IL6 and COVID-19: However, a recent, critically discussed rapid review [40] concluded that IL6, an important proinflammatory cytokine, synthesized in fibroblasts, monocytes, T-cells, and endothelial cells [41], is lower in patients with severe COVID-19 compared with other hyperinflammatory states associated with acute respiratory distress syndrome (ARDS), sepsis, or cytokine release syndrome (CRS) [42].