Updated project metadata. Pancreatic ductal adenocarcinoma (PDA) is a lethal malignancy characterised by a pathologicalfibroinflammatorymicroenvironment. Dichotomous tumour-promoting and -restrictive roles have been ascribed to the tumour microenvironment, however thedisparate effect of individual stromal subsets remains incompletely characterised. Here, we describe how heterocellular OSM-OSMR signalling instructsfibroblast reprogramming,tumourgrowth and metastasis.Macrophage-secreted OSM stimulatesinflammatory gene expression in cancer-associated fibroblasts (CAFs), which in turn induce a pro-tumorigenic environment and engage tumour cellsurvival and migratory signalling pathways. Tumour cells implanted in Osm-deficient (Osm-/-) mice display an epithelial-dominated morphology, reduced tumour growth and did notmetastasise. Moreover, the tumour microenvironment of Osm-/-animals exhibit increased abundance of αSMAposmyofibroblasts and a shift in myeloid and T cell phenotypes, consistent with a more immunogenic environment. Taken together, these data demonstrate how OSM-OSMR signalling coordinates heterocellular interactions to drive a pro-tumorigenic environment in PDA.