Adult-onset Still's disease (AOSD) is a rare, systemic, polygenic, autoinflammatory disorder characterized by the cardinal manifestations of fever, arthralgia or arthritis, skin rash, and increased counts of leukocytes and neutrophils (1–3). Patients may have serious complications such as macrophage activation syndrome (MAS), pulmonary arterial hypertension, and disseminated intravascular coagulopathy (1, 2). As the clinical manifestations of AOSD are consistent with several key common features of autoinflammatory diseases, it has been considered as the archetype of non-familial, polygenic, systemic autoinflammatory diseases in recent years (1).

Danger signals such as pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) are transmitted to macrophages and neutrophils that activate specific inflammasomes, leading to the production of IL-1β and IL-18. Furthermore, this process leads to intense innate immune cell activation and overproduction of several proinflammatory cytokines, including IL-6, IL-8, IL-17, and tumor necrosis factor (TNF)-α. This cytokine storm is the hallmark of AOSD, which plays a crucial role in the pathogenesis of AOSD (4). Meanwhile, we also demonstrated that the levels of anti-inflammatory cytokines IL-10 and IL-37 were increased in the serum of AOSD patients and positively correlated with disease activity (5, 6). Moreover, emerging evidences revealed that a series of mediators were considered as activity markers for AOSD, such as S100A8/9, S100A12, macrophage migration inhibitory factor (MIF), and HMGB1 (7–10). In addition, our previous study showed that microRNAs were potential biomarkers to distinguish AOSD from sepsis (11).

Cysteine-rich angiogenic inducer 61 (Cyr61), also termed cellular communication network factor 1 (CCN1), is a secreted extracellular protein mainly produced by stromal cells (12, 13). Multiple functions have been well-established on Cyr61 with regard to embryonic development, tumorigenesis, fibrosis, angiogenesis, cell proliferation, and migration (13, 14). However, the role of Cyr61 during inflammation is complicated. Our previous work reported that the expression of Cyr61 was increased in the synovium of rheumatoid arthritis (RA) patients and epidermis of psoriasis patients, and Cyr61 contributed to the cross-talk between immune cells and stromal cells during the development of autoimmune and inflammatory diseases (15–17). Cyr61 increases inflammatory cytokines IL-1β, IL-6, IL-8, and TNF-α expression in fibroblasts, keratinocytes, and macrophages via integrin receptors and downstream mitogen-activated protein kinase (MAPK) and nuclear factor (NF)-κB signals (16, 18, 19). Interestingly, Cyr61 also promotes tissue repair, a process accompanied by inflammation resolution (20). Cyr61 induced senescence of fibroblasts via upregulating p53 expression, which contributed to cutaneous tissue repair (21). Moreover, Cyr61 improved neutrophil efferocytosis by serving as a bridge between macrophages and neutrophils during wound healing (22). Recent clinical research suggested that Cyr61 served as a biomarker for RA and was negatively correlated with disease activity (23). Thus, the function of Cyr61 is controversial and undetermined in autoimmune and autoinflammatory diseases. Whether Cyr61 contributes to the pathogenesis of AOSD is still unclear. Here, we explored the levels of Cyr61 in the serum of AOSD patients and discussed the relationship between Cyr61 and clinical and laboratory features of the disease.

Cyr61 is a secreted, matricellular protein, which is the first member of Cyr61-CTGF-NOV protein family (13). Cyr61 contributes to multiple physiological and pathological processes, such as embryonic development, angiogenesis, cell proliferation and migration, tumorigenesis, wound healing, and inflammation, by binging to diverse integrin receptors and activating different downstream signaling pathways (12, 13). In recent years, elevated levels of Cyr61 had been found in many autoimmune and inflammatory diseases, such as RA, systemic lupus erythematosus (SLE), inflammatory bowel disease (IBD), Sjogren's syndrome (SS), and psoriasis (15, 17, 27–29). However, the profiles of Cyr61 in AOSD, a multigenic, systemic autoinflammatory disease, are still unknown. Here, we investigated the serum level of Cyr61 and its association with disease activity in AOSD patients, we proposed that Cyr61 was a potential biomarker of AOSD disease activity.

In the present study, we found that the serum levels of Cyr61 were significantly increased in inactive AOSD patients compared with those in active patients. Moreover, during serial follow-up, serum levels of Cyr61 were elevated in the inactive phase after effective treatment. This inverse correlation was also confirmed in AOSD patients classified by different clinical manifestations. The serum levels of Cyr61 were increased in AOSD patients who had disease remission after fever, skin rash, sore throat, arthralgia, and lymphadenopathy improved. The serum levels of Cyr61 were inversely correlated with systemic score and proinflammatory cytokines, such as IL-1β, IL-6, and IL-17. Taken together, we concluded that the serological Cyr61 level was inversely correlated with AOSD disease activity. Coincidentally, a recent clinical cohort study of RA showed that the levels of Cyr61 were dramatically lower in patients with higher disease activity, and it showed a significant increase at 12 weeks in ACR responders (ACR20/50/70) compared with those in non-responders (23). Similarly, a previous report evaluated the potential value of Cyr61 in SLE-associated pulmonary arterial hypertension patients. It revealed that patients with higher levels of Cyr61 had better survival than those with a lower Cyr61 level. Besides, we noticed that the levels of Cyr61 showed no correlation with the levels of CRP, ESR, and ferritin maybe because Cyr61 is a stromal cell-derived factor, not an inflammatory cytokine in the initial stage of inflammation in AOSD.

The negative correlation of Cyr61 with disease activity is complicated and partially resulted from the diverse functions of Cyr61. It is well-known that Cyr61 triggered inflammation by upregulating the expression of inflammatory cytokines, such as TNF-α, IL-1β, and IL-6 in both immune cells and nonimmune cells via different integrin receptors and MAPK/NF-κB signal pathway (16, 18, 19). However, multiple studies revealed the function of Cyr61 on the limitation of inflammation and tissue repair. In liver inflammation, Cyr61 facilitated the expansion of HLA-DR^−/lowCD33⁺CD11b⁺ myeloid-derived suppressor cells (MDSCs) via α_mβ₂/STAT3 signal and Cyr61/GM-CSF-induced MDSCs inhibited T-cell proliferation significantly (30). In murine experimental autoimmune myocarditis, overexpression of Cyr61 decreased the cardiac disease score and inhibited macrophage and lymphocyte migration without changing the chemokine and cytokine expression (31). As well, in dextran sodium sulfate (DSS)-induced colitis, misfunction of Cyr61 resulted in high mortality and Cyr61 treatment promoted mucosal healing by accelerating intestinal epithelial cell proliferation (27). During the biliary injury, Cyr61 promoted the proliferation and differentiation of cholangiocytes through αvβ5/NF-κB/JAG1 pathway to improve tissue repair (32). In addition, Cyr61 bound both integrins α_vβ₃ and α_vβ₅ on macrophages and phosphatidylserine on neutrophils to promote neutrophil efferocytosis (22) and induced fibroblast senescence by activating p53 and ROS pathway and inhibited fibrosis in wound healing (21). Whether Cyr61 plays a protective role in the pathogenesis of AOSD needs further study. As Cyr61 was increased in inactive AOSD patients, we speculated that Cyr61 participated in the development of AOSD, especially involved in the process of inflammation resolution and tissue repair during AOSD remission.

To our knowledge, the inflammatory stimuli, such as TNF-α, IL-1β, IL-6, and IL-17, which were increased in the serum of AOSD patients, promoted Cyr61 expression (15, 18, 33); however, non-parametric Spearman correlation analysis revealed that the serum levels of Cyr61 were inversely correlated with the levels of IL-1β, IL-6, and IL-17 (Figure 4). The reason why Cyr61 levels were not increased in inflammatory circumstances might be due to the tight control of Cyr61 expression. MicroRNA is one of the key posttranscriptional controllers in regulating gene expression, and several microRNAs were reported to target Cyr61, such as miR-22, miR-181c, miR-155, and miR-142-5p (34–37). In our previous work, the expression of miR-142-5p was dramatically increased in plasma from active AOSD than that from inactive AOSD (11). As Cyr61 is the direct target of miR-142-5p, it might partially contribute to the lower expression of Cyr61 in active patients of AOSD with systemic inflammation.

It is important to acknowledge that there are some limitations in our study. First, the power of the study is limited by a relative small sample size and the retrospective design, as a result, prospective studies with a larger sample size are needed in the future. Second, active AOSD patients enrolled were not all treatment naive, and not every active patient was followed up, which may reduce the power of statistical analysis. Finally, the mechanism of Cyr61 in the pathogenesis of AOSD is still unknown; further experimental study is still needed.

In conclusion, our study first reported the increased levels of Cyr61, a matricellular protein in the serum of inactive AOSD patients, we found that its expression was inversely correlated with disease activity. As Cyr61 is secreted by stroma cells, such as fibroblast cells, keratinocytes, and endothelial cells (15, 17, 38), the injured tissues after inflammation destruction might provide the local source of Cyr61 in remission phase, contributing to tissue repair. However, given the fact that Cyr61 is expressed in distinct cell types and different times, the proinflammatory and anti-inflammatory characters were exhibited diversely, the pathogenesis involved needs to be studied thoroughly. Thus, we propose that Cyr61 might be a potential biomarker for the remission of AOSD.

The datasets generated for this study are available on request to the corresponding author.

The studies involving human participants were reviewed and approved by The Institutional Research Ethics Committee of Ruijin Hospital (identifier 2016-62), Shanghai, China. The patients/participants provided their written informed consent to participate in this study.

YSun and YSu conceived of the study and participated in its design and coordination. ZW and JY carried out the ELISA and performed the statistical analysis. HC, ZZ, QH, HL, XC, HS, and JT collected samples and contributed to data acquisition, analysis, and critical review for intellectual content. TF made the statistics for all data. YSun, YSu, and CY drafted the manuscript and revised the manuscript. All authors read, revised, and approved the final manuscript.