IdeS specifically blocks B cell maturation of IgG-producing cells. PBMCs were treated with PBS or IdeS and stimulated with rIL-2 and R848 to activate memory B cells and differentiate them into Ig-producing cells. ELISPOT filter plates were evaluated for the number of IgG-producing cells. (A) The filter plate was seeded with 50,000 or 100,000 cells that were treated or not with rIL-2/R848 on day 0 and with/without IdeS (30 μg/ml) on day 0 or 3. (B) Number of IgA-, IgM-, and IgG-producing cells after stimulation with rIL-2/R848 in the presence or absence of 30 μg/ml IdeS for 96 h. Data are mean (± SD) of four to six wells. The p value was calculated using GraphPad Prism and an unpaired, two-tailed t test. (C) Number of IgG-producing cells after stimulation with rIL-2/R848 in the presence or absence of 0.3–30 μg/ml IdeS for 72 h. Data are mean (± SD) of two to four wells. (D) Number of IgG-producing cells after pretreating cells for 1 h with 0.3–30 μg/ml IdeS prior to removing IdeS and subjecting cells to 72 h of stimulation with rIL-2/R848. Data are mean (± SD) of two to four wells. (E) Graphical illustration of the ELISPOT experiments in which IdeS was added from the start or at the end of the culturing period. Data in (B)–(D) are representative of two independent experiments. ns, not significant.