Intestinal FDG-PET signals correlated with tissue infiltration by EGFP⁺ donor lymphocytes. Lethally irradiated CB6F1 mice received transplants of BMCs alone (A,C) or with bone marrow and additional splenocytes (B,D) from C57BL/6-Tg(ACTB-EGFP) donors. At 1 hour after application of 50 MBq FDG on day 21 after transplantation, the whole gut was removed and scanned by PET (A,B). Subsequently, fluorescence reflectance imaging was performed to detect EGFP⁺ donor cells (C,D). Colocalized with intestinal FDG uptake, fluorescence imaging demonstrated an accumulation of EGFP⁺ donor cells in Peyer plaques and in the colon of mice with intestinal GVHD. Analyses of these colon specimens by fluorescence microscopy showed tissue infiltration by EGFP⁺ donor cells (E,H). Immunofluorescene staining revealed predominant CD4⁺ (F,I) and CD8⁺ (G,J) lymphocyte infiltration in EGFP⁺ areas. For simplification, spleen images were removed from the original PET and fluorescence images, as they were not relevant for intestinal GVHD. The results are representative of at least 3 independent experiments.