DPP4 and chronic obstructive pulmonary disease: According to ACE2, it is recognized as the main receptor for SARS-CoV-2 and it is a requirement for COVID-19 virus entry combined with TMPRSS2. High expressions patterns of ACE2 and Dipeptidyl Peptidase 4 (DPP4) can be detected in blood and alveolar lavage fluid in patients with chronic obstructive pulmonary disease and asthma [21, 23].