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TITLE: MECHANISM FOR IN VITRO “ACCOMMODATION” OF HUMAN AORTIC ENDOTHELIAL CELLS BY ALLOANTIBODIES SPECIFIC FOR HLA MOLECULES: DOWN REGULATION OF ADHESION MOLECULES AND OVER EXPRESSION OF PROTECTIVE GENES

Kishore Narayanan,¹ Bashoo Naziruddin,¹ Donna Phelan,² T. Mohanakumar.<sup>1

1</sup>Surgery, Pathology and Immunology, Washington University School of Medicine, St. Louis, MO; ²HLA Laboratory, Barnes–Jewish Hospital, St. Louis, MO, USA

Xenografts and allografts have been shown to survive despite the presence of circulating antigraft antibodies and complement. This phenomenon has been referred to as accommodation and endothelial cells are thought to play an important role in this process. Accommodation has been well documented in Xenotransplantation; wherein the role of endothelial cell accommodation in graft survival has been defined using both in vitro and in vivo models. However the mechanisms by which accommodation occurs in allotransplantation are not well understood. In order to define the mechanisms involved in the accommodation of allografts, we developed an in vitro model of accommodation using anti HLA antibodies obtained from highly sensitized (high panel reactive antibodies, PRA) and human aortic endothelial cells. We delineated that exposure of human aortic endothelial cells to low doses of PRA serum induces a change in their phenotype making them resistant to complement mediated lysis. The accommodated phenotype manifests after 72 hours of exposure with low concentrations of PRA serum. The accommodated endothelium bound to fewer peripheral blood mononuclear cells (16%) when compared to their corresponding controls (83%). This was correlated with a significant reduction in the expression of the adhesion receptors VCAM and ICAM (5% and 3% respectively) as compared to their controls (41% and 44% respectively). Further analysis for molecules involved in bringing about such a change in the phenotype revealed an over expression of Bcl–2, Bcl–xL and Heme oxygenase–1 in the accommodated cells when compared to their controls. Based on this data we postulate the existence of a novel signaling mechanism triggered in the endothelial cells by anti–HLA antibodies that results in the down regulation of adhesion receptors and the over expression of protective genes. This process also renders these cells resistant to complement mediated lysis.