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

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 coronavirus disease 2019 (COVID-19) patients. We hypothesized that PROS1 may protect the upper airways by regulating epithelial and myeloid cell responses during severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

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 the lungs of COVID-19. Regulation and secretion of PROS1, along with multiple soluble mediators, were quantified in control and infected cultures using ELISAs.

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 CXCL10/11^high into a regenerative S100A2^posKRT^high 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 toward a phenotype with increased MHC class II.

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.