Serum cytokines and their predictive value in pulmonary involvement of systemic sclerosis

Abstract

Objective: To identify serum cytokines which predict mortality and/or disease progression in patients with systemic sclerosis, especially with pulmonary involvement.

Methods: Serum cytokines (IL-6, IL-7, IL-8, IL-10, CCL2, CCL4, TGF-β, TNF-α) were measured in 125 SSc patients, who were recruited and observed in our outpatient clinic. Of these, 60 had pulmonary involvement, classified as either interstitial lung disease (ILD, 43 patients), pulmonary arterial hypertension (PAH, 7 patients) or pulmonary hypertension and ILD (PH-ILD, 10 patients). The association of serum cytokines with clinical features was analysed and their correlation with BAL cytokines measured in a subset of SSc patients with ILD.

Results: Serum cytokines were detected at different levels: high (TGF-β, median 287.5 pg/ml; CCL2, median 89.7 pg/ml; CCL4, median 104.2 pg/ml), low (IL-6, median 3.2 pg/ml; IL-7 median 2.3 pg/ml; IL-8, median 5.2 pg/ml; TNF-α, median 0 pg/ml but with a bimodal distribution) and very low (IL-10, median 0.4 pg/ml). IL-6 and IL-7 were predictive for death in a Cox regression analysis in all SSc patients as well as in all patients with pulmonary involvement; IL-6 was predictive for mortality in SSc-ILD patients. In a multivariate analysis, cytokine levels could also predict a change in lung function, e.g. IL-7 was a predictor for a decline of diffusion capacity (DLCO) by 20 or 30% in ILD patients. In a subset of ILD patients, serum cytokines were compared to BAL cytokines, but revealed only few correlations.

Conclusion: In conclusion, the analysis of serum cytokines implicates a role as biomarkers, distinct from BAL.

Keywords: chemokines; cytokines; lung involvement; survival; systemic sclerosis.

Fig. 1.

Serum cytokines of 125 SSc patients. Fig. 1A: Serum cytokines measured are displayed as median and interquartile range. Due to the high frequency of values below the detection limit which are not shown, for IL-10 and TNF-alfa the IQR cannot be displayed. Serum cytokines split up for patients without pulmonary involvement (No pulm.), SSc-associated interstitial lung disease (ILD), SSc-associated pulmonary arterial hypertension (PAH) and SSc-associated interstitial lung disease with pulmonary hypertension (PH-ILD) are displayed for IL-6 (Fig. 1B), CCL2 (MCP-1, Fig. 1C), CCL4 (MIP-1β, Fig. 1D), IL-7 (Fig. 1E) and TGF-β (Fig. 1F). Significant differences between the SSc subgroups were calculated using the Mann-Whitney test and are indicated.

Fig. 2.

Predictive capacity of cytokine levels IL-7 and IL-6 for survival in different cohorts. Kaplan-Meier survival curves are shown according to cytokine cut-off levels determined by ROC analysis in different cohorts, restricted to cytokines that were predictive for time to death in Cox regression analysis. Significance is given by the log-rank test comparing survival curves. A) All patients (whole cohort). B) All patients with pulmonary involvement. C) Patients with ILD.

Fig. 3.

Cytokine levels in BAL and serum of 21 SSc-ILD patients. BAL and serum cytokines measured at the same time point are displayed as median and interquartile range; due to the high frequency of values below the detection limit not done for serum IL-10 and TNF-α. Significant differences were calculated using a Wilcoxon signed rank test. Statistically significant differences are indicated.