Serum amyloid A1 in acute GI GVHD and SAA1 levels in the vitamin A and placebo cohorts. (A) Liver (positive control); SAA1 is strongly expressed in hepatocytes as expected. (B-H) No GVHD: immunohistochemical stain for SAA1 in human colon tissue sections; no SAA1 staining observed. (I-P) Acute GI GVHD: immunohistochemical stain for SAA1 in human GVHD colonic tissue sections. (I-J) Strong SAA1 staining of intestinal epithelium and lamina propria. (K-L) Increased SAA1 superficial density and staining of crypts, lamina propria, and stroma. (M-N) Patchy epithelial SAA1 staining of lamina propria and intestinal epithelial cells. (O-P) Weak focal to patchy SAA1 staining of lamina propria. (Q) SAA1 levels in an independent cohort of 21 patients with acute GI GVHD at time of diagnosis (median day of onset of acute GI GVHD was day +42 after HSCT) and 19 recipients of allogeneic HSCT without acute GVHD at ∼day +42 after HSCT. (R) SAA1 levels at day +7, day +14, day +21, and day +42 after HSCT the vitamin A and placebo study arms. ∗P < .05.