ICOS co-stimulation of peripheral blood T cells significantly increased the production of IFN-γ in all three groups (median values: active SLE, 612.8 pg/ml [P < 0.001]; inactive SLE, 1843.1 pg/ml [P < 0.005]; normal control individuals, 174.9 pg/ml [P < 0.05]). This evidence concerns the gene IFNG and systemic lupus erythematosus.