAT1 and STAT3 genes [46], implicated in the IFN pathway, have been proposed to be associated with RA development.

Associations with genetic variants of pattern recognition receptors have also been reported. IFI1H is an RNA cytoplasmic receptor that acts as innate immune sensor for viral infection. It can also activate the IFN pathway and has been associated with aberrant IFN signalling in the so-called interferonopathies [47]. Polymorphisms in IFI1H have also been described as risk loci for RA [48,49]. Likewise, several authors have reported increased RA susceptibility due to some alleles in TLR genes being implicated in IFN responses (TLR2, TLR4, TLR8 and TLR9) [50].

Therefore it must be considered that the IFN pathway and IRGs can have a role in RA susceptibility, at least in subgroups of patients with characteristics of severe disease (shared epitope or erosive course).

IFNs AS A TRIGGER OF RA

The first descriptions of type I IFNs in RA patients go back to 1979, when increased levels were described in plasma [51], and later confirmed in synovial fluid [52]. Further studies associated high IFNα serum levels with extra-articular manifestations [53]. However, decreased IFN levels were also reported [54]. Nevertheless, this controversial evidence suggested the existence of dysregulated IFNs in RA patients and proved worthy of further research.

The notion that type I IFNs could play a role in RA came from a clinical setting, since increased autoimmune reactions and occurrence of autoantibodies were detected in patients with viral infections or haematological malignancies upon IFNα therapy [55]. The immune-stimulatory effect of this treatment can precipitate immune-mediated reactions de novo or exacerbate an existing autoimmune tendency. This is supported by the observed increase in the titre of antibodies and by the development of clinical disease in patients with pre-existing antibodies [56]. Nevertheless, it must be noted that not all of the patients receiving IFNα developed autoantibodies, and the occurrence of autoantibodies did not necessarily mean autoimmune disease development. This feature highlights IFNα as a trigger of autoimmunity, but also the need for additional mechanisms (genetic background, for instance).

Development of RA in the context of IFNα therapy has been widely documented [5762], and was associated with rheumatoid factor occurrence in up to 34% of the cases [56], whereas few cases developed anti-CCP (cyclic citrullinated peptide) antibodies and erosive lesions. However, in most of the cases, RA manifestations remitted after IFNα therapy was withdrawn; this did not happen when the patient exhibited anti-CCP antibodies or harboured the shared epitope, and these patients had predominantly a persistent disease. Another remarkable finding of IFN-induced RA is that almost all the patients exhibited a poly-articular onset. This is especially important, since a large number of joints affected at disease onset was associated with an aggressive outcome. Moreover, most patients were unresponsive or showed weak response to NSAIDs (non-steroidal anti-inflammatory drugs), even after cessation of the therapy [61,6365]. Interestingly though, improvements were reported after hydroxychloroquine treatment [60,65]; however, these cases need to be interpreted with caution, as it is not clear whether these traits can be classified as definite RA by classical criteria.

These findings led us to hypothesize that IFNα may be considered a trigger for RA in predisposed individuals. Furthermore, these patients probably have an aggressive disease phenotype, with a large number of affected joints and a poor response to NSAIDs, but better to antimalarials, in accordance with the inhibitory effect of this therapy on the IFNα pathway.

The idea of IFNs as a trigger of RA is also supported by evidence that infectious agents may not only have a role in the origin of RA, but also in the exacerbation of the disease [66,67]. Accordingly, it has been reported that a subgroup of RA patients exhibited a transcriptional profile that is similar to that of a viral infection [68], with IFN-mediated immunity being strongly up-regulated in these subjects. These patients also exhibit increased anti-CCP positivity, which can be associated with the breakdown of tolerance and increased B-cell activation promoted by IFNα. Moreover, anti-CCP positivity is commonly associated with rapid progression, hence associating IFN-mediated immunity with poor outcome.

Animal models of RA may also provide interesting insights into this field. Defects in DNA clearance were followed by an autoantibody-mediated chronic polyarthritis in mice that resembles human RA [69]. Additionally, nucleic acids by themselves have been described to induce RA development in mouse models [70] because of their ability to induce IFNα synthesis [71]. Although signalling through IFNAR is needed, PRKR (protein kinase R), a well-known downstream mediator of the IFN pathway, is not required for RA development. Therefore a role for IRF3 and IRF7 in IFN-induced RA would be expected. Other studies have also reported a role of TLR7 and TLR8 in nucleic-acid-induced IFNα production [72]. Actually, all of these mediators are also linked with RA susceptibility in humans. Nevertheless, the frequency of RA development differed among different mouse strains even upon the same stimuli [70], thus reinforcing the involvement of additional mechanisms, such as genetic background, for RA development.

All of these findings are in line with the observation that circulating nucleic acids, and autoantibodies directed against nuclear antigens, can be found in RA patients [73,74]. Moreover, the presence of both nucleic acids and increased IFNα levels has been reported in the synovial fluid, as well as the peripheral blood, of some RA patients with erosive disease [75,76]. Actually, synovial pDCs produce large amounts of IFNα [75,77], which induce further maturation of pDCs, and are able to present arthritogenic antigens to T-cells [7880], thereby promoting autoimmune phenomena and RA progression.

IFN SIGNATURES IN RA PATIENTS

Signalling through the type I IFN pathway results in an increased expression of several IFN-stimulated genes. This global expression profile is referred to as the ‘IFN signature’ and it has been profoundly studied in rheumatic diseases. Although the first evidence of rheumatic patients exhibiting an IFN signature came from studies in SLE [81], subsequent studies have demonstrated that several systemic rheumatic conditions, including a subgroup of RA patients, are hallmarked by an IFN signature [82]. This signature in RA was also reported in the synovial membrane, thereby supporting their involvement in the target tissue, similarly to other organ-specific autoimmune disorders [82].

An IFN signature has been reported in 25–65% of RA patients. Some studies reflect that this transcriptomic signature is not different in RA compared with SLE patients [83,84], whereas others have revealed lower IFN activation in RA [82,85]. A previous study of peripheral blood gene expression signatures by Smiljanovic et al. [86] revealed that, despite being shared by SLE and RA patients, the RA IFN signature qualitatively differs from that of SLE patients in terms of target genes and transcription-factor-binding sites and, remarkably, genomic imprints found in RA patients are more heterogeneous, thus suggesting the existence of distinctive transcriptional programmes between SLE and RA. RA patients characterized by this transcriptional profile exhibit increased expression of JAK/STAT mediators, as well as numerous chemokines, chemokine receptors and adhesion proteins, thereby suggesting an underlying enhanced immune activation [68,87]. Additionally, studies by genomic microarrays reported that IFI gene expression patterns are different at the synovial level between SLE patients with arthritic symptoms and RA patients [88]. This finding is in line with the differences observed in joint pathology between these two conditions and supports the idea that type I IFNs are actually more complex than initially considered, being heterogeneous even among different ‘related’ diseases.

As a consequence, the applicability of the IFN signature is still a matter of debate that needs further investigation. It can be calculated from many different IFN-stimulated genes, that, although most of them show similar expression patterns, differences are notable. In fact, a considerable variability and a wide range of IFN-stimulated gene expression have been reported. Somers et al. [89] stated that PRKR expression in SLE patients was completely independent of that of other IFN-induced genes studied (MX-1, IFI44L, IFIT1 and IFI44). Interestingly, PRKR had been reported to be irrelevant for IFN-induced RA [71], thereby emphasizing the complexity of the type I IFN system. On the other hand, data modelling and analysis also differ among studies [85], and this makes the comparison between different studies difficult. Therefore analysis of a IFN-induced profile based on average gene expression levels ignores co-regulation and interactions of such IFN-induced genes [85]. Moreover, it has been reported that the IFN signature in the rheumatoid synovium could also be induced by TNFα [90,91], thus pointing to a role for TNFα as a modulator of the IFN signature and bringing into question the specificity of the IFN signature as a biomarker of type I signalling in RA. However, whether this mechanism occurs in peripheral blood is unknown.

In conclusion, transcriptomic profiles, such as the IFN signature, could provide a direct insight into the genes and pathways involved in a certain disease in a single patient. Additionally, they are non-invasive techniques, thus being valuable tools for the clinical management of complex diseases. However, several concerns also exist and the results need to be interpreted with caution. In RA, apart from providing new insights into pathogenic mechanisms, the IFN signature also has direct implications in two unmet clinical needs, namely early diagnosis and prediction of therapy response.

IFN AND RA DIAGNOSIS

One of the main challenges in the clinical management of RA is early diagnosis. In this sense, the IFN signature has been reported in the pre-clinical phase of RA and it has been demonstrated to exhibit a consistent predictive value [92,93]. In addition, the IFN signature increased the predictive value of traditional markers (anti-CCP and rheumatoid factor) [92]. Moreover, the fact that the IFN signature is found in patients even before the clinical diagnosis reinforces the hypothesis of IFN as a trigger for RA, thereby ruling out the idea of the IFN signature in RA as an epiphenomenon or consequence of the disease itself.

Genetic variants, infections or dysregulation of the IFN pathway could be potential explanations to account for the presence of the IFN signature before the clinical onset of RA [7]. Regardless of the mechanism, the continuous IFN-mediated activation of the immune system will progressively lead to breakdown of tolerance and the development of autoimmunity. The fact that this signature can be detectable before the clinical onset allows an opportunity for the establishment of early treatment, which has been reported to result in a better sustained disease control.

IFN AND THE CLINICAL RESPONSE TO TREATMENT

Another major challenge in RA clinical routine is the prediction of therapy outcome, especially to biological therapies, so as to avoid ineffective treatment in potentially unresponsive patients, thereby keeping these patients away from adverse effects and high costs.

Van Baarsen et al. [94] reported data from whole-blood real-time PCR analysis of IRGs in infliximab-treated patients, concluding that a high IFN signature was associated with poor clinical outcome, as measured as a change in DAS (disease activity score) from baseline and EULAR (European League Against Rheumatism) response criteria after 12 weeks. Similarly, Sekiguchi et al. [95] found that sustained low expression of IFN-regulated genes is associated with a good response [ACR (American College of Rheumatology) 50% criteria] after 22 weeks [95]. Additionally, infliximab treatment decreased anti-CCP titres in patients with a low, but not in those with a high, IFN signature [96], which is in accordance with the differences in anti-CCP antibodies levels depending on the IFN groups [68]. Another plausible explanation could be the existence of an anti-CCP-producing plasma cell clone highly activated by IFN in these patients. However, this latter study did not report clinical differences in response to TNFα blockade. Moreover, contradictory results have been recently reported in a prospective study of RA patients undergoing TNFα-blockade [97] when the IFN signature in neutrophils was analysed. Differences not only in the assayed population, but also in the transcription factors focused on, with more attention paid to the STAT factors in the latter study, may contribute to this controversy.

Similarly, some studies point to a role for the IFN signature as a predictor of a negative response to B-cell therapy in RA. A previous study by Raterman et al. [98] using a whole genome transcript profile approach revealed that, among all of the genes in the human genome, only IRGs were associated with poor clinical response and a change in the DAS28 score after rituximab therapy. Actually, serum IFNα bioactivity at baseline correlated negatively with clinical response after 24 weeks in a similar study [99]. Accordingly, a greater decrease in autoantibody titre was found in patients with a low IFN signature compared with their counterparts with a high IFN signature, thus reinforcing the hypothesis of a resistant B-cell clone probably stimulated by IFNα. Similar results were reported from synovial samples [100], with high IFN-related gene expression being associated with a smaller change in DAS28 score and poor clinical response. Subsequent studies revealed, however, that good responders are characterized by a slight increase in IRG expression early after therapy, whereas this expression was constitutively increased (unchanged) in non-responders [101]. What seems clear is that the lower the IFN signature, the better the clinical response to therapy. Again, a potential explanation of these results could be that high IFN expression is associated with the presence of a B-cell clone insensitive to the effects of rituximab, whose activity and survival may be promoted by B-cell survival factors, such as BLyS [102]. Indeed, BLyS expression has been associated with IFNα in a number of autoimmune diseases [103105].

More recently, a potential role for the IFN signature in the prediction of clinical response to tocilizumab has been proposed [106], although evidence is limited.

Taken together, in general these results suggest a predictive value for the IFN signature in RA patients undergoing commonly used biological therapies, with a high IFN signature at baseline being associated with poor clinical outcome. This effect, however, may not be specific for RA. Comabella et al. [107] have reported a lack of response to IFNβ therapy in a subgroup of multiple sclerosis patients characterized by an overexpression of type I IFN-induced genes, which is linked to increased secretion of IFNα upon stimulation and elevated activation of DCs [107]. This observation highlights a relevant question concerning type I IFNs: should they be considered ‘as a whole’ or could differences be expected among different types, such as α and β? Mavragani et al. [108] have addressed this question. In a prospective study of RA patients treated with TNFα blockers, they found that both IFNα and IFNβ explained the IFN signature, but the better clinical response was associated with an IFNβ-biased balance, presumably via IL-1Ra production [108], a cytokine antagonist up-regulated by IFNs [109]. These results emphasize the differences between both type I IFNs, suggesting that the IFN signature as a whole should be interpreted with caution. However, IL-1Ra is only modestly effective in RA [30,31]. In addition, these results explain the discrepancies observed in IFNβ-treated RA patients [26,27], as it could be not only the dose of IFNβ, but also its balance with IFNα, that really matters in the clinical context of RA. Finally, these results may confirm a pathogenic role for IFNα in a subgroup of RA patients where this cytokine can counteract the ‘protective’ effects of IFNβ [110].

Since both the IFN signature and its role as a predictive biomarker seem to be present in many autoimmune conditions, it has been proposed that the IFN signature is patient-specific rather than disease-specific. This idea provides a new rationale for therapy indications in autoimmunity based on the relevance of molecular profiling, as it has been adopted in other clinical contexts.

In this scenario, the IFN signature could be a key factor for decision-making in RA treatment, with two possible approaches that need to be considered: (i) the IFN signature as a biomarker to optimize and stratify the clinical management; and (ii) the IFN signature as a therapeutic target. In the first strategy, IFN could be considered a useful tool to stratify patients as suitable candidates (or not) for biological therapies, thus aiming to reduce costs and adverse effects. In the second strategy, the IFN signature could be considered as a target itself [111], with a major role in the pathogenesis of some RA patients, so pharmacological inhibition of this pathway may lead to better disease control. In fact, patients with a high IFN signature might benefit from HCQ (hydroxychloroquine), a treatment that ameliorates IFN-induced RA in some patients and which is able to reduce IFNα production [112114]. Moreover, antimalarial treatment has been reported to be able to decrease circulating immunocomplexes in RA patients [115], which are potent IFN stimulators. Moreover, chloroquine was able to block rheumatoid factor production induced by immune complexes in activated B-cells by inhibiting TLR9 signalling [116]. Additionally, HCQ also decreases IL-1, IL-6 and TNFα levels [117], thereby accounting for its beneficial effect in RA patients [118,119]. However, direct evidence for a beneficial effect of HCQ on IFNα production in RA is lacking.

JAK inhibitors are also attractive therapies for these patients not only because of the central involvement of JAK proteins in the IFN pathway, since JAK1 and TYK2 are involved in signal transduction downstream of the IFNAR upon type I IFN ligation [6], but also for their ability to decrease type I IFN synthesis in DCs and synovial cells from RA patients [120,121]. This effect seems to be mediated by suppression of the phosphorylation of STAT1, thereby avoiding broad IRF gene activation.

Finally, IFNα blockade might be advisable in these patients, although there is limited evidence from clinical experience with this therapy. However, despite being initially conceived as a therapy for SLE patients [122], some advances have been made in other type I IFN-mediated conditions [123,124], thus reinforcing again the idea of IFNα as a patient-specific target, rather than a disease-specific one.

Previously, some authors have proposed the use of the type I IFN signature as both a disease target and a pharmacodynamic biomarker [125] in SLE, since differential expression of type I IFN-related genes probably account for different response to anti-IFNα therapy. This personalized approach will possibly benefit a number of patients and it would be also a potential explanation for the lack of success in some clinical trials with biological drugs, since patients are frequently recruited and randomized on the basis of unresponsiveness to traditional therapies, but no attention is paid to the activation pathways, which actually are the underlying mechanisms that need to be interfered with. To the best of our knowledge, no studies have been performed following this scheme, probably because of the lack of standardized and validated procedures for assessment of the type I IFN signature, but pharmacodynamic biomarkers are becoming increasingly important in the context of personalized medicine.

However, despite their clear advantages, expression profiles exhibit a number of limitations that should be considered, such as the differential proportion among leucocyte populations, differ-ential response in expression profiles in these populations, different sources from the isolated RNA, and technical and statistical approaches [126].

IFNα AND VASCULAR DAMAGE

Rheumatic conditions are associated with increased CV (cardiovascular) disease mortality and morbidity [127,128]. Actually, RA patients exhibit a CV risk profile similar to that of a 10-year-older non-RA population or same-aged diabetic patients [129]. This increased risk is unexplained by the effect of traditional CV risk factors [128], but it seems to be influenced by disease parameters [130]. It is clear that chronic inflammation and immune dysregulation underlie, at least in part, this excess risk [130133]. In this scenario, a potential role for type I IFNs has been shown in autoimmune patients. Because of the relevance of the IFN signature in its pathogenesis, most evidence has been obtained from SLE studies, although some authors have also provided new insights in RA patients as well. However, most of these studies have been confirmed in vitro in the presence of IFNα, thereby suggesting that detrimental effects on vascular biology could be attributed to IFNα, whereas the contribution of IFNβ remains unclear.

The IFN signature has been associated with increased and premature direct endothelial damage in murine models of lupus [134] and with endothelial dysfunction in SLE patients [135]. In fact, type I IFNs have been reported to modulate several steps in atherosclerotic plaque progression in murine models of lupus and atherosclerosis [136]. Several studies point to a prominent role of EPC (endothelial progenitor cell) dysfunction mediated by increased type I IFNs. This hypothesis was also confirmed in later studies, thereby revealing detrimental effects of type I IFNs (mainly IFNα) in EPC recruitment, differentiation and functionality. In fact, IFNα skewed the human EPC population to a ‘non-angiogenic’ phenotype, as characterized by their impaired vasculogenic ability in vitro [137]. Interestingly, it has been reported that IFNα treatment down-regulated pro-angiogenic mediators in murine EPC cultures [138], which is in line with decreased VEGF (vascular endothelial growth factor) levels detected in SLE patients [137,139]. Furthermore, IFNα was able to down-regulate angiogenic mediators in several cell types [140,141], thereby promoting a generalized failure of vascular repair. Of note, abrogation of IFN signalling resulted in restoration of normal vascular function. These findings are consistent with an early vascular damage in SLE, suggesting that high IFNα expression could promote CV disease in lupus patients after the onset of the disease, thus explaining the association between the IFN signature and subclinical CV disease reported in SLE patients [89].

Although the IFN signature has not been studied in RA in relation to CV disease, it has been positively correlated with increased immune activation, cell adhesion, blood clotting and fatty acid metabolism pathways [87], which are of relevance in atherosclerotic progression. Our group has recently reported increased IFNα serum levels in a subgroup of RA patients [142]. Of note, these patients presented an EPC imbalance associated with aggressive disease markers and pro-inflammatory cytokines. Probably, this pro-inflammatory environment leads to accelerated EPC maturation and altered vascular repair, which could support the increased rate of CV events observed in these patients [142]. Additionally, we have reported a new detrimental role for IFNα in vascular repair failure in RA patients, since IFNα seems to be associated with lower Tang cell (angiogenic T-cell) counts in patients and in in vitro cultures [143]. Tang cells have recently been discovered by their ability to promote vascular repair through co-operation with EPCs, and lower Tang cell counts have been related to vascular disease [144].

Several mechanisms may explain the vascular damage associated with high IFNα expression (Figure 3). Type I IFNs have been linked to destabilization of atherosclerotic plaques [145] and exhibit widespread negative effects on the vasculature by affecting many cell types in atherosclerosis [146]. Besides the crucial role of local pDCs, IFNα enhances lipid uptake by macrophages, thereby promoting foam cell differentiation, and they have been associated with vascular disease in SLE patients [147]. In addition, since type I IFNs can increase the expression of co-stimulatory molecules and promote a shift towards Th1 responses, they could induce and amplify a local immune response within the plaque, thus promoting increased plaque instability. Finally, vascular repair is impaired by the effect of IFNα on EPCs and Tang cells.

IFNα and vascular damage

Figure 3
IFNα and vascular damage

IFNα is associated with vascular damage through different ways, including by promoting increased endothelial injury (direct endothelial damage, platelet activation, foam cell differentiation and plaque destabilization) and impaired endothelial repair (EPC dysfunction and Tang cell decrease). ROS, reactive oxygen species.

Figure 3
IFNα and vascular damage

IFNα is associated with vascular damage through different ways, including by promoting increased endothelial injury (direct endothelial damage, platelet activation, foam cell differentiation and plaque destabilization) and impaired endothelial repair (EPC dysfunction and Tang cell decrease). ROS, reactive oxygen species.

Apart from VEGF, other molecular mediators behind the negative effects of IFNα on the vasculature have been described. The IFNα pathway has been directly involved in endothelial damage. It has been reported that IFI16, a DNA sensor induced by type I IFNs, promotes endothelial damage and activation and can perpetuate inflammatory responses [148151]. Interestingly, serum levels of soluble IFI16 are increased in autoimmune patients, especially in RA [150]. Another mediator that deserves to be mentioned is ET-1 (endothelin-1), a potent vasoconstrictor involved in cardiopulmonary pathology, that is inducible by IFNα [152,153] and nucleic acids via TLR3. In fact, ET-1 protein levels increase after IFNα treatment [152] and correlated with IFN-induced proteins [153] in systemic sclerosis patients. Therefore IFN-induced ET-1 levels may account for the adverse cardiopulmonary effects of IFNα therapy and could have a role in IFN-mediated vascular damage. Accordingly, ET-1 levels are increased in serum and synovial fluid from RA patients [154,155], mainly in patients with altered capillaroscopy profiles [155] or extra-articular manifestations [156]. In addition, control of disease activity and inflammation resulted in decreased ET-1 levels and improved cardiac function [157]. Hence ET-1 could have a major role in IFN-induced endothelial damage in RA. Parallel quantification of the IFN signature in these studies would provide insights into the actual role of type I IFNs in vascular damage in vivo, but current evidence is limited.

Finally, it is noteworthy that polymorphisms in IRFs have been related to CV disease susceptibility [158,159] and to markers of subclinical atherosclerosis [160,161] in RA patients. Furthermore, pharmacological therapy with IFNα (in non-autoimmune subjects) has also been associated with CV events [162164], thus reinforcing the pathogenic role for type I IFNs, and especially IFNα, on the vasculature even in the absence of autoimmune milieu.

CONCLUSIONS

Despite the role played by type I IFNs in several autoimmune disorders, evidence supporting their relevance as a trigger for RA is only recently emerging. Increased type I IFN levels may characterize a subgroup of RA patients with distinctive genomic expression profile, as well as specific clinical features. The fact that the IFN signature is found even in the pre-clinical phase of the disease and its potential role as a predictive marker for biologic therapies support the use of IFNα as possible biomarker for different steps in the clinical management of RA. Since the choice of biological drugs has increased in recent years, and it is becoming increasingly greater as new targets are characterized, type I IFNs are useful biomarkers that should be taken into account for clinical decision-making. The use of type I IFN signatures as pharmacodynamic biomarkers is a promising area not only in RA, but also in a number of autoimmune conditions. Additionally, the involvement of type I IFNs in endothelial damage and vascular repair failure may account for the increased CV risk found in RA patients and could allow, along with other disease features, the identification of patients at risk. Taken together, the inclusion of type I IFN signatures in algorithms for individualized treatments, as well as for CV risk stratification, would notably benefit a number of RA patients.

Abbreviations

     
  • CCP

    cyclic citrullinated peptide

  •  
  • CV

    cardiovascular

  •  
  • DAS

    disease activity score

  •  
  • DC

    dendritic cell

  •  
  • EPC

    endothelial progenitor cell

  •  
  • ET-1

    endothelin-1

  •  
  • HCQ

    hydroxychloroquine

  •  
  • IFI

    IFN-inducible

  •  
  • IFNAR

    IFNα receptor

  •  
  • IFN

    interferon

  •  
  • IL

    interleukin

  •  
  • IL-1RA

    IL-1 receptor antagonist

  •  
  • IRAK

    interleukin receptor-associated kinase

  •  
  • IRF

    IFN-regulatory factor

  •  
  • IRG

    IFN-response gene

  •  
  • ISRE

    IFN-stimulated response element

  •  
  • JAK

    Janus kinase

  •  
  • NSAID

    non-steroidal anti-inflammatory drug

  •  
  • pDC

    plasmacitoid DC

  •  
  • PRKR

    protein kinase R

  •  
  • RA

    rheumatoid arthritis

  •  
  • SLE

    systemic lupus erythematosus

  •  
  • STAT

    signal transducer and activator of transcription

  •  
  • Tang

    cell, angiogenic T-cell

  •  
  • TLR

    Toll-like receptor

  •  
  • TNF

    tumour necrosis factor

  •  
  • TRAF

    TNF receptor-associated factor

  •  
  • TYK2

    tyrosine kinase 2

  •  
  • VEGF

    vascular endothelial growth factor

FUNDING

Our own work was supported by European Union FEDER funds, Fondo de Investigación Sanitaria (FIS) [grant number PI12/00523]. J.R.-C. is supported by a FPU fellowship from the Ministerio de Educación (Spain).

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