Purpose: Preoperative or neoadjuvant therapy (NT) is increasingly used in patients with locally advanced or inflammatory breast cancer (BC) to allow optimal surgery and aim for pathological response. However, many BCs are resistant or relapse after treatment. Here, we investigated conjunctive chemotherapy-triggered events occurring systemically and locally, potentially promoting a cancer stem-like cell (CSC) phenotype and contributing to tumor relapse. Experimental Design: We started by comparing the effect of paired pre- and post-NT patient sera on the CSC properties of BC cells. Using cell lines, patient-derived xenograft models and primary tumors, we investigated the regulation of CSCs and tumor progression by chemotherapy-induced factors. Results: In human patients and mice, we detected a therapy-induced CSC-stimulatory activity in serum, which was attributed to therapy-associated monocytosis leading to systemic elevation of monocyte chemoattractant proteins (MCPs). The post-NT hematopoietic regeneration in the bone marrow highlighted both altered monocyte-macrophage differentiation and biased commitment of stimulated hematopoietic stem cells towards monocytosis. Chemotherapeutic agents also induce monocyte expression of MCPs through a JNK-dependent mechanism. Genetic and pharmacological inhibitions of the MCP-CCR2 pathway blocked chemotherapy's adverse effect on CSCs. Levels of nuclear Notch and ALDH1 were significantly elevated in primary BCs following NT, whereas higher levels of CCR2 in pre-NT tumors were associated with a poor response to NT. Conclusions: Our data establish a mechanism of chemotherapy-induced cancer stemness by linking the cellular events in the bone marrow and tumors, and suggest pharmacological inhibition of CCR2 as a potential co-treatment during conventional chemotherapy in neoadjuvant and adjuvant settings.
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