Myeloid Derived Suppressor Cells Migrate in Response to Flow and Lymphatic Endothelial Cell Interaction in the Breast Tumor Microenvironment

Simple Summary Myeloid-derived suppressor cells (MDSCs) are a subset of immune cells that contribute to a pro-tumorigenic microenvironment, promoting immunosuppression in multiple tumors. In breast cancer, they have been mostly studied in pre-clinical mouse models. These models allow for examination of the development of pre-metastatic niches and immune infiltration at the primary tumor site, but are difficult to use to extricate complex intercellular signaling events and MDSC migration. Here, we describe the development of a breast tumor microenvironment model that includes primary-derived mouse MDSCs. We use this model to identify interactions that drive MDSC migration through both biophysical and cellular signaling events within the tumor microenvironment. At the site of the tumor, myeloid derived suppressor cells (MDSCs) infiltrate and interact with elements of the tumor microenvironment in complex ways. Within the invading tumor, MDSCs are exposed to interstitial fluid flow (IFF) that exists within the chronic inflammatory tumor microenvironment at the tumor-lymphatic interface. As drivers of cell migration and invasion, the link between interstitial fluid flow, lymphatics, and MDSCs have not been clearly established. Here, we hypothesized that interstitial fluid flow and cells within the breast tumor microenvironment modulate migration of MDSCs. We developed a novel 3D model to mimic the breast tumor microenvironment and incorporated MDSCs harvested from 4T1-tumor bearing mice. Using live imaging, we found that sorted GR1+ splenocytes had reduced chemotactic index compared to the unsorted population, but their speed and displacement were similar. Using our adapted tissue culture insert assay, we show that interstitial fluid flow promotes MDSC invasion, regardless of absence or presence of tumor cells. Coordinating with lymphatic endothelial cells, interstitial fluid flow further enhanced invasion of MDSCs in the presence of 4T1 cells. We also show that VEGFR3 inhibition reduced both MDSC and 4T1 flow response. Together, these findings indicate a key role of interstitial fluid flow in MDSC migration as well as describe a tool to explore the immune microenvironment in breast cancer.