Protection of mice against EAE by IVIg is associated with an expansion of Treg in the peripheral lymphoid organs. (A) IVIg protects mice from EAE: EAE was induced in female C57Bl/6J mice by immunization with MOG_35-55. From day 0 of immunization until the peak of disease (day 21–25), mice received daily intraperitoneal injections of IVIg (□) or PBS alone (■). The severity of the disease was evaluated daily using a clinical score as explained in “EAE induction, assessment, and IVIg treatment.” Mice: n = 6 to 8 for each group from 4 independent experiments. Data are the means plus or minus SD (*P < .05). (B) Increase in percentage of Treg: spleen cells from 3 PBS-treated mice (), and 3 IVIg-treated mice(▭) (open bars) were analyzed at days 10, 21, and 27 after immunization by flow cytometry for the expression of CD4, CD25, and Foxp3. Results of 1 experiment of 4 are shown. Data are the means plus or minus SD (*P < .05). (C) Representative plots from individual mice from each group at day 10, 21, and 27, gated on CD4⁺. The number in each quadrant represents CD25⁺Foxp3⁺ gated on CD4⁺.