Low, intermediate and high serum IL-15 levels describe three groups of systemic sclerosis patients. Low, intermediate and high serum IL-15 levels depict three groups of systemic sclerosis patients with different skin score (white bars), different vital capacity (VC; grey bars) and different diffusing capacity for carbon monoxide (DLCO; black bars). Bars depict the median (25th and 75th percentile), and lines depict the 10th and 90th percentiles. Kruskal–Wallis analysis showed P < 0.05 for VC and DLCO with P = not significant for skin score. *P < 0.05 for VC of high IL-15 serum level versus low and intermediate IL-15 serum levels. *P < 0.05 for DLCO of high IL-15 serum level versus low IL-15 serum level. P = 0.057 for DLCO intermediate IL-15 serum level versus low IL-15 serum level. p%, population percentage.