DPP4 and COVID-19: Moreover, recent studies have shown that some primary receptors for SARS-CoV-2, such as dipeptidyl peptidase 4 (DPP4) and angiotensin converting enzyme 2 (ACE2), are highly expressed in intestinal epithelial cells and may modify the gut microbiome and increase the levels of opportunistic pathogens in COVID-19 patients, which further leads to immune dysregulation and increases the mortality rate of COVID-19 (Olivares et al., 2018; Liskova et al., 2021; Penninger et al., 2021; Posadas-Sánchez et al., 2021).