Although the percentages of ICOS on both CD4+CD45RO+ and CD8+CD45RO+ T cells from more than half of the patients with inactive SLE were relatively low (Figure 2c,d), peripheral blood T cells from these patients with inactive SLE exhibited significantly higher proliferative response (Figure 3) and IFN-γ production (Figure 4) with ICOS co-stimulation than did cells from normal control individuals. This evidence concerns the gene CD4 and systemic lupus erythematosus.