HLA-G inhibits rolling adhesion of activated human NK cells on porcine endothelial cells

Human NK cells adhere to and lyse porcine endothelial cells (pEC) and there fore may contribute to the cell-mediated rejection of vascularized pig-to-h uman xenografts. Since MHC class I molecules inhibit the cytotoxic activity of NK cells, the expression of HLA genes in pEC has been proposed as a pot ential solution to overcome NK cell-mediated xenogeneic cytotoxicity. HLA-G , a minimally polymorphic HLA class I molecule that can inhibit a wide rang e of NK cells, is an especially attractive candidate for this purpose. In t his study we tested whether the expression of HLA-G on pEC inhibits the mol ecular mechanisms that lead to adhesion of human NK cells to pEC and subseq uent xenogeneic NK cytotoxicity. To this end two immortalized pEC lines (2A 2 and PED) were stably transfected with HLA-GI. Rolling adhesion of activat ed human NK cells to pEC monolayers and xenogeneic cytotoxicity against pEC mediated by polyclonal human NK lines as well as NK clones were inhibited by the expression of HLA-G. The adhesion was partially reversed by masking HLA-G on pEC with anti-HLA mAbs or by masking the HLA-G-specific inhibitory receptor ILT-2 on NK cells with the mAb HP-Fl. The inhibition of NK cytoto xicity by HLA-G was only partially mediated by ILT-2,. indicating a role fo r other unknown NK receptors. In conclusion, transgenic expression of HLA-G may be useful to prevent human NK cell responses to porcine xenografts, bu t is probably not sufficient on its own. Moreover, the blocking of rolling adhesion by HLA-G provides evidence for a novel biological function of HLA molecules.