Cancer-associated fibroblasts serve as decoys to suppress NK cell anti-cancer cytotoxicity [NMFs]
Cancer associated fibroblasts (CAFs) are one of the most abundant components of the breast tumor microenvironment (TME) and major contributors to immune modulation in the TME. CAFs are well known to regulate the activity of diverse types of immune cells including T cells, macrophages and dendritic cells, however little is known about their interaction with Natural killer (NK) cells. NK cells constitute an important arm of anti-tumor immunity, yet the regulation of NK cell activity by CAFs in solid tumors is poorly understood. Here we find, using mouse models of cancer and ex-vivo cocultures, that immunosuppressive CAF subsets severely inhibit NK cell cytotoxicity towards cancer cells. We unravel the mechanism by which this suppression occurs, through CAF-mediated downregulation of the NK-surface receptors, Natural Killer Group 2D (NKG2D) and DNAX Accessory Molecule-1 (DNAM-1). Ligands for these receptors are known to be expressed by cancer cells and minimally expressed in healthy tissue. Here we find that CAFs also upregulate ligands for NKG2D and DNAM-1. Ligand-receptor engagement between NK cells and CAFs leads to CAF cytolysis, which in turn diminishes the expression of NKG2D and DNAM-1 on NK cells via a negative feedback loop, and promotes cancer escape from NK cell surveillance. These results reveal a CAF-mediated immunosuppressive mechanism with implications for treatment of solid tumors. Overall design: To investigate the baseline transcriptomic differences between normal mammary fibroblasts (NMFs) from C57BL6 and BALB/C mice, we isolated fibroblasts from the mammary fat pad of healthy female mice using magetic seperation (3 biological replicates each) and subsequently cultured NMFs for four days prior to bulk RNA sequencing