Peripheral blood mononuclear cells (PBMCs) from patients with systemic lupus erythematosus (SLE) have increased expression of genes typically induced by type I interferon (IFN). However, it has been difficult to identify and quantify the factors responsible for activation of the IFN pathway in SLE. To characterize these mediators, we developed an assay that measures the functional effects of plasma or serum components on the gene expression of cultured target cells.

WISH epithelial cell line cells were cultured with medium, with recombinant IFNα, IFNβ, or IFNγ, or with 50% plasma from SLE patients (n = 73), rheumatoid arthritis (RA) patients (n = 19), or healthy donors (n = 30). Real‐time quantitative polymerase chain reaction was used to determine WISH cell expression of IFN target genes, including PRKR, IFIT1, IFI44, MX1, and C1orf29 (preferentially induced by IFNα) and CXCL9 (Mig) (preferentially induced by IFNγ).

IFNα‐regulated genes were induced by SLE plasma samples, but not by most of the RA or healthy control plasma samples. The activity in SLE plasma was inhibited >90% by anti‐IFNα antibody, but not by anti‐IFNβ or anti‐IFNγ antibodies. The expression of each IFNα target gene induced by SLE plasma correlated with the expression of that gene studied ex vivo in PBMCs from the same patients and with the titer of anti–RNA binding protein (anti‐RBP)–specific autoantibodies. Plasma activity paralleled PBMC expression of IFNα‐inducible genes over time.

IFNα in SLE plasma is a major stimulus of IFN target gene expression and is related to expression of those genes in PBMCs from SLE patients and to the titer of anti‐RBP autoantibodies. These data provide additional support for the view that IFNα mediates immune system activation and dysregulation in SLE.