DPP4 and diabetes mellitus: A possible link between diabetes, inflammation, and COVID-19 could be the angiotensin-converting enzyme 2 (ACE2) and the dipeptidyl peptidase-4 (DPP4) receptors, which are expressed by pulmonary cells and can act as receptors for coronaviruses and, at the same time, are expressed in extrapulmonary tissues, playing an important role in the regulation of metabolic and inflammatory homeostasis [30].