In March 2020, we performed a multicenter, randomized, double-blind phase II study of mesenchymal stem cells (MSCs) therapy in patients with severe COVID-19 (ClinicalTrials.gov: NCT04288102). Findings showed that MSC was a potentially effective therapeutic approach for patients with severe COVID-19, but not all patients responded well. To further understand the proteomic biomarkers of response to MSC therapy in patients with severe COVID-19, we characterize the proteomic profile of 58 patient pre-MSC treatment plasma samples (35 responders versus 23 non-responders) by using proteomics assays. The project aims to identify potential biomarkers for predicting response to MSC therapy by pre-MSC treatment plasma proteomic profile in severe COVID-19.