ICOS and systemic lupus erythematosus: These data also suggest that ICOS co-stimulation enhances the expression of ICOS on T cells and amplifies their response to ICOS co-stimulation in both patients with SLE and normal control individuals, and would (at least in part) explain the discrepancy between the relatively low expression of ICOS on peripheral blood T cells (Figure 2) and augmented response to ICOS co-stimulation in inactive SLE (Figures 3 and 4).