Impact of brequinar treatment on gene expression in bone marrow progenitors of tumor-bearing mice

Immune checkpoint inhibitors (ICIs) have transformed the therapeutic landscape in oncology; however, ICIs are effective in subsets of patients. A major limitation of their efficacy is tumor-driven immune suppression, and one key mechanism is the ‘biogenesis’ of myeloid-derived suppressor cells (MDSCs). A fundamental gap in MDSC biology is the dearth of approaches that target their biogenesis. We hypothesized that targeting MDSC biogenesis will mitigate MDSC burden and bolster responses to ICIs. We focused on a class of agents, dihydroorotate dehydrogenase (DHODH) inhibitors, recently found to restore terminal differentiation of leukemic myeloid progenitors and is in clinical trials of AML. One such DHODH inhibitor is brequinar. Using preclinical models of mammary cancer that elicit robust MDSC responses, we demonstrated that brequinar suppressed MDSC biogenesis, enhancing the antitumor and anti-metastatic activity of PD-1-based ICI therapy. In this single-cell RNA transcriptomics study, we sought to determine the impact of brequinar treatment of tumor-bearing mice on altering gene expression in bone marrow myeloid progenitors. Thus, this cutting-edge molecular approach enabled us to test the hypothesis that brequinar acted in the bone marrow at a qualitative level to mitigate MDSC biogenesis. Bone marrow cells enriched for cKit+ progenitors were isolated from 3 separate mice each from the following groups: 1) non-tumor-bearing controls, 2) 4T1 tumor-bearing mice treated with the vehicle control and 3) 4T1 tumor-bearing mice treated with brequinar. Non-tumor-bearing samples served as the reference control