Insights into IL‐29: Emerging role in inflammatory autoimmune diseases

Abstract Interleukin‐29 (IL‐29) is a newly discovered member of type III interferon. It mediates signal transduction via binding to its receptor complex and activates downstream signalling pathways, and therefore induces the generation of inflammatory components. Recent studies reported that expression of IL‐29 is dysregulated in inflammatory autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosus, osteoarthritis, Sjögren's syndrome, psoriasis and systemic sclerosis. Furthermore, functional analysis revealed that IL‐29 may involve in the pathogenesis of the inflammatory autoimmune disorders. In this review, we will systematically review the current knowledge about IL‐29. The information collected revealed the regulatory role of IL‐29 and may give important implications for its potential in clinical treatment.


| INTRODUC TI ON
The newly discovered cytokine interleukin-29 (IL-29) belongs to type III interferon (IFN), which has another two subfamilies IL-28A and IL-28B. They are also known as IFN-λ1, IFN-λ2 and IFN-λ3, respectively. 1  (IL-10R2 and CRF2-4). 5,6 Recently, IL-29 has been focused much attention. Production of IL-29 was found specific to some tissues, and there was a tissue specificity responding to IL-29. 7 IL-29 receptor is expressed in dendritic cells, T cells, intestinal epithelial cells and leukaemia cells. The important role of IL-29 in tumours and its potential use for clinical therapy has been widely discussed. 8 Due to antiviral and immunoregulatory characteristics of IL-29, studies also showed that IL-29 performed significantly in the pathogenesis of inflammatory autoimmune diseases, 9 for instance, systemic lupus erythematosus (SLE), 10 rheumatoid arthritis (RA), 11

psoriasis 12 and
Sjögren's syndrome (SS). 13 The discovery of type III IFNs opens up a new field of IFN research in which IL-29 is considered to be a core member. On account of the strong association between this molecule and inflammatory autoimmune diseases, we systematically review recently published articles on this significant relationship. The regulatory capacity of IL-29 in inflammatory autoimmune diseases has drawn increased attention to these studies. It is hoped that the information collected will contribute to future research on IL-29, and may provide some clues for its role in inflammatory autoimmune diseases. Furthermore, our review may give important implications for its potential in clinical treatment.

| IL-29 S I G NALLING
Studies suggested that IL-29 is able to activate downstream signalling pathways, and therefore induces the generation of inflammatory components. Activation of Janus kinase/signal transduction and activator of transcription (JAK-STAT) signalling pathway might be induced by IL-29 through STAT1 and STAT2. Similarly, activation of STAT3 and STAT5 by IL-29 also proved to affect the JAK-STAT signalling. 7,14 Consistently, IL-29 induced signal transduction through activation of protein kinase B (Akt) and mitogen-activated protein kinase (MAPK). It is notable that the ability of IL-29 to affect signalling pathways may depend on specific cells. Osteoarthritis (OA) fibroblastlike synovial cells (FLS) stimulated with IL-29 activated JAK-STAT, MAPK and Akt signalling pathways, resulted in phosphorylation of the proteins. 15 Mast cell line P815 treated with IL-29 promoted the production of IL-4 and IL-13 through phosphatidylinositol 3-kinase (PI3K)/Akt and JAK-STAT3 signalling pathways. 16 Monocyte-derived macrophage responded to IL-29 treatment via STAT1 phosphorylation. 17 In RAW264.7 cells, an elevation of lipopolysaccharide (LPS)-induced nuclear factor-kappa B (NF-κB) signalling activation was observed following IL-29 stimulation. 11 In addition, IL-29 down-regulated expression of nuclear factor of activated T cell 1 (NFATC1)-mediated osteoclastogenic genes such as tartrate-resistant acid phosphatase (TRAP), cathepsin K (CTSK) and matrix metalloprotein 9 (MMP-9) through activation of c-Jun N-terminal kinase (JNK), and inhibition of c-Fos, and NFATc1 in receptor activator of nuclear factor-κ B ligand (RANKL)-stimulated RAW264.7 cells. 9 Moreover, IL-29 regulated toll-like receptor 3 (TLR3) expression in keratinocytes, but it was hindered following adding JAK inhibitor 1, suggesting that IL-29-induced TLR3 generation may depend on the activation of JAK-STAT pathway. 18 It is recognized that bone erosion in RA correlated with increased production of pro-inflammatory cytokines and accelerated osteoclastogenesis in affected joints.
IL-29 suppressed osteoclastogenesis by activation of STAT signalling pathway and inhibition of NF-κB activation, and NFATc1 translocation. 9 When tyrosine residues on STATs were phosphorylated, some homodimers and heterodimers were formed and translocated into nucleus, and then combined with IFN-stimulated response elements (ISREs) in regulatory regions of the IFN-stimulated genes (ISGs). For example, ISG factor 3 (ISGF3), a transcription complex, consisted of phosphorylated STAT1, STAT2 and IFN regulatory factor (IRF) 9, initiated the transcription of ISGs. 19 Subsequently, IL-29 presented the ability of antiviral protection, anti-proliferative response, antitumour activities and immune regulation. 15 All these revealed that IL-29 may F I G U R E 1 Signal transduction pathways initiated by IL-29. IL-29, a heterodimeric cytokine, binds to receptor complex composed of IL-28R1 and IL-10R2. The induced Janus kinase/signal transduction and activator of transcription (JAK-STAT) pathway leads to the activation of STAT1 and STAT2, and subsequently combines with IRF9 to form ISGF3 transcription factor complex. The formed complex transmits signalling to the nucleus. Similarly, the activating signalling was transmitted to the nucleus via mitogen-activated protein kinase (MAPK), protein kinase B (Akt), nuclear factor-kappa B (NF-κB) and nuclear factor of activated T cell 1 (NFATc1) pathways, presenting the ability of antiviral protection, anti-proliferative response and immune regulation involve in cytokine secretion and regulation of cellular function through modulating the activation and signalling transduction of signalling pathways ( Figure 1).

| Effect of IL-29 on non-immune cells
Human keratinocytes (KCs) were reported to induce expression of antiviral proteins and TLR3. 18

| Significant role of IL-29 in innate immunity
Monocytes respond well to IL-29 and produce several cytokines.  16 These findings suggested that IL-29 had an ability to regulate cytokine release and induce mast cell infiltration.

| IL-29 plays important roles in adaptive immunity
Human CD27 − naive and CD27 + B cells stimulated by IL-29 alone or in combination with the TLR7/8 ligand R848 significantly up-regulated the marker CD69, indicating that both naive and memory B cell populations were responsive to IL-29 stimulation. 6 CD19 + B cells stimulated with IL-29 could significantly up-regulate IFN-stimulated genes myxovirus resistance-1 (Mx1) and OAS1 expression. IL-29 stimulation on CD19 + B cells also up-regulated TLR7 expression. 6 Intriguingly, the combination of IL-29 with R848 enhanced TLR7/8mediated IgG and IgM production, and the production of R848-mediated IL-6 on treated CD19 + B cells, implying the role of IL-29 to regulate TLR7/8-triggered cytokine secretion. 6 Addition of IL-29 enhanced R848-induced proliferation of CD19 + B cell. 6 Collectively, these findings suggested IL-29 to regulate the function of B cells, especially binding to TLR7/8 ligation.
Interleukin-29 directly inhibited Th2 polarization by regulating Th2 restrictive transcription factor GATA3. 24  on CD3 + CD4 + T cells also noted reduction in IL-4 and IL-5 production. 30 Collectively, IL-29 may affect release of inflammatory cytokines in T cells and regulation of T cell differentiation (Figure 2).

| IL-29 expression in inflammatory autoimmune disorders
Rheumatoid arthritis is a type of chronic systemic autoimmune disease. IL-29 was expressed in CD68 + macrophage and FGF-2+ fibroblast in the lining layers of RA synovium. 31 Expression of IL-29 was significantly higher in serum, peripheral blood mononuclear cells (PBMCs) and synovial tissue in RA patients compared with that in healthy controls. 11,32 Levels of IL-29 in synovial fluid (SF) were higher in RA patients compared with that in OA patients. 31 Serum levels of IL-29 positively correlated with rheumatoid factor (RF), anti-cyclic citrullinated peptide (anti-CCP) antibody and disease activity score of 28. Interestingly, anti-CCP-positive RA patients had higher serum IL-29 levels than that in healthy controls and anti-CCP-negative RA patients. 32 In RA synovial fluid, increased level of granzyme M (GrM) could induce the release of IL-29. 33 In SLE patients, IL-29 expression was increased in SLE patients as compared to healthy controls. SLE patients with active disease showed elevated expression of IL-29 than the less active group, suggesting that IL-29 may imply disease activity. 34 SLE patients sometimes showed clinical manifestations, such as alopecia, mucosal ulcer, malar rash, chest affection and thrombocytopenia. 35 SLE patients with renal disease and arthritis in contrast to patients without the manifestations showed increased serum IL-29 levels. 34 Serum levels of IL-29 correlated with SLE disease activity index, anti-dsDNA antibody and C-reactive protein. 34 In OA patients, IL-29 expression was strongly higher in OA PBMCs than healthy controls. Serum levels of IL-29 and IL-29 expression in synovium were elevated in OA patients when compared to healthy controls. 15 Therefore, expression of IL-29 was dysregulated in RA, SLE and OA patients, and correlated with disease activity.
Levels of IL-29 were strongly increased in serum of SS patients with intermediate minor salivary gland tissue (MSG) infiltrates when compared to the controls. 13 Psoriasis and atopic dermatitis (AD) are two common chronic inflammatory skin diseases. IL-29 can be derived from Th17 cells, and IL-29 expression was enhanced in lesion skin of psoriasis patients. 36 Levels of IL-29 in psoriatic lesions positively correlated with antiviral protein expression. 12 Similarly, serum levels of IL-29 were elevated in psoriasis patients as compared to healthy controls. 37 IL-29 levels were significantly higher in the skin of AD patients when compared to healthy controls. 38 Hashimoto's thyroiditis (HT) is a typical autoimmune thyroid disease. Serum levels of IL-29 were higher in HT patients as compared to healthy controls. 39 Furthermore, systemic sclerosis (SSc) patients showed increased IL-29 serum levels as compared to that in healthy controls. 40 Childhood uveitis is a sight-threatening inflammatory eye disease.
Most patients with uveitis are associated with juvenile idiopathic arthritis (JIA). IL-29 levels were down-regulated in ocular fluid from JIA-associated uveitis patients. IL-29 intraocular levels were decreased in JIA-associated uveitis patients when compared to those with idiopathic uveitis. 41 Taken together, IL-29 levels were abnormal in some other inflammatory autoimmune diseases including SS, psoriasis, AD, HT, SSc and uveitis (Table 1).

| IL-29 regulates the production of autoimmunerelated components
TLR4 plays an important role in synovial inflammation and contributes to the pathogenesis of RA. 42 Expression of TLR4 in RAW264.7 cells was increased when exposed to IL-29. 11  antibody treatment down-regulated the expression of antiviral proteins. 12 Overall, above findings provided clues that IL-29 may involve in the pathogenesis of inflammatory autoimmune diseases by regulating expression of the inflammatory components.

| Therapeutic potential of IL-29
Previous investigations have confirmed that IL-29 plays a vital role in several therapeutic fields, including hepatitis C and B, and melanoma, due to its ability of antiviral and antitumour. 3,27 In recent years, attention has been paid to the role of this cytokine in immune regulation. IL-29 is abnormally expressed in some inflammatory autoimmune diseases. Moreover, numerous researches have shown that IL-29-mediated inflammation plays an important role in the pathogenesis of these diseases. Given the biological role of IL-29 in many immune and non-immune cells, including KCs, monocytes, pDC, B cells, mast cells and its regulatory ability in many autoimmune components, this cytokine may be considered as a therapeutic target for the complex inflammatory diseases such as OA and RA. However, more information about the application of IL-29 in the treatment of inflammatory autoimmune disorders needs to be further clarified.

| REG UL ATI ON OF IL-29 E XPRE SS I ON
Above findings showed that IL-29 regulates the production of inflammatory cytokines and chemokines, and therefore may play a potential role in inflammatory autoimmune disease. In turn, regulation of IL-29 expression may reverse the abnormal expression and function of IL-29, giving the possibility to inhibit the generation of inflammatory components and suppress the development of the disorders. In normal conditions, pDCs treated with HSV or imiquimod produced high levels of IL-29. 25 IL-29 is known to induce IL-12p40 production in macrophages. However, the effect can be inhibited in the presence of IFN-α. 21 Exposure of P815 cells to SC2 antibody hindered the capacity of IL-29 to induce IL-4, IL-13 and IL-6 release. 29 After adding rPer, a common allergen, the levels of IL-29 in HMC-I cell were increased. Interestingly, the effect of rPer on IL-29 expression in HMC-1 cell was suppressed by adding PA1 blocking antibody. Poly(I:C) induced significant up-regulation of IL-29 expression. 16 Studies have shown that IL-29 makes a significant effect on the Th2 response via antagonizing the effect of CD4 + T cells and Th2-related cytokine production. Nevertheless, production of IFN-λ in turn was controlled by Th2 cytokines. 24 IL-29 was able to regulate IL-13 release, but was abolished following adding proteinase-K in naive CD3 + T cells. 45 When stimulated PBMCs from volunteers with IL-4, transcription and secretion of IL-29 were increased. 46   is recognized to regulate IL-19 expression. However, the capacity of IL-29 to induce IL-19 was hindered by IL-10 in PBMCs. 20 When stimulated human fibroblasts with purified granzymes, expression of IL-29 was increased. 33 These data indicated that IL-29 can be regulated by different stimulation in normal conditions, such as IFN-α and Th2-related cytokines. In autoimmunity, it was accepted that IL-29 secretion was inhibited by IL-4 stimulation on Th17 cells from psoriasis patients. 12 In arthritis, the role of IL-29 to enhance the expression of TLR4 in macrophage cell line RAW264.7 cells could be strengthened after adding LPS stimulation. Activation of NF-κB signalling became more significant when RAW264.7 cells stimulated with both IL-29 and LPS than IL-29 stimulation alone, suggesting that LPS may regulate the effect of IL-29 and the role of IL-29. 11 In RA patients, serum IL-29 levels were reduced after the treatment with DMARDs. 32 Collectively, these data suggested that IL-29 is also controlled in autoimmunity.

| CON CLUS ION
Interleukin-29 is a newly discovered cytokine and belongs to the IL-10 family. This cytokine has become a research hotspot recently. Compared to IL-28A and IL-28B, IL-29 is more structurally unique.
Furthermore, IL-29 is considered to be an effective IFN molecule in human and seems to be the most abundant IFN molecule in serum. 47 Findings from animal models and human have both proved the important role of IL-29 in inflammation. 31,32,34,35,43,44

ACK N OWLED G EM ENTS
This work was supported by grants from the National Natural Science Foundation of China (81701606), Sichuan Provincial Science and Technology Program (2019YJ0540).

CO N FLI C T O F I NTE R E S T
All the authors who have taken part in this study declared that they have no conflicts of interest to this manuscript.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data are available.