IFN-α serum levels and in vitro IFN-α production after CpG ODN2216-stimulation. (a) Interferon (IFN)-α serum levels were measured in blood samples collected from patients with systemic lupus erythematosus (SLE) and healthy control individuals using ELISA. Patients with a SLE Disease Activity Index above 4 were classified as having active SLE. Although serum IFN-α levels were higher in patients with active SLE than in those with inactive SLE and healthy control individuals, IFN-α levels in all SLE patients combined were not significantly different from those in healthy control individuals (3.72 ± 3.89 pg/ml in SLE patients [n = 43] versus 1.2 ± 3.9 pg/ml in healthy control individuals [n = 26]; P = 0.280 < 0.001). (b) IFN-α concentrations in culture supernatants were measured using ELISA after 24 hours of incubation with CpG ODN2216. IFN-α production in peripheral blood mononuclear cells isolated from SLE patients was significantly lower than that in cells isolated from healthy control individuals (342.46 ± 636.82 pg/ml in SLE patients [n = 43] versus 1,610.35 ± 759.56 pg/ml in healthy controls [n = 26]; P < 0.001). IFN-α production in peripheral blood mononuclear cells isolated from active SLE patients was also significantly lower than in cells isolated from healthy control individuals (P = 0.005). The solid bars represent the mean value in each experimental group. Patients with high IFN-α serum levels are indicated by #, *, and $ in both experiments. Statistical significance was analyzed using Student's t-test.