['Type I interferon (IFN) plays a pivotal role in the pathogenesis of systemic lupus erythematosus (SLE) and is therefore considered a potential therapeutic target. This study was undertaken to establish a feasible biomarker for IFN effects with respect to disease activity and effectiveness of IFN-suppressive therapy in SLE patients.', 'Transcriptomes of purified monocytes from 9 SLE patients and 7 healthy controls were analyzed by Affymetrix GeneChip technology. Levels of sialic acid-binding Ig-like lectin 1 (Siglec-1) (sialoadhesin, CD169) in inflammatory and resident monocytes were determined at the protein level in 38 healthy controls and 52 SLE patients, using multicolor flow cytometry.', 'Transcriptomes of peripheral monocytes from SLE patients revealed a dominant type I IFN signature. Siglec-1 was identified as one of the most prominent type I IFN-regulated candidate genes. At the protein level, the frequency of Siglec-1-expressing monocyte subsets was correlated with disease activity (as measured by the SLE Disease Activity Index) and was inversely correlated with levels of complement factors. Most interestingly, levels of anti-double-stranded DNA (anti-dsDNA) antibodies were highly correlated with the percentage of resident monocytes, but not inflammatory monocytes, expressing Siglec-1. High-dose glucocorticoid treatment resulted in a dramatic reduction of Siglec-1 expression in cells from patients with active SLE.'] Is sialic acid-binding Ig-like lectin 1 expression in inflammatory and resident monocytes a potential biomarker for monitoring disease activity and success of therapy in systemic lupus erythematosus?