ABNORMAL HUMAN MONOCYTE ACTIVATION BY CARBOHYDRATE LIGANDS ON PORCINE ENDOTHELIUM, AND REVERSAL BY OVEREXPRESSION OF FUCOSYL ??1,2 TRANSFERASE:

103 We have shown that primate macrophage lineage cells are of critical importance in the process of delayed porcine xenograft rejection in vivo. In studies investigating the in vitro binding properties of human of monocytes to porcine endothelium (PAEC), we found that purified monocytes demonstrated 49% greater adhesion to PAEC monolayers than to human endothelial monolayers (HUVEC). In subsequent functional assays, human monocytes produced 7.2-fold higher levels of IL-1 beta following exposure to PAEC than to HUVEC and 3.4-fold higher levels of PGE2. Since human monocytes express a variety of carbohydrate-binding proteins, we next investigated the possibility that the adhesive and other functional abnormalities exhibited by human monocytes following exposure to PAEC might be a result of abnormal binding to carbohydrate ligands on porcine endothelium which are not normally present on HUVEC. Since the carbohydrate substrate required for expression of GLYCAM-1, the carbohydrate ligand for the principal lectin on monocytes, CD62L, is the same as that used by fucosyl α1,2-transferase, we studied the effects of endothelial cell overexpression of this enzyme on interactions between human monocytes and PAEC. Following exposure to PAEC transfected with fucosyl α1,2 transferase, human monocytes exhibited 50% lower adhesion than to control PAEC (p<0.01). In a similar fashion, overexpression of fucosyl α1,2 transferase by PAEC was associated with marked reductions in the levels of monocyte-derived IL-1 beta and PGE2, to levels observed following exposure to HUVEC. These results demonstrate that carbohydrate structures on PAEC serve as specific ligands for human monocytes, resulting in functional dysregulation. Transgenic strategies that reduce PAEC expression of carbohydrate ligands for human monocytes may decrease the macrophage-mediated component of delayed pig-to-primate xenograft rejection.