The role of cutaneous type I interferons in autoimmune and autoinflammatory diseases

Abstract

Interferons (IFNs) are well known as mediators of the antimicrobial response but also serve as important immunomodulatory cytokines in autoimmune and autoinflammatory diseases. An increasingly critical role for IFNs in evolution of skin inflammation in these patients has been recognized. IFNs are produced not only by infiltrating immune but also resident skin cells, with increased baseline IFN production priming for inflammatory cell activation, immune response amplification and development of skin lesions. The IFN response differs by cell type and host factors and may be modified by other inflammatory pathway activation specific to individual diseases, leading to differing clinical phenotypes. Understanding the contribution of IFNs to skin and systemic disease pathogenesis is key to development of new therapeutics and improved patient outcomes. In this review, we summarize the immunomodulatory role of IFNs in skin, with a focus on type I, and provide insight into IFN dysregulation in autoimmune and autoinflammatory diseases.

Intro/Background

The skin comprises a critical physical and chemical barrier, and resident skin cells and resident and migratory immune cells secrete immunomodulatory proteins to protect us from colonization and invasion by foreign microorganisms. Interferons (IFNs) are one important class of signaling proteins secreted to combat potential infection. While IFNs can serve a protective role, they also contribute to the pathogenesis of autoimmune and autoinflammatory diseases. Pathognomonic skin lesions frequently herald systemic autoimmune disease onset and represent key features that assist in diagnosis. Critically, an important role for type I IFNs in cutaneous and systemic disease pathogenesis has been recognized.

Types and role of interferons

There are three main classes of IFNs, with type I IFNs representing the largest class. Type I IFNs in humans encompass 17 members, including 13 IFNα subtypes, IFNβ, IFNω, IFNε and IFNκ(1). While most cell types produce IFNβ, the primary producers of IFNα are hematopoietic cells(2). There exists only a single type II interferon, IFNγ, which is produced predominantly by T and NK cells, and also assists in the antiviral immune response(3). The third class of IFNs, type III IFNs, is comprised of 4 IFNλ subtypes (IFNλ 1, 2, 3 or 4), with receptor expression mostly restricted to epithelial cells, myeloid cell subsets and neuronal cells(4). IFNλ is structurally similar to interleukin-10 family cytokines and has similar signaling effects to type I IFNs, exhibiting a role in the antimicrobial response of epithelial cells(5).

Type I interferons serve an important immunomodulatory role in healthy skin, leading to promotion of antigen presentation and NK cell activation through shaping the innate immune response, activation and augmentation of the adaptive immune system and induction of an antimicrobial state(1, 2). Type I IFNs also perform an anti-proliferative and tumor immune surveillance role. Type II IFNs can specifically inhibit keratinocyte proliferation(6). Type III IFNs are induced by nucleic acid signaling but their overall function in the skin requires additional study.

Downstream signaling of type I interferons

All type I IFNs bind to the same heterodimeric transmembrane receptor, the IFNα/β receptor (IFNAR), composed of IFNAR1 and 2; however, binding affinity and tissue-specific receptor expression can influence biological activity of the type I IFNs(7–10). The IFNAR is found on nearly all nucleated cells. Binding of type I IFNs to the IFNAR leads to activation of the Janus activated kinase-signal transducer and activation of transcription (JAK-STAT) pathway(11). IFNAR1 is associated with tyrosine kinase 2 (TYK2), and IFNAR2 is associated with JAK1. Once JAK1 and TYK2 are activated, they phosphorylate tyrosine residues on cytoplasmic tails of the IFNAR, which serve as binding sites for the Src-homology-2 (SH2) domain of STAT proteins 1–6(12, 13). Relative STAT expression also influences specific STAT activation. Classically, a phosphorylated STAT1 and STAT2 dimer translocates to the nucleus, associates with interferon response factor 9 (IRF9), resulting in formation of the IFN-stimulated gene factor 3 (ISGF3) complex(14). ISGF3 then binds to interferon-stimulated response elements (ISREs), resulting in activation of interferon-stimulated genes (ISGs) (Figure 1).

Figure 1.

Pathways of interferon dysregulation in skin lesions of patients with autoimmune and autoinflammatory diseases. Cutaneous lupus erythematosus (CLE) is shown as an example in A. B represents pathways dysfunctional in Aicardi-Goutieres Syndrome (AGS) in red, Chronic Atypical Neutrophilic Dermatosis With Lipodystrophy and Elevated Temperature (CANDLE) in grey and STING-associated vasculopathy with onset in infancy (SAVI) in pink. dsDNA=double stranded DNA, dsRNA=double stranded RNA, ssRNA=single stranded RNA, IC=immune complex.

Localization of interferons in skin

In healthy resident skin cells, keratinocytes produce interferons at baseline, with minimal to no detectible production from fibroblasts or endothelial cells(15). This baseline interferon production is a result of chronic IFNκ production, with no apparent contribution from other type I IFNs(15, 16). IFNκ expression increases upon treatment with type I or II interferons(16), and chronic elevation of baseline IFNκ amplifies basal IFN responses(15). In inflammatory states, exposure to cytokines like TNF-α or antimicrobial peptides prime for additional type I IFN expression, particularly IFNβ(17).

Upon appropriate stimulation, many cell types are capable of type I IFN production. Immune cells, similar to keratinocytes, are poised to respond more rapidly to low levels of interferons. Plasmacytoid dendritic cells (pDCs) are able to rapidly secrete large amounts of IFNα and are known to accumulate in key tissues affected by inflammation in rheumatic disease. In autoimmune skin lesions, pDCs are recruited to the dermal-epidermal junction(18), contributing to interface dermatitis, a defining histopathologic feature in lupus and dermatomyositis. Immune complexes in cutaneous lupus lesions can induce type I IFN production in pDCs(19). Langerhans cells, a specialized subset of dendritic cells residing in skin, also produce interferons and release increased amounts of IFN-induced chemokines upon stimulation with Toll-like receptor (TLR) 3 agonist polyinosinic:polycytidylic acid (poly(I:C)) as compared to monocyte-derived DCs(20). Inflammatory monocytes have also been shown to be critical IFN producers in skin upon ultraviolet B (UVB) stimulation(21). Interestingly, IFNARKO mice demonstrate increased skin inflammation, suggesting a protective role for type I IFNs after UVB radiation in wild-type mice(21).

Triggers and regulation of IFN production in skin

Triggers for IFN production in skin can include ultraviolet radiation, infection, injury and cell death, all of which generate damage or pathogen-associated molecular patterns (DAMPs or PAMPs, respectively). Upon sensing of DAMPs or PAMPs by pattern recognition receptors (PRRs), IFN production is induced. The IFN response can differ depending on the underlying trigger, responding receptor and cell type, immune response and various host modifying factors. Keratinocytes express a wide range of PRRs, including toll-like receptors (TLRs) and cytoplasmic nucleic acid sensors, all with a unique ligand (PAMP or DAMP) preference (Table 1). Altered TLR and increased cytosolic nucleic acid sensor expression is noted in autoimmune skin diseases(22, 23), suggesting a general disease mechanism by which an environment with chronically elevated IFNs may modify IFN response mechanisms. UV radiation exposure to the skin of healthy volunteers and also mice has been shown to stimulate a striking cutaneous type I IFN response(21, 24). Which pathways sense and activate IFNs after UV exposure and how this differs in autoimmune disease are currently being investigated.

Table 1.

Toll-like receptors and cytosolic nucleic acid sensors in keratinocytes.

Pattern recognition receptor (PRR)	Ligand	Location and Expression	Reference
Toll-like receptors (TLRs)
TLR1	TLR2/1 heterodimer recognizes tri-acylated lipoproteins	Cell membrane, constitutively expressed, throughout the epidermis	(22, 136–138)
TLR2	pathogen-derived lipoproteins (tri- or diacyl lipopeptides, lipoteichoic acid, peptidoglycan), fungal components	Cell membrane, constitutively expressed, throughout the epidermis	(22, 136, 138–140)
TLR3	dsRNA, poly(I:C)	Intracellular membranes (Endosome/lysosome), constitutively expressed, basal layer of epidermis, expression increased upon exposure to IFNα + poly(I:C)	(23, 136, 138, 141)
TLR4	Lipopolysaccharide (LPS)	Cell membrane	(136)
TLR5	flagellin	Cell membrane, constitutively expressed, basal layer of epidermis	(22, 136, 138)
TLR6	TLR2/6 heterodimer recognizes di-acylated lipoproteins	Cell membrane	(136, 137)
TLR7	ssRNA	Endosome/lysosome, not expressed at baseline but treatment of keratinocytes with poly(I:C) can upregulate TLR7 expression	(142, 143)
TLR9	dsDNA, chromatin-IgG complexes	Endosome/lysosome	(136, 137)
TLR10	unknown	Constitutively expressed	(136, 138)
Cytosolic nucleic acid sensors
Protein kinase R (PKR)	dsRNA	Expression increased upon exposure to IFNα + poly(I:C)	(23)
Retinoic acid-inducible gene I (RIG-I)	ssRNA, dsRNA	Constitutively expressed, poly(I:C) leads to upregulation of type I IFNs while UVB has an opposing effect, expression increased upon exposure to IFNα + poly(I:C)	(23, 144, 145)
Melanoma differentiation associated gene 5 (MDA-5)	dsRNA	Expression increased upon exposure to IFNα + poly(I:C)	(23)
Interferon-γ-inducible protein 16 (IFI16)	dsDNA, ssDNA	Expression in upper epidermal layers in lesional skin of SLE patients	(146–148)
Cyclic GMP-AMP synthase (cGAS)	dsDNA	cGAS-stimulator of interferon genes (STING) pathway is activated by apoptosis-derived membrane vesicles from SLE patient sera	(146, 149)

Multiple factors serve to modulate the cellular response to type I IFNs, including IFNAR downregulation, negative regulator and microRNA upregulation, differential STAT activation, cooperation of STATs with interferon regulatory factors (IRFs), post-translational modification and chromatin remodeling(2). As an example of potential host modifying factors, commensal microbial flora can serve as a rheostat of IFN responsiveness to viral infections in mice(25). Antibiotic-treated mice have been shown to demonstrate decreased IFN responsiveness after mucosal or systemic viral infection, and expression of IFN and IFN-stimulated genes (ISGs) is reduced in macrophages from antibiotic-treated mice(25). In human keratinocytes, treatment with interferons in vitro leads to decreased barrier gene expression and increased S. aureus adherence which may induce further IFN production(26).

IFN effects and involvement in pathogenesis of autoimmune skin disease

Cutaneous lupus erythematosus (CLE)

Cutaneous disease in SLE can be an isolated feature or associated with underlying systemic manifestations. Skin inflammation is present in the majority of patients and is often the first harbinger of disease onset or a disease flare, offering a crucial opportunity to potentially intervene even prior to onset of systemic inflammation. Multiple subtypes of cutaneous lupus exist, including acute CLE (ACLE), subacute CLE (SCLE), chronic CLE (CCLE, including discoid lupus) and intermittent CLE (ICLE or tumid lupus)(27). Cutaneous lupus lesions demonstrate a hallmark interface dermatitis, or inflammatory infiltrate bordering the dermoepidermal junction (DEJ), characterized by apoptotic keratinocytes, vacuolar changes, CD8+ lymphocytes and pDCs(28, 29). Even in SLE patients with no clinically apparent skin lesions, molecular signatures in non-lesional skin can still indicate an aberrant immune response. Both lesional and non-lesional skin from adults with lupus exhibit chronic upregulation of type I interferons(15, 30), and comparison of isolated CLE vs. systemic lupus associated CLE demonstrate similar gene expression profiles(31).

Although incompletely understood, the pathogenesis of CLE lesions is thought to be driven by IFNs (Figure 1A). CLE patients exhibit an elevated IFN signature in peripheral blood that correlates with clinical cutaneous disease activity as assessed by the Cutaneous Lupus Erythematosus Disease Area and Severity Index (CLASI)(32, 33). IFNĸ production is increased at baseline in non-lesional SLE keratinocytes, leading to increased type I IFN responsiveness and UV light sensitivity(15, 34). Lupus keratinocytes also demonstrate a hypersensitive response to IFN stimulation, with a larger magnitude of change in ISG expression upon IFN treatment as compared to control keratinocytes(35). ISG expression, including Myxovirus resistance gene A (MxA), is upregulated at baseline in both the epidermis and dermis of lesional CLE skin(36). IFN-inducible CXCL9, CXCL10 and CXCL11 are 3 of the 5 most upregulated chemokines in lesional CLE skin, and their receptor (CXCR3) is among the top 3 most differentially regulated chemokine receptors(37, 38). MxA, CXCL9 and CXCL10 cutaneous expression patterns also differ by CLE subtype, suggesting that IFNs may have a role in directing differing clinical phenotypes(39).

Key genetic risk variants involved in IFN signaling pathways have also been described in SLE, including IRF5 and STAT4(40–42), but how each of these relates to skin disease hasn’t been delineated. SLE patients with these genetic risk variants have also been noted to have differences in disease phenotype, which may in part be explained through altered IFN signaling. As an example, SLE patients with high risk IRF5 genotypes were demonstrated to have elevated serum IFNα activity, with the highest levels observed in patients with anti-double-stranded DNA (dsDNA) or anti-RNA binding protein (RBP) autoantibodies(43). SLE patients with STAT4 risk alleles are diagnosed at a younger age and also more likely to have nephritis and anti-dsDNA autoantibodies(44, 45). Genetic risks for CLE have also been linked to IFN signaling as polymorphisms in IFNK(46) are associated with skin disease in African American and European ancestry females with SLE, and mutations in TREX1, a DNA exonuclease that when inhibited leads to accumulation of nucleic acids and increased IFN production, result in familial chilblain lupus (47, 48).

It is well known that UV radiation is a trigger for cutaneous inflammation and disease flare in SLE patients. UV radiation is known to amplify the IFN response to nucleic acids in keratinocytes, and mice lacking TREX1 develop UV-induced skin lesions(49). ISGs are increased in lupus-prone mice and human patients vs. healthy controls after UV radiation (50, 51) and this coincides with enhanced CD123+ dendritic cell and CD68+ macrophage recruitment in SLE skin after UV radiation (50). In C57BL/6J mice, UV radiation induces not only a type I IFN response in skin, but also a type I IFN response in peripheral blood and kidney tissue, suggesting a role for UV radiation and cutaneous IFNs in the initiation of systemic inflammation(24). Interestingly, this type I IFN response is more pronounced in female vs. male mice, lending insight into a potential mechanism by which females may be more susceptible to select autoimmune diseases such as SLE(24). In addition, IFNs repress UVB-mediated Treg induction in lupus-prone mice, which contributes to T cell activation(51). Importantly, persistence of IFN responses in CLE patients after UV exposure correlated with endothelial cell activation, likely contributing to leukocyte recruitment and development of clinical lesions(52). In pDCs, supernatant from UV-treated, apoptotic monocytes induces type I IFN production in combination with SLE total IgG (pooled from plasma of two patients), and both RNase and DNase treatment decrease type I IFN induction(53), suggesting that immune complexes predispose to inflammation following UVB.

Dermatomyositis

Dermatomyositis (DM) is an idiopathic inflammatory myopathy characterized by pathognomonic rash, muscle weakness and variable involvement of other organ systems, including the lungs, gastrointestinal tract and heart. In children, skin inflammation is the most common presenting symptom and most classically manifests as scaly, erythematous, raised lesions over the knuckles, or reddish purple discoloration of the upper eyelids with associated edema(54).

Similar to CLE, skin inflammation can be an important indicator of ongoing disease activity, photosensitivity is common, and lesions exhibit an interface dermatitis(55, 56). However, the pathophysiology of DM skin lesions is not as well understood. Type I IFN signaling is upregulated in DM and juvenile dermatomyositis (JDM) skin(57) as well as in muscle(58) and peripheral blood(59). The type I IFN signature in peripheral blood in DM and JDM has also been reported to correlate with disease activity(60, 61). Immunostaining of DM lesional skin demonstrates increased MxA staining in the epidermis, endothelial cells and inflammatory cell infiltrate(62, 63). CXCL10 expression is also higher in DM lesional skin, predominantly in the upper dermis in the presence of lymphocytic infiltrate and also in the epidermis near areas of interface dermatitis(62). In DM skin disease, similar to CLE, type I IFNs have been purported to lead to recruitment of CXCR3+ lymphocytes, with increased MxA staining correlating with higher numbers of CXCR3+ lymphocytes(62, 64). In anti-melanoma differentiation-associated 5 gene (MDA5) autoantibody-positive DM patients, MxA immunostaining in skin was distributed in blood vessels in the dermis, suggesting a role for IFNs in the vasculopathy that characterizes DM(65). Even non-lesional JDM skin has been described as altered, with increased numbers of pDCs and mast cells(66).

A recent analysis of two dermatomyositis skin microarray datasets revealed both a type I and type II IFN signature(67). Similarly, dermatomyositis muscle has been reported to have both a type I and II IFN signature, although the type I IFN signature may be somewhat more specific to DM versus other idiopathic inflammatory myopathies(68). IFNβ expression in the skin has been shown to correlate with ISGs(57), but whether IFNκ also plays a role in DM skin remains to be determined.

Scleroderma

Scleroderma is an autoimmune disorder with features of fibrosis, vasculopathy, and inflammation, contributing to pathogenesis at various stages of disease(69–71). In systemic sclerosis (SSc), the extent of skin involvement associates with prognosis, with lower survival in patients with a higher baseline skin score and improved survival in those patients with improvement in skin thickening(72).

IFN treatment has been known to trigger both systemic and localized scleroderma, leading to speculation on the role of IFNs in scleroderma pathogenesis. Localized scleroderma (LSc) has been described at IFNβ injection sites(73). SSc has also been reported in multiple sclerosis (MS) patients after IFNα and IFNβ treatment(74, 75). Immunostaining for MxA in lesional LSc biopsies shows expression that is most prominent in the deep dermis and subcutis near inflammatory infiltrates(76). CXCL10 staining is apparent in the dermal perivascular lymphoplasmacytic infiltrate(77). In pediatric LSc, bulk RNA sequencing of lesional skin confirms upregulation of IFNγ and ISGs, including CXCL9, CXCL10 and CXCL11, and LSc patients with more active skin lesions had higher IFN scores(78). A SSc skin microarray study also revealed ISGs as the top upstream transcriptional regulators, with IFNα and IFNγ as the top upstream activated cytokines(79). In this same study, approximately 75% of patients had a fibroinflammatory signature, which included gene expression scores of ISGs, that was found to correlate with the modified Rodnan skin score (mRSS)(79). Cutaneous expression of ISGs IFI44 and SIGLEC-1 has also been demonstrated to correlate with the mRSS(80). Interestingly, IFNκ was shown to be downregulated in SSc keratinocytes, suggesting that there may be different sources of type I IFNs based on cell type and individual autoimmune diseases(81).

Similar to gene expression studies in skin, an IFN signature in peripheral blood has been noted in both localized and systemic scleroderma patients(77, 82). As compared to the peripheral blood IFN signature in SLE, SSc patients were found to have upregulation of endothelial adhesion molecules, suggestive of the underlying vasculopathy that is central to the disease pathogenesis in SSc(82).

It has been suggested that IFN upregulation might play a role in the earlier stages of scleroderma pathogenesis. A study focused on patients with early SSc described that a type I IFN signature is still present in peripheral blood, despite the absence of clinical evidence of fibrosis(83). In fact, in early and non-fibrotic compared to fibrotic SSc patients, the IFN score was higher(83). pDCs have been shown to produce IFNα upon treatment with sera from SSc patients combined with necrotic material(84) in an FcgammaRII and RNA-dependent manner, suggesting a role for immune complexes(85). Indeed, TLR8 overexpression in a murine model of disease exacerbates fibrosis(86), and expression of ISGs also increases in skin and fibroblasts from SSc patients upon TLR3 stimulation(87). In SSc patients treated via hematopoietic stem cell transplantation (HSCT), there is a decrease in type I IFN expression in skin that correlates with decreased fibrosis and capillary regeneration(88).

Sjogren’s syndrome

Sjogren’s syndrome (SS) is characterized by inflammation of the lacrimal and salivary glands, resulting in exocrine dysfunction, with clinical features of keratoconjunctivitis sicca/xeropthalmia and xerostomia. SS can be both a primary disease or secondary to/associated with another underlying rheumatic disease and is associated with hypergammaglobulinemia and production of the classic autoantibodies SSA/Ro and SSB/La. Cutaneous manifestations in SS occur in up to 50% of patients and can include xerosis, angular cheilitis, eyelid dermatitis, pruritis, cutaneous vasculitis and skin lesions with histologic similarity to CLE(89, 90). Gene expression studies from both peripheral blood and salivary gland tissue highlight an IFN signature(91, 92), with a predominant type I IFN signature in peripheral blood and type II IFN signature in salivary gland tissue(92). Intriguingly, the type I IFN signature correlates with apoptotic gene expression(92), but whether this contributes to skin disease remains unknown. Monocytes from patients with primary SS also have a type I IFN signature in 55% of patients as compared to healthy controls(93). The importance of IFNs in SS is also reinforced by evidence in murine models, with SS mice that have a non-functional IFN receptor failing to develop clinical disease(94).

Psoriasis

Type I IFN activation has also been described in psoriasis and psoriatic keratinocytes. Genetic polymorphisms which lead to activation of cytosolic signaling pathways and IFN production are risk factors for psoriasis(95); indeed, DDX58 (RIG-I) activation is required for IL-23 activation and psoriasis in murine models(96). Type I IFNs and ISGs are significantly elevated in psoriatic plaques (97–100). A phase I trial of MEDI-545, an, anti-IFN-α monoclonal antibody was unable to show clinical benefit in patients with chronic psoriatic plaques, which may support the hypothesis that IFNs are involved in initiation of psoriasis but not in chronic plaque formation (101). Further work to understand how IFNs contribute to psoriatic development is required.

IFN effects and involvement in pathogenesis of autoinflammatory skin disease

Interferonopathies

The interferonopathies are autoinflammatory disorders characterized by overproduction of IFN due to mutations in genes involved in regulation of nucleic acid sensing. Through the study of interferonopathies, we have gained insight into the pathogenic role of interferons and underlying disease mechanisms driven by interferons. A spectrum of cutaneous manifestations are seen in the clinical presentation of interferonopathies especially vasculopathy (chilblain-like rash, microangiopathic vasculopathy, gangrene/ulcers/infarcts in acral areas) and skin eruptions of nodular erythema and violaceous plaques in cold-sensitive acral areas(102). Further, undifferentiated autoinflammatory disease patients with elevated IFN-response-gene scores more commonly had neutrophilic panniculitis(103). Further study has suggested that some disorders may favor NF-κB driven pathology over that mediated by interferons(103) but that IFN signature elevation is associated with erythematous, macular skin lesions and Gottron’s papules (skin lesions common in patients with DM). Understanding the balance between IFN-mediated and other inflammatory activation is an important goal for future research.

Aicardi-Goutieres Syndrome (AGS)

AGS patients were first described with progressive encephalopathy, basal ganglia calcifications, white matter hypodensities and persistent cerebrospinal fluid lymphocytosis(104). It was later noted that the most pathognomonic extra neurological symptom of AGS was the cutaneous finding of chilblain-like lesions on the digits and that these patients also had elevated IFNα in cerebrospinal fluid (CSF) and serum(105). Chilblain-like lesions are reported in approximately half of AGS patients, most often on the fingers and toes, but also other acral surfaces, including the ears(106).

Mutations in genes encoding the cellular nucleases TREX1(107), RNASEH2 complex(108), and SAMHD1(109) among others have been discovered in AGS patients. While these mutations lead to increased IFN generation, how these mutations directly lead to skin manifestations isn’t well understood. TREX1 encodes a 3’−5’ exonuclease that degrades ssDNA(110, 111), dsDNA(112), and ssRNA(113). Accumulation of nucleic acids causes a rise in IFN production in a cyclic GMP-AMP synthase (cGAS) and stimulator of IFN genes (STING) dependent manner, and deletion of TREX1 in keratinocytes raises ISG production in keratinocytes(114) (Figure 1B). However, mice with a dysfunctional TREX1 do not get spontaneous skin lesions(112). This suggests that triggers are needed for phenotype. Indeed, mice with dysfunctional TREX1 exhibit increased ear swelling and inflammation when injected with DNA, independent of its oxidation status (wild type mice develop lesions only from UV-oxidized DNA, which is resistant to TREX1 degradation)(115). Other mutations associated with TREX1 may also impact UVB sensitivity. Mutations in RNASEH2 can lead to defective repair of damaged RNA which increases the propensity for UVB-mediated damage and type I IFN production in response(116). Case reports have linked AGS with photosensitivity(117), but how individual mutations contribute remains to be determined. In C57BL/6J mice exposed to UV radiation, both the type I IFN response in skin and peripheral blood is primarily dependent on the cGAS-STING pathway in the early response phase at 6 hours post-radiation, lending insight into a potential role for cGAS-STING in the early type I IFN response and subsequent innate inflammatory cell recruitment(24).

CANDLE (Chronic Atypical Neutrophilic Dermatosis With Lipodystrophy and Elevated Temperature)

CANDLE is categorized as a proteasome-associated autoinflammatory syndrome (PRAAS) and is manifested by recurrent fevers, annular, purpuric rash, lipodystrophy and multisystem inflammation. Skin biopsies from CANDLE patients demonstrate mononuclear cell and neutrophilic infiltrate with dermal collagen degeneration(118). Mutations in the PSMB8 gene were initially found in 8/9 patients from a CANDLE cohort, accompanied by elevated serum levels of CXCL10 and IFN signaling as a top dysregulated pathway on whole blood gene expression analysis (pathway gene list including both type I and type II IFN-induced genes) (118). Additional mutations in genes involved in proteasome activity have since been identified that result in a CANDLE phenotype, including PSMB4, PSMA3, PSMB9 and POMP, which encodes a proteasome maturation protein(119). In patients with proteasome alterations other than in PSMB8, skin biopsies demonstrated increased ubiquitin-positive keratinocytes and ubiquitin-rich inclusions in keratinocytes. CANDLE patient keratinocytes showed impairment in proteasome assembly, and siRNA knockdown of patient proteasome mutations resulted in type I IFN induction(119). Indeed, in CANDLE, IFNs may participate in a feed-forward loop in which normal triggers of type I IFN production, such as UV light or infections, result in cellular stress and oxidized proteins that cannot be degraded, which results in further type I IFN production, upregulation of the immunoproteasome and subsequent inflammation (Figure 1B).

SAVI (STING-associated vasculopathy with onset in infancy)

Another group of patients exhibiting lupus-like malar rash and vasculitic skin lesions in conjunction with interstitial lung disease have been described to harbor TMEM173 mutations, leading to gain-of-function in stimulator of interferon genes (STING) and subsequent IFN overproduction(120, 121) (Figure 1B). At baseline, SAVI patients have maximal upregulation of type I IFN and ISGs with constitutive STAT1 phosphorylation(121). Lesional skin from SAVI patients is characterized by vascular inflammation of capillaries and microthrombosis, and dermal fibroblasts from SAVI patients are hypersensitive to treatment with even low-dose cyclic GMP-AMP (cGAMP), resulting in increased IRF3 phosphorylation and type I IFN transcription(121).

Murine models of SAVI-associated mutations also develop profoundly elevated ISG signatures. However, systemic disease is independent of the type I IFN receptor, suggesting other inflammatory pathways or other types of IFNs contribute to disease, at least in mice(122). Mice harboring SAVI-associated mutations have not been reported to develop skin disease, so how the type I IFN pathways participate in SAVI-associated skin manifestations is not yet known.

Insight into disease mechanisms by targeting the IFN pathway

Anifrolumab

Anifrolumab is a monoclonal antibody that binds to subunit 1 of the type I IFN receptor (IFNAR1), thereby blocking type I IFN activity. Trials of anifrolumab for treatment of SLE have shown promise for improvement in CLE disease activity. In a phase IIb, randomized, double-blind, placebo-controlled study of anifrolumab in adults with moderate-to-severe SLE (MUSE trial), there was greater efficacy of anifrolumab in patients with a higher IFN signature, including improvement in skin disease activity as assessed by the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K), CLASI and British Isles Lupus Assessment Group (BILAG) index(123, 124). Improvement in rash was only significantly improved as assessed by the BILAG in the low IFN signature subgroup(124). In the Treatment of Uncontrolled Lupus via the IFN Pathway (TULIP) trial II, there was ≥50% decrease in CLASI scores in half of the anifrolumab group compared to only 25% of the placebo group (p = 0.04)(125). In systemic scleroderma patients, anifrolumab treatment has also been shown to decrease type I ISG expression in patient skin biopsies collected 28 days after dosing with anifrolumab(126).

Janus Kinase (JAK) inhibitors

JAK inhibitors block one or multiple JAKs (JAK1, JAK2, JAK3, TYK2), which are tyrosine kinases that bind to a wide variety of cytokine receptors (including all three types of IFNs) and thereby affect the immune response(127, 128). CLE lesions have been shown to exhibit high expression of phospho-JAK2 similar to CXCL10 and MxA, and treatment of keratinocytes and a 3d epidermis model with ruxolitinib after poly(I:C) stimulation decreases type I ISG expression(129). Treatment of murine lupus with tofacitinib resulted in improvement of both systemic and cutaneous disease manifestations(130). In DM, skin disease has shown improvement after treatment with ruxolitinib, further supporting a role for IFNs in DM pathogenesis(131). Treatment of 18 interferonopathy patients with baricitinib led to a decrease in IFN scores and clinical symptoms, with improvement in cutaneous disease also reported although not specifically scored(132). Similarly, treatment of cutaneous lesions in familial chilblain lupus with baricitinib leads to improvement in skin disease(133). Liu et al also demonstrated that treatment of SAVI patient T and B cells with JAK inhibitors blocks constitutive phosphorylation of STAT1(121).

Anti-BDCA2 antibody (BIIB059)

BIIB059 is a humanized monoclonal antibody that binds blood DC antigen 2 (BDCA2), a C-type lectin and pDC specific receptor. BIIB059 is believed to inhibit TLR-induced type I IFN and other inflammatory mediator production. In CLE, BIIB059 has been shown to reduce skin inflammation(134). In a randomized, double-blind, placebo-controlled trial of BIIB059 in SLE patients with active skin disease, BIIB059 decreased expression of MxA and IFITM3 and also CD45+ cellular infiltrate in skin biopsies four weeks after treatment and additionally improved CLASI scores(134). BIIB059 has also been described to reduce IFNα production from pDCs of CLE patients after stimulation with TLR agonists, providing an additive therapeutic benefit to hydroxychloroquine(135).

Conclusions

Overproduction of type I IFNs is a unifying theme amongst many autoimmune and autoinflammatory patients with skin manifestations. In SLE/CLE, this contributes to inflammatory cell activation and photosensitivity, a mechanism which likely extends to other diseases, possibly the autoinflammatory diseases as well. Further research is needed to understand the ways in which interferons drive disease and to identify which patients will benefit most from targeting of IFNs.