Cytokine regulation of HLA-G expression in human trophoblast cell lines

HLA class I genes are differentially expressed among subpopulations of cells in first trimester human placentas. In this study, HLA class I protein was detected in extravillous cytotrophoblast cells by immunohistochemistry using the monoclonal antibody W6/32. In the same trophoblast subpopulation, class Ib proteins were identified with two monoclonal antibodies, 87G (anti-HLA-G) and 131 (anti-HLA-A/G) and class Ia protein was detected with the monoclonal antibody, 4E (anti-HLA-B/C). All of the antibodies also identified antigens on the human trophoblast-derived choriocarcinoma cell line, JEG-3. Therefore, the JEG-3 cells were used as a model system to study cytokine regulation of HLA-G in trophoblast cells. Northern blot hybridization studies showed that interferons (IFN-alpha, IFN-beta, IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) modestly enhanced steady state levels of HLA-G mRNA. Yet analysis of HLA-G protein by immunocytochemistry and flow cytometry failed to identify any changes in intracellular or membrane expression of HLA-G protein following cytokine treatment. Resistance to upregulation of HLA class I antigens was not a general feature of JEG-3 cells; IFNs enhanced expression of HLA-B/C as well as HLA class I light chain, beta 2-microglobulin. HLA null Jar choriocarcinoma cells did not contain HLA-G mRNA or antigen and exposure to cytokines had no effect on HLA-G. The results of this study are consistent with the postulate that trophoblast cell expression of HLA-G is stringently regulated and is controlled in part by post-transcriptional mechanisms.