ABSTRACT
Angiotensin-converting enzyme-2 (ACE2) has been recognized as the binding receptor for the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Flow cytometry demonstrated that there was little to no expression of ACE2 on most of the human peripheral blood-derived immune cells including CD4+ T, CD8+ T, activated CD4+ /CD8+ T, Tregs, Th17, NKT, B, NK cells, monocytes, dendritic cells, and granulocytes. There was no ACE2 expression on platelets and very low level of ACE2 protein expression on the surface of human primary pulmonary alveolar epithelial cells. The ACE2 expression was markedly upregulated on the activated type 1 macrophages (M1). Immunohistochemistry demonstrated high expressions of ACE2 on human tissue macrophages, such as alveolar macrophages, Kupffer cells within livers, and microglial cells in brain at steady state. The data suggest that alveolar macrophages, as the frontline immune cells, may be directly targeted by the SARS-CoV-2 infection and therefore need to be considered for the prevention and treatment of COVID-19.