Increased apoptosis is associated with robust immune cell infiltration and cytolytic activity in breast cancer.

Tumor infiltrating immune cells plays a critical role in cancer progression. Apoptosis is an autonomous cell death that counteracts tumor growth. To this end, we hypothesized that increased apoptosis in breast cancer is associated with immune cell killing. Apoptosis score of MSigDB Hallmark collection was used to analyze METABRIC cohort (n=1904) and TCGA (n=1069) as validation cohort. High apoptosis tumors enriched cancer promoting signaling pathways; hypoxia, KRAS, TGF-β, PI3K signaling, and was associated with low MKI67 expression and less cell proliferation gene sets, less homologous recombination defects, and less altered fraction. High apoptosis tumors also enriched angiogenesis and high infiltration of vascular endothelial cells, pericytes and stromal cells and significantly enriched inflammation and immune response-related gene sets and high infiltration of CD8, CD4 memory, dendritic cells, M1 and M2 macrophages and significant elevation of cytolytic activity and immune checkpoint molecules, consistently in both cohorts. In conclusion, breast cancer patients with high apoptosis are associated with angiogenesis, immune response, high immune cell infiltration and cytolytic activity. To the best of our knowledge, this is the first study to utilize in silico translational approach to demonstrate the clinical relevance of apoptosis in breast cancer patients in large cohorts.