BackgroundEarly endothelial cell (EC) activation/damage and profibrotic Th2-associated cytokines play a pivotal role in systemic sclerosis (SSc). Interleukin 33 (IL33) is a novel member of the IL1 family that promotes Th2 responses and inflammation through the ST2 receptor. IL33 is also a chromatin-associated transcriptional regulator in ECs.ObjectiveTo investigate the role of the IL33/ST2 axis in SSc.MethodsSkin biopsies were obtained from 30 patients with SSc (15 early/15 late stage) and 10 healthy subjects. Lung, kidney, heart, oesophagus, stomach, placenta biopsies and bronchoalveolar lavage cells from patients with SSc and controls were also analysed. IL33/ST2 expression was investigated by immunohistology, confocal immunofluorescence microscopy, western blotting and RT-PCR.ResultsIn skin biopsies from control subjects, constitutive nuclear IL33 protein expression was found in dermal ECs and keratinocytes, while ST2 was weakly expressed in ECs and fibroblasts. In skin biopsies from patients with early SSc, IL33 protein was downregulated or absent in ECs and epidermis while IL33 mRNA was normally expressed or even upregulated. Moreover, ECs, perivascular infiltrating mast cells, CD68-positive macrophages, CD3-positive T cells, CD20-positive B cells and activated fibroblasts/myofibroblasts exhibited strong ST2 expression. In skin biopsies from patients with late SSc, IL33 was constitutively found in most ECs while ST2 immunostaining was weaker. In early SSc, the loss of endothelial IL33 protein and the overexpression of ST2 involved all affected organs. Dermal and pulmonary fibroblasts showed IL33 expression in SSc.ConclusionIL33 and ST2 are abnormally expressed in SSc. In early SSc, upon EC activation/damage IL33 may be mobilised from ECs to signal through ST2 in key profibrotic players such as inflammatory/immune cells and fibroblasts/myofibroblasts.