Breast and Ovarian Carcinoma Overexpress HLA-G, a Neglected Cancer Immunosuppressive Protein

Purpose: HLA-G binds to the inhibitory receptors of uterine NK cells and plays an important role in protection of fetal cells from maternal NK lysis. HLA-G also mediates tumor escape, but the immunosuppressive role is often neglected. These studies have focused on the examination of HLA-G expression in human breast and ovarian carcinoma and HLA-G immunosuppressive role in NK cytolysis. Methods: We examined HLA-G expression in breast and ovarian carcinoma cell lines by real time PCR, ELISA and immunofluorescent staining, and in frozen breast and ovarian carcinoma tissues by immunohistochemistry (IHC). We treated the breast cancer cell lines with anti-human HLA-G antibody or progesterone. Then, NK cytolysis was measured by using MTT assay. Results: We find breast and ovarian cancer cell lines increase the expression of HLA-G mRNA and protein, compared to normal cells. IHC shows that 100% of frozen breast and ovarian carcinoma tissues overexpress HLA-G protein. HLA-G IHC scores of breast and ovarian carcinoma are significantly higher than normal breast and ovarian tissues, respectively (both p < 0.01). Blocking HLA-G of the breast cancer cells by the antibody increases NK cytolysis. Progesterone upregulates HLA-G mRNA and protein of human breast cancer cell lines. The increased HLA-G expression by progesterone suppresses the NK cytolysis. Conclusion: Human breast and ovarian carcinoma overexpress HLA-G immunosuppressive molecules. Blocking HLA-G protein by antibody improves the cytolysis of NK cells against human breast cancer cell lines. In contrast, upregulation of HLA-G expression by progesterone impairs NK cytolytic function. Thus, HLA-G is a new immune checkpoint protein and potential cancer immunotherapeutic target.