Histologic pattern of small bowel allograft rejection in the rat. Mucosal biopsies do not provide sufficient information.

Accessory small bowel transplantation was performed in a rat model, and the histologic sequence of acute rejection with and without immunosuppression with cyclosporine defined. The first rejection episodes occurred on the fifth postoperative day, defined as phase I. The mucosa and submucosa of the grafts were infiltrated by mononuclear and polynuclear leukocytes, and lymphocytes were scattered along the myenteric ganglia. Three to five days later (phase II) the infiltrate intensified and was associated with villous flattening and sloughing of epithelial cells. The muscular layer was invaded by numerous lymphocytes and neutrophils. At the end of the rejection episode complete destruction of the mucosa and the muscular layer developed (phase III). As no lesion other than lymphocytic infiltration could be detected in the mucosa in phases I and II of rejection, which may be considered reversible, and this change is known to be unspecific, biopsies of the mucosa alone are inadequate for a purely histologic diagnosis of small bowel allograft rejection.