Onset of intestinal aGVHD is not delayed in Peyer's patch (PP^KO)–deficient mice. Myeloablatively conditioned PP^KO Balb/c mice received transplants of luc⁺ FVB/N-L2G85 splenocytes plus FVB/N wild type bone marrow cells. (A) In vivo BLI of transplanted mice showed expansion of alloreactive T cells over lymphatic and intestinal sites by day 4, intestinal, liver, and skin infiltration by day 6 after aHCT. (B) Ex vivo images of the intestinal tract on day 3 displayed BLI signals from the mLN and the spleen. In some animals small light-emitting foci (arrow) were observed, which were sampled and analyzed (see below). (C) Ex vivo imaging of the gastrointestinal tract confirmed diffuse intestinal infiltration by luc⁺ donor cells on day 5. (D) Histologically, above described foci (arrow) were identified as ill-defined small lymphocytic aggregates. All of the analyzed mice were deficient for PP. (E) hematoxylin and eosin (H&E) analysis on day 6 after aHCT revealed a grade 2 GVHD on intestinal samples, showing apoptotic bodies and extensive crypt destructions in PP^KO recipients.