Histology and immunohistochemistry for CD4⁺, CD8⁺, and CD3⁺FOXP3⁺ T cells of representative colonic biopsies from healthy controls and patients with no GvHD, with GvHD after bone marrow transplantation, and with CMV infection. A normal amount of CD3⁺ T cells (red, membranous) with very few regulatory T cells coexpressing FOXP3 (brown, nuclear) is found in healthy controls, whereas mucosal biopsies of patients without GvHD display a high number of CD3⁺FOXP3⁺ T cells in the lamina propria (arrows, bottom row) as opposed to a low number of regulatory T cells in patients with GvHD. All FOXP3⁺ cells in addition expressed CD25 (inset/bottom row). The number of CD8⁺ T cells is increased in biopsies from patients with GvHD especially in and around crypts displaying many characteristic apoptotic bodies. In CMV infection (inset/upper panel: nuclear staining for CMV antigen), the number of CD3⁺FOXP3⁺ regulatory T cells is also elevated compared with patients with GvHD. Original magnifications, × 200; and × 600 (inset). Microscope: Olympus AX70 (Olympus, Melville, NY); numeric aperture of objective lenses: × 20, 0.70 mm; × 60, 1.40. Stains: hematoxylin and eosin (H&E; top row); APAAP and immunoperoxide. Camera: JVC KY-F70 (JVC, Yokohama, Japan); acquisition software: DISKUS (Koenigswinter, Germany); and software used for image processing: Adobe Photoshop 7.0 (Adobe Systems, San Jose, CA).