Efferocytosis fuels malignant pleural effusion through TIMP1

Malignant Pleural Effusion (MPE) results from the capacity of several human cancers to metastasize to the pleural cavity. The median survival is 3-12 months and no effective treatments are currently available. Immune-based therapies have failed until now, reflecting our insufficient understanding of the basic immunological mechanisms leading to MPE progression. Here, we show that phagocytosis of apoptotic cells in the pleural cavity fuels the progression of MPE. We found that efferocytosis through the receptor tyrosine kinases AXL and MERTK in macrophages led to the production of IL-10. Using single cell RNA-Seq, we revealed that IL-10 is indeed produced by four distinct pleural cavity macrophage subpopulations characterized by different metabolic states and cell chemotaxis properties. In turn, IL-10 acts on dendritic cells (DCs) inducing the production of tissue inhibitor of metalloproteinases 1 (TIMP1). Genetic ablation of AXL and MERTK in macrophages or IL-10 receptor in DCs or TIMP1 significantly reduced MPE progression. Taken together, our results delineate an inflammatory cascade – from the clearance of apoptotic cells by macrophages, to production of IL-10, to induction of TIMP1 in DCs – that facilitates MPE progression. This inflammatory cascade offers a series of targets for therapies which aim at preventing or treating MPE. We induced malignant pleural effusion in mice by injecting tumor cells into the pleural cavity of anaesthesized mice. After 10 days, we euthanized the mice, aspirated the pleural liquid, extracted myeloid cells and performed single cell sequencing.