ACE2 and viral infectious disease: Intravenous transplantation of ACE2-negative mesenchymal stem cells (MSCs) promoted recovery of patients from severe COVID-19 [193], thus supporting the hypothesis that binding of SARS-S protein through ACE2 expressed on MSC-derived small EVs could limit the viral infection through competitively inhibit the binding of SARS-S to ACE2 expressed on alveolar type II cells [194].