One of the possible candidates is Matrix Metallopeptidase 9 (MMP-9) which has significantly elevated expression and activity in COVID-19 patients.63,64 MMP-9 can impair the alveolar epithelial-endothelial capillary barrier by degrading the extracellular matrix, stimulate neutrophil and leukocyte migration, and promote inflammation.65 Keeping consistent with the above transcriptome analysis data (Fig. 5g), the LAD2/RBD supernatant-treated A549 cells had higher MMP9 expression levels, and after blocking LAD2 degranulation with Lor. Here, MMP9 is linked to COVID-19.