Cancer cells communicate with macrophages to prevent T cell activation during development of cell cycle therapy resistance

Cancer cells evolve, acquire resistance to therapy and change their environment. One resistance mechanism involves altering communication with non-malignant cells in the tumor microenvironment (TME). By corrupting the signals for growth and survival, evolving cancer cells can engineer a pro-tumor TME. However, the specific interactions between malignant and non-malignant cells that predispose drug resistance and their changes during treatment remain widely unknown. Here we examine the composition, communication, and phenotypic diversity of tumor-associated cell populations in serial biopsies from early-stage ER+ breast cancers. These patients received either endocrine therapy (letrozole) alone or in combination with a CDK4/6 cell cycle inhibitor ribociclib, and were analyzed using single-cell RNA sequencing (scRNAseq). Our analyses reveal cancer cells from ribociclib resistant tumors stimulate macrophage differentiation towards an immune-suppressive phenotype through upregulation of a broad diversity of cytokines and growth factors. This shift in phenotype leads to reduced macrophage cell IL-15/-18 crosstalk with CD8+ T-cell via IL-2/15RA/18R receptors, resulting in diminished T-cell activation and recruitment. Thus, cancer communication promoting an immune-cold TME predispose tumors to develop CDK4/6 inhibitor resistance, and that the beneficial effects of cell cycle inhibitors through blocking cancer cell proliferation must be balanced against their known inhibitory effect on immune cell division and activation. An optimal treatment strategy will require coupling the prevention of cancer division with activation of an effective cytotoxic T-cell response. ### Competing Interest Statement Ruth O'Regan participates on the advisory board for Cyclacel, PUMA, Biotheranostics, Lilly, Pfizer, Genentech, Novartis; declares research funding from Pfizer, Novartis, Seattle Genetics, PUMA. Priyanka Sharma declares research funding from Novartis, Merck, Bristol Myers Squibb. Consulting: Seattle Genetics, Merck, Novartis, AstraZeneca, Immunomedics, Exact Biosciences. Laura Spring participates on the advisory board for Novartis, Lumicell, Puma Biotechnology and Avrobio. Cynthia Ma declares research funding from Pfizer, Puma; Consulting: Eisai, Athenex, OncoSignal, Agendia, Biovica, AstraZeneca, Seattle Genetics. Kari Wisinski declares research funding and clinical trial involvement with Novartis, Eli Lilly, Astra Zeneca, Sanofi and Pfizer. He participated on an advisory board for Eisai, Pfizer and Astra Zeneca. Kevin Kalinsky is a medical advisor to Immunomedics, Pfizer, Novartis, Eisai, Eli-Lilly, Amgen, Merck, Seattle Genetics and Astra Zeneca; receives institutional support from Immunomedics, Novartis, Incyte, Genentech/Roche, Eli-Lilly, Pfizer, Calithera Biosciences, Acetylon, Seattle Genetics, Amgen, Zentalis Pharmaceuticals, and CytomX Therapeutics; and his spouse is employed by Grail and previously by Array Biopharma and Pfizer. Anne O'Dea Consults for the Pfizer, PUMA Biotechnology, Astra Zeneca, and Daiichi Sankyo. Qamar Khan declares research funding from Novartis. All other authors have no conflicts of interest to disclose.