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Early activation/damage of endothelial cells and multiple profibrotic T helper type 2 (Th2)-associated cytokines play an important role in the pathogenesis of systemic sclerosis (SSc).1
Interleukin 33 (IL-33) was recently identified as a member of the IL-1 family and a ligand for the orphan receptor ST2L, which mediates the action of IL-33 on polarised Th2 lymphocytes and other leucocyte subsets, and tissue-resident cells, such as vascular endothelial cells and fibroblasts/myofibroblasts.2 IL-33 has been shown to induce IL-13-dependent cutaneous fibrosis and stimulate angiogenesis and vascular permeability.3 4 In healthy human tissues IL-33 protein is constitutively highly expressed in the nuclei of resting endothelial cells, but rapidly lost upon angiogenic or proinflammatory activation.2 Indeed, IL-33 is thought to function as an endogenous ‘alarmin’ that is rapidly released after endothelial cell damage to alert the cells of the innate and adaptive immune system.2
Recently, we have shown that microvascular endothelial cells in inflamed skin from patients with early-stage SSc lacked or had decreased nuclear IL-33 protein expression.5 Instead, IL-33 transcript levels were even upregulated suggesting that, in early-stage SSc, IL-33 may be actively synthesised and mobilised from endothelial cells.5 However, whether IL-33 is released into the circulation in SSc remained to be determined. In the present study, we evaluated, in patients with SSc, serum levels of IL-33 and their possible correlation with clinical features and measures of microvascular involvement.
Fifty-eight consecutive patients with SSc entered the study. Patients were classified as having limited cutaneous SSc (lcSSc) or diffuse cutaneous SSc (dcSSc)6 and as being in early- or late-stage SSc according to disease duration.7 SSc patients were phenotypically assessed as previously recommended.8 All patients reported the occurrence of Raynaud's phenomenon and underwent nailfold videocapillaroscopy, based on which they were divided into three different capillaroscopic patterns: ‘early’, ‘active’ and ‘late’.9 Table 1 shows the clinicodemographic characteristics of SSc patients. Thirty age- and sex-matched healthy subjects served as controls. Since elevated serum levels of IL-33 were previously reported in active rheumatoid arthritis (RA),10 we enrolled 20 age- and sex-matched patients with active RA diagnosed according to the American College of Rheumatology 1987 revised criteria, as a disease control group. Serum levels of IL-33 were measured by ELISA (Koma Biotech, Seoul, Korea).
Circulating IL-33 was detectable in 34 (58.6%) SSc cases, 18 (90%) RA cases and 7 (23.3%) healthy controls (both p<0.05 vs controls, χ2 test). Serum levels of IL-33 were significantly higher in SSc (median 148.08 pg/ml, range 0–4791.0) and RA (median 280.44 pg/ml, range 0–3293.95) than in controls (median 0 pg/ml, range 0–814.86; p=0.001 and p<0.001, respectively; figure 1A). IL-33 levels were not significantly different between SSc and RA. No significant difference was detected between lcSSc (median 108.38 pg/ml, range 0–4791.0) and dcSSc (median 260.15 pg/ml, range 0–3971.0). Circulating IL-33 was significantly higher in both lcSSc and dcSSc than in controls (p=0.003 and p=0.001, respectively; figure 1A). IL-33 was significantly raised in early-stage SSc (median 1766.0 pg/ml, range 0–4791.0) compared with controls (p<0.001) and with late-stage SSc (median 0 pg/ml, range 0–4103.0; p<0.001), while it was not different between late-stage SSc and controls (figure 1B). IL-33 levels did not differ significantly between patients with the ‘early’ (median 11.9 pg/ml, range 0–4791.0) and those with the ‘active’ capillaroscopic pattern (median 254.29 pg/ml, range 0–4647.0), but were significantly higher in patients with the ‘active’ than in those with the ‘late’ pattern (median 0 pg/ml, range 0–157.86; p=0.01; figure 1B). No significant association was found with the other clinicodemographic and laboratory parameters.
Our data demonstrate that IL-33 can be released into the circulation in patients with SSc at an early stage, probably in response to proinflammatory triggers and following endothelial cell activation/damage. The increase in IL-33 associated with the ‘active’ capillaroscopic pattern, which is characterised by frequent giant capillaries and capillary haemorrhages, suggests that in SSc IL-33 may participate in the active derangement of the microcirculation. A prospective follow-up investigation of serum IL-33 is warranted.
In early-stage SSc, strong tissue expression of ST2L has been reported in endothelial cells, inflammatory/immune cells and myofibroblasts.5 Therefore, all these ST2L-expressing cells may be highly responsive to the increased levels of IL-33, supporting the idea that the IL-33/ST2L axis may be implicated in different pathogenetic mechanisms of SSc, as disturbed angiogenesis, inflammation, immune abnormalities and fibrosis. Further studies are now required to investigate the functionality of IL-33/ST2L downstream signalling in different SSc effector cells.
Funding This study has been supported by grants from the University of Florence (Progetti di Ricerca di Ateneo, ex 60% to L. Ibba-Manneschi and M. Matucci-Cerinic).
Competing interests None.
Patient consent Obtained.
Ethics approval The study was approved by the Institutional Review Board. All subjects gave written informed consent.
Provenance and peer review Not commissioned; externally peer reviewed.