TYPE I INTERFERON SCORE IN PERIPHERAL BLOOD OF PATIENTS WITH PEDIATRIC RHEUMATIC DISEASES

Introduction: Type I interferons (IFNs) are proteins that provide protection from viral infections, through induction of hundreds of genes implicated in antiviral response, the so-called "IFN signature". In the last years, the term "type I interferonopathy (IFNpathy)" has been used to identify a group of monogenic diseases in which the causative mutation determines a constitutive up-regulation of type I IFN activity directly relevant for pathogenesis. A restricted set of 6 genes has been proposed and widely used to identify patients with potential type I IFNpathies, which is commonly referred to as type I IFN score. Objectives: In this study, we aim to investigate the type I IFN score in peripheral blood of patients affected by paediatric rheumatic diseases characterized by the presence of systemic inflammation. Methods: We evaluated the expression of type I interferon-stimulated genes (ISGs) (IFI27, IFI44L, IFIT1, ISG15, RSAD2, SIGLEC1), by real time PCR in patients with systemic lupus erythematosus (SLE) (n=14), juvenile dermatomyositis (JDM) (n=7), systemic juvenile idiopathic arthritis (sJIA) (n=2), chronic recurrent multifocal osteomyelitis (CRMO) (n=3), adenosine deaminase deficiency (DADA2) (n=3), CANDLE syndrome (n=1), NLRC4-related disease (n=1), familial mediterranean fever (FMF) (n=2), undefined autoinflammatory diseases (UAD) (n=11) and in healthy donors (HDs) (n=4). The median fold change of the ISGs, when compared with the median of the combined HD, was used to create the type I IFN score. The mean interferon score of the controls plus two SD above the mean was calculated. Scores higher than this value (1,61) were defined as positive. Results: We found that a significant up-regulation of the type I IFN score characterized the peripheral blood of SLE, DADA2 and UAD patients compared to HDs (p<0.05). In SLE patients the type I IFN score was significantly related to the disease activity as measured by the SLEDAI score (p<0.01). In JDM, 2 patients with active disease had high type I IFN score. As expected, the CANDLE patient had elevated type I IFN score. We analysed the expression levels of the ISGs in other rheumatic disorders such as sJIA, CRMO, FMF but did not find significant differences with HDs. Interestingly, type I IFN score was elevated in the NLCR4 patient. Some of the UAD patients showed a strong up-regulation of the type I IFN score and clinical manifestations in common with the typical IFNpathies including variable combination of cerebral calcifications, lipodystrophy, interstitial lung disease, cerebral stroke, recurrent fever, low levels of C3 and mild positivity of antinuclear antibodies. Also variable expression of the ISGs was observed: in 2 patients a fold change of IFI27 of 937 and 530 compared to controls, while the expression of the other 5 genes was much lower. We found that one UAD patient with a high type I IFN score carried a de novo undescribed gain of function mutation in the STAT1 gene. Rheumatic disease Median of type I IFN score (IQR) Percentage of positive patients SLE (n=14) 8.8 (1.9-14.7) 78.5 JDM (n=7) 0.7 (0.3-9.5) 28.5 sJIA (n=2) 0.12 (0.11-0.12) 0 CRMO (n=3) 0.5 (0.2-3.5) 33.3 DADA2 (n=3) 3.9 (3.6-10.4) 100 CANDLE (n=1) 4.9 NLRC4 (n=1) 2.5 FMF (n=2) 0.5 (0.32-0.68) 0 UAD (n=11) 2.3 (1.4-5.5) 81.8 Conclusion: Up-regulation of type I IFN score characterized different systemic inflammatory disorders, in addition to classical type I IFNpathies. In the group of UAD we found patients with high expression of the ISGs and characterized by clinical manifestations in common with the typical interferonopathies.