Serum interferon-alpha 2 measured by single-molecule array associates with systemic disease manifestations in Sjogren's syndrome

Objectives Type I IFN (IFN-I) activation is a prominent feature of primary SS (pSS), SLE and SSc. Ultrasensitive single-molecule array (Simoa) technology has facilitated the measurement of subfemtomolar concentrations of IFNs. Here we aimed to measure IFN-alpha 2 in serum from pSS, SLE and SSc using a Simoa immunoassay and correlate these levels to blood IFN-stimulated gene (ISG) expression and disease activity. Methods Serum IFN-alpha 2 was measured in patients with pSS (n = 85 and n = 110), SLE (n = 24) and SSc (n = 23) and healthy controls (HCs; n = 68) using an IFN-alpha Simoa assay on an HD-X analyser. IFN-I pathway activation was additionally determined from serum by an IFN-I reporter assay and paired samples of whole blood ISG expression of IFI44, IFI44L, IFIT1, IFIT3 and MxA by RT-PCR or myxovirus resistance protein 1 (MxA) protein ELISA. Results Serum IFN-alpha 2 levels were elevated in pSS (median 61.3 fg/ml) compared with HCs (median <= 5 fg/ml, P < 0.001) and SSc (median 11.6 fg/ml, P = 0.043), lower compared with SLE (median 313.5 fg/ml, P = 0.068) and positively correlated with blood ISG expression (r = 0.66-0.94, P < 0.001). Comparable to MxA ELISA [area under the curve (AUC) 0.93], IFN-alpha 2 measurement using Simoa identified pSS with high ISG expression (AUC 0.90) with 80-93% specificity and 71-84% sensitivity. Blinded validation in an independent pSS cohort yielded a comparable accuracy. Multiple regression indicated independent associations of autoantibodies, IgG, HCQ treatment, cutaneous disease and a history of extraglandular manifestations with serum IFN-alpha 2 concentrations in pSS. Conclusion Simoa serum IFN-alpha 2 reflects blood ISG expression in pSS, SLE and SSc. In light of IFN-targeting treatments, Simoa could potentially be applied for patient stratification or retrospective analysis of historical cohorts.