Single-cell transcriptome profiling of the myeloid cells repopulating after chemotherapy identifies a neutrophil-like monocyte subset with pro-tumor activities

Patients with cancer often receive chemotherapy to control tumor progression and reduce disease symptoms. Cytotoxic chemotherapeutic agents not only kill rapidly growing cancer cells but also reduce normal cells including myeloid cells, the main innate immune population involved in fighting infections and repairing tissue damages. Rapid loss of myeloid cells caused by chemotherapy triggers myelopoiesis, a process in which the hematopoietic stem and progenitor cells in the bone marrow regenerate myeloid cells, including monocytes, neutrophils, dendritic cells and macrophages, to reconstitute the myeloid cell compartment. We previously reported that chemotherapy with an alkylating agent cyclophosphamide (CTX) in mice leads to repopulation of myeloid cells that acquire immunosuppressive activities within the monocyte subset. However, detailed information on the cellular composition and molecular identity of these chemotherapy- induced immunosuppressive monocytes is lacking. Here, we investigated how the various myeloid cell subsets in the bone marrow of mice respond to CTX chemotherapy through single-cell RNA sequencing analysis (scRNAseq). We found that myeloid progenitor cells and monocytes were reduced 2 days after chemotherapy but rebounded and surpassed their pretreatment levels by day 7. Further scRNAseq analysis of pre-enriched monocytes revealed that the monocyte population was heterogenous, and that chemotherapy tilted myelopoiesis towards the production of neutrophil-like monocytes (NeuMo). We identified Cxcr4 and Cx3cr1 as suitable markers for isolation of chemotherapy-induced NeuMo and demonstrated that these cells were suppressive to T cells. Together with the evidence that CTX-induced monocytes can promote breast cancer metastasis in mice, our data reveal the heterogeneity of the monocytes reemerging after chemotherapy and identify the NeuMo subset as a potential therapeutic target for enhancing the efficacy of chemotherapy in cancer.