Sequential duodenal biopsies from a patient with allogeneic HSCT. (A) Duodenal biopsy with severe GVHD showing partial loss of surface epithelium (H&E) and increased numbers of apoptotic crypt epithelial cells (inset: cleaved caspase-3 staining showing apoptotic epithelial cells at the base of crypts). (B) Predominance of CD8⁺ T cells in the lamina propria with focal infiltration of the crypt epithelium (immunohistochemistry, APAAP method). (C) In contrast, duodenal biopsy in the same patient during NV infection shows villous blunting (H&E) and only a slight increase of apoptotic epithelial cells at the tip of the villi (inset: cleaved caspase-3) and (D) a massive increase of intraepithelial CD8⁺ T cells pronounced in the surface epithelium. Original magnifications, ×200 and ×600 (inset). Microscope: Olympus AX70 (Olympus); numeric aperture of objective lenses: ×20, 0.70 mm; ×60, 1.40. Stains: H&E (panels A and C); APAAP and streptavidin AP (panels B and D, inset). Camera: JVC KY-F70 (JVC); acquisition software: DISKUS; and software used for image processing: Adobe Photoshop 7.0 (Adobe Systems).