ACE2 and susceptibility to pneumonia measurement: Angiotensin-converting enzyme 2 (ACE2), the host receptor of SARS-CoV-2, is a negative regulator of MAPK signalling and thus efficiently prevents both the activation of MAPKs and pneumonia caused by exposure to lipopolysaccharide (LPS) [50], bleomycin [224], cigarette smoke [225] or particulate matter 2.5 (PM2.5) [226].