Medroxiprogesterone acetate drives expansion of CD11b+Gr1high myeloid-derived suppressor cells which suppress NK cell effector functions in tumor-bearing mice (P2059)

Abstract

Abstract Hormone replacement therapy with the progesterone analogue medroxyprogesterone acetate (MPA) is widely used in postmenopausal women for the treatment of endometrial conditions, and as a contraceptive. However, prolonged administration of MPA is associated with increased incidence of breast cancer through ill-defined mechanisms. Our aim was to explore if MPA threats immunosurveillance to tumors by effecting myeloid-derived suppressor (MDSCs; CD11b+Gr1+) and NK cells in mammary tumor-bearing mice. We used the highly metastatic 4T1 breast tumor which does not express classical progesterone or glucocorticoid receptors. MPA did not affect primary tumor growth in 4T1-tumor bearing mice but promoted lung metastasis burden. This effect was accompanied by expansion of splenic CD11b+Gr1+ cells (mostly CD11b+Gr1high cells). Sorted CD11b+Gr1+ cells from MPA-treated tumor bearing mice showed a more pronounced inhibitory activity of NK cell degranulation in response to YAC-1 cells and IFN-γ production in response to cytokines than those isolated from untreated tumor-bearing mice. Also, MPA significantly increased the percentage of spleen NK cells in tumor-bearing mice with similar lung infiltration of CD11b+Gr1+ and NK cells as compared to untreated tumor-bearing mice. We conclude that in breast cancer-bearing hosts MPA promotes the accumulation of CD11b+Gr1+ which suppress NK-cell mediated anti-tumor activity potentially contributing to tumor progression and metastasis.