IL6 and COVID-19: COVID-19–induced inflammation elevates key cytokines such as IFN-γ, IL-1β, and IL-6, which potently induce IDO1 expression in immune cells including macrophages, fibroblasts, and dendritic cells (Figure 4) (Anderson et al., 2021; Chilosi et al., 2022; Dehhaghi et al., 2024).