Small bowel allografts. Sequence of histologic changes in acute and chronic rejection.

Using a rat model of accessory small bowel transplantation, the histologic sequence of both acute and chronic rejection in intestinal allograft rejection has been defined. Histologically, all allografts were normal for the first 5 postoperative days. Allografts with caval venous drainage were subject to acute rejection. By 6 to 7 days postoperatively, plasma cells and lymphocytes infiltrated the lamina propria of these grafts (phase I). By 8 to 9 days postoperatively, the cellular infiltration intensified and was associated with villous blunting and scattered epithelial sloughing (phase II). By the 10th day, complete mucosal destruction developed, with heavy transmural infiltration by lymphocytes, plasma cells, and polymorphonuclear leukocytes (phase III). This histologic end point of acute graft rejection was accompanied by death of the host. Grafts with portal venous drainage underwent a similar, although less rapid, sequence of histologic changes (phase I 6 to 9 days, phase II 10 to 13 days, phase III 13 or more days) resulting in graft fibrosis and encapsulation. Some variability was seen among different areas of a given circumferential cross section taken from grafts in phases I and II. Studying circumferential cross sections allowed correct classification into the appropriate phases.