IDO1 and lung adenocarcinoma: Given that cytokines have the capacity to stimulate the expression of enzymes such as IDO1 and TDO2, and taking into account that lung adenocarcinoma is thought to originate from type-II alveolar epithelial cells that are capable of producing IL-1β [26, 27], we hypothesized that IL-1β stimulates enzymes that facilitate Kyn production in lung adenocarcinoma cells.