IL-21 protein is expressed in the GI tract and skin, but not the serum, of allo-HSCT patients with GVHD. Samples of colon and skin were biopsied from transplant patients and were stained with anti–IL-21 mAb, counterstained with hematoxylin, and then analyzed in a blinded fashion. (A) Representative examples of gut and skin staining with anti–IL-21 mAb or isotype control antibody. Counts of double immunostained cells were independently assessed by 2 pathologists (P.R. and A.J.) on an Olympus AX 70 microscope with wide-field eyepiece number 26.5 using CellSens software (Olympus). At 400× magnification, this wide-field eyepiece provided a field size of 0.344 mm². (B) High magnification example of gut staining showing numerous IL-21–expressing MNCs. Graphs comparing the relative amount of anti–IL-21 staining in the gut (C) and skin (D) with the GVHD pathology grade. n = 14 and 16 for GI and skin biopsies, respectively.