Osteopontin in the development of systemic sclerosis--relation to disease activity and organ manifestation

Sir, SSc is characterized by an interplay between early immunological activation and vascular damage, resulting in the generation of activated fibrogenic fibroblasts and, ultimately, excessive synthesis of extracellular matrix with deposition of increased amounts of structurally normal collagen [1, 2]. The organ involvement in SSc ranges from skin-limited to multi-organ fibrosis. Depending on the degree of skin involvement, SSc is frequently categorized as lcSSc or dcSSc type. Osteopontin (OPN) is a pleiotropic cytokine involved in the recruitment and retention of macrophages and T cells to sites of inflammation [3]. Classical mediators of acute inflammation (TNF-α, IL-1ß) as well as fibrogenic cytokines (angiotensin II, TGF-ß) strongly induce OPN expression [3]. OPN has been implicated as a key factor in the development of interstitial fibrosis [4, 5]. Due to its chemotactic and pro-fibrotic properties, we analysed circulating levels of OPN in patients with SSc and varying degrees of organ involvement by a commercially available ELISA (R&D Systems, Minneapolis, MN). Plasma of a total of 70 patients with SSc was analysed. Twenty age-matched healthy volunteers and 59 patients with idiopathic pulmonary hypertension served as controls. All patients were recruited from the Department of Immunology and Rheumatology and the Department of Dermatology at Hanover Medical School and Charite University Berlin, Germany, between 2008 and 2009. The study was carried out in accordance with the Declaration of Helsinki and approved by the institutional review board of Hanover Medical School. Written informed consent was obtained. Plasma OPN concentrations are elevated 4-fold in patients with SSc [n = 70; 89.1 (52.4) ng/ml] compared with healthy controls [n = 21; 23.8 (3.7) ng/ml; P < 0.0001] and 1.4-fold compared with patients with idiopathic pulmonary hypertension [n = 59; 63.4 (44.2) ng/ml; P = 0.003]. In the group of SSc patients, plasma OPN concentrations correlate significantly with CRP (r = 0.3; P = 0.02). OPN levels showed a weak, but significant association with disease duration (r = 0.27; P = 0.03). Patients with or without the presence of sclerodactyly (P = 0.2), oesophageal involvement (P = 0.1) and cutaneous calcinosis (P = 0.2) did not differ with regard to circulating levels of OPN. Similarly, the presence or absence of autoantibodies such as anti-topoisomerase antibody (P = 0.6) and centromer B antibodies (P = 0.08) did not have an effect on OPN levels. Patients with a diffuse form of scleroderma showed significantly elevated levels of OPN compared with patients presenting with a limited form (P = 0.001). Moreover, OPN levels correlated weakly with the modified Rodnan skin score (r = 0.3; P = 0.05) Plasma OPN levels in patients with pulmonary fibrosis (n = 29) are significantly elevated compared with patients without pulmonary fibrosis (P < 0.0001; Fig. 1A). The same was true for patients with pulmonary hypertension (n = 8) compared with patients without (P = 0.001). (A) Plasma levels of OPN are elevated in patients with pulmonary fibrosis (P < 0.0001) compared with patients without. (B) ROC curve analysis identifies patients with pulmonary fibrosis with good sensitivity and specificity. LF: lung fibrosis. We calculated receiver operating characteristic (ROC) curves to address the diagnostic value of OPN as a novel disease marker for SSc and its complication pulmonary fibrosis. When patients with pulmonary fibrosis were compared with patients without, an OPN cut-off value of >65.1 ng/ml resulted in a specificity of 57.1%, a sensitivity of 90.3% and a likelihood ratio of 2.1 in diagnosing patients with pulmonary fibrosis [area under the curve 0.8 (0.05); 95% CI 0.7, 0.9; P < 0.0001; Fig. 1B]. In the present study, we investigated for the first time circulating plasma levels of OPN in patients with SSc and varying degrees of organ involvement. The pro-fibrotic role of OPN has been described in various experimental studies [4, 5]. As part of the inflammatory response, OPN leads to the recruitment and retention of macrophages and T cells [3]. Ultimately, this results in interstitial fibrosis. The increased level of OPN would support a role for OPN in an autocrine feedback loop, which could potentiate the disease process by promoting the further accumulation of T cells and macrophages to the site of inflammation. Thus, OPN might be central to the perpetuation of disease activity by influencing the availability of the disease-promoting cells in SSc. Moreover, recombinant OPN induced a significant increase of migration and proliferation of fibroblasts [6], another important pathophysiological mechanism in SSc. It was also shown that OPN expression in skin fibroblasts is responsible for inflammation-associated fibrosis [7]. Delivery of OPN antisense oligodeoxynucleotides into mouse skin wounds leads to accelerated healing and reduced granulation tissue formation and scarring [7]. Macrophage-derived platelet derived growth factor-BB seems to be responsible for the OPN expression in wound fibroblasts [7]. In our study, OPN levels in patients with pulmonary fibrosis were significantly elevated compared with patients without. This finding is in line with a previous study, identifying OPN as one of the major genes that significantly distinguished idiopathic pulmonary fibrosis from healthy control lungs [6]. Our results may offer the possibility of using OPN inhibitors as a novel therapeutic target of T-cell chemotaxis, since monoclonal OPN antibodies have been used with great success in CIA [8]. In addition, the serial measurement of OPN in SSc should be studied to clarify whether the increase of OPN parallels or precedes the development of pulmonary fibrosis. Then, OPN measurement would be an attractive tool for early detection of this complication that responds better to treatment in the early stage. Disclosure statement: The authors have declared no conflicts of interest.