Unveiling the clinical relevance of low density neutrophils in Breast Cancer patients

Abstract

Neutrophils are prominent immune components of solid tumors, which can protect against the onset of cancer (N1) or have protumor activity (N2). Circulating neutrophils, divided into high density neutrophils (HDN) and low density neutrophils (LDN), functionally mirror those N1 and N2 cells, respectively. LDN, practically absent in non-pathological conditions, have been extensively studied in cancer, due to their increased frequency in this disease and their protumor phenotype. However, this has been mainly demonstrated in animal models and proper validation in humans is an urgent need. In this thesis, we enlightened the clinical impact of LDN in breast cancer (BC) patients. We observed that LDN were practically absent in healthy donors’ blood, while significantly increased in the blood of BC patients, particularly with metastatic disease. Within the population of non metastatic patients, LDN were more prevalent in patients with poor response to neoadjuvant chemotherapy than in patients with good response. The association of a higher incidence of circulating LDN and the worse prognosis of BC patients could be explained by the protumor features exhibited by these cells. Namely, there are more LDN expressing the immunosuppressive markers PD-L1 and CCR4, than HDN. Additionally, LDN also showed increased expression of activation markers; robust formation of neutrophil extracellular traps; augmented phagocytic activity and higher capacity to release reactive oxygen species, which altogether contribute for tumor development and metastization. Moreover, the percentage of LDN in BC patients’ blood was positively correlated with the immunosuppressive CCR4+ regulatory T cells and negatively correlated with activated cytotoxic T lymphocytes, corroborating the impairment on the antitumor immune responses by LDN, which was further demonstrated ex vivo. Hence, this thesis reveals the potential of LDN as a clinical meaningful biomarker of BC response to treatment and opens new avenues for developing targeted immunotherapies.