Interferons generated during infection, are potent inducers of indoleamine-2,3-dioxygenase and KMO (Carlin et al., 1989, Alberati-Giani et al., 1996, Silva et al., 2002, Brooks et al., 2016), and corticosteroids produced during stress activate TDO (Green and Curzon, 1975, Green et al., 1976, Nakamura et al., 1987, Young, 1981, Zunszain et al., 2012). Here, TDO2 is linked to infection.