LAUSR

Cancer-associated fibroblasts (CAFs) perform diverse roles and can modulate therapy responses 1 . The inflammatory environment within tumours also influences responses to many therapies, including the efficacy of oncolytic viruses 2 ; however, the role of CAFs in this context remains unclear. Furthermore, little is known about the cell signalling triggered by heterotypic cancer cell–fibroblast contacts and about what activates fibroblasts to express inflammatory mediators 1 , 3 . Here, we show that direct contact between cancer cells and CAFs triggers the expression of a wide range of inflammatory modulators by fibroblasts. This is initiated following transcytosis of cytoplasm from cancer cells into fibroblasts, leading to the activation of STING and IRF3-mediated expression of interferon-β1 and other cytokines. Interferon-β1 then drives interferon-stimulated transcriptional programs in both cancer cells and stromal fibroblasts and ultimately undermines the efficacy of oncolytic viruses, both in vitro and in vivo. Further, targeting IRF3 solely in stromal fibroblasts restores oncolytic herpes simplex virus function. Tumour fibroblasts influence oncolytic viral therapy. Arwert et al. show that transcytosis of cancer cells into fibroblasts activates STING and IRF3 to upregulate interferon-β1, eliciting a transcriptional program to reduce the effectiveness of oncolytic viral therapy.