IL6 and COVID-19: Interestingly, IDO1 inhibitors suppress the COVID-19-induced pro-inflammatory cytokine release, including TNF-α, IL-6, IL-1α and IL-1β in isolated PBMCs derived from COVID-19-infected rhesus macaques and lower mortality among critically ill patients with COVID-19 (Song et al., 2020; Agrawal et al., 2021).