Serum CCL18 May Reflect Multiorgan Involvement with Poor Outcome in Systemic Sclerosis

Background: Serum C–C motif chemokine ligand 18 (seCCL18) in systemic sclerosis (SSc) has been primarily associated with progressive interstitial lung disease (SSc-ILD) and mortality. However, its relationship with non-pulmonary organ involvement, disease activity, and long-term outcome has not been comprehensively evaluated. We therefore examined the clinical relevance of seCCL18 in a single-center SSc cohort. Methods: A total of 151 patients with SSc (83 diffuse cutaneous (dcSSc), 68 limited cutaneous SSc (lcSSc); median (IQR) disease duration: 9 (4;16) years) and 47 age- and sex-matched healthy controls (HCs) were enrolled. Serum CCL18 concentrations were measured by enzyme-linked immunosorbent assay. Elevated seCCL18 was defined as >130 ng/mL (mean + 2 SD of the healthy control group). Organ involvement and disease activity (EUSTAR Activity Index, EUSTAR-AI) were assessed at baseline, while survival was analysed longitudinally. Results: Patients with SSc had significantly higher seCCL18 levels than HCs (mean ± SD: 99.9 ± 43.2 vs. 75.0 ± 27.5 ng/mL, p < 0.01). Elevated seCCL18 was associated with SSc-ILD (81.1% vs. 60.5%, p = 0.022), reduced forced vital capacity (FVC < 70%: 16.2% vs. 3.5%, p = 0.006), and reduced diffusing capacity for carbon monoxide (DLCO < 70%: 80.6% vs. 54.4%, p = 0.005). Higher seCCL18 levels were observed in patients with myocardial disease (104.8 ± 41.8 vs. 83.8 ± 44.2 ng/mL, p = 0.008), left ventricular diastolic dysfunction (107.1 ± 40.5 vs. 84.5 ± 45.0 ng/mL, p < 0.001), and oesophageal involvement (110.7 ± 38.3 vs. 93.3 ± 43.1 ng/mL, p = 0.009). SeCCL18 levels above the cut-off were more frequently associated with tendon friction rubs (51.4% vs. 27.4%, p = 0.007), active disease (EUSTAR-AI ≥ 2.5: 73% vs. 44%, p = 0.002), and elevated inflammatory markers (CRP > 5 mg/L: 51.4% vs. 19.3%, p < 0.001; ESR > 28 mm/h: 37.8% vs. 18.4%, p = 0.015). During a median follow-up of 87 months, 22 patients (15%) died. Elevated baseline seCCL18 predicted poorer survival in univariate analysis (log-rank p = 0.013) and remained an independent predictor of mortality in multivariable Cox regression (HR 1.789; 95% CI 1.133–2.824; p = 0.013), together with declining DLCO and reduced six-minute walk test performance. Conclusions: Elevated seCCL18 may identify patients with systemic sclerosis who exhibit a more severe multisystem phenotype, including cardiopulmonary, gastrointestinal, and musculoskeletal involvement, increased inflammatory activity, and reduced long-term survival. These findings suggest that seCCL18 may have some clinical utility as a prognostic biomarker reflecting widespread disease involvement beyond the lungs, even in patients with long-standing disease; however, the lack of an established cut-off value requires further validation in prospective, multicentre studies.