PROS1 released by lung basal cells limits inflammation in epithelial and monocytes during SARS-CoV-2 infection

Introduction Factors regulating the severity of pneumonitis during viral infections remain unresolved. We previously found higher expression of protein S (PROS1) in lung epithelium of mild compared to severe COVID-19 patients. We hypothesised that PROS1 may protect the upper airways by regulating epithelial and myeloid cell responses during SARS-CoV-2 infection. Methods To test this, in vitro air-liquid-interface (ALI) cultures of primary healthy human lung epithelial cells were infected with SARS-CoV-2. This model, validated through immunofluorescent staining, confocal microscopy and single-cell RNA-sequencing, replicated pathogenic changes seen in lungs of COVID-19. Regulation and secretion of PROS1, along with multiple soluble mediators, were quantified in control and infected cultures using ELISAs. Results We found that PROS1 is present in the basal cells of healthy pseudostratified epithelium and is released during SARS-CoV-2 infection through an IFN-mediated process. Transcriptome analysis revealed that PROS1 downregulated the SARS-CoV-2 induced proinflammatory phenotypes of basal cells, transforming pathogenic CXCL10/11high into a regenerative S100A2posKRThigh basal cell phenotype. In parallel, SARS-CoV-2 increased the secretion of M-CSF from epithelial cells, which induced the expression of PROS1 receptor MERTK on monocytes interacting with the lung epithelium. PROS1, in turn, shifted SARS-CoV-2-induced pathogenic monocyte phenotypes towards a phenotype with increased MHC class II. Conclusion These findings highlight the crucial role of PROS1 in protecting against severe lung pathology caused by SARS-CoV-2, by reducing epithelial- and monocyte-derived inflammation, promoting pro-repair epithelial phenotypes, and enhancing antigen presentation in myeloid cells.