HLA-G1 co-expression boosts the HLA class I-mediated NK lysis inhibition

It is now acknowledged that the pattern of HLA-G expression is not restrict ed to extravillous cytotrophoblast cells, as several studies described HLA- G in HLA class If cells, such as thymic epithelial cells, cytokine-activate d monocytes and some tumors, In these situations, HLA-G may provide an addi tional inhibitory signal to escape from NK cell-mediated cytotoxicity, Acco rdingly, the aim of this study was to define the behavior of HLA-G once it is cc-expressed into an HLA-A, -B, -C and -E+ cell line, For this purpose, HLA-G1 cDNA was transfected into an HLA class I+ melanoma cell line which w as used as a target towards freshly isolated peripheral blood NK cells, Cyt otoxic experiments using either anti-HLA-G1 or anti-HLA-G1 inhibitory recep tor mAb show that HLA-G1 boosts the HLA class I-mediated inhibition of poly clonal Nh cells through interaction with ILT-2, which appears as the major HLA-G1 inhibitory receptor involved. Nevertheless, HLA-G1 is also able to i nhibit the cytolytic activity of an ILT-2- NK clone which otherwise express es another HLA-G1 inhibitory receptor belonging to the KIR103 gene family, In order to more precisely define the relative role exerted by HLA-G1 versu s -E on polyclonal NK cells, antibody-blocking assays were carried out usin g either anti-HLA class I or anti-CD94/NKG2A, Results demonstrate that in t he absence of HLA-G1, the naturally expressed HLA class I-mediated NK inhib ition is predominantly exerted by HLA-E through binding with CD94/NKG2A, In contrast, once HLA-G1 is expressed, it becomes the major NK inhibitory lig and.