Immunomodulation by cisplatin chemotherapy in an immunocompetent intraductal model for triple-negative breast cancer

Triple-negative breast cancer (TNBC) is an aggressive breast cancer subtype showing limited immunotherapeutic efficacy due to a highly immunosuppressed tumor microenvironment. Some chemotherapeutics such as the platinum-based agent cisplatin have been reported to alleviate this immunosuppression. More recently, cisplatin was suggested to even synergize with the inhibition of immune checkpoint proteins programmed death (PD)-1 and PD-ligand (L) 1. Our goal was to investigate this immunomodulation by cisplatin at the local tumor and systemic level using an immunocompetent intraductal model that recapitulates the complete TNBC disease process. Lactating female BALB/c mice were inoculated with triple-negative, syngeneic and bioluminescent 4T1 mammary tumor cells through the mammary teat canal of the third mammary gland pair. At 3 weeks after the inoculations, tumor cells broke through the ductal epithelial barrier and invaded the mammary fat pad, providing a clinical indication for chemotherapy. The mice therefore received cisplatin intraperitoneally every 5 days for 3 weeks, after which primary tumors, axillary lymph nodes, lungs, spleen and blood were isolated and immune cell subtypes as well as PD-(L)1 expression were analyzed through flow cytometry. The immune-related biomarkers chitinase 3-like 1 (CHI3L1) and lipocalin 2 (LCN2) were analyzed locally and systemically to verify disease progression. Cisplatin treatment significantly reduced tumor and metastatic growth, corroborated by decreased CHI3L1 and LCN2 levels in primary tumors, spleen and serum. Immunosuppressive CD11b+ myeloid cells that accumulated in body fluids and tissues were not uniformly eliminated by cisplatin. CD11b+Ly6C-Ly6G+ granulocytic myeloid-derived suppressor cells, the most abundant myeloid subpopulation, were significantly reduced in spleen and axillary lymph nodes only, keeping primary tumors, lungs and blood immunosuppressed. Focusing on adaptive immune cell subtypes, axillary lymph nodes showed a significant decrease in CD3ε+CD4+CD8α- helper T cells and even complete absence of CD3ε+CD4-CD8α+ cytotoxic T cells following cisplatin treatment, representing a drawback for immunotherapeutic efficacy. In contrast, helper T cells, cytotoxic T cells and CD19+ B cells were enhanced in the spleen, creating an anti- tumorigenic niche. The expression of PD-(L)1 was also significantly increased in the cisplatin-exposed spleen. In conclusion, our study provides a unique detailed characterization of the modulation of TNBC progression by cisplatin treatment and associated immune cell and immune checkpoint content across multiple tissues, identifying the spleen as most positively affected and potential driver for synergy with immunotherapy