A–F, Spleens from ICOS+/+ (WT) (A–C) or ICOS−/− mice (D–F) after sensitization with S. mansoni were stained with PNA for germinal center B cells. Mice received either anti-CD28 (A, D), HIg (B, E) or PBS (C, F). All images are at 4x magnification. G, Germinal center area was measured using Image J software on pictures of 4x magnification (3–4 mice per group were analyzed). Data are representative of two independent experiments. H, Germinal center area was measured after treatment with 50, 100, 200, 400 μg of anti-CD28 or 400 μg of HIg.