Mechanisms of Tumor-Driven Immune Suppression in Breast Cancer Metastasis

In order to investigate mechanisms of immune regulation of breast cancer, our lab developed a novel syngeneic orthotopic transplantation mouse model of breast cancer metastasis. Comparative microarray analysis of low metastatic vs. highly metastatic tumor cells isolated from these mice revealed several important genetic alterations related to immune control of cancer. Of these, we identified a significant downregulation of CD1d in the highly metastatic tumor cells. CD1d is an MHC class I-like molecule that presents glycolipid antigens to a specialized subset of T cells known as natural killer T (NKT) cells. In this thesis, we demonstrate that tumor downregulation of CD1d inhibits NKT-mediated antitumor immunity and promotes metastatic breast cancer in a CD1d-dependent manner in vitro and in vivo. Using NKT-deficient transgenic mouse models, we demonstrate important differences between type I and type II NKT cells in their ability to regulate antitumor immunity of CD1d-lo vs. CD1d-hi breast tumors. These results emphasize the importance of determining the tumor CD1d expression status and stage of disease when tailoring NKT-based immunotherapies for breast cancer. Additionally, our microarray revealed a significant upregulation of the IFN-gamma transcription factor STAT1 in the highly metastatic tumor cells. While previous studies had implicated proinflammatory STAT1 as a tumor suppressor, accumulating evidence has correlated STAT1 over-activation with increased tumor progression in multiple types of cancer, including breast cancer. Indeed, we demonstrate that STAT1 upregulation promotes aggressive tumor growth of low metastatic TM40D breast cancer cells, while gene knockdown of STAT1 in highly metastatic TM40D-MB cells attenuates their phenotype. We show that STAT1 promotes tumor progression by secreting cytokines that directly recruit myeloid-derived suppressor cells (MDSCs) to the tumor microenvironment. MDSCs, along with recruited regulatory T cells (Tregs), promote tumor tolerance and disease progression though direct inhibition of T cell-mediated antitumor immunity. Importantly, tumor STAT1 expression and tumor infiltration of MDSCs in human biopsies correlates with progression from DCIS to invasive breast cancer. Understanding the interplay between tumor-derived factors, such as CD1d and STAT1, and immune effector cells during breast cancer progression will lead to the identification of improved therapeutic targets for inducing tumor rejection and preventing metastatic disease.