Altered distribution of MHC class II antigens on enterocytes during acute small bowel allograft rejection in rats

Class II major histocompatibility complex (MHC) antigen induction was investigated on enterocytes of heterotopic rat small bowel allografts in the Lewis-Brown Norway strain combination and on isografts in the Lewis-Lewis strain combination. I a antigens were detected with monoclonal antibodies using an immunoperoxidase technique. Generally, MHC class II antigens were not exhibited in the isografted group, with the exception of two long-term isografts that presented the same pattern as normal small bowel. In these cases, I a was expressed in a patchy distribution predominantly in the villi, and only very few enterocytes stained positive in Lieberkühn's crypts. Allografted rats showed a typical pattern of I a expression on the enterocytes during the rejection course. The initial expression was confined to the crypts, indicating a very early stage of rejection when compared to histological findings. More advanced stages of rejection were accompanied by increasing I a biosynthesis in the crypts and I a expression by the epithelium lining the villi. Cyclosporin (CyA) was not able to fully inhibit MHC class II antigen expression; however, the appearance of I a was delayed.