HLA-G expression in human melanoma cells: protection from NK cytolysis.

Expression of the non-classical HLA-G class I antigen is physiologically restricted to a limited number of tissues including trophoblasts, and is thought to play a role in establishing tolerance of the fetus by the maternal immune system. We investigated whether ectopic expression of HLA-G could also be detected in tumor cells and confer them the ability to escape immune cytotoxic responses. High levels of all alternatively spliced HLA-G transcripts could be detected in melanoma cells by RT-PCR. Analysis of biopsies from a melanoma patient revealed a higher HLA-G transcription level in skin metastasis as compared to healthy skin, while specific amplification of the HLA-G5 transcript was only observable in the tumor. HLA-G protein expression could also be detected in two melanoma cell lines. HLA-G-positive tumors inhibit cytotoxic lysis by the NK cell line YT2C2-PR. This inhibition is not observed with B-EBV cell lines bearing matched class I specificities, and is thought to occur through interaction of HLA-G with inhibitory receptors that are distinct from known KIRs interacting with HLA-E or classical class I molecules. Together, these results confirm that HLA-G expression at the surface of tumor cells can participate in the evasion of antitumoral immune responses and favor tumor progression.

Molecular mechanisms controlling constitutive and IFN-gamma-inducible HLA-G expression in various cell types.

HLA-G molecule is thought to play a major role in down-regulating the maternal immune response by inhibiting NK and T cell cytolytic activities. We examined the molecular regulatory mechanisms that may control the restricted expression pattern of the HLA-G gene. We first analyzed protein interactions between nuclear extracts from the HLA-G-positive JEG-3 choriocarcinoma and the HLA-G-negative NK-like YT2C2 cell lines to a 244 bp regulatory element located 1.2 kb from the HLA-G gene, previously shown to direct HLA-G expression in transgenic mouse placenta. This allowed characterization of cell-specific DNA-protein interactions that could account for differential cell-specific expression of the HLA-G gene. In particular two DNA-protein complexes were exclusively observed in YT2C2, suggesting that this HLA-G regulatory element is a target for putative cell-specific repressor factors. We further mapped nuclear factor binding sites to a 70 bp fragment in the upstream region of the regulatory element. We then investigated the effect of IFN-gamma on HLA-G gene expression. HLA-G cell surface expression was enhanced by IFN-gamma treatment in JEG-3 and U937 cell lines and peripheral blood monocytes while no effect was observed in tera-2 teratocarcinoma cell line. HLA-G transcriptional activity was increased only in JEG-3 and U937 cell lines. Activity of the 1.4-kb HLA-G promoter region was unchanged after IFN-gamma treatment in JEG-3 and Tera-2. These results suggest that both post-transcriptional and transcriptional mechanisms implicating IFN-responsive regulatory sequences outside the 1.4 kb-region are involved in IFN-gamma gene activation of the HLA-G gene.

IL-10 selectively induces HLA-G expression in human trophoblasts and monocytes.

HLA-G plays an essential role in feto-maternal tolerance by inhibiting lysis by maternal NK cells. The factors that allow tissue-specific activation of HLA-G gene expression in trophoblasts remain to be characterized. We investigated the potential effect of IL-10, a cytokine which is secreted in placenta, on HLA-G gene transcription in trophoblasts. Using Northern blot, RNase protection assay and RT-PCR analysis, we demonstrated that IL-10 enhances steady-state levels of HLA-G transcription in cultured trophoblast cells. We further tested the effect of IL-10 on HLA-G gene transcription and protein expression in peripheral blood monocytes, showing that IL-10 can up-regulate HLA-G cell surface expression in this cell type. This effect of IL-10 is selective, since classical MHC class I products and MHC class II are down-regulated in monocytes following IL-10 treatment. Induction of HLA-G expression by IL-10 on monocytes may thus play a role in down-regulation of the immune response. We propose that IL-10 secretion by trophoblasts during pregnancy may also influence the HLA class I expression pattern at the feto-maternal barrier, thus protecting the fetus from rejection. This should be taken into consideration in the design of treatment for pathologies of pregnancy.