Fig 1: Soluble SIGLEC-1 is a surrogate for the surface expression of SIGLEC-1 on CD14+ monocytes. a Gating strategy for the delineation of CD14+ monocytes, following single-cell discrimination. Histograms depict the distribution of SIGLEC-1 expression on the surface of CD14+ monocytes obtained by flow cytometry in illustrative donors expressing low normal (blue), normal (green), high (orange) or very high (red) levels of SIGLEC-1. Dotted line represents the background expression of SIGLEC-1 in live lymphocytes, which are known not to express SIGLEC-1. b Scatter plot depicts the frequency (geometric mean ± 95% CI) of SIGLEC-1 expression on the surface of CD14+ monocytes in a discovery cohort (cohort 1) of healthy donors (N = 24; black squares) and SLE patients (N = 34; red triangles). P value was calculated using a two-tailed non-parametric Mann–Whitney test. c Correlation between SIGLEC-1 mean fluorescence intensity (MFI) on CD14+ monocytes obtained by flow cytometry and the corresponding sSIGLEC-1 concentration in the healthy control and SLE patient groups. P value was calculated by linear regression. Illustrative SIGLEC-1 low normal, normal, high and very high SLE patients shown in (a) are highlighted in (b) and (c). SIGLEC-1 sialic acid binding Ig-like lectin 1, sSIGLEC-1 soluble SIGLEC-1, SLE systemic lupus erythematosus