HLA-G Inhibits Rolling Adhesion of Activated Human NK Cells on Porcine Endothelial Cells

Abstract

Human NK cells adhere to and lyse porcine endothelial cells (pEC) and therefore may contribute to the cell-mediated rejection of vascularized pig-to-human 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 potential solution to overcome NK cell-mediated xenogeneic cytotoxicity. HLA-G, a minimally polymorphic HLA class I molecule that can inhibit a wide range of NK cells, is an especially attractive candidate for this purpose. In this study we tested whether the expression of HLA-G on pEC inhibits the molecular mechanisms that lead to adhesion of human NK cells to pEC and subsequent xenogeneic NK cytotoxicity. To this end two immortalized pEC lines (2A2 and PED) were stably transfected with HLA-G1. Rolling adhesion of activated 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-F1. The inhibition of NK cytotoxicity by HLA-G was only partially mediated by ILT-2, indicating a role for other unknown NK receptors. In conclusion, transgenic expression of HLA-G may be useful to prevent human NK cell responses to porcine xenografts, but 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.

HLA‐E Expression on Porcine Cells: Protection from Human NK Cytotoxicity Depends on Peptide Loading

“…Human NK cells have further been reported to lyse porcine target cells in vitro, especially after activation and in the presence of human serum containing xenoreactive natural antibodies (27)(28)(29). This susceptibility of pEC to human NK cytotoxicity may be explained by the failure of swine leukocyte antigen (SLA) class I molecules to interact with human NK inhibitory receptors.…”

Association between two polymorphisms in the HLA-G gene and angiographic coronary artery disease

“…Of interest, an association between HLA-G polymorphisms and other chronic inflammatory disorders, such as inflammatory bowel disease (33,34), asthma (35,36) and multiple sclerosis (37)(38)(39), has been repeatedly reported. Besides the potential role played by HLA-G in inflammation and immunity (15), there is evidence to suggest that this molecule may also modulate endothelial cell activity (40,41). In this regard, it has been previously demonstrated that purified recombinant soluble HLA-G is capable of downregulating endothelial cell proliferation and migration in vitro (42).…”

HLA-G in Transplantation: A Relevant Molecule for Inhibition of Graft Rejection?

“…Forte et al (24,25) have shown that expression of HLA-G at the surface of PEC monolayers inhibits the rolling adhesion of activated human NK cells. In these experiments, rolling adhesion of activated NK cell lines to HLA-G transfected PEC monolayers was inhibited by 50% as compared with their rolling adhesion to nontransfected PEC.…”

“…Briefly, transfection of HLA-G1 cDNA or genomic HLA-G DNA into PEC or Chinese Hamster ovarian cells (CHO) lead to a 'partial' to complete inhibition (ranging from 30% to 100%) of the cytolytic function of human polyclonal NK cells and that of most human NK cell lines (22)(23)(24)(25)(26). Furthermore, two studies showed a correlation between the level of HLA-G1 cell surface expression by transfected PEC and the extent of NK functional inhibition (25,26). The HLA-G receptors primarily involved in this inhibition remain unknown; however, while working with NK clones, Forte et al found no correlation between the ability of HLA-G-transfected PEC to inhibit NK lysis and the expression by the NK clones of the HLA-G receptor ILT2.…”

“…The expression of HLA-G by transplanted tissue might participate in graft protection as it was also shown that (ii) allogeneic CD4 π T cells secrete soluble HLA-G5 (20), and that (iii) membrane-bound HLA-G1 or soluble HLA-G5 both inhibit CD4 π T cell alloreactivity (20,21). The protective effect of HLA-G was investigated in the context of xenotransplantation as well, and authors showed that (iv) HLA-G1 inhibits xenogeneic NK cell cytolytic and migratory functions in transfection systems (22)(23)(24)(25)(26). Finally, (v) a transgenic animal model evidenced a new role for HLA-G, as an inhibitor of APC functional maturation, thus inducing prolonged allograft survival (27).…”