IMMUNOLOGICAL CHANGES WITHIN THE MUCOSA A ND MUSCULARIS OF CHRONICALLY REJECTING INTESTINAL TRANSPLANTS

Clinical experience has shown that chronic rejection of small bowel tr ansplants can hardly be detected by mucosal biopsy before the onset of symptoms such as diarrhea and weight loss. Our previous animal studie s have shown that this is due to the fact that chronic rejection prima rily affects the graft's muscularis externa and myenteric nerve plexus before changes become visible at the mucosal level. Severe functional alterations within the graft caused by chronic rejection can at least partly be reversed by FK 506 rescue therapy. In the present study, we therefore investigated the effect of chronic subclinical rejection on the distribution of immune cells within the gut wall. Subclinical chr onic rejection after allogeneic orthotopic small bowel transplantation in ACI-LEWIS rats was achieved by a 4-week course of immunosuppressio n (CsA 15 mg/kg). Allografts were compared to syngeneic grafts (ACI-AC I) and age-matched control intestine (ACI). Transplanted animals were sacrificed on postoperative day 90. Frozen cross-sections were obtaine d and immunohistochemically stained for a panel of immune cells. Chron ic rejection was associated with complete repopulation of the lamina p ropria by host-derived MHC-1 positive mononuclear cells and an upregul ation of donor specific MHC-1 (anti-ACI) in the muscularis externa. Th e amount of CD8 positive T-suppressor cells, NK-cells (HNK-1), macroph ages (ED 2) and neutrophiles in the lamina propria remained unchanged. T-helper cells (CD4) were markedly decreased but still present wherea s the mucosa appeared completely void of B-cells (MRC OX-33). Chronic rejection showed a massive infiltration of the muscularis externa with macrophages, natural killer cells, and neutrophiles. These infiltrate s were particularly dense around the myenteric plexus. Staining intens ity for CD4 and CD8 positive lymphocytes was only moderately elevated. B-cells were almost not detectable in the muscularis of control or ch ronically rejecting specimen. Syngeneic grafts exhibited no changes in the distribution of intestinal immune cells compared to controls. Our results clearly show that macrophages, natural killer cells and neutr ophiles are the predominant cell types infiltrating both intestinal mu scle layers and the myenteric plexus during chronic rejection. These n atural immune cells, possibly supported by specific immune cells such as CD4 and CD8 lymphocytes, could be responsible for elevated cytokine levels within the muscularis leading to significant structural and fu nctional changes of smooth muscle and nerves. We could also demonstrat e that immunological changes occur within the graft's mucosa far befor e histological changes can be detected. The complete mucosal depletion of B-cells promises to be an excellent marker for early chronic rejec tion as mucosal specimen can easily be obtained by routine endoscopic biopsy.