Coagulation factor V is a marker of tumor-infiltrating immune cells in breast cancer.

Background: Factor (F) V is an essential cofactor in blood coagulation, however, expression in breast tumors has also been linked to tumor aggressiveness and overall survival. The specific role of FV in breast cancer is yet unknown. We therefore aimed at dissecting the biological relevance of FV in breast cancer.

Methods: Gene expression data from a Scandinavian breast cancer cohort (n = 363) and the cancer genome atlas (TCGA) (n = 981) and 12 replication cohorts were used to search for co-expressed genes, followed by gene ontology analysis. Pathological and bioinformatic tools were used to evaluate immune cell infiltration and tumor purity. T cell activation, proliferation and migration were studied in FV treated Jurkat T cells.

Results: co-expressed genes were mainly associated with immune system processes and cell activation. Tumors with high expression of were more infiltrated with both lymphoid (T cells, NK cells, and B cells) and myeloid cells (macrophages and dendritic cells), and expression was negatively correlated with tumor purity (ρ = -0.32). Confirming a prognostic role, data from the Kaplan-Meier plotter showed that high expression was associated with improved relapse-free survival. The strongest association was observed in basal-like breast cancer (HR = 0.55; 95% CI, 0.42-0.71). Exogenous FV did not substantially affect activation, proliferation or migration of human T cells.

Conclusions: was identified as a novel marker of immune cell infiltration in breast cancer, and the prognostic role of was verified. FV emerge as an interesting immunological biomarker with potential therapeutic relevance for the cancer-inflammation-thrombosis circuit.