GDF15 and COVID-19: We also observed that the plasma levels of several proteins mainly derived from respiratory epithelial cells (IL18, WFDC2, GDF15, MUC16, TGFA, MMP7, and LGALS3) and macrophages (IL18, SPP1, PLAUR, CCL23, and LGALS3) were capable of distinguishing moderate and severe COVID-19 significantly and consistently regardless of the overall inflammatory status of the patients (Supplementary Table S5); hence, the damaging response of pulmonary epithelial cells and dysregulated macrophage activation might be critical determinants of severe pneumonic progression.