HUMAN HLA-E AND HLA-G EXPRESSION ON PORCINE ENDOTHELIAL CELLS INHIBIT XENOREACTIVE NATURAL KILLER CELLS THROUGH CD94/NKG2 DEPENDENT AND INDEPENDENT PATHWAYS.
H Sasaki, XC Xu, B Duffy, and T Mohanakumar, Department of Surgery and Pathology, Washington University School of Medicine, St. Louis, MO.

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Previous studies have shown that peptides derived from leader sequence of the HLA-G can bind and upregulate a non-classical HLA-class I molecule, HLA-E, on the cell surface. This upregulation of HLA-E surface expression, but not HLA-G, is thought to be the molecule which provides negative signals to human natural killer (NK) cells. In this study, we demonstrate that the expression of HLA-G, as well as HLA-E in porcine endothelial cells, directly protected these sensitive porcine cells from human NK cell mediated lysis. Antibody blocking assays using F(ab')2 fragments of the HLA class I specific monoclonal antibody PA2.6, which did not crossreact with SLA molecules, indicated that the protection was mediated by the expression of HLA-E or HLA-G, but not by endogenous porcine SLA class I molecules. Further, only the HLA-E mediated protection was blocked by anti-human CD94 antibody. In addition, only the engagement of HLA-E, but not HLA-G, lead to phosphorylation of the CD94/NKG2 complex and recruitment of SHP-1. Therefore, HLA-E protects porcine endothelial cells from human NK cell mediated lysis through CD94/NKG2 dependent pathway. In contrast, HLA-G directly inhibited human NK cells in the absence of CD94/NKG2 phosphorylation or SHP-1 recruitment, and thus through a CD94/NKG2 independent pathway. Our results, therefore, demonstrate that, 1) both HLA-G or HLA-E can block human NK mediated lysis of porcine cells, 2) HLA-G by itself was sufficient to directly block human NK mediated lysis, and 3) HLA-E and HLA-G block NK mediated lysis by two different mechanisms.