Cancer is a deadly disease where malignant cells proliferate and avoid elimination by the immune system. Although natural killer (NK) cells, of the innate immune response, recognise and eliminate cancer cells, their cytotoxic function is often impaired in the tumour microenvironment. Indeed, difficult to treat solid cancers, like pancreatic cancer, are rich in cancer associated fibroblasts (CAF) which support tumour growth and suppress immune cell functions. Overcoming barriers like CAF-derived immune suppression in the solid tumour microenvironment remains a challenge. Successful restraint of CAF has proven challenging due to their complex intratumoral heterogeneity and plasticity. However, the potential exploitation of NK cell cytotoxicity remains attractive in the field of cancer immunotherapy. This work aimed to investigate the crosstalk between CAF and NK cells and to identify potential targets in solid tumours, thus improving NK cell cytotoxicity and reducing tumour growth.

Initial flow cytometry and imaging analysis of mouse models of solid tumours revealed NK cell dysfunction within the tumours, characterised by an immature and less cytotoxic profile as compared to more mature blood circulating NK cells. Moreover, within the tumours, NK cells interacted closely with CAF defined by expression of platelet derived growth factor receptor alpha (PDGFR-⍺), while being distant from myofibroblastic CAF. Investigation of the roles of distinct CAF populations in vitro and ex vivo by flow cytometry and quantitative PCR, revealed that TGF-β-driven CAF (myofibroblastic) were strong NK cell modulators, in contrast to inflammatory CAF and healthy fibroblasts. Furthermore, in vitro assays revealed that pancreatic CAF-derived factors suppressed NK cell functions, in contrast to healthy fibroblasts. Comparative mass spectrometry and ELISA revealed CAF increased secretion of extracellular matrix proteins osteopontin, laminin and perlecan, but also the lipid metabolite prostaglandin E2 (PGE2). Blocking CAF-derived PGE2 in vitro restored NK cell functions such as tumour killing capacity and granzyme B secretion. In vivo, targeting this crosstalk by blocking PGE2 receptors EP2 and EP4 with antagonists reduced tumour growth and restored NK and T cell derived granzyme B.

This work revealed unknown and contrasting roles for CAF populations in NK cell anti-tumour function, suggesting that targeting myofibroblastic CAF and derived factors like PGE2 could be relevant to improving intratumoral NK cell cytotoxicity in difficult to treat cancers such as pancreatic cancer.