Rationale: Macrophage polarization has been extensively studied in different disease models, but the distinct role of macrophage subsets in lung repair, their functional profiles, lineage relation and the related macrophage mediators driving resolution and repair in influenza virus (IV)-induced lung injury are not well understood.

Methods: FACS,single cell transcriptomics to analyse the diversity of macrophage/monocyte populations after IV infection. In vivo adoptive transfer experiments to characterize the functional phenotypes of macrophage subsets and lung organoids were used. Phosphokinome screening and mass spectrometry analysis were applied to find out the putative signaling mechanisms driven by Plet1 and identification of Plet1 receptor.

Results: We demonstrate that during severe viral pneumonia, CX3CR1pos monocytes are mobilized upon infection and undergo co-ordinated trajectories of pro-inflammatory to tissue-healing phenotypes, before differentiating into tissue-resident alveolar macrophages that retain a long-term tissue-protective phenotype.This phenotype is characterized by expression of Plet1.We reveal that, Plet1 is an important mediator of macrophage-epithelial cross-talk during lung barrier repair.MAPK, more specifically Raf-MEK-ERK and Wnt/beta-catenin were identified as two key signal transduction pathways that are regulated by Plet1 in alveolar epithelial cells.

Conclusions: These results suggest that activating the abovementioned signaling pathways could be beneficial in treating lung injury. Therefore, application of recombinant Plet1 in patients could be considered as therapeutic strategy to improve the outcome after severe lung injury.