Monocytes have been implicated in the spread of C. pneumoniae from the primary site of infections (Beagley et al., 2009), but human monocytes were able to suppress C. trachomatis growth via IDO-dependent (Carlin and Weller, 1995) and IDO-independent mechanisms (Koehler et al., 1997; Nettelnbreker et al., 1998; Marangoni et al., 2014). Here, IDO1 is linked to infection.