Background: The alarmin interleukin-33 (IL-33) and its receptor ST2 are suggested contributors to the immunopathology of COVID-19. However, little is known about the distribution of this pathway in diseased lung tissues.

Aims: To reveal the spatiotemporal expression patterns of IL-33 and soluble (sST2) and membrane ST2 (mST2) variants across COVID-19-inflicted lung pathologies.

Methods: Distal lung tissue from 20 fatal COVID-19 cases were obtained by minimally invasive autopsy technique. Non-diseased control tissue was obtained by resection surgery (n=10). IL-33 and ST2 expression patterns were assessed histologically by combined in situ hybridization and multiplex immunohistochemistry (ISH-mIHC) and spatially resolved single cell analysis.

Results: Healthy baseline IL-33 mRNA and protein expression was high and confined to structural cells such as alveolar capillaries. COVID-19 lungs had significantly reduced IL-33 protein (p < 0,0001; 78% reduction). The composition of IL33 expressing cell populations differed between stages of diffuse alveolar damage (DAD). IL-33 mRNA was unchanged at a total tissue level but increased in patchy DAD microenvironments. Mast cells, which represented the dominating mST2 and sST2-expressing cell type in healthy lung tissue, had reduced ST2 expression in COVID19. However, total tissue sST2, but not mST2, increased in COVID-19 (p<0.04). This phenomenon was attributed to a marked upregulation of sST2 in alveolar structural cells in acute DAD regions.

Conclusions: The present study reveals novel spatiotemporal alterations of IL33 and ST2 in COVID-19 and suggests that the timing for pharmacological targeting of this pathway is a critical determinant for therapeutical effect.