Here, we used the MMTV-PyMT transgenic mouse model of breast cancer to explore the role of MDSCs during breast cancer progression. Mice expressing the polyomavirus middle T antigen (PyMT) driven by the mouse mammary tumor virus (MMTV) promoter 13 develop breast tumors that closely resembles human pathogenesis 14 and give rise to MDSCs during tumor progression 2. The spleen is the predominant site of MDSC accumulation in tumor-bearing mice CD84, JAML) for faithful MDSC detection and prospective enrichment. Further interrogation of the distinct MDSC gene expression signature identified several novel surface markers (e.g. Focusing on G-MDSCs, our pseudotemporal analysis delineates the emergence of MDSCs as an aberrant differentiation state that forms a separate branch during the transition of neutrophil progenitors into mature neutrophils. Our computational analysis of 14,646 single cell transcriptomes revealed a unique MDSC gene signature, which is largely shared between G-MDSCs and M-MDSCs, but which strongly differs from their normal myeloid counterparts. Here, we used single-cell RNA sequencing (scRNAseq) to delineate the molecular features of MDSCs in the MMTV-PyMT mouse model of breast cancer. However, these markers overlap with those defining healthy neutrophils and monocytes, which makes it challenging to distinguish MDSCs from normal cells to advance our understanding of MDSC biology and ultimately, to establish novel therapeutic avenues to interfere with their tumor-promoting and immune suppressive roles. In humans, G-MDSCs are defined as CD11b +CD14 −CD15 + or CD11b +CD14 −CD66b +and M-MDSCs as CD11b +CD14 +HLA-DR −/lowCD15 − followed by additional functional characteristics such as T cells suppression and ROS assays 12. In mice, MDSCs are defined through the expression of CD11b +Gr1 + and can be further classified into CD11b +Ly6C lowLy6G + granulocytic MDSCs (G-MDSCs) and CD11b +Ly6C +Ly6G − monocytic MDSCs (M-MDSCs) 12. This limits our ability to determine specific MDSC functions as opposed to bulk-level changes in neutrophils or monocytes during cancer. The unique molecular features of MDSCs are currently unclear and it remains elusive whether MDSCs represent a unique subpopulation of myeloid cells that differ from their normal, healthy counterparts. Besides their immune-suppressive function, MDSCs may also actively shape the tumor microenvironment through complex crosstalk with breast cancer cells and surrounding stroma, resulting in increased angiogenesis, tumor invasion, and metastasis 8, 10, 11.
In addition, MDSCs produce a range of immunosuppressive and cancer-promoting cytokines including IL-10 and TGF-β 9. MDSCs can mediate immune suppression through multiple mechanisms including the production of reactive oxygen species (ROS) and depletion of key amino acids required for T cell proliferation through expression of arginase (Arg) and indoleamine 2,3-dioxygenase (IDO) 6, 7, 8. In cancer patients, increased numbers of MDSCs in circulation correlate with advanced clinical stages, increased metastatic progression and immune suppression 5. These MDSCs are a heterogeneous population of neutrophil- and monocyte-like myeloid cells, which are increasingly recognized as key mediators of immune suppression in various types of cancer 3, 4. During tumor development, breast cancer cells secrete various cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF), which exert systemic effects on hematopoiesis and myeloid cell differentiation promoting the development of myeloid-derived suppressor cells (MDSCs) 2, 3. We establish the MDSC-specific gene signature and identify CD84 as a surface marker for improved detection and enrichment of MDSCs in breast cancers.īreast cancer is one of the most prevalent types of cancer with over 260,000 new cases and over 40,000 deaths in 2018 in the US 1.
Our computational analysis of 14,646 single-cell transcriptomes revealed that MDSCs emerge through an aberrant neutrophil maturation trajectory in the spleen that confers them an immunosuppressive cell state. To identify MDSCs in an unbiased manner, we used single-cell RNAseq to compare MDSC-containing splenic myeloid cells from breast tumor-bearing mice to wildtype controls. Here, we sought to determine the molecular features of breast cancer-associated MDSCs using the widely studied mouse model based on mammary tumor virus (MMTV) promoter-driven expression of the polyoma middle T oncoprotein (MMTV-PyMT). It remains elusive how MDSCs differ from their normal myeloid counterparts, which limits our ability to specifically detect and therapeutically target MDSCs during cancer. Myeloid-derived suppressor cells (MDSCs) are innate immune cells that acquire the capacity to suppress adaptive immune responses during cancer.