Anti–CTLA-4 antibody triggers insulitis in young GM-CSF/IL-3–deficient mice. (A) Normal islets in a wild-type mouse treated with anti–CTLA-4 mAb, original magnification, × 60. (B) Normal islets in a young GM-CSF/IL-3–deficient mouse treated with control Ig, original magnification, × 200. (C) Insulitis in a young GM-CSF/IL-3–deficient mouse treated with anti–CTLA-4 mAb, original magnification, × 400.