Neutrophil extracellular traps promote tumor chemoresistance to anthracyclines.

The microenvironment plays an important role in promoting tumor cell chemoresistance, but the mechanisms responsible for this effect are not clear. Here, using models of multiple myeloma (MM) and solid cancers, we demonstrate a novel mechanism mediated by neutrophils, a major cell population in the bone marrow (BM), that protects cancer cells from chemotherapeutics. We show that in response to tumor-derived soluble factors, BM neutrophils release their DNA in the form of neutrophil extracellular traps (NETs). Cell-free DNA derived from NETs is then taken up by tumor cells via endocytosis and localizes to the cytoplasm. We found that both NETs and cell-free DNA taken up by tumor cells can bind anthracyclines, leading to tumor cell resistance to this class of chemotherapeutic agents. Targeting cell-free DNA with Pulmozyme or blocking NET formation with a PAD4 inhibitor abrogates the chemoprotective effect of neutrophils and restores sensitivity of tumor cells to anthracyclines.