Histamine-degrading enzymes as cellular markers of acute small bowel allograft rejection

Intestinal histamine-degrading enzymes diamine oxidase (DAO) and histamine N-methyltransferase (HNMT) activities are relatively constant per individual and bowel segment, and they reflect the functional integrity of the intestinal mucosa. It was, therefore, hypothesised that a decrease in these enzymes could be indicative of acute rejection of an intestinal allograft. Enzymatic activities of DAO and HNMT were determined in mucosal biopsies of isogeneic (Lewis-to-Lewis, n=48) and allogeneic (Brown Norway-to-Lewis, n=48) heterotopic small bowel transplants in a rat model at various time periods. Allograft recipients were not given any immunosuppression. While no changes in enzyme activities were observed in isografts up to day 8 following transplantation, significantly reduced activities of both enzymes were found in all allografts 6–8 days after transplantation. Activities of both DAO and HNMT exhibited a strong negative correlation with the histological rejection score (P<0.01). We can conclude that DAO and HNMT activities in gut mucosa are reliable quantitative markers of acute intestinal allograft rejection in the rat that support histopathological analysis.