Infiltration by myeloperoxidase-positive neutrophils is an independent prognostic factor in breast cancer

Purpose Myeloperoxidase (MPO) is an enzyme secreted by neutrophil granulocytes as a result of phagocytosis during inflammation. In colorectal cancer, tumour infiltration by MPO expressing cells has been shown to be independently associated with a favourable prognosis. In this study, we explored the role of MPO-positive cell infiltration and its prognostic significance in invasive breast cancer. Methods We performed immunohistochemical staining for MPO on multiple tissue microarrays comprising a total of 928 human breast cancer samples with detailed clinical-pathological annotation and outcome data. Results MPO-positive cell infiltration (>= 5 cells/tissue punch) was found in 150 (16%) of the 928 evaluable breast cancer cases. In univariate survival analyses, infiltration by MPO-positive cells was associated with a significantly better overall survival (p < 0.001). In subset univariate analyses, the infiltration by MPO-positive cells was associated with significantly better overall survival in the Luminal B/HER2-negative subtype (p = 0.005), the HER2 enriched subtype (p = 0.011), and the Triple Negative subtype (p < 0.001). In multivariate analysis, MPO expression proved to be an independent prognostic factor for improved overall survival (p < 0.001). Conclusions This is the first study to show that infiltration of MPO-positive cells is an independent prognostic biomarker for improved overall survival in human breast cancer.