Estradiol-modulation of HLA-II in human breast cancer cells

Immune responses, including those against antigens on tumour cells, require antigen to be processed into peptides, loaded onto HLA-II molecules by the peptide editor, HLA-DM, and subsequently presented at the cell surface for CD4⁺ T-cell recognition. Although HLA-II molecules are not normally expressed on epithelial cells, they can be induced by exposure to interferon-gamma (IFN-γ), which may explain the increased frequency of HLA-DR (a type of HLA-II molecule) on some tumour cells. Our laboratory previously showed that co-ordinate expression of HLA-DR and HLA-DM on tumour cells in human breast carcinoma tissues associated with high levels of IFN-γ and increased survival. Interestingly, estrogen receptor negative (ER^-) tumours from younger women more frequently expressed HLA-DR than did ER⁺ tumours from older women. Since breast tumours contain high levels of estradiol, irrespective of menopausal status, these observations implied that estradiol negatively regulates HLA-DR expression in ERα⁺ but not in ERα^- tumour cells. To test this hypothesis in an experimental model, we analyzed IFN-γ inducible HLA-II in breast cancer cell lines (BCCL) including paired ERα(MC2) and vector (VC5) transfected MDA-MB-231 in the presence and absence of estradiol (E₂) and/or anti-estrogens. We found: i) E₂-treatment significantly diminished IFN-γ inducible HLA-II and the master regulator of HLA-II expression, CIITA, in the ERα⁺ MC2 line but not in the ERα^- VC5 line; ii) depletion of ERα in MC2 by the antiestrogen, ICI or by siRNA reversed the E₂-mediated effect on HLA-II expression, CIITA promoter activity and transcriptional activation of CIITA; iii) various components of the IFN-γ pathway were disrupted in E₂-treated ERα⁺ cells, but not in ERα^- cells. Altogether, these results suggest that E-activation of ERα negatively regulates HLA-II expression in breast cancer cells by interfering with IFN-γ signalling. However, the mechanism by which this occurs remains to be elucidated and is the focus of current and future studies.