Rheumatoid arthritis (RA) is a systemic immune disease characterized by chronic symmetrical synovitis inflammatory polyarthritis of the small joints (1). In addition to arthritis, RA causes various extra-articular complications, such as RA-associated interstitial lung disease (RA-ILD) and skin damage (2, 3). Specific environmental and genetic factors cause immune system disorders in RA patients and determine the occurrence and development of RA (4).

Various immune cells and multiple cytokines produced by immune cells exert essential roles in the development and disease progression of RA. Interleukins (ILs) are a class of cytokines widely involved in immune system modulation and are implicated in RA. Two CD4⁺ T cell subsets, T helper 1 cells (Th1) and T helper 17 cells (Th17), have long been considered risk factors for RA development (5). In contrast, T helper 2 cells (Th2) and regulatory T cells (Tregs) are decreased in number and exhibit functional impairment in RA. IL-1 and IL-17, which are secreted by Th1 and Th17 cells, respectively, have been reported to be proinflammatory factors in RA synovial membranes. In addition, IL-15, secreted by macrophages, promotes T cells in RA synovial membrane tumor necrosis factor (TNF)-α production in a cell contact-dependent manner (6). The cytokine network, constituted by multiple cytokines and feedback loops, subtlety regulates the immune response and inflammation in complex ways and is involved in RA development (7).

IL-27 is a pleiotropic immunoregulatory cytokine that was first identified in 2002 (8). Since the discovery of IL-27, its immune modulatory function has been gradually elucidated by numerous studies. An association between IL-27 single-nucleotide polymorphisms (SNPs) and genetic susceptibility to RA has been described (9). Abnormal IL-27 levels have been well demonstrated in previous studies, and IL-27 participates in RA development via multiple pathways (10–12). Earlier studies found that IL-27 primarily affects the differentiation of helper T cell subsets (13). However, more recent studies suggest that IL-27 might contribute to the pathogenesis of RA through additional direct and indirect regulatory pathways. Current studies have shown that IL-27 is extensively and profoundly involved in immunological imbalance in RA. Furthermore, IL-27 is a potential therapeutic target for the treatment of immune-related diseases. This review discusses updates on the regulatory effects of IL-27 on immune cells and RA development. We expect that our review may provide ideas for future studies, which would promote increased awareness of the relationship between RA and IL-27.

The IL-27 receptor (IL-27) is a member of the IL-12 and IL-6 cytokine families and was identified two decades ago  (8, 14, 15). IL-27 is a heterodimer with p28, and Epstein–Barr virus induced 3 (EBI3) subunits that are encoded by IL27 and EBI3, respectively ( Figure 1 ). The IL-27 p28 subunit is homologous to IL-12 p35 and IL-6  (16). EBI3 was first identified in B lymphocytes transfected with Epstein–Barr virus (EBV), and it is homologous to IL-12 p40  (17). However, the two subunits can exist independently. The IL-27 p28 subunit, also known as IL-30, can be secreted alone  (18). In this review, we use the designation “IL-27 p28 subunit” instead of “IL-30” because the name of the gene encoding the IL-27 p28 subunit protein is IL-27 according to HGNC guidelines  (8).

Existing studies suggest that recombinant IL-27 p28 blocks the activity of cytokines, including IL-6, IL-11, and IL-27  (16), which suppresses hepatic injury and alleviates experimental autoimmune uveitis  (19). However, other studies found that the conformation of IL-27 p28 used in some studies was incorrect. IL-27 p28 can bind with soluble IL-6Rα and signal in the presence of a more appropriate conformation. IL-27 p28 can even signal independently at high concentrations  (20, 21). This feature of IL-27 p28 raises a concern about IL-27, as we do not know whether IL-27 or IL-27 p28 is performing these biological functions. Further investigation is warranted to determine whether another subunit of IL-27, EBI3, can exert some of its biological functions independently. Determining the answers to these questions could help to better understand the roles of IL-27 in diseases.

IL-27R, a heterodimer, is a member of the class I cytokine receptor family, which is composed of α subunits of the cytokine binding protein and β subunits of the signal transduction protein ( Figure 1 ) (14, 15, 22). The two subunits, IL-27Rα, also known as T cell cytokine receptor (TCCR) or WSX-1, and glycoprotein 130 (gp130) are composed of IL-27R, and they are both members of the immunoglobulin superfamily. Gp130 is a receptor subunit that is shared by IL-6 family cytokines. Both IL-27Rα and gp130 are necessary for IL-27 signal transduction. Memory CD8⁺ T cells are not responsive to IL-27 stimulation because they downregulate the expression of gp130  (23). Moreover, research has indicated that IL-27Rα is capable of forming homodimers and activates Janus tyrosine kinase (JAK)/signal transduction and activator of transcription (STAT) signaling  (24, 25). Additionally, soluble IL-27Rα is able to inhibit IL-27 signaling  (26). Similar to IL-27, these features of IL-27R also influence our understanding of IL-27 signaling in diseases.

IL-27Rα is expressed by several varieties of immune cells and non-immune cells, including T and B lymphocytes, NK cells, monocytes, macrophages, and hepatocytes  (27). It has been reported that lymphoid tissues, including the spleen, thymus, and lymph nodes, also express high levels of IL-27Rα  (28). Most cells express gp130, and its expression levels are related to cellular status  (29). Cells in different states may express different levels of IL-27. Cell activation causes CD4⁺ T cells to express increased IL-27Rα but causes NK cells to express reduced levels, which influences cellular responses to IL-27 signaling.

IL-27 is a soluble-secreted cytokine, so understanding its cellular sources is important. The two subunits of IL-27 are expressed by many immune cells, such as monocytes/macrophages, lymphocytes, activated dendritic cells (DCs), plasma cells, natural killer (NK) cells, and other non-immune cells, such as placental trophoblasts and endothelial cells  (30, 31). Specific signaling stimulation also regulates IL-27 expression. Dibra et al. found that coexisting Toll-like receptor (TLR) 9 signaling from macrophages and CD3 signaling from T cells stimulate the expression of IL-27 p28 in splenic cells  (32). Liu et al. determined that lipopolysaccharide (LPS) and interferon-γ stimulation promotes IL-27 p28 expression in macrophages  (30). Moreover, in DCs, expression of EBI3 was induced by TLR signaling via nuclear factor kappa B (NF-κB)  (33).

As a soluble cytokine, IL-27 primarily acts as a first messenger and mediates IL-27-induced signal transduction by binding to cell membrane-bound IL-27R. IL-27R typically activates the JAK family, and the JAK family-induced signaling pathway is considered the classical apex of the IL-27 signaling cascade  (34). The cytoplasmic region of IL-27Rα has a Box1 motif and an SH domain, which is related to JAK1 and JAK2 and may assist in signal transduction  (35). Gp130 is also associated with the JAK family members JAK1, JAK2, and tyrosine kinase (TYK) 2. The IL-27 signaling pathway can be activated by IL-27 and its receptor IL-27R in different cell types and in different cellular states ( Figure 2 ). In mast cells, IL-27 treatment leads to STAT3 phosphorylation and increases mRNA levels of IL-1α, IL-1β, IL-18, and TNF-α  (28). In primary monocytes, IL-27 stimulation induces the expression of IL-6, C–X–C motif chemokine (CXCL) 10, C–C motif chemokine ligand (CCL)3, CCL4, and TNF-α, accompanied by phosphorylation of STAT1 and STAT3  (28, 36). IL-27 also increases the phosphorylation of STAT1 and STAT3 in macrophages  (37). In germinal center B cells, IL-27 induces weak phosphorylation of STAT1 and STAT3. However, IL-27 significantly promotes the phosphorylation of STAT1 and STAT3 and increases the expression of the transcription factor T-bet in naive and memory B cells. In anti-Ig-stimulated-naive or memory B cells, IL-27 also induces CD54, CD86, and CD95 expression. Moreover, IL-27 increases the proliferation of anti-Ig-activated naive B cells and anti-CD40-activated naive and germinal center B cells but not of CD40-activated memory B cells. In CD4⁺ T cells, IL-27 induces expression of the transcription factor T-bet in a STAT1-dependent and STAT1-independent manner, which is crucial for Th1 polarization. IL-27 activates different JAK family members and downstream signaling pathways in different cell types and cell states, which regulates the signaling and biological pattern of immune cells. However, the exact roles and mechanisms of IL-27-induced JAK signal transduction in different cells remain unclear, and further studies are needed.

Given the available studies, we can preliminarily determine that IL-27 signaling could influence RA development by regulating CD4⁺ T cell differentiation, inhibiting monocytes/macrophages and osteoclasts in the joint cavity, interrupting synovial ectopic lymphoid structure (ELS) interactions with Th17 cells, and regulating RA synovial fibroblast (RA-FLS)-mediated inflammation. However, some conclusions from different studies are conflicting, and clinical samples or arthritis models were not utilized in some studies. To clearly present and discuss the mechanisms of IL-27 signaling and RA pathologies, a table is provided ( Table 1 ).

The IL-27 signaling pathway has not been clinically investigated as a potential target for disease treatments except in tumors. Recently, SRF388, a targeted IL-27 p38 antibody, has been evaluated for its safety and therapeutic potential for the treatment of advanced solid tumors in a phase 1, first-in-human, dose-escalation, and expansion study (NCT04374877). Solid tumor animal models and cellular experiments with bioinformatics research have demonstrated that blocking IL-27 signaling is beneficial for the treatment of hepatocellular carcinoma and renal cell carcinoma  (80, 81). The medium-term results of a phase I clinical trial demonstrated that SRF388 inhibited STAT3 phosphorylation in 12 recruited tumor patients with mild adverse reactions  (82). These results indicate that inhibiting IL-27 signaling via SRF388 might represent an alternative tumor treatment.

However, IL-27, a pleiotropic cytokine, yielded confusing results when attempting to alleviate RA patients by intervening in IL-27 signaling. It is difficult to see that the roles of IL-27 in regulating RA pathology are many and even contradictory. Evidence from animal studies suggests that IL-27 exerts pro- or anti-inflammatory roles in different arthritis animal models. Conflicting results have also been observed in numerous cellular experiments. Considering the limitations of animal and cellular studies, it is not currently possible to conclude whether addition or blocking of IL-27 would treat RA according to only one or multiple laboratory studies. The roles of IL-27 in cells or tissues related to RA pathology should continue to be explored using cellular and animal studies in the future. However, further studies on IL-27’s levels and roles in RA blood, joint cavities, and rheumatoid nodules by detecting clinical specimens from RA are also important. Alejandro et al. focused on the roles of IL-27 in RA patients with follicular synovitis and found that agents regulating IL-27, such as SRF388, might only be suitable for RA patients with specific clinical and pathologic characteristics  (68).

Overall, the intricate links between RA immune regulation and inflammatory responses are being actively explored but are still not fully understood. Many cytokines exert beneficial or detrimental effects on RA patients by regulating immune and inflammatory responses. Among these cytokines, IL-27 is undoubtedly one of the most controversial cytokines for RA due to its pleiotropism. The awareness of immunity and inflammation is expanded while exploring the biological functions of IL-27. The pleiotropic regulatory functions of IL-27 serve as a reminder that inflammation involves a series of continuously changing states rather than simply a resting or constant state. IL-27 might be a cytokine that promotes an inflammatory state.

In the present article, we systematically reviewed and discussed the associations between RA and IL-27 by referencing previous studies. IL-27 levels are generally elevated in blood, synovial fluids, synovial fibroblasts, and even rheumatoid nodules. Nevertheless, it remains unclear whether higher levels of IL-27 are beneficial or harmful to RA patients. According to experimental research, IL-27 alleviates RA progression by inhibiting ELS formation and osteoclast differentiation. However, other laboratory evidence indicates that the roles of IL-27 in CD4⁺ T cells and RA-FLSs are complex and even conflicting, so it is difficult to draw a definitive conclusion regarding IL-27’s functions.

Considering that widely used arthritis animal models do not fully reflect the pathological characteristics of RA, use of clinical samples from RA should be considered in future studies. IL-27 might exert different roles in different RA patients; thus, the clinical characteristics, including demographic information, laboratory tests, and medication history, of RA patients should be recorded in detail. Overall, the existing evidence suggests that IL-27 exerts both pro- and anti-inflammatory effects in different ways. It is hard to say whether activating or inhibiting the IL-27 signaling pathway would be effective as an RA treatment. It is possible that some RA patients would benefit from IL-27 signaling pathway activation, while others may benefit from pathway inhibition, and still others might be unsuitable for treatment targeting the IL-27 signaling pathway. Therefore, careful attention should be given to IL-27 signaling-targeted therapy for RA to clearly understand how IL-27 signaling works in RA patients and whether RA patients would benefit from intervening in IL-27 signaling.

ST contributed to the conception of this review. LH wrote the manuscript. LH, ZC, and KY revised the manuscript. LH and KY designed and illustrated the figures. ZC, JY, TL, WL, XB, and PS performed the literature search and interpretation. ZC, YG, YL, YaH, YiH, KQ, YW, and ST reviewed and revised the manuscript. All authors contributed to the article and approved the submitted version.

This study was supported by the National Natural Science Foundation of China (81874383, 82074267, 82174185), the Chinese Medicine Scientific Research Project of the Health Commission of Hubei Province (ZY2021Q024), and the program of Tongji-Rongcheng Center for Biomedicine (HUST).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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