This possibility gains further support from the following results of the present study: expression of ICOS on peripheral blood T cells from patients with active SLE drastically decreased after treatment with high-dose prednisolone; ICOS co-stimulation significantly enhanced expression of ICOS on peripheral blood T cells from patients with inactive SLE and normal control individuals; and dexamethasone, a strong inhibitor of lymphocyte activation, almost completely abrogated the induction of ICOS with ICOS co-stimulation. The gene discussed is ICOS; the disease is systemic lupus erythematosus.