201P - Underlying mechanisms of neutrophil-mediated immunosuppression and resistance to treatment in breast cancer: Further evidence that these cells matter

Background

Despite advances in breast cancer (BC) treatment, current therapies often fail in metastatic settings, underscoring the need for innovative strategies. Neutrophils, known for their dual role in cancer, emerged as potential therapeutic targets. Circulating neutrophils are classified as high density neutrophils (HDN) which are pro-inflammatory and cytotoxic, and low density neutrophils (LDN), which promote tumor growth and metastasis, mirroring their tumor counterparts. Our group found significant LDN accumulation in patients’ blood, correlating with advanced disease and poor prognosis. To clarify the role of neutrophils in BC, we focused on LDN’ immunosuppressive function, namely their arginase activity, known to inhibit T cell antitumor response.

Methods

Both neutrophil populations were obtained from the blood of 146 BC patients by density gradient centrifugation and characterized by flow cytometry. We conducted in vitro assays and co-cultures with patient-derived cells to explore LDN-mediated immunosuppression.

Results

In 3D co-cultures, HDN showed strong cytotoxicity against BC cell lines, while LDN failed to induce tumor cell death. Conversely, LDN exhibited enhanced immunosuppressive traits, including elevated PD-L1 expression, which impairs T cell activation. In addition, T cells cultured in LDN-conditioned medium showed significant activation impairment, suggesting LDN-released soluble factors as mediators of this effect. This suppression was partially reversed by an arginase inhibitor. Although arginase levels were similar in LDN and HDN, we identified a specific LDN subset with heightened arginase activity, which displays other pro-tumor features, including higher PD-L1 and MMP-9 expression, and was linked with poorer prognosis in this BC cohort.

Conclusions

Our findings highlight the heterogeneity of neutrophils and the potential of identifying distinct immunosuppressive LDN subsets that contribute to tumor progression by impairing antitumor T cell responses. Our data advance the understanding of LDN in BC and highlight their potential as biomarkers for disease monitoring, predicting therapy responses, and as targets for novel immunotherapies.

Legal entity responsible for the study

The authors.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.