Abstract #4178: Immunosuppressive role of antiestrogen induced TGF-ß in breast cancer

AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO For endocrine therapy of breast cancer antiestrogens like Tamoxifen are among the most commonly used substances. Nevertheless, during long term antiestrogen treatment resistance is frequently occurring, raising the question for possible mechanisms. Our group could demonstrate that antiestrogens activate the TGF-s system in the breast cancer cell line MCF-7. With respect to the established immunosuppressive role of TGF-s, antiestrogen resistance might be due to immunomodulation in the microenvironment of tumors. To investigate this putative mechanism we developed a co-culture system consisting of MCF-7 cells and different lymphocyte subsets isolated from blood of healthy female donors. The paracrine effects of antiestrogen-induced TGF-s resulted in the transcriptional downregulation of effector molecules like granzyme B, perforin and the Fas ligand in the CD8+ cytotoxic T cell subpopulation as determined by quantitative RT-PCR. Western blot analysis showed similar expression patterns for according proteins. In naive CD4+ T cells we observed an induction of FoxP3-mRNA indicating the differentiation into inhibitory regulatory T cells. All effects were reversible by the application of a TGF-s neutralizing antibody. Furthermore direct antiestrogen treatment of lymphocytes had no effect on the regulation of the mentioned molecules. To confirm these results on the functional level, an allogeneic mixed lymphocyte tumor reaction (MLTR) was established. Either unstimulated CD8+ T cells, natural killer (NK) cells or unstimulated peripheral blood mononuclear cells (PBMC) were directly added to MCF-7 cells in the ratios 1:1, 10:1 and 20:1. After 24 hours cytotoxicity was measured via lactate dehydrogenase release. Pretreatment of MCF-7 with antiestrogens for 3-5 days led to a significant decrease in cytotoxicity. In following RT-PCR analysis the level of mRNAs of the employed lymphocytes showed characteristic decrease in effector molecules like granzyme B and perforin. The diminution of cytotoxicity was reversible with a neutralizing TGF-s antibody. Taken together our data are pointing out the possibility of an immunosuppression in the tumor microenvironment due to the induction of TGF-s by antiestrogens, a mechanism eventually contributing to antiestrogen resistance during therapy. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 4178.