HAS2 and diabetes mellitus: Risk factors for COVID-19 include cardiovascular disease and diabetes, and such high-risk groups exhibit common metabolic features of low levels of Gln, NAD+, and overproduction of hyaluronic acid (HA).159–161 Levels of Gln and NAD+ cause dysregulation of SIRT1, a key negative regulator of the Hyaluronan synthase 2 (HAS2) gene.162 These metabolic alterations eventually lead to the overproduction of HA and Plasminogen activator inhibitor 1 (PAI-1) and the expansion of Tregs and myeloid-derived suppressor cells (MDSCs) populations.